Surface acoustic mode

Figure 2. Schemes for using piezoelectric quartz crystals. A. Quartz crystal microbalance configuration, standing shear wave between facing Au electrode contacts B. Surface acoustical mode configuration, surface undulation caused by bias between metal fingers C. Horizontal shear plate mode.

The first mention of surface phonons is due to Lord Rayleigh (1885), who predicted the existence of a surface acoustic mode with a sound velocity lower than in the bulk. He proved this result, using elasticity theory, by representing the semi-infinite sofid by a continuous and isotropic medium (Landau and Lifehitz, 1967). Considering an infinitesimal volume element, he wrote a Fourier component of its displacement u q, co), in the following form ... 

In addition to the acoustical modes and MSo, we observe in the first half of the Brillouin zone a weak optical mode MS7 at 19-20 me V. This particular mode has also been observed by Stroscio et with electron energy loss spectrocopy. According to Persson et the surface phonon density of states along the FX-direction is a region of depleted density of states, which they call pseudo band gap, inside which the resonance mode MS7 peals of. This behavior is explained in Fig. 16 (a) top view of a (110) surface (b) and (c) schematic plot of Ae structure of the layers in a plane normal to the (110) surface and containing the (110) and (100) directions, respectively. Along the (110) direction each bulk atom has six nearest neighbors in a lattice plane, while in the (100) direction it has only four. As exemplified in Fig. 17, where inelastic... 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Fig. 13.17 shows the structure and principle of a T-bumer, as used to measure the response function of propellants. Two propellant samples are placed at the respective ends of the T-burner. The burner is pressurized with nitrogen gas to the test pressure level. The acoustic mode of the burning established in the burner is uniquely determined by the speed of sound therein and the distance between the burning surfaces of the two samples. When the propellant samples are ignited, pressure waves travel from one end to the other between the burning surfaces of the samples. When a resonance pressure exists for a certain length of the T-bumer, the propellant is sensitive to the frequency. The response function is determined by the degree of amplification of the pressure level. 

Surface acoustic-wave (SAW) elements Plate-mode oscillators Interface impedance elements Fiber optic elements sensitive to elastic constants... 

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

Fig. 12.3. Mercury sensor based on surface acoustic waves (SAW) with shear-horizontal acoustic plate mode. This approach was tested in Ref. [8].

The addition of mass provides the means of transduction for many chemical sensors, including surface acoustic wave (SAW) devices, quartz crystal microbalances (QCM), and microcantilevers. In all these devices, the mass addition either perturbs the vibration, oscillations, or deflection within the transducer. The mode of transduction in an optical interferometer can also be linked to mass addition the sensor s response is altered by refractive index changes in the material being monitored. It is possible that this change can be elicited solely from refractive index changes without the addition of mass, although in sensing a particular... 

Devices based on piezoelectric crystals, which allow transduction between electrical and acoustic energies, have been constructed in a number of conrigurations for sensor applications and materials characterization. This cluqtter examines those devices most commonly utilized for sensing a( licatithickness-shear mode (TSM) resonator, the surface acoustic wave (SAW) device, the acoustic plate mode (APM) device, and the flexural plate wave (FPW) device. Each of these devices, shown schematically in Figure 3.1, uses a unique acoustic mode. 

Figure 3.1 Schematic sketches of the four types of acoustic sensors, (a) Thickness-Shear Mode (TSM) resonator (b) Surface-Acoustic-Wave (SAW) sensor, (c) Shear-Horizontal Acoustic-Plate-Mode (SH APM) sensor, and (d) Flexural-Plate-Wave (FPW) sensor.

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

For liquid-phase sensing, both density and viscosity, as well as the nature of the acoustic mode, play a role in AW perturbations. For TSM and SH-APM devices, in-plane motion of the substrate surface entrains a thin layer of liquid through viscous coupling. Entraiiunent of a liquid layer by the sensor surface constitutes a mass load proportional to the product of thickness and density of the coupled liquid layer, giving rise to a velocity change. 

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