Silicene Oxides and Ag(111)

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Hybridization Effects Between Silicene/Silicene Oxides and Ag(111) Wei Wei, Ying Dai, and Baibiao Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07220 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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The Journal of Physical Chemistry

Hybridization Effects between Silicene/Silicene Oxides and Ag(111)

Wei Wei, Ying Dai,* and Baibiao Huang

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China *E-mail: [email protected], Tel: +86 531 88565569

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ABSTRACT: Hybridization with Ag(111) alters the buckling configuration of silicene with the formation of two half unit cells, in each of which the protruded silicon atoms are in the same sublattice. Such a buckling can enhance the rehybridization of silicene π and σ states, and these rehybridized states mix with silver orbitals arousing the linear band dispersion approximate 1 eV bellow the Fermi level at Γ-point. It is an indication that silicene on Ag(111) loses its π characters as in the free-stranding silicene due to the symmetry breaking and strong hybridization with Ag(111), while the σ states are mildly modulated. Oxygen atom favors a Si−O−Si bond overbridging two neighboring silicon atoms and averages the height of silicon atoms with respect to Ag(111). As the degree of oxidation increases, silicene loses the two-dimensional continuity and, however, the binding energy between silicene oxides and Ag(111) increases. Silicene on Ag(111) is readily to be extensively oxidized in a very lager scale of oxygen pressure and, as the temperature increases, deoxidation is hardly to occur. Hybridization between silicene oxides with Ag(111) preserves some features as in the case of bare silicene on Ag(111), i.e. the linear band dispersion about 1 eV bellow the Fermi level, thus providing new hints for the origin of the linear band dispersion and the nature of hybridization effects with Ag(111). Silicene oxide (Si18O1) peeled off from Ag(111) indicates a finite band gap of 0.16 eV, while the silicene oxides turn out to be metallic as the oxidation degree increases. 1. INTRODUCTION Silicene, a novel silicon allotrope analog of graphene, has attracted extensive attention due to its unique electronic and optical properties.1-4 In light of first-principles density 2


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functional theory (DFT) calculation, free-standing silicene monolayer was predicted to be stable in lightly-buckled hexagonal lattice with Si atoms arranged in a honeycomb structure.1 Although the buckling, due to the mixed sp2−sp3 hybridization, free-standing silicene has enough symmetry to preserve the linear band dispersion relation around the Fermi level at particular symmetry points (Dirac point), K and K′, in the Brillouin zone.2 As in graphene, consequently, electrons in silicene behave as massless Dirac fermions.5 Interestingly, silicene has extra virtues in comparison with graphene: silicene can be a promising candidate for quantum spin Hall effects (QSHE) due to the large spin-orbital coupling.6,7 In the presence of Rashba spin-orbit coupling and exchange field, silicene hosts a quantum anomalous Hall state; while a topological phase transition results in a valley-polarized quantum anomalous Hall state when the Rashba spin-orbit coupling is tuned.8 In addition, comparing with graphene, silicene is more compatible with current silicon-based device technologies. As already noted above, therefore, silicene holds the great promise in applications in such as electronics, spintronics as well as valleytronics, and spurs activities in research community of two-dimensional silicene.9-14 However, silicon does not has any solid phase analogous to graphite and thus silicene cannot be obtained by exfoliation approaches as in the case of graphene. Alternatively, epitaxial growth of silicene has been demonstrated on substrates such as Ag(111),15-21 Ir(111),22 as well as ZrB2(0001).2,23 In view of the immiscibility and the almost perfect commensurability of silicene and silver crystalline structures, Ag(111) was frequently chosen as the substrate for silicene growth. In terms of growth 3



