Dielectric properties of orientationally ordered/disorderedC60(111) films

Abstract

ac electrical measurements are conducted in high vacuum, perpendicular to the plane of C 60 ͑111͒ films, sandwiching these films between parallel and conducting surfaces. The C 60 ͑111͒ films are grown epitaxially on Ag͑111͒ substrates. A flat and smooth gold electrode surface is pressed onto the surfaces of these films. Frequency and temperature are varied from /2ϭ40-100 kHz, and Tϳ85-530 K, respectively. Decreasing T below room temperature, the dielectric permittivity, Ј, increases due to the establishment of orientational order among C 60 molecules. Oxygen desorption studies suggest that Ј may depend on oxygen concentration, as well as the structural environments in which oxygen and C 60 molecules interact with each other. Measurements conducted above room temperature, show that Ј͑,T͒ can be rescaled into a single master function, revealing that ac electrical conductivities of the films are thermally activated across the band gap of the crystalline C 60 semiconductor. ͓S0163-1829͑96͒00347-5͔

Disorder/order phase transition in C60 thin films studied by surface photovoltage spectroscopy

“…This behavior, in comparison with single crystals, may originate from the relatively high density of structural defects ͑including grain boundaries͒, strain and impurities in thin films. In fact, it was previously shown that structural defects, 18 impurities, 18 -21 strain, 22 or an increased surface/ volume ratio [23][24][25] can reduce the T c value of solid C 60 by up to 30 K. The contribution of all these defects could suppress the discontinuity in a at the phase transition. 19,21,24 It should be noted, however, that the changes in the orientational state of C 60 molecules occurring at the phase transition have a much stronger effect on the material's electronic properties than the discontinuity in the lattice parameter.…”

“…In fact, it was previously shown that structural defects, 18 impurities, 18 -21 strain, 22 or an increased surface/ volume ratio [23][24][25] can reduce the T c value of solid C 60 by up to 30 K. The contribution of all these defects could suppress the discontinuity in a at the phase transition. 19,21,24 It should be noted, however, that the changes in the orientational state of C 60 molecules occurring at the phase transition have a much stronger effect on the material's electronic properties than the discontinuity in the lattice parameter. In particular, we have demonstrated recently 8 a strong effect of the phase transition on the conductivity and photoconductivity of a C 60 film with coexisting amorphous and nanocrystalline phases for which no discontinuity in a was observed.…”

Revisiting electronic couplings and incoherent hopping models for electron transport in crystalline C60 at ambient temperatures

“…The outer-sphere contribution was estimated using the Marcus formula l o = (Dq) 2 (1/e op À 1/e s )(1/r À 1/R), where e s and e opt are the static and optical dielectric constants, r is the cavity radius for C 60 and R is the center-to-center distance between two fullerenes. We use the experimental parameters e s = 4.4, 39,40 e op = n 2 = 4.0 (at 1.06 mm), 41,42…”

Effect of Crystalline Structure and Impurity Content of C60 Thin Films on the Order/Disorder Phase Transition

“…The latter originate from the strong texture of our films. Defects of crystalline structure of solid C 60 [9], impurities [9][10][11], strains [12], or an increased surface/volume ratio [13][14] have been demonstrated to result in a reduction of T, up to 25 K. The first two factors are known to lead also to a broadening of the phase transition temperature range (phase transition of a second degree) or even to suppress the phase transition [ 10,14] T,K Figure 3. Results of XRD measurements of the lattice parameter for sample 3.…”