Xu, Hao; (2019) Two-dimensional materials for nanoelectronic and optoelectronic applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Date back to 2004, atomically thin graphene nanosheets were initially discovered with an innovative mechanical exfoliation method, and the research focused on two-dimensional (2D) materials has been triggered thoroughly since the Nobel Prize in Physics was awarded for the ground-breaking work regarding 2D graphene in 2010. Until now, almost one decade has elapsed and witnessed the fast development of 2D family and its expansion. In this PhD research project, the primary effort has been devoted to synthesising, characterising novel 2D materials, uncovering their nature properties and fabricating them for nanoelectronic and optoelectronic applications. The detailed key findings are listed below. Few-layer black phosphorene quantum dots (BPQDs) were grown directly on Si substrates using the molecular beam epitaxy technique. Detailed experiments were carried out to evidence the formation of BPQDs and the results show that BPQDs follow the Frank-van der Merwe growth mode and the favoured few-layer growth trend with pyramid configuration. Self-powered phototransistors enabled by spatially bandgap-graded MoS2(1-x)Se2x homojunctions were achieved. The graded bandgaps, induced by the spatial Se-composition and thickness grading in plane, were varied from 1.83 eV to 1.73 eV, forming a homojunction with a built-in electric field. Consequently, a strong and sensitive photovoltaic effect controlled by gate voltage was demonstrated, enabling the homojunction phototransistors to deliver a photoresponsivity of 500 mA/W under the self-powered working mode. By utilising the self-oxidised edge domains that inevitably formed in air, back-gated field-effect transistors (FETs) on SiO2/Si substrates enabled by long-term air-exposed bilayer WSe2 were fabricated and studied. Detailed material characterisations proved the formation of WOx/WSe2 heterojunctions around edges of WSe2. With air-induced WOx passivating dangling bonds at edges of WSe2, the FETs possessed ohmic contact from 77 K to 300 K, high conductivity up to ~ 2600 S/m and ultra-low leakage gate current as small as ~ 10-12 A. Few-layer MoS2/glassy-graphene heterostructures on quartz were prepared, followed by the detailed material characterisations. Transparent photodetectors enabled by these novel van der Waals heterostructures were excited by monochromatic lasers for probing their photoresponsive features, respectively. Notably, the interface between MoS2 and glassy-graphene is ohmic contact, differing from the previously reported Schottky junction between MoS2 and graphene. The highest specific detectivity of 1.8×1010 Jones and the best photoresponsivity of 12.3 mA/W were achieved.

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