Interaction between Post-Graphene Group-IV Honeycomb Monolayers

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Interaction between Post-Graphene Group-IV Honeycomb Monolayers and Metal Substrates: Implication for Synthesis and Structure Control Nan Gao, Hongsheng Liu, Si Zhou, Yizhen Bai, and Jijun Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00023 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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The Journal of Physical Chemistry

Interaction between Post-Graphene Group-IV Honeycomb Monolayers and Metal Substrates: Implication for Synthesis and Structure Control Nan Gao, Hongsheng Liu, Si Zhou,* Yizhen Bai, Jijun Zhao* Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China

Abstract Beyond graphene, other group IV monolayers with honeycomb lattice, including silicene, germanene and stanene, have attracted much attention due to their peculiar physical properties and potential applications in future electronic devices. However, since sp3 hybridization is more favorable than sp2 hybridization for Si, Ge and Sn, these group IV monolayers have to be stabilized by metal surfaces during epitaxial synthesis. Using systematical first-principles calculations, here we investigate the interactions between these monolayers and various metal surfaces, i.e., Ag(111), Ir(111), Pt(111), Al(111), Au(111), and Cu(111). STM images, charge density difference and partial density of states of these monolayer/metal systems have been calculated and discussed. Combining with the known experimental facts, we find that a moderate strength of interaction at 0.6~0.7 eV/atom is beneficial for the epitaxial growth of silicene and germanene without too much buckling or in-plane distortion. We further propose that Al(111) substrate might be a good choice for synthesis of stanene with low-buckled structure.

*

Corresponding authors. Email: [email protected] (S. Zhou), [email protected] (J. Zhao) 1

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1. INTRODUCTION Graphene, the honeycomb lattice of carbon atoms with unique electronic band structure (e.g., Dirac cone, ultrahigh Fermi velocity), holds great promise for future electronics1 and stimulates significant efforts to explore other two-dimensional (2D) materials with honeycomb lattice.2-4 As early as in 1994, the silicon and germanium analogues of graphene (namely, silicene and germanene) had already been theoretically explored by Takeda and Shiraishi.5 Unlike graphene in which the A and B sublattices are in exactly the same plane, the two sublattices in silicene or germanene are relatively shifted in the direction perpendicular to the basal plane, forming a low-buckled structure. Later in 2009, first-principles calculations by Cahangirov et al. confirmed that silicene and germanene with low-buckled structures are dynamically stable and still possess the unique Dirac cone in their electronic bands.6 Compared to graphene, stronger spin-orbit coupling (SOC) effect in the heavier group IV elements (Si, Ge, Sn) as well as buckled geometry in the monolayer lattice lead to more prominent quantum spin Hall effect (QSHE) in silicene, germanene and stanene (or tinene).7-9 However, experimental synthesis of silicene, germanene and stanene is a great challenge, since none of these group IV elements favors sp2 hybridization or possesses bulk allotrope with graphite-like layered structure in nature. To date, the main method for fabricating the monolayer films of these group IV elements is molecular beam epitaxy (MBE) growth on metal substrates. In 2012, Vogt et al.10 have epitaxially synthesized monolayer silicene sheet on Ag(111) substrate and observed a superstructure of (3×3) silicene lattice on (4×4) Ag(111) surface. In the same year, several other groups11-14 independently reported experimental synthesis of monolayer silicene on Ag(111) substrate with a variety of superstructures, such as 2


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(4×4), (√13×√13)R13.9º, and (2√3×2√3)R30º. In addition to Ag(111), silicene monolayer sheets have been also fabricated on several other substrates, including ZrB2(0001),15 ZrC(111)16 and Ir(111).17 Soon after the experimental success of silicene, fabrications of germanene on metal substrates have been reported by Gao’s group18 and Le Lay’s group,19 who observed a superstructure of 3×3 germanene in conjunction with √19×√19 Pt(111), and a superstructure of √3×√3 germanene and √7×√7 Au(111), respectively. More recently, germanene sheets were also fabricated on Al(111) surface,20-22 Sb(111) surface,23 Ge2Pt crystals,24 hexagonal AlN buffer layer on Ag(111) substrate,25 and MoS2 substrate,26 respectively. Finally, epitaxial growth of 2D stanene film on Bi2Te3(111) substrates by MBE method was also reported by Jia’s group.27 Despite the exciting progresses above, synthesis of large-scale and high-quality silicene, germanene and stanene remains a challenging issue. The as-prepared 2D sheets are usually of limited size (in the magnitude of µm)10, 12-15, 19-21, 24, 27 and contain certain amount of defects.28 To address this issue, interaction of group-IV monolayer sheets on a variety of metal substrates has been theoretically explored by extensive first-principles calculations,29-38 aiming to predict other possible growth substrates and to elucidate the influence of metal substrate on the electronic structure of group-IV monolayers. For instance, the superiority of Ag(111) substrate for MBE growth of silicene has been theoretically explained by the moderate and homogeneous interaction between silicene and Ag surface as well as the low diffusion barrier of silicon atom on Ag(111).29 Recently, Gao et al. proposed that Ag(111) surface is a good choice for the growth of large-scale monolayer stanene with honeycomb 3



