Low-temperature transport properties of individual SnO2 nanowires

Stannic oxide (SnO₂) nanowires have been prepared by Chemical vapor deposition (CVD). The low-temperature transport properties of a single SnO₂ nanowire have been studied. It is found that the transport of the electrons in the nanowires is dominated by the Efros-Shklovskii variable-range hopping (ES-VRH) process due to the enhanced Coulomb interaction in this semiconducting nanowire. The temperature dependence of the resistance follows the relation lnR∼T^−1/2. On the I-V and dI/dV curves of the nanowire a Coulomb gap-like structure at low temperatures appears.     

Large-scale synthesis of rutile SnO2 nanorods

A high yield of tin oxide (SnO₂) nanorods was obtained via annealing a nanoscale precursor in the molten salt flux and surfactant. X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction and infrared spectroscopy showed that the nanorods are composed of SnO₂ with rutile structure. The surfactant and temperature have a profound influence on the production of SnO₂ nanorods.     

Cr-doping induced ferromagnetic behavior in antiferromagnetic EuTiO3 nanoparticles

We have synthesized a series of high quality EuTi_1−xCr_xO₃ (x = 0.0, 0.02, and 0.04) nanoparticles by simple sol-gel technique. The averaged grain size of these obtained nanoparticles displays no obvious change with Cr-doping and is about 100 nm. The structural and magnetic properties of EuTi_1−xCr_xO₃ (x = 0.0, 0.02, and 0.04) samples were detailedly investigated. It is found that the G-antiferromagnetic (G-AFM) ordering of pure EuTiO₃ can be significantly modified with slight Cr-doping, and finally the ferromagnetic behavior is enhanced for EuTi_1−xCr_xO₃ system with Cr-doping.     

Characteristics of photoluminescence and Raman spectra of InP doped silica fiber

We fabricated a silica optical fiber doped with InP sandwiched in the core and the inner cladding layers by using the conventional modified chemical vapor deposition process. We presented the experimental X-ray analysis on the optical properties and found that compound InP was contained in the fiber core after annealing process. Broadband photoluminescence observed in InP doped fiber was well coincided with those coming from the recombination of deep levels in InP. The occurrence of temperature-dependent photoluminescence both at the lower and room temperature would be related with the localized defects and their confinement in the micro-network structure of Si-O-Si. The Raman spectra reveal that Si-O-In vibration would disorder the silica ring structures and enervate their vibrations.     

Photoluminescence from amorphous silica nanowires synthesized using TiN/Ni/SiO2/Si and TiN/Ni/Si substrates

Photoluminescence characteristics of amorphous silica nanowires (a-SiONWs) grown on TiN/Ni/Si and TiN/Ni/SiO₂ substrates have been studied. A-SiONWs grown on TiN/Ni/Si substrates show a Si-rich composition compared to those grown from TiN/Ni/SiO₂/Si. The emission characteristics of the nanowires were found to depend on the type of substrate. By annealing the a-SiONWs grown on TiN/Ni/Si in air, emission bands shift from blue to green bands. It is likely that silicon to oxygen ratio is an important factor in deciding the types of defects and emission bands of amorphous silica nanowires.     

Characterization of semiconductor core-shell nanoparticles by resonant Raman scattering and photoluminescence spectroscopy

Colloidal CdSe nanoparticles (NPs), passivated with CdS and ZnS, were characterized by resonant Raman scattering and photoluminescence (PL). The effect of the passivating shell, its volume and formation procedure on optical and vibrational spectra is discussed. Analyzing the Raman peaks due to optical phonons inside the core and those related to the core-shell interface allows some understanding of the relation between the core-shell structure and its PL properties to be achieved. In particular, a compositional intermixing at the core/shell interface of the NPs was deduced from the Raman spectra, which can noticeably affect their PL intensity.     

Growth of high-density Ru- and RuO2-composite nanodots on atomic-layer-deposited Al2O3 film

Growth of Ru- and RuO₂-composite (ROC) nanodots on atomic-layer-deposited Al₂O₃ film has been studied for the first time using ion-beam sputtering followed by post-deposition annealing (PDA). X-ray photoelectron spectroscopy analyses reveal that RuO₂ and Ru co-exist before annealing, and around 10% RuO₂ is reduced to metallic Ru after PDA at 900 °C for 15 s. Scanning electron microscopy measurements show that well-defined spherical ROC nanodots are not formed till the PDA temperature is raised to 900 °C. The mean diameter of the nanodots enlarges with increasing PDA temperature whereas the nanodot density decreases, which is attributed to coalescence process between adjacent nanodots. It is further illustrated that the resulting nanodot size and density are weakly dependent on the annealing time, but are markedly influenced by the decomposition of RuO₂. In this article, the ROC nanodots with a high density of 1.6 × 10¹¹ cm⁻², a mean diameter of 20 nm with a standard deviation of 3.0 nm have been achieved for the PDA at 900 °C for 15 s, which is promising for flash memory application.     

