Carbon Nanotube Channel Research Articles

Diffusion of One-Dimensional Crystals in Channels of Single-Walled Carbon Nanotubes

The transport of one-dimensional CuI crystals in channels of single-walled carbon nanotubes (SWCNTs) has been studied by high resolution electron microscopy. The diffusion kinetics has been investigated by counting the number of CuI atoms escaping from the nanotube channel. The diffusivity is calculated to be 6.8 × 10–21 m2/s, which corresponds to an activation-barrier height of ~1 eV/atom. A comparison with the theoretically estimated height of the energy barrier for molecular transport through a graphene layer is indicative of mass transfer through vacancy defects in graphene.

Designing Single-Wall Carbon Nanotube Forest Growth for Nanofluidic Applications

Ultrafast fluid transport through the core of single-wall carbon nanotube (SWNT) channels promises to advance membrane applications, from efficient water purification and low-cost separation of high-value components, to next-generation protective garments. For the last application, we recently demonstrated cm2-area, vertically-aligned, SWNT-polymer composite membranes that combined both high breathability and protection in a single functional material, thus breaking the typically elusive trade-off between the two.1 As we seek to further enhance the performance of these membranes, we target vertically-aligned SWNT “forest” growth toward: a) minimizing SWNT diameter to maximize protection via size exclusion; b) maximizing SWNT number density to maximize breathability; and c) scaling up SWNT growth area to eventually incorporate into garments. We perform low-pressure chemical vapor deposition (CVD) in an AIXTRON® Black Magic cold-wall furnace, featuring a wafer-scale, local heater stage and gas showerhead. The combination of a low flux of carbon precursors with sub-nm Fe/Mo catalyst films on alumina-coated Si wafers maintains small-diameter SWNTs. High-resolution TEM and X-ray scattering2 confirm that our forests contain >99% SWNTs below 3.5 nm diameter (on wafers up to 4 inches). Consistent with literature reports, 6 at% Mo optimally preserves small, densely packed particles, templating growth of forests with high number densities up to 1.4x1012 cm-2. In an effort to control the forest thickness and uniformity across large substrate areas, we investigate the area-dependent growth kinetics and temporal density decay. Despite exhibiting a growth rate that is initially uniform across the substrate, growth at the center self-terminates first, while edge growth continues at a sublinear rate for tens of minutes. Understanding these varying reaction lifetimes that arise on a single substrate is key to engineering large-area SWNT membranes with highly uniform transport characteristics. To scale SWNT forest production and overcome non-uniformities in wafer-scale growth, we seek to simplify the catalyst film formulation. While wafer-scale growth from Fe films of equal thickness in the absence of Mo is markedly more robust over multiple sequential batches, the SWNT diameter distributions have larger means and tails, which are detrimental to the rejection properties of our membranes. Tuning the Fe thickness down renders smaller SWNTs, but the range of densities grown from these Fe-only films is 0.2-0.6x1012 cm-2, regardless of thicknesses studied (0.35-0.65 nm). This challenge of accessing a parameter space that co-optimizes growth yield of small, monodisperse diameter, high-density SWNT forests highlights a deficiency in conventional catalyst design and suggests a need for improved designs that could unlock a range of SWNT applications. This work is supported by the Defense Threat Reduction Agency (DTRA) D[MS]2 project under Contract No. BA12PHM123 and was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Bui, N.; Meshot, E. R.; Kim, S.; Peña, J.; Gibson, P. W.; Wu, K. J.; Fornasiero, F., Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores. Advanced Materials 2016, 28 (28), 5871-5877.Meshot, E.; Zwissler, D. W.; Bui, N.; Kuykendall, T. R.; Wang, C.; Hexemer, A.; Wu, K. J. J.; Fornasiero, F. Quantifying the Hierarchical Order in Self-Aligned Carbon Nanotubes from Atomic to Micrometer Scale. ACS Nano 2017, 11 (6), 5405-5416.

Thermal Device Design for a Carbon Nanotube Terahertz Camera.

