Low Temperature Internal Friction of Thin Fullerene Films
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phonon scattering by grain boundaries. We have sought to verify the existence of tunneling states that are intrinsic to fullerene solids by performing internal friction measurements on thin fullerene films, where the disorder due to compaction should be removed. In addition, because the films are prepared at elevated substrate temperatures, problems associated with impurities in the starting powder should be minimized. Below we present the results of these measurements, as well as, results obtained from characterization of the thin films using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Results of internal friction measurements performed on the compacted fullerene solids are also presented. EXPERIMENTAL DETAILS Thin Film Preparation and Characterization Thin fullerene films were produced by sublimation of commercially available C6 0/C 70 powder. The sublimation chamber consisted of a turbo pumped, Pyrex "double tough" glass tube. The chamber's base pressure was limited by the use of Viton O-rings to lxlO- 7 mbar after one day of pumping. The sublimation stage was a small stainless steel furnace which contained a small orifice that could be filled with C60/C 70 powder. Heat to the furnace was provided by 3 mil tungsten wire. The sample stage consisted of a small aluminum block which could also be heated to a temperature of 250 'C. In order to verify that the deposited fullerene films were indeed crystalline, a Nanoscope III operating in the STM mode was used to characterize the films. These films were deposited on substrates that consisted of 110 nm gold films sputtered on mica platelets which were held at 250 "C. The fullerene films were deposited at a rate of 0.02 A sec-1 which was achieved with a furnace temperature of 430 OC. The thickness of these fullerene films ranged from submonolayer coverage to 30 monolayers. Our first images were obtained on films with submonolayer coverage, which were deposited on substrates held at room temperature. They showed spherical protrusions with a diameter of 0.7 ± 0.1 nm, which we interpreted as C60 molecules. The arrangement of the molecules was best described as disordered. Films that were deposited at a substrate temperature of 200 'C showed much more order, with the fullerene molecules being hexagonally arranged in crystalline domains with diameters of 30 nm and nearest neighbor distances of 1.00±.02 nm. These domain sizes were probably limited by the width of terraces that were present on the underlying gold structure. It is interesting to note that in some images, molecules with diameters of 0.8 ± 0.1 nm could be seen. Their heights appeared to be 0.1 ± 0.05 nm above the average height of the other molecules. Because of this small additional height, we could exclude the possibility that these features were caused by a second layer of C60 molecules. We, instead, attributed these features to molecules of C70 . In several images, the number of these protrusions were counted and it was determined that the number of protrusions was about 13% of the
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