exploring bulk insulating surfaces and ultra thin insulating films, at the atomic level by uhv-spm...

TTHHÈÈSSEE

En vue de l'obtention du

DDOOCCTTOORRAATT DDEE LL’’UUNNIIVVEERRSSIITTÉÉ DDEE TTOOUULLOOUUSSEE

Délivré par l'Université Toulouse III - Paul Sabatier Discipline ou spécialité : NANO-PHYSIQUE.

JURY

Louis PORTE, Professeur à l’Université Paul Cézanne Aix-Marseille III. William SACKS, Professeur à l’Université Pierre et Marie Curie Paris VI. Roland CORATGER, Professeur à l’Université Paul Sabatier à Toulouse. Tomaso Zambelli, Habilité à Diriger des Recherches à l’Institut f. Biomedizinische Technik, Zurich. Jacques VIGUÉ, Directeur de recherche CNRS à l'Université Paul Sabatier à Toulouse. Sébastien GAUTHIER, Directeur de recherche CEMES-CNRS à Toulouse.

Ecole doctorale : Sciences de la Matière Unité de recherche : Centre d’Elaboration de Matériaux et d’Etudes Structurales (CEMES),

UPR 8011 CNRS Directeur(s) de Thèse : Sébastien GAUTHIER Rapporteurs : Louis PORTE et William SACKS

Présentée et soutenue par Miguel Angel VENEGAS DE LA CERDA Le 30 Septembre 2008

Titre : Etudes expérimentales de surfaces et de films minces isolants par microscopie à

sonde locale sous ultra vide

UNIVERSITE TOULOUSE III-PAUL SABATIER U.F.R P.C.A

ECOLE DOCTORALE SCIENCES DE LA MATIERE

DOCTORAT DE L’UNIVERSITE DE TOULOUSE III - Paul Sabatier. Discipline : NANO-PHYSIQUE, NANO-COMPOSANT,

NANO-MESURE.

Miguel Angel VENEGAS DE LA CERDA.

Etudes expérimentales de surfaces et de films minces isolants par microscopie à sonde locale sous ultra vide

Thèse dirigée par Sébastien GAUTHIER

Date de la soutenance, le 30 Septembre 2008

Jury :

Louis PORTE, Professeur à l’Université Paul Cézanne Aix-Marseille III,

IM2NP/ L2MP-Marseille, Rapporteur. William SACKS, Professeur à l’Université Pierre et Marie Curie Paris VI,

INSP/STMSG- Paris, Rapporteur. Roland CORATGER, Professeur à l’Université Paul Sabatier,

CEMES/GNS-Toulouse. Tomaso Zambelli, Habilité à Diriger des Recherches à l’Institut f. Biomedizinische Technik,

ETH-Zurich, Switzerland. Jacques VIGUÉ, Directeur de recherche CNRS,

IRSAMC/LCAR-Toulouse. Sébastien GAUTHIER, Directeur de recherche CNRS,

CEMES/GNS-Toulouse.

5

Etudes expérimentales de surfaces et de films minces isolants par microscopie à sonde locale sous ultra vide.

Dans ce travail de recherche nous avons réalisé des études expérimentales de surfaces et

de films minces isolants par microscopie à sonde locale sous ultra vide à température ambiante. En particulier nous avons utilisé la microscopie à effet tunnel (STM) et la microscopie à force atomique en mode non-contact (NC-AFM). Nous présentons des résultats qui concernent deux systèmes : la surface isolante KBr(001) et le film mince isolant d'alumine formé par oxydation de la surface (110) d'un cristal de NiAl.

• Dans un premier temps, nous avons modifié la tête du microscope STM/AFM en changeant le dispositif de détection optique des oscillations du cantilever. L'amélioration importante apportée nous a permis de mener une série d'expériences sur la surface de clivage du cristal ionique KBr(001). Nous avons mis en évidence à partir d'images de marches monoatomiques acquises avec la résolution atomique des changements de contraste réversibles déclenchés par le passage de la pointe sur le bord de marche. Ces observations ont été interprétées en terme de déplacements atomiques à l'extrême apex de la pointe entraînant un changement de signe de l'ion terminal, qui détermine le type d'image observée. Cette hypothèse a été confirmée en analysant les courbes expérimentales donnant la force entre la pointe et la surface en fonction de la distance pointe-surface. Cette étude a été suivie de quelques tentatives pour imager des molécules organiques sur cette surface isolante.

• Le système Pd/Al10O13/NiAl(110) à été étudié par microscopie à effet tunnel. La

couche d'oxyde est formée par l'exposition à O2 à une température particulier (~280°C) de la surface (110) d'un cristal de NiAl sous ultravide. Nous avons obtenu des images en résolution atomique qui nous ont permis de remonter à la structure atomique de la couche isolante d'alumine, de stœchiométrie Al10O13, ainsi que de l'un des types de parois de domaine du film isolant. Nous avons également réalisé de mesures de façon à caractériser ses propriétés électriques à une échelle nanométrique. Ce substrat nous a ensuite servi à faire croître des agrégats métalliques de palladium dans différentes conditions. La répartition des îlots ainsi formés n'est pas homogène, certains défauts étant décorés par le palladium. La possibilité d'utiliser ce substrat pour réaliser des jonctions métal-molécule-métal planaires connectables à un système de mesure extérieur par la méthode du nano-stencil développée au CEMES a ensuite été envisagée.

Les perspectives ouvertes par ce travail dans le domaine de l'électronique moléculaire sont

discutées dans la conclusion du manuscrit. mots clés : Microscopie à effet tunnel, microscopie à force atomique, mode non-contact, spectroscopie de force, physique des surfaces, surfaces isolants, ultra vide

7

Exploring bulk insulating surfaces and thin insulating films, at the atomic level by UHV-SPM techniques.

In this research work we carried out experimental studies of insulating surfaces and insulating thin films surfaces by scanning probe microscopy techniques under ultra vacuum at room temperature. In particular we used Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy in the non contact mode (NC-AFM). We present experimental results on two systems: the insulating surface KBr (001) and the thin insulating alumina film formed by oxidation of the (110) surface of a NiAl crystal.

• Initially, we modified the STM/AFM head by changing the optical device of the detection of the cantilever oscillations system. This crucial improvement enabled us to carry out a series of experiments on the (001) cleaved surface of the ionic crystal KBr at the atomic level. We have evidence obtained from atomic resolution images, that shows a change in contrast when the tip passes through a step edge. Where we could observe a systematic and reversible change in the contrasts of the image. These observations were interpreted in terms of the atomic displacements of the last extremity tip apex, involving the change of last ion sign. This change of ion determines the type of image observed at atomic resolution. This assumption was confirmed by analyzing the experimental curves giving the force between the tip and the surface according to the tip to surface distance. This study was followed of some attempts to images organic molecules on this insulating surface.

• The Pd/Al10O13/NiAl (110) system was explored by STM and scanning tunneling spectroscopy (STS), where the oxide layer is formed by exposing the NiAl(110) surface to an oxygen atmosphere, while keeping the sample temperature at (~280°C) under ultra-high vacuum. The atomic resolution images obtained enabled us to go down into the atomic structure of the insulating alumina layer, with stoichiometry Al 10O13. In addition, it could be possible to atomically resolve a unit cell of one type of defects formed. We also carried out electrical measurements in order to characterize its electric properties on a nanometer scale. This substrate was then used to growth metal palladium aggregates under various conditions. The distribution of the small formed islands is not homogeneous; certain defects of the alumina film are being decorated by palladium aggregates. The possibility to use this substrate to carry out suitable experiment for interconnections of planar junctions involving a metal-molecule-metal junction onto an external measurement system may probably be considered. This can be held by the nano-stencil technique, developed in the GNS at CEMES

Prospects opened by this work in the field of molecular electronics are discussed in the conclusion part of the manuscript. Keywords : Scanning tunnelling microscopy, atomic force microscopy, non contact mode, force spectroscopy, surface physics, insulating surfaces, ultra high vacuum

9

LABORATOIRE DE RATTACHEMENT : Centre d’Elaboration de Matériaux et d’Etudes Structurales (CEMES),

UPR 8011 CNRS 29 rue Jeanne Marvig (BP 94347)

31055 TOULOUSE Cedex 4 FRANCE

11

Remerciements

Tout d’abord, je remercie énormément le Conseil des Sciences et Technologies de

l’état d’Aguascalientes (CONCYTEA) pour tout le soutien qu’il m’a apporté. Grâce à cette

bourse de l’état d’Aguascalientes j’ai eu la possibilité de poursuivre mes études à l’étranger

en nanosciences, merci beaucoup.

J’exprime tous mes remerciements aux rapporteurs, Louis PORTE et William SACKS,

de s’être intéressé à ces travaux.

J’exprime tous mes remerciements aux membres du jury, Roland CORATGER,

Tomaso ZAMBELLI et Jacques VIGUÉ pour avoir accepté de faire partie du jury et pour avoir

pris de leur temps pour lire cette thèse.

Je voudrais aussi beaucoup remercier M. Jean-Pierre LAUNAY, directeur du Centre

d’Elaboration des Matériaux et d’Etudes Structurales (CEMES), pour m’avoir aidé à tout

moment et pour cette grande opportunité d’avoir fait mon doctorat au CEMES.

Je suis très reconnaissant envers Christian JOACHIM pour deux raisons:

Tout d’abord pour sa motivation constante, son énergie transmise, ses conseils, son

attention, sa patience, son amitié, et son courage qu’il m’a consacrés avec grande sympathie,

toujours et depuis notre rencontre aux États Unis à Nanotech2003. Merci de m’avoir transmis

ces éléments clés qui m’ont donné la force de poursuivre mes études dans le domaine

fascinant des nano sciences.

En second, car grâce à lui j’ai eu la belle opportunité de venir en France poursuivre

mes études dans un des meilleurs groupes de recherche du monde en Nano Sciences, le GNS.

J’exprime toute ma reconnaissance et mes plus sincères remerciements de manière très

particulière à mon directeur de thèse, M. Sébastien GAUTHIER. Grâce à sa patience, sa

compréhension, son attention, ses orientations, sa motivation, ses conseils, et ses efforts j’ai

pu finir cette thèse. Merci à lui qui dans les moments les plus difficiles et critiques de ce

travail m’a toujours apporté tout son soutien et son aide. Grâce à ses nombreux enseignements

et connaissances, ce travail a acquis la qualité scientifique nécessaire. Merci toujours !

Je remercie spécialement de tout mon amour ma femme Jessy, qui a toujours été avec

moi et m’a toujours soutenu dans les moments les plus durs de cette aventure, à qui je dédie

tous mes efforts. Ainsi qu’à ma famille pour tout son soutien.

Merci à David MARTROU pour ces apports et conseils partagés pendant la partie

expérimentale de cette thèse.

Mille mercis à Haiming GUO pour m’avoir enseigné à travailler l’Ultra Vide d’une

façon toujours très sympa et très attentive.

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Je remercie également Jose ABAD, « Départamento de Física, Centro de Investigación

Optica y Nanofìsica-CIOyN », de l’Université de Murcia en Espagne pour tout ses apports qui

ont été d’une grande aide pour les expériences en AFM.

Je dois exprimer mes remerciements les plus sincères à Olivier GUILLEMET pour ses

conseils de préparation d’échantillons et déposition de molécules qu’il m’a toujours donnés

d’une façon très amicale et sympathique, sa précieuse aide, dans mes études expérimentales

en microscopie de champ proche sous ultra vide, a été de grand aide.

Je remercie vivement Jérôme POLESEL pour les petites conversations très

enrichissantes, son travail de thèse m’a toujours apporté des éléments indispensables pour la

compréhension de la machinerie d’AFM en mode dynamique.

Je dis merci à Henri- Pierre JAQUOT pour ses conseils en chimie qu’il m’a donné

toujours avec grande sympathie.

Je remercie à Christophe DESHAYES pour sa précieuse collaboration dans les images

en SEM des Cantilevers, et à Théophile Noël pour sa collaboration dans les études

préliminaires pour l’implémentation de la Diode super luminescent.

A E. Meyer, L. Nony pour leurs précieuse conseils et orientations pour effectuer le

changement du système optique, je les dis merci beaucoup.

Un grand merci à tous ceux qui m’ont accueilli au CEMES ainsi qu’à tout le personnel

du laboratoire. Je voudrais remercier dans son ensemble le groupe GNS ainsi que le CEMES

pour ce grand accueil et le soutient lors de l’élaboration de ce travail.

Et finalement merci à tout le monde pour les petites aides qu’ils m’ont toujours

offertes.

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Dedicada a Jessy con todo mi amor: por todo su gran apoyo, esfuerzo, paciencia,

comprensión y amor. Que sin su soporte, no hubiéramos sobrevivido en Toulouse.

Millón de gracias Bebe, te amo.

Contest

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1. General Introduction 19

1.1. Antecedents: the nano-stencil technique……………………….………….………19

1.1.1. Molecular electronics…………………………………………………………19

1.1.2. Connecting a single molecule…………………………………….…..…..…..20

1.1.2.1. The STM metal-vacuum-molecule-substrate junction………........…20

1.1.2.2. Mechanically Breakable Junction………………………….…….....21

1.1.2.3. The CNT–MB–CNT covalent junction approach.……….………….21

1.1.2.4. The 4-ISPM Interconnects approach.………….…….......................22

1.1.3. The Nanostencil technique…………………………………………………....24

1.2. Contributions of this work……………………………………………………....…27

1.2.1. Non – Contact Atomic Force Microscopy…………………………….…...…27

1.2.1.1. NC-AFM history…………………………………...……………......27

1.2.1.2. NC-AFM principle………………………………………………….29

1.2.1.3. Optical beam deflection sensor………………………………..........32

2. Optimization of the AFM beam deflection sensor 35

2.1. Introduction…………………………………………………………………… ...….35

2.2. Sensitivity of the optical beam method………………………………………...….36

2.3. Source of noises………………………………………………………………..……38

2.3.1. The shot noise………………………………………………….………..……39

2.3.2. The Johnson noise……………………………………..…………………...…40

2.3.3. The optical source noise……………………………………………….…...…40

2.3.3.1. Laser noises………………...………………………………………….…40

2.3.4. The cantilever thermal noise………………………………………..….….….41

2.4. Modifications of the RT AFM Omicron head…………………….…….………...42

2.4.1. The superluminescent laser diode……………………………………….....…43

2.4.2. The single mode optical fiber……………………………………………....…44

2.4.3. The focusing optics…………………………………………………….…..…46

2.4.4. New characteristics of the beam deflection sensor…………………….…......47

2.5. Noise spectral analysis………………………………………………………...……48

2.5.1. Estimation of the equivalent input noise of the new beam deflection

system………………….……………………………………………………..52

2.5.2. Estimation of the cantilever temperature at maximum laser power level…….52

Contest

16

2.6. Sensitivity measurement and amplitude calibration…………………………..…54

2.7. Conclusions and perspectives………………………………..……………...…..…56

3. NC-AFM study on KBr(001) 59

3.1. Introduction…………………………… …………………….………………….…..59

3.2. Experimental methods……………………………………….………………….….60

3.2.1. Force spectroscopy…………………………………..……..……………..…..61

3.2.1.1.Extraction of the force from ∆f(z) using the Sader-Jarvis formula.............61

3.2.1.2.Description of the technique……………………………………………...64

3.3. Results…………………………………………………………………...……..……66

3.3.1. Two types of atomic resolution images……………………….……...………66

3.3.2. Atomically resolved force curves……………………………...…..…………67

3.3.3. Spontaneous tip polarity reversal………………………………......................70

3.3.4. Tip polarity reversal on a monoatomic step………………...…………….…..71

3.3.5. Potential energy surfaces of the two-level system………….……….…….….73

3.3.6. Tip polarity reversal on two successive monoatomic steps.………………….74

3.3.7. Discussion of the topography results…………………..………………….….78

3.4. Force spectroscopy with the bistable tip……………………..……………...….…79

3.5. General discussion…………………………………………..……………………...82

3.6. Conclusion………………………………………………………….…………….....84

4. The Pd/Al10O13/NiAl(110) system 87

4.1. Introduction…………………………………………………………………… ....…87

4.2. The NiAl(110) intermetallic alloy………………………………………….…..…..88

4.2.1. Preparation procedure…………………………………………….……..……89

4.3. The alumina film formed on NiAl(110)…………………….………………...……90

4.3.1. Preparation procedure………………………………………….……..………90

4.3.2. The oxide film meso scale morphology……………………....……..…….….91

4.3.3. Intermediate scale images…………………………………………..………...93

4.3.4. Structural relation between the alumina unit cell and

the NiAl(110) substrate………………………………………………………..94

4.3.5. Higher Resolution image, with three domain boundaries………….……...….95

4.3.6. Atomic resolution image of the oxide film surface…………...…….……..….97

4.3.7. The atomic model of the A domain………………………….…….…………98

Contest

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4.3.8. Interpretation of the atomic resolution image of domain A…....….…………99

4.3.9. The IA-Anti Phase Domain Boundary atomistic model……….………...….100

4.3.10. Interpretation of the atomic resolution image of the IA – APDB…..…..…...101

4.3.11. Meshing the surface with the repetitive pattern………………………..……101

4.4. Scanning tunneling Spectroscopy measurements…………………………….....103

4.4.1. Introduction……………………………………………………………….....103

4.4.2. Determination of the electronic gap for a 1700 L alumina film……….........104

4.4.3. Apparent thickness of the oxide film as a function of the imaging

bias voltage…………………………………….…………………………...….105

4.4.4. STS measurements in the gap of two different oxides……………...........…108

4.5. Palladium growth…………..……………….……………………………….……110

4.6. Conclusion………………………………………………………..………….…….111

5. Conclusions and perspectives 113

5.1. Conclusions…..………………………………………………….…………………113

5.2. Perspectives ……………………………………………..………….………..……115

5.3. Molecules visualization attempts…………………..…………………….…………115

5.3.1.1.Deposition by sublimation under UHV………….………………...……115

5.3.1.2.Deposition from a chloroform solution…………...……………………..116

5.3.2. Nano manipulation……………………………………...……………...……118

6. References 123

7. Substantial summary in French 131

Chapter 1 General Introduction

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CHAPTER 1

General Introduction

Introduction

This chapter is divided into two parts. The first one is dedicated to introduce the reader into

the domain of interest. The different methods that have been recently developed to electrically

connect a molecule between electrodes will be shortly discussed. In particular, we will go

more into detail in the developments made in the GNS group. We will show some

achievements already performed and what are the future goals to be achieved. In the second

part of this chapter, we present an overview of how this thesis has contributed to some of the

goals exposed in the first part. The experimental results that will be developed all along this

thesis are then briefly presented.

1.1 Antecedents: The nano-stencil technique.

1.1.1 Molecular electronics

The constant miniaturization of the electronic components driven by the needs of the

microelectronics industry has shown tremendous progress in the construction of smaller and

smaller electronic devices. A common component normally used as a building block of an

electronic circuit is the transistor, which recently could be considerably reduced in size in the

sub-micrometer range, with a gate thickness below 50 nm and limited to 15 nm

[Stephen2006]. However, this miniaturization of electronic components is starting to be

affected by technological and fundamental limits, from where other miniaturization

alternatives have been proposed. Novel solutions provided by the molecular electronics


20

community are being developed in order to propose molecules as new candidates for nano-

meter sized building blocks with the goal of integrating them into future electronic

components. To achieve this goal, new challenges need to be tackled. In particular, it is

required to develop methods to connect single molecules to macroscopic conductive

electrodes. In the following, we discuss the most interesting approaches to address a single

molecule being explored nowadays.

1.1.2 Connecting a single molecule.

1.1.2.1 The STM metal-vacuum-molecule-substrate junction.

Figure 1.1: LT-STM image showing the switching of a single naphthalocyanine molecule on

NaCl/Cu(111) by tunnelling current. From [Liljeroth2007].

The most flexible and easy way to electronically access and to contact a single molecule today

in Ultra High Vacuum (UHV) conditions has been rendered possible thanks to the incoming

of Scanning Probe Microscopy (SPM) techniques and surface science developments. In

particular, Scanning Tunnelling Microscopy (STM) can perform experimental studies of

organic molecules deposited on the following substrates: metallic surfaces, insulating films

grown on metallic substrates, semiconducting surfaces and/or insulating films on

semiconducting substrates. A very nice example can be found in [Liljeroth2007] that shows

the experimental study of a single naphthalocyanine molecule (fig. 1.1a) deposited on a

NaCl(100) bi-layer previously grown on top of Cu(111). Interestingly, it was found that this

molecule displays a two-state behavior due to a tautomerization involving the displacement of

two hydrogen atoms from one pair of nitrogen atoms to the other in the central cavity of the

molecule (shown in fig. 1.1c). This activation was produced by a tunnelling current injected

with the STM tip in the red spot of fig. 1.1b. The corresponding STM images show how these

proton transfers affect the LUMO orbital, inducing a transition between a high- and a low


21

current state. This experiment shows an important progress for novel molecular logic devices

with functional molecules. One major drawback of this STM-based approach is that it is

limited to two electrodes and the geometry of connection cannot be chosen at will.

1.1.2.2 Mechanically Breakable Junctions.

Figure 1.2: The mechanically breakable junction schematic with (a) the bending beam, (b) the counter supports, (c) the notched gold wire, (d) the glue contacts, (e) the piezo element,

and (f) the glass tube containing the molecular solution.

Another approach described in 1997 by M. Reed and J. Tour [Reed1997], shows an

experiment at room temperature with a mechanically controllable break junction (MCB) (Fig.

1.2). In this approach, a notched metal wire is glued onto a flexible substrate and is fractured

due to the bending of the substrate by a piezo element. This mechanism can establish an

adjustable tunnelling gap. A large reduction factor between the piezo elongation and the

electrode separation ensures an inherently stable contact or tunnel junction. The wire contacts

are atomically sharp when broken, as demonstrated in the conductance quantization observed

on the I(V) characteristics of the junction. In the experiments reported here, benzene-1,4-

dithiol molecules were adsorbed from a solution on the surface of the two gold electrodes of

the break junction. This results in the formation of a self-assembled monolayer (SAM) on the

gold electrodes. Characteristic features, detected on the I(V) characteristics of the junction

were attributed to the presence of one or a few molecules positioned between these electrodes.

One major drawback of this approach is that it is not possible to image the junction in order to

establish its precise structure.

1.1.2.3 The CNT–MB–CNT covalent junction approach.

The idea is to use carbon nanotubes as intermediate electrodes to connect a molecule. Carbon

nanotubes have lengths in a range (several thousands of nm) which makes them easy to


22

connect to metallic electrodes and diameters (1 to 2 nm) that are close in dimensions to

smaller objects, like molecules.

Figure 1.3: A molecular bridge composed of two or three molecules connected

between two single wall carbon nanotubes.

