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Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomic force microscopy
Surface Science 604 (2010) 713–717
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomic force 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
Article history: Received 30 November 2009 Accepted for publication 22 January 2010 Available online 2 February 2010 Keywords: Lithium niobate Ferroelectric Surface Gold Atomic force microscopy
a b s t r a c t Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporation onto the positive face of single crystalline, lithium niobate, LiNbO3(0 0 0 1) surface. In order to study the mechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographic and phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uniformly poled LiNbO3 surfaces were prepared via an ambient high temperature anneal and served as a support for the thin gold films. These gold atomic layers were grown using electron bombardment evaporation sources under ultra-high vacuum (UHV) conditions and subsequently characterized under both vacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at the top of LiNbO3 substrate step edges. With increased coverage, island formation proceeds due to local aggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphology, the data is discussed in terms of thin film growth mechanisms as well as the influence of native surface features such as defects and charge distribution. Understanding growth mechanisms for gold layers on ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on patterned poled LiNbO3 surfaces would occur as well as yielding greater insight on the atomic characteristics of metals on ferroelectric interfaces. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Understanding the mechanisms behind atomic and molecular adsorption on ferroelectric oxide surfaces is the precursor to the development of devices that can harness the unique characteristics associated with polarizable materials. A great deal of work has been done on metal/metallic oxide surface systems, but the study of ferroelectric metal oxide surfaces, specifically perovskite or perovskite-like crystals is not nearly as exhaustive. The payoff for utilizing polar or polarizable materials that can influence surface chemistry and interfacial properties via variations in electron density is very large. Of particular interest is the use of controllable surface charge density to influence molecular and atomic adsorption thermodynamics and kinetics. To this end there has been a recent surge in effort to characterize adsorption, from molecules to metallic nanostructures, on ferroelectric surfaces, including both perovskite and related crystals [1–8]. Growth of nanoclusters and nanocrystals of noble metals has been observed at the surfaces of model ferroelectrics such as strontium titanate (SrTiO3) [9–11]. Preferential interactions, with respect to the surface chemistry of the exposed polar faces, were * Corresponding author. E-mail address: [email protected] (T.P. Pearl). 0039-6028/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.01.022
found where the metal was found to interact more favorably with the TiO2 termination in contrast to the SrO termination. In the case of palladium growth on SrTiO3(0 0 1) the size and shape of Pd nanocrystals formed could be tuned as a function of different surface reconstructions [12]. This, coupled with varying the conditions during deposition of the palladium, i.e., varying sample temperature during deposition and annealing of the system after deposition, were reliable techniques to make surfaces that were covered with pyramidal, hexagonal, and hut shaped nanocrystals. This has been replicated for various metals on ferroelectric interfaces such as cobalt, silver, and iron on strontium titanate [13,14]. In some cases, however, it is not very clear where the nucleation of the metal atoms occurs. While a trend has emerged, there is still little known about the specific characteristics of the interaction of nucleating metal with the ferroelectric surface with respect to adsorption and subsequent nucleation. Regardless of what, at times, seem to be fairly thorough analysis of this metal on ferroelectric interface, there is still an incomplete picture of the morphology, chemistry, and structure of the interface at the atomic scale. Furthermore, there has been relatively little work done to try to understand the non-model perovskite crystals like LiNbO3 with respect to metal thin film growth. Lithium niobate, LiNbO3, is currently used in a wide variety of existing device architectures and its very high surface charge density
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(70 lC/cm2) makes it attractive for use in other applications similar to those explored involving other metallic oxides and ferroelectrics. In this letter, we present the results from the study of evaporation of gold and subsequent growth of atomically flat Au layers on the positive face of the Z-cut (0 0 0 1) LiNbO3 surface as a model system for studying the interaction of noble metal adsorbates with a polar interface. Surface supported Au nanostructures on metallic oxide surfaces have been used for studying heterogeneous catalysis, molecular electronics, and chemical sensing [15], but there has not been a detailed study of the thin film growth of Au on a ferroelectric surface. Utilizing non-contact atomic force microscopy (AFM) we have observed nucleation sites for the growth of atomic layers of Au on LiNbO3(0 0 0 1) grown under ultra-high vacuum conditions.
