Solution Based Synthesis of doped ZnO Nanowires with Rare Earth Elements for Optoelectronic Applications

• Autor(a) principal: Pina, Mafalda de Almeida Duarte
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/92226
• topic: Conventional oven; Europium; Microwave; Nanowire; Solvothermal; UV sensors; Domínio/Área Científica::Engenharia e Tecnologia::Nanotecnologia

Semiconductor nanostructures, and in particular ZnO nanowires (NWs), have been intensively studied due to their unique properties that allow its implementation in optoelectronic devices such as UV sensors and 1D TFTs. The doping of these NWs with rare earth elements allows to tune its conductivity and optical properties. The present work reports the synthesis and characterization of undoped and europium doped ZnO NWs, produced through solvothermal methods assisted by microwave irradiation (MW) and conventional oven heating (CO). Different doping concentration levels were this way studied. Both synthesis methods were successful in the production of ZnO nanostructures as confirmed by SEM and XRD, and the doping of Eu lead to the formation of Eu(OH)3 nanostructures attached to the ZnO NWs. The MW synthesis lead to the formation of aggregated NWs forming nanoflowers while in the CO route the aggregation was less evident, with more isolated structures, both with average lengths of 6 μm and a very broad size distribution. Optical characterization allowed to extract the band gap (Eg≈3.2 eV). Electrical characterization was conducted on single ZnO NWs, with different alignment methodologies and in NWs with different post-annealing treatments. An optimization of the alignment strategy to add contacts to isolated NWs using conventional photolithography was achieved. In each substrate 6 individual NWs were aligned. After processing, the yield of working devices was still low as concluded from the electrical characterization where it was possible to extract a resistivity value in just 5 of the measured NWs and no trend for the resistivity values was observed. The use of a passivation layer (Parylene-C) on top of the NWs allowed to improve the stability and adhesion of the NW to the substrate. These nanostructures were tested as UV sensors showing a responsivity of 1.5 mA/W, 1.7 mA/W and 53 μA/W for ZnO NWs without annealing, annealing at 300°C and 700°C.