Single-Layer Tl2O: A Metal-Shrouded 2D Semiconductor with High

Aug 18, 2017 - The first metal-shrouded two-dimensional semiconductor, single-layer Tl2O, is discussed from first principles. It is thermally and dyna...
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Single-Layer Tl2O: A Metal-Shrouded 2D Semiconductor with High Electronic Mobility Yandong Ma, Agnieszka Kuc, and Thomas Heine* Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, 04103 Leipzig, Germany S Supporting Information *

the Tl2O surface and the (001) surface of the hexagonal metallic Tl bulk promises excellent semiconductor−electrode contacts. Stable, low-resistive contacts between 2DS and metals are critical for maximizing electronic injection.20,21 SL Tl2O shows a direct bandgap of 1.56 eV and charge carrier mobilities up to 4.3 × 103 cm2 V−1 s−1, similar to those in black phosphorus monolayers (1.0 × 104 cm2 V−1 s−1).22 Tl2O has been known since 1906,23 but its layered structure was not elucidated until 1971 by Sabrowsky,19 who also suggested three alternative synthesis routes. The material is predominantly crystallized in a layered structure with space group R3m ̅ (No. 166). However, the material reported in 1971 also contained two further polytypes and showed metallic electron transport.19 A structural model of layered Tl2O is shown in Figure S1, and the crystal structure of SL Tl2O is shown in Figure 1. Evidently, this material shares similar structural features with the previously known SL metalshrouded 2DCs.10−18 SL Tl2O adopts a hexagonal lattice with one O and two Tl atoms in each unit cell. The optimized lattice constants for bulk and SL are a = b = 3.551 Å (expt 3.516 Å19) and a = b = 3.589 Å, respectively. In SL Tl2O, one layer of O atoms is sandwiched between two layers of Tl atoms; i.e., its bare surfaces are terminated with Tl atoms. Central O atoms are octahedrally coordinated to six Tl atoms. In the Tl6O octahedron, the six O−Tl bonds are identical (2.568 Å). Micromechanical cleavage and liquid exfoliation are standard techniques to fabricate single layers from layered bulk materials if the cleavage energy is low,24,25 typically below 1 J m−2. Our calculated cleavage energy of SL Tl2O, ∼0.43 J m−2, is similar to those of graphite and MoS 2 (0.37 and 0.42 J m −2 , respectively26) and suggests that cleavage from the layered bulk is possible. To assess the stability of SL Tl2O, we first investigate its phonon spectrum (Figure 2a). All phonon branches are positive in the entire Brillouin zone, indicating a stable minimum at the potential energy surface. We further substantiate the thermal stability of SL Tl2O by simulated annealing in 3×3 supercells (at 300 and 500 K; Figures S2 and S3). We conclude that SL Tl2O is thermally stable. The band structure of SL Tl2O (Figure 2b) shows a semiconductor with a direct bandgap of 1.56 eV, similar in size to silicon (1.1−1.4 eV)27 and phosphorene (1.5 eV).28 Electronic orbital analysis matches chemical intuition, showing Tl 6p domination of the conduction band minimum (CBM), and a valence band maximum (VBM) formed by O 2p and Tl

ABSTRACT: The first metal-shrouded two-dimensional semiconductor, single-layer Tl2O, is discussed from first principles. It is thermally and dynamically stable, has a low cleavage energy calling for exfoliation from layered Tl2O bulk, and has a very small interface mismatch compared to (001) Tl metal. Single-layer Tl2O exhibits a direct bandgap of 1.56 eV and a very high charge carrier mobility of 4.3 × 103 cm2 V−1 s−1. The metal-shrouded 2D semiconductor promises interesting applications in 2D electronics. An intriguing layer-thickness-dependent direct-to-indirect bandgap transition is observed, and contrary to early literature, the bulk is also a semiconductor.

M

etal-shrouded two-dimensional crystals (2DCs) (M2X, M = metal and X = C, N; Figure 1) have aroused

Figure 1. (a) Top and (b) side views of a SL Tl2O. The solid lines in (a) denote the unit cell. Inset in (a) shows the Brillouin zone. Inset in (b) shows the Tl6O octahedral geometry.

