Silicene on Monolayer PtSe2: From Strong to Weak Binding via NH3

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Silicene on Monolayer PtSe2: From Strong to Weak Binding via NH3 Intercalation Shahid Sattar, Nirpendra Singh, and Udo Schwingenschlögl* Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ABSTRACT: We study the properties of silicene on monolayer PtSe2 by first-principles calculations and demonstrate a much stronger interlayer interaction than previously reported for silicene on other semiconducting substrates. This fact opens the possibility of a direct growth. A band gap of 165 meV results from inversion symmetry breaking and large spin-splittings in the valence and conduction bands from proximity to monolayer PtSe2 and its strong spin−orbit coupling. It is also shown that the interlayer interaction can be effectively reduced by intercalating NH3 molecules between silicene and monolayer PtSe2 without inducing charge transfer or defect states near the Fermi energy. A small NH3 diffusion barrier makes intercalation a viable experimental approach to control the interlayer interaction. KEYWORDS: silicene, platinum diselenide, heterostructure, binding energy, intercalation, NH3

I. INTRODUCTION Silicene is a two-dimensional material consisting of Si atoms arranged in a honeycomb lattice similar to graphene. It shows a linear electronic dispersion (π and π* bands forming a Dirac cone) at the corners of the hexagonal Brillouin zone (K points) and mixed sp2−sp3 hybridization due to a characteristic structural buckling.1 As the spin−orbit coupling (SOC) is much stronger than in graphene, a band gap of ∼2 meV is opened. The size of this band gap can be controlled electrically by applying a gate voltage in the out-of-plane direction.2,3 Silicene is proposed to host exotic properties such as the quantum spin Hall effect,4 quantum anomalous Hall effect,5 and valley polarized quantum anomalous Hall effect.6 The material can be prepared on different substrates (even with large lattice mismatch)7−11 but not in freestanding form. Search for additional substrates has lead to theoretical investigations on the interlayer interaction with the insulator h-BN,12 the semiconductor GaAs,13 and the metals Ca,14 Ag,15 and Ir.16 Because the transition metal dichalcogenide MoS2 has important advantages for exploiting and tuning the electronic properties of graphene,17 it also has been explored as a potential substrate for silicene, both experimentally18 and theoretically.19,20 However, a highly buckled structure without Dirac cone is realized in this case due to a large lattice mismatch. Despite many efforts, no substrate has been identified today that simultaneously offers small lattice mismatch, a reasonable band gap, and control of the interlayer interaction. Unlike monolayer MoS2 and many other transition metal dichalcogenides, monolayer PtSe2, which can be grown on various substrates,21−25 does not favor a 2H structure but rather a 1T structure26 in that each Pt atom is octahedrally © XXXX American Chemical Society

coordinated by six Se atoms (Pt−Se bond length of 2.52 Å). While monolayer MoS2 is metallic in the 1T structure,27 monolayer PtSe2 is found to be a semiconductor on Pt(111)21 and SiO2.22 Interestingly, the experimental lattice constant reported in ref 21 is 3.70 Å, which is close to that of silicene (3.86 Å), thus opening, in principle, the possibility of epitaxial growth. For this reason, in the present work, we employ firstprinciples calculations to study the properties of silicene on monolayer PtSe2. It turns out that monolayer PtSe2 is a promising substrate not only due to small lattice mismatch but also due to a much stronger interlayer interaction as compared to other semiconducting substrates (required for growing twodimensional silicene) and the size of the opened band gap. We also provide a route for reducing the interlayer interaction after growth by identifying NH3 as an effective intercalant that avoids charge transfer. In the following, we first discuss the structural and electronic properties of silicene on monolayer PtSe2 and then clarify the effects of NH3 intercalation.

