Carbon Nanotube Junctions Research Articles

Chemical vapor deposition-grown carbon nanotubes/graphene hybrids for electrochemical energy storage and conversion

Chemical vapor deposition (CVD)-grown carbon nanotubes (CNTs)/graphene hybrids produce a novel three-dimensional carbon composite structure with seamless C–C junctions of one-dimensional CNTs and two-dimensional graphene. The CNTs/graphene hybrids promote the advantages of their building blocks, exhibiting large surface area, high electrical conductivity, excellent mechanical and chemical stability. Hence they are becoming an ideal electrode material platform for electrochemical energy storage and conversion including supercapacitors, lithium batteries and fuel cells. In this review, we first summarize the CVD growth for the fabrication of three types of CNTs/graphene hybrids, including CNTs/graphene oxide, CNTs/planar graphene and CNTs/graphene foam. And then we discuss the applications of CVD-grown CNTs/graphene hybrids in supercapacitors, lithium batteries and fuel cells. Finally, we propose the challenges and opportunities in future development of the CNTs/graphene hybrids.

Physically Unclonable Functions Based on Single-Walled Carbon Nanotubes: A Scalable and Inexpensive Method toward Unique Identifiers

A physically unclonable function (PUF) is a physical system that cannot be reproduced or predicted and therefore is a good basis on which to build security and anticounterfeiting applications. The unclonability of PUFs typically stems from the randomness induced in a system during sophisticated fabrication methods. It is precisely this built-in complexity that is the bottleneck hindering scalability and increasing costs. Here, we produce in a simple manner PUFs based on arrays of carbon nanotube junctions simultaneously assembled by dielectrophoresis. We demonstrate that the intrinsic inhomogeneity of carbon nanotubes at the nanoscale, combined with the unpredictability introduced by liquid phase-based fabrication methods, results in unique electronic profiles of easily scalable devices. This approach could be extrapolated to generate PUFs and thus advanced identification systems based on other nanoscale materials.

Tunneling-Limited Thermoelectric Transport in Carbon Nanotube Networks Embedded in Poly(dimethylsiloxane) Elastomer

As an emerging flexible thermoelectric material, polymer composites with embedded carbon nanotube (CNT) networks have shown promising properties, but their thermoelectric transport has not been fully understood. Herein, we embedded CNT networks in poly(dimethylsiloxane) (PDMS) elastomers and analyzed their thermoelectric properties using Landauer theory. We find that the simultaneous increase in Seebeck coefficient and electrical conductivity with increasing CNT content up to ∼40% can be attributed to the tunneling transport at CNT junctions with the gap distance decreasing with increasing CNT content. Beyond 40% CNTs, both properties are reduced and saturated due to the reduced PDMS content and increased material nonuniformity, which effectively replaced the PDMS gap with an air gap at the junction with a higher barrier to reduce the properties. Our results and analysis provide important insights into the material optimization of hybrid thermoelectric composites based on CNT networks and many other nanos...

DNA-Powered Stimuli-Responsive Single-Walled Carbon Nanotube Junctions

Reconfigurable stimuli-responsive molecular materials play an important role in the fabrication of nanoscale systems and devices. Here, we report a bottom-up strategy for the reversible assembly of single-walled carbon nanotube linear junctions in solution. The assembly/disassembly of the nanotubes can be controlled via the intrinsic responsiveness to different stimuli of sequence-specific deoxyribonucleic acid linkers forming the junctions.

Numerical Analysis of the Structural Stability of Ideal (Defect-Free) and Structurally and Morphologically Degenerated Homogeneous, Linearly- and Angle-Adjoined Nanotubes and Cylindrical Fullerenes Under Axial Loading Using Finite Element Method

We report on the structural stability of ideal (defect-free) and structurally and morphologically degenerate carbon nanotubes and nanotube junction systems under axial loading based on the finite element method. We estimated the values for critical buckling load for uncapped and capped single-walled carbon nanotubes (SWCNTs) and linear and angle-adjoined SWCNT heterojunctions in ideal and structurally degenerate systems containing single-, double-, triple-, pinhole- and pentagon–heptagon (i.e., 5–7) structural defects and also containing a substitutional nitrogen (N) atom inclusion under compressive loading. Absolute atomic vacancy (defect) concentration in studied SWCNTs models was assumed to be nil for ideal systems, and was up to 3.0 at.% for structurally and morphologically degenerate systems. It was found that all types of structural defects and the morphological N-defect had reduced the load carrying capacity and mechanical strength in all SWCNT systems studied. The SWCNT models containing physically large vacant sites, such as triple- and pinhole-defects, displayed significantly lower critical load values compared to the systems that contained only a single-, double- or triple-vacancies. In addition, we found that capped SWCNTs performed marginally better in critical load carrying capacity compared to uncapped SWCNT systems. Furthermore, majority of the investigated structures displayed reduced load in SWCNTs with narrower tube widths, proportional to the size and the type of the defect investigated. The effects of chirality, such as zigzag- versus armchair-type, on the structural stability of the investigated SWCNT models were also investigated.

