An experimental UHV AFM-STM device for characterizing surfacenanostructures under stress/strain at variable temperatureY. Nahas, F. Berneau, J. Bonneville, C. Coupeau, M. Drouet et al. Citation: Rev. Sci. Instrum. 84, 105117 (2013); doi: 10.1063/1.4826555 View online: http://dx.doi.org/10.1063/1.4826555 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v84/i10 Published by the AIP Publishing LLC. Additional information on Rev. Sci. Instrum.Journal Homepage: http://rsi.aip.org Journal Information: http://rsi.aip.org/about/about_the_journal Top downloads: http://rsi.aip.org/features/most_downloaded Information for Authors: http://rsi.aip.org/authors

REVIEW OF SCIENTIFIC INSTRUMENTS 84, 105117 (2013)

An experimental UHV AFM-STM device for characterizing surfacenanostructures under stress/strain at variable temperature

Y. Nahas, F. Berneau, J. Bonneville, C. Coupeau, M. Drouet, B. Lamongie, M. Marteau,J. Michel, P. Tanguy, and C. TromasInstitut Pprime, Département Physique et Mécanique des Matériaux, UPR 3346,CNRS-Université de Poitiers-ENSMA, 86962 Futuroscope-Chasseneuil, France

(Received 21 June 2013; accepted 9 October 2013; published online 31 October 2013)

A compression setup fully integrated in an ultra high vacuum chamber is presented. The systemhas been designed to combine in situ mechanical test together with near field microscopy at vari-able temperature, from 90 to 600 K. Compressive stress can be applied on the samples up to500 MPa at different strain rates ranging from 10−6 s−1 to 10−2 s−1. The setup performances arehighlighted through investigations on Au and Ni3(Al,Ta) single crystals. In particular, it is demon-strated that the high mechanical stability of the original apparatus allows us to follow in situ theevolution of the same area of interest over a large range of temperature and to keep the high spatialresolution offered by near field microscopy, even at high strain levels. © 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4826555]

I. INTRODUCTION

Transmission electron microscopy (TEM) is commonlyused, since a few decades, to investigate the dislocation mi-crostructures and to identify the elementary plastic mecha-nisms taking place in crystalline materials. The invention ofthe scanning probe microscope (SPM) by Binnig and Rohrer1

makes possible to finely investigate the slip line structures atsample surfaces down to atomic resolution under specific ex-perimental conditions. TEM provides information of the dis-location organization in the bulk while SPM allows investi-gations of the traces let by the emergence and propagationprocess of dislocations at the nanometer scale (see review inRef. 2). This latter approach has been widely used underatmospheric conditions, for instance to study the develop-ment of slip bands in Al single crystals3, 4 or the yield stressanomaly in Ni3Al intermetallics.5 Such studies using atomicforce microscopy (AFM) in air under atmospheric pressureare however drastically limited by surface contamination, fastoxidation of metallic materials, and/or lateral resolution dueto tip-sample convolution.

Some investigations have already been performed underultra high vacuum (UHV) (<10−10 mbar) by scanning tunnel-ing microscopy (STM). In these experiments, dislocations aregenerally nucleated using the STM tip as an indenter, withno real control of the applied stress to the materials. Spe-cific dislocation configurations have thus been evidenced atthe atomic level such as dislocation dissociations in Au6–8 orAg9, 10 fcc materials and stair-rod recombinations.7

In situ experiments allow investigating the same surfacearea under increasing strain and can be performed both underatmospheric conditions11–13 and in UHV environment.14–16 Inthis latter case, the micro-bending device interfaced with ahomemade STM allows observations at high temperatures,but is limited to deform samples in their elastic stage.17

In this context, we developed an original experimentaldevice that allows following in situ the evolution of strainedmaterials by AFM/STM under UHV environment in a large

temperature range. The main challenges are presented andthe performances of this unique experimental equipment aredemonstrated through preliminary results on specific singlecrystals.

II. EXPERIMENTAL SETUP

A. Requirements

The requirements for this unique experimental device areas follows:

(1) working in UHV environment to avoid any contamina-tion of the sample surfaces, even at high temperature;

(2) keeping the high spatial resolution offered by the nearfield microscopy for strained/stressed samples (implyingthat the deformation setup needs to be interfaced on themicroscope stage);

(3) allowing to explore a whole stress/strain curve in a largerange of temperature, even with tip in contact with thesample surfaces (in situ experiments); and

(4) deforming materials with strain rates ranging from10−6 s−1 to 10−2 s−1.

B. UHV system

The experimental setup is shown in Fig. 1. The setup iscomposed of two UHV chambers and a fast entry lock forsample introduction. Several valves allow isolating one cham-ber from the other.

