Luminescence of Sulfur-Doped ZnO Nanostructures and Powders Characterized

Daniel S. Burgess

To better understand the potential of sulfur-doped ZnO for use in solid-state lighting applications, investigators from Duke University in Durham, N.C., and from the US Army’s Aviation and Missile Research, Development and Engineering Center at Redstone Arsenal in Alabama have performed a comparative analysis of the luminescence of doped ZnO nanostructures and micropowder. Their findings suggest that the former may be suited to the development of electrically driven emitters and that the latter may offer a highly efficient replacement for down-conversion phosphors such as the cerium-doped YAG employed today in white LEDs.

ZnO has potential applications in the development of rugged solid-state light sources. The visible emission from UV-stimulated sulfur-doped ZnO nanowires is shown. The output spectrum overlaps well the response of the dark-adapted human eye. Courtesy of Henry O. Everitt.

Henry O. Everitt, a researcher at the university and at the Army center, said that it has been understood for decades that doping ZnO with sulfur increases the II-VI semiconductor’s output in the visible. Because high-quality ZnO in bulk and in thin films emits much more strongly in the UV at the semiconductor’s band edge than in the visible, the effect typically has been seen as an indicator of material defects — specifically, distortions in the crystal lattice resulting from the substitution of sulfur for oxygen.

The visible output closely matches the response of the dark-adapted human eye, however, so what had been considered a flaw may be a boon for the development of material systems for solid-state lighting, particularly given that ZnO is easy to process, does not oxidize and is more environmentally friendly than current down-conversion phosphors, he noted.

In the work, the researchers characterized the photoluminescence of sulfur-doped ZnO nanowires with lengths of more than 20 μm and with average diameters of approximately 100 nm, doped nanoribbons with similar lengths and with average cross sections of 300 × 100 nm, and a 125-μm-diameter doped powder, all grown by Jie Liu, a professor of chemistry at Duke. To study the effects of doping, they also collected data on a commercially available undoped ZnO micropowder.

A HeCd laser, an optical parametric amplifier producing 100-fs pulses at a repetition rate of 1 kHz, and a 300-W xenon arc lamp served as the UV excitation sources for CW photoluminescence, time-resolved photoluminescence and photoluminescence excitation spectroscopy, respectively. The response from the samples was detected with a cooled CCD camera or with a streak camera, depending on the experiment.

Their results confirm that doping ZnO nanostructures and micropowder with sulfur also greatly enhances visible emission, so that the materials yield quantum efficiencies of 30 and 65 percent, respectively. An optical pump wavelength of approximately 380 nm was found to maximize the visible response. The lower experimental quantum efficiency of the nanostructures came as a surprise to the scientists, given that the high surface-to-volume ratio of the structures should result in the availability of many defect sites to contribute to the visible response. They propose that the nanostructures feature nonradiative relaxation pathways that are not present in the powder and that compete with the process behind defect emission.

Everitt said that the team plans to work on optimizing the growth and doping of the materials to maximize the quantum efficiencies. Further efforts will involve developing a physical model of the nanostructures to explore the underlying relaxation mechanism.

Noting that the Army is interested in rugged, efficient light sources for the battlefield and that the automotive industry similarly would like shock-resistant sources, he expressed optimism about the materials’ potential. The commercial availability of 380-nm LEDs in particular, he suggested, points the way toward high-brightness, high-efficiency white LEDs using sulfur-doped ZnO micropowder as the phosphor.

Nano Letters, June 2006, pp. 1126-1130.