On the Existence of Ordered Phases of Encapsulated Diamondoids into Carbon Nanotubes
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On the Existence of Ordered Phases of Encapsulated Diamondoids into Carbon Nanotubes S. B. Legoas1, R. P. B. dos Santos2, K. S. Troche3, V. R. Coluci4, and Douglas S. Galvao3 1 Departamento de Física, CCT, Universidade Federal de Roraima, Boa Vista - RR, 69304-000, Brazil. 2 Departamento de Física, Universidade Estadual de São Paulo, Rio Claro - SP, 13506-900, Brazil 3 Instituto de Física “Gleb Wataghin, Universidade Estadual de Campinas, Campinas - SP, 13083-970, Brazil 4 Faculdade de Tecnologia, Universidade Estadual de Campinas, Limeira - SP, 13484-332, Brazil ABSTRACT We have investigated some diamondoids encapsulation into single walled carbon nanotubes (with diameters ranging from1.0 up to 2.2 nm) using fully atomistic molecular dynamics simulations. Diamondoids are the smallest hydrogen-terminated nanosized diamond-like molecules. Diamondois have been investigated for a large class of applications, ranging from oil industry to pharmaceuticals. Molecular ordered phases were observed for the encapsulation of adamantane, diamantane, and dihydroxy diamantanes. Chiral ordered phases, such as; double, triple, 4- and 5-stranded helices were also observed for those diamondoids. Our results also indicate that the modification of diamondoids through chemical functionalization with hydroxyl groups can lead to an enhancement of the molecular packing inside the carbon nanotubes in comparison to non-functionalized molecules. For larger diamondoids (such as, adamantane tetramers), we have not observed long-range ordering, but only a tendency of incomplete helical structural formation. INTRODUCTION Recently, carbon nanostructures known as diamondoids, have attracted the attention of the scientific community [1-5]. Diamondoids are essentially hydrogen-terminated nanosized diamond fragments (Fig. 1). Due to their flexible chemistry, pure diamondoids are particularly attractive for preparation of pharmacological active compounds with pronounced membrane activity due to the lipophilicity of the aliphatic diamondoid core structure [2, 3]. On the other hand, functionalized diamondoids [7] are ideal candidates for molecular building blocks in nanotechnology [8], and for molecular electronic applications when the incorporation of functionalized groups allows their attachment to metal surfaces to form self-assembled monolayers [9]. McIntosh et al. [10] have studied the structural, electronic, and encapsulation properties of adamantane and diamantane using density functional theory. The encapsulation was investigated to (n,n) armchair SWCNTs ranging from n = 4 up to 8. They found that the encapsulation of diamantane into armchair SWCNTs with diameters larger than 9.5 Å (n ≥ 7) could occur spontaneously, without any energetic cost. This suggests that carbon nanotubes are promising materials to be used in assembling diamondoid structures. However, investigations of the diamondoid encapsulation into larger diameter SWCNTs (n > 8) have not been fully investigated. For this reason, in this work, we have carried out fully atomistic molecula
