R. Capan

Optical dispersion in spun nanocrystalline titania thin films

Sol–gel derived titanium dioxide (TiO2) films are prepared on silicon substrates using the method of spin-coating with different speeds from 1000 to 6000 rpm. Spectroscopic ellipsometric measurements provide dispersion of the refractive index and extinction coefficient within the wavelength range of 300–1000 nm. The behaviour is in agreement with the single oscillator model. Optical absorption is believed to be due to allowed indirect transition over the optical gap of approximately 3.2 eV.

Sol?gel derived nanocrystalline titania thin films on silicon

Electrical characteristics of sol–gel derived titanium dioxide (TiO2) as insulating layers were investigated by making capacitance and leakage current measurements in metal–insulator–semiconductor configurations. The structure was fabricated by depositing 37 nm thick anatase TiO2 films on p-type silicon (p-Si) substrates. The frequency dispersion of capacitance was attributed to the leaky behaviour of the TiO2 dielectrics. Using an equivalent circuit, values of the frequency-independent dielectric constant, interfacial surface density and threshold voltage were estimated to be 13, 3 × 1014 m−3 and −0.085 V, respectively. The carrier diffusion was found to be primarily responsible for the diode leakage current at room temperature but the increase in the ideality factor with lowering temperature was believed to be due to fluctuations of barrier height at the TiO2/p-Si interface.

Poly(methyl methacrylate) films for organic vapour sensing

Optical constants and fabrication parameters are investigated using surface plasmon resonance (SPR) studies on spun films of poly(methyl methacrylate) (PMMA) derivatives in contact with two different dielectric media. A value of 1.503 for the refractive index of PMMA films produced from a solution having concentration of 1?mg?ml?1 at the speed of 3000?rpm is in close agreement with the data obtained from ellipsometric measurements. The film thickness shows a power-law dependence on the spin speed but the thickness increases almost linearly with the concentration of the spreading solution. These results are in good agreement with the hydrodynamic theory for a low-viscosity and highly volatile liquid. On the basis of SPR measurements under dynamic conditions, room temperature response of PMMA films to benzene vapours is found to be fast, highly sensitive and reversible. The sensitivity of detection of toluene, ethyl benzene and m-xylene is much smaller than that of benzene.

Molecular engineering of pyroelectric polysiloxane Langmuir-Blodgett superlattices: synthesis, film preparation and pyroelectric properties

Abstract A wide range of linear and cyclic polysiloxanes substituted with side chains containing carboxylic acid groups have been synthesized and characterized in terms of their Langmuir/Langmuir-Blodgett (LB) film properties and their pyroelectric activity. The effects on these properties of varying the degree of side-group substitution, the length of aliphatic side groups, the incorporation of polar aromatic side groups and the deposition conditions utilized during the preparation of multilayer assemblies have been investigated. These materials form stable Langmuir layers at the air-water interface which can be transferred onto substrates such as glass and aluminium-coated glass. The alternate layer LB deposition technique, in which each polysiloxane layer is co-deposited in an alternating stacking sequence with monolayers of a monomeric aliphatic amine compound, has been used to fabricate macroscopically polar films which display a temperature-dependent electric polarization, the ‘pyroelectric effect’. Data are presented here for both linear and cyclic substituted polymer backbones showing that both systems provide useful insight into the pyroelectric behaviour of organic materials. Trends in the relationships between the pyroelectric activity and (1) the chemical structure of the materials and (2) the structure of the acid/amine superlattice have been identified and indicate that the optimum pyroelectric coefficient is observed for a linear copolysiloxane compound substituted with a polar aromatic pendant side group. Indeed, the pyroelectric coefficient measured for this material is ∼10μCm−2 K−1 which is currently the highest value reported for an LB assembly to our knowledge.

Spun thin films of poly(methyl methacrylate) polymer for benzene sensing

Thin films of poly(methyl methacrylate) (PMMA) were deposited on suitably prepared substrates by spinning the spreading solution in chloroform at 3000?rpm for atomic force microscopy and surface plasmon resonance studies. Four different types of PMMA were used within a range of molecular weight between 540 and 760?kg?mol?1. The film surface was found to be uniform. Values of refractive index for PMMA film were found to exist in the range of 1.50 ? 0.04 to 1.53 ? 0.02. All films exhibited fast response to the exposure of benzene vapour but the sensitivity of detection depended upon the molecular weight.