In the work reported by Nuckolls and co-workers, single wall carbon nanotubes (SWNT),

deposited on a substrate are locally cut by oxidation. This cut produces a 10 nm gap and

leaves the terminations of the SWNT with specific ending chemical groups (carboxylic acid

groups). These carboxylic acid groups can then create covalent bonds with the oligoaniline

type molecules of the solution in which this system is subsequently introduced. These

molecules serve as molecular bridge connectors in between the 10 nm gap of the nanotube

[Nuckolls2006]. Note that the gap is not small enough to be occupied only by a single

molecule but by a bridge of two or three of them, furthermore the precise structure of the

junction is not known.

The next approaches have the ambition to avoid the drawbacks of these methods by

combining the visualization and manipulation capabilities of STM or AFM with a planar

architecture.

1.1.2.4 The 4-ISPM Interconnects approach.

Another strategy currently being developed is based on the facilities provided by the

commercial UHV four independent scanning probes microscope (4-ISPM), from Omicron

Nanotechnology [Omicron2008a], in combination with recent surface science progress

[Yang2007].


23

The 4-ISPM (figure 1.4a) is integrated in a common stage and equipped with a scanning

electron microscope. The entire equipment is a sophisticated analytical instrument, designed

for local and non-destructive four point contact for electrical measurements and in situ

function tests of mesoscopic devices.

Using the four tips of this device to directly connect a single molecule is not possible, due to

the size of the tips. One needs to use intermediate electrodes. An approach based on the

manipulation of gold islands on MoS2 was recently proposed [Yang2007]. It is illustrated in

figure 1.4b. Four islands are used to connect a single molecule. It is important that the islands

have a small height, to allow STM or AFM imaging in the inter-electrode area. This is the

case for the gold islands that are grown on MoS2. In addition, it turns out that these islands

can be manipulated so that the relative position of the four electrodes can be adapted to the

molecule to be connected.

This strategy faces two major challenges:

Addressing the islands will require building ultra sharp tips.

Finding metal/substrate systems where the substrate has the desired electronic

properties (it should ideally have a large band gap) and the metal grows in a 2D mode to get

large islands with a small height is a difficult task because, generally, for thermodynamic

reasons, metals tend to grow 3D on these low energy surfaces (note that MoS2 is a

semiconductor, but with a rather low gap of ~1.2 eV [Lauritsen2004]).

a) b)

Figure 1.4: a) The four STM tips stage, with the sample in the center and b) atomistic view of

the proposed interconnection.


24

The next approach is similar to the present one, except that fixed electrodes are used instead

of four mobile STM tips. It develops the idea of using intermediate scale electrodes. For this

reason, it suffers from the same limitations concerning the metal/substrate system.

1.1.3 The Nanostencil technique.

The nanostencil technique consists in using a stencil deposition mask embedded in an AFM

cantilever to make nanoelectrodes (figure 1.5a). As shown in figure 1.5b, a membrane is

managed in the side of the hollow tip of a cantilever by Focused Ion Beam (FIB). Different

patterns can then be drilled by FIB (figure 1.5c to f).

Figure 1.5: (a) Schematic view of the geometrical configuration for the AFM nanostencil technique. (b) Scanning electron microscopy image showing a recessed box in the rear face of

AFM tip, which was first thinned down to 80 nm by FIB. (c)-(f) different patterns drilled in the AFM tip wall. From [Guo2007].

Such a cantilever can be used in a static or dynamic way. In the static nanostencil technique,

the pattern defined by the aperture in the tip is simply replicated on the surface, with a

determined deformation, due to the geometry (figure 1.5a) and in certain cases to surface

diffusion. In the dynamic nanostencil technique, the sample is moved in a predefined way

during the deposition through the cantilever aperture, in order to draw the desired pattern on

the substrate surface. A very important specificity of this technique is that the surface can be

imaged by AFM before and after the fabrication of the device, with the tip that was used for


25

the fabrication. This is crucial to achieve nanometer scale alignment between two stencil

levels.

One major drawback of this technique is that the aperture in the cantilever tends to clog

rapidly. The only way to limit this phenomenon is to minimize the amount of deposited

material, hence the size of the deposited pattern. This is done by combining two stencil

lithography steps. The first one uses a static micro stencil mask built in a thin silicon nitride

membrane to fabricate micro-electrodes (figure 1.6a). This is done on a special post in the

preparation chamber. The deposited micro electrodes connect a small area (figure 1.6d and e),

where the subsequent stenciling step can be performed, to micro pads (figure 1.6c) which will

be connected to the I(V) measuring device outside the UHV chamber. This last connection is

established by a movable array of conducting microcantilevers.

Figure 1.6: (a) Optical microscopy image of a static stencil silicon nitride membrane with

microelectrodes patterns. (b) 3D view of the microelectrodes protected by the shadow of the AFM cantilever. (c) Contact AFM image of Au microelectrodes deposited on a SiO2 substrate through a static stencil mask, size: 73µm × 39µm. (d) and (e) A star and two wires have been

deposited in the region delimited by the micro-electrodes. From [Guo2007].

Figure 1.6 shows the schematic view of the setup developed by modifying a variable

temperature AFM/STM Omicron head [Omicron2008b]. The sample is placed on an X-Y

piezo-actuated table from Piezosystem Jena (Germany) equipped with capacitive sensors to

linearize the displacement in a 100 x 100 µm2 area. This table allows to accurately position

the cantilever with respect to the microelectrodes for dynamic stencil deposition. Note that

this table is also useful for large scale AFM imaging, beyond the range of the piezo ceramics

of the omicron head (5 x 5 µm2). We would like to mention that we had the opportunity to

collaborate in this project with a personal contribution that consisted in the conception and

construction of the electronic interface used to move this table.

Metal evaporation for deposition is performed from an effusion cell by collimating the

evaporated beam onto the FIB-drilled AFM cantilever by a series of three diaphragms with


26

decreasing diameters. In addition, a micro-comb made with an array of ten metallic

cantilevers (xyz µPS), can be brought in contact with the micro-pads of the micro-electrodes

thanks to a X-Y-Z micro positioning stage in order to electrically connect the nanodevice to

external measuring instruments [Ondarçuhu2000].

Figure 1.7: The nanostencil machine. On the left, a picture of the X-Y piezo actuated table, mounted on the VT AFM head. On the right, a schematic diagram of the experimental setup.

The substrate is mounted on the X-Y nano-positioning stage; the collimated metal vapor beam is guided from the evaporator to the substrate through the apertures in the AFM cantilever. The X-Y-Z micro positioner (xyz µPS) is used to position the microcantilever array with the

help of an optical microscope.

The size of the patterns that can be fabricated by the dynamic nanostencil method are

presently limited to a few 10 nm. That is the reason why, as for the preceding method

described in 1.1.2.4 intermediate electrodes have to be used, as illustrated in figure 1.8.

Figure 1.8: This figure suggests how a molecule could be electrically addressed using Pd

clusters grown on an alumina/NiAl(110) substrate.


27

This figure is built on an STM image of Pd islands grown on a thin alumina layer on

NiAl(110), a system that will be studied in chapter 4.

1.2 Contributions of this work

This work reported in this thesis participated in the development of the nanostencil project

along two directions:

• Improvement of the AFM

It is very important to visualize all the steps of the fabrication of the device by imaging. The

only technique able to image single molecule on insulating substrate is AFM in the so-called

Frequency Modulation (FM-AFM) or Non-Contact mode (NC-AFM). This technique is

briefly presented in the next part of this chapter.

• Investigation of Pd/alumina/NiAl(110)

As already mentioned, the nanostencil technique requires finding a suitable metal/insulator

system. The ideal system would exhibit a two-dimensional, epitaxial growth of a crystalline

metallic deposit. Unfortunately, the growth mode for most metal/insulator system is 3D,

mainly because the surface free energy of insulators is usually much lower than that of metals.

Nevertheless, one can hope to achieve 2D growth even in these conditions if kinetic

limitations come into play. We have chosen to investigate the alumina/NiAl(110), which is

made by oxidation of NiAl(110), because it was reported that Pd grows on this surface in the

form of flat crystalline islands [Yoshitake2006, Hanssen1999], which, as already suggested

(figure 1.8) could be used as intermediate scale electrodes to electrically address a molecule.

The results which have been obtained from STM and STS measurement on this insulating

layer as well as preliminary studies of Pd deposition on it are reported in chapter 4: "The

Pd/Al10O13/NiAl(110) system".

1.2.1 Non–Contact Atomic Force Microscopy

1.2.1.1 NC-AFM history

Since its invention in 1986 [Bining1986], Atomic Force Microscopy has developed beyond

all expectations. It is nowadays used in many scientific and technological domains to

characterize the surface of a large variety of materials in different environments, ranging from


28

ultra high vacuum (UHV) to liquids. This success relies heavily on the availability of

microfabricated force sensors, which convert the force felt by a sharp tip positioned in the

vicinity of the sample into a displacement, which can be measured by different techniques.

These sensors have generally a diving board, cantilever beam geometry. Today, a large

number of different cantilever types, differing by their resonant frequency f0, stiffness k, and

quality factor Q are commercially available (figure 1.9).

AFM was originally used in the static mode, where the quasi-static deflection of a low k

cantilever (typically 0.1 N/m) is used to get a topography of the surface. While this mode can

produce nice images displaying the atomic periodicity of crystalline surfaces, it is not capable

of true atomic resolution except under very special circumstances [Giessibl1992]. The low k

necessary to improve the force sensitivity makes the static mode prone to jump-to-contact

instabilities and the tip-substrate interaction is generally quite strong, due to long-range van

der Waals forces that press the tip against the substrate.

a) b)

Figure 1.9: A typical NC-AFM cantilever chip, from Nano science Instruments, a) The entire chip top view and cross section views, in b) a zoom on the cantilever, top view and side view.

In dynamic modes, which were developed to limit this tip-surface interaction, the cantilever is

vibrated near its resonance frequency. There are two basic methods for dynamic operation:

amplitude-modulation (AM-AFM) and frequency-modulation (FM-AFM) or NC-AFM. In

AM-AFM [Martin1987], the cantilever is driven at a fixed, near-resonance frequency. The

changes in the amplitude and the phase of the oscillator while the tip scans over the sample

surface are then used as imaging signals. Designed originally to use long-range forces

(electrical or magnetic forces) for intermediate scale imaging (10 nm resolution), this mode

was later used at closer distance, to reach the repulsive tip-sample interaction regime. This

"Tapping Mode" [Zhong1993] is now used routinely for most ambient conditions AFM

investigations. It allows to get true atomic resolution [Erlandsson1997], but is limited by its


29

inherent slowness: the time scale for amplitude changes in AM-AFM, given by τ~2Q/ω0, is

proportional to Q. But the performance of cantilevers, limited by their thermal noise, is

inversely proportional to Q. Increasing Q in AM-AFM improves the signal-to-noise ratio, but

in the same time, leads to prohibitively long acquisition times.

The solution to this problem was proposed by Albrecht and co-workers [Albrecht1991] in the

founding paper of FM-AFM. In this method, the cantilever is embedded in a positive

feedback loop that oscillates at the cantilever resonance frequency while another loop

maintains its oscillation amplitude at a pre-set value. In contrast to the AM method, where the

frequency is externally fixed, the resonance frequency of the cantilever in the FM method

varies under the influence of the tip-sample forces. The time scale for these variations is only

limited by the time scale of the oscillation itself (~1/f0). The FM technique is thus potentially

much faster than the AM method.

True atomic resolution was first obtained in NC-AFM by Giessibl [Giessibl1995], and

Kitamura and Iwatsuki [Kitamura1995] in 1995. It has now been achieved on a wide variety

of surfaces (metals, covalent or ionic semiconductors, covalent or ionic insulators) and the

technique, after been used in UHV for a long time, is now adapted to ambient conditions and

to liquids, especially for applications in biology [Fukuma2007].

1.2.1.2 NC-AFM principle

NC-AFM is compared to STM in figure 1.10. In STM, one uses the tunneling current I to

keep the tip-surface distance at a constant value while scanning the surface with the tip. This

is achieved by a feedback loop in which the tunneling current is compared with a preset value

I0, and the resulting error signal I - I0 is used to act on the tip position D via a Proportional-

Integral (P-I) corrector and a piezoceramic actuator (figure 1.10a). The excellent resolution of

STM stems from the exponential dependence of the tunneling current on the distance.

(a)

(b)

Figure 1.10: Comparison between (a) STM and (b) NC-AFM principles.


30

At this level of description, NC-AFM works in a similar way. Instead of using the I(D)

characteristics of the tunneling junction, one relies on the ωres(D) curve of an oscillator that

uses the cantilever as its frequency determining element. The resonant frequency of the

oscillator ωres depends on the force exerted by the substrate on the tip that in turn depends on

the tip-substrate distance, as illustrated by the curve in figure 1.10b. ωres(D) is compared to a

preset value ωres0 and the resulting error signal ωres - ωres

0 is used to act on the tip position D.

The complexity of NC-AFM comes from the way this oscillator is implemented. In the

following, we built it step by step.

The cantilever can be considered as a one degree of freedom harmonic oscillator, described by

a harmonic transfer function )(ωCl (figure 1.11) that writes:

)(

1

)(

)()(

020

220

ωω

ωωωω

ωω

Q

jk

F

zCl

+−== (1.1)

where 0ω is the resonance frequency of the cantilever, k its stiffness and Q its quality factor.

Figure 1.11: Block representing the harmonic transfer function of the cantilever

The block diagram of figure 1.12 shows how the NC-AFM oscillator is built. The cantilever is

inserted in a positive feedback loop including a block of harmonic transfer function )(ωG .

Figure 1.12: Block diagram of the NC-AFM oscillator

The global transfer function reads then:

)()(1

)(

)(

)()(

ωωω

ωωω

GCl

Cl

F

zT

−==

This system will oscillate at ω = ωc if: 0)()(1 =− ωω GCl , that is:

0)()(

1)()(

=Φ+Φ=

cGcCl

cc GCl

ωωωω


31

where ClΦ and GΦ are the arguments of )(ωCl and )(ωG . Using the second condition and

equation (1.1), one obtains:

( ) 01

1arctan

20

20

=

−+Φ

ωωω

ωωQcG

and:

1)]([tan4

1

)](tan[2 2200 +

Φ+

Φ=

cGcGc QQ ω

ωω

ωω (1.2)

This relation shows that, in general, ωc, the resonant frequency of the oscillator differs from

ω0, the resonant frequency of the cantilever. The phase of the controller G fixes ωc. This

oscillator is a phase-controlled oscillator [Dürig1997]. In addition, for an arbitrary phase

setting GΦ , ωc depends on the quality factor Q. This means that if the tip-substrate force is

non-conservative, but becomes dissipative, "topographic" information, which is supposed to

be carried by the conservative force will be mixed with "dissipation" information carried by

the non-conservative component of the tip-substrate force. Relation (1.2) shows that the only

setting which can decouple the "topographic" and "dissipation" channels is 2/π=ΦG . In this

case, ωc = ω0, the resonant frequency of the oscillator coincides with the resonant frequency

of the cantilever.

But with this phase setting, the system described by the diagram of figure 1.12 is not stable; it

is then necessary to control the oscillation amplitude, as shown in figure 1.13.

Figure 1.13: Implementation of the amplitude controlled positive feedback NC-AFM oscillator

Two feedback loops are involved. The first one, in blue, includes a Phase Locked Loop (PLL)

based frequency demodulator whose role is first to measure the oscillation frequency and


32

second to synthesize a sinusoidal carrier at the oscillation frequency and with a phase

satisfying the 2/π=ΦG condition mentioned previously. The second loop, in red, includes

an amplitude demodulator. The oscillation amplitude A is measured and compared to a preset

value A0. The error signal A - A0 is filtered by a P-I corrector and then multiplied with the

carrier produced by the PLL to produce the excitation force necessary to maintain the

oscillation amplitude at A0. From this excitation force, it is straightforward to extract the

energy which is dissipated to maintain the oscillation amplitude at A0. It includes two parts,

the first one that corresponds to the energy dissipated in the cantilever, due to the finite value

of the Q factor and the second one that corresponds to the non-conservative component of the

tip-substrate interaction force.

Combining this oscillator with the distance regulation loop presented in figure 1.10b gives

finally the diagram for the complete control system shown in figure 1.14. It is seen that two

images are simultaneously produced: A "topographic image" built from the piezo position D

necessary to maintain the frequency shift at ∆fc and a "dissipation" image built from the

excitation force necessary to maintain the amplitude at A0.

Figure 1.14: Complete diagram of the NC-AFM

1.2.1.3 Optical beam deflection sensor

Several methods have been proposed and developed to detect the cantilever deflection: the

original electron tunneling method [Binnig1986], optical interferometry [Rugar1989],

piezoelectrical detection [Giessibl2002], piezoresistive detection [Tortonese1993] or optical


33

beam deflection [Meyer1988]. Our setup is based on the Optical Beam Deflection (OBD)

method, illustrated in fig. 1.15.

Figure 1.15:.The optical beam deflection method.

A light source (laser or light-emitting diode) is focused by an optical system of lenses onto the

back of a cantilever. The reflected light goes to a photo-detector sensor that is split into four

quadrants. The small displacements of the extremity of the cantilever are magnified by an

optical lever effect. By combining the electrical signals from the four quadrants of the

photodiode, it is possible to measure the vertical deflection of the cantilever, but also its

lateral torsion, which is related to the friction of the tip on the substrate.

At the beginning of this thesis, the RT Omicron AFM head was equipped with a light emitting

diode (LED). Previous experiments had convinced us that the performance of this head was

severely limited by this light source, mainly because of its large size, leading to the

impossibility to properly focus it on the cantilever. It was then decided to replace the LED by

a superluminescent laser diode. The way this modification was performed and the

characterization of the resulting improvement in the performance of the AFM head are

reported in detail in chapter 2: "Optimization of the AFM beam deflection sensor".

Following this instrumentation work, experiments were carried on a reference surface for NC-

AFM: KBr(001). The initial goal was to test the new setup, but the work went much farther.

The main result is the demonstration that the atomic contrast observed in the dissipation

images of KBr(001) is related to an adhesion hysteresis phenomenon, which involves a two-

level system localized near the apex of the tip. This demonstration is based on the observation

of a reversible change in the polarity of a particular tip when crossing monoatomic steps

while imaging KBr(001) with atomic resolution, coupled with the observation of bistable


34

∆f(z) curves with the same tip. These studies are described in chapter 3: "NC-AFM study on

KBr(001)".

These experiments were followed by different attempts to adsorb and image molecules on the

same surface. Preliminary observations, which are far from being understood, but which

could suggest other, more precise and controlled experiments are briefly discussed in the last

chapter: "Conclusions and perspectives".

Chapter 2: Optimization of the AFM beam deflection sensor

35

CHAPTER 2

Optimization of the AFM beam deflection sensor

2.1 Introduction

In this chapter, we describe the modifications that were made on the RT STM/AFM head

from Omicron [Omicron1997] during this thesis. The experiments performed with the

optimized setup are described in chapter 3: "NC-AFM study of KBr(001) ".

In 2.1, we derive an analytical expression giving the sensitivity (in Vm-1) of the beam

deflection sensor in terms of the different parameters of the device. This expression is useful

to discuss how the sensor can be optimized. But it is not enough to optimize the sensitivity.

Noise should also be considered, as what we are interested in is to improve the signal to noise

ratio. The different sources that contribute to the noise of the instrument are discussed in 2.2.

The modifications of the RT STM/AFM head are described in 2.3. Measurements of the noise

spectrum of the cantilever displacement were performed in order to characterize quantitatively

the new optical beam deflection system. They are presented and analyzed in 2.4. We show

that, once the stiffness of the lever is known (part 2.5), this analysis provides a non-

destructive method to measure the sensitivity of the instrument that is useful to calibrate the

oscillation amplitude of the cantilever, which is an essential experimental parameter for NC-

AFM.


36

2.2. Sensitivity of the optical beam method

Fig. 2.1: a) Optical beam deflection set-up, b) front view of the photo-detectors and scheme of the pre-amplification electronics.

The basic set-up of the Optical Beam Deflection (OBD) method is illustrated in fig. 2.1,

[Sarid1991, Fukuma2005]. A light source (laser or light-emitting diode) is focused by an

optical system of lenses onto the back of a cantilever. The reflected light goes to a photo-

detector sensor that is split in two parts, PD1 and PD2. l is the length of the lever, S the

distance from the end of the lever to the photo detectors, d × d the surface of the optical spot

on the photo-detector, P the optical power at the entrance of the system, and α the power

attenuation coefficient due to imperfections in the optical path (figure 2.1a). The efficiency of

the photo-detector is given by its responsivity η (in A/W), which is the conversion coefficient

between the incident optical power (in W) and the generated electrical output current (in A).

The photo-induced currents i1 and i2 produced by PD1 and PD2 are converted into voltages V1

and V2 by two transimpedance amplifiers whose gain G is given by the resistor RIV, IVRG −= .

Finally a voltage 21 VVV −=∆ is produced by a difference amplifier (figure 2.1b).

The deflection of the lever under the influence of a downward force F is given by

[Sarid1991]:

( )lyEI

Fyyz 3

6)(

2

−=

Where y is the coordinate along the length of the cantilever, E the Young’s modulus, and I the

area moment of inertia. The angle at the extremity of the lever is then given by:

EI

Fl

dy

dz

ly2

2

−===

θ


37

The spring constant of the cantilever, defined as zFk /−= depends on the Young’s modulus

according to 3/3klEI = . With these expressions, the angular deflection of the optical beam

θ2 can be related to the cantilever linear deflection z by lz /32 =θ . The linear displacement

of the reflected optical beam in the photo-detector plane reads then zzlSd β==∆ )/3( ,

where the amplification factor lS /3=β can easily reach 1000, with the typical values

l=0.1 mm and S= 20 mm.

When 0=z , the optical spot is centered on the photo-detector. Then P1+P2=αP with

P1=P2=αP/2. In general, when the cantilever is deflected:

( )

−+=

−∆+=

)(

61

2

21

21 dsdl

SzP

dsd

dPP

αα and ( )

−−=

−∆−=

)(

61

2

21

22 dsdl

SzP

dsd

dPP

αα

Then the power imbalance 21 PPP −=∆ reads ldPSzP /6α=∆ , considering that ds << d (ds

is typically of the order of 20 µm, while d is usually in the millimeter range). Finally, the

cantilever deflection z is detected at the output of the preamplifier by:

zld

SzPRVVV IV σηα =−=−=∆ 6

21 (2.1)

This expression of the sensitivity σ (in Vm-1) suggests that in order to increase the

displacement sensitivity of the technique, d should be as small as possible. But usually, this

quantity cannot be chosen at will, as shown by Sarid [Sarid 1991]. The light beam should be

focused on the cantilever, whose width is in the range of 20 µm. The spot on the photodiode is

then affected by diffraction effects. For the purpose of estimation, one can use the Airy's

formula, which gives the diameter q of the spot produced at a distance R from a screen with a

circular aperture of diameter a illuminated by light of wavelength λ: aRq /22.1 λ= . For a

light beam spot on the cantilever of diameter a, the far field diffraction limited spot size on the

photo-detector is given approximately by: aSd /λ= . Then:

zl

aPRV IV λ

ηα 6−=∆

Note that this new expression is independent of the distance S from the lever to the photo-

detector.

