Structural Stability of Transparent Conducting Films Assembled from Length Purified Single-Wall Carbon Nanotubes

Abstract

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...

Electronic Durability of Flexible Transparent Films from Type-Specific Single-Wall Carbon Nanotubes

“…A 2-fold reduction in sheet conductivity is observed for films deposited on PDMS as opposed to quartz, which we attribute to SWCNT ARTICLE network damage that occurs as a result of the swelling/ deswelling processes in the PDMS. 9 When this factor is applied to the data in Figure 3, we find reasonable agreement with comparable measurements reported by Green et al 23 For the relevant range of film thicknesses, reflection optical micrographs on transparent substrates appear pink for the metallic films and green for the semiconducting films due to constructive interference effects (insets, Figure 3b,e). Our main interest here is the influence of strain on flexible electronic performance, which is uninvestigated for type-purified SWCNT films.…”

“…The critical role of nanotube aspect ratio, for example, in dictating the percolation behavior of thin SWCNT membranes has recently been demonstrated. 6,9,10 Only very recently have improvements in SWCNT sorting and purification methods 11À15 enabled the production of highly uniform SWCNT ensembles with specific length and/ or electronic type distributions. Thin films made from such materials hold tremendous potential for flexible electronics applications, including the replacement of indium tin oxide (ITO) in liquid crystal displays and photovoltaic devices.…”

“…The density profile of a compressed film measured in TEM (Figure 4b,c) suggests that the SWCNTs align normal to the direction of prestrain. 9,10 Two pieces of the same film were thus deposited on each PDMS substrate; one oriented parallel to the compression axis (x) and the other oriented perpendicular (y). Gold electrodes were then deposited on the surface of the film, as shown in Figure 4f.…”

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

“…[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.…”

Wrinkling of inhomogeneously strained thin polymer films

“…8,[14][15][16] Although inhomogeneities or defects have been qualitatively linked to strain localization events, a systematic understanding of pre-existing strain homogeneity has remained elusive.…”