Transport phenomena and conduction mechanism of single-walled carbon nanotubes (SWNTs) at Y- and crossed-junctions

This letter illustrates the transport phenomena associated with single-walled carbon nanotube (SWNT) junctions of Y- and cross-configurations. Localized gating effect exhibited by Y- and crossed-junctions suggests the resemblance of their electrical characteristics with ambipolar and unipolar p-type FETs, respectively. Temperature dependence of the I-V characteristics reveals that the conduction mechanism in the said SWNT junctions is governed by thermionic emission at temperatures above 100 K and by tunneling at T < 100 K. In-depth analysis of current transport through the crossed- and Y-junction SWNTs is significant in view of their predominant influence on the electrical performance of carbon nanotube networks (CNT-mat).     

Noise characteristics of single-walled carbon nanotube network transistors

The noise characteristics of randomly networked single-walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition (PECVD) are studied with field effect transistors (FETs). Due to the geometrical complexity of nanotube networks in the channel area and the large number of tube-tube/tube-metal junctions, the inverse frequency, 1/f, dependence of the noise shows a similar level to that of a single single-walled carbon nanotube transistor. Detailed analysis is performed with the parameters of number of mobile carriers and mobility in the different environment. This shows that the change in the number of mobile carriers resulting in the mobility change due to adsorption and desorption of gas molecules (mostly oxygen molecules) to the tube surface is a key factor in the 1/f noise level for carbon nanotube network transistors.     

Post annealing effect on the electrical properties of top-gate SWNT network transistors

High performance top-gate single walled carbon nanotube network transistors are fabricated with aluminum oxide (Al2O3) layer as a gate dielectric by atomic layer deposition. It exhibits large on/off ratio (>10(4)) due to selective growth of semiconducting tubes by the plasma enhanced chemical vapor deposition. I-V characteristics show p-type or n-type depending on the deposition temperature. We investigate the type dependent characteristics for the carrier polarities with the post annealing effect on the top-gate SWNT network transistors. The dramatic change in the polarity of the top-gate SWNT network transistors, from n-type to p-type due to conversion of I-V characteristics is observed by post-annealing at 350 degrees C for 30 minutes under vacuum. Our observation suggests that competition between electron transfer from the Al2O3 layers to the SWNT surface and electron capture by oxygen molecules adsorbed on the tube walls seems to be the key point for the V(th) change as a function of Al2O3 deposition temperature.     

Shrinking a carbon nanotube

We report a method to controllably alter the diameter of an individual carbon nanotube. The combination of defect formation via electron irradiation and simultaneous resistive heating and electromigration in vacuum causes the nanotube to continuously transform into a high-quality nanotube of successively smaller diameter, as observed by transmission electron microscopy. The process can be halted at any diameter. Electronic transport measurements performed in situ reveal a striking dependence of conductance on nanotube geometry. As the diameter of the nanotube is reduced to near zero into the carbon chain regime, we observe negative differential resistance.     

Electrical transport properties of peptide nanotubes coated with gold nanoparticles via peptide-induced biomineralization

We present temperature dependent electrical transport measurements of peptide nanotube devices coated with monodisperse arrays of gold nanoparticles (AuNP). As the temperature is lowered, the current-voltage (I-V) characteristics become increasingly nonlinear and below 20 K conduction only occurs above a threshold voltage V(T). The current follows the scaling behavior I ∝ [(V ? V(T))/V(T)]α for V > V(T) with α ～ 2.5 signifying two-dimensional (2D) charge transport. The temperature dependence of the resistance shows thermally activated behavior with an activation energy of 18.2 meV corresponding to the sequential tunneling of charges through 6 nm monodispersed AuNP arrays grown on a peptide surface.     

