Raman Investigation of Fullerene [60] Under hydrostatic Pressure
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Q9.7.1
Raman Investigation of Fullerene [60] Under hydrostatic Pressure
Mostafa El-Ashry, Maher Amer, John F. Maguire* Dept. of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435 *Air Force Research Laboratory, WPAFB, OH 45433
ABSTRACT We report the results of a study of adsorption of small molecules on the surface of buckminsterfullerene, C60. The pressure dependence of the Raman spectrum was investigated over the range 0-10 GPa in methanol-water mixtures that were used as the pressure transmitting fluid (PTF) in a diamond anvil cell. It is found that the spectral shift and its pressure derivative are sensitive to both the applied pressure and to the composition of the PTF. These observations are consistent with an explanation that involves preferential adsorption onto the surface of the C60. In particular, the notion of C60 collapse needs not be invoked to explain the observations. INTRODUCTION Since their discovery in the mid 1980’s buckminsterfullerenes have been a subject of intense research interest. They have the potential to provide new classes of structural materials, sensors, electronic devices, and many other applications [1]. It has not been so widely recognized that these molecules, including the closely related single walled nanotubes, provide unique new experimental models with which to study the statistical mechanics and condensed matter physics of thermodynamically small systems in low dimensionality. A thermodynamically “small” system is defined as a system in which the correlation length is of the same order as the system dimension. Small systems are ubiquitous in nature and include processes that occur at the cell wall and the interactions that govern the formation of long-range structures (sometimes called “self-assembly”) in the mesoscopic regime [2]. They also occur in three common physical situations. First, near a critical point the correlation length diverges giving rise to marked changes in the properties like the isothermal compressibility and diffusivity [3]. Second, within an interface the properties of matter are very different from bulk matter. For example, if we take a mixture of oil and water then there will be a little water in the oil-rich phase and a little oil in the water-rich phase. However, inside the thin interface (~5 nm), between the two bulk phases, there is a huge concentration gradient in which there is a region containing a 50% homogeneous mixture of oil and water, a substance that has very unusual properties but does not exist in any other situation. Here the range of the density correlation function is of order ten molecular diameters, which is about the same as the thickness of the interface[4]. Finally, when particles (e.g. colloids) are very large on the scale of molecules but still much smaller than the size at which the laws of continuum macroscopic mechanics might hold, they are said to be mesoscopic and may display unique and unusual behavior.
Q9.7.2
It is the ratio of the range of the correlation length to the characteristic
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