AgCl Films Research Articles

Hydrogen-photoinjection-induced enhancement of photochromism in AgCl films

The photochromic effect in thin AgCl films can be significantly enhanced using AgCl-WO3 two-layer film structures. Irradiation of the WO3 film surface leads to the detachment of hydrogen atoms from hydrogen-containing molecules that were preliminarily adsorbed on the oxide surface. These atoms pass to the halide film surface and produce its sensitization in the course of irradiation, which leads to accelerated photolysis in the AgCl film.

The chemistry of chlorine on Ag(1 1 1) over the sub-monolayer range: A density functional theory investigation

Chlorine adsorption on Ag(1 1 1) as a function of coverage has been studied by means of periodic density functional theory. Incorporation of Cl into the substrate leading to a surface AgCl film has also been considered. It is concluded that at low coverage ( θ Cl < 0.2 ML) on-surface adsorption is favoured over Cl penetration while at higher coverage on-surface and subsurface adsorption become both thermodynamically and kinetically favoured. Implications for the Cl promoted silver catalyzed ethylene partial oxidation are discussed.

Integration of Selective Area Anodized AgCl Thin Film with AlGaN/GaN HEMTs for Chloride Ion Detection

We have demonstrated a selective area AgCl anodization process, which can be integrated with the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) for chloride ion detection. A limit of detection of chloride ion concentration achieved was 1 X 10 -8 M using a 20 X 50 μm anodized Ag/AgCl layer on the HEMT gate sensing area. Unlike the conventional open-circuit potential measurement used for the electrochemical measurement, the drain current of the HEMT was monitored as the output signal in our sensor. The effects of anodization bias voltage and time on the AgCl film properties were investigated. A continuous anodized AgCl film was achieved with the bias voltage of 0.5 or 1 V. However, AgCl films anodized at higher biases (5 V) were not continuous and a larger grain size was obtained. Energy-dispersive X-ray analysis was used to analyze the composition of the anodized AgCl and showed that slightly chloride deficient films (around Ag:Cl = 57:43) were obtained.

Local structure of thin AgCl films on silver surface

The new structural results revising commonly accepted point of view on role of products of silver chlorination in catalytic reaction of ethylene epoxidation are presented. It was established that AgCl nucleation on Ag(111) mainly occurs on step edges but low temperature reaction could also start on atomic terraces. On atomically resolved images of AgCl nuclei surface obtained with scanning tunneling microscope, the specific features of atomic size were observed and attributed to silver atoms and atomic clusters. We predict that such silver cluster formation could explain a very high activity of chlorinated surface in ethylene epoxidation. Interatomic distances measured in thin AgCl films were found to be equal to corresponding bulk parameters. The photoelectron diffraction patterns observed upon Ag(100) chlorination were explained by the formation of AgCl(111) domains rotated on 90° with respect to each other.

Electrochemical dynamics and structure of the Ag/AgCl interface in chloride-containing aqueous solutions

In this paper we report on electrochemical and structural properties of the Ag/AgCl interface in contact with aqueous solutions containing Cl −. Electrochemical oscillations are connected with the anodic formation of AgCl films on polycrystalline Ag and Ag(111) electrodes in contact with Cl − solutions. The effects of chemical and electrochemical variables on the oscillatory behaviour are taken into account. Persistent oscillations are found for pH values ranging from acidic to alkaline. Oscillatory behaviour is shown to be typical of high-Cl − and low current density conditions. The polycrystalline vs single-crystal nature of the Ag substrate, affecting the AgCl film structure, is shown to have a quantitative bearing on the oscillating behaviour. The electrochemical results are complemented with electron microscopic observations and X-ray diffractometry. Coherent Ag/AgCl interfaces are found after prolonged oscillation experiments and grain-boundary type pore structures are found; lack of porosity in the structure correlates with the absence of potential oscillations. A simple linear dynamic model is devised for the rationalisation of the oscillatory behaviour, based on reaction-diffusion processes within the pores of the anodic AgCl film. Qualitative predictions of the model match experimental evidence in terms of: oscillation amplitude, frequency, phase and conditions for the onset of the oscillations.

