Growth and characterization of molybdenum oxide thin films prepared by photochemical metal–organic deposition (PMOD)

Molybdenum oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of a molybdenum dioxide acetylacetonate complex on Si(1 0 0) substrates. Photodeposited films were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the surface morphology examined by Atomic Force Microscopy (AFM). It was found that as-photodeposited films are uniform and smooth, with thickness of 350 nm, with rms surface roughness of 28 nm and contain non-stoichiometric oxides (MoO_3−x). The results of XRD analysis showed that post-annealing of the films in air at 450 °C transforms the sub-oxides to α-MoO₃ phase with a much rougher surface morphology (rms = 144 nm). The as-photodeposited MoO_3−x films are amorphous, and exhibit better optical quality than annealed films.     

Photoluminescence properties of BaMoO4 amorphous thin films

BaMoO₄ amorphous and crystalline thin films were prepared from polymeric precursors. The BaMoO₄ was deposited onto Si wafers by means of the spinning technique. The structure and optical properties of the resulting films were characterized by FTIR reflectance spectra, X-ray diffraction (XRD), atomic force microscopy (AFM) and optical reflectance. The bond Mo-O present in BaMoO₄ was confirmed by FTIR reflectance spectra. XRD characterization showed that thin films heat-treated at 600 and 200 °C presented the scheelite-type crystalline phase and amorphous, respectively. AFM analyses showed a considerable variation in surface morphology by comparing samples heat-treated at 200 and 600 °C. The reflectivity spectra showed two bands, positioned at 3.38 and 4.37 eV that were attributed to the excitonic state of Ba²⁺ and electronic transitions within MoO²⁻₄, respectively. The optical band gaps of BaMoO₄ were 3.38 and 2.19 eV, for crystalline (600 °C/2 h) and amorphous (200 °C/8 h) films, respectively. The room-temperature luminescence spectra revealed an intense single-emission band in the visible region. The PL intensity of these materials was increased upon heat-treatment. The excellent optical properties observed for BaMoO₄ amorphous thin films suggested that this material is a highly promising candidate for photoluminescent applications.     

Preparation, structure and ultraviolet photoluminescence of ZnO films by a novel chemical method

A novel and simple chemical method was developed for the deposition of ZnO films from aqueous solution, integrating the merits of successive ionic layer adsorption and reaction with the chemical bath deposition technology. By this new method, dense and continuous ZnO thin films with good crystallinity can be prepared in a very short time, e.g., in about 20 min. Results show that as-deposited ZnO films on glass and Si (1 0 0) exhibit hexagonal wurtzite crystalline structure and the preferential orientation along (0 0 2) plane. With a dense and continuous appearance, the film is composed of ZnO particles in even size of 200-300 nm. The strong and sharp emission at 391 nm and several weak emissions at the wavelength band of 440-500 nm indicate the high optical quality and the stoichiometrical nature of obtained film. Mechanism analysis shows that the reaction duration in hot water and the drying process are vital important factors affecting the deposition process and the crystallization behavior of the film prepared via the aqueous solution route.     

Effect of layer structures of gold nanoparticle films on surface enhanced Raman scattering

Using a method of collecting nanoparticles at a water/hexane interface in a close-packed monolayer film and transferring such films onto a solid substrate, three-dimensional multilayer films of nanoparticles were formed. The packed nanoparticles were gold nanospheres (NS) with a 26 nm diameter or gold nanorods (NR) with a 31 nm diameter and 74 nm length. We investigated variations in the surface enhanced Raman scattering (SERS) intensities from such nanoparticle films as the layer compositions were changed. The films stacked with NR layers generated much higher SERS intensity than those of NS layers. The SERS intensities from both kinds of films increased as the number of layers were increased. However, when the NR layer and NS layer were stacked alternately, SERS intensity varied in a zigzag fashion. It was found that the structure of top layer plays a distinguishable role in generating strong SERS enhancement while the lower layers contribute to SERS with less dependency on structures. Interlayer coupling as well as intralayer coupling was considered in order to explain the observations.     