Pyroelectric superlattices based on copolysiloxane/calix[8]arene alternate layer LB films

Abstract The pyroelectric effect in alternate layer LB superlattices incorporating interacting lamellae of carboxyl and amino moieties has been investigated. Three distinct systems have been studied, namely (a) a copolysiloxane carboxylic acid/aliphatic amine alternate multilayer, (b) a calix[8]arene carboxylic acid/calix[8]arene amine alternate multilayer, and (c) a hybrid system consisting of a copolysiloxane carboxylic acid/calix[8]arene amine multilayer. In all superlattices there occurs proton transfer between acid and amine moieties leading to a temperature-dependent electric polarisation which is reinforced by added contributions from dipolar reorientation effects. This paper describes how the pyroelectric coefficient can be enhanced by utilising a hybrid alternate layer LB film superlattice containing a 50:50 copolysiloxane substituted with polar aromatic side-chains terminated with carboxyl groups, which is co-deposited with a calix[8]arene molecular basket substituted with primary amine groups. The pyroelectric activity of the copolysiloxane acid/calix[8]arene amine system is substantially higher and stable over a wider temperature range than either of the other two superlattices, giving a pyroelectric coefficient of 10.2 μC m −2 K −1 at 25°C.

Cd2+ and Mn2+ incorporation in copolysiloxane/eicosylamine alternate layer LB films : influence on pyroelectric activity

Abstract The pyroelectrical activity in copolysiloxane/eicosylamine alternate layer Langmuir–Blodgett (LB) films incorporating Cd2+ and Mn2+ ions is reported. The pyroelectric coefficients and dielectric constants have been measured in order to determine the figure of merit in the context of thermal imaging materials. The pyroelectric results indicate that cadmium ion incorporation leads to a significantly improved performance compared to the system without metal ions. Additionally, preparation of LB films using a metal-doped subphase leads to an extended operating temperature range for the fabricated pyroelectric devices.

Swelling behavior of the 2-(4-methoxyphenylamino)-2-oxoethyl methacrylate monomer LB thin film exposed to various organic vapors by quartz crystal microbalance technique

Abstract In this work FT-IR, 1H and 13C-NMR spectra are used to analyze the structure of a novel 2-(4-methoxyphenylamino)-2-oxoethyl methacrylate (MPAEMA) materials. MPAEMA LB thin films are characterized by UV-Vis spectroscopy and Quartz Crystal Microbalance (QCM) techniques. The sensing behaviors of the films were investigated with respect to volatile organic compounds (VOCs) at room temperature. The sensing responses of the films against VOCs (dichloromethane, chloroform, benzene and toluene) were measured by the QCM method. It was found that the MPAEMA films exhibited good response, reversibility, stability and faster response and recovery characteristic to VOCs. The changes in resonance frequency associated with mass changes can be attributed to the swelling behavior of monomeric thin films during vapor absorption. This swelling is due to the capturing of organic vapor molecules in the sensor environment. Ficks law for early-time diffusion was adopted to quantify real time QCM data for the swelling processes. It was observed that diffusion coefficients ( ) for swelling obeyed the law and could be correlated with the VOCs used. The response of MPAEMA films to the chosen VOCs has been investigated in conditions of physical properties of the solvents, and the films were obtain to be largely sensitive to dichloromethane vapor compared to other studied vapors.

Pyroelectric figure of merit for a heterogeneous alternate layer Langmuir-Blodgett film system

Abstract The pyroelectric effect arises as a result of the change of the polarisation with temperature. Pyroelectric materials, which must have a non-centrosymmetric structure, exhibit an inherent electrical polarisation that changes when their temperature is changed. A heterogeneous monolayer system containing linear copolysiloxanes has been shown to produce a non-centrosymmetric structure in this work. Alternate layer Langmuir–Blodgett (LB) films have been fabricated using LB film deposition technique. The pyroelectric and dielectric properties of these alternate layer LB film assemblies have been measured and used to determine the pyroelectric figure of merit in the context of thermal imaging devices.

A theoretical model for the pyroelectric response in Langmuir-Blodgett films

Abstract Understanding the structure of pyroelectric materials is important to elucidate the nature of the temperature-dependent microscopic and macroscopic electric polarisation. Pyroelectric materials must have a non-centrosymmetric structure. Previously, researchers [C.A. Jones, PhD thesis, University of Durham, (1987); J. Mater. Chem. 1 (1991) 819; Langmuir 11 (1995) 4623] have reported the pyroelectric effect of non-centrosymmetric ultrathin LB films prepared using the alternate layer Langmuir–Blodgett (LB) deposition technique. They have proposed three main mechanisms to explain the origin of the pyroelectric activity, namely, tilting, proton transfer, and ionic processes. Linear and cyclic polysiloxane materials with aliphatic and aromatic side groups have been studied in this work. These materials have been alternately deposited with eicosylamine to form the active pyroelectric material in metal–LB film–metal (MIM) devices, whose pyroelectric coefficients have been measured using a quasi-static measurement technique [W.H. Majid, Abd., PhD thesis, Univ. of Sheffield (1994); Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 3 (1995) 197; Thin Solid Films 327–329 (1998) 369]. The relation between pyroelectric mechanism(s) and microscopic and macroscopic pyroelectric response for polysiloxane/eicosylamine alternate layer LB films will be explained in this work. Results indicate that the physical mechanism by which the pyroelectric activity arises in the LB films is critically dependent upon their structural quality, the ions within the multilayer arrangement and the molecular dipole moments of the molecules.