ZnO nanowires: surface functionalization with colloidal semiconductor quantum dots and excitation-intensity-dependent photoluminescence properties

Abstract

Contemporary nanotechnology has great interest in the assembly and study of hybrid structures composed of different materials that can offer enhanced properties or achieve new functions through the interaction between different constituents. ZnO nanowires, with the wide direct band gap and large exciton binding energy of the material, have attracted intense research interests for their potential optoelectronic applications in the near ultraviolet spectral region. They have also been studied for use in energy conversion and storage. They are promising candidates as photoanodes in nanostructured photovoltaic devices. Their high electron mobility favors a quick collection of the photogenerated charge carriers. Semiconductor quantum dots represent another research focus due to their striking size-tunable band gap, which provides a convenient approach to tune the spectral region of absorption and emission of the materials. In this work, a hybrid nanostructure is built by surface functionalization of ZnO nanowire arrays with colloidal CdSe quantum dots (QDs), which has potential applications in photovoltaics and sensing applications. The QDs are synthesized by a wet-chemical method and stabilized with bifunctional 3-mercaptopropionic acid (MPA) molecules. The growth kinetics and optical properties of the QDs are studied. The average QD size can be tuned in the range of 1.4 - 2.5 nm by adjusting the growth time. The photoluminescence of the dry QD powder is found to be very sensitive to the ambient environment, which is attributed to oxygen-related surface effects. Adsorption of oxygen molecules can passivate the surface defects of the QDs which otherwise act as trap centers during photo-excitation and induce nonradiative and fast Auger recombination processes in the QDs. The attachment of the CdSe QDs on the ZnO nanowire surface is achieved by using the stabilizers of the QDs as molecular linkers, which further favors the charge transfer between these two systems. The photoconductivity of the nanowire/quantum-dot hybrid structure is studied under selective photoexcitation of the QDs. An enhancement of of the photoconductivity up to 10 times is observed in air. The dynamics is further found to strongly depend on the gas environment. Desorption of surface oxygen from the ZnO nanowires, activated by charge tunnelling between the nanowires and the QDs, is found to be the dominating process for the photoconductivity enhancement. The gas environment influences the charge relaxation in the QDs through oxygen-related surface passivation, which impacts the charge tunnelling between the nanowires and the QDs and, hence, the photoconductivity dynamics. Defects in ZnO materials can significantly influence their physical properties such as the electrical conductivity and luminescence spectra. Photoluminescence spectroscopy is a convenient, non-destructive technique for studying the crystal quality and defect states of semiconductors. ZnO generally shows an ultraviolet emission band due to the near-band-edge recombination processes and several defect-related emission bands in the visible spectral region. The dependence of the photoluminescence properties on the excitation intensity of ZnO nanowires and bulk wafers is studied. It is found that the relative strength of the band-edge emission and the defect-related emission dramatically varies with the excitation intensity. The increase of these two emission bands with excitation intensity further exhibits sample-specific behaviors, which depends on the defect species and concentrations and the microscopic origins of the defect-related emission processes. Low-temperature measurements further reveal that biexcitons or even an electron-hole plasma (EHP) may be formed in ZnO nanowires for excitation intensities >10^3 W/cm^2, which causes the broadening of the donor-bound exciton emission peak and the superlinear increase of the band-edge emission with excitation intensity.