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Initial-stage behaviors of tin and lead adsorption on vanadium surface oxide nanomesh on Pd(111)
Surface Science 613 (2013) 35–39
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Initial-stage behaviors of tin and lead adsorption on vanadium surface oxide nanomesh on Pd(111) Lap Hong Chan, Shinji Hayazaki, Kokushi Ogawa, Junji Yuhara ⁎ Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 17 December 2012 Accepted 4 March 2013 Available online 13 March 2013 Keywords: Nanodot Nanomesh Tin Lead Vanadium oxide Palladium Scanning tunneling microscopy DFT calculations
a b s t r a c t The self-organized formation of tin (Sn) and lead (Pb) atoms in a well ordered (4 × 4) vanadium oxide nanomesh on Pd(111) has been investigated using scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory (DFT). At a very low Sn coverage, most of the Sn atoms filled the vanadium oxide nanohole and a few of them were adsorbed on top of the vanadium oxide nanomesh. On the contrary, all of Pb atoms were located in the vanadium oxide nanohole. The ab initio DFT total-energy calculations indicated that the Sn and Pb atoms preferred to adsorb in the nanohole rather than on the nanomesh. The simulated STM images for the Sn and Pb atoms exhibited a local protrusion, while the experimental STM images showed a large, uniform protrusion. The adsorbed Sn and Pb atoms might be mobile in the nanohole at room temperature, possibly as a result of interaction with the STM tip. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The controlled growth of arrayed metal nanostructures on solid surfaces by self-organized method is of great interest both from the fundamental and technological points of view [1]. Using surface nano-templates, nanodots can be fabricated periodically at the nanoscale in a self-assembly process. Of particular interest is the nucleation of adatoms into an ordered array of regular clusters that are small enough to have quantum mechanical properties. So far, surface nanomesh structures, such as boron nitride (BN) on Rh(111), silicon carbide (SiC) surface, and vanadium oxide on Pd(111), have been investigated for the fabrication of controlled nanodots and nanoclusters [2–10]. The BN and SiC nanomeshes have demonstrated the formation of dispersed metal nanoclusters on top of the nanomeshes and the nanoholes have been shown to be the least stable adsorption site for adsorbates [2,3]. On the contrary, the formation of metal nanodots has been reported for the systems of lead (Pd), bismuth (Bi), and silver (Ag) atoms in the vanadium oxide (4 × 4) nanomesh on Pd(111) [4–6]. The Pd atoms form a monomer nanodot, while the Bi and Ag atoms form dimer to tetramer nanodots in addition to a monomer nanodot. All these metals do not form the nanoclusters. In our previous work, we used the group IV elements, silicon (Si) and germanium (Ge), for the semiconducting nanodot formation in the vanadium oxide nanomesh
⁎ Corresponding author. E-mail address: [email protected] (J. Yuhara). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.03.002
on Pd(111). The Si atoms only formed a nanocluster, and Ge atoms formed both nanodots and nanoclusters [7]. Since the group IV elements exhibit wide variety of physical properties depending on the crystal structure and mass number. The properties change from non-metallic in character at the top of the group to metallic at the bottom: carbon is a non-metal, Si and Ge are metalloids, and tin (Sn) and lead (Pb) are typical metals. Therefore, the general reactivity of the group IV elements as a whole is difficult to ascertain, and the reactivity of each element must be considered individually. In this paper, in order to systematically understand the adsorption behaviors of group IV elements in the (4 × 4) vanadium oxide on Pd(111), Sn and Pb are chosen for the characterization of geometric structure using STM and DFT calculations.
