uhv ernst peq

Prof Dr. Karl-Heinz Ernst, ETH ZH Catal. Vorl. 2010

UHV-based techniques for surface science and research in

heterogeneous catalysis

Prof. Dr. Karl-Heinz Ernst

Nanoscale Materials ScienceEmpa, Swiss Laboratories for Materials Science and Technology

08. 11. 2009 9:45-10:30 (HCI H8.1)10. 11. 2009 8:45-10:30 (HCI H2.1)

ftp://ftp.empa.ch/pub/empa/outgoing/KHErnst/

Sunday, November 7, 2010


Contents1. Introduction & Nomenclature1.1. Why ultra-high vacuum? Gas kinetics & vacuum pumps1.2. The concept of model systems (examples from surface science)1.2.1. Single crystal surfaces: Miller indices, adsorption sites, coverage, adsorbate superstructures 1.2.2. Closing the materials and pressure gap: more complex model systemsExamples for application of UHV techniques

2. Probes for surfaces: photons, electrons, ions, etc.

3. Surface structure: Low energy electron diffraction (LEED)

4. Electronic structure and chemical composition4.1. X-ray photoelectron spectroscopy (XPS) 4.2. Auger electron spectroscopy (AES)4.3. High-resolution electron energy loss spectroscopy (HREELS)

5. electronic and geometric surface structure: Scanning tunneling microscopy (STM).



Short introduction of your docent

• Lehre/Berufsfachschule (Professional Training), Degree: Chemisch-Technischer Assisitent (CTA)

• Chemical Engineering at the University of Applied Sciences (Technische Fachhochschule, TFH) Berlin, Degree: Chemie-Ingenieur (grad)

• Chemistry at Freie Universität Berlin (FUB), Degree: Diplom-Chemiker

• Graduate Study at the Inst. f. Physical Chemistry, FU Berlin and at the Berlin Electron Storage Ring for Synchrotron Radiation (BESSY), (subject: Solid state physics related) Supervisor: K. Christmann, 2nd thesis expert reader: G. Ertl Degree: Dr. rer. nat. (PhD)

• Habilitation at University Zurich, Degree: Private Docent

• Professor title (University Zurich 2010)

www.empa.mss


http://www.empa.mss

http://www.empa.mss


1988 - 1990 Lecturer in Physical Chemistry, TFH Berlin1990 - 1991 Post-Doctorate at the University of Washington, Seattle, USA, Group of Prof. C. T. Campbell1992 - 1992 Research Associate with the Special Research Fond No. 6 (SFB 6) of the German National Science Foundation (DFG) at the Free University, Berlin.1992-present Senior scientist at Empa Dübendorf, since 1998 head of Surface Technology Group, www.empa.ch/mss (now Molecular Surface Science Group, MSSG)8/99-5/2000 On sabbatical leave to UC Berkeley, Department of Physics and Lawrence Berkeley National Laboratory, California, USA, Group of Prof. Y.-R. Shen9/2003-8/2004 On sabbatical leave to Univ. of Washington, Seattle, USA, Department of Bioengineering, Group of Prof. Viola Vogel

Scientific Publications (Oct. 2010): 109 (68 peer reviewed)Hobbies: Sailing, Diving, Mountaineering (down-graded now to: hiking)

Professional Experience


http://www.empa.ch/mss

http://www.empa.ch/mss


K. ChristmannIntroduction to Surface Physical ChemistrySteinkopf Verlag Darmstadt 1991

G. Ertl / J. KüppersLow energy electrons and surface chemistryVCH, Weinheim 1985

P. D. Woodruff / T. A. DelcharModern techniques in surface scienceCambridge University Press, Cambridge 1989

M. Henzler / W. Göpel,Oberflächenphysik des Festkörpers, Teubner, Stuttgart 1991

A. ZangwillPhysics at SurfacesCambridge University Press, Cambridge 1989

R. I. Masel, Principles of Adsorption and Reaction on Solid Surfaces, Wiley & Sons, New York, 1996

Books on surface science



Why UHV? Collision frequency of gas particles with a surface

At 300 K and 1 atm for N2, ZA is about 3 x 1023 s-1 cm-2



At 300 K, if every N2 molecule that strikes this surface remains adsorbed, a complete monolayer is formed in about 3 ns.

If p=10-3 torr (1.3 x 10-6 atm), t=3x10-3 sIf p=10-6 torr (1.3 x 10-9 atm), t=3 sIf p=10-9 torr (1.3 x 10-12 atm), t=3000 s or 50 minutes

Requirement for Experiment in Vacuum: Clean surface quickly becomescontaminated through molecular collision - p must be less than about1.3 x 10-12 atm (10-9 torr).

10-10 to 10-11 torr (UHV - ultrahigh vacuum) is lowest pressure routinelyavailable in vacuum chamber.

A single crystal surface has about 1 x 1019 m-2 atoms (1 x 1015 cm-2)



With the assumption of an atom surface density of 3·1014 cm-2, M = 28 (N2 for Air), T = 300 K and a given pressure p, there are ~106·[p[mbar] monolayers] of gas atoms per second adsorbed on the surface. This is with the assumption that each molecule hitting the surface will be adsorbed (sticking coefficient S = 1). As a typical surface analytical measurement will take some minutes up to several hours, a pressure in the range of 10-10–10-11 mbar is required. Such low pressures can be achieved and maintained using Turbo- molecular-, Ion-Getter-, Cryo- and Oil-Diffusion-Pumps. The following table gives the pressure ranges for different vacuum regimes.

Pressure Range Vacuum Regime

1-10-3 mbar Rough Vacuum

10-3-10-5 mbar Medium Vacuum

10-5-10-8 mbar High Vacuum (HV)

Better 10-9 mbar Ultra High Vacuum (UHV)



Vacuum pumps



real catalyst / real conditions

model catalyst / UHV

real catalyst / UHVmodel catalyst / real conditions

A real catalyst is too complex to understand:The concept of model systems



The Noble Prize in Chemistry 2007: Gerhard Ertl Employing well-defined surfaces as model systems for heterogeneous catalysis

Heidelberg, 29.06.2007

N2 + 3H2 → 2NH3



Kinetic Oscillations in CO oxidation

work function measurements



Ernst et al, J. Catal. 1992

r-WGS reaction over Cu(110)

CO2 + H2 → H2O + CO

Single crystals allow normalization to surface sites and a better comparison with other catalysts (if the number of sites is know as well).

