Influence of nanotube length on the optical and conductivity properties of thin single-wall carbon nanotube networks

Recent Advances in Transparent Electrodes and Their Multimodal Sensing Applications

This review examines the recent advancements in transparent electrodes and their crucial role in multimodal sensing technologies. Transparent electrodes, notable for their optical transparency and electrical conductivity, are revolutionizing sensors by enabling the simultaneous detection of diverse physical, chemical, and biological signals. Materials like graphene, carbon nanotubes, and conductive polymers, which offer a balance between optical transparency, electrical conductivity, and mechanical flexibility, are at the forefront of this development. These electrodes are integral in various applications, from healthcare to solar cell technologies, enhancing sensor performance in complex environments. The paper addresses challenges in applying these electrodes, such as the need for mechanical flexibility, high optoelectronic performance, and biocompatibility. It explores new materials and innovative techniques to overcome these hurdles, aiming to broaden the capabilities of multimodal sensing devices. The review provides a comparative analysis of different transparent electrode materials, discussing their applications and the ongoing development of novel electrode systems for multimodal sensing. This exploration offers insights into future advancements in transparent electrodes, highlighting their transformative potential in bioelectronics and multimodal sensing technologies.

Investigation on Junction Contacts of Semiconducting Carbon Nanotube Networks Using Conductive Atomic Force Microscopy

Semiconductor single-walled carbon nanotube (s-SWNT) networks have gained prominence in electronic devices due to their cost-effectiveness, relatively production-naturality, and satisfactory performance. Configuration, density, and resistance of SWNT-SWNT junctions are considered crucial factors influencing the overall conductivity of s-SWNT networks. In this study, we present a method for inferring the lower bounds of the SWNT-SWNT junction resistance in s-SWNT networks based on conductive atomic force microscopy TUNA images. This method further enables the proposal of a classification for SWNT-SWNT junctions based on the current behavior relative to their surroundings. The three types of SWNT-SWNT junctions are denoted as (i) true contact (T), (ii) poor contact (P), and (iii) false contact (F). Of them, the true and poor contacts, respectively, represent good and poor electrical contact for the subject SWNT-SWNT junctions whose electrical conductivity hardly improves under external tip pressure, while that of the false contact can be further improved by external pressure. Statistical analysis demonstrates that while T-type junctions make a significant contribution to network conductivity, their proportion accounts for only approximately 40%. The P-type and F-type junctions, which constitute over 60% of the total, may be a contributing factor that constrains the overall conductivity of the s-SWNT networks. The height ratio of the junction to the sum of two SWNTs was also observed to exhibit variations among the three types. Finally, we propose a three-dimensional model to elucidate the formation mechanism underlying each type of junction. The present study provides insights into the performance of spontaneous contacts between s-SWNTs in the networks, and the systematic image acquisition and junction classification processes may provide support for future advancements in these networks.

Universalized and robust length separation of carbon and boron nitride nanotubes with improved polymer depletion-based fractionation

Partitioning nanoparticles by shape and dimension is paramount for advancing nanomaterial standardization, fundamental colloidal investigations, and technologies such as biosensing and digital electronics. Length-separation methods for single-walled carbon nanotubes (SWCNTs) have historically incurred trade-offs in precision and mass throughput, and boron nitride nanotubes (BNNTs) are a rapidly emerging material analogue. We extend and detail a polymer precipitation-based method to fractionate populations of either nanotube type at significant mass scale for four distinct nanotube sources of increasing average diameter (0.7 nm to >2 nm). Such separations result in a supernant phase containing shorter nanotubes and a pellet phase containing the longer nanotubes, with the threshold length for depletion decreasing with increasing polymer concentration. Cross-comparison through analytical ultracentrifugation, spectroscopy, and microscopy versus applied polymer concentration show tailorable and precise length fractionation for 100 nm through >1 μm rod lengths, with fractionation also designable to remove non-nanotube impurities. The threshold length of depletion is further found to increase for decreasing nanotube diameter at fixed polymer concentration, a finding consistent with scaling attributable to nanotube radial excluded volume. The capabilities demonstrated herein promise to significantly advance nanotube implementation within the scientific community.

Ultra-Mild Fabrication of Highly Concentrated SWCNT Dispersion Using Spontaneous Charging in Solvated Electron System

The efficient dispersion of single-walled carbon nanotubes (SWCNTs) has been the subject of extensive research over the past decade. Despite these efforts, achieving individually dispersed SWCNTs at high concentrations remains challenging. In this study, we address the limitations associated with conventional methods, such as defect formation, excessive surfactant use, and the use of corrosive solvents. Our novel dispersion method utilizes the spontaneous charging of SWCNTs in a solvated electron system created by dissolving potassium in hexamethyl phosphoramide (HMPA). The resulting charged SWCNTs (c-SWCNTs) can be directly dispersed in the charging medium using only magnetic stirring, leading to defect-free c-SWCNT dispersions with high concentrations of up to 20 mg/mL. The successful dispersion of individual c-SWCNT strands is confirmed by their liquid-crystalline behavior. Importantly, the dispersion medium for c-SWCNTs exhibits no reactivity with metals, polymers, or other organic solvents. This versatility enables a wide range of applications, including electrically conductive free-standing films produced via conventional blade coating, wet-spun fibers, membrane electrodes, thermal composites, and core-shell hybrid microparticles.

