Single-walled Carbon Nanotube Thin Films Research Articles

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Young's modulus of SWNT film coating used in this work does not impair the actuation range.

Exploiting the hierarchical morphology of single-walled and multi-walled carbon nanotube films for highly hydrophobic coatings.

Self-assembled hierarchical solid surfaces are very interesting for wetting phenomena, as observed in a variety of natural and artificial surfaces. Here, we report single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) thin films realized by a simple, rapid, reproducible, and inexpensive filtration process from an aqueous dispersion, that was deposited at room temperature by a dry-transfer printing method on glass. Furthermore, the investigation of carbon nanotube films through scanning electron microscopy (SEM) reveals the multi-scale hierarchical morphology of the self-assembled carbon nanotube random networks. Moreover, contact angle measurements show that hierarchical SWCNT/MWCNT composite surfaces exhibit a higher hydrophobicity (contact angles of up to 137°) than bare SWCNT (110°) and MWCNT (97°) coatings, thereby confirming the enhancement produced by the surface hierarchical morphology.

Highly conductive single-walled carbon nanotube thin film preparation by direct alignment on substrates from water dispersions.

A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant attention, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. We present a method that uniquely produces a noncorrosive ink that can be directly applied to a device in situ, which is different from previous fabrication techniques. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g., superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with a rodlike morphology as measured by light scattering. The Mayer rod technique is used to prepare urea-SWNT, highly aligned films (two-dimensional nematic order parameter of 0.6, 5 μm spot size, via polarized Raman) with resistance values as low as 15-1700 Ω/sq in a transmittance range of 2-80% at 550 nm. These values compete with the best literature values for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

High-density metallic nanogap arrays for the sensitive detection of single-walled carbon nanotube thin films.

We have investigated the extraordinary optical transmission of terahertz waves through an array of nanogaps with varying dimensions and periodicities, and used this platform to demonstrate terahertz sensing of a thin film of single-walled carbon nanotubes. We have used atomic layer lithography to fabricate periodic arrays of nanogap loops that have a gap size of 2 nm and a loop length of 100 μm (aspect ratio of 50,000). These sub-mm-scale loops of nanogaps can sustain terahertz electromagnetic resonances along the contour. We have characterized the transmission of terahertz waves through the nanogap arrays and investigated the influence of inter-gap electromagnetic coupling as the array periodicity shrinks from 100 μm to 4 μm. While the gaps occupy only 0.1% of the surface area, we have measured an amplitude (|E|) transmittance of over 50% due to the strong and broadband field enhancement inside the nanogaps. The absolute transmission through the 2 nm gaps along the rectangular loops can be boosted up to 25%, while it is only 1% for annular gaps with the same perimeter. Furthermore, the extremely tight field confinement and strong field enhancement near the 2 nm gap lead to 43% extinction of THz waves in a 10 nm-thick film of single-walled carbon nanotubes over the gaps. On the other hand, THz extinction by the same nanotube film on a bare glass substrate is only 2%. These nanogaps pave the way toward developing sensitive terahertz detectors for biological and chemical targets.

Effect of orientation on the piezoresistivity of mechanically drawn single walled carbon nanotube (SWCNT) thin films

Mechanically drawn SWCNT thin films were prepared and systematically investigated for understanding the effect of SWCNT orientation on the piezoresistive response of SWCNT thin film based piezoresistive sensors. Through polarized Raman spectroscopy analysis and coupled electrical–mechanical test, a quantitative relationship between the piezoresistive sensitivity and SWCNT alignment order parameter was established. This along with the morphological characterization, film area and sheet resistance measurement allow identifying the critical roles of SWCNT orientation, packing density, and micro-crack formation in dictating the piezoresistivity of mechanically drawn SWCNT thin film strain sensors. As compared to the randomly oriented SWCNT thin film sensor, the one with draw ratio of 3.2 exhibited 6times increase on the gauge factor.

Large-area, low-voltage, antiambipolar heterojunctions from solution-processed semiconductors.

