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Polymorphous WO3 micro- and nanostructures have been synthesized by the controlled Joule heating of tungsten wires under ambient conditions in a few seconds. The growth on the wire surface is assisted by the electromigration process and it is further enhanced by the application of an external electric field through a pair of biased parallel copper plates. In this case, a high amount of WO3 material is also deposited on the copper electrodes, consisting of a few cm 2 area. The temperature measurements of the W wire agrees with the values calculated by a finite element model, which has allowed us to establish the threshold density current to trigger the WO3 growth. The structural characterization of the produced microstructures accounts for the γ -WO3 (monoclinic I), which is the common stable phase at room temperature, along with low temperature phases, known as δ -WO3 (triclinic) on structures formed on the wire surface and ϵ -WO3 (monoclinic II) on material deposited on external electrodes. These phases allow for a high oxygen vacancies concentration, which is interesting in photocatalysis and sensing applications. The results could help to design experiments to produce oxide nanomaterials from other metal wires by this resistive heating method with scaling-up potential. [ABSTRACT FROM AUTHOR]

Zn2GeO4 microrods obtained by thermal evaporation of a compacted powder mixture of ZnO and Ge exhibit quite intense UV luminescence at low temperatures. Herein, the luminescence properties of Zn2GeO4 microrods are studied for 2:1 and 1:1 ZnO:Ge ratio in the precursor mixture. In both cases, Zn2GeO4 microrods of high crystal quality produce a 355 nm emission under aforementioned bandgap excitation conditions at low temperatures. However, this emission vanishes at room temperature (RT) in the 1:1 samples while it is kept in the 2:1 ones. Herein this work, the thermal quenching of the UV luminescence is studied by means of steady and time‐resolved photoluminescence techniques from 4 K up to RT for both Zn2GeO4 microrods. The analysis of the results leads us to conclude that although the luminescence mechanisms are the same in both cases, a higher decay rate is observed in the 1:1 in both intensity and lifetime, which explain the observed thermal quenching at RT. [ABSTRACT FROM AUTHOR]

Novel transparent semiconducting oxides have emerged as key materials for a variety of optoelectronic applications due to their ultra-wide band gap. Owing to the critical role of native defects and impurities in the optoelectronical properties, investigation in detail of the electronic structure of these oxides has become a challenge, being luminescence technique a powerful way to gain insight into the electronic levels. Here, the photoluminescence temperature dependence of Li doped Zn 2 GeO 4 microrods is analysed and discussed in the framework of the electronic levels structure related to native defects and Li impurities. Unlike the undoped material, the blue emission (2.8 eV) of Li-doped germanate exhibits a negative thermal quenching as its photoluminescence intensity increases with increasing temperature. The mechanisms underlying this anomalous behaviour are discussed with the aid of X-ray photoemission spectroscopy and first principles calculations. This study has helped us to understand the role of Li in the electronic structure and optical reponse of Zn 2 GeO 4. Our results highlight the versatility of Zn 2 GeO 4 luminescence through Li doping, which could be exploited in optoelectronic devices with bespoke properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The optical properties of zinc germanate microrods are investigated and correlated with vibrational modes of the crystalline structure. The samples are grown by a thermal evaporation method using ZnO and Ge as precursors. Polarized Raman spectroscopy and polarized micro‐photoluminescence techniques are applied in order to shed some light into the origin of the luminescence in Zn2GeO4. Oxygen defects are usually responsible for light emission in most of semiconducting oxides. Here, the authors report two luminescence bands at room temperature, a broad green one at about 2.4 eV and an ultraviolet one at 3.2 eV, which are of composed nature. In addition, the authors observe changes in the Raman and PL spectra recorded with polarizers oriented parallel or orthogonal to the microrod axis. The evolution of the luminescence bands in Zn2GeO4 and their correlation with the observed Raman modes (O‐Ge‐O and Ge‐O‐Zn vibration modes) are discussed in the framework of oxygen defects in the Zn2GeO4 structure. Vibrational modes and luminescence bands of Zn2GeO4 microwires are investigated. The correlation between vibrational modes in polarized Raman measurements and luminescence bands are explored. Besides the orientation of the wires, some insights into the oxygen defects that are involved in the luminescence bands are discussed. [ABSTRACT FROM AUTHOR]