Metal film quartz crystal microbalance

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

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). 

This detector is based on the collective oscillations of the free electron plasma at a metal surface. Typically a prism is coated with a metal film and the film coated with a chemically selective layer. The surface is illuminated by a laser and the amount of material adsorbed by the coating affects the angle of the deflected beam. This platform is theoretically similar in sensitivity to a quartz crystal microbalance. This is another platform whose selectivity is based on the coating. The typical coating is using bound antibodies thus, this device becomes a platform for immuno-sensors (12). 

Quartz crystal microbalance is operated in typical resonant frequencies ranging from 1 to 10 MHz, with most of them operating in 5-10 MHz. A typical QCM is a disk in the size of 10-16 mm in diameter with a thickness of approximately 0.15 mm. A thin metal film, gold, aluminum, or others, is deposited onto the surface of the quartz serving as electrodes. The metal... 

In the gravimetric method, the adsorbent (usually in the form of powder) is placed into a bulb, which is mounted on a sensitive balance and the bulb is then evacuated. Next, the weight increase of the adsorbent solid as a function of the absorptive gas pressure is monitored at constant temperature. More recently, the quartz crystal microbalance (QCM) technique has been applied this is very sensitive to mass increases. Quartz is a piezoelectric material and the thin crystal can be excited to oscillate in a traverse shear mode at its resonance frequency when a.c. voltage is applied across the metal (usually gold) electrodes, which are layered on two faces of the crystal. When the mass on the crystal increases upon adsorption, its resonance frequency decreases. The increase in the mass is calculated from the reduction in resonance frequency. On the other hand, adsorption on single flat surfaces can also be measured by ellipsometry, which measures the film thickness of transparent films optically using the difference between light reflection from bare and adsorbed surfaces. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

The high sensitivity, excellent mechanical stability and the controllable thermal influences make the quartz-crystal microbalance a valuable tool. Determinations of the physical film thickness, however, may be more of a problem since the density pf of a thin film, depending on deposition method, chosen parameters and film thickness, is generally different from the density of the bulk material being considered. At film thicknesses below 100 nm, the density discrepancy is greater than with thicker films. This seems to be valid for metals as well as for dielectrics. It is, however, usually possible to reproduce the film density. For the deposition process, this demonstrates that the chosen vacuum and deposition parameters must be carefully controlled and maintained exactly to always attain the same film density. In its manner and after calibration, the geometrical film thickness can be exactly reproduced by quartz-crystal monitoring. 

QCMs have been used as film-thickness monitors in vacuum deposition of metals and inorganic solids since the 1970s. The monograph by Lu and Czandema [35], while over 20 years old, is still a very good summary of early applications of the quartz crystal microbalance in physics and engineering, as well as applications as thickness monitors in the vacuum deposition industry. Well before the fiill... 

The first subdiscipline of chemistry in which the QCM was widely applied was electrochemistry. In 1992 Buttry and Ward published a review entitled Measurement of interfacial processes at electrode surfaces with the electrochemical quartz crystal microbalance , with 133 references [8]. This is the most widely cited paper on quartz crystal microbalances. After presenting the basic principles of AT-cut quartz resonators, the authors discuss the experimental aspects and relation of electrochemical parameters to QCM frequency changes. In their review of the investigation of thin films, they discuss electrodeposition of metals, dissolution of metal films, electrovalency measurements of anion adsorption, hydrogen absorption in metal films, bubble formation, and self-assembled monolayers. The review concludes with a brief section on redox and conducting polymer films. 

Quartz Crystal Microbalance. This experiment depends on the manner in which the resonant frequency of an oscillating quartz crystal piezoelectric element responds to a change in mass on the Interface perpendicular to the oscillatory motion. Typically the oscillation is excited between facing metal film electrodes on opposite side of the crystal as shown in Figure 2(top). A change in mass at one of the Interfaces produces a proportional oscillatory frequency decrease. 

Almost any metal electrode may be applied. One must take care not to operate too close to the limits of the electrochemical window of the used combination of electrode and electrolyte solution in order to avoid catalytic or undesired side effects such as gas evolution reactions. When using thin film electrodes, such as in combined quartz crystal microbalance (QCMB) studies, extreme care must be taken not to scratch the metal surface, usually gold, in order to maintain an electronically conductive path within the electrode. Examples of such experiments are given in the literature [4-7]. Also, when using metal electrodes in aqueous solutions, the background voltammogram should be examined very closely as the formation of surface oxides or the like may be mistaken for signals of the solid under study. 

As discussed, the electropolymerized PEDOT-PSS (o- = 80 S cm , 1 s/t ratio = 0.68) has a completely different composition than the chemically polymerized PEDOT-PSS (Electrochemical quartz crystal microbalance (EQCM) analyses have shown that the composition of the electropolymerized PEDOT-PSS is not dependent on the anion concentration [135]. This indicates that the mechanism of synthesis of the polymer strongly influences its composition. During electropolymerization, the first formed (and doped) oligomers are very close to the metal electrode as charge transfer occurs at a tunnel distance. Consequently, those doped oligomers interact electrostatically with the closest sulfonate anions of a PSS chain at the vicinity of the electrode. This mechanism leads to a high concentration of PEDOT in the PEDOT-PSS film, independent on the anion concentration. 

Metal island films of controlled mass thickness are deposited in a commercial vacuum system evaporator using a glow discharge evaporation unit. The metal island films are in general fabricated onto preheated glass substrates. The film thickness is monitored using a commercial quartz crystal microbalance. 

Electronic noses The so-called electronic noses consist of chemical gas sensors that are able to monitor changes in the offgas composition of fermentation processes. The different sensors of electronic noses are based on conductive polymers (CP), metal oxide semiconductors (MOS), metal oxide semiconductor field effect transistors (MOSFET), or quartz crystal microbalance (QCM). CP-based sensors use the electrochemical properties of polymers like polypyrrole or polyindole. The absorbance of selected molecules of the off-gas into the polymer film causes changes in the sensors conductivity. MOS sensors possess an electrochemically active surface of metal oxides like tin oxide or copper oxide. The sensitivity... 

Polymerization of pyrrole has also been carried out chemically by mixing the monomer with a homogeneous oxidant (Fe ) in solution. Mermillod et al [61] found that poly(pyrrole) synthesized in water by action of Fe ( 104)3 produces particles in solution as well as films on the reactor walls. The material was electrochemically identical with electropolymerized product. Gregory et al [62] chemically deposited very uniform films of poly(pyrrole) on textile fibres and woven glass cloth. Gottesfeld et al [63] found that chemically deposited films were uniform and could serve as a conducting substrate for metallization structures for microelectronic circuitry. Hillman et al [64] studied the electrodeposition of poly(vinylfer-rocene) with the quartz crystal microbalance. 

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