Formation and spectroscopic characterization of mono-dispersed CdSe nanocrystals

In this article, mono-dispersed hexagonal structure CdSe nanocrystals with polyhedron shape were prepared by an open solvent thermal reaction. They show a discrete excitonic transition structure in the absorption spectra and the minimal photoluminescence （PL） peak full-width at half-maximum of 19nm. The PL quantum yield is about 60%. Transmission electron micrographs, high-resolution transmission electron micrographs, x-ray powder diffraction patterns, UV-vis absorption spectra and PL spectra were obtained for the as-prepared CdSe nanocrystals. The size of the CdSe nanocrystals can be tuned by changing the reaction temperature or time. Due to the improved synthesis method, a different growth mechanism of the CdSe nanocrystals is discussed.     

Growth and optical properties of ultra-long single-crystalline α-Si3N4 nanobelts

Ultra-long single-crystalline -Si₃N₄ nanobelts were synthesized by catalyst-assisted crystallization of polymer-derived amorphous silicon carbonitride (SiCN). The obtained nanobelts were characterized using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and selected-area electron diffraction. The results revealed that the -Si₃N₄ nanobelts are 20 to 40 nm in thickness, 400–600 nm in width and a few hundreds of micrometers to several millimeters in length, and grow along either the [011] or the [100] direction. Intense visible photoluminescence was observed over a spectrum ranging from 1.65 to 3.01 eV, which can be attributed to defects in the -Si₃N₄ structure. PACS 81.07.-b; 78.67.-n; 81.05.Je     

Fabrication and characterization of Cu–CdSe–Cu nanowire heterojunctions

The Cu–CdSe–Cu nanowire heterojunctions were fabricated by sequential electrochemical deposition of layers of Cu metal and CdSe semiconductor within the nano-pores of anodic alumina membrane templates. X-ray diffraction reveals the cubic phase for Cu and hexagonal phase for CdSe in the electrodeposited Cu–CdSe–Cu nanowire heterojunctions. The composition of the nanowire heterojunction segments is characterized by energy dispersive X-ray spectroscopy. The morphological study of nanowire heterojunctions has been made using scanning electron microscope and high resolution transmission microscopy. The nanowire heterojunctions grown in 100 and 300 nm nano-pore size templates have been found to have optical band gaps of 1.92 and 1.75 eV, respectively. The absorption spectra of 100 nm nanowire heterojunctions show a blue shift of 0.18 eV. The collective nonlinear current–voltage (I–V) characteristics of the 300 and 100 nm nanowire heterojunctions show their rectifying and asymmetric behaviour, respectively.     

Dynamic characterization of charge transport in organic and polymer transistors

In this paper we describe three methods that can be used to measure the transient response of organic and polymer field-effect transistors (FETs) and also how such measurements can be used to determine the drift mobility and velocity. The first method measures the response of a FET to a step voltage applied to the source with the gate grounded and the drain held at close to ground, while the second uses a ramp input to the source. The third technique evaluates the frequency response of the FET, connected as a diode, to a large-signal alternating voltage. We show that important information can be obtained from such measurements which can be quantitatively interpreted with the help of models that we are developing. In general, there is good agreement between the drift mobility measured with these approaches and the field-effect mobility calculated from transistor output and transfer characteristics. The specific results we present in this paper are for pentacene devices; however, other recent work by our group indicates that these results are more general.     

Growth and Roughness of the Interface for Ballistic Deposition

In ballistic deposition (BD), (d+1)-dimensional particles fall sequentially at random towards an initially flat, large but bounded d-dimensional surface, and each particle sticks to the first point of contact. For both lattice and continuum BD, a law of large numbers in the thermodynamic limit establishes convergence of the mean height and surface width (sample standard deviation of the height) of the interface to constants h(t) and w(t), respectively, depending on time t. We show that h(t) is asymptotically linear in t, while (w(t))² grows at least logarithmically in t when d=1. We use duality results showing that w(t) can be interpreted as the standard deviation of the height for deposition onto a surface growing from a single point.     

Growth and characterization of InAs quantum dots with low-density and long emission wavelength

The growth parameters affecting the deposition of self-assembled InAs quantum dots(QDs)on GaAs sub- strate by low-pressure metal-organic chemical vapor deposition(MOCVD)are reported,The low-density InAs QDs(～5×10~8 cm~(-2))are achieved using high growth temperature and low InAs coverage.Photolu- minescence(PL)measurements show the good optical quality of low-density QDs.At room temperature, the ground state peak wavelength of PL spectrum and full-width at half-maximum(FWHM)are 1361 nm and 23 meV(35 nm).respectively,which are obtained as the GaAs capping layer grown using triethylgal- lium(TEG)and tertiallybutylarsine(TBA).The PL spectra exhibit three emission peaks at 1361,1280, and 1204 nm,which correspond to the ground state,the first excited state,and the second excited state of the QDs,respectively.     

