EELS Investigation of the Work Function Reduction in Au decorated ZnO Nanotapers

Abstract

The study of field emission (FE) from one dimensional (1D) nanostructures is emerging as a promising technology that can make a considerable contribution in the development of next generation devices such as electron microscopes, x‐ray sources, flat panel displays and microwave devices. Generally, FE from carbon nanotubes has been extensively studied, although these have high work function (~5.2 eV) and low electron emission. The electron emission is also influenced by the density of state (DOS) of the emission site. The DOS can be increased by either some doping or by coating the surface with a much higher DOS material. Owing to the low work function, enhanced currents have been demonstrated in several ZnO systems, for example nanowires, nanopins, nanorods, tetrapod like, nitrogen implanted nanowires, Au coated ZnO nanowires etc. The Au‐coated ZnO nanowires show low turn‐on potential and excellent stability. The work function of ZnO can be altered either by hybridization with a donor organometallic, or inorganic molecule / polymer by the attachment of dipolar self‐assembled monolayers perpendicular to the surface of ZnO, which induces charge transfer between the adsorbate molecule and substrate surface. The incorporation of Au ions on the surface modifies the surface morphology and that influences the enhanced field emission by lowering the Φ. The possibility of tuning the band gap by coating the ZnO surface with Au nanoparticles is very important because of its potential application in light emission devices in the ultraviolet (UV) region. For this work, ZnO nanotapers samples have been coated using different Au particles size and analyzed using means of local conductive atomic force microscopy (CAFM) to measure the local conductance and also electron energy loss spectroscopy (EELS) in a STEM microscope to investigate the chemistry across the Au‐ZnO interface as function of the Au particles size. The surface modification of the ZnO due to the Au decoration can alter the bandgap which might lead to potential applications for light emitting devices. The bandgap was measured using high‐energy resolution EELS analysis and appears to be dependent upon the Au particles size on the ZnO surface. Figure 1 shows the ADF STEM image of the ZnO rod with some Au particles decorating the surface. EELS STEM was carried out across the Au‐ZnO interface along the line shown in Figure 1. Figure 2 shows the O K‐edge EELS spectra extracted from across the Au‐ZnO interface and away in the ZnO region. The two spectra are clearly different. In particular, the peaks labeled as 2 and 3 in Figure 2 are much more defined than those in the spectrum extracted across the Au‐ZnO interface. This is the clear sign that the incorporation of Au particles onto the surface modify the surface morphology and as result the Zn‐O bonding. The same EELS STEM analysis was repeated across the Au‐ZnO interfaces from samples decorated with different Au particles size and we have observed different features in the O K‐edge in the EELS spectrum. This is evidence that the interaction with the Au particles modifies the ZnO surface and the effect is Au particle size dependent.