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lattice structure at low temperature.37 To gain deeper and systematical insights into the interactions of group IV monolayers with metal substrates, herein we perform first-principles calculations on those experimentally synthesized systems as well as several hypothetic monolayer/metal superstructures. Except for very strong interactions as well as severe distortions in silicene/Ir(111) and germanene/Pt(111), the adsorption energies between silicene/germanene and most metal substrates in experiments are moderate and fall in a narrow range of 0.6~0.7 eV per atom. This finding provides important implication for selecting suitable metal substrates for the synthesis of stanene as well as other novel 2D materials.

2. COMPUTATIONAL METHODS Density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP) using the plane-wave basis set39 with an energy cutoff of 450 eV and the projector-augmented wave (PAW)40 potentials. Following previous studies,41, 42 we chose the optB86b-vdW functional43,

44

to describe the long-range dispersion interaction

between group IV monolayers and metal substrates. We adopted the experimental lattice constants for Ag (4.09 Å), Ir (3.84 Å), Pt (3.92 Å), Al (4.05 Å), Au (4.07 Å), Cu (3.61 Å) solids of fcc phase and then built three-layer slab models of (111) surface. During relaxation, the bottom layer of metal atoms in the slab model was fixed to simulate a semi-infinite metal substrate. From our DFT optimization, the equilibrium 2D lattice parameters of silicene, germanene and stanene were 3.87 Å, 4.06 Å, 4.67 Å, respectively. To avoid interaction between periodic images, a 15 Å vacuum space 4


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perpendicular to the slab model was added. The 2D Brillouin zones were sampled by uniform k-point meshes with spacing of about 0.03/Å. With the fixed supercell parameters, all model structures of group IV monolayer on metal substrate were fully relaxed by electronic and ionic degrees of freedom using thresholds of 10−4 eV for total energy and 0.02 eV/Å for force. The scanning tunneling microscopy (STM) images of these monolayer/metal superstructures were simulated using the Tersoff-Hamann approximation45 with a constant height of 2 Å above the uppermost atoms. To characterize the interaction strength between group-IV monolayer (or monomer) and metal substrate, we define the adsorption energy as: Eads = (Esub + Eup – Et )/ N

(1)

where Et is the total energy of hybrid system, Esub is the energy of metal substrate by slab model, Eup is the energy of group-IV monolayer (or monomer), and N is total number of atoms in the group-IV monolayer (or N = 1 for monomer). To compare the adsorption energies by various DFT methods with dispersion correction, we adopt the frequently observed (3×3) silicene phase with respect to (4×4) Ag(111) surface10-12 as a representative. Our results show that the PBE-D2 and PBE-D3 methods give identical adsorption energy of 0.73 eV/atom, which is only 0.05 eV/atom higher than that by optB86b-vdW (see Table S1 of Supporting Information). On the other hand, optPBE-vdW and optB88b-vdW yield moderately lower adsorption energy with regard to optB86b-vdW by 0.11~0.17 eV/atom. To our surprise, vdW-DF and vdW-DF2 underestimate adsorption energy by 69.1% and 48.5% respectively, as compared with optB86b-vdW. Since a previous DFT calculation41 demonstrated that optB86b-vdW functional agrees well with experiments on the 5



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atomic structures and electronic properties of silicene on MoS2 substrate, here we choose optB86b-vdW to describe the interaction between group IV monolayers and metal substrates.

