A New Surface and Structure for Silicene: Polygonal Silicene

Sep 23, 2013 - Chem. C , 2013, 117 (42), pp 22142–22148 ...... Hanf , Didier Dentel , Natalia Massara , Ahmed Mehdaoui , Philippe Sonnet , and Carme...
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A New Surface and Structure for Silicene: Polygonal Silicene Formation on the Al(111) Surface Tetsuya Morishita,*,†,‡ Michelle J. S. Spencer,*,§ Shuhei Kawamoto,†,∥ and Ian K. Snook§ †

Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ‡ Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan § School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia S Supporting Information *

ABSTRACT: Seeking a new substrate for fabrication of silicene is of high importance, since its formation in experiment is extremely difficult and only a few substrates have been found to be suitable so far. Reporting of other possible substrates for silicene, particularly cheaper alternatives, would also open up the possibility of controlled experimental growth and functionalization of this new nanomaterial. Here, we show, based on first-principles molecular dynamics calculations, that the honeycomb silicene monolayer is stable on the 4 × 4 Al(111) surface and that it possesses a binding energy comparable to that on the 4 × 4 Ag(111) surface. It is furthermore demonstrated that a new ordered silicene monolayer structure consisting of 3-, 4-, 5-, and 6-sided polygons can also be formed on the Al(111) surface. This new silicene monolayer, which we call “polygonal silicene”, possesses an almost flat monolayer structure and exhibits a free electron-like density of state, indicating high electronic conduction. The present findings indicate that silicene can take monolayer forms other than the honeycomb lattice as a result of varying the substrate. Silicene is thus expected to exhibit variable electronic properties, which would extend its applicability to possible future nanoelectronics.

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INTRODUCTION Silicon (Si)-based two-dimensional (2D) materials have been a focus of growing interest due to their potential technological applications and their high affinity with existing electronic circuits.1,2 The importance of 2D materials is that they bridge the gap between zero- and one-dimensional materials and three-dimensional structures. As a result, they can be deposited or grown on a substrate for applications as electronic devices, batteries, and sensors, among others. With the worldwide desire to enhance our technological capabilities, the microelectronics industry is dominated by an intense drive to miniaturize and “scale down” electronic devices. In order to keep up with the targets set by the electronics industry and encouraged by Moore’s law to increase functionality and performance and decrease costs, this area of research is of vital importance to society as well as industries worldwide. Hence, it is essential to determine whether miniaturizing current microelectronic devices could be solved with the use of nanoscale silicon. The advantage of using nanosilicon in such devices, as opposed to other 2D materials, is that our current semiconductor technology is already equipped for silicon. A number of experimental and theoretical attempts to fabricate Si-based 2D materials have thus been reported over the past several years.3−14 A Si monolayer having the © 2013 American Chemical Society

honeycomb lattice, which is a graphene analogue and is called silicene, has recently been successfully formed on metallic substrates.15−20 The existence of silicene was first examined by theoretical calculations7−9,11 and was later confirmed by experimentally observing its growth on the Ag(111) surface.15−18 Silicene is known to share several characteristics with graphene; both consist of a monolayer honeycomb lattice and exhibit a linear dispersion relation in their electronic band structure. The Si−Si bond in silicene is, however, considered to be dominated by sp3 hybridization, in contrast to the C−C bond in graphene which is sp2 bonded. It is thus anticipated that the interaction between the silicene monolayer and the substrate is stronger than that for graphene, which plays a more important role in characterizing the silicene structure. In order for silicene to be used in electronic devices, it is imperative to find suitable substrates that are commercially viable and on which the 2D material can adhere and deliver suitable properties. Silver surfaces such as Ag(111) have already been used to grow a silicene monolayer. It is suggested that a moderate interaction between silicene and the substrate is the Received: August 13, 2013 Revised: September 19, 2013 Published: September 23, 2013 22142

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the experimental values. Both of the silicene systems contain 18 Si atoms and 80 Al or Ag atoms.

