A three-step process for epitaxial growth of (111)-oriented C60 films on alkali–halide substrates

Epitaxial (111)-oriented C₆₀ films have been grown on alkali–halide substrates, KCl (100), KBr (100) and NaCl (100) by a three-step process: (1), substrate surface cleaning by high temperature heating; (2), initial deposition with a low deposition rate to grow two or three monolayers (ML); and (3), deposition with a high deposition rate to grow a film with expected thickness. It was found that (111)-oriented epitaxial C₆₀ films could be grown at low temperatures in a wide temperature range, from 40 to 120°C. By this three-step process, we can also grow epitaxial C₆₀ films at deposition rates as high as 35 Å/min.     

Optical properties and quantum efficiency of thin-film alkali halides in the far ultraviolet

The optical constants of thin films of CsI, KI, and KBr and the quantum efficiency (QE) of planar photocathodes made with these alkali halides in the 53.6-174.4-nm spectral range are presented. The optical constants were obtained from measurements of the reflectance as a function of incidence angle. The effect of film heating and exposure to UV irradiation on the optical properties and on the QE of the three alkali halides was investigated. KBr was found to be the most stable material for both heating and UV irradiation. KI appeared to be close to temperature stable, whereas UV exposure affected its optical constants. CsI optical constants changed after 420 K heating and after UV exposure. The changes in the optical constants were related to the QE changes, and a certain correlation between both variations was determined. However, it was also demonstrated that the QE changes cannot be explained solely by the changes in optical constants.     

Chemical-mechanical polishing of SiO2 thin films studied by X-ray reflectivity

Specular X-ray reflectivity from SiO₂ thin films prepared on silicon substrates by plasma-enhanced chemical vapor deposition showed the films to have a characteristic width of the decay in density at the free surface of 17 Å, to be about three-quarters the density of -quartz, and to have an interfacial layer at the silicon interface that was of the order of 100 Å wide and less dense than the bulk of the film. After chemical-mechanical polishing the characteristic width of the decay in density at the free surface was reduced to 10 Å; furthermore, the near-surface region to a depth of 30 Å had a greater density than the as-deposited film. Off-specular reflectivity confirmed that the decrease in characteristic width at the free surface was due to reduced roughness upon polishing and also revealed that the lateral correlation length in the limit of long wavelengths was the same for both polished and unpolished samples. The compression of the near-surface region during polishing is believed to enhance the dissolution of SiO₂ into the slurry which is necessary to achieve smooth surfaces.     

Electrochemical and thermodynamic properties of lanthanides (Nd, Sm, Eu, Tm, Yb) in Alkali metal chloride melts

Electrochemical and thermodynamic properties of Nd, Sm, Eu, Tm, and Yb chlorides in melts of individual LiCl, NaCl, KCl, and CsCl, of 3LiCl-2KCl, LiCl-KCl-CsCl, NaCl-KCl-CsCl, and NaCl-2CsCl eutectics, and of the equimolar NaCl-KCl mixture in the temperature interval 573–1173 K were studied by stationary and nonstationary methods. The mechanism of reduction of lanthanide(III) ions to metal was elucidated, and the main kinetic and thermodynamic characteristics of the process in alkali metal chloride melts were calculated. The influence of the ionic potential and polarizing power on the behavior of lanthanides in molten salts of various cationic compositions was determined.     

Transmission electron microscopy studies of nanostructured TiO2 films on various substrates

A chemical vapour deposition (CVD) synthetic route to the production of nanocrystalline titanium dioxide has been carefully investigated on various substrates. CVD was performed at a relatively low temperature of 320 °C on KCl crystal, Al foil, KBr pellet and freshly sliced MICA substrates. The influence of substrate material on film formation was studied in order to find a titanium dioxide film with good intercalation properties for an electrode in a dye-sensitized solar cell. Intercalation properties depend on average grain sizes and porosity in nanophased materials. These films were thoroughly characterized with respect to their surface morphology, crystal structure and the phase composition. Transmission electron microscopy (TEM) accompanied by selected area electron diffraction (SAED) was employed for structural characterization of TiO₂ films. The studies showed that films deposited on KCl crystal, KBr pellet and MICA are solely composed of an anatase phase whereas in the film deposited onto Al foil, the brookite phase of TiO₂ is also present. The structural parameters of anatase were determined using the Rietveld refinement of electron diffraction data. By comparison of anatase lattice parameters with their corresponding bulk values, the significant deviation in values of lattice parameters a and c in anatase phase was observed and attributed to the thin-film features. The average grain size and the grain size distribution obtained by TEM were compared for TiO₂ films deposited on different substrates.     

