Photophysics of GaSe/InSe nanoparticle heterojunctions

The photophysics of mixed aggregates of GaSe/InSe nanoparticles have been studied using static and time-resolved absorption and emission spectroscopies. The results indicate that the GaSe/InSe interfaces form heterojunctions and exhibit photoinduced direct charge transfer from the GaSe valence band to the InSe conduction band. This results in the electrons and holes being localized separately in these two types of nanoparticles. The energy diagram of the nanoparticle heterojunction can be constructed from the static spectra, known bulk band offsets, and quantum confinement effects. These considerations accurately predict the energy of the observed charge-transfer band. Photoexcitation also produces excitons in the aggregates, away from the heterojunctions. These excitons can undergo diffusion and quench upon reaching a heterojunction. Time-resolved fluorescence kinetics can be modeled to extract an exciton diffusion coefficient. A value of 2.0 nm2/ns is obtained, which is in good agreement with values obtained from previous fluorescence anisotropy decay measurements.     

dl‐Aspartic Acid‐Mediated Hydrothermal Synthesis of β‐ZnS Microspheres and Their Optical Properties

ZnS hollow microspheres were synthesized by a dl ‐aspartic acid mediated hydrothermal route. dl ‐aspartic acid plays an important role as crystal growth soft template, which regulates the release of Zn²⁺ ions for the formation of ZnS hollow spheres. The formation of these hollow spheres was mainly attributed to an Ostwald ripening process. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), electron diffraction (ED), UV/Vis spectroscopy (UV), and photoluminescence (PL). The shells of the microspheres were composed of ZnS quantum dots (QDs) with the average size of 2.31 nm. The average microspheres diameter is 0.5–3.5 μm. The shell thickness of the hollow sphere is ≈?300 nm. The optical bandgap energy increased significantly compared to the bulk ZnS material due to the strong quantum confinement effect. Two strong emissions at ≈?425 nm and ≈?472 nm in the photoluminescence (PL) spectrum of ZnS hollow microspheres indicate strong quantum confinement because of the presence of QDs.     

Transient absorption spectra and dynamics of InSe nanoparticles

Time-resolved fluorescence and transient absorption results have been obtained for small (approximately 3 nm) and large (approximately 5-8 nm) InSe nanoparticles in room-temperature solutions. The large particles are nonfluorescent, indicating that the conduction band is at M and the optical transition is forbidden. For some fraction of the small particles, the bottom of the conduction band is at Gamma and the optical transition is allowed. The small particle fluorescence measurements indicate that hole trapping occurs on the 200-300 ps time scale. The transient absorption spectra are featureless throughout the visible with a broad maximum at 600-650 nm. The transient absorption kinetics of both small and large particles show a 200-300 ps decay component that is assigned to hole trapping. These kinetics also show a 15 ps decay that has a larger amplitude in the case of the large particles and is assigned to an electron Gamma to M relaxation. The amplitude of this decay indicates that the initial electron and hole intraband transitions result in roughly comparable intensities of the initial transient absorption.     

Conformational study of CdS nanoparticles prepared by ultrasonic waves

CdS nanoparticles of about 5 nm in size have been prepared with the aid of ultrasound irradiation to ethylenediamine solution of cadmium acetate dehydrate and elemental S in presence of 1-decanthiol under air and normal laboratory conditions. X-ray diffraction (XRD) and selected area electron diffraction (SAED) studies indicate that the products are nanocrystallites in hexagonal structure. High resolution transmission electron microscopy (HRTEM) image reveals that lattice fringes are clearly visible, conforming their crystallinity with lattice space of 0.27 nm corresponding to (1 0 2) plane of hexagonal CdS. Energy-dispersive X-ray analysis (EDAX) shows that the product are entirely pure and atomic percentage ratio of Cd to S is about 53:47. UV–vis absorption spectroscopy of the as prepared nanoparticles reveals an energy band gap of about 3.8 eV compared to 2.42 eV corresponding to its bulk value; a blue shift of about 1.38 eV, which is understood as quantum size effect due to confinement of electron and hole in a small volume.     

Synthesis,characterization and size dependent energy band gap of binary CdSe quantum dot and its nanoparticle film

Nanocrystals provide a confinement effect for nanoscale engineering. In this work the cadmium selenide quantum dot and its nanoparticle films have been deposited by chemical bath deposition method (CBD). Effects of deposition time, pH and annealing operation on the optical and structural of CdSe nanoparticle film were studied. The energy band gap, structure and morphology of the samples are investigated by X-ray diffraction (XRD), UV-Vis spectrometer and scanning electron microscope (SEM). It was found that the optical band gap, nanoparticle size and thin film configuration are changed by varying pH, deposition time and annealing operation.     

