Carbon Nanotube Heterojunctions Research Articles

Interfacing carbon nanotubes of arbitrary chiralities into linear heterojunctions

Motivated by recent advances in synthesis and characterization of carbon nanotube (CNT) heterojunctions, we introduce a systematic approach for obtaining atomic geometries that connect two carbon nanotubes of different chiralities. Using our approach, it is straightforward to construct atomic interface geometries between two single-walled CNT's of arbitrary chiralities arranged at different orientations and angles. Our method generalizes existing approaches and is readily applicable to joining domains of graphene nanoribbons as well. As an example, we focus on linear heterojunctions, and we postulate the minimum number of simple topological defects required at the interface, and the preferred spatial arrangements, to obtain maximally linear heterojunctions given any two arbitrary chiralities. We also provide a physical picture of the defect structure of the resultant interface geometries using the results of classical force-field simulations.

A novel catalyst-free synthesis of vertically aligned silicon nanowire–carbon nanotube heterojunction arrays for high performance electron field emitters

A novel, catalyst-free strategy for the direct synthesis of vertically aligned silicon nanowire-carbon nanotube (SiNW-CNT) heterojunction arrays is presented. Such a heterojunction with the junction area in the nanoscale displays enhanced field emission characteristics at low turn-on field, with a nearly three times increase in the field enhancement factor.

Unusual deformation mechanisms in carbon nanotube heterojunctions (5,5)–(10,10) under tensile loading

AbstractThe Poisson's ratio effect of single wall carbon nanotube heterojunctions (HJ) of the type (5,5)–(10,10) is investigated using an hybrid finite element atomistic‐continuum approach to simulate the mechanical behaviour of nanostructures. The nanotube HJs provide a peculiar deformation pattern, with combined bending and axial stretching of carbon nanotubes (CNTs), and a broad agreement of their axial stiffness with spring series continuum mechanics and existing molecular dynamics (MD) simulations. A possible auxetic behaviour is observed for the pure HJ with no tubes when subjected to tensile loading.

Axial vibration of carbon nanotube heterojunctions using nonlocal elasticity

In the present study, axial vibration of carbon nanotube heterojunctions is studied using nonlocal rod theory. The nonlocal constitutive equations of Eringen are used in the formulations. The carbon nanotubes with different lengths, chirality and diameters are considered in the heterojunctions. Effect of nonlocality, length of the carbon nanotubes and lengths of each segment are investigated in detail for each considered problem. It is obtained that, by joining carbon nanotubes good vibrational properties are obtained by suitable selection of parameters.

Theoretical study of heat conduction in carbon nanotube hetero-junctions

Thermal conductivity of single-wall carbon nanotubes with linear, “Y” and “X” hetero-junctions is studied using nonequilibrium molecular dynamics method. The three types of junctions with almost equal number of point defects show a similar decrease in thermal conductivity compared to that of the pristine carbon nanotube. Thermal conductivity of the nanotubes with different number of point defects is also calculated for a qualitative comparison. The thermal conductivity decreases with the defect number increasing. When the defects form an interface, they can cause a larger decrease in the thermal conductivity of CNT than when they are distributed dispersedly. The number of defects contained in the nanojunction determines the thermal conductivity to a great extent more than the geometrical forms of the nanojunctions. Phonon spectra of these junctions are also analyzed to explore the mechanism of degradation in thermal conductivity of these junctions.

Al 3+-directed self-assembly and their electrochemistry properties of three-dimensional dendriform horseradish peroxidase/polyacrylamide/platinum/single-walled carbon nanotube composite film

A novel general methodology for protein immobilization and third-generation biosensor construction is demonstrated, which involves Al 3+-directed polyacrylamide (PAM) self-assembly into an ordered dendriform structure, easily immobilizing enzymes and nanoparticles. Platinum/single-walled carbon nanotube (Pt/SWCNT) heterojunction nanomaterials were for the first time fabricated via an EDTA-directed synthesis strategy. The Pt/SWCNTs were employed as a supporting matrix to explore a novel immobilization and biosensing platform of redox proteins through cooperating Al 3+-directed PAM self-assembly. Compared with the almost single-layer horseradish peroxidase (HRP)/PAM film electrode, multilayer HRP/PAM/Pt/SWCNT film electrode exhibited a pair of much stronger redox peaks at −0.22 V (vs. Ag/AgCl). Moreover, with advantages of the ordered multilayer HRP/PAM/Pt/SWCNT film, facilitated direct electron transfer of the metalloenzymes with an apparent heterogeneous electron transfer rate constant ( k s) of 14.94 ± 1.36 s −1 and smaller peak-to-peak separation (Δ E p) of about 37 mV was acquired on the PAM/Pt/SWCNT-based enzyme electrode. The PAM/Pt/SWCNT-based biosensor demonstrated significant electrocatalytic activity for the reduction of hydrogen peroxide with a small apparent Michaelis–Menten constant (87 μM), wide linear range (1–270 μM), very low detection limit (0.08 μM, S/ N = 3), and high sensitivity (372 mA cm −2 M −1). Together, these indicated that the Al 3+-directed HRP/PAM/Pt/SWCNT film was one of ideal candidate materials for direct electrochemistry of redox proteins and the construction of the related enzyme biosensors, and may find potential applications in biomedical, food, and environmental analysis and detection.

