Synthesis of carbon nanotubes by chemical vapor deposition over patterned catalyst arrays leads to nanotubes grown from specific sites on surfaces. The growth directions of the nanotubes can be controlled by van der Waals self-assembly forces and applied electric fields. The patterned growth approach is feasible with discrete catalytic nanoparticles and scalable on large wafers for massive arrays of novel nanowires. Controlled synthesis of nanotubes opens up exciting opportunities in nanoscience and nanotechnology, including electrical, mechanical, and electromechanical properties and devices, chemical functionalization, surface chemistry and photochemistry, molecular sensors, and interfacing with soft biological systems.

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Carbon Nanotubes: Synthesis, Integration, and Properties

This article reviews the electronic and transport properties of carbon nanotubes. The focus is mainly theoretical, but when appropriate the relation with experimental results is mentioned. While simple band-folding arguments will be invoked to rationalize how the metallic or semiconducting character of nanotubes is inferred from their topological structure, more sophisticated tight-binding and ab initio treatments will be introduced to discuss more subtle physical effects, such as those induced by curvature, tube-tube interactions, or topological defects. The same approach will be followed for transport properties. The fundamental aspects of conduction regimes and transport length scales will be presented using simple models of disorder, with the derivation of a few analytic results concerning specific situations of shortand long-range static perturbations. Further, the latest developments in semiempirical or ab initio simulations aimed at exploring the effect of realistic static scatterers chemical impurities, adsorbed molecules, etc. or inelastic electron-phonon interactions will be emphasized. Finally, specific issues, going beyond the noninteracting electron model, will be addressed, including excitonic effects in optical experiments, the Coulomb-blockade regime, and the Luttinger liquid, charge density waves, or superconducting transition.

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Electronic and transport properties of nanotubes

Single wall carbon nanotubes (SWCNTs) have been used as the active channels of field effect transistors (FET). The next development step involves the integration of CNTFETs to form logic gates; the basic units of computers. For this we need to have both p- and n-type CNTFETs. However, without special treatment, the obtained CNTFETs are always p-type:  the current carriers are holes and the devices are ON for negative gate bias. Here we show that n-type CNTFETs can be prepared not only by doping but also by a simple annealing of SWNT-based p-FETs in a vacuum. We use our ability to prepare both p- and n-type nanotube transistors to build the first nanotube-based logic gates:  voltage inverters. Using spatially resolved doping we implemented this logic function on a single nanotube bundle.

Carbon Nanotube Inter- and Intramolecular Logic Gates

We studied various gas molecules (NO2, O2, NH3, N2, CO2, CH4, H2O, H2, Ar) on single-walled carbon nanotubes (SWNTs) and bundles using first principles methods. The equilibrium position, adsorption energy, charge transfer, and electronic band structures are obtained for different kinds of SWNTs. Most molecules adsorb weakly on SWNTs and can be either charge donors or acceptors to the nanotubes. We find that the gas adsorption on the bundle interstitial and groove sites is stronger than that on individual nanotubes. The electronic properties of SWNTs are sensitive to the adsorption of certain gases such as NO2 and O2. Charge transfer and gas-induced charge fluctuation might significantly affect the transport properties of SWNTs. Our theoretical results are consistent with recent experiments.

/pdf/gas-molecule-adsorption-in-carbon-nanotubes-and-nanotube-blls8d2qck.pdf

Gas molecule adsorption in carbon nanotubes and nanotube bundles

Ultrathin films of single-walled carbon nanotubes (SWNTs) represent an attractive, emerging class of material, with properties that can approach the exceptional electrical, mechanical, and optical characteristics of individual SWNTs, in a format that, unlike isolated tubes, is readily suitable for scalable integration into devices. These features suggest the potential for realistic applications as conducting or semiconducting layers in diverse types of electronic, optoelectronic and sensor systems. This article reviews recent advances in assembly techniques for forming such films, modeling and experimental work that reveals their collective properties, and engineering aspects of implementation in sensors and in electronic devices and circuits with various levels of complexity. A concluding discussion provides some perspectives on possibilities for future work in fundamental and applied aspects.