Electrical Transport Mechanisms in Graphene Nanoplatelet Doped Polydimethylsiloxane and Application to Ultrasensitive Temperature Sensors

The temperature effect on electronic transport mechanisms in graphene nanoplatelet (GNP) doped polydimethylsiloxane (PDMS) for temperature sensing applications has been investigated under electrical impedance spectroscopy (EIS) analysis. AC measurements showed a very prevalent frequency-dependent behavior in low filled nanocomposites due to the lower charge density. In fact, 4 wt % GNP samples showed a nonideal capacitive behavior due to scattering effects. Therefore, the standard RC-LRC circuit varies with the substitution of capacitive elements by CPEs, where a CPE is a constant phase element which denotes energy dissipation. In this regard, the temperature promotes a prevalence of scattering effects, with an increase of resistance and inductance and a decrease of capacitance values in both RC (intrinsic and contact mechanisms) and LRC (tunneling mechanisms) elements and, even, a change from ideal to nonideal capacitive behavior as in the case of 6 wt % GNP samples. In this way, a deeper understanding of electronic mechanisms depending on GNP content and temperature is achieved in a very intuitive way. Finally, a proof-of-concept carried out as temperature sensors showed a huge sensitivity (from 0.05 to 11.7 °C^-1) in comparison to most of the consulted studies (below 0.01 °C^-1), proving, thus, excellent capabilities never seen before for this type of application.

Direct Measurement of Folding Angle and Strain Vector in Atomically Thin WS2 Using Second-Harmonic Generation

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Local strain engineering at the nanoscale level is practically achieved via permanently deformed wrinkled nanostructures, which are reported to show photoluminescence enhancement, bandgap modulation, and funneling effect. Folding in 2D materials is reported to tune optoelecronic properties via folding angle dependent interlayer coupling and symmetry variation. The accurate and efficient monitoring of local strain vector and folding angle is important to optimize the performance of optoelectronic devices. Conventionally, the accurate measurement of both strain amplitude and strain direction in wrinkled nanostructures requires the combined usage of multiple tools resulting in manufacturing lead time and cost. Here, we demonstrate the usage of a single tool, polarization-dependent second-harmonic generation (SHG), to determine the folding angle and strain vector accurately and efficiently in ultrathin WS₂. The folding angle in trilayer WS₂ folds exhibiting 1-9 times SHG enhancement is probed through variable approaches such as SHG enhancement factor, maxima and minima SHG phase difference, and linear dichroism. In compressive strain induced wrinkled nanostructures, strain-dependent SHG quenching and enhancement is observed parallel and perpendicular, respectively, to the direction of the compressive strain vector, allowing us to determine the local strain vector accurately using a photoelastic approach. We further demonstrate that SHG is highly sensitive to band-nesting-induced transition (C-peak), which can be significantly modulated by strain. Our results show SHG as a powerful probe to folding angle and strain vector.

Tunable Piezoresistivity from Magnetically Aligned Ni(Core)@Ag(Shell) Particles in an Elastomer Matrix

Core-shell (Ni@Ag) particles are aligned through the thickness of a poly(dimethylsiloxane) (PDMS) film using a magnetic field in a continuous roll-to-roll process. The alignment of the particles dramatically decreases the percolation threshold for electrical conductivity through the thickness of the film by nearly an order of magnitude from 28 vol % without the field to ≈1 vol % with a 52 mT magnetic field during curing. However, the magnetic forces lead to rough surface topography for intermediate Ni@Ag loadings, but confining the Ni@Ag/PDMS composite by a glass constraint provides a smooth surface. This difference in geometry changes the morphology of the vertically aligned "chains" of Ni@Ag particles where the chains are more aggregated when the film is unconstrained. As the Ni@Ag concentration is decreased below 3.6% for the constrained film, breaks in the aligned particles evident from X-ray tomography lead to pressure sensitive resistance across that film with a large decrease in resistance above a threshold pressure. The threshold pressure is demonstrated to be controllable from ≈15 to ≈290 kPa through the loading of aligned Ni@Ag in the PDMS, but this threshold pressure decreases on cyclic loading. These magnetically aligned composites represent a facile route to mechanically responsive films that could be used in a variety of applications where cyclic loading above and below the threshold pressure is not required, such as disposable pressure sensors for ensuring reliability of products through transportation and embedded structural health monitoring for identifying critical displacements.

