T. K. Sham

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.

Double‐crystal monochromator beam line on the Aladdin 1‐GeV storage ring

We report the design, construction, and performance of a soft x‐ray beam line with accessible photon energy 0.8–4.0 keV at the Synchrotron Radiation Center, University of Wisconsin–Madison. The beam line features an ultrahigh‐vacuum (UHV) compatible high‐precision double‐crystal monochromator (DCM) covering Bragg angles 12°–72°, which was designed and built at the Physical Science Laboratory (PSL), University of Wisconsin–Madison. The monochromatic x rays are focused by a bent cylindrical mirror (Ni‐coated fused silica, located 7.5 m from the source) into the experimental chamber (located 5.5 m from the mirror) down to a spot less than 1 mm(h)×3 mm(v). During the initial runs, the DCM used a pair of InSb(111) crystals and covered photon energy 1.75–3.7 keV. At the silicon K edge (1840 eV), the beam line delivered about 4×1011 photons/s with stored electron beam at 100 mA at 1 Gev, among the most intense and stable sources currently available at this energy. The energy resolution is about 0.9 eV at the Si ...

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 ...

M3,2‐edge x‐ray absorption near‐edge structure spectroscopy: An alternative probe to the L3,2‐edge near‐edge structure for the unoccupied densities of d states of 5d metals

We report M3,2‐edge x‐ray absorption near‐edge structure (XANES) measurements for a series of 5d metals, Ta, W, Pt, and Au using a total electron yield technique. It is found that the M3,2 XANES absorption features and their systematics are comparable to their L3,2 counterparts. These results clearly indicate that M3,2‐edge XANES can be used as an alternative probe for the unoccupied densities of d states of the 5d metals and their compounds. The implication of these results is discussed.

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.

Semiconductor Growth and Junction Formation within Nano-Porous Oxides

We report a systematic study of the chemical properties of porous silicon as a moderate reducing agent and a nano-structure template for the reductive formation of nano-metal cluster aggregates. This study investigates the consequences of the interaction of silver ions in aqueous solution with porous silicon. The nature of the redox reaction, especially the role of surface hydrogen and silicon atoms, is established through the study of the morphology, structure, and electronic and optical properties of Ag on porous silicon. The specimens were characterized with SEM and X-ray absorption fine structure (XAFS) spectroscopy using synchrotron radiation

Structure and electronic properties of SiO2/Si multilayer superlattices: Si K edge and L3,2 edge x-ray absorption fine structure study

We report an x-ray absorption fine structure study at the Si K and L3,2 edges of a series of Si/SiO2 superlattices (SL). The SL system comprises four periods of elemental silicon with a spacing of 1, 1.4, 2.2, and 2.6 nm sandwiched by a 1.5 nm silicon oxide and capped by a 3 nm silicon oxide layer. These systems exhibit electroluminescence and photoluminescence. X-ray absorption near edge structure (XANES) at both the Si K and L3,2 edge confirms that the Si layers are amorphous. Polarization dependent measurement at the Si K edge reveals that a distinct Si/SiO2 interface exists with strong Si–O bonding oriented preferentially closer to the surface normal. High resolution XANES at the Si L3,2 edge shows a noticeable blueshift of the edge threshold as the lattice spacing decreases, in good accord with quantum confinement. The results and their implications for the origin (quantum confinement and interface/oxide defects) of luminescence in these superlattice systems are discussed.

X-ray absorption fine structure and electron energy loss spectroscopy study of silicon nanowires at the Si L3,2 edge

X-ray absorption fine structures (XAFS) and electron energy loss spectroscopy (EELS) at the Siu200aL3,2 edge have been used to investigate a series of Si nanowires (as-prepared and HF refreshed). X-ray excited optical luminescence (XEOL) was also used to study the optical properties of these Si nanowires. Although no noticeable edge-jump blueshift (widened band gap) is observed in XAFS, a noticeable change in the edge jump (a less steep rise and the blurring of spectral features) is observed, indicating considerable degradation in the long-range order and size effects. However, EELS with a nanobeam exhibits a threshold blueshift and parabolic behavior for some selected wires indicating that there are grains smaller than the nominal diameter in these nanowires. Thus, XAFS probes the average of a distribution of wires of various sizes of which the majority is too large to exhibit detectable quantum confinement behavior (blueshift) observed and inferred in EELS and XEOL. The results and their implications are di...

Luminescence from porous silicon: an optical X-ray absorption fine structures study at the Si L3,2-edge

Abstract We report optical luminescence from porous silicon (PS) using excitation photons with energies above and below the Si L 3,2 -edge. The yield of the luminescence was in turn used to obtain the X-ray absorption fine structures (XAFS) of PS at these edges. It is shown that the small chemically capped nm nodules and pillars as well as encapsulated Si crystallites attached to the “bulk” of the PS network are primarily responsible for the luminescence, not the entire network. It is also noted that the combined use of the total electron yield, the photo luminescence yield and the fluorescence yield in measuring the Si L 3,2 -edge absorption greatly enhances the capability of the XAFS in site and sampling depth selectivity. K-edge data were also shown for comparison.

Interaction of Au on Si(100) studied by core level binding energy shifts

Abstract From the Au 4f and Si 2p core level photoemission measurements of Au/Si(100), it is found that both core levels exhibit positive binding energy shifts when they form chemical bonds. The Si 2p shift is interpreted in terms of electronegativity difference, while the Au 4f shift is attributed to d-electron depletion to form an sd hybrid.