Microdielectric Measurements of Pristine and Modified Thin Fullerene (C 60 ) Films
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ABSTRACT As developed by Senturia and co-workers [1], microdielectrometry is a technique for measuring complex permittivity utilizing microfabrication technology to incorporate both the interdigital sensing electrodes and associated circuitry on the same microchip. By covering a microdielectrometer chip with a thin layer of C 6 0 , it is possible to dynamically monitor the film's frequency response and dielectric properties as a function of doping with selected gases and other species. It is known that solid C 60 has a substantial amount of interstitial volume. The presence of mobile ions in these spaces compromises the breakdown voltage and makes pure C 60 unsuitable for applications requiring high-quality dielectric films. However, various immobile ions or neutral species (e.g. oxygen) can be made to fill the interstitial volume, changing the characteristics of the C6 0 films and, in some cases, improving the dielectric properties. In-situ microdielectric measurements of pristine and modified C 60 films were performed for frequencies ranging from 0.005 Hz to 100 kHz. Based on the low-frequency behavior of the dielectric constant, a model is proposed for the mechanism of oxygen diffusion into the interstitial spaces of the fullerene material.
INTRODUCTION Since the discovery by Kriitschmer et al. [2J of the method for bulk synthesis of fullerene C6 0 , the complex dielectric function e(w) = 6 1 (W) + i6 2 (w) of solid C6 0 films has been studied over a broad frequency range using a wide variety of techniques [3, 4, 5, 6, 7, 8]. Figure 1 presents a summary of experimental data reported for the real Ei(w) and imaginary c2 (w) parts of the dielectric function at 300 K over the entire optical frequency range from the infrared through the ultraviolet. In Fig. 1, the features in the optical dielectric function below -,0.3 eV are due to molecular vibrations, and can be associated with the four strong first-order F1 , intramolecular modes (at 526, 576, 1183, and 1428 cm-') and two strong combination (wi + W2) modes (at 1539 cm- 1 and 2328 cm-') first observed by Kriitschmer et al. [2] from measurements of the infrared absorption spectrum of a C 60 film. Transmission experiments carried out on thicker (•,-2-3 pm) films reveal numerous (-,100) additional features attributed to other intramolecular combination modes [9], which also enter into e(w). The IR peaks in Fig. 1 have been successfully fit [10] to the standard Drude-Lorentz model [11], in which the complex dielectric function 0w) is given by the sum of contributions from a background core real constant c,, a Drude (or free carrier) term EDOa(w) and a phonon term ph .n...(w) [10]. As the absorption edge of C6 0 solid has been determined to be "-'1.5 eV [12], the structure in e(w) in the visible-UV range below -- 7 eV (see Fig. 1) is primarily of electronic origin, and may be assigned to electronic transitions between carbon 7r (bonding) and lr* (antibonding) electron states. The data in this range were obtained by Ren et aL [5] for films vacuum deposited on Si(100) u
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