Showing papers by "Gg G. Siu published in 2006"

Current transport studies of ZnO/p-Si heterostructures grown by plasma immersion ion implantation and deposition

Abstract: Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.

Luminescence from colloidal 3C-SiC nanocrystals in different solvents

Abstract: We have investigated the role of the solvents in the luminescence from colloidal 3C-SiC suspensions. By dispersing electrochemically etched polycrystalline 3C-SiC wafers in water, ethanol, or toluene, we have fabricated suspensions of 3C-SiC nanocrystals that exhibit intense photoluminescence. By taking into account the quantum confinement effect and observed size distributions of the 3C-SiC crystallites, a simple model is formulated to explain the photoluminescence spectra. Our results show that the colloidal 3C-SiC nanocrystals are robust and intense emitters that have good chemical stability and biocompatibility. They are thus useful in biotechnology and nano-optoelectronics applications.

Synthesis and magnetic properties of Zn1−xCoxO nanorods

Abstract: Magnetic Zn1−xCoxO nanorods were fabricated via direct hydrothermal synthesis. The measurements of x-ray diffraction, x-ray photoemission spectroscopy, and optical absorption spectra demonstrate the presence of cobalt in the +2 state in a tetrahedral crystal field, which indicates that Co ions have been doped into the nanorods. The observations of morphology and microstructure reveal that the Zn1−xCoxO nanorods grow along the [0002] direction through Ostwald ripening not only competing with but being assisted by oriented attachment. The field dependence of magnetization (M-H curves) of the Zn1−xCoxO nanorods measured at 300K shows their ferromagnetic characteristics. The coercive fields (Hc) were obtained to be 98 and 36Oe for nominal x=0.029 and 0.056, respectively. Our experimental results provide one effective method for fabrication of transition metal doped ZnO nanostructures with room-temperature ferromagnetism by direct chemical synthesis.

Intergrowth mechanism of silicon nanowires and silver dendrites

Abstract: The intergrowth mechanism of silicon nanowires and silver dendrites formed by electroless metal deposition has been investigated by scanning electron microscopy. A self-assembled localized microscopic electrochemical cell model can adequately describe the self-organized Si nanowires growth. Using these in situ prepared Si nanowire arrays as templates, a diffusion-limited aggregation process is proposed to explain the formation of the silver dendritic nanostructures.

Temperature-dependent photoluminescence from MEH-PPV and MEH-OPPV containing oxadiazole in the main chain

Abstract: We report the temperature-dependent photoluminescence (PL) behavior of two poly (p-phenylene-vinylene) derivatives with different backbones, a homopolymer poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and a copolymer poly-{[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]-alt-[2,5-diphenylene-1,3,4-oxadiazole-vinylene]} (MEH-OPPV). The PL peak positions of the MEH-PPV in both solid solution and film are blue shifted with increasing measurement temperature because the thermally induced torsion and libration modes reduce the conjugation lengths of the polymer, but the shift rate is smaller in the MEH-PPV film than that in the solid solution. The PL peak position of the MEH-OPPV film is independent of measurement temperature because there is a large dihedral angle between the adjacent monomer units. The large dihedral angle increases the conformational disorder to eliminate the effect of the disorder induced by the thermally induced torsion and libration modes on the conjugation lengths of the MEH-OPPV.

In situ fabrication of alumina nanotube array and photoluminescence

Abstract: Aluminum foil was anodized in aged electrolyte under high voltage. The morphology observation shows that the alumina film has a three-layer structure from bottom to top and the middle layer shows large quantities of individual alumina nanotubes. Their formation mechanism is discussed in detail. Under ultraviolet excitation, the alumina film exhibits an emission centered at ∼400nm. Based on annealing behavior of the emission band and electron paramagnetic resonance result, the origin of the emission is considered to be due to optical transition in single ionized oxygen vacancy (F+ center) in the alumina. The experimental results can be expected to have favorable applications in optoelectronics and biotechnology.

Silver nanocrystal superlattices : Self-assembly and optical emission

Abstract: Unique passivated silver nanocrystal superlattices were fabricated via a conventional hydrothermal method. The method is based on a general phase transfer and separation mechanism occurring at the interfaces of the liquid, solid, and solution phases present during the synthesis. A photoluminescence (PL) band centered at 390nm was recorded from the obtained superlattice samples. Spectral analyses suggest that the PL arises from the radiative recombination of sp-band electrons with d-band holes in the silver nanocrystals and its intensity enhancement is due to the surface plasmon and an electrostatic enhancement mechanism analogous to that in surface enhanced Raman scattering.

Optical emission from the aggregated state in poly [2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene]

Abstract: We report the photoluminescence characteristics of solid solutions of poly[2-methox-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) with different concentrations in polystyrene. Reduction in the distances between the MEH-PPV chains with increased MEH-PPV concentrations makes the conjugation segments aggregated in the solid solution. Absorption by the conjugation segments in the aggregated state leads to a redshift absorption edge. By comparing the photoluminescence and absorption spectra of the solid solutions with those of the 100% MEH-PPV film, it is shown that the emission from the 100% MEH-PPV film originates from the radiative recombination of excitons in the aggregated state.