Synthesis of titania/carbon nanotube heterojunction arrays for photoinactivation of E. coli in visible light irradiation

TiO 2/multi-wall carbon nanotube (MWNT) heterojunction arrays were synthesized and immobilized on Si(0 0 1) substrate as photocatalysts for inactivation of Escherichia coli bacteria. The vertically aligned MWNT arrays were grown on ∼5 nm Ni thin film deposited on the Si by using plasma enhanced chemical vapor deposition at 650 °C. Then, the MWNTs were coated by TiO 2 using dip-coating sol–gel method. Post annealing of the TiO 2/MWNTs at 400 °C resulted in crystallization of the TiO 2 coating and formation of Ti–C and Ti–O–C carbonaceous bonds at the heterojunction. The visible light-induced photoinactivation of the bacteria increased from MWNTs to TiO 2 to TiO 2/MWNTs, in which the bacteria could even slightly breed on the MWNTs. In addition, the TiO 2/MWNTs annealed at 400 °C showed a highly improved antibacterial activity than the TiO 2/MWNTs annealed at 100 °C. The excellent visible light-induced photocatalytic efficiency of the TiO 2/MWNTs/Si film annealed at 400 °C was attributed to formation of the carbonaceous bonds at the heterojunction, in contrast to the 100 °C annealed TiO 2/MWNTs/Si sample which had no such effective bonds.

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions

Carbon nanotube heterojunctions (HJs), which seamlessly connect nanotubes of different chiral structure using a small number of atomic-scale defects, represent the ultimate scaling of electronic interfaces. Here we report the first electrical transport measurements on a HJ formed between semiconducting and metallic nanotubes of known chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD) with approximately 60 meV charging energy and approximately 75 meV level spacing. A detailed atomistic and electronic model of the HJ enables the identification of specific defect arrangements that lead to the QD behavior consistent with the experiment.

TiO 2–carbon nanotube heterojunction arrays with a controllable thickness of TiO 2 layer and their first application in photocatalysis

TiO 2–carbon nanotube (CNT) heterojunction arrays on Ti substrate were fabricated by a two-step thermal chemical vapor deposition (CVD) method. CNT arrays were first grown on Ti substrate vertically, and then a TiO 2 layer, whose thickness could be controlled by varying the deposition time, was deposited on CNTs. Measured by electrochemical impedance spectroscopy (EIS), the thickness of the TiO 2 layer could affect the photoresponse ability significantly. About 100 nm thickness of the TiO 2 layer proved to be best for efficient charge separation among the tested samples. The optimized TiO 2–CNT heterojunction arrays displayed apparently higher photoresponse capability than that of TiO 2 nanotube arrays which was confirmed by surface photovoltage (SPV) technique based on Kelvin probe and EIS. In the photocatalytic experiments, the kinetic constants of phenol degradation with TiO 2–CNT heterojunctions and TiO 2 nanotubes were 0.75 h −1 ( R 2 = 0.983) and 0.39 h −1 ( R 2 = 0.995), respectively. At the same time, 53.7% of total organic carbon (TOC) was removed with TiO 2–CNT heterojunctions, while the removal of TOC was only 16.7% with TiO 2 nanotubes. These results demonstrate the super capability of the TiO 2–CNT heterojunction arrays in photocatalysis with comparison to TiO 2-only nanomaterial.

Quantum transport properties of carbon nanotube with topologic defects

Using the density-functional theory (DFT) combining non-equilibrium Green's function (NEGF) technique, we have studied the electronic transport properties of (7,0)-(8,0), (8,0)-(9,0) and (8,0)-(10,0) single-walled carbon nanotube heterojunctions which contain a pentagon-heptagon (5/7) pair of defect, a pentagon-heptagon (5/7) pair of defect and a pentagon-hexagon-heptagon (5/6/7) topological defect, respectively. Our calculations indicate that the 5/6/7 defect has greater effect on the electronic transport properties than that of the isolated 5/7 defect because there exist three weakly localized states in (8,0)-(10,0) heterojunction while two in (7,0)-(8,0) heterojunction. Furthermore, there appears to be negative differential resistance (NDR) in the semiconductor-metal (8,0)-(9,0) heterojunction in small bias region due to the suppression of conduction channel.