3. RESULTS AND DISCUSSION We start from the adsorption of an individual atom of group-IV (Si, Ge, Sn) on various metal surfaces, including Ag(111), Au(111), Pt(111), Ir(111), Cu(111) and Al(111). All metal substrates are modelled by a 4×4 supercell to avoid the lateral interaction between the adatom and its replica. Four possible adsorption sites, i.e., top, bridge, hcp and fcc sites, are considered for the adatom. However, the adatom at top or bridge site is unstable after relaxation and would move to hcp or fcc site spontaneously. The adsorption energies of Si, Ge and Sn monomers on these metal substrates are plotted in Figure 1a (the detailed data are given by Table S2 of Supporting Information). The adsorption energies on the hcp and fcc sites are very close (differ by about 0.02~0.04 eV/atom). For most systems considered, the adsorption energies for a group-IV adatom on the metal surface range between 3.5 and 4 eV/atom. However, the adsorption energy in the case of Si/Ir(111) or Ge/Pt(111) is as large as 6 eV/atom, implying much stronger interaction between Si (Ge) atom and Ir (Pt) substrate. Mulliken charge analysis shows that pronounced charge transfer occurs from the Ir (Pt) substrate to the Si (Ge) adatom, i.e. 0.36 e (0.35 e), much larger than the values for the monomers on the other metal substrates (0.01~0.22 e) (see Table S2 of Supporting Information). Consequently, silicene on Ir(111) and germanene on Pt(111) may suffer from strong interactions from the substrates and be severely distorted from the original honeycomb lattice, as we will demonstrate in the successive discussions. 6


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To gain a systematical trend, we construct a variety of monolayer/metal superstructures. For silicene and germanene, we adopt the superstructures that have been reported in experiments;10,

11, 17-20, 46

for stanene, we build several hypothetic model structures with

reasonable lattice mismatch. The structural and energetic information of all these models are listed in Table 1. We use two parameters – the average in-plane deformation D and the buckling height h to characterize the structural variations of a group IV monolayer upon adsorption on the metal substrates. The buckling height h is given by the average vertical distance between the top and bottom sublayers of the honeycomb lattice. The in-plane deformation D is defined by

D = ∑ , − + − ⁄

(2)

where ( , ) and (xi, yi) are the coordinates of ith atom in the group IV monolayer of freestanding state and on metal substrate, respectively; N is total number of atoms in the monolayer. The STM images of all these monolayer/metal superstructures are also simulated and they show satisfactory agreement with the experimental ones,10,

14, 17-20

implying the

validity of both our structural models and computational methods (see Figure S1 and S2 of Supporting Information for detailed comparisons). Figure 1b shows the distribution of adsorption energies of group IV monolayers supported by various metal substrates. Overall speaking, the results for the monolayers coincide well with the trend of monomers illustrated by Figure 1a. For silicene (or germanene)/metal superstructures that have been obtained in experiments, the interlayer interactions fall into two ranges: (1) moderate interaction with adsorption energies of 0.6~0.7 eV/atom as for silicene/Ag(111) and germanene/Au(111) or Al(111); (2) strong interaction 7



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with adsorption energies of 1.3~1.8 eV/atom as for silicene/Ir(111) and germanene/Pt(111). Apparently, transition metal substrates with open d shell (Ir, Pt) are more reactive than the metals without d electron (Al) or with closed d shell (Ag, Au), resulting in stronger binding strength with the silicene or germanene sheet. Under such strong interactions, the group IV monolayers are severely distorted from the low-buckled honeycomb lattices of the freestanding sheets, with average in-plane deformation of 0.57 Å for silicene/Ir(111) and 0.42 Å for germanene/Pt(111), respectively. Also, these monolayers exhibit pronounced buckling height, i.e., 1.39 Å for silicene/Ir(111) and 1.43 Å for germanene/Pt(111), much larger than the values of freestanding sheets (0.44 Å for silicene and 0.69 Å for germanene, respectively, see Table S3 of Supporting Information). These structural changes are detrimental for retaining the superior physical properties of silicene and germanene. A previous study even suggested that Si (Ge) atoms on Pt(111) favor the formation of an ordered Pt-Si (Pt-Ge) surface alloy rather than a silicene (germanene) sheet.47 Therefore, transition metal substrates with open d shell such as Ir and Pt should be avoided for experimental synthesis of high-quality group IV 2D materials. On the other aspect, silicene on Ag(111) as well as germanene on Au(111) or Al(111) maintain their original low-buckled honeycomb lattice reasonably well. The average in-plane deformation is rather small (0.08~0.25 Å), and the buckling height is relatively closer to the values of freestanding monolayers: 0.78~1.09 Å for silicene and 0.56~1.28 Å for germanene, respectively. Therefore, the moderate interfacial interaction with Eads = 0.6~0.7 eV/atom may be favorable for the epitaxial growth of group IV monolayers on metal substrates without inducing severe structural deformation. 8


