Electronic properties of Janus MXY/graphene (M = Mo, W; X ≠ Y = S, Se) van der Waals structures: a first-principles study

Unveiling Versatile Electronic Properties and Contact Features of Metal-Semiconductor Graphene/γ-Ge2SSe van der Waals Heterostructures

Recently, searching for a metal-semiconductor junction (MSJ) that exhibits low-contact resistance has received tremendous consideration, as they are essential components in next-generation field-effect transistors. In this work, we design a MSJ by integrating two-dimensional (2D) graphene as the metallic electrode and 2D Janus γ-Ge2SSe as the semiconducting channel using first-principles simulations. All the graphene/γ-Ge2SSe MSJs are predicted to be energetically, mechanically, and thermodynamically stable, characterized by the weak van der Waals (vdW) interactions. The graphene/γ-SGe2Se MSJ-vdWH form the n-type Schottky contact (SC), while the graphene/γ-SeGe2S MSJ-vdWH form the p-type one, suggesting that the switching between p-type and n-type SC in the graphene/γ-Ge2SSe MSJ-vdWHs can occur spontaneously by simply altering the stacking patterns, without requiring any external conditions. Notably, the contact features, including contact types and barriers of the graphene/γ-Ge2SSe MSJs are significant in versatility and can be altered by applying electric gating and adjusting interlayer spacing. Both the applied electric gating and strain engineering induce switchability between p- and n-type and SC to OC in the graphene/γ-Ge2SSE MSJs. This versatility underscores the potential of the graphene/γ-Ge2SSe MSJ for next-generation applications that require low-contact resistance and high performance.

Asymmetric XMoGeY2 (X = S, Se, Te; Y = N, P, As) monolayers as potential flexible materials for nano piezoelectric devices and nanomedical sensors

Highly efficient nano piezoelectric devices and nanomedical sensors are in great demand for high-performance piezoelectric materials. In this work, we propose new asymmetric XMoGeY2 (X = S, Se, Te; Y = N, P, As) monolayers with excellent piezoelectric properties, dynamic stability and flexible elastic properties. The piezoelectric coefficients (d11) of XMoGeY2 monolayers range from 2.92 to 8.19 pm V-1. Among them, TeMoGeAs2 exhibits the highest piezoelectric coefficient (d11 = 8.19 pm V-1), which is 2.2 times higher than that of common 2D piezoelectric materials such as 2H-MoS2 (d11 = 3.73 pm V-1). Furthermore, all XMoGeY2 monolayers demonstrate flexible elastic properties ranging from 96.23 to 253.70 N m-1. Notably, TeMoGeAs2 has a Young's modulus of 96.23 N m-1, which is only one-third of that of graphene (336 N m-1). The significant piezoelectric coefficients of XMoGeY2 monolayers can be attributed to their asymmetric structures and flexible elastic properties. This study provides valuable insights into the potential applications of XMoGeY2 monolayers in nano piezoelectric devices and nanomedical sensors.

Tuning the Electronic Properties of Two-Dimensional Lepidocrocite Titanium Dioxide-Based Heterojunctions

Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2-MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2-MoSSe heterostructure.

Two dimensional Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers: ferromagnetic semiconductors with spontaneous valley polarization and tunable magnetic anisotropy

Two-dimensional (2D) ferromagnetic (FM) materials with valley polarization are highly desirable for use in valleytronic devices. The 2D Janus materials have fascinating physical properties due to their asymmetrical structures. In this work, the electronic structure and magnetic properties of Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers are systematically studied using first-principles calculations. RuBrCl, RuBrF, and RuClF monolayers are all FM semiconductors. The valley polarization is present in the band structure and this is determined by the spin orbit coupling (SOC). The valley splitting energy of the RuClF monolayer is as large as 204 meV, with a perpendicular magnetic anisotropy (PMA) energy of 1.918 mJ m-2 and a Curie temperature of 316 K. Therefore, spontaneous valley polarization at room temperature will be seen in the RuClF monolayer. The Curie temperature of the RuBrF monolayer is higher than that of the RuClF, but the magnetic anisotropy energy (MAE) is in-plane magnetic anisotropy (IMA). The valley splitting energy of the RuBrCl monolayer is higher and the PMA energy is lower than that of the RuClF monolayer. The Curie temperature was only 197 K. The valley polarization was modulated in the RuXY monolayers at different biaxial strains, during which the semiconductor properties are still maintained. The PMA of the RuClF and RuBrCl monolayers is enhanced by the biaxial compressive strains, which are mainly attributed to the variation of the (dyz, d2z) orbital matrix elements of the Ru atoms. The MAE of the RuBrF monolayer is tuned from IMA into PMA at a biaxial strain of -6%. These results show an example of a 2D Janus ferrovalley material.

