Thin liquid film quartz crystal

Quartz crystals have a characteristic oscillation frequency which varies according to their mass. Although crystal wafers have been used as mass sensors in vacuum and gas-phase experiments for many years, it is only recently that they have been employed in contact with liquids or solutions. Quartz crystal wafers can be used as electrodes by depositing a thin film of electrode material on the exposed surface, and interfacial mass changes can then be monitored. It is then known as the electrochemical QCM or EQCM. It is a direct, but non-selective, probe of mass transport. 

Fig.l Schematic presentation of the quartz crystal resonator in contact with a liquid. The contacting medium is a thin film rigidly attached to the crystal surface from one side, at z = d. The opposite surface of the crystal (z = 0) is unconstrained, d is the thickness of the quartz crystal... 

Piezoelectric quartz crystal oscillators function on the basis of the well-established relationship (Sauerbery equation) between the oscillation frequency of a quartz crystal and the mass of a thin film deposited on its surface [501]. QCM has been extensively used in measurements in vacuo and in the gas phase, which includes the studies on gas phase silylation for oxygen RIE development [443] (see 6.2) and on resist outgassing [439,502]. The QCM technique has been extended to measurements in the liquid phase including aqueous media and has found powerful utility in studies of dissolution kinetics of phenolic and other acidic resists in aqueous base [503]. 

The viscosity coefficients may also be determined by studying the reflexion of ultrasonic shear waves at a solid-nematic interface. The technique was developed by Martinoty and Candau. A thin film of a nematic liquid crystal is taken on the surface of a fused quartz rod with obliquely cut ends (fig. 3.7.1). A quartz crystal bonded to one of the ends generates a transverse wave. At the solid-nematic interface there is a transmitted wave, which is rapidly attenuated, and a reflected wave which is received at the other end by a second quartz crystal. The reflexion coefficient, obtained by measuring the amplitudes of reflexion with and without the nematic sample, directly yields the effective coefficient of viscosity. 

Viscoelastic loading The impact of viscoelastic materials on the electromechanical properties of a TSM resonator can be best described by the acoustic load concept. Adding a viscoelastic film with a sufficiently small shear modulus to the quartz crystal results in an additional frequency shift as expected for a thin rigid layer if the environment is air, or a decrease in frequency shift as expected from eqn [1] if the layer is adsorbed in liquid. The so-called extra mass effect occurring in air can be expressed by a plus sign in eqn [5] ... 

The quartz crystal microbalance (QCM) consists of a quartz crystal that is electrically driven into oscillation. The resonance frequency of the crystal is monitored. This frequency is highly dependent on any mass added to the crystal surface. Hence the mass dependence of the QCM resonance frequency can be, in air, used to weigh minute amounts of material with a sensitivity of the order of 1 ng/cm. QCM can also be coupled with electrochemistry here, the quartz crystal surface is coated with an appropriate electrode material, for example, thin film gold. This electrochemical QCM (EQCM) configuration can be used to monitor electrochemically triggered surface processes associated with the deposition (or loss) of material at the working electrode surface. However, in liquid medium the frequency shift of the QCM crystal is not solely sensitive to added mass but is also influenced by changes in the local property of the medium associated with the surface electrochemical process of interest. For example, density or viscosity variation of the medium in the electrode vicinity, in addition to variation in the viscoelastic properties of the deposited layer, can cause shifts in the resonant frequency of QCM. 

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