Self-aggregates formation of 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes and of their binary mixtures with the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been studied. Absorption and fluorescence studies were carried out. Both absorption and fluorescence spectra have revealed the formation of self-aggregates of dye molecules in monomolecular layers. Moreover, information about the molecular organization at the air-solid substrate interface has been obtained.     

Large blue shift in the optical band-gap of sol-gel derived Ba0.5Sr0.5TiO3 thin films

In this paper, we report and analyze the large blue shift in the optical band-gap of sol-gel derived Ba_0.5Sr_0.5TiO₃ (BST) thin films. BST films of different thickness (150 nm, 320 nm and 480 nm respectively) were deposited layer by layer onto fused quartz substrates by a spin coating technique. The drying temperature for individual layers (pre-sintering temperature) was varied as 400, 500 and 600 ^°C. A large blue shift in the band-gap was observed (with a value 4.70 eV compared to the bulk value of 3.60 eV) for films pre-sintered at 400 ^°C, which decreased with increase in the pre-sintering temperature. To date such blue shifts have been attributed to grain size reduction, stress and the amorphous nature of the films. Here, the blue shift has been correlated with the presence of charge carriers generated by oxygen vacancies and explained on the basis of the Burstein-Moss effect.     

Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C₂H₂) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS. Received: 22 October 2001 / Revised version: 16 January 2002 / Published online: 14 March 2002     

Cavity ring-down spectroscopy of H2O in the range 16 570-17 125 cm

Following previous investigations on H₂¹⁶O and H₂¹⁸O by cavity ring-down spectroscopy, this method has now been applied to investigate the energy region of the 5ν polyad in the absorption spectrum of H₂¹⁷O. In the range 16 570-17 125 cm⁻¹, the highest energy range investigated for the H₂¹⁷O isotopologue so far, 516 lines are attributed to H₂¹⁷O and assigned from a newly generated line list.     

Photocatalytic oxidation of natural organic adsorbates on anatase TiO2 films observed by infrared adsorption spectroscopy with a multiple internal reflection geometry

Photocatalytic oxidation of organic adsorbates on anatase TiO₂ films has been examined in different atmospheres of humid air, dry air and vacuum. The photocatalytic oxidation was observed by IR absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry. The photocatalytic oxidation is the fastest in the air at a humidity of 70% where oxygen and water vapor are consuming to produce OH radicals and O₂⁻ anions on the TiO₂ surface with the UV exposure. In the dry air, a rate of the photocatalytic oxidation is almost 30% of that in the humid air, where only O₂⁻ anions oxidize the organic adsorbates. In vacuum, on the other hand, it is negligible, which suggests that adsorbed H₂O molecules do not play an important role in the photocatalytic reaction. It is suggested that an addition of the water vapor is necessary to achieve the higher catalytic activity.     

Combinatorial synthesis and optical characterization of alloy and superlattice films based on SrTiO3 and LaAlO3

We have grown alloy and superlattice films consisting of SrTiO₃ (STO) and LaAlO₃ (LAO) by pulsed laser deposition using composition-spread technique. All the (STO)_x(LAO)_1−x (0 ≤ x ≤ 1) alloy and superlattice films exhibited a single-phase perovskite structure. The optical properties of these films were characterized by absorption spectroscopy at room temperature. The spectra show a broad absorption due to O 2p-Ti 3d(t_2g) transition in an ultraviolet region. We found that absorption edges of both alloy and superlattice films systematically shifted to higher energy with increasing LAO composition. Clear difference was observed in the composition dependence of the indirect and a direct band edges.     

New observation and analysis of the E(4)Σ state in NaLi

Polarization labeling spectroscopy is applied to study the excited E(4)¹Σ⁺ state of NaLi in the energy range 25 850-27 750 cm⁻¹ above the bottom of the ground state. The potential curve of the E state is constructed using the Inverted Perturbation Approach method.     

Isotopic disorder effect in the infrared reflection spectra of LixLi1−xY F4 single crystals

Polarized infrared reflection spectra of LiY F₄ single crystals with different compositions of ⁷Li and ⁶Li isotopes were measured and analysed using the model of independent oscillators. Lattice dynamics calculations were performed. It was found that, for some modes, the contribution of the isotopic disorder to the damping constant is comparable to the anharmonic width. Results of simulations agree qualitatively with the experimental data. Possible reasons for the remaining discrepancies are discussed.     

Optical band gap of zinc nitride films prepared on quartz substrates from a zinc nitride target by reactive rf magnetron sputtering

Polycrystalline zinc nitride films have been synthesized onto quartz substrates from the zinc nitride target and the nitrogen working gas by reactive rf magnetron sputtering at room temperature. X-ray diffraction study indicates that polycrystalline zinc nitride films are of cubic structure with the lattice constant a = 0.979(1) nm and have preferred orientations with (3 2 1) and (4 4 2). Its absorption coefficients as well as the film thickness are calculated from the transmission spectra, which are measured with a double beam spectrophotometer. The optical band gap has been determined from the photon energy dependence of absorption coefficient, an indirect transition optical band gap of 2.12(3) eV has been obtained.     