Turn-over rates can be related to surface structure and change of such (reconstruction).

Change in stady-state H-coverage lifts reconstruction and induces lower catalyst activity

673 K, PH2 = 760 torr

573 K, PH2 = 760 torr

673 K, PH2 = 110 torr



Topography of catalysts via scanning tunneling microscopy (STM)



z

x

y y

x

z

y

x

zc)b)a)

e) f)d)

Close-packed FCC surfaces are common model systems

Single crystal surfaces are ordered systems that allow the use of diffraction methods

1.2.1. Single crystal surfaces: Miller indices, adsorption sites, coverage, adsorbate superstructures



Miller indicesz

x

y y

x

z

y

x

zc)b)a)

e) f)d)

(100) (110) (111)



C0

C0 2

= 6.696ÅC0

1.4215 Å

Surface science at the (0001) graphite surface(mainly for self-assembly phenomena at the liquid-solid interface, but many studies of metal clusters - vacuum and liquid - are known)



Stepped surfaces as model systems



The structure of surfaces



422 S.J. Jenkins, S.J. Pratt / Surface Science Reports 62 (2007) 373–429

Table 11Corresponding figure numbers in which top-down views of fcc, bcc and hcpsurfaces from the master hierarchy are presented in the Gazetteer

fcc Fig. bcc Fig. hcp Fig.

{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43

Fig. 30. The only flat fcc surfaces. The {111} and {100} surfaces are triply-reflexive (X3) and quadruply-reflexive (X4) respectively.

Fig. 31. Selected stepped fcc surfaces. All are reflexive (X) apart from {110}, which is doubly-reflexive (X2).




{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43






{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43






{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43






{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43






{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43






{111} 30 {110} 34 {0001} 38{100} 30 {3032} 39

{211} 35 {3034} 39{110} 31 {100} 36 {1120} 40{311} 31 {111} 36 {1122} 40{210} 32{531} 33 {321} 35 {1010} 41

{310} 36 {1011} 41{331} 31 {332} 36 {3038} 41{211} 31 {411} 36 {3 0 3 10} 41{511} 31 {433} 36 {4154} 42{310} 32 {611} 36 {5272} 42{320} 32 {521} 37 {4156} 42{321} 33 {543} 37 {1121} 44{731} 33 {631} 37 {1124} 43{751} 33 {653} 37 {1126} 43{421} 33 {4150} 43{753} 33 {7188} 43

::

{4152} 46{2131} 45{2130} 43



Selected fcc surfaces

S.J. Jenkins, S.J. Pratt / Surface Science Reports 62 (2007) 373–429 423

Fig. 32. Selected achiral kinked fcc surfaces. All are reflexive (X).

Fig. 34. The only flat bcc surface. It is doubly-reflexive (X2).

Fig. 35. Selected stepped bcc surfaces; note that the {211} surface is reflexive(X) and therefore achiral, whereas all other stepped bcc surfaces including{321} are chiral (D, L).

Fig. 33. Selected chiral (D, L) kinked fcc surfaces.


















fcc surfaces without mirror planes








Selected bcc surfacesS.J. Jenkins, S.J. Pratt / Surface Science Reports 62 (2007) 373–429 423






Fig. 36. Selected achiral kinked bcc surfaces. All are reflexive (X) except for {111} and {100}, which are triply-reflexive (X3) and quadruply-reflexive (X4)respectively.




Fig. 37. Selected chiral (D, L) kinked bcc surfaces.

Fig. 38. The only flat hcp surface. It is a bayonet surface and designated XX6o.

Selected hcp surfaces


Fig. 39. The only interrupted-flat hcp surfaces. These surfaces are uniterminated-reflexive and designated XXu.

Fig. 40. The only meandering row hcp surfaces. The {1122} surface is glissadic (XXø), whereas {1120} is reflexive-glissadic (XX2ø).


Fig. 41. Selected stepped hcp surfaces. Note that {1010} is doubly-reflexive (XX2τ , XX2

τ ) with (τ, τ ) ⇒ (σ, λ), whereas {1011} is reflexive (XXτ , XXτ ) with(τ, τ ) ⇒ (λ, σ ). In contrast, {3038} and {3 0 3 10} are both uniterminated-reflexive surfaces (XXu).

Fig. 42. Selected geminal hcp surfaces. All such surfaces are pure one-chiral (DXu, LXu).



2.1 GaAs-Oberflachen

fs1(a) s2 v s(b)

Abbildung 2.2: (a) Facettierte Oberflache. Die ideale Oberflache f zerfallt in die benachbarten

Flachen s1 und s2. (b) Vizinalflache. Die Orientierung der idealen Oberflache v weicht um

einen kleinen Winkel von der Orientierung der stabilen Oberflache s ab.

Eine ubersichtliche Darstellung der verschiedenen Orientierungen laßt sich durch

eine stereografische Projektion in die Ebene erreichen (ausfuhrlich z. B. in Ref. [32]).

Die stereografische Projektion ist winkeltreu, so daß sich Winkel zwischen verschie-

denen Flachen direkt bestimmen lassen. Aufgrund der Kristallsymmetrie sind viele

Ebenen aquivalent, insbesondere solche, deren Indizes durch zyklisches Vertauschen

auseinander hervorgehen [(11n) entspricht (n11)]. Daher genugt es fur viele Zwecke,

die Projektion auf das stereografische Dreieck zwischen (001), (011) und (111) zu be-

schranken (Abb. 1.1 auf S. 11).

Die drei niederindizierten Flachen an den Ecken des Dreiecks bilden eine Basis der

Kristallstruktur. Alle hoherindizierten Oberflachen lassen sich aus diesen drei Flachen

aufbauen. Orientierungen, die auf den Seiten des Dreiecks liegen, sind Kombinationen

der Flachen an den jeweiligen Ecken. So sind z. B. die Flachen mit den Miller-Indizes

(11n) aus (111) und (001) zusammengesetzt (vgl. Abb. 5.1(b) auf S. 56). Aufgrund

dieser Uberlegungen sieht man auch leicht, daß (1 1 25) = 1 · (111) + 24 · (001) vizinal

zu (001) liegt. Allgemeingultig ist das in diesem Absatz Dargestellte jedoch nur fur

die idealen Oberflachen. Auf GaAs(113) etwa bildet sich eine stabile Struktur, die

unabhangig von niederindizierten Oberflachen ist [2, 3].

