Synthesis and optical properties of SnO2-CuO nanocomposite

SnO₂-CuO nanocomposite was synthesized by impregnating SnO₂ nanowires with CuCl₂ solution and subsequent calcination. SEM and XRD were used to characterize the morphology and structure of the product. The optical properties were analyzed by Raman and photoluminescence (PL) spectra at room temperature. Except the strong orange emission of SnO₂, the PL spectrum showed a red shoulder at 678 nm which originated from the interface between SnO₂ and CuO.     

Emission characteristics of diameter controlled SnO2 nanowires

SnO₂ nanowires with controlled diameters were grown by chemical vapor deposition process for which four different diameters ranging from 50 to 140 nm were grown by the controlling thickness of gold-thin-films as catalysts. The influence of the diameter-to-thickness ratio as well as the mechanism of its formation was studied. The relationship between photoluminescence intensities and aspect ratio with considering surface effects of SnO₂ nanowires was also investigated. The room temperature luminescence intensity was diminished with decreasing the diameter of nanowires due to the increasing surface/volume ratio. The transition energy and emission intensity show abnormal behavior as temperature decreased from room temperature to 5 K.     

SnO2微纳米材料的合成及其生长机理研究

利用简单的化学气相沉积法,以Sn粉为源材料合成不同形貌的一维SnO₂纳米棒、纳米线和纳米花等纳米结构,并通过减小载气中的氧含量获得新颖的SnO₂亚微米环状结构.通过调节Sn粉的量和载气中的氧含量、升温速率等试验条件,有效实现SnO₂一维纳米结构的控制生长.采用扫描电子显微镜、能谱仪和X射线衍射仪表征产物形貌、成分和物相结构,并探讨了SnO₂微纳米材料的生长机理. 关键词： 2')" href="#">SnO₂ 纳米结构 亚微米环 生长机理     

Stimulated emission from trapped excitons in SnO2 nanowires

The pump fluence dependent photoluminescence (PL) spectra of SnO₂ nanowires were investigated, which were synthesized with a high-temperature chemical reduction method. The integrated intensity of the narrower peak at 3.2 eV experiences a strong superlinear dependence on the pump fluence, and the narrowest width of the sharp peak is only 19 meV. Moreover, under high excitation fluence, an ultrafast decay time (less than 20 ps) appears in the time-resolved PL spectra. The emission of these SnO₂ nanowires shows strong apparent stimulated emission behaviors although the SnO₂ is a dipole forbidden direct gap semiconductor. The stimulated emission should relate to the localized islands on the surface of nanowire, which was observed through the high resolution transmission electron microscopy (HRTEM) image. The giant-oscillator-strength effect of bound exciton generated from the localized islands was considered to induce the stimulated emission of SnO₂ nanowires.     

Ultraviolet absorption spectrum and cross-sections of ethynyl (C2H) radicals

This work reports the ultraviolet absorption spectrum and cross-sections of ethynyl (C₂H) radicals in the wavelength range 235-260 nm, determined at T=298 K and at a total pressure of about 20 kPa (150 Torr). Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C₂HCF₃ or C₂H₂ in He. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples showed diacetylene (C₄H₂) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane, products of the reaction of ethynyl radicals with methane. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of a transient absorption centered at about 243 nm. The spectra obtained from the 193-nm photolysis of both C₂HCF₃/He and C₂H₂/He radical precursors were nearly identical, suggesting that the absorption feature can be attributed to the ethynyl radicals. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and an absorption peak at about 243.5 nm. The absorption cross-sections for ethynyl radicals have been determined in this work for the first time. The C₂H cross-section, at the maximum absorption, at 243.5 nm is . The stated uncertainty includes the random and systematic measurement errors. The UV absorption feature, detected in this work, can be assigned to the transitions from the ground electronic state and also possibly from coupled ground and lowest electronic states to the electronic state B (or 32A′) of C₂H. This assignment is based on previously reported high level ab initio molecular orbital calculations and results of recent laser-induced fluorescence studies of the ethynyl radical.     

