Scanning Photoelectron Microscopy Research Articles

Scanning photoelectron microscopy discloses the role of Cr and Mo in the selective corrosion of hardmetal grades with Co-Ni binders

Advanced applications of hardmetal (HM) in chemically aggressive ambients call for the improvement of their corrosion resistance, combined with optimal abrasion and toughness properties. This goal is chiefly achieved with binder alloying. In particular, Cr and Mo additions to Co-Ni matrices have proved effective in many cases, but the physico-chemical mechanisms underlying this functional improvement are poorly understood. This situation, on the one hand does not enable the fine-tuning of the composition, and, on the other hand, does not allow to reliably extrapolate the performance of these grades to new ambients. The aim of this work is to propose a molecular-level approach to the understanding of the electrochemical corrosion of HM, based on space-resolved compositional and chemical-state analysis of the surface of HM subjected to controlled electrochemical corrosion. This type of analysis is enabled by synchrotron-based scanning photoelectron microscopy (SPEM) and photoelectron microspectroscopy (μ-XPS). Specifically, we studied a WC-based grade with 15 w% binder with 4.67 Co/Ni ratio, alloyed with 0.78 w% Cr and 0.94 w% Mo, corroded potentiostatically in neutral sulphate aqueous solution in the pseudopassive and transpassive ranges. SPEM and μ-XPS allowed to locate and distinguish the chemical state of the binder and carbide elements in the respective phases and to distinguish the surface stoichiometry brought about by corrosion in different conditions. In particular, we found that pseudopassive conditions lead to the formation of surface films, enriched in W(VI) and Cr(III). Transpassive conditions cause notable surface enrichment in Cr(III) as well as to the formation of mixed Cr-W oxides in the WC regions. Mo is leached from the binder at all potentials investigated, while Mo(0) is present in the WC region under pseudopassive conditions, while transpassivity leads to its oxidative removal. Chloride in the electrolyte favours dissolution of oxidized metals, in particular causing the leaching of Cr(III), while Mo(III) is found at the surface of WC grains.

Effect of Strain and Surface Proximity on the Acceptor Grouping in ZnO.

According to the present knowledge, the level of zinc oxide conductivity is determined by donor and acceptor complexes involving native defects and hydrogen. In turn, recently published low-temperature cathodoluminescence images and scanning photoelectron microscopy results on ZnO and ZnO/N films indicate grouping of acceptor and donor complexes in different crystallites, but the origin of this phenomenon remains unclear. The density functional theory calculations on undoped ZnO presented here show that strain and surface proximity noticeably influence the formation energy of acceptor complexes, and therefore, these complexes can be more easily formed in crystallites providing appropriate strain. This effect may be responsible for the clustering of acceptor centers only in certain crystallites or near the surface. Low-temperature photoluminescence spectra confirm the strong dependence of acceptor luminescence on the structure of the ZnO film.

Synchrotron-Based X-ray Photoelectron Microscopy of LMO/LAGP/Cu Thin-Film Solid-State Lithium Metal Batteries

Solid-state batteries (SSB), characterized by solid-state electrolytes—in particular inorganic ones (ISSE)—are an ideal option for the safe implementation of metallic Li anodes. Even though SSBs with ISSEs have been extensively investigated over the last two decades, they still exhibit a series of technological drawbacks. In fact, mechano-chemical issues, mainly the stability of the electrolyte/anode interface, hinder their widespread application. The present investigation focusses on a thin-film LMO (Lithium-Manganese-Oxide)/LAGP (LiAlGe Phosphate)/Copper, anodeless Lithium-metal battery and explores the morphochemical evolution of the electrode/electrolyte interfaces with synchrotron-based Scanning Photoelectron Microscopy (SPEM) of intact pristine and cycled cells. Chemical images were acquired with submicrometer resolution, to highlight the coupled geometrical and chemical-state changes caused by electrochemical ageing. Geometrical changes of the electrolyte/cathode interface were induced by periodic volume changes, causing de-cohesion of the solid-solid contact, but no chemical-state changes accompany the cathodic damaging mode. Instead, shape changes of the electrolyte/anode region pinpoint the correlation between mechanical damaging with the decomposition of the LAGP ISSE, due to the reduction of Ge, triggered by the contact with elemental Li. The micro-spectroscopic approach adopted in this study enabled the assessment of the highly localized nature of the cathodic and anodic degradation modes in SSB devices and to single out the chemical and mechanical contributions.

