Surface Science 604 (2010) 713–717

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Surface Science

journal homepage: www.elsevier .com/ locate/susc

Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomicforce microscopy

Satyaveda C. Bharath, Thomas P. Pearl *

Department of Physics, North Carolina State University, Raleigh, NC 27695-7518, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 November 2009Accepted for publication 22 January 2010Available online 2 February 2010

Keywords:Lithium niobateFerroelectricSurfaceGoldAtomic force microscopy

0039-6028/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.susc.2010.01.022

* Corresponding author.E-mail address: tppearl@ncsu.edu (T.P. Pearl).

Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporationonto the positive face of single crystalline, lithium niobate, LiNbO3(0 0 0 1) surface. In order to study themechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographicand phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uni-formly poled LiNbO3 surfaces were prepared via an ambient high temperature anneal and served as asupport for the thin gold films. These gold atomic layers were grown using electron bombardment evap-oration sources under ultra-high vacuum (UHV) conditions and subsequently characterized under bothvacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at thetop of LiNbO3 substrate step edges. With increased coverage, island formation proceeds due to localaggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphol-ogy, the data is discussed in terms of thin film growth mechanisms as well as the influence of native sur-face features such as defects and charge distribution. Understanding growth mechanisms for gold layerson ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on pat-terned poled LiNbO3 surfaces would occur as well as yielding greater insight on the atomic characteristicsof metals on ferroelectric interfaces.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Understanding the mechanisms behind atomic and molecularadsorption on ferroelectric oxide surfaces is the precursor to thedevelopment of devices that can harness the unique characteristicsassociated with polarizable materials. A great deal of work hasbeen done on metal/metallic oxide surface systems, but the studyof ferroelectric metal oxide surfaces, specifically perovskite orperovskite-like crystals is not nearly as exhaustive. The payoff forutilizing polar or polarizable materials that can influence surfacechemistry and interfacial properties via variations in electron den-sity is very large. Of particular interest is the use of controllablesurface charge density to influence molecular and atomic adsorp-tion thermodynamics and kinetics. To this end there has been a re-cent surge in effort to characterize adsorption, from molecules tometallic nanostructures, on ferroelectric surfaces, including bothperovskite and related crystals [1–8].

Growth of nanoclusters and nanocrystals of noble metals hasbeen observed at the surfaces of model ferroelectrics such as stron-tium titanate (SrTiO3) [9–11]. Preferential interactions, with re-spect to the surface chemistry of the exposed polar faces, were

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found where the metal was found to interact more favorably withthe TiO2 termination in contrast to the SrO termination. In the caseof palladium growth on SrTiO3(0 0 1) the size and shape of Pdnanocrystals formed could be tuned as a function of different sur-face reconstructions [12]. This, coupled with varying the conditionsduring deposition of the palladium, i.e., varying sample tempera-ture during deposition and annealing of the system after deposi-tion, were reliable techniques to make surfaces that werecovered with pyramidal, hexagonal, and hut shaped nanocrystals.This has been replicated for various metals on ferroelectric inter-faces such as cobalt, silver, and iron on strontium titanate[13,14]. In some cases, however, it is not very clear where thenucleation of the metal atoms occurs. While a trend has emerged,there is still little known about the specific characteristics of theinteraction of nucleating metal with the ferroelectric surface withrespect to adsorption and subsequent nucleation.

Regardless of what, at times, seem to be fairly thorough analysisof this metal on ferroelectric interface, there is still an incompletepicture of the morphology, chemistry, and structure of the inter-face at the atomic scale. Furthermore, there has been relatively lit-tle work done to try to understand the non-model perovskitecrystals like LiNbO3 with respect to metal thin film growth. Lith-ium niobate, LiNbO3, is currently used in a wide variety of existingdevice architectures and its very high surface charge density

714 S.C. Bharath, T.P. Pearl / Surface Science 604 (2010) 713–717

(�70 lC/cm2) makes it attractive for use in other applications sim-ilar to those explored involving other metallic oxides and ferroelec-trics. In this letter, we present the results from the study ofevaporation of gold and subsequent growth of atomically flat Aulayers on the positive face of the Z-cut (0 0 0 1) LiNbO3 surface asa model system for studying the interaction of noble metal adsor-bates with a polar interface. Surface supported Au nanostructureson metallic oxide surfaces have been used for studying heteroge-neous catalysis, molecular electronics, and chemical sensing [15],but there has not been a detailed study of the thin film growthof Au on a ferroelectric surface. Utilizing non-contact atomic forcemicroscopy (AFM) we have observed nucleation sites for thegrowth of atomic layers of Au on LiNbO3(0 0 0 1) grown under ul-tra-high vacuum conditions.