key to forming a stable silicene layer and that the Ag substrate is one suitable substrate for its growth.1,21,22 Silver itself is, however, a noble metal with high cost for frequent use in experiment and commercial production. Many experimental groups have thus been trying to find other suitable substrates for silicene: in fact, Meng et al. have just recently reported the formation of silicene on the Ir(111) surface.20 Iridium, however, is also not a cheap metal; hence, there is still a need to find other possible substrates for silicene. Furthermore, it is crucial to see how structural and electronic properties are affected by varying the substrate. Several factors such as strain, external electronic field, and type of substrate are found to be effective in controlling the physical properties of 2D nanomaterials.23 Since the properties of silicene could be more easily controlled by substrate than, for example, graphene, exploring suitable substrates for silicene is of great importance. In this paper, we demonstrate using first-principles molecular dynamics (FPMD) calculations that the 4 × 4 honeycomb silicene structure, that is formed on the Ag(111) surface, can also be stably formed on the Al(111) surface. The advantage of forming silicene on Al surfaces is that they are much more easily handled in experiment than the more expensive Ag and Ir substrates. We also show that silicene can take a new monolayer structure on Al(111), consisting of a regular arrangement of triangles, rectangles, pentagons, and hexagons, which is likely to exhibit a higher electronic conduction than the honeycomb silicene. Such a highly metallic silicene overlayer is a suitable material for electrodes of, e.g., secondary batteries and Si-based field effect transistors. The present study demonstrates the inherent flexibility of the structure and properties of silicene, which can be controlled by varying the substrate, indicating that there are a myriad of potential applications for this novel nanomaterial.

Figure 1. Silicene formed on Al(111). (a) The honeycomb silicene and (b) the polygonal silicene. The yellow spheres represent Si atoms, and the blue spheres represent Al atoms. The upper and lower panels show the top and side views, respectively. (c) Schematic configuration of the polygonal silicene showing its tessellation. The unit cell for the polygonal layer is shown in red.

COMPUTATIONAL METHOD Our FPMD calculations based on density functional theory were performed using the Vienna ab initio Simulation Package (VASP).24 The exchange-correlation functional in the Perdew− Burke−Ernzerhof form25 was used, and the ion−electron interaction was described by the projector augmented wave method.26 A plane-wave basis set with an energy cutoff of 250 eV was used with a k-point mesh of 13 × 13 × 1 for the electronic structure calculations, 3 × 3 × 1 for the geometry optimization and MD calculations, and 1 × 1 × 1 (Γ-point) for the logarithmic mean-force dynamics (LogMFD) calculations (see refs 27 and 28 and Supporting Information for details on LogMFD). The present silicene system was prepared by depositing a honeycomb Si monolayer on a 4 × 4 supercell of the Al(111) surface with dimensions of 11.4221 Å × 11.4221 Å × 21.326 Å. The Al substrate is composed of five layers with the bottom layer fixed. We tried an eight-layer slab but found that five layers was sufficient for the present calculations by observing that the binding energy (BE) of the silicene is converged to within 0.01 eV/Si atoms using the five-layer slab. We also performed SCF calculations using a supercell whose vacuum spacer is twice as large as the original one and confirmed that the energy is converged to within 3 × 10−4 eV/atom using the original vacuum spacer. The silicene/Ag(111) system was also prepared in the same way with supercell dimensions of 11.6496 Å × 11.6496 Å × 21.512 Å. The lattice constants of the Al or Ag substrates were determined by relaxing the bulk Al or Ag crystal under zero pressure, giving values that are very close to

structure for the silicene/Al(111). We observed neither significant structural deformation nor breakdown during the heating and quenching process (300−700 K) mentioned above. The honeycomb lattice was thus firmly maintained on the Al(111) surface as shown in Figure 1a. We also performed FPMD calculations for a larger system (392-atom system) for 6 ps with a 2 × 2 × 1 k-point mesh and confirmed that the size effect on the structural properties and the stability of the silicene/Al(111) is negligible. The bond lengths for the optimized silicene structure on Al(111) are 2.36−2.40 Å, being slightly longer than but comparable to those for the 4 × 4 silicene/Ag(111) (2.32−2.38 Å).15,21,29 Since the buckling in the honeycomb lattice is retained, six of 18 Si atoms are outer atoms, as shown in the lower panel of Figure 1a, and sit higher on the surface. The other 12 atoms sit closer to the surface, which we refer here to as surface-side atoms. The buckling distance is 0.9−1.4 Å, which is slightly longer than that for the 4 × 4 silicene/Ag(111) (0.7−0.8 Å).15,21,30 While the lattice constant of the Al substrate is smaller than that of the Ag substrate, the Si−Si bonds are longer in the silicene/Al(111), thus making the buckling distance longer than that in the silicene/Ag(111). The buckling, in fact, induces corrugation of the outermost (surface) Al layer, which is also visible in the silicene/Ag(111). We found that the relative z-positions of the Al atoms in the topmost surface layer are highly correlated with the positions of the Si atoms; the Al atoms that are closest to the outer Si atoms are located above the mean height of the topmost Al surface atoms, while the Al atoms that are closest to the lower-level Si atoms