Measuring in Situ Length Distributions of Polymer-Wrapped Monochiral Single-Walled Carbon Nanotubes Dispersed in Toluene with Analytical Ultracentrifugation

The length of a carbon nanotube is an important dimension that has to be adjusted to the requirements of an experiment or application, e.g., through sorting methods. So far, atomic force microscopy (AFM) has been the method of choice for measuring length distributions, despite being an ex situ method with apparent shortcomings. In this work, we explore analytical ultracentrifugation (AUC) as an in situ method for measuring the length distribution of polymer-wrapped (7, 5) single-walled carbon nanotubes dispersed in toluene. This is an AUC study of nanotubes in nonaqueous media, the preferred media for nanotubes used in device fabrication. In AUC, the temporally and spatially dependent change in optical absorption of a sample is measured under centrifugation. The resulting sedimentation curves can be deconvoluted with a standard data processing procedure (SEDFIT), to yield the sedimentation coefficient distribution. However, the conversion of the sedimentation coefficient distribution into a length distribution is nontrivial and requires finding a suitable model for the nanotube friction coefficient. Also, since AUC is based on optical absorption, it yields a volume distribution and not a number distribution as obtained from AFM reference data. By meeting these challenges and finding a surprisingly simple empirical flexible-chain-like model to describe the sedimentation behavior of one specific chiral structure, we suggest AUC as a viable method for measuring in situ nanotube length distributions of nonaqueous dispersions.

Carbon Nanotube-Doped Adhesive Films for Detecting Crack Propagation on Bonded Joints: A Deeper Understanding of Anomalous Behaviors

A novel nanoreinforced adhesive film has been developed to detect adhesive deformation and crack propagation along the bonding line by means of the electrical response of the material. Adhesive films were doped by spraying an aqueous dispersion of carbon nanotubes (CNTs) over the surface. To determine the sensitivity of bonded joints, single lap shear (SLS) and mode-I fracture energy tests have been carried out while their electrical response has been measured. It has been found that CNT-doped adhesive films are able to detect adhesive deformation and final failure for SLS specimens and crack initiation and propagation along the bonding line for mode-I specimens with a high sensitivity. Sudden increases on electrical resistance are correlated to a rapid growing of the crack length due to instability on crack propagation in a tick-slip case, whereas specimens with a more uniform crack propagation are linked to a steadier increase on electrical resistance, and both of them are properly correlated to the mechanical response. By analyzing more in detail the electrical response and comparing with theoretical approaches, the stick-slip behavior is associated with the presence of porosity and lack of adhesives because of possible manufacturing issues such as adhesive overflowing. These statements are also validated by microstructural analysis. Therefore, the potential and applicability of the proposed adhesive films for evaluating the structural integrity has been demonstrated.

Chiral-index resolved length mapping of carbon nanotubes in solution using electric-field induced differential absorption spectroscopy

The length of single-walled carbon nanotubes (SWCNTs) is an important metric for the integration of SWCNTs into devices and for the performance of SWCNT-based electronic or optoelectronic applications. In this work we propose a rather simple method based on electric-field induced differential absorption spectroscopy to measure the chiral-index-resolved average length of SWCNTs in dispersions. The method takes advantage of the electric-field induced length-dependent dipole moment of nanotubes and has been verified and calibrated by atomic force microscopy. This method not only provides a low cost, in situ approach for length measurements of SWCNTs in dispersion, but due to the sensitivity of the method to the SWCNT chiral index, the chiral index dependent average length of fractions obtained by chromatographic sorting can also be derived. Also, the determination of the chiral-index resolved length distribution seems to be possible using this method.

Carbon nanotube based transparent conductive films: progress, challenges, and perspectives

Developments in the manufacturing technology of low-cost, high-quality carbon nanotubes (CNTs) are leading to increased industrial applications for this remarkable material. One of the most promising applications, CNT based transparent conductive films (TCFs), are an alternative technology in future electronics to replace traditional TCFs, which use indium tin oxide. Despite significant price competition among various TCFs, CNT-based TCFs have good potential for use in emerging flexible, stretchable and wearable optoelectronics. In this review, we summarize the recent progress in the fabrication, properties, stability and applications of CNT-based TCFs. The challenges of current CNT-based TCFs for industrial use, in comparison with other TCFs, are considered. We also discuss the potential of CNT-based TCFs, and give some possible strategies to reduce the production cost and improve their conductivity and transparency.

