Role of Chalcogen Defect Introducing Metal-Induced Gap States and Its Implications for Metal-TMDs' Interface Chemistry

A DFT study of superior adsorbate-surface bonding at Pt-WSe2 vertically aligned heterostructures upon NO2, SO2, CO2, and H2 interactions

This study investigates the potential of platinum (Pt) decorated single-layer WSe2 (Pt-WSe2) monolayers as high-performance gas sensors for NO2, CO2, SO2, and H2 using first-principles calculations. We quantify the impact of Pt placement (basal plane vs. vertical edge) on WSe2's electronic properties, focusing on changes in bandgap (ΔEg). Pt decoration significantly alters the bandgap, with vertical edge sites (TV-WSe2) exhibiting a drastic reduction (0.062 eV) compared to pristine WSe2 and basal plane decorated structures (TBH: 0.720 eV, TBM: 1.237 eV). This substantial ΔEg reduction in TV-WSe2 suggests a potential enhancement in sensor response. Furthermore, TV-WSe2 displays the strongest binding capacity for all target gases due to a Pt-induced "spillover effect" that elongates adsorbed molecules. Specifically, TV-WSe2 exhibits adsorption energies of - 0.5243 eV (NO2), - 0.5777 eV (CO2), - 0.8391 eV (SO2), and - 0.1261 eV (H2), indicating its enhanced sensitivity. Notably, H2 adsorption on TV-WSe2 shows the highest conductivity modulation, suggesting exceptional H2 sensing capabilities. These findings demonstrate that Pt decoration, particularly along WSe2 vertical edges, significantly enhances gas sensing performance. This paves the way for Pt-WSe2 monolayers as highly selective and sensitive gas sensors for various applications, including environmental monitoring, leak detection, and breath analysis.

MoS2 Field-Effect Transistor Performance Enhancement by Contact Doping and Defect Passivation via Fluorine Ions and Its Cyclic Field-Assisted Activation

MoS2-based field-effect transistors (FETs) and, in general, transition metal dichalcogenide channels are fundamentally limited by high contact resistance (RC) and intrinsic defects, which results in low drive current and lower carrier mobilities, respectively. This work addresses these issues using a technique based on CF4 plasma treatment in the contacts and further cyclic field-assisted drift and activation of the fluorine ions (F-), which get introduced into the contact region during the CF4 plasma treatment. The F- ions are activated using cyclic pulses applied across the source-drain (S/D) contacts, which leads to their migration to the contact edges via the channel. Further, using ab initio molecular dynamics and density functional theory simulations, these F- ions are found to bond at sulfur (S) vacancies, resulting in their passivation and n-type doping in the channel and near the S/D contacts. An increase in doping results in the narrowing of the Schottky barrier width and a reduction in RC by ∼90%. Additionally, the passivation of S vacancies in the channel enhances the mobility of the FET by ∼150%. The CF4 plasma treatment in contacts and further cyclic field-assisted activation of F- ions resulted in an ON-current (ION) improvement by ∼90% and ∼480% for exfoliated and CVD-grown MoS2, respectively. Moreover, this improvement in ION has been achieved without any deterioration in the ION/IOFF, which was found to be >7-8 orders.

Defects and Defect Engineering of Two-Dimensional Transition Metal Dichalcogenide (2D TMDC) Materials

As layered materials, transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials. Interestingly, the characteristics of these materials are transformed from bulk to monolayer. The atomically thin TMDC materials can be a good alternative to group III-V and graphene because of their emerging tunable electrical, optical, and magnetic properties. Although 2D monolayers from natural TMDC materials exhibit the purest form, they have intrinsic defects that limit their application. However, the synthesis of TMDC materials using the existing fabrication tools and techniques is also not immune to defects. Additionally, it is difficult to synthesize wafer-scale TMDC materials for a multitude of factors influencing grain growth mechanisms. While defect engineering techniques may reduce the percentage of defects, the available methods have constraints for healing defects at the desired level. Thus, this holistic review of 2D TMDC materials encapsulates the fundamental structure of TMDC materials, including different types of defects, named zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D). Moreover, the existing defect engineering methods that relate to both formation of and reduction in defects have been discussed. Finally, an attempt has been made to correlate the impact of defects and the properties of these TMDC materials.