Empirical evaluation of attractive van der Waals potentials for type-purified single-walled carbon nanotubes

Erik K. Hobbie, Thomas Ihle, John M. Harris, Matthew R. Semler
2012 Physical Review B  
Van der Waals forces play a critical role in the structure and stability of single-wall carbon nanotube (SWCNT) materials. Thin films assembled from SWCNTs purified by electronic type show particular promise for flexible electronics applications, but mechanical durability remains an unresolved issue. Using transition resonances determined from spectroscopic measurements of typepurified SWCNTs deposited on quartz, coupled with analogous spectroscopic characterization of polydimethylsiloxane
more » ... ) substrates, we use the Lifshitz theory of van der Waals dispersion interactions developed by Rajter and co-workers [R. F. Rajter et. al., Phys. Rev. B 76, 045417 (2007)] to examine the influence of electronic type on van der Waals contact potentials in polymer supported nanotube networks. Our results suggest a significantly stronger nanotube-nanotube and nanotube-polymer attraction for the semiconducting SWCNT fractions, consistent with recent measurements of the electronic durability of flexible transparent SWCNT coatings. PACS numbers: 61.48.De, 78.67.Ch, 82.35.Np, 78.20.Bh 2 I. INTRODUCTION Van der Waals (vdW) forces play a significant role in the structural stability of matter across a broad range of chemistry, physics, and biology. 1-6 They also play a particularly important role in nanotechnology, where they dominate the short-range attraction between nanoparticles and can hinder their dispersion and manipulation. 7-11 For particles lacking a permanent dipole moment, vdW dispersion forces arise solely from small fluctuations in the electromagnetic field -or more precisely, the dielectric permittivity -across the space between the particles, 12,13 which is dominated by the zero-point energy of quantum vacuum fluctuations. The quantum-field nature of such a mundane, ubiquitous and sometimes macroscopic force is quite remarkable, if on occasion not fully appreciated. Single-wall carbon nanotubes (SWCNTs) are nanometer thick tubes of graphene 100 nm to 100 µm in length. They can be either metallic or semiconducting, depending on the chiral vector (n, m) that characterizes the symmetry of rolling a 2D graphene sheet into a hollow tube. 14 They are one of the most studied materials within the realm of modern nanotechnology, with exceptional physical properties that herald the possibility of significant technological potential. 15 The importance of vdW forces in SWCNT materials cannot be overstated. The high aspect ratio and strong anisotropy create potential wells thousands of k B T in depth, but SWCNTs have yet to realize their full potential as mechanical reinforcing agents. This is primarily due to the mechanical failure of interfacial contacts, which are largely governed by vdW forces. Although chemical crosslinking can help mitigate such effects, this often occurs at the expense of the intrinsic SWCNT properties of interest, 16 which can limit the potential impact of applications. Raw nanotube materials typically contain a broad distribution of lengths and a mixture of the two distinct electronic species, usually 1/3 metallic and 2/3 semiconducting. Recent advances in the separation of SWCNTs by length and electronic type, however, have ushered in a new era of research focused on the physical attributes and potential applications of highly monodisperse nanotube materials. [17] [18] [19] [20] [21] [22] In particular, such purified SWCNTs show tremendous promise for flexible electronics applications, 23 where the mechanics of vdW contact forces will play a potentially profound role in dictating device durability and performance. 24,25 One recent study, for example, offered compelling evidence that electronic type can have a significant influence on the durability of flexible transparent SWCNT films, with metallic nanotubes offering improved performance over semiconducting nanotubes. 26 The Lifshitz framework of vdW interactions has recently been cast in a form that can be readily applied to SWCNTs. 27 Building on the extensive work of French 28 and Parsegian, 29 Rajter et. al., 27 have developed a formalism that offers considerable insight into the nature of vdW interactions in nanotubes. The scope of the work presented in Ref. [27] is ambitious, in that it employs a first-principles ab initio scheme to compute the band structure of distinct chiral species. There are, however, some subtle but important issues that are not fully resolved by a purely theoretical approach. These include a dramatic depolarization effect that favors polarizations parallel the nanotube axis, 30 the rather large influence of excitonic effects in quasi-1D systems, 31 and the overall challenge of accurately describing experimentally observed absorption spectra with an ab initio scheme. As an alternative, the approach we adopt here therefore relies on spectral data as a way to correctly, if only empirically, overcome these issues. By exploiting recent advances in the Lifshitz theory of vdW interactions applied to nanotubes, our goal is thus to address the role of SWCNT electronic type in dictating the magnitude of vdW contact forces, both between nanotubes and between nanotubes and a polymer substrate. Our results suggest a significantly stronger nanotube-nanotube and nanotube-polymer attraction for semiconducting SWCNTs on polydimethylsiloxane (PDMS), and we discuss these findings in the context of the electronic durability of flexible SWCNT coatings. 26
doi:10.1103/physrevb.85.245439 fatcat:bvlels4jnzejxpnvd7ulznefbm