Crystallographic shear planes in nanocrystalline SnO2 thin films by high-resolution transmission electron microscopy

Atomic structures of crystallographic shear planes (CSPs) in nanocrystalline thin films of semiconductor SnO₂ were investigated by high-resolution electron microscopy. The films were prepared by electron beam evaporation in high vacuum (10^–6 torr) and followed by annealing in synthetic air at 700 °C for 1–2 H. CSPs with the displacement vector of [1/2 0 1/2] were observed in the planes parallel to (¯101), (110) and (¯3¯21). Most of the CPSs were found to terminate or interact with each other within SnO₂ crystallites. Partial dislocations exist at terminal places of CSPs or along intersecting lines of CSPs. CSP steps were also observed. Structural models of these defects have been proposed. Based on analysis of experimental data, it has been suggested that the Sn/O ratio at CSPs which are not parallel to their displacement vector, at cores of partial dislocations and at CSP steps, is higher than that of the perfect structure, that is, these defects are able to provide extra free electrons with the films.     

Structural, electronic, and optical properties of nanocrystalline As-doped ZnO films on quartz substrates determined by Raman scattering and infrared to ultraviolet spectra

Nanocrystalline As-doped ZnO films with different laser power energy (40 mJ and 55 mJ) and As doping concentrations (C_As from 1% to 3%) have been grown on quartz substrates by pulsed laser deposition. The average grain size of the films was calculated from the (002) peak of x-ray diffraction patterns and is estimated to vary from 9 to 13 nm. Electronic transitions and optical properties of the films have been investigated by Raman scattering, far-infrared reflectance, and infrared-ultraviolet spectral transmittance technique. With increasing doping concentration, the A₁ longitudinal optical phonon mode shifts towards the lower energy side and can be described by (564-75C_As) cm^-1 owing to the increment of free carrier concentration. The E₁ transverse optical phonon frequency is located at about 415 cm^-1 and does not show an obvious decreasing trend with the C_As. The optical constants in the photon energy range of 0.5-6.5 eV have been extracted by fitting the experimental data with the Adachi's model. The refractive index dispersion in the transparent region can be well expressed by a Sellmeier's single oscillator function. Due to different doping concentration and hexagonal crystalline structure, the optical band gap of the films grown at 40 mJ linearly decreases with increasing As concentration. The phenomena agree well with the results from the theoretical calculations.     

Dependence of electrical properties on thermal temperature in nanocrystalline SnO2 thin films

Nanocrystalline SnO2 thin films were prepared by pulsed laser deposition techniques on clean glass substrates, and the films were then annealed for 30 min from 50 to 550 degrees C with a step of 50 degrees C, respectively. The investigation of X-ray diffraction confirmed that the various SnO2 thin films were consisted of nanoparticles with average grain size in the range of 23.7-28.9 nm. Root-mean-square surface roughness of the as-prepared SnO2 thin film was measured to be 25.6 nm which decreases to 16.2 nm with thermal annealing. Electrical resistivity and refractive index were measured as a function of annealing temperature, and found to lie between 1.24 to 1.45 momega-cm, and 1.502 to 1.349, respectively. The results indicate that nearly opposite actions to root-mean-square surface roughness and electrical resistivity make a unique performance with thermal annealing temperature. The post annealing shows greater tendency to affect the structural and electrical properties of SnO2 thin films which composed of nanoparticles.     

Preparation of nanocrystalline SnO2 thin films used in chemisorption sensors by pulsed laser reactive ablation

Nanocrystalline SnO₂ thin films were fabricated by pulsed laser reactive ablation using a metallic Sn target. Oxidation of Sn to SnO₂ occurred principally on the substrate surface and was negligible during transportation of Sn atoms in the ablated plume from the target to the film. Therefore, the substrate temperature was the most important parameter to influence the phase constitution of the films. When the substrate temperature was higher than the melting point of metal Sn (230 °C), SnO₂ phase was obtained. Otherwise the films were β-Sn dominant. X-ray diffraction and transmission electron microscopy techniques were used to determine the grain size in the films, which was in the range 10–30 nm, depending upon the substrate temperature and the subsequent annealing. For chemisorption performance, films with a thickness up to 24 nm showed a higher sensitivity than the films 38 nm and 96 nm thick. Excellent chemisorption properties have been achieved on the very thin nanocrystalline films. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of nanocrystalline SnO2 powder by amorphous citrate route

Nanocrystalline SnO₂ has been synthesized by liquid mix technique using citric acid as the complexing agent. The tin oxide powder obtained at different calcination temperatures (773–1223 K) is characterized using powder X-ray diffraction (XRD), SEM, TEM, TG-DTG and UV spectroscopic techniques. The material obtained is nanocrystalline, having particle size in the range of 10–14 nm. The technique is cost-effective and yields the desired product at temperatures as low as 773 K.     

Frequency-dependent electrical conductivity of nanocrystalline SnO2

Nanocrystalline SnO₂ with a crystallite size of 3–4 and 5–6 nm has been prepared by a sol-gel process in aqueous solution. Its ac electrical conductivity has been measured in dry air at a temperature of 200°C. The observed frequency dependence of its conductivity has been interpreted in terms of the random potential barrier model. The data obtained indicate that the transport properties of the material are dominated by hopping conduction through disordered crystallite boundaries.     

Temperature dependence of fractal dimension of grain boundary region in SnO2 based ceramics

Fractal dimensions of grain boundary region in doped SnO₂ ceramics were determined based on previously derived fractal model. This model considers fractal dimension as a measure of homogeneity of distribution of charge carriers. Application of the derived fractal model enables calculation of fractal dimension using results of impedance spectroscopy. The model was verified by experimentally determined temperature dependence of the fractal dimension of SnO₂ ceramics. Obtained results confirm that the non-Debye response of the grain boundary region is connected with distribution of defects and consequently with a homogeneity of a distribution of the charge carriers. Also, it was found that C−T ⁻¹ function has maximum at temperature at which the change in dominant type of defects takes place. This effect could be considered as a third-order transition.     