The first UHV chamber (❶ in Fig. 1) is dedicated to sam-ple preparation. It includes a resistive oven (❷ in Fig. 1) al-lowing annealing up to 1200 K associated with an Ar ionbeam (5 keV) (❸ in Fig. 1). This allows carrying out sev-eral heating/sputtering cycles to obtain a crystalline surfaceof good quality for AFM-STM investigations. A LEED (LowEnergy Electron Diffraction)/Auger (Auger spectroscopy)

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2

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FIG. 1. Photograph of the setup: ❶ preparation chamber; ❷ 1200 K oven;❸ ion gun; ❹ LEED-Auger; ❺ mechanical test chamber; ❻ video camera; ❼long focus optical instrument; ❽ STM tip preparation stage; and ❾ externalmotors.

setup (❹ in Fig. 1) is also available in the preparation chamberfor structural and chemical characterizations of the samples,respectively.

The second chamber is especially dedicated to mechani-cal testing (❺ in Fig. 1). A near field microscope at variabletemperature (VT) is interfaced with a home-built compressiondevice described in details in the following. The compressiondevice is located on the AFM/STM stage, so that the clas-sical damping system by eddy current stays efficient duringthe in situ mechanical tests in order to ensure a high spatialresolution. The VT microscope comes from Omicron Nan-oTechnology GmbH, which has fully adapted its commercialVT AFM-STM apparatus for fitting the home-built compres-sion setup. The switching between the two AFM/STM modesis allowed just by changing the tip; tip holder is however thesame. A video camera helps to perform the rough manual ap-proach of the tip close to the sample surface (❻ in Fig. 1). Along focus optical instrument is also installed for positioningthe tip on a specific area of the sample surface with a mi-crometer accuracy and allows adjusting the laser position inthe AFM mode (❼ in Fig. 1). The STM tips are usual 300 μmdiameter tungsten wires that can be flashed to improve theirimaging quality (the STM tip preparation stage (❽ in Fig. 1)allows applying about 3 A).

UHV is obtained using successive pumping devices. Aprimary pump connected to a turbomolecular one allowsreaching a pressure of 10−9 mbar after initial baking of all theUHV setup at 120◦C. Titanium sublimators and ionic pumpsin each chamber allow then to reach a pressure lower than10−10 mbar. A working pressure of 3 × 10−11 mbar into bothchambers is routinely achieved.

C. Home built compression setup

The compression device is composed of two differentparts.

1 2

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FIG. 2. Technical drawing of the internal compression setup (a) overview in-cluding the external motors and the UHV chamber: ❶ rotary feedthrough; ❷sample localization; ❸ AFM/STM stage also used as mechanical structure forthe internal compression device; and ❹ UHV chamber. (b) Technical draw-ing of various elements composing the compression setup: ❺ piezoelectricactuators; ❻ mechanical guides; ❼ optical position sensors; ❽ load cells; ❾sample holder; and ❿ conical stressors (compression plates) acting as thermalbarriers. (c) Technical drawing of the sample holder. The sample is scannedby AFM/STM tips on its lower part.

An external part is directly linked to the UHV chamberand consists in two external motors in a symmetrical configu-ration (❾ in Fig. 1; ❶ in Fig. 2(a)). The external compressionsetup is aimed to reach the elastic or plastic stage of interestover a whole strain/stress curve. The dimensions of the sam-ple are nominally 2 × 2 × 6 mm3; the sample is located in themiddle of the compression device (❷ in Fig. 2(a); Fig. 2(b))and compression is achieved by screw-nut based elements.In this configuration, the microscope stage is mechanicallylinked to the external environment and AFM/STM investiga-tions are not possible.

Once the specific strain/stress level is reached, the ex-ternal motors are declutched from the microscope stage, inorder to allow the eddy current vibration isolating to be effec-tive. It was demonstrated that the change in stress induced bythe external declutch never exceeds 7% of the total load (seea characteristic stress/strain curve on Fig. 3 for Ni3(Al,Ta)deformed at 300 K). The damping system has been adaptedby Omicron NanoTechnology GmbH to receive the additional20 kg weight of the home-built internal compression device.The smooth and regular displacements using two piezoelec-tric actuators (PZT) attached to the microscope stage (❺ inFig. 2(b)) allow in situ AFM/STM imaging of the sample sur-face under both increasing and decreasing strain/stress. Thefull extension of the two PZTs is 120 μm, which correspondsto an additional straining of approximately 2%.

