Showing papers by "Deren Yang published in 2009"

General Layer-By-Layer Approach To Composite Nanotubes and Their Enhanced Lithium-Storage and Gas-Sensing Properties

Abstract: A novel and versatile layer-by-layer approach has been developed to synthesize composite metal oxide nanotubes on carbon-nanotube (CNT) templates. We demonstrate this approach by the preparation of CeO2−SnO2 and Ag−NiO composite nanotubes. The electrostatic attraction between the CNTs and metal ions play the most important role in the formation of CNT-based core−shell nanotubes, from which the composite metal oxide nanotubes are subsequently synthesized. It is found that the CeO2−SnO2 and Ag−NiO composite nanotubes lead to excellent device performance when they are used in gas sensors and Li-ion batteries, respectively.

Labeling transplanted mice islet with polyvinylpyrrolidone coated superparamagnetic iron oxide nanoparticles for in vivo detection by magnetic resonance imaging.

Abstract: Superparamagnetic iron oxide nanoparticles (SPIO) are emerging as a novel probe for noninvasive cell tracking with magnetic resonance imaging (MRI) and have potential wide usage in medical research. In this study, we have developed a method using high-temperature hydrolysis of chelate metal alkoxide complexes to synthesize polyvinylpyrrolidone coated iron oxide nanoparticles (PVP-SPIO), as a biocompatible magnetic agent that can efficiently label mice islet β-cells. The size, crystal structure and magnetic properties of the as-synthesized nanoparticles have been characterized. The newly synthesized PVP-SPIO with high stability, crystallinity and saturation magnetization can be efficiently internalized into β-cells, without affecting viability and function. The imaging of 100 PVP-SPIO-labeled mice islets in the syngeneic renal subcapsular model of transplantation under a clinical 3.0 T MR imager showed high spatial resolution in vivo. These results indicated the great potential application of the PVP-SPIO as an MRI contrast agent for monitoring transplanted islet grafts in the clinical management of diabetes in the near future.

A Versatile Approach for the Synthesis of ZnO Nanorod-Based Hybrid Nanomaterials via Layer-by-Layer Assembly

Abstract: A layer-by-layer (LBL) assembly approach has been developed to synthesize ZnO nanorod-based hybrid nanomaterials such as ZnO/CeO2, ZnO/CdS, and ZnO/Ag on ZnO nanorod templates at room temperature. The morphology, structure, and composition of the ZnO nanorod-based hybrid nanomaterials have been characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis. It is indicated that a uniform coating layer consisting of homogeneous nanoparticles is deposited on the surface of ZnO nanorods due to the strong electrostatic attraction between metal ions and polyelectrolyte modified ZnO nanorods. The general approach presented here can be extended to the synthesize other one-dimensional (1D) nanostructure-based hybrid nanomaterials.

Localized surface plasmon enhanced photoluminescence from ZnO films: Extraction direction and emitting layer thickness

Abstract: The light emission from ZnO films is enhanced by localized surface plasmon (LSP) of Ag island films. The dependence of LSP coupled emission on light-extracting direction and the thickness of ZnO films is investigated. It is found that the photoluminescence (PL) enhancement factors for upward extraction are much smaller than those for downward extraction. This is ascribed to the low transmittance and nonradiative absorption loss of Ag films. The near-field effect of LSP results in the dependence of the PL enhancement of ZnO films on the film thickness. A maximum PL enhancement factor of more than 15 is obtained when the thickness of ZnO was about 40 nm.

Fabrication and characterization of CuInS2 films by chemical bath deposition in acid conditions

Abstract: Non-crystalline copper indium disulphide (CuInS2) thin films had been deposited on ITO glass by chemical bath deposition (CBD) in acid conditions. Then polycrystalline CuInS2 films were obtained after sulfuration in sulfur atmosphere at 450 °C for 1.5 h. The films had been characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), Raman scattering measurements and energy dispersive X-ray analysis (EDX). The optical and electrical property of the thin films was also measured. The results showed that the pure, flatness, and well crystallized CuInS2 thin films with good electrical and optical property had been obtained, meaning that the chemical bath deposition in acid conditions is suitable for the deposition of CuInS2 thin films.

A General Approach for Uniform Coating of a Metal Layer on MWCNTs via Layer-by-Layer Assembly

Abstract: A layer-by-layer (LBL) approach has been developed to synthesize uniform CNT−metal nanocomposites at room temperature. As the representative examples, CNT−Pt, CNT−Pd, and CNT−Sn nanocomposites have been fabricated to illustrate the basic idea presented here. The morphology, structure, and composition of the as-synthesized products have been characterized by transmission electron microscopy, high-resolution transmission electron microscopy, field emission scanning electron microscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy analyses. The effects of the charge density on the CNTs, the reduction rate, and the sonication process on the formation of uniform CNT−metal nanocomposites have been investigated.