The emergence of semiconducting materials with inert or dangling bond-free surfaces has created opportunities to form van der Waals heterostructures without the constraints of traditional epitaxial growth. For example, layered two-dimensional (2D) semiconductors have been incorporated into heterostructure devices with gate-tunable electronic and optical functionalities. However, 2D materials present processing challenges that have prevented these heterostructures from being produced with sufficient scalability and/or homogeneity to enable their incorporation into large-area integrated circuits. Here, we extend the concept of van der Waals heterojunctions to semiconducting p-type single-walled carbon nanotube (s-SWCNT) and n-type amorphous indium gallium zinc oxide (a-IGZO) thin films that can be solution-processed or sputtered with high spatial uniformity at the wafer scale. The resulting large-area, low-voltage p-n heterojunctions exhibit antiambipolar transfer characteristics with high on/off ratios that are well-suited for electronic, optoelectronic, and telecommunication technologies.

Temperature-dependent nonlinear phonon behavior in high-density carbon nanotube thin films

We report the temperature-dependent Raman spectra for high-density single-walled carbon nanotube thin films. We show that the position of the main Raman mode (G) softens as the temperature increases and is nonlinear in the range of 70–270 K. This effect is explained by optical phonon decay. In the linear regime, the first-order temperature coefficient (χT) equals −0.02 cm−1/K, which is lower than for any other carbon nanotubes. Importantly, we found that local laser-induced temperature change shows a nonlinear trend as a function of global temperature with a minimum at 270 K. Our results contribute to understand the thermal properties of carbon nanotube thin films that could be applied, for example, in photovoltaic or thermoelectric devices.

High‐frequency impedance of single‐walled carbon nanotube networks on transparent flexible substrate

Transparent and flexible networks with randomly distributed single‐walled carbon nanotubes (SWCNTs) are emerging as novel materials for various applications, particularly as electronic materials. We investigate the frequency‐dependent impedance measurements up to 20 GHz, of single‐walled carbon nanotube networks on transparent and flexible substrates, by using two different techniques: two probe and Corbino reflectometry setups. The thickness and roughness of the thin film of SWCNT are examined by AFM. The impedance measurements show the cut‐off frequency increases with increasing the density of SWCNT. The log–log plot of low frequency impedance as a function of the cut‐off frequency shows the same slope in both measuring techniques. The slope is independent on sample geometries, which is characteristic of a totally disordered system. The cut‐off frequencies observed by the Corbino reflectometry technique are 3 orders of magnitude higher than those observed by two‐probe technique even for the same SWCNTs density network.

Intrinsic diameter dependent degradation of single-wall carbon nanotubes from ion irradiation

The structural response of single-wall carbon nanotube (SWCNT) thin-films to ion irradiation has been systematically studied with increasing fluence between 5×1012 and 1×1015 (150keV11B+)/cm2 as a function of SWCNT diameter distribution. SEM analysis reveals that the SWCNT morphology remains intact after radiation exposure. The optical absorbance intensity decreases with increasing fluence, and near complete suppression is observed at approximately 2.5×1014(11B+)/cm2, resulting in an equivalent reduction in signal intensity for resonant Raman spectroscopy. Changes observed in the optical response with increasing fluence demonstrate that defective SWCNTs behave similarly to other strongly doped or physically shortened SWCNTs. The Raman D/G′ increases with fluence for all samples, yet the magnitude of increase is strongly dependent on the SWCNT diameter distribution. The diameter dependence stems from differences in the SWCNT mass per unit length, which is incorporated into an inter-vacancy length (Lv) model that describes the radiation-induced changes. Ultimately, all SWCNT data converge to show an equivalent change in D/G′ at a corresponding Lv, even though the fluence required to achieve a particular Lv varies with SWCNT diameter. Thus, the radiation-induced changes in SWCNTs are intrinsically linked to their diameter, representing a fundamental property unique to this class of nanomaterials.

Fully solution-processed transparent conducting oxide-free counter electrodes for dye-sensitized solar cells: spray-coated single-wall carbon nanotube thin films loaded with chemically-reduced platinum nanoparticles.

We report fully solution-processed fabrication of transparent conducting oxide-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) by combining spray-coating of single-wall carbon nanotubes (SWCNTs) and chemical reduction of chloroplatinic acid precursor to platinum nanoparticles (Pt NPs) with formic acid. The power conversion efficiency of a semitransparent DSSC with such SWCNT-based CE loaded with Pt NPs is comparable to that of a control device with a conventional CE. Quantification of Pt loading shows that network morphology of entangled SWCNTs is efficient in forming and retaining chemically reduced Pt NPs. Moreover, electron microscopy and electrochemical impedance spectroscopy results show that mainly Pt NPs, which are tens of nanometers in diameter and reside at the surface of SWCNT CEs, contribute to electrocatalytic activity for triiodide reduction, to which we attribute strong correlation between power conversion efficiency of DSSCs and time constant deduced from equivalent-circuit analysis of impedance spectra.