Terahertz (THz) wave detectors are increasingly expected to serve as key components of powerful nondestructive and noncontact inspection tools in a large variety of fields. In contrast to conventional THz detectors based on rigid solid materials, we previously developed an uncooled and bendable THz camera based on the THz-induced photothermoelectric effect of carbon nanotube (CNT) array devices and demonstrated omnidirectional THz imaging of three-dimensional curved samples. Although this development opened a pathway to flexible THz electronics, the physical parameters that determine the performance of the CNT THz camera have not been fully investigated. As a result, the thermal device design has not been optimized in terms of the camera sensitivity and spatial resolution. In this work, we studied the underlying mechanism of the THz-induced photothermoelectric effect of the CNT camera and found physical factors related to the detector performance. Through simulation and experiments, we observed that the detection sensitivity and response time strongly depend on the CNT channel width and film thickness. We further identified that the irradiated wave penetration into the CNT film through the electrode materials deteriorates the detection area, which is directly linked to the camera spatial resolution. By utilizing the improved CNT device design fabricated based on these findings, we eliminated undesired signals generated via thermal diffusion and THz wave penetration and achieved higher-sensitivity THz detection and higher imaging resolution compared to our previously reported THz camera. The presented technologies are expected to contribute to future flexible THz imaging applications and will also be applicable to other types of photothermoelectric devices.

Oxidative desulfurization of model fuel in the presence of molecular oxygen over polyoxometalate based catalysts supported on carbon nanotubes

With the emergence of environmental problems such as acid rain and haze, the study on fuel desulfurization has become an important subject of environmental protection. Oxidative desulfurization (ODS) becomes the promising method due to its mild reaction condition, low cost and high efficiency. In this work, two series of catalysts supported by carbon nanotubes (CNTs) are prepared by two different procedures: a) the polyoxometalate (POM) is impregnated into the channel of CNTs by impregnation method, b) the modified CNTs is wrapped by polyoxometalate through ion exchange method. The novel catalysts are investigated by FT-IR, XRD, XPS, SEM and N2 adsorption–desorption isotherms. The desulfurization performance of all catalysts is carried out under a certain condition, and the catalysts have positive effects on dibenzothiophene (DBT) conversion. The effect of factors on desulfurization is evaluated, and the favorable conditions are determined. Based on optimal conditions, the DBT conversion reached up 99.4%, and the desulfurization system could be recycled 8 times without noticeable decrease in activity. In addition, the impact of the critical parameters (temperature, mass of catalyst and oxygen quantity) was investigated using Box–Behnken experimental design. The optimum simulation condition is consistent with the experimental investigation, which further proves the accuracy of the experimental conclusion.

Detection of Individual Molecules and Ions by Carbon Nanotube-Based Differential Resistive Pulse Sensor.

This paper presents a new method of sensing single molecules and cations by a carbon nanotube (CNT)-based differential resistive pulse sensing (RPS) technique on a nanofluidic chip. A mathematical model for multichannel RPS systems is developed to evaluate the CNT-based RPS signals. Individual cations, rhodamine B dye molecules, and ssDNAs are detected successfully with high resolution and high signal-to-noise ratio. Differentiating ssDNAs with 15 and 30 nucleotides are achieved. The experimental results also show that translocation of negatively charged ssDNAs through a CNT decreases the electrical resistance of the CNT channel, while translocation of positively charged cations and rhodamine B molecules increases the electrical resistance of the CNT. The CNT-based nanofluidic device developed in this work provides a new avenue for single-molecule/ion detection and offers a potential strategy for DNA sequencing.

Facile construction of Ag nanoparticles encapsulated into carbon nanotubes with robust antibacterial activity

A facile glucose assisted one-step method for preparing highly dispersed Ag nanoparticles (Ag-NPs) encapsulated into carbon nanotubes (CNTs) was developed. The CNTs with open tips are employed as the nano-reactor and support, silver nitrite (AgNO3) is used as the precursor and glucose is employed both as the reducing and protecting agent in the whole preparation process. The Ag-NPs with diameters of 6–10 nm are uniformly assembled into the channel of CNTs. The detailed structure and morphology of Ag-NPs encapsulated into CNTs (Ag-NPs@CNTs) were systematically characterized. The obtained Ag-NPs@CNTs exhibits an enhanced antibacterial activity against Escherichia coli, comparing with those of Ag-NPs supported on activate carbon (Ag-NPs/AC) and on CNTs (Ag-NPs/CNTs) prepared by the traditional wetness impregnation method.

