Optical properties of citrate-stabilized CdS nanoparticles

Citrate-stabilized CdS nanoparticles of size 4 nm are obtained by varying the sulfide:citrate ion concentration in a simple aqueous synthesis method. The optical absorption and photoluminescence properties of the nanoparticles are studied. The size of the crystallites is found to be less affected by sulfide:citrate ratio. At lower concentrations of S²⁻, trap state emission is favoured and at higher concentrations excitonic transition is predominant as shown by optical absorption and photoluminescence spectra. Effective surface capping and optimum concentration of S²⁻ leads to the quenching of surface-defect-related emission. Increase in citrate ion concentration is found to increase the intensity of photoluminescence band arising from trap state emission revealing the role of sulfide:citrate ratio on surface modification of CdS nanocrystals. The nanoparticles are hexagonal as shown by the X-ray diffraction and selected area electron diffraction pattern.     

Growth mechanism and optical property of ZnO nanoparticles synthesized by sonochemical method

ZnO nanoparticles have been synthesized by ultrasonic irradiation of an aqueous-alcoholic/aqueous-alcoholic-ethylenediamine (EDA) solutions of zinc nitrate and sodium hydroxide. ZnO nanoparticles possess hexagonal wurtzite structures and they exhibit special photoluminescence properties with a red-shift of 22 nm in UV emission band. It is found that the ultrasonic irradiation time and the solvents both influence the growth mechanism and optical properties of ZnO nanoparticles. The possible growth mechanism of ZnO nanoparticles formation by sonochemical method has been tried to discuss.     

Synthesis and Characterization of CdS Nanoparticles with Strong Electrolyte Behavior

The CdS nanoparticles whose structure is similar to a strong electrolyte were synthesized by the colloidal chemical method. The CdS nanoparticles with Cd²⁺-rich surface are capped by the electrically neutral ligand of 2,2-bipyridine (bpy), and the counterion, BPh₄ ^-, is adsorbed around the particle as balance charge. The donation from 2,2-bipyridine at 2-position to the Cd²⁺-rich surface of the CdS nanoparticles was characterized by X-ray photoelectron spectroscopy (XPS). These CdS nanoparticles can redisperse in pyridine (py) or DMF, and have high stability. The determination of electroconductivity and the electrophoresis deposition in dilute solution containing the CdS nanoparticles further prove the rationality of the above electrolyte structure of the CdS nanoparticles.     

Fabrication of nanopatterned germanium surface by laser-induced etching: AFM, Raman and PL studies

Fabrication of the nanopatterned germanium (Ge) surface is done by laser-induced etching. Atomic force microscopy is utilized here to study the surface and sizes of Ge nanoparticles. Raman and photoluminescence (PL) spectroscopy have been used to characterize their vibrational and light emission properties. Wavelength-dependent Raman investigations of these nanopatterned Ge surface reveal spatial distribution of sizes of nanoparticles. Nanopatterned Ge structures (etched for 60 min) emit a broad PL band having two maxima at 2.1 and 2.35 eV.     

Ultrasonic irradiation as a tool to modify the H-desorption from hydrides: MgH2 suspended in decane

Effects of ultrasonic irradiation on magnesium hydride (MgH₂) suspended in decane were investigated with the purpose of improving its hydrogen desorption process. Firstly, we have found that the presence of MgH₂ improves the sonolysis of decane enhancing the amount of hydrogen evolved during the sonication process. The sonicated-MgH₂ maintains its microstructural properties practically unaltered but a drastic reduction of the particle size of MgH₂ (down to 20 μm) as well as a high pressure MgH₂ phase are observed. However, no substantial modifications of H-kinetic properties of hydride occur as is determined by thermal desorption measurements. This could be attributed to decomposition of decane during sonication which leads to the formation of carbon compounds that hinder the thermal decomposition of MgH₂.     

Formation of CdS nanoparticles using starch as capping agent

CdS nanoparticles have been synthesized using starch as capping agent in aqueous solution. The morphology and crystalline structure of such samples were measured by high-resolution transmission electron microscopy and X-ray diffraction, respectively. The average grain size of the nanoparticles determined by these techniques was of the order of 5 nm. Photoluminescence of CdS nanoparticles shows a strong emission peak below to the band gap bulk semiconductor attributed to center trap states, also the broadening peak was interpreted in terms of electron-phonon interaction.     

