Gunnar Moos

Interaction of molecular oxygen with single-wall carbon nanotube bundles and graphite

The adsorption of oxygen on highly oriented pyrolytic graphite (HOPG) and bundles of single-wall carbon nanotubes (SWNTs) at 28 K is studied using thermal desorption spectroscopy and by a measurement of sticking probabilities. The low-coverage binding energy of oxygen adsorbed on SWNT bundles, 18.5 kJ/mol, is 55% higher than the low-coverage binding energy on HOPG, 12.0 kJ/mol. Molecular mechanics calculations reveal that such an increase can be attributed to the higher effective coordination of binding sites on SWNT bundles. The character of the oxygen-SWNT interaction should therefore be van der Waals type which suggests that the observed oxygen species is physisorbed and does not facilitate bulk doping of SWNT samples.

Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles

We present a study on the kinetics of xenon desorption from single-wall carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS). Desorption features from SWNT samples are broadened and peaked at higher temperature if compared to graphite. This can be explained using a coupled desorption–diffusion (CDD) model, which yields the low-coverage binding energy for Xe adsorption on SWNT bundles, 27 kJ mol−1. The latter is about 25% higher than the monolayer binding energy on graphite, 21.9 kJ mol−1. Using molecular mechanics calculations we find that this increase is consistent with adsorption in highly coordinated groove-sites on the external bundle surface or in endohedral sites inside of SWNTs.

Influence of excited electron lifetimes on the electronic structure of carbon nanotubes

Abstract We have studied the dynamics of electrons in single-wall carbon nanotube (bucky paper) samples using femtosecond time-resolved photoemission. The lifetime of electrons excited to the π ∗ bands is found to decrease continuously from 130 fs at 0.2 eV down to less than 20 fs at energies above 1.5 eV with respect to the Fermi level. This should lead to a significant lifetime-induced broadening of the characteristic van Hove singularities in the nanotube density of states.

Adsorption and desorption of weakly bonded adsorbates from single-wall carbon nanotube bundles

We present an investigation of the kinetics of adsorption and desorption of some weakly bonded gases from single wall carbon nanotube bundles and—for comparison—from graphite. Thermal desorption spectra from CH4, Xe and SF6 as well as those of MeOH, EtOH and H2O can be used to obtain adsorbate binding energies. The observed trends in the binding energies of gases with different van der Waals radii suggest that so-called groove sites on the external bundle surface are the preferred low coverage adsorption sites due to their higher binding energy. These results shed new light on the wetting properties of SWNT bundles. In addition we find that measured sticking coefficients can be related to the diffusion kinetics of adsorbates into the bulk of the nanotube samples.

Wetting of single-wall carbon nanotube ropes and graphite

We have studied the interaction of a variety of adsorbates, ranging from inert gases to polar molecules, solvents and aromatic compounds with single‐wall carbon nanotube and graphite surfaces using thermal desorption spectroscopy (TDS). These studies allow to investigate the wetting properties of such surfaces on the microscopic scale without complications due to microscopic surface roughness arising with some other techniques. The overwhelming majority of adsorbates studied here is found to wet graphite and nanotube surfaces completely while both are found to be wet only partially by water.

Interaction of O2 and C60 with Single‐Wall Carbon Nanotube Bundles from Thermal Desorption Spectroscopy

We have studied the kinetics of oxygen desorption from single‐wall carbon nanotube (SWNT) bundles and highly oriented pyrolytic graphite (HOPG) using thermal desorption spectroscopy (TDS). The binding energies for adsorption on SWNT bundles and HOPG at low coverages are 18.5 kJ/mol and 12.0 kJ/mol, respectively. Molecular mechanics calculations using van der Waals pair‐potentials show that the higher binding energy found for adsorption on SWNT bundles can be attributed to van der Waals interactions and is due to higher coordinated sites available for adsorption on the tube bundles. We find no evidence for a stronger bound, chemisorbed oxygen species or for dissociative adsorption. We also present preliminary results on the kinetics of desorption of C60 from nanotube bundles and from HOPG.

Femtosecond time-resolved photoemission as a probe of electronic transport in single wall carbon nanotubes

We have performed the first time-domain measurements of the electron-electron (e-e) and electron-phonon (e-ph) dynamics in single-wall carbon nanotube samples (bucky paper) using time-resolved two-photon photoemission. In these room temperature experiments the absorption of a visible femtosecond pump pulse creates a non-equilibrium electron distribution whose evolution in time can be probed by a second UV-pulse. The decay of the excited electron distribution is characterized by a fast channel on the subpicosecond time-scale—associated with thermalization of the non-equilibrium distribution—and a slower channel which can be attributed to e-ph interaction. Once thermalized the electron distribution cools down to the lattice temperature as determined by the electron-phonon coupling constant g which was found to be 1×1015 Wm−3 K−1.