Janus Type Monolayers of S-MoSiN2 Family and Van Der Waals Heterostructures with Graphene: DFT-Based Study

Novel representative 2D materials of the Janus type family X-M-ZN2 are studied. These materials are hybrids of a transition metal dichalcogenide and a material from the MoSi2N4 family, and they were constructed and optimized from the MoSi2N4 monolayer by the substitution of SiN2 group on one side by chalcogen atoms (sulfur, selenium, or tellurium), and possibly replacing molybdenum (Mo) to tungsten (W) and/or silicon (Si) to germanium (Ge). The stability of novel materials is evaluated by calculating phonon spectra and binding energies. Mechanical, electronic, and optical characteristics are calculated by methods based on the density functional theory. All considered 2D materials are semiconductors with a substantial bandgap (>1 eV). The mirror symmetry breaking is the cause of a significant built-in electric field and intrinsic dipole moment. The spin−orbit coupling (SOC) is estimated by calculations of SOC polarized bandstructures for four most stable X-M-ZN2 structures. The possible van der Waals heterostructures of considered Janus type monolayers with graphene are constructed and optimized. It is demonstrated that monolayers can serve as outer plates in conducting layers (with graphene) for shielding a constant external electric field.

First-principles investigation of potential water-splitting photocatalysts and photovoltaic materials based on Janus transition-metal dichalcogenide/WSe2 heterostructures

Two-dimensional materials have been shown to exhibit exotic properties that make them very interesting for both photo-catalytic and photo-voltaic applications. In this study, van der Waals corrected density functional theory calculations were carried out on heterostructures of MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2. The heterostructures are semiconductors with type II band alignments which are advantageous for electron-hole pair separation. The HSE06 level electronic band gap was found to be 1.093 eV, 1.427 eV and 1.603 eV for MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 respectively. We have considered eight high symmetry stacking patterns for each of the heterostructures, and among them the most stable stacking orders were ascertained based on the interlayer binding energies. The binding energies of the most stable MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 heterostructures were found to be -0.0604 eV, -0.1721 eV, and -0.3296 eV with an equilibrium interlayer space of 5.75 Å, 4.05 Å, and 4.76 Å respectively. The Power Conversion Efficiency (PCE) was found to be 20, 19.98, and 18.24 percent for the MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 heterostructures, respectively. The results show that they can serve as suitable photovoltaic materials with high efficiency, thus, opening the possibilities of developing solar cells based on 2D Janus/TMD heterostructures. The most stable heterostructures are also tested for photocatalytic water splitting applications and WSeTe/WSe2 shows excellent photocatalytic activity by being active for full water splitting at pH = 7 and pH = 14, the MoSSe/WSe2 heterostructure is good for the oxygen evolution reaction and WSSe/WSe2 is active for the hydrogen evolution reaction.

Study of Two-Dimensional Janus WXY (X≠Y= S, Se, and Te) Trilayer Homostructures for Photovoltaic Applications Using DFT Screening of Different Stacking Patterns

Based on the first-principles density functional theory, Janus WXY (X ≠ Y = S, Se, and Te) trilayer homostructures for different stacking patterns are studied in this work to analyze their appropriateness in fabricating photovoltaic (PV) devices. A total of fifteen trilayer homostructures are proposed, corresponding to the suitable five stacking patterns, such as AAA, AA'A, ABA, AB'A, and A'BA' for each Janus WXY (X ≠ Y = S, Se, and Te) material. Structural and energetic parameters for all the fifteen structures are evaluated and compared to find energetically stable structures, and dynamic stability is confirmed by phonon dispersion curves. All these configurations being homostructure, lattice mismatch is found to be very low (∼0.05%), unlike heterostructure, making them feasible for optoelectronics and PV applications. WSSe AAA, WSSe AA'A, and WSeTe AA'A are dynamically stable along with negative binding energy and show type-II band alignment, enabling effective spatial carrier separation of photogenerated carriers. The optical properties of dynamically stable WSSe AAA and WSSe AA'A structures are also calculated, and the absorption coefficients at the visible light region are found to be ∼3.5 × 10⁵ cm^-1, which is comparable to the perovskite material absorption coefficient. Moreover, we have compared the optical characteristics of dynamically stable WSSe AAA and WSSe AA'A structures with their monolayer structures to realize the significance of stacking trilayer structures. Electrical properties such as mobility and conductivity for dynamically stable WSSe AAA and WSSe AA'A structures are evaluated to suggest them as a probable efficient material in PV technology.