Luca Gregoratti

Photoelectron microscopy and applications in surface and materials science

We review the recent achievements of photoelectron microscopy (PEM), which is a rapidly developing technique that is significantly advancing the frontiers of surface and materials science. The operation principles of scanning photoelectron microscopes (SPEM), using different photon optic systems to obtain a micro-probe of sub-micrometer dimensions, and of the full-field imaging microscope, using electrostatic lenses for magnification of the irradiated sample area, are presented. The contrast mechanisms, based on photon absorption and photon-induced electron emission, are described and the expected development in the photon and electron optics and detection systems are discussed. Particular attention is paid to the present state-of-art performance of the microscopes collecting photoelectrons (PEs), which carry specific information about the lateral variations in the chemical, magnetic and electronic properties of the material under investigation. Selected results, obtained recently with instruments installed at synchrotron light facilities, are used to illustrate the potential of PEM in characterising micro-phases and dynamic processes with a lateral resolution better than 100 nm.

Graphene oxide windows for in situ environmental cell photoelectron spectroscopy

The performance of new materials and devices often depends on processes taking place at the interface between an active solid element and the environment (such as air, water or other fluids). Understanding and controlling such interfacial processes require surface-specific spectroscopic information acquired under real-world operating conditions, which can be challenging because standard approaches such as X-ray photoelectron spectroscopy generally require high-vacuum conditions. The state-of-the-art approach to this problem relies on unique and expensive apparatus including electron analysers coupled with sophisticated differentially pumped lenses. Here, we develop a simple environmental cell with graphene oxide windows that are transparent to low-energy electrons (down to 400 eV), and demonstrate the feasibility of X-ray photoelectron spectroscopy measurements on model samples such as gold nanoparticles and aqueous salt solution placed on the back side of a window. These proof-of-principle results show the potential of using graphene oxide, graphene and other emerging ultrathin membrane windows for the fabrication of low-cost, single-use environmental cells compatible with commercial X-ray and Auger microprobes as well as scanning or transmission electron microscopes.

ESCA Microscopy at ELETTRA: what it is like to perform spectromicroscopy experiments on a third generation synchrotron radiation source

Abstract We present ESCA Microscopy, the first X-ray microscopy beamline operating on ELETTRA, the third generation synchrotron radiation source in Trieste, Italy. ESCA Microscopy is an advanced user facility open to the international scientific community; its operation is based on the use of a Fresnel zone plate to demagnify to submicrometre dimensions the photon beam emitted by an undulator in the 200–1000 eV energy range. ESCA Microscopy was designed as a scanning photoemission microscope especially suited for surface analysis; it also operates in transmission mode and should find many applications in materials science, chemistry and physics. We describe here the beamline experimental setup and present some recent results obtained during the first months of operation. While so doing, we will outline certain spectroscopic aspects and point out some operational problems that are typical of scanning photoemission microscopy, a technique which should find in advanced sources such as ELETTRA the right conditions to achieve maturity.

Mechanism of dark-spot degradation of organic light-emitting devices

Using chemically sensitive x-ray photoelectron microscopy, we investigate the mechanism of dark-spot formation and degradation of organic light-emitting devices. The morphological and chemical evolution of the Al cathode surface under operation conditions reveals the formation of “domelike” structures, followed by local disruptions of the cathode, exposing microareas of the underlying indium tin oxide anode. The chemical maps and microspot spectra identify a release of volatile In-, Sn-, and C-containing species, including metallic In, which is clear evidence that the degradation is driven by local decomposition of the anode∕organic interface.

Spectral and spatial anisotropy of the oxide growth onRu(0001)

Scanning photoelectron spectromicroscopy has been used to study the onset and the initial stages of oxidation of Ru(0001) at three oxidation temperatures, 625, 675, and 775 K, and oxygen exposures of about 105 Langmuir. The lateral heterogeneity developed during oxide nucleation and growth and the local composition of the coexisting phases have been determined using as fingerprints the O 1s and Ru 3d spectra, thus combining chemical mapping with spectroscopy from selected features from the maps. The onset of oxide formation is characterized by the appearance of randomly distributed small islands (⩾0.5 μm) identified as germinal patches exhibiting some spectral features of bulk RuO2. The following anisotropic growth of the RuO2 phase and in particular the shape of the oxide islands shows a strong dependence on the oxidation temperature. The spectroscopic information obtained for the areas surrounding the oxide islands reveals an intermediate oxygen state characterized by distinct O 1s and Ru 3d features di...

