Dielectric spectroscopy of nanoparticulate semiconductors in thin films

Abstract

Very little has been published on dielectric spectra, i.e., a9 (u), a0 (u) (dielectric permittivity and loss, respectively), of thin ®lms, aside from studies of neat polymeric ®lms. It also appears that little has been published on the sub-optical dielectric properties of nanoparticulates, particularly thin ®lms of such nanoparticles. Interestingly, Hamon's dielectric study [1] of relatively large (1 im) copper phthalocyanine particles in paraf®n wax is among the `classical' dielectric spectroscopy studies available in the literature, but such studies do not appear to have been followed up in the nanoparticulate semiconductor literature. Very thin ®lms of nanoparticulate semiconductors, such as antimony-doped tin oxide in this study, can be prepared by coating colloidal dispersions of such nanoparticulates dispersed in aqueous binders such as gelatin or other aqueous dispersible polymers. Such thin ®lms are useful as antistatic layers in photographic and packaging technology where electrostatic charge dissipation is warranted [2±5]. We present here the ®rst sub-optical dielectric loss spectra for thin ®lms of nanoparticulates. Using such tin oxide nanoparticulates dispersed with gelatin at several different tin oxide-to-gelatin weight ratios, we have discovered that these thin ®lm coatings exhibit pronounced peaks in the dielectric loss spectra. These spectra are correlated with surface electrical resistances, and a simple heterogeneous model is used to derive a proportionality relating the loss peak frequency and the thin ®lm conductivity. Infrared optical properties of nanoparticulate gold by Harris and co-workers [6, 7] have been related to high frequency conductivity, and these measurements have been re-analyzed in terms of conduction on fractal structures by Niklasson and Granqvist [8]. Fractal scaling analysis has also been applied to the analysis of conduction percolation in thin discontinuous Au ®lms based upon a.c. conductivity measurements over the region of 100 Hz to 10 MHz [9], and a.c. admittances over 0.01 Hz to 10 MHz have been reported by Morris [10] for discontinuous metal thin ®lms. Roy and co-workers [11] reported frequency-dependent resistivities (100 Hz±100 kHz) for sputtered Au-Eikonel (an aromatic polyester) ®lms and did not report any loss spectra. The nanoparticulate tin oxide used in this work was doped with antimony (8 mol %). This material was obtained from Ishihara Sangyo Kaisha Ltd. (Yokkaichi, Japan) as an aqueous dispersion at 30% (w=w) solids. Particle sizing was done by image analysis of transmission electron micrographs. Although clusters of particles were prevalent, the sizing was done by focusing on the primary particle size (diameters of individual particles in clusters). Thin ®lms were prepared by using gelatin as a binder and by coating aqueous gelatin and tin oxide colloidal dispersions on a 100 im thick polyethylene terephthalate (PET) support. Thin ®lms at tin oxideto-gelatin weight ratios of 65=35, 75=25 and 85=15 were prepared by combining the tin oxide colloidal dispersion and aqueous gelatin at these respective solid component ratios and then coating this mixture at 16.1 ml my2 on the PET support. The ®lms were dried immediately after coating. Surface electrical resistance (SER) in units of U squarey1 was measured after equilibrating the thin ®lms for 24 h at 5% relative humidity (RH). These SER were obtained by measuring the direct current between two parallel stainless-steel electrodes (25.4 mm long and spaced 6.35 mm apart) held at a potential difference of 200 V, similar to a method described previously [5]. Dielectric spectroscopy was done using an automated Novocontrol system comprising a Solartron± Schlumberger SI 1260 frequency response analyzer and a Chelsea high-impedance pre-ampli®er of variable gain. Heated nitrogen was used to maintain a constant temperature of 25 8C. Circular samples approximately 2.9 cm in diameter were excised from the coatings and placed in the spectrometer bridge circuit between two brass electrodes 2.0 cm and 2.5 cm in diameter, respectively. Measurements of permittivity and dielectric loss were made over the 1 Hz to 1 MHz range. The RH was recorded but not controlled during dielectric measurements. Examination of the doped tin oxide nanoparticulates by transmission electron microscopy (TEM) and image analysis showed that the primary particle size was about 8 nm. TEM of cross-sections (,75 nm thick) of the thin ®lms suggested that particle aggregation in the dried ®lms was not uniform. Differential interference contrast light microscopy normal to the thin ®lms showed that the ®lm surface appeared fairly uniform over large areas. Atomic force microscopy showed that the surface roughness corresponded to the approximate diameter of the tin oxide particles.