Single-walled Carbon Nanotubes Layers Research Articles

Enhancing the sensitivity of chemiresistor gas sensors based on pristine carbon nanotubes to detect low-ppb ammonia concentrations in the environment

The possibility of using novel architectures based on carbon nanotubes (CNTs) for a realistic monitoring of the air quality in an urban environment requires the capability to monitor concentrations of polluting gases in the low-ppb range. This limit has been so far virtually neglected, as most of the testing of new ammonia gas sensor devices based on CNTs is carried out above the ppm limit. In this paper, we present single-wall carbon nanotube (SWCNT) chemiresistor gas sensors operating at room temperature, displaying an enhanced sensitivity to NH3. Ammonia concentrations in air as low as 20 ppb have been measured, and a detection limit of 3 ppb is demonstrated, which is in the full range of the average NH3 concentration in an urban environment and well below the sensitivities so far reported for pristine, non-functionalized SWCNTs operating at room temperature. In addition to careful preparation of the SWCNT layers, through sonication and dielectrophoresis that improved the quality of the CNT bundle layers, the low-ppb limit is also attained by revealing and properly tracking a fast dynamics channel in the desorption process of the polluting gas molecules.

Enhanced electroluminescence performances by controlling the position of carbon nanotubes

Alternating current driven powder electroluminescence (EL) devices with single-walled carbon nanotubes (SWCNTs) were fabricated to utilize the field enhancement by controlling the position of SWCNTs. The SWCNT conditions, which could lead to increased EL performance, were optimized by examining the characteristics of EL devices after controlling the length of the SWCNTs, where the SWCNT layer was positioned at the interface between the dielectric and emitting layers. The EL device with a short SWCNT layer for a crushing time of 20 min exhibited the highest EL performance of the samples examined. In addition, SWCNTs with the optimized SWCNT condition were inserted at various interfaces to determine the effective position of the SWCNT layer in the EL device. The highest EL performance, such as a 51% and 65% increase in luminance and efficiency, respectively, was achieved by inserting SWCNTs at the interface between the dielectric and emitting layers together with SWCNTs between the emitting layer and bottom electrode. Large increase in EL performance was attributed to the field enhancement by short SWCNTs, leading to increased charge carrier tunneling.

Antenna resonances in terahertz photoconductivity of single wall carbon nanotube fibers

Electromagnetic properties of single wall carbon nanotube (SWCNT) film on the surface of silica fibers have been studied numerically and experimentally. Optical properties of SWCNT layers are strongly influenced by the interference effects and antenna resonances in SWCNTs. Experimentally, the antenna resonances have been verified with low temperature photoconductivity measurements in terahertz range.

Single-walled carbon nanotubes wrapped poly-methyl methacrylate fiber optic sensor for ammonia, ethanol and methanol vapors at room temperature

Part of the outer layer of a commercial two-layer poly-methyl methacrylate optical fiber was mechanically removed and replaced with a coating of single-walled carbon nanotubes (SWCNT) by a dip-coating technique. The resulting fiber was used as a fiber optic sensor for the detection of ammonia, methanol and ethanol vapors in the concentration range 0–500 ppm at room temperature. Different thicknesses of the SWCNT layer on the fiber were prepared by repeated dip coating. All the sensors were found to show good response for all three vapors used even at lower vapor concentrations (<50 ppm). The results show that a lower SWCNT thickness is good for ammonia sensing whereas a higher thickness is good for alcohol sensing. The interaction between the analyte and SWCNT is responsible for the sensing of the volatile organic compounds.

Photovoltaic Device Performance of Single-Walled Carbon Nanotube and Polyaniline Films on n-Si: Device Structure Analysis

In this paper, we explore the use of two organic materials that have been touted for use as photovoltaic (PV) materials: inherently conducting polymers (ICPs) and carbon nanotubes (CNTs). Due to these materials' attractive features, such as environmental stability and tunable electrical properties, our focus here is to evaluate the use of polyaniline (PANI) and single wall carbon nanotube (SWNT) films in heterojunction diode devices. The devices are characterized by electron microscopy (film morphology), current-voltage characteristics (photovoltaic behavior), and UV/visible/NIR spectroscopy (light absorption). We have found that both PANI and SWNT can be utilized as photovoltaic materials in a simple bilayer configuration with n-type Silicon: n-Si/PANI and n-Si/SWNT. It was our aim to determine how photovoltaic performance was affected utilizing both PANI and SWNT layers in multilayer devices: n-Si/PANI/SWNT and n-Si/SWNT/PANI. The short-circuit current density increased from 4.91 mA/cm(2) (n-Si/PANI) to 12.41 mA/cm(2) (n-Si/PANI/SWNT), while an increase in power conversion efficiency by ~91% was also observed. In the case of n-Si/SWNT/PANI and its corresponding device control (n-Si/SWNT), the short-circuit current density was decreased by an order of magnitude. The characteristics of the device were affected by the architecture and the findings have been attributed to the more effective transport of holes from the PANI to SWNT and less effective transport of holes from PANI to SWNT in the respective multilayer devices.