Evidence of Universal Temperature Scaling in Self-Heated Percolating Networks

During routine operation, electrically percolating nanocomposites are subjected to high voltages, leading to spatially heterogeneous current distribution. The heterogeneity implies localized self-heating that may (self-consistently) reroute the percolation pathways and even irreversibly damage the material. In the absence of experiments that can spatially resolve the current distribution and a nonlinear percolation model suitable to interpret them, one relies on empirical rules and safety factors to engineer these materials. In this paper, we use ultrahigh resolution thermo-reflectance imaging, coupled with a new imaging processing technique, to map the spatial distribution ΔT(x, y; I) and histogram f(ΔT) of temperature rise due to self-heating in two types of 2D networks (percolating and copercolating). Remarkably, we find that the self-heating can be described by a simple two-parameter Weibull distribution, even under voltages high enough to reconfigure the percolation pathways. Given the generality of the phenomenological argument supporting the distribution, other percolating networks are likely to show similar stress distribution in response to sufficiently large stimuli. Furthermore, the spatial evolution of the self-heating of network was investigated by analyzing the spatial distribution and spatial correlation, respectively. An estimation of degree of hotspot clustering reveals a mechanism analogous to crystallization physics. The results should encourage nonlinear generalization of percolation models necessary for predictive engineering of nanocomposite materials.

A facile and low-cost length sorting of single-wall carbon nanotubes by precipitation and applications for thin-film transistors

Semiconducting single-wall carbon nanotubes (SWCNTs) with long lengths are highly desirable for many applications such as thin-film transistors and circuits. Previously reported length sorting techniques usually require sophisticated instrumentation and are hard to scale up. In this paper, we report for the first time a general phenomenon of a length-dependent precipitation of surfactant-dispersed carbon nanotubes by polymers, salts, and their combinations. Polyelectrolytes such as polymethacrylate (PMAA) and polystyrene sulfonate (PSS) are found to be especially effective on cholate and deoxycholate dispersed SWCNTs. By adding PMAA to these nanotube dispersions in a stepwise fashion, we have achieved nanotube precipitation in a length-dependent order: first nanotubes with an average length of 650 nm, and then successively of 450 nm, 350 nm, and 250 nm. A similar effect of nanotube length sorting has also been observed for PSS. To demonstrate the utility of the length fractionation, the 650 nm-long nanotube fraction was subjected to an aqueous two-phase separation to obtain semiconducting enriched nanotubes. Thin-film transistors fabricated with the resulting semiconducting SWCNTs showed a carrier mobility up to 18 cm(2) (V s)(-1) and an on/off ratio up to 10(7). Our result sheds new light on the phase behavior of aqueous nanotube dispersions under high concentrations of polymers and salts, and offers a facile, low-cost, and scalable method to produce length sorted semiconducting nanotubes for macroelectronics applications.

Intrinsic conductivity of carbon nanotubes and graphene sheets having a realistic geometry

The addition of carbon nanotubes (CNTs) and graphene sheets (GSs) into polymeric materials can greatly enhance the conductivity and alter the electromagnetic response of the resulting nanocomposite material. The extent of these property modifications strongly depends on the structural parameters describing the CNTs and GSs, such as their shape and size, as well as their degree of particle dispersion within the polymeric matrix. To model these property modifications in the dilute particle regime, we determine the leading transport virial coefficients describing the conductivity of CNT and GS composites using a combination of molecular dynamics, path-integral, and finite-element calculations. This approach allows for the treatment of the general situation in which the ratio between the conductivity of the nanoparticles and the polymer matrix is arbitrary so that insulating, semi-conductive, and conductive particles can be treated within a unified framework. We first generate ensembles of CNTs and GSs in the form of self-avoiding worm-like cylinders and perfectly flat and random sheet polymeric structures by using molecular dynamics simulation to model the geometrical shapes of these complex-shaped carbonaceous nanoparticles. We then use path-integral and finite element methods to calculate the electric and magnetic polarizability tensors (αE, αM) of the CNT and GS nanoparticles. These properties determine the conductivity virial coefficient σ in the conductive and insulating particle limits, which are required to estimate σ in the general case in which the conductivity contrast Δ between the nanoparticle and the polymer matrix is arbitrary. Finally, we propose approximate relationships for αE and αM that should be useful in materials design and characterization applications.

Engineering nanostructured polymer blends with controlled nanoparticle location for excellent microwave absorption: a compartmentalized approach

