Disorder/order phase transition in C[sub 60] thin films studied by surface photovoltage spectroscopy (original) (raw)
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Disorder'order phase transition in C60 thin films studied by surface photovoltage spectroscopy
2003
The electronic properties of C 60 thin films have been studied using surface photovoltage ͑SPV͒ spectroscopy at 120-300 K. Temperature variations of the SPV spectra are correlated with temperature-dependent x-ray diffraction patterns of the same samples, which indicate the first-order phase transition at T c ϭ250 K. Absolute values of both the band-to-band and band ͑or band tail͒ -to-band tail SPV signals are shown to exhibit a clear minimum at T c ϭ250 K in contrast with the well-known increasing background of the SPV intensity with decreasing temperature. The energy positions of the thresholds of the band-to-band and band ͑or band tail͒ -to-band tail regions in the SPV spectra also exhibit nonmonotonic behavior with a mirror symmetry and distinct extrema near T c . On the basis of the presented results, we discuss possible reasons behind the effect of rotational and orientational states of C 60 molecules on the electronic structure of the C 60 fullerite and charge carrier transport in C 60 thin films.
Journal of Applied Physics, 2003
This article reports the temperature dependence of the structural and photoelectrical properties of C 60 thin films with varying crystal structure and oxygen content near the orientational disorder/order phase transition at about 260 K. X-ray diffraction data demonstrate that highly crystalline oxygen-free C 60 films undergo the first-order phase transition at 252 K with a lattice parameter discontinuity ⌬a/a of 0.22%; no discontinuity was observed in C 60 films with coexisting amorphous and nanocrystalline phases. A strong effect of the phase transition on the dark-and photoconductivity in those films is presented: In highly crystalline C 60 films both dark-and photoconductivity increase as the temperature is lowered through the transition region. However, opposite temperature changes of the conductivity are observed in nanocrystalline/amorphous C 60 films. In situ exposure of samples to oxygen gas suppresses any critical behavior. The experimental results are interpreted in terms of trap-controlled mobility of the dominant charge carriers such that the temperature changes of the conductivity are attributed to variations in the concentration, energy distribution, and population of the carrier traps at the phase transition. Differences in the parameters of the traps, which are associated with the freeze-out of the molecular orientations, may also explain the strong variations in the temperature dependence of the conductivity observed in our C 60 films and reported by other authors.
Journal of Applied Physics, 2003
This article reports the temperature dependence of the structural and photoelectrical properties of C 60 thin films with varying crystal structure and oxygen content near the orientational disorder/order phase transition at about 260 K. X-ray diffraction data demonstrate that highly crystalline oxygen-free C 60 films undergo the first-order phase transition at 252 K with a lattice parameter discontinuity ⌬a/a of 0.22%; no discontinuity was observed in C 60 films with coexisting amorphous and nanocrystalline phases. A strong effect of the phase transition on the dark-and photoconductivity in those films is presented: In highly crystalline C 60 films both dark-and photoconductivity increase as the temperature is lowered through the transition region. However, opposite temperature changes of the conductivity are observed in nanocrystalline/amorphous C 60 films. In situ exposure of samples to oxygen gas suppresses any critical behavior. The experimental results are interpreted in terms of trap-controlled mobility of the dominant charge carriers such that the temperature changes of the conductivity are attributed to variations in the concentration, energy distribution, and population of the carrier traps at the phase transition. Differences in the parameters of the traps, which are associated with the freeze-out of the molecular orientations, may also explain the strong variations in the temperature dependence of the conductivity observed in our C 60 films and reported by other authors.
2001
Near the temperature of 260 K, C 60 crystal is known to undergo a first order phase transition, associated with changes in molecular rotations. The present paper reports the effect of the crystalline structure and impurity content of C 60 thin films on their structural behavior near this phase transition. Polycrystalline C 60 films with different grain sizes and oxygen content were obtained by varying the conditions of their vacuum deposition and post-grown exposure. Temperature-resolved X-ray diffraction in the range 300-15 K was used to determine the lattice parameter and its changes near the phase transition temperature. Decrease in grain sizes and increase in oxygen content of the films are found to lead to a gradual reduction in the discontinuity in lattice parameter and the transition temperature.
