Scanning Photoelectron Spectromicroscopy Research Articles

Catalytically Active Materials Visualized by Scanning Photoelectron Spectro-Microscopy

Catalytically Active Materials Visualized by Scanning Photoelectron Spectro-Microscopy

Scanning photoelectron spectromicroscopy: From static to operando studies of functional materials

The scanning photoelectron microscope (SPEM), developed more than 30 years ago, has undergone numerous technical developments, providing an incredibly vast kind of feasible sample environments, which span from the traditional high spatial resolution core level based chemical analysis to insitu and operando complex experiments, including also electrochemical setups and operational electronic devices at various temperatures. Another important step ahead is overcoming the so-called pressure gap for operando studies, recently extended to near ambient values by building special environmental cells. Using recent results of conventional and unconventional experiments, obtained with SPEM at the ESCA Microscopy beamline at Elettra Sincrotrone Trieste the present review demonstrates the current potential of this type of photoelectron spectromicroscopy to explore the interfacial properties of functional materials with high spatial resolution.

Radiation damage of liquid electrolyte during focused X-ray beam photoelectron spectroscopy

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has evolved into an effective tool to analyze the chemical states of interfaces relevant to realistic environments and operating conditions. Herein we employ a graphene-capped microvolume arrays sample platform for scanning photoelectron spectro-microscopy (SPEM) of electrified liquid-solid electrochemical interfaces. By using highly electron and X-ray transparent graphene membrane as a working electrode, we probed the electronic structure of a model electrochemical system under operando conditions within the first few nanometers of the electrode/liquid electrolyte interface. We report the conditions when highly focused X-ray irradiation affects the chemistry at the liquid–solid interface due to solvent radiolysis by primary radiation, photoelectrons as well as secondary electrons. We recorded radiolytic products by photoemission spectroscopy and characterized their impact on the chemical speciation at the electrified solid-liquid interface. Three different exposure regimes were tested to elucidate the onset and dependence of XPS radiolytic signatures of the dose rate. The observed effects highlight the need for careful consideration of radiolytic processes for artifact-free liquid phase XPS measurements and data interpretation.

Mesoscopic Stripes in Antiferromagnetic Fe Chalcogenide Probed by Scanning Photoelectron Spectromicroscopy

We have performed scanning photoelectron spectromicroscopy measurements in the paramagnetic (PM) and the antiferromagnetic (AFM) phases of Fe1+δTe (δ = 0.09). The spectromicroscopy images reveal stripe modulation of the electronic structure in the AFM monoclinic phase at low temperature, that disappears at high temperature in the PM phase. The stripes, running along the monoclinic crystal axis, are characterized by different density of states around the Γ point and near the Fermi level (EF). Since the near EF electronic states around the Γ point have strong electron–lattice coupling in Fe1+δTe, the observed stripes are likely to be related to the strain modulation introduced by the monoclinic lattice distortion.

Imaging and characterization of piezoelectric potential in a single bent ZnO microwire

We achieved direct visualization of the piezoelectric potentials in a single bent ZnO microwire (MW) using focused synchrotron radiation (soft x-ray) scanning photoelectron spectro-microscopy. Using radial-line scan across the bent section of ZnO MW, the characteristic core-level shifts were directly related to the spatial distribution of piezoelectric potentials perpendicular to the ZnO polar direction. Using piezoelectric modeling in ZnO, we delineated the band structure distortion and carrier concentration change from tensile to compressed sides by combining the spatial resolved cathodoluminescence characteristics in an individual microwire. This spectro-microscopic technique allows imaging and identification of the electric-mechanical couplings in piezoelectric micro-/nano-wire systems.

Microcontact printing pattern as a mask for chemical etching: A scanning photoelectron microscopy study

Synchrotron-based scanning photoelectron spectromicroscopy and microspectroscopy were used to monitor the outcome of the etching process involving the transfer of a lithographic pattern produced by microcontact printing (μCP) of self-assembled monolayers (SAMs) to the underlying metal (gold) substrate. As a test system, octadecanethiolate (ODT) SAMs on gold substrates were chosen. The μCP ODT SAMs were found to protect the underlying gold against the wet-chemical etching, ensuring the effective transfer of the μCP pattern to the substrate. These SAMs exhibited only a slight degradation upon their exposure to the Au-etching solution. In contrast, a significant degradation of the edges of the printed features was observed. This degradation was predominantly related to a lateral diffusion of the active etching agents across these edges, along the SAMs-Au interface. This process can result in a blurring and narrowing of the printing features of a μCP SAM pattern at its transfer to the underlying substrate.