High temperature response capability in carbon nanotube/polymer nanocomposites

High temperature response capability is critical for multifunctional carbon nanotube polymer nanocomposites. In this work, a CNT/polymer nanocomposite with high temperature coefficients of resistance (Average value = −3%/K, Peak value = −7%/K, Response range = 323 K–473 K) was fabricated successfully. By developing a percolation network model, this work reveals theoretically that the observed high temperature response is mainly resulted from glass transition of polymer matrix. During this process, the electron transport at CNT junctions switched from quantum tunneling to thermally activated hopping. The simulation results also reveal that the temperature response capability can be further improved (Average value = −4.7%/K, Peak value = −10%/K, Response range = 380 K–500 K) by using Polytetrafluoroethylene as polymer matrix with 0.5 wt% CNT loadings. These highly sensitive, low cost composites have a great potential for wide range applications, such as temperature sensors and temperature threshold switches.

Promoting Dual Electronic and Ionic Transport in PEDOT by Embedding Carbon Nanotubes for Large Thermoelectric Responses.

Thermoelectric (TE) energy conversion with nontraditional organic materials is promising in wearable electronics and roll-to-roll manufacturing because of mechanical flexibility, lightweight, and easy processing. Although typical organic materials have a benefit of low thermal conductivity that creates a large temperature gradient, relatively small thermopower (or Seebeck coefficient) often requires copious number of TE legs to fabricate practical TE devices. Here, we show that hybrids of poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) and carbon nanotubes (CNTs) can produce extremely large thermopower, ∼14 mV/K at room temperature by a chemical reduction. With decent electrical conductivity, an extraordinary power factor of ∼1200 μW/m K2 at room temperature was observed. The large power factor could be attributed to prominent dual electronic and ionic conduction, which islikely to be promoted by embedding the CNTs in PEDOT due tothe improvementin thecarrier mobility, in comparison with the inferior and widely varyingTE properties of PEDOT-only samplesin the literature. While a higher CNT concentration gave a larger electronic contribution, a longer reduction or a lower CNT concentration provided a larger ionic contribution. Meanwhile, well-separated CNTs created CNT junctions intervened by PEDOT-Tos, suppressing the thermal transport. Further research utilizing the high TE responses could greatly help to develop practical wearable and/or mass-producible thermal energy harvesting and storage devices.

Investigations on the transformation of vertically aligned CNTs to intramolecular junctions by atmospheric pressure PECVD

Atmospheric pressure glow discharge plasma assisted chemical vapor deposition process is used for the growth of carbon nanotubes (CNTs) on Inconel without using any external catalyst. Four different sets of samples are prepared by varying the growth time. The grown nanostructures are then characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Field emission measurements are also done on the grown structures. Our investigations revealed that vertically aligned CNTs transformed into intramolecular junctions of CNTs with an increase in deposition time from 4 to 10 min. This time dependence for the transformation is investigated and correlated with other characterization results. It is seen that field emission behavior is the best for the vertically aligned CNTs but it also improves when the junctions of CNTs are present in the matrix after increasing growth time to 15 min. The investigations reveal that branching brings favorable defects in the grown CNTs that can give rise to impressive field emission current density. A schematic model has been prepared based on the experimental findings to find out the reason behind the growth of these junctions and how did they help in improving field emission.