2. Experimental

Uniformly poled, single crystalline, LiNbO3 wafers (75 mmdiameter, 0.5 mm thick, polished, Z-cut; Crystal technology, Inc.)were diced into 3 mm � 10 mm pieces using a stainless steel wiresaw (South Bay Technology). The surface preparation of these LiN-bO3(0 0 0 1) samples was identical to that highlighted elsewherewith the only difference being the size of the sample used duringthe experiment [8]. Briefly, samples underwent two ultrasonicbath cleaning cycles, in acetone followed by methanol. The LiNbO3

samples were then annealed in a Lindberg high temperature fur-nace (ambient gas environment) in two steps: first at a very lowramp rate (�1.5 �C/min) to 200 �C and then at a faster ramp rate(�30 �C/min) to 1000 �C where it dwelled for 2 h. Samples werethen transferred via fast entry load-lock to an Omicron NanoTech-nology Multiprobe ultra-high vacuum (UHV) system (3 �10�11 Torr base pressure) equipped with surface preparation andcharacterization tools. The clean LiNbO3(0 0 0 1) surface morphol-ogy was characterized with low energy electron diffraction (LEED)and AFM (Omicron, beam deflection VT-SPM) prior to deposition ofAu. Prior to microscopy and diffraction measurements in UHV,samples were subjected to a 5 min in situ anneal at 100 �C to en-sure that the surface was free of any ambient species which mayhave adsorbed to the surface during the cool down from the annealcycle.

After confirmation of the LiNbO3 surface crystallinity and clean-liness, we deposited Au atoms onto the sample at very low flux,with coverage on the order of a few monolayers to a partial mono-layer. This deposition occurred using a triple cell evaporator (Omi-cron NanoTechnology GmbH, EFM 3T) attached to the UHV systemdescribed above. High purity gold slugs (99.995% metals basis, AlfaAesar) 3.175 mm in diameter � 3.175 mm long were used in con-junction with an alumina crucible in the evaporator. The distancefrom the end of the evaporator to the sample was approximately94 mm and a shutter attached to a rotary feed through was usedto control exposure of the evaporation flux to the sample. A fluxmonitor integrated into the evaporator measures the Au flux priorto opening the shutter.

The Au slug in the evaporator was degassed by slowly increas-ing the evaporator filament current to �1.5 A and increasing theelectron bombardment voltage to �600 V while monitoring thepressure in the chamber (maintained below 1 � 10�8 Torr). This al-lowed for a complete degassing of the Au slug prior to depositionto the surface. Water cooling to the evaporator housing was thenturned on and the filament current was further increased untilemission started and the flux monitor indicated an ion flux (posi-tive polarity) above 1 nA. To optimize the emission and flux output,the voltage and current were adjusted along with the evaporantposition in a linear motion (forward or backward) being cautiousnot to move too far forward or back, because of the danger of

destroying the heating filament. When the preferred conditionswere realized the shutter was opened and the deposition began,with the shutter being closed when the desired deposition timewas attained. All Au deposition on the LiNbO3 surface was doneat a sample temperature of 300 K. Generally a filament current of1.98–1.99 A was used with an electron beam voltage range ofabout 750–780 V. These conditions yielded an ion flux of �17 nAand an emission current in the range of 16–17 mA. Deposition timefor Au on LiNbO3 varied from 150 s to about 30 s.

The equation governing evaporation fluxes from effusive lowpressure sources is: deposition rate [atoms/(cm2 s)] �8 � 1021 (pr2)/(L2 p(M T)). Since this formula does not contain a sticking coef-ficient, it is an arrival rate to the surface. In this formula r is the ra-dius of the evaporant bar or rod, L is the distance from the tip of theevaporant to the substrate surface, M is the molecular weight ofthe evaporant, T is the temperature of the evaporant, and p is thevapor pressure which is a function of temperature for a givenmaterial [16]. With this calculation, the flux for our depositionparameters is �2 � 10�3 monolayers per second, assuming 1015

