In the following, a brief description of main SPELEEM imaging modes is provided.

X-ray Photoemission Electron Microscopy (XPEEM) – energy filtered imaging

The technique can be used as for slow secondaries electrons (utilizing a work function contrast) as well as for electrons whose energy is characteristic for the material studied. When primary electrons from the atomic core level are excited by the X-rays at a fixed energy, hν, they escape from the sampleThese photoelectrons with a certain kinetic energy, Ekin= hνEbinφ, are selected by the hemispherical energy analyzer to form an XPEEM image, where Ebin is a core level binding energy and φ the work function. By varying the Ekin, one can probe the chemical state of the emitting atoms for a specific element allowing performing elemental/chemical mapping.

XPEEM example of a GaAs(100) surface heated up to a temperature where Ga droplets start to form. According to the photoemission electron spectra (graph on the left) for GaAs surface and Ga droplets, the GaAs and Ga peaks are 75.6 eV and 76.8 eV (Ga3d5/2 ) respectively. Thus, the Ga droplets appear dark when taking the XPEEM image at GaAs peak (E1), while they are bright when imaging the metallic Ga peak (E2). FoV= 25 µm for both XPEEM images.

 

Micro-X-ray Photoemission Spectroscopy (µ-XPS)

Photoelectron spectroscopy from extremely small areas down to a fraction of a micron can be performed. The high flux on the samples allows both high spatial and high energy resolution.

Performing photoelectron microscopy on an insulating material is possible by taking advantage of a patterned mask used to deposit a gold layer on the surface. (Left) Crystal structure of KTiOPO₄: by differently polarizing the sample, two different domains (namely “+” and “-“) can be imaged in XPEEM (right). The K3p core level µ-XPS spectra are acquired from the selected areas (5 µm2) indicated on the XPEEM image. The “+” domain is visibly richer in potassium content than the “-” domain. FoV=50 µm, hv=130 eV.

 

Micro-X-ray Absorption Spectroscopy (µ-XAS)

The microscope images the secondary electron emission at fixed kinetic energy as a function of the photon energy hv. In combination with linear and circular chroism, XPEEM has become the main tool for imaging the magnetic state of surfaces, thin films, and buried interfaces.

Metal-insulator transition (MIT) of VO₂(110) sample made of high quality 110 nm strain thin films of VO₂ grown on (110)-oriented substrate of rutile TiO₂. (Left) The crystal structure of the VO₂(110) sample varies from an insulating monoclinic structure at low temperature to a metallic tetragonal rutile structure isostructural with the underlying TiO₂ at high temperature. (Right) O1s μ-XAS spectra are acquired from the VO₂(110) sample at T=20 °C and T= 90 °C (blue and red curve, respectively). The peak intensity at 530.8 eV is associated to V-V dimers: this contribution vanishes as soon as the temperature is increased to 90 °C and at the same time a shift in the leading slope of the O K edge towards lower energy is observed thus revealing the metallicity of electrons.

 

X-ray Magnetic Linear Dichroism (XLCD) and X-ray Magnetic Circular Dichroism (XMCD)

Taking advantage of elliptically polarized undulator, ferromagnetic and antiferromagnetic domains in magnetic materials can be studied with spatial resolution down to a couple of nanometers. It is important to note that, due to the experimental configuration with normally incident X-ray beam, the beamline provides high sensitivity to the out-of-plane component of the magnetic moment.

