Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.     

Structural and optical properties of InGaN/GaN nanowire heterostructures grown by PA-MBE

The structural and optical properties of InGaN/GaN nanowire heterostructures grown by plasma-assisted molecular beam epitaxy have been studied using a combination of transmission electron microscopy, electron tomography and photoluminescence spectroscopy. It is found that, depending on In content, the strain relaxation of InGaN may be elastic or plastic. Elastic relaxation results in a pronounced radial In content gradient. Plastic relaxation is associated with the formation of misfit dislocations at the InGaN/GaN interface or with cracks in the InGaN nanowire section. In all cases, a GaN shell was formed around the InGaN core, which is assigned to differences in In and Ga diffusion mean free paths.     

Controlled synthesis and multifunctional properties of FePt-Au heterostructures

In this work, FePt-Au heterostructured nanocrystals (HNCs) such as tadpole-, dumbbell-, bead-, and necklace-like nanostructures were synthesized by a facile heteroepitaxial growth of Au NCs onto FePt nanorods (NRs). A study of the growth mechanism revealed that the morphology control of the final products can be correlated with the adsorption sites of hydrogen onto the FePt NRs, which can be manipulated by the amount of the forming gas (Ar/7% H₂) added. Not only the optical characteristic and magnetic properties of the intrinsic materials were retained in the products, but also the FePt-Au HNCs showed the tunable multifunctional properties resulted from the interactions between Au and FePt. Moreover, for methanol oxidation, the FePt-Au HNCs exhibited enhanced catalytic activity and CO tolerance on the catalyst surface compared to commercial Pt catalysts. It is worth noting that as multifunctional units, the FePt-Au HNCs also possess a heterogeneous surface, which could potentially enable their site-specific functionalization for targeting or imaging purposes in biomedical applications. More interestingly, the catalytic properties of the FePt-Au HNCs also endow this material with application potentials in nanocatalysis.      

Benzene-thermal synthesis and optical properties of CdS nanocrystalline

Solvent thermal process has been successfully applied to prepare nanocrystalline CdS via the reaction CdSO₄ with Na₂S₃ in benzene at 80–120 °C. It was found that solvent, temperature, and reactant have significant effect on crystal structure and particle size of CdS nanocrystalline. The results show that increasing of temperature and using of solvent of low dielectric constant are beneficial to prepare CdS nanocrystalline in hexagonal phase. The water content in the system not only induces the presence of cubic phase CdS nanocrystalline, but also leads to a particle size increase. The results demonstrate that both phase and particle size can be well controlled. As-prepared CdS nanocrystalline powders display obvious blue-shift characteristics.     

Synthesis of Cu/SiO2 composite films via gamma-irradiation route and their optical absorption properties

The Cu particles or clusters dispersed mesoporous SiO₂ composite films were prepared by a new method: first the matrix SiO₂ films were prepared by sol–gel process combined with the dip-coating technique, and then they were soaked in Cu(NO₃)₂ solutions followed by γ-ray irradiation at room temperature and in ambient pressure. Thus, the prepared Cu/SiO₂ composite films were examined by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and optical absorption spectroscopy. It has been shown that the Cu particles grown within the porous SiO₂ films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With the Cu particles size changing, an interesting peak shift was observed in the optical absorption measurement. Possible mechanisms of this behavior are discussed in this paper.     

Controlled synthesis of CdS nanobelts and the study of their cathodoluminescence

Cadmium sulfide nanobelts were synthesized on an Au-capped Si substrate via chemical vapor deposition by annealing CdS powder at 860 °C. Two types of nanobelts were found. While both types grew towards [101¯0], only one type had Au particle at its tip. Room-temperature cathodoluminescence (CL) was performed on the nanobelts which were prepared between 840 and 1060 °C. Each CL spectrum exhibited two peaks. The maximum band edge emission (BEE) was found at 510 nm in all spectra while that of the deep level emission (DLE) was located between 706 to 756 nm. The ratio of intensity of BEE to that of DLE increased with increasing annealing temperature. The variation of the DLE intensity in the samples was due to the variation of defect density. A blue shift up to 49 nm was detected for DLE as the annealing temperature was increased by 160 °C. Such a shift is due to the presence of more than one type of defects in these CdS nanobelts.     