atoms per square centimeter per surface layer.After completing the Au deposition, the sample was left to settle

for about 1 h. The deposited surface was then moved to the analy-sis chamber and the surface morphology was probed using AFM innon-contact mode. A set of low-pass and high-pass filters (StanfordResearch Systems, SIM965 analog filters) on the cantilever deflec-tion signal was used in an effort to eliminate the significantamount of noise which disrupted the AFM during approach andnormal operation due to the high surface charge density at the sur-face. Samples were also taken out of vacuum and examined usingan ambient AFM (CP Research Thermomicroscopes) in non-contactmode. Both the UHV and ambient microscopes use beam deflectionfor measurement of cantilever motion and both microscopes usethe same type of NCAFM cantilever (Mikromasch NSC-15). NCAFMcantilevers used have a resonance frequency in the range 265–400 kHz and a force constant of �40 N/m. When using the UHVAFM, topography images were recorded (non-contact, frequencymodulation mode) and when using the ambient AFM both topog-raphy and phase images of the surface were recorded (non-contact,amplitude modulation mode). The phase signal measures the lagbetween the periodic signal that drives the cantilever to oscillateand the cantilever oscillation signal. All images used in this letterwere processed using WSxM image processing software, with nofiltering applied [17].

3. Results and discussion

After preparing the LiNbO3 sample by annealing in an ambientfurnace, the surface was imaged using NCAFM in order to verify theexistence of a flat surface with atomic steps. A surface with atomicsteps indicates a single crystalline material which is an ideal mor-phology for studying adsorption at the molecular and atomic scale.This surface can be seen in Fig. 1 which displays a LiNbO3(0 0 0 1)surface with monatomic steps. The figure shows a series of steps,�2.5 Å in height, which is consistent with a monatomic step onthe LiNbO3(0 0 0 1) surface. AFM phase images of the bare pre-pared LiNbO3 surface have no relative contrast as expected. For thisparticular study, only the positive face of the single domain LiN-bO3(0 0 0 1) surface was utilized. Previous experiments involvingmetal on ferroelectric oxide surfaces referenced earlier have alsoused the positive face for deposition, so it is appropriate to usethe positive face for comparative reference.

The sample surface of the LiNbO3 was aligned to face the aper-ture of the evaporation cell using a line-of-sight view port. Initially,deposition of gold was done for a relatively long time, 150 s, toguarantee a clearly observable effect on the LiNbO3 surface. After

Fig. 1. Non-contact AFM image of clean LiNbO3(0 0 0 1), positive crystal face,recorded under ambient conditions, 2.5 lm � 2.5 lm. Average step height: 0.25 nmfor steps shown, in agreement with crystal structure.

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this, the deposition time was gradually optimized to obtain a lowcoverage outcome for the purpose of identifying the initial nucle-ation site. Fig. 2 shows a NCAFM image of Au deposited on LiNbO3

for 150 s. From this image it can be noted that there is partial, ormore specifically, incomplete coverage of Au atoms on the LiNbO3

terraces. It is not obvious where the nucleation of the Au began butit is clear that there is a relatively non-uniform wetting or growthof Au on the terraces. The voids in the Au layers, between �2.5–5 Ådeep, that cover the substrate terraces are primarily localized nearthe bottom of each step edge. Nevertheless it is important to notethe tendency of the gold atoms to begin nucleation on another stepedge before completely covering a terrace, meaning partial cover-age for each terrace. It was assumed that this incomplete coverage

Fig. 2. Non-contact AFM (1875 nm � 1875 nm) image of Au deposited on LiNbO3

showing single atomic layers of the metal on the ferroelectric terraces. Thedownstairs direction for the step edges shown in this image is left to right. Thevoids present in the Au layers on the lithium niobate terraces are localized aboutthe bottom of the step edges shown.

was related to the fact that it was more energetically favorable forgold atoms to adsorb on or near a step edge or step base ratherthan completely adsorbing across an entire terrace. Additionally,as shown for many most heterogeneous thin film growth, thereis a significant barrier for Au adatom motion across step edges,so that Au atoms are confined to the individual terraces on whichthey initially adsorb. Also of significance in Fig. 2 is the height ofthe terraces that contained Au atoms. The height profile of theimages taken, including the depth of the voids in the Au layer, isindicative of more than one layer of Au, possibly two, comprisingthe layer on the LiNbO3 terrace.

