Highly sensitive ethanol sensors based on nanocrystalline SnO2 thin films

This paper reports on a simple and inexpensive ultrasonic spray pyrolysis method to synthesize agglomerate-free nanosized SnO₂ particles with a size smaller than 10 nm. Scanning electron microscopy, transmission electron microscopy and high resolution X-ray diffraction studies were used to characterize the morphology, crystallinity, and structure of the SnO₂ particles. Under the optimized experimental conditions, the prepared SnO₂ sensor shows the high response (S = 491) towards 100 ppm ethanol gas at 300 °C, linearity in the range of 100–500 ppm, quick response time (2 s), recovery time (60 s) and selectivity against other gases. The response of the sensor was monitored in a 250–450 °C temperature range. The seven fold enhancement in gas response and selective detection of C₂H₅OH in the presence of other gases such as CH₃OH and CH₃CHOHCH₃ are the significant points in this investigation. These results demonstrate that pure nanocrystalline SnO₂ thin film can be used as the sensing material for fabricating high performance ethanol sensors.     

Superheating of confined Pb thin films

In this work we report, for the first time, an experimental observation of a superheating phenomenon in metal thin films. By means of cold rolling, Pb thin films of about 20 nm thick were sandwiched by Al layers, and between them semicoherent epitaxial Pb/Al interfaces were formed. In situ x-ray diffraction analysis indicated that the confined Pb thin films could be superheated for at least 6 degrees C. Thermodynamic analysis indicated that such a substantial superheating in the confined two-dimensional thin films may originate from suppression of growth of the molten droplets by the epitaxial Al/Pb/Al confinement, instead of suppression of melt nucleation for the confined particle superheating.     

Thickness effect on the band gap of magnetron sputtered Pb45Se45O10 thin films on Si

Oxygen doped PbSe thin films with different thickness were grown on the Si (100) substrates by magnetron sputtering, and characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and physical properties measurement system. As the film thickness increased, the intensity of the (200) PbSe prominent diffraction peak increased, while the (220) peak almost vanished, indicating the primary growth direction. The change rate between the light and dark resistance increased with the film thickness, and the maximum of 64.76% was obtained. According to the density functional theory calculations and the experimental results, the band gap of the PbSe thin films decreased from 0.278 eV to 0.21 eV when doped with oxygen. Doping with oxygen during the deposition process is a viable way to prepare PbSe thin films with a tunable band gap. The band gap increased almost linearly with the lattice constant, confirmed by the calculated and experimental results.     

Role of heat treatment on structural and optical properties of thermally evaporated Ga10Se81Pb9 chalcogenide thin films

Amorphous chalcogenides, based on Se, have become materials of commercial importance and were widely used for optical storage media. The present work deals with the structural and optical properties of Ga₁₀Se₈₁Pb₉ ternary chalcogenide glass prepared by melt quenching technique. The glass transition, crystallization and melting temperatures of the synthesized glass were measured by non-isothermal DSC measurements at a constant heating rate of 30 K/min. Thin films of thickness 4000 Å were prepared by thermal evaporation techniques on glass/Si (1 0 0) wafer substrate. These thin films were thermally annealed for two hours at three different annealing temperatures of 345, 360 and 375 K, which were in between the glass transition and crystallization temperatures of the Ga₁₀Se₈₁Pb₉ glass. The structural, morphological and optical properties of as-prepared and annealed thin films were studied. Analysis of the optical absorption data showed that the rules of the non-direct transitions predominate. It was also found that the optical band gap decreases while the absorption coefficient, refractive index and extinction coefficient increase with increasing the annealing temperature. Due to the higher values of absorption coefficient and annealing dependence of the optical band gap and optical constants, the investigated material could be used for optical storage.     

Characterization of vacuum evaporated In - Se thin films

The InSe films of different thicknesses (290–730 mm) were deposited onto glass substrates under a pressure of 3×10^?5 Torr by vacuum evaporation method. The composition (In=53.50%, Se=46.50%) of this film was confirmed using Auger Electron Spectroscopy (AES). Thicknesses of the deposited films have been measured using a Multiple Beam Interferometry. The amorphous nature of the film is confirmed with X-ray diffractogram. From the transmittance spectra in the range of 500 nm-1200 nm, it is observed that the film showed direct allowed transition. Effect of thickness on the optical parameters such as the fundamental band gap, absorption constant, refractive index of InSe thin films are reported. Under low electric field (～ 1.5×10⁵ Vcm^?1), the results of DC conductivity measurements revealed that the variable range hopping is the dominant conduction mechanism. The values of localized states density, localization radius and hopping energy of this film are estimated as 5.57×10²⁰ cm^?3eV^?1, 0.84 Å and 0.247 eV, respectively.     

