Crystal, structure microbalance

Lincot D, Ortega-Borges R (1992) Chemical bath deposition of cadmium sulfide thin films. In situ growth and structural studies by Combined Quartz Crystal Microbalance and Electrochemical Impedance techniques. J Electrochem Soc 139 1880-1889... 

Given the efforts in this group and others (Table 1) to form the Cd based II-VI compounds, studies of the formation of Cd atomic layers are of great interest. The most detailed structural studies of Cd UPD have, thus far, been published by Gewirth et al. [270-272]. They have obtained in-situ STM images of uniaxial structures formed during the UPD of Cd on Au(lll), from 0.1 M sulfuric acid solutions. They have also performed extensive chronocoulometric and quartz crystal microbalance (QCM) studies of Cd UPD from sulfate. They have concluded that the structures observed with STM were the result of interactions between deposited Cd and the sulfate electrolyte. However, they do not rule out a contribution from surface reconstructions in accounting for the observed structures. 

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. 

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. 

As discussed above in Chapter 3, ellipsometry and quartz crystal microbalance (QCM) approaches provide a useful insight into the adsorption of both the supporting interfacial assembly and the proteins themselves. Beyond monitoring the adsorption dynamics and the structural integrity of the biomolecule, the orientation of the active site is of prime importance. For example, if the active site itself binds to the self-assembled monolayer, transport of the substrate or co-enzyme may be blocked. 

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

The strong dependence of the layer structure on the nature of the contacting electrolyte has been further investigated by using the electrochemical quartz crystal microbalance (EQCM). As discussed above in Chapter 3, this technique is based on the measurement of the frequency with which a coated quartz crystal vibrates, and this frequency can then be related to the mass of this crystal provided that the material attached to the surface is rigid. In this way, the changes that occur in thin films as a result of redox processes can be monitored. 

The techniques and difficulties involved in the preparation and characterization of single crystal metal surfaces have been considered here in detail because the evaluation of chemical activity in terms of surface structure, particularly of the crystallographic surface structure, is one of the most promising applications of the vacuum microbalance. The preparation of flat, clean, undistorted single crystal samples suitable for surface studies is a difficult and tedious assignment. 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

Liebau M, Hildebrand A, Neubert RHH (2001) Bioadhesion of supramolecular structures at supported planar bilayers as studied by the quartz crystal microbalance. Eur Biophys J Biophys Lett 30 42-52... 

The electrochemical reduction of Ti02 is known to be accompanied by the intercalation of small cations. This finding has been explored in sensitizing anatase films for battery applications [149]. Cation coordination to titanium alkoxide sol-gel precursors is also well known [150]. Lyon and Hupp used quartz crystal microbalance techniques to determine the mass of intercalating cations as the TiOa film is reduced [151]. Hagfeldt and co-workers have studied Li+ and Na intercalation into anatase Ti02 both theoretically and experimentally [152, 153). They found that the diffusion constants for Li and Na+ are temperature dependent with an activation barrier of 0.4 eV for insertion and 0.5 eV for extraction. The Li+ diffusion coefficient at 25 °C into the nanoporous structure was approximately 2 X 10 cm s for insertion and 4 x 10 cm s for extraction. 

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