All the parameters that influence the displacement sensitivity of the method appear in this

expression. To increase the sensitivity, one can:

• Increase the light source power P. Current sources have powers in the mW range. A

limitation is the heating of the cantilever by the absorbed light.


38

• Increase α, the light transmission coefficient of the system. This includes minimizing

reflections at the different interfaces, optical absorption by the different components

and using reflective (metallized) cantilevers.

• Use an efficient photodiode, with a as high as possible value for the responsivity

η. There is not much to gain here, as most of the Si PIN photodiodes used for AFM

have comparable responsivities, which, depending on the wavelength, vary between

0.4 and 0.7 A/W.

• Maximize a, the size of the light spot on the cantilever to minimize the diffraction

which tends to enlarge the size of the spot on the photo-detector. Of course a should

stay below w, the width of the cantilever.

• Minimize l, the length of the cantilever. The length of typical commercial cantilevers

varies between 100 µm and 200 µm. This choice is related to the desired resonance

frequency of the cantilever.

• Minimize λ, the wavelength of the light used. Usual diodes used in AFM cover a

range of wavelengths between about 600 nm and 1000 nm.

• Increase the gain RIV of the preamplifier.

Sensitivity is of course an important parameter for the instrument, but it cannot be used

without considering in the same time the noise that will inevitably perturb the measurements.

Optimizing the sensitivity can lead to an increase of the noise. In the following, we consider

the main sources of noise that affect the measurements in NC-AFM.

2.3. Sources of noise

Two types of noise can be distinguished in NC-AFM, the thermal noise of the cantilever, and

the noise of the deflection sensor. The thermal noise of the cantilever is a fundamental noise

arising from the thermal fluctuations of the cantilever. It is intrinsic, and does not depend on

the type of deflection sensor used. The most important noises present in the OBD method

according to [Fukuma2005] are the Johnson noise of the conversion resistors of the

preamplifier, the shot noise of the photo-detectors and the noise of the optical source. In the

following, we discuss first the noise of the OBD sensor, then the thermal noise of the

cantilever. To get the expressions that will be used to interpret the experiments discussed in

the following of this chapter, we need first to introduce the Omicron preamplifier, which in


39

contrast to the basic preamplifier shown in figure 2.1 uses a four-quadrants photodiode (figure

2.2).

Figure 2.2: The Omicron Pre-Amplifier electronic circuit with the four-quadrants photo-detector used in the AFM/STM RT-UHV head.

Each of these four quadrants has its own I-V converter. Three different signals can be

extracted from different combinations of their outputs A, B, C and D:

• The friction signal, related to the lateral displacement of the optical spot on the

photodiode, given by 5× [(A+B) – (C+D)].

• The average (A+B+C+D )/4, which reads:

4/IVtotal PRV ηα= (2.2)

• The topography signal, 5×[(B+D)-(A+C)] expressed from (2.1) as:

zld

SPRV IVFN

65ηα= (2.3)

or:

zVzVld

SV totaltotalFN µ== 120

(2.4)

where totalVµ is the displacement sensitivity.

2.3.1 The shot noise

The shot noise affecting an electrical current is related to the discreteness of the electron

charge. It appears each time the discrete nature of electrons has to be taken into account in the

description of the phenomenon. For instance, it affects the tunneling current across a barrier,

but not the current in a good conductor. In the case of photo-detection, the electrons


40

contributing to the photocurrent are generated by discrete events, described as the absorption

of a photon of the incident light.

The current shot noise Power Spectral Density (PSD) of the photocurrent I is expressed

as ( ) eIfSshotI 2= , where e is the magnitude of the electron charge [Schottky1918]. Each

quadrant of the photo-detector generates its own noise. The corresponding voltage noise after

the transimpedance amplifiers of the circuit presented in figure 2.2 is given by:

( ) PeRRIIIIefS IVIVDCBAshotV ηα22 2)(2 =+++=

The corresponding noise voltage at the output of the preamplifier is then given by:

( ) totalIVIVIVshotV eVRPeRPeRfS

FN

2222 2005025 ==×= ηαηα (2.5)

2.3.2 The Johnson noise

Johnson noise [Johnson1928, Nyquist1928] is the electronic noise generated by the thermal

agitation of the charged carriers inside an electrical conductor at equilibrium. It is a

consequence of the fluctuation-dissipation theorem [Callen1951]. To each of the converting

resistors RIV is associated a voltage PSD:

IVBjohnson

V TRkfS 4)( = (2.6)

where kBT is thermal energy. The corresponding noise voltage at the output of the

preamplifier is then given by:

( ) IVBjohnson

VBjohnson

VBjohnson

VBjohnson

VAJohnson

V TRkSSSSfSFN

4005)( 2 =×+++= (2.7)

2.3.3 The optical source noise

Light emitting diodes (LED) and lasers are the two most common light sources used in AFM.

Lasers are affected by specific noise, which we briefly describe in the following. LED have

also their specific noise and suffer from a major drawback: the size of their equivalent source

is usually quite large, making them difficult to focus. This is the main reason for the head

modification that is described in part 2.4.

2.3.3.1. Laser noises

It is well known that the beam intensity, the beam direction, and the profile of the beam of a

laser fluctuate. The main source of these fluctuations is mode hopping which corresponds to

transitions between different modes of the laser resonator. These hops are generally provoked

by external disturbances, such as a variation of the temperature or, in the case of optical


41

feedback, when some light is reflected back into the laser cavity. It has been shown

[Ojima1986] that this noise can be largely reduced by modulating the laser diode current at

high frequency, which causes the diode to "average" over its accessible modes. This trick is

commonly used in videodisc players [Ojima1986] and has been adapted to laser diodes used

for AFM [Kassies2004, Fukuma2005]. It has the other advantage of eliminating the

interferences between the light reflected by the cantilever and the light scattered by the

sample, which are commonly observed in AFM, because it reduces drastically the coherence

length of the laser. Another type of source, which presents the same advantages, is the

superluminescent diode (SLD), which will be introduced in 2.4.1.

Let us note that independently of these considerations, the noise generated by the intensity

fluctuations is generally negligible in AFM because what is measured is the difference

(B+D)-(A+C): most of the laser intensity fluctuations are eliminated as a common mode noise

by the preamplifier. Furthermore, the noise induced by beam direction and profile fluctuations

can be diminished by using a single mode optical fiber to couple the laser diode to the

focusing optics of the OBD sensor.

2.3.4 The cantilever thermal noise

The energy of a system in contact with a thermostat at finite temperature T fluctuates, by an

amount which is very small for a macroscopic object, but that becomes significant for a

microscopic and relatively soft object such as an AFM cantilever. To a very good

approximation, a cantilever can be considered as a one degree of freedom harmonic oscillator.

As stated by the equipartition theorem, the average energy for each quadratic term in its

Hamiltonian should be given by TkB2/1 , where Bk is the Boltzmann constant. The mean

value of the potential energy is then given by:

Tkkz B2

1

2

1 2 =

from which the variance of the displacement z, which characterizes the fluctuations of the

cantilever can be extracted:

k

Tkz B=2 (2.8)

This variance can be related to )( fSThermalz , the power spectral density of the displacement z,

by:

∫∞

=0

2 )( dffSz Thermalz (2.9)


42

It is usual to express these displacement fluctuations in terms of the input of the force-

displacement harmonic transfer function of the cantilever:

)()()(2

fSfCfS ThermalForce

Thermalz = (2.10)

Where C(f) is the free cantilever harmonic transfer function and )( fSThermalForce is the power

spectral density of the force that has to be applied to the ideal, noiseless cantilever to produce

the displacement fluctuations characterized by )( fSThermalz . C(f) is readily derived from

Newton's equation: )(tFzzkzm +−−= &&& γ . Were γ is a viscosity, by injecting

)2exp()( tfjfzz π= and )2exp()()( tfjfFtF π= :

Qffjff

kffC

oo

o

+−= 22

2

)( (2.11)

where mkf /2/10 π= is the resonance frequency of the cantilever and γπ /2 0fmQ = is its

quality factor. Combining (2.8-11), we get:

∫∞

+−==

0

2

22

22 )( dffS

Qffjff

kf

k

Tkz Thermal

Forceoo

oB

It turns out that )( fSThermalForce is independent of frequency. The integral can then be evaluated

giving finally:

TkQf

TkkS B

o

BthermalForce γ

π4

2 == (2.12)

The thermal force fluctuations depend only on the dissipation. This is another expression of

the fluctuation-dissipation theorem and the expression (2.12) is analogous to the expression

(2.6) we already used for the voltage noise in a resistor, where R has been replaced by γ.

Finally, the contribution of the cantilever thermal noise at the output of the preamplifier reads,

from (2.4, 2.10, 2.11, 2.12):

[ ]2

22222

32222

)()(

2)( total

oo

oBtotal

thermalForce

ThermalV V

ffffQk

QfTkVSfCS

FN +−==

πµµ (2.13)

2.4. Modifications of the RT AFM Omicron head

The RT Omicron AFM head was used before this thesis, in particular during the PhD work of

Jérôme Polesel Maris [Polesel2005] to investigate different surfaces (Al2O3(0001), TiO2(110),


43

KBr(100),...). During these experiments, a number of problems were identified, which lead to

the conclusion that this set-up was far from state-of-the-art, rendering impossible certain

experiments. In particular:

• It was impossible to work below a frequency shift |∆f|≈10 Hz

• It was impossible to work with an amplitude below A0≈5 nm.

• It was difficult to get atomic resolution on KBr(001).

• It was impossible to see the thermal noise peak on the frequency spectrum of

the cantilever displacement.

All these observations point to a bad signal-to-noise ratio. Discussions with E. Meyer and L.

Nony (working at this period as a post-doctoral fellow) from the Institute of Physics in Basel

convinced us that these problems originate from the deflection sensor and in particular from

the LED used in this system. The main reason seemed to be that this diode is difficult to

focus. The size of its spot is much larger than the typical width of a cantilever and most of the

light power is lost at this level. It was then decided to replace the LED by a laser. We

gratefully acknowledge the help and advices from E. Meyer and L. Nony during this

modification, which is described in the following. Some of the work reported here was done

by Théophile Noël (student at ISEN-Lille) during his stay in CEMES from June to September

2006.

2.4.1 The superluminescent laser diode

Figure 2.3: Schematic of the AFM setup


44

The superluminescent diode is a special type of laser diode that combines the high output

power and brightness of a laser diode with the low coherence of a LED [SLDshort]. It is

easier to focus than a LED due to its smaller equivalent source size and the low coherence

length eliminates completely the interference effects between the light reflected by the

cantilever and the light reflected by the sample that generally affect the approach curves in

AFM. We choose a diode from SuperLumDiodes, Ltd. [SuperLum] (ref: SLD-26-HP) with

the characteristics summarized in table 2.1.

Output power at the end of the monomode fiber 5 mW

Maximum SLD current 155 mA

Spectral center 681 nm

Spectral bandwidth, FWHM 9.6 nm

Table 2.1: SLD characteristics

The output of the diode is connected to a single mode optical fiber equipped with a FC/APC

connector. The diode is also equipped with an internal photodiode, which is used by the

power driver (ref: Pilot-4AC) to regulate its optical power and with a thermistor and a

thermoelectric cooler to regulate the temperature of the diode. The diode is pressed on a heat

sink to avoid excessive heating.

The LED that was installed on the AFM head before the modifications had a power of

1.2 mW. The SLD diode we chose is more powerful and is expected to dissipate more. In

addition, SLDs are very sensitive to heating and would not support the outgassing procedure

that has to be applied to the vacuum chamber to reach the UHV regime, which consists in

heating the chamber at about 120°C for days. It was then necessary to let the SLD outside the

vacuum chamber and to guide the light with an optical fiber.

2.4.2 The single mode optical fiber

Commercial single mode optical fibers are generally protected by a polymer coating. To avoid

outgassing problems, we ordered special gold-coated fibers from Fiberguides Industries

(Stirling, NJ, USA). It was then necessary to equip the fiber with a FC/APC connector in

order to connect it to the laser. We asked to a specialized company to do it, but unfortunately,

after many trials and despite specialized equipment, we had to conclude that it was impossible

to get a connector with a transmission better than a few percents on this fiber. The 4 µm size


45

of the core of these fibers is quite small, meaning that it should be centered in the connector

with a better than 1 µm precision to obtain a sufficient transmission coefficient. We then

decided to use a commercial, acrylate-coated fiber. It has been in use for more than one year

without any significant effect on the vacuum level in the chamber.

The fiber cable has been purchased from Thorlabs (item P2-620A.FC-5). It is equipped with 2

FC/APC connectors. When connected on the SLD, the transmission was always better than

50%, reaching sometimes 80%, a figure that is not attainable without industrial assembling

capabilities. The single mode optical fiber (SM600 from Thorlabs) has a 4 µm core, a 125 µm

cladding and a 245 µm acrylate coating. The operating wavelength is 622/680nm and the

numerical aperture 0.12.

The following steps were followed to mount the fiber on the vacuum chamber and the

microscope:

- After removing one of the connectors, the fiber external protective jacket is stripped

off over a 1 m length.

- This section of the fiber is then introduced into a 0.5 mm hole managed in the center

of a CF40 flange that is subsequently filled with an epoxy glue.

- The acrylate coating is then stripped off over a 0.1 m length at the extremity of the

fiber.

- The extremity of the fiber is cleaved with a fiber cleaver (XL410 from Thorlabs) in

order to minimize the optical reflection at the end of the fiber and get a perfect circular optical

spot shape.

- The fiber extremity is glued with a cyanoacrylate glue in a 2.5 mm diameter zirconia

ferrule, which will be positioned afterwards in the optical assembly of the AFM (figure 2.4b).

This procedure is rather tedious and delicate, but the final result is satisfying. The system has

been performing satisfactorily during more than one year.


46

2.4.3 The focusing optics

(a)

(b)

Figure 2.4: (a) Path of the optical beam in the optical block of the AFM head. (b) Optical focusing set-up.

The optical system to focus the beam on the cantilever has been designed with the free

software Oslo edu Edition 6.2.4 [Oslo]. It was chosen, for the sake of simplicity, to replace

only the LED, without changing the optical block of the AFM, in particular, the two moveable

mirrors (fig. 2.4a). The only degree of freedom was then the distance between the extremity

of the fiber and the first mirror (figure 2.4a), which was used to adjust the focus on the

cantilever. The constraints in the design of the optical system were the distances between the

first mirror, the cantilever, the second mirror and the photodiode, which are imposed, the size

of the spot on the cantilever, which should be smaller than its width (25 µm) and the size of

the spot on the photodiode, which should be smaller than the size of its active area. The main

problem was to minimize the spot diameter on the cantilever, which is enlarged by diffraction

and spherical aberration. The optimal solution is built with two planar convex lenses, this

arrangement being known to minimize spherical aberration. With suitable lenses

(EPCX45230 and EPCX45355 in BK7 with a MgF2 anti-reflective coating from Edmund

Optics [Edmund], with focal lengths 10 mm and 15 mm), diffraction contributes to less than

15 µm and spherical aberration to less than 10 µm to the size of the optical beam on the

cantilever. The minimal size of the spot is then 18 µm.


47

The dimensions of the mechanical system shown in figure 2.4b are such that it can replace the

LED without any other modification on the head. The distance between the whole system and

the mirror 1 (figure 2.4a) can be roughly adjusted. The distance between the fiber extremity

and the first lens can be finely tuned with a screw, in order to adjust easily the spot diameter

on the cantilever.

2.4.4 New characteristics of the beam deflection sensor.

The first tests after these modifications showed a tremendous improvement in the

performance of the head. In particular:

• It is now possible to work at a frequency shift |∆f|≈1 Hz

• It is now possible to work with an amplitude below A0≈1 nm.

• We now get atomic resolution on KBr(001) routinely.

• It is easy to see the thermal noise peak on the frequency spectrum of the

cantilever displacement.

These improvements originate in two combined factors. αP, the optical power incident on the

photodiode is much higher than before the modifications, increasing from approximately 20

µW for the LED to more than 1 mW for the SLD. This is not only due to the more powerful

light source (5 mW for the SLD instead of 1.2 mW for the LED), but also to the much better

focusing on the cantilever. This gain by a factor of 50 gives a √50∼7 improvement in the S/N

ratio if one considers that the measurements are shot noise limited (as will be demonstrated in

the following). But this evaluation is an underestimate, since, due to the bad focusing with the

LED, much scattered light was detected by the photodiode. The power of 20 µW mentioned

above includes optical power that is not useful for the cantilever displacement measurement.

Overall, we estimate that the S/N ratio improved by at least a factor of 10.

In the following, we discuss in more detail the measurements of the noise made in order to

better characterize the new set-up.


48

2.5 Noise spectral analysis

Figure 2.5: Noise measurement setup

In order to characterize the new optical beam deflection system, measurements of the noise

spectrum of the cantilever displacement, given by VFN (2.4), were performed. In our analysis,

the expression of the total PSD considers the addition of the thermal noise, the shot noise and

the Johnson noise power spectral densities:

)()()()( fSfSfSfS JohnsonV

ShotV

ThermalVV FNFNFNFN

++= (2.14)

Each of these terms has a different dependence in the laser power P, which can be adjusted

with the SLD driver and measured from the Vtotal signal. One can rewrite eq. (2.14) in terms of

the Vtotal, using (2.5, 7 and 13):

IVBtotalIVtotalthermalForceV RTkVeRVfSfCS

FN400200)()( 222 ++= µ (2.15)

It is seen that each of these terms has a different dependence in Vtotal. This behavior will be

used to separate the different contributions to the noise cantilever displacement spectrum.

Figure 2.5 shows the measurement set-up. The spectrum analyzer (Hewlett Packard

HP2582A) we used is limited to the 0 to 25 kHz frequency range. It was then necessary to

shift the frequency range of the signal of interest in this range. This was done by heterodyning

the signal with a multiplier and a waveform generator. The VFN output signal of the pre-

amplifier and the output of the wave generator were connected to the inputs of the multiplier

circuit. The PSD of the output of the multiplier is obtained from the spectrum analyzer.

The following measurements were performed in UHV, with a NCH cantilever from

NanoSensors [Nanosensors2000] with the following nominal characteristics: n+-Si, 0.01-

0.025 ohm.m, thickness 2.5-4.5 µm, width 20-40 µm, length 125 µm, tip height 10-15 µm,


49

fo=280-265 kHz, k=25-50 N/m. The gain of the preamplifier was determined by RIV =

90.9 kohm, with a bandwidth of 400 kHz.

Two types of spectrum were measured while varying the laser power P:

• Around the resonance frequency of the cantilever, on a 500 Hz frequency range to

characterize the thermal noise contribution. Each spectrum corresponds to the average

of 256 spectra.

• Far from the resonance peak around a frequency of 170 kHz on a 25 kHz frequency

range, to extract the shot noise and Johnson noise contributions. Each spectrum

corresponds to the average of 64 spectra.

Figure 2.6: Background noise as a function of Vtotal.

Around 170 kHz, the spectra are flat. The average of each of these spectra varies linearly with

Vtotal as shown by the linear adjustment displayed in fig. 2.6. This behavior is a clear signature

of the shot noise. This linear dependence is rather specific and it is difficult to imagine other

sources of noise, which would have not been considered in our analysis, giving such a linear

dependence in the laser power. For this reason, we will use the data of fig. 2.6 to calibrate our

measuring set-up. The signal at the input of the spectrum analyzer VHP is related to VFN by VHP

= δ VFN. δ depends on the gain of the multiplier and the amplitude of the waveform generator

signal. The PSD given by the analyzer reads then SVHP = δ 2 SVFN (2.16). From (2.15), the slope

B of the linear adjustment in fig. 2.6 should be given by 2200 δ⋅⋅= eRB IV , from which δ 2 =

6.15.

Using this value to evaluate the Johnson noise term gives =2400 δIVBTRk 8.9 10-12 V2Hz-1

(with T=300 K), much smaller than the measured value A = 4.97 10-12 V2Hz-1 (fig. 2.6). This

is not surprising as many other sources of white noise are present in the system. The resistors

are not ideal, and most electronic components generate white noise. Furthermore, measuring


50

the thermal noise of a resistor of relatively low value (RIV = 90.9 kohm) requires special

arrangements that we did not implement for these experiments.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2.7: Spectra in the vicinity of the cantilever resonance for different values of totalV .

Open squares: experimental data, red lines: analytical adjustments.

Note that for these spectra, 01.0/2 ≈= kTkz B nm.

The spectra in the vicinity of the cantilever resonance are plotted in figure 2.7 for different

values of the laser power, as measured by Vtot. Considering that the shot and Johnson noise

background (fig. 2.6) should be negligible compared to the thermal noise in this frequency

range, an adjustment of these spectra was performed using the following analytical

expression:


51

[ ]22222

3222

)()(

2)()(

ffffQk

QfTkfSfC

oo

oBthermalForce

+−=

πξξ (2.17)

where T=300 K, k=30 N/m, and f0, Q and ξ have been chosen in order to reproduce as well as

possible the experimental peak position, width and amplitude.

The spectra are very well described by the analytical expression (2.17) except in (e) and (f)

where the neglect of the white noise background becomes perceptible. Then, from equations

(2.13, 16 and 17): 2222totalVµδξ = . This dependency in 2

totalV is confirmed in figure 2.8, from

which we extract 1522 1056.1=µδ m-2 and 715 1059.115.6/1056.1 ==µ m-1. The sensitivity

is then given by: totaltotal VV 9.15=µ in mV.nm-1.

It can be checked that this value has the good order of magnitude for our system. From

equation (2.4): ldS/120=µ . d can be estimated from the Airy's formula: aSd /22.1 λ=

where a is the size of the beam spot on the cantilever. As seen in 2.2, a=18 µm when the

system is perfectly focused. Then taking l = 125 µm (NCH cantilever) and λ=680 nm :

71008.222.1/120 == la λµ m-1, which compares quite well with the measured value, taking

into account the crudeness of this evaluation.

Figure 2.8: Dependence of ξ 2 on Vtotal: Black squares: experimental data, blue crosses: fit with ξ 2= 2151056.1 totalV .


52

2.5.1. Estimation of the equivalent input noise of the new beam deflection system.

To characterize a sensor, it is customary to express its noise as an equivalent input noise, that

is the noise that has to be applied to the input of an ideal noiseless sensor to produce the

measured output noise. This figure-of-merit facilitates the comparison between different types

of sensors. In our case, this equivalent input noise should be expressed in m2Hz-1, since we

deal with a displacement sensor. Figure 2.7 shows that our system is dominated at high laser

power by the shot noise: at 52.3=totalV V, the shot noise contributes to 92% of the total noise.