To get a better understanding of the initial nucleation site of theAu atoms on the LiNbO3 surface, lower coverage was necessary. Fora very similar deposition rate, Au atoms were deposited onto theLiNbO3 surface for 60 s. The resulting structures produced fromthe 60 s deposition were very similar to that of the 150 s deposi-tion and therefore not shown. Furthermore, for the sample pre-pared with 60 s Au exposure, a ridge of Au forms starting at thetop of each substrate step edge and parallel to each step edge, thattapers off towards the bottom of the neighboring step edge with noAu on the rest of the terrace, similar to that of the 150 s deposition.This specific ridge feature was not as prominent when the deposi-tion was done for 150 s but the incomplete coverage of the terracewas definitely a common theme in both cases. Also, common toboth cases was the inability to distinguish whether the nucleationof the Au layer began at the step edge or the step base of the LiN-bO3 terrace.

Lastly, the deposition time was reduced to 30 s of Au exposurein order to more precisely identify the initial nucleation site of theAu atoms. Fig. 3 shows ambient NCAFM topography and phaseimages of Au adsorbed on the LiNbO3 surface for this depositionduration. The AFM phase image is a measure of energy dissipationdue to the AFM-tip interaction with the surface [18–20]. The phaseimage is a map of the difference in phase between the resonantcantilever and its excitation source. In general, the amplitude ofthe phase shift increases with increasing surface viscosity or withdecreasing surface stiffness. Very clearly visible in the images inFig. 3 is the similar feature of incomplete coverage of Au atomsbut with a much lower amount of Au atoms present. Also includedin the phase images recorded was the very evident difference incontrast of the Au layer with respect to the surrounding LiNbO3

terraces. The greater intensity shown in the phase images corre-sponds to a less stiff material on the surface which would corre-spond to the adsorbed Au in contrast to the ferroelectric terraces.This obvious contrast difference at the step edge was not evidentat all for images taken on the bare LiNbO3 surface before depositionof Au atoms. From these phase images it was much easier to assessthe initial nucleation site of the Au atoms, which was undoubtedlyat the top of the LiNbO3 step edges. The topographic height of theAu regions or bands in Fig. 3 is �2 Å higher than the terrace, whichwould be in agreement with a single layer of atoms at the top ofthe step edge as opposed to a thicker layer of Au nucleated atthe bottom of the step edge since the data shown is for a relativelylow coverage of Au. This observation and the structures shown inFig. 2 support the conclusion of preferential location of Au at thetop of the substrate step edges. There is also evidence that theAu band on LiNbO3 terrace tapers off to the actual LiNbO3 surfaceon the terrace. This implies that the Au atoms accumulate or orderat the very end of the step edge before accumulating laterally alongthe terrace.

A possible explanation for the existence of an accumulation ofadsorbed Au at the step edges is the variation in surface electronaffinity associated with the surface layers formed by surface adsor-bates. It has been demonstrated that the polarization dependentelectron affinity of LiNbO3 surfaces exhibits a difference in photo-threshold for electron ejection between the opposite domains of a

Fig. 3. Non-contact AFM (730 nm � 1100 nm) images, topography (left) and phase (right), of Au deposited on LiNbO3 for 30 s of exposure. The downstairs direction for thestep edges shown in this image is left to right. The band of Au that is forming on the top of the lithium niobate step edges is difficult to resolve in topography but prominent inrelative phase contrast.

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periodically poled LiNbO3 sample [21]. This was attributed to a var-iation in the electron affinity for opposite surface dipoles for differ-ent surface adsorbates. Also screening of polarization charges byadsorbates was found to give rise to additional surface dipoles,which implies a variation in the electron affinity. Following thesearguments, it would not be surprising for certain adsorbates to ini-tially attach at polar step edges of the LiNbO3 surface to screen thepolarization charge. Assuming that the charge variation is greatestat crystallographic step edges, then it follows that there is a surfacedipole that further attracts Au atoms to step edges and hencecauses an aggregation or pile-up there before Au atoms fill in therest of the terrace. For this polar surface, atomic planes terminatingin step edges present an opportunity for a steep gradient in surfacecharge, where the crystallography dictates the charge imbalanceacross the step edge as opposed to the charge smearing across stepedges that occurs for metallic surfaces.

