Localized luminescence of zinc oxide nanostructures

Abstract

The photoluminescence (PL) of materials has been widely used to investigate their optical properties. Recently, cathodoluminescence (CL) spectroscopy has also attractedmuch attention for the characterization of nanostructures such as nanowires, nanorods, and nanoribbons.1–3 This technique records luminescence after creating pairs of negative and positive charge carriers (so-called electron-hole pairs) in a material by high-energy electron bombardment. CL spectra are typically captured using a scanning electron microscope. The highly focused beam of electrons impinges on the sample and induces it to emit light from a localized area, with the result that CL achieves higher spatial resolution than conventional photoluminescence (PL) techniques. Furthermore, CL can also provide additional information on local stress and impurities in nanostructures, which is usually beyond the capability of other methods. Recently, we used CL to investigate zinc oxide (ZnO) nanostructures.2 ZnO has a direct bandgap of∼3.3eV and an excitonic binding energy (i.e., of the electron-hole pair) of 60meV. It has become a focus of interest owing to its potential for a wide variety of optoelectronic applications, including nanowires and nanorods. A ZnO luminescence spectra typically consists of a sharp band at ∼380nm, due to near band edge excitonic recombinations, and a broad green emission band in the visible at ∼510nm due to deep states inside the bandgap.4 The stronger luminescence of the ∼380nm-band relative to that of the visible emission is often considered indicative of defect-free, crystalline ZnO structures. Figure 1(a) shows a CLmeasurement recorded for a single isolated ZnO nanowire with a diameter of ∼25nm. Monochromatic CL images were also simultaneously recorded at 380 and 520nm, as shown in Figure 1(b) and (c). These images clearly demonstrate that low-level luminescence signals from a nanostructure can be obtained. Figure 1(d) shows that the nanowire spectrum displays the two characteristic ZnO luminescence peaks. The Figure 1. (a) Scanning electron microscope image of a ZnO nanowire on a silicon substrate. (b) Monochromatic CL image of (a) at 380nm. (c) Monochromatic CL image of (a) at 520nm. (d) CL spectrum obtained from the same area as in (a).