Let us recall that the thermal noise of the cantilever is not a noise of the sensor but is used

there as a known noise source to characterize it. To calculate the equivalent input noise, we

just need to divide the measured shot noise (in V2/Hz) by the square of the sensitivity totalVµ

in V2m-2:

27

27221027.3

)1059.1(

200200 −===total

IV

total

totalIVz

V

eR

V

VeRS

µ m2Hz-1

for the maximum optical power, corresponding to totalV =3.52 V (figure 2.7a) or Sz1/2 =

57 fm.Hz-1/2.

This experiment was performed with a non-coated cantilever, for which totalV was limited to

about 4 V. With a NCHR cantilever (coated with Al), we could achieve the equivalent of a 3-

fold increase in totalV (we had to reduce the gain of the preamplifier to avoid saturation at

10V). In this situation, the equivalent input noise becomes: Sz1/2 = 57/√3 = 22 fm.Hz-1/2.

This figure can be compared with the best value for a beam deflection system obtained so far,

reported by Fukuma and Jarvis [Fukuma2006] for a NCHR cantilever in air where

Sz1/2 = 11 fm.Hz-1/2.

2.5.2. Estimation of the cantilever temperature at maximum laser power level.

Figure 2.9: Dependence of f0 and Q in Vtotal. f0: Black squares: experimental data, red line: fit with f0= -6.77 Vtotal.+ 271200.5, Q: red circles and black line.


53

Figure 2.9 reveals that f0 decreases linearly when the optical power increases. This

observation can be related to the heating of the cantilever by the optical beam. In the

following, we use these data to estimate the temperature increase of the cantilever at

maximum optical power.

The resonant frequency of the cantilever can be calculated from:

( )ct mm

kf

24.02

10 +

=π

(2.18)

Where mt is the mass of the tip and mc is the mass of the cantilever [Sarid1991]. The stiffness

k can be calculated from the dimensions of the cantilever and the Young's modulus of silicon

E = 1.69 1011 Nm-2. As already mentioned at the beginning of the chapter, k can be expressed

as 3/3 lEIk = . The area moment of inertia of a rectangular lever is related to its width w and

thickness t by 12/3wtI = [Sarid1991]. Then:

33 4/ lEwtk = (2.19)

showing that the resonance frequency depends on the temperature via (1) the change in the

dimension of the cantilever and (2) the temperature dependence of the Young's modulus of

silicon. A detailed study of the temperature dependence for comparable cantilevers

[Gysin2004] showed recently that the variations of the resonant frequency are largely

dominated by the variations of the Young's modulus. As explained in [Gysin2004], the

temperature dependence of the Young's modulus of Si can be estimated from the temperature

dependence of its bulk modulus B, using E=3B. Then:

5

0

00

0

0 10770.15.271200

5.2712005.2711762

)0(

)0()52.3(22 −×−=−=−=∆=∆=∆=∆

Vf

VfVf

f

f

k

k

E

E

B

B

Figure 2.10: Si Bulk modulus as a function of temperature. Black squares: experimental values from [Nandanpawar1978], red line: fit with B=995.6 - 0.058 T Gpa.


54

From the temperature dependence of B, plotted in figure 2.10: )(058.0 GPaTB ∆−=∆ .

Combining these two relations, we get:

KKB

T 3058.0

9783109.5

058.0

3003109.5 55

≈−

××−=−

×××−=∆−−

This crude estimate shows that the temperature increase of the cantilever is easily detectable,

but is negligible for room temperature experiments. Nevertheless, it shows that the upper limit

for the laser power P is not very far from our experimental conditions.

The decrease of Q with increasing temperature displayed in figure 2.9 is more difficult to

interpret. It is related to internal friction phenomena, which depend on several distinct

mechanisms and are highly material specific.

2.6. Sensitivity measurement and amplitude calibration.

In the preceding paragraph, we have used a value k = 30 N/m, somewhat arbitrarily, to extract

the sensitivity. In fact, the noise measurements can be considered either as a way to calibrate

the amplitude of oscillation, once the stiffness is known or alternatively as a way to calibrate

the stiffness, once the amplitude is known. The amplitude can be determined by using the

cantilever in the contact mode, using the known calibration of the z piezoelectric ceramics to

calibrate the cantilever displacement under a z increment, but this technique damages the tip.

Our strategy is to avoid destructive measurements. It is then necessary to use a non-

destructive method to measure the stiffness.

Several methods have been proposed to determine k [Burnham2003]:

• The first group of methods uses known analytical formulas, which relate k to the

dimensions of the cantilever and the elastic moduli of the cantilever material

[Cleveland1992]. These methods are better for cantilevers of simple shape made from

well-characterized material, for instance our cantilevers (diving board geometry and

Si).

• The second group of methods in based on the measurement of the static deflection

under the application of a force, via a calibrated cantilever [Cumpson2002] or by the

addition of known masses [Cleveland1992]. These methods are very tedious and can

be destructive.

• The third group of methods uses the thermal noise in vacuum [Hutter1992] or in a

medium of known viscosity and density [Sader1999].


55

We chose the simplest method, which belongs to the first group, where we calculate the value

of k from (2.19): 33 4/ lEwtk = . 3 cantilevers were examined by SEM, two NCH cantilevers

extracted from the same wafer and one NCHR cantilever (same type as NCH, but with a 20

nm Al coating) from another wafer. These SEM observations were performed by Christophe

Deshayes in CEMES. The images and the measurement of the dimensions of the first NCH

cantilever are shown in fig. 2.11. In fig. 2.11a, the length has been measured between the

attachment point of the cantilever to its support and the apex of the tip. This convention is

somewhat arbitrary, but is consistent with approximating the shape of the cantilever by a

rectangle. In fig. 2.11b, as the section of the cantilever is trapezoidal, the width is calculated

as the mean between the widths of the two faces of the cantilever. In fig. 2.11c, the cantilever

had to be aligned rather precisely to get a precise value for its thickness. Applying relation

(2.19) with the dimensions reported in fig. 2.11 gives k=30.3 N/m.

(a)

(b)

(c)

(d)

Figure 2.11: SEM images of a NCH cantilever.

To check that this value is reasonable, the resonant frequency of the cantilever can be

calculated from equation (2.18). The tip can be modeled as a square-base pyramid of volume

V=Ah/2, where h=15.6 µm is the height of the tip and A=(12.9 µm)2/2 the area of its base

(fig. 2.11d). We get mc = 1 pg and f0 = 305 kHz. This evaluation gives an error below 10%


56

since the resonance frequency of this cantilever was measured at 280 kHz. This corresponds

to an error below 20% on the value of k, in agreement with the Nanosensors Product Guide

[Nanosensors2000].

The same measurements were performed for the other two cantilevers. The results are

summarized in the following table:

l (µm) w (µm) t (µm) k (N.m-1)

NCH1 127.5 26.7 3.82 30.3

NCH2 130.8 29.6 3.74 28.5

NCHR 121.2 25.0 3.80 32.5

Table 2.2: Results of the measurements for 3 cantilevers

The contribution of the Al coating was neglected in the calculation of k for the NCHR

cantilever as it is expected to lead to a minor (<1%) correction, estimated from a calculation

of effective Young's modulus for a bi-layer cantilever [Roark2002]. It is seen that the value of

k does not vary significantly from one cantilever to the other. The cantilever used for the

noise measurements discussed in 2.4 was taken from the same wafer than NCH1 and 2. This

is why we took k=30 N/m for the quantitative evaluation of the results of this study.

Note that this method for measuring k could be improved by a better description of the shape

of the cantilever, which could be done by numerical methods.

2.7. Conclusions and perspectives.

The two main achievements reported in this chapter are the improvement of our RT AFM

head and the development of the noise measurements, which happen to constitute a useful and

practical method to measure the oscillation amplitude of the cantilever.

We have shown that the main factors in the head performance improvement are the increase

of the optical power incident on the photodiode and the better focusing of the optical beam.

There is now a limited room for further improvements. The laser power cannot be much

increased because of the heating of the cantilever. The focusing and quality of the optics can

certainly be further improved, but the gain there will be modest. Another improvement that is

planned is to replace the actual photodiode by a smaller one, of smaller capacitance. This will

decrease the background noise generated by the preamplifier.


57

We have developed a non-destructive and simple method to evaluate the oscillation amplitude

from the noise spectra. Its main limitations in terms of precision are in the measure of k where

a precision of 20% is estimated. Increasing this precision to 10% is certainly feasible by a

better modelization of the cantilever by numerical methods. It is important to measure as

precisely as possible the experimental parameters to allow quantitative measurements in NC-

AFM.


58

Chapter 3 NC-AFM study on KBr(001)

59

CHAPTER 3

NC-AFM study on KBr(001)

3.1 Introduction

The surfaces of ionic insulators, such as alkali halides [Bennewitz2002a] or CaF2

[Reichling2002] have played a major role in the development of atomic resolution NC-AFM.

They are easily prepared by cleavage and atomic resolution images are routinely obtained. In

addition, the interpretation of their atomic contrast is now well understood. This is

particularly true for the KBr(001) surface, which was extensively investigated both

experimentally [Hoffmann2004, Hoffmann2002] and theoretically [Pakarinen2006] by a

number of groups and can, for these reasons, be considered as one of the reference surfaces

for NC-AFM studies.

The starting point of the experiments described in this chapter was the need to validate the

improvements made on our AFM head, described in chapter 2. We then naturally choose

KBr(100) for the reasons outlined above and also because this substrate was investigated

previously with the AFM head before its modifications, during the thesis of J. Polesel-Maris

[Polesel2005]. While performing these experiments, new results were obtained, which can all

be related to changes in the atomic structure of the tip apex under the influence of tip-

substrate interactions.

The experimental methods needed to perform reliable measurements in NC-AFM are

described in part 3.2. The observation of a systematic and reversible change in the atomic

contrast when the tip crosses a monoatomic step of the KBr(001) surface is presented in part

3.3. These experiments are complemented by force and dissipation spectroscopy

measurements performed with the same tip, reported in part 3.4. These results are discussed in

part 3.5 before concluding the chapter.


60

3.2. Experimental methods

The KBr crystals were cleaved in air, quickly transferred to the UHV system, and finally

heated at 400 K for 1 hour to remove the charges produced during the cleavage process. This

preparation method produces an atomically well-ordered surface with (001) terraces separated

by monoatomic steps, as shown in figure 3.1. Under standard UHV conditions (pressure in the

10-10 Torr range), the surface remains clean for a few days.

(a)

(b)

Figure 3.1: (a) Constant-frequency detuning image of the KBr(001) surface. ∆f=-50 Hz,

App=7.7 nm, (γ=-1.3 fN m1/2). (b) Profile along the white line drawn on (a) showing 3 monoatomic steps of height 0.33 nm. Arrows point to atomic kinks on the steps. Parameters

of the cantilever: f0=269 800 Hz, k≈30 N/m, Q≈25 000.

We use NCH silicon cantilevers provided by NanoSensors (Neuchatel, Switzerland). The

work discussed here was performed with a cantilever that was characterized by thermal noise

measurements from which the values of the resonance frequency f0, the spring constant k and

the quality factor Q were deduced (see the legend of figure 3.1). These values are used to

calibrate the dissipation scale and to calculate the value of the "normalized frequency shift" γ

= kA3/2∆f/f0 introduced by Giessibl [Giessibl1997] as a useful parameter to compare images

obtained under different experimental conditions.

Residual long-range electrostatic forces were compensated by applying a bias voltage

between the plate supporting the sample and the tip as described in [Guggisberg2000]: the


61

distance regulation feedback loop is disabled and the frequency shift is recorded as a function

of the bias voltage (figure 3.2). The maximum of the parabola (which corresponds to the

minimal force) gives then the voltage that has to be applied to compensate the contact

potential difference between the tip and the sample.

Figure 3.2: ∆f(V) curve on KBr(100). f0=269 800 Hz, App = 6.1 nm. The red curve is a parabolic fit giving the position of the maximum at V=-0.78 V.

Great care was taken to set the time constants of the different feedback loops of the control

system. Improper settings of these loops can induce artifacts, especially when measuring

dissipation signals [Nony2006]. The phase in the AGC loop was set to its optimal value,

where the f∆ channel is decoupled from the dissipation channel by minimizing the

dissipation signal.

3.2.1. Force spectroscopy

3.2.1.1. Extraction of the force from ∆f(z) using the Sader-Jarvis formula

The frequency shift f∆ can be related to the tip-substrate interaction force by the following

relation, first derived by Giessibl [Giessibl1997]:

[ ]∫ ++−=∆ 0

1

0

00int0

0

)2sin()2sin(1(f

dttftfAbFkA

f

f

f ππ (3.1)

For a cantilever of resonance frequency 0f oscillating with an amplitude A. b is the distance of

closest approach to the surface. This expression, derived using the Hamilton-Jacobi approach,


62

is valid for any amplitude of oscillation provided that 1/ 0 <<∆ ff , a condition that is always

met in our measurements.

To determine the interaction force from the observed frequency shift, eq. (3.1) must be

inverted. This inversion is simple only in the small amplitude limit. In this case, intF can be

linearized:

[ ] ( ) )2sin()2sin(1( 0int

int0int tfAz

FAbFtfAbF

Abz

ππ+=∂

∂++≈++

and eq. (3.1) reduces to:

Abzz

F

kf

f

+=∂∂−=∆ int

0 21

The frequency shift is proportional to the gradient of the interaction force, which plays the

role of an effective additional stiffness for the cantilever harmonic oscillator.

In the general case, when the amplitude A is comparable or larger than the scale on which intF

varies significantly, the inversion is more challenging. An analytical expression, valid only in

this large amplitude limit has been derived by Durig [Durig1999]. Numerical schemes for

arbitrary amplitude have been proposed by Durig [Durig2000] and Giessibl [Giessibl2001].

An accurate analytical approximation, valid for arbitrary amplitudes, has been derived by

Sader and Jarvis [Sader2004]. It consists in injecting the interaction force, expressed in terms

of its Laplace transform in eq (3.1) and approximating the resulting Bessel function by a Padé

approximant. The resulting formula reads:

( ) ( )( )

( )( )

zdf

zf

zd

d

zz

A

f

zf

zz

AkzF

z

′

′∆′−′

−′∆

−′+= ∫

∞

0

2

3

0

2

1

int28

12π

(3.2)

Three remarks are in order:

• To get the value of the force at a distance z from the surface requires integrating from

z to infinity. This means that, in principle, the ( )zf∆ curve must reach the region

where the interaction force becomes negligible, i.e. ( ) 0≈∆ zf . But due to the

denominators in ( )zz −′ in the integrand, and to the fact that for large amplitude it is

the second term in the integrand that dominates, this condition is not very stringent.


63

• This expression involves the derivative of ( )zf∆ . Differentiating an experimental

curve generally generates noise. This noise can be reduced by a suitable smoothing

procedure of the initial data.

• The precision of the force extraction is affected by the precision on the values of k and

A, demonstrating the importance of the precise measurement of these quantities to

make quantitative force measurements, as stressed in chapter 2.

Figure 3.3 shows a simple example of application of the Sader-Jarvis (SJ) formula (3.2),

starting from the following analytical expression for a Morse-van der Waals tip-substrate

force:

( ) [ ] [ ]{ })(exp)(2exp26 2int σκσκκ −−−−−+−= zzE

z

HRzF b (3.3)

where the first term on the right side corresponds the van der Waals force, with a tip radius R

and a Hamaker constant H and the second term corresponds to the Morse contribution,

described by the parameters κ, Eb and σ. Expression (3.3) is plotted in figure 3.3a (black

squares), with H=1 eV, R=10 nm, and the parameters extracted from [Giessibl2003], which

have been optimized to describe a Si-Si bond, κ=15.5 nm-1, Eb=2.15 eV and σ=0.235 nm.

The corresponding frequency shift for a cantilever oscillating at f0=270 kHz with an

amplitude A=5 nm can be calculated analytically from eq. (3.1) [Nony2006] and is plotted in

figure 3.3a (black line).

(a) (b)

Figure 3.3: Sader and Jarvis formula applied to the simple case of a calculated Morse-van der Waals tip-substrate force. (a) Black square: force, continuous black line:

frequency shift calculated using (3.1), red line: force calculated using the SJ expression (3.2) from the frequency shift. (b) Zoom of (a) for the forces.


64

The SJ formula was then applied to this frequency shift curve by numerical integration. The

resulting force curve, plotted in figure 3.3a (red line) is very close to the original one,

showing the efficiency of the SJ formula.

3.2.1.2. Description of the technique

The procedure to get ∆f(z) and D(z) curves consists in choosing the position on the surface

where the data should be taken, disabling the distance regulating feedback loop and making

the distance vary according to a pre-defined pattern while monitoring the ∆f and D signals.

We generally use patterns of the type illustrated in figure 3.4a. In this example, the tip is first

retracted by 2nm (black line), then approached by about 2.4 nm (red line) and this sequence is

repeated in time-reversed order (green and blue lines). ∆f is recorded as a function of time and

can be plotted as a function of z, as shown in figure 3.4b.

(a)

(b)

(c)

(d)

Figure 3.4: (a ) z(t) pattern (right scale) and corresponding ∆f(t) curve (left scale). (b) ∆f(z) curves extracted from (a). (c) Zoom of (b). (d) Corrected ∆f(z) curves. App=7 nm, f0=271,900

Hz, ∆f=-144 Hz.

We adopted this type of pattern essentially because it allows identifying some of the

numerous artifacts that can spoil the data in NC-AFM. Because the second half of the data

corresponds to a time-reversed sequence displacement, all the artifacts related to the different


65

response times of the apparatus can be avoided or identified. These include an improper

setting of the feedback loops parameters and the history-dependent response of the z

piezoelectric ceramics. In the zoom shown in figure 3.4c, one can see that the four ∆f(z)

curves do not coincide, especially in the 0-0.3 nm z domain. The black and green curves,

measured when the tip retracts from the substrate are on the right of the red and blue curves

measured when approaching the tip toward the substrate. Because this effect is systematic, it

cannot be the consequence of thermal drifts. It is more important for larger z excursions. A

systematic study of spectra acquired in different conditions convinced us that it is related to

the response of the z piezoelectric ceramics, which is affected by creep. It is well know that

the creep of PZT ceramics follows a logarithmic law. More precisely, the response of a PZT

to a voltage step is given by:

+=

00 1)( t

tLogLtL γ , where L0 is the elongation at time t0

after the application of the voltage [Jung2000]. We applied this relation to correct the data of

figure 3.4 by considering the voltage ramp as a succession of voltage steps and adjusting the

parameter γ to make the four curves of figure 3.4c coincide. The result, shown in figure 3.4d

is satisfying: The separation between the forward and backward curves, which was of the

order of 0.05 nm before correction has been reduced to about 0.01 nm, which is comparable

to the noise level in these experiments. This correction procedure has been applied to the data

when necessary.

∆f(z) and D(z) curves can be also affected by piezo creep and thermal drifts in the x-y plane.

In this case the location on which the spectrum is measured becomes uncertain. This effect

can be minimized by waiting for the stabilization of the instrument and choosing positions in

close proximity to the scanning line to avoid large piezo excursions.


66

3.3. Results

3.3.1 Two types of atomic resolution images

The constant-frequency shift atomic resolution images we obtained on the KBr (001) surface

can be classified in two types, displayed in figure 3.5.

Figure 3.5: Constant-frequency detuning images of KBr (001). Size: 3 nm x 3 nm (a) ∆f=-340

Hz, Ap-p=5 nm (γ=-4.7 fN m1/2), (b) ∆f=-230 Hz, Ap-p= 7 nm (γ=-5.3 fN m1/2), and e) a front view of the KBr bulk crystal atomistic model.

Both show a square lattice with a period of 0.47 +/-0.02 nm close to a/√2=0.466 nm, which

corresponds to the interatomic distance between two ions of the same species, where a=0.66

nm is the bulk lattice parameter of KBr (figure 3.5e). Similar images have been reported and

compared to calculated images in [Pakarinen2006, Hoffmann2004]. Detailed comparisons

between experimental data and image calculations have established that the atomic contrast

formation on ionic surfaces is generally dominated by the short-range electrostatic interaction

between a polar ionic tip and the substrate. The sign of the tip apex ion determines two types

of images, which differ by specific features of the contrast as observed in most experimental

studies [Foster2002, Hoffmann2002, Pakarinen2006]. Of special importance in this

interpretation is the role played by the relaxations of the substrate and tip ions under the

influence of the tip-substrate interaction. Ions displaced in the direction perpendicular to the

surface extend the range of the electrostatic interaction, hereby enhancing the atomic contrast

[Livshits1999]. This effect is even more pronounced on defects, as analyzed, for instance, for

NaCl steps in reference [Bennewitz2000].


67

These works strongly suggest that the image of figure 3.5a was obtained with a Br--

terminating tip, while the image of figure 3.5b was obtained with a K+-terminating tip. In

figure 3.5a, the Br- tip is attracted by the K+ ions, which therefore appear as white bumps, and

repelled by the Br- ions, which appear as dark disks. The characteristic features which connect

neighboring K+ ions, seen on the profile of figure 3.5c as secondary maxima, result from the

bending of the tip due to the strong repulsive force it feels when it is above Br- ions

[Pakarinen2006]. In figure 3.5b, the K+ tip is attracted by the Br- ions, which therefore appear

as white bumps. The profile in figure 3.5d shows a slight asymmetry, not seen in the

calculated images [Pakarinen2006], which is most likely due to a non-ideal tip. The observed

corrugations are quite high, above 0.1 nm. This could be related to a relatively flat or round

tip which, due to increased long-range forces, allows stable imaging closer to the surface,

where the stronger short-range interaction is expected to produce higher corrugations

[Bennewitz2002b].

3.3.2 Atomically resolved force curves

Figure 3.6 shows an atomic resolution image with a weak "K+ tip" contrast (corrugation

≈ 0.03 nm). Spectra have been taken either on Br ions (black dots) or on K ions (white dots).

Figure 3.6: 3 x 3 nm image of KBr(001). App=7 nm, f0=271900 Hz, ∆f=-144 Hz.

These 14 spectra are displayed in figure 3.7a and b, and show the raw data. It is seen that the

two types of spectra are highly reproducible. The approach and retract curves are non

distinguishable. But the spectra differ significantly between the two types of ions showing an

atomic contrast. As expected, the difference is concentrated in close proximity to the surface,

where the chemical interaction responsible for the contrast is supposed to play a role. The

average curves for the two sites have been inverted using the SJ formula.