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RESULTS AND DISCUSSION We performed a geometry optimization for a 4 × 4 silicene/ Al(111) [more precisely, a 3 × 3 honeycomb silicene on the 4 × 4 Al(111) surface21] followed by a series of MD calculations in which a heating and quenching cycle of 9 ps was performed. The final structure from the MD calculation (300 K) was again relaxed to a 0 K structure, for which we will show the structural and electronic properties below. Figure 1a shows the resultant

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The original honeycomb lattice of the silicene starts to deform, and then it transforms to a new monolayer structure containing rectangular, pentagonal, and hexagonal rings as shown in Figure 2c. Geometry optimization was performed for the new structure after the transition, and the resultant structure is presented in Figure 1b. It is clear that the new structure no longer consists of the honeycomb lattice, but it preserves an ordered configuration; it forms a network of a central hexagonal polygon, which is surrounded by only pentagons, which are then surrounded by a combination of 3-, 4-, and 5-sided polygons. From this, we call the new Si monolayer “polygonal silicene”. Figure 1c shows the schematic configuration of the polygonal silicene. We clearly see that the 2D space can be tiled by this structural motif. This tiling, in fact, bears a resemblance to the rhombitrihexagonal tiling and is commensurate with the 4 × 4 Al(111) surface. The side view shows that the polygonal silicene is almost flat apart from one atom that sits slightly higher on the surface of each of the central hexagons. The bond lengths associated with the rectangles and pentagons range from 2.48 to 2.71 Å, while those associated with the hexagon range from 2.36 to 2.4 Å. The former are much longer than the average bond length of the honeycomb silicene. The distance between the protruding atoms and the flat layer is about 2 Å. The protruding atoms were found to return to their elevated position during MD run at 300 K, even when initially placed at the lower level of the other atoms. We did find, however, that a perfectly flat polygonal monolayer was stable at 300 K on an Al(111) substrate with a lattice constant expanded by ∼7%. A perfectly flat polygonal silicene could thus be formed under tensile conditions or on a substrate having a larger lattice constant than that of the Al(111) substrate. It is known that the Si−Si bonds in crystalline and amorphous Si tend to become longer and metallic through a pressure-induced structural transformation,31−33 which results from degradation of the sp3 hybridization under pressure.34 It is thus expected that the characteristics of the Si−Si bonds in the honeycomb silicene would also dramatically change after the structural transformation to the polygonal structure, in which the corrugated honeycomb structure consisting of the sp3 tetrahedral structural motif becomes a flat monolayer structure having longer Si−Si bonds. Interestingly, 2D structures containing rectangular or pentagonal polygons have also been observed in MD simulations for quasi-2D Si systems under pressure.35 It is therefore likely for our system that the internal stress is increased locally at some stage during the “tugging” process, which leads to formation of the polygonal silicene on Al(111). In contrast to the honeycomb silicene system, the topmost Al surface layer under the polygonal silicene overlayer shows a very weak corrugation (a maximum of 0.18 Å). This can be attributed to the highly flat monolayer structure of the polygonal silicene, which also results in little correlation between the relative z positions of the Al and Si atoms in the polygonal silicene system. The polygonal silicene is found to be less stable than the original honeycomb silicene by only 0.0135 eV/atom. We have confirmed that the polygonal silicene in a larger system size (392-atom system) is also stable during the 300 K MD calculations. It is thus likely that the polygonal silicene can be formed on Al(111) as a metastable conformation. It is worth noting that, though we also tried to pull one of the outer Si atoms in the 4 × 4 silicene/Ag(111), we found the structural transformation was unlikely to occur. In fact, a dislocation was

(i.e., the Si atoms closest to the surface) are below the mean height, reflecting some degree of interaction between the Si and surface Al atoms. The surface corrugation induced by the Si buckling is more enhanced on the Al surface than on the Ag surface because of the longer buckling distance of Si in the former. The amplitude of the surface corrugation in the Al substrate is a maximum of 0.55 Å, while that in the Ag substrate is a maximum of 0.43 Å (this value agrees well with the value reported by Enriquez et al.29). We calculated the binding energy (BE) for silicene/Al(111) and silicene/Ag(111), the latter being directly comparable to the former. The BE was estimated by BE = E(silicene/sub) − [E(silicene) + E(substrate)], where E(silicene/sub) is the total energy for the optimized silicene/Al(111) [or silicene/ Ag(111)], and E(silicene) and E(substrate) are the total energy for the isolated silicene and substrate, respectively. The calculated BE is −0.43 eV/Si for silicene/Al and −0.46 eV/Si for silicene/Ag, the latter being consistent with the previously calculated BE by Enriquez et al. (−0.46 eV)29 and by Vogt et al. (−0.48 eV).15 The BE for silicene/Al(111) is slightly lower but is close to that for silicene/Ag(111). It is thus considered that a silicene monolayer is stable on Al(111) as well as on Ag(111). To further check the stability of the silicene monolayer on Al(111), we attempted to pull one of the outer Si atoms toward the vacuum space, as in an AFM manipulation experiment. What we have actually done is to run a logarithmic mean-force dynamics (LogMFD)27,28 calculation in which we treated the z coordinate of the “tugged” Si atom as the collective variable having the velocity that drives the atom to move toward the vacuum space (the z-axis is taken perpendicular to the surface). In the LogMFD run, the z coordinate of the “tugged” Si atom is allowed to move on the free energy surface, while its x and y coordinates are constrained to the original positions. Since the effective free energy barrier is substantially reduced in LogMFD, the Si atom can easily move toward the vacuum space with the initial velocity (300 K), yielding the free energy profile on the fly27,28 [note that the displacement of the atom toward the vacuum space is very slow, which allows us to estimate the mean force and, hence, the free energy curve as a function of the displacement in the z direction (presented in the Supporting Information)]. Figure 2a−c shows the time evolution during the process of “tugging” the Si atom outward.