Understanding carbon nanotube channel formation in the lipid membrane

Carbon nanotubes (CNTs) have been considered a prominent nano-channel in cell membranes because of their prominent ion-conductance and ion-selectivity, offering agents for a biomimetic channel platform. Using a coarse-grained molecular dynamics simulation, we clarify a construction mechanism of vertical CNT nano-channels in a lipid membrane for a long period, which has been difficult to observe in previous CNT-lipid interaction simulations. The result shows that both the lipid coating density and length of CNT affect the suitable fabrication condition for a vertical and stable CNT channel. Also, simulation elucidated that a lipid coating on the surface of the CNT prevents the CNT from burrowing into the lipid membrane and the vertical channel is stabilized by the repulsion force between the lipids in the coating and membrane. Our study provides an essential understanding of how CNTs can form stable and vertical channels in the membrane, which is important for designing new types of artificial channels as biosensors for bio-fluidic studies.

Negative Capacitance Carbon Nanotube FETs

As continued scaling of silicon FETs grows increasingly challenging, alternative paths for improving digital system energy efficiency are being pursued. These paths include replacing the transistor channel with emerging nanomaterials (such as carbon nanotubes), as well as utilizing negative capacitance effects in ferroelectric materials in the FET gate stack, e.g., to improve sub-threshold slope beyond the 60 mV/decade limit. However, which path provides the largest energy efficiency benefits—and whether these multiple paths can be combined to achieve additional energy efficiency benefits—is still unclear. Here, we experimentally demonstrate the first negative capacitance carbon nanotube FETs (CNFETs), combining the benefits of both carbon nanotube channels and negative capacitance effects. We demonstrate negative capacitance CNFETs, achieving sub-60 mV/decade sub-threshold slope with an average sub-threshold slope of 55 mV/decade at room temperature. The average ON-current ( ${I}_{ \mathrm{ON}}$ ) of these negative capacitance CNFETs improves by $2.1\times $ versus baseline CNFETs, (i.e., without negative capacitance) for the same OFF-current ( ${I}_{ \mathrm{OFF}}$ ). This work demonstrates a promising path forward for future generations of energy-efficient electronic systems.

Improving subthreshold swing to thermionic emission limit in carbon nanotube network film-based field-effect

In this letter, we explore the vertical scaling-down behavior of carbon nanotube (CNT) network film field-effect transistors (FETs) and show that by using a high-efficiency gate insulator, we can substantially improve the subthreshold swing (SS) and its uniformity. By using an HfO2 layer with a thickness of 7.3 nm as the gate insulator, we fabricated CNT network film FETs with a long channel (>2 μm) that exhibit an SS of approximately 60 mV/dec. The preferred thickness of HfO2 as the gate insulator in a CNT network FET is between 7 nm and 10 nm, simultaneously yielding an excellent SS (<80 mV/decade) and low gate leakage. However, because of the statistical fluctuations of the network CNT channel, the lateral scaling of CNT network film-based FETs is more difficult than that of conventional FETs. Experiments suggest that excellent SS is difficult to achieve statistically in CNT network film FETs with a small channel length (smaller than the mean length of the CNTs), which eventually limits the further scaling down of this kind of CNT FET to the sub-micrometer regime.

Asymmetric dipole distribution of single-file water molecules across nanochannels under high pressure gradient

The dipole distributions of water molecules across the channel of single-walled carbon nanotubes (SWCNTs) under high pressure gradient P are studied using molecular dynamics (MD) simulations. It is found that the profiles of the dipole distributions are highly symmetrical with two peak values around 30° and 150° at zero or low pressure gradient (P ≤ 6MPa), and remarkably asymmetric with the main peak around 30° under high pressure gradient (P ≥ 12MPa). The reason is mainly that molecules can enter into the SWCNTs from two ends under low pressure water, whereas the water molecules can only enter into the tube from one end under high pressure. Further, it also implies that water molecules prefer to retain the hydrogens forward when entering into and passing through the nanotube. Our observations may be instructive for future applications in nanometer scale fluid devices and biological channels.