Spontaneous S_ gratings and specific features of their formation and evolution in photosensitive AgCl-Ag films

The formation and evolution of spontaneous S_ gratings in photosensitive waveguide AgCl-Ag films under the action of a laser beam are investigated experimentally and theoretically. It is found that in the cases of s and p polarizations of laser radiation these gratings differ significantly not only in the period but also in the structure and spatial and temporal stability. The different character of the gratings is explained in detail by the competition between the S_ gratings and other gratings evolving simultaneously in the film and the change in the scattering indicatrix in the case of p polarization as a result of intense evolution of degenerate C gratings. The regularity of S_ gratings on TM0 modes in the case of p polarization and large angles of incidence makes it possible to increase the accuracy of determining the refractive indices of substrates using AgCl films corresponding to the cutoff thicknesses of TM0 waveguide modes.

The role of FA:K+ and FA:Na+ defects in laser light generation and color image formation at the (100) and (110) surface sites of AgCl and AgBr.

FA:K+ and FA:Na+ color centers at the low coordination (100) and (110) surface sites of AgCl and AgBr thin films play important roles in providing tunable laser oscillation and color image sensitization. Double-well potentials at these sites are investigated using ab initio molecular electronic structure calculations. Quantum clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that are the nearest neighbors to the FA defect site were allowed to relax to equilibrium. The calculated Stokes shifted optical transition bands suggest that laser light generation is sensitive to (i) the lattice anion, (ii) the coordination number of surface ions, (iii) the dopant cation, (iv) the vibrational coupling mode, and (v) the choice of the basis set centered on the anion vacancy. An attempt has been made to explain these results in terms of Madelung potentials, electron affinities and optical–optical conversion efficiencies. All relaxed excited states of the defect-containing surfaces were deep below the lower edges of the conduction bands of the ground state defect-free surfaces, suggesting that FA:K+ and FA:Na+ are suitable laser defects. The probability of orientational destruction of the two centers, attributed to the assumed saddle point ion configurations along the 〈110〉 axis, was found to be dependent on the lattice anion, the coordination number of surface ions, and the dopant cation. For optical memories, a high recording sensitivity was assigned to FA:Na+ relative to FA:K+, to the (110) surface relative to the (100) surface, and to AgCl relative to AgBr. The dependence of exciton (energy) transfer on the lattice anion, the coordination number of surface ions, and the dopant cation was clarified. The Glasner–Tompkins empirical rule was generalized to include the surface coordination number and the dopant cation in addition to the lattice anion. As far as color image formation is concerned, the examined supersensitizer was found to increase the sensitizing capabilities of two primary dyes in the excited states by increasing the relative yield of quantum efficiency. The (110) surfaces of AgBr and AgCl were more sensitive than the corresponding (100) surfaces, and the AgBr thin film was found to be more sensitive than that of AgCl. Based on quasi Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states was determined.

Small-angle scattering during formation of periodic structures in light-sensitive films subjected to laser radiation

Patterns of small-angle scattering that appear in thin AgCl films containing granular silver upon the formation of spontaneous periodic structures by p-polarized and circularly polarized laser beams (He-Ne laser, P=8 mW, λ = 632.8 nm) are studied. It is found that, at angles of incidence exceeding 4° regardless of the type of polarization, the diffraction of the waveguide modes excited by the dominant C gratings from neighboring microgratings with the vectors K ≠ K c dominantly contributes to the small-angle scattering. In the case of circularly polarized light, the pattern of small-angle scattering becomes more complicated as a result of the formation of S − gratings and related secondary regular gratings.