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temperature, silicene on Ag(111) exhibits a variety of superstructures, such as 4×4,15-20 (2√3×2√3)R30°19,21 and (√13×√13)R13.9°18-20 with respect to the 1×1 Ag(111). It is worth noting that the linear band dispersion near the Fermi level is regarded as the signature of Dirac electrons for silicene on Ag(111). On one hand, angular-resolved photoelectron spectroscopy results suggest the existence of linear energy dispersion near Fermi level with a Fermi velocity of 1.3×106 ms-1 for silicene on 4×4 Ag(111);16 while a linear band dispersion near Fermi level with a Fermi velocity of 1.2×106 ms-1 has been illustrated for √3×√3 silicene on Ag(111) by observed interference patterns of scanning tunneling spectroscopy.4 On the other hand, no Landau-level sequences particular to the Dirac electrons have been found in scanning tunneling spectroscopy measurements is an strong indication of the absence of linear band dispersion for silicene on silver substrate.15 It is therefore highly desirable to verify the origin of linear band dispersion in silicene on Ag(111). It is of great interest that, for silicene, controlling structures and properties by surface functionalization for various electronic applications is straightforward since the silicon atom’s sp3 hybridization allows the silicene surface more active. It has been illustrated in an experiment that field-effect transistor can be realized in silicene on Ag(111), that do emphasize the importance of silicene functionalization.24 In recent, functionalization of silicene on Ag(111) has been demonstrated, such as hydrogenation,25-27 chlorination as well as oxidation.28,29 As reported, hydrogenation of silicene on Ag(111) announces the successful access of half-silicane, enabling the tunability of properties. In particular, dehydrogenation occurs at a moderate 4


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temperature, providing a feasible possibility for controllable hydrogen storage based on silicene.25-27 In addition, chlorination of silicene can also be an effective method to protect silicene or tune the electronic state of silicene for device applications.28 In the case of silicene oxidation on Ag(111), tunable band gap from semimetallic to semiconducting attributed to the adsorption of oxygen atoms was demonstrated.29 Oxidation of silicene can be regarded as not only a strategy to realize various electronic structures in silicene, but also a possibility for exploring the silicene oxide that can be readily used in silicene-based electronic devices. Indeed, oxidation of silicene is an important step towards introduction of oxygenated functional groups onto silicene monolayer for application purpose. However, there still some details are in urgent need to complete the picture of silicene oxidation on Ag(111), for example, the configuration of silicene oxides and oxidation thermodynamics. In particular, hybridization with Ag(111) should play a crucial role in modulating the electronic structures of silicene oxides, as in the case of pristine silicene on Ag(111). It is consequently of importance to unravel the structural and electronic properties of oxidized silicene on Ag(111) with the interfacing interactions taken into account to provide a clear description of silicene functionalization on substrates. In this work, the 4×4 silicene/Ag(111) superstructure has been employed, since it is the silicene phase with the simplest and best characterized atomic structure, to address the issues proposed above. We clarify the absence of linear band dispersion of silicene near Fermi level on Ag(111) due to the symmetry breaking and strong hybridization 5



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effects between silicene and Ag(111). In conjunction with DFT total energy calculations, ab initio atomistic thermodynamics method is adopted to evaluate the (p, T)-surface phase diagrams and determine the stability of a particular surface configuration as a function of the chemical potential of growth environment.30,31 On the basis of orbital-projected band structures, resultant electronic properties of the silicene oxides with the hybridization effects with Ag(111) taken into account are presented and discussed.

2. COMPUTATIONAL DETAILS The first-principles DFT calculations were performed using the projector augmented wave method, as implemented in the plane-wave basis code Vienna ab initio simulation package.32,33 In respect to the exchange-correlation functional, the Perdew, Burke and Ernzerhof34 in the framework of generalized gradient approximation35 was employed. A cutoff energy of 470 eV for the plane-wave expansion of wave functions was adopted and the conjugate-gradient algorithm was used to fully relax the lattice parameters and atomic positions (up to < 0.02 eV/Å). In the integration over the first Brillouin zone the Monkhorst and Pack scheme of k-point sampling was performed:36 a 17×17×1 surface grid was used to relax the two-dimensional hexagonal unit cells and obtain the lattice constants. A slab model was used to simulate the 4×4 silicene/Ag(111) system, while a 5×5×1 surface grid was used for the geometry optimization and total energy calculation. In this slab model, bottom two Ag layers were fixed as their bulk parameters while the upper two Ag layers and silicene were 6