Synthesis and characterization of axially periodic Zn2SnO4 dendritic nanostructures

Zn2SnO4 （ZTO） nanowires with a unique dendritic nanostructure were synthesized via a simple one-step thermal evaporation and condensation process. The morphology and microstructure of the ZTO nanodendrite have been investigated by means of field emission scanning electron microscopy （SEM）, x-ray diffraction （XRD） and high-resolution transmission electron microscopy （HRTEM）. SEM observation revealed the formation of branched nanostructures and showed that each branch exhibited a unique periodic structure formed by a row of overlaid rhombohedra of ZTO nanocrystals along the axis of the nanobranch. HRTEM studies displayed that the branches grew homoepitaxially as single-crystalline nanowires from the ZTO nanowire backbone. A possible growth model of the branched ZTO nanowires is discussed. To successfully prepare branched structures would provide an opportunity for both fundamental research and practical applications, such as three-dimensional nanoelectronics, and opto-electronic nanodevices.     

The effect of the addition of carbon black and the increase in film thickness on the sensing layers of ZnO / ZnFe2O4 in polymer thick film gas sensors

The aim of this work is to investigate the effect of increasing the carbon black and the sensing layer thickness on the response of a sensor. Three sensors of 60/40 mol% ZnO/Fe₂O₃ with different percentages of carbon black (1.5, 2 and 2.5 wt%) were fabricated on alumina substrates and copper thin film electrodes. The base resistance of the 1.5 wt% carbon black sensor was while for the 2 wt% carbon black sensors it was . The lowest base resistance was recorded with the 2.5 wt% carbon black sensor to be . The sensors were used to detect propanol in the concentration range 2500–5000 ppm, increasing with a step size of 500 ppm, at room temperature. The responses of the sensors were determined as ((R_gas−R_air)/R_air)×100 while the sensitivity was calculated as the slope of the graphs. The sensitivity was increased as the amount of carbon black decreased. The sensitivities of the sensors to propanol at room temperature were 0.0105, 0.005 and 0.002%/ppm for the three sensors. On the other hand, four sensors have different thickness were fabricated used by the same manner. It was found that the response of the sensor increased relatively as the thicknesses of the sensing layer decreased. Results show that the one-layer sensor has the highest response, followed by a decrease in the responses for sensors with higher numbers of layers, 2, 3, 4 and 5 successively.     

In-situ bridging of SnO2 nanowires between the electrodes and their NO2 gas sensing characteristics

A simple and efficient way of making highly sensitive SnO₂ nanowire-based gas sensors without an individual lithography process was studied. The SnO₂ nanowires network was floated upon the Si substrate by separating the Au catalyst layer from the substrate. As the electric current is transported along the networks of the nanowires, not along the surface layer on the substrate, the gas sensitivities could be maximized in this networked and floated structures. The sensitivity was 5-30 when the NO₂ concentration was 1-10 ppm. The response time was ca. 20-60 s.     

Absorption spectra of AsH2 radical in 435-510 nm by cavity ringdown spectroscopy

The absorption spectra of jet-cooled AsH₂ radicals were recorded in the wavelength range of 435-510 nm by cavity ringdown spectroscopy. The AsH₂ radicals were produced by pulsed DC discharge in a molecular beam of a mixture of AsH₃, SF₆, and argon. Seven vibronic bands with fine rotational structures have been identified and assigned as the , , and (n = 1-3) bands of the electronic transition. Based on the previous studies of AsH₂ radical, rotational assignments and rotational term values for each band were obtained, and the molecular parameters including vibrational constants, rotational constants, centrifugal distortion constants, and spin-rotation interaction constants were also determined.     

Thermal infrared cross-sections of C2F6 at atmospheric temperatures

Spectral absorption cross-sections, k_ν (), have been measured in the 8.0 and bands of C₂F₆. Temperature and total (N₂-broadening) pressure have been varied to represent the conditions specified in various models of the terrestrial atmosphere so that the absorption cross-sections can be applied directly to the optical remote-sensing of C₂F₆ in the atmosphere. The measured absolute intensities of the 8.0 and the bands are (1.636±0.003)×10⁻¹⁶ and (, respectively.     

