Predicting Single-Layer Technetium Dichalcogenides (TcX2, X = S, Se

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Predicting Single-Layer Technetium Dichalcogenides (TcX2, X=S, Se) with Promising Applications in Photovoltaics and Photocatalysis Yalong Jiao, Liujiang Zhou, Fengxian Ma, Guoping Gao, Liangzhi Kou, John M. Bell, Stefano Sanvito, and Aijun Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12606 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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ACS Applied Materials & Interfaces

Predicting Single-Layer Technetium Dichalcogenides (TcX2, X=S, Se) with Promising Applications in Photovoltaics and Photocatalysis Yalong Jiao1, Liujiang Zhou2, Fengxian Ma1, Guoping Gao1, Liangzhi Kou1, John Bell1, Stefano Sanvito3 and Aijun Du1,* 1

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia 2

Bremen Center for Computational Materials Science, University of Bremen, Germany 3

School of Physics and CRANN, Trinity College, Dublin 2, Ireland

Abstract One of the least known compounds among transition metal dichalcogenides (TMDCs) is the layered triclinic technetium dichalcogenides (TcX2, X=S, Se). In this work we systematically study the structural, mechanical, electronic and optical properties of TcS2 and TcSe2 monolayers based on density functional theory (DFT). We find that TcS2 and TcSe2 can be easily exfoliated in a monolayer form because their formation and cleavage energy are analogous to those of other experimentally realized TMDCs monolayer. By using a hybrid DFT functional, the TcS2 and TcSe2 monolayer are calculated to be indirect semiconductors with band gaps of 1.91 and 1.69 eV, respectively. However, bilayer TcS2 exhibit directbandgap character and both TcS2 and TcSe2 monolayer can be tuned from semiconductor to metal under effective tensile/compressive strains. Calculations of visible light absorption indicate that 2D TcS2 and TcSe2 generally possess better capability of harvesting sunlight compared to single-layer MoS2 and ReSe2, implying their potential as excellent lightabsorbers. Most interestingly, we have discovered that the TcSe2 monolayer is an excellent photocatalyst for splitting water into hydrogen due to the perfect fit of band edge positions with respect to the water reduction and oxidation potentials. Our predictions expand the 2D family of TMDCs and the remarkable electronic/optical properties of monolayer TcS2 and TcSe2 will place them among the most promising 2D TMDCs for renewable energy application in the future.

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KEYWORDS: two-dimensional materials, technetium dichalcogenides, photovoltaics, photocatalysis, strain effect

Introduction Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are an important class of inorganic materials displaying excellent optical properties,1 possessing high mobility2 and exhibiting a wide range of electronic properties such as half-metallicity,3 and both semiconducting4 and superconducting states.5 Traditional TMDCs6 such as MoS2, adopt a sandwich-type structure (S-Mo-S) in which the metal atoms (Mo) are located in between two layers of chalcogen atoms (S). The in-plane atoms are bonded covalently, while the neighbouring sheets are weakly coupled via van der Waals forces. The typical TMDCs are considered to be promising for the next-generation flexible optoelectronics, photovoltaics, batteries and energy harvesting owning to their unique properties.7-9 Among the large family of TMDCs, the structural configurations of technetium and rhenium dichalcogenides10-13 are by no means the typical TMDCs. They have larger and asymmetrical unit cells, corrugated surfaces, highly anisotropic structures and metal-metal bonds, which are totally different from the traditional TMDCs. Since the lattice structure and symmetry are of vital importance in determining the fundamental properties of materials, these TMDCs with low symmetry could probably bring interesting physical or chemical properties of both scientific and technological importance. For example, single-layers ReS2 (space

group P1 ), which have been fabricated14 recently, are found to exhibit competitive

performance with large current on/off ratios and low subthreshold swings, demonstrating potential applications in large-scale 2D logic circuits. The investigation of the ReSe2 monolayer12 reveals it has a rich Raman spectrum with good signal strength, which even does not change as the layer thickness increases. Therefore, revealing the novel properties of 2D technetium and rhenium dichalcogenides would be of great significance for shedding new light on these structures. In this work, we theoretically explore the structural, mechanical, electronic and optical properties of the TcS2 and TcSe2 monolayers. The possibility of extracting TcS2 and TcSe2 monolayers from their bulk counterparts is estimated by calculating the mechanical


