Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing

The role of surface morphology of MoO₃-doped SnO₂ thin film on the gas sensing properties is analyzed. SnO₂ thin films doped with 1, 3, 5 and 10 wt% MoO₃ are prepared by sol-gel spin coating process. Structural and morphological properties are studied using glancing angle X-ray diffractometer, atomic force microscopy, transmission electron microscopy and high resolution transmission electron microscopy. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies are used for chemical analysis. A good correlation is found between the characteristics of the surface and gas sensing properties of these films. MoO₃ addition is found responsible for increase in acidic nature of films which in turn increases their sensitivity and selectivity towards NO₂ gas.     

<Emphasis Type="Italic">h</Emphasis>-MoO<Subscript>3</Subscript> microrods grown on wood substrates through a low-temperature hydrothermal route and their optical properties

Molybdenum trioxide (MoO₃) films with optical properties were successfully grown on wood substrates using a low-temperature hydrothermal method. Scanning electron microscopy analysis shows that MoO₃ thin films were composed of rods-like microstructures and the rod sizes increased as the initial pH value of the solution increased. X-ray diffraction studies indicate crystallinity was greatly improved by increasing the pH value of the precursor solution. Raman spectrum, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analysis further proves that pure MoO₃ films could be grown on a wood substrate. The UV–Vis analysis suggests that the films exhibited better in response to UV light. The band gap energy estimated from the Kubelka–Munk function was found to be in the range of 2.847–2.974 eV. The results from thermogravimetric analysis and differential scanning calorimetry experiments imply that the films possessed good thermal stability.     

A.c. conduction in evaporated MoO3/SiO amorphous thin films

The dielectric properties of vacuum-deposited MoO₃/SiO films of different compositions studied in the frequency range 10² to 10⁶ Hz at various temperatures (193 to 393 K) are reported. The properties of the film capacitor are found to be temperature and frequency dependent. The decrease in a.c. conductance with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres generated in MoO₃/SiO films during the evaporation process.     

Laser molecular beam epitaxy growth and properties of SrTiO3 thin films for microelectronic applications

Dielectric SrTiO₃ thin films were deposited on LaAlO₃ and Si substrates using laser molecular beam epitaxy. The correlations between the deposition parameters of SrTiO₃ thin films, their structural characteristics, and dielectric properties were studied. The conditions for achieving epitaxial SrTiO₃ thin films were found to be limited to deposition conditions such as deposition temperature. We show that the SrTiO₃ films with single (110) orientation can be grown directly on Si substrates. The nature of epitaxial growth and interfacial structures of the grown films were examined by various techniques, such as Laue diffraction and X-ray photoelectron spectroscopy. The SrTiO₃/Si interface was found to be epitaxially crystallized without any SiO₂ layer. Furthermore, we have measured dielectric properties of the grown SrTiO₃ multilayer suitable for tunable microwave device. A large tunability of 74.7%, comparable to that of SrTiO₃ single-crystal, was observed at cryogenic temperatures. Such STO thin films will be very promising for the development of microelectronic device applications.     

The study of optical properties of In2O3 and of mixed oxides In2O3−MoO3 system deposited by coevaporation

A discussion of the optical properties of two systems of dielectric films i.e. In₂O₃ and of mixed oxides In₂O₃−MoO₃ system is presented. Film thickness, substrate temperature, annealing and composition (in molar%) have a profound effect on the structure and optical properties of these films. The decrease in optical band gap with the increase in film thickness of In₂O₃ is interpreted in terms of incorporation of oxygen vacancies in the In₂O₃ lattice. The decrease in optical band gap with the increase in substrate temperature and annealing of In₂O₃ thin films is ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperature. For the mixed oxides In₂O₃−MoO₃ system the results are found to be compatible with the reduction in the value of optical band gap of these materials as the molar fraction of MoO₃ increases in the In₂O₃ thin films and is attributed to the incorporation of Mo(VI) ions in an In₂O₃lattice that causes the indium orbital to become a little less tightly bound. The decrease in optical band gap of mixed oxides In₂O₃−MoO₃ system, with increasing film thickness is interpreted in terms of incorporation of oxygen vacancies in both In₂O₃ and MoO₃ lattice which are also believed to be the source of conduction electrons in In₂O₃–MoO₃ complex. The decrease in optical band gap with increasing substrate temperature and annealing of mixed oxides In₂O₃−MoO₃ system is due to the increasing concentration of oxygen vacancies, formation of indium and molybdenum species of lower oxidation state and indium interstitials. The blue colouration of mixed oxides In₂O₃–MoO₃ samples is due to the inter-electron transfer from oxygen 2p to molybdenum 4d level due to which Mo species of lower oxidation states are formed.     