Thermal-protective and oxygen-resistant nanocoating using silica-nanocomposites for laser thinning of polymorphic molybdenum ditellurides

Laser irradiation has been investigated as a method of fabricating atomically thin crystals from layered two-dimensional (2D) materials with precise thickness control. However, during laser irradiation, inevitable structural changes can occur in the 2D materials due to the high thermal energy, altering their physical and chemical properties and introducing disorder, which has hindered the study and use of the intrinsic properties of atomically thin materials. In this study, we report a laser thinning method using a new coating material, a layered silica-based nanocomposite (SNC), that preserves the pristine crystal structure of a polymorphic 2D material, MoTe2, down to an atomically thin geometry. We confirmed that the nano-porous coating of layered silica-based nanocomposite (SNC) behaved as a good thermal-protective and oxygen-resistant material during the laser thinning process using various experimental techniques, atomic force microscopy, Raman spectroscopy and scanning photoelectron microscopy with synchrotron radiation. Accordingly, our SNC coating of 2D materials is a promising way to engineer the phase and geometry of polymorphic 2D materials, enabling versatile applications in various optoelectronic, spintronic, and electronic devices.

Operando photoelectron spectromicroscopy of nanodevices: Correlating the surface chemistry and transport in SnO2 nanowire chemiresistors

With size reduction of active elements in microelectronics to tens of nanometers and below, the effect of surface and interface properties on overall device performance becomes crucial. High resolution spectroscopic and imaging techniques provide a metrological route for characterization of these properties relevant to device diagnostics and failure analysis. With its roughly 100 nm spatial resolution, superior surface sensitivity, and approximately 200 meV spectral resolution, scanning photoelectron microscopy (SPEM) stands out as a comprehensive tool to access the surface/interface composition of nanodevices, as well to provide chemical state designations and materials property evolutions upon treatment by thermal, electrical, chemical, radiative and other stimuli. Here we present a SPEM-on-device setup that combines X-ray spectromicroscopy with advanced NIST microhotplate technology to demonstrate new combined analytical and electrical measurements capabilities of this metrology platform for operando nanodevice characterization. Using model integrated SnO2 nanowire (NW) chemiresistor devices, the chemically induced alterations in the chemical state of the nanowire surface are correlated to the observed conductance changes, thus directly testing the receptor and transduction mechanisms for SnO2 NW conductometric chemical sensors.

Soft X-ray spectroscopy study of Fe impurities in Mn[formula omitted]CoxGa Heusler films

Co-substituted Mn3−xCoxGa Heusler films, exhibiting the magnetic anisotropy transition with x, are considered as promising spintronic materials. We have investigated the local electronic structures of Mn3−xCoxGa films as well as the chemical distributions of the substituted ions employing photoemission spectroscopy (PES), scanning photoelectron microscopy (SPEM), soft X-ray absorption spectroscopy (XAS), and soft X-ray magnetic circular dichroism (XMCD). SPEM measurements show that Co ions are distributed homogeneously in Mn3−xCoxGa films. On the other hand, some Fe impurities are detected in the capping layers of Mn3−xCoxGa films. Despite the existence of ferromagnetic metallic Fe impurities, however, it is found that the magnetic anisotropy transition in Mn3−xCoxGa is determined mainly by the substituted Co ions but hardly by Fe impurities. This work demonstrates that, in searching for new functional materials, special attention should be paid on the magnetic Fe impurities that might affect their magnetic properties.

