Macroscale alignment of CdSe/CdS nanorods by porous anodic alumina templates

Centimeter‐scale assemblies of highly ordered CdSe/CdS core‐shell nanorods have been successfully fabricated by infiltration of the nanorods into the pores of a transparent porous anodic alumina membrane. The high degree of the nanorod ordering is proved by the demonstration of both highly polarized PL and absorption of the aligned nanorods. The measured PL linear polarization degree is 53%. We have found that both the elongated CdS shell and the nearly spherical CdSe core exhibit a strong dependence of absorption on polarization angle with respect to the nanorod axis. We conclude that both CdS shell and CdSe core absorb more efficiently light with polarization along the axis of the nanorod. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A comparison between optically active CdZnSe/ZnSe and CdZnSe/ZnBeSe self-assembled quantum dots: effect of beryllium

The composition and size of optically active Cd_xZn_1−xSe/ZnSe quantum dots are estimated with a previously developed method. The results are then compared with those obtained for Cd_xZn_1−xSe/Zn_0.97Be_0.03Se QDs. We show that introducing Be into the barrier material enhances both Cd composition and quantum size effect of optically active quantum dots.     

Temperature-induced line broadening, line narrowing and line shift in the luminescence of nanocrystalline ZnS:Mn

Emission spectra of nanocrystalline ZnS:Mn²⁺ have been recorded as a function of temperature between 4 and . For the two emission bands observed (a defect-related emission band around and a Mn²⁺-related emission band around ), the temperature dependence of the width and the position of the emission bands has been analyzed. The shift and the broadening of the Mn²⁺ emission can be satisfactorily explained by theoretical models and parameters for electron-phonon coupling that are similar to those for bulk ZnS. For the defect-related emission, the shift to lower energies follows the decrease in bandgap of bulk ZnS with increasing temperature. The width of the defect-related emission decreases as the temperature is raised. This anomalous behavior is explained by inhomogeneous broadening at low temperatures.     

Growth and optical characterization of single quantum well structure of submonolayer ZnS/ZnTe

Single quantum wells of submonolayer ZnS/ZnTe were grown between ZnTe layers using hot wall epitaxy method with fast-movable substrate configuration. As ZnS well widths decrease from 1 to 0.15 monolayer, the photoluminescence peaks shift to higher energies from 2.049 to 2.306 eV, and the photoluminescence intensities increase. As ZnS well width decrease, the PL spectra show the lower-energy tails and consequently the increased PL FWHMs. This is a result of a convolution of two PL peaks from two-dimensional and zero-dimensional quantum islands, supported by a still lived lower-energy peaks of zero-dimensional quantum islands above 50 K. The energy shift in the power dependence of photoluminescence spectra is proportional to the third root of the excitation density. These behaviors can be described by the formation of submonolayer type-II ZnS/ZnTe quantum well structure, and the coexistence of two-dimensional and one-dimensional islands in ZnS layers.     

Mechanism of the growth of ZnSe nanowires with Fe catalysts

ZnSe nanowires of high optical quality have been grown with Fe catalysts at the usual temperatures for the fabrication of ZnSe-base optoelectronic devices (about 600 K). It is found that only Fe nanoparticles of 10-50 nm diameter act as catalysts in the narrow temperature range around 600 K (with a minimum temperature of 473-523 K, and a maximum temperature of 623-673 K). The growth mechanism is discussed in terms of the vapour-liquid-solid mechanism.     

Preparation of ZnO Nanospheres and Their Applications in Dye-Sensitized Solar Cells

ZnO nanospheres are synthesized by a two-step self-assembly method. X-ray diffraction pattern and Raman scattering spectra measurements show that all the samples present a typical wurtzite structure. A regular sphere shape is inspected by field emission scanning electron microscope and transmission electron microscope for the samples. It is shown that the as-synthesized ZnO nanosphere is composed of numbers of primary particles with size of around 1Ohm. A possible growth mechanism for the two-step self-assembly ZnO nanosphere is proposed. After applying the ZnO nanospheres to dye-sensitized solar cells （DSSCs）, a 117% increase of the overall light to electricity conversion efficiency η is observed compared with that of the ZnO nanopartieles based DSSCs. Associated with the UV-vis results, light scattering is assigned to the great improvement of η.     

