Aligned Single-walled Carbon Nanotubes Research Articles

High performance flexible pH sensor based on carboxyl-functionalized and DEP aligned SWNTs

The detection and control of the pH is very important in many biomedical and chemical reaction processes. A miniaturized flexible pH sensor that is light weight, robust, and conformable is very important in many applications, such as multifunctional lab-on-a-chip systems or wearable biomedical devices. In this work, we demonstrate a flexible chemiresistive pH sensor based on dielectrophoresis (DEP) aligned carboxyl-functionalized single-walled carbon nanotubes (SWNTs). Decorated carboxyl groups can react with hydrogen (H+) and hydroxide (OH−) ions, enabling the sensor to be capable of sensing the pH. DEP is used to deposit well-organized and highly aligned SWNTs in desired locations, which improves the metal-nanotube interface and highly rapid detection of the pH, resulting in better overall device performance. When pH buffer solutions are dropped onto such SWNTs, the H+ and OH− ions caninteract with the carboxyl groups and affect the generation of holes and electrons in the SWNTs, leading to resistance variations in the SWNTs. The results shows that the relative resistance variations of the sensor increases linearly with increasing the pH values in the range from 5 to 9 and the response time ranges from 0.2s to 22.6s. The pH sensor also shows high performance in mechanical bendability, which benefited from the combination of flexible PET substrates and SWNTs. The SWNT-based flexible pH sensor demonstrates great potential in a wide range of areas due to its simple structure, excellent performance, low power consumption, and compatibility with integrated circuits.

Evaluation of interface trap densities and quantum capacitance in carbon nanotube network thin-film transistors

The interface trap density in single-walled carbon nanotube (SWNT) network thin-film transistors (TFTs) is a fundamental and important parameter for assessing the electronic performance of TFTs. However, the number of studies on the extraction of interface trap densities, particularly in SWNT TFTs, has been insufficient. In this work, we propose an efficient technique for extracting the energy-dependent interface traps in SWNT TFTs. From the measured dispersive, frequency-dependent capacitance–voltage (C–V) characteristics, the dispersive-free, frequency-independent C–V curve was obtained, thus enabling the extraction and analysis of the interface trap density, which was found to be approximately 8.2 × 1011 eV−1 cm−2 at the valence band edge. The frequency-independent C–V curve also allows further extraction of the quantum capacitance in the SWNT network without introducing any additional fitting process or parameters. We found that the extracted value of the quantum capacitance in SWNT networks is lower than the theoretical value in aligned SWNTs due to the cross point of SWNTs on the SWNT network. Therefore, the method proposed in this work indicates that the C–V measurement is a powerful tool for obtaining deep physical insights regarding the electrical performance of SWNT TFTs.

Decoration of zinc oxide nanoparticles on vertically aligned single wall carbon nanotubes: An efficient field emitter

In present work, zinc oxide (ZnO) nanoparticles are decorated on the surface of Vertically Aligned-Single Wall Carbon Nanotubes (VA-SWCNTs) to enhance field emission properties of SWCNTs. VA-SWCNTs synthesized by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique are decorated with ZnO nanoparticles by thermal evaporation technique. Modified surface morphologies of as-prepared films were characterized by Atomic Force Microscope (AFM) and Raman spectroscopy to confirm the attachment of ZnO nanoparticles on VA-SWCNTs. Remarkable increase in current density, stability as well as reduction in turn on field were observed in ZnO nanoparticles coated VA-SWCNTs. Attachment of ZnO nanoparticles reduce the turn-on field of VA-SWCNTs field emitters from 2.0 to 1.0V/μm, while current density increase from 3.6 to 16.8mA/cm2 with long term stability and repeatability. The observed results are explained in terms of increased emitting area by modified surface with high field enhancement factor of VA-SWCNTs field emitters.

Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes.

The one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm(2)) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 10(6) nanotubes in a cross-sectional area of 1 μm(2). The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.

Polarization-selective alignment of a carbon nanotube film by using femtosecond laser ablation

We report on a considerable alignment of single-walled carbon nanotubes (SWCNTs) in randomly oriented bundles, by using a simple drop-and-dry method and irradiation with high-intensity femtosecond laser pulses. A remarkable third-harmonic generation was observed after irradiation with the laser pulses, whereas a narrow-band white-light continuum was generated in the as-prepared films. This observation, combined with scanning electron microscopy images, confirmed the high degree of alignment of the SWCNTs. In contrast to the pulsed irradiation of carbon soot, the powerdependent laser irradiation of a highly-purified SWCNT film show polarization-dependent ablation of individual nanotubes caused by polarization-dependent absorption. Raman spectroscopy results confirmed the presence of fractured nanotubes caused by the ablation processes. Polarizationresolved absorption spectroscopy results revealed that the aligned SWCNT film had potential usage in optical polarizers.