The geometrical effect of single walled carbon nanotube network transistors on low frequency noise characteristics

The noise characteristics of randomly networked single walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition with field effect transistor. Geometrical complexity due to the large number of tube-tube junctions in the nanotube network is expected to be one of the key factors for the noise power of 1/f dependence. We investigated low frequency noise as a function of channel length (2-10 microm) and found that increased with longer channel length. Percolational behaviors of nanotube network that differs from ordinary semiconducting and metallic materials can be characterized by a geometrical picture with electrical homo- and hetero-junctions. Fixed nanotube density provides a test conditions to evaluate the contributions of junctions as a noise center. Hooge's empirical law is applied to investigate the low frequency noise characteristics of single walled carbon nanotube random network transistors. The noise power shows the dependence of the transistor channel length. It is understood that nanotube/nanotube junctions act as a noise center. However, the differences induced by channel length in the noise power are observed as not so significant. We conclude that tolerance of low frequency noise is important property for SWNT networks as an electronic device application.     

Electronic properties of a single-walled carbon nanotube/150mer-porphyrin system measured by point-contact current imaging atomic force microscopy

The electronic properties of a single-walled carbon nanotube/150mer of porphyrin polymer wire system were investigated. Current-voltage (I-V) curves were measured simultaneously along with topographic observations using point-contact current imaging atomic force microscopy. Symmetric I-V curves were obtained at bare single-walled carbon nanotubes but characteristic asymmetrical rectifying behavior was found at the single-walled carbon nanotube/150mer-porphyrin junctions. This finding is of key importance for the development of new nanoscale molecular electronic devices.     

Scalable fabrication of carbon nanotube/polymer nanocomposite membranes for high flux gas transport

We present a simple, fast, and practical route to vertically align carbon nanotubes on a porous support using a combination of self-assembly and filtration methods. The advantage of this approach is that it can be easily scaled up to large surface areas, allowing the fabrication of membranes for practical gas separation applications. The gas transport properties of thus constructed nanotube/polymer nanocomposite membranes are analogous to those of carbon nanotube membranes grown by chemical vapor deposition. This paper shows the first data for transport of gas mixtures through carbon nanotube membranes. The permeation of gas mixtures through the membranes exhibits different properties than those observed using single-gas experiments, confirming that non-Knudsen transport occurs.     

Multiscale modeling of nanowire-based Schottky-barrier field-effect transistors for sensor applications

We present a theoretical framework for the calculation of charge transport through nanowire-based Schottky-barrier field-effect transistors that is conceptually simple but still captures the relevant physical mechanisms of the transport process. Our approach combines two approaches on different length scales: (1) the finite element method is used to model realistic device geometries and to calculate the electrostatic potential across the Schottky barrier by solving the Poisson equation, and (2) the Landauer-Büttiker approach combined with the method of non-equilibrium Green's functions is employed to calculate the charge transport through the device. Our model correctly reproduces typical I-V characteristics of field-effect transistors, and the dependence of the saturated drain current on the gate field and the device geometry are in good agreement with experiments. Our approach is suitable for one-dimensional Schottky-barrier field-effect transistors of arbitrary device geometry and it is intended to be a simulation platform for the development of nanowire-based sensors.     

The influence of carbon nanotube functionalization route on the efficiency of dispersion in polypropylene by twin-screw extrusion

The joint effect of chemical functionalization and polymer melt blending conditions on carbon nanotube dispersion in polypropylene, as well as its influence on the electrical and mechanical properties of the resulting composites were investigated. Melt blending was performed using a prototype twin screw extruder enabling sampling along the barrel. The carbon nanotube dispersion was assessed by optical and scanning electron microscopy. The functionalization reaction was tailored for compatibility with the polymer, and characterized by X-ray photoelectron spectroscopy. In particular, nanotubes covalently bonded to polypropylene showed distinctive dispersion ability, while the carbon nanotube dispersion remained stable even after re-melting. However, the polypropylene-functionalized nanotubes produced composites with higher electrical resistivity, possibly due to the insulating effect of the polymer bonded to the nanotubes surface.     