Combinatorial ion beam synthesis of CdSxSe1−x nanocrystals

In this presentation we focus on the synthesis of buried multielemental semiconductor nanoparticles by sequential high dose ion implantation and post-implantation annealing. Nanocluster formation and alloying was studied by Raman-, Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction analysis (XRD) on a materials library of CdS_xSe_1−x nanoclusters buried in thermally grown SiO₂ on silicon. Characteristic peak shifts of the LO-Raman signal and XRD-peaks due to varying S- and Se-fraction indicate that the ion beam synthesized clusters consist of a solid solution of Cd, S and Se. In addition the influence of the implanted dose ratios on the structural evolution of the nanocluster-SiO₂ system will be discussed.     

Photoluminescence properties of Co-doped ZnO nanocrystals

We performed photoluminescence experiments on colloidal, Co²⁺-doped ZnO nanocrystals in order to study the electronic properties of Co²⁺ in a ZnO host. Room temperature measurements showed, next to the ZnO exciton and trap emission, an additional emission related to the Co²⁺ dopant. The spectral position and width of this emission does not depend on particle size or Co²⁺ concentration. At 8 K, a series of ZnO bulk phonon replicas appear on the Co²⁺-emission band. We conclude that Co²⁺ ions are strongly localized in the ZnO host, making the formation of a Co²⁺d-band unlikely. Magnetic measurements revealed a paramagnetic behaviour.     

Structural, Optical and Electrical Properties of ZnS/Porous Silicon Heterostructures

ZnS films are deposited by pulsed laser deposition on porous silicon （PS） substrates formed by electrochemical anodization of p-type （100） silicon wafer. Scanning electron microscope images reveal that the surface of ZnS films is unsmoothed, and there are some cracks in the ZnS films due to the roughness of the PS surface. The x-ray diffraction patterns show that the ZnS films on PS surface are grown in preferring orientation along cubic phase β-ZnS （111） direction. White light emission is obtained by combining the blue-green emission from ZnS films with the orange-red emission from PS layers. Based on the I-V characteristic, the ZnS/PS heterojunction exhibits the rectifying junction behaviour, and an ideality factor n is calculated to be 77 from the I-V plot.     

Synthesis and photoluminescence of aligned straight silica nanowires on Si substrate

Aligned straight silica nanowires (NWs) have been synthesized on Si wafer by thermal evaporation of mixed powders of zinc carbonate hydroxide and graphite at 1100 °C and condensation on Si substrate without using any catalyst. The straight silica NWs have diameters ranging from 50 to 100 nm, and lengths of several micrometers, with cone-shaped tips at their ends. High deposition temperature and relatively high SiO_x vapor concentration near the growth substrate would be beneficial to the formation of the aligned straight silica NWs. Different morphologies of silica nanostructures have also been obtained by varying the deposition temperature and the vapor concentration of the SiO_x molecules. Room temperature photoluminescence measurements on the oriented silica NWs show that two green emission bands at 510 and 560 nm, respectively, revealing that the aligned straight silica NWs might have potential applications in the future optoelectronic devices.     

Optical properties of Zn0.5Cd0.5Se thin films grown on InP by molecular beam epitaxy

We report photoluminescence (PL) and reflectivity measurements of Zn_0.5Cd_0.5Se epilayers grown by molecular beam epitaxy on InP substrates. The low-temperature PL spectra are dominated by asymmetric lines, which can be deconvoluted into two Gaussian peaks with a separation of ∼8 meV. The behavior of these peaks is studied as a function of excitation intensity and temperature, revealing that these are free exciton (FE) and bound exciton emission lines. Two lower energy emission lines are also observed and assigned to the first and second longitudinal optical phonon replicas of the FE emission. The temperature dependence of the intensity, line width, and energy of the dominant emission lines are described by an Arrhenius plot, a Bose-Einstein type relationship, Varshni's and Bose-Einstein equations, respectively.     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

Surface effects on photoluminescence of single ZnO nanowires

The influence of surface effects on the temperature dependent photoluminescence (PL) spectra from individual ZnO nanowires has been studied. It is found that the surface effects of the nanowire are very important in both ultraviolet (UV) and visible emission. We propose a new luminescence mechanism based on the recombination related to oxygen vacancies to explain the temperature dependent visible emission, which is significantly influenced by the carrier depletion and band bending caused by surface effects. In addition, the observed attenuation of UV emission with increasing temperature is ascribed to the decreasing depletion region and the increasing surface states related nonradiative recombination.     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