Using probe beam deflection (PBD) to investigate the electrochemical oxidation of silver in perchlorate media in the presence and absence of chloride ions

The electrochemical oxidation of silver in 0.1 M KClO 4 solutions containing KCl were investigated by cyclic voltammetry (CV) and electrochemical probe beam deflection (PBD). Ag + (aq) ions were the main product of the silver oxidation in the absence of the halide. The formation of Ag + (aq) provoked a beam deflection towards the electrode surface. A beam deflection away from the electrode surface was then observed during the reduction of the Ag + (aq) ions. A convolution analysis yielded a diffusion coefficient of 1.2×10 −9 m 2 s −1 for Ag + (aq) in this medium. An anodic peak due to the formation of AgCl (s) film was observed for the oxidation of silver in solutions containing Cl − (aq). As the applied potentials were made more positive in media containing chloride (after the peak due to the AgCl (s) formation), a flux of ions away from the electrode surface was clearly detected by PBD. This was assigned to the formation of Ag + (aq) ions through the porous AgCl (s) film structure. Oscillations on the position of the laser beam were present during the oxidation at high chloride concentrations, due to the precipitation of AgCl (s) from the solution phase. The electrochemical and PBD data were consistent with a dissolution-precipitation mechanism for the AgCl (s) film formation.

Recording of the polarization of light specified by anisotropic crystals with the aid of photoinduced dichroism in photosensitive films

It is shown that photoinduced dichroism in photosensitive AgCl films with Ag granules can be used to record the spectral dependence of the state of polarization specified by anisotropic crystals. The recording of the state of polarization throughout the visible spectral region is favored by the frequency photoadaptation, which is typical of AgCl–Ag films. This effect also determines the good reproduction of color corresponding to the spectrum of the irradiating beam. The results show the feasibility of determining the magnitude of photoinduced dichroism and its dispersion by analyzing the pattern recorded in a film in polarized light.

Instability and optical turbulence in the formation of spontaneous gratings in single-mode photosensitive AgCl-Ag films

In experiments on illumination of thin photosensitive AgCl films containing granular Ag by a focused laser beam (λ=633 nm), self-pulsations in the form of optical turbulence were found and studied. In the course of illumination, the turbulence was observed in the pattern of small-angle scattering in the form of “boiling” of light spots separated by dark spacings. The formation of turbulence is associated with spontaneous microgratings, whose origin involves waveguide TE modes scattered in a film, and their competition in the course of illumination, which leads to the growth of one microgratings and the decay of the other.

Recording of the optical rotation spectrum using the Weigert effect in AgCl-Ag films

Due to its spectral properties a dichroism, induced in thin light-sensitive AgCl films, doped by granular Ag, makes it feasible to record spectra of the optical rotation in the visible region. In this paper the experimental data are presented for the recorded rotation spectrum of the optically active crystal. It is shown that the spectrogram recorded contains information about the angle of rotation and the value of the induced dichroism. A new method of determining the dichroism and its dispersion is suggested based on the study of the spectrogram in white light using a pair of polaroids with different crossing angles.

Epitaxial growth of AgCl layers on the Ag(100) surface

Layers of silver chloride were formed by exposing the Ag(100) surface to gaseous chlorine at 250 K. The growth and structure of the AgCl layers have been characterized by XPD, LEIS and LEED. The formation of AgCl was monitored by looking at the line shape and position of the Ag M 4,5 VV transitions. The chlorination process was also followed by LEIS. The XPD data, collected keeping the sample at 150 K, indicate that AgCl layers ca. 20 A thick grow with the (111) plane parallel to the substrate surface but randomly oriented with respect to the Ag(100) surface. Instead, the XPD patterns show that thicker AgCl films grow epitaxially on Ag(100).

Influence of chlorine on planar optical guide cladded with RbCl and AgCl

Thin films of RbCl and AgCl are prepared using vacuum evaporation technique and are characterized for Cl 2 effect through a number of techniques like XRD analysis, optical absorption and ellipsometry measurements. The time-series characterization with these techniques do show the expected differences with alternate pulsed chlorine and ambient air exposures. These are used as sensitive films on planar optical waveguide for Cl 2 gas. It is observed that, as cladding, they possess very good and selective chlorine sensing behaviour in ambient air over a wide concentration range of few (3) ppm-1.5%.

Structure and modification of silver halide thin films using scanning tunneling and atomic force microscopy

Atomic force and scanning tunneling microscopy were used to determine the surface morphology of vapor-deposited thin films of AgBr and AgCl. The film structures were consistent with epitaxial growth. Scanning tunneling microscope was found to produce significant modification of the Ag halide films. One type of modification appeared as a slight etching of the surface in the vicinity of the tunneling tip. Another was the production of protrusions extending far above the original surface of the film. Models to explain each of these modifications are presented.