Ordered growth and local workfunction measurements of tris(8-hydroxyquinoline) aluminium on ultrathin KBr films

In this work we investigate the growth of tris(8-hydroxyquinoline) aluminium (Alq(3)) on single-crystal Ag(111) substrates partially covered by an ultrathin KBr film. Noncontact atomic force microscopy is used to determine the molecular ordering of 0.8 monolayer Alq(3) evaporated onto these substrates. The simultaneous measurement of the local surface potential by means of Kelvin probe force microscopy yields the local workfunction difference between the pure Ag(111) surface and the one covered by an ultrathin KBr film, by pure Alq(3), or by both (KBr|Alq(3)). The molecular ordering and the interface dipole formation are discussed with respect to experiments described in the literature in which electron diffraction and photoelectron spectroscopy were used, respectively.     

Photoelectric determination of the equilibrium of thin Pd films (80–120 Å)

The dependence of the work function φ of thin Pd films (80–120 Å) on the temperature of subsequent annealing has been studied. In agreement with previous papers φ increased up to a temperature T_t=400 K. Above this temperature an anomalous decrease of φ was observed. From electrical resistance measurements and electron micrographs, it was observed that agglomeration of the film occured above 400 K.

The anomalous behaviour of φ is explained as an enrichment of the lower coordinated crystal faces on the surface.     

Structural correlations in light irradiated Ge36Se64 amorphous films—Radial distribution function study

The effect of laser irradiation on the electronic structure of amorphous Ge₃₆Se₆₄ films has been detected by studying the variation of the bond length (r) and the coordination number C_N. The total distribution function T(r) of the as deposited film is characterized by the first coordination sphere corresponding to the superposition of the correlation Ge–Se and Se–Se situated at 2.53 Å. The average estimated C_N is 2.519. The second peak ascribed to the correlation Se–Se lies at 3.85 Å showing good agreement with other published data. After irradiation, the first peak of T(r) shows a considerable shift towards a small r and a reduction of C_N. On the contrary, the second neighbor data shows a slight increase of r and a great increment of C_N value (5.11 before irradiation against 6.59 after irradiation). Study of the variation of both r and C_N values induced by subsequent annealing of the film is also given. The relative concentrations of the GeGe, GeSe and SeSe bonds, as well as, the number of the GeSe₄ tetrahedral per atom are calculated using the continuous random network (CRN) and the chemically order continuous random network (COCRN) models. These calculations argue the presence of Ge₂(Se_1/2)₆ ethane like unit in addition to Ge(Se₄)_1/2 even with the COCRN model. The formation of dynamical bonds during irradiation of the film under study is suggested. Correlation to volume changes during illumination studied by tight binding molecular dynamics computer simulation has been also considered.     

A scanning electron microscope study of the degradation of electron channelling effects in alkali halide crystals during electron irradiation

The effects ofin-situ, 25 kV electron irradiation on SEM electron channelling patterns from alkali halide crystals at room temperature are reported. Patterns were generated using wide beam and narrow beam methods. It is observed that after an initial period of irradiation, during which well defined high resolution patterns can be generated, degradation occurs. This is marked either by channelling line difiuseness or by waviness, depending on the method of generation. After prolonged irradiations, patterns can no longer be detected. Subsequent annealing experiments on irradiated NaCl show that patterns return to their original clarity if the annealing temperature is between 285 and 325‡C. Initial distortions (i.e. diffuseness and waviness) are attributed principally to the effects on the incident beam of irradiation induced surface electric fields; pattern loss, and subsequent recovery on annealing, is attributed to lattice distortion effects arising from beam-induced defect clusters.     

Mass-Spectrometric Study of the Temperature Variation in the Dimer-to-Monomer Ratio in the Free-Surface Vaporization Fluxes from Alkali Halide Single Crystals

A mass-spectrometric method is used to investigate the kinetics of the vaporization of LiF (970–1070 K), NaCl (722–889 K), KCl (780–900 K), KBr (724–918 K), and CsI (656–838 K) single crystals. A dimer-to-monomer ratio, J_D/J_M, in the fluxes vaporizing from free surfaces of these crystals are calculated, using the currents of M⁺, MX⁺, and M₂X⁺ ions formed via electron impact ionization of the molecules MX and M₂X₂ (M is the alkali atom; X is the halogen). It is found that the dimer-to-monomer ratio increases with temperature at a continually increasing rate in the LiF and NaCl cases. In the KCl, KBr, and CsI cases, this rate passes through a maximum. Such a specific temperature variation in J_D/J_M is discussed through the analysis of proposed mechanisms of dimer formation in light of the terrace–ledge–kink and surface charge models.     