Spectromicroscopic evidence for epitaxial poly-Si thin film formed on amorphous Si substrate by nickel induced lateral crystallization

The spatial variations in the chemical composition and electronic structure of nickel metal induced lateral crystallization (Ni-MILC) of amorphous silicon film with in situ vacuum annealing were investigated by scanning photoelectron spectromicroscopy. Lateral chemistry variations of the Ni-MILC of amorphous silicon were directly imaged. Via a system study of the correlation between the core and valence level spectra, the nature of chemical bonding for the different chemical phases was examined. Our results clearly show that the Ni-MILC in UHV leads to the formation of a high-crystallinity poly-Si film, which was laterally grown over ∼20μm at 500°C for 1h.

In situ spectromicroscopic study of nickel induced lateral crystallization of amorphous silicon thin film using SPESM

Scanning photoelectron spectromicroscopy (SPESM) has been used to study nickel metal induced lateral crystallization (Ni-MILC) of amorphous silicon (a-Si) thin films, produced by in situ annealing of vacuum deposited Ni patterned films on a-Si. The spatial variations in the chemical composition of the Ni-MILC of a-Si were directly imaged. High-resolution photoemission spectra of both Si 2p and Ni 3p core levels and valence band were used to evaluate morphological changes and chemical interactions. Our direct spectromicroscopic characterization clearly shows that the Ni-MILC process in UHV leads to the lower crystallization temperature and a faster crystallization speed of a-Si, and a poly-Si film with high-crystallinity can be obtained. A unified mechanism for the enhanced growth rate of the high-crystallinity poly-Si film produced by Ni-MILC in UHV is proposed.

Silicon nanowires grown on Si(1 0 0) substrates via thermal reactions with carbon nanoparticles

The effect of thermal processing at 1050 °C of a dispersed film of carbon nanoparticles deposited on a Si substrate with a native SiO 2 layer has been studied by scanning electron microscopy and scanning photoelectron spectromicroscopy. It has been found that the thermal processing results in formation of pyramidal-shaped defects of 2–7 μm with strongly reduced SiO 2 content with silicon wires of diameter ranging between 30 and 50 nm decorating the pyramid walls. The nucleation of the Si nanowires occurs via reduction of the native oxide layer by the nanosized carbon particles, without the need of metal catalysts and at temperatures relatively lower than that used in similar techniques.

Surface analyses of In–V oxide films aged electrochemically by Li insertion reactions

The surface of a Li-insertion In–V mixed-oxide electrode and the evolution of its composition and morphology upon prolonged intercalation/deintercalation cycles have been studied by a combination of the following bulk and surface-sensitive analytical techniques: Electrochemical analysis (cyclic voltammetry, impedance spectroscopy), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning photoelectron spectromicroscopy (SPEM). The In–V oxide film, obtained by rf sputtering as a thin film on conductive glass for use as a transparent electrode in electrochromic windows, is a Li-insertion material with good reversibility and high charge capacity, having a surface morphology characterised by a low initial roughness. XPS results showed that Li+-insertion brought about the reduction of V5+ to V4+ and V3+, with the onset of new structures in the valence band, related to the formation of Li2CO3 on the electrode surface. Such chemical changes were largely reversed upon Li-deinsertion and only a barely detectable effect on the surface morphology was seen after up to ten cycles. Extensive Li-charge–discharge cycling induced larger changes in the electrode morphology (increase in grain size and roughness), in the surface composition and microscopic aspect, with a partial passivation effect on the film electrochemical behaviour. The formation of surface deposits with a characteristic, elongated shape has been revealed to occur after 500 Li charge–discharge cycles. Submicron chemical analysis of such deposits onto aged oxide electrodes has been performed by SPEM, which enabled the location and assignment of the surface composition.