2. Experimental and computational methods The experiments were carried out under ultrahigh-vacuum (UHV) conditions with a base pressure of less than 5.0 × 10 −10 mbar in the sample preparation chamber and less than 2.0 × 10 −11 mbar in the analysis chamber. The analysis chamber was equipped with a rear view LEED system operating with a LaB6 filament and a UHV STM system (Omicron STM-1). Four-grid optics enabled Auger analysis in a retarding field. All STM images were acquired with W tips electrochemically etched in a KOH solution using constant current mode with a positive sample bias voltage at room temperature (RT). To prepare a clean Pd(111) surface, the crystal was sputtered by 2-keV Ar+ ions at RT and annealed at 1050 K. The sample was annealed
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using electron bombardment from the backside. The Pd(111) surface was also exposed to molecular oxygen in a 3.0 × 10−8 mbar O2 background pressure at 750 K followed by a flash to 1050 K to remove impurities. A vanadium film was deposited on the clean Pd(111) at RT from a water-cooled electron beam evaporator. The amount of deposited vanadium was fixed at 0.3 ML (we define one monolayer (ML) as overlayer having the atomic density of a Pd(111) layer: 1.52 × 1015 atoms/cm2). After deposition, the vanadium films on Pd(111) were annealed at 550 K at an O2 partial pressure of 3 × 10 −7 mbar and cooled to RT at the same oxygen pressure. A LEED study of this vanadium oxide surface on Pd(111) exhibited a distinct (4 × 4) pattern. The two different levels of vanadium sites were observed in the STM images and the height difference was 0.04 nm. The height difference was caused by the difference in the oxygen atom adsorption sites below, which are on-top (V1) and threefold hollow (V2) sites of Pd surface atom [4]. This perfectly ordered nanomesh was used in this study as a nano-template to examine the initial stage behaviors of metal adsorption. Tin or lead was then deposited on this nanomesh-structured surface at RT using boron nitride crucible evaporators at a deposition rate of 0.01 ML/min. The amount of adatom deposited was 0.001 ML. A theoretical study of the local structure of Sn and Pb atoms on a (4 × 4) vanadium oxide nanomesh on Pd(111) has been performed by ab initio DFT total-energy calculations using the Vienna ab initio simulation package (VASP) [11,12]. These calculations were based on DFT with a plane-wave basis set [13,14]. The electron–ion interaction was described by the projector augmented wave (PAW) method [15,16]. The calculations were performed at an energy cutoff of 250 eV, resulting in an accurate description of the Pd bulk, Pd surface, vanadium oxide film, and adatoms. We used the generalized gradient approximation (GGA) of Perdew and Wang [17,18], commonly referred to as PW91. To calculate the fractional occupancies, the
broadening approach introduced by Methfessel and Paxton was used with N = 1 and σ = 0.02 eV [19]. The Pd(111) surface comprised the (4 × 4) super cell and fourlayer slab. The structural parameters for the vanadium oxide (4 × 4) nanomesh were obtained from the structural models proposed by reference [4]. In the c direction, we prepared a 15 Å vacuum. As a result, the calculated lattice parameters used were a = 10.995 Å, b = 10.995 Å, and c = 25.000 Å. A Monkhorst–Pack mesh with 2 × 2 × 1 k-points was used with the supercell. Because the structural model of the vanadium oxide nanomesh was obtained by MD simulation, the positions of the Pd, V, and O atoms were fixed and only Sn or Pb atoms were relaxed for the ab initio total energy calculations. Simulated STM images were calculated by Extended Hückel theory and utilize a Green's function matching using the Nt_STM package [20]. The tip was an apex structure of W(100). All simulated STM images were calculated for a sample bias of of + 1.5 V and a tunneling current of 1.0 nA with a lateral resolution of 0.04 nm. The simulation parameters for Fermi energy, cut-off radius for atomic orbital interactions, and the inverse of the minimum length in the k-sampling grid were − 10 eV, 0.9 nm, and 3 nm, respectively. 3. Results and discussion A small amount of Sn atoms was deposited onto the vanadium oxide nanomesh on Pd(111) at RT to examine the geometric structure. Most of the Sn atoms occupied the nanohole while a few were adsorbed on top of vanadium in the nanomesh, as shown in the STM image of Fig. 1(a). Fig. 1(b)–(d) shows the high-resolution STM images and it is clearly seen that the Sn atoms filled the nanoholes or adsorbed on the V1 site of the nanomesh. The Sn atoms always adsorbed on the V1 site, but not V2, of the nanomesh. When the Sn
Fig. 1. STM images of Sn deposition onto the vanadium oxide nanomesh on Pd(111): (a) large-scale image (Vs = 1.5 V, It = 0.45 nA) and (b)–(d) high-resolution images (Vs = 2.0 V, It = 0.6 nA) of three different Sn adsorption sites.
L.H. Chan et al. / Surface Science 613 (2013) 35–39
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Fig. 2. STM images of Pb deposition onto the vanadium oxide nanomesh on Pd(111): (a) large-scale image and (b) high-resolution image of Pb adsorption site.
atoms filled the nanoholes, the brightness area is much larger than the single atom. Therefore, the Sn atom might be mobile in the nanohole or the STM scanning tip induced Sn migration. Since the brightness is not totally uniform but slightly off-centered toward V2 site, the Sn atoms in the nanohole seem to have a preferential site. The vanadium oxide nanomesh itself seems to be chemically stable against Sn deposition. The coverage of large protrusion is in good agreement with the Sn coverage deposited. Therefore, it is concluded that each large protrusion contains one Sn atom. In contrast to the Sn deposition, all of Pb atoms deposited onto the nanomesh filled the nanoholes and none of the Pb atoms were adsorbed on the nanomesh, as shown in the STM image of Fig. 2(a). High-resolution STM image in Fig. 2(b) exhibits that the Pb atoms were observed as the large protrusions with uniform height. Because the brightness was constant and the area was much larger than the single atom, the Pb atom might be mobile in the nanohole. The STM image for the Pb atoms in the nanohole looks very much similar to the Bi atoms [5]. Since the coverage of the large protrusions is consistent with the Pb coverage deposited, it is concluded that each large protrusion contains one Pb atom.