Electrically pumped ultraviolet random lasing from ZnO-based metal-insulator-semiconductor devices: Dependence on carrier transport

Abstract: The electrically pumped ultraviolet (UV) random lasing and carrier transport of ZnO-based metal-insulator-semiconductor (MIS) structures on Si substrates have been systematically investigated. With the increase of positive bias voltage on the gates of the MIS devices, the current-voltage (I-V) characteristics manifest a normal curved I-V region where the current increases with the bias, followed by a negative differential resistance (NDR) region. Moreover, the UV electroluminescence from the devices in the normal region is transformed from spontaneous emission into increasingly intensive random lasing; while, that in the NDR region is transformed from random lasing into very weak spontaneous emission. The reason for the effect of NDR on the random lasing from the devices has been tentatively explored.

Enhanced electroluminescence of silicon-rich silicon nitride light-emitting devices by NH3 plasma and annealing treatment

Abstract: Silicon-rich silicon nitride (SRSN) films were deposited on p-type silicon substrates using a conventional plasma-enhanced chemical vapor deposition (PECVD) system. Before deposition, silicon substrate was pre-treated by NH 3 plasma in the PECVD system. And devices with metal–insulator–semiconductor (MIS) structure were fabricated using indium tin oxides (ITO) as anode and aluminum (Al) film as cathode. It was found that after 1100 °C annealing the electroluminescence (EL) intensity of NH 3 plasma pre-treated MIS devices was increased greatly comparing with that of without NH 3 plasma pre-treated devices. It is due to the passivation or reducing of interfacial states and nonradiative defects in SRSN films by the NH 3 plasma pre-treatment and high-temperature annealing that enhanced the EL intensity of the SRSN MIS devices.

Enhancement of electroluminescence from TiO2/p+-Si heterostructure-based devices through engineering of oxygen vacancies in TiO2

Abstract: We report that electroluminescence (EL) from TiO2/p+-Si heterostructure-based devices can be significantly enhanced through a prior treatment of TiO2 films in argon (Ar) plasma. It is found that the Ar-plasma treatment introduces excess oxygen vacancies within a certain depth of TiO2 films. The increase in the concentration of oxygen vacancies leads to the enhancement of EL from TiO2/p+-Si heterostructure-based devices because oxygen vacancies are the light-emitting centers. This work demonstrates the use of defect engineering to improve the performance of oxide-based optoelectronic devices.

Bidirectional direct-current electroluminescence from i-MgxZn1−xO/n-ZnO/SiOx double-barrier heterostructures on Si

Abstract: We report bidirectional direct-current electroluminescence (EL) from i-MgxZn1−xO/n-ZnO/SiOx double-barrier heterostructures on Si. When the heterostructure-based device is forward biased with negative voltage applied on Si, ultraviolet (UV) emission is more intense than visible emissions. The visible emissions appear only at sufficiently high currents. As the device is reverse biased, the visible emissions dominate the EL at low currents. However, they are gradually overridden by the UV emission with increasing current. The EL mechanism has been discussed in terms of energy band structures of the device under forward and reverse biases.

Electroluminescence from TiO2/p+-Si heterostructure

Abstract: Titanium films sputtered on heavily boron-doped (p+) silicon substrates were thermally oxidized to form electroluminescent TiO2/p+-Si heterostructures. The electroluminescence (EL) features a broad spectrum covering red, green, and blue regions. We believe that in TiO2 recombinations between electrons at oxygen-vacancy-related energy levels and holes in the valence band result in the EL. Furthermore, the EL mechanism has been explained in terms of the energy band diagram of the TiO2/p+-Si heterostructure, which possesses an intermediate ultrathin SiOx layer revealed by high resolution transmission microscopy.

Impurity engineering for germanium-doped Czochralski silicon wafer used for ultra large scale integrated circuit

Abstract: Internal gettering (IG) technology has been challenged by both the reduction of thermal budget during device fabrication and the enlargement of wafer diameter. Improving the properties of Czochralski (Cz) silicon wafers by intentional impurity doping, the so-called ‘impurity engineering (IE)’, is defined. Germanium has been found to be one of the important impurities for improving the internal gettering effect in Cz silicon wafer. In this paper, the investigations on IE involved with the conventional furnace anneal based denudation processing for germanium-doped Cz silicon wafer are reviewed. Meanwhile, the potential mechanisms of germanium effects for the IE of Cz silicon wafer are also interpreted based on the experimental facts. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Impurity Engineering of Czochralski silicon

Abstract: The novel concept of “impurity engineering in CZochralski (CZ) silicon ” for large scaled integrated circuits has been reviewed. By doping with a certain impurities into CZ silicon materials intentionally, such as nitrogen (N), germanium (Ge) and even carbon (C, with high concentration), internal gettering ability of CZ silicon wafers could be improved. Meanwhile, void defects in CZ silicon wafer could be easily eliminated during annealing at higher temperatures. Furthermore, it was also found that the mechanical strength could be increased, so that breakage of wafers decreased. Thus, it is believed that by impurity engineering CZ silicon wafers can satisfy the requirment of ultra large scale integrated circuits.

