The coupling between mechanical flexibility and electronic performance is evaluated for thin films of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) deposited on compliant supports. Percolated networks of type-purified SWCNTs are assembled as thin conducting coatings on elastic polymer substrates, and the sheet resistance is measured as a function of compression and cyclic strain through impedance spectroscopy. The wrinkling

SWCNT Purification and Sample Preparation: Nanotube fractions were prepared using a number of approaches to yield equivalent results. Initially, type purification was achieved using density-gradient ultracentrifugation (DGU) 1 and (6,5) enrichment was achieved using nonlinear DGU. 2 These techniques are well documented and details can be found elsewhere. [1][2][3] Length-enriched CoMoCat fractions were prepared by dispersing the SWCNTs (Southwest nanotechnologies lot SG65EX or SG65i lot 046) in water via ultrasonication with 2 % sodium deoxycholate (DOC). The suspensions were coarsely centrifuged to remove impurities and bundles and then purified by length using transient DGU. 4,5 For AFM characterization of the fractions, silicon wafers were functionalized with 3-(ethoxydimethylsilyl)-propylamine (APDMES, Sigma-Aldrich) by 15 min UVO followed by immersion in 1 % APDMES/isopropanol for 20 min and subsequent rinsing with isopropanol and DI water. The substrates were dried at 70 o C for 20 min. SWCNT samples in 10 g/L DOC were diluted 100× with an aqueous solution of 0.2 wt. % sodium cholate and 20 mM sodium thiocyanate and deposited on a small piece of silicon wafer

The evolution of wrinkles and folds in compressed thin films of type-purified single-wall carbon nanotubes (SWCNTs) on polydimethylsiloxane (PDMS) substrates is used to study the mechanical response of pristine nanotube networks. While the low-strain moduli are consistent with the exceptional mechanical properties of individual nanotubes, the films are remarkably fragile, exhibiting small yield strains that decrease with increasing thickness. We find significant differences in the mechanical response of semiconducting as compared to metallic SWCNT networks, and we use simple scaling arguments to relate these differences to previously determined Hamaker constants associated with each electronic type. A comparison with conductivity measurements performed on identical films suggests more than a two-fold variation in the onset of rigidity vs. connectivity percolation, and we discuss the potential implications of this for both rigid-rod networks and the use of type-purified SWCNTs in flexible electronics.

ABSTRACT:Single-wall carbon nanotube (SWCNT) films show significant promise for transparent electronics applications that demand mechanical flexibility, but durability remains an outstanding issue. In this work, thin membranes of length purified single-wall carbon nanotubes (SWCNTs) are uniaxially and isotropically compressed by depositing them on prestrained polymer substrates. Upon release of the strain, the topography, microstructure, and conductivity of the films are characterized using a combination of optical/fluorescence microscopy, light scattering, force microscopy, electron microscopy, and impedance spectroscopy. Above a critical surface mass density, films assembled from nanotubes of well-defined length exhibit a strongly nonlinear mechanical response. The measured strain dependence reveals a dramatic softening that occurs through an alignment of the SWCNTs normal to the direction of prestrain, which at small strains is also apparent as an anisotropic increase in sheet resistance along the same direction. At higher strains, the membrane conductivities increase due to a compression-induced restoration of conductive pathways. Our measurements reveal the fundamental mode of elasto-plastic deformation in these films and suggest how it might be suppressed. ' INTRODUCTIONIn the past two decades, single-wall carbon nanotubes (SWCNTs) have received considerable attention due to their outstanding mechanical, optical, and electronic properties, and a vast amount of research has been devoted to the characterization of these attributes 1 and the potential applications they suggest. 2 Promising applications are rapidly emerging in such areas as high performance composites, 3 thermoelectric materials, 4 and conducting polymer composites.5 Thin SWCNT films, in particular, show exceptional promise for applications that require transparent coatings with superior mechanical, electronic, and optical qualities. [6][7][8][9] The natural tendency for SWCNTs to form flexible, transparent networks with high electrical conductivity at remarkably low surface coverage is a direct consequence of the magnitude of the typical SWCNT aspect ratio, 10 suggesting that nanotube length is a critical factor in dictating the physical properties of such membranes.The electronic and optical properties of SWCNTs are determined by their electronic band structure, 1 which is specified by the chiral vector (n, m) characterizing the symmetry of rolling a two-dimensional graphene sheet into a hollow tube of diameter a. All existing synthetic techniques therefore produce raw material that contains a distribution of electronic types, ranging from semiconducting to metallic, as well as a broad range of lengths, from 10 nm up to hundreds of micrometers. Since scalable processes for purifying lab-grade quantities of SWCNTs have only recently been formulated, 11-13 thin films of exemplary purified materials have just now become readily available for fundamental research, pointing toward a number of promising applications. Thin SWCNT membranes have rece...

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 (PDMS) 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. INTRODUCTIONVan 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][8][9][10][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...