Ordered arrays of II/VI diluted magnetic semiconductor quantum wires: formation within mesoporous MCM-41 silica

We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.     

Photochemistry of semiconductor colloids: Properties of extremely small particles of Cd3P2 and Zn3P2

Extremely small particles of Zn₃P₂ and Cd₃P₂ were prepared as colloids in aqueous solution and in the solid state. The onset of absorption and the fluorescence band, which lie in the infrared for the macrocrystalline materials, can be shifted through the whole range of the visible by changing the particle size. Fluorescence quantum yields of 15% were obtained. The optical effects are due to the strong perturbations in the electronic energy levels as a consequence of carrier confinement (Q-type materials). The solid particles, which fluoresce strongly, can be redissolved without changes in size. Methylviologen quenches the fluorescence and promotes the photoanodic dissolution.     

Facile one pot synthesis of PbS nanosheets and their characterization

Metal-sulfides semiconductor nanosheets are talented entrant to be applied in electro-optic devices. Hence, the synthesis of PbS nanosheets is achieved in the current work using a simple route. The synthesized nanosheets were characterized by X-ray diffraction (XRD), FT-Raman, scanning electron microscope (SEM), UV-Visible, Photoluminescence (PL) and impedance spectroscopy techniques. XRD pattern and Raman spectrum confirms the formation of crystalline structure of PbS nanosheets. SEM study shows that the synthesized PbS is well defined nanosheets of <5 nm thicknesses. The absorption band edge is found to be remarkably blue shifted in nanosheets compare to bulk. The energy gap is calculated to be 1.16 eV which is about 3 times superior than the bulk value (0.41 eV). The enhancement of band gap indicates the occurrence of quantum confinement effect in PbS nanosheets. A strong violet emission band at ∼405 nm is observed in PL spectrum which is assigned to electrons transition from conduction-band edge to holes, ensnared at interstitial Pb²⁺ sites.     

Incorporation of CoS nanoparticles into ZSM-5 zeolite by hydrothermal and ion exchange methods

Cobalt sulfide nanoparticles were introduced into the medium-pore zeolite ZSM-5 (Zeolite Scony Mobil Five) by ion exchange in aqueous suspension and also by the addition of cobalt sulfate to the synthesis gel in hydrothermal zeolite synthesis. The latter method was systematically studied in the presence of tetraethylammonium ions as organic agents. The materials were characterized by chemical analysis, x-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), energy dispersion x-ray (EDX), IR, BET and diffuse reflectance spectroscopy (DRS). SEM picture and BET were used to discriminate between CoS nanoparticles in the zeolite pores and on the outer crystal surface. Their crystalline structure and morphology were studied by XRD and scanning electron microscopy. The results showed that in hydrothermal method zeolite acts as a template. CoS nanoparticles with an approximate size of 22 nm grow on the surface of zeolite. In ion exchange method, however, the majority of CoS nanoparticles are about 6 nm in diameter, located on the surface of the MFI (type materials ZSM-5) structure. Exciton absorption peaks at higher energy than the fundamental absorption edge of bulk CoS indicate quantum confinement effect in nanoparticles as a consequence of their small size. The absorption spectra show that the optical band gap for CoS nanoparticles produced by hydrothermal and ion exchange methods is 3.68 and 4.1 eV, respectively.     

Optical absorption and valence band photoemission from uncapped CdTe nanocrystals

CdTe nanocrystals have been successfully fabricated by a mechanical alloying process. X-ray diffraction (XRD) patterns demonstrate that a single-phase CdTe compound with a zinc blende structure has been formed after ball milling elemental Cd and Te mixture powders for 27 h. The large broadening effect for the width of the {111} diffraction peak of uncapped CdTe nanocrystals on smaller size was observed in slowly scanned XRD patterns. The X-ray photoelectron spectrum was used to study the surface of the uncapped CdTe nanocrystals within both core level and valence band regions. The presence of tellurium oxide film on the surface of the uncapped CdTe nanocrystals has been detected in the X-ray photoelectron spectrum of the Te 3d core level, which was comparable to the observed amorphous oxide thin layer on the surface of uncapped CdTe nanocrystals in a high resolution transmission electron microscopy (HRTEM) image. The energy of the valence band maximum for uncapped CdTe powders blue shifts to the higher energy side with smaller particle sizes. In UV-visible optical absorption spectra of the suspension solution containing uncapped CdTe nanocrystals, the absorption peaks were locating within the ultraviolet region, which shifted toward the higher energy side with prolonged ball milling time. Both blue shifts of valence band maximum energy and absorption peaks with decreasing particle size provide a unique pathway to reveal the quantum confinement effect of uncapped CdTe nanocrystals.     