Dielectric modeling of transmittance spectra of thin ZnO:Al films

A dielectric model comprising band gap transitions and free electron excitations (Drude model) is successfully applied to simulate transmittance spectra of ZnO films doped with 0.5%, 1% and 2% Al. The Drude formula contains a frequency-dependent damping term in order to get a good fit in the visible spectral region. Useful physical parameters obtained from the fit are electron density and mobility within the grains, film thickness, band gap and refractive index. The optically determined film thickness agrees with that obtained with the stylus method within 2%. The optically determined electronic parameters are compared with those obtained by electrical measurements. Contrary to thin In₂O₃:Sn films, the Drude mobility inside the grains is similar to the direct current Hall mobility indicating more perfect film growth without forming pronounced grain boundaries. Maximum value is 35 cm²/V s. The effective electron mass is estimated to be about 0.6 of the free electron mass. The refractive index at 550 nm decreases with increasing electron density.     

Structure transition and multiferroic properties of Mn-doped BiFeO3 thin films

Pure BiFeO₃ (BFO) and Mn-doped BiFe_1?yMn_yO₃ thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel method. The effects of Mn-doping on the structure and electric properties of the BFO thin films were studied. The X-ray diffraction (XRD) analysis reveals a structure transition in the Mn-doped BiFe_0.96Mn_0.04O₃ (BFMO) thin film. The Rietveld refined XRD patterns conform the trigonal (R3c: H) and tetragonal (P422) symmetry for the BFO and BFMO thin films, respectively. The structure transition and the mixed valences of Mn ions substantially improve the electric properties of the BFMO thin film. The remnant polarization (P _r) of the BFMO thin film was 105.86 μC/cm² at 1 kHz in the applied electric field of 865 kV/cm. At an applied electric field of 150 kV/cm, the leakage current density of BFMO thin film is 1.42 × 10^?5 A/cm². It is about two orders of magnitude lower than that of the pure BFO thin film (1.25 × 10^?3 A/cm²). And the enhanced saturated magnetization of the BFMO thin film is 4.45 emu/cm³.     

SnO2 films prepared by activated reactive evaporation

Transparent conducting films of SnO₂ doped with antimony were prepared on glass substrates by activated reactive evaporation for the first time. The sheet resistance and optical transmittance in the wavelength range 0.4–1.6 μm were studied as functions of various deposition parameters such as the ambient pressure of an 85%Ar15%O₂ mixture, the substrate temperature and the antimony doping concentration in the SnSb alloys. The sheet resistance and optical transmittance showed a strong dependence on the above-mentioned deposition parameters. The best results were obtained for a 90at.%Sn10at.%Sb alloy evaporated in 85%Ar15%O₂ at a partial pressure of about 5 × 10^?4 Torr with a substrate temperature about 350°C. These films, with a sheet resistance of 10 μ/□ had an average transmittance of 95% over the wavelength range 0.4–1.8 μm. The film thickness was about 0.25 μm. Thicker films (about 0.5 μm) had a sheet resistance as low as 1.5 ω/□ with an average transmittance 85% in the wavelength range 0.4–1.6 μm.     

Structural and magnetic properties of nanocrystalline Fe-doped SnO2

Single phase Sn_1?xFe_xO₂ (x = 0.1, 0.3 and 0.5) nanoparticles having size in the range 9–17 nm were prepared by a chemical route. XRD analysis of the samples confirmed the formation of cassiterite phase without any impurity. The nanocrystalline nature of the samples and their crystallinity were confirmed by TEM measurements. Raman and Mössbauer spectroscopy studies indicate structural disorder and vacancies in the nanostructures. M–H and M–T data recorded by a SQUID magnetometer show that the samples are essentially paramagnetic with a weak ferromagnetic component. Ferromagnetism is argued to be associated with the vacancies and defects present in the grain boundaries and interfaces of the nanoparticles.     

用激光烧结法制备的SnO2薄膜的气敏性质

用溶胶-凝胶法制备SnO2前驱液,然后用提拉法分别在单晶Si和Al2O3基片表面制备出SnO2前驱膜,再用脉冲Nd:YAG激光烧结前驱膜使其转变为晶体SnO2薄膜.用XRD分析了单晶Si表面的SnO2薄膜,研究激光功率对SnO2薄膜相组成的影响.TEM观察表明,激光烧结后的薄膜SnO2颗粒均匀,直径约为10 nm.用激光烧结法制备的SnO2薄膜对浓度为1.80×10-4丙酮的最高灵敏度为30～40,明显高于用传统烧结法制备的SnO2薄膜的灵敏度.激光烧结能降低薄膜具有最高灵敏度的工作温度.     

A study of structural transition in nanocrystalline titania thin films by X-ray diffraction Rietveld method

Structural and microstructural analyses of nanocrystalline titania thin films prepared by pulsed laser deposition have been carried out. At lower oxygen partial pressures (≤10⁻⁴ mbar), rutile films were formed, whereas at 1.2 × 10⁻³ mbar of oxygen partial pressure, the thin films contained both rutile and anatase phases. At 0.04 and 0.05 mbar of oxygen partial pressure, the film was purely anatase. Addition of oxygen has also shown a profound influence on the surface morphology of the as deposited titania films. Modified Rietveld method has been used to determine crystallite size, root mean square strain and fractional coordinates of oxygen of the anatase films. The influence of crystallite size and strain on the rutile to anatase phase transition is investigated.