Fig. 3(a) shows the cross-sectional profile of the Sn atom, corresponding to the white line in Fig. 1(b). The height of the Sn atom located on the V1 site was only 0.05 nm higher than that of the V1 site without Sn adsorption. The result indicated that the Sn atoms adsorbed to the V atoms might have no dangling bond, while the V atoms of V1 site have a directional dangling bond. Fig. 3(b) and (c) shows cross-sectional profiles of the Sn atoms along the white lines in Fig. 1(c) and (d), respectively. The heights of these Sn atoms in the nanohole were higher than that of the V2 sites that have a directional dangling bonding [4]. Therefore, the Sn atoms in the nanohole may also have a dangling bond. Fig. 3(d) shows a cross-sectional profile of the Pb atom along the white line in Fig. 2(b). The height of the Pb atom was uniform in the nanohole and was similar to that of V2 site. Because the atomic size of Pb is 0.36 nm and the height of the V2 site from the Pd(111) surface is 0.32 nm [4], the Pb atoms in the nanohole have non-directional metallic bonding. The three structure-optimized models, simulated STM images, and cross-sectional profiles of Sn atoms in the vanadium oxide nanomesh with/without oxygen in the center of the nanohole are displayed in Fig. 4(a)–(c). The simulated STM image of the Sn atom agreed only
Fig. 3. The cross-sectional profiles of STM image: (a)–(d) corresponding to the solid line in Figs. 1(b)–1(d) and 2(b), respectively.
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Fig. 4. Structural models, simulated images, and line profiles for Sn atoms located on (a) the V1 sites in the vanadium oxide nanomesh (b) the fcc hollow site, and (c) the fcc hollow site with oxygen in the center of the nanohole.
qualitatively with the experimental STM image, shown in Fig. 4(a). The simulated image looks much wider and brighter than the experimental image. Therefore, the parameters for Extended Hückel theory are not appropriate for such an isolated atom, since the bonding environment is very different from the bulk Sn crystal. Fig. 4(b) shows the structure-optimized model of the Sn nanodot without the oxygen atom in the center of the nanohole on Pd(111). The structure optimized calculations indicated that the Sn atom was energetically stable for both fcc and hcp hollow sites. The simulated STM image and its cross-sectional profile showed that the height of the Sn atom was similar to that of V atom in the V2 site on Pd(111). This is consistent with the experimental cross-sectional profile. The barrier height energy of the surface migration for Sn atom in the nanohole was calculated to be 0.17 eV. The small barrier height energy clearly explains the Sn migration in the nanohole, which is consistent with wide and uniform brightness for the Sn atom in the STM image. However, small surface barrier height energy contradicts the off-centered small protrusion in the experimental STM image. Fig. 4(c) shows the structure optimized model that Sn atom was located next to the center of oxygen atom on Pd(111). The simulated image showed local protrusion at the position of the Sn atom, and the height for the Sn atom was higher than the V1 site. In this
model, Sn atoms could not easily migrate in the nanohole because of the limited space. Therefore, there might be no oxygen atom in the nanohole, but this model clearly explains the STM image of the Sn atoms that showed the off-centered protrusion. Fig. 5(a) shows the structure-optimized model, simulated STM images, and cross-sectional profiles that the Pb atom was located on Pd(111) hcp hollow site. The simulated image showed a local protrusion at the position of the Pb atom and the cross-sectional profile showed that the height for the Pb atom was similar to the V atom in the V2 site. As discussed above, the disagreement of the appearance of the STM image between experiment and simulation can be explained by the small energy barrier height of Pb atom migration in the nanohole, which was calculated to be 0.34 eV. The adsorption energies of Sn and Pb atoms determined using total-energy DFT calculations were summarized in Table 1. The vanadium oxide nanomesh structures, with/without an oxygen atom in the center of the nanohole, were calculated, since the structural model of the vanadium oxide proposed by Kresse et al. implies the possibility of the oxygen atom [4]. The calculation results indicated that the adsorption in the nanohole was much more stable than the adsorption onto the nanomesh regardless of the oxygen atom. Nevertheless, the Sn atom was observed at V1 sites in the experimental STM image. The
Fig. 5. Structural models, simulated images, and line profiles for Pb atoms located on the hcp hollow site on Pd(111).