Highly stretchable and transparent electrode film based on SWCNT/Silver nanowire hybrid nanocomposite

Abstract Highly stretchable, flexible, and transparent electrode films were fabricated based on a composite of silver nanowire (Ag NW) networks hybridized with single-walled carbon nanotubes (SWCNTs) in a sandwich structure, wherein a Ag NW-SWCNT hybrid network electrode was embedded between layers of polydimethylsiloxane and transparent polyurethane films. When 0.025 wt% SWCNTs were hybridized with Ag NWs, a Ag NW-SWCNT network electrode, which had excellent optical properties (very low haze value of less than 1% and 95% transmittance) and a low sheet resistance value of 30 Ω/sq, was obtained through the effective electrical transportation of the NW–NW, CNT–NW, and CNT–CNT junctions. The fabricated film heaters showed environmental endurance, and no significant performance degradation was observed after the films were kept at a high temperature of 85 °C and humidity of 85%. They also exhibited high stretchability up to 20%. Effective Joule heating can increase the temperature of the electrode films (30 Ω/sq) up to 68 °C at a bias of 11 V.

Characterizing the surface forces between two individual nanowires using optical microscopy based nanomanipulation

The adhesion and friction between two Al2O3 nanowires (NWs) was characterized by the use of optical microscopy based nanomanipulation, with which peeling, shearing and sliding was performed. The elastically deformed shape of the NWs during peeling and shearing was used to calculate the adhesion and frictional forces; force sensing was not required. The obtained adhesion stress between two Al2O3 NWs varied from 0.14 to 0.25 MPa, lower than that observed for carbon nanotube junctions, and was attributed to van der Waals attraction. Stick-slip was observed during the shearing and sliding of two NWs, and was the consequence of discrete contact between surface asperities. The obtained static and kinetic frictional stresses varied from 0.7 to 1.3 MPa and 0.4 to 0.8 MPa, respectively; significantly greater than the obtained adhesion stress.

Ab Initio Study of Iodine-Doped Carbon Nanotube Conductors

The widespread use of copper in power and data cabling for aircraft, ships, and ground vehicles imposes significant mass penalties and limits cable ampacity. Experimental research has suggested that iodine-doped carbon nanotubes (CNTs) can serve as energy efficient replacements for copper in mass sensitive cabling applications. The high computational costs of ab initio modeling have limited complimentary modeling research on the development of high specific conductance materials. In recent research, the authors have applied two modeling assumptions, single zeta basis sets and approximate geometric models of the CNT junction structures, to allow an order of magnitude increase in the atom count used to model iodine-doped CNT conductors. This permits the ab initio study of dopant concentration and dopant distribution effects, and the development of a fully quantum based nanowire model which may be compared directly with the results of macroscale experiments. The accuracy of the modeling assumptions is supported by comparisons of ballistic conductance calculations with known quantum solutions and by comparison of the nanowire performance predictions with published experimental data. The validated formulation offers important insights on dopant distribution effects and conduction mechanisms not amenable to direct experimental measurement.

Developing nanotube junctions with arbitrary specifications.

Experimentally synthesized carbon nanotube (CNTs) junctions (either single or with 2D/3D CNT network topology) are expected to have random orientation of defect sites (non-hexagonal rings) around the junction. This random and irregular nature of the junction topology and defect characteristics is expected to affect their strength and durability as well as impact the associated mesoscopic and macroscopic properties. On the contrary, theoretical and computational studies often investigate structure-property relationships of pristine and regular junctions of carbon nanostructures. In this study, we developed a computational framework to model a variety of junction structures between CNTs with arbitrary spatial (orientation and degree of overlap) and intrinsic (chirality) specifications. The developed computational model also has the ability to tune the degree of topological defects around the junction via a variety of defect annihilation approaches. Our method makes use of the primal/dual meshing concept, where the development and manipulation of the junction nodes occur using triangular meshes (primal mesh), which is eventually converted to its dual mesh (honeycomb mesh) to render a fully covalently bonded CNT junction. Here each carbon atom has 3 bonded neighbors (mimicking sp2 hybridization). Under a given set of CNT orientation, overlap and chirality specifications, the approach creates a number of CNT junction configurations with varying degrees of energetic stability, offering an opportunity to investigate the effect of topological arrangement of defects around the junction on mechanical, electrical and thermal properties. In addition, it is shown via few examples that the discussed methodology can easily be extended to create multi-junction nanotube clusters, multi-wall nanotube junctions, as well as true 3D random network structures.

Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest.

