20 research outputs found
Probing carrier dynamics in photo-excited graphene with time-resolved ARPES
The dynamics of photo-generated electron-hole pairs in solids are dictated by
many-body interactions such as electron-electron and electron-phonon
scattering. Hence, understanding and controlling these scattering channels is
crucial for many optoelectronic applications, ranging from light harvesting to
optical amplification. Here we measure the formation and relaxation of the
photo-generated non-thermal carrier distribution in monolayer graphene with
time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we
identify impact ionization as the primary scattering channel, which dominates
the dynamics for the first 25fs after photo-excitation. Auger recombination is
found to set in once the carriers have accumulated at the Dirac point with time
scales between 100 and 250fs, depending on the number of non-thermal carriers.
Our observations help in gauging graphene's potential as a solar cell and
TeraHertz lasing material.Comment: 10 pages, 4 figure
Investigation of Two-Dimensional Electron Gases with Angular Resolved Photoemission Spectroscopy
In this thesis we study the electronic structure of different two-dimensional (2D) electron systems with angular resolved photoemission spectroscopy (ARPES). This technique is based on the photoelectric effect and directly probes the electronic structure of a system. By carefully analyzing the measured band structure with respect to peak position and line width we can determine the complex self-energy Σ that describes the renormalization of the electron's energy and the change in lifetime due to many-body interactions. The 2D electron systems investigated in this work are surface alloys on Ag(111), bismuth trimers on Si(111) and epitaxial graphene monolayers grown on SiC(0001). Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth, lead or antimony (alloy atoms) on the clean silver surface. Although (Bi,Pb,Sb) and Ag atoms are immiscible in the bulk they form long-range ordered surface alloys, where every third Ag atom is replaced by an alloy atom. These systems as well as the Bi trimers on Si(111) show a spin splitting of the 2D band structure due to the Rashba-Bychkov (RB) effect. The RB model states that in a symmetry broken environment (such as the surface of a semi-infinite crystal) the spin-orbit interaction will lift the spin-degeneracy of the band structure. Such a spin-split band structure bares great potential for applications in the field of spintronics, e.g. in a Datta-Das spin field effect transistor. In the present work we investigate the origin of the observed giant spin splitting in surface alloys, especially the interplay between structural parameters and the atomic spin-orbit interaction. Furthermore, we will show that it is possible to transfer these concepts to a semiconducting substrate, which is better suited for spintronics applications. The third system under investigation — graphene — is an ideally two-dimensional crystal. It consists of a single layer of carbon atoms arranged in a honeycomb lattice, and its charge carriers are confined within a plane that is just one atom thick. These charge carriers behave like massless Dirac particles and possess extremely high carrier mobilities. This makes graphene a promising material system for high-speed electronic devices. In order to reach this ambitious goal one needs reliable methods for the large-scale production of high quality graphene films. Epitaxial growth on silicon carbide (0001) substrates is the method of choice in this case, as it offers the advantage of a precise thickness control and a semiconducting substrate at the same time. However, the presence of the substrate reduces the carrier mobility of graphene's charge carriers considerably. Therefore, it is necessary to decouple the graphene layer from the substrate after epitaxial growth. A second issue that needs to be addressed, are viable doping methods for graphene. As graphene's peculiar band structure results from a sensible interplay between electrons and crystal lattice it is not an option to replace single atoms of the graphene lattice by dopants as is common practice when doping silicon. In order to preserve its band structure, graphene is usually doped by adsorbing atoms or molecules on its surface. As graphene grown on SiC is n-doped due to charge transfer from the substrate, appropriate means for p-type doping are clearly required. In this thesis, we will present a new growth method for quasi free-standing graphene on SiC(0001) and viable means for p-type doping
Illuminating the dark corridor in graphene: polarization dependence of angle-resolved photoemission spectroscopy on graphene
We have used s- and p-polarized synchrotron radiation to image the electronic
structure of epitaxial graphene near the K-point by angular resolved
photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is
suppressed due to the interference of photoelectrons emitted from the two
equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we
show that by rotating the polarization vector, we are able to illuminate this
'dark corridor' indicating that the present theoretical understanding is
oversimplified. Our measurements are supported by first-principles
photoemission calculations, which reveal that the observed effect persists in
the low photon energy regime.Comment: 5 pages, 4 figure
Population Inversion in Monolayer and Bilayer Graphene
The recent demonstration of saturable absorption and negative optical