L.H. Chan et al. / Surface Science 613 (2013) 35–39 Table 1 Adsorption energy of adatom on the vanadium oxide nanomesh on Pd(111). Adatom
Surface model
Adsorbed position
Eadsorption (eV)
Sn
V5O14 V5O14 V5O14 V5O14 V5O14 V5O14 V5O14 V5O14
V1 on-top Close to fcc site hcp hollow site fcc hollow site V1 on-top Close to fcc site hcp hollow site fcc hollow site
−0.83 −4.79 −5.06 −5.06 −0.95 −4.23 −4.78 −4.83
Pb
+O +O
+O +O
present results indicate that some of the Sn atoms are trapped at the V atom in V1 sites, when a few of the evaporated Sn atoms are heading toward for the V1 site during the deposition. Our results indicated the group IV elements have different reactivity depending on the local bonding properties. In other words, the elements that show metallic character such as Pb and Sn atoms like to fill a nanohole and they are not easily bound to V atoms on the nanomesh. On the contrary, the elements with non-metallic character such as Si and Ge atoms have a tendency to form a chemical bond with V atoms on the nanomesh. The local bonding properties such as directional or non-directional seem to play a key role for the growth mechanism. 4. Conclusions The adsorption of very low Sn and Pb coverages on the (4 × 4) vanadium oxide nanomesh on Pd(111) has been investigated using STM and DFT calculations. The Sn atoms occupied the nanohole or adsorbed on the V1 site of the nanomesh and all of Pb atoms occupied the nanohole. The DFT calculations indicated that the adsorption in the nanohole was more stable than that on the nanomesh for both Sn and Pb atoms. The simulated images indicated a local protrusion, while the STM images showed large, uniform protrusions for Pb and Sn atoms adsorbed in the nanohole. Although the differences can be explained by the small barrier height energies of surface migration
39
for the Sn and Pb atoms, the large protrusion with uniform height might be related to interaction with the STM tip during scanning. Acknowledgments This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas “New Materials Science Using Regulated Nano Spaces — Strategy in Ubiquitous Elements” (no. 22013008) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and by a Research Grant Program of Specially Promoted Project from the Toyota Physical and Chemical Research Institute. References [1] H. Brune, M. Giovannini, K. Bromann, K. Kern, Nature 394 (1998) 451. [2] Martina Corso, Willi Auwärter, Matthias Muntwiler, Anna Tamai, Thomas Greber, Jürg Osterwalder, Science 303 (2004) 217. [3] W. Chen, K. Ping Loh, H. Xu, A.T.S. Wee, Langmuir 20 (2004) 10779. [4] C. Klein, G. Kresse, S. Surnev, F.P. Netzer, M. Schmid, P. Varga, Phys. Rev. B 68 (2003) 235416. [5] S. Hayazaki, T. Matsui, H.L. Zhang, W. Chen, A.T.S. Wee, J. Yuhara, Surf. Sci. 602 (2008) 2025. [6] J. Yuhara, S. Hayazaki, K. Ogawa, T. Matsui, J. Appl. Phys. 112 (2012) 034902. [7] Lap Hong Chan, Shinji Hayazaki, Kokushi Ogawa, Junji Yuhara, Appl. Surf. Sci. 265 (2013) 291. [8] S. Surnev, G. Kresse, M. Sock, M.G. Ramsey, F.P. Netzer, Surf. Sci. 495 (2001) 91. [9] S. Surnev, J. Schoiswohl, G. Kresse, M.G. Ramsey, F.P. Netzer, Phys. Rev. Lett. 89 (2002) 246101. [10] S. Surnev, M. Sock, G. Kresse, J.N. Andersen, M.G. Ramsey, F.P. Netzer, J. Phys. Chem. B 107 (2003) 4777. [11] G. Kresse, J. Hafner, Phys. Rev. B 48 (1993) 13115. [12] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [13] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. [14] R. Car, M. Parrinello, Phys. Rev. Lett. 55 (1985) 2471. [15] P.E. Blochl, Phys. Rev. B 50 (1994) 17953. [16] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [17] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [18] Y. Wang, J.P. Perdew, Phys. Rev. B 44 (1991) 13298. [19] M. Methfessel, A.T. Paxton, Phys. Rev. B 40 (1989) 3616. [20] J. Cerda, M.A. Van Hove, P. Sautet, M. Salmeron, Phys. Rev. B 56 (1997) 15885.