II. COMPUTATIONAL METHODS We employ density functional theory (DFT) and the projectoraugmented plane wave method as implemented in the Vienna ab initio simulation package.28 For structural relaxation (lattice constants and atomic positions), we use the generalized gradient approximation of the exchange−correlation potential in the Perdew−Burke−Ernzerhof (PBE) parametrization. The band structure is calculated by the HSE0629 functional before intercalation and by the PBE functional after intercalation. The underestimation of the band gap of PtSe2 in the latter case is not critical for our conclusions, as only silicene states Received: November 13, 2017 Accepted: January 5, 2018

A

DOI: 10.1021/acsami.7b17304 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a, c, e) Top and (b, d, f) side views of silicene on monolayer PtSe2 for different lateral positions. The Si, Pt, and Se atoms are shown in blue, gray, and green, respectively. Configuration (a, b) has minimal energy. appear in the vicinity of the Fermi energy, and thus determine the properties discussed in the following. We choose the optB86b-vdW density functional30,31 to describe the long range van der Waals interaction because it provides agreement with the experimental results for silicene on MoS2.32 The DFT-D3 method33 is employed to draw a comparison for the binding energies and interlayer distances. The plane wave cutoff energy is set to 500 eV. We ensure an energy convergence of 10−6 eV in the iterative solution of the Kohn−Sham equations and a force convergence of 10−3 eV/Å in the structural relaxation. Because monolayer PtSe2 on Pt(111)21 and SiO222 reproduces the semiconducting nature of freestanding monolayer PtSe2 (whereas bulk PtSe2 is metallic), we can neglect the Pt(111) or SiO2 substrate in our model. For 1 × 1 silicene on monolayer PtSe2, a Γ-centered Monkhorst−Pack 24 × 24 × 1 k-mesh is used (16 × 16 × 1 for the structural relaxation) and for 2 × 2 silicene on monolayer PtSe2 with one intercalated NH3 molecule a Γ-centered Monkhorst−Pack 16 × 16 × 1 k-mesh (8 × 8 × 1 for the structural relaxation). Moreover, a 15 Å thick layer of vacuum is added to each heterostructure to exclude spurious interaction between periodic images in the out-of-plane direction, as three-dimensional periodic boundary conditions are applied. The NH3 diffusion barrier is calculated by means of the nudged elastic band method,34 with 10 images between the initial and final states.

where Esilicene+PtSe2, EPtSe2, and Esilicene are the total energies of the heterostructure and its components. We find Eb = −279 meV, which is more than double as compared to the values of the semiconducting substrates investigated previously (−120 meV on MoS220 and −126 meV on GaS,35 for example), and the obtained interlayer distance of 2.58 Å confirms an exceptionally strong binding. We note that on monolayer PtSe2 the buckling of silicene is increased from its pristine value of 0.46−0.69 Å (difference in z-coordinates of the lowermost and uppermost Si atoms). A strong interlayer interaction is also obtained by the DFT-D3 method, which gives Eb = −306 meV, with an interlayer distance of 2.54 Å. To evaluate whether silicene can grow on monolayer PtSe2, we calculate the cohesive energy per Si atom for bulk silicon, μSi − Esilicon/NSi, for freestanding silicene, μSi − Esilicene/NSi, and for silicene on monolayer PtSe2, μSi − (Esilicene+PtSe2 − EPtSe2)/NSi. In these relations, μSi is the chemical potential of Si (total energy of an isolated Si atom), Esilicon is the total energy of bulk silicon, and NSi is the number of Si atoms. The cohesive energy of freestanding silicene (4.77 eV) turns out to be lower than that of bulk silicon (5.40 eV), i.e., the bulk structure is favorable. The fact that we obtain for silicene on monolayer PtSe2 also a value of 5.40 eV indicates that the growth of silicene on monolayer PtSe2 is possible. Figure 2a shows the electronic band structure obtained for silicene on monolayer PtSe2, without considering the SOC. A remainder of the Dirac cone of silicene is evident at the K point. A broken inversion symmetry due to the presence of monolayer PtSe2 gives rise to a band gap of 165 meV, which is too large for a topological state to be induced by SOC. It amounts to more than twice the value reported for silicene−MoS2 heterobilayers (70 meV).20 To obtain insight into the interlayer interaction, we show partial densities of states in Figure 2b. Despite the large band gap of monolayer PtSe2 (1.74 eV), we find strong hybridization between the Si p states and the Pt d and Se p states at the band edges. The regions of almost linear dispersion (remainder of the Dirac cone) are also subject to this hybridization. Addressing now the effect of SOC, we show in Figure 2c the electronic band structure obtained when the SOC (which is of significant strength in monolayer PtSe2) is taken into account in the calculations. It turns out that the hybridization between silicene and monolayer PtSe2 results in SOC-induced spin-splittings at the edges of the valence (231 meV) and conduction (17 meV) bands, reducing the band gap