In order to obtain better materials, control over the precise location of nanoparticles is indispensable. It is shown here that ordered arrangements of nanoparticles, possessing different characteristics (electrical/magnetic dipoles), in the blend structure can result in excellent microwave absorption. This is manifested from a high reflection loss of ca. -67 dB for the best blend structure designed here. To attenuate electromagnetic radiation, the key parameters of high electrical conductivity and large dielectric/magnetic loss are targeted here by including a conductive material [multiwall carbon nanotubes, MWNTs], ferroelectric nanostructured material with associated relaxations in the GHz frequency [barium titanate, BT] and lossy ferromagnetic nanoparticles [nickel ferrite, NF]. In this study, bi-continuous structures were designed using 50/50 (by wt) blends of polycarbonate (PC) and polyvinylidene fluoride (PVDF). The MWNTs were modified using an electron acceptor molecule, a derivative of perylenediimide, which facilitates π-π stacking with the nanotubes and stimulates efficient charge transport in the blends. The nanoscopic materials have specific affinity towards the PVDF phase. Hence, by introducing surface-active groups, an ordered arrangement can be tailored. To accomplish this, both BT and NF were first hydroxylated followed by the introduction of amine-terminal groups on the surface. The latter facilitated nucleophilic substitution reactions with PC and resulted in their precise location. In this study, we have shown for the first time that by a compartmentalized approach, superior EM attenuation can be achieved. For instance, when the nanoparticles were localized exclusively in the PVDF phase or in both the phases, the minimum reflection losses were ca. -18 dB (for the MWNT/BT mixture) and -29 dB (for the MWNT/NF mixture), and the shielding occurred primarily through reflection. Interestingly, by adopting the compartmentalized approach wherein the lossy materials were in the PC phase and the conductive materials (MWNT) were in the PVDF phase, outstanding reflection losses of ca. -57 dB (for the BT and MWNT combination) and -67 dB (for the NF and MWNT combination) were noted and the shielding occurred primarily through absorption. Thus, the approach demonstrates that nanoscopic structuring in the blends can be achieved under macroscopic processing conditions and this strategy can further be explored to design microwave absorbers.

Confronting the complexity of CNT materials

The morphology of commercially available carbon nanotube materials is often much more complex than the term "carbon nanotube" (CNT) would imply. Commercial CNT materials are typically composed of roughly spherical CNT domains having a highly ramified internal structure and a size on the order of microns. Clearly, such structures cannot reasonably be modeled as "rods". To address this problem, we first perform molecular dynamics simulations (MD) to generate structures similar to those measured experimentally, based on the presumptions that CNT domains are composed of worm-like cylinders having observed persistence lengths and that these CNTs are confined to spherical domains having the observed average domain size. This simple model generates structures remarkably similar to those observed experimentally. We then consider numerical path-integral computations to calculate the self-capacitance C and intrinsic conductivity [σ]∞ of these CNT rich domains. This information is then incorporated in a generalized effective medium theory to estimate the conductivity of bulk composite materials composed of these complex-shaped "particles". We term these CNT structures "tumbleweeds", given their evident morphological similarity to this naturally occurring growth form. Based on this model, we find that the conductivity percolation threshold of the tumbleweeds can be quite low, despite their quasi-spherical average shape. We also examine the structure factor S(q) of the CNT-rich domains as function of the number N of CNTs within them, to aid in the structural characterization of CNT nanocomposites. The structure factor S(q) of our model tumbleweed is found to resemble that of hyperbranched, star and dendrimer polymers, and also domain structures observed in polyelectrolytes. Commercial CNT materials at high loading should then have physical features in common with suspension of "soft" colloidal particles by virtue of their deformability and roughly spherical shape.

Controlling exfoliation in order to minimize damage during dispersion of long SWCNTs for advanced composites

We propose an approach to disperse long single-wall carbon nanotubes (SWCNTs) in a manner that is most suitable for the fabrication of high-performance composites. We compare three general classes of dispersion mechanisms, which encompass 11 different dispersion methods, and we have dispersed long SWCNTs, short multi-wall carbon nanotubes, and short SWCNTs in order to understand the most appropriate dispersion methods for the different types of CNTs. From this study, we have found that the turbulent flow methods, as represented by the Nanomizer and high-pressure jet mill methods, produced unique and superior dispersibility of long SWCNTs, which was advantageous for the fabrication of highly conductive composites. The results were interpreted to imply that the biaxial shearing force caused an exfoliation effect to disperse the long SWCNTs homogeneously while suppressing damage. A conceptual model was developed to explain this dispersion mechanism, which is important for future work on advanced CNT composites.

Biocatalytic carbon nanotube paper: a ‘one-pot’ route for fabrication of enzyme-immobilized membranes for organophosphate bioremediation

A generic methodology for a rapid and direct fabrication of enzymatically-active carbon nanotubes (CNTs) paper for organophosphates bioremediation is presented. The enzyme organophosphate hydrolase is immobilized onto CNTs simultaneously to membrane formation process.

Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.

Aligned SWNT films from low-yield stress gels and their transparent electrode performance

Carbon nanotube films are promising for transparent electrodes for solar cells and displays. Large-area alignment of the nanotubes in these films is needed to minimize the sheet resistance. We present a novel coating method to coat high-density, aligned nanotubes over large areas. Carbon nanotube gel dispersions used in this study have aligned domains and a low yield stress. A simple shearing force allows these domains to uniformly align. We use this to correlate the transparent electrode performance of single-walled carbon nanotube films with the level of partial alignment. We have found that the transparent electrode performance improves with increasing levels of alignment and in a manner slightly better than what has been previously predicted.