Photo-catalytic H2 evolution over a series of Keggin-structure heteropoly blue sensitized Pt/TiO2 under visible light irradiation

We report the visible light-induced hydrogen generation over a series of Keggin-structure heteropoly blue (HPB) anions (PW₁₂O₄₀³⁻, phosphotungstic blue (PTB), GeW₁₂O₄₀⁴⁻ (GTB), SiW₁₂O₄₀⁴⁻ (STB), BW₁₂O₄₀⁵⁻ (BTB)) sensitized Pt/TiO₂ photo-catalysts. The sensitization of TiO₂ by HPB was certified using photo-electrochemical measurements and UV-vis absorption spectra. PTB showed the most pronounced sensitization effect for TiO₂ in those HPB anions and Pt/TiO₂-PTB showed the highest hydrogen generation activity. The sensitization of TiO₂ was significantly dependent on the reduction potential of HPA, which was determined by the kind of central atom in HPA.     

Enhanced activity of mesoporous Nb2O5 for photocatalytic hydrogen production

The mesoporous Nb₂O₅ photocatalysts were synthesized via an evaporation-induced self-assembly (EISA) method. The mesoporous structure of the as-made samples was studied by small-angle X-ray diffraction, N₂ adsorption-desorption isothermal and transmission electron microscopy. The increase of the calcination temperature during the synthesis resulted in enhanced crystallization, but decreased mesoporosity of the samples. The later was found to have a crucial influence on the photocatalytic activity by bringing on decreased BET surface area and especially increscent pore wall thickness. The advantage of the mesoporous Nb₂O₅ was also proved by performing 20 times higher photocatalytic activity than a bulk Nb₂O₅ without any porosity. A model was given to describe the effect of mesoporosity on the transportation and recombination of carriers.     

DRIFTS study of surface reactivity to NO2 by zinc nanoparticle aggregates and zinc hollow nanofibers

Zinc nanostructures synthesized with different morphologies from the same evaporation/condensation technique are studied with concern to surface reactivity to NO₂ by Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS). Synthesis of nanopowders is obtained, according to previous work, by gas flow thermal evaporation at 540 °C of bulk Zn grains. Two types of Zn powders are obtained and studied in experiments. The first one is collected on the cold walls of the reactor as a deposit produced by thermophoretic effect. It is constituted by grains (∼10 μm) originated by the stratification of smaller aggregates (∼200 nm) and isolated primary particles (∼50 nm) born in the gas flow. The second type of powder is grown from the condensation of Zn chemical vapors within the expansion orifice of the quartz reactor after relatively long time (∼1 h) deposition process. It is constituted mainly by hollow Zn nanofibers with external and internal diameter about 100 and 60 nm. Preliminary characterization of the two types of powders is made by SEM, TEM, XRD. Thereafter, the two types of samples are studied by DRIFTS at variable temperature (VT). Comparison is made between the home-synthesized nanopowders with respect to commercial Zn standard dust. The Zn hollow nanofibers when exposed to NO₂ are found to exhibit dramatic reactivity, which is not observed at all either in the case of clustered aggregate zinc or of commercial Zn dust powders. Results indicate that, at increasing temperature from RT to 300 °C, the hollow nanofibers surface reacts distinctively with adsorbant gas NO₂, with contemporary formation of a progressively growing narrow absorption band at 2500 cm⁻¹ and contemporary depression of a doublet (∼1600-1628 cm⁻¹) band. In order to justify this striking spectral feature, we propose the occurring of a possible polymerization process at nanofibers surface where most probably as a consequence of pre-treatment and exposure to gas NO₂ a very thin film of ZnO is formed. The possible role of huge specific surface of hollow nanofibers as inferred by preliminary SEM, TEM, XRD studies is discussed.     

Synthesis of ZnO nanowires by pulsed laser deposition in furnace

ZnO nanowires were fabricated on Au coated (0 0 0 1) sapphire substrates by using a pulsed Nd:YAG laser with a ZnO target in furnace. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The length and the diameter of these ZnO nanowires were around 3-4 μm and 120-200 nm, respectively, confirmed by scanning electron microscopy (SEM). The diameter control of the nanowires was achieved by varying the position of substrates. The ultraviolet emission of nanowires from the near band-edge emission (NBE) was observed at room temperature. The formation mechanism and the effect of different position of substrates on the structural and optical properties of ZnO nanowires are discussed.     

Scalable synthesis of Sb(III)Sb(V)O4 nanorods from Sb2O5 powder via solvothermal processing

Scalable Sb(III)Sb(V)O₄ nanorods from Sb₂O₅ powder were prepared using solvothermal route. XRD and HRTEM demonstrate that the nanorods are single-crystal orthorhombic-Sb₂O₄ phase with several micrometers long and 200-300 nm diameter size. XPS result further shows that the antimony cations in the nanorods are composed of three valence and five valence antimony ions. The emission of the nanorods appears around 450 nm wavelength. The formation mechanism of the Sb(III)Sb(V)O₄ nanorods was discussed in detail.     

