Li, Xiao; (2022) Surface Effects of Low-dimensional Materials for Electronic and Optoelectronic Applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Owing to the reduced volume, the properties of low-dimensional materials can be significantly modified or even predominated by surface effects, which are correlated with intrinsic defects, environmental factors, and surface functionalisation. Fully understanding and reasonably controlling the surface effects are critical and essential for the development of advanced structures by taking advantage of the fascinating properties of low-dimensional materials. This work is devoted to the investigation of unclear but crucial surface effects, including the edges, surface adsorbates, and surface passivation of low-dimensional materials in their electronic and optoelectronic applications. These results are expected to provide useful insights into controlling the surface properties of low-dimensional materials for promising applications. Initially, the effects of edges on the hysteresis of graphene electrical transport are explored. An unusual hysteresis of electrical transport at room temperature is observed through the introduction of longer edges in graphene via patterning. It is suggested to be caused by the edge-induced local capacitive gating enhancement. The larger hysteresis under laser illumination reveals that the longer edges enable more charge exchange between the graphene and the environment. Then, the environmental effects on the electrical properties of two-dimensional materials in contact with chemical volatile organic compounds (VOCs) are examined. The surface adsorbates are found to play a crucial role in the adsorption of VOCs on glassy graphene in ambient conditions. The electrical response of glassy graphene to acetone is clarified to be mainly attributed to the carrier transfer with the assistance of the adsorbed oxygen species on the surface. Additionally, the effects of surface passivation on the optoelectronic properties of GaAs nanowire photodetectors are studied. The surface passivation not only reduces the surface recombination of GaAs nanowires, importantly, also changes the Schottky barrier height at the interface. As a result, the designed GaAs nanowire-based photodetectors demonstrate one order of magnitude higher photosensitivity, up to 4.5× 104 AW-1, and a significantly shortened response time after surface passivation. Future work will continue to explore the surface effects in a more quantitative manner. In addition, decreasing undesirable surface effects in 2D heterostructures also has the potential to be investigated.

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