Spin coated carbon nanotubes as the hole transport layer in organic photovoltaics

Spin coated single-walled carbon nanotubes (SWNTs) are incorporated in organic bulk heterojunction photovoltaic devices as the hole transport layer (HTL). The SWNTs films were repetitively spin coated with dichloroethane to various thicknesses, and were evaluated as the HTL in poly(3-hexylthiophene) (P3HT) and 1-(3-methoxy-carbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) photovoltaic devices. Insertion of a 12.2 nm thick SWNTs layer leads to power conversion efficiencies as high as 3.04%, compared to 1.16% and 2.84% for the devices without and with the traditional PEDOT:PSS as the HTL, respectively.

Conductivities of graphite fiber composites with single-walled carbon nanotube layers

The present study investigates the potential of air-spraying method in enhancement of electrical and thermal conductivities. Single-walled carbon nanotubes (SWCNTs) functionalized with carboxylic acid groups were air-sprayed on the surface of carbon fiber prepreg. The prepregs were stacked up and processed to carbon fiber laminates with SWCNTs layered between plies. From scanning electron microscopy, it was discovered that SWCNTs formed thin bands between carbon fiber layers and did not transport deep into the thickness direction due to low resin flow during cure. As it was hypothesized prior to experiments, the electrical conductivity was significantly enhanced by using a higher concentration of SWCNTs, while no amelioration for thermal conductivity was observed.

Fluctuation-induced tunneling dominated electrical transport in multi-layered single-walled carbon nanotube films

Low temperature measurements may give some insight into the transport mechanism of single-walled carbon nanotube (SWCNT) films, which could lead to an optimal SWCNT film with designed photoelectric properties. Despite intense research efforts on the low temperature transport in SWCNT films, it is still an open question for the low temperature transport in multi-layered SWCNT films. In this work, the multi-layered SWCNT films were prepared with a layer by layer vacuum filtration. It suggests that the space between different layers of the multi-layered SWCNT can be ignored. For deposition of different-layered SWCNT films using the same total amount of SWCNT suspension, the increase of the layer numbers can reduce the density of the resulting films, which may account for the low temperature transport. The effect of thermal annealing and subsequent nitric acid treatment on the electrical properties of the SWCNT films has also been investigated. At the temperature range of 80–300 K, the transport of the multi-layered SWCNT films can be explained by a fluctuation-induced tunneling model. Our results could build a bridge connecting measured temperature coefficient of resistance and the microscopic tunneling barrier.

Kinetics of Photoactuation in Single Wall Carbon Nanotube−Nafion Bilayer Composite

Cantilevers made of a bilayer composite of single walled carbon nanotubes (SWNTs) deposited onto Nafion exhibit substantial mechanical motion upon exposure to visible or near-infrared light. The kinetics of the tip displacement are studied as a function of SWNT thickness, mass loading, and humidity level. The rate constants strongly depend upon the SWNT layer thickness. During the light-on cycle the absorption of the light by the SWNTs drives the motion while in the light-off cycle the stiffness of the SWNTs retards the relaxation. At low humidity levels a second, fast, but small, response in the opposite direction of the net major displacement is observed. The kinetics of the photoactuation was also monitored by IR spectroscopy with similar kinetic parameters. The primary mechanism responsible for the actuation appears to be migration of water and hydrated hydrogen ions in the Nafion film toward the interface and into the SWNT layer.

Efficient fabrication of wafer scale thin film of individualized single-walled carbon nanotubes by dual-nozzle spin casting

In this paper, a dual-nozzle spin casting method was proposed to form a thin film of individualized single-walled carbon nanotubes (SWNTs) at the wafer scale. Each nozzle simultaneously ejected the SWNT solution and methanol, respectively. During the ejection process, two solutions were mixed at the contacting end of the nozzles and then dropped onto the substrate. Functionalization of the wafer substrate with the amine group improved the uniformity of the SWNT thin film as well as the adhesion between the individualized SWNTs and the substrate. The best condition of the spin casting involved the substrate functionalization using 3-aminopropyltriethosilane aqueous solution with a concentration of approximately 10 mM and a deposition velocity of approximately 5000 rpm. The root-mean-square roughness of the fabricated SWNT layer over the wafer substrate was found to be 1.4-1.8 nm, which indicated that the resultant thin film was one or two layers of SWNTs. The wafer scale SWNT thin film formed by dual-nozzle spin casting can be further used for the mass production and high integration of the SWNT nanoelectronic devices.