Imaging the variation in band bending across a silicon pn junction surface using spectromicroscopy

We present a characterization of lateral silicon pn junction arrays fabricated on a Si(001) surface using a synchrotron-based scanning photoelectron microscope (SPEM). The Si 2p images show energy dependent contrast which varies continuously across the space charge region between regions of different doping. Combined with measurements of the changes in the Si 2p spectra across the pn junction, we demonstrate the capacity of SPEM in imaging variations in dopant concentration, the width of the charge depletion zone, and variations in band bending with oxide preparation.

Selforganization of alkali metal on a catalytic metal surface

The spatial distribution of potassium on an Rh(110) surface during the catalytic O2+H2 reaction is investigated employing photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) as spatially resolving in situ methods. Depending on the reaction conditions, potassium condenses reversibly into macroscopic islands where it is coadsorbed with oxygen. Mass transport of potassium with the reaction fronts is observed. Differences in the mobility and in the bonding strength of potassium on the “reduced” and on the oxygen-covered surface areas are considered to be the key factors for the formation of the stationary concentration patterns.

X-ray photoelectron microscopy of the C 1s core level of free-standing single-wall carbon nanotube bundles

Core level photoemission spectra from a free-standing bundle of single-wall carbon nanotubes have been measured using a high-flux soft x-ray spectromicroscope. The good signal-to-noise ratio for the C 1s emission provides information on fundamental quantities such as the core-hole lifetime and binding energy, free from uncontrolled interactions between the nanotubes and the substrate or between the nanotubes and contaminants. We show that it is possible to distinguish chemically different nanotubes from the binding energy and line shape of the C 1s core level. This finding opens unique opportunities to probe in situ the response of the nanotube electronic properties and chemical activity to mechanical actions, doping, and functionalization.

Soft X‐ray Imaging and Spectromicroscopy: New Insights in Chemical State and Morphology of the Key Components in Operating Fuel‐Cells

Fuel cells are one of the most appealing environmentally friendly devices for the effective conversion of chemical energy into electricity and heat, but still there are key barriers to their broad commercialization. In addition to efficiency, a major challenge of fuel-cell technology is the durability of the key components (interconnects, electrodes, and electrolytes) that can be subject to corrosion or undesired morphology and chemical changes occurring under operating conditions. The complementary capabilities of synchrotron-based soft X-ray microscopes in terms of imaging, spectroscopy, spatial and time resolution, and variable probing depths are opening unique opportunities to shed light on the multiple processes occurring in these complex systems at microscopic length scales. This type of information is prerequisite for understanding and controlling the performance and durability of such devices. This paper reviews the most recent efforts in the implementation of these methods for exploring the evolving structure and chemical composition of some key fuel cell components. Recent achievements are illustrated by selected results obtained with simplified versions of proton-exchange fuel-cells (PEFC) and solid-oxide fuel-cells (SOFC), which allow in situ monitoring of the redox reactions resulting in: 1) undesired deposits at interconnects and electrodes (PEFC); 2) material interactions at the electrode-electrolyte interface (PEFC); 3) release of corrosion products to the electrolyte phase (PEFC, and 4) mass-transport processes and structural changes occurring at the high operation temperatures of SOFC and promoted by the polarization.

Direct writing of fluorescent patterns on LiF films by x-ray microprobe

Fluorescent patterns with submicron dimensions have been obtained by creating stable F3+ and F2 color centers in LiF films using a focused x-ray beam provided at the ELETTRA synchrotron radiation facility. The patterns were written by scanning the LiF specimen with respect to the x-ray microprobe. In these attempts, using an x-ray microspot with a diameter of 100 nm and a flux density ⩾109 photons/s, we generated ∼500-nm-wide lines efficiently emitting in the visible spectral region when excited by blue light at 458 nm. Preliminary results indicate that the spectral distribution of the emitted luminescence can be changed by varying the photon dose delivered to the sample.