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

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

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 ͑SWNT͒ and highly oriented pyrolytic graphite ͑HOPG͒ samples. Thermal-desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K, which is shifted to a significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O 2 binding energy on SWNT bundles ͑18.5 kJ/mol͒ is 55% higher than that for adsorption on HOPG ͑12.0 kJ/mol͒. 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

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

Embryonic Stem Cell-Derived L1 Overexpressing Neural Aggregates Enhance Recovery after Spinal Cord Injury in Mice

An obstacle to early stem cell transplantation into the acutely injured spinal cord is poor survival of transplanted cells. Transplantation of embryonic stem cells as substrate adherent embryonic stem cell-derived neural aggregates (SENAs) consisting mainly of neurons and radial glial cells has been shown to enhance survival of grafted cells in the injured mouse brain. In the attempt to promote the beneficial function of these SENAs, murine embryonic stem cells constitutively overexpressing the neural cell adhesion molecule L1 which favors axonal growth and survival of grafted and imperiled cells in the inhibitory environment of the adult mammalian central nervous system were differentiated into SENAs and transplanted into the spinal cord three days after compression lesion. Mice transplanted with L1 overexpressing SENAs showed improved locomotor function when compared to mice injected with wild-type SENAs. L1 overexpressing SENAs showed an increased number of surviving cells, enhanced neuronal differentiation and reduced glial differentiation after transplantation when compared to SENAs not engineered to overexpress L1. Furthermore, L1 overexpressing SENAs rescued imperiled host motoneurons and parvalbumin-positive interneurons and increased numbers of catecholaminergic nerve fibers distal to the lesion. In addition to encouraging the use of embryonic stem cells for early therapy after spinal cord injury L1 overexpression in the microenvironment of the lesioned spinal cord is a novel finding in its functions that would make it more attractive for pre-clinical studies in spinal cord regeneration and most likely other diseases of the nervous system