Both external and internal deformation modes allowkeeping constant strain rate ranging from 10−6 to 10−2 s−1

and stress-strain curves can be recorded during loading and

105117-3 Y. Nahas et al. Rev. Sci. Instrum. 84, 105117 (2013)

0.0 0.2 0.4 0.6 0.8 1.0 1.20

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FIG. 3. Stress/strain curve obtained on a Ni3(Al,Ta) single crystal for com-pression at 300 K along the [1̄23] axis. First straining carried out by externalmotors (blue filled triangles) and second straining using the internal piezo-electric actuators (green open squares). The stress change (7.9 MPa in thiscase) induced by the declutch is about 6%. Some oscillations are visible dur-ing plastic deformation: they are due to sample relaxation while taking sev-eral AFM images at different strain values.

unloading of the sample. A mechanical guide (❻ in Fig. 2(b))is fixed along the compression device to ensure the uniaxialdeformation of the samples. Two optical rulers allow mea-suring displacements of the compression plates with a 5 nmresolution (❼ in Fig. 2(b)). Two 2000 N full-scale load cellsare located on both sides of the internal compression device(❽ in Fig. 2(b)), leading to a maximum measured stress of500 MPa associated with a resolution ±0.05 MPa for thenominal 4 mm2 cross section. Dedicated software written inLabView (National InstrumentsTM) monitors the mechanicaltests and collects the data, in order to relate each AFM/STMimage to the strain/stress curve.

In situ compression tests can be carried out at differ-ent temperatures. The specific sample holder (❾ in Fig. 2(b);Fig. 2(c)) allows increasing the temperature up to 600 K withthe help of a resistive heater. A cryostat mounted on the up-per part of the sample holder enables carrying out in situ ex-periments down to a limit temperature of 90 K using liquidHelium. Several thermocouples are dispatched along the com-pression device to ensure a continuous control of temperature.Finally, two conical stressors (❿ in Fig. 2(b)) on both sides ofthe sample holder act as thermal barriers to avoid any workingtroubles on the PZTs.

III. PERFORMANCES

Investigations were performed on {111} and {541} facesof Au and Ni3(Al,Ta) single crystals, respectively, to highlightthe performances of the experimental device. Before introduc-tion in UHV chambers, the sample surfaces were polished toreach a “rms” roughness lower than 1 nm. For Ni3(Al,Ta), theprocedure consists in several steps of mechanical-chemicalpolishing as follows: a first rough polishing during 1 h us-ing Al2O3 9 μm particle size diluted at 10% in distilled water,

a second 1 h polishing with Al2O3 particle size reduced to3 μm, a further fine polishing step using successively silicaparticles with diameter of 40 nm for 12 h and aluminium ox-ide particles with diameter of 20 nm for 20 min, before beingcleaned by soft distilled water-jet and CO2 blast.

A surface preparation step is also performed in UHV toremove elements adsorbed in air and to obtain clean and ho-mogeneous surfaces at the atomic scale. It consists in repeatedcycles of Ar ion sputtering and annealing (for Au, sputter-ing at 900 V and 5 × 10−6 mbar, annealing at 800 K; forNi3(Al,Ta), sputtering at 2.5 kV and 4 × 10−7 mbar, anneal-ing at 1200 K).

In Fig. 4 is shown a characteristic Au(111) surface. TheSTM investigations have been performed at 300 K for differ-ent applied stresses. As expected, the surface exhibits stepswith elementary height equal to 0.22 nm (that corresponds

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FIG. 4. Au(111) surface obtained by STM investigations at 300 K and atdifferent straining (a) 4.25 MPa, and (b) 4.40 MPa. Grey arrows indicate thestress-induced modifications of some nanometer scale steps. The profile isassociated with the grey dashed line.

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to the distance between two successive octahedral planes)(Fig. 4(a)). Under stress, the monoatomic steps are alwaysvisible. Moreover, it is observed some local reorganizationscharacterized by steps tending to align along specific crystal-lographic directions (see arrows on Fig. 4(b)). Investigationsof this phenomenon are in progress but strongly suggest somestress-induced diffusion mechanisms, which gives insight tothe large field of investigations now offered by this new ex-perimental apparatus.

It is well-known that the Au(111) surface exhibits at theatomic scale a herringbone pattern resulting from succes-sive fcc/hcp stackings at the free surface.18–21 This specificatomic structure is shown in Fig. 5 for two stress values. Theheight profile shows a value close to the usual 0.03 nanome-ters; this low (picometer scale) contrast is still clearly distin-guishable under an applied stress. Moreover, the herringbonepattern is observed to be locally destabilized with increas-ing applied stress. A zipping-unzipping of herringbone pat-

(b)

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FIG. 5. Herringbone patterns on Au(111) surface obtained by STM investi-gations at 300 K and at different straining (a) 4.25 MPa, and (b) 4.40 MPa.The profile is associated with the grey dashed line.