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Initial-stage behaviors of tin and lead adsorption on vanadium surface oxide nanomesh on Pd(111) Lap Hong Chan, Shinji Hayazaki, Kokushi Ogawa, Junji Yuhara ⁎ Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 17 December 2012 Accepted 4 March 2013 Available online 13 March 2013 Keywords: Nanodot Nanomesh Tin Lead Vanadium oxide Palladium Scanning tunneling microscopy DFT calculations
a b s t r a c t The self-organized formation of tin (Sn) and lead (Pb) atoms in a well ordered (4 × 4) vanadium oxide nanomesh on Pd(111) has been investigated using scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory (DFT). At a very low Sn coverage, most of the Sn atoms filled the vanadium oxide nanohole and a few of them were adsorbed on top of the vanadium oxide nanomesh. On the contrary, all of Pb atoms were located in the vanadium oxide nanohole. The ab initio DFT total-energy calculations indicated that the Sn and Pb atoms preferred to adsorb in the nanohole rather than on the nanomesh. The simulated STM images for the Sn and Pb atoms exhibited a local protrusion, while the experimental STM images showed a large, uniform protrusion. The adsorbed Sn and Pb atoms might be mobile in the nanohole at room temperature, possibly as a result of interaction with the STM tip. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The controlled growth of arrayed metal nanostructures on solid surfaces by self-organized method is of great interest both from the fundamental and technological points of view [1]. Using surface nano-templates, nanodots can be fabricated periodically at the nanoscale in a self-assembly process. Of particular interest is the nucleation of adatoms into an ordered array of regular clusters that are small enough to have quantum mechanical properties. So far, surface nanomesh structures, such as boron nitride (BN) on Rh(111), silicon carbide (SiC) surface, and vanadium oxide on Pd(111), have been investigated for the fabrication of controlled nanodots and nanoclusters [2–10]. The BN and SiC nanomeshes have demonstrated the formation of dispersed metal nanoclusters on top of the nanomeshes and the nanoholes have been shown to be the least stable adsorption site for adsorbates [2,3]. On the contrary, the formation of metal nanodots has been reported for the systems of lead (Pd), bismuth (Bi), and silver (Ag) atoms in the vanadium oxide (4 × 4) nanomesh on Pd(111) [4–6]. The Pd atoms form a monomer nanodot, while the Bi and Ag atoms form dimer to tetramer nanodots in addition to a monomer nanodot. All these metals do not form the nanoclusters. In our previous work, we used the group IV elements, silicon (Si) and germanium (Ge), for the semiconducting nanodot formation in the vanadium oxide nanomesh
⁎ Corresponding author. E-mail address: [email protected] (J. Yuhara). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.03.002
on Pd(111). The Si atoms only formed a nanocluster, and Ge atoms formed both nanodots and nanoclusters [7]. Since the group IV elements exhibit wide variety of physical properties depending on the crystal structure and mass number. The properties change from non-metallic in character at the top of the group to metallic at the bottom: carbon is a non-metal, Si and Ge are metalloids, and tin (Sn) and lead (Pb) are typical metals. Therefore, the general reactivity of the group IV elements as a whole is difficult to ascertain, and the reactivity of each element must be considered individually. In this paper, in order to systematically understand the adsorption behaviors of group IV elements in the (4 × 4) vanadium oxide on Pd(111), Sn and Pb are chosen for the characterization of geometric structure using STM and DFT calculations.
