Localized luminescence of zinc oxide nanostructures

The photoluminescence (PL) of materials has been widely used to investigate their optical properties. Recently, cathodoluminescence (CL) spectroscopy has also attracted much 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) 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).

Figure 1(a) shows a CL measurement 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 spatial resolution of these simultaneous real-space luminescence images is high and represents a significant advantage over conventionally acquired PL spectra. The high spatial resolution is even more evident in Figure 2, showing similar measurements obtained for aggregated ZnO nanostructures.

Figure 2. (a) SEM image of a ZnO nanowire on a silicon substrate. (b) CL spectrum obtained from the same area as in (a). (c) Monochromatic CL image of (a) at 380nm. (d) Monochromatic CL image of (a) at 520nm.

Figure 3. (a) Close-up view of the area enclosed by the square in Figure 2(a). (b) Same for the square shown in Figure 2(c).

The luminescence spectrum shown in Figure 2(b) is similar to that of Figure 1(d). We observed that the ∼520nm luminescence was not homogeneous for the nanostructures shown in Figure 2(c), even though most can emit strongly at 380nm, as shown in Figure 2(b). The observed luminescence inhomogeneities could not have been distinguished without the high spatial resolution afforded by CL. This result demonstrates that a luminescence spectrum such as the one displayed in Figure 2(b), which is typically obtained with conventional PL spectroscopy, may not represent the optical characteristics of all the nanostructures included in the probe area. The high spatial resolution of CL measurements is evident in Figure 3(a) and (b), which present zoomed views of the area enclosed by the squares in Figure 2(a) and (c). Comparing panels (a) and (b) in Figure 3 clearly shows that luminescence from nanowires separated by less than 50nm can be resolved, thus emphasizing that nanostructure spatial resolution is different from that of bulk materials, which have larger generation volumes. We also investigated the correlation between luminescence spectra and electronic transport characteristics by performing CL measurements on field-effect transistor devices consisting of individual ZnO nanostructures. Details are provided elsewhere.²

Our findings show that local luminescence measurements using CL can complement conventional PL techniques and provide more information on the inhomogeneities of semiconductor-based nanomaterials. Accordingly, future work should focus on further understanding the correlation between the PL and CL properties of nanostructures, with the goal of developing broader applications for these localized approaches.

The author acknowledges support from the Korea Research Foundation (MOEHRD grant KRF-2005-041-C00168).