Preparation of CaWO4:Ln@SiO2 (Ln=Tb, Dy and Ho) nanoparticles by a combustion reaction and their optical properties

The CaWO₄:Ln³⁺@SiO₂ (Ln=Tb, Dy and Ho) nanoparticles were synthesized via a combustion process at 800 °C, using citric acid as chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports. The persistent phosphor nanoparticles were characterized by X-ray diffraction (XRD), reflectance UV-vis and fluorescence spectroscopy (PL) and transmission electron microscopy (TEM) techniques. XRD patterns indicated that crystalline calcium tungstate with scheelite structure was produced. The reflectance UV-vis spectra showed the broad absorption band of groups and the PL spectra showed the wide excitation band, broad emission band of and characteristic emissions of Ln³⁺ ions. The average particle sizes were determined by TEM, which are about 50 nm.     

Optical and IR study of CdS nanoparticles dispersed in a new confined p-phenylenevinylene

In the present study we have synthesized CdS semiconducting quantum dots by reverse micelle method using dodecanthiol (C₁₂H₂₆S) as the capping agent. The synthetic medium consists of a quaternary water/Sodium dodecyl sulfate (SDS)/buta-1-ol/hexane microemulsion. The size of the particles was controlled by changing the molar ratio W=[H₂O]/[SdS], where [H₂O] and [SdS] are the molar concentrations in hexane of water and SdS respectively. The CdS nanoparticles were embedded in a new PPV derivate named Fluorinated Bisphenol A P-phenylenevinylene (BPAF-PPV). Fourier transform infrared spectroscopy (FT-IR) showed a strong interaction of thiol groups with CdS nanoparticles. Blue shift of the optical absorption onset is observed due to quantum size effect. The band gap and particle sizes of the nanoparticles were deduced from optical absorption spectra and the use of an effective mass approximation (EMA) model. Photoluminescence spectroscopy evidenced a charge transfer process via the interface between BPAF-PPV/CdS nanoparticles.     

Shape-dependent plasmon resonances of Ag nanostructures

Different silver nanostructures have been rapidly synthesized under microwave irradiation from a solution of silver nitrate (AgNO₃) and β$\beta$-D glucose; neither additional reducing nor capping agent were required in this soft green solution approach. Not only spherical nanoparticles, but also necklace and wires have been synthesized. The plasmon resonances of the synthesized silver nanostructures were tuned by varying the irradiation time and hence by changing size and morphology of nanostructures. The obtained nanostructures were characterized by X-Ray diffraction (XRD), Uv–Vis spectroscopy (Uv–Vis), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The change of peak position and the shape of the absorption spectra were clearly observed during the whole reaction process; in fact, it was evidenced that initially Ag nanoparticles were formed, which, as reaction time elapsed, self-assembled and fused with each other to yield nanowires.     

CdS/ZnO纳米复合结构的制备、表征及其光催化活性的改善

利用简单的水热法在ZnO纳米棒表面合成CdS纳米粒子.用扫描电镜(SEM)和X射线衍射(XRD)对CdS/ZnO异质结构进行表征.实验结果表明,在生长CdS的过程中ZnO被逐渐地腐蚀.选择CdS/ZnO纳米复合材料作为光催化剂在紫外光和绿光照射的条件下降解甲基橙(MO).CdS/ZnO纳米复合材料纳米棒作为光催化剂降解...     