Optical fiber evanescent wave absorption spectrometry of nanocrystalline tin oxide thin films for selective hydrogen sensing in high temperature gas samples

SnO₂ nanocrystalline material was prepared with a sol-gel process and thin films of the nanocrystalline SnO₂ were coated on the surface of bent optical fiber cores for gas sensing. The UV/vis absorption spectrometry of the porous SnO₂ coating on the surface of the bent optical fiber core exposed to reducing gases was investigated with a fiber optical spectrometric method. The SnO₂ film causes optical absorption signal in UV region with peak absorption wavelength at around 320 nm when contacting H₂-N₂ samples at high temperatures. This SnO₂ thin film does not respond to other reducing gases, such as CO, CH₄ and other hydrocarbons, at high temperatures within the tested temperature range from 300 °C to 800 °C. The response of the sensing probe is fast (within seconds). Replenishing of the oxygen in tin oxide was demonstrated by switching the gas flow from H₂-N₂ mixture to pure nitrogen and compressed air. It takes about 20 min for the absorption signal to decrease to the baseline after the gas sample was switched to pure nitrogen, while the absorption signal decreased quickly (in 5 min) to the baseline after switching to compressed air. The adhesion of tin oxide thin films is found to be improved by pre-coating a thin layer of silica gel on the optical fiber. Adhesion increases due to increase interaction of optical fiber surface and the coated silica gel and tin oxide film. Optical absorption spectra of SnO₂ coating doped with 5 wt% MoO₃ were observed to change and red-shifted from 320 nm to 600 nm. SnO₂ thin film promoted with 1 wt% Pt was found to be sensitive to CH₄ containing gas.     

Thickness dependence of PL efficiency of organic thin films

Thickness dependence of photoluminescence (PL) efficiency and spectral shape of phosphorescent organic thin films is investigated and theoretically analyzed. The PL efficiency increases with increasing thickness to reach the maximum 92% at around 50 nm. It reduces to 77% at the thickness of 130–140 nm and oscillates between the values upon further increment of thickness. The quenching of excitons at the surface of organic layer significantly reduces the PL efficiency when the film is very thin. If the film is thicker than the critical thickness for the waveguiding of emitted light, the waveguided power is absorbed during the propagation through the organic layer so that apparent PL efficiency is reduced by the amount. Microcavity effect formed by quartz/organic layer/air also affects the PL efficiency. The appropriate thickness to obtain the PL efficiency close to the intrinsic value of a film is just the critical thickness for waveguiding through the film.     

溶胶-凝胶方法制备ZnO:Al薄膜及其制备工艺条件研究

采用Sol-Gel工艺在玻璃基片上制备出C轴择优取向性、高可见光透过率以及高电导率的Al³⁺离子掺杂的ZnO透明导电薄膜ZnO:Al(ZAO薄膜).并研究了退火温度、Al掺杂量等对其光电性能的影响.结果表明,溶胶-凝胶法制备ZAO薄膜的最佳工艺条件为:溶胶浓度0.75 mol/L、掺杂量1.5 atm%,镀膜层数10层(厚度约为136 nm)、退火温度600℃.     

Thickness dependent physical properties of close space evaporated In2S3 films

In recent years, In₂S₃ is considered as a promising buffer layer in the fabrication of heterojunction solar cells. Film thickness is one of the important parameters that alters the physical characteristics of the grown layers significantly. The effect of film thickness on the structural, morphological, optical and electrical properties of close space evaporated In₂S₃ layers has been studied. In₂S₃ thin films with different thicknesses in the range, 100–700 nm were deposited on Corning glass substrates at a constant substrate temperature of 300 °C. The films were polycrystalline exhibiting strong crystallographic orientation along the (103) plane. The deposited films showed mixed phases of both cubic and tetragonal structures up to a thickness of 300 nm. On further increasing the film thickness, the layers showed only tetragonal phase. With increase of film thickness, both the crystallite size and surface roughness in the films were found to be increased. The optical constants such as refractive index and extinction coefficient of the as-grown layers have been calculated from the optical transmittance data in the wavelength range, 300–2500 nm. The optical transmittance of the films was decreased from 82% to 64% and the band gap varied in the range, 2.65–2.31 eV with increase of film thickness. The electrical resistivity as well as the activation energy was evaluated and found to decrease with film thickness. The detailed study of these results was presented and discussed.     

