Recent advances in 2D TMD circular photo-galvanic effects

Two-Dimensional Transition Metal Dichalcogenides: A Theory and Simulation Perspective

Two-dimensional transition metal dichalcogenides (2D TMDs) are a promising class of functional materials for fundamental physics explorations and applications in next-generation electronics, catalysis, quantum technologies, and energy-related fields. Theory and simulations have played a pivotal role in recent advancements, from understanding physical properties and discovering new materials to elucidating synthesis processes and designing novel devices. The key has been developments in ab initio theory, deep learning, molecular dynamics, high-throughput computations, and multiscale methods. This review focuses on how theory and simulations have contributed to recent progress in 2D TMDs research, particularly in understanding properties of twisted moiré-based TMDs, predicting exotic quantum phases in TMD monolayers and heterostructures, understanding nucleation and growth processes in TMD synthesis, and comprehending electron transport and characteristics of different contacts in potential devices based on TMD heterostructures. The notable achievements provided by theory and simulations are highlighted, along with the challenges that need to be addressed. Although 2D TMDs have demonstrated potential and prototype devices have been created, we conclude by highlighting research areas that demand the most attention and how theory and simulation might address them and aid in attaining the true potential of 2D TMDs toward commercial device realizations.

Room-Temperature Geometrical Circular Photocurrent in Few-Layer MoS2

Valleytronics, i.e., the manipulation of the valley degree of freedom, offers a promising path for energy-efficient electronics. One of the key milestones in this field is the room-temperature manipulation of the valley information in thick-layered material. Using scanning photocurrent microscopy, we achieve this milestone by observing a geometrically dependent circular photocurrent in a few-layer molybdenum disulfide (MoS2) under normal incidence. Such an observation shows that the system symmetry is lower than that of bulk MoS2 material, preserving the optical chirality-valley correspondence. Moreover, the circular photocurrent polarity can be reversed by applying electrical bias. We propose a model where the observed photocurrent results from the symmetry breaking and the built-in field at the electrode-sample interface. Our results show that the valley information is still retained even in thick-layered MoS2 at room temperature and opens up new opportunities for exploiting the valley index through interface engineering in multilayer valleytronics devices.

Asymmetric contact-induced selective doping of CVD-grown bilayer WS2 and its application in high-performance photodetection with an ultralow dark current

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are excellent candidates for high-performance optoelectronics due to their high carrier mobility, air stability and strong optical absorption. However, photodetectors made with monolayer TMDs often exhibit a high dark current, and thus, there is a scope for further improvement. Herein, we developed a 2D bilayer tungsten disulfide (WS2) based photodetector (PD) with asymmetric contacts that exhibits an exceptionally low dark current and high specific detectivity. High-quality and large-area monolayer and bilayer WS2 flakes were synthesized using a thermal chemical vapor deposition system. Compared to conventional symmetric contact electrodes, utilizing metal electrodes with higher and lower work functions relative to bilayer WS2 aids in achieving asymmetric lateral doping in the WS2 flakes. This doping asymmetry was confirmed through the photoluminescence spectral profile and Raman mapping analysis. With the asymmetric contacts on bilayer WS2, we find evidence of selective doping of electrons and holes near the Ti and Au contacts, respectively, while the WS2 region away from the contacts remains intrinsic. When compared with the symmetric contact case, the dark current in the WS2 PD with asymmetric (Au, Ti) contact decreases by an order of magnitude under reverse bias with a concomitant increase in the photocurrent, resulting in an improved on/off ratio of ∼105 and overall improved device performance under identical illumination conditions. We explained this improved performance based on the energy band alignment showing a unidirectional charge flow under light illumination. Our results indicate that the planar device structure and compatibility with current nanofabrication technologies can facilitate its integration into advanced chips for futuristic low-power optoelectronic and nanophotonic applications.

Tunable optical bistability of two-dimensional tilted Dirac system

We study the phenomenon of controlling the light by light known as the optical bistability for the two-dimensional tilted Dirac system. Using the Boltzmann approach under relaxation time approximation, we find that the optical bistability can be controlled by the nonlinear response of the system. For the prototype, we consider an inversion symmetry broken system. We find that the optical bistability associated with the nonlinear response is tunable with the strength of the tilt, gap and chemical potential. The resulting features suggest the inputs for the development of future-generation optical devices.