Conduction mechanism in codeposited AgSe thin films

Thin films of AgSe of varying compositions and thicknesses have been formed on glass substrates employing the three-temperature method.I–V characteristics and thermoelectric power, α, of annealed samples have been measured as functions of composition, thickness and temperature of the films. Films exhibitn-type conductivity. Nonohmic conduction in films of Ag _x Se_1−x (0<x<0·5) and Ag _x Se_1−x (0>x>0·5) have been accounted for on the basis of the theory of Rose of defect insulator containing shallow traps and on Schottky emission respectively.     

Conduction and dielectric polarization in Se thin films

Se films were prepared by thermal evaporation technique in thickness range 150-8500 Å. X-ray diffraction measurements showed that Se films are in the amorphous state. The ac conductivity and dielectric properties of the amorphous Se films have been investigated in the frequency range 100-100 KHz and 100-400 K temperature range. The ac conductivity σ_ac(ω) is found to be proportional to ω^s where s<1. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier (CBH) model. The dc conductivity at the room temperature was also studied in the same thickness range. It was concluded that the same mechanism of carrier motion might be dominant in both ac polarization and dc conduction. This carrier transport mechanism might be electronic.     

Conduction mechanism in sputtered polycrystalline zinc oxide thin films

Conductivity and Hall effect data are reported for columnar polycrystalline zinc oxide films between 1 and 4 μm thick deposited onto insulating substrates by r.f. reactive sputtering. Crystallites are (00.1) oriented, typically a few hundred nanometers in diameter and 98%–99% packed. Resistivities between 10⁷ and 10¹¹ Ω cm at 300 K typically decrease by between one and three orders of magnitude for a 100 K temperature increase. The shallow oxygen vacancy donor is the only impurity level present in significant densities; IR absorption indicates densities greater than 10¹⁷ cm^-3, but Hall measurements show apparent carrier concentrations of between 10⁴ and 10⁸ cm^-3.This anomaly is resolved by invoking the Petritz diode model of conduction in discontinuous materials, modified to include the case of highly doped crystallites and applied to zinc oxide with deplectively chemisorbed intercrystalline states. Quantitative agreement is found for conductivity against temperature. The measurable Hall carrier concentration, deduced from the empirical intracrystallite carrier densities of between 1 × 10¹⁵ and 1 × 10¹⁹ cm^-3 and barrier heights of between 0.6 and 1.2 eV, agrees well with Hall effect measurements. Crystallite surface state densities of about 10¹³ cm^-2 are indicated as responsible for these effects.     

Photoresponsive self-assembly motions in polymer thin films

New developments in research of photoresponsive polymer thin film systems that exhibit new performances are reviewed. Topics introduced here involve surface photoalignment of mesostructured materials, photomechanical responses, photo-triggered efficient mass migrations, photoalignment and patterning of microphase separated nanostructures in block copolymers and preparation of surface-grafted photoresponsive polymers. Most of the systems utilize liquid crystalline materials and are designed as to exert cooperative self-assembling motions to provide uniform or patterned structures and orientations. These prototypes are anticipated to provide significant clues to create new smart responsive systems for the future science and technology of soft materials.     

Conduction and dielectric polarization in thin anodic aluminium oxide films

The a.c. polarization of anodic Al₂O₃ films was studied in the thickness range 30–1500 Å and the frequency range 300 Hz to 15 kHz. The interfacial polarization mechanism involving distinct regions separated by flaws and/or voids in the bulk was thought to become dominant with decreasing thickness because of an increasing concentration of defects. The d.c. conductivity at the room temperature was also studied in the same thickness range. The observed linear log J versus E^1/2 characteristics for several thicknesses showed that d.c. conduction in these films obeys the Poole-Frenkel law over a certain range of applied electric fields. Anomalies observed in the electric field and the thickness dependences of the d.c. conductivity are considered to arise from the thickness dependence of the defect concentration. It was concluded that the same mechanism of carrier motion might be dominant in both a.c. polarization and d.c. conduction.     

Conduction mechanisms in co-evaporated mixed Mn/Siox thin films

The thermoelectric power and d.c. conductivity of co-evaporated Mn/SiOx films, 100 nm thick, containing 20 to 100 at% Mn have been measured over the temperature ranges 275 to 580 K and 110 and 575 K, respectively. Three conduction regions have been identified: p-type, via non-polaronic holes and small polarons; intrinsic and metallic. The d.c. conductivity arises from a combination of non-activated and activated processes.     

Viscoelastic properties of thin polymer films

Thin polymer films have been prepared by electron beam irradiation of adsorbed layers in vacuum. Electrodes have been applied so that capacitance measurements can be made in the presence of a d.c. bias. It is found that the polymer layer deforms under the influence of the electrostatic forces and that the deformation follows the extended Voigt model for viscoelastic deformation, indicating a combination of pure elastic and viscoelastic behaviour. Values of the elastic moduli and retardation time have been obtained.     

