Helium-4 adsorption to fullerenes

Since the discovery and purification of C$\sb{60}$ there has been intense research interest in fullerenes. We are exploring the adsorption behavior of $\sp4$He on fullerenes and the possibility that several of the newly discovered carbon materials provide a host environment for $\sp4$He in which the $\sp4$He properties are substantially different from the three-dimensional case. The deeper interest in this question lies in the possibility of realizing a quantum fluid in such an environment. We are examining the possibilities of $\sp4$He intercalation into C$\sb{60}$ and into bundles of Single Wall Carbon Nanotubes (SWCNs). The former provides a cubic lattice with potential intercalation into the interstitial sites. The results presented here argue that deep penetration into the C$\sb{60}$ matrix does not occur under the conditions of this study. We do however find evidence for some excess adsorption that is independent of the C$\sb{60}$ film thickness. SWCNs might allow $\sp4$He intercalation into the insides of the tubes (if the endcaps are removed) or in between the tubes (if they are arranged parallel to each other), thus allowing for quasi one-dimensional systems of $\sp4$He. The results presented here show that $\sp4$He atoms are strongly bound to SWCNs with substantial $\sp4$He presence in the vicinity of the SWCNs for T $\sim$ 15K. A model of a 1-dimensional ideal gas with a binding energy of $\sim$330K is consistent with the low temperature data presented here. Our experiments on $\sp4$He with C$\sb{60}$ films utilize Quartz Crystal Microbalances (QCMs) that were coated with C$\sb{60}.$ We measured the $\sp4$He mass adsorption as $\sp4$He was added under isothermal conditions or under conditions where the $\sp4$He charge in the sample cell was held constant as the temperature was varied. The experiments of $\sp4$He in the vicinity of SWCNs use bundles of SWCNs that were stored in vacuum since purification. We measured the $\sp4$He desorption as the temperature was linearly increased from T = 14K to T = 320K