Ultra-Low Powered CNT Synaptic Transistor Utilizing Double PI:PCBM Dielectric Layers

We demonstrated a CNT synaptic transistor by integrating 6,6-phenyl-C61 butyric acid methyl ester(PCBM) molecules as charge storage molecules in a polyimide(PI) dielectric layer with carbon nanotubes(CNTs) for the transistor channel. Specifically, we fabricated and compared three different kinds of CNT-based synaptic transistors: a control device with Al2O3/PI, a single PCBM device with Al2O3/PI:PCBM(0.1 wt%), and a double PCBM device with Al2O3/PI:PCBM(0.1 wt%)/PI:PCBM(0.05 wt%). Statistically, essential device parameters such as Off and On currents, On/Off ratio, device yield, and longterm retention stability for the three kinds of transistor devices were extracted and compared. Notably, the double PCBM device exhibited the most excellent memory transistor behavior. Pulse response properties with postsynaptic dynamic current were also evaluated. Among all of the testing devices, double PCBM device consumed such low power for stand-by and its peak current ratio was so large that the postsynaptic current was also reliably and repeatedly generated. Postsynaptic hole currents through the CNT channel can be generated by electrons trapped in the PCBM molecules and last for a relatively short time(~ hundreds of msec). Under one certain testing configuration, the electrons trapped in the PCBM can also be preserved in a nonvolatile manner for a long-term period. Its integrated platform with extremely low stand-by power should pave a promising road toward next-generation neuromorphic systems, which would emulate the brain power of 20W.

Universal model of bias-stress-induced instability in inkjet-printed carbon nanotube networks field-effect transistors

We propose a universal model for bias-stress (BS)-induced instability in the inkjet-printed carbon nanotube (CNT) networks used in field-effect transistors (FETs). By combining two experimental methods, i.e., a comparison between air and vacuum BS tests and interface trap extraction, BS instability is explained regardless of either the BS polarity or ambient condition, using a single platform constituted by four key factors: OH− adsorption/desorption followed by a change in carrier concentration, electron concentration in CNT channel corroborated with H2O/O2 molecules in ambient, charge trapping/detrapping, and interface trap generation. Under negative BS (NBS), the negative threshold voltage shift (ΔVT) is dominated by OH− desorption, which is followed by hole trapping in the interface and/or gate insulator. Under positive BS (PBS), the positive ΔVT is dominated by OH− adsorption, which is followed by electron trapping in the interface and/or gate insulator. This instability is compensated by interface trap extraction; PBS instability is slightly more complicated than NBS instability. Furthermore, our model is verified using device simulation, which gives insights on how much each mechanism contributes to BS instability. Our result is potentially useful for the design of highly stable CNT-based flexible circuits in the Internet of Things wearable healthcare era.

Designing hybrid gate dielectric for fully printing high-performance carbon nanotube thin film transistors

The electrical characteristics of carbon nanotube (CNT) thin-film transistors (TFTs) strongly depend on the properties of the gate dielectric that is in direct contact with the semiconducting CNT channel materials. Here, we systematically investigated the dielectric effects on the electrical characteristics of fully printed semiconducting CNT-TFTs by introducing the organic dielectrics of poly(methyl methacrylate) (PMMA) and octadecyltrichlorosilane (OTS) to modify SiO2 dielectric. The results showed that the organic-modified SiO2 dielectric formed a favorable interface for the efficient charge transport in s-SWCNT-TFTs. Compared to single-layer SiO2 dielectric, the use of organic–inorganic hybrid bilayer dielectrics dramatically improved the performances of SWCNT-TFTs such as mobility, threshold voltage, hysteresis and on/off ratio due to the suppress of charge scattering, gate leakage current and charge trapping. The transport mechanism is related that the dielectric with few charge trapping provided efficient percolation pathways for charge carriers, while reduced the charge scattering. High density of charge traps which could directly act as physical transport barriers and significantly restrict the charge carrier transport and, thus, result in decreased mobile carriers and low device performance. Moreover, the gate leakage phenomenon is caused by conduction through charge traps. So, as a component of TFTs, the gate dielectric is of crucial importance to the manufacture of high quality TFTs from the aspects of affecting the gate leakage current and device operation voltage, as well as the charge carrier transport. Interestingly, the OTS-modified SiO2 allows to directly print horizontally aligned CNT film, and the corresponding devices exhibited a higher mobility than that of the devices with the hybrid PMMA/SiO2 dielectric although the thickness of OTS layer is only ∼2.5 nm. Our present result may provide key guidance for the further development of printed nanomaterial electronics.

