Hole doping and surface functionalization of single-walled carbon nanotube chemiresistive sensors for ultrasensitive and highly selective organophosphor vapor detection

We developed a chemiresistive sensor based on doped and functionalized semiconducting single-walled carbon nanotube (SWNT) networks for ultrasensitive and rapid detection of dimethyl methylphosphonate (DMMP) (simulant of nerve agent sarin) vapor. The semiconducting SWNT network was deposited between interdigitated electrodes and modified by solid organic acid tetrafluorohydroquinone (TFQ). The TFQ molecules could not only selectively bind DMMP onto the sidewalls of SWNTs via the strong hydrogen bonding interaction, but also tailor the electronic properties of SWNTs via heavy hole doping. This synergetic effect significantly improved the sensitivity of the devices, and enabled the sensors to easily detect DMMP at 20 parts-per-trillion (ppt) concentration with a response time of less than 2 min, without the need for pre-concentration of the analytes. This sensitivity is about five orders of magnitude higher than that of the unmodified SWNT chemiresistor, and also significantly higher than that of the functionalized SWNT chemiresistors previously reported. Moreover, the SWNT-TFQ sensors could be recovered when DMMP is replaced with referencing gas. The SWNT-TFQ sensors also show excellent selectivity toward DMMP over some interfering organic vapors. The response mechanism, i.e. charge transfer and dedoping was investigated.     

Flexible carbon nanotube sensors for nerve agent simulants

Chemiresistor-based vapour sensors made from network films of single-walled carbon nanotube (SWNT) bundles on flexible plastic substrates (polyethylene terephthalate, PET) can be used to detect chemical warfare agent simulants for the nerve agents Sarin (diisopropyl methylphosphonate, DIMP) and Soman (dimethyl methylphosphonate, DMMP). Large, reproducible resistance changes (75-150%), are observed upon exposure to DIMP or DMMP vapours, and concentrations as low as 25?ppm can be detected. Robust sensor response to simulant vapours is observed even in the presence of large equilibrium concentrations of interferent vapours commonly found in battle-space environments, such as hexane, xylene and water (10?000?ppm each), suggesting that both DIMP and DMMP vapours are capable of selectively displacing other vapours from the walls of the SWNTs. Response to these interferent vapours can be effectively filtered out by using a 2?μm thick barrier film of the chemoselective polymer polyisobutylene (PIB) on the SWNT surface. These network films are composed of a 1-2?μm thick non-woven mesh of SWNT bundles (15-30?nm diameter), whose sensor response is qualitatively and quantitatively different from previous studies on individual SWNTs, or a network of individual SWNTs, suggesting that vapour sorption at interbundle sites could be playing an important role. This study also shows that the line patterning method used in device fabrication to obtain any desired pattern of films of SWNTs on flexible substrates can be used to rapidly screen simulants at high concentrations before developing more complicated sensor systems.     

Dimethyl methylphosphonate detection with a single-walled carbon nanotube capacitive sensor fabricated by airbrush technique

Single-walled carbon nanotube (SWNT) films were prepared on interdigitated electrodes by airbrush technique, and their sensing properties to dimethyl methylphosphonate (DMMP) were studied. The SWNT films were characterized by field-emission scanning electron microscope. The response to different concentrations of DMMP vapors were investigated at room temperature. The results showed that the capacitance of airbrush SWNT sensor decreased rapidly in varying concentrations ranging from 12 to 60 mg/m³ (2.4–12 ppm). The capacitance sensitivity was about 12.5 % when exposed to 12 mg/m³ DMMP vapor. The capacitance sensitivity was higher when the initial capacitance and loss tangent were higher and the SWNT film was denser. It was found that the capacitance sensitivity was nearly 10 times to the resistance sensitivity. The airbrush SWNT sensor exhibited highly and fast capacitance response, good repeatability and selectivity for DMMP vapor.     