Large amplitude free vibration of nanotube-reinforced composite doubly curved panels resting on elastic foundations in thermal environments

A large amplitude vibration analysis is presented for nanocomposite doubly curved panels resting on elastic foundations in thermal environments. The doubly curved nanocomposite panels are studied with the consideration of different types of distributions of uniaxial aligned single-walled carbon nanotubes (SWCNTs). The material properties of the functionally graded carbon nanotube-reinforced composites (FG-CNTRCs) are assumed to be graded in the thickness direction according to linear distributions of the volume fraction of CNTs and are estimated through a micromechanical model. The motion equations are based on a higher order shear deformation theory and von Kármán strain-displacement relationships. The thermal effects are also included and the material properties of CNTRCs are assumed to be temperature-dependent. The motion equations are solved by a two-step perturbation approach to determine the nonlinear frequencies of the CNTRC doubly curved panel. The numerical illustrations cover small- and large-amplitude vibration characteristics of CNTRC doubly curved panels resting on Pasternak elastic foundations. The present solutions also highlight the effects of CNT volume fraction, temperature variation, foundation stiffness, panel curvature ratio as well as in-plane boundary conditions on the nonlinear free vibration behaviors of CNTRC doubly curved panels.

Free vibration analysis of thick disks with variable thickness containing orthotropic-nonhomogeneous material using finite element method

Free vibration analysis of thick functionally graded nanocomposite annular and solid disks with variable thickness reinforced by single-walled carbon nanotubes (SWCNTs) is presented. Four types of distribution of uniaxial aligned SWCNTs are considered: uniform and three kinds of functionally graded (FG) distribution through radial direction of the disk. The effective material properties of the nanocomposite disk are estimated by a micro mechanical model. The axisymmetric conditions are assumed and employing the graded finite element method (GFEM), the equations are solved. The solution is considered for four different thickness profiles, namely constant, linear, concave and convex. The achieved results show that the type of distribution and volume fraction of CNTs and thickness profile have a great effect on normalized natural frequencies.

Controlling the diameter of aligned single-walled carbon nanotubes on quartz via catalyst reduction time

One of the main objectives of chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNT) is control over their diameter and type (metallic or semiconducting). Here, we investigate the evolution of iron catalyst particles on quartz substrates depending on the duration of the reduction step with hydrogen and its effect on the growth of horizontally aligned SWCNT. We find a strong dependence of catalyst particle size and size distribution on the initial iron film thickness and the reduction time at 630 °C. Initial decrease of the particle size is followed by an unexpected increase. Statistical analysis of the Raman radial breathing modes of the SWCNT over large areas gave reliable and reproducible diameter distributions that correlated directly with the catalyst particle size distributions. By changing the reduction time it was possible to reproducibly shift the average SWCNT diameter from 1.5 (±0.3) nm to 1.2 (±0.2) nm while maintaining a nanotube density of 5–6 SWCNT/μm. In order to describe the evolution of the particle size during the reduction process at this particular temperature, we propose a model that includes the diffusion of iron into the quartz and its re-emergence.

Controlling the Diameter of Single-Walled Carbon Nanotubes by Improving the Dispersion of the Uniform Catalyst Nanoparticles on Substrate.

To have uniform nanoparticles individually dispersed on substrate before single-walled carbon nanotubes (SWNTs) growth at high temperature is the key for controlling the diameter of the SWNTs. In this letter, a facile approach to control the diameter and distribution of the SWNTs by improving the dispersion of the uniform Fe/Mo nanoparticles on silicon wafers with silica layer chemically modified by 1,1,1,3,3,3-hexamethyldisilazane under different conditions is reported. It is found that the dispersion of the catalyst nanoparticles on Si wafer surface can be improved greatly from hydrophilic to hydrophobic, and the diameter and distribution of the SWNTs depend strongly on the dispersion of the catalyst on the substrate surface. Well dispersion of the catalyst results in relatively smaller diameter and narrower distribution of the SWNTs due to the decrease of aggregation and enhancement of dispersion of the catalyst nanoparticles before growth. It is also found that the diameter of the superlong aligned SWNTs is smaller with more narrow distribution than that of random nanotubes.

Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy is an all-optical, contact free technique that allows directly measuring the transient mobile carrier dynamics and terahertz conductivity over picosecond time scales, and is uniquely suitable as a probe of conductivity in nanomaterials. We have applied time-resolved THz spectroscopy to investigate ultrafast carrier dynamics to a variety of nanostructured semiconducting systems such as silicon nanocrystal films, nanogranular iron pyrate (FeS2) and vanadium dioxide (VO2). Furthermore, terahertz emission spectroscopy of nanomaterials, where the sample itself emits THz radiation in response to optical excitation, provides important insights into carrier dynamics. We have recently generated THz pulses from optically excited macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs). We propose that top-bottom asymmetry present in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized THz radiation by a photocurrent surge.

Revealing distortion of carbon nanotube walls via angle-resolved X-ray spectroscopy

Arrays of aligned single-walled carbon nanotubes (SWCNTs) produced by supergrowth method were studied by scanning electron microscopy (SEM) and angle-resolved near-edge X-ray absorption fine structure spectroscopy, which defined that nanotube disorder is 10–13° and 23–27°, respectively. The latter value was confirmed by X-ray fluorescent spectroscopy. The difference in the obtained angular deviations was attributed to distortion of the SWCNT walls, because the X-ray spectroscopy methods are sensitive to a local environment of probing atoms, while the SEM examines the nanotubes at a substantially larger length scale. Significant distortion (20–24°) of SWCNT walls could be related to the defects introduced during the growth process.

(Invited) Post-Growth Manipulation of Horizontally Aligned Single-Walled Carbon Nanotubes

Horizontally aligned single-walled carbon nanotubes (SWNTs) are promising for electronic applications such as field-effect transistors (FETs). In addition to the density-controlled CVD growth [1,2] and chirality characterization of aligned SWNTs on quartz substrates, we will discuss two manipulation techniques of SWNT arrays. First is the full-length removal of metallic SWNTs by using organic film-assisted electrical breakdown, which gives semiconducting SWNT arrays suitable for scalable device fabrication [3]. We applied the electrical breakdown technique to SWNT array covered with organic films such as poly(methyl methacrylate) (PMMA) and realized that metallic nanotubes continues to burn as fuses for fireworks. This method yielded not only a high on/off ratio (~10,000) and fine spatial resolution (~55 nm) in the same way as in-air breakdown, but also long removal length (>10 µm) which is unattainable by conventional breakdown. Thermal calculations revealed that burned edges of a SWNT keep high temperature with oxidation heat generation of surrounding organic materials even after Joule heat stops. The second manipulation technique is the bundling of selected parts of SWNT arrays. Horizontally aligned SWNTs were peeled off from original substrates by using polymer films with lithographically-defined resist structures. Then, the resist was dissolved to give partially suspended SWNTs over trenched polymer films, and neighboring suspended SWNTs formed bundles by capillary process. Finally the structures were placed on target substrates to give partially bundled SWNTs. This technique would be available for controlling percolative transport of SWNT networks. Reference: [1] S. Chiashi, H. Okabe, T. Inoue, J. Shiomi, T. Sato, S. Kono, M. Terasawa, S. Maruyama, J. Phys. Chem. C, 116 (2012) 6805. [2] T. Inoue, D. Hasegawa, S. Badar, S. Aikawa, S. Chiashi, S. Maruyama, J. Phys. Chem. C, 117 (2013) 11804. [3] K. Otsuka, T. Inoue, S. Chiashi and S. Maruyama, Nanoscale, 6 (2014) 8831.

Generation of terahertz radiation by optical excitation of aligned carbon nanotubes.

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis.

Chemiresistive sensor based on polythiophene-modified single-walled carbon nanotubes for detection ofNO2

In the present investigation, a chemiresistive sensor based on conducting polythiophene-modified single-walled carbon nanotubes (SWNTs) for NO2detection has been reported. The SWNTs were aligned across 3 μm gap between two gold microelectrodes on Si / SiO2substrate. Alignment of SWNTs was carried out by using AC dielectrophoretic technique and it was confirmed by current–voltage (I–V) measurement. Aligned SWNTs were modified by conducting polythiophene using charge controlled potentiostatic deposition and it was confirmed by field-effect transistor (FET) and electrochemical measurements. Polythiophene-modified SWNT devices were used for the NO2detection in chemiresistive modality. This sensor exhibited very good linear response range from 10 ppb to 10 ppm.