Dynamics of photoexcited electrons in metals and semimetals

Titel, Inhalts-, Abbildungs-, und Tabellenverzeichnis 1\. Einleitung 1 2\. Relaxationsdynamik photoangeregter Elektronen 5 3\. Experimenteller Aufbau 19 4\. Anisotropie der Quasiteilchen-Lebensdauern in Graphit (HOPG) 47 5\. Elektronendynamik in Magnesiumdiborid 69 6\. Elektron-Phonon-Kopplung und optische Absorption in Kohlenstoff- Nanoröhren 81 7\. Nicht-thermische Elektronenverteilungen in Kupfer und Graphit 103 8\. Der Ag(111)-Oberflächenzustand: Elektronendynamik in zwei Dimensionen? 113 9\. Gas-Oberflächen-Wechselwirkung: Adsorption und Desorption von HOPG und Nanoröhren 125 10\. Abschließende Zusammenfassung und Ausblick 137 Literaturverzeichnis 141 Kurzfassung, Publikationen, Danksagung 159Die Arbeit soll einen Beitrag zum Verständnis der Dynamik photoangeregter Elektronen an Metall- und Halbmetall-Oberflächen leisten. Die Elementarprozesse der Relaxationsdynamik, Elektron-Elektron-Streuung, Elektron-Phonon-Streuung und Streuung an Defekten, werden an geeigneten Modellsystemen untersucht. Dazu wurden Experimente mit zeitaufgelöster Photoemission aufgebaut und durchgeführt, um die Prozesse, die auf einer Zeitskala von 10-14 s bis 10-11 s stattfinden, analysieren zu können. Die durch Elektron-Elektron-Streuung bestimmten Quasiteilchen-Lebensdauern an einer Graphit (HOPG)-Oberfläche werden im Energiebereich von -0,11 eV bis 2,3 eV bezüglich des Fermi-Niveaus untersucht. Die Energieabhängigkeit der Quasiteilchen-Lebensdauer zeigt eine Anomalie zwischen 1,1 eV und 1,5 eV bezüglich des Fermi-Niveaus, also im Energiebereich eines Sattelpunktes in der elektronischen Bandstruktur. Die Messungen geben einen klaren Hinweis auf Anisotropien der Quasiteilchen-Lebensdauer. Die beobachtete Anomalie kann schon anhand eines einfachen Modells einer einzelnen Graphitschicht, nämlich durch das für Elektronen mit unterschiedlichem Impuls bei einem Elektron- Elektron-Streuprozess zur Verfügung stehende Phasenraumvolumen, verstanden werden. Die Interpretation wird durch ab-initio-Selbstenergie-Rechnungen [Spa01b] und einen Vergleich mit Experimenten an defektreichen HOPG- Oberflächen unterstützt. Letztere zeigen einen Anstieg der Relaxationsraten durch Fehlordnung des Gitters. Darüber hinaus wurde die Zwei-Photonen-Photoemission methodisch so weiterentwickelt, dass mit ihr nicht nur die Elektron-Elektron- sondern auch die Elektron-Phonon-Kopplung an Oberflächen quantifiziert werden kann. Auch hier wird das Gleichgewicht zwischen Elektronengas und Gitter durch einen ultrakurzen Laserpuls gestört, und anschließend wird der Energietransfer zwischen Elektronen und Phononen über Photoemissionsspektren als Funktion der Zeit gemessen. Der Elektron-Phonon-Kopplungsparameter λ kann mit der Theorie von Allen [All87] anhand der gemessenen Energietransfer-Rate bestimmt werden. Die Kopplungsparameter werden sowohl für Systeme sehr schwacher Kopplung, wie Kohlenstoff-Nanoröhren (λ = (4 +- 1) 10-4), als auch für das 2001 als Supraleiter mit außergewöhnlich hoher Sprungtemperatur (Tc = 39 K) identifizierte [Nag01] Magnesiumdiborid (λ = 0,73 +- 0,45) bestimmt. Die Messung der Energietransfer-Rate beruht hier nicht auf der Annahme thermalisierter Elektronenverteilungen und kann somit auch zur Analyse von Systemen eingesetzt werden, in denen die Relaxation über Elektron-Elektron- Streuung und über Elektron-Phonon-Streuung auf ähnlichen Zeitskalen abläuft. Die genaue Form der Photoemissionsspektren von optisch angeregtem Graphit wird mit Referenzmessungen an einer Cu(111)-Oberfläche verglichen. Die Intensitätsverteilungen der nach der Anregung entstandenen Elektronen- und Lochverteilungen können anhand eines einfachen thermischen Modells weitgehend erklärt werden, das auf einer dynamischen Verschiebung des chemischen Potentials μ(T) durch die optische Anregung aufbaut. Die angeregten Ladungsträger in Kupfer relaxieren über die direkt konkurrierenden Prozesse Elektron-Elektron-Streuung und Elektron-Phonon-Streuung. Neben dem Studium der Elektronendynamik wurden Messungen der Adsorptions- Eigenschaften von Inertgasen auf Kohlenstoff-Nanoröhren durchgeführt, welche einen ersten Schritt zur gezielten Modifizierung der elektronischen Eigenschaften von Nanoröhren darstellen könnten.This thesis aims at widening the understanding of the dynamics of photoexcited electrons at metal and semimetal surfaces. The elementary relaxation processes electron-electron-scattering, electron-phonon-scattering and scattering with defects are studied in model systems. A time-resolved two-photon-photoemission experiment was set up and performed for the investigation of these dynamics occurring on a timescale of 10-14 s to 10-11 s. Quasiparticle (QP) lifetimes of photoexcited electrons in highly oriented pyrolytic graphite have been studied in the energy range from -0.11 eV to 2.3 eV with respect to the Fermi level. A pronounced anomaly in the energy dependence of the QP lifetimes between 1.1 eV and 1.5 eV provides strong evidence for lifetime anisotropies. The anomaly can be associated with electrons near a saddle point in the graphite band structure at the M point of the Brillouin zone and can be explained even in a simple model for the phase space available for electron-electron-scattering in a graphene sheet considering constrains by momentum and energy conservation. This interpretation is supported by recent ab initio calculations [Spa01b] and experiments on defect-enriched HOPG as well as temperature dependent measurements. The method of two-photon-photoemission was developed into a tool to quantify the electron-phonon-interaction at surfaces. After perturbing the equilibrium between electron gas and lattice the energy transfer between the two subsystems is measured as a function of time from photoemission spectra. By using the theory from Allen [All87] the electron-phonon mass enhancement parameter can be determined. The analysis is presented for a sample of single wall carbon nanotubes revealing a weak coupling of λ = (4 +- 1) 10-4 and for magnesiumdiboride - recently discovered to be superconducting at temperatures up to 39 K - giving a coupling parameter of λ = 0.73 +- 0.45. The measurement of the energy transfer between electrons and lattice is not based on the assumption of a thermalized electron gas and can therefore be applied to systems with directly competing electron-electron and electron-phonon- dynamics. Photoemission spectra from the excited electronic system in graphite are compared with reference measurements of a Cu(111) surface. The detailed intensity distribution can be explained within a simple thermodynamic model taking the temperature dependence of the chemical potential μ(T) into account. In addition to the investigation of electron dynamics this thesis includes results from adsorption and desorption measurements of inert gases on graphite and porous carbon nanotube samples; which serves as preparatory work for a modification of the electronic properties of nanotubes

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 SAINT 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 C-60 from nanotube bundles and from HOPG