68

(a) (b)

(c) (d)

Figure 3.7:(a) Spectra obtained on the points labeled in figure 3.6. Black lines: spectra on Br, red lines: spectra on K. (b) Zoom of (a) and average curves (shifted downward by -80 Hz for clarity). (c) Average of the approach and retract curves. The green curve is the spectrum on K shifted to the left by 0.03 nm to correct for the atomic corrugation. (d) Force-distance

curves on the two sites. App=7 nm, f0=271,900 Hz, ∆f=-144 Hz.

These results show that one can obtain an atomic contrast not only on the images but also on

the force curves, even at room temperature. Two results extracted from the literature, which

can be used to compare with our results are displayed in figure 3.8. In a are shown

the ∆f and the forces curves obtained on KBr(001) at low temperature (7 K) [Hoffmann2002].

The overall shape and the order of magnitude of the forces are comparable to our results. Note

nevertheless that the van der Waals contribution, evaluated by fitting an analytical expression

on the data obtained far from the surface, was subtracted from the data, in order to isolate the

short-range contribution to the force. It was not possible to apply the same procedure to our

data because the chosen z range does not extend far enough form the surface to allow a

precise determination of the van der Waals contribution. In b are shown the force curves


69

obtained at room temperature on NaCl(001) [Schiermeisen2006]. This substrate is expected to

behave qualitatively as KBr(001).

(a)

(b)

Figure 3.8: (a) ∆f and force curves obtained on KBr(001) at 7 K by [Hoffmann2002]. The black line corresponds to the topographic maximum (presumably a Br ion) and the grey line corresponds to the topographic minimum (presumably a K ion). (b) Force curves obtained on

NaCl(001) at RT by [Schiermeisen2006].

An important difference between the low temperature (LT) data (fig. 3.8a) and the room

temperature (RT) data (fig. 3.7d and 3.8b) is that the repulsive part of the force curves is

much steeper at LT. In addition, the RT curves present deviations from the expected smooth

behavior that have been interpreted in [Schiermeisen2006] in terms of mechanical relaxation

of the tip apex under the influence of the tip-substrate interaction.

As recently proposed by [Ghamesi2008], real tips generally do not have the perfect structure

that has been used in most of the calculations done so far. Realistic tips can take different

structures that can be close in energy so that the structure can evolve during the approach of

the tip toward the surface, especially at RT, where the barriers separating these different

energy minima are more easily crossed. This type of interpretation could account for the

differences mentioned in the preceding paragraph.

In the following, we detail another example, which shows that indeed, the structure of the tip

can change during imaging.


70

3.3.3 Spontaneous tip polarity reversal

Figure 3.9: Observation of a spontaneous tip polarity change. (a) z, (b) ∆f and (c) dissipation (W) image. Image size: 8 nm x 6.2 nm; ∆f=-210 Hz, Ap-p=7 nm (γ=-4.8 fN m1/2). The contrast

was enhanced in the Br- part of the image (a) for clarity. (d), (e), and (f) Profiles corresponding to the lines drawn on the images.

Figure 3.9 shows images in which the contrast abruptly changed while imaging the surface.

This phenomenon is routinely observed when imaging ionic crystal surfaces and is attributed

to a change of the tip polarity [Reichling2002, Hoffmann2002]. The discussion of figure 3.5

shows that in this particular instance, the tip switches from a K+ termination in the upper part

of the images to a Br- termination in the lower part. This switching event happened

spontaneously since it was not induced by a deliberate change in the experimental conditions

or by the presence of a defect in the immediate proximity of the tip. It is inherently stochastic

and may be induced, for instance, by an accidental fluctuation of the tip-surface distance. The

change of the structure of the tip modifies the tip-substrate interaction potential, meaning that

it should be accompanied by a frequency jump. In the constant-frequency detuning mode, this

jump is rapidly corrected by a change in the tip-surface distance. The profiles displayed in

figure 3.9 show that the K+-tip is approximately 0.3 nm closer to the surface than the Br—tip

(figure 3.9d), while the average frequency detuning keeps the same value on the two parts of

the image, except for a small residual oscillation due to imperfect distance regulation, as

presented in figure 3.9e. The change of the structure of the tip also induces a lateral

displacement of the apex of the tip, as seen in figure 3.9, where the atomic rows on the two

sides of the transition are not in the position expected from the crystallographic structure of

KBr. Due to the crystalline nature of the sample, it is not possible to quantify this

displacement up to a lattice period of the surface from this image. Finally, it is observed that

atomic resolution also appears in the damping image with the K+-tip, with an increase in the


71

dissipation level of approximately 20 meV/cycle relative to the Br--tip. Note that the

dissipation is minimal on the Br ions, as shown by the profiles in figure 3.9. Atomic

resolution damping images were reported, for instance, in [Loppacher2000].

3.3.4 Tip polarity reversal on a monoatomic step

During the same experiment, we sometimes observed a systematic and reversible change of

the polarity of the tip apex when imaging monoatomic steps.

Figure 3.10: (a) z, (b) ∆f and (c) W image of a monoatomic step with a Br--terminating tip. Image size: 18 nm x 6.4 nm; ∆f=-120 Hz, Ap-p=7.1 nm (γ=-2.8 fN m1/2). (d), (e), (f) Profiles

corresponding to the lines drawn on the images.

Figure 3.10 shows an example where a step is imaged "normally", with a Br- tip, while figure

3.11 shows an example where the tip changes its polarity on the step. Notice that the absolute

value of the frequency shift was increased between figure 3.10 and 3.11, meaning that the tip

is closer to the surface in figure 3.11. The results of figure 3.10 are comparable to images

taken on NaCl steps [Bennewitz2000]. The height of the step measured on the topographic

profile [fig. 3.10d] is around 0.33 nm, in agreement with the lattice parameter of KBr. In

addition, one recovers the normal structure where the K+ <110> rows are laterally shifted by

half the nearest K+-K+ distance when crossing the step, as expected. The step induces a small

dip on the frequency detuning profile [figure 3.10e] and a small peak on the damping profile

[figure 3.10f].


72

Figure 3.11: Observation of tip polarity switching on a step edge. (a) Forward z, (b) backward z, (e) ∆f and (f) W image. Image size: 10.3 nm x 6.9 nm; ∆f=-210 Hz, Ap-p=7 nm

(γ=-4.8 fN m1/2). (c), (d), (g) and (h) Profiles corresponding to the lines drawn on the images.

In figure 3.11, the monoatomic step separates a lower terrace on the left from a higher one on

the right. The tip polarity changes from Br- on the left terrace to K+ on the right one. In

contrast to the tip switching analyzed in figure 3.9, this change is systematic. It is also

reversible, the tip commuting from Br- to K+ during the forward scan (from left to right) and

from K+ to Br- during the reverse, backward scan. Notice that the forwards scanned

topographic image [figure 3.11a] is extremely similar to the backwards scanned topographic

image [figure 3.11b]. This observation demonstrates that the scanning speed is low enough to

prevent any distortion of the image and that the tip switching is not induced by a too slow tip-

substrate distance regulation when crossing the step. We also checked that the frequency

detuning and damping images are not affected by the scanning direction and more generally

that the four images do not depend on the angle between the scanning direction and the step

edge.

Here again, similarly to figure 3.9, the relative position of the Br- ions imaged on the lower

terrace and the K+ ions imaged on the upper terrace is not the position expected from the


73

crystallographic arrangement of KBr. Finally, notice that the steps are decorated by triangle-

like structures, which are not observed in the "normal" imaging case of figure 3.9. They will

be discussed later in this chapter.

Interestingly, in the topographic profile of figure 3.11c and d, the tip height increases by only

0.03 nm at the step, instead of 0.33 nm in the case of normal step imaging as in figure 3.10.

But, as argued for the spontaneous tip polarity reversal of figure 3.9, the change in the

structure of the tip modifies the tip-substrate interaction potential and induces a change in the

tip height. The measured step height in figures 3.11c and d should then be corrected by this

amount. This leads to the conclusion that the distance between the K+-tip and the upper

terrace is smaller by 0.33-0.03=0.3 nm than the distance between the Br--tip and the lower

terrace. This is precisely what is observed in figure 3.9d. In addition, a similar contrast in

dissipation is observed when the tip is K+, with a minimum on the Br ions and the dissipation

profile indicates that for a given tip polarity, the dissipation level shown in figure 3.11h is the

same as in figure 3.9f. These similarities between figures 3.9 and 3.11 strongly suggest that

the same change in the tip structure is at the origin of both the spontaneous tip polarity change

in figure 3.9 and the deterministic step-induced tip polarity change in figure 3.11.

3.3.5 Potential energy surfaces of the two-level system

Two different mechanisms can be invoked to explain a reversal of the tip polarity. Ions can be

transferred during an accidental contact between the tip and the surface. Indeed, it is believed

that the ionic tips invoked in the interpretation of images of ionic substrates are formed by the

transfer of a cluster from the substrate during the first stage of imaging [Reichling2002]. A

second possibility is that the structure of the tip apex evolves without any material transfer

from the substrate. The repetitive and reversible character of the step-induced tip switching

observed in figure 3.11 strongly favors the second explanation since a transfer of material

between the tip and the substrate would leave visible defects on the surface, which are not

observed. The two states of the tip correspond then to two local minima in a potential energy

surface, as shown schematically in figure 3.12. Such two-level systems (TLS) have been

invoked in the context of DFM to relate the energy dissipated between the tip and the surface

to adhesion hysteresis [Sasaki2000, Kantorovich2004] and to interpret abrupt jumps in ∆f(D)

approach curves on KBr (001) in terms of structural changes of the tip apex [Hoffmann2007].

In both cases, these changes are related to the deformation of the potential energy surface of

the total tip-substrate system as the tip approaches the substrate. The step-induced switching


74

of figure 3.11 can be explain in a similar way if one assumes that the double well potential is

deformed by the interaction of the tip with the monoatomic step according to the scheme of

figure 3.12. In the dual well potential of figure 3.12, one well corresponds to a tip terminating

with a K+ ion while the other well represents a tip terminating with a Br- ion.

Figure 3.12: Qualitative sketch of the deformation of the potential energy curve of the tip-substrate system as a function of the tip position relative to a monoatomic step.

In states A and D, far from the step, the potential energy surface of the system is the same, but

in A the tip is in the Br- state, while in D it is in the K+ state. These two states are separated by

a barrier that hinders the tip switching at RT. We consider in figure 3.12 that the K+ tip is

more stable than the Br- tip, but this assumption is arbitrary. Close to the step, the Br-→K+

transition happens when the potential energy surface takes the shape shown in state C; the K+

state is stabilized relative to the Br- state; the inter-well barrier is low enough to allow the

transition. In contrast, the K+→Br- transition is associated with state B, where the Br- state is

stabilized. This model, which is is the minimal way to explain the observations of figure 3.11,

requires C to be on the right of B. But, as already mentioned, no significant difference can be

observed between the forward and the backward images in figure 3.11-a and 3.11-b and in the

profiles of figure 3.11-c and 3.11-d. This observation implies that the locations of B and C are

too close to be distinguished in the images of figure 3.11. The tip polarity reversal should then

happen on a very short length scale, well below 1 nm.

3.3.6 Tip polarity reversal on two successive monoatomic steps

More insight can be gained by considering the images of two successive steps like in the

upper part of figure 3.13. In this area, on the middle terrace, the state of the tip depends on the

direction of scanning. It is K+ for the forward scan and Br- for the backward scan. In contrast,


75

in the lower part of the images, the left step is no more in the image frame and the images

become similar to the images of figure 3.11.

Figure 3.13: Observation of tip polarity switching on two successive step edges. (a) Forward z, (b) backward z, (c) ∆f and (d) W image. Image size: 20 nm x 13 nm, ∆f=-210 Hz, Ap-p=7 nm

(γ=-4.8 fN m1/2).

These observations can be rationalized with the help of figure 3.14a, which generalizes the

drawing of the potential energy curves of the TLS (figure 3.12) to the two steps cases.

Figure 3.14: (a) Deformation of the potential energy curve of the tip-substrate system in the two steps case of figure 3.13. (b) Profiles corresponding to the lines drawn on the images of figure 3.13. Continuous line: forward z profile (left scale), dash line: backward z profile (left

scale) and dotted line damping profile (right scale).


76

At the beginning of the forward scan, the tip is in the Br- state A. It crosses the L step exactly

as in the single step case, to reach the K+ state Df on the middle terrace. Then something new

happens: the system has to switch two times on the R step to end in the K+ state G at the end

of the scan. This is unavoidable, if one wants to remain consistent with the potential energy

curves proposed in figure 3.14. The tip has to switch from the K+ state on the middle terrace

to the Br- state close to the step edge, before being switched back to the K+ state by the R step.

This last polarity reversal is similar to the single step case. The only difference with the single

step case for this forward scan is the switching event from Df to E in figure 3.14a. A closer

look at figure 3.13a shows indeed that the R step is bordered on its left side by a fluctuating

line [labelled α in figure 3.13a], which as will be confirmed later by the analysis of the

topographic forward profile of figure 3.14b is precisely the locus of this K+→Br- transition.

The backward scan can be followed in a similar way. The tip starts on the right terrace in the

K+ state G, it crosses the R step as in the single step case, to reach the Br- state Db on the

middle terrace; then, the system has to switch two times to reach the initial state A. The new

switching event, specific to the two steps case is from Db to C. Here again, this transition can

be localized in figure 3.13b. It appears as a fluctuating line near the top of the L step [β in

figure 3.13b].

The location of these switching events can be determined by a detailed examination of the

profiles shown in figure 3.14b. On the left of the forward topographic profile the tip, in its Br-

state begins to climb the L step until it switches to the K+ state on the dashed line I. The tip

height z then decreases, but as in figure 3.11a and c, this decrease is compensated by the step

climbing. This Br→K+ transition produces a sharp peak in the topographic profile which can

also be observed in the profile of figure 3.11c. At the right of the middle terrace, on the

dashed line III, the profile shows an abrupt upwards jump that is located on the fluctuating

line α mentioned previously. Its value of approximately 0.3 nm is consistent with a K+→Br-

transition as was suggested by the sequence of figure 3.14a. The tip is then in its Br- state and

the profile becomes similar to the profile observed previously on the L step: it shows the same

sharp peak (dashed line IV) that we attribute to the Br-→K+ transition. On the backward

profile, the tip describes the same peak on the R step (corresponding now to a K+→Br-

transition on the dashed line IV), but stays in the Br- state for imaging the middle terrace.

Here again, the profile has the same shape as in figure 3.11d. Then, at the left side of the

middle terrace, on the dashed line II [corresponding to the fluctuating line β in figure 3.13b],

the tip height decreases abruptly by an amount of approximately 0.3 nm, which is consistent


77

with a Br- to K+ polarity reversal. The tip is then in its K+ state and describes a profile similar

to the profiles observed previously, switching back to the Br- state on the line I.

In figure 3.14b, the gray lines are qualitative extrapolations between the trajectories of the K+

tip between the middle and upper terrace (R step) for the forward profile and of the Br- tip

between the middle and lower terrace (L step) for the backward profile. The resulting

completed profiles allow us to position the switching events relative to the steps.

A detailed analysis of the images shows that the position and the height of the peaks which

are generated by the Br-→K+ transition during the forward topographic scan and by the

K+→Br- transition during the backward topographic scan depend on the position along the

step edge. This observation can be understood if one assumes that the transition probability is

modulated by the atomic structure of the step edge. This modulation is at the origin of the

triangular-like patterns mentioned previously, as shown in figure 3.15. The right apex of the

triangle belongs to the Br- ions lattice of the upper terrace (disks in figure 3.15) while the left

two apices (arrows in figure 3.15) correspond to the transition peaks. These peaks have no

clear relation with the ions positions of the substrate. There are two peaks in a period along

the step edge, suggesting that they are related to the bridge sites between the two types of ions

that constitute the step edge.

Figure 3.15: Detail of the triangular-like patterns (black triangles). The position of the Br-

ions of the upper terrace is indicated by disks.

Finally, notice the perfect correlation between the state of the tip and the level of dissipation

shown in figure 3.13d and 3.11b. The dissipation level is higher when the tip is in the K+

state, even for a short time as for instance between lines I and II on the profiles of figure

3.11b, confirming our analysis of these profiles.


78

3.3.7 Discussion of the topography results

The topographic profile of figure 3.9d shows that the tip-substrate distance decreases by 0.3

nm when the tip switches from Br- to K+. A naive interpretation would be to assume that the

switch results from the diffusion of a K+ ion between a site at the apex and a site on a side of

the tip, leading to its shortening by an amount of the order of 0.28 nm, the ionic diameter of

K+. But this is not exact, since the data of figure 3.9 being obtained in the constant-frequency

detuning mode, the change in the tip-substrate interaction force resulting from this structural

modification has to be taken into account. Nevertheless, this 0.3 nm distance jump suggests

that the tip polarity reversal is associated with a minor change of the tip apex structure,

involving the rearrangement of only a few ions.

The tip switching also induces a lateral displacement of the image, corresponding to the

displacement of the outermost atoms of the tip, which dominate the imaging process. In figure

3.13, the same object -the middle terrace- is imaged with the two types of tip. The two images

of this object are separated by less than 1 nm, in the direction perpendicular as well as in the

direction parallel to the steps (the kink on the left step edge can be taken as a reference). This

observation confirms our previous conclusion that the tip switching should involve very few

atoms.

As already mentioned, the transition from B to C in figure 3.9 happens on a very short length

scale. This indicates that the interactions involved in the tip switching vary significantly on

this scale. This observation, together with the fact that the images showing tip switching are

in the atomic resolution regime, suggests that the forces responsible for switching are the

short-range electrostatic forces between the polar tip apex and the ions of the surface. It is

likely that relaxation effects, which are more pronounced on the step edges, play a role by

extending the interaction range in their immediate vicinity [Livshits1999]. This would

produce the highly localized interaction necessary to account for the sharpness of the

transition from B to C.

The observation of the fluctuating lines in figure 3.13 indicates that the probability of the

transitions from Df to E and Db to C varies from 0 to 1 on a length scale that is given by the

amplitude of the fluctuations of these lines, which corresponds to a few Angstroms. These

transitions have a probabilistic character, in contrast to the B to C transition that is

deterministic at the observed scale. In the regions covered by the fluctuating lines, the

potential energy surface is deformed in such a way that the probability to cross the inter-well

barrier is finite, but less than unity.


79

These examples give an idea of the structural modifications that could be at the origin of the

switching phenomenon we observe. It is not possible to propose a more specific model,

without the help of calculations of the activation barriers involved, which should satisfy the

conditions mentioned in discussing the deformations of the potential energy surface in the

vicinity of a step (figures 3.9 and 3.13) and in the previous paragraph.

In the following, we show that this bistability is also observable in the force-distance

measurements performed with the same tip.

3.4. Force spectroscopy with the bistable tip

Figure 3.16a shows an image of a monoatomic step, which displays the tip switching from a

Br- to a K+ tip discussed previously. The tip stays Br- on a few scan lines, as seen on the

profile in figure 3.16b.

(a)

(b)

Figure 3.16 (a) Image of a monoatomic step with the bistable tip ∆f = -230 Hz, A=3.2 nm. Size:10 nm x 7.8 nm. (b) Profiles corresponding to the lines drawn in figure a.

The ∆f spectra obtained at the places indicated in figure 3.16a are displayed in figure 3.17.

Figure 3.17: ∆f spectra obtained at the places indicated in figure 3.16a. The colors

corresponds to that indicated in Fig. 3.16a.


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The tip was retracted and approached from the imaging set point twice, with a z amplitude of

1.14 nm. The 7 spectra demonstrate that the approach and retract curves are very

reproducible. No tip irreversible evolution could be detected during the acquisition of these

data.

3 types of spectra can be distinguished: s1 and s4, which are clearly obtained with a Br tip, s3

and s7 which are obtained with a K+ tip and s2, s5 and s6 obtained in proximity to the step,

meaning that it is not possible to specify unambiguously from the image the type of tip at the

beginning of the spectrum.

(a)

(b)

Figure 3.18: (a) Average ∆f spectra obtained at the places indicated in figure 3.16a. The colors corresponds to that indicated in Fig. 3.16a. (b) s1 and s4 have been shifted by 0.27

nm toward the right, s7 has been shifted by 0.03 nm toward the right.

s1 and s4 present the usual, monotonous decrease of the frequency when approaching the

substrate. The average of the corresponding 4 spectra for s1 and s4 is plotted in figure 3.18a.

In contrast, s3 and s7 present a strongly anomalous non-monotonous behavior. The

corresponding averages are also plotted in figure 3.18a. When the tip approaches the surface,

the resonant frequency decreases, as usual in response to attractive forces. Around z=2.5 nm

(D in fig. 3.18b), the frequency reaches a minimum and grows again, suggesting the onset of a

repulsive interaction. ∆f then rises to a maximum (B in fig. 3.18b) and decreases again to

reach A.

There are 3 points of intersection of s3 and s7 with ∆f=-230 Hz, the frequency shift used for

the image of figure 3.16a (figure 3.18). The middle point C, near z=0.2 nm cannot be used for

imaging because around this point the frequency shift increases when the tip gets closer to the

surface, leading to an instability. The two remaining points could be used for imaging. It turns

out that A is used for imaging with the K+ tip, as demonstrated by the location of s3 and s7 in

the image of figure 3.16a. The other point (E) is not used for the distance regulation in this

case.


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The clue to the understanding of this experiment is that A and E are separated by a z distance

of approximately 0.27 nm. This z change corresponds clearly to the switching from a Br- tip

to a K+ tip measured previously. In addition, as shown in figure 3.18b, the s1 and s4 ∆f curves

are very similar to s3 and s7 when they are shifted by this quantity toward the right on the z

axis. These observations suggest that the point E in figure 3.18b is the regulation point for the

Br- tip in the image of figure 3.16a. In other words, the tip switching can be related to a

unique ∆f(z) curve presenting two stable operating points, A for the K+ tip and E for the Br-

tip. When working with the K+ tip, the tip structure switches back and forth between a K+

termination and a Br- termination during each oscillation cycle, while the Br- tip stays

unchanged.

Note that from the point of view of the control system these two operating points are

equivalent. What is measured is the frequency shift. That the tip switches or not does not

matter. The intuitive idea that the tip would switch from K+ to Br- during its first retraction

from the surface and stay Br- afterwards is wrong!

The s3 and s7 curves are separated by 0.03 nm. This separation, which is lower than the

corrugation observed in figure 3.16a (around 0.07 nm according to the profile shown in figure

3.16b), is probably related to the fact that the locations of s3 and s7 relative to the atomic

contrast are different. The shape of s6 strongly suggests that this curve corresponds to a K+

tip, even if this assumption is not clearly supported by the image of figure 3.16. The

separation between s6 and s1 is of the order of magnitude of the corrugation observed on the

profile of fig. 3.16b.

Figure 3.19:Force (right scale) and dissipation (left scale) curves corresponding to s1 (black), s3 (green) and s6 (magenta).

