Photoemission Mapping Research Articles

Geometry-Induced Spin Filtering in Photoemission Maps from WTe_{2} Surface States.

We demonstrate that an important quantum material WTe_{2} exhibits a new type of geometry-induced spin filtering effect in photoemission, stemming from low symmetry that is responsible for its exotic transport properties. Through the laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping, we showcase highly asymmetric spin textures of electrons photoemitted from the surface states of WTe_{2}. Such asymmetries are not present in the initial state spin textures, which are bound by the time-reversal and crystal lattice mirror plane symmetries. The findings are reproduced qualitatively by theoretical modeling within the one-step model photoemission formalism. The effect could be understood within the free-electron final state model as an interference due to emission from different atomic sites. The observed effect is a manifestation of time-reversal symmetry breaking of the initial state in the photoemission process, and as such it cannot be eliminated, but only its magnitude influenced, by special experimental geometries.

Design and performance of a new setup for spatially resolved transmission X-ray photoelectron spectroscopy at the Swiss Light Source.

The successful design, installation and operation of a high spatial resolution X-ray photoelectron spectrometer at the Swiss Light Source is presented. In this instrument, a Fresnel zone plate is used to focus an X-ray beam onto the sample and an electron analyzer positioned at 45° with respect to the incoming beam direction is used to collect photoelectrons from the backside of the sample. By raster scanning the sample, transmitted current, X-ray absorption and X-ray photoemission maps can be simultaneously acquired. This work demonstrates that chemical information can be extracted with micrometre resolution; the results suggest that a spatial resolution better than 100 nm can be achieved with this approach in future. This kind of photoelectron spectromicroscope will allow in situ measurements with high spatial resolution also under ambient pressure conditions (in the millibar range). Element-specific X-ray photoemission maps can be obtained before and while exposing the sample to gas/gas mixtures to show morphological and chemical changes of the surface.

Reversible graphene decoupling by NaCl photo-dissociation

We describe the reversible intercalation of Na under graphene on Ir(1 1 1) by photo-dissociation of a previously adsorbed NaCl overlayer. After room temperature evaporation, NaCl adsorbs on top of graphene forming a bilayer. With a combination of electron diffraction and photoemission techniques we demonstrate that the NaCl overlayer dissociates upon a short exposure to an x-ray beam. As a result, chlorine desorbs while sodium intercalates under the graphene, inducing an electronic decoupling from the underlying metal. Low energy electron diffraction shows the disappearance of the moiré pattern when Na intercalates between graphene and iridium. Analysis of the Na 2p core-level by x-ray photoelectron spectroscopy shows a chemical change from NaCl to metallic buried Na at the graphene/Ir interface. The intercalation-decoupling process leads to a n-doped graphene due to the charge transfer from the Na, as revealed by constant energy angle resolved x-ray photoemission maps. Moreover, the process is reversible by a mild annealing of the samples without damaging the graphene.

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 ...

Spatially Resolved Photoemission and Electrochemical Characterization of a Single-Chamber Solid Oxide Fuel Cell

The present paper pinpoints the feasibility of in operando near-ambient pressure characterization of an operating single chamber solid oxide fuel cell (SC-SOFC) by means of spatially resolved X-ray photoelectron spectroscopy. The results are based on an innovative and unique setup expanding the capabilities of the scanning photoelectron microscopes. In particular, the impact of getting access to the low mbar range on the very observability of chemical-state changes in an LSM cathode have been highlighted. Ex situ and in situ electrochemical measurements at OCP and close circuit cell conditions complemented the photoemission mapping and spectroscopy. The latter has evidenced the evolution of the chemical state of Sr upon the exposure of the typical SC-SOFC CH4–O2 gas feed.

