Parallel Carbon Nanotubes Research Articles

1/f Noise in Carbon Nanotube Devices—On the Impact of Contacts and Device Geometry

We report on the 1/f noise in various ballistic carbon nanotube devices. A common means to characterize the quality of a transistor in terms of noise is to evaluate the ratio of the noise amplitude A and the sample resistance R. By contacting semiconducting tubes with different metal electrodes we are able to show that a small A/R value by itself is no indication of a suitable metal/tube combination for logic applications. We discuss how current in a nanotube transistor is determined by the injection of carriers at the electrode/nanotube interface, while at the same time excess noise is related to the number of carriers inside the nanotube channel. In addition, we demonstrate a substantial reduction in noise amplitude for a tube transistor with multiple carbon nanotubes in parallel

A Compact SPICE Model for Carbon-Nanotube Field-Effect Transistors Including Nonidealities and Its Application—Part I: Model of the Intrinsic Channel Region

This paper presents a circuit-compatible compact model for the intrinsic channel region of the MOSFET-like single-walled carbon-nanotube field-effect transistors (CNFETs). This model is valid for CNFET with a wide range of chiralities and diameters and for CNFET with either metallic or semiconducting carbon-nanotube (CNT) conducting channel. The modeled nonidealities include the quantum confinement effects on both circumferential and axial directions, the acoustical/optical phonon scattering in the channel region, and the screening effect by the parallel CNTs for CNFET with multiple CNTs. In order to be compatible with both large-(digital) and small-signal (analog) applications, a complete transcapacitance network is implemented to deliver the real-time dynamic response. This model is implemented with an HSPICE. Using this model, we project a 13 times CV/I improvement of the intrinsic CNFET with (19, 0) CNT over the bulk n-type MOSFET at the 32-nm node. The model described in this paper serves as a starting point toward the complete CNFET-device model incorporating the additional device/circuit-level non-idealities and multiple CNTs reported in the paper of Deng and Wong.

Transport through the intertube links between two parallel carbon nanotubes

Quantum transport through a junction between two metallic zigzag carbon nanotubes connected by intertube links has been studied within the tight-binding method and Landauer formula. It is found that the conductance oscillates with both the coupling strength and length. The close relation between conductance oscillations and the corresponding local densities of states is clearly shown. It indicates that the size and coupling strength effects of a nanojunction on the conductance will be important for the design of nanodevices in the future.

Formation and stability of parallel carbon nanotube junctions

Parallel single-walled carbon nanotube (SWCNT) junctions are investigated using tight-binding Monte Carlo simulations. SWCNTs of different size and type, including zigzag (3,0)–(8,0) and armchair (2,2)–(6,6) without defects, are investigated. The calculation results show that the stable parallel junctions can be formed by small size nanotubes, when the two tubes close to each other. The coalescence of tubes to larger ones and interlinking of two tubes have been obtained. The discussion of the effect of size and type of SWCNT, as well as the contact mode, on the formation of junctions shows that thin zigzag tubes with bond crossing hexagon interface are favorable for forming junctions.

Flow-Induced Planar Assembly of Parallel Carbon Nanotubes and Crossed Nanotube Junctions

We report the in-situ assembly of carbon nanotubes by chemical vapor deposition of hydrocarbon precursor (a solution of ferrocene dissolved in isopropyl alcohol). We utilized the vapor stream inside the reaction chamber to comb carbon nanotubes along the same direction and obtained two-dimensional (planar) assembly of nanotubes with tunable distributions. When the carbon source was flowing at a relatively higher rate of approximately 0.2 ml/min, most of nanotubes were driven along the vapor flow direction during their growth process and formed a thin freestanding mat featured with a parallel arrangement, whereas a lower flowing rate (approximately 0.05 ml/min) only resulted in random spider-web structures consisting of crossed nanotube junctions with a variety of configurations (e.g., "+", "Y", "T" shapes and twists). The measured direction-dependent electrical resistance of these two assemblies was in agreement with respective structures, which was anisotropic for parallel nanotubes and nearly isotropic for random networks. Such large-area planar carbon nanotube arrays with controlled orientation and various junction configurations will facilitate the design and fabrication of electronic and mechanical devices.

