September 11 | Edgars Butanovs

This Dissertation is dedicated to the development and characterization of various transition metal dichalcogenide based core-shell nanowire (NW) heterostructures. Novel synthesis methods of highly-crystalline core-shell NWs have been demonstrated, the nanomaterials structure, composition, morphology and photoelectrical properties have been characterized. In this work, ZnO-WS2, ZnO-MoS2, GaN-ReS2, ZnS-ReS2 and ZnO-ReS2 core-shell, as well as PbI2-decorated ZnO NWs were synthesised. Surface of ZnO NWs was passivated using thin PbI2 or WS2 coatings, and photodetectors of such hybrid materials exhibit reduced dark current and photoresponse time in comparison to pure ZnO NWs. The developed synthesis methods are not limited to the demonstrated heterostructures and can be applied for other materials.

Thesis:

1. Few-layer WS2 can be epitaxially grown on ZnO nanowires at 800°C by sulfurization of a WO3 coating, pre-deposited on nanowires via reactive magnetron sputtering. The WS2 shell enhances ZnO nanowire photosensitivity in the short wavelength range (spectral responsivity increases from 1.5 A/W to 7.0 A/W at 405 nm, dark current remains similar) and extends the spectral range to the red part of the spectrum. ZnO nanowire surface passivation with few-layer WS2 improves photoresponse time for almost two orders of magnitude from several seconds to tens of miliseconds.
2. Few-layer PbI2 was synthesised using a new method – iodination at 420°C of a PbOx coating, pre-deposited via reactive magnetron sputtering. ZnO nanowire decoration with few-layer PbI2 leads to significantly decreased dark current (from several nanoamperes to tens of picoamperes) at a cost of slightly reduced spectral responsivity from 1.5 A/W to 0.6 A/W at 405 nm. ZnO nanowire surface passivation with few-layer PbI2 decreases photoresponse time by more than one order of magnitude from several seconds to tens of miliseconds.
3. Decomposition and sulfurization at 700°C of an ammonium heptamolybdate tetrahydrate coating, pre-deposited on ZnO nanowires via solution immersion, was found to be a viable method for ZnO-MoS2 core-shell nanowire synthesis. Compared with similar two-step method of magnetron-sputtered MoO3 coating conversion, the immersion method of molybdenum precursor deposition yields significantly thinner and more uniform few-layer MoS2 shell compared to sputter-deposited MoO3.
4. Preparation of novel GaN-ReS2, ZnS-ReS2 and ZnO-ReS2 core-shell nanowire heterostructures was demonstrated using new few-layer ReS2 synthesis method – sulfurization at 800°C of a ReOx coating, pre-deposited via reactive magnetron sputtering. ZnO nanowire recrystallization and conversion to ZnS phase was observed at the temperature necessary for highly-crystalline ReS2 growth. GaN and ZnS nanowires were found to be stable for the core-shell heterostructure synthesis, however, the ReS2 shell crystalline quality was lower than that on the recrystallized ZnO substrate.