Large-scale synthesis of TiO2 nanorods via nonhydrolytic sol-gel ester elimination reaction and their application to photocatalytic inactivation of E. coli

A simple method of synthesizing a large quantity of TiO(2) nanorods was developed. A nonhydrolytic sol-gel reaction between titanium(IV) isopropoxide and oleic acid at 270 degrees C generated 3.4 nm (diameter) x 38 nm (length) sized TiO(2) nanocrystals. The transmission electron microscopic image showed that the particles have a uniform diameter distribution. X-ray diffraction and selected-area electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the TiO(2) nanorods are highly crystalline anatase crystal structure grown along the [001] direction. The diameters of the TiO(2) nanorods were controlled by adding 1-hexadecylamine to the reaction mixture as a cosurfactant. TiO(2) nanorods with average sizes of 2.7 nm x 28 nm, 2.2 nm x 32 nm, and 2.0 nm x 39 nm were obtained using 1, 5, and 10 mmol of 1-hexadecylamine, respectively. The optical absorption spectrum of the TiO(2) nanorods exhibited that the band gap of the nanorods was 3.33 eV at room temperature, which is 130 meV larger than that of bulk anatase (3.2 eV), demonstrating the quantum confinement effect. Oleic acid coordinated on the nanorod surface was removed by the reduction of the carboxyl group of oleic acid, and the Brunauer-Emmett-Teller surface area of the resulting naked TiO(2) nanorods was 198 m(2)/g. The naked TiO(2) nanorods exhibited higher photocatalytic activity than the P-25 photocatalyst for the photocatalytic inactivation of E. coli.     

Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.     

Modulating the Electronic and Optical Properties of Tetragonal ZnSe Monolayers by Chalcogen Dopants

The recently proposed three‐atom‐thick single‐layer ZnSe sheet demonstrates a strong quantum confinement effect by exhibiting a large enhancement of band gap relative to the zinc blende (ZB) bulk phase. In this work, we aim at investigating the electronic and optical properties of this ultrathin tetragonal ZnSe single‐layer sheet with various chalcogen dopant atoms, based on density functional theory (DFT). We find that these single‐layer sheets with dopant atoms are still direct‐band semiconductors with tunable band gaps, which can lead to strong light absorption and potential applications in solar energy harvesting. Theoretical optical absorbance results show that the S‐doped ZnSe monolayer exhibits a higher absorption performance compared to other doped and undoped ZnSe monolayers. These findings pave a way for the modulation of novel ultrathin tetragonal ZnSe monolayers for a wealth of potential optoelectronic applications.     

Hydrogen surface passivation of Si and Ge nanowires: A semiempirical approach

A semiempirical nearest‐neighbor tight‐binding approach, that reproduces the indirect band gaps of elemental semiconductors, has been applied to study the electronic and optical properties of Si and Ge nanowires (NWs). The calculations show that Si‐NWs keep the indirect bandgap whereas Ge‐NWs changes into the direct bandgap when the wire cross section becomes smaller. Also, the band gap enhancement of Si‐NWs showing to quantum confinement effects is generally larger than that of similar‐sized Ge‐NWs, confirming the larger quantum confinement effects in Si than in Ge when they are confined in two dimensions. Finally, the dependence of the imaginary part of the dielectric function on the quantum confinement within two different schemes: intra‐atomic and interatomic optical matrix elements are applied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2448–2454, 2010     

Optical Properties of Pure ZnSe Nanocrystals Synthesized by Laser Ablation in Organic Liquids

Synthesis of the zinc selenide (ZnSe) nanocrystals (NCs) by pulsed laser ablation approach is reported in two distinct liquid media (ethanol and acetone) by means of the 1st harmonic of high frequency Nd:YAG laser. Based on the X-ray diffraction (XRD) pattern, ZnSe NCs have both wurtzite and zinc blende structures with some overlapping peaks. Transmission electron microscopy (TEM) reveals that as-synthesized NCs are relatively monodispersed and spherical in shape. UV–Vis spectra indicate that the band gap of ZnSe NCs in acetone and ethanol is blue shifted comparing to the band gap of bulk ZnSe which is due to quantum confinement effect. According to the XRD results, TEM observations and UV–Vis spectroscopy, as-synthesized ZnSe NCs in ethanol are larger than the ones in acetone. Two kinds of band edge and deep level emissions are identified by means of the room temperature photoluminescence spectroscopy.     