Local structure determination for surface chlorination with EELFS

In this paper the results of an investigation of the local structure of a layer of molecular chlorine condensed on Si(1 0 0) at T = 110 K and an unbroken AgCl film evaporated on Ag(1 1 1) at T = 160 K are presented. It was found that the nearest neighbor ClAg distance in the AgCl film of 2.75 ± 0.05 A ̊ is in good agreement with bulk values (2.77 Å). The ClCl distance of 2.00 ± 0.05 Å in a chlorine film condensed on Si(1 0 0) is also close to the bulk value (1.98 Å).

Spatially Resolving Impedance Spectroscopy

ABSTRACTTwo techniques for measurement of local electrical conductivity of inhomogeneous materials are described. I) A novel variant of the impedance technique for thin/thick film characterization was developed; due to the two-dimensionality of the cell set up, at different frequencies different parts of the material are probed. The technique was experimentally verified by measuring the position coordinate of a Ag strip artificially added to a AgCl film. Its application to the Ag/AgCl boundary is touched upon. II) Micro-electrodes were used to probe surface conductances and local subsurface conductivities. The technique was implemented by a home-made high impedance adapter and combined with AFM to measure the contact area. Several examples of application are shown, viz. measurements of a) the enhanced surface conductivity of mechanically polished AgCl crystals, b) the interdiffusion coefficient of Cd2+ in AgCl, and c) the grain conductivity of polycrystalline AgCl.

Thin film growth of AgCl on NaCl(001)

The film growth processes occuring during the deposition of AgCl on NaCl substrates are studied by means of the gold decoration technique and Rutherford backscattering spectrometry. At substrate temperatures between 20 and 100°C, first an intermediate monolayer forms by the nucleation and coalescence of 2D islands. Further growth proceeds by the formation of secondary islands on top of the monolayer and a layer-by-layer growth of the continuous AgCl film after their coalescence. At substrate temperatures above 100°C, mixed films form due to interdiffusion with the NaCl substrate. At all temperatures applied, single-crystalline films form with the epitaxial relationship [100](001)AgCl ‖[100](001)NaCl.

Conductance effects of ammonia on silver chloride boundary layers

The influence of the Lewis base NH3 on the conductivity of AgCl films is investigated. Three types of films are studied: epitaxial films on NaCl (100), polycrystalline films on sapphire, as well as 'freestanding' films. In the case of films in contact with s substrate, annealing is necessary in order to achieve equilibrium values for the conductivity even under an inert atmosphere. Two counteracting effects of NH3 are discussed: core effects due to increased transfer resistivities and space-charge effects due to an increase of the Ag+-vacancy concentration. The latter phenomenon can lead to an increase in the overall conductance, if the interstitial contribution to the total conductivity is not too large. As far as the first effect is concerned, the conductance change involved can be quantitatively correlated with the ammonia partial pressure using a simple adsorption model. The conductance response is below 5 min in the temperature range 100–170 °C (103 Pa < p(NH3) < 105 Pa) for the freestanding films and is reproducible within the limits of experimental error.

The ionic conductivity profile of thin evaporated AgCl films on a planar sapphire substrate

The ionic conductivity profile of AgCl-films stepwise evaporated onto a sapphire-substrate has been measured. After each step the films have been annealed. A bombardment of the uncompleted films by argon-ions (just before the successive evaporation processes) enhances the ionic conductivity. The first sheet (about 350 nm thick) of these films adjacent to the interface shows extremely high conductivity values up to about 10−4 S/cm at 25°C. But also the remaining part of each film between about 350 and 800 nm away from the interface still shows an unexpectedly good conductivity of the same magnitude as the highest conductivity value (of 6×10−6 S/cm at 25°C) found so far for disperse systems AgCl/Al-oxide. The measureds activation energies (between 0.34 and 0.47 eV) of these films are markedly higher than the migration enthalpy of Ag-vacancies (0.28 eV). The discussion suggests that the high conductivity and its profile are essentially determined by the microscopical structure of the films.