Preparation of alumina nanowires, nanorods, and nanowalls by chemical etching

Alumina (Al₂O₃) nanowires, nanorods, and nanowalls have been prepared from anodic aluminum oxide (AAO) templates by chemical etching in NaOH solution. Heating the template prior to etching is crucial to the morphology of subsequent prepared alumina nanostructures, which greatly depend on the phases of the AAO templates. It has been found that the templates with amorphous Al₂O₃, γ-Al₂O₃, and α+γ-Al₂O₃ phases will grow nanowires, nanorods, and nanowalls, respectively. A possible mechanism for forming different alumina nanostructures is proposed.     

Solid solutions of bismuth-based Aurivillius oxides: structural and dielectric characterization

Bismuth layered perovskite structures show interesting physical properties varying as a function of external parameters (temperature, frequency, electric and magnetic fields). When a magnetic ion is incorporated in some of these materials, some of the structures show simultaneous ferroelectricity and ferro/antiferromagnetism. Thus, they exhibit magnetoelectric properties under the influence of an external magnetic field. This paper compares the structural (XRD and SEM) and electrical properties of two eight-layered Aurivillius oxides.     

Photocatalytic activity of 6.5 MeV electron-irradiated ZnO nanorods

The microwave-synthesized zinc-oxide (ZnO) nanonorods of average length of ～ 1500 nm and diameter ～ 100 nm were irradiated with 6.5 meV electrons. From sample to sample, the electron fluence was varied over the range 5×10¹⁴ to 2.5×10¹⁵ e-cm^?2. The pre- and post-electron-irradiated ZnO nanorods were characterized by X-ray diffraction, UV–VIS, EDAX, scanning electron microscopy, transmission electron microscopy, and BET methods. The results show that after electron irradiation, the ZnO nanorods could retain the hexagonal phase with the wurtzite structure; however, the average length of the ZnO nanorods reduced to ～ 800 nm. Moreover, the oxygen atoms from a fraction of ZnO molecules were dislodged, and the process contributed to the formation of Zn–ZnO mixed phase, with increased zinc to oxygen ratio. In the photo-degradation of Rhodamine-B, a significant enhancement in the photocatalytic activity of the electron-irradiated ZnO nanorods was observed. This could be attributed to the induced defects, reduced dimensions, and increased surface area of the ZnO nanorods, in addition to the formation of the Zn–ZnO phase. All these could collectively contribute to the effective separation of the photogenerated electrons from the holes on the ZnO nanorods, and therefore enhance the photocatalytic activity under UV exposure.     

Synthesis and characterization of β-Ga2O3 nanorods

A novel method was applied to prepare β-Ga₂O₃ nanorods. In this method, β-Ga₂O₃ nanorods have been successfully synthesized on Si(1 1 1) substrates through annealing sputtered Ga₂O₃/Mo films under flowing ammonia at 950 °C in a quartz tube. The as-synthesized nanorods are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). The results show that the nanorod is single-crystalline Ga₂O₃ with monoclinic structure. The β-Ga₂O₃ nanorods are straight and smooth with diameters in the range of 200-300 nm and lengths typically up to several micrometers. The growth process of the β-Ga₂O₃ nanorods is probably dominated by conventional vapor-solid (VS) mechanism.     

Diagnostic and characterization of the VCSEL diodes based on GaSb

Vertical-Cavity Surface-Emitting Laser (VCSEL) diodes are among the youngest members of the semiconductor laser diode family. The main aim of our work focuses on the measurement of the basic properties (the spectral range of the laser emission, temperature and current tunability) of experimental VCSEL diode lasers based on GaSb operating in the infrared region around 4250 cm^?1. A high-resolution FTIR Bruker IFS 120 HR spectrometer with a maximum resolution of 0.0035 cm^?1 was used in the emission setup for the laser diagnostic research. The absorption spectra of atmospheric pollutants like methane, carbon monoxide and ammonia have been measured using these VCSELs for the first time.     

Growth and characterization of nanostructured anatase phase TiO<Subscript>2</Subscript> thin films prepared by DC reactive unbalanced magnetron sputtering

Titanium dioxide thin films were deposited on three different unheated substrates by unbalanced magnetron sputtering. The effects of the sputtering current and deposition time on the crystallization of TiO₂ thin films were studied. The TiO₂ thin films were deposited at three sputtering current values of 0.50, 0.75, and 1.00 A with different deposition times of 25, 35, and 45 min, respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The film thickness was determined by field emission scanning electron microscopy (FE-SEM), and the optical property was evaluated with spectroscopic ellipsometry. The results show that polycrystalline anatase films were obtained at a low sputtering current value. The crystallinity of the anatase phase increases as the sputtering current increases. Furthermore, nanostructured anatase phase TiO₂ thin films were obtained for all deposition conditions. The grain size of TiO₂ thin films was in the range 10–30 nm. In addition, the grain size increases as the sputtering current and deposition time increase.     

