Effects of Pressure on the Vibrational, Structural and Electronic Properties of C 60 Powder and Thin Films
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Effects of Pressure on the Vibrational, Structural and Electronic Properties of C60 Powder and Thin Films S. C. Sharma*, B. Ha, J. H. Rhee, Y. Li, D. Singh, and R. Govinthasamy Department of Physics and Materials Science & Engineering Program The University of Texas at Arlington, Arlington, TX 76019, USA *
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ABSTRACT We present results from a study of the vibrational, structural, and electronic properties of C60 powder and thin films. Raman spectroscopy and diamond anvil cell have been used to study pressure dependence of the Raman active modes of C60 powder. The material undergoes structural phase transition between 9 and 15 GPa. Some of the Raman modes soften, while others harden with increasing pressure. Thin films of C60 and La-doped C60 have also been studied by using Raman scattering, x-ray diffraction, x-ray photoelectron spectroscopy and uv photoemission spectroscopy. Whereas the powder and La-doped C60 films exhibit fcc crystalline structure, the C60 film appears disordered. Further, we observe a significant difference in the electronic valence bands of the doped and undoped films. INTRODUCTION The vibrational, structural and electronic properties of C60 are the subject of an increasing level of interest [1, 2]. The C60 molecule consists of a total of 60 equivalently bonded carbon atoms making a soccer-ball cage structure. Each carbon atom in this cage structure is bonded with three other carbon atoms through three bonds and one bond; with intercarbon distances of 0.145 nm and 0.140 nm for single and double bonds, respectively. In crystalline C60, the molecules are bound by weak van der Waals forces in a closed packed face-centered-cubic (fcc) lattice with a lattice constant of 0.142 nm. The material undergoes a structural phase transition, from fcc to simple cubic structure, at Tc = 260 K. Above Tc, the molecules rotate around their lattice positions, whereas this motion is restricted below Tc; being frozen at 90 K. It has been reported that C60 can be polymerized above Tc by optical excitations as well as by the application of high pressure and high temperature [3, 4]. In the polymer phase, the adjacent molecules are covalently bonded by a four-membered ring joining two molecular cages. A photochemical [2 + 2] cycloaddition reaction has been proposed for the photoinduced dimerization in crystalline C60. Details of the photo-induced formation of the intermolecular covalent bonds, however, still remain the subject of discussion. Excellent reviews on the C60 based materials and high-pressure techniques are available in the literature [1, 2, 5, 6]. Here we present results from a series of experiments designed to further study the vibrational, structural, and electronic properties of C60 powder and thin films by using Raman scattering, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and uv photoemission spectroscopy (PES). L3.10.1
EXPERIMENTAL C60 powder of 99.9% purity and containing 1-10 ยต m particles was obtained from MER [7]. Thin films w
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