Extremely high-frequency piezoelectroacoustic transducer based on BN-tube/SiC-whiskers rope

Innovative idea of piezoelectric electroacoustic transducer in extremely high-frequency terahertz range on the basis of BN-tube/SiC-whiskers rope is suggested and substantiated. Unlike an acoustic spectrum of solid rectangular pins and films used so far in ultrasonic pulsers and receivers, in the acoustic spectrum of circular hollow nanotubes, the peculiar squash E_2g and the subsequent E_ng modes of starlike chain belonging to a gallery of whispering acoustic modes was shown by ab initio RHF/6-31G calculations to exist in the Raman spectra. Inherent important feature of these standing vibrations is their weak attenuation and high frequency, which, as depended on the nanotube diameter, fall in the range of about 1 GHz–1 THz. Hypersound was suggested to be excited by resonant microwaves using the piezoelectric properties of BN heteropolar nanotubes and then to transmit it into a sample by high modulus encapsulated SiC-whiskers. Such BN-tube/SiC-whiskers of 100–800 nm in diameter and with 20 aspect ratio were synthesized by carbothermal and CVD techniques. Cactus-like arrays of SiC nanowhiskers were synthesized by CVD technique. A sketch of the hypersound generator/detector, with the piezoelectroacoustical transducer on the basis of the BN-tube/SiC-whisker assembly serving as hypersonic antenna, was advanced.     

Effect of ratios of Cd:Se in CdSe nanoparticles on optical edge shifts and photoluminescence properties

This paper addresses the issue related to morphology of CdSe nanoparticles capped with organic molecules. Semiconducting CdSe nanoparticles of 5–16 nm are synthesized using CdO precursor, capped with trioctyl phosphine (TOP)/trioctyl phosphine oxide (TOPO) using different starting precursor ratios of Cd:Se. At an optimum ratio of Cd/Se-2:1, highly luminescent and small sized (5 nm) nanoparticles are obtained. At other Cd/Se precursor ratios (0.5:1, 1:1, 3:1) larger particles are formed with varying photoluminescence (PL) intensity and optical absorption (UV–VIS). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to determine the crystallinity and stoichiometry of the system, respectively. It is shown that the blue shifts of the optical absorption edge concurrent with the CdSe nanocrystal size reduction, for sizes measured by XRD with respect to the bulk semiconductor, agree perfectly with the strong quantum confinement model. The optical edge shifts are significantly higher for CdSe nanocrystallite as measured by transmission electron microscopy (TEM) than the theoretical prediction based on the strong quantum confinement model. This is understood on the basis of agglomeration effects as observed by TEM for CdSe nanocrystallites. The nano-sized CdSe growth island thus formed comprises of several TOP/TOPO passivated nanocrystals.     

Quantum confinement effect in ZnO nanoparticles synthesized by co-precipitate method

Quantum mechanical effects such as an increased bandgap of semiconductors with reduction of size are viewed as having strong potential for future applications. In the present work, zinc oxide (ZnO) nanoparticles (NPs) were synthesized via the co-precipitate method. Very narrow particle size distribution of the ZnO nanoparticles was achieved through careful control of the synthesis conditions. The structural, morphological, and optical characterization was carried out using X-ray diffraction, atomic force microscopy, and UV–vis reflectance techniques, respectively. The results indicated that increasing the temperature from 60 to 65 °C caused a subsequent increase in particle size from 4 to 12 nm. An associated increase in bandgap with decrease in particle size was also noticed which is a strong indication of the quantum confinement effect.     

Optical properties of ZnO nanorods realised by aqueous chemical growth

Photoluminescence investigations of ZnO nanorods realised by an advanced two-step aqueous chemical growth process have been carried out revealing well-resolved near-band-edge emission accompanied by phonon replicas. The optical properties of nanorods with different lengths and diameters are quite similar indicating a good control of the growth process without influencing the optical properties even on plastic substrate. The near-band-edge emission has a very broad line-width of 10 meV. Annealing in Ar atmosphere reduces the deep-level emission with a corresponding increase of the near-band-edge emission.     

