Encapsulation of 2-3-nm-Sized ZnO Quantum Dots in a SiO2 Matrix and Observation of Negative Photoconductivity

Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO2 matrix using aqueous chemically grown ZnO nanoparticles in a precursor of tetraethyl orthosilicate The microstructure shows almost a uniform embedment of the QDs in the SiO2 matrix, resulting in a ZnO QDs-SiO2 composite structure The phorocurrent transients of the composite show an instant fall in the Current followed by an exponential decay under ultraviolet (UV) illumination, causing negative photoconductivity (NPC), in contrast to the positive photoconductivity in only ZnO nanoparticles The interface defect states due to the presence of the SiO2 network around ZnO act as charge trap centers for the photoexcited electrons and are responsible for the NPC The presence of interface-trapped charges under UV illumination has been further confirmed from capacitance-voltage measurements