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Two-dimensional (2D) Janus materials with extraordinary properties are promising candidates for utilization in advanced technologies. In this study, new 2D Janus XWSiP2 (X = S, Se, Te) monolayers were constructed and their properties were systematically analyzed by using first-principles calculations. All three structures of SWSiP2, SeWSiP2, and TeWSiP2 exhibit high energetic stability for the experimental fabrication with negative and high Ecoh values, the elastic constants obey the criteria of Born–Huang, and no imaginary frequency exists in the phonon dispersion spectra. The calculated results from the PBE and HSE06 approaches reveal that the XWSiP2 are semiconductors with moderate direct band-gaps varying from 1.01 eV to 1.06 eV using the PBE method, and 1.39 eV to 1.44 eV using the HSE06 method. In addition, the electronic band structures of the three monolayers are significantly affected by the applied strains. Interestingly, the transitions from a direct to indirect semiconductor are observed for different biaxial strains εb. The transport parameters including the carrier mobility values along the x direction μx and y direction μy were also calculated to study the transport properties of the XWSiP2. The results indicate that the XWSiP2 monolayers not only have high carrier mobilities but also anisotropy in the transport directions for both holes and electrons. Together with the moderate and tunable energy gaps, the XWSiP2 materials are found to be potential candidates for application in the photonic, photovoltaic, optoelectronic, and electronic fields. [ABSTRACT FROM AUTHOR]

Motivated by the successful synthesis of two-dimensional MoSi2N4 [Y.-L. Hong et al., Science, 2020, 369, 670–674] and Janus MoSSe [A.-Y. Lu et al., Nat. Nanotechnol., 2017, 12, 744–749], in this work, we propose novel 2D Janus XMoGeN2 (X = S, Se and Te) monolayers using first-principles prediction. The controllable electronic features of Janus XMoGeN2 (X = S, Se and Te) monolayers under an external electric field and strain are also examined. Our predictions demonstrated that 2D XMoGeN2 materials are structurally and dynamically stable. All these 2D XMoGeN2 materials are indirect semiconductors with band gaps of 1.60/2.10, 1.54/2.07 and 1.05/1.56 eV obtained by the PBE/HSE functional for SMoGeN2, SeMoGeN2 and TeMoGeN2 monolayers, respectively. Furthermore, the electronic band gap and band structures of these monolayers are controllable under an external electric field and strain, making them promising candidates for flexible optoelectronics and nanoelectronics. The electric field tunes the TeMoGeN2 monolayer from semiconductor to metal and leads to a change in the band gap. While strain modifies the band gap of the TeMoGeN2 monolayer, giving rise to a shift in the CB from the Γ–M path to the M point and a tendency to transform from semiconductor to metal. Our findings suggest that these novel 2D XMoGeN2 materials are potential candidates for use in future high-performance applications. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) Janus materials with extraordinary properties are promising candidates for utilization in advanced technologies. In this study, new 2D Janus XWSiP 2 (X = S, Se, Te) monolayers were constructed and their properties were systematically analyzed by using first-principles calculations. All three structures of SWSiP 2 , SeWSiP 2 , and TeWSiP 2 exhibit high energetic stability for the experimental fabrication with negative and high E coh values, the elastic constants obey the criteria of Born-Huang, and no imaginary frequency exists in the phonon dispersion spectra. The calculated results from the PBE and HSE06 approaches reveal that the XWSiP 2 are semiconductors with moderate direct band-gaps varying from 1.01 eV to 1.06 eV using the PBE method, and 1.39 eV to 1.44 eV using the HSE06 method. In addition, the electronic band structures of the three monolayers are significantly affected by the applied strains. Interestingly, the transitions from a direct to indirect semiconductor are observed for different biaxial strains ε b . The transport parameters including the carrier mobility values along the x direction μ x and y direction μ y were also calculated to study the transport properties of the XWSiP 2 . The results indicate that the XWSiP 2 monolayers not only have high carrier mobilities but also anisotropy in the transport directions for both holes and electrons. Together with the moderate and tunable energy gaps, the XWSiP 2 materials are found to be potential candidates for application in the photonic, photovoltaic, optoelectronic, and electronic fields.

Motivated by the successful synthesis of two-dimensional MoSi 2 N 4 [Y.-L. Hong et al. , Science , 2020, 369 , 670-674] and Janus MoSSe [A.-Y. Lu et al. , Nat. Nanotechnol. , 2017, 12 , 744-749], in this work, we propose novel 2D Janus XMoGeN 2 (X = S, Se and Te) monolayers using first-principles prediction. The controllable electronic features of Janus XMoGeN 2 (X = S, Se and Te) monolayers under an external electric field and strain are also examined. Our predictions demonstrated that 2D XMoGeN 2 materials are structurally and dynamically stable. All these 2D XMoGeN 2 materials are indirect semiconductors with band gaps of 1.60/2.10, 1.54/2.07 and 1.05/1.56 eV obtained by the PBE/HSE functional for SMoGeN 2 , SeMoGeN 2 and TeMoGeN 2 monolayers, respectively. Furthermore, the electronic band gap and band structures of these monolayers are controllable under an external electric field and strain, making them promising candidates for flexible optoelectronics and nanoelectronics. The electric field tunes the TeMoGeN 2 monolayer from semiconductor to metal and leads to a change in the band gap. While strain modifies the band gap of the TeMoGeN 2 monolayer, giving rise to a shift in the CB from the Γ-M path to the M point and a tendency to transform from semiconductor to metal. Our findings suggest that these novel 2D XMoGeN 2 materials are potential candidates for use in future high-performance applications.

In this work, we systematically examine the electronic features and contact types of van der Waals heterostructures (vdWHs) combining single-layer boron phosphide (BP) and Janus Ga 2 SSe using first-principles calculations. Owing to the out-of-plane symmetry being broken, the BP/Ga 2 SSe vdWHs are divided into two different stacking patterns, which are BP/SGa 2 Se and BP/SeGa 2 S. Our results demonstrate that these stacking patterns are structurally and mechanically stable. The combination of single-layer BP and Janus Ga 2 SSe gives rise to an enhancement in the Young's modulus compared to the constituent monolayers. Furthermore, at the ground state, the BP/Ga 2 SSe vdWHs possess a type-I (straddling) band alignment, which is desired for next-generation optoelectronic applications. The interlayer separation and electric field are effectively used to tune the electronic features of the BP/Ga 2 SSe vdWH from the type-I to type-II band alignment, and from semiconductor to metal. Our findings show that the BP/Ga 2 SSe vdWH would be appropriate for next-generation multifunctional optoelectronic and photovoltaic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)