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 different from both the chemisorbed and the oxide form. It is assigned to an intermediate state acting as precursor of the oxide. The experimental data are discussed in the framework of the oxidation pathway and of core level fingerprints for the various oxygen–Ru phases suggested in recent theoretical models.

Interfacial reactions and Schottky barrier properties of composite patterned metal/GaN interfaces

Scanning photoelectron spectromicroscopy has been applied to study the interfacial chemistry and electronic properties of adjacent patterned Au and Ti/Au regions on GaN, fabricated by metal deposition at room temperature through appropriate masks. The lateral variations in the composition of the interfacial phases before and after annealing to 800 °C and their effect on local band bending are determined. The very small difference in the Schottky barrier heights measured in the adjacent Au/GaN and Ti/Au/GaN regions is identified as a property of the metal/GaN interfaces not predicted by the existing theoretical models.

Anisotropy of the electronic structure of low-dimensional Sr(Ca)CuO 2 single crystals studied by scanning photoelectron spectromicroscopy

The energy band structure of superconducting ( T c onset=80 K), low-dimensional Sr(Ca)CuO 2 single crystals was studied by photoelectron spectromicroscopy. The sample consists of double parallel Cu–O chains, and spectra show a strong anisotropy for electron momenta k parallel and perpendicular to the chain direction q . Spectra were collected with two experimental setups: k‖ q ( A⊥ q ) and k⊥ q ( A‖ q ), where A is the electric field vector of the incident photon beam. By imaging the surface on the micrometer scale we find significant variations in the valence band intensity in the vicinity of about 2 eV below the Fermi level for k‖ q . For this geometry, A⊥ q and the spectra are very sensitive to beam damage. This effect can be related to the breaking of weak inter-chain bonds with the high flux density (∼10 18 photons s −1 cm −2) beam focused on the sample surface.

Spectromicroscopy of interfacial interactions between thin Ni films and a Au–Si surface

The interaction between a (√3×√3)R30°-Au/Si surface with thin (≤2 ML) Ni films with different dimensions deposited at 300 K is studied by means of a scanning photoelectron spectromicroscopy. The interfacial processes occurring at different Ni coverages and annealing temperatures are probed with lateral resolution of 0.12 μm. The results have revealed that Ni displaces Au from the Si surface and the presence of Au lowers the formation temperature of the Ni disilicide phase. It has been found that the behavior of the Ni/Au–Si interfaces at high temperatures, in particular the changes in the morphology of the interface and the formation of well defined Ni x (Au 1− x )Si 2 islands, is strongly influenced by the dimensions of the Ni patches. The observed evolution of the Ni/Au–Si interfaces is interpreted considering the high solubility of Ni in Si, the miscibility between Ni and Au and the Au–Si alloying at elevated temperatures.

Inhomogeneous and thickness-dependent chemical reactivity at GaSe–Si interfaces

Scanning photoelectron spectromicroscopy experiments on the edge of patterned Si overlayers on GaSe revealed the presence of an inhomogeneous reaction with a metallic-like Ga phase separation. The Si–Se chemically shifted components show lateral variations and a thickness-related SiSe2/SiSex (x=0.5, 1, and 1.5) ratio on a micrometer scale. The dependence of the peak intensities on the overlayer thickness suggests an initial layer-by-layer coverage until ≈2 Å, followed by a clustering growth mode.

The Soft X-ray Scanning Photoemission Microscopy Project at SRRC.

The Synchrotron Radiation Research Center (SRRC) and the Institute of Atomic and Molecular Sciences (IAMS) have initiated a project to construct a scanning photoelectron spectromicroscopy end station at SRRC (SRRC-SPEM). High-brightness soft X-rays will be provided by the U5 undulator beamline. Zone-plate-based soft X-ray optics will be used to focus the beam to form the microprobe. A hemispherical sector analyser with multichannel detection capability will collect the photoelectrons. A total of up to 32 images can be acquired concurrently. The apparatus is also equipped with a sample distribution system for in situ sample preparation and characterization in conjunction with other surface spectroscopic techniques.