Facile Electrochemical Synthesis of 2D Monolayers for High-Performance Thin-Film Transistors

In this paper, we report high-performance monolayer thin-film transistors (TFTs) based on a variety of two-dimensional layered semiconductors such as MoS₂, WS₂, and MoSe₂ which were obtained from their corresponding bulk counterparts via an anomalous but high-yield and low-cost electrochemical corrosion process, also referred to as electro-ablation (EA), at room temperature. These monolayer TFTs demonstrated current ON–OFF ratios in excess of 10⁷ along with ON currents of 120 μA/μm for MoS₂, 40 μA/μm for WS₂, and 40 μA/μm for MoSe₂ which clearly outperform the existing TFT technologies. We found that these monolayers have larger Schottky barriers for electron injection compared to their multilayer counterparts, which is partially compensated by their superior electrostatics and ultra-thin tunnel barriers. We observed an Anderson type semiconductor-to-metal transition in these monolayers and also discussed possible scattering mechanisms that manifest in the temperature dependence of the electron mobility. Finally, our study suggests superior chemical stability and electronic integrity of monolayers even after being exposed to extreme electro-oxidation and corrosion processes which is promising for the implementation of such TFTs in harsh environment sensing. Overall, the EA process proves to be a facile synthesis route offering higher monolayer yields than mechanical exfoliation and lower cost and complexity than chemical vapor deposition methods

Three-Dimensional Integrated X‑ray Diffraction Imaging of a Native Strain in Multi-Layered WSe₂

Emerging two-dimensional (2-D) materials such as transition-metal dichalcogenides show great promise as viable alternatives for semiconductor and optoelectronic devices that progress beyond silicon. Performance variability, reliability, and stochasticity in the measured transport properties represent some of the major challenges in such devices. Native strain arising from interfacial effects due to the presence of a substrate is believed to be a major contributing factor. A full three-dimensional (3-D) mapping of such native nanoscopic strain over micron length scales is highly desirable for gaining a fundamental understanding of interfacial effects but has largely remained elusive. Here, we employ coherent X-ray diffraction imaging to directly image and visualize in 3-D the native strain along the (002) direction in a typical multilayered (∼100–350 layers) 2-D dichalcogenide material (WSe₂) on silicon substrate. We observe significant localized strains of ∼0.2% along the out-of-plane direction. Experimentally informed continuum models built from X-ray reconstructions trace the origin of these strains to localized nonuniform contact with the substrate (accentuated by nanometer scale asperities, i.e., surface roughness or contaminants); the mechanically exfoliated stresses and strains are localized to the contact region with the maximum strain near surface asperities being more or less independent of the number of layers. Machine-learned multimillion atomistic models show that the strain effects gain in prominence as we approach a few- to single-monolayer limit. First-principles calculations show a significant band gap shift of up to 125 meV per percent of strain. Finally, we measure the performance of multiple WSe₂ transistors fabricated on the same flake; a significant variability in threshold voltage and the “off” current setting is observed among the various devices, which is attributed in part to substrate-induced localized strain. Our integrated approach has broad implications for the direct imaging and quantification of interfacial effects in devices based on layered materials or heterostructures