Single-walled Carbon Nanotube Arrays Research Articles

Sowing Clean-Release Salt Catalyst for the Synthesis of Contamination-Free Single-walled Carbon Nanotube Arrays.

Horizontal arrays of single-walled carbon nanotubes (SWCNTs) have shown immense potential for application in emerging devices due to their excellent electrical and thermal properties. The direct growth of SWCNTarrays using high-activity metal catalysts is one of the promising methods to approach the mass production of dense SWCNT arrays. However, an inevitable obstacle lies in the post-purification of metal residual. Herein, a sowing strategy to prepare size-tunable potassium chloride (KCl) catalysts for the efficient growth of the SWCNT array with a density of 10 tubes per micron is reported. Through a controllable etching process, numerous surface defects (e.g., vacancies and kinks) are uniformly generated on the substrate as seed pit-like sites for the accommodation and anchoring of catalysts. The well-distributed KCl catalysts with a homogeneous size of ≈1.4nm enable the growth of ≈1.3nm SWCNTs through a vapor-liquid-solid mechanism. Importantly, 94 at.% KCl catalysts can be dramatically removed through a simple water-washing process, thus leaving contamination-free SWCNT arrays behind. Interestingly, 85% of nanotubes show metallic properties, which is demonstrated by the combination of electrical characterization and the multi-laser Raman spectroscopy. This sowing strategy contributes to the direct growth of uncontaminated high-density SWCNT arrays.

Nonideality in Arrayed Carbon Nanotube Field Effect Transistors Revealed by High-Resolution Transmission Electron Microscopy.

High density and high semiconducting-purity single-walled carbon nanotube array (A-CNT) have recently been demonstrated as promising candidates for high-performance nanoelectronics. Knowledge of the structures and arrangement of CNTs within the arrays and their interfaces to neighboring CNTs, metal contacts, and dielectrics, as the key components of an A-CNT field effect transistor (FET), is essential for device mechanistic understanding and further optimization, particularly considering that the current technologies for the fabrication of A-CNT wafers are mainly laboratory-level solution-based processes. Here, we conduct a systematic investigation into the microstructures of A-CNT FETs mainly via cross-sectional high-resolution transmission electron microscopy and tentatively establish a framework consisting of up to 11 parameters which can be used for structure-side quality evaluation of the A-CNT FETs. The parameter ensemble includes the diameter, length (or terminal), and density distribution of CNTs, radial deformation of CNTs, array alignment defects, surface crystallography facets of contact metal, thickness distribution of high-k dielectrics (HfO2), and the contact ratios for the CNT-CNT, CNT-metal, CNT-dielectric, and CNT-substrate interfaces. Enriched array alignment defects, i.e., bundle, stacking, misorientation, and voids, are observed with a total ratio sometimes up to ∼90% in pristine A-CNTs and even up to ∼95% after the device fabrication process. Thus, they are suggested as the prevalent performance-limiting factors for A-CNT FETs. Complex interfacial structures are observed at the CNT-CNT, CNT-metal contact, and CNT-high-k dielectric interfaces, making the local environment and the property of each component CNT involved in an A-CNT FET distinct from others in terms of the diameters, radial deformation, and interactions with the local surroundings (mainly through van der Waals interactions). The present study suggests further improvements on the fabrication technology of A-CNT wafers and devices and mechanistic investigations into the impacts of complex array alignment defects and interface structures on the electrical performance of A-CNT FETs as well.

Study on dynamic mechanism of controllable SWCNTs arrays prepared by self-assembly

Single walled carbon nanotubes (SWCNTs) are seamless hollow tubular structures formed by a layer of graphite curled along a specific spiral vector. Individual SWCNT is restricted by the inconvenient manipulation in its nanoscale, and unable to maximize its functionality in the macro field. As a result, the two-dimensional array is the most basic structure for manufacturing SWCNTs devices in practical application. However, the fidelity of the array pattern and the distribution morphology of individual SWCNT in the large-scale array directly affect its performance, which also poses a challenge to the research on assembly methods and their underlying mechanisms. In this paper, the dynamic mechanism of controllable SWCNTs arrays prepared by self-assembly method has been studied from molecular dynamics to hydrodynamic by a combination of experiments and simulations. Some key factors limiting the fidelity of the arrays and the distribution morphology of individual SWCNT in the array, such as the self-assembly template, the concentration of the SWCNTs dispersion, are investigated and optimized by experiments. Finally, inspired by the results of the mechanism research, a blade-coating procedure is applied in self-assembly process to improve the alignment of SWCNTs in the array. The improved alignment of SWCNTs arrays are verified through morphology analysis and volt-ampere (V-A) characteristics compared with the SWCNTs randomly distributed arrays. The explicitly results are not only helpful to understand dynamic phenomena during self-assembly process and the influencing factors of the self-assembly method but also will provide meaningful guidance in the design, fabrication of SWCNTs arrays to prevent distortion in large scale and further promote their industrial application in manufacturing next-generation micro-nano devices.