Carbon Nanotube Flexible Electronics

Being mechanically flexible with superior electrical properties, thin-films of single-wall carbon nanotubes are proven to be one of the most promising material platforms for high performance flexible electronics. In this talk, I will discuss our solution-based platform for wafer-scale assembly of high-purity semiconducting carbon nanotube networks, which leads to low-cost fabrication of large quantity of thin-film transistors (TFTs) with excellent yield, uniformity, and respectable performance on flexible substrates. Based on the semiconducting carbon nanotube TFTs, a wide range of macro-scale system-level electronics have been demonstrated including flexible integrated circuits, flexible active-matrix organic light-emitting diode displays, and a user-interactive electronic skin that can simultaneously map and respond to various forms of stimuli. With emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required, our demonstrations present a unique niche for nanotube electronics by taking full advantage of their superior electrical and physical properties. Our work shows carbon nanotubes’ immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent performance.

Meniscus-dragging deposition of single-walled carbon nanotubes for highly uniform, large-area, transparent conductors

We report the simple production of highly uniform single-walled carbon nanotube (SWCNT) thin films with significant reduction of the conducting material consumption by applying an effective microliter-scale solution process. The transparent thin film conductors were formed directly on a rigid or flexible substrate within a short process time by dragging a meniscus of an aqueous SWCNT solution with a volume of tens of microliters. The easily controllable coating parameters enabled the fine-tuning of the film thickness on a nanometer scale. This coating process is well suited for large-area, bendable SWCNT thin films whose randomly interconnected networks exhibit two-dimensional percolation behavior.

Effect of atomic interconnects on percolation in single-walled carbon nanotube thin film networks.

The formation of covalent bonds to single-walled carbon nanotube (SWNT) or graphene surfaces usually leads to a decrease in the electrical conductivity and mobility as a result of the structural rehybridization of the functionalized carbon atoms from sp(2) to sp(3). In the present study, we explore the effect of metal deposition on semiconducting (SC-) and metallic (MT-) SWNT thin films in the vicinity of the percolation threshold and we are able to clearly delineate the effects of weak physisorption, ionic chemisorption with charge transfer, and covalent hexahapto (η(6)) chemisorption on these percolating networks. The results support the idea that for those metals capable of forming bis-hexahapto-bonds, the generation of covalent (η(6)-SWNT)M(η(6)-SWNT) interconnects provides a conducting pathway in the SWNT films and establishes the transition metal bis-hexahapto organometallic bond as an electronically conjugating linkage between graphene surfaces.

Fabrication of stable pn junction single-walled carbon nanotube thin films by position selective Cs plasma irradiation method

Stable n-type thin film transistors (TFTs) are fabricated with Cs encapsulated single-walled carbon nanotubes (Cs@SWNTs). The transport property of SWNTs-TFTs clearly changes from p- to n-type characteristic after the Cs plasma irradiation. Based on the systematic investigations, it is revealed that there is an optimum ion energy for the effective Cs encapsulation, which is around 50 eV. Furthermore, it is also found that the n-type feature is stable even in water and high temperature (< 400 °C) conditions. The pn junction structure is also realized by position selective doping of Cs. This very stable pn junction TFT is important for the practical application of SWNTs-based thin film electronics.

Analyte-Dependent Sensing Mechanisms: The Fabrication and Characterization of a 32-Channel Array of SWCNT-TF Chemiresistive Sensors