Screen-printed single-walled carbon nanotube networks and their use for dimethyl methylphosphonate detection

Single-walled carbon nanotube (SWNT) films were prepared on silicon/silica substrates by screen-printed technique at a wafer scale, and their sensing properties to dimethyl methylphosphonate (DMMP) were studied. The SWNT networks were characterized by field-emission scanning electron microscope. The resistance responses to different concentrations of DMMP vapors were investigated at room temperature. The results showed that the resistance changes of the screen-printed SWNT films increased rapidly in varying concentrations ranging from 20 to 200?ppm. The sensor exhibited high resistance responses, good reproducibility and excellent long-term stability for DMMP vapor detection. The screen-printed SWNT networks would be potentially extended to large-scale, low cost and simple manufacturing sensor applications.     

Single-walled carbon nanotube network ultramicroelectrodes

Ultramicroelectrodes (UMEs) fabricated from networks of chemical vapor deposited single-walled carbon nanotubes (SWNTs) on insulating silicon oxide surfaces are shown to offer superior qualities over solid UMEs of the same size and dimensions. Disk shaped UMEs, comprising two-dimensional "metallic" networks of SWNTs, have been fabricated lithographically, with a surface coverage of <1% of the underlying insulating surface. The electrodes are long lasting and give highly reproducible responses (either for repeat runs with the same electrode or when comparing several electrodes with the same size). For redox concentrations 相似文献   

Fabrication and characterization of single walled nanotube supercapacitor electrodes with uniform pores using electrophoretic deposition

Well dispersed aqueous suspensions containing single walled carbon nanotubes (SWNTs) were prepared without surfactants by functionalizing SWNTs in an acid treatment. SWNT coated electrodes were prepared from the SWNT aqueous suspensions using various methods to create uniform nanoporous networks of SWNTs on stainless steel (SST) current collectors. The EPD process was identified as the primary tool for reliably producing uniform SWNT nanoporous networks on SST substrates. Optical and scanning electron microscopic images and the BET surface area analysis were used to evaluate the SWNT dispersion quality of the electrodes. The average SWNT nanopore size produced from the EPD process was found about 1 nm and was nearly unaffected by extended EPD processing times. The SWNT coated electrodes were characterized using the cyclic voltammetry and their capacitance was determined. A correlation between the extended EPD processing time and the electrode capacitance was quantified.     

Percolation in networks of aligned SWNTs formed with laminar flow deposition

Macroscopic networks of highly aligned SWNTs have been fabricated at room temperature by laminar flow deposition from aqueous suspensions. This deposition method allows the growth of a macroscopic two dimensional SWNT network through successive deposition cycles. AFM image analysis showed that each deposition cycle puts down a reproducible density of SWNTs, with the final density being directly proportional to the number of deposition cycles for a given solution. The macroscopic electronic behavior of these networks was characterized by DC conductance measurements taken after each deposition cycle. This showed that these networks could be described by two dimensional percolation models throughout the growth process.     

Assessment of the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only approximately 1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.     

Factors controlling the electrodeposition of metal nanoparticles on pristine single walled carbon nanotubes

The electrodeposition of metal (Pd and Pt) nanoparticles on networks of pristine single walled carbon nanotubes (SWNTs) has been investigated using a microelectrochemical cell. A microcapillary containing electrolyte solution and a reference electrode is contacted with a silicon oxide substrate bearing a SWNT network, connected as the working electrode. Electrodeposition is promoted by applying a potential between the SWNT network and the reference electrode. By combination of analysis of the resulting current-time curves with atomic force microscopy and field emission scanning electron microscopy imaging of the network surfaces after electrodeposition, the nature of metal nanoparticle formation on SWNTs has been elucidated. In particular, the parameters controlling nanoparticle number density, distribution, and size have been identified, with short deposition times and high driving forces favoring the formation of ultrasmall particles at high density. Capacitance and network resistance effects are minimized in the microcapillary configuration, making it possible to accurately analyze short time-scale deposition processes (millisecond time scale). Furthermore, it is also possible to make many measurements on a pristine sample, simply by moving the position of the microcapillary to a new location on the substrate.     

