Molecular recognition on acoustic wave devices

Microporous thin films composed either of zeolite crystals embedded in sol-gel derived glass or of a molecular coupling layer, zeolite crystals and a porous silica overlayer, were formed on the gold electrodes of Quartz Crystal Microbalances (QCM). The microporosity of the thin films was characterized by in situ nitrogen and vapor sorption isotherms. Both preparation methods result in thin films with substantial microporosity. Selective adsorption based on molecular size exclusion from the microporous films could be achieved

Molecular recognition on acoustic wave devices

Microporous thin films composed of a molecular coupling layer, zeolite crystals, and a porous silica overlayer, were formed on the gold electrodes of quartz crystal microbalances (QCMs). The silica overlayer enhances the mechanical stability of the zeolite films, and results in additional surface area and porosity as characterized by the sorption isotherms and transient sorption of vapors with different molecular diameters and different polarities. The protecting silica glass layer is gas permeable such that the regular zeolite micropores with molecular sieving capability are still accessible in the composite film. A novel surface tailoring technique for the microporous thin films was developed, in which organosilane molecules were chemisorbed on the silica overlayer via siloxane linkages, forming a molecular "gate" at the gas thin film interface. The adsorption of vapors into the microporous zeolite films is therefore controlled by the permeability of the gate layer. Selective adsorption based on kinetic or equilibrium exclusion from the microporous films could be achieved, as demonstrated by discrimination of molecules with similar polarity but different molecular diameters (water vs. ethanol), and effective exclusion of larger molecules such as rt-hexane. As a result of the increase in the vapor sorption selectivity and reduction of the external surface area of the thin films, the modified QCMs show high selectivity towards water over other molecules. Keywords: acoustic wave device; sensor; zeolite film; organosilane coating; humidity sensin

Influence of viscoelasticity and interfacial slip on acoustic wave sensors

Acoustic wave devices with shear horizontal displacements, such as quartz crystal microbalances (QCM) and shear horizontally polarised surface acoustic wave (SH-SAW) devices provide sensitive probes of changes at solid-solid and solid- liquid interfaces. Increasingly the surfaces of acoustic wave devices are being chemically or physically modified to alter surface adhesion or coated with one or more layers to amplify their response to any change of mass or material properties. In this work, we describe a model that provides a unified view of the modification in the shear motion in acoustic wave systems by multiple finite thickness loadings of viscoelastic fluids. This model encompasses QCM and other classes of acoustic wave devices based on a shear motion of the substrate surface and is also valid whether the coating film has a liquid or solid character. As a specific example, the transition of a coating from liquid to solid is modelled using a single relaxation time Maxwell model. The correspondence between parameters from this physical model and parameters from alternative acoustic impedance models is given explicitly. The characteristic changes in QCM frequency and attenuation as a function of thickness are illustrated for a single layer device as the coating is varied from liquid-like to that of an amorphous solid. Results for a double layer structure are given explicitly and the extension of the physical model to multiple layers is described

Applications of Acoustic Wave Devices for Sensing in Liquid Environments

Acoustic wave devices such as thickness shear mode (TSM) resonators and shear horizontal surface acoustic wave (SH-SAW) devices can be utilized for characterizing physical properties of liquids and for chemical sensor applications. Basic device configurations are reviewed and the relationships between experimental observables (frequency shifts and attenuation) and physical properties of liquids are presented. Examples of physical property (density and viscosity) determination and also of chemical sensing are presented for a variety of liquid phase applications. Applications of TSMs and polymer-coated guided SH-SAWs for chemical sensing and uncoated SH-SAWs for “electronic tongue” applications are also discussed

Stroboscopic X-Ray topography of surface acoustic wave devices

Stroboscopic X-ray topography using the time structure of synchrotron radiation has been used to investigate 38 MHz lithium niobate, LiNB0(_3), surface acoustic wave devices. The methods of topography are discussed with particular reference to synchrotron radiation and stroboscopic techniques. Surface acoustic wave devices are described and relevant aspects of their performance are discussed. A detailed review is given of previous investigations of surface acoustic wave devices, including investigation of the contrast mechanism of the imaging of surface acoustic waves. Experimental evidence relating to the performance of surface acoustic wave devices and to the interaction of surface acoustic waves with defects such as grain boundaries is discussed; it is suggested that the use of ultrasonic wave propagation theory could allow grain boundaries to be considered as infinitely thin cracks. The effects of input voltage and incident X-ray wavelength are investigated. Computer programs are presented which simulate image positions for stroboscopic investigations of lithium niobate and Laue patterns for cubic and hexagonal crystals. An assessment is made of the perfection of crystals of lithium tetraborate, Li (_2) B (_4) O (_7) a possible future surface acoustic wave substrate material. Possibilities for future investigations are discussed