Als Zone bezeichnet man die Menge aller Flachen, die durch Drehung um eine be-

stimmte Achse ineinander ubergehen. Die Richtung dieser Achse gibt der Zone ihren

Namen. Untersuchungen auf Oberflachen der [110]-Zone waren der Ausgangspunkt der

vorliegenden Arbeit. In Abb. 2.3 ist diese Zone dargestellt. In ihr liegen die Ebenen

mit den Indizes (11n). Die Winkel zwischen diesen Flachen sind in Tab. 2.1 zusam-

mengefaßt.

Bei Galliumarsenid sind die beiden durch Aufschneiden eines Kristalls erzeugten

Oberflachen, also Vorder- und Ruckseite, in der Regel nicht aquivalent, da der Stoff aus

17

A surface of arbitrary orientation usually has a high surface energy. It is common that the real surface deviates substantially from its orientation and forms facets of the next most stable face (a) or close to the original orientation exists a stable “vicinal” surface.

Faceting and vicinal surfaces



More realistic surface model



Rekonstruktion, Relaxation

Relaxation: Veränderung derGitterkonstante senkrecht zurOberfläche

Rekonstruktion: Parallel zurOberfläche werden Atome neuAngeordnet (grössere Einheitszelle)z.B. 2x1 oder 7x7

Reconstructions and relaxation:In reality the surface deviates from the bulk plane periodicity.



missing row reconstruction of clean surfaces

Pt(110) – 1x2 reconstruction Pt(110) – 1x1

[001]

[110]

(111) micro facet

(CO adsorption lifts reconstruction and creates (1x1) surface!)



Relaxation directly under a single molecule

The Cu step atom of a (211) surface is lifted under a propene molecule (DFT calculation)Rekonstruktion, Relaxation

Relaxation: Veränderung derGitterkonstante senkrecht zurOberfläche

Rekonstruktion: Parallel zurOberfläche werden Atome neuAngeordnet (grössere Einheitszelle)z.B. 2x1 oder 7x7



46 W. Reimer et aL / LEED analysis of the (2 X 1)H-Ni(110) structure

0 = 1.5 [5]. At T>~ 200 K a local reconstruction into a one-dimensionally disordered phase starts already at rather low coverages [6]; it equally exhibits a (1 • 2) pattern.

The present work is concerned with a LEED structural analysis of the low temperature (2 X 1) phase at 0 = 1.0 (There is strong evidence that the derived local geometry of the adsorption site equally holds for all of the lattice gas p h a s e s a t O H ~< 1.0, e.g. by the coverage independent magnitudes of the adsorption energy, the frequency of the N i - H vibration and the change in work function in this coverage range).

Before starting the analysis of the adsorbate covered system, also the structure of the clean Ni ( l l0 ) surface was determined. A full account of the LEED structure analysis itself will be given elsewhere [7], in addition a similar analysis of the (1 • 2) reconstructed surface was reported recently [8].

The LEED pattern of the (2 x 1)H phase is characterized by a systematic extinction of the (0, n / 2 ) beams at normal incidence indicating the presence of a glide symmetry plane [9] and thus of two H atoms per unit cell [2]. Based on He diffraction experiments Rieder and Engel [2] proposed a structural model which is reproduced in fig. l a and whereafter the adsorbed atoms form

a) [110]

YH

]( % ( o,,

,,' [0011 , [0011

b) [110] yH

dl

Z H d12

[110] d23

L,,,[l'lo] , 1001] - ' I001]

Fig. 1. Models of the (2 x 1)H-Ni(110) structure: (a) for H atoms on quasi-threefold sites, top view; (b) for H atoms on long-bridge sites, top view; (c) side view illustrating the structural

parameters; (d) geometry of the local Ni3-H adsorption cluster.

d12 = 1.18 Åd23 = 1.32 Åd34 = 1.25 Å

dbulk = 1.25 Å

Relaxation p2mg (2x1)2H / Ni(110)



Superstructures: Wood notation



€

b1

b 2

⎛

⎝ ⎜

⎞

⎠ ⎟ =

m11 m12

m21 m22

⎛

⎝ ⎜

⎞

⎠ ⎟

a1

a2

⎛

⎝ ⎜

⎞

⎠ ⎟

c)

a2

a1

b1

b2

a)

a2a1

b1

b2

b)

a2a1

b1 b2

€

2 00 2⎛

⎝ ⎜

⎞

⎠ ⎟

Superstructures: Matrix notation

€

1 −11 1⎛

⎝ ⎜

⎞

⎠ ⎟

€

2 1−1 1⎛

⎝ ⎜

⎞

⎠ ⎟



Adsorption sites



Coverage



E(x,y)

ad

(x,y) E(x,y)

repulsive

a) b) c)

attractive

x,y x,y x,y

diff

Ordered layers can only form with sufficient surface diffusion (a)

Adsorption energy is modulated via substrate corrugation (a) plus by lateral interaction between adsorbed species (b)

When repulsion matches adsorption energy, saturation coverage is reached.



1.2.2. Closing the materials and pressure gap: more complex model systemsExamples for application of UHV techniques



Closing the pressure and the materials gap



Stable configurations of small clusters

Side length of the cube 1 nm 1 μm 1mm

Number of bulk atoms (NB) 1 2.7·1010 2.7·1019

Number of surface atoms (NS) 29 5.4·107 5.4·1013

NS/(NB+NS) 0.96 2·10-3 2·10-6

Number of edge atoms 12 3.6·104 3.6·107

Number of corner atoms 8 8 8



Normalization of turnover rate w/surface sites



Electronic structure related to cluster size: XPS



Melting Point of Gold (Au)



Relevant interactions for surface analytical methods

2. Probes for surfaces: photons, electrons, ions, etc

Photons



Surface sensitive methods for elemental, chemical and structural analysis can be achieved by using particles for excitation or detection, which interact strongly with matter. With other words using particles with a very short mean free path in matter.



Inelastic mean free path of electrons λ as a function of the electron

energy for different materials. As can be seen λ is very energy but not

very material dependent.



