Pressure-Driven One-Dimensional Superlattices in Monolayer Crystals on a Vicinal Surface

A 1D superlattice can be imposed on monolayer materials by the periodic atomic steps of a vicinal crystalline surface. By means of the rotational anisotropy of second harmonic generation, we demonstrate that the periodic modulation of monolayer WS2 by the diamond (230) surface gives rise to an orientation-dependent breaking of crystal symmetry, a fundamental prerequisite for generation of a one-dimensional superlattice from a two-dimensional lattice, when subjected to external pressure. These observations contrast with the case of monolayer WS2 on the diamond (100) surface, which largely retains its 3-fold rotational symmetry when pressure is applied. Vicinal surfaces offer a potentially versatile approach to the nanoscale periodic in-plane modulation of layered crystals and engineering their electronic and optical functionality.

Junction Field-Effect Transistors Based on MoSe2/WSe2 Heterostructures for High-Performance Photodetection

Two-dimensional (2D) materials show great potential in creating high-performance ultracompact photodetectors. Existing 2D photodetectors are usually designed based on a photogating effect or photovoltaic effect. However, achieving a balance between photodetectivity and photoresponsivity presents a significant challenge due to increased dark currents at trap level recombination or the lack of a gain mechanism. Herein, we rationally design a gate-tunable junction field-effect transistor photodetector based on MoSe2 and WSe2. With proper modulating the depletion layer and Schottky barrier using source-drain and gate bias, the device can effectively reduce dark current, resulting in an ultrahigh photodetectivity of 1.55 × 1013 Jones and an ultrahigh optical switching ratio of 104. Furthermore, our photodetector exhibits a high photoresponsivity of 476 A/W and an ultrafast response time of 50 μs under 635 nm laser irradiation with an extended detection capability to the 1550 nm band. These outstanding performances highlight the potential of 2D heterojunctions in addressing the growing demands of next-generation photonic sensors.

Tailored Growth of Transition Metal Dichalcogenides' Monolayers by Chemical Vapor Deposition

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the "standard" TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD1-TMD2 ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.