Surface characterization of the Zn-Ni-Al2O3 nanocomposite coating fabricated under ultrasound condition

Zn-Ni-Al₂O₃ nanocomposite coating, which was fabricated by eletrodeposition technique with the aid of ultrasound, was investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The results reveal that 7.2 wt.% nano-alumina particles uniformly dispersed in the matrix of the composite coating. The XPS analyses demonstrate that the outermost layer of Zn-Ni-Al₂O₃ coating was composed of nano-alumina and Zn(OH)₂, while the transition layer between the outermost layer and the Zn-Ni matrix consisted of nano-alumina, metallic Zn, ZnO and metallic Ni. In order to investigate the influences of ultrasonic agitation and the incorporation of nano-alumina on the composition and surface structure of Zn-Ni matrix, the comparison studies of Zn-Ni-Al₂O₃ nanocomposite coating with Zn-Ni coatings fabricated with and without ultrasound were conducted. The results indicate that ultrasonic agitation resulted in a decrease of Ni content in the Zn-Ni matrix and an increase of the thickness of surface oxide layer; while the incorporation of nano-α-Al₂O₃ increased the Ni content in the Zn-Ni matrix.     

A comparative study of microstructure of RuO2 nanorods via Raman scattering and field emission scanning electron microscopy

Raman scattering (RS) and field emission scanning electron microscopy (FESEM) have been used to extract microstructural information of RuO₂ nanorods (NRs) and a two-phase system comprising NRs embedded in polycrystalline matrix deposited on different substrates by the metal-organic chemical vapor deposition method. The red shifts and asymmetric broadening of the Raman line shape for the NRs are analyzed by the spatial correlation model. The deduced spatial correlation length l is found to be much smaller than that of the average size L₀ estimated from the FESEM images. The Raman features for the two-phase system can be resolved into two parts: a Lorentzian line shape feature corresponding to the polycrystallite at higher frequency side and an asymmetrically broadened NRs' signature located at lower frequency end. The volume fraction of NRs in the two-phase system can be determined from the analysis. These results demonstrate the significance of RS as a structural characterization method when used in conjunction with FESEM.     

Raman and photoluminescence spectroscopy of zinc tungstate powders

ZnWO₄ powders, synthesized using co-precipitation technique and annealed in air at different temperatures in the range of 80-, were studied by Raman and photoluminescence spectroscopy. ZnWO₄ single crystal was used for comparison. The interpretation of the observed variations of the Raman spectra and intrinsic photoluminescence band upon annealing is suggested.     

Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light

A novel and simple method for preparing F-doped anatase TiO₂ (defined as FTO) film with high photocatalytic activity was developed using titanium-n-butoxide and NH₄F as TiO₂ and fluorine precursors under mild condition, i.e. low temperature (lower than 373 K) and ambient pressure. The prepared samples were characterized by XRD, SEM, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectrum (DRS), photoluminescence spectrum (PL) and TG-DSC analysis. The photocatalytic activity was evaluated by decomposing X-3B under artificial solar light. The results showed that the crystallinity of TiO₂ was improved by F-doping. F⁻ ions can prevent the grain growth, and the transformation of anatase to rutile phase was also inhibited. The doped fluorine atoms existed in two chemical forms, and the ones incorporated into TiO₂ lattice might take a positive role in photocatalysis. Compared with surface fluorination samples, FTO film exhibited better photocatalytic activity. The high photocatalytic activity of FTO may due to extrinsic absorption through the creation of oxygen vacancies rather than the excitation of the intrinsic absorption band of bulk TiO₂. Furthermore, the FTO can be recycled with little photocatalytic activity depression. Without any further treatment besides rinsing, after 6 recycle utilization, the photocatalytic activity of FTO film was still higher than 79%.     

Photoluminescence of Si from Si nanocrystal-doped SiO2/Si multilayered sample

A multilayered Si nanocrystal-doped SiO₂/Si (or Si-nc:SiO₂/Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO₂ results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers (N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or d_Si). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and d_Si = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO₂ prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements.     

Cooperative emission study in ytterbium-doped Y2SiO5

Ytterbium ions infrared and visible cooperative luminescences, resulting from YAG laser and selective site excitations, in (6%) Yb-doped Y₂SiO₅ thin film are analyzed. Magnetically coupled Yb-Yb ion pairs seem to play a major role in energy transfer and cooperative emission, confirming the prevalence of superexchange mechanisms.     

Synthesis, structure, and photoluminescence of Zn2SnO4 single-crystal nanobelts and nanorings

A large quantity of single-crystal Zn₂SnO₄ (ZTO) nanobelts is synthesized by using a thermal evaporation method. The lengths of the nanobelts are up to several hundreds of micrometers, and the average width and thickness are about 400 and 30 nm, respectively. Some ring-like nanobelts, called nanorings here, are also observed. The nanobelts are characterized in detail with scanning electron microscope, X-ray powder diffraction, transmission electron microscope, high-resolution transmission electron microscope and selected area electron diffraction. Possible growth mechanisms for the ZTO nanobelts and nanorings are proposed. In addition, the photoluminescence spectrum (PL) of the nanobelts at room temperature shows a stable broad blue-green emission around the 400-600 nm wavelengths with a maximum center at 490 nm. The strong PL emission of the nanobelts may find potential applications in nano-scale optoelectronic devices.