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cleavage energy, while their dynamical stability is evaluated by phonon spectra. The electronic properties of TcS2 and TcSe2 in bulk and monolayer forms are systematically studied at the level of general gradient approximation (GGA) and hybrid functional. In an attempt to search for potential candidates for photovoltaics,15-16 the optical properties of TcS2 and TcSe2 monolayer are evaluated by computing the light absorption spectrum and by making comparison with other excellent light absorbers. Furthermore, the band-edge positions of single-layer TcS2 and TcSe2 are examined and compared to the redox potentials of water for the purpose of estimating the possibility of water splitting,17-21 and the thermal properties of these technetium dichalcogenides are predicted via phonon frequencies. Moreover, thickness and strain effects on the band gap modulation and mechanical properties are investigated to tailor the electronic and optical properties of 2D TcS2 and TcSe2 for potential applications in novel electronic nanodevices.22 One important issue that cannot be neglected is the radioactive nature of Tc element that may hinder their realistic applications. However, a very recent work by Halter et al.23 demonstrates that the radioactive element uranium is able to be used as uranium-based catalyst to obtain the electrocatalytic hydrogen production from water. This is a direct experimental validation that one radiative element has the potential of energy production. Therefore, the potential applications in photovoltaics and photocatalysis for TcX2 in our work are not expected to be impossible.

Computational Details First-principles calculations were performed based on the density functional theory (DFT) using the plane-wave VASP code.24-25 The generalized gradient approximation in the Perdew, Burke, and Ernzerhof form (GGA-PBE) was used as exchange correlation functional26 for the calculations of the geometries and band structures. Hybrid density functional theory based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional27-28 was adopted to calculate the optical absorption spectra as well as to correct the well-known underestimation of band gap in the PBE calculations. A damped van der Waals correction was incorporated using Grimme’s scheme29 to better describe non-bonding interactions. The projector-augmented-wave (PAW) method30 was used to describe the electron−ion interaction and the plane-wave energy cutoff was set to 500 eV. In order to study 2D systems under the periodic boundary condition, a vacuum layer with a thickness at least 15 Å was set to minimize the spurious interaction between neighbouring layers. A



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Monkhorst−Pack k-points grid 31 of 5 x 5 x 5 and 5 x 5 x 1 was used for sampling in the first Brillouin zone during geometry optimizations of TcS2 (or TcSe2) bulk and monolayer, respectively. All the atoms have been fully relaxed until residual force and energy were converged to 0.005eV/ Å and 10-6eV, respectively. The reliable band alignments were derived from the HSE06 calculations that were used to predict the vacuum potential as a common reference and to align the average electrostatic potential in the unit cell to the vacuum potential. Lattice-dynamical calculations including phonon spectra and thermal properties were computed using the Phonopy code32 interfaced with density functional perturbation theory (DFPT)33 implemented in the VASP. The same method was also employed by the previous study regarding technetium dichalcogenides.11 For the phonon calculations, a 2 x 2 x 1 supercell was employed for TcS2 and TcSe2 along with more stringent convergence criteria on the forces (0.001 eV/ Å) and energy (10-8eV). For the calculations of thermal properties, the expression of the entropy was as follows: = − ln 1 − ,

ℏ,

−

1 ℏ, ℏ, , −1 +

Where T is the temperature of the system, ћ is the reduced Planck constant, kB is the Boltzmann constant and ћω(q,s) is the energy of a single phonon with angular frequency ω. The Helmholtz free energy was computed by the formula: 1 = ℏ, + ln[1 − ℏ#,/% & ] 2 ,

,

The heat capacity at constant volume was calculated by using the following expression: ℏ, * () = [ ] ,

[

ℏ,

ℏ,

− 1]*

Results and Discussions In contrast to traditional TMDCs such as 1T-TiS2 and 2H-MoS2, bulk Tc dichalcogenides, which have been synthesised experimentally,34-35 possess much different geometries. The

crystal structure of bulk TcS2 is triclinic with space group P1, adopting a distorted 1T ACS Paragon Plus Environment

