The NanoESCA end-station (Figure 1) is a photoemission electron microscope (PEEM) with energy and spin resolving capabilities. It can be used for both real space and momentum space imaging of condensed phase samples in order to reveal the electronic structure of solid surfaces through static and dynamic (time resolved) studies. The system consists of a Preparation Chamber and an Analysis Chamber.
The relevant scientific fields include interfacial charge transfer, ultrafast demagnetisation, orbital tomography, (ultrafast) molecule-substrate interactions, ultrafast processes in topological insulators, band structure evolution, phase transition dynamics, etc.
The NanoESCA endstation (Figure 1) is a photoemission electron microscope (PEEM) with energy and spin resolving capabilities in both real and momentum space (momentum microscopy). The system consists of a Preparation Chamber and an Analysis Chamber.
Figure 1: The NanoESCA Endstation at the ELI-ALPS
The information depth is around 5 nm, therefore the sample surface must be clean, properly prepared and characterised. The preparation chamber is equipped with:
X-ray photoelectron spectroscopy (XPS) for chemical surface analysis:
with a Scienta Omicron XM 1000 monochromatic Al Kα X-ray source and an Argus CU hemispherical electron analyser
FWHM for Ag 3d5/2 is better than 0.6 eV
with imaging capability (lateral resolution ~100 micron)
Low energy electron diffraction (LEED) for structural surface analysis (Figure 2) using the BDL 600IR 4-grid rear view LEED of OCI Vacuum Microengineering, capable also for Auger electron spectroscopy (AES)
Ar+ sputtering gun for surface cleaning (IS 40C1 from Prevac)
Preparation manipulator with low and high temperature stages 100 to 1500 K
Mass spectrometer up to 300 amu (RGA300, Stanford Research Systems)
Metal evaporator for the preparation of ultrathin films, monolayers etc. (EFM3, Focus)
Gas dosing with a microcapillary
array even for liquids with low vapor pressure
Figure 2: LEED pattern of Au(111) with the herringbone reconstruction
Photoemission Electron Microscopy (PEEM) mode: laterally resolved microscopy of the sample surface
Energy filtered real space imaging mode (Imaging Photoelectron Spectroscopy mode): laterally and energy resolved microscopy of the sample surface (Figure 3-4)
Momentum microscopy (k-space) mode: measuring the momentum distribution of photoelectrons originated from a preselected small area (~40 µm) of the sample surface with energy resolution (Fig. 3-5)
Spin filtered imaging mode: with a state-of-the-art Au/Ir(100) imaging spin filter. The detected electrons can be filtered according to their spin state at both real space and momentum space (k-space) imaging (Figure 6).
The analysis chamber has a 4-axis LHe cooled sample stage for low temperature studies (≤30 K).
|Lateral resolution||35 nm|
|K-space resolution||0.03 Å-1|
|Energy resolution||20 meV|
NanoESCA is a sort of electron microscope, which uses the sample as the electron source. The electrons excited by a photon source are collected by the immersion lens. At the back focal plane contrast apertures can be positioned to limit the acceptance angle, thus reducing spherical aberrations and yielding better lateral resolution. In the first image plane an iris is located by which the region of interest can be limited (Figure 3). In PEEM mode this image is projected on a 2D detector by projective lenses without any energy filtering.
For energy filtered imaging a hemispherical analyser can be used and the PEEM column can be considered as the entrance lens of the analyser. Hemispherical electron analyser select the kinetic energy of photon-emitted electrons by a spherical potential bending the electron pathways such that the kinetic energy distribution becomes dispersed in one direction.
Figure 3: Schematic design of the IDEA (Imaging Dispersive Energy Analyser). (Ref: Physics’ Best, April 2019)
Evidently the energy dispersion introduces severe aberrations in the image, which is compensated by a second hemisphere (IDEA design) analogous to Kepler’s orbitals. In real space imaging the first image plane is projected onto the entrance slit of the first hemisphere, while in k-space imaging the back focal plane is projected onto the analyser entrance (Figure 3-4). The aberration compensated electron beam passed through the analysers is then imaged on the second 2D detector.
Figure 4: Illustration of real space and k-space imaging; left: Polycrystalline iron sample real space image (field-of-view, FOV 114 µm); right: Fermi energy slice of Rh(111) at 300 K.
By sweeping the energy E of the imaged electrons it becomes possible to get a three-dimensional kx-ky-E image stack. This image stack represents the band structure of a crystalline sample. By cutting the obtained 3D information package along any chosen azimuthal direction, E-k dispersion relations can be visualised, such as in ARPES, as illustrated for Au(111) in Figure 5. The absolute work function can be determined in both real space and k space measurements.
Figure 5: The surface band structure of Au(111) at 300 K.
Spin filtered imaging mode:
The spin filter is positioned after the exit slit of the IDEA double hemispherical. The method is based on spin polarisation dependent reflection. An Ir(100) scattering crystal covered by a monolayer of Au is used. The inert nature of Au allows longer measurement times without cleaning. The filter is selective along one spin direction parallel to the sample surface. This powerful method can be used for both modes of imaging: direct space for the analysis of magnetic domains and their orientations (Figure 6) as well as determining the spin orientation of the electron states in momentum space.
Figure 6: Demonstration of the spin contrast on a polycrystalline iron sample in real space mode (FOV 130 µm).
For static studies a focused VUV source (He lamp) is available. For ultrafast time-resolved studies, pulses of the HR GHHG Condensed Beamline can be applied with a monochromatisation option.
A Venteon CEP5 CEP stable Ti:Sa oscillator is also available (6 fs, 830 nm, 80 MHz).
|GHHG Condensed Beamline (100kHz)|
|CEP stabilised Ti:Sa oscillator (Venteon CEP5)|
|Pulse length||<6 fs|
|Central wavelength||830 nm|
|Average power @ laser output||250 mW|
|Repetition rate||80 MHz|
|Photon energy (main lines)
|21.22 ev - 58 nm|
|He-II mode||40.81 ev - 30 nm|
|Mercury arc lamp (CW)|
|Photon energy||<5.2 eV – 238 nm|
Rectangular or circular samples can be studied. Lateral sample size: max 10 mm. Sample thickness: 1 mm – 1.8 mm. Metals and semiconductors can be measured. Insulators cannot be used due to sample charging. For momentum microscopy experiments a ~40 μm spot is chosen on the sample. On that scale the surface must be atomically ordered.
Ar ion sputtering
Energy filtered real space imaging mode
Momentum microscopy (k-space) mode
Spin filtered imaging mode with a Au/Ir(100) imaging spin filter at both real space and momentum space (k-space) imaging.
Static and time resolved experiments