Resonant microwave absorption in thermally deposited au nanoparticle films near percolation coverage

We observe a resonant transition in the microwave absorption of thin thermally deposited Au nanoparticle films near the geometrical percolation transition pc where the films exhibit a 'fractal' heterogeneous geometry. Absorption of incident microwave radiation increases sharply near pc, consistent with effective medium theory predictions. Both the theory and our experiments indicate that the hierarchical structure of these films makes their absorption insensitive to the microwave radiation wavelength λ, so that this singular absorption of microwave radiation is observed over a broad frequency range between 100 MHz and 20 GHz. The interaction of electromagnetic radiation with randomly distributed conductive scattering particles gives rise to localized resonant modes, and our measurements indicate that this adsorption process is significantly enhanced for microwaves in comparison to ordinary light. In particular, above the percolation transition a portion of the injected microwave power is stored within the film until dissipated. Finally, we find that the measured surface conductivity can be quantitatively described at all Au concentrations by generalized effective medium theory, where the fitted conductivity percolation exponents and pc itself are consistent with known two-dimensional estimates. Our results demonstrate that microwave measurements provide a powerful means of remotely measuring the electromagnetic properties of highly heterogeneous conducting films, enabling purposeful engineering of the electromagnetic properties of thin films in the microwave frequency range through fabrication of 'disordered' films of conducting particles such as metal nanoparticles or carbon nanotubes.

Variable range hopping in single-wall carbon nanotube thin films: a processing-structure-property relationship study

By varying the ultrasonication and ultracentrifugation conditions, single-walled carbon nanotube (SWCNT) dispersions with a broad range of SWCNT length and diameter (L = 342-3330 nm; d = 0.5-12 nm) were prepared and characterized by a preparative ultracentrifuge method (PUM) and dynamic light scattering (DLS) technique. The well-characterized dispersions were then fabricated into SWCNT thin films by spray coating. Combined optical, spectroscopic, and temperature-dependent electrical measurements were performed to study the effect of SWCNT structures on the charge transport behavior of SWCNT thin films. Regardless of SWCNT size in the dispersion and the thin film thickness, the three-dimensional variable range hopping (3D VRH) conduction model was found to be appropriate in explaining the temperature-dependent sheet resistance results for all SWCNT thin films prepared in this study. More importantly, with the SWCNT structural information determined by the PUM method, we were able to identify a strong correlation between the length of SWCNTs and the 3D VRH parameter T0, the Mott characteristic temperature. When the SWCNT length is less than ∼700 nm, the T0 of SWCNT thin films shows a drastic increase, but when the length is greater than ~700 nm, T0 is only weakly dependent on the SWCNT length. Under the framework of traditional VRH, we further conclude that the electron localization length of SWCNT thin films shows a similar dependence on the SWCNT length.

Structural Properties of Chemically Functionalized Carbon Nanotube Thin Films

Buckypapers are thin sheets of randomly entangled carbon nanotubes, which are highly porous networks. They are strong candidates for a number of applications, such as reinforcing materials for composites. In this work, buckypapers were produced from multiwall carbon nanotubes, pre-treated by two different chemical processes, either an oxidation or an epoxidation reaction. Properties, such as porosity, the mechanical and electrical response are investigated. It was found that the chemical pretreatment of carbon nanotubes strongly affects the structural properties of the buckypapers and, consecutively, their mechanical and electrical performance.

Graphene oxide as a multi-functional p-dopant of transparent single-walled carbon nanotube films for optoelectronic devices

Modulation of electronic structures and surface properties of transparent carbon nanotube films is a challenging issue for their application in optoelectronic devices. Here, we report, for the first time, that graphene oxide (GO) nanosheets play the role of a p-doping agent and surface energy modifier of single-walled carbon nanotube (SWCNT)-based transparent conducting electrodes (TCEs). The deposition of highly oxidized, small-sized (i.e., diameter of less than 500 nm) GO nanosheets onto a SWCNT network film reduces the sheet resistance of the pristine film to 60% of its original value by p-doping. The modified TCEs exhibit an outstanding optoelectronic feature of high conductivity with high transparency. Moreover, the wettability of the electrode surface was also noticeably increased, which is advantageous for the solution-based processing of organic electronics. Furthermore, the organic photovoltaic (OPV) cells with the GO-doped SWCNT anodes on flexible substrates were successfully demonstrated. In stark contrast to a power conversion efficiency of 0.44% for pristine SWCNT anodes, GO-doped SWCNT anodes show a drastically enhanced power conversion efficiency of 2.7%.

