Single-walled Carbon Nanotube Junctions Research Articles

Charge and Thermoelectric Transport in Polymer-Sorted Semiconducting Single-Walled Carbon Nanotube Networks.

Understanding the charge transport mechanisms in chirality-selected single-walled carbon nanotube (SWCNT) networks and the influence of network parameters is essential for further advances of their optoelectronic and thermoelectric applications. Here, we report on charge density and temperature-dependent field-effect mobility and on-chip field-effect-modulated Seebeck coefficient measurements of polymer-sorted monochiral small-diameter (6,5) (0.76 nm) and mixed large-diameter SWCNT (1.17-1.55 nm) networks (plasma torch nanotubes, RN) with different network densities and length distributions. All untreated networks display balanced ambipolar transport and electron-hole symmetric Seebeck coefficients. We show that charge and thermoelectric transport in SWCNT networks can be modeled by the Boltzmann transport formalism, incorporating transport in heterogeneous media and fluctuation-induced tunneling. Considering the diameter-dependent one-dimensional density of states (DoS) of the SWCNTs composing the network, we can simulate the charge density and temperature-dependent Seebeck coefficients. Our simulations suggest that scattering in these networks cannot be described as simple one-dimensional acoustic and optical phonon scattering as for single SWCNTs. Instead the relaxation time is inversely proportional to energy (τ ∝ (E - EC)s, s = -1, EC being the energy of the first van Hove singularity), presumably pointing toward the more two-dimensional character of scattering events and the necessity to include scattering at the SWCNT junctions. Finally, our observation of higher power factors in trap-free, 1,2,4,5-tetrakis(tetramethylguanidino)benzene-treated (6,5) networks than in the RN networks emphasizes the importance of chirality selection to tune the width of the DoS. To benefit from both higher intrinsic mobilities and a large thermally accessible DoS, we propose trap-free, narrow DoS distribution, large-diameter SWCNT networks for both electronic and thermoelectric applications.

Thermoelectric properties of single-wall carbon nanotube networks

The Seebeck coefficient S and power factor P of single-wall carbon nanotube (SWCNT) networks were systematically investigated as a function of chemical potential μ, based on theoretical simulations employing non-equilibrium Green’s function theory. The results focused on the gap regions of semiconducting (s-)SWCNTs. The thermoelectric properties of individual SWCNTs were classified into three groups: s-SWCNTs, metallic (m-)SWCNTs, and pseudo-metallic SWCNTs. The maximum values of P for individual s-SWCNTs was independent of SWCNT diameter. In parallel and serial networks of SWCNTs, S and P were found to be very sensitive to the amount of m-SWCNTs, as well as the SWCNT diameter distributions. A comparison with experimental results suggested that an SWCNT bundle can be modeled as a “rope” with an equivalent S calculated for parallel circuits. The presence of SWCNT junctions in the films substantially reduced the P value from those of the composing SWCNTs while S was almost unvaried.

A One-Step Chemical Strategy for the Formation of Carbon Nanotube Junctions in Aqueous Solution: Reaction of DNA-Wrapped Carbon Nanotubes with Diazonium Salts.

A single-step chemical strategy allows the formation of single-walled carbon nanotube (SWCNT) molecular junctions in aqueous solution. SWCNTs were first wrapped with DNA to be water soluble and solution processable. Diazonium salts, which have been shown to react spontaneously with carbon nanotubes in water at room temperature, were then employed to covalently link SWCNT segments. The DNA wrapping of the nanotubes acted as a protective layer that limits the functionalization predominantly to the nanotube terminal ends, therefore allowing the assembly of linear SWCNT junctions. Upon increasing the concentration of the linker, we observed first the formation of side-to-end junctions, and eventually the assembly, through side-to-side interactions, of SWCNTs into bundles. This approach demonstrates the possibility of tuning the formation of linear and branched carbon nanotube junctions that in turn is of importance for the sustainable fabrication of solution-processable CNT-based nanoscale systems and devices.

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.

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.

Fingerprinting seamless single-walled carbon nanotube junctions via the migration of encapsulated N2molecules from bottom to top: are arrays of VA-SWNTs continuous?

Structure control such as diameter changes along single-walled carbon nanotubes (SWNTs) can be achieved in arrays of vertically aligned (VA-) SWNTs by switching the feedstock during growth. The local nature of the macroscopic transition from one diameter to another is then questioned as one can either envisage seamless transitions or discontinuous individual SWNTs. Here, we demonstrate that encapsulated molecules can serve as markers to doubtlessly identify seamless interconnections in macroscopic samples. A migration of nitrogen molecules inside the continuous SWNTs is observed using bulk scale measurements on double-layered SWNT arrays synthesized from different carbon/nitrogen feedstocks. The existence of N2 molecules at the top of the SWNT arrays proves that there are continuous SWNTs throughout the double layered arrays with different diameters.