Effect of a ZnSe Layer on the Thermochromic Properties of MoO3 Thin Films

The effect of a ZnSe layer on the thermochromic properties of MoO₃ thin films was investigated. ZnSe/MoO₃ and MoO₃ thin films were deposited by thermal evaporation in vacuum of MoO₃ and ZnSe powders. Photoacoustic spectra of the films indicated that the thermochromic response for the ZnSe/MoO₃ bilayer system was more accentuated, with respect to the MoO₃ sample, for temperature treatments above 150?°C. This result is explained in terms of the injection of electrons from the II?CVI semiconductor to the oxide film.     

An XPS study of amorphous MoO3/SiO films deposited by co-evaporation

X-ray photoelectron spectroscopic (XPS) core-level spectra of MoO₃/SiO thin films are presented. The effects of changes in composition, substrate temperature during deposition and annealing on the binding energy of Mo(3d) and Si(2p) core lines in mixed films are compared with those of MoO₃ and SiO. Appreciable changes in Mo(3d) peak positions and slight changes in Si(2p) peak positions are observed. The change in binding energy of the Mo(3d) doublet may be attributed to the effective incorporation of silicon ions in an MoO₃ lattice which may cause the molybdenum orbital to be a little less tightly bound. This helps in the internal electron transfer from the oxygen (2p) to the molybdenum (4d) level as a result of which the molybdenum is readily changed to lower oxidation states during heat treatment. XPS spectra show that the position of the Si(2p) core state shifts monotonically with increasing oxygen concentration from the value of 101.8 to 102.6 eV.     

Effect of substrate temperature and film thickness on the surface structure of some thin amorphous films of MoO3 studied by X-ray photoelectron spectroscopy (ESCA)

X-ray photoelectron spectroscopy (XPS) core level spectra of MoO₃ substoichiometric amorphous thin films in the thickness range 100 to 670 nm were studied as a function of thickness. Some samples 500 nm thick were studied for different substrate temperatures in the range 293 to 543 K. It was observed that with the increase of thickness of the samples no change in the electron spectrum was observed in the material. Under vacuum conditions, MoO₃ turned blue when the substrate temperature was higher than 373 K. XPS spectra supported the formation of the Mo⁵⁺ oxidation state in the blue samples. Blue coloration was observed after heating in vacuum and this was attributed to an internal electron transfer from oxygen to metallic orbitals by thermal ionization creating an Mo⁵⁺ oxidation state.     

Bi2MoO6 dielectric thin films fabricated by thermal oxidation of Bi/Mo thin films at ultra-low temperature

Bi/Mo multilayer thin films are deposited on Si/SiO₂/Pt substrates by direct current magnetron sputtering. The effect of annealing temperature on the microstructure, dielectric and electrical properties of the as-sputtered films is characterized systematically. X-ray diffraction data indicate that the films annealed at 450–600 °C are a mixture of diphase with the main phase Bi₂MoO₆ and secondary phase Bi₂Mo₂O₉. Results of scanning electron microscope observation show that the films annealed at 500–550 °C are dense and uniform, in particular the films annealed at 500 °C exhibit optimal dielectric and electrical properties with dielectric constant as high as 37.5, dielectric loss 1.06 %, temperature coefficient of dielectric constant ?10.86 ppm °C^?1 at 1 kHz, and leakage current density of 1.46 × 10^?7 A mm^?2 at an electric field of 18.2 kV mm^?1. With the advantages of ultralow densification temperature (500 °C) and very high sputtering deposition rate (76 nm min^?1), it is anticipated that thermal oxidation method of the sputtered Bi/Mo thin films could be a promising technique for fabrication of Bi₂MoO₆ ceramic thin film embedded-capacitors.     

Electronic conduction in thin amorphous MoO3/SiO films deposited by co-evaporation

A study of the effects of changes in composition and temperature on the electrical properties of MoO₃/SiO thin amorphous films is presented. The high-field conduction is probably due to the Poole-Frenkel effect as it is in simple SiO. At low temperature and low field the electron hopping process is dominant but conduction at higher temperatures is a contact-limited process. The decrease in conductivity with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres introduced in MoO₃/SiO films during the evaporation process. The increase in conductivity in MoO₃/SiO films with increasing temperature is attributed to the increasing concentration and higher mobility of charge carriers.     