Thickness determination for Cu and Ni nanolayers: Comparison of completely reference-free fundamental parameter-based X-ray fluorescence analysis and X-ray reflectometry

Monochromatized synchrotron radiation of the electron storage ring BESSY II has been used for the non-destructive thickness determination of nanolayered materials by two different methods. The aim of these investigations was the comparison of completely reference-free fundamental parameter-based X-ray fluorescence analysis with X-ray reflectometry to validate the quantification of X-ray fluorescence analysis as an absolute method. For this purpose, Cu and Ni layers with a thickness varying between 5 nm and 50 nm as well as double layers of both metals deposited on Si have been studied. In X-ray reflectometry characterization experiments, the tunability of the photon energy allows the determination of not only the total layer thickness but also the individual layer thicknesses of the Cu/Ni double-layer systems. Reference-free X-ray fluorescence analysis involves both the fundamental parameter approach and the knowledge of all relevant experimental parameters obtained by instrumentation calibrated absolutely.The layer thickness determined by both methods agreed within their combined uncertainties. In view of the limits of X-ray reflectometry for very thin layers, laterally inhomogeneous samples, and multi-elemental layer compositions, reference-free X-ray fluorescence analysis offers the potential for the thickness determination of such samples.     

Wavelength dependencies in interferometric measurements of thin protein films

Summary The evaluation of the spectral modulation in reflectance measurements at thin dielectric films allows the characterisation of layers in the m-range. Diode array spectrometers make this technique a useful tool in sensor development. The variation of the optical pathlength in thin films due to change of refractive index or to change of physical thickness of a homogeneous layer has been applied to refractometry and detection of hydrocarbons by polymer swelling. More recently, the monitoring of solid-phase affinity reactions, like immunoassays has successfully been demonstrated. While the homogeneous layer concept is useful for data evaluation in the first two cases, it has its limitations for the treatment of adlayer formation as in the latter case. This is discussed with respect to results from affinity reactions carried out at optically different interference layers.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Quantitative Analysis of Thin Aluminium-Oxynitride Films by EPMA

 Thin films of aluminium oxynitride with diverse composition were prepared by dc-magnetron sputtering of aluminium, utilising sputtering power as well as argon, oxygen and nitrogen gas flows to vary the composition. Since film properties depend mainly on the content of incorporated oxygen and nitrogen, a method for quantitative analysis of the main constituents based on electron probe micro analysis with energy dispersive detection was developed. The excellent precision of the quantitative results for aluminium as well as oxygen and nitrogen are shown. Furthermore, a film layer analysis program was applied for the quantification of several films deposited under the same deposition parameters on silicon wafers, from 520 nm down to 40 nm thickness, showing that electron probe micro analysis with energy dispersive detection is a reliable method for quantitative compositional analysis of thin aluminium oxynitride films down to approximately 20 nm thickness. Since this method of analysis provides only bulk information, expected inhomogeneities of the depth distribution of the film components were checked by secondary ion mass spectrometry depth profiles of two thin films and correlated to the EPMA results. The thickness of the films was determined by ellipsometry. Received September 1, 1998     

Principles of quartz crystal microbalance/heat conduction calorimetry: Measurement of the sorption enthalpy of hydrogen in palladium

A sensitive new measurement technology is described which combines calorimetry, gravimetry, and rheology applied to chemical reactions in thin films: quartz crystal microbalance/heat conduction calorimetry (QCM/HCC). The quartz crystal microbalance/heat conduction calorimeters constructed so far simultaneously measure heat generation, mass uptake or release, and viscoelastic property changes in the same, sub-milligram solid film sample when gases interact with the film in an isothermal surrounding. It is possible to measure the energetics of formation of a single layer of adsorbed molecules on a gold surface with this technique. The principles of operation of both the mass and the heat flow sensor are described, and one implementation of the combined sensor and apparatus and its electronics is presented. Methods for calibration and the preparation of thin sample films are summarized. As an illustrative example, the determination of the sorption enthalpy of hydrogen in a 25 °C palladium film of 140 nm thickness is discussed in detail. Other examples of the operation of the QCM/HCC are tabulated.     