http://www.ifc.dicp.ac.cn/library/cailiao/pdf1/11.pdf

Ultrathin Films of Single-Walled Carbon Nanotubes for Electronics and Sensors: A Review of Fundamental and Applied Aspects

Degassing of bundles of single-walled carbon nanotubes in vacuum at 500 K is found to drive the thermoelectricpower (TEP) strongly negative, indicating that the degassed metallic tubes in a bundle are n type. The magnitude of the negative TEP indicates that important asymmetry in the electronic carbon pi bands exists near the Fermi energy. Easily measurable increases in the TEP ( approximately 5-10 mV/K) and resistivity ( 2%-10%) are observed at 500 K upon exposure to N2 and He, suggesting that even gas collisions with the nanotube wall can contribute significantly to the transport properties.

Effects of gas adsorption and collisions on electrical transport in single-walled carbon nanotubes

We show that the long-range dipolar interactions in a crystalline cubic polar semiconducting nanowire give rise to an important splitting of the Raman-active transverse optic (TO) and longitudinal optic (LO) phonons at the center of the Brillouin zone The dipole sums that determine the two LO and two TO phonon frequencies in the nanowire are sensitive to the aspect ratio (L/D), where L and D are the length and diameter, respectively In the limit L/D →∞, we predict the phonon frequencies for several important polar semiconducting nanowires Our calculated results are also compared with Raman scattering data obtained on crystalline GaP and GaAs semiconducting wires

Optical phonons in polar semiconductor nanowires

Thermoelectric properties of single wall carbon nanotubes (SWNT) are quite sensitive to gases in contact with the tube walls. This effect makes possible a thermoelectric chemical sensor. Large, reversible swings in thermoelectric power (S), sometimes even involving sign changes in S, have been observed. Even contact of the SWNTs with He and N 2 and H 2 result in easily detectable and reversible changes in S. Smaller, polar alcohol molecules stimulate a large thermoelectric response, although H 2 O has no effect. For adsorption of six membered ring molecules C 6 H n in SWNTs, the large thermoelectric response observed for Benzene (n = 6) is seen to decrease as the π electrons in the molecule are removed, and the coupling between the molecules and the SWNT is thereby reduced. These effects are discussed in terms of the diffusion thermopower for a rope, and a new scattering channel associated with adsorbed molecules.

Thermoelectric chemical sensor based on single wall carbon nanotubes

In situ resistivity (p) and thermoelectric power (S) have been used to study the nature of the adsorption of hydrogen in bundles of single-walled carbon nanotubes for H 2 pressure P ≤ 1 atm and temperatures 77 K <T<500 K.Isothermal plots of S vs Δρ/ρ 0 are found to exhibit linear behavior as a function of gas coverage, consistent with a physisorption process. Studies of S, p at T = 500 K as a function of pressure exhibit a plateau at a pressure P∼40 Torr, the same pressure where the H wt. % measurements suggest the highest binding energy sites are being saturated. The effects of H 2 exposure at 500 K on the thermoelectric transport properties are fully reversible.

Thermoelectric study of hydrogen storage in carbon nanotubes

In situ resistivity and thermoelectric power (S) have been used to study the nature of the adsorption of hydrogen in bundles of single-walled carbon nanotubes for H2 pressure P <1 atm and temperatures 77 K<T<500 K. Isothermal plots of S vs. Δρ/ρ0 are found to exhibit linear behavior as a function of gas coverage, consistent with a physisorption process. Studies of S, ρ at T = 500 K as a function of pressure exhibit a plateau at a pressure P~40 Torr, the same pressure where the H % measurements suggest the highest binding energy sites are being saturated. The effects of H2 exposure at 500 K on the thermoelectric transport properties are fully reversible.

http://eri.louisville.edu/papers_pres/Sumanasekera%20PRB%202002.pdf

C. K. W. Adu

Papers

Effects of gas adsorption and collisions on electrical transport in single-walled carbon nanotubes

Optical phonons in polar semiconductor nanowires

Thermoelectric chemical sensor based on single wall carbon nanotubes

Thermoelectric study of hydrogen storage in carbon nanotubes

Thermoelectric Study of Hydrogen Storage in Carbon Nanotubes