Optical spectra and photoluminescence of C 60 thin films
We report on transmission and reflectance spectra of fullerite Cm thin films deposited on quartz substrate. An Urbach tail of states is present below the absorption edge due to the disordered structure of the films. Photoluminescence (PL) spectra have been measured from 10 to 300 K. Recombination of self-trapped excitons and their phonon replicas are present in the PL spectra. Temperature dependence of the integrated intensity of each emission line is also reported. Copyright 0 1996
Solid State Communications, 2003
The electrical resistance of thin silver films deposited on C 60 crystals shows anomalies near 261, 240, and 100 K. These temperatures coincide, respectively, with the bulk rotational, surface rotational, and quenched disorder structural phase transitions of crystalline C 60 . Films of other metals on C 60 show similar behavior. Our findings demonstrate that thin metal film overlayers are sensitive probes of the structural phase transitions in C 60 , and also provide evidence for a novel structuralelectronic interaction at the metal/C 60 interface. q
Conductance and Polarisability of C60 Films
Journal of Nanoscience and Nanotechnology, 2007
Thin films of C 60 deposited in vacuum are studied using current-voltage (I-V) measurements and atomic force microscopy (AFM). In situ electrical measurements give an average resistivity of ca. 30 M cm for the as-deposited films at room temperature. The I-V dependences are found to correspond to ohmic behaviour but they have a hysteresis shape attributed to remnant polarisation due to the domain structure of the films. AFM images show a grainy surface morphology for the deposited C 60. Temperature dependent measurements in the range 290-365 K provide evidence for a variable range hopping mechanism of conductance with an activation energy of 0.8-1.0 eV. With further temperature increase the C 60 films restructure leading to an increase in grain size and a change of the electrical properties with I-V dependences showing Schottky barrier formation. The effect of oxygen on the conductance of the C 60 films under their exposure to an ambient atmosphere is considered and discussed.
Conductance and polarisability of C60 films
Journal of Nanoscience and Nanotechnology, 2007
Thin films of C 60 deposited in vacuum are studied using current-voltage (I-V) measurements and atomic force microscopy (AFM). In situ electrical measurements give an average resistivity of ca. 30 M cm for the as-deposited films at room temperature. The I-V dependences are found to correspond to ohmic behaviour but they have a hysteresis shape attributed to remnant polarisation due to the domain structure of the films. AFM images show a grainy surface morphology for the deposited C 60. Temperature dependent measurements in the range 290-365 K provide evidence for a variable range hopping mechanism of conductance with an activation energy of 0.8-1.0 eV. With further temperature increase the C 60 films restructure leading to an increase in grain size and a change of the electrical properties with I-V dependences showing Schottky barrier formation. The effect of oxygen on the conductance of the C 60 films under their exposure to an ambient atmosphere is considered and discussed.
Observation and identification of the molecular triplet in C60 thin films
Chemical Physics Letters, 2001
The excited state properties of C 60 thin ®lms have been probed at 77 K using Raman spectroscopy. The change in the Raman, A 2g mode of C 60 , whose position is largely independent of temperature was monitored as a function of the excitation intensity at 514.5 nm. This mode, normally positioned at 1469 cm À1 , was seen to shift to a lower Raman frequency with increasing laser intensity. Two excited state species have been identi®ed. The ®rst, at 1466 cm À1 , has been associated with the molecular triplet of C 60 , while the second species, at 1463 cm À1 , has been speculated to be an excited state cooperative in the solid.
Conduction-band dispersion in heteroepitaxial C60
Applied Surface Science, 1993
The heteroepitaxial growth of C,,, thin films has been studied on various layered substrates. Recausc ot a good Iatticc match and a favourable corrugation of its (001) cleaved surface, GeS was chosen as substrate. Under optimized sublimation conditions, WC have obtained multilayer films of C,,, fullerite, highly ordered on a large scale, as it is proved. for the first time. hy a very sharp LEED pattern. In the case of one monolayer, spots characteristic of both C,,, and substrate are visible. therchy allowing the geometry of the epitaxy to be specified. The empty electronic states of these C,,, films have been studied by I, i-rc~olved invcr\t' photoelectron spectroscopy (KRIPES) and the observed structures show a slight hut significant dispersion with respect to the uavc vector component parallel to the surface. This effect. which crucially depends on the sample thickness. confirm> that the empty conduction-hand r states are partly delocalized