Single-Molecule Methane Sensing Using Palladium-Functionalized nIR Fluorescent Single-Walled Carbon Nanotubes.

There has been considerable interest in detecting atmospheric and process-associated methane (CH4) at low concentrations due to its potency as a greenhouse gas. Nanosensor technology, particularly fluorescent single-walled carbon nanotube (SWCNT) arrays, is promising for such applications because of their chemical sensitivities at single-molecule detection limits. However, the methodologies for connecting the stochastic molecular fluctuations from gas impingement on such sensors require further development. In this work, we synthesize Pd-conjugated ss(GT)15-DNA-wrapped SWCNTas near-infrared (nIR) fluorescent, single-molecule sensors of CH4. The complexes are characterized using X-ray photoelectron spectroscopy (XPS) and spectrophotometry, demonstrating spectral changes between the Pd2+ and Pd0 oxidation states. The nIR fluctuations generated upon exposure from 8 to 26 ppb of CH4 were separated into high- and low-frequency components. Aggregating the low-frequency components for an array of sensors showed the most consistent levels of detection with a limit of 0.7 ppb. These results advance the hardware and computational methods necessary to apply this approach to the challenge of environmental methane sensing.

Potential of Carbon Nanotube Chemiresistor Array in Detecting Gas-Phase Mixtures of Toxic Chemical Compounds.

Toxic industrial chemicals (TICs), when accidentally released into the workplace or environment, often form a gaseous mixture that complicates detection and mitigation measures. However, most of the existing gas sensors are unsuitable for detecting such mixtures. In this study, we demonstrated the detection and identification of gaseous mixtures of TICs using a chemiresistor array of single-walled carbon nanotubes (SWCNTs). The array consists of three SWCNT chemiresistors coated with different molecular/ionic species, achieving a limit of detection (LOD) of 2.2 ppb for ammonia (NH3), 820 ppb for sulfur dioxide (SO2), and 2.4 ppm for ethylene oxide (EtO). By fitting the concentration-dependent sensor responses to an adsorption isotherm, we extracted parameters that characterize each analyte-coating combination, including the proportionality and equilibrium constants for adsorption. Principal component analysis confirmed that the sensor array detected and identified mixtures of two TIC gases: NH3/SO2, NH3/EtO, and SO2/EtO. Exposing the sensor array to three TIC mixtures with various EtO/SO2 ratios at a fixed NH3 concentration showed an excellent correlation between the sensor response and the mixture composition. Additionally, we proposed concentration ranges within which the sensor array can effectively detect the gaseous mixtures. Being highly sensitive and capable of analyzing both individual and mixed TICs, our gas sensor array has great potential for monitoring the safety and environmental effects of industrial chemical processes.

Dispersion in Single-Wall Carbon Nanotube Film: An Application of Bogoliubov–Valatin Transformation for Hamiltonian Diagonalization

We present a theoretical study on the energy dispersion of an ultrathin film of periodically-aligned single-walled carbon nanotubes (SWCNTs) with the help of the Bogoliubov–Valatin transformation. The Hamiltonian of the film was derived using the many-particle green function technique in the Matsubara frequency formalism. The periodic array of SWCNTs was embedded in a dielectric with comparatively higher permittivity than the substrate and the superstrate such that the SWCNT film became independent with the axis of quantization but keeps the thickness as the variable parameter, making the film neither two-dimensional nor three-dimensional, but transdimensional. It was revealed that the energy dispersion of the SWCNT film is thickness dependent.

Anisotropic Growth of One-Dimensional Carbides in Single-Walled Carbon Nanotubes with Strong Interaction for Catalysis.