Controlled synthesis and optical properties of BaFBr:Eu2+ crystals via ethanol/water solutions

BaFBr:Eu²⁺ crystals with different structures were successfully fabricated via a simple precipitation method using ethanol/water mixtures as solvents. The amount of ethanol in the solvent mixtures played a significant role in the formation of final products, enabling the well-controlled growth of the BaFBr crystals. A possible formation mechanism was proposed based on the results of controlled experiments. The phases and morphologies of the resulting samples were systematically investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED) and elementary analysis. The optical properties of the annealed BaFBr:Eu²⁺ nano-cuboids were investigated using photoluminescence (PL), photo-stimulated luminescence spectroscopy (PSL) and kinetic decays. Faster decay behaviors demonstrate that these BaFBr:Eu²⁺ phosphors are promising materials for applications in optical storage fields. Furthermore, it is envisaged that this environmentally benign method can be extended to prepare other fluoride halides.     

Chemical synthesis of CdS nanoparticles and their optical and dielectric studies

Cadmium sulphide nanoparticles were synthesized by chemical displacement reaction method using cadmium nitrate as cadmium source and ammonium sulphide as sulphur source. The CdS samples are characterized using X-ray diffraction, UV–Vis spectroscopy, FTIR spectroscopy, scanning electron microscopy and impedance spectroscopy. CdS nanoparticles are found to possess cubic structure with the crystallite size ~10 nm. The absorption spectra of synthesized CdS nanoparticles revealed the blue shift in excitonic transitions with respect to CdS bulk material, clearly confirming the formation of nanoparticles. The dielectric properties of CdS nanoparticles are studied in the frequency range 10³–10⁷ Hz at room temperature. The dielectric properties of CdS nanoparticles are found to be significantly enhanced specially in the low frequency range due to confinement.     

Electrical and optical properties of p-SiC/n-GaN heterostructures

We report on the optical, electrical and structural properties of GaN films heteroepitaxially grown by low pressure chemical vapor deposition on 6H-SiC substrates. We employed photoluminescence (PL), Hall effect measurements, scanning tunneling microscopy (STM) and X-ray analysis to determine the quality of our films. Heterojunction diodes were fabricated on p-type SiC and characterized by temperature dependent current–voltage and capacitance–voltage techniques. The results are interpreted within the thermionic emission model and the barrier found is attributed to the conduction band offset between 6H-SiC and wurtzite GaN. The diodes show electroluminescence of the donor-acceptor pair recombination type of 6H-SiC at room temperature. By analysis of the injection behavior we can interpret our data, determining the high valence band offset between 6H-SiC and -GaN to 0.67 eV. This high valence band offset favors applications for hetero-bipolar transistors (HBT).     

Optical properties of rotationally twinned InP nanowire heterostructures

We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.     

Growth of Ge-Si nanowire heterostructures via chemical vapor deposition

The growth of Ge-Si and Ge-Si nanowire (NW) heterostructures was demonstrated via chemical vapor deposition. Due to the influence of interface energy, differing topographies of the heterostructures were observed. On initially grown Ge NWs, numerous Si NW branches were grown near the tip due to Au migration. However, on initially grown Si NWs, high-density Ge nanodots were observed.     

CdS/ZnO核壳结构纳米微粒的合成及其发射光谱研究

采用单分子前驱体热分解的方法合成了单分散CdS纳米晶,以CdS纳米晶作为核,在CTAB辅助下,对其表面进行修饰,荧光光谱表明CdS/ZnO核壳结构被成功合成。考查了温度对包覆的影响,结果表明,随着温度的升高晶体结晶越好,包覆越来越完全,ZnO包覆在CdS纳米晶的表面而掩盖了CdS纳米晶的缺陷,使得缺陷发光减弱而带隙发光增强。     

离子液体中微波辅助制备ZnO纳米棒及光学性能研究

在离子液体1-丁基-3甲基咪唑六氟磷酸盐[BMIM][PF6]水溶液中通过微波加热10min制备出ZnO纳米棒。用X射线衍射仪、场发射扫描电镜、紫外分光光度计和荧光分光光度计对其形貌、结构和性能进行了表征。研究表明,产物结晶性良好,产率高,大小均匀,平均直径为20nm,长度为400～500nm。通过对ZnO纳米棒形成机理和实验条件进行系统探讨,提出了三步反应机理,同时发现离子液体对产物的形貌起着关键作用。该方法简便、快速、环保,可推广运用于其它一维纳米功能材料的制备。     