conductivity in the Terahertz range in graphene has opened up new opportunities
for optoelectronic applications based on this and other low dimensional
materials. Recently, population inversion across the Dirac point has been
observed directly by time- and angle-resolved photoemission spectroscopy
(tr-ARPES), revealing a relaxation time of only ~ 130 femtoseconds. This
severely limits the applicability of single layer graphene to, for example,
Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived
population inversion in bilayer graphene. The effect is attributed to the small
band gap found in this compound. We propose a microscopic model for these
observations and speculate that an enhancement of both the pump photon energy
and the pump fluence may further increase this lifetime.Comment: 18 pages, 6 figure
Electronic decoupling of an epitaxial graphene monolayer by gold intercalation
The application of graphene in electronic devices requires large scale
epitaxial growth. The presence of the substrate, however, usually reduces the
charge carrier mobility considerably. We show that it is possible to decouple
the partially sp3-hybridized first graphitic layer formed on the Si-terminated
face of silicon carbide from the substrate by gold intercalation, leading to a
completely sp2-hybridized graphene layer with improved electronic properties.Comment: 7 pages, 4 figures, 1 tabl
Phonon-pump XUV-photoemission-probe in graphene: evidence for non-adiabatic heating of Dirac carriers by lattice deformation
We modulate the atomic structure of bilayer graphene by driving its lattice
at resonance with the in-plane E1u lattice vibration at 6.3um. Using time- and
angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet
(XUV) pulses, we measure the response of the Dirac electrons near the K-point.
We observe that lattice modulation causes anomalous carrier dynamics, with the
Dirac electrons reaching lower peak temperatures and relaxing at faster rate
compared to when the excitation is applied away from the phonon resonance or in
monolayer samples. Frozen phonon calculations predict dramatic band structure
changes when the E1u vibration is driven, which we use to explain the anomalous
dynamics observed in the experiment.Comment: 16 pages, 8 figure
Snapshots of non-equilibrium Dirac carrier distributions in graphene
The optical properties of graphene are made unique by the linear band
structure and the vanishing density of states at the Dirac point. It has been
proposed that even in the absence of a semiconducting bandgap, a relaxation
bottleneck at the Dirac point may allow for population inversion and lasing at
arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by
impact ionization has been discussed in the context of light harvesting
applications. However, all these effects are difficult to test quantitatively
by measuring the transient optical properties alone, as these only indirectly
reflect the energy and momentum dependent carrier distributions. Here, we use
time- and angle-resolved photoemission spectroscopy with femtosecond extreme
ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the
non-equilibrium response of Dirac electrons near the K-point of the Brillouin
zone. In lightly hole-doped epitaxial graphene samples, we explore excitation
in the mid- and near-infrared, both below and above the minimum photon energy
for direct interband transitions. While excitation in the mid-infrared results
only in heating of the equilibrium carrier distribution, interband excitations
give rise to population inversion, suggesting that terahertz lasing may be
possible. However, in neither excitation regime do we find indication for
carrier multiplication, questioning the applicability of graphene for light
harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the
technique of choice to assess the suitability of new materials for
optoelectronics, providing quantitatively accurate measurements of
non-equilibrium carriers at all energies and wavevectors.Comment: 16 pages, 7 figure
Link between interlayer hybridization and ultrafast charge transfer in WS-graphene heterostructures
Ultrafast charge separation after photoexcitation is a common phenomenon in
various van-der-Waals (vdW) heterostructures with great relevance for future
applications in light harvesting and detection. Theoretical understanding of
this phenomenon converges towards a coherent mechanism through charge transfer
states accompanied by energy dissipation into strongly coupled phonons. The
detailed microscopic pathways are material specific as they sensitively depend
on the band structures of the individual layers, the relative band alignment in
the heterostructure, the twist angle between the layers, and interlayer
interactions resulting in hybridization. We used time- and angle-resolved
photoemission spectroscopy combined with tight binding and density functional
theory electronic structure calculations to investigate ultrafast charge
separation and recombination in WS-graphene vdW heterostructures. We
identify several avoided crossings in the band structure and discuss their
relevance for ultrafast charge transfer. We relate our own observations to
existing theoretical models and propose a unified picture for ultrafast charge
transfer in vdW heterostructures where band alignment and twist angle emerge as
the most important control parameters.Comment: 8 pages, 5 figure
Link between interlayer hybridization and ultrafast charge transfer in WS2-graphene heterostructures
Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters