Carbon nanotube based gas sensor

We carry out first-principles density-functional calculations to investigate the electronic structure of the gold-carbon nanotube contact. It is found that a pressure applied on the gold-nanotube contact shifts the Fermi level from the valence edge to the conduction edge of the carbon nanotube. This can explain the n-type transport behavior frequently observed in the nanotube field-effect transistor using the gold as electrodes. An atomistic model is proposed for a possible origin of the pressure when the nanotube is embedded in the gold electrode.

Tin oxide nanoparticles are synthesized using solution combustion technique and tin oxide-carbon composite thick films are fabricated with amorphous carbon as well as carbon nanotubes (CNTs). The x-ray diffraction, Raman spectroscopy and porosity measurements show that the as-synthesized nanoparticles are having rutile phase with average crystallite size ∼7 nm and ∼95 m 2 /g surface area. The difference between morphologies of the carbon doped and CNT doped SnO 2 thick films, are characterized using scanning electron microscopy and transmission electron microscopy. The adsorption-desorption kinetics and transient response curves are analyzed using Langmuir isotherm curve fittings and modeled using power law of semiconductor gas sensors.

This work reports the development and study of a resistive gas sensor for benzene detection, based on multi-walled carbon nanotubes (MWCNTs). The MWCNTs were covered with a nanoporous SiO2 layer whose function is to concentrate the pollutant in order to enhance the sensor performances. In this paper, we will describe the preparation of the sensor and highlight the beneficial effects of both the preconcentration layer and the operating temperature. MWCNT/SiO2-based sensors operating at 125°C are able to detect 50 ppb of benzene in air. Further developments are in progress in order to improve this detection limit.

We report on a study of molecular modifications of the electronic characteristics of single-walled carbon nanotube ͑SWNT͒ field-effect transistors ͑FETs͒ through insertion of different organic self-assembled monolayers ͑SAMs͒ between the SWNT and an electrode. The changes induced by the molecular interface were elucidated using a device structure created by directed assembly of a single SWNT over three prepatterned electrodes, one of which had a SAM deposited via dip-pen nanolithography. The resulting direct comparison of two FETs sharing the same SWNT revealed pronounced modification of the transfer characteristics, on/off ratio, and threshold voltages due to the SWNT/molecule/metal junction. The effects are attributed primarily to the alteration of the electronic bands in the Au electrode and the SWNT, and the resulting changes in the effective Schottky barrier height/thickness, by the ordered and well-aligned molecular SAM.

Clear positive temperature coefficient resistor ͑PTCR͒ dc behavior has been shown in Pr-doped ZnO ͑0001͉͒͑0001͒ bicrystals by electrical characterization over an unprecedentedly wide temperature range between 40 and 1070 K. With subtraction of the PTCR dc, the admittance can be described by a deep trap level at 0.26 eV but no clue to the origin of the PTCR behavior is provided. Capacitance-voltage characteristics revealed a maximum in the Schottky barrier heights consistent with the PTCR behavior. The PTCR behavior in Pr-doped ZnO c-axis oriented bicrystals is thus phenomenologically analogous to that of the ferroelectric BaTiO 3 .

A model is proposed for the previously reported lower Schottky barrier ⌽ Bh for hole transport in air than in vacuum at a junction between the metallic electrode and semiconducting carbon nanotube ͑CNT͒. We consider the electrostatics in a transition region between the electrode and CNT in the presence or absence of oxygen molecules ͑air or vacuum͒, where an appreciable potential drop occurs. The role of oxygen molecules is to increase this potential drop with a negative oxygen charge, leading to lower ⌽ Bh in air. The Schottky barrier modulation is large when a CNT depletion mode is involved, while the modulation is negligible when a CNT accumulation mode is involved. The mechanism prevails in both p-and n-CNT's, and the model consistently explains the key experimental findings.

A model is proposed for the previously reported lower Schottky barrier ⌽ Bh for hole transport in air than in vacuum at a junction between the metallic electrode and semiconducting carbon nanotube ͑CNT͒. We consider the electrostatics in a transition region between the electrode and CNT in the presence or absence of oxygen molecules ͑air or vacuum͒, where an appreciable potential drop occurs. The role of oxygen molecules is to increase this potential drop with a negative oxygen charge, leading to lower ⌽ Bh in air. The Schottky barrier modulation is large when a CNT depletion mode is involved, while the modulation is negligible when a CNT accumulation mode is involved. The mechanism prevails in both p-and n-CNT's, and the model consistently explains the key experimental findings.

We have characterized the fully self-consistent electronic properties of a prototypical metal/nanotube interface using a combined nonequilibrium Green's function and density-functional-theory-based formalism, under different conditions of gate and bias voltages. Both carbon and boron nitride nanotubes between Al electrodes were considered. The electronic properties of the interface are dominated both by a dipole and by metalinduced gap states formed through the transfer of charge between the metal and the nanotube. In addition, first-principles estimates-within the local density approximation-of the Schottky barrier heights are given.

Carbon nanotube field-effect transistors ͑CNTFETs͒ fabricated out of as-grown nanotubes are unipolar p-type devices. Two methods for their conversion from p-to n-type devices are presented. The first method involves conventional doping with an electron donor, while the second consists of annealing the contacts in vacuum to remove adsorbed oxygen. A comparison of these methods shows fundamental differences in the mechanism of the transformation. The key finding is that the main effect of oxygen adsorption is not to dope the bulk of the tube, but to modify the barriers at the metal-semiconductor contacts. The oxygen concentration and the level of doping of the nanotube are therefore complementary in controlling the CNTFET characteristics. Finally, a method of controlling individually the contact barriers by local heating is demonstrated.

Oxide charge on the sidewalls of SiO 2 embedded silicon wires with 20ϫ 20 nm 2 cross section is shown to influence the Schottky barrier height for Pd 2 Si/ Si junctions positioned on the end surfaces of the wires. Compared with results on planar silicon surfaces, the electron barrier height is 0.3 eV lower for wires investigated as fabricated. By increasing the oxide charge through irradiation by ultraviolet light, the electron barrier decreases by an additional 0.15 eV and the hole barrier correspondingly increases by about the same amount. The phenomenon is explained by assuming an oxide charge density in the range of 10 12 cm −2 .

The temperature dependence of 1 / f noise in individual semiconducting carbon nanotube ͑CNT͒ field-effect transistors is used to estimate the distribution of activation energies of the fluctuators D͑E͒ responsible for the noise. D͑E͒ shows a rise at low energy with no characteristic energy scale, and a broad peak at ϳ0.4 eV. The peak, responsible for the majority of noise at room temperature, cannot be due to electronic excitations, carrier number fluctuations, or structural fluctuations of the CNT, and likely results from the motion of defects in the dielectric or at the CNT-dielectric interface, or very strongly physisorbed species ͑binding energy ϳ0.4 eV͒ on the CNT or dielectric surface.

The temperature dependence of 1 / f noise in individual semiconducting carbon nanotube ͑CNT͒ field-effect transistors is used to estimate the distribution of activation energies of the fluctuators D͑E͒ responsible for the noise. D͑E͒ shows a rise at low energy with no characteristic energy scale, and a broad peak at ϳ0.4 eV. The peak, responsible for the majority of noise at room temperature, cannot be due to electronic excitations, carrier number fluctuations, or structural fluctuations of the CNT, and likely results from the motion of defects in the dielectric or at the CNT-dielectric interface, or very strongly physisorbed species ͑binding energy ϳ0.4 eV͒ on the CNT or dielectric surface.

A model is proposed for the previously reported lower Schottky barrier for holes ΦBh in air than in vacuum at a metallic electrode-semiconducting carbon nanotube (CNT) junction. We consider that there is a transition region between the electrode and the CNT, and an appreciable potential can drop there. The role of the oxidation is to increase this potential drop with negatively charged oxygen molecules on the CNT, leading to lower ΦBh after oxidation. The mechanism prevails in both p- and n-CNTs, and the model consistently explains the key experimental findings.

A model is proposed for two observed current–voltage (I–V) patterns in a recent experiment with a scanning tunneling microscope tip and a carbon nanotube [Collins et al., Science 278, 100 (1997)]. We claim that there are two mechanical contact modes for a tip (metal)–nanotube (semiconductor) junction (1) with or (2) without a tiny vacuum gap (0.1–0.2 nm). With the tip grounded, the tunneling case in (1) would produce large dI/dV with V>0, small dI/dV with V<0, and I = 0 near V = 0 for an either n or p nanotube; the Schottky mechanism in (2) would result in I ≠ 0 only with V<0 for an n nanotube, and the bias polarities would be reversed for a p nanotube. The two observed I–V patterns are thus entirely explained by a tip–nanotube contact of the two types, where the nanotube must be n-type.

A model is proposed for the previously reported lower Schottky barrier ΦBh for hole transport in air than in vacuum at a junction between the metallic electrode and semiconducting carbon nanotube (CNT). We consider the electrostatics in a transition region between the electrode and CNT in the presence or absence of oxygen molecules (air or vacuum), where an appreciable potential drop occurs. The role of oxygen molecules is to increase this potential drop with a negative oxygen charge, leading to lower ΦBh in air. The Schottky barrier modulation is large when a CNT depletion mode is involved, while the modulation is negligible when a CNT accumulation mode is involved. The mechanism prevails in both p- and n-CNT’s, and the model consistently explains the key experimental findings.

An equivalent circuit model is proposed for the Schottky barrier at the junction between a metallic electrode and a semiconducting carbon nanotube (NT). We have applied the model to a gold-NT junction under the presence of neutral polarized NH3 molecules, and have shown that visible Schottky barrier modulation is possible for the gas densities as low as 3 x 10^13 cm−2, which is quite feasible experimentally.