2. Experimental Uniformly poled, single crystalline, LiNbO3 wafers (75 mm diameter, 0.5 mm thick, polished, Z-cut; Crystal technology, Inc.) were diced into 3 mm 10 mm pieces using a stainless steel wire saw (South Bay Technology). The surface preparation of these LiNbO3(0 0 0 1) samples was identical to that highlighted elsewhere with the only difference being the size of the sample used during the experiment [8]. Briefly, samples underwent two ultrasonic bath cleaning cycles, in acetone followed by methanol. The LiNbO3 samples were then annealed in a Lindberg high temperature furnace (ambient gas environment) in two steps: first at a very low ramp 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 were then transferred via fast entry load-lock to an Omicron NanoTechnology Multiprobe ultra-high vacuum (UHV) system (3 1011 Torr base pressure) equipped with surface preparation and characterization tools. The clean LiNbO3(0 0 0 1) surface morphology was characterized with low energy electron diffraction (LEED) and AFM (Omicron, beam deflection VT-SPM) prior to deposition of Au. Prior to microscopy and diffraction measurements in UHV, samples were subjected to a 5 min in situ anneal at 100 °C to ensure that the surface was free of any ambient species which may have adsorbed to the surface during the cool down from the anneal cycle. After confirmation of the LiNbO3 surface crystallinity and cleanliness, we deposited Au atoms onto the sample at very low flux, with coverage on the order of a few monolayers to a partial monolayer. This deposition occurred using a triple cell evaporator (Omicron NanoTechnology GmbH, EFM 3T) attached to the UHV system described above. High purity gold slugs (99.995% metals basis, Alfa Aesar) 3.175 mm in diameter 3.175 mm long were used in conjunction with an alumina crucible in the evaporator. The distance from the end of the evaporator to the sample was approximately 94 mm and a shutter attached to a rotary feed through was used to control exposure of the evaporation flux to the sample. A flux monitor integrated into the evaporator measures the Au flux prior to opening the shutter. The Au slug in the evaporator was degassed by slowly increasing the evaporator filament current to 1.5 A and increasing the electron bombardment voltage to 600 V while monitoring the pressure in the chamber (maintained below 1 108 Torr). This allowed for a complete degassing of the Au slug prior to deposition to the surface. Water cooling to the evaporator housing was then turned on and the filament current was further increased until emission started and the flux monitor indicated an ion flux (positive polarity) above 1 nA. To optimize the emission and flux output, the voltage and current were adjusted along with the evaporant position in a linear motion (forward or backward) being cautious not to move too far forward or back, because of the danger of
destroying the heating filament. When the preferred conditions were realized the shutter was opened and the deposition began, with the shutter being closed when the desired deposition time was attained. All Au deposition on the LiNbO3 surface was done at a sample temperature of 300 K. Generally a filament current of 1.98–1.99 A was used with an electron beam voltage range of about 750–780 V. These conditions yielded an ion flux of 17 nA and an emission current in the range of 16–17 mA. Deposition time for Au on LiNbO3 varied from 150 s to about 30 s. The equation governing evaporation fluxes from effusive low pressure sources is: deposition rate [atoms/(cm2 s)] 8 1021 (p p r2)/(L2 (M T)). Since this formula does not contain a sticking coefficient, it is an arrival rate to the surface. In this formula r is the radius of the evaporant bar or rod, L is the distance from the tip of the evaporant to the substrate surface, M is the molecular weight of the evaporant, T is the temperature of the evaporant, and p is the vapor pressure which is a function of temperature for a given material [16]. With this calculation, the flux for our deposition parameters is 2 103 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 analysis chamber and the surface morphology was probed using AFM in non-contact mode. A set of low-pass and high-pass filters (Stanford Research Systems, SIM965 analog filters) on the cantilever deflection signal was used in an effort to eliminate the significant amount of noise which disrupted the AFM during approach and normal operation due to the high surface charge density at the surface. Samples were also taken out of vacuum and examined using an ambient AFM (CP Research Thermomicroscopes) in non-contact mode. Both the UHV and ambient microscopes use beam deflection for measurement of cantilever motion and both microscopes use the same type of NCAFM cantilever (Mikromasch NSC-15). NCAFM cantilevers used have a resonance frequency in the range 265– 400 kHz and a force constant of 40 N/m. When using the UHV AFM, topography images were recorded (non-contact, frequency modulation mode) and when using the ambient AFM both topography and phase images of the surface were recorded (non-contact, amplitude modulation mode). The phase signal measures the lag between the periodic signal that drives the cantilever to oscillate and the cantilever oscillation signal. All images used in this letter were processed using WSxM image processing software, with no filtering applied [17].
3. Results and discussion After preparing the LiNbO3 sample by annealing in an ambient furnace, the surface was imaged using NCAFM in order to verify the existence of a flat surface with atomic steps. A surface with atomic steps indicates a single crystalline material which is an ideal morphology 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 on the LiNbO3(0 0 0 1) surface. AFM phase images of the bare prepared LiNbO3 surface have no relative contrast as expected. For this particular study, only the positive face of the single domain LiNbO3(0 0 0 1) surface was utilized. Previous experiments involving metal on ferroelectric oxide surfaces referenced earlier have also used the positive face for deposition, so it is appropriate to use the positive face for comparative reference. The sample surface of the LiNbO3 was aligned to face the aperture of the evaporation cell using a line-of-sight view port. Initially, deposition of gold was done for a relatively long time, 150 s, to guarantee a clearly observable effect on the LiNbO3 surface. After
S.C. Bharath, T.P. Pearl / Surface Science 604 (2010) 713–717
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 nm for steps shown, in agreement with crystal structure.
