Symmetry Breaking and Spin-Orbit Coupling for Individual Vacancy-Induced In-Gap States in MoS2 Monolayers.

Spins confined to point defects in atomically thin semiconductors constitute well-defined atomic-scale quantum systems that are being explored as single-photon emitters and spin qubits. Here, we investigate the in-gap electronic structure of individual sulfur vacancies in molybdenum disulfide (MoS2) monolayers using resonant tunneling scanning probe spectroscopy in the Coulomb blockade regime. Spectroscopic mapping of defect wave functions reveals an interplay of local symmetry breaking by a charge-state-dependent Jahn-Teller lattice distortion that, when combined with strong (≃100 meV) spin-orbit coupling, leads to a locking of an unpaired spin-1/2 magnetic moment to the lattice at low temperature, susceptible to lattice strain. Our results provide new insights into the spin and electronic structure of vacancy-induced in-gap states toward their application as electrically and optically addressable quantum systems.

Nanoironing van der Waals Heterostructures toward Electrically Controlled Quantum Dots.

Assembling two-dimensional van der Waals (vdW)-layered materials into heterostructures is an exciting development that sparked the discovery of rich correlated electronic phenomena. vdW heterostructures also offer possibilities for designer device applications in areas such as optoelectronics, valley- and spintronics, and quantum technology. However, realizing the full potential of these heterostructures requires interfaces with exceptionally low disorder which is challenging to engineer. Here, we show that thermal scanning probes can be used to create pristine interfaces in vdW heterostructures. Our approach is compatible at both the material- and device levels, and monolayer WS2 transistors show up to an order of magnitude improvement in electrical performance from this technique. We also demonstrate vdW heterostructures with low interface disorder enabling the electrical formation and control of quantum dots that can be tuned from macroscopic current flow to the single-electron tunneling regime.

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition.

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications.

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

A first-principles study on strain engineering of monolayer stanene for enhanced catalysis of CO2 reduction.

Using first-principles calculations, we investigated the changes in the lattice structure, electronic structures and catalytic performance for CO2 reduction reaction (CO2RR) of stanene under applied strain. Our calculations showed that the initial buckled honeycomb structure of free-standing stanene becomes increasingly flat upon the increase of tensile strain. Stanene remains its gapless semiconductor characteristic within the strain range of -2% and 2%, beyond which a semiconductor-to-metal transition occurs. Under strain, the adsorption of CO is weakened, which can facilitate the desorption of product CO, enabling a strained stanene to be a better catalyst for CO2RR to CO than strain-free stanene. In particular, the stanene with 4% strain may give rise to the best performance because of the weakest CO adsorption (Eadsorp= -0.15 eV). The adsorption of intermediate product COOH on stanene is tunable with strain. We also evaluated the overall catalytic performance of the strained stanene based on the adsorption of CO and COOH and the selectivity against HER. If the reduction of COOH is governed by adsorption of the intermediate, a 10% strain may give a stronger COOH adsorption, weaker CO adsorption and better selectivity against HER, leading to an enhanced catalytic performance for CO2RR to CO. On the other hand, if the reduction of COOH is governed by desorption, a tensile strain higher than 4% may result in an enhanced catalytic performance. Our study here suggests that strain-tuned stanene might serve as an optimal electrocatalyst for CO2RR to CO with a high activity and selectivity.