Local vibration at the surface of a Ge nanocrystal embedded in a silicon oxide matrix

Abstract: A low-frequency Raman vibration mode, whose peak position and linewidth are independent of the sizes of Ge nanocrystals and the polarization configuration of incident excitation light, was observed in silicon oxide films embedded with Ge nanocrystals which were prepared using magnetron cosputtering of SiO2–Ge–Si targets. The peak position of the Raman mode is sensitive to the content of Si in the matrix. After the sample is annealed above a special temperature that increases with the content of Si, the Raman mode disappears. Microstructural observations and spectral analyses disclose that this low-frequency Raman mode arises from a local structure which is positioned at the surfaces of Ge nanocrystals and consists of Ge, Si, and O atoms. High-temperature annealing leads to the removal of Ge atoms from the local structure. As a result, the local vibration mode vanishes.

Si-based solid blue emitters from 3C-SiC nanocrystals

Abstract: A procedure for preparing 3C-SiC/SiO2 composite nanocrystals embedded in Si matrix that emit blue light is reported. Through electrochemical etching of polycrystalline 3C-SiC wafers followed by ultrasonic treatment in water bath, we fabricated luminescent colloidal 3C-SiC nanocrystals. Porous Si samples that have been naturally oxidized in air for 12 h were immersed in agitated aqueous suspension of 3C-SiC nanocrystals for 10 min and then dried in air, followed by annealing in argon atmosphere to form core-shell structured 3C-SiC/SiO2 nanocrystals embedded in Si matrix. Our result shows that the luminescence of 3C-SiC/SiO2 composite nanocrystals is very stable over time or under high temperature. As robust and stable Si-based solid blue-emitters, they have important implications for engineering photonic components in optoelectronics and photonics.

Resonant electron transfer and luminescent enhancement in a toluene suspension of Si nanocrystals

Abstract: Efficient resonant electron transfer from the surface bonding structure to the conduction band of quantum confined Si nanocrystals is observed by Si nanocrystals in a toluene suspension. Based on the electron transfer mechanism, the enhancement of photoluminescence originates from the band-to-band recombination in the p-type Si nanocrystals suspended in a toluene solution. The energy levels of the electrons in the Si nanocrystals chemisorbed with toluene molecules are calculated using the method of linear combination of atomic orbitals, and the characteristics of the obtained density of states is in good agreement with the observed photoluminescence properties.

Photoluminescence from C60-coupled porous structures formed on Fe+-implanted silicon.

Abstract: ⟨111⟩-oriented p-type Si wafer with a resistivity of 1–5Ωcm was implanted with Fe+ and then annealed at 1100°C in N2 for 60min, followed by anodization in a solution of HF to form porous structure with β-FeSi2 nanocrystallites. Photoluminescence (PL) spectral measurements show that a strong PL peak appears in the range of 610–670nm. The position of the PL peak remains unchanged, but its intensity increases with the storage time in air until about three months and then saturates. C60 molecules were chemically coupled on the porous structure through a kind of silane coupling agent to form a nanocomposite. It is revealed that the stable PL peak monotonically shifts to a pinning wavelength at 570nm. Experimental results from PL, PL excitation, Raman scattering, and x-ray diffraction measurements clearly show that the pinned PL originates from optical transition in C60-related defect states, whereas the photoexcited carriers occur in the β-FeSi2 nanocrystallites formed during anodization. This work opens a new...

Electronic states and luminescence in higher fullerene/porous Si nanocrystal composites.

Abstract: Photoluminescence (PL) measurements have been performed on the nanocomposites of higher fullerene-coupled porous silicon (PS) nanocrystals. For the C70PS and C76(78)PS nanocomposites, the PL spectra show a pinning wavelength at approximately 565 nm and for the C84PS and C94PS nanosystems the pinning wavelength is at approximately 590 nm. The PL pinning property is closely related to the sorts of the coupled fullerenes. A band mixing model of direct and indirect gaps in a nanometer environment consisting of nc-Si core, SiO2 surface layer, and coupled fullerene has been proposed for calculation of electronic states. Good agreement is achieved between the experiments and theory.

Energy band mixing in core-shell-structured Si∕FeSi2 nanocomposites

Abstract: Using the bandmixing model of direct and indirect gaps, we calculate the energy levels of electrons and holes confined in the Si∕FeSi2 nanocomposite consisting of nanocrystalline Si core, interfacial FeSi2 layer, and outer Fe2O3 crust. The obtained results explain very well the intensity enhancement and pinning behavior of the observed photoluminescence peaks after the nanocomposite is stored in air. We also theoretically obtain the photoluminescence excitation characteristics. Good agreement is achieved between the theory and experiments.

Si nanowires sheathed with thin diamondlike carbon films

Abstract: Via electroless metal deposition and industrial plasma technology, we have successfully synthesized unique composite silicon nanowire structures with diamondlike carbon (DLC) sheaths acting as inorganic passivation layers. We have also discussed their growth mechanism in terms of a self-organization process. The thickness of the DLC sheaths were quantified with Raman spectroscopy. The Raman analytical result is in good agreement with direct microstructure observations.

Stable optical emission from annealed poly (p-phenylene vinylene) derivative films containing oxadiazole

Abstract: The structure and optical properties of poly {[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene]-alt-[2,5-diphenylene-1,3,4-oxadiazole vinylene]} (MEH-OPPV) are studied by examining and analyzing the photoluminescence, absorption, and Fourier transform infrared spectra of the MEH-OPPV films annealed at different temperatures It is revealed that MEH-OPPV is of high thermal stability due to aryl-substituted oxadiazole groups introduced in the polymer backbone, which increases the conformational disorder to result in short conjugation lengths in MEH-OPPV Because the interchain distance and the conjugation length are restricted by the large dihedral angle between the adjacent monomer units, the relaxation of the polymer chains cannot change the electron structures during thermal annealing As a result, the optical properties of MEH-OPPV in the film are nearly unaffected by thermal annealing