Flame synthesis of 5 V spinel-LiNi0.5Mn1.5O4 cathode-materials for lithium-ion rechargeable-batteries

The lithium transition metal oxide LiNi_0.5Mn_1.5O₄ with an space group (SG) structure has shown great potential as a cathode material for 5 V lithium-ion rechargeable-batteries. In this work, a flame-assisted spray technology (FAST) was developed to produce nanostructured LiNi_0.5Mn_1.5O₄ powder in a continuous manner. The as-synthesized powder had a uniform morphology, was spherical in shape and had a nanocrystalline structure, as observed by SEM and TEM. The XRD pattern of the as-synthesized powder matched the spectrum of spinel-LiNi_0.5Mn_1.5O₄. The average grain size was about 16 nm, as calculated by XRD. However, XRD also indicated the impurity Mn₂NiO₄ in the powder. By varying flame temperature, it was possible to show that the impurity was formed due to the high temperature of the flame. While flame temperature was minimized by lowering the H₂/N₂ ratio, it was not possible to completely eliminate Mn₂NiO₄ from the as-synthesized powder. After annealing at 800 °C for 2 h, the impurity was eliminated, and the XRD pattern of the powder indicated a pure-phase spinel structure with an SG. The electrochemical performance of the flame-synthesized LiNi_0.5Mn_1.5O₄ powder was tested in coin-type test batteries that were charged and discharged at constant current under a 5 V potential. The test cells showed the characteristic voltage plateaus of spinel-LiNi_0.5Mn_1.5O₄ ( SG). The material proved to be electrochemically active as a cathode material for lithium-ion rechargeable-batteries.     

Relationships between surface compositions and properties of surfaces of mixed fumed oxides

Fumed oxides SiO₂/Al₂O₃ (SA), SiO₂/TiO₂ (ST) and Al₂O₃/SiO₂/TiO₂ (AST) at different content of alumina and titania were investigated by one-pass temperature-programmed desorption (OPTPD) time-of-flight mass-spectrometry (TOFMS), Auger electron spectroscopy (AES), NMR, FTIR, thermally stimulated depolarization current (TSDC), microcalorimetry, adsorption of nitrogen, water, (dimethylamino)azobenzene (DMAAB) and metal ions (Pb(II) and Ni(II)). It was shown that all the studied adsorption/desorption and energetic properties of mixed fumed oxides depend strongly on the surface content of alumina (shown as a surface content of aluminum, ) in SA and AST and titania (shown as a surface content of titanium, ) in ST and AST. Many of these properties demonstrate clear correlations with the and values over the total range of alumina and titania content in the materials.     

Field suppression of the modulated phase of Ce2Pd2Sn

Low temperature magnetic (M) and thermal (C_P) properties of the intermetallic compound Ce₂Pd₂Sn have been investigated at zero and different magnetic fields. Two transitions were recognized at and , with latter nearly coinciding with the extrapolated Curie-Weiss temperature . The Curie factor evaluated from T≥T_M, is ≈2μ_B. The positive value of θ_P, the triangular coordination of the magnetic (Ce) atoms and the weak effect of applied magnetic field, reveal that T_M cannot be considered as a canonic antiferromagnetic transition like claimed in the literature. M(T) measurements under moderate magnetic fields () show T_C(B) increasing while T_M(B) is practically not affected. Both transition merge in a critical point at for , where the intermediate phase is suppressed. At , the cusp of a first order transition is observed in C_P(T). According to the proposed ferromagnetic ground state, it is followed by a C_P(T)∝T^3/2exp(-E_g/T) dependence, with a gap of anisotropy .     

The effect of carrier gas flow on structural and optical properties of TiO2 nanowires

Optical properties and photoluminescence of TiO₂ nanowires, synthesized by two-step thermal evaporation process, under different Ar gas flow as carrier have been studied. The gas flow was varied from 50 to 150 sccm in order to find the optimum gas flow to growth TiO₂ nanowires. As evidenced by X-ray diffraction patterns, our synthesized nanowires, were found to be crystalline rutile TiO₂. Our results indicated a convenient gas flow for controlling diameter size of nanowires was about 100 sccm. In this case, diameters and lengths were, respectively, within the ranges of 40–100 and 400–1800 nm. The experimental data of the reflectance of TiO₂ nanowires have been obtained through using spectrometer of wavelength 250–800 nm that has been indicated reflectance decreasing with increasing the gas flow, due to the scattering from the surface of the nanowires and also an increase in voids’ roughness. Under excitation 370 nm, the TiO₂ nanowires can emit light peaked at 435 nm. It is believed that peak 435 may be due to free excitons emission.     

Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures in the range 550-600 °C under flow of Ar or Ar/O₂ as carrier gas. Uniform ZnO nanowires with diameter 15-25 nm and tetrapod nanowires with diameter 30-50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that the concentration of O₂ in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up to about 5.3 at the operating temperature 300 °C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the ZnO nanostructures is also discussed and their potential application is indicated accordingly.