Effect of Crystalline Structure and Impurity Content of C 60 Thin Films on the Order/Disorder Phase Transition
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Effect of Crystalline Structure and Impurity Content of C60 Thin Films on the Order/Disorder Phase Transition Eugene A. Katz1, David Faiman1, 2, Svetlana Shtutina2, Aleksandra P. Isakina3 and Konstantin A. Yagotintsev3 1 The National Solar Energy Center, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer, 84990 Israel 2 Department of Physics, Ben- Gurion University of the Negev, Beersheba, 84105 Israel 3 Verkin Institute for Low Temperature Physics & Engineering, National Academy of Science of Ukraine, 47 Lenin Ave., Kharkov 310164, Ukraine ABSRACT Near the temperature of 260 K, C60 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 C60 thin films on their structural behavior near this phase transition. Polycrystalline C60 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.
INTRODUCTION Solid C60 is a molecular crystal with C60 molecules occupying the lattice sites of a facecentered cubic (fcc) structure at room temperature [1]. C60 molecules have a rotational degree of freedom in the crystal. The C60 crystal is known to undergo a phase transitions associated with changes in the molecular rotations. Near the temperature Tc = 260 K, C60 crystal undergoes a first order phase transition from the fcc structure above Tc to a simple cubic (sc) structure below Tc [2-4]. C60 molecules have been found to rotate freely in the fcc phase while rotation locks into specific orientations in the sc phase. However, the published data dealing with this phase transition in C60 thin films are very contradictory because of the fact that the films grown under different deposition conditions and/or subjected to different post-growth exposures may have substantially various crystalline structure and impurity content. On the other hand, no systematic study of the effect of the crystalline structure and impurity content of C60 films on their behavior near the phase transition have been performed. This paper reports the first results of such kind of study. EXPERIMENTAL DETAILS C60 thin film was deposited by a vacuum deposition technique on substrates of mica, optical glass and optical glass predeposited with an Ag sub-layer. The starting C60 powder (‘Super Gold
W7.6.1
Grade’, > 99.9%) was commercially obtained from Hoechst AG. Detailed description of the deposition conditions was given elsewhere [5-8]. The thickness of all C60 films under the present study was about 100 nm. The crystalline structure of the C60 films was
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