Domain images on NiO(100). (a) Twin domain LEEM image taken with a (1/2,0) beam with 24 eV electrons, (b) spin domain XMLDPEEM image taken at the Ni L₂ edge, (c) twin domain XLDPEEM image taken at the O K edge.                                                              E.Bauer “Surface Microscopy with Low Energy Electrons” Springer Ed.2014
NiO(100) surface. (a,c,e) Ni L₂ edge XMLDPEEM images, (b) O K XLDPEEM image, (d,f) Co L₂ XMCDPEEM images of a 3 nm thick Co layer on the NiO(100) surface. (a) and (b) show the spin and twin domain structure before Co deposition, respectively, (c,d) the domain structure in NiO and Co in the as-prepared state, (e,f) the corresponding structures after annealing above the Neél temperature and cooling to room temperature. The arrows indicate the spin orientation. Annealing caused some domain growth in NiO and significant domain changes in Co film, but the relative spin orientation in Co and NiO was retained.      E.Bauer “Surface Microscopy with Low Energy Electrons” Springer Ed.2014

 

PhotoElectronDiffraction (PED)

The intensity of a core level line as a function of energy and emission angle is measured. The technique can provide spatially resolved information on the surface crystallographic structure and it is therefore complementary to LEED and STM. If the valence band electrons form the diffraction pattern, the band and Fermi surface mapping in the full cone become possible (µ-ARPES technique).

(Left) μ-ARPES (kx,ky) map is acquired at EB= ‒2.25 eV from Sn intercalated graphene sample. The photon energy used is hν = 45 eV. The Dirac cones emerge after Sn intercalation. (Right) E(k) for SnSix intercalated graphene (A) and SnOx intercalated graphene (B) extracted from the μ-ARPES (kx,ky) map.  Y.R. Niu et al. Ultramicroscopy 183, 49-54 (2017)

 

Low Energy Electron Microscopy (LEEM)

This is the most powerful technique for the study of the morphology of crystalline surfaces. Several contrast mechanisms (including Dark Field Imaging) allow the determination of the lateral dimensions of regions with a given crystal structure, the thickness distribution of thin overlayers with monolayer resolution, the imaging of monoatomic surface steps and other morphological features.

(Left) Bright field LEEM image of GaP(111) surface showing three trails (STV =4.3 eV). (Right) Dark field LEEM image of the same area of GaP(111). The surface reconstruction consists of six different domains. All domains are present inside and outside the trail but their size and shape are changed by the passing Ga droplet. Inside the trails the domains are larger and more elongated. The domain colours correspond to the diffraction spots marked in the LEED pattern (STV=10.3 eV) shown in the centre. The sixfold domain structure is found in the trails as well as on the surrounding surface.                                                A.A. Zakharov et al. J. Electron Spectros. Relat. Phenomena 185, 10 (2012)

LEEM image of several graphene layers grown on SiC(0001). (Left) LEEM image shows a strong contrast due to graphene different thickness of one to four monolayers. (Right) The reflectivity oscillations resulting from the quantum size effect can be used to count the number of graphene layers from such IV curves.                        C. Virojanadara et al. Phys. Rev. B 78, 245403 (2008)

 

Micro-Low Energy Electron Diffraction (µ-LEED)

By simply switching one lens and removing the contrast aperture the LEED pattern of the imaged area can be obtained. The imaged area can be as small as 250 nm, so the diffraction pattern from such a small area can be obtained.

(Top) LEEM image of a pentacene island grown on a BN nanomesh on top of a Rh(111) crystal. Electron energy STV=3.4 eV, FoV=20 µm. (Bottom) µ-LEED pattern from the pentacene layer (left) and the BN nanomesh (right).                                              M.Ling Ng et al. Phys. Rev. B  81, 115449 (2010)

 

Mirror Electron Microscopy (MEM) mode

In this mode the specimen is more negative than the electron source so that the electrons are reflected in front of the surface.  Contrast is determined by field distribution above the surface which depends upon the surface topography and the charge distribution on the surface.  In the case of magnetic specimens also the magnetic field distribution above the surface can be imaged by proper illumination and imaging conditions.

KTiOPO₄ surface imaged in a mirror mode. A periodic domain structure is fabricated by electric poling: the different contrast between the domains corresponds to variations in the charge distribution related to the different amount of potassium at the domains´ surfaces. STV= 1.5 eV and FoV=50 µm. The positive STV is due to the large bandgap of the dielectric KTiOPO₄ surface.