Structural distortion and enhanced ferroelectric properties of Tb and Cr co-doped BiFeO3 thin films

BiFeO₃ and Bi_0.92Tb_0.08Fe_1?xCr_xO₃ (BTFCO) thin films were successfully prepared on SnO₂: F (FTO)/glass substrates by a chemical solution deposition technique. The influences of Tb and Cr co-doping on the structure, the leakage current, charge defects, the dielectric and the ferroelectric properties of the BTFCO thin films were investigated systematically. X-ray diffraction and Raman spectra results clearly reveal the structural distortion in the co-doped thin films. The X-ray photoelectron spectroscopy measurements show the absence of Fe²⁺ ions indicating the suppression of oxygen vacancies due to Tb and Cr co-doping. The electrical conduction mechanism of the BTFCO thin film is dominated by the Ohmic conduction in the low resistance state and trap-controlled space charged limited current in the high resistance state. With 8 %Tb and 1 %Cr co-doping, the film exhibits the superior ferroelectric (2P _r  = 105 μC/cm²) and dielectric properties. All the results show the film is very promising in the practical application.     

Thermal modification of MoO3 nanofilms

Optical spectroscopy results demonstrate that heat treatment of 2- to 60-nm-thick MoO₃ films for 1 s to 120 min in the temperature range 573–873 K reduces their absorbance in the range λ = 300–480 nm (λ_max = 350 nm) and increases it in the range λ = 480-1100 nm (λ_max = 870 nm). The degree of conversion of the MoO₃ films increases with increasing heat-treatment time and temperature and with decreasing film thickness. A mechanism is proposed for the thermal modification of MoO₃ films, which involves the formation of a [(V _a)⁺⁺ e] center during the preparation of the MoO₃ film and a thermally activated electron transition from the valence band to the level of the [(V _a)⁺⁺ e] center, resulting in the formation of an [e (V _a)⁺⁺ e] center.     

Gasochromic switching of reactively sputtered molybdenumoxide films: A correlation between film properties and deposition pressure

Molybdenumoxide (MoO_x) thin films can change their optical properties upon exposure to hydrogen. Since the film properties strongly depend on process parameters we have studied how the films are affected by the total pressure during deposition. Stoichiometric and sub-stoichiometric MoO_x films were prepared by reactive direct current magnetron sputtering in an atmosphere of argon and oxygen. Substoichiometric films were coated with platinum as a catalyst and were colored in diluted hydrogen atmosphere and bleached in air. Optical spectroscopy, X-ray reectometry, spectroscopic ellipsometry and simulations of the measured spectra were used to characterize the films ex situ. In situ switching characteristics as revealed by optical spectroscopy and changes in stress were measured as well. We find that the total pressure during sputter deposition has a strong influence on the optical constants, the film density, and the sputter rate. The mechanical stresses and switching Preprint submitted to Elsevier Science 10 March 2006 cycles during the film coloration and bleaching also strongly depend on the total pressure. The influence of the sputter pressure on film properties is explained by the kinetics during the sputter process.     

Comparison study of microstructure, chemical composition and optical properties between resistive heating and electron beam evaporated LaF3 thin films

LaF₃ thin films were deposited by electron beam (EB) and resistive heating (RH) evaporation, respectively. Properties such as microstructure, chemical composition, surface morphology and optical constants of the LaF₃ thin films were characterized by measurements of X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and spectrophotometer, then comparison was made between this two deposition methods. It's found that the microstructure properties of the LaF₃ films deposited by these two methods were different, and slight content of oxyfluoride films was formed during deposition according to the result of chemical composition analysis. The microstructure of LaF₃ bulk materials after interaction with electron beam and resistive heating was also characterized to analyze how the two deposition processes affect the formation of LaF₃ thin films and their microstructure properties. When it was for the laser resistance of the films, although the EB evaporated LaF₃ thin films occupied lower absorption and optical loss than those of the RH films, they showed slightly smaller laser induced damage thresholds at 355 nm, which was thought to be related to their much more rougher surface and higher tensile stress.     

Oxidation states of molybdenum in oxide films formed in sulphuric acid and sodium hydroxide

X-ray photoelectron spectroscopy is used to investigate the oxidation states of molybdenum in thin films formed potentiostatically, over a range of potentials, in either 1 mol dm^− 3 H₂SO₄ or 10 mol dm^− 3 NaOH at 20 °C. Mo 3d spectra suggested that MoO₂ and Mo(OH)₂ were the main components of the films, with smaller amounts of MoO₃ and possibly Mo₂O₅. O 1s spectra indicated the presence of oxygen as oxide and hydroxide species and as bound water. Ion beam analysis revealed the formation of thin films at all potentials, with significant losses of oxidized molybdenum to the electrolyte.     