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fully relaxed. In order to eliminate the spurious interaction between periodic imagines, a vacuum space of larger than 23 Å was added. We confirmed that including van der Waals interaction leads to negligible differences with respect to the total energy and electronic properties due to the covalent binding of silicene and Ag(111), as also stated in previous works about silicene on Ag(111).15,37 As a result, we presented the following results without considering the semiempirical corrections. As can be seen later, our calculations reproduce the results of previous works that confirms the reliability of our models.

3. RESULTS AND DISCUSSION As indicated in Figure 1(a), free-standing silicene manifests itself with two sublattices buckling alternatively upward and downward. In a 4×4 silicene/Ag(111) structure, 3×3 silicene supercell is commensurate with the 4×4 Ag(111) with a lattice mismatch below 0.3% and slight biaxial strain is induced to silicene therefore. As shown in Figure 1(b), symmetry of silicene on Ag(111) is strongly destroyed with a distinctive buckled form: six silicon atoms shift upward being the top layer and the rest twelve silicon atoms shift downward to make up a bottom layer. It is in good agreement with the observations in scanning tunneling microscope measurements. In Figure 1, the dashed lines divided the silicene unit cell to two parts and each one can be referred to as half unit cell. In light of the interaction with Ag(111), top-layer silicon atoms in two half unit cells belong to different sublattices. In consequence of the inequivalence between two sublattices, the linear band dispersion should be absent. 7



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In prior to discuss the band structure, band folding should be clarified. In Figure 2(a), surface Brillouin zones for 1×1 Ag(111), 1×1 silicene as well as 4×4 silicene/Ag(111) are compared. As can be seen, the K-point of 3×3 silicene is folded onto the Γ-point of 1×1 silicene and therefore the Dirac point appears at the Γ-point of 3×3 silicene, see the band structure shown in Figure 2(b). In Figure 2(b), silicon pz orbital spreads over an energy range larger than 2 eV, while the px,y orbital is mainly localized 1.25 eV below the Fermi level in valence band. In addition, it shows in conduction band about 1.25 eV above the Fermi level the hybridization (or rehybridization) between the π and σ states. In respect to the origin of experimentally observed linear band dispersion, a great debate exists.37-42 We present in Figure 2(c) the band structure of 4×4 silicene/Ag(111) system, which is drawn in different colors denoting the contributions weight of silicon and silver atoms. At the first glance, we can see the strong hybridization between silicene adlayer and Ag(111), and a linear band dispersion can be traced at the Γ-point about 1 eV below the Fermi level. In previous works,15,16 this linear band dispersion is assigned to the π states of silicene. In comparison with the band structure of free-standing silicene shown in Figure 2(b), nevertheless, such a linear band could be probably attributed to the σ states of silicene. Indeed, it has been illustrated from DFT Kohn-Sham orbitals that this linear band is from rehybridized π and σ states of silicene, which are mixed with silver states in silicene/Ag(111) structure.37 The energy splitting between bonding and anti-bonding states is approximate 2 eV. It can be unambiguously noticed from the band structure of 4×4 silicene/Ag(111) that the silicene π symmetry loses its resemblance to that in 8


