About defect phenomena in ZnO nanocrystals.

ZnO nanocrystals are receiving renewed attraction due to their multifunctional properties. Selective enhancement and tuning of their optical and electrical properties are essential for achieving novel devices with accurate sensing and conducting capabilities. The nature and type of intrinsic defects that occur in ZnO influence these properties. In this work, we investigate the intrinsic defect structure of ZnO via electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy and correlate the results with existing computational works. Mainly, the defects are analysed by taking the microscopic defect structure of the lattice into account. The results manifest the core-shell model of the defect structure in ZnO. By default, specifically for nanocrystals, oxygen vacancies localise on the surface, while zinc vacancies localise in the core. The investigations in this report demonstrate that the concentration of the intrinsic defects and their position can be tuned merely by changing the size of the nanocrystal. Additionally, the UV, green, orange and red emissions can be tuned by nanocrystal's size and post-annealing treatments.

Increasing bulk photovoltaic current by strain tuning.

Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO3 bulk single crystal the existence of a purely intrinsic "piezophotovoltaic" effect that leads to a linear increase in photovoltaic current density. The increase reaches 75% under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented.