Thomas Fauster

Femtosecond two-photon photoemission studies of image-potential states

Abstract High-resolution two-photon photoemission studies with femtosecond time resolution permit the accurate determination of decay and dephasing processes for image-potential states. The influence of adsorbates on the respective inelastic and quasielastic scattering processes is investigated for Cu on Cu(100) and Cu(111). The results are discussed in relation to previous work for CO on Cu(100).

Image-potential states and work function of graphene

Image-potential states of graphene on various substrates have been investigated by two-photon photoemission and scanning tunneling spectroscopy. They are used as a probe for the graphene-substrate interaction and resulting changes in the (local) work function. The latter is driven by the work function difference between graphene and the substrate. This results in a charge transfer which also contributes to core-level shifts in x-ray photoemission. In this review article, we give an overview over the theoretical models and the experimental data for image-potential states and work function of graphene on various substrates.

Watching the life of electrons at surfaces with two-photon photoemission

Abstract An electron put on a metal surface is attracted by the image force. A Rydberg-like series of image-potential states is formed which describes loosely bound electrons which can move freely parallel to the surface. With time-resolved two-photon photoemission spectroscopy the electronic band structure and the femtosecond dynamics of these states are studied in the energy- and time-domain. The decay of the image-potential-state population is attributed to electron–electron interaction with metal electrons, which constitutes an inelastic scattering process. Defects (or phonons) may lead to elastic scattering which changes the phase of the electronic wave function without changing the population. With time-resolved two-photon photoemission inelastic and elastic scattering processes can be separated. From these experiments we can extract information on the lifetime of excited electrons at surfaces in excellent agreement with theoretical calculations by Echenique and co-workers. The interaction of image-potential states with adsorbates or steps gives insight into the inelastic and elastic scattering by the various surface defects which are not accessible by any other technique.

Time-resolved photoemission from image-potential states

Abstract Electrons attracted by the image force to a metal surface are loosely bound and form a Rydberg-like series of states converging towards the vacuum level. Energies and lineshapes of these image-potential states are studied by two-photon photoelectron spectroscopy. The use of Ti–sapphire lasers as light sources has raised the count rates by several orders of magnitude and provided the opportunity to study the lifetime in the femtosecond range. Coherent excitation of several higher image-potential states of the Rydberg-like series leads to the observation of quantum beats in the time domain. The decay of the beat pattern and the lifetime are strongly influenced by adsorbates. The dependence on the quantum number of the image-potential states and the adsorbate will be discussed. The model system of image-potential states can be studied extremely well in present experiments and the results contribute to the understanding of photochemical processes and hot-electron dynamics at surfaces.

Two-photon photoemission from image-potential states of epitaxial graphene

Using angle- and time-resolved two-photon photoelectron spectroscopy we observe a single series of image-potential states of graphene on monolayer (MLG) and bilayer graphene (BLG) on SiC(0001). The first image-potential state on MLG (BLG) has a binding energy of 0.93 eV (0.84 eV). Lifetimes of the first three image-potential states of MLG are 9, 44 and 110 fs. On hydrogen-intercalated, quasi-freestanding graphene no unoccupied states are observed. We attribute this to the absence of occupied initial states for direct transitions into image-potential states at photon energies below the work function used in two-photon photoemission. The work function varies between 4.14 and 4.79 eV, but the vacuum level stays ~4.5 eV above the Dirac point for all surfaces studied. This finding suggests that direct excitation of image-potential states cannot be achieved by doping and the electron dynamics for free-standing graphene is not accessible by two-photon photoemission using photon energies below the work function.

Mapping the band structure of GeSbTe phase change alloys around the Fermi level

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.Phase change alloys are used as components for optical data storage like DVD and Blue-Ray disks but many of their conductive properties are only known phenomenologically. This paper reports detailed band structure mapping of the prototype alloy GST-225 using photoemission spectroscopy, and the observation of a Dirac-like surface state above the Fermi level.

Interaction of free-base tetraphenylporphyrin with magnesium oxide: Influence of MgO morphology on metalation

Using x-ray photoemission spectroscopy, we investigated the self-metalation of free-base tetraphenylporphyrin (2HTPP) on thin MgO(100) films on Ag(100). The deposition of one monolayer 2HTPP on MgO results in the formation of magnesium(ii) tetraphenylporphyrin (MgTPP) at room temperature. We demonstrate that the efficiency of the reaction drastically depends on the morphology of the oxide layers. The latter is changed by varying the substrate temperature during the oxide growth. We observe the complete metalation of the 2HTPP monolayer when the MgO films are grown at 393 K. The increase of the growth temperature to 573 K leads to the reduction of the percentage of metalated molecules to ∼50%. We ascribe these results to the fact that MgTPP formation takes place through the hydroxilation of steps and defects on the MgO surface, which leads to an increase of the OH component in the O 1s line.

Two-photon photoemission from CoO layers on Ir(1 0 0).

Two-photon photoelectron spectroscopy is used to study the unoccupied electronic states of cobalt oxide layers on Ir(1 0 0). For thicker layers of (1 0 0) orientation the conduction band minimum is found 2 eV above the Fermi level. Layers with (1 1 1) orientation and thickness ≤4 bilayers show a peak around 3.4 eV energy and no evidence for the conduction band minimum. This is attributed to the metallic character of thin CoO(1 1 1) layers.

Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect

Significance Lead halide perovskites are successfully used in thin-film solar cells, with efficiencies on the laboratory scale exceeding 22%. The electronic structure underlying their exceptional phototransport properties is complex because of the organic–inorganic character of the materials, their mechanical softness, and the strong spin–orbit coupling induced by the constituting heavy elements. Calculations predict that a dynamical Rashba effect could enhance the lifetimes and diffusion lengths of photocarriers in perovskite solar cells. The dynamical Rashba effect is characterized by spin splittings in the band structure at elevated temperatures, induced by local structural disorder. The mechanism should be general to structurally flexible materials composed of heavy elements, making those potentially attractive for both optoelectronics and spintronics. Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline (CH3NH3)PbI3 with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal (CH3NH3)PbI3. At room temperature, the effect peaks for excitation photon energies ΔE=110 meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic–tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both ΔE and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.Long carrier lifetimes and diffusion lengths form the basis for the successful application of the organic-inorganic perovskite (CH3NH3)PbI3 in solar cells and lasers. The mechanism behind the long carrier lifetimes is still not completely understood. Spin-split bands and a resulting indirect band gap have been proposed by theory. Using near band-gap left-handed and righthanded circularly polarized light we induce photocurrents of opposite directions in a single-crystal (CH3NH3)PbI3 device at low temperature (4 K). The phenomenom is known as the circular photogalvanic effect and gives direct evidence for phototransport in spin-split bands. Simultaneous photoluminecence measurements show that the onset of the photocurrent is below the optical band gap. The results prove that an indirect band gap exists in (CH3NH3)PbI3 with broken inversion symmetry as a result of spin-splittings in the band structure. This information is essential for understanding the photophysical properties of organic-inorganic perovskites and finding lead-free alternatives. Furthermore, the optically driven spin currents in (CH3NH3)PbI3 make it a candidate material for spintronics applications.

Elektronen nahe Metalloberflächen: Bildpotentialzustände

Elektronen werden von Metalloberflachen durch eine „Bildkraft” angezogen. So entstehen Bindungszustande, die Bildpotentialzustande heisen. Sie haben viele vom Wasserstoffatom bekannte Eigenschaften. So werden sie zu einem einfachen Modellsystem fur grundlegende Phanomene der Quantenmechanik wie Quantisierung und Interferenz. Die Zweiphotonen-Photoemission eroffnet einen experimentellen Zugang zu den Bildpotentialzustanden mit hoher Energieauflosung im Millielektronenvoltbereich und hoher Zeitauflosung im Femtosekundenbereich. Sie macht die Bildpotentialzustande zu empfindlichen Sonden fur die Messung unterschiedlicher Eigenschaften von Oberflachen und Adsorbaten.