Optical properties of CdIn2Se4 thin films

The optical constants (the refractive index n, the absorption index k, and the absorption coefficient ) of CdIn₂Se₄ thin films were determined in the spectral range of 500–2000 nm. Graphical representation of log() as a function of log(1/) shows two distinct, linear parts indicating the existance of both direct and indirect optical transitions. The corresponding forbidden energy gaps E _g ^d and E _g ^Emphasis>d were determined. The effect of both annealing temperature as well as the substrate temperature on the optical constants of CdIn₂Se₄ thin films was also investigated.     

Pb1－xMnxSe薄膜的光学特性

采用分子束外延的方法在BaF₂(111)衬底上制备出了高质量的Pb_1-xMn_xSe(0≤x≤0.0681)薄膜.X射线衍射结果表明，Pb₁-xMn_xSe薄膜为立方相NaCl型结构，没有观察到MnSe相分离现象，薄膜的取向为平行于衬底(111)晶面.晶格常数随着Mn含量的增加逐渐减小，Mn含量由Vegard公式得到.通 关键词： 1-xMn_xSe外延薄膜')" href="#">Pb_1-xMn_xSe外延薄膜 透射光谱 带隙 折射率     

Phosphoric acid and pH sensors based on polyaniline films

Polyaniline (PANI)-chemically coated the electrode of quartz-crystal microbalance (QCM) has been developed for the determination of phosphoric acid (H₃PO₄) in the liquid phase. The sensing mechanism is based on the fact that the QCM devices produce a change in the oscillating frequency when the PANI coating the electrode of QCM interacts with different concentration of the acid. This was made during the subsequent redoping–dedoping processes of the PANI film in acid and ammonia solutions, respectively. Also, this was made during the successive redoping in different acid solutions. The QCM sensor demonstrated a rapid response to the acid with an excellent reversibility. A linear response of the sensor for different concentrations of H₃PO₄ was found in region of 1 M. The conductivity at different concentration of the acid was also determined. Finally, pH dependence on the electronic absorption of PANI phosphate film was studied which indicates that the film can be used as a sensor over a wider pH range of 3–12. This is in comparison to a PANI-sulphate film which shows limited pH absorption dependence (5–8).     

Electric field-induced birefringence in Pb0.94Ba0.06Sc0.5Nb0.5O3 (PBSN-6) solid-solution single crystals

The temperature dependence of birefringence induced by an electric field in PBSN-6 relaxors was studied. It was shown that the birefringence measured under different conditions of applying an electric field reveals a number of features not characteristic of the behavior of birefringence in conventional relaxors, more specifically, additional anomalies observed under sample cooling and heating in an electric field, a decrease in the thermal hysteresis width with an increase in the electric field strength, and the absence of birefringence saturation (impossibility to reach single-domain state of the sample during the experiment duration) at low temperatures in fields of up to 3 kV/cm. The results obtained are attributed to inhomogeneity of the cubic paraelectric relaxor phase caused by the coexistence of various polar regions having different dimensions and properties.     

Doped thin metal oxide films for catalytic gas sensors

TiO₂ and Pt doped TiO₂ thin films were grown by pulsed laser deposition on 〈0 0 1〉 SiO₂ substrates. The doped films were compared with undoped ones deposited in similar experimental conditions. An UV KrF* (λ = 248 nm, τ_FWHM ≅ 20 ns, ν = 2 Hz) excimer laser was used for the irradiation of the TiO₂ or Pt doped TiO₂ targets. The substrate temperatures were fixed during the growth of the thin films at values within the 300-500 °C range. The films’ surface morphology was investigated by atomic force microscopy and their crystalline quality by X-ray diffractometry. The corresponding transmission spectra were recorded with the aid of a double beam spectrophotometer in the spectral range of 400-1100 nm. No contaminants or Pt segregation were detected in the synthesized anatase phase TiO₂ thin films composition. Titania crystallites growth inhibition was observed with the increase of the dopant concentration. The average optical transmittance in the visible-infrared spectral range of the films is higher than 85%, which makes them suitable for sensor applications.     

Ferroelectric phase transitions in Pb0.96Ba0.04Sc0.5Nb0.5O3 and Pb0.94Ba0.06Sc0.5Nb0.5O3 solid-solution single crystals

The optical properties of Pb_0.96Ba_0.04Sc_0.5Nb_0.5O₃ (PBSN-4) and Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ (PBSN-6) single-crystal solid solutions were studied for the first time. It was shown that the spontaneous phase transition occurring in PBSN-4 with no electric field present is accompanied by a sharp minimum in optical transmission, which indicates the percolation nature of the transition. No sharp changes were observed in the temperature dependence of optical transmission in PBSN-6 single crystals with no electric field applied. However, a very weak electric field, ～0.4 kV/cm, is sufficient to induce the ferroelectric state in PBSN-6 single crystals. It was shown that the destruction of the induced ferroelectric state is a first-order phase transition which is accompanied by an anomalously narrow peak in the small-angle light scattering intensity (or by a minimum in optical transmission) and occurs through the percolation mechanism.     

