G. G. Siu

Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle.

Strong blue emission from anodic alumina membranes with ordered nanopore array

We have investigated the photoluminescence (PL) from anodic alumina membranes with an ordered nanopore array formed on bulk Al foils. Most of the membranes fabricated by anodization in oxalic acid showed a strong PL peak in the blue. Due to an obvious asymmetry, the PL peak can be Gaussian divided into two bands around 405 and 455 nm, having a slight shift with the sample formed in different acid concentrations. The PL excitation (PLE) spectral examinations and analyses revealed that the two blue PL bands originate from optical transitions in two kinds of different oxygen-deficient defect centers, F (oxygen vacancy with two electrons) and F+ (oxygen vacancy with only one electron) centers. Their distributions were discussed on the basis of the observed PL and PLE behaviors. Our experimental results improve the understanding of the blue-emitting property of anodic alumina membranes.

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

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.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.

Raman scattering of alternating nanocrystalline silicon/amorphous silicon multilayers

Nanocrystallite size distribution and structural properties in alternating hydrogenated nanocrystalline silicon/amorphous silicon multilayers were investigated by means of Raman scattering. The obtained Raman spectra show a broad peak at ∼480 cm−1 from amorphous Si and some small peaks superposed on the broad peak. According to the positions of the crystallite peak, the mean crystallite size and volume fraction of the crystalline were calculated. Since these small peaks have strong size dependence of their relative intensities, an effect induced by the atomic vibrations from the near‐surface region of nanocrystals is considered to be responsible for the modification of the vibrational properties and the stable photoluminescence from our samples.

Luminescence from colloidal 3C-SiC nanocrystals in different solvents

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

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.

Formation mechanism of alumina nanotube array

Abstract Formation mechanism of alumina nanotubes and their array was analysed on the basis of voids in both anodic porous alumina membrane and the tube walls of alumina nanotubes. Circular, crack-like, and wheel-like voids were observed and considered to be responsible for the formation of aluminian nanotubes and their array. Based on microstructural observation of individual alumina nanotubes, the morphology of the tube wall filled with the voids was experimentally determined, which will help to understand the formation mechanism of alumina nanotubes. Our observations and analyses give further information on self-organized mechanism of anodic porous alumina membrane.

Blue-emitting β-SiC fabricated by annealing C60 coupled on porous silicon

C60 molecules were chemically coupled in the pores of porous Si through a coupling agent and then coated with a layer of Si, and followed by N2 annealing. X-ray diffraction results indicate that the fabricated samples contain β-SiC particles which may exist in the pores, in addition to Si, SiO2, and graphite. The photoluminescence (PL) spectra show an asymmetrical broadband, which can be Gaussian divided into two bands at 380 (3.26 eV) and 454 (2.73 eV) nm. Spectral analyses and the experimental results from infrared spectroscopy and PL excitation measurements suggest that the 380 nm PL band is related to oxygen-vacancy defects in the SiO2 matrix, whereas the blue PL band is closely connected with the β-SiC particles. Our experiments provide a way for fabricating stable blue-emitting β-SiC materials.

Defect-related infrared photoluminescence in Ge+-implanted SiO2 films

SiO2 films with Ge+ implantation at an energy of 60 keV and a dose of 1×1016 cm−2, followed by annealing at different temperature, exhibit a broad infrared photoluminescence (PL) at room temperature under an excitation of the 514.5 nm line of Ar+ laser. With increasing the annealing temperature, the intensity of the infrared PL band decreases, its full width at half maximum increases, and its energy redshifts. Spectral analysis and some experimental results from Raman scattering, electron spin resonance, and infrared spectroscopy strongly suggest that the infrared PL is mainly related to interfacial oxygen-deficient-type defects between the oxide and Ge nanocrystals.

Enhanced ultraviolet photoluminescence from SiO2/Ge:SiO2/SiO2 sandwiched structure

SiO2/Ge:SiO2/SiO2 sandwiched structure was fabricated for exploring efficient light emission. After annealed in N2 (O2<1%), this structure shows three photoluminescence (PL) bands at 293, 395, and 780 nm. The intensity of the 395 nm band is largely enhanced in comparison with that from the monolayered Ge:SiO2 film. Spectral analyses suggest that the three PL bands originate from S1→S0, T∑(T∏)→S0, and T∏′→S0 optical transitions in GeO color centers, respectively. The improvement of the GeO density resulting from the confinement on Ge diffusion is responsible for the enhanced ultraviolet PL. This structure is expected to have important applications in optoelectronics.