单壁碳纳米管量子点的传导电流与电流噪声谱

近来，碳纳米管尤其是单壁碳纳米管越来越引起人们的注意，并已经广泛应用于实践中。本文采用开放量子系统的研究方法，利用量子主方程给出了与扫描隧道显微镜耦合的单壁碳纳米管量子点的输运电流与电流噪声谱的一般性计算方法，并研究了强弛豫条件下输运电流的性质。     

Production of a high dispersion of silver nanoparticles on surface-functionalized multi-walled carbon nanotubes using an electrostatic technique

Multi-walled carbon nanotube/silver nanoparticles hybrid materials were easily synthesized by a mixing method of acid-oxidized nanotubes and the colloidal dispersion of silver nanoparticles. The silver colloidal dispersion was pre-synthesized by a chemical reduction of silver nitrate by dimethyl sulfoxide in the presence of trisodium citrate dihydrate as capping agents. In the mixing method, approx 5.0 nm diameters of silver nanoparticles with face-centered cubic crystal structures are highly dispersed on the acid-oxidized nanotubes due to the surface reactivity resulting from the enhancement of oxygen-containing functional groups. The results emphasize that the anchoring effect of the functionalized nanotube surface on silver nanoparticles originates from electrostatic metal-support interactions.     

Manipulation and Imaging of Individual Single‐Walled Carbon Nanotubes with an Atomic Force Microscope

The tip of an atomic force microscope is used to create carbon nanotube junctions by changing the position and shape of individual single‐walled carbon nanotubes on a SiO₂ surface. With this manipulation technique, we are able to bend, buckle, cross (see Figure), and break nanotubes, and to unravel a nanotube “crop circle” into a single tube. Tapping‐mode atomic force microscopy measurements of the height of a carbon nanotube on the surface always yield values smaller than the nanotube diameter. Variation of the scan parameters shows that this is due to a tapping deformation by the tip. The tapping deformation of manipulated nanotube crossings and buckles is discussed as well.     

Effect of the geometry of the anodized titania nanotube array on the performance of dye-sensitized solar cells

Highly ordered TiO2 nanotube arrays are superior photoanodes for dye-sensitized solar cells (DSSCs) due to reduced intertube connections, vectorial electron transport, suppressed electron recombination, and enhanced light scattering. Performance of the cells is greatly affected by tube geometry, such as wall thickness, length, inner diameter and intertube spacing. In this paper, effect of geometry on the photovoltaic characteristics of DSSCs is reviewed. The nanotube wall has to be thick enough for a space charge layer to form for faster electron transportation and reduced recombination. When the tube wall is too thin to support the space charge layer, electron transport in the nanotubes will be hindered and reduced to that similar in a typical nanoparticle photoanode, and recombination will easily take place. Length of the nanotubes also plays a role: longer tube length is desired because of more dye loading, however, tube length longer than the electron diffusion length results in low collecting efficiency, which in turn, results in low short-circuit current density and thus low overall conversion efficiency. The tube inner diameter (pore size) affects the conversion efficiency through effective surface area, i.e., larger pore size gives rise to smaller surface area for dye adsorption, which results in low short-circuit current density under the same light soaking. Another issue that may seriously affect the conversion efficiency is whether each of the tube stands alone (free from connecting to the neighboring tubes) to facilitate infiltration of dye and fully use the outer surface area.     

A Field Effect Transistor Fabricated with Metallic Single-Walled Carbon Nanotubes

We report novel transport properties of the individual single-walled carbon nanotube (SWNT) field effect transistors (FETs) decorated with the protein (streptavidin)-coated nanoparticles. Upon adsorption of the protein-coated nanoparticles at the metal-nanotube contact, the metallic SWNT devices abruptly exhibit a p-type semiconducting behavior. In the case of semiconducting SWNT devices, the adsorptions of protein-coated nanoparticles make the gating more effective, resulting in a far suppressed off-state leakage current as well as an enhanced on-state p-channel current. Through the ab initio electronic structure calculations, it is suggested that such an apparent metal-semiconductor transition may be due to the intervening charged species in the contact area, originated from the surface of the proteins. Noting the separation of the semiconducting nanotubes from metallic ones would be a formidable task; we suggest that the device concept here could be another breakthrough for the nanotube-based electronic devices, in which the nanotubes are not necessarily semi-conducting.     