Rational Design on Polymorphous Phase Switching in Molybdenum Diselenide-Based Memristor Assisted by All-Solid-State Reversible Intercalation toward Neuromorphic Application.

In this work, a low-power memristor based on vertically stacked two-dimensional (2D) layered materials, achieved by plasma-assisted vapor reaction, as the switching material, with which the copper and gold metals as electrodes featured by reversible polymorphous phase changes from a conducting 1T-phase to a semiconducting 2H-one once copper cations interacted between vertical lamellar layers and vice versa, was demonstrated. Here, molybdenum diselenide was chosen as the switching material, and the reversible polymorphous phase changes activated by the intercalation of Cu cations were confirmed by pseudo-operando Raman scattering, transmission electron microscopy, and scanning photoelectron microscopy under high and low resistance states, respectively. The switching can be activated at about ±1 V with critical currents less than 10 μA with an on/off ratio approaching 100 after 100 cycles and low power consumption of ∼0.1 microwatt as well as linear weight updates controlled by the amount of intercalation. The work provides alternative feasibility of reversible and all-solid-state metal interactions, which benefits monolithic integrations of 2D materials into operative electronic circuits.

Photoelectron spectromicroscopy analysis of graphene during gate-controlled photo-oxidation process

The analysis of chemical reactions in two-dimensional (2D) materials is becoming increasingly important for device applications. Typically, 2D materials are in the form of small flakes; therefore, local inhomogeneities between or within the flakes should be investigated. In this study, we investigated the gate-controlled UV photo-oxidation reaction of monolayer graphene sheets using field-effect transistor structures. We performed a surface characterization of the photo-oxidized graphene micro-sheets using an original scanning photoelectron microscopy system that enables element-selective electronic and chemical state analysis with a high spatial resolution. We succeeded for the first time in experimentally revealing the progression of the photo-oxidation reaction in graphene from the edge. During the photo-oxidation process, hole doping from the adsorbed molecules occurred first. The reaction proceeded selectively to the edges of the graphene sheets. Defects were then introduced by chemical reactions with adsorbed molecules, and finally the graphene sheets decomposed and disappeared.

Understanding carbide evolution and surface chemistry during deep cryogenic treatment in high-alloyed ferrous alloy

The study investigates the effect of deep cryogenic treatment (DCT) on a high-alloyed ferrous alloy (HAFA) and its effectiveness on carbide evolution and chemical shifts of alloying elements. With ex-situ and in-situ observations ranging from the microscopic to the nanoscopic level, we uncover the atomistic mechanism by which DCT affects carbide precipitation, resulting in a 50% increase in carbide volume fraction. Synchrotron-based scanning photoelectron microscopy provides insight into the agglomeration of carbon during exposure to DCT. We find that Mo plays a crucial role in DCT through its modification of chemical bonding states, which is postulated to originate from the loosely-formed primordial Mo2C carbides formed during exposure to cryogenic temperatures. These in turn provide energetically favorable nucleation zones that accelerate the formation of M7C3 carbides, which serve as intermediate states for the formation of M23C6 carbides, which most strongly impact the mechanical properties. These results are supported by atom probe tomography, showing the preferential formation of Mo-rich M7C3 carbides in DCT samples, resulting from greater solute mobility. This work clarifies the fundamental mechanisms on how DCT affects HAFA, solving a long-elusive problem.

Valence-Band Electronic Structure of ZnO and ZnO:N: Experimental and Theoretical Evidence of Defect Complexes