III. RESULTS AND DISCUSSION For 1 × 1 silicene on monolayer PtSe2, we obtain 3.80 Å as the optimized lattice constant, which falls between the individual optimized lattice constants of 3.75 and 3.86 Å. The minimal energy configuration is identified by considering six cases, in which the Si atoms are placed either on top of the Pt and upper Se atoms, on top of the Pt and lower Se atoms, or on top of the upper and lower Se atoms. Figure 1 shows top and side views of three of these cases. The other three cases are given by inversion of the buckling in silicene. After structural relaxation, the configuration of Figure 1a,b with the Si atoms located on top of the Pt and upper Se atoms turns out to have 86 meV lower energy than the second lowest configuration and 260 meV lower energy than the highest configuration. Because of this large energy advantage, only the configuration of Figure 1a,b can be realized experimentally and is, therefore, studied in the rest of this work. The binding energy per Si atom of silicene on monolayer PtSe2 is calculated as E b = (Esilicene + PtSe2 − E PtSe2 − Esilicene)/2

(1) B


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Figure 2. Silicene on monolayer PtSe2: (a) band structure without SOC and (b) corresponding partial densities of states (inset to clarify the orbital contributions in the vicinity of the band edges); (c) band structure with SOC.

to 36 meV. The fact that the contributions of monolayer PtSe2 are higher in the valence than in the conduction band, see the inset in Figure 2b, explains why the spin-splitting is more pronounced in the former than in the latter. We have recalculated Figure 2 using the lattice constant of monolayer PtSe2 for the heterostructure instead of the optimized lattice constant and find only minor differences, which shows that the inplane strain does not alter the electronic properties significantly. Strain, therefore, is not an efficient tool for tuning the interaction between silicene and monolayer PtSe2. The interlayer interaction between silicene and monolayer PtSe2 potentially can be reduced by introducing intercalants that enlarge the van der Waals gap between the two materials. Alkali metals, such as Li or Na, have been proposed to be suitable for this purpose.36 However, it turns out that silicene becomes electron doped due to charge transfer from the alkali metal atoms. To avoid this detrimental effect, we propose NH3 molecules as intercalant because NH3 intercalation has been demonstrated experimentally to be possible in transition metal dichalcogenides.37,38 The key advantage of NH3 is that it binds strongly to monolayer PtSe2 and that it is not expected to induce any charge transfer or defect states near the Fermi energy. In addition, due to its gaseous nature, the diffusion barrier is anticipated to be small. NH3 reacts with silicene only at elevated temperatures,39 in contrast to NO2 and CO2, for example. We first study different absorption sites for an NH3 molecule on monolayer PtSe2: on top of the Pt atom, on top of the upper Se atom, on top of the lower Se atom, and on top of the Pt−Se bond. Structural relaxation reveals that location on top of the lower Se atom is the favorable configuration with an absorption energy of −245 meV, which clearly exceeds the thermal fluctuations even at elevated temperatures and therefore indicates thermal stability. We next place silicene on top of this configuration (using a 2 × 2 × 1 supercell with one NH3 molecule) and search for the favorable lateral position, considering the same six configurations as described earlier in the case without NH3 intercalation. It turns out that the energy is minimized when a hollow site of silicene is located on top of the NH3 molecule. Top and side views of the final structure are shown in Figure 3. The binding energy per Si atom

Figure 3. Top and side views of silicene on monolayer PtSe2 after NH3 intercalation.