纳米碳管电化学储氢的研究进展

纳米碳管的储氢是近年来纳米碳管领域研究的一个热点。纳米碳管储氢研究有两种方法，一种是气相法，另一种是电化学法。本文对纳米碳管电化学储氢的基本原理、纳米碳管电化学储氢的理论计算以及氢与纳米碳管的相互作用机制，特别是目前单壁和多壁纳米碳管电化学储氢的实验研究进展进行了综述，展望了利用其电化学储氢特性作为高性能电池的可能性。     

Electric field-assisted deposition of nanowires on carbon nanotubes for nanoelectronics and sensor applications

Manipulation and control of matter at the nanoscale and atomic scale levels are crucial for the success of nanoscale sensors and actuators. The ability to control and synthesize multilayer structures using carbon nanotubes that will enable the building of electronic devices within a nanotube is still in its infancy. In this paper, we present results on selective electric field-assisted deposition of metals on carbon nanotubes realizing metallic nanowire structures. Silver and platinum nanowires have been fabricated using this approach for their applications in chemical sensing as catalytic materials to sniff toxic agents and in the area of biomedical nanotechnology for construction of artificial muscles. Electric field-assisted deposition allows the deposition of metals with a high degree of selectivity on carbon nanotubes by manipulating the charges on the surface of the nanotubes and forming electrostatic double-layer supercapacitors. Deposition of metals primarily occurred due to electrochemical reduction, electrophoresis, and electro-osmosis inside the walls of the nanotube. SEM and TEM investigations revealed silver and platinum nanowires between 10 nm and 100 nm in diameter. The present technique is versatile and enables the fabrication of a host of different types of metallic and semiconducting nanowires using carbon nanotube templates for nanoelectronics and a myriad of sensor applications.     

Modelling the nucleation and chirality selection of carbon nanotubes

The selection of chiralities of single-walled carbon nanotubes is one of the key problems of nanotube science. We suggest that the chirality-selective growth of SWNTs could be achieved using chemical vapour deposition (CVD) by controlling the type of caps that form during the nucleation stage. As the catalyst can be solid during CVD, the formation of particular caps may be favoured by an epitaxial relationship to the catalyst surface. The corresponding tubes would then grow preferentially. We show by ab-initio calculations that the formation energy of some lattice-matched caps and tubes are 1-2 eV lower than the non lattice-matched structures.     

Probing electrostatic potentials in solution with carbon nanotube transistors

We have used single-walled carbon nanotube transistors to measure changes in the chemical potential of a solution due to redox-active transition-metal complexes. The interaction of the molecules with a gold electrolyte-gate wire changes the electrostatic potential sensed by the nanotube, which in turn shifts the gate-voltage dependence of the nanotube conductance. As predicted by the Nernst equation, this shift depends logarithmically on the ratio of oxidized to reduced molecules.     

Silicon adsorption in defective carbon nanotubes: a first principles study

The electronic and structural properties of an (8, 0) single-walled carbon nanotube (SWNT) with a single vacancy and interacting with a Si atom are studied using first principles calculations based on the density-functional theory. Initially, the Si atom is positioned in the site above the vacancy, with its position fixed until the nanotube geometry is fully relaxed. After that, the Si atom approaches the tube and it is shown that one C atom is displaced outwards forming a bump. The final configuration, as well as each step of the process, is studied in detail and the resulting band structures and the total charge densities are systematically analysed.