Mar 3, 2010
Turning ZnO nanowires into ultra-bright UV emitters
ZnO nanowires are being promoted as future nanoscale UV emitters for a wide variety of applications, but there are challenges to overcome. Although ZnO is well known for its wide band gap and exciton stability even above room temperature, the efficient exploitation of these properties to generate strong UV emission is hindered by the presence of optically active defect centres within the band gap. As a result, room-temperature luminescence of ZnO is dominated in many cases by a broad green defect emission.
Reporting their results in the journal Nanotechnology, researchers at the Institute of Solid State Physics in Bremen and at the Institute of Solid State Physics in Jena, Germany, have demonstrated a very simple and inexpensive way to passivate these defect centres and enhance the UV emission by a factor of almost 50.
The ZnO nanowires were grown using a vapour-liquid-solid (VLS) process and treated by a mild argon plasma at a pressure of 10–1 mbar. Measurements performed by the team of the absolute photoluminescence intensities revealed an almost complete quenching (~97%) of the green luminescence and a giant enhancement (46 times) of the UV emission at room temperature after the treatment.
Further investigations by the group showed that in such an environment the argon plasma produces large amounts of hydrogen ions, which get incorporated into the ZnO crystal. Clear signatures of hydrogen-bound exciton emission were observed in the photoluminescence spectra of the treated samples recorded at low temperature (10 K). Additionally, the team demonstrated that a direct implantation of H2+ or H+ ions produces qualitatively the same results, thus confirming the above interpretation.
The scientists observed that the enhancement in UV emission was stable at room temperature even after one month. This makes the process very promising for the realization of efficient UV emission from ZnO nanowires.
About the author
Dr Apurba Dev is a postdoc in the Nanowire Optoelectronics Group of Dr Tobias Voss (part of the Semiconductor Optics Group of Prof. J Gutowski) at the Institute of Solid State Physics, University of Bremen, Germany. His research interests include the growth of ZnO nanowires and their optical and electro-optical properties. The experimental techniques for his investigations are mainly photoluminescence studies at room temperature and at low temperature. Jan-Peter Richters is a PhD student in the same group. His research interests include the optical properties and carrier dynamics of functionalized ZnO nanowires. The experimental techniques for his investigations are mainly microphotoluminescence, time-integrated and resolved microphotoluminescence and fs pump-probe spectroscopy.