Fabrication of silicon carbide nanowires/carbon nanotubes heterojunction arrays byhigh-flux Si ion implantation

An array of silicon carbide nanowire (SiCNW)–carbon nanotube (CNT) heterojunctionswas fabricated by high-flux Si ion implantation into a multi-walled carbon nanotube(MWCNT) array with a metal vapor vacuum arc (MEVVA) ion source. Under Siirradiation, the top part of a CNT array was gradually transformed into an amorphousnanowire array with increasing Si dose while the bottom part still remained aCNT structure. X-ray photoelectron spectroscopy (XPS) analysis shows that theSiC compound was produced in the nanowire part even at the lower Si dose of5 × 1016 ions cm−2, and the SiC amount increased with increasing the Si dose. Therefore, the fabrication of aSiCNW–CNT heterojunction array with the MEVVA technique has been successfullydemonstrated. The corresponding formation mechanism of SiCNWs was proposed.

Effect of length and size of heterojunction on the transport properties of carbon-nanotube devices

By applying nonequilibrium Green’s functions in combination with the density-functional theory, we investigate the electronic transport properties of molecular junctions constructed by the mirror symmetrical straight carbon-nanotube heterojunctions. The results show that the length and size of heterojunction play an important role in the electronic transport properties of these systems. The negative differential resistance behavior can be observed in such devices with certain length and size of heterojunction. A mechanism is suggested for the negative differential resistance behavior.

Fabrication of “Tadpole”-like Magnetite/Multiwalled Carbon Nanotube Heterojunctions and Their Self-Assembly under External Magnetic Field

Novel "tadpole"-like Fe3O4/multiwalled carbon nanotube (MWCNT) heterojunctions were successfully synthesized by position-selectively attaching Fe3O4 sphere on the tips of MWCNTs through a straightforward and effective polyol-medium solvothermal method. Transmission and scanning electron microscopy (TEM and SEM) and X-ray diffraction (XRD) investigations show these Fe3O4 spheres are constructed with tiny nanocrystallites (approximately 5 nm in average diameter), which were preferentially aggregated in an oriented pattern on the open ends of the MWCNT template. Magnetic investigation indicates this novel Fe3O4/MWCNT hybrid presents superparamagnetic behavior. The size and corresponding magnetic performance of these magnetite/MWCNT hybrids can be adjustable to some extent for specific applications through altering the reaction parameters. Furthermore, these tadpolelike nanocomposites can orient and self-assemble into one-dimensional structure under external magnetic field, displaying great potential in precise manipulation and organization of carbon nanotube-based structures into integrated functional system.

Size dependence in one-dimensional nano-materials and one-dimensional heterojunctions

ABSTRACTOne-dimensional (1D) nano-materials have attracted a plenty of attention due to their novel structures and properties. Our group has carried out researches on synthesis, structure and property of 1D nano-materials, which are introduced in this paper. First, size effects on the crystal structure of Ag nanowires and on Young's modulus in [0001] oriented ZnO nanowires, respectively, have been revealed and modeled. The former is concerning the systemic energy of an individual Ag nanowire. The latter is caused by the surface stiffening effect arising from surface relaxation induced bond length contractions in the ZnO nanowires. Second, structures of 1D helical nano-materials including SWCNT (single-walled carbon nanotube), B-DNA and MWCNT (mutli-walled carbon nanotube) have been studied. It is shown that there is strong orientation dependence of diffraction intensities from SWCNT and B-DNA, which can even result in certain layer lines missing in their diffraction patterns. Also, it is demonstrated that high-resolution transmission electron microscope (TEM) images of sidewall regions of MWCNTs are not structural ones and from the interference of the {0002} and the {1011} diffraction waves. Third, arrays of four types of 1D heterojunctions have been synthesized. Among these 1D heterojunctions, the interfacial structures of the Ni/MWCNT/a-CNT(amorphous carbon nanotube) heterojunctions show that multiple outer walls in the MWCNTs can simultaneously participate in electrical transport. The electrical properties of the Ni/MWCNT/a-CNT and the Ag/a-CNT heterojunctions have been measured. As a result, it is found that the contacts between the Ag nanowires and the a-CNTs are ohmic ones with universal significance, and that each Ni/MWCNT/ a-CNT contains two diodes connected in series face-to-face. Moreover, most of the diodes have the most nearly ideal characteristics of Schottky contacts, indicated by quantitative analysis with the thermionic emission theory. Last, our group has developed a novel technique for rapidly producing large-area highly-oriented Si nanowire arrays on Si wafers by scratching the Si surface with metal nanoparticles near room temperature in HF solution. By this method, Si nanowires with desirable axial crystallographic directions, desirable doping characteristics and remarkable antireflection property can be readily obtained. The Si nanowire arrays have the potential applicability as an antireflective layer for photovoltaic devices and optical detectors. Furthermore, a combination of this method and the nanosphere lithography has been developed to fabricate large-scale Si and Si1−xGex quantum dot arrays with controllable height, diameter and center-to-center distance.