Thermal conductivity measurements of semitransparent single-walled carbon nanotube films by a bolometric technique

We introduce a new technique for measurement of the thermal conductivity of ultrathin films of single-walled carbon nanotubes (SWNTs) utilizing IR radiation as heat source and the SWNT film as thermometer. The technique is applied to study the temperature dependence of the thermal conductivity of an as-prepared SWNT film obtained in the electric arc discharge process and a film of purified SWNTs prepared by vacuum filtration. The interplay between thermal and electrical transport in SWNT networks is analyzed in relation to the type of intertube junctions and the possibility of optimizing the thermal and electrical properties of SWNT networks for specific applications is discussed.     

Wafer‐Scale Fabrication of Suspended Single‐Walled Carbon Nanotube Arrays by Silver Liquid Dynamics

Suspended single‐walled carbon nanotubes (SWNTs) have advantages in mechanical resonators and highly sensitive sensors. Large‐scale fabrication of suspended SWNTs array devices and uniformity among SWNTs devices remain a great challenge. This study demonstrates an effective, fast, and wafer‐scale technique to fabricate suspended SWNT arrays, which is based on a dynamic motion of silver liquid to suspend and align the SWNTs between the prefabricated palladium electrodes in high temperature annealing treatment. Suspended, strained, and aligned SWNTs are synthesized on a 2 × 2 cm² substrate with an average density of 10 tubes per micrometer. Under the optimal conditions, almost all SWNTs become suspended. A promising formation model of suspended SWNTs is established. The Kelvin four‐terminal resistance measurement shows that these SWNT array devices have extreme low contact resistance. Meanwhile, the suspended SWNT array field effect transistors are fabricated by selective etching of metallic SWNTs using electrical breakdown. This method of large‐scale fabrication of suspended architectures pushes the study of nanoscale materials into a new stage related to the electrical physics and industrial applications.     

Controlled growth and characterization of two-dimensional single-walled carbon-nanotube networks for electrical applications

We demonstrate the reproducible fabrication of single-walled carbon nanotube (SWNT) networks, via catalyzed chemical vapor deposition (cCVD). Fe nanoparticles are employed as the catalyst, with methane as the carbon-containing gas. cCVD growth under these conditions results in the formation of multiply interconnected, random, two-dimensional networks of SWNTs. Investigation of the effect of parameters such as methane flow rate and temperature on the growth process enables control over the density of the network, which controls the network conductivity. Low-density networks demonstrate p-type semiconductor behavior, whilst high-density networks exhibit semimetallic behavior. In both cases conductance is demonstrated over macroscopic length scales, up to millimeters, much longer than the individual SWNTs, despite the surface coverage being <1 %. The networks can be defined in regions of a surface by photolithography before or after growth. Controlled growth of SWNT networks thus enables the application of SWNTs as macroscale conductors with controllable, predictable, and reproducible characteristics.     

Horizontally aligned single-walled carbon nanotube on quartz from a large variety of metal catalysts

Horizontally aligned single-walled carbon nanotubes (SWNTs) are highly desired for SWNT device applications. A large variety of metals including Fe, Co, Ni, Cu, Pt, Pd, Mn, Mo, Cr, Sn, Au, Mg, and Al successfully catalyzed the growth of such tubes on stable temperature (ST)-cut quartz by lattice guidance. In addition, Mg and Al were presented to produce random and aligned SWNTs for the first time. A hypothesis is proposed in which the precipitated carbon shell on the outer surface of the metal catalysts guides the alignment along the crystal lattice but not the catalysts themselves. By elucidating the role of the catalysts, an understanding of the aligned growth mechanism on quartz is further improved. Moreover, a simple "scratch" method by a razor blade such as the carbon steel and tungsten carbide (with 9% cobalt) is presented to pattern the "catalysts" without any complex processing steps such as lithography for the aligned SWNT growth.     

