SPELEEM microscope

The true strength of SPELEEM is the unique combination of complementary imaging and diffraction methods available. By using photons or electrons as probe, the SPELEEM allows a real multi-technique approach to the study of surfaces and thin films. In XPEEM operation, the microscope exploits the characteristics of synchrotron radiation to implement laterally resolved version of x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). In combination with magnetic linear and circular dichroism (MLD and MCD respectively), the SPELEEM can image the magnetic state of surfaces, thin films and buried interfaces. Due to the wide energy range provided by the beamline, 50-1000 eV, the microscope can access most of the relevant absorption edges of transition metals. By using the hemispherical electron energy analyser the microscope allows core level and valence band imaging, thus probing the local chemical state and electronic structure respectively. The SPELEEM can perform energy-filtered XPEEM with an energy resolution of ~ 300 meV in imaging mode, achieving routinely a lateral resolution of about 40 nm. LEEM is often used as a complementary technique, being well suited to tackle dynamical processes in force of the high temporal and structural sensitivity.

Typical applications

Magnetic domain imaging The combination of PEEM with x-ray magnetic dichroism has been the warhorse of magnetic microscope at Synchrotron Facilities. The high lateral resolution of the SPELEEM enables the Nanospectroscopy users to image magnetic domains down to mesoscopic length scales, well below 100 nm. read more	From chemical mapping to nano-ESCA Energy-filtered XPEEM discloses new opportunities in the study of nanostructured composite surfaces. Laterally resolved XPS and core level spectro-microscopy are the most powerful tools currently available to characterize the chemistry of the topmost surface layers of in-situ prepared specimens. read more
Microprobe-ARPES Diffraction imaging in a LEEM-XPEEM micro- scope enables the access to the electronic structure of a surfaces and interfaces, probing an area of just 3 square microns. This operation mode, still largely unexploited, is expanding the application of XPEEM to intriguing fields such as layered materials and Graphene. read more	Video rate imaging Low energy electron microscopy and related methods adds structure sensitivity to the capabilities of synchrotron based spectromicroscopy. With a lateral resolution of just 10 nm, our LEEM is the ideal tool to unveil the dynamics of processes such as growth and self-organization. read more

Operating principle and methods

The SPELEEM images a specimen that is illuminated with soft x-rays, ultra-violet (UV) radiation or low energy electrons. The emitted photoelectrons, or the elastically reflected electrons, are accelerated by a strong field in the objective lens, of which the specimen is an integral part. The objective produces a magnified image of the specimen, which is further magnified by several additional lenses in the imaging column of the instrument. The image is energy-filtered by the hemispherical analyser and finally projected onto an imaging MCP detector with phosphorous screen. This is imaged in real-time by a computer controlled CCD camera.

Energy filtered X−PEEM: The PEEM detects electrons emitted from atomic core levels with kinetic energy E_kin = hν - E_bin -φ, where E_bin is the core level binding energy, hν the photon energy and φ the work function. Typically hν is kept fixed, with energies in the range provided by the beamline (50-1000 eV). The energy filter is used to select the kinetic energy E_kin of photoelectrons, which allows measuring the binding energies of emitting atoms or accessing the surface electronic structure, including surface states and resonances. Typical kinetic energies of the photoelectrons are at the minimum of the inelastic mean free path of the electrons in the matter, ensuring good surface sensitivity through the very reduced mean free path of electrons in the matter. Kinetic energies are in the range 50 to 150 eV, typically well beyond the broad peak of secondary emission. Collecting a series of images at different kinetic energies allows obtaining laterally resolved (or angle resolved) spectroscopic information. The intensity of the photoemission signal is proportional to the number of emitters in the topmost layers within their energy-dependent escape depth, and thus provides straightforward and quantitative information about the surface chemical composition. Optimal elemental sensitivity is achieved by tuning the photon energy to maximise photoionisation cross-section.

UV−PEEM: Threshold microscopy can be performed when using a UV light source (such as an Hg lamp) as illunination. The photon energy must be higher then the lowest local work function on the surface. This method is extremely sensitive to small differences in the local work function, which are directly induced by molecules and atoms adsorbed on the surface. Threshold UV-PEEM has been extensively employed to study dynamical processes such as spatio-temporal pattern formation in surface catalytic reactions.