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geometry as the stable phase. The TcS2 unit cell (Fig. 1a) only contains one “sandwich” and the octahedral Tc coordination units form Tc diamond-shaped chains (Fig. 1c) with Tc in the +4 charge state. These chains are derived from the metal-metal bonds and they reduce the symmetry of the monolayer crystal from the 1T structure to the distorted 1T phase.36-37 It should be noted that such distorted 1T structure is also different from that of the 1T’MoS238-39 which displays a rectangular P21/m space group. The corresponding cell parameters for TcS2 and TcSe2 are listed in the Table 1, where we can find the lattice constant a and b of the TcS2 unit cell to be 6.37 and 6.47 Å, which are almost twice as large as those of 2H-MoS2. When compared to the calculated bulk phase, the lattice constant for TcS2 monolayer (Fig. 1b and 1c) are relatively smaller while the angle α, β and γ are slightly larger. In addition to TcS2, the bulk TcSe2 is also a layered lattice with triclinic symmetry similar to TcS2, and the crystal data for the bulk and monolayer forms are presented in the table. In general, the calculated structural parameters are very close to the experimentally measured values. Moreover, the lattice parameters (a, b and c) for TcS2 and TcSe2 bulk in current work are relatively smaller than those in previous DFT calculations, especially a significant difference occurs on c parameter for TcS2. This is mainly due to the fact that the van der Waals forces are considered in our calculation, which is able to better describe the layer-layer interaction in materials compared with previous theoretical study using the standard DFT method.

Figure 1. (a) Crystal structure of TcS2, (b) side and (c) top views of TcS2 monolayer Table 1. Calculated structural parameters (space group, lattice constant a, b and c, angle α, β and γ) and formation energy (ΔE) of the TcS2 and TcSe2 bulk and monolayer using the DFT-PBE method



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along with previous standard DFT calculations (pre cal.)11 and corresponding experimental values (exp.)34-35

Chemical formulas

TcS2 bulk cal. (pre cal.)

TcS2 bulk exp.

TcS2 1-layer cal.

TcSe2 bulk cal. (pre cal.)

TcSe2 1-layer cal.

Space group a(Å)

P1 (P1)

P1

P1

(P1) P1

P1

6.37 (6.42)

6.371

6.348

6.62 (6.67)

6.593

b(Å)

6.47 (6.52)

6.464

6.461

6.75 (6.80)

6.726

c(Å)

6.62 (7.07)

6.654

N/A

6.96 (7.42)

N/A

α(degree)

62.88 (64.7)

62.94

62.96

63.85 (65.0)

62.02

β(degree)

103.67 (102.9)

103.61

103.50

103.56 (102.9)

103.92

γ(degree)

118.77 (118.9)

119.00

118.75

118.78 (118.8)

118.71

ΔE (meV)

N/A

N/A

76

N/A

87

We evaluate the possibility to obtain TcS2 and TcSe2 monolayers by using two computational strategies. The first method consists in calculating the formation energy, Ef, which determines the strength of the interlayer van der Waals interactions in the bulk Tc dichalcogenides. A low formation energy indicates the undemanding cleavage of the TcX2 monolayer from bulk crystals.40 The expression for Ef is defined as41

,- =

,*. ,0. − /*. /0.

where the E2d and n2d are the total energy and the number of atoms in the 2D unit cell, and the E3d and n3d are the corresponding energy and atom number for the 3D bulk. In our calculations, Ef for TcS2 and TcSe2 monolayers is calculated to be 76 and 87 meV, which is relatively smaller compared to that of ReSe2 (94 meV) (Supporting Information Table S1) and analogous to that of MoS2 (77 meV),41 demonstrating the ease of extracting singlelayers from the bulk form.