Impact of solution chemistry on viral removal by a single-walled carbon nanotube filter

This study investigates the effectiveness of a single-walled carbon nanotube (SWNT) filter for removal of viruses from water. MS2 bacteriophage viral removal was examined over a range of environmentally relevant solution chemistries, spanning various ionic strengths, monovalent and divalent salts, pH, and natural organic matter (NOM) concentrations. Viral removal by the SWNT filter was governed by physicochemical (depth) filtration. The removal of viruses increased at higher ionic strengths (NaCl) due to suppression of repulsive electrostatic interactions between viruses and SWNTs. Addition of divalent salts, however, had varying impacts. While CaCl 2 increased virus removal, likely due to complexation of calcium ions to viral surfaces, addition of MgCl 2 reduced viral removal by the SWNT filter. Solution pH also had significant impact on viral removal as the interactions between viral particles and SWNTs changed from attractive below the virus isoelectric point (about pH 3.9) to repulsive at higher pH. Suwannee River NOM was shown to be detrimental to filter viral removal. Reduction of viral removal by NOM was attributed to adsorption of NOM macromolecules to viruses and SWNTs, thereby resulting in steric repulsive forces. Modifications of the filter to incorporate thicker SWNT layers mitigate the negative impacts of NOM on filter performance. This study has shown that while it is possible to attain high levels of viral removal over a broad range of solution chemistries, the extent of viral removal will be highly dependent on the specific solution chemistry of the treated water.

The electric field enhancements by single-walled carbon nanotubes in In2S3/In2O3 photoelectrochemical solar cells

Using reverse-field emission (FE) current measurements, we demonstrate enhancement of the electric field by single-walled carbon nanotubes (SWNTs) in In2S3/In2O3 photoelectrochemical solar cells (PECs). In reverse-FE measurements, anode and cathode consists of In2S3/In2O3/(with or without) SWNTs on indium-doped tin oxide substrate and microtexturized tip silicon, respectively. The enhanced FE results for In2S3/In2O3/SWNTs show an electric field approximately two times more than In2S3/In2O3. The β value (i.e., electric field enhancement factor) of In2S3/In2O3 PECs with SWNT layers is 4950, which is ∼35.2% higher than that of In2S3/In2O3 PECs without SWNTs (3660). In PECs, the enhanced electric field intensifies the power of electron transfer, which accelerates the electron transfer rate in the cell.

Multifunctional nanocomposite membrane for chemomechanical transducer

We fabricated a novel chemomechanical transducer by using the multilayered nanocomposite membrane composed of layer-by-layer assembled polymers and single-walled carbon nanotubes (SWNTs). The nanomembrane with SWNT layer embedded in it plays multifunctional roles in chemomechanical sensing such as deformable structure under surface stress, electrode for capacitance measurement, and self-heating for biochemical reactions. The thickness of the nanomembrane is 26 nm and it can be suspended over a large square opening with the side dimension of 120 μm. The fabrication is entirely based on the conventional micromachining without any cumbersome processes such as floating and picking up nanomembranes. The nanomembrane transducer integrated with sensor components for capacitive readout and microfluidic channels is utilized to detect anthrax DNA. The nanomembrane transducer is able to selectively detect up to 0.1 μM of target DNA. The assay platform of the nanomembrane transducer offers valuable attributes such as short detection time (a few hundred seconds) and portability. It is demonstrated that the nanomembrane transducer has an applicability to a fast and portable DNA detection platform with self-heating capability for chemical and biological reaction.

