Gamini Udaya Sumanasekera

Carbon nanotubes: A thermoelectric nano-nose

Abstract A thermoelectric `nano-nose has been built from tangled bundles of single-walled carbon nanotubes (SWNT). The response is specific to the details of the interaction of the adsorbed molecule with the nanotube wall; even gases such as He, N2 and H2 can be easily detected. Plots of ΔS vs. ρ a are sensitive to whether oxidation or reduction of the tube wall is taking place, and to whether the gas molecule is physisorbed or chemisorbed.

In situ Raman scattering studies of alkali-doped single wall carbon nanotubes

Electrochemical doping and in situ Raman scattering were used to study charge transfer in K- and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm ˇ1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC24 and Li1:25C6. The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly aAected by long-range order within the ropes. ” 2001 Elsevier Science B.V. All rights reserved.

Experimental probes of the molecular hydrogen–carbon nanotube interaction

Electrical transport (resistance R and thermoelectric power S), Raman scattering, and hydrogen adsorption are used to study the interaction of hydrogen molecules with ropes of single-walled carbon nanotubes. The data are consistent with H2 physisorption under the experimental conditions investigated (4 KoTo500 K; 0.1 atmoPo20 atm). The response of S; R to 1 atm hydrogen at 500 Kis consistent with the introduction of a new scattering channel for electrons/holes in the metallic tubes. Raman scattering from the Q-branch of hydrogen molecules adsorbed on the surface is found shifted only by 1–2 cm � 1 from their frequencies in the free molecule and indicates that two different adsorption sites can be detected. Finally, H2 wt% storage in heavily processed ropes of SWNTs are found to exceed 6% at B1 atm and T ¼ 77 Kand the isosteric heat of adsorption is found to be 120 meV. r 2002 Elsevier Science B.V. All rights reserved.

Thermoelectric chemical sensor based on single wall carbon nanotubes

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.

Single-wall carbon nanotubes from coal

Abstract We demonstrate that single-wall carbon nanotubes (SWNT) can be produced by an arc-discharge method using a catalyzed coal carbon source. Characterization by resonant Raman scattering and electron microscopy confirm that this material contains SWNT with properties similar to graphite-derived SWNT. `Web-like material from our growth process has an estimated yield of 45 wt.% of carbon as SWNT, while the average chamber yield is approximately 20 wt.% C, as determined by FT-Raman and HRSEM measurements.

Charge transfer and weak chemisorption of oxygen molecules in nanoporous carbon consisting of a disordered network of nanographene sheets

The adsorption/desorption processes of oxygen are investigated in nanoporous carbon (activated carbon fiber (ACF)) consisting of a disordered network of nanographene sheets. The heat-induced desorption at 200u2009°C shows the decomposition of oxygen-including functional groups weakly bonded to nanographene edges. The removal of these oxygen-including negatively charged functional groups brings about a change in the type of majority carriers, from holes to electrons, through charge transfer from the functional groups to the interior of nanographene sheets. The oxygen adsorption brings ACF back to the electronic state with holes being majority carriers. In this process, a large concentration of negatively charged O(2)(δ-) molecules with δ ∼ 0.1 are created through charge transfer from nanographene sheets to the adsorbed oxygen molecules. The changes in the thermoelectric power and the electrical resistance in the oxygen desorption process is steeper than that in the oxygen adsorption process. This suggests the irreversibility between the two processes.

Low-temperature thermoelectrical power measurements using analogue subtraction

A simple, inexpensive analogue subtraction method for measuring thermoelectrical power from 4 to 300 K using two chromel (KP)/Au-7 at% Fe (Au:Fe) thermocouples is described. The average sample temperature is varied by lowering a small insert probe into a liquid-He-storage container or raising the probe. Details concerning the interface of the experiment to a personal computer are presented. Data collected on constantan, Bi2 Sr2 CaCu2 O7+ and carbon-nanotube samples show the utility of the method.

Thermoelectric study of hydrogen storage in 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.

Raman-active modes of single-walled carbon nanotubes derived from the gas-phase decomposition of CO (HiPco process).

Here we report Raman scattering studies of ropes of Single-walled carbon nanotubes (SWNTs) grown by a high CO pressure process. Five samples from five different batches were studied as a function of excitation wavelength. Three of these samples exhibited Raman spectra similar to that found for SWNTs made by pulsed laser vaporization of arc-discharge methods. The other two samples were found by Raman scattering to contain a significant fraction of tubes with diameter < 1.0 nm. These samples exhibited unusual spectra that, however, can be well understood within the existing models for the electronic and phononic states in SWNTs. Spectra recorded with 1064 nm for the sample having a significant fraction of smaller diameter tubes shows strong modes present between 500 and 1200 cm-1. We suggest these modes arise due to the enhancement of Raman cross-section for small diameter tubes.

Physical properties of CVD-grown Se—carbon films

Abstract A novel chemical vapor deposition (CVD) approach based on the thermal decomposition of an aromatic hydrocarbon in the presence of Se vapor is used to grow films of layered Se—carbon compounds. In principle, this technique can produce a wide variety of new carbon-based materials, for example, graphite intercalation compounds (GICs) which, for kinetic reasons, cannot be made if the intercalant vapor has to diffuse large distances into a pre-existing graphitic host. In particular, homogeneous oriented submicron films of either pure stage-3 (Se 24 C) or mixed-stage Se—carbon layer compounds have been successfully grown on Ni substrates in evacuated sealed quartz tubes. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering and the c -axis electrical transport measurements are discussed in terms of both covalent Se—carbon bonding and an ionic model assuming the formation of an acceptor-type Se—GIC with electron transfer from carbon to Se. Our CVD-grown Se—carbon films exhibit the largest thermoelectric power reported in the open literature among carbon-based compounds. However, the value is at least a factor of 10 less than reported for these materials in patents by Sharp Corp.