R. A. Rosenberg

One-dimensional zigzag gallium nitride nanostructures

Two one-dimensional (1D) single-crystalline gallium nitride (GaN) nanostructures with periodic zigzag (type I) and diameter-modulated (type II) shapes have been synthesized by passing through ammonia over a mixture of gallium and gallium oxide (Ga2O3) powders held at elevated temperature. The process was catalyzed by the dispersion of thio-capped Au nanoparticles on the substrate onto which GaN nanostructures were condensed. The transformation between these two nanostructure morphologies was also observed. A possible growth model for the zigzag-shaped nanostructures is proposed, in which the formation of the zigzag nanostructures results from the construction of two different nanoscale unit cells. This work provides an avenue to a group of 1D nanostructures with a zigzag shape. The possibility to form 1D nanostructures yet to be discovered by changing the stacking direction of the (0001) plane will facilitate the fabrication of nanoscale functional devices as well as our understanding of the growth behavi...

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.

Observations on the surface and bulk luminescence of porous silicon

Using the x‐ray excited optical luminescence technique, we have investigated the soft x‐ray induced photoluminescence of porous silicon in the optical region (200–900 nm) and the Si K‐edge x‐ray absorption fine structures of porous silicon in the near edge region. It is found that while porous silicon prepared at low current density (20 mA/cm2 for 20 min) exhibits a single broad luminescence band, porous silicon prepared at high current density (200 mA/cm2 for 20 min) exhibits three optical luminescence channels; i.e., in addition to the broad peak characteristic of all porous silicon, there are at least two additional optical luminescence channels at shorter wavelengths, one with modest intensity at ∼460 nm and the other a weak and very broad peak at ∼350 nm. These optical channels have been used to monitor the Si K‐edge absorption of porous silicon in the near edge structure region. Analysis of the data shows that while the band at ∼627.5 nm corresponds to the bulk emission, the other channels are of a ...

Synchrotron-radiation-induced, selective-area deposition of gold on polyimide from solution

Room-temperature photoinduced, selective-area deposition of gold films on polyimide from gold salt solution using synchrotron radiation x rays is described. A film growth rate as high as 40 nm/min is obtained. For thickness <50 nm, the films consist of nanograins with a grain size of about 200 nm. For thicker films, gold deposit forms a ramified morphology on top of the nanograin film. The change in morphology is discussed in terms of the change in the yield of the photoelectrons generated by x rays as a function of growth.

Thermal effect on the oxides on Nb(100) studied by synchrotron-radiation x-ray photoelectron spectroscopy

Synchrotron-radiation glancing-incidence and angle-resolved photoelectron spectroscopy (PES) is used to study the oxides grown in air on a single-crystal Nb(100) surface. Both core-level and valance-band PES are measured for various heat treatments. Glancing-incidence excitation was used to characterize the outer 2-nm Nb2O5 layer without influence from the substrate and to profile through various oxides by varying the incidence angle. Immediately adjacent to this amorphous layer is a layer of NbO2, which contributes to the density of states at 1.2eV below the Fermi level. Thereafter, there exist other oxides that interface with the metal substrate. Annealing at 430K induces a “mild” chemical reaction, which is contained essentially in the oxide layer. Annealing at 550K causes oxygen diffusion into the metal and leaves an oxide layer consisting primarily of Nb2O and a roughened surface, as evidenced by angle-resolved photoemission measurements. Oxide composition alteration caused by annealing at these mode...

Synchrotron radiation induced chemical vapor deposition of thin films from metal hexacarbonyls

We have studied the initial stages of synchrotron radiation [SR] induced chemical vapor deposition [CVD] of metal‐containing thin films from metal hexacarbonyl gases. We have measured the dependence of the initial deposition rate upon gas pressure at room temperature. Substrates were exposed to SR for single fills of the electron storage ring at constant pressure of Mo(CO)6, Cr(CO)6, or W(CO)6 gas. Deposition was monitored in situ by Auger electron spectroscopy using the SR as the excitation source. The presence of metal, carbon, and oxygen in the deposited films was observed, and the results are consistent with previous studies. Deposition was found to be isolated to areas of the substrate exposed to the SR beam. We discuss these results as they relate to the use of SR as a means to induce CVD and the possibility of patterned deposition using a masked SR source.

Depth resolved luminescence from oriented ZnO nanowires

We have utilized the limited penetration depth of x-rays to study the near-surface properties of vertically aligned ZnO nanowires. For an energy of 600 eV the penetration depth varies between 3 and 132 nm as the incidence angle changes from 2° to 33°. Thus, by obtaining optical luminescence spectra as a function of incidence angle, it is possible to probe the near-surface region with nanometer-scale resolution. We will present angle dependent optical luminescence data from oriented ZnO nanowires. By fitting the results to a simple model, we extract a depth for the surface defect regions of ∼14 nm.

Anisotropic x-ray absorption effects in the optical luminescence yield of ZnO nanostructures

The authors have found that the directionality of the orbital populated following core-level x-ray absorption of a hexagonal nanostructure has a strong influence on the resulting optical luminescence yield spectra. For ZnO, there is an enhancement of the band gap exciton luminescence following O 1s to 2pz relative to 2px,y excitation. The defect luminescence O 1s excitation spectrum also shows sensitivity to the nature of the defect (surface or bulk).

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.

Adsorption of saturated hydrocarbons on the Si(111)-7 × 7 surface studied by photoelectron and photon stimulated desorption spectroscopies

Abstract Photoelectron spectroscopy (PES) and photon stimulated desorption (PSD) experiments were carried out to follow the thermal chemistry of methane- (CH 4 ), neopentane- (C 5 H 12 ), and adamantane- (C 10 H 16 )-dosed-Si(111)-7 × 7 surfaces. Both methane and adamantane adsorb molecularly on the 7 × 7 reconstructed Si surface at temperatures of 30 and 85 K, respectively. In contrast, at low coverages ( 15 molecules/cm 2 , 85 K; 0.4 ML), a fraction of the neopentane adlayer adsorbs dissociatively; at higher coverages, neopentane adsorption is predominantly molecular. Conversely, the adamantane and neopentane adlayers desorb at temperatures of ∼200 and 115 K, respectively, for heating rates of ∼1 K/min. No desorption temperature was determined for methane, but the methane adlayer was observed to desorb below 100 K. As determined by both PES and PSD, annealed adamantane- and neopentane-dosed surfaces react to form nearly identical surfaces. Si 2p core-level spectra show chemically shifted components of 0.48±0.05, 1.00±0.05, and 1.50±0.05 eV with respect to the bulk component. This demonstrates formation of Si(CH y ) x ( x =1-3; y =0-3) type surface species. The H + PSD spectra of the thermally reacted surfaces were measured and show chemical shifts of ∼0.7 eV with respect to bulk Si and two sharp resonances at 100.7 and 101.3 eV. The edge shift and associated structure highlights the chemical specificity of PSD and demonstrates its utility for following surface chemical reactions.