3. Surface structure: Low energy electron diffraction (LEED)



fluorescentscreen

diffracted beam

gun

incidentbeam

two dimensionalcrystal lattice(magnified)

diffractionspots

Surface structure: Low energy electron diffraction (LEED)



Primary electron energy 20 - 1000 eV

Ni(111)

sinφ ≈ 1/√E)

λ = h/√(2 me e ·U)



150 eV 250 eV

650 eV



125

DEUTSCHE BUNSEN-GESELLSCHAFTUNTERRICHT

respect to the exact positions of atoms at a surface is some-what more complicated and requires fully dynamical quantum mechanical scattering calculations.

The use of LEED as a standard technique for surface analysis started in the early 1960’s when large enough single crystals and commercial instruments became available for surface studies. At fi rst the technique was only used for qualitative characterization of surface ordering and the identifi cation of two-dimensional superstructures. The quantitative information about the positions of the atoms within the surface is hidden in the energy-dependence of the diffraction spot intensities, the so-called LEED I-V, or I(E), curves. Computer programs and the computer power to analyze these data became available in the 1970’s. With the ever growing speed of modern computers LEED-IV structure determination has been applied to increas-ingly complex surface structures. To date LEED is the most pre-cise and versatile technique for surface crystallography.

For further information about the history, experimental setup, and theoretical approaches of LEED refer to the books by Pen-dry, [Pend74], Van Hove and Tong [Vanh79], Van Hove, Wein-berg and Chan [Vanh86], and Clarke [Clar85]. The present ar-ticle makes extensive use of these works.

2 BASIC PRINCIPLES

The basic principle of a standard LEED experiment is very sim-ple: a collimated mono-energetic beam of electrons is directed towards a single crystal surface and the diffraction pattern of the elastically back-scattered electrons is recorded using a position-sensitive detector. For electrons, like for all wave-like objects, the angular intensity distribution due to the interfer-ence of partial waves back-scattered from a periodic array is described by Bragg’s law or, more conveniently, by a set of Laue equations, one for each dimension of periodicity, which predict a regular pattern of diffraction spots.

2.1 SURFACE PERIODICITY AND RECIPROCAL LATTICE

Because of the short penetration depth of low-energy elec-trons the diffraction process is determined by a small number of atomic layers at the crystal surface. The electrons do not probe the full crystal periodicity perpendicular to the surface. Therefore, the array of relevant scatterers is only periodic in two dimensions. The surface lattice can be described by a pair of lattice vectors a1 and a2, which are parallel to the surface plane, and the surface unit cell, i.e. the contents of the paral-lelogram spanned by a1 and a2. The surface consists of identi-cal copies of the unit cell at every point

R = m1 a1 + m2 a2 (3)

with integer numbers m1 and m2. The left hand side of Figure 1 illustrates common square, rectangular and hexagonal sur-faces and the lattice vectors defi ning their unit cells.

The two-dimensional Laue equations are based on reciprocal lattice vectors within the surface plane which are defi ned by

the real space lattice vectors through a set of four simultane-ous equations:

a1 ! a*1 = 2p a2 ! a*2 = 2p (4a) a1 ! a*2 = 0 a2 ! a*1 = 0 (4b)

Figure 1 (left from top to bottom) arrangement of atoms in the {100} (square) {110} (rectangular) and {111} (hexagonal) surfaces of a simple face cen-tered cubic crystal lattice and a p(2x1) superstructure on a square surface; the diagrams include lattice vectors defining the surface unit cell and the corresponding reciprocal lattices (right).

a1

a2

a1

a2

a*1

a*2

a*1

a*2

b*1

b*2

a1

a2

a*1

a*2

b2

b1

In order for the scalar products in (4a) to be dimensionless, the reciprocal lattice vectors must have units of inverse length, nm-1.As a consequence of (4b) a*2 and a*1 must be perpendicular to a1 and a2, respectively, which means that a rectangular real-space lattice will also have a rectangular reciprocal lattice. For non-rectangular lattices the angles are different in real space and reciprocal space. The right-hand column of Figure 1 shows the corresponding reciprocal lattices for each of the surfaces on the left. The reciprocal lattice vectors defi ne the positions of the diffraction maxima through the Laue equation (5).

k||,out (n1,n2) = k||,in + n1 a*1 + n2 a*2 (5)

k||,out is the component of the wave vector of the diffracted electrons, which is parallel to the surface plane (by conven-

62 3 Experimentelles

!"

#$

!"

#$%&!'

%

Abbildung 3.16:Adsorptionsstruktur von COauf Ru(001). Oben: Strukturmo-dell und LEED-Bild (k-Raum)der (

!3"

!3)R30! CO-Bedeckung

(0.33 ML). Weitere Adsorptionvon CO fuhrt zur Komprimierungdieser Struktur bis zur CO-Satti-gungsbedeckung (0.68 ML), derenLEED-Bild im unteren Teil darge-stellt ist.

!"

#$#$

#$

#$

3.4.3 Das Koadsorbatsystem CO/O/Ru(001)

Sauersto! adsorbiert dissoziativ, also atomar auf Ruthenium.[Ove98, Ove96] DieBindungsenergie ist abhangig von der Bedeckung und betragt fur 0.5 ML proSauersto!atom EB=4.9 eV.[Ove98, Sta96] Die Adsorption erfolgt an dreifach ko-ordinierten (hcp-“hollow”-)Platzen. Im Folgenden soll nur die (2"1)O-Bedeckung(0.5 ML) bzw. die entsprechende Mischbedeckung mit CO diskutiert werden, dadiese Ausgangspunkt der durchgefuhrten Experimente ist. Sie wird anhand vonReferenzspektren prapariert, siehe hierzu Abbildung 3.18. Das LEED-Bild der(2"1)O-Bedeckung ist im rechten Teil von Abbildung 3.17 dargestellt, es zeigteine (2"2)-Symmetrie da es uber drei mogliche Ausrichtungen der (2"1)-Strukturgemittelt ist.

Die maximale CO-Bedeckung auf dieser O-Bedeckung betragt ca. 0.2 ML, waseiner (2"2)-Struktur entspricht. Fur das Gesamtsystem werden zwei unterschied-liche Strukturen diskutiert.[Hof91, Kos92] Die sogenannte Honigwabenstrukturist im linken Teil von Abbildung 3.17 dargestellt. Gegenuber einer (2"1)O-Struktur hat jedes zweite O-Atom einen Platzwechsel vollzogen, der auf der nurmit O-Bedeckten Oberflache energetisch ungunstig ware. Die Gesamtenergie der



128

BUNSEN-MAGAZIN · 12. JAHRGANG · 4/2010UNTERRICHT

the fl uorescent screen have become commercially available in recent years for applications that require low incident beam currents, either to avoid beam damage (e.g. organic molecules) or charging of insulating samples (e.g. oxides). These systems can be operated with electron currents as low as 1 nA. Typical LEED systems have diameters of around 140 mm.

