Sandwich Films Research Articles

Thermally stable dynamic piezoelectricity in sandwich films of porous and nonporous amorphous fluoropolymer

Porous amorphous fluoropolymer films of very low stiffness were produced by a solvent evaporation technique. Corona-charged sandwich films consisting of a porous and a nonporous layer exhibit piezoelectric thickness-extension resonances in their dielectric spectrum, through which the temperature dependence of their dynamic stiffness, coupling factor, and piezoelectric coefficient could be determined. Their strong piezoelectricity with coefficients of up to 600 pC/N at temperatures of at least 120 °C could make these polymer electret films interesting candidates for sensor and actuator applications in elevated temperature environments.

Carbazole‐based electroluminescent devices obtained by vacuum evaporation

AbstractTransparent conductive oxide (SnO2)/organic layers/aluminum thin film sandwich structures were obtained by vacuum evaporation. The organic component was either a thin carbazole film or a bilayer. In that case, the carbazole film was deposited onto a thin insulating polymer film. The polymer used was the poly(tetrabromo‐p‐phenyleneselenide) (PBrPDSe). Photoluminescence measurements have shown that the carbazole thin films emit blue light. (I‐V) measurements have shown that the structures exhibit diode characteristics. The forward direction is obtained when the transparent conductive oxide (TCO) is positively biased. However, the reproducibility of the results obtained with a single carbazole layer is poor. It appears that the stability of the sample is improved when a thin PBrPDSe film (40 nm) is introduced between the carbazole and the SnO2. The polymer film avoids the short circuit effect. In that case, the turn‐on voltage of the diode is about 3 V, when the thickness of the carbazole film is around 250 nm and the electroluminescence appears at a voltage of about 5 V. It is shown that the thermionic effect cannot be used to explain the I‐V characteristics, which are interpreted with the help of the Fowler–Nordheim tunnel effect. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2042–2055, 2001

Dielectric and conduction properties of pure polyimide films

AbstractCapacitors with an Al–polyimide–Al sandwich film structure have been fabricated: the top and the bottom aluminium electrodes were deposited by vacuum evaporation and the polyimide film was deposited by an isothermal immersion method. An X‐ray diffractogram of the film indicates the amorphous nature of the polyimide film. Dielectric and ac conduction properties of polyimide capacitors in the frequency range 10 kHz to 10 MHz at various temperatures (303–423 K) are reported; the dc conduction at different voltages and at various temperatures (303–423 K) is given. The capacitance of the film decreases with increasing frequency but increases with increasing temperature. The ac conduction studies suggest that electron hopping is responsible for conduction while the dc conduction studies reveal that Poole–Frenkel conduction is predominant at high fields. The activation and zero field activation energies are also calculated.© 2001 Society of Chemical Industry

Characteristics of Indium-Tin-Oxide/Silver/Indium-Tin-Oxide Sandwich Films and Their Application to Simple-Matrix Liquid-Crystal Displays

We developed low-resistivity transparent conductive films having the structure of indium-tin-oxide/silver/indium-tin-oxide (ITO/Ag/ITO). The thin silver film was sandwiched by ITO films. Our goal was to study the characteristics of the sandwich films and the display characteristics of simple-matrix liquid-crystal displays (LCDs) fabricated using the sandwich film. The electrical and optical characteristics of the sandwich films depended greatly on the thickness of the ITO and silver layers. Low resistivity and high transmittance were obtained when the film structure had a thickness of ITO/Ag/ITO: 40 nm/15 nm/40 nm. The simple-matrix LCD fabricated using a sandwich of ITO/Ag/ITO exhibited 27%–48% reduction in the level of crosstalk compared to the conventionally available simple-matrix LCDs fabricated using a single-layer ITO film; thus, the display performance was improved.