Structural determination of bilayer graphene on SiC(0001) using synchrotron radiation photoelectron diffraction

In recent years there has been growing interest in the electronic properties of ‘few layer’ graphene films. Twisted layers, different stacking and register with the substrate result in remarkable unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based photoelectron diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 Å and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 Å. Our work generalises the use of a versatile and precise diffraction method capable to shed light on the structure of low-dimensional materials.

Tunable ultrafast extreme ultraviolet source for time- and angle-resolved photoemission spectroscopy

We present a laser-based apparatus suitable for visible pump/extreme UV (XUV) probe time-, energy-, and angle-resolved photoemission spectroscopy utilizing high-harmonic generation from a noble gas. Tunability in a wide range of energies (currently 20-36 eV) is achieved by using a time-delay compensated monochromator, which also preserves the ultrashort duration of the XUV pulses. Using an amplified laser system at 10 kHz repetition rate, approximately 10(9)-10(10) photons/s per harmonic are made available for photoelectron spectroscopy. Parallel energy and momentum detection is carried out in a hemispherical electron analyzer coupled with an imaging detector. First applications demonstrate the capabilities of the instrument to easily select the probe wavelength of choice, to obtain angle-resolved photoemission maps (GaAs and URu(2)Si(2)), and to trace ultrafast electron dynamics in an optically excited semiconductor (Ge).

High-resolution photoemission mapping of the three-dimensional band structure of Bi(111)

Angle-resolved photoemission spectra of Bi(111) excited with synchrotron radiation at photon energies from 9 to 100 eV have been used to determine the initial- and final-state band structures along $\mathrm{\ensuremath{\Gamma}}T$. Deviations from the free-electron dispersion for energies below about 60 eV can be accounted for by an energy-dependent extension function to the final-state bands. The experimentally determined dispersion of the top three bulk valence bands is compared with literature data and various band-structure calculations, none of which satisfactorily describes the observed band width, the critical-point energies, or their specific dispersion relations ${E}_{i}(k)$. At a photon energy of 29 eV, direct transitions lead to a pronounced final-state gap. In this region, the experimental band-structure map consists of features resembling the one-dimensional density-of-states projection of the Bi bulk band structure onto the (111) surface.

Electronic valence bands in decagonal Al-Ni-Co

Valence-band photoemission from the $s\ensuremath{-}p$ region of the tenfold and the two inequivalent twofold surfaces of quasicrystalline decagonal ${\mathrm{Al}}_{71.8}{\mathrm{Ni}}_{14.8}{\mathrm{Co}}_{13.4}$ reveals strongly dispersing bands. These exhibit a free-electron-like dispersion along quasiperiodic and periodic directions of the decagonal quasicrystal. The experimental photoemission maps are reproduced in detail by a model in which parabolic bands emanate from a set of reciprocal lattice vectors. A parity rule for the principal zone centers is observed.

THREE-DIMENSIONAL BAND MAPPING BY COMBINED VERY-LOW-ENERGY ELECTRON DIFFRACTION AND PHOTOEMISSION

Resolving the 3D wave vector k in photoemission mapping of the band structure E( k ) requires knowledge of the unoccupied final states. Both dispersions and lifetimes of these states can be achieved by very-low-energy electron diffraction (VLEED). By incorporating the non-free-electron and excited-state effects in the final states, combining VLEED with photoemission provides accurate mapping of the valence E( k ) resolved in the 3D wave vector and under control of the intrinsic accuracy. We here concentrate on the most accurate combined method which uses angle-dependent VLEED and photoemission measurements. It provides access to many Brillouin-zone lines using one crystal surface, and benefits from an intensity gain and a better intrinsic accuracy near the Fermi level.

Photon emission by scanning tunneling microscopy in air

A photon emission experiment has been done by Scanning Tunneling Microscopy in air. The emitted photons from a gold surface are collected in order to print side by side the STM topography and the corresponding photoemission mapping. Significant correlations can be observed between both images. The emission spectra are also collected while scanning the surface, thus yielding simultaneously a third piece of information. The results are markedly different from those obtained in ultra high vacuum.