Xingtai Zhou

Time-resolved x-ray excited optical luminescence from SnO2 nanoribbons : Direct evidence for the origin of the blue luminescence and the role of surface states

Time-resolved x-ray excited optical luminescence (XEOL) and x-ray absorption near edge structures have been employed to study the origin of the multicolor luminescence from SnO2 nanoribbons. The authors find that the yellow-green luminescence has a long lifetime while the blue luminescence a short one. The luminescence is attributed to the radiative decay of trapped electrons in oxygen vacancies just below the conduction band and electrons in the conduction band to intrinsic surface states in the band gap.

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.

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.

Optical emission of biaxial ZnO–ZnS nanoribbon heterostructures

The electronic structure and optical properties of biaxial ZnO-ZnS heterostructure nanoribbons (NRs) have been investigated using x-ray absorption near-edge structures (XANES) and x-ray excited optical luminescence (XEOL). The XANES were recorded in total electron yield and wavelength-selected photoluminescence yield across the K- and L(3,2)-edges of zinc and sulfur and the K-edge of oxygen. The XEOL from the NRs exhibit a very weak band-gap emission at 392 nm and two intense defect emissions at 491 and 531 nm. The synchrotron x-ray pulse ( approximately 100 ps, 153 ns repetition rate) was used to track the optical decay dynamics from ZnO-ZnS NR, which can be described by two lifetimes (7.6 and 55 ns). Comparison with similar measurements for ZnO and ZnS nanowires reveals that the luminescence from ZnO-ZnS NRs was dominated by the ZnO component of the NR as the ZnS component contributes little. The implication of this observation is discussed.

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.

Interaction between nuclear graphite and molten fluoride salts: a synchrotron radiation study of the substitution of graphitic hydrogen by fluoride ion.

The interaction between nuclear graphite and molten fluoride salts (46.5 mol % LiF/11.5 mol % NaF/42 mol % KF) is investigated by synchrotron X-ray diffraction and C K-edge X-ray absorption near-edge structure (XANES). It is found that there are a large number of H atoms in IG-110 nuclear graphite, which is attributed to the residual C-H bond after the graphitization process of petroleum coke and pitch binder. The elastic recoil detection analysis indicates that H atoms are uniformly distributed in IG-110 nuclear graphite, in excellent agreement with the XANES results. The XANES results indicate that the immersion in molten fluoride salts at 500 °C led to H atoms in nuclear graphite partly substituted by the fluorine from fluoride salts to form C-F bond. The implications of these findings are discussed.

Dynamic View on Nanostructures: A Technique for Time Resolved Optical Luminescence using Synchrotron Light Pulses at SRC, APS, and CLS

We present an experimental technique using the time structure of synchrotron radiation to study time resolved X‐ray excited optical luminescence. In particular we are taking advantage of the bunched distribution of electrons in a synchrotron storage ring, giving short x‐ray pulses (10–102 picoseconds) which are separated by non‐radiating gaps on the nano‐ to tens of nanosecond scale — sufficiently wide to study a broad range of optical decay channels observed in advanced nanostructured materials.

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.

Effect of substrate surface on the structure and electronic properties of cubic boron nitride films

Cubic boron nitride (c-BN) films were prepared by mass-selected ion beam deposition (MSIBD) technique. The effects of substrate surface roughness were investigated by boron and nitrogen k-edge x-ray absorption near-edge structure, x-ray diffraction, and atomic force microscopy. All the films are a mixture of nanocrystalline sp3-bonded c-BN and sp2-bonded BN phases. The substrate with a rough surface causes a decrease of the c-BN phase content of the film on it. A significant large lattice contraction of the c-BN crystallites in the films, relative to the bulk, is observed. It is also found that the electronic structure of the nanocrystalline c-BN films by MSIBD technique is somewhat different from that of microcrystalline c-BN∕h-BN references. We attribute the effect of the nature of the substrate on the morphology and structure of the c-BN films to the orientation of sp2-bonded graphitic BN basal plane on the top surface of the films during their growth, and the lattice contraction and energy band structure modification of c-BN films to the large compressive stress, respectively.Cubic boron nitride (c-BN) films were prepared by mass-selected ion beam deposition (MSIBD) technique. The effects of substrate surface roughness were investigated by boron and nitrogen k-edge x-ray absorption near-edge structure, x-ray diffraction, and atomic force microscopy. All the films are a mixture of nanocrystalline sp3-bonded c-BN and sp2-bonded BN phases. The substrate with a rough surface causes a decrease of the c-BN phase content of the film on it. A significant large lattice contraction of the c-BN crystallites in the films, relative to the bulk, is observed. It is also found that the electronic structure of the nanocrystalline c-BN films by MSIBD technique is somewhat different from that of microcrystalline c-BN∕h-BN references. We attribute the effect of the nature of the substrate on the morphology and structure of the c-BN films to the orientation of sp2-bonded graphitic BN basal plane on the top surface of the films during their growth, and the lattice contraction and energy band struct...