Carbazole based electroluminescent devices obtained by vacuum evaporation

Transparent conductive oxide (SnO2)/organic layers/aluminum thin film sandwich structures have been obtained by vacuum evaporation. The organic component was either a thin carbazole film or a bilayer. In that case, the carbazole film was deposited on a thin insulating polymer film. The polymer used was the poly(tetrabromo-p-phenylenediselenide) (PBrPDSe). Photoluminescence measurements have shown that the carbazole thin films emit blue light. The I–V measurements have shown that the structures exhibit diode characteristics. The forward direction is obtained when the TCO is positively biased. However, the reproducibility of the results obtained with a single carbazole layer is poor. It appears that the stability of the sample is improved when a thin PBrPDSe film (50nm) is introduced between the carbazole and the SnO2. The polymer film avoids short circuit effect. In that case, the turn-on voltage of the diode is about 3V, when the thickness of the carbazole film is around 250nm and the electroluminescence appears at a voltage of about 4V.

The giant-magneto-impedance effect in FM/Ag/FM (FM≡FeCuCrVSiB) sandwich films

Sandwich films with the structure FM/Ag/FM have been prepared byradio-frequency sputtering and then annealed at an optimizedtemperature (here FM represents the FeCuCrVSiB amorphoussoft magnetic layer). The giant-magneto-impedance (GMI) effects forthe samples have been studied as a function of alternating-currentfrequency (50 kHz to 13 MHz) and external magnetic field. When thefrequency increases above a critical frequency f* (about150 kHz), the GMI ratio increases quickly with increasing frequency,and reaches a maximum value at a characteristic frequency fm ofabout 4 MHz. GMI ratios at fm as large as 109% and 135% wereobtained for longitudinal and transverse applied magnetic fields,respectively. These results are much better than those obtained forsingle-layer films, and the frequencies used in producing the effectare very low.

Domain wall coupling in magnetic thin film sandwiches

Se ha estudiado el acoplamiento entre paredes magneticas en peliculas tipo sandwich de permalloy/Cu/permalloy, obtenidas por pulverizacion catodica, de espesores comprendidos entre 2000A/200A/2000A, para evaluar el campo de acoplamiento entre las paredes. Para realizar este estudio se han cortado la muestras de un tamano lo suficientemente pequeno para conseguir que la energia magnetostatica de la muestra sea del mismo orden de magnitud que la energia de la pared magnetica. Los resultados obtenidos muestran la posibilidad de conseguir dots magneticos de tamanos de micrometros con un comportamiento biestable.

Asymmetrical magneto-impedance in a sandwich film with a transverse anisotropy using an AC bias

A new method of obtaining asymmetrical magneto-impedance in a film system consisting of two outer magnetic layers and an inner conductive lead is proposed, which utilizes a high-frequency longitudinal bias field. For a certain magnetic structure, as in the case of antisymmetrical transverse magnetization in the outer layers, in the presence of a longitudinal DC magnetic field H ex the AC bias field induces a high-frequency circulatory magnetization which contributes to the voltage measured across the film. Depending on the sign of H ex, this voltage is in phase or counter-phase with that induced by the current flowing along the film layers. As a result, the total voltage does not respond in the same way to positive and negative H ex. This process is described in terms of the surface impedance tensor. The contribution to the voltage due to the current and the bias field is given by the diagonal and off-diagonal components of this tensor, respectively, which have a different symmetry with respect to H ex.

Co-immobilization of cholesterol oxidase and horseradish peroxidase in a sol–gel film

Physical adsorption, physically entrapped sandwich and microencapsulation techniques have been utilized for the immobilization of cholesterol oxidase (ChOx) and horseradish peroxidase (HRP) on tetraethyl orthosilicate (TEOS) derived sol–gel films. The enzyme activity studies indicate no significant leaching of enzyme from the microencapsulated and physically entrapped enzyme sandwich films. The ChOx activity has been assayed as a function of cholesterol concentration in these films, and spectrophotometric and electrochemical techniques have been used for the estimation of cholesterol concentration in each instance. Spectrophotometric measurements exhibit a response time of 10, 30 and >100 min for physically entrapped, physisorbed and microencapsulated films, respectively. A similar order of response times has been found for these enzyme sol–gel films using an electrochemical technique. A fast response time of 50 s has been observed amperometrically with a physically entrapped enzyme sandwich sol–gel film. The results obtained from spectrophotometric and electrochemical measurements using the three systems yield linearity for 2–10 mM cholesterol. All enzyme sol–gel films are found to be stable for about 8 weeks at 25°C and 12 weeks at 4–5°C.