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The interaction between silicene/germanene and various metal substrates can be better visualized from their charge density differences. As depicted by Figure 2, electron accumulation is more pronounced in the interfacial region of silicene on Ir(111) as well as germanene on Pt(111) or Au(111) compared to the other superstructures. To gain further insight into the interaction mechanism, we calculate the local density of states (LDOS) of the silicene (or germanene)/metal superstructures, as displayed in Figure 3. All the superstructures show significant electron coupling between the group IV monolayers and the metal substrates. For instance, for (3×3) Si on (4×4) Ag, the 3p states of Si atoms coincide with the 4d states of Ag atoms at the vicinity of Fermi level, revealing p-d hybridization, which has been reported by previous theoretical calculations29, 31, 32, 48, 49 and also confirmed in experiments.50 The orbital hybridization is attributed to the unpaired electrons in p orbital of Al (3s23p1), the inner-shell d electrons of Ag (4d105s1) and Au (5d106s1), and the open-shell d electrons of Ir (5d76s2) and Pt (5d96s1), all of which are active to participate in chemical bonding with the adsorbed group IV monolayers. The interaction from the metal substrates helps stabilize the monolayers and is beneficial for their growth,29, 48 however, may also destroy the intrinsic band structures of these Dirac 2D materials.33, 38, 51 Compared to silicene and germanene that have been synthesized on various metallic and semiconducting substrates,10-26 stanene has only been prepared on Bi2Te3(111) surface.27 To explore the possible metal substrates for the growth of stanene, we build several supercell models of stanene on the (111) surface of Cu, Au, Ag, and Al crystals in the fcc phase. For each metal substrate, we consider two possible superstructures with reasonable lattice mismatch (see Table 1 and Figure 4). The stanene sheet adsorbed on Cu(111), Ag(111) or 9



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Au(111) surfaces becomes either flat with very small bulking height (h ≤ 0.03 Å) (referred to as “planar-type”) or highly buckled (h ≥ 1.75 Å) (referred to as “buckled-type”), compared to h = 0.85 Å for freestanding stanene.38 In the planar-type superstructures, the stanene lattice is highly stretched (lattice mismatch δ = 6.35~8.46%) and each Sn atom sits on the hollow site of the metal surfaces, resulting in a very small in-plane deformation of 0.03~0.16 Å with regard to the freestanding sheet. On the contrary, the Sn atoms in the buckled-type superstructures locate on various sites of the metal surfaces, showing large in-plane deformation of D = 0.48~0.74 Å. Both types of superstructures have large adsorption energies of 0.92~1.36 eV/atom, indicating strong interactions between stanene and the Cu, Ag or Au substrates. The distinct adsorption behaviors of stanene on Ag(111) and Au(111) surfaces compared to silicene (or germanene) arise from the relatively stronger metallicity of stanene. Cu, Ag and Au substrates with active d orbitals result in too strong binding with the stanene sheet. On the contrary, Al metal without d electron is less reactive and provides moderate interaction with stanene (Eads ~ 0.7 eV/atom). The 1×1 stanene on (√3×√3) Al(111) has a flat structure (h = 0.03 Å), while √3×√3 stanene on (3×3) Al(111) retains the low-buckled honeycomb structure with h = 0.71 Å, close to that of freestanding stanene. For both two superstructures, the Sn atoms always sit on the hollow sites of Al(111) surface, yielding almost perfect honeycomb lattice for the stanene sheet (D = 0.06~0.08 Å). Note that Al(111) was also predicted as a possible growth substrate for silicene36 and has been used for synthesizing germanene.20-22 Our results, consistent with these earlier experimental and theoretical studies, reveal the moderate reactivity of Al metal and its suitability for the preparation of group IV 2D 10