Figure 2. Snapshots of the silicene/Al(111) from the process of “tugging” an outer Si atom indicated by the red circle. The structural transformation proceeds from (a) to (c). Note that all the snapshots are from the transient state [the transition has not yet completed at (c)]; thus, each yellow sphere in (c) does not indicate the final position of the corresponding Si atom. 22144

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Figure 3. Comparison of the electronic properties of the honeycomb and polygonal silicene: (a, d) the band structures; (b, c) the DOS; and (e, f) the ELF contour plot. (a), (b), and (e) are for the honeycomb silicene, while (c), (d), and (f) are for the polygonal silicene. The band energy in (a− d) is measured from the Fermi level. Note that the band structures in (a) and (d) were calculated for the silicene forms shown in Figure 1 without the substrate. Also shown for comparison is the band structure for the honeycomb silicene solely relaxed in vacuum (gray small circles), which exhibits the linear dispersion relation.

plots show the ELF along the plane which runs parallel to the substrate and is located between the outer and substrate-side Si atoms. The formation of the covalent bonds in the honeycomb structure is clearly indicated in the ELF plot (Figure 3e), which is manifested by the distinct color contrast (red and blue). The ELF plot for the polygonal structure, in contrast, shows a less localized arrangement of the electrons with less distinct color contrast (Figure 3f). This is consistent with the possible degradation of the sp3 bonding and the structureless profile of the DOS for the polygonal silicene. The charge redistribution, which was analyzed by Bader charge calculations, gives deeper insights into the effect of the monolayer structure and substrate on the electronic properties of silicene. We found an appreciable transfer of electrons from the outermost (surface) Al layer to the silicene; −6.42e and −7.26e to the honeycomb and polygonal silicene, respectively (for the 18-Si systems). The fact that the polygonal silicene gains more electrons is consistent with its ELF profile. A closer look reveals that the amount of charge on each Si atom depends on its location (see Table 1). Each of the surface-side

instead created in the Ag substrate, with the silicene being deformed, while maintaining the honeycomb lattice (see Supporting Information for details). It thus appears that the slightly larger BE of the silicene on Ag(111) hampers the silicene from forming the polygonal monolayer structure. [We show later that metallicity in the honeycomb silicene on Al(111) is highly enhanced, which is also likely to enhance the structural transformation on Al(111) more so than on Ag(111).] The band structures were calculated for the honeycomb and polygonal silicene (Figures 3a and 3d, respectively). As seen in Figure 3a, the honeycomb silicene (without the Al substrate) loses the linear dispersion in the same manner as the 4 × 4 silicene/Ag(111).18 The contribution from the K-point in the Brillouin zone (BZ) for the hexagonal unit lattice is included in the Γ-point in Figure 3 due to the smaller BZ for the present silicene system.18 We confirmed that the free-standing silicene, which was obtained by further relaxing the honeycomb silicene in vacuum without the substrate, regained the symmetric honeycomb lattice and the linear dispersion relation. (The band structure of the free-standing silicene is also presented in Figures 3a and 3d for comparison, with the gray circles.) It is thus clear that the symmetry breaking due to the substrate significantly affects the electronic structure of silicene/Al(111) as was reported in ref 18 for the case of silicene/Ag(111). The band structure for the polygonal silicene (without the Al substrate) also shows no linear dispersion. It, however, shows a stronger dispersion relation around the Fermi level, and thus we expect higher electronic conduction in the polygonal silicene than in the honeycomb silicene. Figures 3b and 3c compare the electronic density of states (DOS) for the honeycomb and polygonal silicene. We see that, while the DOS for the honeycomb silicene still possesses a sharp dip around −8 eV in the s-orbital band, that for the polygonal silicene exhibits a much more structureless profile, resulting in a free electron-like DOS as a whole. The electron localization function (ELF) for the two silicene structures is presented in Figures 3e and 3f. (The ELF plots give an indication of the probability of bonding or nonbonding electron pairs, with 0 indicating little or no probability and 1 high probability of highly localized electrons.) The contour