this, the deposition time was gradually optimized to obtain a low coverage outcome for the purpose of identifying the initial nucleation 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, or more specifically, incomplete coverage of Au atoms on the LiNbO3 terraces. It is not obvious where the nucleation of the Au began but it is clear that there is a relatively non-uniform wetting or growth of Au on the terraces. The voids in the Au layers, between 2.5–5 Å deep, that cover the substrate terraces are primarily localized near the bottom of each step edge. Nevertheless it is important to note the tendency of the gold atoms to begin nucleation on another step edge before completely covering a terrace, meaning partial coverage 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. The downstairs direction for the step edges shown in this image is left to right. The voids present in the Au layers on the lithium niobate terraces are localized about the bottom of the step edges shown.
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was related to the fact that it was more energetically favorable for gold atoms to adsorb on or near a step edge or step base rather than completely adsorbing across an entire terrace. Additionally, as shown for many most heterogeneous thin film growth, there is a significant barrier for Au adatom motion across step edges, so that Au atoms are confined to the individual terraces on which they initially adsorb. Also of significance in Fig. 2 is the height of the terraces that contained Au atoms. The height profile of the images taken, including the depth of the voids in the Au layer, is indicative of more than one layer of Au, possibly two, comprising the layer on the LiNbO3 terrace. To get a better understanding of the initial nucleation site of the Au atoms on the LiNbO3 surface, lower coverage was necessary. For a very similar deposition rate, Au atoms were deposited onto the LiNbO3 surface for 60 s. The resulting structures produced from the 60 s deposition were very similar to that of the 150 s deposition and therefore not shown. Furthermore, for the sample prepared with 60 s Au exposure, a ridge of Au forms starting at the top of each substrate step edge and parallel to each step edge, that tapers off towards the bottom of the neighboring step edge with no Au on the rest of the terrace, similar to that of the 150 s deposition. This specific ridge feature was not as prominent when the deposition was done for 150 s but the incomplete coverage of the terrace was definitely a common theme in both cases. Also, common to both cases was the inability to distinguish whether the nucleation of the Au layer began at the step edge or the step base of the LiNbO3 terrace. Lastly, the deposition time was reduced to 30 s of Au exposure in order to more precisely identify the initial nucleation site of the Au atoms. Fig. 3 shows ambient NCAFM topography and phase images of Au adsorbed on the LiNbO3 surface for this deposition duration. The AFM phase image is a measure of energy dissipation due to the AFM-tip interaction with the surface [18–20]. The phase image is a map of the difference in phase between the resonant cantilever and its excitation source. In general, the amplitude of the phase shift increases with increasing surface viscosity or with decreasing surface stiffness. Very clearly visible in the images in Fig. 3 is the similar feature of incomplete coverage of Au atoms but with a much lower amount of Au atoms present. Also included in the phase images recorded was the very evident difference in contrast of the Au layer with respect to the surrounding LiNbO3 terraces. The greater intensity shown in the phase images corresponds to a less stiff material on the surface which would correspond to the adsorbed Au in contrast to the ferroelectric terraces. This obvious contrast difference at the step edge was not evident at all for images taken on the bare LiNbO3 surface before deposition of Au atoms. From these phase images it was much easier to assess the initial nucleation site of the Au atoms, which was undoubtedly at the top of the LiNbO3 step edges. The topographic height of the Au regions or bands in Fig. 3 is 2 Å higher than the terrace, which would be in agreement with a single layer of atoms at the top of the step edge as opposed to a thicker layer of Au nucleated at the bottom of the step edge since the data shown is for a relatively low coverage of Au. This observation and the structures shown in Fig. 2 support the conclusion of preferential location of Au at the top of the substrate step edges. There is also evidence that the Au band on LiNbO3 terrace tapers off to the actual LiNbO3 surface on the terrace. This implies that the Au atoms accumulate or order at the very end of the step edge before accumulating laterally along the terrace. A possible explanation for the existence of an accumulation of adsorbed Au at the step edges is the variation in surface electron affinity associated with the surface layers formed by surface adsorbates. It has been demonstrated that the polarization dependent electron affinity of LiNbO3 surfaces exhibits a difference in photothreshold for electron ejection between the opposite domains of a
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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 the step 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 in relative phase contrast.