2. Experimental and computational methods The experiments were carried out under ultrahigh-vacuum (UHV) conditions with a base pressure of less than 5.0 × 10 −10 mbar in the sample preparation chamber and less than 2.0 × 10 −11 mbar in the analysis chamber. The analysis chamber was equipped with a rear view LEED system operating with a LaB6 filament and a UHV STM system (Omicron STM-1). Four-grid optics enabled Auger analysis in a retarding field. All STM images were acquired with W tips electrochemically etched in a KOH solution using constant current mode with a positive sample bias voltage at room temperature (RT). To prepare a clean Pd(111) surface, the crystal was sputtered by 2-keV Ar+ ions at RT and annealed at 1050 K. The sample was annealed
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using electron bombardment from the backside. The Pd(111) surface was also exposed to molecular oxygen in a 3.0 × 10−8 mbar O2 background pressure at 750 K followed by a flash to 1050 K to remove impurities. A vanadium film was deposited on the clean Pd(111) at RT from a water-cooled electron beam evaporator. The amount of deposited vanadium was fixed at 0.3 ML (we define one monolayer (ML) as overlayer having the atomic density of a Pd(111) layer: 1.52 × 1015 atoms/cm2). After deposition, the vanadium films on Pd(111) were annealed at 550 K at an O2 partial pressure of 3 × 10 −7 mbar and cooled to RT at the same oxygen pressure. A LEED study of this vanadium oxide surface on Pd(111) exhibited a distinct (4 × 4) pattern. The two different levels of vanadium sites were observed in the STM images and the height difference was 0.04 nm. The height difference was caused by the difference in the oxygen atom adsorption sites below, which are on-top (V1) and threefold hollow (V2) sites of Pd surface atom [4]. This perfectly ordered nanomesh was used in this study as a nano-template to examine the initial stage behaviors of metal adsorption. Tin or lead was then deposited on this nanomesh-structured surface at RT using boron nitride crucible evaporators at a deposition rate of 0.01 ML/min. The amount of adatom deposited was 0.001 ML. A theoretical study of the local structure of Sn and Pb atoms on a (4 × 4) vanadium oxide nanomesh on Pd(111) has been performed by ab initio DFT total-energy calculations using the Vienna ab initio simulation package (VASP) [11,12]. These calculations were based on DFT with a plane-wave basis set [13,14]. The electron–ion interaction was described by the projector augmented wave (PAW) method [15,16]. The calculations were performed at an energy cutoff of 250 eV, resulting in an accurate description of the Pd bulk, Pd surface, vanadium oxide film, and adatoms. We used the generalized gradient approximation (GGA) of Perdew and Wang [17,18], commonly referred to as PW91. To calculate the fractional occupancies, the
broadening approach introduced by Methfessel and Paxton was used with N = 1 and σ = 0.02 eV [19]. The Pd(111) surface comprised the (4 × 4) super cell and fourlayer slab. The structural parameters for the vanadium oxide (4 × 4) nanomesh were obtained from the structural models proposed by reference [4]. In the c direction, we prepared a 15 Å vacuum. As a result, the calculated lattice parameters used were a = 10.995 Å, b = 10.995 Å, and c = 25.000 Å. A Monkhorst–Pack mesh with 2 × 2 × 1 k-points was used with the supercell. Because the structural model of the vanadium oxide nanomesh was obtained by MD simulation, the positions of the Pd, V, and O atoms were fixed and only Sn or Pb atoms were relaxed for the ab initio total energy calculations. Simulated STM images were calculated by Extended Hückel theory and utilize a Green's function matching using the Nt_STM package [20]. The tip was an apex structure of W(100). All simulated STM images were calculated for a sample bias of of + 1.5 V and a tunneling current of 1.0 nA with a lateral resolution of 0.04 nm. The simulation parameters for Fermi energy, cut-off radius for atomic orbital interactions, and the inverse of the minimum length in the k-sampling grid were − 10 eV, 0.9 nm, and 3 nm, respectively. 3. Results and discussion A small amount of Sn atoms was deposited onto the vanadium oxide nanomesh on Pd(111) at RT to examine the geometric structure. Most of the Sn atoms occupied the nanohole while a few were adsorbed on top of vanadium in the nanomesh, as shown in the STM image of Fig. 1(a). Fig. 1(b)–(d) shows the high-resolution STM images and it is clearly seen that the Sn atoms filled the nanoholes or adsorbed on the V1 site of the nanomesh. The Sn atoms always adsorbed on the V1 site, but not V2, of the nanomesh. When the Sn
Fig. 1. STM images of Sn deposition onto the vanadium oxide nanomesh on Pd(111): (a) large-scale image (Vs = 1.5 V, It = 0.45 nA) and (b)–(d) high-resolution images (Vs = 2.0 V, It = 0.6 nA) of three different Sn adsorption sites.
L.H. Chan et al. / Surface Science 613 (2013) 35–39
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Fig. 2. STM images of Pb deposition onto the vanadium oxide nanomesh on Pd(111): (a) large-scale image and (b) high-resolution image of Pb adsorption site.
atoms filled the nanoholes, the brightness area is much larger than the single atom. Therefore, the Sn atom might be mobile in the nanohole or the STM scanning tip induced Sn migration. Since the brightness is not totally uniform but slightly off-centered toward V2 site, the Sn atoms in the nanohole seem to have a preferential site. The vanadium oxide nanomesh itself seems to be chemically stable against Sn deposition. The coverage of large protrusion is in good agreement with the Sn coverage deposited. Therefore, it is concluded that each large protrusion contains one Sn atom. In contrast to the Sn deposition, all of Pb atoms deposited onto the nanomesh filled the nanoholes and none of the Pb atoms were adsorbed on the nanomesh, as shown in the STM image of Fig. 2(a). High-resolution STM image in Fig. 2(b) exhibits that the Pb atoms were observed as the large protrusions with uniform height. Because the brightness was constant and the area was much larger than the single atom, the Pb atom might be mobile in the nanohole. The STM image for the Pb atoms in the nanohole looks very much similar to the Bi atoms [5]. Since the coverage of the large protrusions is consistent with the Pb coverage deposited, it is concluded that each large protrusion contains one Pb atom.
