Browsing by Author "Li, Wei, Ph. D."

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Charge and energy transport properties of two-dimensional transition metal dichalcogenides
(2018-08-14) Li, Wei, Ph. D.; Akinwande, Deji; Zheng, Yuebing; Lai, Keji; Chen, Ray
Since the first discovery of graphene, two-dimensional (2D) materials have drawn tremendous attention due to their atomic thickness and superior properties. Among the family of 2D materials, transition metal dichalcogenides (TMDs) with sizable bandgaps, which differ from pristine graphene with semi-metallic character, have attracted interest for use in building next generation electronics and optoelectronics devices. However, the notable gap between the theoretically predicted behaviors and experimental observations of 2D TMD based devices has been reported, due to their sensitivity to extrinsic factors. It is crucial to study and understand the charge and energy transport properties of 2D TMDs, which can establish strategies for achieving desired device functionalities and improving their performance. In this dissertation, we report the successful access of monolayer and few layer TMDs using mechanical exfoliation, and large area MoS₂ atomic layer sheets by CVD. In Chapter 3, the electrical mapping of the local conductance in MoS₂ FETs is obtained by microwave impedance microscopy (MIM), and the spatial evolution of the insulator-to-metal transition is clearly resolved as a function of the back-gate voltages. Furthermore, vertical heterostructures consisting of alternate n-type and p-type TMD semiconductors are fabricated using custom dry-transfer setup. The MIM measurement on TMD heterostructure FETs demonstrates unusual charge density and energy band modulation at the heterostructure region. The transport properties show a "anti-ambipolar" behavior in the MoS₂/WSe₂ heterojunction FETs, which is induced by the strong interlayer charge recombination between two type TMDs. In Chapter 4, we demonstrate the plasmon-trion and plasmon-exciton resonance energy transfer (RET) from a single AuNT to monolayer MoS₂ at room temperature. Switching between the plasmon-trion and plasmon-exciton RET has been achieved by varying the dielectric constant of the surrounding media of AuNT-MoS₂ hybrids, which results in tuning of trion population in the MoS₂ via dielectric screening. Moreover, we develop the opto-thermoplasmonic nanolithography (OTNL), an all-optical lithographic technique, to achieve high-throughput, versatile and maskless patterning of different atomic layers. The low-power and high-resolution patterning of both graphene and MoS₂ monolayers has been achieved through exploiting thermal oxidation and sublimation at the highly localized thermoplasmonic hot spots.