The LEED pattern is recorded using a video camera with suit-able image processing software. As with all methods that use electrons as probes, vacuum conditions are required because electrons cannot penetrate a gas atmosphere at normal pres-sures. In general, however, the vacuum conditions required to avoid contamination of clean surfaces are more rigorous (typi-cally < 10-9 mbar) than those imposed by the use of electrons (typically < 10-6 mbar).

4 APPLICATIONS

In this section we will discuss a small selection of typical ap-plications of LEED in order to illustrate the different levels at which this technique yields information about surface ge-ometries.

4.1 LEED PATTERN: CO ON NI{111}

The adsorption of carbon monoxide on the {111} surface of nickel is a good example how LEED diffraction patterns can be used for a simple characterization of adsorbate structures. With increasing coverage of CO adsorbed on Ni{111} four dif-ferent LEED patterns are observed between about 0.30 and 0.62ML (1 ML corresponds to 1 molecule per substrate sur-face atom):

• a diffuse [2 1; -1 1] or p(!3 x !3) R30° pattern between 0.3 and 0.4 ML,

• a sharp [2 0; 1 2] or c(2 x 4) pattern for coverage around 0.5 ML,

• a sharp [3 1; -1 2] or p(!7 x !7) R19° pattern between 0.56 and 0.60 ML,

• a more complicated [3 2; -1 2] pattern at the maximum cov-erage of 0.62 ML, which is described as “c(2!3 x 4)rect” in non-standard Wood notation.

Images of the fi rst three patterns are depicted in Figure 5 to-gether with the corresponding real-space unit cells (red arrows and dashed lines). The middle part of the Figure also shows the complete (2x4) unit cell (in black). Note that the “c” in the Wood notation c(2 x 4) means that the center and the corners of the (2x4) unit cell are lattice points. Therefore the primitive unit cell is only half the size, as indicated by the red arrows. The matrix notation always refers to the primitive unit cell. The yellow arrows in the LEED patterns (left) indicate the reciprocal lattice vectors corresponding to the unit cells marked in red.

For the c(2 x 4) and p(!7 x !7) R19° structures it is not pos-sible to reach all diffraction spots by adding integer multiples of these two vectors. This is because the observed pattern is a superposition of LEED patterns arising from different parts

of the surface, where the ordered arrangements of molecules are the same in principle but may have different orientations. Such rotation or mirror domains are usually observed if the superstructure has lower symmetry than the underlying sub-strate alone. Any symmetry operation of the substrate surface (rotation or mirror) that is not shared with the superstructure will therefore convert the superstructure unit cell into a unit cell that is equivalent but has a different orientation. This new unit cell has a different reciprocal lattice with a new set of dif-fraction spots. All orientation domains are equivalent and will, therefore, cover equal areas of the surface. In the case of the c(2 x 4) superstructure, which has a rectangular unit cell, the missing symmetry is the three-fold rotation of the hexagonal substrate surface; therefore there are two additional rotational

Figure 5: Experimental LEED patterns formed by CO adsorbed on Ni{111} (left) and corresponding real-space unit cells (right): p(!3 x !3) R30° (top, Ekin = 98eV) c(2 x 4) (middle, Ekin = 129eV) and p(!7 x !7) R19° (bottom, Ekin = 117eV). Note that the real space diagrams are rotated by about 30° with respect to the crystal orientation of the experiment; the dark structure extending from the top left to the middle of the LEED patterns is the shadow of the electron gun [Held98].

!"="0.33ML:"""p(#3"x"#3)"R30°

a1

a2

2"""1

$1"""1M"="

!"="0.50ML:"""c(2"x"4)

a1

a2

2"""0

1"""2M"="

!"="0.57ML:"""p(#7"x"#7)"R19°

a1

a2

3"""1

$1"""2M"="

CO / Ni(111)



a) b)

!

[001]

[110]_

Mirror domains



Superstructures



The adsorption site is not available via static LEED



Complete surface structure via dynamical I-V LEED and multiple scattering theory









A

Fig. 1. (a) Top- (b) side-view of the two possible terminations (A&) of a hccp (10%) surface.

ature could be controlled by a NiCi-Ni thermo- couple spot-welded to the edge of the sample.

Further crystal preparatian in vacuum con- sisted of cycles of argon sputtering at 3~-500 eV ion energy and beam currents of 3-6 PA/cm’ whereby the sample temperature was successively increased up to 500 K followed by cycles of annealing at 680 K. Since a phase transition from the hcp to the fee phase occurs at 700 K much

Fig. 2. I&ED-pattern of the clean co(lOiO) surface at an electron energy E, = 66 eV.

care was taken not to reach sample temperatures above 680 K. After numerous sputter.-annealing cycles only small amounts of carbon remained at the surface which could be reactively removed by oxygen at 650 K. Traces of oxygen impurity could be reacted off by hydrogen at 680 K. The cleanli- ness of the surface could be controlled very sensi- tively by HREELS. A LEED pattern of the clean surface at an electron energy E, = 66 eV is shown in fig. 2.

LEED I/ 1/ curves were measured and recorded by means of a video (“auto”) LEED system devel-

e ne r gy ieV)

Fig. 3. Experimental I/Y curves of four s~mmet~-equivalent LEED-beams: (1.2). (I.?). f%.2) and ii.?).

H. Over et al. / A LEED structural analysis of the Co(lOi0) surface

Comparison of multilayer relaxations Ad,, and Ad,, for some rectangular single crystal surfaces

Cu(ll0) [24] Al(110) [25] Re(lOi0) [l] Fe(211) [26] c0(1010)

- 8.5( * 0.7)% - 8.4( * O.S)% - 17% - 10% - 12.8( ?0.5)% + 2.3( k 0.9)% + 4.9( f 1 .O)% + l-2% 1-58 + 0.76( k 0.2)%

for both crystals Re(lOi0) and Co(lOi0) indicating a similar relaxation mechanism for these hcp surfaces.