Fig. 3(a) shows the cross-sectional profile of the Sn atom, corresponding to the white line in Fig. 1(b). The height of the Sn atom located on the V1 site was only 0.05 nm higher than that of the V1 site without Sn adsorption. The result indicated that the Sn atoms adsorbed to the V atoms might have no dangling bond, while the V atoms of V1 site have a directional dangling bond. Fig. 3(b) and (c) shows cross-sectional profiles of the Sn atoms along the white lines in Fig. 1(c) and (d), respectively. The heights of these Sn atoms in the nanohole were higher than that of the V2 sites that have a directional dangling bonding [4]. Therefore, the Sn atoms in the nanohole may also have a dangling bond. Fig. 3(d) shows a cross-sectional profile of the Pb atom along the white line in Fig. 2(b). The height of the Pb atom was uniform in the nanohole and was similar to that of V2 site. Because the atomic size of Pb is 0.36 nm and the height of the V2 site from the Pd(111) surface is 0.32 nm [4], the Pb atoms in the nanohole have non-directional metallic bonding. The three structure-optimized models, simulated STM images, and cross-sectional profiles of Sn atoms in the vanadium oxide nanomesh with/without oxygen in the center of the nanohole are displayed in Fig. 4(a)–(c). The simulated STM image of the Sn atom agreed only
Fig. 3. The cross-sectional profiles of STM image: (a)–(d) corresponding to the solid line in Figs. 1(b)–1(d) and 2(b), respectively.
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L.H. Chan et al. / Surface Science 613 (2013) 35–39
Fig. 4. Structural models, simulated images, and line profiles for Sn atoms located on (a) the V1 sites in the vanadium oxide nanomesh (b) the fcc hollow site, and (c) the fcc hollow site with oxygen in the center of the nanohole.
qualitatively with the experimental STM image, shown in Fig. 4(a). The simulated image looks much wider and brighter than the experimental image. Therefore, the parameters for Extended Hückel theory are not appropriate for such an isolated atom, since the bonding environment is very different from the bulk Sn crystal. Fig. 4(b) shows the structure-optimized model of the Sn nanodot without the oxygen atom in the center of the nanohole on Pd(111). The structure optimized calculations indicated that the Sn atom was energetically stable for both fcc and hcp hollow sites. The simulated STM image and its cross-sectional profile showed that the height of the Sn atom was similar to that of V atom in the V2 site on Pd(111). This is consistent with the experimental cross-sectional profile. The barrier height energy of the surface migration for Sn atom in the nanohole was calculated to be 0.17 eV. The small barrier height energy clearly explains the Sn migration in the nanohole, which is consistent with wide and uniform brightness for the Sn atom in the STM image. However, small surface barrier height energy contradicts the off-centered small protrusion in the experimental STM image. Fig. 4(c) shows the structure optimized model that Sn atom was located next to the center of oxygen atom on Pd(111). The simulated image showed local protrusion at the position of the Sn atom, and the height for the Sn atom was higher than the V1 site. In this
model, Sn atoms could not easily migrate in the nanohole because of the limited space. Therefore, there might be no oxygen atom in the nanohole, but this model clearly explains the STM image of the Sn atoms that showed the off-centered protrusion. Fig. 5(a) shows the structure-optimized model, simulated STM images, and cross-sectional profiles that the Pb atom was located on Pd(111) hcp hollow site. The simulated image showed a local protrusion at the position of the Pb atom and the cross-sectional profile showed that the height for the Pb atom was similar to the V atom in the V2 site. As discussed above, the disagreement of the appearance of the STM image between experiment and simulation can be explained by the small energy barrier height of Pb atom migration in the nanohole, which was calculated to be 0.34 eV. The adsorption energies of Sn and Pb atoms determined using total-energy DFT calculations were summarized in Table 1. The vanadium oxide nanomesh structures, with/without an oxygen atom in the center of the nanohole, were calculated, since the structural model of the vanadium oxide proposed by Kresse et al. implies the possibility of the oxygen atom [4]. The calculation results indicated that the adsorption in the nanohole was much more stable than the adsorption onto the nanomesh regardless of the oxygen atom. Nevertheless, the Sn atom was observed at V1 sites in the experimental STM image. The
Fig. 5. Structural models, simulated images, and line profiles for Pb atoms located on the hcp hollow site on Pd(111).