Large-area assembly of densely aligned single-walled carbon nanotubes using solution shearing and their application to field-effect transistors.

Dense alignment of single-walled carbon nanotubes over a large area is demonstrated using a novel solution-shearing technique. A density of 150-200 single-walled carbon nanotubes per micro-meter is achieved with a current density of 10.08 μA μm(-1) at VDS = -1 V. The on-current density is improved by a factor of 45 over that of random-network single-walled carbon nanotubes.

Bolometric Properties of Semiconducting and Metallic Single-Walled Carbon Nanotube Composite Films

Single-walled carbon nanotubes (SWNTs) have shown interesting bolometric properties, making them good candidates for the detection of infrared and terahertz radiation. However, little has been reported on the bolometric characteristics of SWNT as a function of their chirality or the possible influence of composite morphology on these properties. The separation of SWNTs based on chirality allows for almost purely semiconductive or metallic SWNTs to be studied. The current study focuses on the bolometric performance of self-assembled composite films of SWNTs. The dependence of these properties on the chirality of the SWNTs was evaluated. To this end, metallic, semiconducting, and a 1:1 mixture of metallic and semiconductive were studied. Also, a theoretical model based on the Wiedemann–Franz law is used to explain the resistance of the SWNT composite films as a function of temperature. Results show that the composite morphology has a significant impact on bolometer performance, with cracked composite films containing highly aligned SWNT arrays suspended over a silicon substrate showing superior responsivity values due to higher thermal isolation. Uncracked composite films showed superior thermal coefficient of resistance values (α = −6.5%/K), however, the responsivity was lower due to lower thermal isolation.

Photoluminescence enhancement of aligned arrays of single-walled carbon nanotubes by polymer transfer.

The photoluminescence of as-grown, aligned single-walled carbon nanotubes (SWNTs) on quartz is strongly quenched and barely detectable. Here we show that transferring these SWNTs to another substrate such as clean quartz or glass increases their emission efficiency by up to two orders of magnitude. By statistical analysis of large nanotube arrays we show at what point of the transfer process the emission enhancement occurs and how it depends on the receiving substrate and the employed transfer polymer. We find that hydrophobic polystyrene (PS) as the transfer polymer results in higher photoluminescence enhancement than the more hydrophilic poly(methyl methacrylate) (PMMA). Possible mechanisms for this enhancement such as strain relief, disruption of the strong interaction of SWNTs with the substrate and localized emissive states are discussed.

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.

On the vibration of aligned carbon nanotube reinforced composite beams

Carbon nanotubes have exceptional mechanical, thermal and electrical properties, and are considered for high performance structural and multifunctional composites. In the present study, the natural frequencies of aligned single walled carbon nanotube (CNT) reinforced composite beams are obtained using shear deformable composite beam theories. The Ritz method with algebraic polynomial displacement functions is used to solve the free vibration problem of composite beams. The Mori-Tanaka method is applied to find the composite beam mechanical properties. The continuity conditions are satisfied among the layers by modifying the displacement field. Results are found for different CNT diameters, length to thickness ratio of the composite beam and different boundary conditions. It is found that the use of smaller CNT diameter in the reinforcement element gives higher fundamental frequency for the composite beam.

Nonlinear analysis of functionally graded nanocomposite rotating thick disks with variable thickness reinforced with carbon nanotubes

In this paper the nonlinear elasticity solution of functionally graded nanocomposite rotating thick disks with variable thickness reinforced with single-walled carbon nanotubes (SWCNTs) is presented. Four distribution types of uniaxial aligned SWCNTs are considered: uniform and three types of functionally graded (FG) distributions along radial direction of the disk. The effective material properties of the nanocomposite disk are estimated by a micro-mechanical model. The governing nonlinear equations are based on the axisymmetric theory of elasticity with the geometric nonlinearity in axisymmetric complete form. The nonlinear graded finite element method (NGFEM) based on Rayleigh–Ritz energy formulation with the Picard iterative scheme is employed to solve the nonlinear equations. The solution is considered for four different thickness profiles, namely constant, linear, concave and convex. The effects of different types of distributions and volume fractions of CNTs and various types of thickness profiles on the displacement and stresses of the rotating disks as well as comparison between linear and nonlinear responses are investigated. The achieved results show that the displacement and stress fields can be controlled by changing the type of distribution and volume fraction of CNTs as well as the thickness profile. Moreover, the difference between linear and nonlinear results are noticeable in high angular velocities; thus, for obtaining accurate results, the geometric nonlinearity must be considered.