Tungsten and molybdenum carbides have shown great potential in catalysis and superconductivity. However, the synthesis of ultrathin W/Mo carbides with a controlled dimension and unique structure is still difficult. Here, inspired by the host-guest assembly strategy with single-walled carbon nanotubes (SWCNTs) as a transparent template, we reported the synthesis of ultrathin (0.8-2.0 nm) W2C and Mo2C nanowires confined in SWCNTs deriving from the encapsulated W/Mo polyoxometalate clusters. The atom-resolved electron microscope combined with spectroscopy and theoretical calculations revealed that the strong interaction between the highly carbophilic W/Mo and SWCNT resulted in the anisotropic growth of carbide nanowires along a specific crystal direction, accompanied by lattice strain and electron donation to the SWCNTs. The SWCNT template endowed carbides with resistance to H2O corrosion. Different from normal modification on the outer surface of SWCNTs, such M2C@SWCNTs (M = W, Mo) provided a delocalized and electron-enriched SWCNT surface to uniformly construct the negatively charged Pd catalyst, which was demonstrated to inhibit the formation of active PdHx hydride and thus achieve highly selective semihydrogenation of a series of alkynes. This work could provide a nondestructive way to design the electron-delocalized SWCNT surface and expand the methodology in synthesizing unusual 1D ultrathin carbophilic-metal nanowires (e.g., TaC, NbC, β-W) with precise control of the anisotropy in SWCNT arrays.

Determine the Complete Configuration of Single-Walled Carbon Nanotubes by One Photograph of Transmission Electron Microscopy.

Developing a convenient method to determine the complete structure of single-walled carbon nanotubes (SWNTs) is important to achieve the fully controlled growth of this nanomaterial. However, approaches that can identify handedness at the atomic level with simple equipment, operation, and data analysis are still lacking. Here, the SWNTs/graphene (Gr) vertical heterostructures are artificially constructed with aligned interfaces to realize the lattice interpretation of SWNT upper and lower walls separately by only one transmission electron microscopy image, thus transforming the 3D handedness information to projected 2D space. Gr displays prominent out-of-plane deformation at the interface, promoting the energetic advantage for the aligned interface construction. The interfacial alignment between the SWNT and Gr shows no obvious dependence on either the helical angle or diameter of SWNTs. The half-wrapping of SWNTs by deformed Gr also triggers diversified alterations in electronic structures based on theoretical calculations. 27 specimens with SWNTs prepared by two disparate methods are examined, implying equal handedness distribution in the randomly aligned SWNTs grown on quartz and potential handedness enrichment in horizontal SWNT arrays grown on a-sapphire. This work provides a simple strategy for chiral discrimination and lays a characterization foundation for handedness-selective growth of nanomaterials.

Growth of high-density horizontal SWNT arrays by pre-cracking of carbon source

Obtaining high-density horizontal single-walled carbon nanotube (SWNT) arrays is the key point of developing SWNT-based integrated circuits. However, some catalysts showing good control over SWNT structures possess relatively low activity to grow SWNT, which restricts the density of the grown arrays. Therefore, we developed a rational method to grow high-density SWNT arrays by iron (Fe)-assisted pre-cracking of carbon source in gas phase. A tiny amount of Fe, decomposed from ferrocene, was introduced into the chemical vapor deposition system with the gas flow without forming Fe nanoparticles on the substrates. The density of the SWNT arrays grown from Mo2C, WC and Cu catalysts can be significantly increased by 3–5 times with the help of Fe, reaching 40, 35 and 24 tubes μm−1, respectively. Through gas chromatography and Raman spectrum characterization, it is proved that carbon source decomposed more completely with Fe in the gas phase, which resulted in higher utilization efficiency of carbon source and better quality of the grown SWNTs. This pre-cracking approach is an effective way to improve the density of SWNT arrays with controlled structures.