periodically poled LiNbO3 sample [21]. This was attributed to a variation in the electron affinity for opposite surface dipoles for different surface adsorbates. Also screening of polarization charges by adsorbates was found to give rise to additional surface dipoles, which implies a variation in the electron affinity. Following these arguments, it would not be surprising for certain adsorbates to initially attach at polar step edges of the LiNbO3 surface to screen the polarization charge. Assuming that the charge variation is greatest at crystallographic step edges, then it follows that there is a surface dipole that further attracts Au atoms to step edges and hence causes an aggregation or pile-up there before Au atoms fill in the rest of the terrace. For this polar surface, atomic planes terminating in step edges present an opportunity for a steep gradient in surface charge, where the crystallography dictates the charge imbalance across the step edge as opposed to the charge smearing across step edges that occurs for metallic surfaces. In an effort to characterize the type of nucleation that is occurring during the deposition of Au on the LiNbO3 surface three main modes of growth were considered: Volmer-Weber which defines island formation, Frank-van der Merwe which defines layer by layer growth, and Stranski–Krastanov which defines layer then island formation of films. What can be seen from the data shown is that the preference of adsorbing Au atoms to attach to a particular surface site, even in the limit of only a few layers, leads to the formation of a non-uniform film. The data from our deposition experiments suggests that the Au layer that forms incomplete monolayer to bilayers in the limit of low exposure eventually leads to further islanding. As confirmation of this, for growth experiments performed where the thickness of the Au layers is 30– 60 Å, large island-like features occur on the surface, roughly 20 nm high and 125 nm wide [22]. Since the low coverage limit shows a non-uniform film with subsequent islanding, we conclude that the Au metal growth on LiNbO3 proceeds via a Volmer-Weber mechanism. The attraction of the Au atoms to the step edge versus any other adsorption site such as the step base can be explained in a few ways using the logic of similar systems of metal adsorbed on an oxide surface. On MgO(0 0 1) surfaces, it was found that Au and other transition metals prefer to bind to defective sites, like atomic steps [23]. This attraction has been explained in terms of orbital mixing. It was found that transition metal electrons interact more strongly with defect sites on the MgO surface because of stronger orbital mixing of the transition metal states with the defect states when compared to other surface sites. Consequently, 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 the attraction of the metal adsorbates to the LiNbO3 steps is the interfacial charge transfer. A few items generally need to be considered when considering this phenomenon: the metal work function, the electron affinity of the oxide, the band gap energy of the oxide, and other interface characteristics. This charge transfer occurs commonly through covalent bonding of the metal atoms to the oxide surface but also can occur through interadsorbate, metallic bonding. For thin MgO films grown on metallic surfaces, Au clusters were found to form on the surface and this was explained by partial charge transfer from the substrate atoms to the Au atoms [24,25]. In the case of low coverage Pd atoms on ultrathin alumina, Al2O3, grown on NiAl(1 1 0) substrates, it was very apparent that the Pd atoms preferred to form small clusters along the step edges of the Al2O3/NiAl(1 1 0) surface over other point defect sites [26]. Many other systems involving metal atoms on oxide surfaces, including ultrathin oxides grown on metals, also experience a greater attraction for the metal atoms to bind at the step edges rather than at any other defect sites [24,27–30]. On ferroelectric LiNbO3 surfaces, which share some similarities with the other oxide surfaces mentioned, these effects may also be present. Orbital mixing and charge transfer can occur on LiNbO3 due to the existence of step edge defects and available electron rich sites. The difference with the LiNbO3 substrate compared to the other substrates is that the LiNbO3 surface contains very high surface charge and for the uniformly poled material, is composed of two charged interfaces (positive and negative crystal faces) of opposite sign. This charge is most likely not uniformly distributed across the surface and as a result may be higher in some areas than others, for instance step edges. This surface charge may enhance, exaggerate, or modify the defined step adsorption effects creating the very interesting results observed in these experiments.
4. Summary In an effort to decipher the nucleation site of Au atoms adsorbed on the positive face of single domain LiNbO3(0 0 0 1) careful surface preparation was done to produce a flat surface with monatomic steps, ideal for adsorption of atomic and molecular species. Low coverage of Au atoms was analyzed using AFM and it was found that the Au atoms do not uniformly cover and fill LiNbO3 terraces. Also at much lower coverage we conclude that the Au atoms prefer to adsorb directly on and in the vicinity of
S.C. Bharath, T.P. Pearl / Surface Science 604 (2010) 713–717
the monatomic steps of the LiNbO3 surface. This step edge preference is attributed to a possible charge transfer between the Au and LiNbO3 step interface and also the alignment of Au atomic orbitals with the step edge (which is a type of defect) atomic orbitals. Further it was observed that the Au atoms accumulate at the very edge of the LiNbO3 steps to form a band that gradually tapers off across the LiNbO3 terrace away from the top of the step edge. Acknowledgments The authors would like to thank the NCSU Department of Materials Science and Engineering for the use of a high temperature furnace, as well as the following individuals for fruitful discussions and access to sample preparation equipment: Gerd Duscher, Joseph Tedesco, Jiyoung Choung, Robert J. Nemanich, J.E. Rowe, Christopher B. Gorman, Laura I. Clarke, Angus Kingon, Alexei Gruverman, Cheng Wang, and Harald Ade. We likewise wish to thank financial support from the National Science Foundation (NSF DMR-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.