Rapid-thermal-anneal-based internal gettering for germanium-doped Czochralski silicon

Abstract: The internal gettering (IG) effects involved with a rapid thermal anneal (RTA) in germanium-doped Czochralski silicon (GCz-Si) wafer have been investigated. It was found that germanium doping could enhance the oxygen precipitation in bulk while shrinking the denuded zone width near the surface through pre-RTA at high temperature plus low–high temperature conventional furnace anneals. Rapid cooling rate after RTA was clarified to be beneficial for oxygen precipitation for GCz-Si wafer. It was suggested that the germanium doping could increase the vacancy concentration in Cz-Si during RTA by forming the germanium–vacancy complexes. In contrast to that in Cz-Si wafer, the smaller-sized higher-density oxygen precipitates were presented in the nucleation anneals, then followed RTA pretreatment while more oxygen precipitates survived during ramping processes after nucleation anneals in the GCz-Si wafer. Enhanced heterogeneous nucleation and reduced critical radius of precipitates associated with the germanium–vacancy complexes have been proposed for the oxygen precipitation enhancement.

Structure of Ge–O complexes in Czochralski silicon

Abstract: The structures of Ge–O complexes in germanium-doped Czochralski (CZ) silicon wafers have been investigated by means of density functional theory (DFT). The calculations present the fact that the Ge–O complexes can be formed with the absence of vacancy during low-temperature thermal cycles so that they can enhance oxygen precipitation. Furthermore, the total energy of different Ge–O complexes is calculated, and then optimized and stable structure of Ge–O complexes is suggested.

Electrophotoluminescence of sol-gel derived ZnO film: effect of electric field on near-band-edge photoluminescence.

Abstract: The effect of electric field on near-band-edge (NBE) photoluminescence (PL) of a sol-gel derived ZnO film has been investigated via a SiO2/ZnO/SiOx(x<2) double-barrier structure on Si under different forward biases. A forward current-voltage curve is characterized by a negative-differential-resistance (NDR) region, which follows a normal region. With an increase of forward bias the NBE PL of the ZnO film is enhanced in the normal region, but it is attenuated in the NDR region. The increase of forward bias also causes the NBE PL of the ZnO film to blueshift from ~377.6 to ~374.9 nm no matter how current changes. The mechanism for the effect of bias on the intensity and position of NBE PL of the ZnO film is discussed.

Oxygen precipitation in heavily phosphorus-doped silicon wafer annealed at high temperatures

Abstract: Oxygen precipitation in heavily phosphorus-doped (P-doped) Czochralski (CZ) silicon subjected to single-step annealing at high temperatures in the range of 1050–1150 °C has been investigated. It was indicated that in the heavily P-doped CZ silicon there were more grown-in oxygen precipitates, thus promoting the generation of induced defects and accelerating the Ostwald ripening of oxygen precipitates during the high temperature annealing, in comparison with the control lightly P-doped CZ silicon with comparable initial oxygen concentration and thermal history. Moreover, it was found that, during the annealing at 1050 °C, oxygen precipitation in the outer region about 2.5 cm in width was noticeably retarded with respect to that in the inner region across the heavily P-doped CZ silicon wafer. The mechanism for the enhanced formation of grown-in oxygen precipitates and the retardation of oxygen precipitation at high temperature, as mentioned above, has been tentatively explained.

Electric-field-induced random lasing from ZnO and Mg0.1Zn0.9O films optically pumped with an extremely low intensity.

Abstract: Ultraviolet random lasing from ZnO and Mg0.1Zn0.9O films have been achieved under optical pump of a continuous wave He-Cd laser as the films are simultaneously exerted by an electric-field with appropriate polarity and amplitude. With the aid of electric-field, the optical pump intensity for generating random lasing can be reduced at least five orders of magnitude in comparison with conventional pulse-laser pumped ones. It is believed that the role of electric-field exerted on the films is similar to that of increasing optical pump intensity, in terms of increasing the amount of non-equilibrium electrons in the near-surface regions of films.