The effect of SWCNT with the functional group deposited on the counter electrode on the dye-sensitized solar cell

This study investigated the applicability of single wall carbon nanotubes (SWCNT) with the functional group deposited on the FTO-glass (Fluorine doped tin oxide, SnO 2:F) substrate of the counter electrode for a dye-sensitized solar cell (DSSC). A nanocrystalline TiO 2 layer was fabricated on the FTO-glass substrate of the working electrode, and then sintered in a high-temperature furnace. The working electrode with a TiO 2 thin film was immersed in the solution of N-719 (Ruthenium) dye for 12 h. Moreover, the counter electrode with a layer of Ag (or without a layer of Ag) and a layer of SWCNT, which were (or was) fabricated in that order on the FTO-glass substrate, was subsequently prepared. Finally, the DSSC was assembled, the power conversion efficiency of the DSSC was measured using an I–V measurement system, and the incident photo conversion efficiency (IPCE) of the DSSC was obtained using the phase-locked loop optical chopper. This study also examined the effects of a layer of Ag deposited on the FTO-glass substrate, the type of organic solvent (such as DMAC and acetylacetone), and the sintering temperature on the performance of the DSSC. This film of SWCNT/Ag markedly increased the IPCE from 3.9% (conventional DSSC with a thin film of platinum on the FTO-glass substrate of the counter electrode) to 15.3% (DSSC with SWCNT/Ag/acetylacetone), as the wavelength of the light was 380 nm. Furthermore, as the wavelength of the light is 550 nm, the IPCE of the DSSC with SWCNT/Ag/acetylacetone (6.8%) becomes nearly equal to that of conventional DSSC (7.2%). Most interestingly, this study shows that the power conversion efficiency of the DSSC with SWCNT/Ag/acetylacetone (1.3037%) is not inferior to that of DSSC with a thin film of platinum on the counter electrode (1.25%).

Heterogeneous electron transfer kinetics and electrocatalytic behaviour of mixed self-assembled ferrocenes and SWCNT layers

The electron transfer dynamics and electrocatalytic behaviour of ferrocene-terminated self-assembled monolayers (SAMs), co-adsorbed with single-walled carbon nanotubes (SWCNTs) on a gold electrode, have been interrogated for the first time. Ferrocene monocarboxylic acid (FMCA) or ferrocene dicarboxylic acid (FDCA) was covalently attached to the cysteamine (Cys) monolayer to form Au-Cys-FMCA and Au-Cys-FDCA, respectively. The same covalent attachment strategy was used to form the mixed SWCNTs and ferrocene-terminated layers (i.e. Au-Cys-SWCNT/FMCA and Au-Cys-SWCNT/FDCA). Using cyclic voltammetry and electrochemical impedance spectroscopy, the impact of neighbouring SWCNTs on the electron transfer dynamics of the ferrocene molecular assemblies in an acidic medium (0.5 M H(2)SO(4)) and in a solution of an outer-sphere redox probe ([Fe(CN)(6)](4-)/[Fe(CN)(6)](3-)) was explored. The electron transfer rate constants in both media essentially decreased as Au-Cys-FMCA > Au-Cys-SWCNT/FDCA > Au-Cys-FDCA > Au-Cys-SWCNT/FMCA. This trend has been interpreted in terms of several factors such as the locations of the ferrocene species in a range of environments with a range of potentials, the proximity/interactions of the ferrocenes with one another, and electrostatic interaction or repulsion existing between the negatively-charged redox probe and the modified electrodes. The thiocyanate ion (SCN(-)) was used as a model analyte to examine the influence of the neighbouring SWCNTs on the electrocatalytic ability of the ferrocene assemblies. The Au-Cys-SWCNT/FDCA showed the best catalytic activity (in terms of onset potential and catalytic peak current height) for the oxidation of SCN(-), possibly due to the repulsive interactions between the negatively charged SCN(-) and high number of surface -COOH species at the SWCNT/FDCA. This study has provided some useful insights as to how CNTs co-assembled with ferrocene-terminated thiols could impact on the electron transfer kinetics as well as the electrocatalytic detection of the self-assembled ferrocene layers.

Cellular behavior of human mesenchymal stem cells cultured on single-walled carbon nanotube film

The influences of carboxylic functionalized single-walled carbon nanotubes (SWCNTs) on cell adhesion, spreading and cell lineage commitment of human mesenchymal stem cells (hMSCs) were evaluated. hMSCs were cultured on a thin mesh like layer of SWCNTs with a vertical height of less than 100 nm. The influence of the SWCNT film was significant on the cell spreading and focal adhesion distribution. Cells spread better on a SWCNT film as compared to cover slip (control), resulting in larger cell area and have higher occurrence of filopodia (microspikes) at the cell boundaries. Cytoskeleton arrangement was observed to be less orientated in the cells cultured on a SWCNT film as compared to control. Neurogenic markers such as nestin, glial fibrillary acidic protein and microtubule associated protein 2 genes were transiently upregulated (a process where cellular components, in this case RNA, is increased in response to external variable) over the first week while genes indicative of osteogenesis remained at its nominal level. These results suggest that nano roughness alone is sufficient to modulate cellular behavior and early stage of stem cell lineage commitment without the aid of an induction medium.

