X-ray excited optical luminescence studies or ZnS and ZnO nanostructures.
Due to their potential as optoelectronic devices, luminescing nanostructures have been among the most studied in the recent past. Room-temperature UV lasing has been demonstrated in ZnO nanowires. For highly asymmetric wurtzite structures, the orientation of the emitting luminescent dipole with respect to the excited state polarization can play a role in the luminescence yield. ZnS is an important, wide bandgap (E{sub g} = 3.54 eV for the thermodynamically stable zinc blende form at room temperature) II-VI semiconductor. It has been developed for a number of applications including UV light-emitting diodes, injection lasers and phosphors. In this presentation we will discuss results of a study on ZnS nanostructurees using synchrotron-radiation-based, x-ray-excited optical luminescence (XEOL). Results on ZnO will be presented elsewhere. The experimental approach has been described previously. All measurements were performed on beamline 4-ID-C at the Advanced Photon Source. Samples were prepared by a high-temperature growth technique described previously. Briefly, ZnS powder was placed in the center of a horizontal alumina tube upstream of a Si wafer, which was covered with 2 nm thiol-capped gold nanoparticles (used to catalyze the growth). The tube was heated to 1000 C while an Ar/H{sub 2} gas mixture flowed through the tube. This process resulted in the formation of nanoribbons of lengths in the range 10-100 {micro}m and widths less than 100 nm. The samples were characterized by high-resolution TEM images, which showed large areas of hexagonal wurtzite structure interspersed by nanosized regions with cubic sphalerite structure. Using XEOL, we have determined the local phase of the luminescing sites in ZnS nanowires. The inset of the accompanying figure shows the temperature-dependent optical spectrum obtained when exciting the nanowires with 1100 eV x-rays. There are three main peaks: a band-edge, exiton state at 338 nm, a defect-related emission at 430 nm, and a Au-related feature at 520 nm. (Au is used as a catalyst to grow the wire.) By monitoring the yields of each of these peaks as a function of energy at the Zn L edge, we are able to determine the phase associated with them. The x-ray fluorescence (XRF) and 338 nm peak has a wurtzite signature, while the 430 and 520 nm curves resemble sphalerite. These results show that the 338 nm exciton state originates in the dominant, wurtzite phase of the wire, while the other two peaks occur in dilute ({approx}0.1%) regions of sphalerite.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); Natural Sciences and Engineering Research Council of Canada (NSERC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 971926
- Report Number(s):
- ANL/XFD/CP-118962; TRN: US1001417
- Resource Relation:
- Conference: 5th International Conference on Synchrotron Radiation in Materials Science (SRMS 5); Jul. 30, 2006 - Aug. 2, 2006; Chicago, IL
- Country of Publication:
- United States
- Language:
- ENGLISH
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