Studies of the chemical and electrical properties of fullerene and 3-aminopropyltrimethoxysilane based low-k materials

Among an extremely large number of possible fullerene applications in the field of electronics, optics and photovoltaics, C₆₀-cages are also considered as a promising dopant for low dielectric constant (low-k) materials. In this study, we incorporated C₆₀ species into a 3-aminopropyltrimethoxysilane (APTMS) based material. We prepared thin films by spin coating. Using X-ray photoelectron spectroscopy we analyzed the time-related interactions between the components of the prepared samples and the influence of the C₆₀ replacement by its better soluble derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) on the chemical properties of the material. We applied atomic force microscopy to investigate the surface texture and thicknesses of the obtained films. In order to obtain information concerning the electrical properties of the material we performed capacitance-voltage characterization. We have proven that the increase of C₆₀ species realized by PCBM incorporation within the APTMS-based matrix reduces the dielectric constant of the examined films while preserving its homogeneity.     

Structural and optical properties of ZnO films grown on silicon and their applications in MOS devices in conjunction with ZrO<Subscript>2</Subscript> as a gate dielectric

Photoluminescence (PL) properties of undoped ZnO thin films grown by rf magnetron sputtering on silicon substrates have been investigated. ZnO/Si substrates are characterized by Rutherford backscattering (RBS), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). ZrO₂ thin films have been deposited on ZnO using microwave plasma enhanced chemical vapour deposition at a low temperature (150°C). Using metal insulator semiconductor (MIS) capacitor structures, the reliability and the leakage current characteristics of ZrO₂ films have been studied both at room and high temperatures. Schottky conduction mechanism is found to dominate the current conduction at a high temperature. Good electrical and reliability properties suggest the suitability of deposited ZrO₂ thin films as an alternative as gate dielectric on ZnO/n-Si heterostructure for future device applications.     

A facile one-step hydrothermal method to produce graphene–MoO3 nanorod bundle composites

We describe a facile in situ hydrothermal fabrication of graphene–MoO₃ nanorod bundle composites utilizing sodium salicylate. The structure, morphology and composition of graphen–MoO₃ composites were investigated by means of field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and thermogravimetric-differential scanning calorimetry (TG-DSC). FESEM and TEM studies show that the presence of ordered MoO₃ nanorod bundles in composites, the characterization results of XRD, Raman spectra and TG-DSC analysis confirm the reduction of graphite oxide (GO) to graphene accompanying by the formation of MoO₃ nanorod bundles in the hydrothermal process. Due to characteristics of MoO₃ and graphene–MoO₃ composites, our findings may have implications in the synthesis and fabrication of well-defined functional graphene–MoO₃ hybrid materials. It may also provide a general approach for the preparation of graphene–metal oxide hybrid materials.     

Novel microwave assisted sol–gel synthesis (MW-SGS) and electrochromic performance of petal like h-WO3 thin films

Use of domestic microwave oven is first time employed for chemical deposition of nanocrystalline hexagonal WO₃ (h-WO₃) thin films. Low cost precursors like sodium tungstate, hydrochloric acid, oxalic acid and potassium sulfate signifies cost effectiveness of this thin film fabrication route. Scanning electron microscopy images reveal formation of petal like nanodisks. A number of analytical techniques were used to characterize the WO₃ petal like nanodisks, including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy, FT-IR spectroscopy, Raman scattering spectroscopy, UV–visible spectrophotometry and cyclic voltammetry (CV). The X-ray photoelectron spectroscopic studies revealed 2.89 O/W atomic ratio. The electrical transport studies on WO₃ thin films show semiconducting behavior with n-type semiconductivity. The value of determined coloration efficiency is 57.90 cm²/C. The mechanism of Li⁺ intercalation and deinercalation in h-WO₃ matrix is proposed for enhanced electrochromism.     

Spray pyrolyzed β-In2S3 thin films: Effect of postdeposition annealing

Indium sulfide thin films prepared using spray pyrolysis, with In/S ratio 2/3 in the solution, were annealed in vacuum at 300 and 400 °C. The effect of this treatment on properties of the films was studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, optical absorption, transmission and electrical measurements. Optical constants of the films were calculated using the envelope method. Annealing did not affect the optical properties of the film much, but the resistivity of the films showed a drastic decrease and the grain size increased. In₂S₃ thin films have potential use as buffer layer in photovoltaic heterojunction devices.