EQCM and Quartz Crystal Impedance Measurements for the Characterization of Thiophene-Based Conducting Polymers
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acoustic wavelength is twice the crystal thickness. The deposition of a mass layer on one quartz crystal surface can be described as a change in thickness of the material in which the acoustic wave propagates and results in a change in the resonant frequency of the quartz crystal. If a change in thickness of the mass layer can be considered as a change in thickness of the quartz crystal, as in the case of thin films rigidly coupled to the quartz crystal, the Sauerbrey equation relates the mass layer change, Am, to the quartz crystal resonant frequency change, Af, as follows: f= - 202 Am /[A (pqpq)112]
(1)
where f0 is the resonant frequency of the quartz crystal prior to the mass change and A, pq and Pq are the piezoelectrically active area, the shear modulus, and the density of the quartz,
respectively. In interpreting EQCM data for polymer layers, it becomes vital to determine whether or not the films are rigidly coupled to the quartz crystal. As a diagnostic of the rigidity of the polymer film, we performed quartz crystal impedance measurements with a Frequency
Response Analyzer (FRA), and we carried out admittance analyses of the polymer coated quartz crystal referring to the models proposed for viscoelastic films. The FRA is generally used to
characterize materials by electrochemical impedance studies in the region from lmHz to 197
Mat. Res. Soc. Symp. Proc. Vol. 600 © 2000 Materials Research Society
100kHz. We used the FRA equipment for a completely different purpose, and we investigated the quartz crystal impedance in the resonant frequency region, i.e. 9MHz, to verify the rigid film condition. In this paper quartz crystal impedance measurements performed for different thickness of polydithieno[3,4-b:3',4'-d]thiophene (pDTT1) [2,3] films are presented and discussed. EQCM data collected during pDTT1 p-doping are also reported. EXPERIMENT The quartz crystals were 9 MHz AT-cut (SEIKO), coated with Pt electrodes of active area 0.196 cm 2 . The crystal was clamped in the electrochemical cell by means of O-rings and with only one face in contact with the electrolyte. EQCM data were collected with a SEIKO EG&G QCA917 quartz crystal analyzer. Quartz crystal impedance measurements were carried out with a PC interfaced Solartron 1255 Frequency Response Analyzer (FRA) with 10 mV AC voltage, in the 9MHz region. The polymer pDTT1 was electrosynthesized in galvanostatic conditions at 1 mAm"2 at room temperature in acetonitrile (ACN) - 0.1 M tetraetylammonium tetrafluoborate (Et4NBF 4) 0.015 M DTT1. Polymer p-doping and undoping was performed by cyclic voltammetry (CV) with a EG&G PAR M270A potentiostat/galvanostat in propylene carbonate (PC) - 0.2 M Et4NBF 4. An Ag electrode (-200 mV vs. saturated calomel electrode) was used as quasi reference electrode. All chemicals were reagent-grade products, dried and purified before use. RESULTS AND DISCUSSION For each frequency swept the FRA gives the real and imaginary components of the impedance of the system under study (ZA). The impedance of the quartz crystal perturbed
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