Dissipation curves taken simultaneously with the data of figure 3.16a are presented in figure

3.19 as well as the force curves derived from the ∆f spectra by the Sader-Jarvis method. Note

that this derivation was done only for the purpose of qualitative evaluation, because, as can be


82

seen in figures 3.17 and 3.18, the data were not measured far enough from the surface to reach

the region where the tip-substrate interaction could be neglected. A clear correlation between

these two sets of curves can be observed. The dissipation is negligible for the s1 curves, when

no switch is active, while the dissipation signal increases for the s3 and s6 curves, when the

tip switching is activated at each oscillation cycle. Interestingly, the dissipation levels for s3

and s6 near the imaging operation point are comparable to what is observed in fig. 3.9f, 3.11h

and 3.14b for the mean dissipation level (≈30 meV/cycle) as well as for the contrast in

dissipation atomic (≈20 meV/cycle). We can then relate the origin of the atomic contrast in

the dissipation image to an atomic site-sensitive tip switching effect.

3.5. General discussion

Let's first summarize the results obtained so far:

• We have shown that in certain imaging conditions, the polarity of the tip can change

systematically and reversibly when crossing a monoatomic step. This was interpreted

in terms of a TLS.

• The experimental observations suggest that the change of the structure concerns a very

small region of the apex of the tip, possibly a few atoms.

• The force and dissipation curve obtained with the same tip show that the tip explore its

two states at each oscillation cyle when it is K+ but not when its is Br-

• A contrast is observed on the dissipation images for the K+ tip in correlation with the

exploration of these two states.

An example of a specific model for a TLS near the apex of a KBr tip was recently proposed in

[Hoffmann2007]. One of the corners of a cubic KBr cluster constitutes the tip. The TLS is

associated with the displacement of one the ions of a KBr "molecule" adsorbed on one of

(001) facets in the immediate vicinity of the tip apex in such a way that the tip polarity is

reversed from one state to the other. That such a tip can indeed form can be appreciated from

the simulations of contact between an ionic tip and the surface that have been reported for LiF

[Shluger1997] or NaCl [Lantz2006]. In these works, it is shown that the contact is followed

by the stretching of a string of ions from the surface as the tip separates from the substrate.


83

We believe that the breaking of such a string could produce a tip with a few unstable adsorbed

molecules that could be a good candidate to explain our observations.

Figure 3.20: From [Hoffmann2007]

More recently, a theoretical study showed that the potential energy surfaces of realistic Si tips

exhibit many energetically close local minima that correspond to different structures

[Ghasemi2008], with the consequence that these tips easily deform in a reversible manner

under the influence of the tip-substrate interactions, causing an increase in the tip-substrate

dissipation.

Analogous phenomena were already observed on force curves (see figure 3.8b and

[Schiermeisen2006]), and invoked to explain the observation of atomic contrast on dissipation

images [Sasaki2000, Kantorovich2004] by the so-called adhesion hysteresis mechanism. A

work is performed by the tip at each oscillation cycle if the distance at which the switch

occurs is not on the average the same when the tip approaches or retracts from the surface.

The energy balance that should be respected shows that the work is given by:

∫−= dzzFW )(

Figure 3.21 shows where the dissipation cycle could be involved in our measurements. We

consider that the Br- tip is described by the right part of the ∆f curve extrapolated by the dash

red line in the left part of the graph and that the K+ tip corresponds to the dash black curve.


84

Figure 3.21: Tentative positioning of the hysteresis cycle on the ∆f experimental curves. The

dash red (resp. black) line corresponds to the Br- (resp. K+) tip.

If the operating point is A, the tip, which has a Br- ion at its apex when far from the substrate,

switches to a K+ terminated tip from the point labeled 1 on the red curve to the point labeled 2

on the black curve in figure 3.21, presumably under the influence of a repulsive force, as

indicated by the increase of ∆f in this part of the red curve. When the tip retracts it switches

back from the black to the red curve near 3. Note that only the part of this cycle that is

described counter clockwise, that is 1->2->3->1 will dissipate energy [Sasaki2000,

Kantorovich2004]. This is consistent with figure 3.19, where it can be seen that for the s3 and

s6 curves, the dissipation starts to increase near the maximum of the force curves, which

corresponds approximately to point 3 in figure 3.21. Note also that the distances at which the

tip switches are determined by the thermal crossing of an energy barrier, which is a random

event. The switching probability becomes important only in the neighborhood of point 3 and

the measured ∆f curve is an average between the two extrapolated ∆f curves corresponding to

the two states of the tip in this distance range.

3.6. Conclusion

The main result of this chapter is the demonstration that the atomic contrast observed in the

dissipation images of KBr(001) is related to an adhesion hysteresis phenomenon, which

involves a TLS localized near the apex of the tip. No direct experimental evidence for the

contribution of a TLS as the main source of dissipation has yet been obtained, because

accessing to an individual dissipating event is difficult due to its stochastic nature and the


85

high frequency of the cantilever oscillation. Nevertheless, the detailed investigation of the tip

switching on the monoatomic steps of KBr, coupled with the observation of bistable ∆f curves

provide a consistent picture which fully confirms the adhesion hysteresis mechanism.

We believe that this type of phenomenon is quite general and that it should be possible to

reproduce the observations that were made in this chapter with other tips. More precise

experiments are under way to provide a better basis to a modelization of the phenomenon.


86

Chapter 4. The Pd/Al10O13/NiAl(110) system

87

CHAPTER 4

The Pd/Al10O13/NiAl(110) system

4.1 Introduction

Aluminum oxide films grown from aluminum rich metallic alloys play an important role in

technology and surface science. For the automotive industry as supports in heterogeneous

catalysis [Khan2006], for the electronic industry in metal-insulator-metal architectures

oriented toward the development of electron-emitting devices [Yoshitake2006] and for STM

studies of adsorbed single molecules [Mikaelian2006], among many other applications.

Nowadays, the film formed by oxidation of the NiAl(110) surface is one of the most

intensively studied surface oxide.

One particular attractive characteristics of this system is that, if the film is thin enough, it can

be studied by STM and STS. Numerous STM studies have led to significant advances in the

understanding of the structural and electronic properties of this complex system.

We have explored this system for the following reasons: firstly, the oxide film grown on

NiAl(110) provides a suitable support for the deposition of organic molecules [Qiu2004].

Secondly, for very low rate of metal deposition, metal growth on this surface exhibits a

pseudo two-dimensional growth mode [Yoshitake2006]. These characteristics make this

system an attractive candidate that can be explored with aims to construct metal-molecule-


88

metal junctions with the AFM-nano-stencil experiments [Guo2007]. Thirdly, one can perform

comparative experimental studies by STM and by NC-AFM.

In this chapter, we show the experimental results obtained by STM and STS techniques on the

Pd/Al10O13/NiAl(110) system. In section 4.2 we describe the NiAl(110) surface at the atomic

level and the preparation procedure to atomically clean the crystal under UHV conditions.

Section 4.3 is dedicated to the aluminum oxide film. We describe the preparation procedure,

the formation of the oxide film, and the structural features extracted from the STM images,

from the mesoscopic scale down to the atomic level. STS measurements on the oxide film are

detailed in section 4.4. First, the influence of the oxide film electronic band gap on the STM

images is discussed. Then, we describe our measurements for two different alumina film

fabrication procedures. Finally, the last part of the chapter, section 4.5, is devoted to the

palladium growth experiments on alumina. In this section, we discuss the formation of

palladium particles and their morphology on top of the alumina film for two coverage. These

results are then compared with those found in recent published papers.

4.2 The NiAl(110) intermetallic alloy

NiAl is an ordered binary intermetallic alloy that has been studied by many groups

[Davis1985, Lui1989, Hansen2001a, Jaeger1991]. The NiAl crystal has the cesium chloride

(CsCl) structure, cubic centered type with nickel atoms in every corner enclosing one

aluminum atom in the center and with a lattice parameter of 2.887 Å.

Figure 4.1a shows the NiAl(110) surface top view atomistic model of a 3x3 super cell, where

the pink balls are the aluminum atoms and the blue balls the nickel atoms. The (110) surface

unit cell dimensions are a1=2.887 Å and a2=4.083 Å, as shown in figure 4.1a. This surface is

affected by a relaxation of the last atomic layer, the aluminum atoms are displaced toward the

vacuum and the nickel atoms are displaced toward the bulk producing a ripple of 0.22 Å

[Davis1985], as shown in the side view of the atomistic model in figure 4.1b. An atomic

resolution STM image of a 4 nm by 4 nm area is displayed in figure 4.1c. It gives the

characteristic directions of the NiAl(110) surface, which will be used in the following since

all the images presented in this chapter have been obtained on the same sample. According to

[Hansen2001a], aluminum atoms appear higher, with white contrast while nickel atoms

appear lower, with black contrast. Finally, on figure 4.1d, we show a tunneling spectrum,

which exhibits the characteristic behavior of a metallic substrate.


89

a)

c)

b)

d)

Figure 4.1: Atomistic model of NiAl(110) surface. (a) Top view. (b) Side view showing the relaxation. Atomic resolution STM image of a 4 nm x 4 nm area,

V=-0.6 V, I=0.09 nA. d) Tunnel spectrum.

4.2.1. Preparation procedure

The first step to prepare a clean NiAl surface is to remove the contaminated surface layer by

Ar+ sputtering. Normally the energy used is 1.5 keV but sometimes it is required to increase

this parameter, with caution, because sputtering at 5 keV produces dislocations in the crystal.

We have found that sputtering the sample at 1.5 keV to 3 keV during 40 minutes with an

emission current of 3 µA gives satisfactory results.

The next step is to anneal the NiAl(110) crystal. The key for a good preparation is to use the

right temperature of annealing, which is around 1100 °C [Song2005a]. Figure 4.2 shows the

progression from an insufficiently annealed surface to a surface that presents large terraces

separated by monoatomic steps. Four preparations at ~ 750 ºC, ~ 806 °C, ~ 970 °C, and

~ 1150 °C show that annealing with temperatures around 1000 to 1100 °C is necessary to heal


90

the defects created by ion bombardment. At least 10 to 20 sputtering-annealing such cycles

are needed to perfectly clean a new sample.

54nm 52nm 55nm

a) ~ 750 ºC b) ~ 806 °C c) ~ 970 °C d) 3D view at ~ 1150 °C

02468

1012

1 31 61 91 121

151

181

211

241

Distance [nm]

Hei

ght [

Å]

e)

Figure 4.2: (a) to (d) shows a series of annealing temperatures from which it is possible to follow the formation of the NiAl(110) terraces as well as the disappearance of the white balls

present in the first image. e) Profile of image d.

The mean terrace width in figure 4.2d is 60 nm and the monoatomic step height is measured

at ~2 Å as expected from the NiAl structure (figure 4.1).

To clean organic contamination, it is sometimes useful to enter oxygen during the annealing

process at a pressure of 1x10-7 Torr for 10 minutes. But this procedure shortens the lifetime of

the filament of the electron bombardment heater.

When starting from an oxidized, but otherwise clean sample, ion bombardment is usually not

required, an annealing step is sufficient to get a clean well-ordered surface.

4.3. The alumina film formed on NiAl(110)

4.3.1. Preparation procedure

The preparation procedure can be achieved by a two steps process [Jaeger1991] or by a single

step oxidation [Lay2005], both of them can oxidize the surface at 1200 L dose, where

1L=1x10-6 Torr s:


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• The two step process: In the preparation procedure reported in [Jaeger1991], the

sample is first oxidized, and the resulting film is annealed. Once the NiAl(110) crystal

has been cleaned, oxygen is introduced into the preparation chamber at a pressure of

1x10-6 Torr during 20 minutes for a 1200 L dose, while keeping the sample

temperature at ~280 °C. The second step consists in annealing the sample at

temperatures around ~695 °C to ~927 °C [Lykhach2005, Jaeger1991] to induce the

crystallization of the oxide film. Alumina layers produced by this method have a

thickness of about 5 Å. This method is the most used nowadays, and is the one chosen

for our experiments. We will describe it in more details in further sections.

• The one step process: The alumina thin film can also be formed directly at

crystallization temperatures, around ~797 °C [Lay2005], where the formation of the

alumina is in competition with the decomposition of the film. This process requires a

lower oxygen partial pressure in the chamber, which can produce a better oxide film

[Yoshitake2006, Yoshitake2004, Lay2002]. However, longer oxidation times are

required for the same dose of 1200 L. For this preparation it is required to find the

equilibrium temperature point between three important kinetic processes: oxidation,

crystallization and decomposition. This method can produce alumina films with a

thickness greater than ~8.5 Å [Yoshitake2006].

To summarize, the complete preparation procedure of the alumina film comprises typically

the following steps. The NiAl(110) surface is cleaned by one or several cycles of argon

sputtering at 1.5 to 3 keV, with an ion current of 3 µA, during 40 minutes, followed by

annealing at 1100 °C during 20 minutes. Once the NiAl(110) surface is clean and well

structured, we keep it at a temperature that can go from 155 °C to 600 °C to oxidize it with a

1200 L oxygen dose at a pressure of 1x10-6 Torr, during 20 minutes. Finally, an annealing at

800 °C for five minutes is performed to crystallize the oxide film.

This fabrication method leads to the formation of a well crystallized alumina layer that

decorates the NiAl(110) terraces all over the surface. In the following, we describe the general

structural features of the alumina film, starting from the mesoscopic scale.

4.3.2. The oxide film meso scale morphology

Figure 4.3 shows a 200 nm x 140 nm STM image, obtained at a sample bias voltage of +2.88

V and a tunnelling current of 8.2 pA. Four terraces can be distinguished, separated by steps.


92

The single steps present an apparent height of about ~ 2.9 Å, while double steps, marked with

black arrows, show an apparent height of about ~ 6.2 Å.

The surface displays flat defectless areas, separated by characteristic lines. These lines have

an apparent height around 1.5 Å and are organized in approximately periodic networks.

According to the bibliography [Müller1990, Libuda1994], they have two distinct origins:

• Due to the two fold symmetry of the NiAl(110) surface, the alumina film grows in

two reflection domains (RD), designated as domain A and domain B, which are

separated by Reflection Domains Boundaries (RDB). RDB show no characteristic

order.

• The alumina film unit cell is commensurate with the NiAl(110) unit cell in a

direction which is close to the NiAl[ ]011 direction, because of a "row matching"

phenomenon, which will be explained in section 4.3.4. This commensurability

generates stress along the [ ]011 direction and leads to the formation of line

defects, known as Antiphase Domains Boundaries (APDB), which separate two

domains of the same type. They exhibit two different morphologies: (1) straight

shape, denoted as IA or IB, and (2) zigzag shape, denoted as IIA or IIB

[Kulawik2003].

Figure 4.3: STM image of a sample prepared by the two steps process consisting in a double (2 x 1200 L) oxidation at ~285 °C followed by crystallization at ~800 °C during 10 minutes.

200 nm x 140 nm, +2.88 V, 8.2 pA.


93

In the image of figure 4.3, the lower and upper terraces, mainly covered by domain B, exhibit

APDBs, while the A and B domains that coexist on the large intermediate terrace are

separated by a RDB. An area labeled as DD (for Different Domains) seems to result from a

bad crystallization of the alumina, probably due to the presence of impurities during the

oxidation process.

4.3.3. Intermediate scale images

The formation of the oxide film on top of the NiAl(110) crystal can take place as well at a

slightly lower oxidation temperature, around ~155 °C. Figure 4.4 shows a STM image of the

alumina film when imaging at a relatively high voltage, outside the band gap of the oxide

film, when the unoccupied states of the oxide participate in the tunneling current. At a sample

bias voltage of +4.2 V and a tunneling current of 10 pA the main structural features of the

oxide film can be imaged.

Figure 4.4: STM image of a double 1200L oxidation (2x1200L) at ~155°C, subsequent annealing at 812°C and 850°C during 10 minutes for each one. Scanning zone of 28.5 nm x

19 nm, + 4.2 V of bias voltage and 10 pA for the tunneling current.


94

In Figure 4.4, the STM tip was positioned in a region of the surface that was mainly covered

by the B domain. The image shows an area of 28.5 nm x 19 nm, where well formed straight

IB-APDB are present. They exhibit a height of about ~0.8 Å with a width of ~2 nm (see the

profile in figure 4.4). In addition, the surface presents a network of dark straight lines, parallel

to the IB-APDB. These features are typical characteristics when imaging in this voltage range

[Nilius2004]. A unit cell is positioned on the image of figure 4.4. One of its sides is parallel to

the dark lines and its size in the perpendicular direction corresponds to 2 lines (as it can be

distinguished in the upper right corner of the image, the period of the dark lines network

corresponds in fact to two lines).

Figure 4.5 shows a higher resolution STM image of a domain boundary (RDB) separating a A

and a B domain. A careful examination of the atomic features which are discernible in this

image allows to distinguish the periodicities of the alumina film and to position the unit cells

for the two A and B domains.

Figure 4.5: 17 nm x 20 nm STM image at -2.11 V, 15 pA. The alumina film preparation is the same as for figure 4.4 (2x1200 L at ~155°C and annealed at 812°C and 850°C).

In the following, we would like to schematically describe the alumina unit cell and the

orientation of the two reflection domains with respect to the NiAl(110) support.

4.3.4. Structural relation between the alumina unit cell and the NiAl(110) substrate

Some characteristics of the alumina film are summarized in figure 4.5, which shows the

alumina unit cells of the A and B domains and their orientation with respect to the substrate as

well as the different types of domain boundaries.


95

Figure 4.5: (a) Orientation of the alumina unit cells of A and B domains relative to the NiAl(110) substrate and orientation of the domain boundaries. (b) Illustration of the near

coincidence between the diagonal of the unit cell and a characteristic distance of the NiAl(110) substrate.

The straight APDB (IA or IB) run parallel to the short side of the oxide unit cell (b1), while

the zigzagged APDB (IIA or IIB) are parallel to its diagonal. A RDB is present along the

[ ]011 direction, but these boundaries are also observed in different directions. The alumina

unit cell has a rhomboidal shape, which is almost rectangular, with dimensions b1=10.55 Å,

b2=17.88 Å and angle α=88.6°, as shown in figure 4.5a. It covers 16 NiAl(110) units cells.

The long side of the unit cell makes an angle +/-θ with respect to the [ ]011 direction, as

indicated in figure 4.5. Values of θ found in the literature range from ~24.1° to ~24.74°.

Figure 4.5b shows that the length of the diagonal of the unit cell nearly coincides with a

characteristic distance of the NiAl(110) substrate. Indeed, the alumina film is commensurate

to the substrate in this direction. This commensurability generates stress along this direction

and leads to the formation of the APDB.

We now proceed to go into more detail by increasing the resolution of the STM images in

order to access the atomic structure of the film.

4.3.5. Higher Resolution image, with three domain boundaries

After reducing gradually the sample bias voltage and the tunneling current, atomic features

started to appear. Figure 4.6 shows a higher resolution STM image of a 18 nm by 20 nm area

of the alumina film. The sample bias voltage is +150 mV, and the tunneling current 4 pA.

This image shows weakly the atomic characteristics of the film. In this part of the surface, the

alumina film exhibits three IA-APDB that separate four A domains. These APDB are


96

composed of straight parts, which belong to the IA types and kinks, which reveal small

sections of IIA-APDB. For these tunneling parameters of imaging, the APDB appear darker

than the A domains, in contrast to the image presented in figure 4.4, which was obtained at

+4.2 V, where the IB/IIB-APDB presented double lines with a brighter contrast. This

inversion of the contrast as a function of the bias was systematically observed in our

experiments.

In this image (fig.4.6) the drift of the STM was minimal and the imaging conditions were

stable. As a result, it was possible to position and to measure the unit cells in the A domain

and in the IA domain boundaries. The six rectangles that have been drawn in figure 4.6

include a row of 4 A domain unit cells, labeled A (blue color), limited by two IA-APDB unit

cells, labeled IA (black color). Their dimensions are ~17.7 Å x 10.6 Å for the A domain and ~

20.46 Å x ~10.6 Å for IA domain boundary.

Figure 4.6: High resolution image, 18 nm x 20 nm area at 150 mV and 4 pA. Same preparation conditions as for figure 4.4.

If we look carefully, this image can give us the atomic structure of the IA and IIA-APDB,

however from now on we will focus only on the A-RD and IA-APDB unit cells. Let’s take a

look at the structure with a better atomic resolution image, obtained by lowering again the

applied bias voltage and reducing the scanning area.


97

4.3.6. Atomic resolution image of the oxide film surface.

We decreased again the applied bias voltage to +50 mV while keeping the same tunneling

current of 4 pA. For these tunneling conditions the atomic features of the oxide film could be

imaged more precisely.

The image presented in figure 4.7 shows a 10 nm x 10 nm surface area. An alumina A domain

goes from the lower left part to the upper right part of the image, in the middle of two IA anti

phase domain boundaries. Two repetitive patterns can be identified, especially over the bright

contrast zone. They correspond to the repetition of the unit cells of the A-RD and the IA-

APDB, indicated with rectangles and labeled as “A” and “IA” respectively. Note that the

corners of these cells are located on a dark spot surrounded by a clear circle, which can be

used as a guide to identify the unit cells.

Figure 4.7: Atomic resolution image of a 10 x10 nm surface at +50 mV and 4 pA. Same preparation conditions as for figure 4.4.

As will be discussed in the following, this type of image has been used to determine the

atomic structure of the film [Kresse2005, Schmid2006], by considering that the position of

the oxygen atoms is approximately given by the white bumps that are observable in the

image. Before this discussion, we present the resulting model in the following section.


98

4.3.7. The atomic model of the A domain

The bulk alumina chemical formula is Al2O3. Alumina can exist in a number of crystalline

phases, labeled for instance κ, γ, δ, θ and α. The most stable phase is α-Al 2O3 (sapphire),

which crystallizes in the corundum structure. In the α-Al 2O3, the oxygen atoms are arranged

in hexagonal closed-packed planes and the aluminum atoms are placed in 2/3 of the

octahedral interstitial sites between the oxygen planes [kresse2005]. The oxide film formed

on top of NiAl (110) has been previously described considering the gamma and kappa phases

of the bulk alumina structure in order to propose some structural models [Jaeger1991,

Stierle2004]. However, it was found that the film structure differs from all the other alumina

phase structures, as revealed in [kresse2005] by STM experiments coupled to Density

Functional Theory (DFT) calculations.

Figure 4.8: Atomistic model of the alumina unit cell, from reference [kresse2005]

The room temperature STM images reported in this work presented square and triangular

features given by the arrangement of the bright contrast detected, and it was attributed to the

oxygen surface atoms positions. In other experiments [Kulawik2003], it was suggested that

the aluminum atoms appear as bright protrusions. This contrast change depends on the bias

voltage and the tip condition. An initial model was built from these positions and

subsequently refined by DFT calculations. The final result is shown in figure 4.8. Square and

Oxygen atoms

Aluminum atoms


99

triangular features are marked in green color. The oxygen atoms keep a tetrahedral or

pyramidal coordination with the oxygen tip pointing toward the substrate as shown by the

white lines outside the unit cell in figure 4.8. These building blocks are responsible for the

triangular and square features in STM images. It was proposed that the existence of square

pyramids can be understood as truncated octahedra, knowing that oxygen octahedra with a

central metal atom are the most important building blocks in metal oxides structures.