Synthesis and Characterization of SnO2 Nanopowder Prepared by Precipitation Method

Tetragonal SnO₂ nanopowder of the range ～8 nm has been successfully synthesized by precipitation method. The prepared powder was characterized by thermogravimetry analysis (TGA), x-ray diffraction (XRD), transmission electron microscopy (TEM), Infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and room temperature photoluminescence (PL) spectroscopy. Experimental results show that the prepared powder was phase pure SnO₂ of tetragonal rutile structure without any impurities. The optical band gap was determined to be 4.26 eV, using diffuse reflectance technique with the aid of Kubelka-Munk relation. The blue shift of the band gap was attributed to the quantum size confinement effect.     

Size-dependent ultrafast electronic energy relaxation and enhanced fluorescence of copper nanoparticles

The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles.     

Low temperature synthesis of cubic phase zinc sulfide quantum dots

In this study, we report on a new method for the synthesis of ZnS quantum dots (QDs). The synthesis was carried out at low temperature by a chemical reaction between zinc ions and freshly reduced sulfide ions in ethanol as reaction medium. Zinc chloride and elemental sulfur were used as zinc and sulfur sources, respectively and hydrazine hydrate was used as a strong reducing agent to convert elemental sulfur (S₈) into highly reactive sulfide ions (S²⁻) which react spontaneously with zinc ions. This facile, less toxic, inexpensive route has a high yield for the synthesis of high quality metal sulfide QDs. Transmission electron microscopy (TEM) image analysis and selected area electron diffraction (SAED) reveal that ZnS QDs are less than 3 nm in diameter and are of cubic crystalline phase. The UV-Vis absorption spectrum shows an absorption peak at 253 nm corresponding to a band gap of 4.9 eV, which is high when compared to the bulk value of 3.68 eV revealing strong quantum confinement. PL emission transitions are observed at 314 nm and 439 nm and related to point defects in ZnS QDs.     

Boundary lubrication by sodium salts: a Hofmeister series effect

This article describes the preparation and the physico-chemical characterization of a new host-guest system consisting of zeolite beta nanoparticles as host and mitoxantrone as guest. The resulting host-guest system mitoxantrone@beta is characterized in terms of morphology (transmission electron microscopy, dynamic light scattering), structure (powder wide-angle X-ray diffraction, nitrogen sorption), surface charge (ξ-potential measurements), and optical properties (UV-visible absorption, steady-state fluorescence). Mitoxantrone@beta particles are monodisperse in size with a mean diameter centered around 100 nm. Mitoxantrone guest molecules are adsorbed at the micropore entrances of zeolite host. Resulting nanoparticles retrieve the interesting optical properties of guest molecules with a fluorescence emission band in the near-infrared region. Mitoxantrone loading is comparatively evaluated by three different means (elemental analysis, direct and indirect UV-visible absorption studies) showing a loading level of 6.8 μmol/g. Mitoxantrone@beta nanoparticles also show a noticeable cytotoxic effect when applied to cancer cells.     

类石墨烯C3N4纳米片光催化分解水制氢中的量子限域效应

从层状化合物获得的纳米片是一类新型纳米结构材料,这种二维各向异性的纳米甚至亚纳米级的材料具有独特的物理化学性能,其中最好的一个例证就是从石墨烯C₃N₄到石墨烯C₃N₄纳米片的转变。通过高温氧化热刻蚀方法将体相g-C₃N₄剥离成g-C₃N₄纳米片,应用于染料敏化可见光分解水产氢,表现出了较体相g-C₃N₄高于2.6倍的产氢速率。通过X射线衍射（XRD）、傅里叶变换红外（FTIR）光谱、扫描电子显微镜（SEM）、Brunauer-Emmett-Teller（BET）、荧光光谱和光电化学等表征研究了g-C₃N₄纳米片的结构及曙红（EY）和g-C₃N₄纳米片之间的电子迁移过程。热剥离后的g-C₃N₄纳米片具有较高的比表面积,不仅可以更为有效地吸附染料分子,还因其量子限域效应大大增强了光生电荷的分离效率和电子转移效率,改善了电子沿平面方向的传输能力以及光生载流子的寿命,从而显著提高g-C₃N₄纳米片的光催化产氢活性。