In this article, we discuss the fabrication and characterization of a 32-channel array of single-walled carbon nanotube thin film (SWCNT-TF) chemiresistor sensors. By simply varying the SWCNT-TF device geometry from a length-to-width (L/W) ratio of 1.5:1?15:1, the device switching characteristics were tuned, which provided a unique platform for the investigation of analyte-dependent sensing mechanisms. Our results indicate that the dominant mechanisms for pH sensing and M13 bacteriophage sensing are distinctively different. The percentage resistance change (?R/R0) of the SWCNT-TF devices in response to pH change can be well explained by chemical doping due to OH?/H+ adsorption on nanotubes. In contrast, electrostatic gating was found dominant in the sensing of M13 bacteriophage, where a decrease in the ?double-layer capacitance by M13 captured on the sensor?s surface significantly reduced the overall SWCNT-TF conductance. The of all devices was found to exhibit a logarithmic dependence on M13 concentration. An experimental detection limit of 0.5 pM M13 was demonstrated on all devices, with a theoretical detection limit down to the fM range on high-aspect-ratio (15:1) devices. Using M13 bacteriophage as a model virus, this study demonstrated the potential of SWCNT-TF chemiresistor sensors for real-time electrical virus detection, provided better understanding of sensing mechanisms, and outlined the importance of film/device geometry in the optimization of sensitivity and reproducibility.

Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells.

There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future.

SWCNT-thin-film-enabled fiber sensors for lifelong structural health monitoring of polymeric composites - From manufacturing to utilization to failure

With glass, polyaramid, nylon and polyethylene terephthalate as the fiber substrate, a continuous spray coating and winding process was developed for fabricating SWCNT-FibSen sensor – single-walled carbon nanotube (SWCNT) thin film enabled 1D fiber sensor. The microstructures of the SWCNT-FibSen and its piezoresistive behavior were respectively characterized by scanning electron microscopy and Raman spectroscopy and evaluated by the coupled electromechanical tensile test. With embedding the SWCNT-FibSen sensor in the fiberglass prepreg laminates at different orientations and locations, its use for multipurpose sensing in life-long (from manufacturing to failure) structural health monitoring of high-performance polymeric composites was demonstrated. During the manufacturing process, the sensor was able to provide valuable local resin curing information. After the manufacturing process, the sensor was useful in mapping the stress/strain states of the host composite under different deformation modes – tension, bending, compression and failure. The integrity of the sensing performance of the embedded SWCNT-FibSen sensor was demonstrated upon a 10,000-cyclic coupled electromechanical test.

Paper-based ultracapacitors with carbon nanotubes-graphene composites

In this paper, a paper-based ultracapacitors were fabricated by the rod-rolling method with the ink of carbon nanomaterials, which were synthesized by arc discharge under various magnetic conditions. Composites of carbon nanostructures, including high-purity single-walled carbon nanotubes (SWCNTs) and graphene flakes were synthesized simultaneously in a magnetically enhanced arc. These two nanostructures have promising electrical properties and synergistic effects in the application of ultracapacitors. Scanning electron microscope, transmission electron microscope, and Raman spectroscopy were employed to characterize the properties of carbon nanostructures and their thin films. The sheet resistance of the SWCNT and composite thin films was also evaluated by four-point probe from room temperature to the cryogenic temperature as low as 90 K. In addition, measurements of cyclic voltammetery and galvanostatic charging/discharging showed the ultracapacitor based on composites possessed a superior specific capacitance of up to 100 F/g, which is around three times higher than the ultracapacitor entirely fabricated with SWCNT.

Diameter distribution control of single-walled carbon nanotubes by etching ferritin nanoparticles

We focused on controlling the diameter of single-walled carbon nanotubes (SWNTs) by controlling the ferritin catalyst diameter using reactive ion etching (RIE). The average diameter of ferritin decreased by about 26.5% upon RIE treatment, which resulted in the decrease of the mean diameter of the SWNTs by about 34% with a small diameter distribution. This was demonstrated by both atomic force microscopic and Raman spectroscopic measurements. We found that the current on/off ratio of the SWNT thin film transistors was significantly improved after the catalyst treatment. It was estimated that the purity of the semiconducting SWNT also increased from 58.8 to 92.4%.

Fabrication of Single-Walled Carbon Nanotube/Si Heterojunction Solar Cells with High Photovoltaic Performance

The photovoltaic properties of carbon nanotube/Si heterojunction solar cells were investigated using network films of high-quality single-walled carbon nanotubes (SWNTs) grown by atmospheric-pressure floating-catalyst chemical vapor deposition. Because of the optimization of the device window size and the utilization of SWNT thin films with both low resistivity and high transparency, a high photovoltaic conversion efficiency of greater than 12% was achieved for SWNT/Si heterojunction solar cells without any postprocessing, such as carrier doping treatment. In addition, the high reproducibility of the photovoltaic performance of these devices in air was demonstrated.