XAS−PEEM, XMC(L)D-PEEM: In x-ray absorption spectroscopy PEEM images the secondary electron emission at fixed kinetic energy as a function of the photon energy hν. When hν matches a core level energy a strong increase in the secondary emission intensity is observed. Such resonances arise from transitions from core levels into unoccupied valence states via excitation processes occurring during the filling of the core holes. They are characteristic fingerprints of the emitter chemical state, so that elemental sensitivity is acheived. X-ray absorption near-edge spectroscopy (XANES) provides a wealth of information about the emitter, such as site location and valence state. Due to the very low energy of the secondary electrons (less than a few eV), their mean free path is relatively large. XAS and XANES can thus probe buried interfaces or films up to a depth of ~10nm. The resolving power of the monochromator determines the attainable energy resolution and for this reason energy filter is not required. In combination with magnetic linear and circular dichroism (MLD and MCD respectively) XPEEM can image the magnetic state of surfaces, thin films and buried interfaces.

LEEM: LEEM is a structure sensitive technique which uses elastically backscattered electrons to image a crystalline surface with a lateral resolution of few tens nm. The use of a contrast aperture positioned in the diffraction plane allows employing primary or secondary diffracted beams for imaging. When the primary diffracted beam (or "00" beam) is selected, we perform bright−field LEEM. Here, the contrast is purely structural (diffraction contrast) and depends on the local differences in diffraction for the different surface phases present on the sample. By selecting a secondary diffracted beam, a darkfield image of the surface is produced. Here all areas that contribute to the formation of the selected beam appear bright. Other methods available in LEEM are phase contrast and quantum size contrast. In the first, the height difference between terraces at different heights on the surface leads to a phase difference in the backscattered waves. Defocusing can convert such phase difference into an amplitude difference, allowing to image steps at surfaces. The second method is based on the interference of waves that are backscattered at the surface and at the interface of a thin film, producing maxima and minima in the backscattered intensity depending on the local thickness of the film.

MEM: in mirror electron microscopy (MEM) the surface is illuminated with electrons at very low energy. The sample bias is adjusted so that the electrons interact very weakly with the surface (this occurs at the transition MEM−LEEM). Under these conditions the contrast is due to work function differeces and topography variations. MEM allows non-crystalline samples to be imaged.

Imaging mode

The sample is illuminated with x-rays or UV radiation, to excite photoemission, or with electrons. IL and P1 image the specimen image produced by the objective. A slit can be inserted in the dispersive plane of the analyzer, in order select the desired energy of the photoelectrons. The specimen image is finally projected onto the detector by the action of P2 and P3. The contrast aperture in the diffraction plane limits the angular acceptance for optimum lateral resolution.

Microprobe diffraction

The sample is illuminated with x-rays or UV radiation, to excite photoemission, or with electrons. IL and P1 image the specimen image produced by the objective. A slit can be inserted in the dispersive plane of the analyzer, in order select the desired energy of the photoelectrons. The specimen image is finally projected onto the detector by the action of P2 and P3. The contrast aperture in the diffraction plane limits the angular acceptance for optimum lateral resolution.

µ−LEED
The SPELEEM is operated as a LEED instrument. Reflection of the e-beam by a crystalline surface results in the formation of a diffraction pattern in the back-focal plane of the objective lens. The beam energy is varied by changing the bias voltage between sample and electron emitter. The probed area can be restricted to 2µm either by inserting an aperture in the image plane in the input or exit side of the beam separator, respectively. LEED data yields information about the surface structure.

µ−PhD and µ−ARPES
In micro x-ray photoelectron diffraction (PhD) the photons are used as probe. The exit slit of the electron analyser must be used, in order to allow energy filtering. The field limiting aperture is used to select the probed area. By imaging diffraction from a core level, one can get information on the local order around the emitter. Diffraction imaging using photelectrons emitted from the valence band gives access to the electronic structure in the momentum space.
 

Microprobe spectroscopy (mu-XPS).

The last two projectors, P2 and P3, are used to image the dispersive plane of the analyser. The dispersive plane appears as a line, and its intensity profile represents the photoemission spectrum. Note that the energy slit is not inserted. The probed area is selected by the field limiting aperture inserted in the image plane after the objective lens.Micro-XPS mode allows fast acquisition times while reaching the best energy resolution of the SPELEEM, 0.2eV.

Microscope performance