Phonon scattering at SWCNT–SWCNT junctions in branched carbon nanotube networks

In this research article, we analyze phonon scattering in branched single-walled carbon nanotube (SWCNT) networks with SWCNT–SWCNT T- and X- junctions using the wave packet method. Five phonon branches including the longitudinal acoustic, twisting, transverse acoustic, radial breathing, and flexural optical modes are selected to study energy reflection, ramification, and transmission through T- and X-junctions with (6,6) and (4,4) SWCNTs. The results of the simulations indicate that the diameter of SWCNTs affects phonon scattering at carbon nanotube junctions; T-junctions of (6,6) SWCNTs transmit energy more efficiently when compared to T-junctions with (4,4) SWCNTs. In addition, T-junctions of both (6,6) and (4,4) SWCNTs transmit vibrational energy more efficiently when compared to X-junctions in the same phonon frequency range—for example, in the case of the longitudinal acoustic branch, the average energy transmission at T-junctions for low-frequency phonons (lower than 6 THz) was found to be 1.8–2.4 times higher [for the case of (6.6) and (4,4) SWCNTs, respectively] when compared to the X-junctions. It is also observed that energy transmission at the T-junctions shows a dependency on the phonon group velocity with the higher group velocity phonons showing higher energy transmission; however, for the case of the X-junctions, there is little or no correlation observed between the group velocity and energy transmission indicating a complete energy redistribution of the incoming phonons at the junction. Moreover, for the SWCNT–SWCNT branched networks, the energy ramification at the T-junctions was found to be very similar to that at the X-junctions for both (6,6) and (4,4) SWCNTs indicating transverse thermal transport at the X-junctions to be as efficient as the T-junctions.

Photochemical generation of bis-hexahapto chromium interconnects between the graphene surfaces of single-walled carbon nanotubes

The linkage of single-walled carbon nanotube junctions by the photochemistry of organometallic chromium reagents produces dramatic increases in network conductivity.

In search of molecular scale devices: Theoretical study of linearly fused straight single-walled carbon nanotube junctions based on the pentagon/heptagon pair defects

The structural and electronic properties of topological defect of straight intramolecular junctions between two zigzag single-walled carbon nanotubes (SWCNT-IMJ) were studied theoretically. The interfacial junctions can be constructed by fusing two zigzag tubes that differ in helicities and diameters. In the present study, one segment is kept constant by using the (5,0) nanotube, while varying another segment from (6,0) to (10,0). Practically, these (5,0)//(m,0) structures are composed of one or more pentagon/heptagon pairs as “defect” functioning to fuse other perfect hexagonal lattices. It was found that, at the interfacial junctions the 5/7 pair defects induce modified rehybridization of the carbon–carbon networks, which directly affect the geometric and electronic properties. Consequently, the electronic structures are dependent on the variation of diameter and length of carbon nanotubes. Analyses of the electronic structures reveal that the HOMO and LUMO levels exhibit the even–odd “quantum size” oscillation versus varying nanotube chiralities, even with increasing length of nanotubes. The energy gap that indicates metallic or semi-conducting behavior depends on correlation between the tubes indices and tubule length. The NBO analysis has revealed that topological pair defects variation could be an important role for altering the electronic characteristics on these intramolecular junctions. By screening all structures based on their electronic structure diagrams and orbital visualization, we have found that the straight (5,0)//(7,0) junction should be suitable as a molecular rectifying diode. However, other junction structures can also be useful with appropriate tuning of the bias voltage. The outcome of this study would be of help as a knowledge base in the field of carbon nanomaterials molecular electronics.

Thermal transport in 3D pillared SWCNT–graphene nanostructures

Abstract

Thickness effects on the quantum conductance of single wall carbon nanotube junctions

This research aims to investigate the influence of the tube thickness in the zigzag-semiconductor carbon nanotube junction on the quantum conductance using the tight binding model and the Green's function technique. The conductance of (12,0)/(6,6) carbon nanotube junction decreases with the thickness of the tube. However, the conductance behavior of (10,0)/(5,5) depends on the thickness of each part. These junctions could be used as the building blocks of nanoscale diodes.

Rectifying behavior in nitrogen-doped zigzag single-walled carbon nanotube junctions

Using first-principles density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of (8,0), (9,0) and (13,0) zigzag single-walled carbon nanotube junctions with one undoped and one nitrogen-doped zigzag carbon nanotube electrode. Our results show that the transport properties are strongly dependent on the magnitude of energy gap of carbon nanotube. Large rectifying behavior can be obtained in the junction with large energy gap. The observed rectifying behavior are explained in terms of the evolution of the transmission spectra and energy band structures with applied bias voltage combined with molecular projected self-consistent Hamiltonian eigenstates analysis.