Photochlorination to prepare semiconducting single-walled carbon nanotube and its intramolecular junction

As the key to manufacturing large scale integrated circuits, horizontally aligned semiconducting single-walled carbon nanotube (s-SWNT) arrays play a key role in the next-generation electronic device. Though various techniques have been developed, obtaining pure s-SWNT horizontal arrays is still the greatest challenge. Here, we propose an effective and facile method to produce s-SWNT arrays from a diverse perspective by implementing photochemical chlorination on the as-grown aligned SWNT arrays. The electrical measurements demonstrate a high fraction of s-SWNTs (>98.1%) after photochlorination, and its exceptional performance as a field-effect transistor with an on-off ratio up to 3730. Meanwhile, through the well-controlled mask-assisted photochlorination, a type of metallic-semiconducting SWNT intramolecular junction with rectifying diode and non-linear transport characteristics has also been successfully obtain. Our work provides a new method for the preparation of high-purity s-SWNT arrays and SWNT intramolecular junction, which has great significance for the carbon-based nano-electronic devices in applications.

Diameter‐Selective Density Enhancement of Horizontally Aligned Single‐Walled Carbon Nanotube Arrays by Temperature‐Mediated Method

AbstractHigh‐density single‐walled carbon nanotube (SWNT) arrays with specific diameters are pursued as potential building blocks in future nanoelectronic devices. The direct growth of SWNT arrays on the single‐crystal substrate has proved offering densely packing high‐quality SWNTs. However, further increase of SWNT density is limited by the deactivation of the catalyst nanoparticles during chemical vapor deposition process. Moreover, an increase of array density without any structure control of SWNTs would make the benefit effect very limited. Here, a temperature‐mediated method is proposed to achieve a high‐density SWNT array with a specific diameter via continual growth of SWNTs after reactivating poisoned catalysts with high carbon capacity and appropriate size. The density of obtained SWNT array on the quartz substrate is increased by more than three times, with diameter distribution controlled ≈2 nm. This approach paves the way for the integration of SWNTs into nanodevices.

Manipulating Single-Walled Carbon Nanotube Arrays for Flexible Photothermoelectric Devices.

Flexible photothermoelectric (PTE) devices possess great application prospects in the field of light energy and thermoelectric energy harvesting which are some of the cornerstones of modern green renewable energy power generation. However, the low efficiency of PTE materials and lack of suitable manufacturing processes remain an impediment to restrict its rapid development. Here, we designed a flexible PTE device by printing a highly integrated single-walled carbon nanotubes (SWCNTs) array at intervals that were surface-functionalized with poly(acrylic acid) and poly(ethylene imine) as p–n heterofilms. After the introduction of a mask to give a selective light illumination and taking advantage of the photothermal effect of SWCNTs, a remarkable temperature gradient along the printed SWCNTs and a considerable power density of 1.3 μW/cm2 can be achieved. Meanwhile, both experimental data and COMSOL theoretical simulations were adopted to optimize the performance of our device, showing new opportunities for new generation flexible PTE devices.

Single layer aligned semiconducting single-walled carbon nanotube array with high linear density

Highly ordered semiconducting single-walled carbon nanotubes(sc-SWCNTs) array with high purity, high linear density and controllable manner is strongly desired for carbon-based integrated circuits, yet it remains a big challenge. Herein, close-packed single layered and controllably aligned sc-SWCNTs arrays were obtained through dielectrophoresis using a high purity sc-SWCNT dispersion. Under optimized condition of length and average number of interconnecting junctions across the channel full of aligned sc-SWCNTs, field effect transistors (FETs) with high performance were achieved with both a high on/off current ratio and large carrier mobility. Based on the optimized channel length, by systematically optimizing the dielectrophoresis parameters of the frequency and duration of applied AC voltage (V pp), the highly ordered sc-SWCNTs arrays with an ultra-high linear density of 54 ± 2 tubes μm−1 showed relatively high device performance of FET. The fabrication process optimized in this report can be further extended and applied in large-area, low-cost carbon-based integrated circuits.

Topologically Tuned Obliquity of Klein-Tunnelling Charged Currents Through Graphene Electrostatically-Confined p - n Junctions

Problem of control over Klein-tunnelling states from electrostatically-confined graphene p - n junctions has been discussed. The lack of quasi-bound states, being the states with a finite life time, in a pseudo-Dirac-fermion model for the graphene quantum dot (GQD) is theoretically predicted as inapplicability of the so-called "resonance condition" leading to an inconsistent linear system corresponding to matching conditions. Within a pseudo-Dirac-Weyl fermion model GQD, the graphene charge carriers are topologically nontrivial and can be confined by a staircase-type potential due to competition between Zak curvature and centrifugal-force actions. The predicted topological effects elucidate experimentally observed resonances created by electron beam and laser pulse in crystalline arrays of single-walled carbon nanotubes as the Klein-tunnelling resonant states in the p - n graphene junctions. We present a robust approach to fabricate stable graphene p - n junctions by fine-tuning the topological effects.