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materials. Again, the interactions between stanene and various metal substrates can be characterized by the charge density differences, as presented by Figure 2 and Figure S3. Less interfacial electron accumulation occurs for stanene on Al(111) than that on Cu(111), Ag(111) and Au(111), indicating the weaker interaction between stanene and Al surface. Moreover, LDOS in Figure 3 shows conspicuous orbital hybridization in the stanene/Al(111) superstructures, that is, the electronic states near the Fermi level are dominated by both 5p orbitals of Sn atoms and 3p states of Al atoms. It is known that stanene as well as silicene and germanene are all intrinsically buckled.2 Flat honeycomb lattice of silicene or germanene is dynamically unstable and has never been observed in experiments even on metal substrates (see Table 1). For stanene, our presented models suggest that the buckling height h can be modulated by the choice of substrate and the phase of superstructure. In particular, a new class of flat honeycomb lattice of stanene may be synthesized on the (111) surfaces of Cu, Ag, Au, and Al metals. The out-of-plane buckling has significant impact on the electronic band structure of stanene. Our calculations show that freestanding stanene has a SOC-induced band gap of 73.5 meV (Figure S4a of Supporting Information). The flat stanene sheet (with geometry taken from our stanene/metal superstructure without inclusion of metal substrate) exhibits metallic behaviors in the electronic band structures with bands crossing the Fermi level (Figure S4b of Supporting Information). The standalone low-buckled stanene sheet with geometry taken from the √3×√3 stanene/(3×3) Al(111) superstructure holds the main features of the band structure of the freestanding monolayer. The Dirac cone remains and the charge neutral point is shifted 11



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slightly below the Fermi level, corresponding to n-type doping (Figure S4c of Supporting Information). For the standalone high-buckled stanene sheets with geometry taken from stanene/metal superstructure, the bands become flat due to the severe buckling as well as large in-plane deformation, indicating the deterioration to the carrier mobility (Figure S4d of Supporting Information). Previous theoretical studies by Liu et al.7 emphasized that the low-buckled honeycomb structure of group-IV element possesses larger SOC-induced band gap and pronounced quantum spin Hall effect than the flat counterpart. Hence, it is desirable to fabricate low-buckled stanene in order to retain its fascinating electronic properties for future device applications, and Al(111) substrate is promising for the epitaxial growth of such high-quality stanene sheets. We simulate the STM images of stanene on Al(111) as displayed by Figure 4. For 1×1 stanene on (√3×√3) Al(111), the stanene sheet is almost planar and hence shows its entire honeycomb lattice in the STM image. The superstructure of √3×√3 stanene on (3×3) Al(111), on the other hand, has a low-buckled structure with the top sublayer Sn atoms located at nearly the same height above the Al surface. As a result, they give rise to light spots in a rhomboid pattern in STM. These simulated STM images would help recognize the stanene/Al(111) superstructures in future experiments. From the experimentally available silicene(germanene)/metal heterostructures, we extract the interaction range for the growth of group IV 2D materials on metals. For device applications, however, our suggested moderate strength of 0.6~0.7 eV per atom may be still too strong for the as-prepared monolayers to be peeled off from the metal substrates. Nevertheless, within this moderate interaction range, the group IV monolayers can retain most 12


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structural features of the freestanding sheets, and thus their original superior physical properties may not be severely destroyed by the metal substrates. A possible route to utilize these superstructures is by oxygen intercalation of bilayer sheets with preferential oxidation of the underneath layer to form an oxide buffer layer between metal substrate and top layer,52 which hence yields quasi-freestanding sheets. On the other aspect, it is possible to prepare group IV monolayers on semiconductor substrates, i.e. germanene on MoS2 substrate26, 42 and stanene on anion-terminated (111) surfaces of SrTe, PbTe, BaSe, and BaTe,53 respectively. Future experimental and theoretical studies are of high necessity for fabrication of high-quality group IV monolayers and make their device applications viable.

4. CONCLUSION We explore the interactions between group IV monolayers — silicene, germanene and stanene on the (111) surfaces of various fcc metals by considering a series of superstructures reported by experiments as well as hypothetic ones. The experimentally synthesized superstructures, i.e. silicene on Ag(111), germanene on Au(111) and Al(111), have moderate adsorption energies falling into a narrow range of 0.6~0.7 eV per Si/Ge atom; two exceptions — silicene on Ir(111) and germanene on Pt(111) show strong interactions and severe distortions in the honeycomb lattice. Using the criteria of moderate interaction, we propose to grow stanene on Al(111) surface, on which the stanene sheet can retain the low-buckled honeycomb structure with moderate adsorption energy of ~0.7 eV per Sn atom. Cu, Ag and Au substrates are less suitable as they provide stronger binding with stanene and induce large structural deformation in the honeycomb lattice. These theoretical results help uncover the 13



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interaction mechanism between group IV monolayers and metal substrates, and prescribe the fundamental principle for the choice of substrates for synthesizing these novel 2D materials.