In an effort to characterize the type of nucleation that is occur-ring during the deposition of Au on the LiNbO3 surface three mainmodes of growth were considered: Volmer-Weber which definesisland formation, Frank-van der Merwe which defines layer bylayer growth, and Stranski–Krastanov which defines layer then is-land formation of films. What can be seen from the data shown isthat the preference of adsorbing Au atoms to attach to a particularsurface site, even in the limit of only a few layers, leads to the for-mation of a non-uniform film. The data from our deposition exper-iments suggests that the Au layer that forms incompletemonolayer to bilayers in the limit of low exposure eventually leadsto further islanding. As confirmation of this, for growth experi-ments performed where the thickness of the Au layers is 30–60 Å, large island-like features occur on the surface, roughly20 nm high and 125 nm wide [22]. Since the low coverage limitshows a non-uniform film with subsequent islanding, we concludethat the Au metal growth on LiNbO3 proceeds via a Volmer-Webermechanism.

The attraction of the Au atoms to the step edge versus anyother adsorption site such as the step base can be explained ina few ways using the logic of similar systems of metal adsorbedon an oxide surface. On MgO(0 0 1) surfaces, it was found thatAu and other transition metals prefer to bind to defective sites,like atomic steps [23]. This attraction has been explained in termsof orbital mixing. It was found that transition metal electronsinteract more strongly with defect sites on the MgO surface be-cause of stronger orbital mixing of the transition metal stateswith the defect states when compared to other surface sites. Con-sequently, defects play an important role in the alignment of the

energy levels of transition metal atoms on the MgO(0 0 1) surface.Another possible explanation that can be used to describe theattraction of the metal adsorbates to the LiNbO3 steps is the inter-facial charge transfer. A few items generally need to be consid-ered when considering this phenomenon: the metal workfunction, the electron affinity of the oxide, the band gap energyof the oxide, and other interface characteristics. This charge trans-fer occurs commonly through covalent bonding of the metalatoms to the oxide surface but also can occur through interadsor-bate, metallic bonding. For thin MgO films grown on metallic sur-faces, Au clusters were found to form on the surface and this wasexplained by partial charge transfer from the substrate atoms tothe Au atoms [24,25]. In the case of low coverage Pd atoms onultrathin alumina, Al2O3, grown on NiAl(1 1 0) substrates, it wasvery apparent that the Pd atoms preferred to form small clustersalong the step edges of the Al2O3/NiAl(1 1 0) surface over otherpoint defect sites [26]. Many other systems involving metal atomson oxide surfaces, including ultrathin oxides grown on metals,also experience a greater attraction for the metal atoms to bindat the step edges rather than at any other defect sites [24,27–30].

On ferroelectric LiNbO3 surfaces, which share some similaritieswith the other oxide surfaces mentioned, these effects may also bepresent. Orbital mixing and charge transfer can occur on LiNbO3

due to the existence of step edge defects and available electron richsites. The difference with the LiNbO3 substrate compared to theother substrates is that the LiNbO3 surface contains very high sur-face charge and for the uniformly poled material, is composed oftwo charged interfaces (positive and negative crystal faces) ofopposite sign. This charge is most likely not uniformly distributedacross the surface and as a result may be higher in some areas thanothers, for instance step edges. This surface charge may enhance,exaggerate, or modify the defined step adsorption effects creatingthe very interesting results observed in these experiments.

4. Summary

In an effort to decipher the nucleation site of Au atoms ad-sorbed on the positive face of single domain LiNbO3(0 0 0 1) care-ful surface preparation was done to produce a flat surface withmonatomic steps, ideal for adsorption of atomic and molecularspecies. Low coverage of Au atoms was analyzed using AFM andit was found that the Au atoms do not uniformly cover and fillLiNbO3 terraces. Also at much lower coverage we conclude thatthe Au atoms prefer to adsorb directly on and in the vicinity of

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the monatomic steps of the LiNbO3 surface. This step edge prefer-ence is attributed to a possible charge transfer between the Auand LiNbO3 step interface and also the alignment of Au atomicorbitals with the step edge (which is a type of defect) atomicorbitals. Further it was observed that the Au atoms accumulateat the very edge of the LiNbO3 steps to form a band that graduallytapers off across the LiNbO3 terrace away from the top of the stepedge.