Thin-film transistors with controllable mobilities based on layer-by-layer self-assembled carbon nanotube composites

We report thin-film transistors (TFTs) capable of controlling mobilities in a broad range using self-assembled nanocomposite multilayers. Single-walled carbon nanotubes (SWNTs) and SiO 2 nanoparticles are vertically stacked on a substrate as the semiconducting and dielectric materials, respectively. The number of assembled layers can adjust the nanotube interconnection and tune the mobilities of the transistors. Our experiments show that the mobility can be enhanced to 35 times, and the highest observed mobility is 333.04 cm 2/V s. Furthermore, we find that the reliability of the devices is increased with the increasing number of SWNT layers in the film. Our results demonstrate an effective technique to produce reliable and high-performance thin-film micro/nanoelectronic devices.

P H -dependent conductance behaviors of layer-by-layer self-assembled carboxylated carbon nanotube multilayer thin-film sensors

The authors report pH-dependent conductance behaviors of single-walled carbon nanotube (SWCNT) multilayer thin-film sensors. SWCNTs are functionalized with carboxylic groups, and layer-by-layer assembled alternatively with polycation, polydiallyldimethyammonium chloride on the microfabricated metal electrodes. Current-voltage (I-V) characteristics show that the conductance of SWCNT multilayer thin-film sensors decreases with increase of pH values. On the other hand, the multilayer resistors with polymethylmethacrylate (PMMA) as a passivation layer demonstrate the increasing conductance with pH, opposite to the case of the absence of PMMA layer. The conductance change in the absence of PMMA attributes to doping/undoping of holes as charge carriers by molecular protonation/deprotonation of p-type semiconducting SWCNTs. With PMMA layer, concentration gradient of hydrogen ions on the dielectric forms the gate bias voltage, which changes the conductance of underlying semiconducting SWCNT layer. Each pH-dependent behavior has versatile applications for chemical and biological detections.

Carbon Nanotube Electrodes for Hot‐Wire Electrochemistry

The use and preparation of single-walled carbon nanotubes (SWCNTs) at thin metallic wire electrodes for hot-wire electrochemical studies is described. The nanotubes were deposited on metal substrates such as gold by electrophoresis from a dispersion containing sodium dodecyl sulphate as an anionic surfactant. The formation of a layer of pure SWCNTs is achieved by thermal treatment at 350 degrees C. When heated in situ by a strong ac current, the electrodes can be used for electrochemical studies of nanotubes at increased temperatures. The state and functionality of the electrodes were characterized by Raman spectroscopy, scanning electron microscopy, and cyclic voltammetry with both anionic and cationic redox systems (dopamine, ferrocene carboxylic acid). First time experiments at the heated SWCNT electrodes demonstrated an excellent suitability of these as-prepared electrodes for thermoelectrochemical studies.

Photoelectrochemical Properties of Carbon Nanotube/SnO2 Nanostructures Prepared by Three Different Methods

AbstractIn this work, we employed three different methods to fabricate solar cell structures on indium tin oxide (ITO) substrates. For the first method, multi-layered structures were prepared by using single walled carbon nanotubes (SWCNTs) and tin oxide (SnO2). First, a SWCNT layer was deposited on the ITO substrate; and photoactive material was then coated on the top of the SWCNT layer. For the second method, photoactive particles were added to a solution of SWCNTs. The SWCNT/SnO2 solution was mechanically stirred and then deposited on the ITO substrate. For the third method, we synthesized photoactive particles on SWCNTs through a chemical-solution routine using SnCl4 as a precursor. We characterized the morphology and structure of the SWCNTs coated with SnO2 nanoparticles prepared with the three different methods by using a field emission scanning electron microscope equipped with an X-ray energy dispersive spectrometer. We characterized the photoelectrochemical properties of all electrodes by using an electrochemical station; mainly, we examined the photocurrent generated under periodic illumination. Our results indicate that there are significant differences in the photocurrent in the presence of SWCNTs. We propose the following hypothetical mechanism: without carbon nanotubes, generated electrons (when light is absorbed by SnO2 particles) must cross the particle network to reach an electrode. Many electrons never escape this network to generate an electrical current. The carbon nanotubes “collect” the electrons and provide, therefore, a more direct route to the electrode, thus improving the efficiency of the solar cells.