Molecular recognition on acoustic wave devices

Microporous thin films composed of a molecular coupling layer, zeolite crystals, and a porous silica overlayer, were formed on the gold electrodes of quartz crystal microbalances (QCMs). The silica overlayer enhances the mechanical stability of the zeolite films, and results in additional surface area and porosity as characterized by the sorption isotherms and transient sorption of vapors with different molecular diameters and different polarities. The protecting silica glass layer is gas permeable such that the regular zeolite micropores with molecular sieving capability are still accessible in the composite film. A novel surface tailoring technique for the microporous thin films was developed, in which organosilane molecules were chemisorbed on the silica overlayer via siloxane linkages, forming a molecular "gate" at the gas thin film interface. The adsorption of vapors into the microporous zeolite films is therefore controlled by the permeability of the gate layer. Selective adsorption based on kinetic or equilibrium exclusion from the microporous films could be achieved, as demonstrated by discrimination of molecules with similar polarity but different molecular diameters (water vs. ethanol), and effective exclusion of larger molecules such as rt-hexane. As a result of the increase in the vapor sorption selectivity and reduction of the external surface area of the thin films, the modified QCMs show high selectivity towards water over other molecules. Keywords: acoustic wave device; sensor; zeolite film; organosilane coating; humidity sensin

Fabrication of high temperature surface acoustic wave devices for sensor applications

Surface acoustic devices have been shown to be suitable not only for signal processing but also for sensor applications. In this paper high temperature surface acoustic wave devices based on gallium orthophosphate have been fabricated, using a lift-off technique and tested for high frequency applications at temperatures up to 600 ºC. The measured S-parameter (S11) has been used to study the mass loading effect of the platinum electrodes and turnover temperature of GaPO4 with a 5? cut. The analysis of these results shows that the mass loading effect can be used to predict the desired resonant frequency of the SAW devices. Also two different adhesion layers for Pt metallisation were studied. Our results show that Zirconium is a more suitable under layer than Titanium

Electron dynamics in surface acoustic wave devices

Gallium arsenide is piezoelectric, so it is possible to generate coupled mechanical and electrical surface acoustic waves (SAWs) by applying a high-frequency voltage to a transducer on the surface of GaAs. By combining SAWs with existing low-dimensional nanostructures one can create a series of dynamic quantum dots corresponding to the minima of the travelling electric wave, and each dot carries a single electron at the SAW velocity (∼ 2800 m/s). These devices may be of use in developing future quantum information processors, and also offer an ideal environment for probing the quantum mechanical behaviour of single electrons. This thesis describes a numerical and theoretical study of the dynamics of an electron in a range of geometries. The numerical techniques for solving the time-dependent Schr ̈dinger equation with an arbitrary time-dependent potential will be described in Chapter 2, and then applied in Chapter 3 to calculate the transmission of an electron through an Aharonov-Bohm (AB) ring. It will be seen that an important property of the techniques used in this thesis is that they can be easily adapted to study realistic geometries, and we will see features in the AB oscillations which do not arise in simplified analytic descriptions. In Chapter 4, we will then study a device consisting of two parallel SAW channels separated by a controllable tunnelling barrier. We will use numerical simulations to investigate the effect of electric and magnetic fields upon the electron dynamics, and develop an analytic model to explain the simulation results. From the model, it will be apparent that it is possible to use this device to rotate the state of the electron to an arbitrary superposition of the first two eigenstates. We then introduce coherent and squeezed states in Chapter 5, which are ex- cited states of the quantum harmonic oscillator. Coherent and squeezed electronic states may be of use in quantum information processing, and could also arise due to unwanted perturbations in a SAW device. We will discuss how these states can be controllably generated in a SAW device, and also discuss how they could then be detected. In Chapter 6 we describe how to use the motion of a SAW to create a rapidly- changing potential in the frame of the electron, leading to a nonadiabatic excita- tion. The nonadiabatically-excited state oscillates from side to side within a 1D channel on a few-picosecond timescale, and this motion can be probed by placing a tunnelling barrier at one side of the channel. Numerical simulations will be performed to show how this motion can be controlled, and the simulation results will be seen to be in good agreement with recent experimental work performed by colleagues. Finally, we will show that this device can be used to measure the initial state of an electron which is an arbitrary superposition of the first two eigenstates.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC

Signal Processing with Surface Acoustic Wave Devices

Our interest is in analog signal processing as it might be applied to NDE, carrying out some sophisticated signal processing using an inexpensive, real-time analog system based on the surface acoustic- wave technology, or perhaps the CCD technology. To date we have considered an inverse filter for correcting for the frequency response of an NDT transducer: we have shown the feasibility of .making such a device and will show some experimental results here. We are making and testing some filters that have been designed to go with a commercial NDT transducer that we have purchased. We are also looking at the possibility of realizing the same sort of filter with a charge-coupled device. In the next year we should like to look at a more complicated analog signal processor - the convolver or correlator - to see how it can be realized and applied to defect signature recognition. At the end of my talk I\u27d like just to mention some work being done by another faculty member at Berkeley on an interesting receiving transducer which I don\u27t think has come to the attention of this particular audience