Synthesis and Characterization of Transition Metal Dichalcogenide Nanoribbons Based on a Controllable O2 Etching.

Although the synthesis of monolayer transition metal dichalcogenides has been established in the last decade, synthesizing nanoribbons remains challenging. In this study, we have developed a straightforward method to obtain nanoribbons with controllable widths (25-8000 nm) and lengths (1-50 µm) by O2 etching of the metallic phase in metallic/semiconducting in-plane heterostructures of monolayer MoS2. We also successfully applied this process for synthesizing WS2, MoSe2, and WSe2 nanoribbons. Furthermore, field-effect transistors of the nanoribbons show an on/off ratio of larger than 1000, photoresponses of 1000%, and time responses of 5 s. The nanoribbons were compared with monolayer MoS2, highlighting a substantial difference in the photoluminescence emission and photoresponses. Additionally, the nanoribbons were used as a template to build one-dimensional (1D)-1D or 1D-2D heterostructures with various transition metal dichalcogenides. The process developed in this study offers simple production of nanoribbons with applications in several fields of nanotechnology and chemistry.

Versatile Post-Doping toward Two-Dimensional Semiconductors.

We have developed a simple and straightforward way to realize controlled postdoping toward 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic-energy dopant beams and a high-flux chalcogen beam simultaneously, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. The electronic properties of doped TMDs (Nb-doped WSe2) have shown drastic change and p-type action with more than 2 orders of magnitude increase in current. Position-selective doping has also been demonstrated by the postdoping toward TMDs with a patterned mask on the surface. The postdoping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

Enhanced Exciton-Exciton Collisions in an Ultraflat Monolayer MoSe2 Prepared through Deterministic Flattening.

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 1012 cm-2 µN-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φph ∼ 2.3 × 1019 cm-2 s-1) due to the enhanced collisions between excitons.

Stabilization of metallic phases through formation of metallic/semiconducting lateral heterostructures.

In this study, we develop a new approach for stabilization of metallic phases of monolayer MoS2 through the formation of lateral heterostructures composed of semiconducting/metallic MoS2. The structure of metallic (a mixture of T and T') and semiconducting (2H) phases was unambiguously characterized by Raman spectroscopy, x-ray photoelectron spectroscopy, photoluminescence imaging, and transmission electron microscope observations. The amount of NaCl, reaction temperature, reaction time, and locations of substrates are essential for controlling the percentage of metallic/semiconducting phases in lateral heterostructures; loading a large amount of NaCl at low temperatures with short reaction times prefers metallic phases. The existence of the semiconducting phase in MoS2 lateral heterostructures significantly enhances the stability of the metallic phases through passivation of reactive edges. The same approach can be applied to other transition metal dichalcogenides (TMDs), such as WS2, leading to boosting of basic research and application of TMDs in metallic phases.