Nanoindentation studies of sublimed fullerene films using atomic force microscopy
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Nanoindentation studies of sublimed fullerene films have been conducted using an atomic force microscope (AFM). Transfer of fullerene molecules from the as-deposited films to the AFM tip was observed during the indentation of AFM tip into some of the samples, whereas such a transfer was not observed for ion-bombarded films. The fullerene molecules transferred to the AFM tip were subsequently transported to a diamond surface when the diamond sample was scanned with the contaminated tip. This demonstrates the capability of material manipulation on a molecular scale using AFM. Atomic-scale friction of the fullerene films was measured to be low. Ability of fullerene films to form transfer film on the mating AFM tip surface may be partly responsible for low friction.
Since its invention in 1986, atomic force microscopy1 has been widely used for various applications. For example, Mate et al.2 have measured atomic-scale friction using friction force microscopy for a tungsten tip sliding against a graphite surface; Burnham and Colton3 have studied nanomechanical properties of surfaces and thin films; Blackman et al.4 have measured the lubricantfilm thickness on thin-film magnetic disks; O'Shea et al.5 have measured solvation forces near a graphite surface. In this paper, we show that material manipulation, in particular, material transfer using AFM, can be achieved on an atomic-scale and observed in real time. We have chosen fullerene films for this study partly because the intermolecular forces of fullerene molecules are the weak van der Waals. As a result, it may be relatively easy to manipulate these molecules. A polished natural diamond (Ha) was used as a reference and for the purpose of calibrating the cantilever deflection. Fullerene powders were prepared by evaporating graphite rods in a 150-Torr He atmosphere using an alternating current electric arc welder.6"8 The soot produced by vaporization of the graphite rods was collected from the arc chamber. Raw fullerene material was extracted from the soot by dissolving the soot in toluene and filtering it to yield a red solution. The toluene extract from the carbon arc soot contains 70-85% C60 and 10-15% C70 with the remainder being higher fullerenes and impurities. To obtain Ceo free from the higher molecular weight fullerenes, a flash chromatography purification method was used as described by Scrivens et al.9 The C6o fullerene films were deposited on polished (111) silicon wafers by sublimation in a vacuum of 10"6 Torr. Both the Cgo powder and the substrate were heated by the same oven with the C6o powder at the center of the heating oven where the temperature is at around 450 °C. J. Mater. Res., Vol. 8, No. 12, Dec 1993 http://journals.cambridge.org
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Some of the evaporated films were bombarded with Ar+ ions (ion energy: 100 keV, ion dose: 1016 ions/cm 2 ) at room temperature and at a 10"6 Torr vacuum. Each of the bombarded films had a masked area to serve as an as-deposited reference film for comparison. AFM studies were performed using a m
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