Synthesis and characterization of silica-silver core-shell composite particles with uniform thin silver layers

Silica-silver core-shell composite particles with uniform thin silver layers were successfully synthesized by a facile and one-step ultrasonic electrodeposition method. By electrolysis of the slurry consisting of preformed silica spheres and silver perchlorate without any additives, the homogenous composite particles can be prepared. The average size of single silver crystals in the composite is about 12 nm and the thickness of silver layer is 14±2 nm. Moreover, the continuity of Ag distribution, the surface roughness and the thickness of silver layer are controllable by adjusting the current density (I), the concentration of electrolyte (C) and the reaction time (t). Optical properties of the composite particles with different silver content were also investigated.     

Enhanced Fluorescence Using an Optical Interference Mirror Overlaid with Silver Island Film

Fluorophores overlaid on an optical interference mirror composed of a metal and thin dielectric layer demonstrate enhanced fluorescence. Fluorescence is also enhanced by silver nanostructures such as silver island films, which excite localized surface plasmon resonance. An optical interference mirror surface was overlaid with a silver island film to amplify the fluorescence enhancement. The optimal thickness of the silver island film (100 nm) was evaluated from transmittance and surface roughness measurements. At this thickness, the fluorescence was amplified sixteen-fold. The thickness of the interference layer was optimized at 40 nm providing a one hundred-sixty fold fluorescence enhancement of rhodamine B. However, only a four-fold improvement in sensitivity was achieved for the determination of a labeled streptavidin using biotin immobilized on the silver island film interference mirror.     

Reagentless system for sulphite determination based on polyaniline

A new reagentless system for sulphite (or sulphur dioxide) determination is reported based on the use of an organic conducting polymer, polyaniline, and its absorbance variation at 550 nm, depending on the sulphite concentration. After chemical polymerisation of aniline a very thin film of polyaniline is obtained. Although the response is not fully reversible, each film can be used for at least 10 measurements for low analyte concentrations (up to 0.5 mg l⁻¹) and five measurements for higher sulphite concentrations. Moreover, the reproducibility, ease of preparation and low cost of the films, permit the use of a new disposable system for each measurement. When the change in absorbance at 550 nm was measured for 210 s (stabilisation time), the system showed a linear response, which ranged from 0.025 to 1.50 mg l⁻¹ of sulphite. A theory with regard to the reaction mechanism between the polyaniline films and sulphite is also proposed. The system was applied to sulphite determination in wine samples and the results were in agreement with those obtained by the Official Method of Analysis (iodometric titration).     

In-depth profile analysis of thin films deposited on non-conducting glasses by radiofrequency glow-discharge–optical emission spectrometry

The potential of radiofrequency glow-discharge–optical emission spectrometry (rf-GD–OES) for quantification of thin films on non-conducting materials has been investigated. A commercial rf-GD chamber from Jobin Yvon operated at 13.56 MHz with Ar as discharge gas was used. The signal integration time was 0.1 s. The effect on emission yields of thin conducting layers on glasses of different thickness was studied in detail, using the rf-GD in the common operating mode “constant pressure–constant forward power”. Calibration curves were obtained for two types of material—conducting reference materials and a set of non-conductors comprising homogeneous glass of known composition and three different thicknesses coated (or not) with thin layers of gold. Qualitative and quantitative in-depth profile analyses of different coated non-conducting samples were investigated. A variety of samples, including different thick glass substrates (from 1.8 to 5.8 mm), different thin films deposited on homogeneous glasses (from 6 nm to 35 nm), and different kinds of coating (conductors such as Fe, Ni, Cr, Al, and Nb and non-conductors such as Si₃N₄) were studied at 450 Pa pressure and 20 W forward power. The quantitative in-depth profiles proved satisfactory and results for depths and concentrations were similar to nominal values.     

