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

We have studied the dynamics of electrons in single wall carbon nanotubes using femtosecond time-resolved photoemission. The lifetime of electrons excited to the pi* 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 DOS.Comment: 6 pages, 4 figure

Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite

We present a study on the kinetics of oxygen adsorption and desorption from single-wall carbon nanotube (SWNT) and highly oriented pyrolytic graphite (HOPG) samples. Thermal desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K which is shifted to significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O2 binding energy on SWNT bundles, 18.5 kJ/mol, is 55% higher than that for adsorption on HOPG, 12.0 kJ/mol. In combination with molecular mechanics calculations we show that the observed binding energies for both systems can be attributed to van der Waals interactions, i.e. physisorption. The experiments provide no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption.Comment: 7 pages, 5 figures, 1 tabl

Electronic Structure and Scaling of Coulomb Defects in Carbon Nanotubes from Modified H\"uckel Calculations

Controlled doping and understanding its underlying microscopic mechanisms is crucial for advancement of nanoscale electronic technologies, especially in semiconducting single-wall carbon nanotubes (s-SWNTs), where adsorbed counterions are known to govern redox-doping levels. However, modeling the associated 'Coulomb defects' is challenging due to the need for large-scale simulations at low doping levels. Using modified H\"uckel calculations on 120 nm long s-SWNTs with adsorbed $\rm Cl^-$ ions, we study the scaling properties of shallow Coulomb defect states at the valence band edge and quantum well (QW) states in the conduction band. Interestingly, the QW states may underlie observed exciton band shifts of inhomogeneously doped semiconductors. Binding energies of Coulomb defects are found to scale with counterion distance, effective band mass, relative permittivity and counterion charge according to $d^{\alpha-2}m^{\alpha-1}\epsilon_r^{-\alpha}|z_j|^{\alpha}$, with $\alpha$ as an empirical parameter, deepening our understanding of s-SWNT doping

Charge-carrier dynamics in single-wall carbon nanotube bundles: A time-domain study

We present a real-time investigation of ultrafast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow to study the processes governing the subpicosecond and the picosecond dynamics of non-equilibrium charge-carriers. On the subpicoseond timescale the dynamics are dominated by ultrafast electron-electron scattering processes which lead to internal thermalization of the laser excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi-level - the highest energy studied experimentally. The subsequent cooling of the laser heated electron gas down to the lattice temperature by electron-phonon interaction occurs on the picosecond time-scale and allows to determine the electron-phonon mass enhancement parameter lambda. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper.Comment: 17 pages, 19 igure

Probing the ultrafast dynamics of excitons in single semiconducting carbon nanotubes

Excitonic states govern the optical spectra of low-dimensional semiconductor nanomaterials and their dynamics are key for a wide range of applications, such as in solar energy harvesting and lighting. Semiconducting single-walled carbon nanotubes emerged as particularly rich model systems for one-dimensional nanomaterials and as such have been investigated intensively in the past. The exciton decay dynamics in nanotubes has been studied mainly by transient absorption and time-resolved photoluminescence spectroscopy. Since different transitions are monitored with these two techniques, developing a comprehensive model to reconcile different data sets, however, turned out to be a challenge and remarkably, a uniform description seems to remain elusive. In this work, we investigate the exciton decay dynamics in single carbon nanotubes using transient interferometric scattering and time-resolved photoluminescence microscopy with few-exciton detection sensitivity and formulate a unified microscopic model by combining unimolecular exciton decay and ultrafast exciton-exciton annihilation on a time-scale down to 200 fs

Influence of the type of alkaline activator on the reactions and mechanical properties of autoclaved bauxite residue-based monoliths

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On the time-analytic behavior of particle trajectories in an ideal and incompressible fluid flow

This (Diplom-) thesis deals with the particle trajectories of an incompressible and ideal fluid flow in ≥ 2 dimensions. It presents a complete and detailed proof of the surprising fact that the trajectories of a smooth solution of the incompressible Euler equations are locally analytic in time. In following the approach of P. Serfati, a complex ordinary differential equation (ODE) is investigated which can be seen as a complex extension of a partial differential equation, which is solved by the trajectories. The right hand side of this ODE is in fact given by a singular integral operator which coincides with the pressure gradient along the trajectories. Eventually, we may apply the Cauchy-Lipschitz existence theorem involving holomorphic maps between complex Banach spaces in order to get a unique solution for the above mentioned ODE. This solution is real-analytic in time and coincides with the particle trajectories

Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode

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). TD-spectra from SWNT samples show a broad desorption feature peaked at significantly higher temperature than the corresponding low-coverage desorption feature on graphite. The observations are explained using a coupled desorption-diffusion (CDD) model, which allows the determination of the low-coverage Xe binding energy for adsorption on SWNT bundles, 27 kJ/mol. This energy is about 25% higher than the monolayer binding energy on graphite, 21.9 kJ/mol. By comparison with molecular mechanics calculations we find that this increase of the binding energy is consistent with adsorption in highly coordinated groove-sites on the external bundle surface.Comment: 8 pages, 5 figure