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[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 Surface Science 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.
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomic force 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
Article history: Received 30 November 2009 Accepted for publication 22 January 2010 Available online 2 February 2010 Keywords: Lithium niobate Ferroelectric Surface Gold Atomic force microscopy
a b s t r a c t Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporation onto the positive face of single crystalline, lithium niobate, LiNbO3(0 0 0 1) surface. In order to study the mechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographic and phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uniformly poled LiNbO3 surfaces were prepared via an ambient high temperature anneal and served as a support for the thin gold films. These gold atomic layers were grown using electron bombardment evaporation sources under ultra-high vacuum (UHV) conditions and subsequently characterized under both vacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at the top of LiNbO3 substrate step edges. With increased coverage, island formation proceeds due to local aggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphology, the data is discussed in terms of thin film growth mechanisms as well as the influence of native surface features such as defects and charge distribution. Understanding growth mechanisms for gold layers on ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on patterned poled LiNbO3 surfaces would occur as well as yielding greater insight on the atomic characteristics of metals on ferroelectric interfaces. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Understanding the mechanisms behind atomic and molecular adsorption on ferroelectric oxide surfaces is the precursor to the development of devices that can harness the unique characteristics associated with polarizable materials. A great deal of work has been done on metal/metallic oxide surface systems, but the study of ferroelectric metal oxide surfaces, specifically perovskite or perovskite-like crystals is not nearly as exhaustive. The payoff for utilizing polar or polarizable materials that can influence surface chemistry and interfacial properties via variations in electron density is very large. Of particular interest is the use of controllable surface charge density to influence molecular and atomic adsorption thermodynamics and kinetics. To this end there has been a recent surge in effort to characterize adsorption, from molecules to metallic nanostructures, on ferroelectric surfaces, including both perovskite and related crystals [1–8]. Growth of nanoclusters and nanocrystals of noble metals has been observed at the surfaces of model ferroelectrics such as strontium titanate (SrTiO3) [9–11]. Preferential interactions, with respect to the surface chemistry of the exposed polar faces, were * Corresponding author. E-mail address: [email protected] (T.P. Pearl). 0039-6028/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.01.022
found where the metal was found to interact more favorably with the TiO2 termination in contrast to the SrO termination. In the case of palladium growth on SrTiO3(0 0 1) the size and shape of Pd nanocrystals formed could be tuned as a function of different surface reconstructions [12]. This, coupled with varying the conditions during deposition of the palladium, i.e., varying sample temperature during deposition and annealing of the system after deposition, were reliable techniques to make surfaces that were covered with pyramidal, hexagonal, and hut shaped nanocrystals. This has been replicated for various metals on ferroelectric interfaces such as cobalt, silver, and iron on strontium titanate [13,14]. In some cases, however, it is not very clear where the nucleation of the metal atoms occurs. While a trend has emerged, there is still little known about the specific characteristics of the interaction of nucleating metal with the ferroelectric surface with respect to adsorption and subsequent nucleation. Regardless of what, at times, seem to be fairly thorough analysis of this metal on ferroelectric interface, there is still an incomplete picture of the morphology, chemistry, and structure of the interface at the atomic scale. Furthermore, there has been relatively little work done to try to understand the non-model perovskite crystals like LiNbO3 with respect to metal thin film growth. Lithium niobate, LiNbO3, is currently used in a wide variety of existing device architectures and its very high surface charge density
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(70 lC/cm2) makes it attractive for use in other applications similar to those explored involving other metallic oxides and ferroelectrics. In this letter, we present the results from the study of evaporation of gold and subsequent growth of atomically flat Au layers on the positive face of the Z-cut (0 0 0 1) LiNbO3 surface as a model system for studying the interaction of noble metal adsorbates with a polar interface. Surface supported Au nanostructures on metallic oxide surfaces have been used for studying heterogeneous catalysis, molecular electronics, and chemical sensing [15], but there has not been a detailed study of the thin film growth of Au on a ferroelectric surface. Utilizing non-contact atomic force microscopy (AFM) we have observed nucleation sites for the growth of atomic layers of Au on LiNbO3(0 0 0 1) grown under ultra-high vacuum conditions.
