Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

In Situ Atomic-Scale Observation of Monolayer MoS2 Devices under High-Voltage Biasing via Transmission Electron Microscopy

2D materials have great potential for not only device scaling but also various applications. To prompt the development of 2D electronics and optoelectronics, a better understanding of the limitation of materials is essential. Material failure caused by bias can lead to variations in device behavior and even electrical breakdown. In this study, the structural evolution of monolayer MoS₂ with high bias is revealed via in situ transmission electron microscopy at the atomic scale. The biasing process is recorded and studied with the aid of aberration-corrected scanning transmission electron microscopy. The effects of electron beam irradiation and biasing are also discussed through the combination of experiments and theory. It is found that the Mo nanoclusters result from disintegration of MoS₂ and sulfur depletion, which are induced by Joule heating. The thermal stress can also damage the MoS₂ layer and form long cracks in both in situ and ex situ biasing cases. Investigation of the results obtained with different applied voltages helps to further verify the mechanism of evolution and provide a comprehensive study of the function of biasing.

Open-source automated chemical vapor deposition system for the production of two- dimensional nanomaterials

The study of two- dimensional (2D) materials is a rapidly growing area within nanomaterials research. However, the high equipment costs, which include the processing systems necessary for creating these materials, can be a barrier to entry for some researchers interested in studying these novel materials. Such process systems include those used for chemical vapor deposition, a preferred method for making these materials. To address this challenge, this article presents the first open-source design for an automated chemical vapor deposition system that can be built for less than a third of the cost for a comparable commercial system. The materials and directions for the system are divided by subsystems, which allows the system to be easily built, customized and upgraded, depending upon the needs of the user. We include the details for the specific hardware that will be needed, instructions for completing the build, and the software needed to automate the system. With a chemical vapor deposition system built as described, a variety of 2D nanomaterials and their heterostructures can be grown. Specifically, the experimental results clearly demonstrate the capability of this open-source design in producing high quality, 2D nanomaterials such as graphene and tungsten disulfide, which are at the forefront of research in emerging semiconductor devices, sensors, and energy storage applications.