Shen Lai

Plasma-Treated Thickness-Controlled Two-Dimensional Black Phosphorus and Its Electronic Transport Properties

We report the preparation of thickness-controlled few-layer black phosphorus (BP) films through the modulated plasma treatment of BP flakes. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with poly(methyl methacrylate) (PMMA) immediately after the plasma etching process. With these techniques, a high field-effect mobility was achieved, 1150 cm(2)/(V s), with an Ion/Ioff ratio of ∼10(5) at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its I-V characteristics and thus to exhibit excellent environmental stability over several weeks.

Large-Area Highly Conductive Transparent Two-Dimensional Ti2CTx Film

We report a simple and scalable method to fabricate homogeneous transparent conductive thin films (Ti2CTx, one of the MXene) by dip coating of an Al2O3 substrate in a colloidal solution of large-area Ti2CTx thin flakes. Scanning electron microscopy and atomic force microscopy images exhibit the wafer-scale homogeneous Ti2CTx thin film (∼5 nm) covering the whole substrate. The sheet resistance is as low as 70 Ω/sq at 86% transmittance, which corresponds to the high figure of merit (FOM) of 40.7. Furthermore, the thickness of the film is tuned by a SF6+Ar plasma treatment, which etches Ti2CTx film layer by layer and removes the top oxidized layer without affecting the bottom layer of the Ti2CTx flake. The resistivity of plasma-treated Ti2CTx film is further decreased to 63 Ω/sq with an improved transmittance of 89% and FOM of 51.3, demonstrating the promise of Ti2CTx for future transparent conductive electrode application.

Probing graphene defects and estimating graphene quality with optical microscopy

We report a simple and accurate method for detecting graphene defects that utilizes the mild, dry annealing of graphene/Cu films in air. In contrast to previously reported techniques, our simple approach with optical microscopy can determine the density and degree of dislocation of defects in a graphene film without inducing water-related damage or functionalization. Scanning electron microscopy, confocal Raman and atomic force microscopy, and X-ray photoelectron spectroscopy analysis were performed to demonstrate that our nondestructive approach to characterizing graphene defects with optimized thermal annealing provides rapid and comprehensive determinations of graphene quality.

Tunneling field effect transistor integrated with black phosphorus-MoS2 junction and ion gel dielectric

We report an interband tunneling field effect transistor (TFET) integrated with a Black Phosphorus (BP)-MoS2 junction and ion gel as a top gate dielectric. The operation of the BP-MOS2 TFET is based on the modulation of the energy band alignment of the BP-MoS2 junction with electrostatic gating control on the MoS2 channel from the top gate through the ion gel dielectric and the supply of tunneling carriers from the BP source, which is degenerately doped with ion gel. The obtained subthreshold swing of the BP-MoS2 TFET reached 65 mV/dec at room temperature and 51 mV/dec at 160 K, maintaining low SS values in more than 2 orders of drain current range. The demonstrated interband TFET based on the BP-MoS2 junction shows significant promise for further application to a new class of two-dimensional functional devices.

Epitaxial Growth of Large-Grain NiSe Films by Solid-State Reaction for High-Responsivity Photodetector Arrays

Film-based photodetectors have shown superiority for the fabrication of photodetector arrays, which are desired for integrating photodetectors into sensing and imaging systems, such as image sensors. But they usually possess a low responsivity due to low carrier mobility of the film consisting of nanocrystals. Large-grain semiconductor films are expected to fabricate superior-responsivity photodetector arrays. However, the growth of large-grain semiconductor films, normally with a nonlayer structure, is still challenging. Herein, this study introduces a solid-state reaction method, in which the growth rate is supposed to be limited by diffusion and reaction rate, for interface-confined epitaxial growth of nonlayer structured NiSe films with grain size up to micrometer scale on Ni foil. Meanwhile, patterned growth of NiSe films allows the fabrication of NiSe film based photodetector arrays. More importantly, the fabricated photodetector based on as-grown high-quality NiSe films shows a responsivity of 150 A W-1 in contrast to the value of 0.009 A W-1 from the photodetector based on as-deposited NiSe film consisting of nanocrystals, indicating a huge responsivity-enhancement up to four orders of magnitude. It is ascribed to the enhanced charge carrier mobility in as-grown NiSe films by dramatically decreasing the amount of grain boundary leading to scattering of charge carrier.

Water-penetration-assisted mechanical transfer of large-scale molybdenum disulfide onto arbitrary substrates

The transfer of two-dimensional (2D) material layers to arbitrary substrates from growth substrates is critical for many applications. Although several studies of transfer processes have been reported, a transfer method that does not degrade 2D layers and damage growth substrates is still required. In this paper, we report a method that with the assistance of water penetration enables the mechanical transfer of MoS2, one of the most widely studied 2D materials, from the growth substrate to a target substrate without any etching of the growth substrate or leaving any polymer residue on the MoS2 layer. The difference between the adhesion forces of the MoS2 and carrier films and the difference between the hydrophobicities of MoS2 and the growth substrate means that water can easily penetrate the interspace at the MoS2/growth substrate interface generated by the peeling off process. We also experimentally confirmed the usefulness of Cu carrier films as a contact material for MoS2 that enables its clean separation. Our transfer method protects the original quality and morphology of large area MoS2 without leaving any polymer residue, and enables the reuse of the growth substrate. This clean transfer approach is expected to facilitate the realization of industrial applications of MoS2 and other 2D materials.