2. Experimental Uniformly poled, single crystalline, LiNbO3 wafers (75 mm diameter, 0.5 mm thick, polished, Z-cut; Crystal technology, Inc.) were diced into 3 mm 10 mm pieces using a stainless steel wire saw (South Bay Technology). The surface preparation of these LiNbO3(0 0 0 1) samples was identical to that highlighted elsewhere with the only difference being the size of the sample used during the experiment [8]. Briefly, samples underwent two ultrasonic bath cleaning cycles, in acetone followed by methanol. The LiNbO3 samples were then annealed in a Lindberg high temperature furnace (ambient gas environment) in two steps: first at a very low ramp 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 were then transferred via fast entry load-lock to an Omicron NanoTechnology Multiprobe ultra-high vacuum (UHV) system (3 1011 Torr base pressure) equipped with surface preparation and characterization tools. The clean LiNbO3(0 0 0 1) surface morphology was characterized with low energy electron diffraction (LEED) and AFM (Omicron, beam deflection VT-SPM) prior to deposition of Au. Prior to microscopy and diffraction measurements in UHV, samples were subjected to a 5 min in situ anneal at 100 °C to ensure that the surface was free of any ambient species which may have adsorbed to the surface during the cool down from the anneal cycle. After confirmation of the LiNbO3 surface crystallinity and cleanliness, we deposited Au atoms onto the sample at very low flux, with coverage on the order of a few monolayers to a partial monolayer. This deposition occurred using a triple cell evaporator (Omicron NanoTechnology GmbH, EFM 3T) attached to the UHV system described above. High purity gold slugs (99.995% metals basis, Alfa Aesar) 3.175 mm in diameter 3.175 mm long were used in conjunction with an alumina crucible in the evaporator. The distance from the end of the evaporator to the sample was approximately 94 mm and a shutter attached to a rotary feed through was used to control exposure of the evaporation flux to the sample. A flux monitor integrated into the evaporator measures the Au flux prior to opening the shutter. The Au slug in the evaporator was degassed by slowly increasing the evaporator filament current to 1.5 A and increasing the electron bombardment voltage to 600 V while monitoring the pressure in the chamber (maintained below 1 108 Torr). This allowed for a complete degassing of the Au slug prior to deposition to the surface. Water cooling to the evaporator housing was then turned on and the filament current was further increased until emission started and the flux monitor indicated an ion flux (positive polarity) above 1 nA. To optimize the emission and flux output, the voltage and current were adjusted along with the evaporant position in a linear motion (forward or backward) being cautious not to move too far forward or back, because of the danger of
destroying the heating filament. When the preferred conditions were realized the shutter was opened and the deposition began, with the shutter being closed when the desired deposition time was attained. All Au deposition on the LiNbO3 surface was done at a sample temperature of 300 K. Generally a filament current of 1.98–1.99 A was used with an electron beam voltage range of about 750–780 V. These conditions yielded an ion flux of 17 nA and an emission current in the range of 16–17 mA. Deposition time for Au on LiNbO3 varied from 150 s to about 30 s. The equation governing evaporation fluxes from effusive low pressure sources is: deposition rate [atoms/(cm2 s)] 8 1021 (p p r2)/(L2 (M T)). Since this formula does not contain a sticking coefficient, it is an arrival rate to the surface. In this formula r is the radius of the evaporant bar or rod, L is the distance from the tip of the evaporant to the substrate surface, M is the molecular weight of the evaporant, T is the temperature of the evaporant, and p is the vapor pressure which is a function of temperature for a given material [16]. With this calculation, the flux for our deposition parameters is 2 103 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 analysis chamber and the surface morphology was probed using AFM in non-contact mode. A set of low-pass and high-pass filters (Stanford Research Systems, SIM965 analog filters) on the cantilever deflection signal was used in an effort to eliminate the significant amount of noise which disrupted the AFM during approach and normal operation due to the high surface charge density at the surface. Samples were also taken out of vacuum and examined using an ambient AFM (CP Research Thermomicroscopes) in non-contact mode. Both the UHV and ambient microscopes use beam deflection for measurement of cantilever motion and both microscopes use the same type of NCAFM cantilever (Mikromasch NSC-15). NCAFM cantilevers used have a resonance frequency in the range 265– 400 kHz and a force constant of 40 N/m. When using the UHV AFM, topography images were recorded (non-contact, frequency modulation mode) and when using the ambient AFM both topography and phase images of the surface were recorded (non-contact, amplitude modulation mode). The phase signal measures the lag between the periodic signal that drives the cantilever to oscillate and the cantilever oscillation signal. All images used in this letter were processed using WSxM image processing software, with no filtering applied [17].
3. Results and discussion After preparing the LiNbO3 sample by annealing in an ambient furnace, the surface was imaged using NCAFM in order to verify the existence of a flat surface with atomic steps. A surface with atomic steps indicates a single crystalline material which is an ideal morphology 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 on the LiNbO3(0 0 0 1) surface. AFM phase images of the bare prepared LiNbO3 surface have no relative contrast as expected. For this particular study, only the positive face of the single domain LiNbO3(0 0 0 1) surface was utilized. Previous experiments involving metal on ferroelectric oxide surfaces referenced earlier have also used the positive face for deposition, so it is appropriate to use the positive face for comparative reference. The sample surface of the LiNbO3 was aligned to face the aperture of the evaporation cell using a line-of-sight view port. Initially, deposition of gold was done for a relatively long time, 150 s, to guarantee a clearly observable effect on the LiNbO3 surface. After
S.C. Bharath, T.P. Pearl / Surface Science 604 (2010) 713–717
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 nm for steps shown, in agreement with crystal structure.