Table 1. Charge Redistribution of the Si Atoms in the Silicene/Al(111)a hollow site bridge site on-top site outer site a

honeycomb silicene

polygonal silicene

−0.46e to −0.70e −0.48e to −0.63e −0.43e +0.02e to +0.18e

−0.47e to −0.75e −0.31e to −0.45e −0.24e to −0.32e +0.18e

The Bader charges are given in units of the elementary charge, e.

Si atoms is located either at a hollow site, bridge site, or on-top site on the Al(111) surface (7, 4, and 1 of the 12 surface-side Si atoms, respectively in the honeycomb silicene, and 7, 3, and 7 of the 17 surface-side Si atoms, respectively, in the polygonal silicene). The Si atoms at hollow sites tend to possess more electrons as shown in Table 1: −0.46e to −0.7e in the honeycomb silicene and −0.47e to −0.75e in the polygonal silicene. The Si atoms at bridge sites hold charges of −0.48e to −0.63e in the honeycomb silicene and −0.31e to −0.45e in the polygonal silicene. A relatively smaller amount of electrons is 22145

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accumulated on the Si atoms at on-top sites: −0.43e in the honeycomb silicene and −0.24e to −0.32e in the polygonal silicene. While all of the surface-side Si atoms are negatively charged, the outer Si atoms are, in contrast, positively charged. The charges of the six outer atoms in the honeycomb silicene range from +0.02e to +0.18e. The protruding atom in the polygonal silicene is also positively charged by +0.18e. Taking account these positively charged atoms, each of the surface-side Si atoms gains, on average, −0.58e in the honeycomb silicene and −0.44e in the polygonal silicene. It is, however, stressed that the total charge transferred from the Al surface is larger in the polygonal silicene as already demonstrated. This means that the transferred charges are more uniformly distributed in the polygonal silicene than in the honeycomb silicene, as is also reflected in the ELF plot (recall that 17 of the 18 Si atoms are the surface-side atoms in the polygonal silicene that has a flat monolayer structure). The Al surface is also positively charged as a consequence of the charge transfer, but the transfer mainly takes place between the silicene and the topmost surface Al layer only. While each of the surface Al atoms gains a charge of +0.3e to +0.6e in both the silicene systems, the atoms in the second Al layer gain only +0.07e or less. It is thus considered that the charge redistribution occurs only between the topmost surface Al layer and the silicene monolayer. This suggests that atomically thin electronic devices utilizing electronic double layers or Schottky barrier junction could be designed based on silicene/ Al(111). To gain further insight into the charge redistribution, we also performed Bader charge analysis for the honeycomb silicene on Ag(111). Interestingly, little charge is transferred from the Ag surface to the honeycomb silicene. The outermost Si atoms are positively charged by +0.08e to +0.15e, but the net charge on each of the surface-side Si atoms ranges from −0.03e to +0.02e, resulting in the total charge transfer of about 0.55e in the 18 Si system. [This is only 8% (= 0.55e/6.42e) of the total charge transferred from the Al surface to the silicene monolayer in the honeycomb silicene on Al(111) system.] The different profiles of the charge redistribution on the Al and Ag surfaces are considered to come from the different electronegativities of Al and Ag; the electronegativity of Ag is close to that of Si, while the electronegativity of Al is lower than that of Si. This finding clearly demonstrates that the substrate plays a significant role in characterizing the silicene structure that can be formed. The different profiles of the charge redistribution are also reflected in the electronic DOS for the honeycomb silicene. Figure 4 compares the DOS for the honeycomb silicene on Al(111) with that on Ag(111). We clearly see that the dips around −8 and 0 eV are less pronounced in the DOS for silicene/Al(111). In particular, the higher density of states around the Fermi level suggests less covalency in the silicene/ Al(111). In contrast, on Ag(111), it has been shown that the band gap can open up for specific geometries of the silicene on this surface.18 From these findings, we consider that the metallicity is enhanced in silicene/Al(111) and that this enhancement accounts for the slightly longer Si−Si bonds in silicene/Al(111) than those in free-standing silicene and silicene/Ag(111). This, in turn, suggests that the silicene monolayer on Al(111) can transform to the polygonal structure more easily than on Ag(111) by applying external work, due to the enhanced metallicity.

