No abstract

We have monitored the changes that occur in the x-ray-excited optical luminescence, absorption, and photoemission spectra as a function of vacuum annealing time and temperature for ZnS nanowires. All measurements were done in situ. Initial heating causes desorption of surface oxides and a concurrent reduction in the intensity of all the luminescence peaks, which we attribute to the creation of surface states that quench the luminescence. Extended annealing causes diffusion of Au from the particle used to nucleate the wire growth, which results in an increase in intensity of its associated luminescent band at 520nm. Changes were also observed in the ZnL- and SK-edge x-ray absorption spectra, which are consistent with this interpretation.

One-dimensional semiconductor nanoribbons of hexagonal wurtzite sulfides (ZnS and CdS) have been prepared in bulk quantity by a thermal evaporation technique using thiol-capped gold nanoparticles as catalysts. Compared to their starting materials, ZnS and CdS powders, the band-gap photoluminescence excited by ultraviolet light from ZnS and CdS nanoribbons at room temperature was significantly enhanced. X-ray-excited optical luminescence at the S K edge confirms the near-band-gap and the defect origin of the luminescence.

X-ray excited optical luminescence (XEOL) using tunable soft x rays from a synchrotron light source, together with x-ray absorption fine structure spectroscopy has been used to study the electronic structure and optical properties of thin films of poly(N-vinylcarbazole). It is found that carbon core level excitation enhances the formation of excimers emitting at 380 and 410 nm. A third excimer at 310 nm is also noted. In addition, excitations across the C K edge and the N K edge show noticeably different optical response. These results are interpreted in terms of the site specificity of the XEOL technique.

We have monitored the optical luminescence from ZnS nanowires as a function of x-ray energy at the Zn L edge (1022 eV). The x-ray absorption spectrum obtained using the 338 nm, band edge emission as a signal resembles that of the wurtzite form of ZnS, while that obtained using the 430 and 520 nm defect emissions, resembles that of the sphalerite phase. Wurtzite is the dominant phase of the wire, while sphalerite is only found at the end of the wire adjacent to the gold particle used for nucleation and in small, highly localized regions of the wire. Therefore, the present results support the idea that the defect luminescence centers are caused by Au ions (520 nm) and vacancies (430 nm), which are located in regions of sphalerite and show how x-ray excited optical luminescence may be used to probe the local environment of such centers.