To summarize, our LEED analysis of the clean Co(lOi0) surface reveals that only termination A represents the stable atomic configuration in the surface, exhibiting a contraction of the first sub- plane spacing by 123 *OS)% and no further relaxation deeper in the bulk. We note that the same termination (A) was found previously for the clean Re(lOi0) surface [l].

We thank Dr. A. Preusser for technical assis- tance.

Note added in proof

After acceptance of our Letter a recent LEED analysis from Lindroos et al. [27] concerning “The termination and multilayer relaxation at the Co- (1010) surface” was brought to our knowledge. The Z/V curves of Lindroos et al. are in good agreement with our results, but a discrepancy occurs with respect to the magnitude of the first layer contraction. While Lindroos et al. ascer- tained a contraction of -6.5( *2)‘% we determined a larger contraction of - 12.8( f 0.5)s.

References

[l] H.L. Davis and D.M. Zehner, J. Vat. Sci. Technol. 17 (1980) 190.

[2] G.L.P. Bertring, Surf. Sci. 61 (1976) 673. [3] G.L.P. Berning, G.P. Alldredge and P.E. Viljoen, Surf.

Sci. 104 (1981) L225. [4] B.W. Lee, R. Alsenz, A. Ignatiev and M.A. Van Hove,

Phys. Rev. B 17 (1978) 1510.

[5] M. Welz, W. Moritz and D. Wolf, Surf. Sci. 125 (1983) 473.

[6] G. Kleinle, W. Moritz, D.L. Adams and G. Ertl. Surf. Sci. 219 (1989) L637.

[7] K.-H. Ernst, E. Schwarz and K. Christmann. in prepara- tion.

[8] K. Mliller and K. Heinz, in: The Structure of Surfaces, Eds. S.Y. Tong and M.A. Van Hove (Springer, Berlin. 1986) p.105.

[9] M.A. Van Hove and S.Y. Tong, Surface Crystallography by LEED, Springer Series in Chemical Physics (Springer. Berlin, 1979).

[lo] W. Moritz, J. Phys. C 17 (1984) 353. [ll] M. Maglietta, E. Zanazzi, F. Jona, D.W. Wepsen and

P.M. Marcus, Appl. Phys. 15 (1978) 409. [12] L. Hedin and B.I. Lundquist, J. Phys. G 4 (1971) 2064. [13] D.W. Marquardt, J. Sol. Indust. Appl. Math. 11 (1963)

431. [14] G. KIeinle. W. Moritr and G. Ertl, Surf. Sci. 238 (1990)

119. [15] G. Kleinle, Thesis, Freie Universitlt Berlin (1989). [16] (a) P.J. Rous, J.B. Pendry, D.K. Saldin. K. Heinz, K.

Mliller and N. Bickel, Phys Rev. Lett. 57 (1986) 2950; (b) P.J. Rous and J.B. Pendry, Comput. Phys. Commun. 54 (1989) 137.

[17] W. Moritz, H. Over, G. Kleinle, G. Ertl, in: The Structure of Surfaces III, Eds. M.A. Van Hove, S.Y. Tong and Xie Xide (Springer, Berlin, 1990) in press.

[18] H. Over, U. Ketterl, W. Moritz and G. Ertl, m prepara- tion.

[19] G. Kleinle, J. Wintterlin, G. Ertl, R.J. Behm, F. Jona and W. Moritz, Surf. Sci. 225 (1990) 171.

[20] E. Zanazzi and F. Jona, Surf. Sci. 62 (1977) 61. [21] J.B. Pendry, J. Phys. C 13 (1980) 937. [22] A. Preusser, Comput. Aided Geom. Design 3 (1986) 267. [23] D.L. Adams, W.T. Moore and K.A.R. Mitchell, Surf. Sci.

149 (1985) 407. [24] D.L. Adams, H.B. Nielsen, J.N. Andersen, 1. Steensgaard,

R. Feidenhans’l and J.E. Sorensen, Phys. Rev. Lett. 49 (1982) 669.

[25] H.B. Nielsen, J.N. Andersen. L. Petersen and D.L. Adams, .I. Phys. C. 15 (1982) L1113.

[26] J. Sokolov, H.D. Shih, U. Bardi, S. Jona and P.M. Marcus, Solid State Commun. 48 (1983) 739.

[27] M. Lindroos, C.J. Barnes, P. Hu and D.A. King, Chem. Phys. Lett. 173 (1990) 92.



Absorption very fast - ~10-16 s- no photoemission for - KE of photoelectron increases as BE decreases- intensity of photoemission goes with intensity of photons- need monochromatic (x-ray) incident beam- a range of KEs can be produced if valence band is broad- since each element has unique set of core levels, KEs can be used to fingerprint elementBinding energy (BE) represents strength of interaction between electron (n, l, m, s) and nuclear charge.



Binding energy for different electron core levelsas a function of the atomic number



EKin = ħω – EB

EB = Eftot(N-1) - Ei

tot(N)

EKin

EB

but relaxation occurs!

N-1 electrons react on the created hole

RELAXATION! EB smaller as E0

Koopmans Theorem:(„frozen orbital approximation“)

N-1 electrons do not react on the hole creation

EB = Orbital eigenenergy = E0

EKin

EB



Koopman's TheoremThe BE of an electron is simply difference between initial state (atom with n electrons) and final state (atom with n-1 electrons (ion) and free photoelectron) BE = Efinal(n -1)- Einitial (n)If no relaxation followed photoemission, BE = - orbital energy which can becalculated from Hartree-Fock



Spin-orbit coupling



l = 1 (p)l = 0 (s)

l = 2 (d)

l = 3 (f)

1

2

3

4

5

6

7

n



As the energy depends on n, l and j it is convenient to label the electron states according to this three quantum numbers in the following way:The value of n followed by the Label of the shell and as subscript the value of j.

Quantum numberQuantum numberQuantum number X-raylevel

Spectroscopicleveln l j

X-raylevel

Spectroscopiclevel

1 0 1/2 K 1s1/2

2 0 1/2 L1 2s1/2

2 1 1/2 L2 2p1/2

2 1 3/2 L3 2p3/2

3 0 1/2 M1 3s1/2

3 1 1/2 M2 3p1/2

3 1 3/2 M3 3p3/2

3 2 3/2 M4 3d3/2

3 2 5/2 M5 3d5/2



XPS survey spectrum of a clean Silver surface



Ag3d detail spectrum of a clean Silver surface



PES/XPS: Apparatus



Electron Spectroscopy: Energy Detector



Schematic representation of theInteraction of an electron beamimpinging in normaldirection on a surface.