Quantitative equivalence between polymer nanocomposites and thin polymer films

The thermomechanical responses of polymers, which provide limitations to their practical use, are favourably altered by the addition of trace amounts of a nanofiller. However, the resulting changes in polymer properties are poorly understood, primarily due to the non-uniform spatial distribution of nanoparticles. Here we show that the thermomechanical properties of 'polymer nanocomposites' are quantitatively equivalent to the well-documented case of planar polymer films. We quantify this equivalence by drawing a direct analogy between film thickness and an appropriate experimental interparticle spacing. We show that the changes in glass-transition temperature with decreasing interparticle spacing for two filler surface treatments are quantitatively equivalent to the corresponding thin-film data with a non-wetting and a wetting polymer-particle interface. Our results offer new insights into the role of confinement on the glass transition, and we conclude that the mere presence of regions of modified mobility in the vicinity of the particle surfaces, that is, a simple two-layer model, is insufficient to explain our results. Rather, we conjecture that the glass-transition process requires that the interphase regions surrounding different particles interact.     

Orientation of polymer functionalized nanorods in thin films

A directed self assembly of anisotropic nanostructures offers a possibility to provide unique functional materials, which are e.g., important in optoelectronic devices. We use the liquid crystalline behavior of polymer functionalized TiO2 and ZnO nanorods to apply methods well known for low molecular liquid crystals to achieve oriented thin films. Convective forces in the meniscus on a structured substrate obtain thin layers of oriented nanoparticles with a ordering parameter of S = 0.7. As another method we present the orientation of polystyrene covered ZnO nanorods under an applied electric field. The method offers a perpendicular alignment of the rods to the surface.     

Inhibition of dewetting of thin polymer films

Strategies for the inhibition of dewetting of thin polymer films are reviewed. First, a brief introduction to the theory and the dynamics of dewetting of thin polymer films is given. Methods for the inhibition of dewetting of thin polymer films, including the modification of the substrate and the polymer, the cross-linking of the polymer and the addition of an additive as well as their mechanisms for inhibition are discussed. The chemical modification of the substrate or the polymer itself and the phase separation increase the thermodynamic stability of the system, while the cross-linking of the polymers reduces the mobility of the polymer chains, kinetically arresting the dewetting. The addition of an additive appears to thermodynamically and kinetically inhibit the dewetting. Finally, the future outlook in this research field is highlighted.     

Conduction mechanism in amorphous In2III X3VI thin films

Amorphous In₂ ^IIIX₃ ^VIfilms (X = Te or Se) are obtained by vacuum thermal evaporation of bulk materials on glass substrates. The current - voltage characteristics in the temperature range 298–378 K and in the thickness range 212–652 nm exhibited a transition from an ohmic region in the lower field followed by non-ohmic region in the high field region, which has been explained by the anomalous Poole-Frenkel effect. The temperature dependence of current in the ohmic region is of thermally activated process. The variation of dielectric constant with temperature is investigated for the two compounds.     

Thermally stimulated discharge conductivity in polymer composite thin films

This paper describes the results of thermally stimulated discharge conductivity study of activated charcoal-polyvinyl chloride (PVC) thin film thermoelectrets. TSDC has been carried out in the temperature range 308–400°K and at four different polarizing fields. Results are discussed on the basis of mobility of activated charcoal and polyvinyl chloride chains.     

Complexities in STEM analyses of polymer blend thin films

Polystyrene (PS)-polyether sulphone (PES) polymer blend thin films were prepared for examination in a scanning transmission electron microscope. The microstructures observed in 75 wt % PS-25 wt % PES films consisted of spherical inclusions, ranging from 0.2 to 1.2m in diameter. X-ray spectrometric analysis in the microscope revealed that the inclusions were PES-rich, while the matrix contained only PS. Attention in this paper is paid to the contrast in the annular dark-field detector (ADF) images from these thin films. This image contrast has a complicated dependence on both the angular range subtended by the dark-field detector and mass-thickness variations within the films. On microscopes with appropriate lens controls which permit the acceptance angle of the ADF detector to be varied, it becomes possible actually to reverse the contrast between the two phases.     

Optical properties of pyrene doped polymer thin films

Thin films of pyrene in polystyrene matrix have been prepared by spin coating technique. The concentration of polystyrene is kept constant to 1 wt.% while that of pyrene dopant varied in the range 2.30×10⁻⁴-2.30×10⁻¹ wt.%. Thickness of the films was found to depend upon concentration of pyrene and varies from 90 to 782 nm. The results of X-ray diffraction analysis reveal the crystalline nature of the films. The optical properties were studied by absorption, excitation and fluorescence spectroscopy. The band gap energy of pyrene in polymer films was calculated from absorption results. A transition from monomer to excimer is observed with thickness variation of the films. The structured part of the spectrum is assigned to the monomer emission while the broad emission band is attributed to well known pyrene excimer-like emission.