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free-standing silicene. However, the residual silicene π character is located in bands closely following the silver interface state that is described instead as free-electron-like silver state.41 In 4×4 silicene/Ag(111), we conclude that silicene loses its π character due to symmetry breaking and strong hybridization with Ag(111), while the σ states are mildly modulated. As demonstrated in Figure 2(d) of the projected band structure on silicene on Ag(111), silicene states exhibit identical dispersion as that of the total band structure, confirming the resonant behavior between silicon and silver states. It could be expected that structure buckling will enhance the rehybridization between silicene π and σ states, which is verified by the band structure of the peeled off silicene from Ag(111) presented in Figure 2(e). In accompanied with the enhanced rehybridization in valence band, band gap of 0.303 eV is introduced in the unsupported silicene due to the symmetry breaking. In this section, we emphasize that the linear band dispersion originates from strongly hybridized states of silver and silicene, and states from silicene is mainly composed of rehybridized π and σ states. Interfacing effects between silicene and Ag(111) makes silicene to lose its π symmetry and, in particular, makes silicene π and σ states rehybridized. In addition, we recalculated the band structures for 4×4 silicene/Ag(111) and peeled off silicene with spin-orbit coupling (SOC) taken into account. As in the case of silicene on Ir(111),30 however, the SOC shows negligible influence on the band structures. In previous theoretical works,13,15,37,41,42 the origin of this linear band dispersion has been discussed. However, the debate still exists. In spite of the consensus that silicene 9



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loses its Dirac fermion behavior due to the significant symmetry breaking, the origin of the experimentally observed linear band dispersion is ascribed to Ag(111), or to the new states from hybridized silicene and Ag(111). In addition, silicene states hybridized with Ag(111) are attributed to the π orbital. In the point of view of rehybridization of silicene π and σ states, we further learned the role the silicene σ states playing. As the silicene on Ag(111) is oxidized, the configurations, electronic properties as well as the hybridization feature between silicene and Ag(111) should be significantly changed. In addition, degree of oxidation of silicene should also play a crucial role. In order to determine the site to accommodate an oxygen atom on silicene, four positions were picked out as initial adsorption sites: above a top-layer silicon atom (T1), above a bottom-layer silicon atom (T2), a bridging site over two neighboring silicon atoms (B) and a hollow position over silicon hexagon (H). After geometry relaxation, the oxygen atom prefers a site overbridging two neighboring silicon atoms, giving rise to a Si−O−Si epoxy bonding. In Figure 3, we present the structure evolution of silicene as it is oxidized. As shown in Figure 3(a), a Si18O1 group forms with one Si−O−Si bond. Introduction of an oxygen atom on silicene leads to relatively small configuration distortion; the top-layer silicon atom decreases in height while the bottom-layer one increases in height with respect to Ag(111). However, Si18O1 on Ag(111) basically retains the two-dimensionality. As the oxygen concentration increases, all oxygen adatoms prefer to form the Si−O−Si bonds and silicene oxides tend to lose the two-dimensional 10


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continuity, as indicated in Figures 3(b)-3(e). As revealed in Figure 3(e) showing a Si18O5 group on Ag(111), for example, six-atom hexagons in silicene are seriously damaged. In a previous work, scanning tunneling microscope and corresponding fast Fourier transform patterns for 2√3×2√3 silicene/Ag(111) were presented.29 It identified that silicene oxides retained the hexagonal honeycomb structure when exposed to a low oxygen dose, while a typical amorphous feature was observed as the oxygen dose was increased implying the destroy of silicon-atom hexagons. In this work, results are in agreement with the experimental observation. In Figure 4, we present silicene oxides peeled off from Ag(111) without and with full relaxation, from which we can see the effects of Ag(111) on the structural properties of silicene oxides. In order to check the stability for silicene oxides on Ag(111), binding energy per silicon atom is defined as following =

1 − + 18

where is the total energy of the whole structure, while and are the total energies of silicene oxides peeled off from Ag(111) and the Ag(111) substrate, respectively. The denominator 18 corresponds to the total number of silicon atoms in the 4×4 silicene/Ag(111) system. As summarized in Figure 5, bare silicene on Ag(111) gives rise to binding energy of −0.58 eV per silicon atom, supporting the strong coupling with Ag(111). As an oxygen atom adsorbed, binding energy between silicene oxide and Ag(111) increases to −0.61 eV. In Si−O−Si group, covalent bonds form between silicon and oxygen atoms, which is confirmed by the inset in Figure 5. On account of the Si−O−Si bond, bonding within silicon atoms is reduced and interaction 11