Many attempts to obtain high current rectification ratios (RRs) in molecular electronics are triggered by a potentiometer rule argument, which predicts that a strongly asymmetric location of the dominant molecular orbital yields large RR-values. Invoking this argument, molecular junctions based on alkane monothiols (CnT) can be expected to exhibit high RRs; the HOMO of these molecules is localized on the thiol terminal group bonded to one electrode. The extensive current-voltage (I-V) results for CP-AFM (conducting probe atomic force microscope) CnT junctions of various molecular lengths (n = 7, 8, 9, 10, and 12) and different metallic contacts (Ag, Au, and Pt) are consistent with conduction dominated by the HOMO, but the measured RR ∼ 1.5 is much smaller than that predicted by the potentiometer rule framework. Further, the linear shift in the HOMO position with applied bias, γ, which gives rise to rectification, is also smaller than expected, and critically, γ has the opposite sign from potentiometer rule predictions. Companion ab initio OVGF (outer valence Green's function) quantum chemical calculations provide important insight. Namely, a linear Stark shift γm is calculated for the HOMO of CnT molecules for electric field strengths (106-107 V cm-1) typical of molecular junctions, and the sign of γm matches the sign of the experimental γ for junctions derived from transport measurements, suggesting that the Stark effect plays an important role. However, the magnitude of the measured γ is only 10-15% of the computed value γm. We propose that this implies that the contacts are far from optimal; they substantially screen the effect of the applied bias, possibly via molecule-electrode interface states. We predict that, with optimized contacts, the rectification ratios in CnT-based junctions can reach reasonably high values (RR ≈ 500). We believe that Stark shifts and limited current rectification due to non-ideal contacts discussed here for the specific case of alkane monothiol junctions are issues of general interest for molecular electronics that deserve further consideration.

A First-Principle Theoretical Study of Mechanical and Electronic Properties in Graphene Single-Walled Carbon Nanotube Junctions.

The new three-dimensional structure that the graphene connected with SWCNTs (G-CNTs, Graphene Single-Walled Carbon Nanotubes) can solve graphene and CNTs′ problems. A comprehensive study of the mechanical and electrical performance of the junctions was performed by first-principles theory. There were eight types of junctions that were constituted by armchair and zigzag graphene and (3,3), (4,0), (4,4), and (6,0) CNTs. First, the junction strength was investigated. Generally, the binding energy of armchair G-CNTs was stronger than that of zigzag G-CNTs, and it was the biggest in the armchair G-CNTs (6,0). Likewise, the electrical performance of armchair G-CNTs was better than that of zigzag G-CNTs. Charge density distribution of G-CNTs (6,0) was the most homogeneous. Next, the impact factors of the electronic properties of armchair G-CNTs were investigated. We suggest that the band gap is increased with the length of CNTs, and its value should be dependent on the combined effect of both the graphene’s width and the CNTs’ length. Last, the relationship between voltage and current (U/I) were studied. The U/I curve of armchair G-CNTs (6,0) possessed a good linearity and symmetry. These discoveries will contribute to the design and production of G-CNT-based devices.

Tuning thermal conductance of CNT interface junction via stretching and atomic bonding

In this work, various effects of stretching and bonding strength on the thermal transport at CNT junctions are comprehensively studied by classical molecular dynamics (MD) simulations. The modeling is performed on a typical parallel-aligned junction formed by two single-walled (10, 10) CNTs. The overlap length is the first condition we studied and it is found that thermal conductance is significantly increased from 1.00 to 11.76 nW K−1 with overlap length from 0.982 to 6.877 nm. Surprisingly, the thermal conductance per unit overlap length is increased rather than a constant value. The van der Waals interaction in non-bonded CNTs has a positive correlation on thermal conductance, which means thermal conductance can be effectively enhanced by applied force in the inter-tube direction. In the axial direction, the applied force is an important condition to adjust thermal conductance at bonded junction. Results show that the thermal conductance for overlap length of 1.966 nm can be enhanced from 2.81 to 3.42 nW K−1 at the initial stage because of the combined squeezing force. However, as applied force approaches the breaking value, the atomic bonding at the junction is greatly weakened with a rapidly dropping thermal conductance from 3.42 to 1.88 nW K−1.

Structure of junctions of multiwalled carbon nanotubes with tetragonal cross section and flattened nanotubes revealed by electron-beam tomography

Multiwalled carbon nanotubes with tetragonal cross section frequently form junctions with flattened multi-walled carbon nanotubes, a kind of carbon nanoribbon. The three-dimensional structure of the junctions is revealed by transmission-electron-microscopy-based tomography. Two types of junction, parallel and diagonal, are found. The formation mechanism of these two types of junction is discussed in terms of the origami mechanism that was previously proposed to explain the formation of carbon nanoribbons and nanotetrahedra.