How to make characteristic X-rays



For XPS



For first order diffraction of the AlKα line, with λ = 8.3 Å, it is found

that quartz crystal spacing of the [10-10] planes is 4.25 Å and the Bragg angle is thus 78.5°. Quartz has many advantages, since it can be obtained in perfect crystals of very large size, which can easily be bent elastically and can be backed to high temperatures without damage or distortion.

A crystal to monochromatethe MgKα line is not known!!



Electron energyanalyzer

Mono-energeticX-ray source

TurbomolecularPump

TurbomolecularPump

AnalyzerChamber

AnalyzerChamber



The Swiss Light Source (SLS) in Villigen



!"#$%&'(&'#



Chemical shift of PES lines



Poly(vinyl fluoride) -(CHF-CH2)-

In this case we can see that all the fluorine atoms are equivalent but that there are two inequivalent carbon atoms in this polymer, where we should expect a strong charge transfer due to the C-F bond and therefore a strong chemical shift.



Poly(vinyltrifluorocetate)



Ultraviolet photoelctron spectroscopy (UPS)



Auger electron spectroscopy (AES)



Schematic illustration of the KLL Auger electron emission process



Typical representationof an Auger spectrum

Auger Spectrum



Diagram showing the energyof the main Auger lines



Scanning Auger Microscopy (SAM)



Molecular Structure:

Angle-scanned X-ray Photoelectron Diffraction

x-rays

• x-ray tube• synchrotron

1st orderinterference

‚forward-focusing‘

analyzer

polar angle Θ

azimuthalangle ϑ



Absolute configuration of molecules

Fasel, Ernst, et al., Angew. Chem. Int. Ed. 43 (2004) 2853



The adsorption site is not available via static LEED



Electron Energy Loss Spectroscopy (EELS)

also known as high resolution EELS (HREELS)Based on inelastic scattering of monoenergetic beam of low energy electrons (Ei = 1-10 eV) from surface.

Es = Ei - Evib


106

Heinrich Rohrer

Gerd Binnig

Nobel Prize (1986)

Scanning tunneling microscopy (STM)



In classical science if a particle impinges a barrier (such as small ball hitting a solid wall) there is no chance of the particle being found beyond the barrier.

In quantum mechanics this is not correct and we speak of a probability that the electron can be found beyond the barrier.

Classical barrier – no transmission, only reflection

quantum barrier – part transmission, Part reflection



In quantum mechanics one must think of the electron has having wave-like as well as particle-like properties – wave-particle duality.

We speak of the wavefunction of the electron and we are interested in wavefunction penetration and overlap.



When two materials (one of which is often in the form of a metal tip) are brought close to one another and a potential difference V applied a tunnelling current I occurs.

This can be expressed as

z is the separation between the two materials and

V is the potential difference applied and

E is the energy of the electron

is read as ‘h bar’ and is Planck’s constant (actually h is Planck’s constant and h bar is h/2π)m* is the effective electron mass

To a first approximation, STM is like doing an I-V measurement but with the electrodes very close together.



However things are a little (a lot) more complicated thanthat.

Firstly, the tip is often a sharpened tip using a strong solution of KOH and a voltage applied.

An ideal tip has oneatom at the end of the tip.



0.25 mm

0.00025 mm

The distance of the tip from the surface is 1 nm = 0,000.001 mm while the height regulation is accurate down to 1 pm = 0,000.000.001 mm

Ag(111)

For an analogous height resolution using the Eiffel tower one must position it at 1 mm above the Champs Élysées and scan it with an accuracy of at least 0,001 mm.



electrons

When the tip is biased positive relative to the sample electrons flow from the sample to the tip – note the tunnel current is in the opposite direction.



There are two common modes for STM operation:

Constant Current Mode By using a feedback loop the tip is vertically adjusted in such a way that the current always stays constant.

Constant Height Mode In this mode the vertical position of the tip is not changed. The current as a function of lateral position represents the surface image.



Constant current contour

Bias voltage

Distance z

eSample

eee

Tip

Tunneling current ≈ e -2κz

VDC

Conductive partLess Conductive part



If the tip-surface interaction is so important how do we maintain accurate separation?

The electronics uses a feedback system. The sample is mounted on a stage which is attached to a piezoelectric material.

A piezoelectric material is a material that expands or contracts when a voltage is applied to it.

During constant current STM when the feedback sensor detects a change in the tunnel current, the voltage to the piezoelectric is changed to adjust the height to keep the tunnel current the same



So if we have a metal tip and a metallic sample, what exactly are we measuring/probing?

Band alignment under Zero bias

In the sample, there may be bands present, which may be occupied or empty.



Band alignment under positive sample bias

Electrons flow from the tip to the empty sample bands or unoccupied sample states.



Band alignment under negative sample bias

Electrons flow from the filled sample bands or occupied sample states.



This also applies to semiconductors with their states in the band gap



Annealing of Cu(111) under the STM



Step fluctuation on Cu(111)



A tool beyond imaging:Manipulation of atoms and molecules


Manipulations with the STM @ 4K



Experiment:

- Adsorption on Cu(211) @ 30 K

- LT-STM @ 6 K

Propene

H3CH C=C H

H

Cu(211)

CO

CO


The smallest Swiss Cross in the world (or PR comes first)

propene on Cu(211)


INELASTIC ELECTRON TUNNELING (IET) - INDUCED ACTION

e -

Translation

e -

Desorption

e -

Chemical Reaction

+ X

e -Flipping (?)

Rotation

e -


Cu(211) propene

6.1nm x 4.9 nmSTM contrast from DFT

Adsorption at 40 K; STM at 7 K


Daniele Passerone, Empa

DFT results for single propene: two configurations

D. Passerone, Empa


Rotamers & Enantiomers


2

2*

1

*1

1

1

*22

*11

6.1nm x 4.9 nm


“IET action spectroscopy”: Rotation

ν (C=C) = 204.2 meV νs(CH3) = 361.5 meVνa(CH3) = 364.3 meVνa(CH3) = 367.0 meV

1 2*

2* 12*

1

Sample bias (mV)

Rea

ctio

nyi

eld

pere

lect

ron

a c

b

10-12

200

200 mV

400300

360 mV

10-11

10-10

10-9

10-8

500

Parschau et al., Angew. Chem. Int. Ed. 48 (2009) 4065

threshold correlates with vibration!