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Figure 2. (a) The cleavage energy Ecl in J/m2 (blue line) and the cleavage strength σ in GPa (red line) as a function of the separation distance d for a fracture TcS2 (solid line) and TcSe2 (dot line) bulk. Inset: Separating a monolayer from its neighboring trilayer. (b, c) Phonon spectra of 2 x 2 x 1 supercells of TcS2 and TcSe2 monolayer along the high-symmetric points in the Brillouin zone

The second approach to estimate the feasibility of obtaining TcS2 and TcSe2 monolayers is by calculating the cleavage energy Ecl, which is defined as the minimum energy required to overcome the interlayer van der Waals force during the exfoliation process. 42-43 (see more computing details in the Supporting Information). Figure 2a (blue curve) illustrates that the calculated Ecl for TcS2 and TcSe2 is 0.89 and 1.04 J/m2, respectively. By further taking the derivative of Ecl with respect to the distance, the cleavage strength σ (red curve) is obtained and estimated to be 2.62 GPa for TcS2 and 3.08 GPa for TcSe2, respectively. It should be noted that the calculated cleavage energy of TcS2 and TcSe2 are smaller than the cleavage energy of ReSe2 (1.10 J/m2, see Supporting Information Fig. S1), whereas they are larger than the experimentally estimated value of graphite (0.37 J/m2).44 Moreover, the cleavage strengths of TcS2 and TcSe2 are lower than that of ReSe2 (3.59 GPa) and comparable to that of graphite (2.10 GPa).43 Considering that ReSe2 monolayers and graphene have been experimentally exfoliated, it is expected that the same should be possible for TcS2 and TcSe2.



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Having confirmed the high possibility of mechanically exfoliating single-layers TcS2 and TcSe2 from their bulk crystals, then the dynamical stabilities of the monolayers are tested by calculating their phonon spectra, as shown in Fig. 2b and 2c. Notice that the DFPT phonon calculations have been used for the study of the bulk phase technetium dichalcogenides by Weck et al..11 It can be clearly seen from Fig. 2b and 2c that no imaginary frequency phonons are found at any wave vector, which demonstrates that TcS2 and TcSe2 monolayers are both dynamically stable.

Figure 3. The thermal properties including (a) entropy S, (b) Helmholtz free energy A, and (c) heat capacity Cv versus temperature for TcS2 and TcSe2

Based on the phonon frequencies of single-layers TcS2 and TcSe2, the thermal characteristics including entropy S, Helmholtz free energy A and heat capacity Cv were obtained within the framework of DFPT as shown in Fig. 3. Fig. 3a shows that the entropy, S, increases steadily with rising the temperature in both TcS2 and TcSe2, the reason of which has been interpreted by Weck et al..11 The entropy S for monolayer TcS2 and TcSe2 are relatively higher than that of their bulk counterparts. In contrast the Helmholtz free energy, A (Fig. 3b),


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decays with increasing T (and S) at a certain temperature according to its own definition (A(T) = U(T) - TS(T), U(T) is the lattice internal energy). Furthermore, in Fig. 3c we plot the heat capacity at constant volume Cv for single-layer TcS2 and TcSe2 based on the calculated vibrational entropies as a function of temperature. At room temperature (300 K) Cv of TcS2 and TcSe2 monolayers is about 63.7 and 68.7 J/mol/K, respectively. These values are relatively larger than some excellent thermoelectric materials such as PbTe and PbSe,45 suggesting the better ability of TcS2 and TcSe2 to retain heat. By contrast, in previous calculations for the bulk phases, the heat capacity Cp at 300 K is approximately 63 and 69 J/mol/K for TcS2 and TcSe2, respectively. In the temperature range, 0-700 K, the heat capacities of TcSe2 are distinguishably higher than those of TcS2. Above 700 K both the heat capacities for TcS2 and TcSe2 at constant volume Cv, approach the value of Dulong–Petit law Cv = 3NR, where R is the universal gas constant and N is the number of atoms. This is different form their bulk counterparts, whose heat capacity approaches the Dulong–Petit asymptotic value at around 450 and 500 k for bulk TcS2 and TcSe2, respectively.11

Figure 4. The stress in the (a) TcS2 and (b) TcSe2 monolayer subjected to biaxial strain. Zone I (green) represents the elastic region while zone II (red) is the plastic region. Insets: The structural snapshots under (a) 16% and (b) 19% strain.

