Two-dimensional Janus Si2OX (X = S, Se, Te) monolayers as auxetic semiconductors: theoretical prediction.

The auxetic materials have exotic mechanical properties compared to conventional materials, such as higher indentation resistance, more superior sound absorption performance. Although the auxetic behavior has also been observed in two-dimensional (2D) nanomaterials, to date there has not been much research on auxetic materials in the vertical asymmetric Janus 2D layered structures. In this paper, we explore the mechanical, electronic, and transport characteristics of Janus Si2OX (X = S, Se, Te) monolayers by first-principle calculations. Except for the Si2OTe monolayer, both Si2OS and Si2OSe are found to be stable. Most importantly, both Si2OS and Si2OSe monolayers are predicted to be auxetic semiconductors with a large negative Poisson's ratio. The auxetic behavior is clearly observed in the Janus Si2OS monolayer with an extremely large negative Poisson's ratio of -0.234 in the x axis. At the equilibrium state, both Si2OS and Si2OSe materials exhibit indirect semiconducting characteristics and their band gaps can be easily altered by the mechanical strain. More interestingly, the indirect-direct bandgap phase transitions are observed in both Si2OS and Si2OSe monolayers when the biaxial strains are introduced. Further, the studied Janus structures also exhibit remarkably high electron mobility, particularly along the x direction. Our findings demonstrate that Si2OS and Si2OSe monolayers are new auxetic materials with asymmetric structures and show their great promise in electronic and nanomechanical applications.

Crystal lattice and electronic and transport properties of Janus ZrSiSZ2 (Z = N, P, As) monolayers by first-principles investigations.

From the extending requirements for using innovative materials in advanced technologies, it is necessary to explore new materials for relevant applications. In this work, we design new two-dimensional (2D) Janus ZrSiSZ2 (Z = N, P, As) monolayers and investigate their crystal lattice and dynamic stability by using density functional theory investigations. The two stable structures of ZrSiSP2 and ZrSiSAs2 are then systematically examined for thermal, energetic, and mechanical stability, and electronic and transport properties. The calculation results demonstrate that both the ZrSiSP2 and ZrSiSAs2 monolayers have good thermal stability at room temperature and high energetic/mechanical stabilities for experimental synthesis. The studied structures are found to be in-direct semiconductors. Specifically, with moderate band-gap energies of 1.04 to 1.29 eV for visible light absorption, ZrSiSP2 and ZrSiSAs2 can be considered potential candidates for photovoltaic applications. The applied biaxial strains and external electric fields slightly change the band-gap energies of the monolayers. We also calculate the carrier mobilities for the transport properties based on the deformation potential method. Due to the lower effective masses, the carrier mobilities in the x direction are higher than those in the y direction. The carrier mobilities of the ZrSiSP2 and ZrSiSAs2 monolayers are anisotropic not only in transport directions but also for the electrons and holes. We believe that the results of our work may stimulate further studies to explore more new 2D Janus monolayers with novel properties of the MA2Z4 family materials.

Moderate direct band-gap energies and high carrier mobilities of Janus XWSiP2 (X = S, Se, Te) monolayers via first-principles investigation.

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.

Janus structures of the C 2h polymorph of gallium monochalcogenides: first-principles examination of Ga2XY (X/Y = S, Se, Te) monolayers.

Group III monochalcogenide compounds can exist in different polymorphs, including the conventional D 3h and C 2h phases. Since the bulk form of the C 2h-group III monochalcogenides has been successfully synthesized [Phys. Rev. B: Condens. Matter Mater. Phys. 73 (2006) 235202], prospects for research on their corresponding monolayers have also been opened. In this study, we design and systematically consider a series of Janus structures formed from the two-dimensional C 2h phase of gallium monochalcogenide Ga2XY (X/Y = S, Se, Te) using first-principles simulations. It is demonstrated that the Janus Ga2XY monolayers are structurally stable and energetically favorable. Ga2XY monolayers exhibit high anisotropic mechanical features due to their anisotropic lattice structure. All Janus Ga2XY are indirect semiconductors with energy gap values in the range from 1.93 to 2.67 eV. Due to the asymmetrical structure, we can observe distinct vacuum level differences between the two surfaces of the examined Janus structures. Ga2XY monolayers have high electron mobility and their carrier mobilities are also highly directionally anisotropic. It is worth noting that the Ga2SSe monolayer possesses superior electron mobility, up to 3.22 × 103 cm2 V-1 s-1, making it an excellent candidate for potential applications in nanoelectronics and nanooptoelectronics.

New C 2h phase of group III monochalcogenide monolayers AlX (X = S, Se, and Te) with anisotropic crystal structure: first-principles study.