Figure 4. Electronic DOS for the (honeycomb) silicene on Al(111) and Ag(111). The band energy is measured from the Fermi level.

Markedly different characteristics in the dynamical properties of the different silicene overlayers are also disclosed. The vibrational density of states (VDOS) for the honeycomb and polygonal silicene were calculated by Fourier transformation of the velocity autocorrelation functions obtained from the 300 K MD simulations for the 392-atom systems. Figure 5a compares

Figure 5. Vibrational DOS for the three silicene systems: (a) total VDOS and (b−d) directionally decomposed VDOS.

the VDOS for the honeycomb and polygonal silicene. While LO- or TO-like peaks (which are observed above 400 cm−1 in the crystalline and amorphous Si36) are preserved in the VDOS for the honeycomb silicene (red and green lines), no distinct peak is found above 400 cm−1 in the VDOS for the polygonal silicene (blue line). In fact, the VDOS for the honeycomb and polygonal silicene are very similar to that for low- and highdensity amorphous Si, respectively;32−34 the former consists of a tetrahedral network formed by covalent bonds, while the latter consists of overcoordinated Si atoms exhibiting a high electronic conductivity. It is thus indicated that the characteristics of the Si−Si bonds in the polygonal silicene are markedly different from those in the honeycomb silicene, being consistent with the highly metallic electronic structure of the polygonal silicene. We also see in Figure 5a that the overall profile of the VDOS for the silicene/Ag(111) (green line) is similar to that for the 22146

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possible. Hence, different approaches could also assist in the formation of a new silicene structure on substrates other than the Al(111) surface.

honeycomb silicene/Al(111) (red line) but that the former shifts to higher ω, suggesting there is an enhanced covalency in silicene/Ag(111), as is indicated in the electronic DOS (Figure 4). The directionally decomposed VDOS are shown in Figure 5b−d. An extremely low intensity above 300 cm−1 is found in the z-component of the VDOS (blue lines in Figure 5b−d) for all the systems, reflecting the absence of stretching modes from covalent-like bonds in the direction perpendicular to the substrate. This confirms that the bonding between the substrate Al (or Ag) and Si atoms is not as strong as compared to the Si− Si covalent bond. Below 200 cm−1, in contrast, the z-component of the VDOS exhibits distinct peaks. There are, however, more structures for the Ag system than the Al systems, with the primary difference being the greater intensity of the peak at ∼180 cm−1, in particular, for silicene/Ag(111). We found, by further decomposing the VDOS for the silicene/Ag(111) into the contributions from each of the Ag layers and the silicene monolayer, that the peak around 180 cm−1 mainly comes from the surface-side Si atoms only, while the peaks around 40 and 100 cm−1 are mainly attributed to the dynamics of the outer Si atoms. Furthermore, the dynamics of the surface-side and outer Si atoms are found to equally contribute to the peak around 25 cm−1. Detailed analysis shows that the dynamical modes at about 25 and 40 cm−1 are coupled with the dynamics of the Ag atoms; the latter is particularly coupled with the surface Ag atoms. In contrast to the silicene/Ag(111), the dynamics of the surface-side Si atoms in the honeycomb silicene on Al(111) is found to contribute to almost all the peaks in the z-component of the VDOS (blue line in Figure 5c), while the contribution from that of the outer Si atoms is extremely low above ∼100 cm−1. We also found that the dynamical modes of the Si and Al atoms are not tightly coupled as they are in the silicene/ Ag(111). These findings indicate that the vibrational properties of silicene are also significantly affected by varying the substrate material.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the LogMFD calculations for the AFM-like manipulation processes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.M.); http://staff.aist.go.jp/tmorishita/. *E-mail [email protected] (M.J.S.S); http://www. rmit.edu.au/staff/michelle-spencer. Present Address ∥

S.K.: Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computations were performed on the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government, the iVEC Pawsey Centre, MASSIVE, and VPAC, Australia, and on the computational facilities at the Research Center for Computational Science, National Institute of Natural Sciences, and at Research Institute for Information Technology, Kyushu University, Japan. This research was also supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We gratefully acknowledge the contributions that Prof. Ian K. Snook made to the work in this publication. He sadly passed away this year, but his enthusiasm, leadership, and knowledge of condensed matter physics, and science in general, will always be remembered and carried on in future work.