Thickness Determination of Ultra-Thin Films on Si Substrates by EPMA

Results from thickness determination of single-element ultra-thin (<10nm) films by electron probe microanalysis (EPMA) are presented. The studied samples were Ge, Sn, Ag and Au thin films deposited by resistive evaporation on Si substrates. The thickness of the films was controlled during evaporation by means of a quartz crystal, previously calibrated using samples with overlayers of different thicknesses (>20nm) measured by Rutherford backscatter spectrometry and optical interferometry. EPMA measurements were performed on an electron microprobe CAMECA SX-50, with incident electron energies ranging from 4keV to 20keV. Film thicknesses were derived from the measured k-ratios using the analytical programs X-Film and Layerf and the Monte Carlo simulation code Penelope. The ionization cross sections used in the simulations were calculated with the distorted-wave Born approximation. Film thicknesses obtained from the EPMA measurements using the various computational methods are compared with those measured with the quartz crystal. The maximum relative difference between results from the different techniques does not exceed 5%.     

Thickness and morphology of polyelectrolyte coatings on silica surfaces before and after protein exposure studied by atomic force microscopy

Analyte–wall interaction is a significant problem in capillary electrophoresis (CE) as it may compromise separation efficiencies and migration time repeatability. In CE, self-assembled polyelectrolyte multilayer films of Polybrene (PB) and dextran sulfate (DS) or poly(vinylsulfonic acid) (PVS) have been used to coat the capillary inner wall and thereby prevent analyte adsorption. In this study, atomic force microscopy (AFM) was employed to investigate the layer thickness and surface morphology of monolayer (PB), bilayer, (PB-DS and PB-PVS), and trilayer (PB-DS-PB and PB-PVS-PB) coatings on glass surfaces. AFM nanoshaving experiments providing height distributions demonstrated that the coating procedures led to average layer thicknesses between 1 nm (PB) and 5 nm (PB-DS-PB), suggesting the individual polyelectrolytes adhere flat on the silica surface. Investigation of the surface morphology of the different coatings by AFM revealed that the PB coating does not completely cover the silica surface, whereas full coverage was observed for the trilayer coatings. The DS-containing coatings appeared on average 1 nm thicker than the corresponding PVS-containing coatings, which could be attributed to the molecular structure of the anionic polymers applied. Upon exposure to the basic protein cytochrome c, AFM measurements showed an increase of the layer thickness for bare (3.1 nm) and PB-DS-coated (4.6 nm) silica, indicating substantial protein adsorption. In contrast, a very small or no increase of the layer thickness was observed for the PB and PB-DS-PB coatings, demonstrating their effectiveness against protein adsorption. The AFM results are consistent with earlier obtained CE data obtained for proteins using the same polyelectrolyte coatings.     

Effects of ZnO Buffer Layer Thickness on Properties of MgxZn1-xO Thin Films Deposited by MOCVD

High-quality MgxZn1-xO thin films were grown on sapphire（0001 ） substrates with a ZnO buffer layer of different thicknesses by means of metal-organic chemical vapor deposition. Diethyl zinc, bis-cyclopentadienyl-Mg and oxygen were used as the precursor materials. The crystalline quality, surface morphologies and optical properties of the Mg, Zn1-xO films were investigated by X-ray diffraction, atomic force microscopy and photoluminescence spectrometry. It was shown that the quality of the MgxZn1-xO thin films depends on the thickness of the ZnO buffer layer and an Mg, Zn1-xO thin film with a ZnO buffer layer whose thickness was 20 nm exhibited the best crystal-quality, optical properties and a flat and dense surface.