Preferential growth of short aligned, metallic-rich single-walled carbon nanotubes from perpendicular layered double hydroxide film

Direct bulk growth of single-walled carbon nanotubes (SWCNTs) with required properties, such as diameter, length, and chirality, is the first step to realize their advanced applications in electrical and optical devices, transparent conductive films, and high-performance field-effect transistors. Preferential growth of short aligned, metallic-rich SWCNTs is a great challenge to the carbon nanotube community. We report the bulk preferential growth of short aligned SWCNTs from perpendicular Mo-containing FeMgAl layered double hydroxide (LDH) film by a facile thermal chemical vapor deposition with CH(4) as carbon source. The growth of the short aligned SWCNTs showed a decreased growth velocity with an initial value of 1.9 nm s(-1). Such a low growth velocity made it possible to get aligned SWCNTs shorter than 1 μm with a growth duration less than 15 min. Raman spectra with different excitation wavelengths indicated that the as-grown short aligned SWCNTs showed high selectivity of metallic SWCNTs. Various kinds of materials, such as mica, quartz, Cu foil, and carbon fiber, can serve as the substrates for the growth of perpendicular FeMoMgAl LDH films and also the growth of the short aligned SWCNTs subsequently. These findings highlight the easy route for bulk preferential growth of aligned metallic-rich SWCNTs with well defined length for further bulk characterization and applications.

Effects of inter-tube distance and alignment on tunnelling resistance and strain sensitivity of nanotube/polymer composite films

A model for carbon nanotube (CNT)/polymer composite conductivity is developed, considering the effect of inter-tube tunnelling through the polymer. The statistical effects of inter-tube distance and alignment on the tunnelling are investigated through numerical modelling, to highlight their role in the conductance and piezoresistance of the composite film. The impact of critical parameters, including the concentration, alignment and aspect ratio of the CNTs and the tunnelling barrier height of the polymer is statistically evaluated using a large number of randomly generated CNT/polymer composite films. A numerical model is presented for the tunnelling resistance as a function of CNT concentration and polymer properties, which provides good agreement with the reported conductance in the literature. In particular, for a low concentration of CNTs close to the percolation threshold, we demonstrate how tunnelling dominates the conductance properties and leads to significant increase in the piezoresistance of the composite. This is important for gaining insight into the optimum concentration and alignment of the CNTs in the composite film for applications such as strain sensors, anisotropic conductive films, transparent electrodes and flexible electronics.

Electronic durability of flexible transparent films from type-specific single-wall carbon nanotubes

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 topography, microstructure and transparency of the films are independently characterized using optical microscopy, electron microscopy, and optical absorption spectroscopy. Thin films made from metallic SWCNTs show better durability as flexible transparent conductive coatings, which we attribute to a combination of superior mechanical performance and higher interfacial conductivity.

Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films

The use of carbon nanotubes (CNTs) as transparent conducting films is one of the most promising aspects of CNT-based applications due to their high electrical conductivity, transparency, and flexibility. However, despite many efforts in this field, the conductivity of carbon nanotube network films at high transmittance is still not sufficient to replace the present electrodes, indium tin oxide (ITO), due to the contact resistances and semi-conducting nanotubes of the nanotube network films. Many studies have attempted to overcome such problems by the chemical doping and hybridization of conducting guest components by various methods, including acid treatment, deposition of metal nanoparticles, and the creation of a composite of conducting polymers. This review focuses on recent advances in surface-modified carbon nanotube networks for transparent conducting film applications. Fabrication methods will be described, and the stability of carbon nanotube network films prepared by various methods will be demonstrated.

Carbon nanotubes: measuring dispersion and length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Modulating conductivity, environmental stability of transparent conducting nanotube films on flexible substrates by interfacial engineering

We have characterized the previously undescribed parameters for engineering the electrical properties of single-walled carbon nanotube (SWCNT) films for technological applications. First, the interfacial tension between bare SWCNT network films and a top coating passivation material was shown to dictate the variability of the films' sheet resistance (R(s)) after application of the top coating. Second, the electrical stability of the coated SWCNT films was affected by the mismatch between the CTE of the supporting substrate and the SWCNT network film. An upshift in the Raman G-band spectrum of SWCNTs on bare PET suggested that compressive strain was induced by the CTE mismatch after heating and cooling. These findings provide important guidelines for the choice of substrate and passivation coating materials that promote environmental stability in SWCNT-based transparent conductive films.