Photosensitive-Stamp-Inspired Scalable Fabrication Strategy of Wearable Sensing Arrays for Noninvasive Real-Time Sweat Analysis.

Wearable sweat sensing is essential to the development of personalized health monitoring in a noninvasive manner with molecular-level insight. Hence, there is an increasing demand for convenient, facile, and efficient fabrication of wearable sensing arrays. Inspired by a photosensitive stamp (PS), we present herein a simple, low-cost, and eco-friendly vacuum filtration-transfer printing method (termed PS-VFTP) for the scalable preparation of single-walled carbon nanotube (SWCNT) based flexible electrode arrays. This method can economically yield customized flexible SWCNT arrays with praiseworthy performance, such as high reproducibility, precision, uniformity, conductivity, and mechanical stability. In addition, the flexible SWCNT arrays can be easily functionalized into high-performance electrochemical sensors for the simultaneous monitoring of sweat metabolites (glucose, lactate) and electrolytes (Na+, K+). The integration of wearable sensing arrays with a signal acquisition and processing circuit system in the intelligent wearable sensors empowers them to realize noninvasive, real-time, and in situ sweat analysis during exercise. More meaningfully, such a PS-VFTP strategy can be easily expanded to the economical manufacturing of other flexible electronic devices.

The change in the thermal conductivity of a multilayer array of carbon nanotubes during its lateral compression

Numerical simulation of thermal conductivity across a multilayer array of single-walled carbon nanotubes is carried out. The effect of transverse compression of the array on thermal conductivity has been studied. It is shown that the compression of the array can occur uniformly when all the nanotubes of the array are compressed equally, and it can occur inhomogeneously when a part of the nanotubes is strongly compressed, and the other part is weakly compressed. With homogeneous compression, the thermal conductivity of the array increases, but with inhomogeneous compression, it does not change and may even decrease in case of a large number of layers. This effect is especially pronounced for arrays of nanotubes of small diameter (D<2 nm). Keywords: carbon nanotubes, nanotube arrays, transverse compression, heat conductivity.

Изменение теплопроводности многослойного массива углеродных нанотрубок при его поперечном сжатии

Numerical simulation of thermal conductivity across a multilayer array of single-walled carbon nanotubes is carried out. The effect of transverse compression of the array on thermal conductivity has been studied. It is shown that the compression of the array can occur uniformly when all the nanotubes of the array are compressed equally, and it can occur inhomogeneously when a part of the nanotubes is strongly compressed, and the other part is weakly compressed. With homogeneous compression, the thermal conductivity of the array increases, but with inhomogeneous compression, it does not change and may even decrease in case of a large number of layers. This effect is especially pronounced for arrays of nanotubes of small diameter (D < 2 nm)

Average SWCNT bundle length estimated by resistance measurement

The length of single-walled carbon nanotubes (SWCNTs) affects the optoelectronic and mechanical properties of macroscopic SWCNT layers. Modern methods are capable to measure the length of short nanotubes, and also require complex sample preparation procedures. In this work we show that the average length of SWCNTs can be estimated by measuring the resistance of randomly oriented SWCNTs array. We observe the change in the slope of the resistance dependence on the distance between the contacts with the interval between 100 and 200 μm. The change of resistance slope indicates a change in the path of current flow through the SWCNT. The change in the conduction path can be associated with the “effective bundle length”, which should be related to the average nanotube length. Thus, we have demonstrated a simple and quick technique to measure SWCNT bundle length, which can be used in-situ and does not require special sample preparation.