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CONCLUSIONS We have shown that silicene can be stably formed on the Al(111) substrate, taking not only the honeycomb structure but also a new regular monolayer structure called polygonal silicene. The structural, electronic, and vibrational properties of the honeycomb and polygonal silicene were investigated, and the properties of the latter are found to be quite different to those of the former, shedding light on new aspects of silicene. A notable finding is that charge transfer from the Al substrate to the silicene monolayer deposited on it is more substantial than from the Ag substrate. It appears that this substantial charge transfer enhances the metallicity of silicene on Al(111), compared to on Ag(111), enabling the silicene to easily transform to the polygonal structure. The present findings show that the type of substrate and the inherent flexibility of Si to overcoordination play a key role in characterizing the silicene layer that is formed. We believe these two factors render silicene a candidate with high potential for applications in a variety of nanoscale technologies. While the polygonal silicene was formed in a process like an AFM manipulation, it has been suggested from our preliminary calculations that impurity diffusion may also trigger the formation of the polygonal monolayer. Other monolayer structures, such as those reported in ref 35, may also be

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REFERENCES

(1) Kara, A.; Enriquez, H.; Seitsonen, A. P.; Lew Yan Voon, L. C.; Vizzini, S.; Aufray, B.; Oughaddou, H. A Review on Silicene - New Candidate for Electronics. Surf. Sci. Rep. 2012, 67, 1−18. (2) Okamoto, H.; Sugiyama, Y.; Nakano, H. Synthesis and Modification of Silicon Nanosheets and Other Silicon Nanomaterials. Chem. Eur. J. 2011, 17, 9864−9887. (3) Nakano, H.; Mitsuoka, T.; Harada, M.; Horibuchi, K.; Nozaki, H.; Takahashi, N.; Nonaka, T.; Seno, Y.; Nakamura, H. Soft Synthesis of Single-Crystal Silicon Monolayer Sheets. Angew. Chem. 2006, 118, 6451−6454. (4) Aufray, B.; Kara, A.; Vizzini, S. B.; Oughaddou, H.; Léandri, C.; Ealet, B.; Le Lay, G. Graphene-Like Silicon Nanoribbons on Ag(110): A Possible Formation of Silicene. Appl. Phys. Lett. 2010, 96, 183102. (5) Sugiyama, Y.; Okamoto, H.; Mitsuoka, T.; Morikawa, T.; Nakanishi, K.; Ohta, T.; Nakano, H. Synthesis and Optical Properties of Monolayer Organosilicon Nanosheets. J. Am. Chem. Soc. 2010, 132, 5946−5947. (6) Spencer, M. J. S.; Morishita, T.; Mikami, M.; Snook, I. K.; Sugiyama, Y.; Nakano, H. The Electronic and Structural Properties of Novel Organomodified Si Nanosheets. Phys. Chem. Chem. Phys. 2011, 13, 15418−15422.

22147

dx.doi.org/10.1021/jp4080898 | J. Phys. Chem. C 2013, 117, 22142−22148

The Journal of Physical Chemistry C

Article

(30) Lin, C. L.; Arafune, R.; Kawahara, K.; Tsukahara, N.; Minamitani, E.; Kim, Y.; Takagi, N.; Kawai, M. Structures of Silicene Grown on Ag(111). Appl. Phys. Exp. 2012, 5, 045802. (31) Mujica, A.; Rubio, A.; Muňoz, A.; Needs, R. J. High-Pressure Phases of Group-IV, III-V, and II-VI Compounds. Rev. Mod. Phys. 2003, 75, 863−912. (32) Morishita, T. High Density Amorphous Form and Polyamorphic Transformations of Silicon. Phys. Rev. Lett. 2004, 93, 055503. (33) Daisenberger, D.; Wilson, M.; McMillan, P. F.; Cabrera, R. Q.; Wilding, M. C.; Machon, D. High-Pressure X-Ray Scattering and Computer Simulation Studies of Density-Induced Polyamorphism in Silicon. Phys. Rev. B 2007, 75, 224118. (34) Morishita, T. Structural, Electronic, and Vibrational Properties of High-Density Amorphous Silicon: A First-Principles MolecularDynamics Study. J. Chem. Phys. 2009, 130, 194709. (35) Johnston, J. C.; Phippen, S.; Molinero, V. A Single-Component Silicon Quasicrystal. J. Phys. Chem. Lett. 2011, 2, 384−388. (36) Kamitakahara, W. A.; Soukoulis, C. M.; Shanks, H. R.; Buchenau, U.; Grest, G. S. Vibrational Spectrum of Amorphous Silicon: Experiment and Computer Simulation. Phys. Rev. B 1987, 36, 6539−6542.

