Heat Capacity of 1D Chains of Atom/Molecule Adsorbates in the Grooves of c-SWNT Bundles

Abstract

Since the discovery of carbon nanotubes in 1991 [1], investigations of the physical properties of these novel materials have been rated as a fundamentally important trend in physics of condensed matter [2,3]. Of special interest is the study of properties of low-dimensional systems [4][5][6][7][8][9]. The unique structure of bundles of single-walled carbon nanotubes (SWNTs) permits obtaining quasi-one-dimensional (1D), 2D, and 3D structures formed by adsorbates [5,10]. Technologically, most of the nanotubes in the prepared bundles have closed ends unless special steps are taken to open them. Figure 15.1 demonstrates the possible sites of adsorption of a relatively small admixture atoms or molecules in a bundle of closed SWNTs (c-SWNTs): interstitial channels (IC), grooves (G) at the outer surface, and the outer surface (OS). These positions differ in geometrical size and energy of adsorbate binding to c-SWNT bundles [11]. At low adsorbate concentrations, quasi-1D chains of admixture atoms/molecules are formed in the IC-and G-sites. One or several layers of atoms/molecules adsorbed at the OS of the c-SWNT bundle form quasi-2D or quasi-3D systems. The 1D, 2D, and 3D systems have different properties at low temperatures [12][13][14][15][16][17][18][19]. Physical properties (adsorption, thermal, and structural) of simple gas admixtures deposited in c-SWNT bundles were investigated in experimental and theoretical works [5, 6, 9-11, 15, 20-39].The heat capacity of 4 He adsorbed on c-SWNT bundles was investigated at temperatures below 6 K [6,33]. It was observed [6] that the heat capacity of the adsorbed helium exhibited the behavior of 1D and 2D structures depending on the technique of sample preparation (laser evaporation or arc discharge). Structures of 4 He adsorbates in the grooves and at the OS of c-SWNT bundles were studied by the neutron diffraction method [18]. At low coverage, the 4 He atoms formed a 1D single line lattice along the grooves. As the concentration of the adsorbed helium atoms increased,