tremendous interest in the past years, as their properties, including metallic conductivity and mechanical properties, offer promising applications as sensors, catalysts, and energy storage materials.1−9 These 2DCs have a trigonal structure with two layers of M atoms covering one layer of X atoms. Over the past few years, the family of metal-shrouded 2DCs has expanded rapidly; examples include MXenes2 (Ti2C,10 Nb2C, V2C,11,12 Zr2C, Hf2C, Ta2C,13 Y2C,14 Ti2N, V2N,15 Cr2N, Zr2N16) and alkaline earth sub-nitrides (Ca2N,1 Sr2N,17 Ba2N18). To date, all reported metal-shrouded 2DCs are metallic. We report a metal-shrouded 2DC that marks a notable exception: according to first-principles calculations, single-layer (SL) Tl2O is a semiconductor. While this finding is interesting by itself, this 2D semiconductor (2DS) also shows high carrier mobilities, and its fabrication appears to be possible by mechanical exfoliation from the experimentally available layered bulk,19 as its low cleavage energy is similar to those of graphene and MoS2. A very small difference in lattice constants between © 2017 American Chemical Society

Received: June 21, 2017 Published: August 18, 2017 11694

DOI: 10.1021/jacs.7b06296 J. Am. Chem. Soc. 2017, 139, 11694−11697

Communication

Journal of the American Chemical Society

Figure 2. (a) Harmonic phonon analysis of SL Tl2O. (b) Band structure of SL Tl2O. The Fermi level is shifted to the VBM.

6s orbitals. This is illustrated by the density of states (DOS) in Figure S4 and the partial charge density in Figure S5. The electronic structure reveals ionic bonding, where the Tl 6p electrons are donated to the oxygen atoms. For comparison, we have investigated the band structures of isoelectronic Group 13 SLs M2O, with M = Ga and In, both of which are unstable metals (Figures S6 and S7), probably as these metals prefer the 3+ oxidation state. To get deeper insight into the semiconducting properties of SL Tl2O, we analyze the electron localization function (ELF), Hirshfeld charges, crystal orbital Hamilton population (COHP), and Bader critical points. ELF shows two localization areas (Figure 3a): one is located around the Tl and the other around the O atoms. There is almost no electron localization between Tl and O atoms, reflecting ionic bonding, where Tl atoms donate electrons to O atoms. The resulting dianionic character of the O atoms is confirmed by a Hirshfeld charge of −0.43, similar to that of O2−, e.g., in BaO. These results are supported by the Bader analysis and the plots of the Laplacian of density (Figure 3b). Positive values of the Laplacian between Tl and O atoms indicate electron depletion, confirming the ionic character. No accumulation of electron density is observed between Tl atoms, suggesting no chemical bond between them. COHP for SL Tl2O (Figure S8) shows that, close to the VBM, the occupied orbitals between Tl and O atoms are antibonding, again confirming ionic bonding between the two elements. Elemental Tl can feature two stable oxidation states, Tl+ and Tl3+. Even though Tl+ can form stable compounds, Tl2O is prone to oxygen or water attacks, requiring inert conditions.19 Given the small lattice mismatch between the (001) surface of hcp Tl (metallic) and SL Tl2O, the material is expected to form excellent metal−semiconductor contacts. This is confirmed by the calculated Tl (001)−Tl2O interface (Figure S9), where we observe electron injection to the Tl 6p orbitals of Tl2O from the metal, and the corresponding projected DOS shows accumulation of Tl 6p electrons in the surface atoms of SL Tl2O facing the metal. We expect similar electron injection from other metals into the Tl 6p orbitals, facilitating the integration of SL Tl2O in electronic devices.

Figure 3. (a) Electron localization function (ELF); ELF = 1 (red) and 0 (blue) indicate accumulated and vanishing electron density, respectively. (b) Laplacian of electron density (Δρ) and Bader critical points (red, bond; green, ring). The green line in the middle scheme denotes the 2D cut plane for the ELF and Bader visualizations.

SL Tl2O is a stable semiconductor because it is stoichiometrically balanced; that is, no counterions are needed to compensate for excess charge. In contrast, the electrons in the previously reported metal-shrouded 2DCs need to be chargecompensated by ion intercalates. For example, for 2D alkaline earth sub-nitrides, each unit cell of M2X should have one excess electron from formal valence consideration, which causes their metallic character.1,17 MXenes are obtained from M2AX materials (A denotes Groups 13 or 14 elements) by removing their A−M bonds,29 which results in a redistribution of the M d states or “dangling bonds” and, hence, leads to metallic nature.9 SL Tl2O offers outstanding electronic transport properties. Table S1 lists its effective masses along the x and y directions (cf. Figure 1a), which are generally smaller than those in TMDCs30−32 and comparable to those in phosphorene.33,34 The low effective masses appear to be counterintuitive for a compound with strong ionic character. However, we note that the ionic interaction is mainly between Tl and O, thus along the z direction, whereas the potential in the Tl and O layers is only weakly varying, resulting in strong band dispersion with low effective masses in the 2DC plane. The carrier mobilities (Table 1) in the x direction are about 15 times higher than those of SL Table 1. Carrier Mobilities along the x and y Transport Directions in SL Tl2O