this, the deposition time was gradually optimized to obtain a low coverage outcome for the purpose of identifying the initial nucleation 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, or more specifically, incomplete coverage of Au atoms on the LiNbO3 terraces. It is not obvious where the nucleation of the Au began but it is clear that there is a relatively non-uniform wetting or growth of Au on the terraces. The voids in the Au layers, between 2.5–5 Å deep, that cover the substrate terraces are primarily localized near the bottom of each step edge. Nevertheless it is important to note the tendency of the gold atoms to begin nucleation on another step edge before completely covering a terrace, meaning partial coverage 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. The downstairs direction for the step edges shown in this image is left to right. The voids present in the Au layers on the lithium niobate terraces are localized about the bottom of the step edges shown.
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was related to the fact that it was more energetically favorable for gold atoms to adsorb on or near a step edge or step base rather than completely adsorbing across an entire terrace. Additionally, as shown for many most heterogeneous thin film growth, there is a significant barrier for Au adatom motion across step edges, so that Au atoms are confined to the individual terraces on which they initially adsorb. Also of significance in Fig. 2 is the height of the terraces that contained Au atoms. The height profile of the images taken, including the depth of the voids in the Au layer, is indicative of more than one layer of Au, possibly two, comprising the layer on the LiNbO3 terrace. To get a better understanding of the initial nucleation site of the Au atoms on the LiNbO3 surface, lower coverage was necessary. For a very similar deposition rate, Au atoms were deposited onto the LiNbO3 surface for 60 s. The resulting structures produced from the 60 s deposition were very similar to that of the 150 s deposition and therefore not shown. Furthermore, for the sample prepared with 60 s Au exposure, a ridge of Au forms starting at the top of each substrate step edge and parallel to each step edge, that tapers off towards the bottom of the neighboring step edge with no Au on the rest of the terrace, similar to that of the 150 s deposition. This specific ridge feature was not as prominent when the deposition was done for 150 s but the incomplete coverage of the terrace was definitely a common theme in both cases. Also, common to both cases was the inability to distinguish whether the nucleation of the Au layer began at the step edge or the step base of the LiNbO3 terrace. Lastly, the deposition time was reduced to 30 s of Au exposure in order to more precisely identify the initial nucleation site of the Au atoms. Fig. 3 shows ambient NCAFM topography and phase images of Au adsorbed on the LiNbO3 surface for this deposition duration. The AFM phase image is a measure of energy dissipation due to the AFM-tip interaction with the surface [18–20]. The phase image is a map of the difference in phase between the resonant cantilever and its excitation source. In general, the amplitude of the phase shift increases with increasing surface viscosity or with decreasing surface stiffness. Very clearly visible in the images in Fig. 3 is the similar feature of incomplete coverage of Au atoms but with a much lower amount of Au atoms present. Also included in the phase images recorded was the very evident difference in contrast of the Au layer with respect to the surrounding LiNbO3 terraces. The greater intensity shown in the phase images corresponds to a less stiff material on the surface which would correspond to the adsorbed Au in contrast to the ferroelectric terraces. This obvious contrast difference at the step edge was not evident at all for images taken on the bare LiNbO3 surface before deposition of Au atoms. From these phase images it was much easier to assess the initial nucleation site of the Au atoms, which was undoubtedly at the top of the LiNbO3 step edges. The topographic height of the Au regions or bands in Fig. 3 is 2 Å higher than the terrace, which would be in agreement with a single layer of atoms at the top of the step edge as opposed to a thicker layer of Au nucleated at the bottom of the step edge since the data shown is for a relatively low coverage of Au. This observation and the structures shown in Fig. 2 support the conclusion of preferential location of Au at the top of the substrate step edges. There is also evidence that the Au band on LiNbO3 terrace tapers off to the actual LiNbO3 surface on the terrace. This implies that the Au atoms accumulate or order at the very end of the step edge before accumulating laterally along the terrace. A possible explanation for the existence of an accumulation of adsorbed Au at the step edges is the variation in surface electron affinity associated with the surface layers formed by surface adsorbates. It has been demonstrated that the polarization dependent electron affinity of LiNbO3 surfaces exhibits a difference in photothreshold for electron ejection between the opposite domains of a
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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 the step 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 in relative phase contrast.