(7) Takeda, K.; Shiraishi, K. Theoretical Possibility of Stage Corrugation in Si and Ge Anologs of Graphite. Phys. Rev. B 1994, 50, 14916−14922. (8) Guzmán-Verri, G. G.; Lew Yan Voon, L. C. Electronic Structure of Silicon-Based Nanostructures. Phys. Rev. B 2007, 76, 075131. (9) Morishita, T.; Nishio, K.; Mikami, M. Formation of Single- and Double-Layer Silicon in Slit Pores. Phys. Rev. B 2008, 77, 081401(R). (10) Lebègue, S.; Eriksson, O. Electronic Structure of TwoDimensional Crystal from Ab Initio Theory. Phys. Rev. B 2009, 79, 115409. (11) Cahangirov, S.; Topsakal, M.; Aktürk, E.; Sahin, H.; Ciraci, S. Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium. Phys. Rev. Lett. 2009, 102, 236804. (12) Morishita, T.; Russo, S. P.; Snook, I. K.; Spencer, M. J. S.; Nishio, K.; Mikami, M. First-Principles Study of Structural and Electronic Properties of Ultrathin Silicon Nanosheets. Phys. Rev. B 2010, 82, 045419. (13) Morishita, T.; Spencer, M. J. S.; Russo, S. P.; Snook, I. K.; Mikami, M. Surface Reconstruction of Ultrathin Silicon Nanosheets. Chem. Phys. Lett. 2011, 506, 221−225. (14) Spencer, M. J. S.; Morishita, T.; Snook, I. K. Reconstruction and Electronic Properties of Silicon Nanosheets as a Function of Thickness. Nanoscale 2012, 4, 2906−2913. (15) Vogt, P.; De Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Le Lay, G. Silicene: Compelling Experimental Evidence for Graphene-Like Two-Dimensional Silicon. Phys. Rev. Lett. 2012, 108, 155501. (16) Feng, B.; Ding, Z.; Meng, S.; Yao, Y.; He, X.; Cheng, P.; Chen, L.; Wu, K. Evidence of Silicene in Honeycomb Structures of Silicon on Al(111). Nano Lett. 2012, 12, 3507−3511. (17) Jamgotchian, H.; Colignon, Y.; Hamzaoui, N.; Ealet, B.; Hoarau, J. Y.; Aufray, B.; Bibérian, J. P. Growth of Silicene Layers on Ag(111): Unexpected Effect of the Substrate Temperature. J. Phys.: Condens. Matter 2012, 24, 172001. (18) Lin, C. L.; Arafune, R.; Kawahara, K.; Kanno, M.; Tsukahara, N.; Minamitani, E.; Kim, Y.; Kawai, M.; Takagi, N. Substrate-Induced Symmetry Breaking in Silicene. Phys. Rev. Lett. 2013, 110, 076801. (19) Fleurence, A.; Friedlein, R.; Ozaki, T.; Kawai, H.; Wang, Y.; Yamada-Takamura, Y. Experimental Evidence for Epitaxial Silicene on Diboride Thin Films. Phys. Rev. Lett. 2012, 108, 245501. (20) Meng, L.; Wang, Y.; Zhang, L.; Du, S.; Wu, R.; Li, L.; Zhang, Y.; Li, G.; Zhano, H.; Hofer, W. A.; Gao, H.-J. Buckled Silicene Formation on Ir(111). Nano Lett. 2013, 13, 685−690. (21) Gao, J.; Zhao, J. Initial Geometries, Interaction Mechanism and High Stability of Silicene on Ag(111) Surface. Sci. Rep. 2012, 2, 861. (22) Kaltsas, D.; Tsetseris, L.; Dimoulas, A. Structural Evolution of Single-Layer Films during Deposition of Silicon on Silver: A FirstPrinciples Study. J. Phys.: Condens. Matter 2012, 24, 442001. (23) Kou, L.; Frauenheim, T.; Chen, C. Nanoscale Multilayer Transition-Metal Dichalcogenide Heterostructures: Band Gap Modulation by Interfacial Strain and Spontaneous Polarization. J. Phys. Chem. Lett. 2013, 4, 1730−1736. (24) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (26) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (27) Morishita, T.; Itoh, S. G.; Okumura, H.; Mikami, M. FreeEnergy Calculation via Mean-Force Dynamics Using a Logarithmic Energy Landscape. Phys. Rev. E 2012, 85, 066702. (28) Morishita, T.; Itoh, S. G.; Okumura, H.; Mikami, M. On-the-Fly Reconstruction of Free Energy Profiles Using Logarithmic MeanForce Dynamics. J. Comput. Chem. 2013, 34, 1375−1384. (29) Enriquez, H.; Vizzini, S.; Kara, A.; Lalmi, B.; Oughaddou, H. Silicene Structures on Silver Surfaces. J. Phys.: Condens. Matter 2012, 24, 314211. 22148

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