Next, we explore the mechanical properties of TcS2 and TcSe2 monolayers by computing the variation of stress subject to biaxial tensile tension. The yield point of TcS2 is at 11% strain as shown in Fig. 4a. Before this point, there is the elastic range where the deformation is reversible and the stretched TcS2 monolayer can return to its original geometry when the tension is released. Further extension after the yield point would induce an irreversible



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plastic deformation and the TcS2 monolayer structure will eventually rupture after reaching the critical breaking strain (15%). Similarly, the yield point and breaking strain for singlelayer TcSe2 (Fig. 4b) are 10% and 18%, respectively. We also examine the strain-stress relation for the experimentally reported ReS2 and ReSe2 monolayers (Supporting Information Fig. S2) and find that their breaking strains (12 % and 16%) are relatively smaller than that of single-layer TcSe2, but comparable to that of 2D TcS2. Additionally, the maximum stresses that TcS2 and TcSe2 can withstand while being stretched are 10.76 and 8.39 N/m, which are similar to the values for ReS2 (10.50 N/m) and ReSe2 (9.54 N/m). It should be noted that adding strain will disturb the system away from equilibrium, thus it would increase its energy as presented in the Supporting Information Fig. S3. The higher is the energy the more difficult is to apply strain in the 2D sheets.

Figure 5. Band structures and the projected density of state for (a, b) TcS2 and (c, d) TcSe2 monolayer. (e) Band gaps of TcS2 and TcSe2 as the dependence of layer number (1, 2, 3 layers and the Bulk) by PBE calculations. The band structures of TcS2 (f) bilayer and (g) trilayer. The Fermi energy was set to zero.

After studying the elastic characteristics of TcS2 and TcSe2, we now turn our focus to their electronic properties. The band structures obtained by PBE calculations demonstrate that bulk TcS2 and TcSe2 (Supporting Information Fig. S4) are both indirect semiconductors with a band gap of 1.02 and 0.85 eV, respectively, which are both slightly larger than previous DFT calculations (0.90 and 0.80 eV)11 and in consistence with experimental values (1.00 and 0.88


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eV). 33 Their monolayer form (Fig. 5a and 5c) displays a significant band gap opening to 1.28 eV for TcS2 and 1.14 eV for TcSe2. By looking at the projected density of state (PDOS) of TcS2 (Fig. 5b), we can find that the 3p orbital of S (abbreviated as S-3p), and the Tc-4d orbital exhibit strong hybridization below the Fermi energy, while the S-3p, Tc-3p and Tc-4d mainly contributes the DOS above the Fermi energy, and orbital hybridization is still significant. Similarly, the highest occupied molecular orbital (HOMO) of TcSe2 (Fig. 5d) is predominantly composed of Se-4p and Tc-4d orbitals, and the lowest unoccupied molecular orbital (LUMO) derives from the hybridization of Se-4p, Tc-3p, and Tc-4d orbitals. Current results with regard to PDOS for monolayer TcX2 are generally similar with those in the bulk phase, in which Tc-4d mixed with S-3p/Se-4p orbitals dominate the HOMO and LUMO for TcS2/TcSe2 bulk. The variation of the band gap with thickness, ranging from 1 layer (1L) to the bulk is evaluated for TcS2 and TcSe2 as shown in Fig. 5e. The bi- and tri-layer TcX2 along the c-axis shows AA-stacking order. It can be found that the band gaps of both TcS2 and TcSe2 undergo a dramatic decrease when the number of layer changes from 1 to 2, while the values just drop slightly for thicker layers. Most interestingly, we find a thickness-induced indirect to direct gap transition in TcS2. Specifically, TcS2 monolayers (Fig. 5a) show indirect-gap character with the CBM at the Γ point and the VBM at a midpoint along the Γ-B (0.5, 0.0, 0.0) symmetry line. As the number of layers increases to 2 (Fig. 5f), the position of VBM moves to the Γ point, producing the indirect-to-direct gap transition and the bilayer TcS2 displays the direct–gap character. With any further increase of the thickness, the CBM of TcS2 (Fig. 5g) shifts to a midpoint along the Γ-B symmetry line while the VBM moves towards the F point (0.0, 0.5, 0.0), leading to the direct-to-indirect gap transition and the trilayer TcS2 becomes again an indirect-gap semiconductor. Such direct-to-indirect gap crossover phenomenon demonstrates potential for tuning the TcS2 electronic properties by thickness control. We also checked the band structures of TcS2 4- and 5-layer structures, finding that they both exhibit indirect gap characters and no gap transition was further observed. Apart from the TcS2, the band structures of TcSe2 bilayers and trilayers (Supporting Information Fig. S5) displays similar topology compared to their monolayer counterpart and the indirect -gap nature is maintained for all layer numbers. Since the GGA is well known to underestimate the band gap of small-gap materials by 0.6– 1.0 eV,46-47 we now employ the advanced hybrid density-functional (HSE06) to predict more