Photoluminescence in anatase titanium dioxide nanocrystals

Titanium dioxide (TiO₂) nanocrystals were prepared by a hydrolysis process of tetrabutyl titanate. X-ray diffraction and Raman scattering showed that the as-prepared TiO₂ nanocrystals have anatase structure of TiO₂, and that the monophase anatase nanocrystals can be achieved through a series of annealing treatments below 650 °C. We measured photoluminescence (PL) spectra of the TiO₂ nanocrystals. Under 2.41–2.71 eV laser irradiation, the TiO₂ nanocrystals displayed strong visible light emission with maxima of 2.15–2.29 eV even at excitation power as low as 0.06 W/cm². To identify the PL mechanism in the TiO₂ nanocrystals, the dependences of the PL intensity on excitation power and irradiation time were investigated. The experimental results indicated that the radiative recombination is mediated by localized levels related to surface defects residing in TiO₂ nanocrystallites. Received: 7 April 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

Photoluminescence and Raman scattering of Cu-doped ZnO films prepared by magnetron sputtering

The Cu-doped ZnO films were prepared by direct current reactive magnetron sputtering using a zinc target with various Cu-chips attached. The influences of Cu-doping on the microstructure, photoluminescence, and Raman scattering of ZnO films were systematically investigated. The results indicate that ZnO films doped with moderate Cu dopant (2.0-4.4 at.%) can obtain wurtzite structure with strong c-axis orientation. The near band edge (NBE) emission of ZnO film can be enhanced by Cu dopant with a concentration of 2.0 at.% and quench quickly with further increase of doping concentration. Two additional modes at about 230 and 575 cm⁻¹, which could be induced by Cu dopant, can be observed in Raman spectra of the Cu-doped ZnO films.     

Water-induced stabilization of ZnS nanoparticles

Surface states become important in particles with reduced size as the surface to volume ratio increases. Here we provide direct evidence of water-induced structure buildup and stabilization of nanocrystalline ZnS. The ZnS nanoparticles, prepared employing a non-equilibrium route, provide an ideal platform to investigate microscopic details of water induced structural transformation and thus demonstrate the role of S-H interactions in the time-evolution of crystalline behaviour in these particles. The colossal changes observed make these materials ideal for water sensing.     

Raman scattering characterization of vertical aligned 1D IrO2 nanocrystals grown on single crystal oxide substrates

Raman scattering (RS) has been used as a technique for characterization of IrO₂ one dimensional (1D) nanocrystals (NCs) deposited on sapphire(100) and LiNbO₃(100) substrates under various conditions. The IrO₂ NCs were grown via metalorganic chemical vapor deposition method using (MeCp)(COD)Ir as the precursor and reactive magnetron sputtering using Ir metal target. The red-shifts and asymmetric broadening of the Raman lineshape for the NCs were analyzed by a modified spatial correlation (MSC) model, which includes the factor of stress induced shift. The proposed MSC model showed that the effects of stress and nanometric size can be separated in analyzing the observed Raman features. The usefulness of the experimental RS together with the MSC model analysis as a residual stress and structural characterization technique for 1D NCs has been demonstrated.     

Effect of zinc nitrate concentration on the structural and the optical properties of ZnO nanostructures

ZnO nanorods and nanodisks were formed on indium-tin-oxide-coated glass substrates by using an electrochemical deposition method. Scanning electron microscopy images showed that the ZnO nanorods were transformed into nanodisks with increasing Zn(NO₃)₂ concentration. X-ray diffraction patterns showed that the ZnO nanostructures had wurzite structures. The full widths at half maxima of the near band-edge emission peak of photoluminescence spectra at 300 K for ZnO nanorods were small, indicative of the high quality of the nanorods. These results indicate that the structural and the optical properties of ZnO nanostructures vary by changing Zn(NO₃)₂ concentration.     

The effect of 3-aminopropyltrimethoxysilane on the formation of NiFe2O4/SiO2 nanocomposites

NiFe₂O₄/SiO₂ nanocomposites were prepared using a sol–gel method with the addition of 3-aminopropyltrimethoxysilane (APS). Different phases and morphologies of NiFe₂O₄/SiO₂ nanocomposites were obtained when different amounts of APS were used. The structural properties of the products were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Sheet-like morphology was observed at higher molar ratio of APS to NiFe₂O₄, while spherical NiFe₂O₄/SiO₂ nanoparticles separated from each other were formed at lower molar ratio of APS to NiFe₂O₄. The magnetic properties of the nanocomposites were also investigated, indicating that the interparticle interactions exhibit strong dependence on the molar ratio of APS to NiFe₂O₄.