UCLA

The discovery of graphene, made of single-layer carbon atoms, defines the starting point in the research and development of stable two-dimensional layer materials (2DLMs). Graphene has been one of the most extensively studied materials due to its unique band structure, the linear dispersion at the K point. It gives rise to novel phenomena, such as the anomalous quantum Hall effect, and has opened up a new category of "Fermi-Dirac" physics. Graphene has also attracted enormous attention for future electronics because of its exceptional high carrier mobility, high carrier saturation velocity, and large critical current density. Graphene's success has shown that other 2D materials beyond graphene may also exhibit fascinating properties and be used for accelerate the development of technology. Two-dimensional transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), are emerging as an exciting material system for future electronics due to their unique electronic properties and atomically thin geometry.

In this dissertation, I will firstly present my research studies on the first experimental observation of a dramatic enhancement of the conductance in a GNR field-effect transistor by a perpendicular magnetic field. Very large negative MR of nearly 100% with conductance enhanced over 10,000 times was observed at low temperatures; and more than 50% remained at room temperature. Then I will show a new approach for the scalable fabrication of high performance graphene transistors with transferred gate stacks. This unique device structure enables scalable fabrication of self-aligned graphene transistors with unprecedented performance including a record high cut-off frequency up to 427 GHz. Taking a step further, I will demonstrate the best performed MoS2 transistors with an on-off ratio exceeding 107, excellent current saturation and a highest intrinsic gain over 30. On-chip microwave measurements demonstrate a highest intrinsic cut-off frequency fT of 42 GHz and a maximum oscillation frequency fMAX of 50 GHz. Furthermore, I will, for the first time, present the integration of multiple MoS2 transistors on quartz and flexible substrates to form a logic inverter or radio frequency amplifier with voltage gain in the gigahertz regime. Finally I will present the first demonstration of an atomically thin heterojunction p-n diode by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurement demonstrates excellent diode characteristics with well defined current rectification behaviour, and photocurrent mapping shows clear photoresponse in the entire overlapping region with an external quantum efficiency as large as 12 %. Electroluminescence studies show prominent emission with both excitonic and hot electron luminescence peaks. A systematic investigation of the emission spectra reveals distinct layer-number dependent and temperature dependent characteristics and can offer important insight about the electron-orbital interaction in the layered materials. These findings can open up exciting new opportunities for 2DLMs in the application of electronics and optoelectronics.