In this paper, we introduce a new phase of two-dimensional aluminum monochalcogenide, namely C 2h-AlX (X = S, Se, and Te). With the C 2h space group, C 2h-AlX possesses a large unit cell containing 8 atoms. The C 2h phase of AlX monolayers is found to be dynamically and elastically stable based on the evaluation of its phonon dispersions and elastic constants. The anisotropic atomic structure of C 2h-AlX leads to a strong anisotropy in its mechanical properties with Young's modulus and Poisson's ratio strongly dependent on the directions examined in the two-dimensional plane. All three monolayers of C 2h-AlX are found to be direct band gap semiconductors, which are compared with the indirect band gap semiconductors of available D 3h-AlX. Particularly, the transition from direct to indirect band gap is observed in C 2h-AlX when a compressive biaxial strain is applied. Our calculated results indicate that C 2h-AlX exhibits anisotropic optical characteristics and its absorption coefficient is high. Our findings suggest that C 2h-AlX monolayers are suitable for applications in next-generation electro-mechanical and anisotropic opto-electronic nanodevices.

Two-dimensional Janus MGeSiP4 (M = Ti, Zr, and Hf) with an indirect band gap and high carrier mobilities: first-principles calculations.

Novel Janus materials have attracted broad interest due to the outstanding properties created by their out-of-plane asymmetry, with increasing theoretical exploration and more reports of successful fabrication in recent years. Here, we construct and explore the crystal structures, stabilities, electronic band structures, and transport properties - including carrier mobilities - of two-dimensional Janus MGeSiP4 (M = Ti, Zr, or Hf) monolayers based on density functional theory calculations. From the cohesive energies, elastic constants, and phonon dispersion calculations, the monolayers are confirmed to exhibit structural stability with high feasibility for experimental synthesis. All the structures are indirect band-gap semiconductors with calculated band-gap energies in the range of 0.77 eV to 1.01 eV at the HSE06 (Heyd-Scuseria-Ernzerhof) level. Interestingly, by applying external biaxial strain, a semiconductor to metal phase transition is observed for the three Janus structures. This suggests potential for promising applications in optoelectronic and electromechanical devices. Notably, the MGeSiP4 monolayers show directionally anisotropic carrier mobility with a high electron mobility of up to 2.72 × 103 cm2 V-1 s-1 for the ZrGeSiP4 monolayer, indicating advantages for applications in electronic devices. Hence, the presented results reveal the novel properties of the 2D Janus MGeSiP4 monolayers and demonstrate their great potential applications in nanoelectronic and/or optoelectronic devices. This investigation could stimulate further theoretical and experimental studies on these excellent materials and motivate further explorations of new members of this 2D Janus family.

Rashba-type spin splitting and transport properties of novel Janus XWGeN2 (X = O, S, Se, Te) monolayers.

We discuss and examine the stability, electronic properties, and transport characteristics of asymmetric monolayers XWGeN2 (X = O, S, Se, Te) using ab initio density functional theory. All four monolayers of quintuple-layer atomic Janus XWGeN2 are predicted to be stable and they are all indirect semiconductors in the ground state. When the spin-orbit coupling (SOC) is included, a large spin splitting at the K point is found in XWGeN2 monolayers, particularly, a giant Rashba-type spin splitting is observed around the Γ point in three structures SWGeN2, SeWGeN2, and TeWGeN2. The Rashba parameters in these structures are directionally isotropic along the high-symmetry directions Γ-K and Γ-M and the Rashba constant αR increases as the X element moves from S to Te. TeWGeN2 has the largest Rashba energy up to 37.4 meV (36.6 meV) in the Γ-K (Γ-M) direction. Via the deformation potential method, we calculate the carrier mobility of all four XWGeN2 monolayers. It is found that the electron mobilities of OWGeN2 and SWGeN2 monolayers exceed 200 cm2 V-1 s-1, which are suitable for applications in nanoelectronic devices.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides.

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In2XY(X/Y= S, Se, Te withX≠Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In2XYare dynamically and thermally stable at room temperature. At equilibrium, both In2STe and In2SeTe are direct semiconductors while In2SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In2XYand their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In2XYis negligible. Meanwhile, the optical absorbance intensity of the Janus In2XYmonolayers is remarkably increased by compressive strain. Also, In2XYmonolayers exhibit very low lattice thermal conductivities resulting in a high figure of meritZT, which makes them potential candidates for room-temperature thermoelectric materials.

Spin-orbit coupling effect on electronic, optical, and thermoelectric properties of Janus Ga2SSe.

In this paper, we investigate the electronic, optical, and thermoelectric properties of Ga2SSe monolayer by using density functional theory. Via analysis of the phonon spectrum and ab initio molecular dynamics simulations, Ga2SSe is confirmed to be stable at room temperature. Our calculations demonstrate that Ga2SSe exhibits indirect semiconductor characteristics and the spin-orbit coupling (SOC) effect has slightly reduced its band gap. Besides, the band gap of Ga2SSe depends tightly on the biaxial strain. When the SOC effect is included, small spin-orbit splitting energy of 90 meV has been found in the valence band. However, the spin-orbit splitting energy dramatically changes in the presence of biaxial strain. Ga2SSe exhibits high optical absorption intensity in the near-ultraviolet region, up to 8.444 × 104 cm-1, which is needed for applications in optoelectronic devices. By using the Boltzmann transport equations, the electronic transport coefficients of Ga2SSe are comprehensively investigated. Our calculations reveal that Ga2SSe exhibits a very low lattice thermal conductivity and high figure of merit ZT and we can enhance its ZT by temperature. Our findings provide further insight into the physical properties of Ga2SSe as well as point to prospects for its application in next-generation high-performance devices.