ZnO Nanowires on Graphite with Improved UV Photoluminescence

Abstract

INTRODUCTIONBroadband semiconductors (such as ZnO) stand out inopto and microelectronics as they allow the developmentof transparent electronics. ZnO, with its diversenanostructured morphologies (nanowires, nanorods,nanocrystalline thin films, etc.) is an extremely attractivecompound to use in a great variety of nanotechnologicalapplications. ZnO also exhibits a high exciton bidingenergy of 60 meV and a prohibited bandwidth in the UV(3,37 eV), making it a great candidate for applications inoptoelectronics1,2. In this work, a detailed characterizationof the morphology and photoluminescence (PL)properties of ZnO nanowires (NWs) grown on differentcarbon substrate is presented.EXPERIMENTAL STUDYZnO NWs were grown through the vapor-phase transportmethod, on carbon substrates (compacted graphite andcarbon fibers) in a tubular furnace under Ar and O flow.In contrast to NWs grown on Si or other semiconductorsubstrates, metallic catalysers3 are not necessary, as theNWs growth occurs directly on the surface of the carbonsubstrates. The morphology, chemical composition andthe stoichiometry of the NWs grown, as well as the maincharacteristics of the valence band density of states closeto Fermi level, were studied by x-ray photoelectronspectroscopy (XPS). Photoluminescence spectra were alsomeasured.RESULTS AND DISCUSSIONPL dependence with excitation power was measured andanalyzed in the different samples: ZnO NWs oncompacted graphite; ZnO NWs on carbon fibers; ZnONWs transferred on Si; and ZnO crystal (see Fig. 1) aswell as the evolution of the ultraviolet (UV) and greenemission intensities for all the samples (see Fig. 2). Theresults show an increase by 3 orders of magnitude in theUV emission intensity for the NWs grown on compactedgraphite with respect to the UV intensity measured in theothers samples and also in comparison to results obtainedin a previous work4.CONCLUSIONIncreasing the emission efficiency is of great interest forapplications in photonics and UV optoelectronics, and isusually achieved from the inhibition of visible emission,with the consequent increase in UV emission5. However,in the ZnO NWs grown on compacted graphite it wasfound that the high efficiency is produced by aconsiderable increase in UV intensity without the need tosuppress the visible emission.