L.H. Chan et al. / Surface Science 613 (2013) 35–39 Table 1 Adsorption energy of adatom on the vanadium oxide nanomesh on Pd(111). Adatom
Surface model
Adsorbed position
Eadsorption (eV)
Sn
V5O14 V5O14 V5O14 V5O14 V5O14 V5O14 V5O14 V5O14
V1 on-top Close to fcc site hcp hollow site fcc hollow site V1 on-top Close to fcc site hcp hollow site fcc hollow site
−0.83 −4.79 −5.06 −5.06 −0.95 −4.23 −4.78 −4.83
Pb
+O +O
+O +O
present results indicate that some of the Sn atoms are trapped at the V atom in V1 sites, when a few of the evaporated Sn atoms are heading toward for the V1 site during the deposition. Our results indicated the group IV elements have different reactivity depending on the local bonding properties. In other words, the elements that show metallic character such as Pb and Sn atoms like to fill a nanohole and they are not easily bound to V atoms on the nanomesh. On the contrary, the elements with non-metallic character such as Si and Ge atoms have a tendency to form a chemical bond with V atoms on the nanomesh. The local bonding properties such as directional or non-directional seem to play a key role for the growth mechanism. 4. Conclusions The adsorption of very low Sn and Pb coverages on the (4 × 4) vanadium oxide nanomesh on Pd(111) has been investigated using STM and DFT calculations. The Sn atoms occupied the nanohole or adsorbed on the V1 site of the nanomesh and all of Pb atoms occupied the nanohole. The DFT calculations indicated that the adsorption in the nanohole was more stable than that on the nanomesh for both Sn and Pb atoms. The simulated images indicated a local protrusion, while the STM images showed large, uniform protrusions for Pb and Sn atoms adsorbed in the nanohole. Although the differences can be explained by the small barrier height energies of surface migration
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for the Sn and Pb atoms, the large protrusion with uniform height might be related to interaction with the STM tip during scanning. Acknowledgments This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas “New Materials Science Using Regulated Nano Spaces — Strategy in Ubiquitous Elements” (no. 22013008) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and by a Research Grant Program of Specially Promoted Project from the Toyota Physical and Chemical Research Institute. References [1] H. Brune, M. Giovannini, K. Bromann, K. Kern, Nature 394 (1998) 451. [2] Martina Corso, Willi Auwärter, Matthias Muntwiler, Anna Tamai, Thomas Greber, Jürg Osterwalder, Science 303 (2004) 217. [3] W. Chen, K. Ping Loh, H. Xu, A.T.S. Wee, Langmuir 20 (2004) 10779. [4] C. Klein, G. Kresse, S. Surnev, F.P. Netzer, M. Schmid, P. Varga, Phys. Rev. B 68 (2003) 235416. [5] S. Hayazaki, T. Matsui, H.L. Zhang, W. Chen, A.T.S. Wee, J. Yuhara, Surf. Sci. 602 (2008) 2025. [6] J. Yuhara, S. Hayazaki, K. Ogawa, T. Matsui, J. Appl. Phys. 112 (2012) 034902. [7] Lap Hong Chan, Shinji Hayazaki, Kokushi Ogawa, Junji Yuhara, Appl. Surf. Sci. 265 (2013) 291. [8] S. Surnev, G. Kresse, M. Sock, M.G. Ramsey, F.P. Netzer, Surf. Sci. 495 (2001) 91. [9] S. Surnev, J. Schoiswohl, G. Kresse, M.G. Ramsey, F.P. Netzer, Phys. Rev. Lett. 89 (2002) 246101. [10] S. Surnev, M. Sock, G. Kresse, J.N. Andersen, M.G. Ramsey, F.P. Netzer, J. Phys. Chem. B 107 (2003) 4777. [11] G. Kresse, J. Hafner, Phys. Rev. B 48 (1993) 13115. [12] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [13] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. [14] R. Car, M. Parrinello, Phys. Rev. Lett. 55 (1985) 2471. [15] P.E. Blochl, Phys. Rev. B 50 (1994) 17953. [16] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [17] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [18] Y. Wang, J.P. Perdew, Phys. Rev. B 44 (1991) 13298. [19] M. Methfessel, A.T. Paxton, Phys. Rev. B 40 (1989) 3616. [20] J. Cerda, M.A. Van Hove, P. Sautet, M. Salmeron, Phys. Rev. B 56 (1997) 15885.