Cheng-Hua Liu - Electrical and Optoelectronic Properties of the Nanodevices Composed of Two-Dimensional Materials: Graphene and Molybdenum (IV) Disulfide (Springer Theses)

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The high-quality graphene p-n and p-n-p junction devices are achieved by controlling the metal diffusion locally. The metal deposited on graphene surface can introduce substantial carrier scattering, limiting the mobility of intrinsic graphene. On the other hand, the weakly functionalization with small carrier scattering can achieve the p-type doping on graphene, enabling us to fabricate the graphene p-n-p junctions. Specifically, the p-type doping regions are contributed by with metal diffusion while he n-type doping is intrinsic. By engineering the lateral diffusion of metallic contacts, the graphene p-n and p-n-p junction device can be realized in one-step with resist-free fabrication. The high-quality of graphene p-n and p-n-p junctions is further substantiated by a pronounced fractional number of quantum Hall (QH) plateau.

Distinctive magnetotransport properties of high-quality graphene p-n and p-n-p junction device described above have been further investigated. For both devices, the temperature dependence of resistance follows a power law and the analysis of the exponent indicates the dominant role of electronhole puddles in the transport behavior. We have also utilized asymmetric method to achieve lateral diffusion in one of the two-terminal electrodes, resulting in graphene p-n junction, as evidence of pronounced QH effect. In addition, the interesting QH effect with a fractional-valued plateau as well as the Shubnikov-de Haas oscillations are demonstrated in our high-quality graphene p-n-p junction device. We observed a well-defined QH plateau-plateau transition of zeroth Landau level, yielding a scaling exponent of . Furthermore, the graphene p-n-p junctions exhibit weak localization behavior, and the coherence length was found to be correlated to carrier scattering in the graphene devices.

We demonstrate a giant persistent photoconductivity (PPC) effect in monolayer MoS 2 in which the photocurrent robustly persists after illumination has ceased. The PPC effect in monolayer MoS 2 FET fabricated on organic-molecule-functionalized substrates sustains up to room temperature and can be highly suppressed by applying a sourcedrain or back gate voltage to the transistors. The persistency and controllability of the PPC effect enable us to achieve a bistable conductance at room temperature by utilizing optical and electrical pulses, paving the way to the applications in memory devices. The observed giant PPC effect in MoS 2 can be attributed to a large electron-capture barrier of trap states, which is estimated to be as high as 390 meV.