Effect of Crystalline Structure and Impurity Content of C60 Thin Films on the Order/Disorder Phase Transition (original) (raw)
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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.
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.
Structural alteration and chemical stability of heat treated C60 films
Synthetic Metals, 1995
The effects of postdeposition annealing of C60 films on the structure and chemical stability of the solid-state material are investigated. An initial study of the change of the photoconductive response during a heating cycle reveals an anomalous irreversible increase of the photocurrent. In a subsequent study, non-oxygenated polycrystalline C60 films are subject to postdeposition heating at 200"C under vacuum. IR analyses reveal no new bond formation, indicating no chemical alteration of the sample. Ultra Violet/Visible absorption spectroscopy shows a decrease in the intensities of the allowed transitions and a significant red-shift of up to 50nm. In Scanning Electron Microscopy, larger grains and island growth are observed. X-ray diffraction shows the presence of mixed phases identified as cubic and hexagonal, which is a closer packed lattice, not seen in unheated films. Time of illumination dependent, low intensity room temperature Raman studies of the pentagonal pinch mode reveal the presence of two spectral components, which are associated with the two phases observed in X-ray diffraction. The intensity of the first component identified with the 1468cm "1 Raman line decreases with time. The intensity of the second spectral component identified with the 1464cm "1 Raman line remains stable under prolonged low intensity illumination. The first component is seen to be highly unstable and undergoes a light indt.w, ed chemical transformation, while the second appears largely insensitive to prolonged illumination. The results suggest that the effect of film annealing is structural in nature and leads to a repacking of the lattice.
Disorder/order phase transition in C[sub 60] thin films studied by surface photovoltage spectroscopy
Journal of Applied Physics, 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.
Surface morphology of C 60 polycrystalline films from physical vapor deposition
Thin Solid Films, 2001
C polycrystalline films grown from physical vapor deposition are obtained. A surface morphology diagram in terms of the 60 degree of supersaturation and the relative strength of bonding energy vs. thermal energy is determined from scanning electron microscopy, and discussed. The size of the grains in the high quality polycrystalline films is observed to depend sensitively on supersaturation and substrate temperature. X-Ray diffraction is applied, and the preferential orientations of these grains are in the (111), (220) and (311) directions. ᮊ
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
Deposition and structural characterization of high quality textured C60 thin films
Thin Solid Films, 2000
We present some features of the growth of C 60 thin ®lms on different substrates at temperatures up to 573 K. The crystalline structure of the ®lms was studied by X-ray diffraction, atomic force microscopy and scanning tunneling microscopy. An effect of selective C 60 deposition on an Ag/glass substrate held at 523±573 K is demonstrated. C 60 was found to be deposited only on the glass part of the substrate but not on the part of the same substrate which had been predeposited with an Ag or Au layer. The potential use of such selective deposition for micro-and nanoscopic C 60 pattering and fabrication of C 60 -based devices is suggested. We report, for the ®rst time, on the possibility of extremely rapid (10±20 A Ê /s) growth of high quality k111l-textured C 60 ®lms onto Ag and Au substrates held at 523±573 K using a simple vacuum deposition technique. Temperature-resolved XRD experiments revealed that textured C 60 ®lms undergo a ®rst order phase transition at 250 K. q 2000 Elsevier Science S.A. All rights reserved.
Transmission electron diffraction of the ordering transformation in crystalline C60
Physical review. B, Condensed matter, 1992
Transmission electron diffraction is used to define the ambient-temperature structure of sublimated pure C60 crystals and examine the ordering transformation that occurs below 249 K. Convergent-beam electron-diffraction analysis of the symmetry of the [100] pattern for the ambient-temperature phase indicates an Fm 3m structure. Analysis of [100], [110],and [111]high-symmetry zone-axis diffraction patterns of the low-temperature ordered phase indicate that the structure belongs to space group Pa3. Crystals with (111)(hexagonal plates) as well as those with other surface normals (rectangular (110)or (100) crystals) were found in the sublimated material. Electron-energy-loss near-edge fine structure demonstrates that significant bonding changes do not accompany the ordering. A twin with a (110) habit plane is identified in the ordered phase.