Electronic transport properties of single-wall carbon nanotube with pentagon-heptagon-pair defect

Using the density-functional theory in combination with nonequilibrium Green's function technique, we studied the densities of states and transport properties of the(7，0)—(8，0)and(8，0)—(9，0)single-wall carbon-nanotube heterojunctions containing one pentagon-heptagon-pair (5/7) topological defect. The result suggests that the effect of pentagon-heptagon-pairs (5/7) topological defects on carbon nanotubes is remarkable.In addition, the heterojunctions constituted by different types of carbon nanotubes also have different characteristics.

Synthesis of Gold Nanorod/Single-Wall Carbon Nanotube Heterojunctions Directly on Surfaces

This contribution describes the synthesis of gold nanorod (Au NR)/single-wall carbon nanotube (SWCNT) heterojunctions assembled directly on Si/SiOx substrates. SWCNTs are attached to amine-functionalized Si/SiOx substrates, and Au monolayer-protected clusters (MPCs) are adsorbed to the surface of SWCNTs through hydrophobic interactions. Seed-mediated reduction of HAuCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB) onto the Au MPCs leads to the growth of larger Au nanostructures directly on the SWCNTs. Au NRs account for 19% of the nanostructures, some of which are attached directly to the sidewall and some at the ends of the SWCNTs. Raman spectroscopic measurements of SWCNTs before and after growth of the Au nanostructures reveal that the presence of Au leads to an approximately 50-fold enhancement of the Raman scattering signal. Combining 1D nanostructures of different materials (Au and carbon in this example) is of fundamental interest and may find use in nanoelectronics, chemical sensing, electrochemical, and spectroscopy applications.

Coherent transport between two interface states in single-walled carbon nanotube quantumdots

The resonance tunnelling of states at two interfaces are investigated through the threemodels of two-terminated carbon nanotube heterojunctions: (5, 5)/(10, 0)/(5, 5), (9, 0)/(10,0)/(9, 0) and (5, 5)/(10, 0)/(9, 0). In (5, 5)/(10, 0)/(5, 5) CNQD, we observed the existenceof interface states in LDOS, and the resonant tunnelling peak at the position ofinterface states also emergent in quantum conductance. With the increase ofQDs size, the resonant tunnelling peak gradually disappears. Comparing theirquantum conductance for different sizes, when the size of the (5, 5)/(10, 0)/(5,5) model is larger than 23 layers, there is not a quantum conductance peak at−0.26 eV. Therefore, the largest size for resonant tunnelling occurring between two interfacestates in (5, 5)/(10, 0)/(5, 5) is about 23 layers (about 45 Å).

Synthesis and characterization of CdS/multiwalled carbon nanotube heterojunctions

CdS nanoparticles were successfully immobilized on multiwalled carbon nanotubes (MWNTs)via a simple solvothermal approach using dimethyl sulfoxide (DMSO) both as medium and sulfursource. The dependences of size and morphology of CdS nanoparticles on the reaction duration,Cd2+ concentration, and medium were investigated. IR, thermogravimetry–differential scanningcalorimetry (TG–DSC), and UV–vis spectrometry were performed to understand theinteraction between CdS nanoparticles and MWNTs.

Vertically aligned carbon nanotube heterojunctions

The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.

Band gap engineering of a carbon nanotube by hydrogen functionalization

Realization of carbon nanotube (CNT)-based transistors requires preexisting semiconducting CNTs, which are not selectively grown by the conventional synthesis approaches. While CNT heterojunction and cross-junction formed with different chiralities have demonstrated the required performance for CNT transistors, these relied on an junction formation accidentally obtained during sample preparation. We propose that hydrogen functionalization of CNTs can transform the electronic structure systematically from metallic (narrow-gap semiconducting) to semiconducting (large-gap semiconducting). We visualize this phenomenon by fabricating a heterojunction between the pristine and the functionalized CNTs that clearly shows both rectifying and gating effects even at room temperature.