Confinement effect of carbon nanotubes on the product distribution of selective hydrogenation of cinnamaldehyde

The catalytic activity of metal catalysts can be modulated by confinement within the channels of carbon nanotubes (CNTs). Here, we show that the product distribution of cinnamaldehyde hydrogenation can be modified by confinement of Ru nanoparticles in CNTs. A catalyst composed of Ru nanoparticles dispersed on the exterior walls of CNTs gave hydrocinnamaldehyde as product. In contrast, confinement of the Ru nanoparticles within CNT channels facilitated hydrogenation of C=O bonds and complete hydrogenation, and both cinnamyl alcohol and hydrocinnamyl alcohol formed in addition to hydrocinnamaldehyde. High-resolution transmission electron microscopy, Raman spectroscopy, hydrogen temperature-programmed reduction, and hydrogen temperature-programmed desorption were used to investigate the characteristics of the catalysts. The results indicate that the different interactions between the confined Ru nanoparticles and the exterior and interior walls of the CNTs, as well as spatial restriction and enrichment within the narrow channels likely play important roles in modulation of the product distribution.

Fischer–Tropsch Synthesis: Influence of Acid Treatment and Preparation Method on Carbon Nanotube Supported Ruthenium Catalysts

The influences of nitric acid treatment on a carbon nanotube (CNT) support and the preparation method, incipient wetness impregnation (IWI) versus chemical vapor deposition (CVD), on catalytic performance during Fischer–Tropsch (FT) synthesis were examined using a slurry phase reactor. Acid treated CNT (ACNT) supported Ru catalysts exhibited higher activities compared to Ru supported on untreated CNTs (UCNTs). The acid-treated CVD catalyst had higher initial CO conversion (smaller average Ru particle size) but sintered more due to a greater tendency for Ru to be deposited exterior to CNT channels relative to IWI. After the initial decline and leveling off period, the ACNT IWI catalyst had the highest steady activity among the catalysts tested. The ACNT IWI catalyst also displayed greater oxygenate selectivity (∼17%) compared to ACNT CVD (∼12%) and UCNT IWI (∼10%) catalysts at similar conversions. Acid treatment created adsorption sites on the CNT surface that anchor Ru precursors and promote CO insertion ...

Heterogeneous Integration of Carbon-Nanotube-Graphene for High-Performance, Flexible, and Transparent Photodetectors.

Low-dimensional carbon materials, such as semiconducting carbon nanotubes (CNTs), conducting graphene, and their hybrids, are of great interest as promising candidates for flexible, foldable, and transparent electronics. However, the development of highly photoresponsive, flexible, and transparent optoelectronics still remains limited due to their low absorbance and fast recombination rate of photoexcited charges, despite the considerable potential of photodetectors for future wearable and foldable devices. This work demonstrates a heterogeneous, all-carbon photodetector composed of graphene electrodes and porphyrin-interfaced single-walled CNTs (SWNTs) channel, exhibiting high photoresponse, flexibility, and full transparency across the device. The porphyrin molecules generate and transfer photoexcited holes to the SWNTs even under weak white light, resulting in significant improvement of photoresponsivity from negligible to 1.6 × 10-2 A W-1 . Simultaneously, the photodetector exhibits high flexibility allowing stable light detection under ≈50% strain (i.e., a bending radius of ≈350 µm), and retaining a sufficient transparency of ≈80% at 550 nm. Experimental demonstrations as a wearable sunlight sensor highlight the utility of the photodetector that can be conformally mounted on human skin and other curved surfaces without any mechanical and optical constraints. The heterogeneous integration of porphyrin-SWNT-graphene may provide a viable route to produce invisible, high-performance optoelectronic systems.