Crystallization of D2O thin films on Ru(0 0 1) surfaces

The phase conversion of amorphous solid water (ASW) to crystalline ice (CI) has been investigated in the very thin (∼10 monolayers) film regime on a Ru(0 0 1) surface. We analyze the converted CI fraction with the Avrami model, and recognize that one-dimensional CI growth occurs, which can be contrasted to the three-dimensional CI growth generally established in the thick (≥50 monolayers) film regime. We evaluate activation energy for the ASW crystallization to be about 1.0 eV. We suggest that the ASW crystallization is not influenced by the substrate even near the substrate-ice interface.     

Effect of argon ion activity on the properties of Y2O3 thin films deposited by low pressure PACVD

Yttrium oxide thin films are deposited by microwave electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapour deposition process using an indegeneously developed Y(thd)₃ {(2,2,6,6-tetramethyl-3,5-heptanedionate)yttrium} precursor. Depositions were carried out at two different argon gas flow rates keeping precursor and oxygen gas flow rate constant. The deposited coatings are characterized by X-ray photoelectron spectroscopy (XPS), glancing angle X-ray diffraction (GIXRD) and infrared spectroscopy. Optical properties of the films are studied by spectroscopic ellipsometry. Hardness and elastic modulus of the films are measured by load depth sensing nanoindentation technique. Stability of the film and its adhesion with the substrate is inferred from the nanoscratch test.It is shown here that, the change in the argon gas flow rates changes the ionization of the gas in the microwave ECR plasma and imposes a drastic change in the characteristics like composition, structure as well as mechanical properties of the deposited film.     

Influences of Temperature on Proton Conductivity in the Hydrogen-Bond Molecular Systems with Damping

Influences of temperature of medium on proton conductivity in hydrogen-bonded systems exposed in an electricfield are numerically studied by the fourth-order Runge-Kutta method with our model. The results obtained show that the proton soliton is very robust against thermal perturbation and damping of medium, and is thermally stable in the temperature range T ≤ 273 K. From the simulation we find out that the mobility （or velocity） of proton conduction in ice crystal is a nonmonotonic function of temperature in the temperature range 170-273 K： i.e., it increases initially, reaches a maximum at about 191 K, subsequently decreases to a minimum at about 211 K, and then increases again. This changed rule of mobility is qualitatively consistent with its experimental data in ice in the same temperature range. This result provides an evidence for existence of solitons in the hydrogen-bonded systems.     

Optical properties of Zn0.5Cd0.5Se thin films grown on InP by molecular beam epitaxy

We report photoluminescence (PL) and reflectivity measurements of Zn_0.5Cd_0.5Se epilayers grown by molecular beam epitaxy on InP substrates. The low-temperature PL spectra are dominated by asymmetric lines, which can be deconvoluted into two Gaussian peaks with a separation of ∼8 meV. The behavior of these peaks is studied as a function of excitation intensity and temperature, revealing that these are free exciton (FE) and bound exciton emission lines. Two lower energy emission lines are also observed and assigned to the first and second longitudinal optical phonon replicas of the FE emission. The temperature dependence of the intensity, line width, and energy of the dominant emission lines are described by an Arrhenius plot, a Bose-Einstein type relationship, Varshni's and Bose-Einstein equations, respectively.     

Development of polycrystalline silicon films on flexible metallic substrates by aluminium induced crystallization

Thin film silicon solar cells on low cost foreign substrates could be attractive for highly efficient and low cost production of photovoltaic electricity. An attempt has been made to synthesise high-quality continuous polycrystalline silicon (pc-Si) layers on flexible metallic substrates using aluminium induced crystallization (AIC) for the first time. Amorphous silicon films deposited by ECR-PECVD were crystallized on diffusion barrier coated metallic substrates at lower temperatures (<577°C). The crystallization was studied using Raman as well as UV reflectance spectroscopy. The as-grown AIC pc-Si films were found to be continuous and densely packed without amorphous phase. The migration of impurities from the substrate to the pc-Si films and the conformability of the barrier layer with the substrate and pc-Si films were studied systematically in terms of chemical and stress level analysis, which are the important aspects to be considered when metallic foils are used as substrates. It was observed that the barrier layer also serves as a buffer layer to minimise the stress level enormously in the AIC grown pc-Si layer, though the supporting material has a thermal expansion coefficient of higher order at higher annealing temperatures. The present investigation proves the possibility to grow better-quality polycrystalline silicon films on flexible metallic foils and further demonstrates the steps that need to be considered to improve the quality of AIC pc-Si films as well as the strength of the barrier layer.     