Reducing electrical resistance in single-walled carbon nanotube networks: effect of the location of metal contacts and low-temperature annealing

The ability to control the density of single-walled carbon nanotubes (SWNTs) during the formation of 2D networks allows one to tune the electrical properties of these thin-films from semiconductive to metallic conduction, allowing their use in numerous new materials applications. However, the resistances of such thin-films are generally non-optimal, dominated by the effects of inter-SWNT tunneling junctions, metal/SWNT contacts, sidewall defects, and the presence of residual dopants. These studies provide insight into the relative contributions of these various items to the overall resistance of an SWNT network contacted by Ti electrodes, and ways to reduce these effects via changing the structure of the metal/SWNT contact, and annealing at low temperature. Further, the addition of a mild-acid treatment was found to cause a 13-fold reduction in resistance and much greater reproducibility in inter-network conductivity.     

Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks

The current percolation in polymer‐sorted semiconducting (7,5) single‐walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field‐effect measurements, atomic force microscopy (AFM), and conductive AFM (C‐AFM) techniques. From AFM measurements, the nanotube length in the as‐processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field‐effect charge transport measurements in the SWNT network using a 2D homogeneous random‐network stick‐percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C‐AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as‐spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field‐effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.     

Copper catalyzing growth of single-walled carbon nanotubes on substrates

Metallic copper, which is normally considered as a contaminant in the growth of single-walled carbon nanotubes (SWNTs), was found to be an efficient catalyst to grow SWNTs under suitable conditions. It showed very high catalytic activity for the growth of both random SWNT networks and horizontally aligned SWNT arrays. Especially, high-quality SWNT arrays were obtained when monodispersed copper nanoparticles were used. The catalytic behavior of copper for the growth of SWNTs was discussed. The weaker interaction between the copper and silica surfaces plays an important role in the growth of high-quality horizontally aligned SWNT arrays. This new synthesis process of SWNTs with a non-ferromagnetic catalyst brings more convenience to the study of magnetic properties of SWNTs and gives more insight in structure-controlled synthesis of SWNTs.     

Generation of single-walled carbon nanotubes from alcohol and generation mechanism by molecular dynamics simulations

Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.     

Single-walled carbon nanotubes based chemicapacitive sensors

Carboxylated single-walled carbon nanotubes (SWNTs) based chemicapacitive gas sensors were fabricated by AC dielectrophoretically aligning SWNTs across microfabricated gold electrodes with controlled density/device resistance. Two different sensing configurations (i.e., horizontal/in-plane and vertical/out-of-plane) were utilized to compare their sensing performance. Upon exposure to water vapor at room temperature, the response (R = [(C--C0)/C0] x 100%) increased with an increase in water vapor concentration similar to that of resistance response. In horizontal configuration, the response was increased with an increase in device resistance which might be attributed to preferentially alignment of semiconducting SWNTs during initial phase of alignment. However, the response was independent of device resistance in vertical/out-of-plane configuration which indicated that the sensing mechanism is based on the change of dielectric constant of gate and atmosphere.     

Single-walled carbon nanotube thermopile for broadband light detection

We designed a thermopile based on a PN doping profile engineered in a suspended film of single-walled carbon nanotubes (SWNTs). Using estimates of the film local Seebeck coefficients, the SWNT thermopile was optimized in situ through depositions of potassium dopants. The overall performances of the thermopile were found to be comparable to state-of-the-art SWNT bolometers. The device is characterized at room temperature by a time response of 36 ms, typical of thermal detectors, and an optimum spectral detectivity of 2 × 10(6) cm Hz(1/2)/W in the visible and near-infrared. This paper presents the first thermopile made of a suspended SWNT film and paves the way to new applications such as broadband light (including THz) detection and thermoelectric power generation.     

旋涂法制备碳纳米管发电薄膜

采用多次旋涂法在玻璃衬底表面涂覆单壁碳纳米管（SWNT）悬浊液,形成均匀的SWNT薄膜,测量碳纳米管的发电特性,利用扫描电子显微镜（SEM）观测SWNT的分布情况,并用四探针电阻仪测量薄膜不同区域的方块电阻和薄膜的电流-电压（1-V）特性,结果表明,薄膜的I-V曲线线性度和重复度很高。不同浓度的氯化钠（NaCl）溶液以不同的流速流过SWNT薄膜表面,研究薄膜两端感应电势和电路中的电流变化情况。通过分析该变化情况,对SWNT发电机理作进一步阐释。