Thermally conductive polymeric composites incorporating 3D MWCNT/PEDOT:PSS scaffolds

Abstract Although several types of stabilizers have been investigated recently to disperse carbon nanotubes (CNTs), the most common of them are thermal insulators that increase the thermal resistance and hinder the heat conduction across the CNT junctions. In this study, we dispersed multi-wall carbon nanotubes (MWCNTs) using the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a functional surfactant. Upon sonication, PEDOT:PSS covered the surfaces of the MWCNTs, thereby preventing their π-stacking and increasing their dispersion. Moreover, PEDOT:PSS acted as a thermally conductive bridge that connected the MWCNTs and decreased their thermal resistance. We employed a freeze-drying method to prepare MWCNT/PEDOT:PSS composites with hierarchical microstructures. After introducing polydimethylsiloxane (PDMS) into the MWCNT/PEDOT:PSS foam, the thermal conductivity of the MWCNT/PEDOT:PSS and PDMS composite reached 1.16 W/mK—over 550% higher than that of pure PDMS. These results suggest that such PDMS composites might be useful as thermal interface materials for thermal management in electronic and photonic applications.

Effect of temperature on the electrical property of epoxy composites with carbon nanotube

Eliminating the influence of environment temperature is critical for high-accuracy carbon nanotube polymer nanocomposites sensors. In this work, the temperature effects on the nanocomposite are studied by both experiment investigation and simulation calculation. Nearly constant resistance values were found at a CNT loading around 3 wt%. By considering the temperature effect and CNT structural distortion in the developed percolation network model, simulation results agree well with experimental data. On this basis, results show that the thermally assisted tunneling on CNT junctions and thermal expansion of polymer matrix are the two core mechanisms, relaxed CNT junctions in CNT/polymer nanocomposite depressed the temperature effect, and the influence of environment temperature could be significantly reduced by adjusting CNT loadings and choosing a matching polymer matrix. All of these findings will benefit for the design of high-accuracy sensors.

A Molecular Turnstile as an E‐Field‐Triggered Single‐Molecule Switch: Concept and Synthesis

A new single‐molecule switching concept relying on the E‐field‐dependent orientation of a push‐pull system (via its dipole moment) is presented. As first steps towards realizing the E‐field‐triggered single‐molecule motion, the synthesis of turnstile 1, which is designed to bridge a gap within a carbon nanotube junction, is reported. Turnstile 1 consists of a hexakis(m‐phenyleneethynylene) macrocycle decorated with phenanthrene‐functionalized oligo(phenyleneethynylene) (OPE) wires at opposite ends, as well as a para‐connected push‐pull rod as rotator unit. The similarity of the rotor dimension with the surrounding macrocycle guarantees efficient π‐stacking between both subunits when an E‐field is applied. To introduce a dipole moment into the rotator, the structure is terminally functionalized with a nitrile group on one side and with a dimethylamino group on the opposite end. Synthetic protocols based on Sonogashira–Hagihara couplings were developed to build up the macrocycle scaffold. The highly functionalized open precursor A is the key building block of the sequence as it allows a twofold intramolecular palladium‐catalyzed cyclization reaction to be performed to obtain the target turnstile in a very good yield of 68 %. The target structure was fully characterized by NMR spectroscopy and mass spectrometry. Furthermore, 1H‐1H NOESY NMR experiments pointed to a pseudo‐rotation of the push‐pull rod within the turnstile structure on the time scale of the NMR experiment.

Variable electronic transport properties of double carbon-nanotube junctions via substitutional Fe, Co, and Mn atom doping

Electronic and transport properties of carbon-nanotube (CNT) hybrid junction CNT(3,3)/CNT(6,0)/CNT(3,3) are investigated and compared with substitutionally doped junction with transition metal atoms, iron, cobalt, and manganese; using density functional theory combined with non-equilibrium Green's function formalism. Transmission coefficient and current–voltage (I–V) characteristics are calculated for all configurations. The results indicate a smooth increase of current with voltage in pristine junction up to 0.75 V while in the case of higher voltages, the increase accelerates significantly and the conductance for the voltages between 1 and 1.5 becomes almost constant, about 40μA V. The replacement of carbon atoms with Fe, Co, and Mn in the scattering region causes rather substantial reduction of the transmission and conductance of the hybrid junction. It causes the current to reduce nearly 25% at the voltage of 1.5 V compared to the pristine one. It also appears that, there is no serious difference in the I–V characteristics for different impurity atoms, except some deviation in manganese conductance from the other two impurities’ curves. These results can be useful for future CNTs based nano-electronic applications.