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accurate band gap values. Our HSE06 calculations correct the indirect band gap of TcS2 and TcSe2 monolayer to 1.91 eV (Fig. 5a) and 1.69 eV (Fig. 5c), respectively. It should be noted that the fundamental band gaps of TcS2 and TcSe2 monolayer are comparable to some highefficiency photovoltaics materials such as MoS2 monolayer (1.8 eV)1, demonstrating the great potential to form efficient light-absorbers.

Figure 6. The HSE06 method calculated light absorption spectra of TcS2 (black solid line), TcSe2 (blue solid line), ReSe2 (green dashed line) and MoS2 (red dashed line) monolayer, overlapped to the incident AM1.5G solar flux.

Our HSE06 band structure calculations illustrate the potential of single-layer TcS2 and TcSe2 to perform as a light harvesting compounds. This can be further evaluated by computing the optical adsorption spectra in the visible light region (390 - 700 nm) as shown in Fig. 6. Results for the excellent sunlight absorber MoS2 monolayer, previous reported single-layer ReSe2, and the incident AM1.5G solar spectrum are also included in this figure for comparison. It can be clearly seen that the absorption onset of single-layer TcSe2 is at 700 nm, which is smaller (in energy) than that of ReSe2 (670 nm), TcS2 (630 nm) and MoS2 (600 nm) monolayer in the current HSE06 calculations, implying the extraordinary light harvesting ability for TcSe2 monolayer in the long wavelength range 550 - 700 nm. Except for the high peak of MoS2 at 520 nm, the TcS2 and TcSe2 monolayer in short wavelength region (390 - 550 nm) also display better light absorption performance than that of ReSe2 and MoS2. Generally, over the entire sunlight spectrum TcS2 and TcSe2 monolayers absorb more light than 2D MoS2 and ReSe2 and so they appear as promising solar cell materials. As it was mentioned previously, the band gaps of single-layer TcS2 and TcSe2 are 1.91 and 1.69


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eV, respectively. Increasing the layers number reduces the gap, driving those to reach the ideal gap value for solar cell materials (1.5 eV). Thus, the TcS2 and TcSe2 in their multi-layer forms are expected to exhibit enhanced ability of light absorption in comparison to their monolayer counterparts. It is well known that strain plays a significant role in tailoring the electronic and optical properties of semiconductor nanostructures.48-49 Experimentally, external strain can be exerted via numerous strategies including adlayer-substrate lattice mismatch,50 external load,51 bending,52 or by applying stress on the material.53 As highly polarized materials, the electronic and optical properties of TcS2 and TcSe2 are expected to be effectively modulated by strain. In this context, we now study the variation of the band gap as a function of the applied strain (see Fig. 7a). It can be clearly seen that by applying either tensile or compressive strain the band gap of TcS2 and TcSe2 monolayers is reduced, because of the downward shift of the CBM and the upward change of the VBM (see Supporting Information Fig. S6 and S7). With sufficient expansion (+12%) or compression (−8%), a single-layer TcSe2 can be transformed from a semiconductor to a metal, while the critical strains for semiconductor-to-metal transition are relatively larger in single-layer TcS2, namely +14% and -10%, respectively. In general, such strain-tunable band gaps in the two Tc-based dichalcogenides suggest their potential applications in nanoelectronic devices constructed by strain engineering techniques. Noticeably the HSE06 method is not employed to study the strain effect on TcS2 and TcSe2 monolayer, not only because such functional is computationally demanding, but also since the current PBE calculations are capable to predict similar trend for the change in band gaps as a function of strain.