Fabrication and characterization of hollow metal waveguides for optical interconnect applications

As data rates continue to increase in high-performance computer systems and networks, it is becoming more difficult for copper-based interconnects to keep pace. An alternative approach to meet these requirements is to move to optical-based interconnect technologies which offer a number of advantages over the legacy copper-based solutions. In order to meet the stringent requirements of high performance and low cost, manufacturable waveguide technologies must be developed. Past solutions have often employed polymer waveguide technologies, which can be expensive and limited by modal dispersion. In the present work, hollow metal waveguides (HMWGs) are investigated as a potential alternative. These waveguides demonstrate very low optical losses of <0.05 dB/cm and the capability to transmit at extremely high data rates. The fabrication, modeling, characterization of the HMWGs are discussed to enable photonic interconnect solutions for future generations of computer and server products.     

Sooting turbulent jet flame: characterization and quantitative soot measurements

Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C₂H₄/air turbulent jet flame to fill this role and presents an extensive database for soot model validation.     

Growth and Characterization of CdSe1 − y S y Nanofilms Obtained by Chemical Bath Deposition

In this work, the growth and characterization of cadmium selenide sulphur (CdSe_1???y S _y ) deposited by chemical bath deposition (CBD) technique at the reservoir temperature of 20?±?2 °C are presented, varying the thiourea volume added to the growth solution in the range of 0–30 ml. The films chemical stoichiometry was determined by energy-dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) analysis and Raman scattering reveal that CdSe_1???y S _y -deposited films showed hexagonal wurtzite crystalline phase. The average size of the crystalline grain in relation to the sulphur volume varies in the range of 1.48–9.2 nm that was determined by using the Debye-Scherrer equation for the direction (100), which is confirmed by analyzing the grain average diameter by high-resolution transmission electron microscopy (HRTEM). Raman scattering shows that the lattice dynamics is characteristic of bimodal behaviour and the multipeak adjust of the first optical longitudinal mode for the CdSeS denotes, in all cases, the Raman shift of the characteristic peak in the range of 177–181 cm^?1 of the CdSe crystals associated with the sulphur incorporation. CdSe_1???y S _y band gap energy can be varied from 1.86 to 2.11 eV by varying the thiourea volume added in the growth solution measured by transmittance at room temperature.     

One- and Two-Photon Excited Fluorescence of CdSe and CdSe/ZnS Quantum Dots in n-Hexane

One- and two-photon absorption and excited fluorescence of the CdSe and the core-shell structure CdSe/ZnS quantum dots （QDs） in n-hexane is investigated. The linear and nonlinear absorption coefficients are measured and the two-photon-absorption cross sections of the QDs are also obtained. For both one-photon fluorescence and two-photon fluorescence, the emission efficiency of CdSe/ZnS is much higher than that of CdSe, originating from the effective surface passivation of the core-shell structure.     

Optical and structural characterization of iron oxide and cobalt oxide thin films at 800 nm

We report on optical and structural properties of α-Fe₂O₃ and Co₃O₄ thin films, grown by direct oxidation of pure metal films deposited on soda-lime glass. Structural characteristics and morphology of the films were investigated by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Linear optical absorption, and linear refraction as well as nonlinear optical properties were investigated. The third-order optical susceptibilities were measured applying the Thermally managed Z-scan technique using a Ti: sapphire laser (150 fs; 800 nm). The results obtained for the Co₃O₄ film were \( \text{Re} \chi^{\left( 3 \right)} \) = ?(5.7 ± 2.4) ×10^?9 esu and \( \text{Im} \chi^{(3)} \) = ?(1.8 ± 0.2) ×10^?8 esu while for the α-Fe₂O₃ film we determined \( \text{Re} \chi^{(3)} \) = +(6.6 ± 2.4) ×10^?10 esu and \( \text{Im} \chi^{(3)} \) = +(2.2 ± 0.4) ×10^?10 esu.     

Growth,spectral properties,and laser demonstration of Nd:GYSO crystal

A generic nonlocal nonlinear optical system with a diffusive type of nonlinearity is investigated analytically, using the homogeneous balance principle and the F-expansion technique. Exact traveling wave and soliton solutions are discovered. Numerical simulation of their propagation and interaction properties is carried out. Our results demonstrate that the nonlocal solitary waves can be manipulated and controlled by changing the nonlocality parameter.