Aligned MWNT channels in free standing polymer nanocomposite as an electrode for Li-ion battery

The investigation and development of flexible power sources has motivated to develop flexible, light weight, binder free, and current collector free electrodes for lithium ion batteries. We have therefore investigated the alignment and pattern of multiwall carbon nanotube (MWNT) channels in polystyrene (PS) matrix using magnetic field (5 kOe) for lithium ion storage and transport. In the presence of magnetic field, anisotropic MWNTs rotate towards the direction of the magnetic field and stabilized due to its magnetically susceptible nature. MWNTs suspended in PS were trapped and then aligned into the direction of the applied field. The alignment of MWNT channels was optimized by Raman spectroscopy and scanning electron microscopy. The charging–discharging capacity and rate capability were found to be increased for aligned MWNT/PS nanocomposite electrodes.

Experimental measurements in single-nanotube fluidic channels

Abstract

Metal-Free Fully Solution-Processable Flexible Electrolyte-Gated Carbon Nanotube Field Effect Transistor

Carbon nanotube-based field effect transistors (CNTFETs) are an interesting alternative to organic FETs in the growing field of printed electronics. Solution processed CNTFETs can be fabricated at low temperatures, are compatible with roll to roll processes and with flexible substrates. Usually metal electrodes for CNTFETs are deposited using standard techniques (e.g., evaporation or sputtering) which require expensive equipment and a high thermal budget. The elimination of such deposition step would allow a fully solution-based process for the CNTFETs fabrication. In this paper, we demonstrate an all carbon nanotube (CNT) transistor which is entirely solution processable without sacrificing the device performance. Performed detailed contact resistance analysis shows that CNT electrodes make better contacts to semiconducting CNTs channel than gold electrodes. The device performance is shown for an electrolyte-gated CNTFET fabricated on a flexible substrate. Such transistors are used as low cost biosensors for vivo implants by exploiting better interaction of flexible substrates to cells.

Anomalous Ionic Conductance in Carbon Nanotube Nanochannels

Simulations and experimental studies have reported an unusually high ionic conductance in carbon nanotube (CNT) nanochannels. The origin of this phenomenon is, however, poorly-understood: literature reports often disagree in the magnitude of the different transport mode contributions to the measured ionic current and even in what ions are actually carrying the current; moreover, results obtained with single pore measurements differ frequently from those with membranes containing billions of open CNT channels, i.e. the average CNT behavior. Toward shedding light on these phenomena, we fabricated a novel nanofluidic platform having a >30-nm wide, FIB-nanomachined silicon nitride nanopore (SiNx) in series with vertically-aligned sub-5 nm carbon nanotubes, the number of which can be controlled from one to billions. By employing this platform with only one or a few open CNTs, we observed giant ionic currents in CNT channels and a power-law increase of conductance with KCl concentration (G ∼ cn, n=0.1-0.4), a dependence that seems to be unique of CNT pores. A few literature reports attributed this giant ionic current in CNTs to a strong electro-osmotic flow. To quantify electro-osmosis in CNT pores, we investigated translocation of neutral molecules in a single CNT nanochannel with the resistive pulse technique. Furthermore, we employed finite element analysis to elucidate the relationship between CNT pore characteristics (e.g., size, surface charge, slip length) and the resulting ionic transport and magnitude of the electro-osmotic component. We confirmed computationally that the unusually large conductance in our experimental platform results from the presence of a strong electro-osmotic coupling between the CNT channel and the SiNx nanopore in series with it. Finally, we apply first-principle simulations to show that cation-CNT interactions may help explain the origin of this electro-osmotic flow.