Sumei Wang

Vacuum electron field emission from SnO2 nanowhiskers synthesized by thermal evaporation

Rod-shaped and wire-shaped SnO2 nanowhiskers were synthesized by thermal evaporating of tin powders at 900 °C. Three Raman peaks (474, 632, 774 cm−1) showed the typical feature of the rutile phase of as-synthesized SnO2 nanowhiskers, which was consistent with the result of x-ray diffraction. A relatively low turn-on field of 1.37 V µm−1 at a current density of 0.1 µA cm−2 was obtained. The dependence of emission current density on the electric field followed a Fowler–Nordheim relationship. Our results indicated that SnO2 nanowhiskers had an interesting FE property as a wide band gap semiconductor.

Raman spectroscopy studies of Ce-doping effects on Ba0.5Sr0.5TiO3 thin films

Ba0.5Sr0.5TiO3 (BST) thin films are among the best-known ferroelectric and dielectric materials. Ce-doped BST films have been fabricated by pulsed laser deposition in order to enhance their dielectric properties. X-ray diffraction, atomic force microscopy, and Raman spectroscopy have been used to study variations of crystal structure, surface morphologies, and phase stability of Ce-doped BST films, respectively. A strong influence of Ce doping on the properties of the BST films has been observed. First, a small amount of Ce dopant makes easy epitaxial growth of a BST film with a smooth surface on a MgO substrate. Second, residual stress in a BST film on a MgO substrate can be reduced by Ce doping, as demonstrated by the blueshift of phonon peaks in Raman spectroscopy.

Large-area uniform nanodot arrays embedded in porous anodic alumina

A three-step method combining physical vapour deposition, grazing ion milling and thermal annealing techniques for large-area fabrication of nanodot arrays in anodic alumina oxide (AAO) templates is demonstrated. By this method, ordered arrays of Ni and Au nanodots with highly uniform diameters and a density of 1.2 ? 1010?cm?2 were fabricated. The diameter and packing density of the nanodots can be controlled by the cell parameters of the AAO template. The fabrication method presented here can be extended to many other materials and allows further investigation of the size related characteristics of nanodot structures.

Large-area fabrication of periodic Fe nanorings with controllable aspect ratios in porous alumina templates.

Highly uniform Fe nanoring arrays in porous anodic alumina templates are fabricated by physical vapour deposition and grazing ion milling techniques. The nanorings have aspect ratios ranging from 0.8 to 4, depending on the deposition conditions. The outer diameter of the individual nanorings, and the area density and distribution patterns are completely determined by the template used. Selected-area electron diffraction reveals that these nanorings have a polycrystalline microstructure. The nanoring fabrication method demonstrated here can be extended to other materials.

Influence of stress on Raman spectra in Ba1−xSrxTiO3 thin films

Ba1-xSrxTiO3 (x = 0.3, 0.5, 0.7) thin films have been prepared on (001) MgO substrates by pulsed laser deposition. Temperature-dependent permittivity and Raman spectra of the Ba1-xSrxTiO3 films are compared with those of the corresponding bulk samples. Raman spectra indicate that a tetragonal structure is present in the Ba0.5Sr0.5TiO3 thin film while the Ba0.5Sr0.5TiO3 bulk sample shows a cubic structure. Temperature-dependent permittivity shows that the phase transition in the film occurs over a wide temperature range, which results in the co-existence of the cubic phase and tetragonal phase in the Ba0.5Sr0.5TiO3 film. The smooth change of phase transition is attributed to the residual stress in the film. We demonstrate that the residual stress releases gradually with increasing thickness of the film. Furthermore, the effects of the composition and thickness on Raman spectra have also been discussed systematically.

Selective phase synthesis of a high luminescence Gd2O3:Eu nanocrystal phosphor through direct solution combustion.

A Gd(2)O(3):Eu nanocrystal phosphor has been directly synthesized by a mild solution combustion method with a single step approach while avoiding further thermal annealing. The as-combusted Gd(2)O(3):Eu powders have been characterized by x-ray diffraction, infrared spectroscopy, transmission electron microscopy and photoluminescence spectra. The ratio of citric acid to metal nitrate (C/M) has a critical impact on the phase composition and crystallization of as-combusted Gd(2)O(3):Eu. An optimal C/M ratio of 0.7 gave highly crystalline powders with a single cubic phase, and a high luminescence intensity comparable with that of a commercial Y(2)O(3):Eu phosphor, even without further thermal annealing. This direct solution combustion method can be used to prepare a variety of high quality oxide nanocrystals.

Dielectric properties of Si-Ba0.5Sr0.5TiO3 composite thin films elaborated by pulsed laser deposition

Si–Ba0.5Sr0.5TiO3 composite thin films were deposited by pulsed laser deposition technique on (100) Nb:SrTiO3 substrates. The crystal structure and surface morphology of the thin films have been characterized by x-ray diffraction and atomic force microscopy, respectively. Experimental results suggested that the Si composition has played a positive role in improving the dielectric properties of the thin films. With the increased Si concentration, the dielectric loss was significantly reduced, accompanied with a tolerable reduction of tunability. In particular, the 14% Si composite film exhibited a dielectric loss of 0.005, which is much lower than that of the undoped Ba0.5Sr0.5TiO3 (BST) film. Moreover, a proper concentration of Si in the film improved the figure of merit from 9 of the undoped BST to 23 of 1.4% Si composite films.

Ce-doping effects on electronic structures of Ba0.5Sr0.5TiO3 thin film

In order to clarify the basic reason why Ce doping can dramatically decrease the leakage current in Ba0.5Sr0.5TiO3 (BST) as reported in our previous work (Wang et al 2005 J. Phys. D: Appl. Phys. 38 2253), we have employed x-ray photoelectron spectroscopy (XPS) and the optical transmittance technique to study the electronic structure of undoped and 1.0 at% Ce-doped BST (CeBST) films fabricated by pulsed laser deposition. XPS results show that Ce doping has a strong influence on the valence band and core levels of BST films, and that the Fermi level is lowered by about 0.35 eV by Ce doping. Optical transmittance measurements demonstrate that the energy gap is expanded with Ce doping. These Ce-doping effects can induce an increase in the barrier height for the thermionic emission and eventually reduce leakage current in CeBST thin films.

Buried tungsten silicide layer in silicon on insulator substrate by Smart-Cut®

A single-crystalline Si/SiO2/poly-WSix/Sub-Si structure has been successfully fabricated by a new method incorporating a standard smart-cut® technique and a high temperature reaction between tungsten and silicon. Annealing at 800–1100 °C does not only strengthen the bonding of the wafers but also induces solid phase reaction of deposited tungsten and silicon. A poly-crystalline WSix (1 < x < 2) layer with a tetragonal structure is formed below the buried oxide layer. Cross section images of TEM show three steep interfaces of the four layers. It is found that increasing the annealing temperature is in favour of decreasing the sheet resistance of tungsten silicide and improving the crystal quality of the top silicon layer. However, a spreading resistance profile measurement shows that annealing under high temperature (≥1000 °C) will induce diffusion of tungsten into the Si substrate which is confirmed by the EDX results and the reason is presented.