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with Ag(111) could be enhanced, which explains the decreased height of silicon atom with respect to Ag(111). It refers to the height of the top-layer silicon atom bonded to the oxygen atom, which decreases by approximate 0.5 Å. As the degree of oxidation improving, more silicon atoms shift toward Ag (111) and the binding energy decreases monotonously. In silicene oxidation process, reaction can be described as follows silicene/Ag(111) + O → O, silicene/Ag(111)#; = 1,2,3,4,5 In order to evaluate the stability of oxidized surfaces, we employed ab initio atomistic thermodynamics approach in combination with DFT total energy calculations to acquire the adsorption free energy ∆*+,- . When silicene/Ag(111) system is in equilibrium with oxygen atmosphere, characterized by pressure p and temperature T, the corresponding adsorption free energy can be defined as ∆*+,- = ∆*+,- (., /, O) = (-)

1 (-) ( ) (-) {* − *-34565/ + ∙ 89 (., p)#} 0 (2 ,-34565/ ) (-)

with A is the surface area; *(2 ,-34565/ ) and *-34565/ as the Gibbs free energies of the oxidized silicene/Ag(111) structure and the clean silicene/Ag(111), respectively; ( )

and 89 (., p) is the chemical potential of an isolated oxygen atom in gas phase. The Gibbs free energy for solid phase can be written as * (-) = + ?@ + AB where and ?@ are the configuration free energy and vibration free energy, respectively; PV is the volume work. It has been identified in previous works that the last three terms in above equation contribute very small to the Gibbs free energy and we consequently 12


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replaced the Gibbs free energies by DFT total energies for our systems.43,44 At high temperature, 4=6> and ?@ may contribute more to the Gibbs free energy. In accordance with the equation for adsorption free energy, however, the extra contributions from 4=6> and ?@ can be canceled. It the case of silicene oxidation on Ag(111), chemical potential of oxygen as a function of temperature T and pressure p can be defined as ( )

8 ., / C = ( )

1 ( ) / 8 C ., / D C + EF . ∙ ln G D H# 2 /

with 8 C ., / D C being the chemical potential of molecular O2 in gas phase and EF is the Boltzmann constant. At standard pressure p0, chemical potential of O2 reads ( )

( )

8 C ., / D C = 8 C (0 K) + K., / D C − K0 K, / D C − .L(., / D C ) and at 0 K it can be defined as ( )

C 8 C (0 K) = 53C + MNO

C the zero-point vibration energy and 53C was calculated to be −8.58 where is MNO

eV. Here, we used experimental values from the JANAF thermodynamics table for all the entropy contributions to the standard chemical potential.45 As shown in Figure 6 the adsorption free energy for silicene oxidation on Ag(111) as a function of oxygen chemical potential, a distinguished feature is that silicene is readily to be oxidized on Ag(111). In a large range of oxygen chemical potential, adsorption free energies for silicene oxides are negative, indicating that oxidation of silicene with different oxygen concentrations is feasible and the resultant oxidized surfaces are stable. It is of interest that surface of silicene/Ag(111) turns out to be more stable as oxidation degree increases. As the oxygen chemical potential drops to 13



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be smaller than −7.5 eV, corresponding to oxygen poor condition, oxidation of silicene on Ag(111) becomes to be difficult with the adsorption free energies being positive. In order to enable a direct comparison with experiments, oxygen chemical potential is converted to a pressure scale at certain temperature. In the case of 500 K, silicene on Ag(111) prefers higher degree of oxidation in a very large scale of pressure from Q

−48 to 4 (A = ln PQRS), corresponding to the oxygen chemical potential range from −7.17 eV to −4.59 eV. As the temperature increases to 700 K, the conclusion received above holds. It is also an indication that silicene on Ag(111) is easy to be oxidized even with low oxygen pressure. In has been demonstrated that dehydrogenation effects of silicene on Ag(111) occur when the temperature increases.26,27 However, deoxidation is hardly to take place for silicene even at very high temperature according to our surface phase diagrams. It spite of the absence of experimental results about the oxidation at different temperatures, the fact that silicene on Ag(111) is easily to be oxidized is experimentally supported.29,46 As discussed above, symmetry and electronic properties of silicene are significantly altered by the hybridization effects with Ag(111). It is therefore can be expected that the resultant configurations of silicene oxides should be modulated by Ag(111) and the nature of orbital hybridization effects will be different as in the case of bare silicene on Ag(111). In comparison to the band structure of silicene/Ag(111), see Figure 2(c), the hybridization between silicene oxide and Ag(111) near Fermi level is reduced, as shown in Figure 7(a) of the band structure of oxidized silicene/Ag(111). It 14