Triangular CdS nanostructure: effect of Mn doping on photoluminescence,electron spin resonance,and magneto-optical properties

In this paper, we report synthesis and study of magneto-optic Faraday effect for dilute magnetic semiconductor nanostructure. The colloidal CdS nanocrystals were prepared via hot injection method and successfully doped with Mn²⁺ cations. The synthesized nanoparticles were characterized by using UV–Vis spectroscopy, X-ray diffraction, photoluminescence spectroscopy, transmission electron microscopy, and electron spin resonance spectroscopy. Systematic studies on effect of Mn²⁺ doping on photoluminescence, electron spin resonance, and magneto-optical properties are carried out. UV–Vis spectral analysis confirms blue shift in bandgap of CdS nanoparticles due to quantum confinement effect. The X-ray diffraction study confirms hexagonal wurtzite phase formation of CdS nanoparticles without any impurity phases. TEM analysis confirms uniform particle size, having particle size distribution around 5 nm. As-synthesized undoped CdS shows triangular-shaped nanocrystals with hexagonal structure; however, triangular shape of CdS nanoparticles is not conserved after Mn²⁺ doping. The photoluminescence characteristic spectra of Mn²⁺-doped CdS nanocrystals showed emission band at 660 nm and its intensity was found to increase with increasing Mn²⁺ concentration. Electron spin resonance signal, with six-line hyperfine structure splitting, confirmed doping of Mn²⁺ ions in CdS lattice. Magneto-optic measurements showed linear variation of Faraday rotation with respect to applied magnetic field, indicating paramagnetic behavior of Mn-doped CdS. The highest Verdet constant 24.81 deg/T cm was observed for 2% Mn-doped CdS nanocrystals, which further decreases with increasing Mn²⁺ concentration.

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Graphical abstract Illustration of Magneto-optic Faraday effect using dilute magnetic semiconductors (TEM image of triangular CdS nanoparticles)

     

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range 1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of 3.35–3.68 eV with decreasing thickness in agreement with the calculated values.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be 4.32 and 4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit 0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

核壳结构CdS/ZnS纳米微粒的制备与光学特性

用微乳液法制备CdS纳米微粒 ,以ZnS对其进行表面修饰 ,得到具有核壳结构的CdS/ZnS纳米微粒 .采用X射线衍射 (XRD)、透射电镜 (TEM )表征其结构、粒度和形貌 ,紫外 可见吸收光谱 (UV)、光致发光光谱(PL)表征其光学特性 .制得的CdS近似呈球形 ,直径为 3.3nm ;以XRD和UV证实了CdS/ZnS核壳结构的实现 .研究了不同ZnS壳层厚度对CdS纳米微粒光学性能的影响 ,UV谱表明随着壳层厚度的增加纳米微粒的吸收带边有轻微的红移 ,同时短波吸收增强 ;PL谱表明壳层ZnS的包覆可减少CdS纳米微粒的表面缺陷 ,带边直接复合发光的几率增大 ,具有合适的壳层厚度时发光效率大大提高 .     

Modification of photonic properties in porphyrin-infiltrated opal crystals

Structural, optical and magnetic properties of porphyrin-infiltrated opal hybrid structures were investigated. Bulk samples of synthetic opal were grown by sedimentation technique from colloidal solution of SiO₂ spheres of diameter 250 nm. The structure of the samples was examined by atomic force microscopy. The photonic properties of crystals were investigated by optical measurements in transmission and reflection modes. The stop band was observed in the region 510–550 nm. The photonic properties of synthetic opal crystals were modified by infiltration with aqueous basic solution of iron–porphyrin (FeTPPS) of concentration 1.0 mM. In hybrid samples the absorption bands typical of FeTPPS were observed in the vicinity of the opal stop band. Magnetic properties of FeTPPS-infiltrated opal samples have been studied at 5–300 K in magnetic fields up to 5 T. The FeTPPS-infiltrated opal crystals can be considered as the structures perspective for magnetophotonic devices.     

Scale-free network provides an optimal pattern for knowledge transfer

We study numerically the knowledge innovation and diffusion process on four representative network models, such as regular networks, small-world networks, random networks and scale-free networks. The average knowledge stock level as a function of time is measured and the corresponding growth diffusion time, τ is defined and computed. On the four types of networks, the growth diffusion times all depend linearly on the network size N as τN, while the slope for scale-free network is minimal indicating the fastest growth and diffusion of knowledge. The calculated variance and spatial distribution of knowledge stock illustrate that optimal knowledge transfer performance is obtained on scale-free networks. We also investigate the transient pattern of knowledge diffusion on the four networks, and a qualitative explanation of this finding is proposed.