Thermal annealing dependence of the optical and electrical properties of amorphous Se79Te15Sb6 thin films

Glassy substrates Se₇₉Te₁₅Sb₆ thin films are thermally evaporated onto chemically cleaned glass. Optical absorption measurements are carried out on as-deposited and thermal annealed Se₇₉Te₁₅Sb₆ films. It is found that the mechanism of the optical absorption follows the rule of non-direct transition. The annealed Se₇₉Te₁₅Sb₆ films show an increase in the optical energy gap with increasing temperature of annealing higher than the glass transition temperature (363 K). The electrical conductivity of the as-deposited and annealed films is found to be of Arrhenius type with temperature in the range 300–360 K. The effect of thermal annealing on the activation energy for conduction is also studied. The results are discussed on the basis of amorphous–crystalline transformations. PACS 61.40; 61.40.D; 64.70.D; 72.80.N; 78.65.M     

Interface effects on the quantum well states of Pb thin films

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.     

Electronic structure and dynamics of quantum-well states in thin Yb metal films

Quantum-well states above the Fermi energy in thin Yb(111) metal films deposited on a W(110) single crystal were studied by low-temperature scanning tunneling spectroscopy. These states are laterally highly localized and give rise to sharp peaks in the tunneling spectra. A quantitative analysis of the spectra yields the bulk-band dispersion in the Gamma-L direction as well as quasiparticle lifetimes. The quadratic energy dependence of the lifetimes is in quantitative agreement with Fermi-liquid theory.     

Thermal decomposition of nickel carbide thin films

Thin films of nickel carbide are produced by evaporating fourteen atomic layers (1.8 × 10¹⁶ atoms cm^?2) of nickel onto the (0001) surface of graphite at room temperature. The presence of nickel carbide is indicated by the characteristic carbide Auger electron signal. LEED shows that no ordered structural change takes place on the graphite (0001) surface when nickel carbide is produced in this manner. Isothermal heating of the sample leads to an irreversible change of the carbon Auger signal. The times required for this change range from 150 min at 150°C to 45 min at 185°C. The times required for decomposition yield an activation energy for decomposition equal to 50 kJ mole^?1. The decomposition of nickel carbide thin films obeys zero order kinetics. Depth profiling of the film after decomposition reveals that the observed decomposition is limited only to the top two or three atomic layers.     

Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

Infrared light-emitting diodes based on colloidal Pb Se/Pb S core/shell nanocrystals

Colloidal Pb Se nanocrystals(NCs) have gained considerable attention due to their efficient carrier multiplication and emissions across near-infrared and short-wavelength infrared spectral ranges. However, the fast degradation of colloidal Pb Se NCs in ambient conditions hampers their widespread applications in infrared optoelectronics. It is well-known that the inorganic thick-shell over core improves the stability of NCs. Here, we present the synthesis of Pb Se/Pb S core/shell NCs showing wide spectral tunability, in which the molar ratio of lead(Pb) and sulfur(S) precursors, and the concentration of sulfur and Pb Se NCs in solvent have a significant effect on the efficient Pb S shell growth. The infrared light-emitting diodes(IR-LEDs) fabricated with the Pb Se/Pb S core/shell NCs exhibit an external quantum efficiency(EQE) of 1.3 % at 1280 nm. The ligand exchange to optimize the distance between NCs and chloride treatment are important processes for achieving high performance on Pb Se/Pb S NC-LEDs. Our results provide evidence for the promising potential of Pb Se/Pb S NCs over the wide range of infrared optoelectronic applications.     

Thermal evolution and aging of Co-sputtered iron-tin thin films

The composition, structure, magnetic properties and stability of iron-tin thin films produced by co-evaporation with de magnetron sputtering are studied. Rutherford backscattering. Auger electron spectroscopy. X-ray diffraction and conversion electron Mössbauer spectroscopy indicate the formation of a homogeneous solid solution of tin in iron with a concentration well above the limit of equilibrium solid solubility. Measurements with a vibrating-sample magnetometer show that the films present saturation magnetization and remanence similar to those of pure iron, but a much smaller coercivity. Thermal treatment or aging lead to second-phase precipitation, which is accompanied by a growth of coercivity and remanence.