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is worth noting that orbitals of silicene oxide appear to contribute at −1.2 and −1.75 eV at the Γ-point and, interestingly, a linear band dispersion primarily contributed from silver orbital appears about 1 eV bellow the Fermi level, as in the case of bare silicene on Ag(111). It is thus an indication that Ag(111) will present a detectable linear band dispersion as adsorbate is placed thereon. In the present work, we have identified that the linear band dispersion probably originates from the mixed states of silicene and Ag(111). In accordance to our results, however, such a silver derived linear band dispersion can also be a result of the formation of silicene oxides on Ag(111). As a consequence, further studies in respect to the hybridization effects between silicene and Ag(111) are still in need since silicene oxides also causes a linear band dispersion. As shown in Figure 7(b), the band structure projected on silicene oxide (Si18O1) indicates small contribution to the states near Fermi level in energy range from −1 to 1 eV. As the silicene oxide is peeled off from Ag(111), its band structure demonstrates large effective mass and a band gap of 0.16 eV due to the significant symmetry damage, as revealed in Figure 7(c). As comparing Figures 7(b) with 7(c), we can notice the hybridization effects between silicene oxide and Ag(111) that shift downward the states of silicene oxide and change the band dispersion relation. It is indicative that the hybridization effects between silicene oxide and Ag(111) are not merely charge transfer and silver d orbital probably participates the hybridization with silicene. In a previous work,46 a randomly oxidized silicene structure on Ag(111) was theoretically discussed. It also demonstrated the formation of Si−O−Si bonds and the easiness of oxidation of silicene on Ag(111). 15



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As shown in Figures 7(d) and 7(e) of the band structures of Si18O2 group on Ag(111), similar results are obtained as the oxidation degree increases, namely, the linear band dispersion about 1 eV bellow the Fermi level. However, the peeled off Si18O2 gives rise to a metallic band structure, as shown in Figure 7(f). In comparison to the band structure of Si18O1, the Fermi level seems to shift downward in Si18O2. As the oxidation degree increases further, metallic band structures reserves when the silicene oxides are peeled off from Ag(111). In experiment in regard to the oxidation of silicene superstructures on silver substrate, scanning tunneling spectroscopy measurements illustrated the band gap opening in silicene due to the adsorption of oxygen atoms.29 However, effects of Ag(111) substrate on the electronic properties were not taken into account and the band gap opening was assigned solely to the isolated silicene oxides.

4. CONCLUSION In the present work, we have found that hybridization effects with Ag(111) will change the buckling configuration of silicene with the formation of two half unit cells, in each of which three silicon atoms in the same sublattice turn out to be the bright spots in the scanning tunneling microscope images. In silicene, π and σ states are rehybridized and this rehybridization will be enhanced in the buckled silicene peeled off from Ag(111). In essence, the rehybridized states will be mixed with silver states to contribute a linear band dispersion approximate 1 eV bellow the Fermi level at Γ-point in silicene/Ag(111). In the superstructure of 4×4 silicene/Ag(111), π orbital 16


