Adsorption of sulfur-containing contaminant gases by pristine, Cr and Mo doped NbS2monolayers based on density functional theory

The adsorption and sensor performance of hazardous gases containing sulfur (SO2, H2S and SO3) on pristine, Cr and Mo doped NbS2monolayers (Cr-NbS2and Mo-NbS2) were investigated in detail based on density functional theory. The comparative analysis of the parameters such as density of states, adsorption energy, charge transfer, recovery time and work function of the systems showed that the pristine NbS2monolayer have poor sensor performance for sulfur-containing hazardous gases due to weak adsorption capacity, insignificant charge transfer and insignificant changes in electronic properties after gas adsorption on the surface. After doping with Cr atoms, the adsorption performance of Cr-NbS2was significantly improved, and it can be used as a sensor for SO2and H2S gases and as an adsorbent for SO3gas. The adsorption performance of Mo-NbS2is also significantly improved by doping with Mo atoms, and it can be used as a sensor for H2S gas and as an adsorbent for SO2and SO3gas. Therefore, Cr-NbS2and Mo-NbS2are revealed to be sensing or elimination materials for the harmful gases containing sulfur (SO2, H2S and SO3) in the atmosphere.

Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX2 (X = S, Se)

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe₂ exhibits CDW order at low temperature, while bulk NbS₂ displays no CDW instability. The CDW transitions in single-layer NbS₂ and NbSe₂ are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS₂, resulting in distinct CDW features in the NbS₂ and NbSe₂ systems.

Experimental and Theoretical Study of Possible Collective Electronic States in Exfoliable Re-Doped NbS2

Metallic transition-metal dichalcogenides (TMDs) are rich material systems in which the interplay between strong electron-electron and electron-phonon interactions often results in a variety of collective electronic states, such as charge density waves (CDWs) and superconductivity. While most metallic group V TMDs exhibit coexisting superconducting and CDW phases, 2H-NbS₂ stands out with no charge ordering. Further, due to strong interlayer interaction, the preparation of ultrathin samples of 2H-NbS₂ has been challenging, limiting the exploration of presumably rich quantum phenomena in reduced dimensionality. Here, we demonstrate experimentally and theoretically that light substitutional doping of NbS₂ with heavy atoms is an effective approach to modify both interlayer interaction and collective electronic states in NbS₂. Very low concentrations of Re dopants (<1%) make NbS₂ exfoliable (down to monolayer) while maintaining its 2H crystal structure and superconducting behavior. In addition, first-principles calculations suggest that Re dopants can stabilize some native CDW patterns that are not stable in pristine NbS₂.

Highly Efficient and Stable Photoelectrochemical Hydrogen Evolution with 2D-NbS2/Si Nanowire Heterojunction

In recent days, 2-dimensional (2D) niobium disulfide (NbS₂) with near-zero Gibbs free energy and superlative acid electrolyte stability has provoked a great deal of interest toward hydrogen evolution reaction (HER) electrocatalyst due to its active basal and edge sulfur sites. Herein, we developed a single step method for the direct deposition of 2D-NbS₂ on high-aspect-ratio topographies of silicon nanowires (NWs) by chemical vapor deposition for the applications in HER electrocatalyst. The resultant 2D-NbS₂ electrocatalyst demonstrates the HER overpotential of ∼74 mV vs RHE (reversible hydrogen electrode) @ 1 mA/cm² under acidic conditions and stable for more than 20 h. More importantly, we developed the Si NWs array based photoelectrochemical water-splitting system with the direct deposition of 2D-NbS₂ as HER catalyst. The resultant 2D-NbS₂-Si NWs photocathode system demonstrates improved charge transfer characteristics at the Si-NbS₂ interfaces that leads to an enhanced turn on potential (from 0.06 to 0.34 V vs RHE) with the current density of -28 mA/cm² at the 0 V vs RHE. The results evidence the synergistic effect of 2D-NbS₂ electrocatalysts that addresses poor surface kinetics of Si toward solar water electrolysis. Our comprehensive studies reveal NbS₂ as a new class of photoelectrochemical cocatalyst for efficient solar HER performance by promoting the charge transfer process with prolonged acid stability.

Vertical and In-Plane Current Devices Using NbS2/n-MoS2 van der Waals Schottky Junction and Graphene Contact

A van der Waals (vdW) Schottky junction between two-dimensional (2D) transition metal dichalcogenides (TMDs) is introduced here for both vertical and in-plane current devices: Schottky diodes and metal semiconductor field-effect transistors (MESFETs). The Schottky barrier between conducting NbS₂ and semiconducting n-MoS₂ appeared to be as large as ∼0.5 eV due to their work-function difference. While the Schottky diode shows an ideality factor of 1.8-4.0 with an on-to-off current ratio of 10³-10⁵, Schottky-effect MESFET displays little gate hysteresis and an ideal subthreshold swing of 60-80 mV/dec due to low-density traps at the vdW interface. All MESFETs operate with a low threshold gate voltage of -0.5 ∼ -1 V, exhibiting easy saturation. It was also found that the device mobility is significantly dependent on the condition of source/drain (S/D) contact for n-channel MoS₂. The highest room temperature mobility in MESFET reaches to approximately more than 800 cm²/V s with graphene S/D contact. The NbS₂/n-MoS₂ MESFET with graphene was successfully integrated into an organic piezoelectric touch sensor circuit with green OLED indicator, exploiting its predictable small threshold voltage, while NbS₂/n-MoS₂ Schottky diodes with graphene were applied to extract doping concentrations in MoS₂ channel.