νs(CH2) = 369 meVν (CH) = 373 meVνa(CH2) = 381 meV

Lateral hopping “action spectroscopy”

νs(CH2) = 369 meVν (CH) = 373 meVνa(CH2) = 381 meV

375 mV

10-12

10-11

10-10

300 400 500 600

*

*

a

bR

eact

ion

yiel

dpe

rele

ctro

n

Sample bias (mV)

c


“enantio-conversion”: from right to left and vice versa

10

1

10

1

101

N = 1.0 ± 0.1

N = 1.9 ± 0.2

Tunneling current (nA)

Hop

ping

rate

(s- 1

) Flipping rate(s -1)ca

b


- 2 V pulse

Oxidation: IET-induced chemistry

400 mV / 100 nA

+ 2 V pulse


allene

propyne ν (C=C) = 266.0 meV-ν (C=C=C) = 242.5 meV

Identification of the reaction product: IET hopping action spectroscopy

CH3 and C=C can be excited propyne-

270 mV

370 mV



A Modern Tool for Studying Surface Processes:Scanning Force Microscopy



!"#$%&%#%&'()"$*(+,%#$*&$-)./)$01234)

5(,.$#$0()/+(6"($078)!"

9&*%:)./)+(,.$#$0()0"+'();39<4=8))

!1/#0%.+8

!" !

#

!" fQ

>.$,(+'#%&'()/.+0(,)) ,:&/%)./)+(,.$#$0()0"+'()) /)?&,,&@#%&'()/.+0(,))))) A+.#*($&$-)./)0"+'())

#%%+#0%&'()&$%(+#0%&.$/

B



Quarzuhren sind aufgrund der darin enthaltenen Quarzoszillatoren hochpräzise. Das zeitgebende Element ist eine Stimmgabel aus einkristallinem Quarz, die bei einer

Frequenz von f0 = 32 768 = 215 Hz schwingt.Scanning force microscopy using a quartz tuning fork

Figure 2: Experimental setup. a) The NC-AFM/STM tuning fork sensor is glued onto the carrier

made of macor. Contacts P1 and P2 are the contacts of the excitation piezo. The signal from the

tuning fork is detected via contact T1 and T2. The µm wire attached to the tip conducts the tunnel-

ing current. b) The walker unit is a tripod situated on shear stack piezos for the coarse approach.

The x-, y- and z-piezos are used during the scan process. The tuning fork is located opposite the

sample (only half of the sample is drawn to keep the view free to the sensor carrier). Further ex-

planations are given in the text. Schematic of the microscope on its support stage: (A) walker unit,

(B) x-, y-piezo and (C) z-piezo of the tripod scanner unit, (D) z dither piezo, (E) sensor carrier, (F)

tuning fork assembly, (G) sample (not fully drawn), (H) sample holder (not fully drawn), (I) sam-

ple stage, (J) microscope stage, (K) walker support and (L) shear stack piezos. The base plate has a

diameter of 10 cm.

amplifier has to be placed nearby to improve the signal-to-noise ratio. The change of the tuning93

fork frequency ∆ f is used as a feedback signal for NC-AFM.94

The tip is electrically connected to a Pt0�9Rh0�1 wire, 50 µm in diameter (see figure 2 a). Using this95

electrical contact, a bias voltage can be applied between tip and sample and a tunneling current can96

be measured. This tunneling current serves as a feedback signal when operating in the STM mode.97

However, while operating in one of the modes, AFM or STM, the other channel can always be co-98

recorded. Much care has been taken to ensure that both channels, AFM and STM, are electrically99

separated from each other, preventing cross talk.100

5



(NiAl)2�substrate (Al19O28Al28O32)2�. An oxygen deficiency with unoccupied electronic states in the411

aluminum oxide band gap was proposed.412

Figure 10: Atomic resolution NC-AFM image of a straight antiphase domain boundary (type I) inthe aluminum oxide on NiAl(110). The scan area is 6.4 nm � 6.4 nm in a) and b). b) An adjustedmodel [42] has been superimposed in. The unit cell is extended by 3 Å along the long edge of theunit cell. Inserted sites are given in lighter colors. Dashed lines indicate the extension. The dottedline highlights wave-like oxygen rows along the unit cell. c) shows an enlarged section of the im-age for better visibility (3.5 nm � 3.5 nm). Yellow arrows denote the direction and length (3 Å) ofthe Burgers vector. Yellow loops indicate spacious arrangements of oxygen sites that are differentfrom all domain sites. ∆ f = -2.75 Hz, Aosc = 3.8 Å, Ubias = -220 mV.

An atomically resolved NC-AFM image of a straight APDB (type B I) is shown in figure 10.413

Clearly visible, the boundary is marked by a fairly wide linear depression. The adjusted model414

for the lateral positions at the APDB [42] is superimposed in figure 10b) and found to be in per-415

fect agreement. From this we see that NC-AFM images the surface oxygen sites of the film with416

high accuracy. The above model is based on a unit cell that has been split up in the middle accord-417

ing to STM images. Important structural elements of the oxygen sub-lattice are highlighted as well418

as the extended unit cell and two equivalent lines between which the inserted new sites are visible.419

Inserted sites are marked in a slightly different color to distinguish them from the usual sites in the420

oxide unit cell: orange and light blue as compared to red and blue. In figure 10c) an enlarged sec-421

tion of the elongated unit cell at the APDB is given. In the middle of the APDB a broken block of422

8 O atoms appears, which is of the kind that is almost aligned with the NiAl[001] direction. A par-423

ticularly spacious arrangement of oxygen atoms in the shape of a quadrangle (yellow dotted loops)424

is formed at this block at the boundary. This is in agreement with DFT calculations [42], which as-425

22

Insulator surfaces: Al2O3 / NiAl(111) and NaCl

!"#$%&'()*+,-.#"/0

123$4$526$(78

!"#$%&'()$ *+,-./0-"12)1+$3),


uhv ernst peq

Documents

eth zh catal

scanning tunneling

shear stack

clean silver

space unit

reciprocal

complex model

leed structural