(b) (a)

FIG. 6. STM image of Au(111) surface at 90 K under an applied stress of1.70 MPa. Two slip lines are indicated by dashed arrows. (b) is a blow-up ofa large terrace shown in (a).

terns induced by tensile stress was already reported by Schaffet al.14 but on larger terraces. Works are also in progressto have a better understanding of this phenomenon, in par-ticular concerning surface stress and nanostep effect on itsoccurrence.

Au single crystals have been deformed using the He cryo-stat. A temperature of 90 K was estimated using a dummysample. Even at such low temperature, the herringbone pat-tern is experimentally evidenced, the monoatomic steps aswell running from upper left to lower right of the STM im-age (Fig. 6). Two elementary slip lines (see dashed arrowsin Fig. 6(a)) are also observed on the right part of the imageresulting from two different slip systems. These experimen-tal results confirm the ability of our apparatus to investigateat low temperature the evolution of atomic structures in theplastic stage.

Ni3(Al,Ta) single crystals have been deformed along the[1̄23] crystallographic direction at three different tempera-tures, 300 K, 400 K, and 600 K, respectively. This inter-metallic compound exhibits a yield stress anomaly character-ized by an increase of resolved yield stress with increasingtemperature in the temperature range 300–800 K. This macro-scopic mechanical behavior has been extensively studiedby the past but the involved elementary plastic events arestill questioned.5, 22–24 The experimental results will be pre-sented and discussed in details in a forthcoming paper, inrelation with the plastic mechanisms.25 For convenience,only the surface evolution at 600 K and increasing plas-tic strain is presented in the following, from εp = 0.15%(Fig. 7(a)) to εp = 0.50% (Fig. 7(c)). The sample was first me-chanically deformed using the external compression device;its evolution at increasing plastic strain was then finely ex-plored by means of the two PZTs. As expected,26 the surfacemainly exhibits slip lines lying at approximately 60◦ fromthe [1̄23] direction, corresponding to primary (111) planes.Some extensions along cube cross-slip planes are also clearlyobserved (see arrows in Fig. 7(b)), in agreement with thehigh deformation temperature. A characteristic step profile isshown in Fig. 7(d). The height of the step is equal to 0.25 nm,which corresponds to the emergence process of only onesuperdislocation.27 All these results definitively demonstratethe ability of the experimental apparatus to follow in situthe evolution of strained materials, even at high temperature,

105117-5 Y. Nahas et al. Rev. Sci. Instrum. 84, 105117 (2013)

(a)

1 µm

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X 0 50 100 150 200 250 3000.00

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FIG. 7. AFM images of the evolution of Ni3(Al,Ta) surface at increasing plastic strain deformed at 600 K along the [1̄23] axis (fixed point indicated by a circle).(a) εp = 0.15%, (b) εp = 0.25%, (c) εp = 0.50%, and (d) profile extracted from the slip line labelled X in (c). Some cube cross slips are shown by arrowson (b).

and evidence its high potential to extract new experimentaldata at the nanometer scale, that can give new insights onthe elementary plastic mechanisms taking place in crystallinematerials.

IV. CONCLUSION

We presented a new experimental device that allowsfollowing in situ, by near field scanning probe microscopyunder ultra high vacuum environment, the evolution ofstressed/strained materials over a large range of temperature,ranging from 90 K to 600 K. The main challenge was to de-velop an internal compression device that can be directly in-terfaced on the UHV AFM/STM stage. The first results ob-tained on Au(111) and Ni3(Al,Ta)(541) single crystals havedemonstrated the high mechanical stability of the apparatus,allowing to follow in situ the evolution of the same area ofinterest over a large range of strain and temperature. The highspatial resolution offered by scanning probe microscopy ispreserved at large strains and high temperatures. It opens newinsights not only to investigate the elementary plastic mecha-nisms taking place in crystalline materials but also to explorenew ways of patterning at the nanometer scale that can be

controlled by stress-induced diffusion phenomena at the freesurfaces.

ACKNOWLEDGMENTS

Authors thank A.-M. Archambault for her help in samplecutting and G. Beney for fruitful discussions regarding Ni3Alpolishing. Financial supports from the French CNRS (with theCPER), from the ANR program, and from European FEDERare gratefully acknowledged. More precisely, this work wasfinancially supported by the ANR Nanoplast 2008. This studyalso pertains to the French Government program “Investisse-ments d’Avenir” (LABEX INTERACTIFS, ANR-11-LABX-0017-01).

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