Fractionation of single wall carbon nanotubes by length using cross flow filtration method

A novel system for fractionating single wall carbon nanotubes (SWCNTs) by length via a three-step cross-flow filtration has been developed in which three membrane filters of different pore sizes, 1.0, 0.45, and 0.2 microm, were used. SWCNTs dispersed in water with the help of sodium carboxymethylcellulose (CMC) detergents were successfully sorted into four samples, and the atomic force microscopy (AFM) observation of those samples confirmed that their length distribution peaks are within the expected ranges from pore sizes of used filters. However, the result of the similar filtration process using a different detergent, sodium dodecylbenzenesulfonate (SDBS), showed no pronounced correlation between the length distribution of SWCNTs and the pore size. The observed difference in the sorting phenomena caused by the detergent type suggests that the permeation property depends on the complex structure resulting from the dispersed SWCNTs and detergent molecules.

Wrinkling and strain softening in single-wall carbon nanotube membranes

The nonlinear elasticity of thin supported membranes assembled from length purified single-wall carbon nanotubes is analyzed through the wrinkling instability that develops under uniaxial compression. In contrast with thin polymer films, pristine nanotube membranes exhibit strong softening under finite strain associated with bond slip and network fracture. We model the response as a shift in percolation threshold generated by strain-induced nanotube alignment in accordance with theoretical predictions.

A two-step shearing strategy to disperse long carbon nanotubes from vertically aligned multiwalled carbon nanotube arrays for transparent conductive films

A surfactant-free two-step shearing strategy was applied to disperse vertically aligned carbon nanotube (VACNT) arrays into individually dispersed CNTs. First, big blocks of VACNT arrays were sheared into fluffy CNTs. The fluffy CNTs were composed of CNT bundles with a diameter of 1-10 microm and a length of several millimeters. After that, the fluffy CNTs were further sheared in liquid phase to obtain individually dispersed CNTs. As comparison, sonication and grinding were also employed for further dispersion of the fluffy CNTs. The length of CNTs dispersed by shearing method was the longest and up to several hundred micrometers. The CNT dispersions from the three methods can be used to fabricate transparent conductive films (TCFs). The TCFs from CNTs dispersed by shearing method showed the highest conductivity at the same transparency. VACNT arrays with a small diameter (approximately 10 nm) were dispersed by the shearing method as well, from which the TCF with a surface resistance of 2.5 kohm/square and a transparency of 78.6% (at 500 nm) was obtained. The ratio of dc to optical conductivity (sigma(dc)/sigma(op)) of the as-dispersed CNT array was 0.711, which can compare beauty with that of single-walled CNTs and double-walled CNTs grown by the CVD process.

Growth velocity and direct length-sorted growth of short single-walled carbon nanotubes by a metal-catalyst-free chemical vapor deposition process

We report on the observation of a very low growth velocity of single-walled carbon nanotubes (SWNTs) and consequently the direct length-sorted growth and patterned growth of SWNTs by using a metal-catalyst-free chemical vapor deposition (CVD) process proposed recently by our group, in which SiO(2) serves as catalyst. We found that the growth velocity of the SWNTs from SiO(2) catalyst is only 8.3 nm/s, which is about 300 times slower than that of the commonly used iron group catalysts (Co as a counterpart catalyst in this study). Such a slow growth velocity renders direct length-sorted growth of SWNTs, especially for short SWNTs with hundreds of nanometers in length. By simply adjusting the growth duration, SWNTs with average lengths of 149, 342, and 483 nm were selectively obtained and SWNTs as short as approximately 20 nm in length can be grown directly. Moreover, comparative studies indicate that the SiO(2) catalyst possesses a much longer catalytic active time, showing sharp contrast with the commonly used Co catalyst which quickly loses its catalytic activity. Taking advantage of the very slow growth velocity of the SiO(2) catalyst, patterned growth of SWNT networks confined in a narrow region of <5 microm was also achieved. The short SWNTs may show intriguing physics owing to their finite length effect and are attractive for various practical applications.

Electrical connectivity in single-walled carbon nanotube networks

Transport in single-walled carbon nanotubes (SWCNTs) networks is shown to be dominated by resistance at network junctions which scale with the size of the interconnecting bundles. Acid treatment, known to dope individual tubes, actually produces a dramatic reduction in junction resistances, whereas annealing significantly increases this resistance. Measured junction resistances for pristine, acid-treated and annealed SWCNT bundles correlate with conductivities of the corresponding films, in excellent agreement with a model in which junctions control the overall network performance.

In situ assembly of multi-sheeted buckybooks from single-walled carbon nanotubes

We report a simple approach for the direct and nondestructive assembly of multi-sheeted single-walled carbon nanotube book-like macrostructures (buckybooks) with good control of the nanotube diameter, the sheet packing density, and the book thickness during the floating catalytic growth process. The promise of such buckybooks is highlighted by demonstrating their high capacitance and high-efficiency molecular separation by directly using them as a binder-free electrode and as a filter, respectively. Our approach also provides a flexible and reliable way to easily assemble various other types of nanotubes into book-like or even more sophisticated sandwich-like hybrid macrostructures, realizing the shape-engineering of one-dimensional nanostructures to macroscopic well-defined architectures for various applications.