Single-walled Carbon Nanotube Chemiresistive Sensors for the Identification and Quantification of Disinfectants

Disinfectants are essential to keep water safe as they kill pathogens by oxidizing the cell membrane. Free chlorine, potassium permanganate, monochloramine, hydrogen peroxide, chlorine dioxide, ozone, and hypobromous acid are the most commonly used disinfectants for the disinfection of water.1 Monitoring the concentration of the disinfectant is crucial as the effectiveness mainly depends on the amount of disinfectants present in water.2 Insufficient levels of disinfectant in drinking water could impose health risks as pathogens could regrow in the distribution system. Standard methods for measuring the disinfectant levels are colorimetric and titrimetric.3 For free chlorine, DPD colorimetric method is well established.1 , 4 For potassium permanganate (KMnO4), colorimetric methods (persulfate and periodate) and spectroscopic methods are available.1 , 5 Currently, the monitoring of disinfectants is done on samples collected at the treatment plant and in different locations throughout the distribution system. This reagent-based measurement of discrete samples requires reagents and spectroscopic readout devices and can suffer from interference. Therefore, these methods are not suitable for the continuous monitoring of the disinfectant.6 Furthermore, there is no known method that can differentiate between multiple disinfectants in real samples without any previous knowledge.Two of the most important water quality parameters are pH and oxidation-reduction potential (ORP). Disinfectants undergo different reactions at different pH, resulting in different active species for different pH. ORP measurement can be used to monitor whether the disinfection was successful as oxidants’ reactivity in water depends on redox conditions.7 ORP of a solution is associated with the pH and the concentration of the disinfectant. At a fixed pH, the ORP of a disinfectant will depend on the concentration. Therefore, just by knowing the ORP, we cannot distinguish disinfectant unless the pH or concentration of the oxidant is known.8 , 9 To sort out these parameters, we propose to introduce a chemiresistive sensing array capable of distinguishing and quantifying disinfectants. Due to simpler fabrication techniques, lower cost and the ability to sense various analytes through appropriate ligand, chemiresistors have great promise in water quality sensing.6 , 10 Previously reported chemiresistive sensor for continuous measurement of free chlorine used carbon nanotube (CNT) substrate functionalized with a redox-active aniline oligomer named phenyl capped aniline tetramer (PCAT).11 Oxidation of PCAT attached to the surface of CNT by free chlorine leads to a change in the oxidation state of the oligoaniline; this change in oxidation state in the molecule changes the doping characteristics of CNT leading to a change in resistance of the CNT film. This resistance change was used to quantify the concentration of free chlorine.11 In this work, we have constructed an array of single-walled carbon nanotubes (SWCNT) sensors functionalized with five redox-active molecules. Structurally different from each other, these molecules will create different active sites. Upon interaction with analytes, each molecule will give different magnitudes of responses. Sensor responses are collected for free chlorine and KMnO4 over a range of concentrations for pH 6.5 and 7.5. In general, sensors give increasing responses to the increasing concentration and decreasing pH of the solution. Though the responses from the sensors have a similar trend to the change of the parameters their varied magnitudes make them suitable for an array. Sensor responses are then analyzed with principal component analysis (PCA). PCA analysis shows clear separation between the analytes and as well as to different pH and concentrations of the solutions (figure). We have therefore demonstrated an array of chemiresistive sensors that can distinguish and quantify disinfectants.

Toward an Intelligent Synthesis: Monitoring and Intervening in the Catalytic Growth of Carbon Nanotubes.

The bottom-up approach to directly synthesizing low-dimensional materials with outstanding performance has extended the material basis for the next generation integrated circuit industry. All the low-dimensional semiconductors, metals, dielectrics, and their heterojunctions are very promising bricks to build faster and more efficient chips because of their atomically smooth surface and interfaces. The greatest challenge in the synthesis of nanomaterials is how to precisely control the structure, crystalline orientation, defects, dimensions, etc. In past decades, both the methodology and the mechanism of synthesis have been systematically investigated to improve the controllability. However, few studies focused on sensing the synthesis processes in situ and responding to the synthesis immediately. Here, we propose the concept of intelligent synthesis in which the final product can be automatically fine-controlled by a closed loop including in situ monitoring and real-time interventions. As a model system, a high-temperature-tolerant circuit is fabricated on the single-walled carbon nanotube (SWCNT) growth substrate for sensing and responding to the synthesis processes. As a result, either highly pure semiconducting (s-) SWCNT arrays or metallic-semiconducting (m-s) junction arrays with different junction positions is simply synthesized by programming the responding signal. The intelligent synthesis shows much higher efficiency and controllability compared to conventional methods and will lead to the next leap in nanotechnology.