Figure 4.8 shows that at the surface, the unit cell is composed of 28 oxygen atoms, indicated

with orange color and red contour. Slightly below this oxygen plane are 24 Al atoms, with

light blue color and contoured with dark blue. The oxide film also contains another oxygen

layer with 24 atoms in the unit cell and then the aluminum interface layer with one atom by

the substrate unit cell, that is 16 atoms. The interface Al atoms have a strong preferential

position above the Ni atoms of the substrate.

The arrangement of the atoms in the oxide film is close to p2gg planar symmetry. The

alumina unit cell covers 16 NiAl(110) unit cells so that the stoichiometry of the film in the

reflection domain is (NiAl)16 Al16O24 Al24O28, or (Al6O7)interface /(Al4O6)surface or simply as

Al 10O13, which is different from the usual Al2O3 bulk stoichiometry originally proposed.

While the composition of the surface layer corresponds to Al2O3, the interface layer is more

Al rich. This is related to the fact that the interface Al atoms are strongly bound to the Ni

substrate.

4.3.8. Interpretation of the atomic resolution image of domain A

Figure 4.9 shows a 3nm x 3nm zoom on the A domain of figure 4.7. The corrugation is of the

order of 0.9 Å in the unit cell. Assuming that the bright features correspond to the oxygen

atom, it was possible to approximately position 28 oxygen atoms, indicated with red circles in

figure 4.9, in the alumina unit cell. These atoms were then gathered in order to identify the

square and triangular features that appear in the model of figure 4.8. It is seen that all the

features of the atomic model are found in the image, with relative position that are in

agreement with the model. These data confirm the analysis of [Kresse2005].

We now introduce the structural model for a IA-APDB.


100

Figure 4.9: 3 nm x 3 nm zoom of image 4.7 on two alumina unit cells of the A domain showing atomic resolution. Bias voltage +50 mV and tunneling current 4 pA.

4.3.9 The IA-Anti Phase Domain Boundary atomistic model

Figure 4.10 shows the atomic structure of the anti phase domain boundary of type IA,

obtained by LT and RT STM experiments and refined with DFT calculations [Schmid2006].

The cell is the same as that of the A-RD previously described but now it is split in the middle

Figure 4.10: Atomistic model of the IA-APDB from [Schmid2006]

Oxygen atoms

Aluminum atoms

New O xygen atoms

New aluminum atoms

Missing Oxygen atoms


101

of the unit cell (in the b2 side marked with a white dashed single line in the A-RD previously

shown in figure 4.8). The two pieces are moved apart by 3 Å, this is indicated here by the

double white dashed line. This separation provides an additional space that allows a reduction

of the compressive stress in the alumina film. Nevertheless, new atoms are inserted in this gap

as shown with yellow and light blue colors in figure 4.10. A partial rearrangement of the

interface aluminum atoms is also necessary to stabilize the structure. Two oxygen atoms are

missing at the positions marked by the red dash circle lines. This oxygen deficiency behaves

like an electron donor. It produces a favorable adsorption site for electronegative species

[Schmid2006].

4.3.10. Interpretation of the atomic resolution image of the IA – APDB

Figure 4.11 shows a zoom on the IA domain boundary of figure 4.7. As for figure 4.9, it was

possible to position the O atoms by assuming that they correspond to bright contrast. The

resulting atomic arrangement confirms the atomic model of figure 4.10.

Figure 4.11: Zoom of image 4.7 on an IA-APDB. Bias voltage +50 mV and tunneling current 4 pA.

4.3.11. Meshing the surface with the repetitive pattern.

Finally, we would like to present in figure 4.12, the image already shown in figure 4.7 but

with a mesh of unit cells. We have superimposed the unit cells along one column that goes

from one IA-APDB to the other. This construction shows that it is possible to arrive at a


102

consistent interpretation of the atomic structure of a complex surface like the alumina film on

NiAl(110).

Figure 4.12: Image already shown in fig. 4.13, with the models of the A domain and the IA domain boundary superimposed.

The second part of this chapter will be focused on the electronic properties of the alumina

film that can be extracted from STS measurements.


103

4.4. Scanning tunneling Spectroscopy measurements

4.4.1. Introduction

Even though many studies have been reported on the alumina film on NiAl(110) by several

surface science experimental techniques, like angular resolved ultraviolet or X-ray

photoemission spectroscopy, low energy electron diffraction, ion scattering spectroscopy, and

high resolution electron energy loss spectroscopy, among others, we have not found much

information regarding experimental results using Scanning Tunnelling Spectroscopy (STS)

measurements to characterize a specific rate of NiAl(110) oxidation, and then compare these

results to another different oxidation rate. Important information can be obtained from STS

measurements and from the comparative results obtained. For that, it is important to briefly

introduce the electronic characteristics of the alumina film explored by STM.

The alumina film presents a well-defined two dimensional band structure due to its small

thickness [Jaeger1994]. The gap between occupied and unoccupied states in the oxide layer

was determined by X-ray absorption spectroscopy, and core and valence photoelectron

spectroscopy to be around ~6.7 eV, slightly smaller than the gap of bulk alumina, which is ~8

eV. The valence band (VB), which derives mainly from the O2p orbitals, localized at -4.5 eV

below the Fermi level. The conduction band (CB), which is dominated by the Al3s levels, is

localized around +2.2 eV above the Fermi level. This experimental study was carried out on a

600L alumina film specimen [Andersson1999].

In contrast, a NiAl(110) sample oxidized with a dose of 900 L studied by STM in

[Hansen2001b] presented an onset of tunneling near the CB side that started at a bias voltage

of ~ +1.3 V instead of +2.2 V. The origin of the discrepancy could be due to the presence of

defects in the film.

Finally, calculations of the density of states for a perfect oxide film and for a I type of APDB

were reported in [Schmid2006]. For the perfect film, the VB starts around ~-3.6 eV, while the

CB starts at ~ +2.95 eV, resulting in a total gap of ~6.6 eV. For the domain boundary, the VB

and the CB are a shifted towards negative values, the VB at ~-4V and the CB at ~+2eV,

giving a reduced total gap of ~6 eV. Three unoccupied states appear along the domain

boundary at 2.3 eV, 2.9 eV and 3.9 eV, in good agreement with the experimental results of

Nilius et al [Nilius2004] who found states at 2.5 eV, 3.0 eV and 4.5 eV by STS.

More generally, the literature shows that the electronic properties of the alumina film are

influenced by defects in the domain boundary, but also in the domains between two


104

boundaries. The presence of these defects is likely to depend on the preparation conditions in

a manner that has not yet been precisely investigated.

In the following, we use these references to discuss our measurements of the gap on a 1700 L

alumina film. Then, we show how the electronic structure of this film influences the thickness

measurement when the imaging bias voltage varies. Finally, we briefly compare the STS

measurements performed on a double 1200 L (2x1200 L) and a 7500 L film.

4.4.2. Determination of the electronic gap for a 1700 L alumina film.

Figure 4.14: 57 nm x 57 nm image of a 1700 L oxidation dose, at a sample bias voltage of +2.3 V and a tunneling current of 5 pA. The NiAl(110) was cleaned by heating at 1100°C for 20 minutes. Then, the 1700 L oxidation was performed at 1x10-6 Torr during 28

minutes, with a sample temperature of 650°C, followed by a final annealing at 800°C during 10 minutes.

The image presented in figure 4.14 shows the oxide film formed with a 1700 L oxygen dose.

APDB appear on the two terraces, separated by a monoatomic NiAl(110) step. While the

upper terrace shows IA and IIA domain boundaries, the lower one exhibits only IIA APDB.

The STS measurements were performed in a voltage range of -5 V to 5 V, in order to span the

entire gap. The I-V and differential conductance curves are shown in figure 4.15. The

tunneling set point (+3 V, 3 pA) is marked with a pink filled square on the blue curve.


105

-50

-30

-10

10

30

50

70

90

110

5.00

3.99

2.98

1.97

0.96

-0.0

5

-1.0

6

-2.0

7

-3.0

8

-4.0

9

Volts[V][eV]

tunn

elin

g cu

rren

t [p

A]

-0.5

4.5

9.5

14.5

19.5

24.5

29.5

34.5

39.5D

iffer

entia

l con

duct

ance

[pA

/V]

EF CBVB

Gap ~ 6.97eV

~ - 3.88eV ~ + 3.08eV

0eV

+3.1

+3.6 9

+4.3

+3.99

+4.8 0

Figure 4.15: I(V) and dI/dV(V) curves on the alumina film of fig. 4.14.

From these curves we can extract the following data:

• The onsets of the valence and conduction bands can be roughly estimated at ~-

3.88 eV and ~3.08 eV, giving an apparent total gap of ~6.97 eV. Obviously this is

only a simple approximation where we consider that the bands begin at the first

important onset in the dI/dV curve. With this limitation in mind, these values are in

good agreement with the calculation of Schmidt et al [Schmid2006] discussed

previously.

• Interestingly, some states are detected near the conduction band edge, at +3.1 eV,

+3.6 eV, +3.99 eV, +4.3 eV and +4.8 eV. Here again, these values are in good

agreement with the calculations of Schmidt et al [Schmid2006], where they obtained

characteristic states after the CB onset at +3.2 eV, +3.6 eV, +4.2 eV and +4.8 eV,

except that we detect an extra feature around +3.99 eV.

This two-dimensional electronic band gap produces an apparent thickness of the film on the

STM images that strongly depends on the sample bias voltage, as described in the following.

4.4.3. Apparent thickness of the oxide film as a function of the imaging bias voltage

The inset of fig. 4.16 shows a 140 nm x 200 nm image of a 1200 L film obtained at +3.2 V

and 10 pA where a pinhole, that is a small area where the NiAl surface is not covered by the

oxide was formed. Pinholes are known to appear during the crystallization of the amorphous

oxide, as reported in [McCarty2001]. The profiles extracted from images taken at different


106

bias voltages from -2.2 V to 4.2 V, along the green line indicated in the image are presented

in fig. 4.16. This line starts from the NiAl clean terrace, crosses a NiAl monoatomic step and

continues on the upper terrace, which is covered by the alumina bilayer. To measure the

alumina film thickness, it is then necessary to subtract the height of a monoatomic NiAl step,

which is approximately 2 Å.

Figure 4.16: Apparent height of the alumina film. The inset shows the 200 nm x 140 nm image of a 1200 L alumina film where these measurements were performed. This image

shows a pinhole in the alumina film where the clean NiAl(110) is present. The profiles were obtained on the horizontal green line. Tunneling parameters: +3.2 V at 10 pA.

We find that the apparent film thickness is of the order of 1 Å from -2.2 to +0.4 V and that it

begins to increase between +1.2 V and +2.2 V to reach a maximum of more than 6 Å at

+4.2 V. The apparent height of the film as a function of the applied bias voltage has been

discussed in [Hansen2001b]. It was found that from -10 V to around +1.5 V, the film presents

a very small or even null height, while from +1.5 V to +4.2 V its thickness starts to increase to

reach a maximum height of 3.5 Å. No further increment was observed from +4.2 V to +10 V.

These results are in qualitative agreement with ours, except that we find significantly higher

thickness. We think that this discrepancy is related to the different work functions of the NiAl

and the alumina-covered NiAl surfaces. The work function is lower on the alumina covered

surface [Song2005], meaning that the current decreases slower than on NiAl. One expects

then to measure a higher apparent height of the alumina film when the tip-substrate distance

increases, that is when the tunneling current is smaller. We performed these experiments with

a 10 pA current while Hansen et al [Hansen2001b] used a 600 pA current.

The small apparent height of the film measured when the bias voltage is in the gap of the

oxide is expected because the density of states of the film is very low at these energies. The


107

increase of the apparent thickness, which begins around +1.5 V and saturates near +3 V can

be clearly related with the onset of the conduction band of the oxide. The fact that this

increase starts well before the conduction band edge indicates that defects participate in this

phenomenon. In contrast, it is surprising that the valence band of the oxide does not seem to

influence the measurement of the alumina film height when the bias voltage is below the VB

edge, that is ~-4 eV. Two explanations have been proposed:

• In [Hansen2001b], a model based on the decomposition of the tunneling current in two

additive contributions, one from the NiAl substrate, and the other from the oxide film

is proposed. No physical justification for the additive assumption is given. We think

that this model is wrong.

• Subsequently, Iwasaki et al [Iwasaki2002] proposed another explanation. A one-

dimensional calculation of the tunneling current shows that the combined effect of the

lowering of the work function of the sample by the alumina film [Song2005] and of its

high dielectric constant leads to a strong asymmetry of the I(V) curve, as suggested in

figure 4.17. This asymmetry could explain that the alumina valence band does not

contribute significantly to the apparent height of the film.

Figure 4.17 (extracted from [Iwasaki2002]): The one dimensional model for electron tunneling between a W tip and the alumina/NiAl(110). a) For positive sample bias (+4.2 V), b) negative sample bias (-4 V). The potential energy diagrams for tunneling between a W tip

and the bare NiAl(110) are also plotted (broken lines). The tip-NiAl distance is 0.7 nm.

In the following, we present STS data obtained in the gap of two samples that were prepared

differently.


108

4.4.4. STS measurements in the gap of two different oxides

Figure 4.18-a shows a high-resolution STM image at -1.55 V and 8 pA. The specimen was

oxidized twice with a 1200 L dose at ~285°C and subsequent annealed at ~800°C during ten

minutes. Both reflection domains are shown, with I and II APDB. The two domains are

separated by a reflection boundary (RDB).

b) Figure 4.18: a) 50 nm x 50 nm image of a doubly oxidized sample at -1.55 V and 8 pA. b) STS

measurements on different parts of the oxide.

Three sets of I(V) and dI/dV curves have been measured on the A and B domain, on the RDB

and on the I APDB in the bias range from -1.8 V to +2 V. The overall shape of the average

curves does not depend on the location on the surfaces. Peaks appear on the conductance

curves at different positions that depend on the location. This type of peaks was

systematically observed on different samples, prepared with different experimental

conditions. We always observe an influence of the domain boundaries on the conductance

spectra, meaning that at least some of these peaks are related to the presence of defects.

Unfortunately, it was not possible to establish a clear correlation between the peaks presence

and positions and the preparation procedure.

a)


109

To illustrate this point, we present the measurements made on an oxide film prepared with a

higher partial oxygen pressure. The image of a sample oxidized at a pressure of 4.85 x 10-6

Torr during 25 minutes at 280 °C and annealed 800 °C during 6 min is shown in figure 4.19a.

Two large terraces separated by a monoatomic NiAl step are covered by mainly the A

domain. The averages of the STS measurements obtained on this image are shown in figure

4.19b.

STS average measurements on the RD

-0.05

0

0.05

0.1

-2-1

.8-1

.6-1

.4-1

.2 -1-0

.7-0

.5-0

.3-0

.10.

09 0.3

0.51

0.72

0.93

1.14

1.34

1.55

1.76

1.97

eV

-0.05

0

0.05

0.1

It dI/dV

nA nA/V

EF

-1.74+1.76

+1.2-1.16

a) b)

Figure 4.19: (a) Image of a sample oxidized at 7500 L and 280 °C (+1.86 V, 8 pA), and (b) corresponding STS measurements.

The spectrum of figure 4.19b is similar to that of figure 4.18b taken on a RD but with

different peak positions. This evolution could be related to the higher oxygen dose used to

prepare the sample of fig. 4.19, but this assumption should be completed by other

experiments.


110

4.5 Palladium growth

Figure 4.22 shows images of Pd deposits for two different coverages. As investigated in

previous works [Hanssen1999, Napetschnig2007], two types of clusters can be seen. The

largest ones appear on the domains of the oxide films while the others decorate either the

domain boundaries (fig. 4.22a) or the upper edge of NiAl steps (fig. 4.22b). The growth in the

domain is considered as resulting from a homogeneous nucleation, while the growth on the

domain boundaries is likely to be related to a nucleation of the Pd islands on the reactive sites

that were mentioned in paragraph 4.3.9, where the atomic model of the APDB was presented.

The decoration of the upper step edge could be related to a barrier preventing the diffusing Pd

atoms to go down on the lower terrace.

Figure 4.22: (a) 35 nm x35nm image at -2.3 V, 10 pA, of a 0.09ML Pd deposition, at room temperature on a 1200L oxide film, at ~250°C and subsequent annealing at 800°C. (b) 50 nm

x 50 nm, +3 V, 2.6 pA image. 0.58ML of Pd deposited on a 1200L at~ 650°C, annealed at 812°C for 10 minutes.

We did not investigate the growth of Pd in detail. Our purpose was to look if this system

could be a good candidate for electrically addressing molecules in a planar configuration with

the nanostencil experiment described in chapter 1. Figure 4.22 shows that the islands which

grow either on the domain boundary or on the step edges are separated by quite small gaps,

which could be suitable to connect a molecule, as suggested in figure 1.8. A precise

estimation of the gap size is difficult due to the finite resolution of the tip. Their height is also

difficult to measure, because it depends strongly on the imaging voltage, in a way that is not

well understood [Napetschnig2007]. Following this reference, and our own evaluations based

on the calibration of the Pd deposition, we estimate a height of the order of 1 nm, which is


111

low enough for allowing the STM visualization of a molecule connected between two islands.

A problem is that the small lateral size of these islands makes them not easily compatible with

state-of-the-art nanostencil capabilities. Exploring in detail the growth conditions could allow

growing larger islands.

An important point that should be investigated to go farther in this direction is the alumina

film quality at the scale of the microelectrodes that will be deposited by static stencil. The

leakage induced by defects should be low enough to allow measurements of the current in the

connected molecule. Note that it is possible to increase the alumina film thickness by the one

step process as reported by Yoshitake and coworkers [Yoshitake2006] (figure 4.23).

Figure 4.23 Thickness variation of the alumina film during the oxidation process for the

single step oxidation. From [Yoshitake2006].

4.6 Conclusion

The structural and electronic properties of the alumina film grown on NiAl(110) by oxidation

were investigated by STM and STS. Most of the results already published were confirmed, in

particular the atomic model for the perfect domain and for the anti phase domain boundaries

of type I. The film presents a large gap, even on the domain boundaries, which make it

suitable as a substrate to decouple molecules from the metallic substrate. Pd deposition leads

to flat islands that decorate the NiAl steps and the alumina domain boundaries, leading to

inter island gap sizes of molecular dimensions. Further work is needed before deciding to use


112

this system for the nanostencil experiments. First, ways to grow larger (but not thicker)

islands have to be found; second, the electrical properties at the scale of the microelectrodes

have to be investigated.

Chapter 5. Conclusions and perspectives

113

Chapter 5

Conclusions and perspectives

5.1 Conclusions

The main contributions of this work can be summarized as follows:

• Our RT AFM head was considerably improved by changing its light source. This

improvement was characterized by careful noise measurements, which in addition

provide a non-destructive method to measure the oscillation amplitude of the

cantilever, which is an important experimental parameter in NC-AFM. This

instrumentation work was essential for a better mastering of this complex instrument

and for establishing the basis for quantitative measurements. It opened the way to the

experiments reported in the following.

• This improved head was used to investigate the cleaved KBr(001) surface, which is a

reference surface for NC-AFM. Atomic resolution is now obtained routinely on this

surface. The methodology to get reliable ∆f(z) curves and to extract force curves from

them was developed. A set of experiments was performed with a tip that presented an

interesting behavior. On topographic, constant ∆f atomic resolution images, the atomic

contrast was observed to change in a deterministic and reversible manner when the tip

crosses monoatomic steps: the images formed on the lower and the upper terraces

present the same atomic periodicity, but with important differences in the basis of the

lattice. In addition, atomic contrast in dissipation is observed on the upper terrace but

not on the lower one. These observations published in [Venegas2008] were interpreted

as a reversible evolution of the structure of the tip, resulting in a change of the sign of


114

the tip terminating ion, in reference to the now accepted imaging mechanism on ionic

surfaces. ∆f(z) curves, obtained with the same tip, present two distinct behaviors: One

the lower terrace, where the dissipation is negligible, the ∆f(z) curve presents the

standard, monotonous, globally attractive character. On the upper terrace, where an

atomic contrast in dissipation is observed, the ∆f(z) curve is such that two imaging

operation points are accessible: the curve is bistable. Our observations establish a

direct link between this bistability, the switching behaviour of the tip on the

monoatomic steps and the appearance of atomic contrast in the dissipations images on

the upper terrace. These results strengthen the adhesion hysteresis hypothesis that was

proposed as the main source of dissipation in NC-AFM [Kantorovich2004]. In

addition they show that the tip can be a relatively "soft" object, generating dissipation,

as recently established by modelling of Si tips [Ghasemi2008].

• Different attempts were made to observe indigo molecules on this KBr(001) surface.

Deposition by UHV sublimation or from a solution were used. These experiments are

not sufficiently advanced to justify a detailed report. Some preliminary observations

are discussed in the "perspectives" part of this chapter.

• The structural and electronic properties of the alumina film grown on NiAl(110) by

oxidation were investigated by STM and STS. Most of the results already published

were confirmed, in particular the atomic model for the perfect domain and for the anti

phase domain boundaries of type I. The film presents a large gap, even on the domain

boundaries, which make it suitable as a substrate to decouple molecules from the

metallic substrate. Pd deposition leads to flat islands that decorate the NiAl steps and

the alumina domain boundaries, leading to inter island gap sizes of molecular

dimensions. Further work is needed before deciding to use this system for the

nanostencil experiments, as suggested in fig. 1.8. First, ways to grow larger (but not

thicker) islands have to be found; second, the electrical properties at the scale of the

microelectrodes have to be investigated.


115

5.2 Perspectives

One of the most exciting capabilities provided by the NC-AFM technique is the visualization

of organic molecules on insulating surfaces. As previously described in chapter 1, this is one

of the critical steps involved in the nanostencil technique. For that it is required to deposit

molecules on a surface. Here we have explored two methods of molecules deposition. The

first one consisted in deposition of indigo molecules by conventional sublimation from the

molecular solid under UHV. The second one consisted in the deposition of a drop of indigo in

a chloroform solution at ambient atmosphere. We briefly describe our findings in the

following.

5.2.1. Molecules visualization attempts

5.2.1.1 Deposition by sublimation under UHV

a) b) c)

Figure 5.1: Structure observed on the top of an indigo terrace. a) A larger scale image, 82.4 nm x 47.7 nm, of the terrace edge, height of 9.29 Å, ∆f = -180 Hz, amplitude = 4.4 nm, b)

the repetitive pattern found with some defects in the upper right corner of the terrace, ∆f=-225 Hz, amplitude = 3.3 nm, c) this image was obtained with a very low oscillation amplitude

A0 = 1.4 nm in a zoomed area over the terrace, ∆f=-433 Hz.