The influence of the substrate and the substrate temperature on the crystallinity of tin films

Thin films of tin were prepared on various substrates (NaCl, KCl, KBr and KI) and at various substrate temperatures (28–90°C) and were studied using transmission electron microscopy and selected area electron diffraction techniques. To determine the dependence on cleanness, tin was deposited on single-crystal surfaces prepared by vacuum cleaving. Films were prepared on substrate surfaces to which a parallel d.c. electric field (0–1000 V cm⁻¹) was applied. It was observed that the crystallites of tin films deposited on KBr were more preferentially oriented than those deposited on NaCl, KCl or KI. The effects of vacuum cleaving and of d.c. electric fields are discussed.     

The role of electrolytes during aerosol synthesis of TiO2

Titania powders are synthesized by gas phase oxidation of TiCl₄ in an electrically heated hot wall aerosol flow reactor. Aqueous solutions of ionic additives are sprayed into the process stream and the effect of NaCl, KCl and CsCl (in the presence of AlCl₃ or SiCl₄) on titania particle characteristics is studied as a function of temperature, residence time, water vapor and ion concentration. Reduction of the aggregate size is obtained with alkali halides only in the presence of either AlCl₃ or SiCl₄ at 1673 K. Of all the alkali halides, CsCl is the most effective in reducing aggregate size while NaCl is the least effective. These changes are explained on the basis of complex ion formation, particle charging and the subsequent retardation of coagulation. The ionic radii of the alkali metals is too large to form solid solution with titania and hence they do not affect titania morphology and phase composition.     

Introduction of a planar defect into colloidal photonic-crystal films and their optical properties

A defect mode in the bandgap of photonic crystals is a key factor for potential applications, emission, bandpass filter, sensor, and low throughput laser. A Fabry-Perot type cavity was known as a multilayer film with a planar defect or a one-dimensional colloidal photonic crystal film with a planar defect. In this work, we have developed a simple and easy method by two colloidal crystals, i.e., face center cubic (fcc) structure, films bonded together by hot pressing to form a sandwich structure, and clear defect mode was observed in the photonic bandgap of fcc (1 1 1) direction. We have investigated the effect of the thickness of the defect layer in the sandwich structure on the optical properties. A single or double dips appeared in Bragg's diffraction peak at different planar defect thicknesses. In addition, a simulation of the reflection spectra of multilayer film calculation showed the defect mode is much influenced by the planar defect thickness.     

Photocatalytic degradation of methylene blue using novel flower-like zirconium bismuth molybdate thin film,grown by temperature-assisted CBD method

Thin films of a novel metal molybdate are successfully coated on soda lime glass by one step chemical bath deposition method. The deposition is carried out under different bath temperature ranging from 30 to 70 °C. In order to analyze the crystalline structure, X-ray diffraction study is carried out. Film thickness and morphology are examined through atomic force microscope and scanning electron microscope. Optical studies are done by UV–Vis–NIR absorbance spectra and photoluminescence spectra. All the samples have absorbance in the UV–Vis region and possess direct allowed transition with bandgap values ranging from 2.17 to 2.83 eV. Direct current show good ohmic response and possess resistivity in the range characteristic of wide bandgap materials. The material possesses three major photoluminescence emission peaks in the green and blue wavelength regions. Thin film grown at 50 °C showed increased absorbance width, minimum bandgap energy, larger electron–hole separation efficiency and more porous flower-like morphology which indicates its higher photocatalytic ability. The photocatalytic degradation process for the removal of methylene blue from waste water, under sunlight irradiation using thin film grown at 50 °C as photocatalyst has been discussed in detail.

     

Calculated binding of gold atoms to alkali halide substrates

The contributions to the atomic desorption energy and activation energy for surface diffusion from electrostatic polarization forces and van der Waals dispersion forces for the deposition of gold on alkali halide substrates have been calculated assuming hard-sphere repulsion between adatom and ion. The results for different substrates are compared with each other and with experimental values for gold on ultrahigh-vacuum-cleaved NaF, NaCl, KCl and KBr surfaces. The magnitude of the polarization term is nearly equal to that of the van der Waals term and can be important in determining the adsorption site and diffusion path. This appears to be the case for NaF but not for NaCl.     

Characteristics of wide-gap hydrogenated amorphous SiC films prepared using electron cyclotron resonance CVD from acetylene

The deposition of hydrogenated amorphous silicon carbide (a-SiC:H) films from a mixture of silane, acetylene and hydrogen gas using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) process is reported. The variation of the deposition and film characteristics such as the deposition rate, optical bandgap, photoluminescence and the infra-red (IR) absorption as a function of the hydrogen dilution is investigated. The deposition rate increases to a maximum value of 250 Å/min at a moderate hydrogen dilution ratio of 20 [hydrogen flow (sccm)/acetylene + silane flow (sccm)], and decreases in response to a further increase in the hydrogen dilution. There is no strong dependence of the optical bandgap of the hydrogen dilution within the dilution range investigated (10 to 60), and the optical bandgap calculated from the E₀₄ method varied marginally from 2.85 eV to 3.17 eV. The room temperature photoluminescence (PL) peak energy and intensity showed a prominent shift to a maximum value of 2.17 eV corresponding to maximum PL intensity at a moderate hydrogen dilution of 30. The PL intensity showed a strong dependence on the hydrogen dilution variation. IR absorption results show that films deposited at higher hydrogen dilution have more Si---C bonding.     