Acknowledgments

The authors would like to thank the NCSU Department of Mate-rials Science and Engineering for the use of a high temperature fur-nace, as well as the following individuals for fruitful discussionsand access to sample preparation equipment: Gerd Duscher, Jo-seph Tedesco, Jiyoung Choung, Robert J. Nemanich, J.E. Rowe,Christopher B. Gorman, Laura I. Clarke, Angus Kingon, Alexei Gruv-erman, Cheng Wang, and Harald Ade. We likewise wish to thankfinancial support from the National Science Foundation (NSFDMR-0403871).

References

[1] J. Garra, J.M. Vohs, D.A. Bonnell, Surface Science 603 (2009) 1106.[2] J. Garra, J.M. Vohs, D.A. Bonnell, Journal of Vacuum Science & Technology A 27

(2009) L13.[3] S. Habicht, R.J. Nemanich, A. Gruverman, Nanotechnology 19 (2008) 495303.[4] J.N. Hanson, B.J. Rodriguez, R.J. Nemanich, A. Gruverman, Nanotechnology 17

(2006) 4946.[5] Y. Yun, N. Pilet, U.D. Schwarz, E.I. Altman, Surface Science 603 (2009) 3145.[6] Y. Yun, L. Kampschulte, M. Li, D. Liao, E.I. Altman, Journal of Physical Chemistry

C 111 (2007) 13951.

[7] Y. Yun, E.I. Altman, Journal of the American Chemical Society 129 (2007)15684.

[8] S.C. Bharath, K.R. Pimputkar, A.M. Pronschinske, T.P. Pearl, Applied SurfaceScience 254 (2008) 2048.

[9] C.M. Wang, V. Shutthanandan, Y. Zhang, L.E. Thomas, D.R. Baer, G. Duscher,Journal of Applied Physics 95 (2004) 5060.

[10] F. Silly, M.R. Castell, Physical Review Letters 96 (2006) 086104.[11] F. Silly, M.R. Castell, Applied Physics Letters 87 (2005) 213107.[12] F. Silly, M.R. Castell, Physical Review Letters 94 (2005) 046103.[13] F. Silly, M.R. Castell, Applied Physics Letters 87 (2005) 063106.[14] Q. Fu, Surface Science Reports 62 (2007) 431.[15] H.J. Freund, Surface Science 500 (2002) 271.[16] L. Holland, W. Steckelmacher, J. Yarwood (Eds.), Vacuum Manual, London,

1974.[17] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero,

A.M. Baro, Review of Scientific Instruments 78 (2007) 013705.[18] J. Tamayo, R. Garcia, Langmuir 12 (1996) 4430.[19] R. Garcia, J. Tamayo, A. San Paulo, Surface and Interface Analysis 27 (1998) 312.[20] R. Garcia, C.J. Gomez, N.F. Martinez, S. Patil, C. Dietz, R. Magerle, Physical

Review Letters 97 (2006) 016103.[21] W.C. Yang, B.J. Rodriguez, A. Gruverman, R.J. Nemanich, Applied Physics Letters

85 (2004) 2316.[22] S.C. Bharath, PhD thesis, North Carolina State University, 2009.[23] J. Park, I. Park, B.D. Yu, Journal of the Korean Physical Society 54 (2009) 109.[24] M. Sterrer, T. Risse, U.M. Pozzoni, L. Giordano, M. Heyde, H.P. Rust,

G. Pacchioni, H.J. Freund, Physical Review Letters 98 (2007) 096107.[25] G. Pacchioni, L. Giordano, M. Baistrocchi, Physical Review Letters 94 (2005)

226104.[26] K.H. Hansen, T. Worren, S. Stempel, E. Laegsgaard, M. Bäumer, H.J. Freund,

F. Besenbacher, I. Stensgaard, Physical Review Letters 83 (1999) 4120.[27] M. Schmid, M. Shishkin, G. Kresse, E. Napetschnig, P. Varga, M. Kulawik,

N. Nilius, H.P. Rust, H.J. Freund, Physical Review Letters 97 (2006) 046101.[28] M.S. Chen, D.W. Goodman, Accounts of Chemical Research 39 (2006) 739.[29] K. Luo, X. Lai, C.W. Yi, K.A. Davis, K.K. Gath, D.W. Goodman, Journal of Physical

Chemistry B 109 (2005) 4064.[30] V.R. Cooper, A.M. Kolpak, Y. Yourdshahyan, A.M. Rappe, Physical Review B 72

(2005) 081409.