Undoped and N-doped $\mathrm{Zn}\mathrm{O}$ films are grown by atomic layer deposition under O-rich conditions. Scanning photoelectron microscopy studies carried out on the films cross sections with a state-of-the-art resolution of 130 nm, allowed the study of the electronic structure of individual crystallites. In has been found that crystallites can be divided into two types, that differ in the electronic structure of the valence band. This finding, together with the cathodoluminescence images showing clustering the acceptor- and donor-related emission, unveiled that acceptors and donors are grouped in separate regions of the $\mathrm{Zn}\mathrm{O}$ and $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$ films. Density-functional-theory (DFT) calculations reveal that the complexes involving zinc vacancy, hydrogen, and nitrogen (in the case of $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$) modify the density of states in the valence-band region, so the experimentally observed differences in photoelectron spectra between crystallites evidence the grouping of acceptor complexes in some crystallites. DFT calculations also suggest that shallow acceptor states might be of $n\ensuremath{\cdot}{V}_{\mathrm{Zn}}$ origin. Nitrogen willingly joins such complexes and facilitates their formation. The separation of donors and acceptors and its tendency to group along the growth columns is of great relevance for future applications, as it indicates that eventual electronic devices should rather be constructed in a vertical architecture.

Synthesis and Characterization of MWCNT-COOH/Fe3O4 and CNT-COOH/Fe3O4/NiO Nanocomposites: Assessment of Adsorption and Photocatalytic Performance.

In this study the adsorption and photodegradation capabilities of modified multi-walled carbon nanotubes (MWCNTs), using tartrazine as a model pollutant, is demonstrated. MWCNT-COOH/Fe3O4 and MWCNT-COOH/Fe3O4/NiO nanocomposites were prepared by precipitation of metal oxides in the presence of MWCNTs. Their properties were examined by X-ray diffraction in powder (XRD), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, synchrotron-based Scanning PhotoElectron Microscopy (SPEM), and Brunauer-Emmett-Teller (BET) analysis. It was found that the optimal adsorption conditions were pH 4 for MWCNT-COOH/Fe3O4 and pH 3 for MWCNT-COOH/Fe3O4/NiO, temperature 25 °C, adsorbent dose 1 g L−1, initial concentration of tartrazine 5 mg L−1 for MWCNT-COOH/Fe3O4 and 10 mg L−1 for MWCNT-COOH/Fe3O4/NiO and contact time 5 min for MWCNT-COOH/Fe3O4/NiO and 15 min for MWCNT-COOH/Fe3O4. Moreover, the predominant degradation process was elucidated simultaneously, with and without simulated sunlight irradiation, using thermal lens spectrometry (TLS) and UV–Vis absorption spectrophotometry. The results indicated the prevalence of the photodegradation mechanism over adsorption from the beginning of the degradation process.

Morphology and composition of nanoinclusions in (Fe, Ni)0.25TiSe2

A nanocomposite material consisting of (Fe,Ni)4Se5 and Ti3Se4 nanocrystals coherently bounded with the (Fe,Ni)0.25TiSe2 substrate was obtained during the synthesis of the (Fe, Ni)xTiSe2 bi-intercalation compound from the mixture of Fe0.25TiSe2 and Ni0.25TiSe2. The morphology of these composite crystals has been studied using optical, scanning electron and atomic force microscopy. The electronic structure of the inclusions was studied using X-ray scanning photoelectron microscopy (SPEM). We have shown that (Fe,Ni)4Se5 and Ti3Se4 inclusions grow simultaneously with (Fe,Ni)xTiSe2 substrate crystal on its (001) plane. The lateral growth of the (Fe,Ni)xTiSe2 crystal is accompanied by the “capturing” of the (Fe,Ni)4Se5 and Ti3Se4 inclusions by the (Fe,Ni)0.25TiSe2 substrate resulting in the formation of the composite crystal.

Severe carbon accumulation on black phosphorous flakes induced by synchrotron x-ray radiation

Two-dimensional (2D) materials, such as black phosphorus (BP) and transition metal dichalcogenide, recently gained much interest due to their unique electronic, photonic, and mechanical properties. Synchrotron-based scanning photoelectron microscopy (s-SPEM) with a spatial resolution (down to 70 nm) has actively applied for exploring the relationship between chemical information and electronic structure of 2D materials on concerned local surface area. Here, we compare carbon accumulation on the surface of BP and MoS2 during synchrotron beam irradiation. During the BP characterization, carbon was rapidly accumulated on BP surface as irradiation progress, while MoS2 demonstrated better resistance to carbon accumulation in comparison to BP. Our work suggests that the higher surface reactivity of BP results in the formation of even BP-carbide structure upon the irradiation, while MoS2 with relatively low reactivity is rather stable against carbon accumulation. This photon-induced carbon accumulation causes the misleading result of surface chemical information and electronic structure in highly reactive 2D materials or nanostructures.