Figure 7. (a) Band gaps as a function of biaxial strain calculated with the PBE functional for TcS2 (red line) and TcSe2 (blue line) monolayers. (b) Band-edge positions of monolayer semiconducting TMDCs



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compared to redox potentials of water calculated by the HSE06 functional. The vacuum energy level is at 0 eV.

In order for a 2D semiconductor to be a candidate for photocatalytic water splitting, this must simultaneously satisfy some criteria. Firstly, the band gap of the 2D material must be greater than 1.23 eV to drive the kinetics of the hydrogen and oxygen evolution reactions, and smaller than 3.00 eV to enhance solar absorption. Secondly, a low formation energy is required to facilitate the fabrication of the single-layer form. The most important criterion is that the band edges must straddle the redox potentials of water. Specifically, The CBM of the 2D semiconductor is required to be higher than the reduction potential of H+/H2 (4.44eV) and the VBM energy to be lower than the oxidation potential of O2/H2O (−5.67 eV). Since the first two conditions for water splitting are met by TcS2 and TcSe2, we now evaluate the third criterion by calculating the band alignments relatively to the vacuum energy level as shown in Fig. 7b. We can find that the band-edge position of TcSe2 perfectly straddles the redox potentials of water, suggesting its potential application for photocatalytic water splitting. The band edges of TcS2 seem to be unfavorable for water splitting, but a moderate 2% compressive strain enables it to straddle the redox potentials of water as well. By contrast, strain-free ReSe2 monolayers are not suitable for driving the kinetics of the hydrogen and oxygen evolution reactions. In short, among the newly discovered rhenium and technetium dichalcogenides, TcSe2 monolayer is expected to be a promising candidate for photocatalysts, while a 2% compressive strain is able to make also TcS2 monolayers suitable for water splitting.

Conclusions In conclusion, a first-principle investigation has been performed to study the structural, thermal, mechanical, electronic and optical properties of TcS2 and TcSe2 monolayers. The calculated phonon spectra prove the dynamical stability of the single-layer TcS2 and TcSe2, and the similar formation energy and cleavage energy compared with graphite and ReSe2 demonstrate that exfoliation of their bulk forms to achieve freestanding monolayers is highly feasible. Moreover the calculated band gaps of 2D TcS2 and TcSe2 (1.91 and 1.69 eV) are close to the ideal band gap of solar cell materials (1.5 eV), suggesting potentially


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remarkable performance in light harvesting. By analysing the band alignment relative to the vacuum level, the TcSe2 monolayer is found to be an ideal candidate for water splitting while a 2% compressive strain can also make TcS2 monolayer suitable for photocatalysts. Furthermore, an indirect-to-direct band gap transition is revealed in TcS2 as its layer number increases from 1 to 2. The semiconducting-to-metal transition can be triggered as well when 2D TcS2 and TcSe2 undergo effective tension or compression, demonstrating their flexible electronic properties manipulated by thickness and external strain. This work is expected to shed new light on the less-known technetium dichalcogenides and fully explore their physical and chemical properties that would pave the way for the future applications in photovoltaics, photocatalysts and other nanodevices.

Supporting Information: Computing details and cleavage energy for exfoliation of ReS2 monolayer from bulk counterpart (Figure S1) and the stress-strain curve (Figure S2), strain energy of the TcS2 and TcSe2 monolayer under biaxial strain (Figure S3), bulk energy band structure for TcS2 and TcSe2 (Figure S4), band structure for bilayer and trilayer TcSe2 (Figure S5), bandgap change as a function of strain for TcS2 (Figure S6) and TcSe2 (Figure S7) monolayer and the calculated structural parameters (Table S1).

AUTHOR INFORMATION Corresponding Author Aijun Du, Email: [email protected] Author Contributions All authors have given approval to the final version of the manuscript.

Acknowledgements: We acknowledge generous grants of high-performance computer time from computing facility at Queensland University of Technology and Australian National Facility. A.D. greatly appreciates the Australian Research Council QEII Fellowship (DP110101239) and financial support of the Australian Research Council under Discovery Project (DP130102420). S.S. acknowledges support from the European Research Council (QUEST project).



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Predicting Single-Layer Technetium Dichalcogenides (TcX2, X = S, Se

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