periodically poled LiNbO3 sample [21]. This was attributed to a variation in the electron affinity for opposite surface dipoles for different surface adsorbates. Also screening of polarization charges by adsorbates was found to give rise to additional surface dipoles, which implies a variation in the electron affinity. Following these arguments, it would not be surprising for certain adsorbates to initially attach at polar step edges of the LiNbO3 surface to screen the polarization charge. Assuming that the charge variation is greatest at crystallographic step edges, then it follows that there is a surface dipole that further attracts Au atoms to step edges and hence causes an aggregation or pile-up there before Au atoms fill in the rest of the terrace. For this polar surface, atomic planes terminating in step edges present an opportunity for a steep gradient in surface charge, where the crystallography dictates the charge imbalance across the step edge as opposed to the charge smearing across step edges that occurs for metallic surfaces. In an effort to characterize the type of nucleation that is occurring during the deposition of Au on the LiNbO3 surface three main modes of growth were considered: Volmer-Weber which defines island formation, Frank-van der Merwe which defines layer by layer growth, and Stranski–Krastanov which defines layer then island formation of films. What can be seen from the data shown is that the preference of adsorbing Au atoms to attach to a particular surface site, even in the limit of only a few layers, leads to the formation of a non-uniform film. The data from our deposition experiments suggests that the Au layer that forms incomplete monolayer to bilayers in the limit of low exposure eventually leads to further islanding. As confirmation of this, for growth experiments performed where the thickness of the Au layers is 30– 60 Å, large island-like features occur on the surface, roughly 20 nm high and 125 nm wide [22]. Since the low coverage limit shows a non-uniform film with subsequent islanding, we conclude that the Au metal growth on LiNbO3 proceeds via a Volmer-Weber mechanism. The attraction of the Au atoms to the step edge versus any other adsorption site such as the step base can be explained in a few ways using the logic of similar systems of metal adsorbed on an oxide surface. On MgO(0 0 1) surfaces, it was found that Au and other transition metals prefer to bind to defective sites, like atomic steps [23]. This attraction has been explained in terms of orbital mixing. It was found that transition metal electrons interact more strongly with defect sites on the MgO surface because of stronger orbital mixing of the transition metal states with the defect states when compared to other surface sites. Consequently, 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 the attraction of the metal adsorbates to the LiNbO3 steps is the interfacial charge transfer. A few items generally need to be considered when considering this phenomenon: the metal work function, the electron affinity of the oxide, the band gap energy of the oxide, and other interface characteristics. This charge transfer occurs commonly through covalent bonding of the metal atoms to the oxide surface but also can occur through interadsorbate, metallic bonding. For thin MgO films grown on metallic surfaces, Au clusters were found to form on the surface and this was explained by partial charge transfer from the substrate atoms to the Au atoms [24,25]. In the case of low coverage Pd atoms on ultrathin alumina, Al2O3, grown on NiAl(1 1 0) substrates, it was very apparent that the Pd atoms preferred to form small clusters along the step edges of the Al2O3/NiAl(1 1 0) surface over other point defect sites [26]. Many other systems involving metal atoms on oxide surfaces, including ultrathin oxides grown on metals, also experience a greater attraction for the metal atoms to bind at the step edges rather than at any other defect sites [24,27–30]. On ferroelectric LiNbO3 surfaces, which share some similarities with the other oxide surfaces mentioned, these effects may also be present. Orbital mixing and charge transfer can occur on LiNbO3 due to the existence of step edge defects and available electron rich sites. The difference with the LiNbO3 substrate compared to the other substrates is that the LiNbO3 surface contains very high surface charge and for the uniformly poled material, is composed of two charged interfaces (positive and negative crystal faces) of opposite sign. This charge is most likely not uniformly distributed across the surface and as a result may be higher in some areas than others, for instance step edges. This surface charge may enhance, exaggerate, or modify the defined step adsorption effects creating the very interesting results observed in these experiments.
4. Summary In an effort to decipher the nucleation site of Au atoms adsorbed on the positive face of single domain LiNbO3(0 0 0 1) careful surface preparation was done to produce a flat surface with monatomic steps, ideal for adsorption of atomic and molecular species. Low coverage of Au atoms was analyzed using AFM and it was found that the Au atoms do not uniformly cover and fill LiNbO3 terraces. Also at much lower coverage we conclude that the 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 preference is attributed to a possible charge transfer between the Au and LiNbO3 step interface and also the alignment of Au atomic orbitals with the step edge (which is a type of defect) atomic orbitals. Further it was observed that the Au atoms accumulate at the very edge of the LiNbO3 steps to form a band that gradually tapers off across the LiNbO3 terrace away from the top of the step edge. Acknowledgments The authors would like to thank the NCSU Department of Materials Science and Engineering for the use of a high temperature furnace, as well as the following individuals for fruitful discussions and access to sample preparation equipment: Gerd Duscher, Joseph Tedesco, Jiyoung Choung, Robert J. Nemanich, J.E. Rowe, Christopher B. Gorman, Laura I. Clarke, Angus Kingon, Alexei Gruverman, Cheng Wang, and Harald Ade. We likewise wish to thank financial support from the National Science Foundation (NSF DMR-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.
717
[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 Surface Science 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.