The quartz crystal microbalance

The quartz balance uses a thin quartz crystal, a few hundred /xm thick, with thin, vapor-deposited gold films on the two sides. Such a crystal has a fundamental mode for shear waves with a frequency in the 1-15 MHz region, which can be excited by application of a corresponding alternating voltage on the two electrodes. The resonance frequency is very sensitive to small mass changes of the system. One 

The redox electrochemistry of thin polymer films is a particularly useful field of application for the quartz microbalance. As an example, we review experiments on poly(xylylviologen) films [15]. The viologen groups can be reversibly reduced in two discrete one-electron steps. 

While the mass is not a specific quantity for a chemical species, the quartz microbalance is a useful device in cases where the chemical identity of the participating species is known, or where there are only a few candidates, which can be distinguished by their molecular masses. 

Nichols, and R. J. Behm, in Electrified Interfaces in Physics, Chemistry and Biology, NATO ASI Series C, ed. by R. Guidelli, Kluwer Academic Publishers, Dordrecht, 1992. 

Porter, in Electrochemical Interfaces, ed. by H. D. Abruna, VCH, New York, 1991 (b) A. Bewick, in Trends in Interfacial Electrochemistry, ed. by A. F. Silva, NATO ASI Series,D. Reidel Publishing Company, Dordrecht, 1986 (c) B. Beden and C. Lamy, in Spectroelectrochem-istry, Theory and Practice, ed. by R. J. Gale, Plenum Press, New York, 1988 (d) A. Bewick and B. S. Pons, in Advances in Infrared 

---

FIGURE 2-20 The quartz crystal microbalance a, the quartz crystal b, the gold electrode c... 

Buttry, D. A., The quartz crystal microbalance as an in situ tool in electrochemistry, in H. D. Abmna, Ed., Electrochemical Interfaces, VCH, Weinheim, Germany, 1991, p. 529. 

The quartz crystal microbalance has a long history of application as a means of determining film thickness in vacuum deposition techniques and more recently as a means of detecting trace constituents in the gas phase. In essence, it is an extremely sensitive sensor capable of measuring mass changes in the nanogram range. 

The first application of the quartz crystal microbalance in electrochemistry came with the work of Bruckenstein and Shay (1985) who proved that the Sauerbrey equation could still be applied to a quartz wafer one side of which was covered with electrolyte. Although they were able to establish that an electrolyte layer several hundred angstroms thick moved essentially with the quartz surface, they also showed that the thickness of this layer remained constant with potential so any change in frequency could be attributed to surface film formation. The authors showed that it was possible to take simultaneous measurements of the in situ frequency change accompanying electrolysis at a working electrode (comprising one of the electrical contacts to the crystal) as a function of the applied potential or current. They coined the acronym EQCM (electrochemical quartz crystal microbalance) for the technique. 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

The photoablation behaviour of a number of polymers has been described with the aid of the moving interface model. The kinetics of ablation is characterized by the rate constant k and a laser beam attenuation by the desorbing products is quantified by the screening coefficient 6. The polymer structure strongly influences the ablation parameters and some general trends are inferred. The deposition rates and yields of the ablation products can also be precisely measured with the quartz crystal microbalance. The yields usually depend on fluence, wavelength, polymer structure and background pressure. 

Vapour and Gas Measurements in Vacuum with the Quartz Crystal Microbalance... 

Variations on the vertical dipping technique have been utilized to construct films containing divalent metal ions. For example, the quartz crystal microbalance (QCM) has been used to evaluate the horizontal lifting method of CdSt LB Film construction (26). In this method, the QCM quartz plate was touched to monolayers compressed on a subphase and lifted horizontally. Y-type transfer (transfer ratio of 1) was demonstrated with two centrosymmetric monolayers deposited for each cycle. A combination of the vertical and horizontal dipping techniques has been utilized to prepare multilayer films from an amphiphilic porphyrin compound (27). 

The quartz crystal microbalance (QCM) is a piezoelectric device consisting of a thin (e.g.) quartz wafer sandwiched between two electrodes. A potential applied across the electrodes results in an oscillation of the quartz. The frequency of the oscillation, which can be measured accurately, is sensitive to mass loading. The relationship between frequency and mass loading is described by the Sauerbray equation ... 

Tzou TZ. Aerodynamic particle size of metered-dose inhalers determined by the quartz crystal microbalance and the Andersen cascade impactor. Int J Phann 1999 186(1) 71—79. 

A related, but less sensitive technology, uses the quartz crystal microbalance, in which the crystal has dimensions of millimeters. When analyte binds to a chemically sensitive layer on a gold coating on the surface of quartz, the oscillating frequency of the crystal is altered. See, for example, B. Zuo, S. Li, Z. Guo,... 

Figure 6.3 (a) Schematic illustration of the Quartz Crystal Microbalance (QCM), and... 

More recently methods have also been developed to measure the adsorbed amount on single surfaces and not onto powders. Adsorption to isolated surfaces can, for instance, be measured with a quartz crystal microbalance (QCM) [383]. The quartz crystal microbalance consists of a thin quartz crystal that is plated with electrodes on the top and bottom (Fig. 9.11). Since quartz is a piezoelectric material, the crystal can be deformed by an external voltage. By applying an AC voltage across the electrodes, the crystal can be excited to oscillate in a transverse shear mode at its resonance frequency. This resonance frequency is highly sensitive to the total oscillating mass. For an adsorption measurement, the surface is mounted on such a quartz crystal microbalance. Upon adsorption, the mass increases, which lowers the resonance frequency. This reduction of the resonance frequency is measured and the mass increase is calculated [384-387],... 

Another tool used to study friction on the molecular scale is the quartz crystal microbalance (QCM) introduced in Section 9.4.1. The QCM has been used to monitor the adsorption of thin films on surfaces via the induced frequency shift [385], In the years since 1986, Krim and coworkers could show that the slippage of adsorbed layers on the QCM leads to a damping of the oscillator [472], This damping is reflected as a decrease in the quality factor Q of the oscillator. From the change in Q, a characteristic time constant rs, the so-called slip-time, can be derived. This corresponds to the time for the moving object s speed to fall to 1 /e, i.e. 

Exchange of species between a solution and a polymer film is an established means of probing solution composition. The quartz crystal microbalance can monitor such exchange processes with high sensitivity. When combined with selectivity via electrochemical control and appropriate choice of polymer, the EQCM becomes an attractive sensor. In order that the potential advantages of the EQCM can be realised, certain criteria must be met. 

First, the underlying principles upon which bulk acoustic wave (BAW) devices operate are described. When a voltage is applied to a piezoelectric crystal, several fundamental wave modes are obtained, namely, longitudinal, lateral and torsional, as well as various harmonics. Depending on the way in which the crystal is cut, one of these principal modes will predominate. In practice, the high-frequency thickness shear mode is often chosen since it is the most sensitive to mass changes. Figure 3.4 schematically illustrates the structure of a bulk acoustic wave device, i.e. the quartz crystal microbalance. 

Fig. 6.10 The principle of the quartz crystal microbalance gold surface modification (left) and analyte detection by frequency modulation (right)... 

Measurement of mass change the quartz crystal microbalance (QCM)5 59... 

Fig. 12.19. Design of the quartz crystal microbalance for electrochemistry, showing the applied electric field and external contact.

---