Indigo molecules were evaporated under UHV from an alumina crucible on KBr(001) at room

temperature. While imaging, we found a large terrace with a height of about 10 Å, shown in

figure 5.1a. Note that the shape of the terrace edge is quite different from that presented by a

KBr(001) step edge. Figure 5.1b shows an image obtained on this terrace. A periodic lattice,

which seems to be hexagonal can be distinguished. The length of the side of the unit cell is

measured to be 4.5 nm. This length has no obvious relation with the size of the indigo

molecule or with a length appearing in the unit cell of the indigo crystal, which is known.

Nevertheless we assume that this object is a crystalline indigo island where the arrangement

of the molecules is different from the arrangement in the bulk because of its small thickness

(of the order of 1 nm). This assumption is partially supported by the image of a defect on this


116

island presented in figure 5.1c where small objects with a shape and a size compatible with

single indigo molecules are distinguishable, as demonstrated by the superposition of the

atomic model of the molecule on the image.

5.2.1.2 Deposition from a chloroform solution

Figure 5.2: The KBr(001) surface after deposition of a drop of indigo in a chloroform solution. A0 = 4.5 nm, 200 nm x 200 nm.

We prepared a solution with chloroform and the indigo powder. Then we put a drop of

solution onto a clean KBr(001) in ambient atmosphere. After that the sample was rapidly

transferred to UHV where an annealing step was performed at ~150°C for one hour.

Figure 5.2 shows a 200 nm x 200 nm image of the sample with an amplitude = 4.5 nm and

∆f = -27 Hz. We can observe that the molecules decorate the step edges, as exhibited by the

brighter contrast areas in the image. In addition, it is possible to distinguish small balls in

certain locations on the surface.

To understand better, we performed two others experiments. The first one consisted in the

deposition of only one drop of chloroform, as shown in figure 5.3a. For the second one, we

deposited again a drop of the solution containing the indigo molecules (figure 5.3b). For each

preparation we followed the same surface treatment under UHV.

40nm


117

(a)

4.0nm

(b)

Figure 5.3: a) After deposition of one drop of chloroform, ∆f=-189Hz, amplitude is 5nm. The inset shows an NC-AFM atomic resolution image of 3.9 nm x 4.1 nm of one defect found on the surface, image obtained at ∆f=-204Hz and same amplitude. b) After deposition of one

drop of indigo solution, amplitude 2.7 nm, ∆f = -100 Hz.

Figure 5.3a shows a high resolution image, after deposition of the chloroform drop. Here we

can clearly see three darker objects. It is difficult to identify them. The inset shown in figure

5.3a shows a zoom on one of these defects with atomic resolution, with a K+ terminated tip.

As we never observed this type of objects on clean KBr(001) surfaces, we suppose that the

chloroform creates these defects.

Figure 5.3b shows an atomic resolution image of a 20 nm x 20 nm area of the sample after

deposition of the solution containing the molecules, the tip has a K+ termination. Now we

found six nano-objects exhibiting a brighter contrast in form of elongated lobes that seem to

be in pairs. The cross shape formed around the nano-object is probably due to the convolution

of the object and the shape of the tip. It is important to say that we could find a different nano

object than those shown in figure 5.3b that will be shown and described in the next image (see

figure 5.4).


118

(a)

(b)

Figure 5.4. a, b) Two images of the KBr(001) surface after deposition of a drop of indigo in a

chloroform solution. A0 = 2.7 nm.

A zoom on each of the different nano-objects found is now presented in figure 5.4. In figure

5.4a, the object exhibits two lobes that are connected at the lower right side end and around

the middle of them. The right lobe seems to be higher than the left one and its dimensions are

very similar to that of the indigo molecule, ~ 1.3 nm in length by 0.37 nm in width, the

measured height is 1.15 Å. The entire width of both lobes is 0.94 nm which is very small

compared with two indigo molecules coupled by their hydrogen bonds, around ~1.19 nm.

This difference suggests that one of these lobes is produced by the same effect of tip shape

convolution. Figure 5.4b shows a single nano object with ~ 1.3 nm in length and ~ 0.52 nm in

width, and height of about ~ 0.78 Å. Here we can appreciate the same tip convolution with a

cross shape around the object.

However all these arguments are only suppositions and further experiments are required in

order to better understand what are the objects present in the last figures. However after the

chloroform deposition, it seems to be that some defects are formed that could perhaps provide

suitable absorption sites for molecules.

5.2.2. Nano manipulation

Small objects like atoms or molecules can be manipulated in two ways: in a lateral or in a

vertical way. Lateral manipulation consists in pushing, pulling or dragging the object without

separating it from the surface [Oyabu2005, Sugimoto2007, Sugimoto2005, Hirth2006].

Vertical manipulation implies to pick the object with the tip and to put it in another location

[Oyabu2003, Morita2]


119

Defects formed on the surface like vacancies can also be manipulated by lateral methods as

reported in [Hirth2006] on CaF2(111).

Here we could experience the same phenomenon. After deposition of a drop of chloroform,

while imaging with atomic resolution at ∆f=-157Hz, in a down direction and amplitude=

5 nm, we found a defect that we could manipulate. This first image exhibiting the

manipulation is shown in figure 5.5a.

(a)

(b)

(c)

Figure 5.5: Lateral manipulations exhibited after the KBr(001) surface with a drop of chloroform. amplitude is 5 nm, 40 nm/s, 10 nm x10 nm in a) the manipulation of a lagoon, b) the manipulated lagoon now static, when going up no manipulation was expressed, only when

scanning in the down direction c) multiple manipulation of three lagoons, note that they continue their respective directions after collision.

In this image, we started to manipulate the defect by laterally controlling the scanning area,

we moved the scanning cursor so we can keep the defect in motion. Then while going up we

decreased the tip to surface distance by setting up ∆f = -86 Hz so an image of the defect can

be obtained, see fig.5.5b. We keep moving this defect until we found two other defects that

were manipulated at the same time. Interestingly we found that while moving the three

defects at the same time they followed a specific direction, and even though the first defect

found crosses the other two defects, all of them continued with their own direction. It is

important to say that this lateral manipulation was exhibited only when scanning in the down

direction.

Vertical manipulation could be also experienced, after deposition of the indigo solution,

where an object was picked up by the tip, inducing an increase in the dissipation signal. Then

by approaching to the surface and scanning over a defect, it appeared that the object was left

on the defect, with a simultaneous reduction of the dissipation signal to its original value.

These experiments need to be explored in order to reproduce what we found, so that a better

reference can be given. If chloroform creates defects then they can be used as anchor sites to


120

deposit molecules on this insulating surface. On the other hand manipulations are very

important for the nanostencil project that requires in some part of the construction procedure

to take a single molecule and to place it in a specific location. This system

Indigo+Chloroform/KBr(001) could be a good candidate to practice vertical and lateral

manipulations in order to define the correct protocols to manipulate a single molecule that

could be exploited in future nanostencil experiments.

.

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Chapitre 1 Introduction,

- le Project de nanostencil et l’électronique moléculaire.

La miniaturisation constante des composants électroniques, conduit par des besoins de

l'industrie de la microélectronique, a montré un énorme progrès dans la construction de

dispositifs électroniques qui sont de plus en plus petits. Un composant commun

normalement utilisé comme un bloc constitutif d'un circuit électronique c’est le transistor,

qui a été considérablement réduit en taille. Cependant, cette miniaturisation des

composants électroniques commence à être affectée par des limites technologiques et

fondamentales. C’est pourquoi que d’autres alternatives ont été proposées par la

communauté scientifique de l'électronique moléculaire. Ces idées proposent l’utilisation

des molécules comme des blocs actifs et fonctionnels dans les circuits électroniques en

vue de les intégrer dans un dispositif électronique dans le futur. Pour atteindre cet objectif

il faut résoudre la problématique de connexion électrique entre une molécule unique et des

électrodes métalliques. Des approches intéressantes qui sont explorées aujourd’hui, sont :

� La jonction métal-vide-molécule-métal proportionnée par la

microscopie à effet tunnel (STM).

� Les Jonctions mécaniquement cassables.

� La Jonction covalente Nano tube de carbone-Molécule-Nano tube de

carbone.

� L’utilisation des îlots métalliques connectés par quatre pointes de STM

indépendants.

� La méthode de nanostencil, (cette approche sera expliquée dans le

prochain paragraphe).

o La méthode de nanostencil utilise une sonde locale ; notamment un cantilever

de Microscopie de force atomique (AFM), qui est percé et utilisé comme

masque dynamique pour collimer un faisceau des atomes métalliques. Dans

une architecture planaire, différentes techniques de déposition de métal sous

ultra vide sont requise. La première qui se sert d’un micro stencil statique pour

la construction de plots métalliques ayant une taille micrométrique, la

deuxième consiste à faire la croissance métallique pour déposer sur la surface

des petits agrégats métalliques ayant les dimensions en taille de l’ordre d’une

molécule. En suit, le dépôt métallique par nanostencil dynamique pour relier le

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132

micro électrode avec l’îlot métallique. Finalement, les microélectrodes son

connectes vers l’extérieur de l’environnement ultravide pour pouvoir faire des

mesures électriques.

Ce travail de recherche apporté dans cette thèse à participé au développement du

projet de nanostencil dans deux directions:

• L’amélioration de l'AFM : Il est très important de visualiser toutes les étapes de la

fabrication du dispositif construit par la méthode de nanostencil à partir des images

d’AFM. La seule technique capable d'image à la molécule unique sur un substrat isolant

est la microscopie de force atomique (AFM) dans le mode de modulation en fréquence

(FM-AFM) aussi appelle non contact (NC-AFM). Cette technique sera brièvement

présentée dans la deuxième partie du chapitre d’introduction.

• La recherche effectuée dans le système Pd/Alumina/NiAl (110) : Comme nous

avons déjà mentionné, la technique de nanostencil exige de trouver un système physique

de métal/isolant approprié pour pouvoir faire une croissance bidimensionnelle et epitaxial

d'un dépôt métallique cristallin. Malheureusement, le mode de croissance pour la plupart

de systèmes métal/isolant est de 3D, principalement parce que l'énergie libre des isolants

est habituellement beaucoup plus inférieure à celle des métaux. Néanmoins, on peut

espérer réaliser la croissance en deux dimensions, même dans ces conditions, si des

limitations cinétiques sont présentes. Nous avons choisi d'étudier l'alumine sur la surface

NiAl (110), qui est formé par oxydation de NiAl (110), parce que le palladium peut faire

la croissance d'îlots cristallines bien facettes et avec un surface plate [Yoshitake2006,

Hanssen1999]. Comme est suggéré dans la figure 1.8 dans le chapitre 1, ces îlots

pourraient être employés comme électrodes intermédiaires pour adresser électriquement

une molécule. Les résultats des mesures électriques obtenus sur cette couche d'oxyde par

microscopie à effet tunnel (STM) et par spectroscopie à effet tunnel (STS) ainsi comment

les études préliminaires du dépôt de palladium sont présentes dans le chapitre 4 : " ; Les

system Pd/Al10O13/NiAl (110) ".

Dans la deuxième partie de ce chapitre nous présentons un bref historique de la microscopie à

force atomique, les différents modes de fonctionnement de cette technique (le mode de

contact et le mode de contact intermittent (Tapping)) sont mentionnés. Le mode de non-

contact sera plus détaillé parce que c’est la technique utilisée dans cette thèse.

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Chapitre 2, Optimisation du capteur de déflection d’AFM

Dans ce chapitre nous présentons les améliorations de la tête AFM qui ont été

effectuées pendant le parcours de cette thèse. Nous commençons pour décrire la méthode

de levier optique et sa sensibilité, qui est la méthode appliquée pour la détection de la

flexion du cantilever. Ensuit nous faisons une brève description de la source de bruits dont

nous parlons de bruits qui apportent plus de contribution :

o Le bruit de grenaille

o Le bruit Johnson

o La source de bruit optique

� Le bruit du laser

o Le bruit thermique du cantilever

La modification de la tête omicron AFM comprend le remplacement de la diode

infrarouge par une diode laser super luminescente. Nous commençons par la description

de La diode laser super luminescente. Cette diode à été connectée vers l’intérieure de

l’ultra vide par une fibre optique de mode simple. Celle-là arrive à un système de

focalisation optique pour amener le faisceau laser jusqu’au la partie derrière du cantilever.

Les nouvelles caractéristiques du capteur de detection du levier optique son présentées,

celles-ci nous ont permit d’effectuer d’analyses spectrales du bruit existant dans le

système, pour caractériser le system de detection. Nous avons ainsi effectué des

estimations de bruit équivalent à l'entrée du nouveau système de detection du levier

optique et des estimations de la température du cantilever au niveau maximum de

puissance du laser. Enfin, nous avons mesuré la sensibilité et calibrage d'amplitude de

l’oscillation du cantilever.

Chapitre 3, Etude de la surface KBr(001) par AFM de non contact.

Nous avons exploré la surface isolant KBr(001) à l’échelle atomique. D’abord pour

tester les nouvelles améliorations effectuées dans la tête, mais nos résultats sont allés plus

loin. Des comportements intéressants d’un point en particulier lors de l’interaction avec la

surface, ont été étudies et publiés [venegas2008].

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La manière de préparer la surface est par clivage mécanique à l’aire et tout de suit la

surface est rentrée dans l’ultra vide pour poursuivre un dégaussage à une température de

150°C pendant une heure.

Nous avons effectue de la spectroscopie de force qui consiste en approchant la ponte

vibrante (oscillateur harmonique dissipé et actionné) nous avons eu la possibilité

d’enregistrer le signal de décalage en fréquence par rapport à la distance. Nous avons

utilisé cette information pour faire l’extraction de la force d’interaction à partir du

décalage en fréquence avec la formule de Sader-Jarvis.

Les résultats obtenus:

o Deux types d’images en résolution atomique. La terminaison de la pointe joue

un rôle capital pour le type de contraste montré dans une image de NC-AFM.

Lors les premiers approches de la pointe sur la surface, la pointe prend des

petites morceaux de KBr qui donnent la possibilité à la pointe d’avoir deux

types de terminassions possibles: une terminaison en K+ et une autre en Br-.

En dépendant du type de terminaison à l’extrême apex de la pointe, de

différents contrastes sont observés. Par exemple, si la pointe est en terminaison

K+ la pointe sera attirée par les ions de Br- sur la surface de sorte que la

machine rétractera la pointe. Ce réaction de la machine, produise un contraste

blanc sur l’image à l’écran, de manière que les images en résolution atomique

montrent les ions de Br- avec un contraste haut tandis que les ions K+ serons

montrés en contraste noir, parce que la pointe sera repoussée par les ions K+ et

la machine va approcher la sonde local.

o Courbes de force déterminées au niveau Atomique. Nous avons fait quelques

mesures pour extraire des courbes de force dans les différents types d’ions et

avec une pointe en terminaison K+. Nous avons observé une petite différence

dans la force.

o Inversion de polarité spontanée à l’extrême apex de la pointe : La polarité de la

pointé changeait d’une façon spontanée dans quelques expériences. Nous

avons obtenu des images en résolution atomique qui montrent ce changement

d’ion en faisant l’inversion de contraste dans l’image.

o Inversion de polarité de la pointe provoque par une marche monoatomique.

Nous avons noté que cette changement de polarité pouvait être induit par la

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interaction d’une marche monoatomique. Avec une pointe qui a été

fonctionnalisé d’une façon très particulière. Les images montrent la terrace à

gauche avec une pointe en terminaison Br- et la terrace de droit avec une

pointe en terminaison K+.

o De ce fait, Nous avons proposé donc un model schématique pour expliquer

l’énergie de potentiel de surface de un système à deux niveaux.

o Aussi, cette inversion de polarité de la pointe à été observé sur deux marches

monoatomiques successives.

- Ainsi, nous avons effectué de l’spectroscopie de force avec une pointe bistable d’où

nous avons remarqué la différence entre une pointe K+ et Br-.

Chapitre 4, Le système Pd/Al10O13/NiAl(110)

L’alliage intermétallique NiAl(110), description du Crystal à une échelle atomique, la

cellule unitaire, le paramètre de maille et la relaxation de la surface sous l’environnement

ultra vide. Ensuite nous présentons la procédure de préparation de cette surface pour avoir

un surface atomiquement propre, en faisant de cycles des bombardements ioniques à 1.5

keV et recuits par bombardement électronique jusqu’au atteindre des températures de

l’ordre de ~ 1100°C.

Le film mince isolant d’alumina formé sur la surface propre de NiAl (110) est faite par

oxydation direct de l’échantillon à une température spécifique. Il y a deux méthodes de

préparation :

� Le processus de deux étapes qui consiste en oxyder la surface et puis

après faire un recuit pour cristalliser la coche d’oxyde.

� Le processus à une seule étape, qui consiste en faire l’oxydation à

température de cristallisation

Morphologie du film d’oxyde à une échelle mesoscopic, le processus de cristallisation

produise deux différents domaines de réflexion, de paroir domaine apparaissent et laissent un

réseau caractéristiques de lignes de default.

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136

o Nous présentons des image à échelles intermédiaires pour commencer à décrire

la cellule unitaire de l’oxyde et pouvoir décrire la relation structural entre la

cellule unitaire d’alumina et le substrat de NiAl (110)

o Image de haut résolution avec trois paroirs domaines qui nous permettent de

visualiser la formation de paroirs domaines. Deux types de défauts sont

produits désignés comme I et II.

o Les Images en résolution atomique de la surface d’oxyde est en accord avec le

Model atomique connu, l’interprétation de l’image en résolution atomique du

domaine de réflexion est décrit. Une comparassions est fait aussi pour Le

modèle atomistique de la Frontière antiphase de domaine du type IA avec son

image en résolution atomique. Finalement nous faisons un maillage de la

surface avec le modèle atomique répétitive.

o Mesures de spectroscopie à effet tunnel. Nous commençons à parler des

propriétés électroniques de la couche d’oxyde en décriant le gap qui présent cet

oxyde et les énergies, où ils apparaissent les bandes de conduction et de

valence. Après nous présentons les mesures électriques par STS sur la alumina

prépare a 1700L (1L=1x10-6Torr s). Ici nous présentons une discussion de

l’hauteur apparente de l’épaisseur de la couche d’oxyde en fonction de la

tension d’imagerie. Ensuit nous montrons de mesures électriques dans le gap

par STS deux différents oxydes. Finalement on discute quelques expériences

par rapport à la croissance cristalline de palladium sous ultra vide.

Chapitre 5, Conclusions et perspectives

- Conclusions

o La tête RT-AFM a été considérablement améliorée en changeant sa source

lumineuse. Cette amélioration a été caractérisée par les mesures de bruit

soigneusement faites, une méthode non destructive pour mesurer l'amplitude

d'oscillation de cantilever a été développée. Ce paramètre expérimental est très

important pour la microscopie de force atomique en mode de non contact (

NC-AFM). Ce travail d'instrumentation était essentiel pour une meilleure

maîtrise de cet instrument qui est très complexe, et pour établir une base pour

des mesures quantitatives. Il a ouvert la voie aux expériences rapportées dans

le suivant.

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137

o Cet amélioré de la tête AFM a été utilisé pour étudier la surfaces KBr (001),

qui est une surface de référence pour NC-AFM. Maintenant, la résolution

atomique de cette surface est obtenue facilement. Une méthodologie fiable

pour obtenir des courbes de décalage en fréquence V.S. distance pointe-

surface, df(z), pour obtenir les courbes de force a été développée. Un ensemble

d'expériences a été exécuté avec une pointe qui a présenté un comportement

intéressant. Sur des images atomiques topographiques et df constantes on a

observé le changement de contraste atomique façon déterministe et réversible

lorsque la pointe croisait sur des marches monoatomiques: les images obtenus

formé sur les terrasses inférieures et les terrasses supérieures présentent la

même périodicité atomique, mais avec des différences importantes dans la base

de la structure cristalline. En outre, on observe le contraste atomique dans la

dissipation sur la terrasse supérieure mais pas sur l’inférieure. Ces

observations, publies dans [Venegas2008] ont été interprétées pendant qu'une

évolution réversible de la structure du bout de la point, ayant pour résultat un

changement du signe du bout terminant de la point, autrement dite de

changement d'ion terminal, ayant comment référence le mécanisme de

formation d’image sur les surfaces ioniques. Les courbes df(z), obtenues avec

la même point présentent deux comportements distincts : Un sur la terrasse

inférieure, où la dissipation est négligeable, et la courbe df(z) présente le même

caractéristique standard, monotone, et globalement attractive. Sur la terrasse

supérieure, où on observe un contraste atomique dans la dissipation, la courbe

df(z) est telle que deux points de fonctionnement de formation d’image sont

accessibles : la courbe est bistable. Nos observations établissent un lien direct

entre cette bi stabilité, le comportement de commutation du bout sur les

marches monoatomiques et l'aspect du contraste atomique dans les images de

dissipations sur la terrasse supérieure. Ces résultats renforcent l'hypothèse

d'hystérésis d'adhérence qui a été proposée comme source principale de

dissipation dans NC-AFM [Kantorovich2004].

o Différentes tentatives ont été faites d'observer des molécules d'indigo sur cette

surface de KBr(001). Le dépôt par sublimation d'UHV ou d'une solution ont

été employés. Ces expériences ne sont pas suffisamment avancées pour

justifier un rapport détaillé. Quelques observations préliminaires sont discutées

dans la partie de " perspectives" a la fin de ce chapitre.

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o Les propriétés structurales et électroniques du film d'alumine forme à ma

surface de cristal NiAl(110) par oxydation ont été étudiées par STM et STS. La

plupart des résultats déjà publies ont été confirmées, en particulier le modèle

atomique pour le domaine parfait et pour la frontière antiphase de domaine du

type I. Le film présente un gap, même sur les frontières de domaine. Cette

propriété lui rend approprié comme substrat pour découpler des molécules du

substrat métallique. Le dépôt de palladium qui forme des ilots plates et facettés

décorent les borde de marches et frontières antiphase de domaine de l'alumine,

ceux-ci sont formes aux petites tailles et avec des espaces entre eux avec des

dimensions moléculaires.

- Perspectives

o Tentatives pour Visualiser des molécules: nous avons explore deux dépositions

de molécules :

� Déposition par sublimation sous ultra vide

� Déposition de molécules à partir d’une solution de chloroforme

o Nous avons expérimente de la nanomanipulation aussi :

• Manipulation horizontal: présente après d’avoir faite le dépôt

d’un goute de chloroforme

• Manipulation vertical: présente après d’avoir faite le dépôt

d’indigo+chloroforme.

exploring bulk insulating surfaces and ultra thin insulating films, at the atomic level by uhv-spm...

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