Theoretical study of crossed and parallel carbon nanotube junctions and three-dimensional grid structures

This work presents a first-principles study of parallel and crossed junctions of single-wall carbon nanotubes (SWNT). The crossed junctions are modeled by two-dimensional grids of zigzag SWNTs. The atomic and electronic structure, stability, and energetics of the junctions are studied for different magnitudes of contact forces pressing the tubes towards each other and hence inducing radial deformations. Under relatively weak contact forces the tubes are linked with intertube bonds which allow a significant conductance through the junction. These interlinking bonds survive even after the contact forces are released and whole structure is fully relaxed. Upon increasing contact force and radial deformation the tube surfaces are flattened but the interlinking bonds are broken to lead to a relatively wider intertube spacing. The intertube conductance through such a junction diminish because of finite potential barrier intervening between the tubes. The linkage of crossing tubes to form stable junctions is enhanced by a vacancy created at the contact. The three-dimensional grid structure formed by SWNTs is also investigated as a possible framework in device integration.

High-Current Nanotube Transistors

Planar field effect transistors (FET) consisting of a large number of parallel single-walled carbon nanotubes (SWCNT) have been fabricated that allow very high on-currents of the order of several milliamperes and on/off ratios exceeding 500. With these devices it is demonstrated, for the first time, that SWCNTs can be used as transistors to control macroscopic devices, e.g., light emitting diodes and electromotors. Those transistors were fabricated by a very simple process that is based on the catalytic chemical vapor deposition (CCVD) growth of SWCNTs at low temperatures, a single lithographic step to define the source and drain contacts, and a bias pulse to eliminate the metallic SWCNTs.

Binding energy of parallel carbon nanotubes

We study the interaction between two separate carbon nanotubes that bond together by examing their geometry in transmission electron microscopy (TEM). The TEM image, in conjunction with a continuum mechanics model interpretation, provides an estimate of 0.36 nN binding energy between the two parallel carbon nanotubes. This result agrees well with the calculated binding energy using an atomistic model.

Monte Carlo simulations of H 2 physisorption in finite-diameter carbon nanotube ropes

We report the results of classical, grand canonical Monte Carlo simulations of the physisorption of H 2 in finite-diameter `ropes' of parallel single-walled carbon nanotubes. The strong dependence of the gravimetric adsorption on the diameter of the rope is correlated with computed values of the specific surface area. The grooves on the external surfaces of the ropes are shown to provide a high binding-energy adsorption site, comparable in strength to those in the endohedral pores. Our results suggest that delamination of nanotube ropes should increase the gravimetric storage capacity.

Highly-ordered carbon nanotube arrays for electronics applications

Highly-ordered arrays of parallel carbon nanotubes were grown by pyrolysis of acetylene on cobalt within a hexagonal close-packed nanochannel alumina template at 650 °C. The nanotubes are characterized by a narrow size distribution, large scale periodicity, and high densities. Using this method ordered nanotubes with diameters from 10 nm to several hundred nm and lengths up to 100 μm can be produced. The high level of ordering and uniformity in these arrays is useful for applications in data storage, field emission displays and sensors, and offers the prospect of deriving computational functions from the collective behavior of symmetrically coupled nanotubes. The fabrication method used is compatible with standard lithographic processes and thus enables future integration of such periodic carbon nanotube arrays with silicon microelectronics.

Monte Carlo simulations of hydrogen adsorption in single-walled carbon nanotubes

Within the framework of a study on the properties of carbon nanotubes, a promising new material, we performed numerical simulation of hydrogen adsorption at room temperature in single-walled nanotubes. The structure of this material is favorable to the adsorption phenomenon because of the narrow size distribution of the nanotube diameters, which have dimensions on the order of the range of the carbon attractive interaction. We discuss the influence of the single-walled carbon nanotube diameters on the relative arrangement of carbon atoms and hydrogen molecules within an array of parallel single-walled carbon nanotubes. We also studied the influence on adsorption of the distance between the nearest-neighbor nanotubes.