11695

carrier

μx (cm2 V−1 s−1)

μy (cm2 V−1 s−1)

electron hole

3.342 × 10 4.302 × 103

0.404 × 103 0.016 × 103

3

DOI: 10.1021/jacs.7b06296 J. Am. Chem. Soc. 2017, 139, 11694−11697

Communication

Journal of the American Chemical Society MoS2 (∼0.20 × 103 cm2 V−1 s−1).35 Similar to the case of phosphorene,22 the carrier mobility of SL Tl2O is strongly anisotropic in the 2DC plane: the electron mobility along the x direction is 8 times larger than that along the y direction, and the hole mobility along the x direction is 268 times the value along the y direction, which suggests strongly directiondependent conductivity. The variation of the electronic properties of SL Tl2O upon increasing layer thickness is summarized in Figures 4 and S10,

Methods. Density functional theory calculations were performed using the VASP code.36,37 Ion-electron interactions are described by projected augmented wave (PAW) approximation.38 Exchange and correlation interactions are treated with Perdew−Burke−Ernzerhof (PBE)39 functional. Correction for London dispersion is included using Grimme’s D3 approach40 for multilayer systems. For band structure calculations, we used Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional.41 Vacuum spacing is set to about 20 Å. Cutoff energy is set to 500 eV and the convergence threshold for residual force is 0.01 eV Å−1. Brillouin zone integration is carried out at 13×13×1 Monkhorst−Pack k-grids. Spin−orbit coupling (SOC) is included in electronic calculations. All results presented herein are based on HSE06+SOC level, unless otherwise stated. The PBE results are given in the Supporting Information for comparison. Bader analyses were performed using ADF-BAND with PBE functional, TZVP basis set, and scalar relativistic effects (ZORA).42 COHP was obtained using LOBSTER code 43 on the files obtained from VASP calculations. For more details on computational methods, see the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06296. Computational methods; MD simulation results; projected DOS; electronic properties of SL Ga2O and In2O; COHP results; electronic properties of Tl (001)−Tl2O interface; band structures of multiple-layer and bulk Tl2O; band structures of SL Tl2O under different strain; PBE results (PDF)

Figure 4. Band structures of (a) bilayer (2L), (b) trilayer (3L), and (c) bulk Tl2O. Fermi level is shifted to VBM. (d) Brillouin zone of bulk Tl2O.

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AUTHOR INFORMATION

Corresponding Author

*[email protected]

which show that Tl2O preserves its semiconducting behavior when going from SL to bulk. However, multiple-layer and bulk Tl2O display indirect bandgaps, similar to TMDCs.30−32 The detailed band dispersions near the band edges are plotted in Figure S11. The bandgap increases persistently from ∼1.0 to 1.56 eV when decreasing the layer thickness from bulk to SL. It should be noted that bulk Tl2O was found to be conductive in experiment, probably due to residual Tl3+ in the samples. Finally, we examine the strain effect on the electronic properties of SL Tl2O (Figure S12). For tensile strain, SL Tl2O bandgap increases by 50 meV for 1% strain and 140 meV for 3% strain, and preserves its direct character, while under compression it is pushed into an indirect-gap semiconductor, where the optical gaps are only slightly larger than their corresponding fundamental gaps (by 1−11 meV). In conclusion, we report a novel metal-shrouded 2DC, SL Tl2O, which is dynamically and thermally stable, and has low exfoliation energy from the bulk. Unlike other reported metalshrouded 2DCs, which are all metallic, SL Tl2O is a natural 2D semiconductor with direct bandgap of 1.56 eV and high carrier mobility of 4.3 × 103 cm2 V−1 s−1. Due to the metal-shrouded structure, SL Tl2O has the special potential to form lowresistance contacts with electrode materials. These properties suggest SL Tl2O as excellent candidate for high-performance electronic and optoelectronic applications and call for experimental investigations.

ORCID

Thomas Heine: 0000-0003-2379-6251 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft FLAG-ERA (HE 3543/27-1) and computer time at ZIH Dresden are gratefully acknowledged. We thank Prof. Udo Schwingenschlö gl and Dr. Vladimir Bačić for inspiring discussions. We thank Dr. M. Franchini for help on Bader calculations.

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REFERENCES

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DOI: 10.1021/jacs.7b06296 J. Am. Chem. Soc. 2017, 139, 11694−11697

Communication

Journal of the American Chemical Society

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DOI: 10.1021/jacs.7b06296 J. Am. Chem. Soc. 2017, 139, 11694−11697