The Effect of Mechanochemical Treatments on Clay Minerals and the Mechanochemical Adsorption of Organic Materials onto Clay Minerals

The present paper reviews investigations on the mechanochemical reactions occurring during the grinding of kaolinite and the mechanochemical adsorption of polar organic compounds by expanding and nonexpanding clay minerals, carried out in our laboratory by IR spectroscopy, supplemented by XRD and TG-DTA techniques. Kaolinite was ground in the absence and presence of diluents (alkali halides) and IR spectra were recorded after different grinding periods. The mechanochemical reactions can be classified into four groups, diffusion of atoms (mainly protons, "prototropy"), delamination, layer breakdown, and adsorption of atmospheric water by the amorphous product. In the mechanochemical adsorption study of phenol and dichlorophenol by smectites (montmorillonite, beidellite, saponite, and Laponite), the mineral was ground with excess phenol. Spectra of ground phenol–smectite mixtures were recorded in KBr disks, after leaving the disks for 3 days in a vacuum furnace at 115°C for the evolution of the excess phenol. In most cases, the first 5 min of grinding led to an increase in the adsorption of phenol. An additional 5 min of grinding resulted in a decrease of the phenol adsorption. In the mechanochemical adsorption study of stearic acid by nonexpanding clay minerals (sepiolite, playgorskite, talc, pyrophyllite, and allophane), the mineral was treated with molten acid. Disks were prepared with NaCl, KCl, and CsCl. Each disk was crushed, reground, and repressed an additional four times. Grinding the products converted some of the adsorbed acid into the stearate anion.     

Growth and characterization of silicon carbide thin films on silicon using a hollow cathode pulse sputtering technique

Investigations of thin film depositions of silicon carbide (SiC) from pulse sputtering a hollow cathode SiC target are presented. The unique feature of the hollow cathode technique is that germanium can be added to the films. This changes the properties of the SiC. Such changes include evidence of GeC bonds, lowering of the resistivity, and lowering of the bandgap. The analysis includes crystallographic and morphological studies of the deposited films and their quality using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy data. Basic electrical properties are also presented along with optical bandgap information gathered from spectroscopic ellipsometry data.     

Graphite formation on Ni film by chemical vapor deposition

Thin film formation of graphite by chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material was carried out on Ni film substrates. On Ni films directly deposited on quartz glass, the graphite films were obtained when the Ni film thickness was above 1 000 Å and above 5 000 Å at 700 °C and 1 000 °C, respectively. Depositions on thinner Ni film substrates comprise amorphous carbon (a-C) or graphite tubes which was owing to the thermal coagulation of the Ni film into droplets. On the other hand, graphite film was obtained on the Ni film with thickness 10 Å when a-C was inserted between the Ni film and the quartz glass. The coagulation of the Ni film is considered to be avoided by inserting a-C layer.     

Oxidation-reduction dynamics in layer-by-layer self-assembled redox polyelectrolyte multilayer modified electrodes

The oxidation-reduction dynamics of layer-by-layer (LbL) self-assembled redox polyelectrolyte multilayer films on electrodes has been studied by cyclic voltammetry, chrono-amperometry, electrochemical quartz crystal microbalance (EQCM), ellipsometry, and Fourier transform reflection-absorption infrared spectroscopy (FT-IRRAS). Thin layer electrochemistry with fast electron transfer at the underlying metal-film interface and charge propagation by electron hopping between adjacent redox sites in the finite thin film has been observed. An almost ideal cyclic voltammetry for a fixed number of redox sites in the thin surface film suggests that the multilayer can be fully oxidized and reduced in the time scale of the experiment (RT/vF > or = 0.05 sec). The electron hopping diffusion coefficient 3 x 10(-10) cm2 s(-1) was obtained from the chronoamperometric current transient and the ellipsometric thickness. Both cyclic voltammetry and potential step yield a surface osmium bipyridyl redox concentration of gamma Os = 4 x 10(-10) mol x cm(-2) for (PAH-Os)5(PVS)4 film. Exchange of ions and solvent occur simultaneously to the charge injection as revealed by the EQCM mass change and the ellipsometric thickness change. From the end-to-end mass-to-charge linear relationship, the molar mass of the ionic and neutral species exchanged largely exceeds the molar mass of any ions or solvent which suggests an important flux of solvent during redox switching. An initial "break in" effect is observed for the first oxidation-reduction cycles when a newly self-assembled film equilibrates with the electrolyte as charge is injected during the electrochemical perturbation.