P.-S. G. Kim

Fabrication, morphology, structure, and photoluminescence of ZnS and CdS nanoribbons

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.

Amorphous carbon nanowires investigated by near-edge-x-ray-absorption-fine-structures

The structure and bonding of amorphous carbon nanowires and amorphous carbon nanowires-converted multiwalled carbon nanotubes have been investigated with carbon K-edge near-edge x-ray absorption fine structure using surface-sensitive total electron yield, and bulk-sensitive fluorescence yield. The results strongly support that amorphous carbon nanowire is a precursor to multiwalled carbon nanotubes.

Effects of in situ vacuum annealing on the surface and luminescent properties of ZnS nanowires

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.

Nanostructured CdS prepared on porous silicon substrate: Structure, electronic, and optical properties

Nanostructured CdS was deposited electrochemically on porous silicon (PS) substrate/cathode. The PS-supported CdS deposits were found to be uniformly packed particles of ∼100 nm, each of which is an aggregate of smaller clusters of several nanometers as revealed by scanning electron microscope and confirmed by glancing incidence x-ray powder diffraction. No significant CdS deposition into the pore of porous silicon is found under the reported experimental conditions. X-ray absorption fine structures (XAFS), both extended x-ray absorption fine structure and x-ray absorption near edge structures, across the S and the Si K edge of the samples have been investigated. The structure and electronic properties of the CdS/PS composite are discussed on the basis of the XAFS results obtained using multichannel detection (total electron and x-ray fluorescence yields) from a series of CdS and PS samples. The optical luminescence behavior of CdS/PS was also investigated utilizing a technique often referred to as synchr...

Determination of the local structure of luminescent sites in ZnS nanowires using x-ray excited optical luminescence

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.

Soft x-ray excited optical luminescence: Some recent applications

X-ray excited optical luminescence (XEOL) studies of several classes of light emitting materials excited using soft x rays (photon energy ranging from 10 to 2500 eV) are presented. We show that XEOL with soft x rays (short penetration depths) is often site specific and is ideally suited for the study of light emitting thin films and devices. Several examples including porous silicon, organic light emitting diode materials, and CdS based nanostructures are used to illustrate the unique properties of XEOL and its applications in the soft x-ray energy region.

Electrochemical route for the fabrication of alkanethiolate-capped gold nanoparticles

Nearly monodispersed gold nanoparticles (NPs) of a few nanometers were fabricated and immobilized simultaneously on silicon surface by the galvanostatic reduction of HAuCl4 in the presence of dodecanethiol. X-ray absorption spectra at both sulfur K edge and gold L3 edge confirm the existence of the alkanethiolate-protected Au NPs and reveal their structural and bonding characteristics. Alkanethiolate-capped Au NPs were also successfully fabricated, with the same technique, on porous silicon, a functional substrate with high surface area. The hybrid thiol-capping/electrodeposition method reported here offers an attractive approach to the fabrication of highly stable gold NPs on various functional substrates with a simple, fast, and easily controlled experimental procedure.

XANES studies of CdS nano-structures on porous silicon

Abstract CdS nano-structures were produced electrochemically using porous silicon (PS) as a template/substrate. AFM and SEM images show that the CdS deposit are grains of ∼100 nm, each of which is an aggregate of smaller particles of several nanometers. XANES studies of the CdS nano-structures together with bulk CdS were conducted. The electronic behavior and optical properties of the CdS/PS composite, particularly those relevant to nano-size effects, were discussed.