Interfacial and adhesional aspects in polyurethane (PUR) membrane coating on Si3N4 surface of ISFET gate for REFET fabrication

Polyurethane membrane coatings on: (i) packaged ISFET (ion-sensitive field-effect transistor) devices and (ii) those in the form of chips have been investigated. The study has revealed that for reliable membrane deposition, proper film adhesion at polyurethane-silicon nitride and polyurethane-cured epoxy interfaces of ISFET was necessary. Membranes formed on packaged ISFETs were found to lift off after a short time period (<1 h) when immersed in pH 6.86 buffer solution. This was because the adhesion between polyurethane and surrounding cured epoxy was insufficient. If the membrane coating was performed on the chip, and the chip was subsequently packaged (protecting the coating on the sides with epoxy and curing the epoxy to fasten the membrane), the coating was not attacked in buffer solution. Based on these studies, a process for depositing reliable, long-lasting polyurethane membranes on ISFET gate surfaces has been proposed. Membrane reliability has been confirmed by storing the membrane-coated ISFET in buffer solution over seven days and periodically observing it microscopically. Membranes have been characterized through pH sensitivity and gate-source impedance measurements. pH sensitivities of ISFETs with polyurethane membranes were measured to be ≤4.2 mV/pH.     

On the use of a O2:SF6 plasma treatment on GaAs processed surfaces for molecular beam epitaxial regrowth

Preparation of processed GaAs surface cleaning in view of molecular beam epitaxy regrowth by means of a O₂SF₆ microwave plasma has been investigated. Photoemission, Auger electron spectroscopy, atomic force microscopy and secondary ion mass spectrometry have been used for characterization. The O₂SF₆ plasma treatment was found to be very efficient for decontaminating the GaAs surface and leads to the formation of an oxide layer that can be taken off by a thermal or low-temperature H-plasma-assisted deoxidation. The levels of oxygen and carbon contaminants at the regrowth interface were measured to be in the range of a standard homoepitaxial layer-epiready substrate interface. Fluorine was observed to be eliminated upon deoxidation while sulphur is present, particularly in the case of low temperature grown layers. This plasma treatment was found to be efficient for preparation of processed GaAs surfaces for molecular beam epitaxial regrowth.     

Plume temperature in the laser ablation of polyimide films measured by infrared emission spectroscopy

The infrared emission spectrum of the plume produced by KrF excimer laser ablation of polyimide films in air and in He was measured in the 680 to 1580 cm^–1 wavenumber range. Using 400 mJ/cm² laser pulses of 248 nm and 35 ns duration yielded a strong emission band characteristic of thev ₂ transitions of hot HCN molecules. Band counters calculations were carried out of thev ₂ emission expected from HCN in thermal equilibrium at various temperatures. They indicate that except for a slight deviation of the measured data from thermal equilibrium, the best fit of the observed results is obtained at a plume temperature of 2250±150K.     

Surface roughening in Si1−xGex alloy films by 100 MeV Au: Composition dependency

Using a 100 MeV Au beam, the surface roughening kinetics of relaxed Si_1−xGe_x alloy films for x=0.5 and 0.7 are studied by means of ex-situ atomic force microscopy (AFM). Swift heavy ion (SHI) irradiation induced surface roughening behavior is demonstrated using the trend in variation of β as a function of fluence when the data are analyzed in terms of the Edwards-Wilkinson (EW) model. By employing the EW model, the observed surface roughening is explained on the basis of the competition between SHI induced sputtering and smoothening through redeposition of the sputtered atoms. The composition dependent variation of surface morphology with increasing fluence is discussed in the light of the strain distribution along the sample surface.     

The properties of free polymer surfaces and their influence on the glass transition temperature of thin polystyrene films

We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T_g) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378K( > T_g), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T_g ( 363K < T < 370K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T_g are also performed on thin spin-cast PS films with thicknesses in the range 8nm < h < 290nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T_g values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T_g that is the same as that of bulk PS (370±1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T_g in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5±1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T_g reductions in thin PS films.     

Layer uniformity in glucose oxidase immobilization on SiO2 surfaces

The goal of this work was the characterization, step by step, of the enzyme glucose oxidase (GOx) immobilization on silicon oxide surfaces, mainly by means of X-Ray photoelectron spectroscopy (XPS). The immobilization protocol consists of four steps: oxide activation, silanization, linker molecule deposition and GOx immobilization. The linker molecule, glutaraldehyde (GA) in this study, must be able to form a uniform layer on the sample surface in order to maximize the sites available for enzyme bonding and achieve the best enzyme deposition. Using a thin SiO₂ layer grown on Si wafers and following the XPS Si2p signal of the Si substrate during the immobilization steps, we demonstrated both the glutaraldehyde layer uniformity and the possibility to use XPS to monitor thin layer uniformity. In fact, the XPS substrate signal, not shielded by the oxide, is suppressed only when a uniform layer is deposited. The enzyme correct immobilization was monitored using the XPS C1s and N1s signals. Atomic force microscopy (AFM) measurements carried out on the same samples confirmed the results.