溶液法制备形貌可控的硫化铜纳米结构

采用溶液法制备出多种形貌的硫化铜纳米结构。经XRD分析，产物为CuS且无其它物相存在。利用TEM、SEM等手段对产物的形貌和结构进行了表征。采用高倍视频显微镜对反应的初期阶段进行了原位观察，考察了反应物浓度对产物形貌和结构的影响，简要分析了硫化铜纳米结构的形成机理，并采用紫外可见光谱分析对所制得的硫化铜纳米结构进行了光学性能的研究。     

Investigation of structural and optical properties of the CdS and CdS/PPy nanowires

In this study, cadmium sulfide (CdS), polypyrrole (PPy) nanowires, and their heterojunctions have been electrochemically synthesized. Morphology of the nanowires has been investigated by scanning electron microscopy. Energy dispersed X-ray, X-ray diffraction, UV–Vis, and FTIR analyses have been used to confirm structure of both CdS and PPy nanowires. For the first time with this study, CdS/PPy nanowire heterojunctions have been integrated into photoelectrochemical (PEC) cells. It has been also demonstrated that PEC performance of the nanowires was strongly function of production conditions, such as deposition time and voltage. The maximum power conversion efficiency of the CdS nanowires obtained in this study was 1.36%. Moreover, efficiencies of the CdS/PPy nanowires have been reached to 5.00%, which makes them very favorable for PEC applications.     

Ultrasonic-template method synthesis of CdS hollow nanoparticle chains

A facile and efficient ultrasonic-template method has been developed for the fabrication of CdS hollow nanoparticle chains. The structures and morphologies of products were characterized by XRD and TEM. UV-Vis and photoluminescence (PL) spectra recorded the optical properties of CdS hollow nanoparticle chains, which showed obvious blue shift relative to the CdS bulk materials. Systematic studies found that the ultrasonic irradiation, concentration of template (polyacrylicamide) and injection method of reaction solution in the system were important factors on the controlled synthesis of hollow nanoparticle chains. The possible mechanism for the formation of CdS hollow nanoparticle chains was also discussed.     

Controlled synthesis and optical properties of Au and Au@PS nanoparticles

In this study, uniform gold (Au) nanoparticles (NPs) were prepared using seed-mediated growth method. The particle size was controlled by tuning the dosage of seed solution. Au@PS core–shell NPs were then synthesized by introducing a polystyrene (PS) shell (2–3 nm thick) around the core of Au NPs (115 nm). Evaluation of the surface plasmon (SP) optical properties indicated that wavelength of SP resonance of Au NPs increased gradually with increase in the particle size. This red shift was about 0.92 nm per 1 nm increase in particle size. The results also indicated that the zeta potential and optical properties of Au NPs could be adjusted by coating PS on the outside. Therefore, surface modifications and surface coating were effective ways to control the optical properties of Au NPs.     

Controlled synthesis and photoluminescence properties of BaXO4 (X = W, Mo) hierarchical nanostructures via a facile solution route

Shape-controlled synthesis of BaWO₄ hierarchical nanostructures has been achieved in a mixed solvent of water and ethanol at room temperature. By simply adjusting the volume ratio of C₂H₅OH and H₂O (R ratio), the size and shape of BaWO₄ nanostructures, such as shuttle-like and ellipsoid-like, are successfully controlled. This simple method has been extended to synthesize BaMoO₄ hierarchical nanostructures. Both BaWO₄ and BaMoO₄ hierarchical nanostructures exhibited new green emission peaks at 558 and 560 nm, respectively.     

A combined template and closed space sublimation approach to the synthesis of single-crystalline CdS nanowires and their optical properties

High-quality single-crystalline CdS nanowires about 40 nm in diameter have been successfully synthesized without any catalyst at ambient pressure by combining the closed space sublimation (CSS) technique with porous anodic alumina membrane (AAM) template method. Extensive characterizations of the nanowires have been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and UV-Visible absorption spectroscopy. A band gap of 2.38 eV is determined from the UV-Vis absorption spectrum obtained from the nanowire sample. Our facile technique may in principle also be used for synthesizing other one-dimensional (1D) materials with high vapor pressure and axial nanowire heterostructures.