Exchange coupling in Nd2Fe14B/Fe/Nd2Fe14B sandwich films and their magnetic properties

Sandwich magnetic films of Nd2Fe14B/Fe/Nd2Fe14B have been grown by a KrF excimer laser (λ=248 nm) ablation technique. Magnetic properties were characterized by varying the thickness and volume fraction of hard (Nd2Fe14B) and soft (Fe) magnetic layers. In the (y)nm[NdFeB]/(x)nm[Fe]/(y)nm[NdFeB]/(100)Si structure, the thickness (y) varied from 3.6 to 54 nm, and (x) from 15 to 112 nm. At (x)=15–23 nm where the volume fraction of Fe corresponded to 51–75%, the sandwich structure exhibited an enhanced Mr/Ms and Hci from the result of the exchange coupling between the magnetic layers. Experimentally calculated exchange constant (As) of As=2.5×10−10 J/m was estimated using the measured thickness (tcs) and reference crystalline anisotropy constant (Kh) of Nd2Fe14B crystal.

Size effect on magneto-impedance in layered films

The analysis of magneto-impedance (MI) in a sandwich film having an inner conductive lead and two soft ferromagnetic layers is concerned with the role of the film dimensions in determining the MI ratio. The equation for the AC magnetic field distribution in the sandwich is solved making the assumption of a weak skin-effect and a large transverse permeability of the magnetic layers. The flux leakage through the conductive layer results in a considerable drop in MI ratio if the film width is less than a certain critical value depending on the transverse permeability and the layer thickness. This process is similar to that known in inductive recording head, for example, in contributing to head efficiency.

Enhanced spin-valve giant magneto-resistance in non-exchange biased sandwich films

A large giant magnetoresistance (GMR) value of 7.5% has been measured in simple NiFeCo(1)/Cu/NiFeCo(2) sandwich films grown on a 30 /spl Aring/ Cr seed layer. This spin-valve GMR effect is consistent with the differential switching of the two NiFeCo layers due to an enhanced coercivity of the NiFeCo(1) layer grown on the Cr seed layer. A change in growth texture of the NiFeCo(l) layer from fcc [111] to bcc [110] crystallographic orientation leads to an increase in magnetic anisotropy and an enhancement in coercivity. The GMR value increases to 8.7% when a thin CoFe interfacial enhancing layer is incorporated. Further enhancement in GMR values up to 14% is seen in the sandwich films by nano-oxide layer formation. The specular reflection at the oxide/magnetic layer interface further extends the mean free path of spin-polarized electrons.

Domain-wall magnetostatic coupling in permalloy/Cu sandwiches

We have studied, by means of numerical simulation, the structure of domain walls in permalloy thin film monolayers and sandwiches consisting of two permalloy layers separated by a Cu interlayer. The calculated transition thicknesses between Bloch and Néel domain walls increase notably for the sandwiches with respect to monolayers. The structures obtained for uncoupled walls show quite similar patterns for the Néel walls and their corresponding quasi-walls. When coupled, the quasi-walls disappear and so the energy of the pair is reduced approximately to half the energy of the uncoupled pair. Results have been tested in thin film permalloy sandwiches obtained from a sputtering system.

Layer-By-Layer Assembly of Core-Shell Magnetite Nanoparticles: Effect of Silica Coating on Interparticle Interactions and Magnetic Properties

Layered magnetic materials have been prepared using layer-by-layer deposition, a technique that allows engineering of the structure of individual particles while remaining simple to carry out. Organic–inorganic sandwich films of superparamagnetic Fe3O4 thus prepared consisted of densely packed assemblies of Fe3O4 nanocrystallites that showed good adhesion to a variety of substrates and excellent mechanical properties. Coating the nanoparticles with a shell of SiO2 was found to greatly reduce the cooperative magnetization switching between adjacent nanoparticles.

Dielectric properties of Al ∣ V 2O 5 ∣ Al thin film sandwich structures

Vanadium pentoxide thin films were prepared by the electron beam evaporation technique onto Corning 7059 glass substrates kept at a temperature of T s=423 K. The dielectric properties of Al ∣ V 2O 5 ∣ Al thin film sandwich structures were studied in the frequency range 0.1–100 kHz and in the temperature range 125–450 K. Both the dielectric constant and the dielectric loss factor were found to depend on frequency and temperature. The activation energy obtained for the dielectric relaxation process was about 0.36 eV.