Supporting Information The atomic structures and STM images of silicene on Ag(111) and Ir(111) (Figure S1) and germanene on Pt(111), Al(111) and Au(111) (Figure S2), which have been observed in experiments. The charge difference between stanene on Au/Ag/Cu/Al(111) surfaces (Figure S3). The electronic band structures stanene sheets of various situations (Figure S4). Adsorption energies of (3×3) silicene on (4×4) Ag(111) surface by different dispersion correction methods (Table S1). Adsorption energies and charge transfer of single adatoms (Si, Ge and Sn) on various metal (111) surfaces (Table S2). The structural information for honeycomb lattice of freestanding silicene/germanene/stanene (Table S3)

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11504041, 11574040), the China Postdoctoral Science Foundation (2015M570243, 2016T90216), the Fundamental Research Funds for the Central Universities of China (DUT16-LAB01), and the Supercomputing Center of Dalian University of Technology.

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Table 1. Lattice mismatch (δ), interlayer distance (d), average in-plane deformation (D), buckling height (h) and adsorption energy (Eads) for various group IV monolayers on metal substrates. Si/Ag (√7/2√3) represents √7×√7 silicene on 2√3×2√3 Ag(111) surface and so forth. Positive and negative δ values represent expansion and compression of the group IV monolayers, respectively. The interlayer distance is defined by the vertical distance between the metal surface and the bottom sublayer of the group IV lattice. Superstructure

δ (%)

d (Å)

D (Å)

h (Å)

Eads (eV/atom)

Si/Ag (√7/2√3)

−2.29

2.39

0.11

1.09

0.64

Si/Ag (√7/√13)

1.73

2.25

0.08

0.78

0.67

Si/Ag (3/4)

−0.44

2.27

0.08

0.91

0.68

Si/Ir (√3/√7)

6.67

1.92

0.57

1.39

1.79

Ge/Pt (3/√19)

−0.75

2.00

0.42

1.43

1.35

Ge/Al (2/3)

5.47

2.14

0.25

1.28

0.61

Ge/Au (√3/√7)

7.68

1.89

0.09

0.56

0.67

Sn/Cu (1/2)

8.46

2.35

0.16

0.01

1.36

Sn/Cu (2/√13)

−1.55

2.43

0.74

1.75

1.10

Sn/Au (1/√3)

6.35

2.47

0.03

0.01

1.15

Sn/Au (√7/√19)

1.55

2.58

0.48

1.82

1.04

Sn/Ag (1/√3)

6.68

2.52

0.10

0.00

1.01

Sn/Ag (√7/√19)

1.89

2.48

0.64

1.82

0.92

Sn/Al (1/√3)

5.84

2.64

0.08

0.03

0.68

Sn/Al (√3/3)

5.85

2.38

0.06

0.71

0.71

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Figure 1. (a) Adsorption energies of single atom on various metal substrates. Green and yellow columns indicate the hcp and fcc adsorption sites, respectively. (b) Adsorption energies of silicene, germanene and stanene monolayers on various metal substrates. Blue and red column represent the superstructures that have been fabricated in experiments and the hypothetic structural models, respectively. The range of moderate adsorption energy (0.6~0.7 eV/atom) is highlighted by the shadow region. Si/Ag (√7/2√3) represents √7×√7 silicene on 2√3×2√3 Ag(111) surface and so forth.

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Figure 2. Sideview plots of spatial distributions of charge density difference of six representative superstructures of group IV monolayers on metal substrates. Si/Ag (3/4) represents 3×3 silicene on 4×4 Ag(111) surface and so forth. Si, Ge, Sn, Ag, Au, Al, Ir and Pt atoms are shown in yellow, blue, grey, light blue, orange, violet, dark green and dark blue, respectively. The green and light purple colors represent the electron accumulation and depletion regions, respectively, with isosurface of 0.005 e/Å3.

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Figure 3. Local density of states (LDOS) of six representative superstructures of group IV monolayers on metal substrates (only the top layer of metal atoms). Si/Ag (3/4) represents 3×3 silicene on 4×4 Ag(111) surface and so forth. The Fermi level is set as zero and shown in dashed line.

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Figure 4. Atomic structures and simulated STM images of stanene on various metal substrates. The protrusive atoms are highlighted by red balls, and rhombus defines the unit cell. Sn/Cu (1/2) represents 1×1 stanene on 2×2 Cu(111) surface and so forth. Sn, Cu, Ag, Au, and Al atoms are shown in grey, green, light blue, orange, and violet, respectively.

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Ordered

Silicide:

Atomic

and

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Structure

of

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TOC Graphic

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Interaction between Post-Graphene Group-IV Honeycomb Monolayers

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