E b′ = (Esilicene + PtSe2 + NH3 − E PtSe2 + NH3 − Esilicene)/8

(2)

now is found to be −68 meV (−64 meV for the DFT-D3 method), which is a massive reduction as compared to direct contact between silicene and monolayer PtSe2. Moreover, the interlayer distance has grown from 2.58 to 4.77 Å and the buckling of silicene is reduced from 0.69 to 0.53 Å (i.e., close to the value of freestanding silicene, 0.46 Å). The electronic band structure in the case of NH3 intercalation is presented in Figure 4, showing linear dispersions of the π and π* bands with a band gap of 15 meV at the K point (no charge transfer). The partial densities of states in Figure 4b provide insight into the nature of the Dirac cone. Virtually, only silicene contributes in a broad energy range around the Fermi energy. SOC therefore results

Figure 4. (a) Silicene on monolayer PtSe2 after NH3 intercalation: (a) band structure without SOC and (b) corresponding partial densities of states; (c) band structure with SOC (inset to clarify the spin-splittings in the valence and conduction bands at the K point). C


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molecules. NH3 intercalation therefore becomes a viable experimental approach for tuning the interlayer interaction between silicene and monolayer PtSe2.

only in small spin-splittings of 4 meV (valence band) and 3 meV (conduction band), see Figure 4c and its inset. To evaluate charge redistribution effects on silicene, we calculate the charge density difference Δρ = ρsilicene + PtSe (+NH ) − ρPtSe (+NH ) − ρsilicene 2

3

2

3

IV. CONCLUSIONS In conclusion, we have demonstrated that silicene experiences a strong interlayer interaction on monolayer PtSe2. The binding energy (similar values being obtained for the optB86b-vdW and DFT-D3 approaches) turns out to be much higher than previously reported for silicene on other semiconducting substrates and the interlayer distance accordingly is much smaller. As a consequence, a substantial band gap of 165 meV is opened in silicene due to breaking of the inversion symmetry in the presence of monolayer PtSe2. In addition, significant spinsplittings appear in the valence and conduction bands because of SOC, which is transferred from monolayer PtSe2 to silicene by unexpectedly strong orbital hybridizations. To reduce the interlayer interaction between silicene and monolayer PtSe2 after growth, we have proposed NH3 molecules as potential intercalant, because we find that the electronic states of silicene are preserved in the vicinity of the Fermi energy and no charge transfer (doping of silicene) is induced. Effects of charge redistribution within the van der Waals gap turn out to be removed by NH3 intercalation, resulting in a heterostructure governed by a weak van der Waals interaction. A small diffusion barrier indicates that it is possible to intercalate NH3 molecules between silicene and monolayer PtSe2.

(3)

with ρsilicene+PtSe2(+NH3) referring to the heterostructure in the absence or presence of NH3, ρPtSe2(+NH3) to monolayer PtSe2 in the absence or presence of NH3, and ρsilicene to silicene. A significant charge redistribution appears in the case of direct contact between silicene and monolayer PtSe2, see Figure 5a.

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Figure 5. Charge density difference maps of silicene on monolayer PtSe2 (a) before and (b) after NH3 intercalation. Yellow and red represent charge accumulation and depletion, respectively. The isosurfaces refer to an isovalue of 1.5 × 10−3 electrons/bohr3.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +966(0) 544700080.

Distinct regions of charge accumulation and depletion (yellow and red, respectively) are observed, particularly within the van der Waals gap, reflecting a strong interlayer interaction. After NH3 intercalation, these regions vanish, see Figure 5b, because the interlayer interaction is much smaller. We also do not observe any other significant charge redistribution. Finally, the diffusion barrier of NH3 between silicene and monolayer PtSe2 is studied for the path from one minimum energy site to the next minimum energy site, see Figure 6a. According to Figure 6b, the diffusion barrier is 190 meV, with the transition state located on top of the Pt−Se bond. It is likely that this barrier is sufficiently low to make intercalation feasible and, at the same time, sufficiently high to control the amount of intercalated

ORCID

Shahid Sattar: 0000-0003-4409-0100 Nirpendra Singh: 0000-0001-8043-0403 Udo Schwingenschlögl: 0000-0003-4179-7231 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). Fruitful discussions with Vasudeo Babar and Hakkim Vovusha are gratefully acknowledged.

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REFERENCES

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Figure 6. (a) NH3 diffusion path and (b) barrier between silicene and monolayer PtSe2. D


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Silicene on Monolayer PtSe2: From Strong to Weak Binding via NH3

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