Thin-Film Transistors with Length-Sorted DNA-Wrapped Single-Wall Carbon Nanotubes

The present experimental study provides a detailed characterization of the single-wall carbon nanotube (SWNT) length effect on the device characteristics of DNA-wrapped SWNT-network thin-film transistors (TFTs). DNA-assisted dispersion and length separation by high-performance liquid chromatography are used to prepare the SWNTs with average lengths of 400 and 200 nm. The TFTs of both SWNTs exhibit high on/off current ratios of 104−106 and mobilities of 0.4−1.3 cm2/(V s) by optimizing the film density of SWNTs. The optimized density of 200 nm SWNTs is found to be higher than that of 400 nm SWNTs as expected by the percolation theory of two-dimensional random networks, where shorter length SWNTs can maintain high on current despite increasing in the number of SWNT junctions. The present result is an important indicator for realizing high-performance TFTs constructed from SWNTs having desired length.

Influence ofO2andN2on the conductivity of carbon nanotube networks

We have performed experiments on single-wall carbon nanotube (SWNT) networks and compared with density-functional theory (DFT) calculations to identify the microscopic origin of the observed sensitivity of the network conductivity to physisorbed ${\text{O}}_{2}$ and ${\text{N}}_{2}$. Previous DFT calculations of the transmission function for isolated pristine SWNTs have found physisorbed molecules have little influence on their conductivity. However, by calculating the four-terminal transmission function of crossed SWNT junctions, we show that physisorbed ${\text{O}}_{2}$ and ${\text{N}}_{2}$ do affect the junction's conductance. This may be understood as an increase in tunneling probability due to hopping via molecular orbitals. We find the effect is substantially larger for ${\text{O}}_{2}$ than for ${\text{N}}_{2}$, and for semiconducting rather than metallic SWNTs junctions, in agreement with experiment.

Ab initio study on the structural, energetic and electronic features of the asymmetric armchair SWCNT junctions

The structural, energetic and electronic features of asymmetric armchair single-walled carbon nanotube (SWCNT) junctions have been studied by ab initio calculations at the B3LYP/6-31G ∗//HF/3-21G ∗ levels. The junctions are composed of two SWCNTs with different radius, which are connected by a set of 5-membered and 7-membered carbon rings. The results show that the metallic–metallic junction is more energetically favorable if the junction is formed with a hexagon inserted between the pentagon–heptagon (5/7) pair defects in the armchair nanotube. The shift of the spatial distribution of HOMO and LUMO shows that the asymmetric electronic structure of the junction could be used as a molecular rectifier.

Electronic structure of straight semiconductor-semiconductor carbon nanotube junctions

We calculate the scanning tunneling microscopy images and the scanning tunneling spectroscopy of straight semiconductor-semiconductor single-wall carbon nanotube junctions with two different diameters. Two kinds of localized states associated with topological defects, one donorlike and the other acceptorlike, arise in the energy gap of the junctions, whose levels strongly depend on the number of topological defects and their spatial arrangement. We find that defects on one side of the tube surface influence the electronic distribution on the other (back) side sufficiently strongly that they may be detected by the scanning of the opposite side as well.

Structural and energetic features of single-walled carbon nanotube junctions: a theoretical ab initio study

Abstract Theoretical investigations on single-walled carbon nanotubes (SWCNT) have been carried out using ab initio calculations at the HF/4-31G and B3LYP/3-21G//HF/3-21G levels of theory. Several junctions between different size nanotubes have been constructed with the insertion of pentagon–heptagon pairs into the perfect hexagonal lattice. The structural and energetic characteristics of the junctions are reported.

From straight carbon nanotubes to Y-shaped junctions and rings

We explore quantum interference effects in the electronic transport properties of single-walled carbon nanotube junctions. Remarkable changes in the electrical conductance are found by varying the length of one of the junction's arms. Owing to the relatively high electron mobility in carbon nanotubes, we show that a double-slit-like electron interferometer may be fabricated by joining two Y-shaped carbon nanotube junctions. These nanostructures have very interesting electrical characteristics that may be manipulated by altering geometrical aspects of the junctions.

Branched Carbon Nanotube Junctions Predicted by Computational Nanotechnology and Fabricated through Nanowelding

Examples of progress in fabricating branched multi-terminal multi-wall and single-wall carbon nanotube junctions as predicted by nanotechnology simulations, using template growth and nanowelding techniques, respectively, have been briefly reviewed in this report. It is argued that similar general progress in computational nanotechnology-driven fabrication of applications in other nanomaterials such as nanotubes, fullerenes, nanowires, quantum dots, DNA molecules, and nanoparticle-based systems and devices are also feasible. This is because, at nanometer length scale, system sizes have shrunk sufficiently small such that it is feasible to simulate the structural, stability, and physical and chemical characteristics with very high accuracy predictive simulations.

Ambipolar electrical transport in semiconducting single-wall carbon nanotubes.

Ambipolar electrical transport is reported in single-wall carbon nanotube (SWNT) field-effect transistors. In particular, the properties of SWNT junctions to TiC are discussed in detail. The carbide-nanotube junctions are abrupt and robust. In contrast to planar junctions, these contacts present low resistance for the injection of both p- and n-type carriers--the apparent barrier height of the junction is modified by the gate field. Thus SWNTs offer the novel possibility of ambipolar Ohmic contacts.