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loses its linearity near the Fermi level due to symmetry breaking and strong hybridization with Ag(111); however, the σ states are mildly modulated. As the silicene on Ag(111) is oxidized, oxygen atom is in favor of a Si−O−Si epoxy bonding with the oxygen atom overbridging two neighboring silicon atoms and averaging the height of silicon atoms with respect to Ag(111). As the oxidation degree increases, silicene loses its two-dimensional continuity. However, the binding energy between silicene oxides and Ag(111) increases as the oxidation degree increases. In conjunction with DFT total energies, ab initio atomistic thermodynamics has been adopted to obtain the adsorption free energy of oxygen atoms, which was then converted to pressure scale at certain temperature. It has been identified that silicene on Ag(111) is readily to be oxidized and prefers higher oxidation degree even at extremely low oxygen pressure. As temperature increases, deoxidation seems not to occur. It is of interest that some features are preserved in oxidized silicene/Ag(111) as in the case of bare silicene on Ag(111), providing new aspects for the origin of linear band dispersion and the nature of hybridization with Ag(111). When the silicene oxide (Si18O1) is peeled off from Ag(111), it manifests a band gap of 0.16 eV, while turns out to be metallic as the oxidation degree increases.

ACKNOWLEDGEMENTS W. W. thanks Dr. Z. M. for help. This work is supported by the National Basic Research Program of China (973 program, 2013CB632401), the National Natural Science foundation of China (11404187, 11374190 and 21333006), the Taishan 17



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Scholar Program of Shandong Province and the Fundamental Research Funds of Shandong University.

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Figure Captions Figure 1. Top view of (a) free-standing 3×3 silicene cell and (b) 4×4 silicene/Ag(111) superstructure. The big (gray) and small (yellow and green) spheres represent silver and silicon atoms, respectively. Top-layer silicon atoms are indicated in green color and the dashed lines are drawn to show the half unit cells.

Figure 2. (a) Two-dimensional Brillouin zones for 1×1 Ag(111) (the largest one), 1×1 silicene (the middle one) as well as the 4×4 silicene/Ag(111) (the smallest one). Band structure of (b) free-standing 3×3 silicene, (c) 4×4 silicene/Ag(111) superstructure, (d) silicene supported on Ag(111) and (e) silicene peeled off from Ag(111), the horizontal dashed lines represent the Fermi level. In (b) and (e), px,y and pz orbitals of silicene are plotted in blue and red colors, respectively. In (c), the band structure is demonstrated in different colors changing from blue to red, denoting atomic contribution weight from silicon and silver atoms, respectively.

Figure 3. Top and side views of silicene oxide configurations with different oxidation degree on Ag(111). (a) Si18O1, (b) Si18O2, (c) Si18O3, (d) Si18O4 and (e) Si18O5. The red, yellow and gray spheres represent oxygen, silicon and silver atoms, respectively.

Figure 4. Configuration of silicene oxides peeled off from Ag(111) without relaxation: (a) Si18O1, (b) Si18O2, (c) Si18O3, (d) Si18O4 and (e) Si18O5, and those with full relaxation: (f) Si18O1, (g) Si18O2, (h) Si18O3, (i) Si18O4 and (j) Si18O5. The red (small) 24


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and yellow (big) spheres represent oxygen, silicon and silver atoms, respectively.

Figure 5. Binding energy of silicene oxide on Ag(111) as a function of oxygen number. The inset shows the charge density difference within a Si−O−Si bond.

Figure 6. Surface phase diagram for 4×4 silicene/Ag(111) superstructure with different oxidation degree indicating the adsorption free energy as a function of oxygen chemical potential. The dependence of adsorption free energies on oxygen chemical potential is converted to oxygen pressure scale at temperature of 500 and 700 K. The dashed line means the half of total energy of an oxygen molecule. The inset amplifies the crossing parts.

Figure 7. Band structures of oxidized silicene/Ag(111) with different oxidation degree (upper panel, Si18O1; bottom panel, Si18O2). (a), (d) Band structures of the total oxidized superstructures, (b), (e) Atom-projected band structures on silicene oxides on Ag(111). (c), (f) Band structures of peeled off Si18O1 and Si18O2 from Ag(111) without relaxation. The horizontal dashed lines represent the Fermi level.

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Figure 1

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Figure 2

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Figure 3

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Figure 5

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Figure 7

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Silicene Oxides and Ag(111)

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