February 26 | Edgars Butanovs

Combining nanowires (NWs) and transition metal dichalcogenides (TMDs) in core-shell heterostructures could lead to new knowledge about the interface formation between different materials and solid-state reactions in such systems, to novel nanostructures with enhanced properties, and development of new TMDs synthesis methods as NWs are a convenient template to study materials growth. This Dissertation is dedicated to the development and characterization of various material NW and TMD core-shell heterostructures and to investigation of their applicability as photodetectors. Novel synthesis methods of core-shell NWs have been demonstrated. The methods developed in this work are not limited to the demonstrated heterostructures but can be applied for other materials, if their compatibility is considered. Furthermore, this work provides an insight into how a few-atomic-layer thin coating can change the photoelectric properties of a ZnO NW.

Thesis:

1. Synthesis of novel ZnO-WS₂ core-shell nanowire heterostructures by sulfurization of a WO₃ coating, pre-deposited on nanowires via reactive magnetron sputtering, at 800°C was demonstrated. ZnO-WS₂ single-nanowire photodetectors exhibit enhanced photosensitivity in the short wavelength range and improved photoresponse time in comparison to pure ZnO nanowires and have spectral responsivity up to 7 A/W at 405 nm wavelength illumination.

E. Butanovs, S. Vlassov, A. Kuzmin, S. Piskunov, J. Butikova, B. Polyakov. Fast-response single-nanowire photodetector based on ZnO/WS2 core/shell heterostructures. ACS Appl. Mater. Interfaces. 10, 13869-13876 (2018)

B. Polyakov, K. Smits, A. Kuzmin, J. Zideluns, E. Butanovs, J. Butikova, S. Vlassov, S. Piskunov, Y. Zhukovskii. Unexpected epitaxial growth of a few WS2 layers on {11 00} facets of ZnO nanowires. J. Phys. Chem. C. 120, 21451-21459 (2016)

2. Synthesis of novel PbI₂-decorated ZnO nanowire heterostructures was demonstrated. New few-layer PbI₂ synthesis method – iodination of a PbO_x coating, pre-deposited via reactive magnetron sputtering, at 420°C – was developed. ZnO-PbI₂ single-nanowire photodetectors exhibit reduced dark current and decreased photoresponse time in comparison to pure ZnO nanowires and have spectral responsivity up to 0.6 A/W at 405 nm wavelength illumination.

E. Butanovs, S. Piskunov, A. Zolotarjovs, B. Polyakov. Growth and characterization of PbI2-decorated ZnO nanowires for photodetection applications. J. Alloys Compd. 825, 154095 (2020)

3. Synthesis of novel ZnO-MoS₂ core-shell nanowire heterostructures by decomposition and sulfurization of an ammonium heptamolybdate tetrahydrate coating, pre-deposited on nanowires via solution immersion, at 700°C was demonstrated, and compared with similar two-step method of magnetron-sputtered MoO₃ coating conversion. Both methods produce crystalline MoS₂ shell, however, the immersion method of molybdenum precursor deposition yields significantly thinner few-layer MoS₂ shell compared to sputter-deposited MoO₃. Formation of ZnS interlayer was observed between the ZnO core and the MoS₂ shell.

E. Butanovs, A. Kuzmin, J. Butikova, S. Vlassov, K. Smits, B. Polyakov. Synthesis and characterization of ZnO/ZnS/MoS2 core-shell nanowires. J. Crys. Growth. 459, 100-104 (2017)

4. Synthesis of novel GaN-ReS₂, ZnS-ReS₂ and ZnO-ReS₂ core-shell nanowire heterostructures was demonstrated. New few-layer ReS₂ synthesis method – sulfurization of a ReO_x coating, pre-deposited via reactive magnetron sputtering, at 800°C – was developed. ZnO nanowire recrystallization and conversion to ZnS phase was observed below the temperature necessary for highly-crystalline ReS₂ growth, while GaN and ZnS nanowires were found to be stable for the core-shell heterostructure synthesis.

E. Butanovs, A. Kuzmin, S. Piskunov, A. Kalinko, B. Polyakov. Growth of a few-layer ReS₂/semiconductor nanowire core/shell heterostructures by rhenium oxide sulfurization. Submitted manuscript.