LASER-INDUCED PROCESS OF BINARY ALLOYING OF Ge–Se

Thin films of chalcogenides are synthesized by CW argon ion laser irradiation of separate Ge/Se and Se/Te film sandwiches according to near stoichiometric proportions. The compound formation and crystalline transition of amorphous Se film are characterized by observing photomicrographs and optical absorption spectra. The composite film of bilayer Ge/Se does not form compounds but deposits as separate elements under equivalent laser fluence.

Mechanical behavior and sound generation efficiency of prestressed, elastically clamped and thermomechanically driven thin film sandwiches

A model of thermomechanically driven plates as used, e.g., for acoustic microresonators is reported. The model describes the quasibuckling, excitation, fundamental frequency and vibration mode of multilayered plates under plane strain. The plates consist of stacked thin films with individual elastic modulus, Poisson's ratio, prestress, heat capacity and coefficient of thermal expansion. Membrane edges are elastically clamped to lateral supports. In thin plates two driving mechanisms are identified: the first couples the thermomechanical bending moment to the curvature of the vibration mode; the second couples the thermomechanical line stress to the periodic length change of the vibrating quasibuckled structure. Results of the model include vibration amplitude, sound field pressure, emitted power and quality factor as functions of the membrane properties. Maximum output is obtained close to the quasibuckling transition of the structures, where their resonance frequency is minimum. Largest sound pressures are generated by structures with rigid supports or with vanishing initial bending moment. The model shows excellent agreement with experimental data from micromachined resonant silicon membranes.

Ion beam induced magnetic nanostructure patterning

Using results from previous work on the initial stages of ion beam mixing in metallic multilayers, we developed a method allowing us to pattern the magnetic properties of Pt/Co thin film sandwiches and multilayers without affecting their optical or topographical properties. Excellent control of local coercivity or magnetic anisotropy is obtained in the irradiated areas. We produced arrays of 1 μm wide stripes with perpendicular magnetic anisotropy, separated by 1 μm wide paramagnetic stripes, and studied domain wall motion in these structures. Calculations show that nanometer scale resolution can be achieved. Besides its interest for micromagnetics studies, the technique holds promise for applications in high density magnetic recording and magnetic sensor technologies.

Electrical, optical and photovoltaic effect in pyronine G (Y) based thin film sandwich devices

The optical, electrical and photovoltaic properties of pyronine G (Y) have been investigated in the form of thin film devices. The molecular system of the pyronine shows the phenomenon of extended conjugation which is in fact responsible for its absorption in the longer wavelength, i.e. 540 nm of the visible spectrum. Pyronine G (Y) was employed in the form of thin film for the fabrication of sandwich devices having ITO/pyronine/Al and ITO/pyronine/In structures. The present communication deals with the study of optical, electrical and photoelectrical properties of ITO/PYR/Al and ITO/PYR/In devices by measuring the J–V characteristics in dark as well as under illumination and C–V characteristics in dark. The photoaction spectra of the devices and absorption spectra of PRY thin films were also employed for evaluating the photogeneration process in the device. The observations reveal the formation of a Schottky barrier at Al–PYR and In–PYR interfaces and ohmic contact at the ITO–PYR interface, showing a diode ideality factor greater than unity. From the detail analysis of J–V characteristics of the device at different temperatures, it is found that the position of the Fermi level lies above the valence band indicating the semiconductivity of PYR as p-type. The photogeneration mechanism of charge carriers in the device involves the dissociation of excitons at Al–PYR and In–PYR interfaces in their respective devices. Various electrical and photovoltaic parameters were also calculated from the analysis of experimental results and discussed in detail. The mechanism of the photoconductivity process in the PYR thin film is been discussed in detail.

DC Electrical measurements on evaporated thin films of vanadium pentoxide

The current–voltage characteristics of the Au–V 2O 5–Au thin film sandwich system at different temperatures were studied. The DC conduction mechanism was explained using Jonscher–Ansari modified Poole–Frenkel type. Both freshly evaporated and annealed vanadium pentoxide films are amorphous.