Fig. 14.5. The resistivity (filled circles) and the activation energy (open triangles) of the conductivity of a 212 A thick Ba^C«, film at 60° C in UHV as a function of x after annealing at 220°C. The stoichiometry was determined by use of a quartz crystal microbalance and ex situ RBS measurements [14.48],

14.1.4. Alkali Metal-Doped C70

Electrical resistivity measurements have also been reported on MiC70 [14.16,41], Pristine C70 is insulating, just as is undoped C60, and this has been established both electrically and optically. Optical measurements have yielded an edge in the optical absorption at ~1.4 eV [14.54], Unlike the behavior found for p{x) in MjC60 compounds, where there is only one minimum, and this minimum is at x = 3 (see Fig. 14.2), two minima in p(x) are found for MXC70 [14.26,27], in good agreement with the electron paramagnetic resonance (EPR) studies on KjC70 [14.55], where the spin concentration was found to go through two maxima as a function of doping. The susceptibility at the second maximum in the doping process showed Pauli-like behavior and the stoichiometry at this doping level was found to be K^Qq. These results are also consistent with band structure calculations [14.55,56], which show that the D5h symmetry of the C70 molecule results in unfilled A'[ and E" levels above the Fermi level (see Fig. 12.3), which are orbitally singly and doubly degenerate, respectively [14.56]. One thus expects from one-electron band theory that K^Qq and K^o should have conductivity minima, due to the half-filling of their respective conduction bands. If the A'[ states are indeed lower in energy, as indicated by band calculations (see Fig. 12.3), then K4C70 is supposed to be more conducting, owing to its higher density of states at the Fermi level [14.56]. Values observed for pmin at 300 K for the two resistivity minima are Pmin ~ 0-5 il-cm and 1.7 mil-cm at the first (K,C70) and second (K4C70) resistivity minimum, respectively, consistent with the trends predicted by the theoretical calculations.

At the second, much deeper, resistivity minimum, the temperature dependence of p(T) was investigated [14.26] and a nonmetallic T dependence was found for disordered film K^Qo samples. The observed functional form of p(T) for K4C 70 [14.26] was well fit by the fluctuation-induced-tunneling (FIT) model [14.47] over the temperature range 4 < T < 300 K. These results were interpreted in terms of a microstructure characteristic of a heterogeneous system in which small insulating barriers separate metallic regions, and within the metallic regions, the electron states are weakly localized. No superconducting transition was observed for T > 1.35 K. No electrical resistivity measurements are yet available on alkali metal-doped C70 single crystals.

14.1.5. Transport in Metal-Cm Multilayer Structures

Metal-Qo multilayers provide another class of fullerene-based materials (see §8.7.4) with interesting transport properties [14.57]. Several experiments, including x-ray scattering, in situ resistance, and Raman scattering measurements, have now been performed on metal-Qo multilayers, showing charge transfer between the metal layers and the CM layers and a general decrease in resistance with increasing numbers of superlattice unit cells.

To illustrate the transport behavior of metal-Qo multilayers, consider the Al/Qo system shown in the inset of Fig. 14.6. The metal species for all multilayer systems studied thus far was selected to have stronger bonding to itself than to C60, so that the metal does not intercalate into the Q, lattice. The inset to Fig. 14.6 shows C60 deposited on a substrate, in this case a polished (100) single crystal of yttrium-stabilized zirconia (YSZ), onto which contacts are deposited for four-terminal Van der Pauw transport measurements. The resistance measurements in Fig. 14.6 show that the presence of a C60 layer between the A1 and the substrate significantly changes the resistance of a given thickness of A1 deposited on the substrate. Figure 14.7 shows the decrease in resistance measured after depositing increasing numbers (N) of Al/C60 superlattice unit cells. When A1 is deposited on Qo, it is likely that some A1 diffuses into the Q0 layers (marked in the inset to Fig. 14.6 as Al^Qo). The mixed AlxQo layer is conducting because of the A1 clusters and the charge transfer to the Q0, arising through the strong Al-C 60 interaction. At the Q0-A1 interface a doped monolayer (DML) of

Was this article helpful?

0 0

Post a comment