Coexisting multi-states in catalytic hydrogen oxidation on rhodium

Catalytic hydrogen oxidation on a polycrystalline rhodium foil used as a surface structure library is studied by scanning photoelectron microscopy (SPEM) in the 10−6 mbar pressure range, yielding spatially resolved X-ray photoemission spectroscopy (XPS) measurements. Here we report an observation of a previously unknown coexistence of four different states on adjacent differently oriented domains of the same Rh sample at the exactly same conditions. A catalytically active steady state, a catalytically inactive steady state and multifrequential oscillating states are simultaneously observed. Our results thus demonstrate the general possibility of multi-states in a catalytic reaction. This highly unusual behaviour is explained on the basis of peculiarities of the formation and depletion of subsurface oxygen on differently structured Rh surfaces. The experimental findings are supported by mean-field micro-kinetic modelling. The present observations raise the interdisciplinary question of how self-organising dynamic processes in a heterogeneous system are influenced by the permeability of the borders confining the adjacent regions.

Direct Observation of Charge Transfer between NOx and Monolayer MoS2 by Operando Scanning Photoelectron Microscopy

Atomically thin transition-metal dichalcogenides (MoS2, WSe2, etc.) have long been touted as promising materials for gas detection because of their tunable band gaps; however, the sensing mechanism...

Near ambient pressure photoelectron spectro-microscopy: from gas–solid interface to operando devices

Near ambient pressure scanning photoelectron microscopy adds to the widely used photoemission spectroscopy and its chemically selective capability two key features: (1) the possibility to chemically analyse samples in a more realistic gas pressure condition and (2) the capability to investigate a system at the relevant spatial scale. To achieve these goals the approach developed at the ESCA Microscopy beamline at the Elettra Synchrotron facility combines the submicron lateral resolution of a Scanning Photoelectron Microscope with a custom designed Near Ambient Pressure Cell where a gas pressure up to 0.1 mbar can be achieved. In this manuscript a review of experiments performed with this unique setup will be presented to illustrate its potentiality in both fundamental and applicative research such as the oxidation reactivity and gas sensitivity of metal oxides and semiconductors. In particular, the capability to perform operando experiments with this setup opens the possibility to study operating devices and to properly address the real nature of the studied systems, because if microscopy and spectroscopy are simultaneously combined in a single technique it can yield to more conclusive results.

How the anisotropy of surface oxide formation influences the transient activity of a surface reaction

Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) allow local surface analysis and visualising ongoing reactions on a µm-scale. These two spatio-temporal imaging methods are applied to polycrystalline Rh, representing a library of well-defined high-Miller-index surface structures. The combination of these techniques enables revealing the anisotropy of surface oxidation, as well as its effect on catalytic hydrogen oxidation. In the present work we observe, using locally-resolved SPEM, structure-sensitive surface oxide formation, which is summarised in an oxidation map and quantitatively explained by the novel step density (SDP) and step edge (SEP) parameters. In situ PEEM imaging of ongoing H2 oxidation allows a direct comparison of the local reactivity of metallic and oxidised Rh surfaces for the very same different stepped surface structures, demonstrating the effect of Rh surface oxides. Employing the velocity of propagating reaction fronts as indicator of surface reactivity, we observe a high transient activity of Rh surface oxide in H2 oxidation. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty of well-defined surface structures within one sample.

Strain Induced Phase Transition of WS2 by Local Dewetting of Au/Mica Film upon Annealing

Here, we present a proof-of-concept experiment where phase engineering at the nanoscale of 2D transition metal dichalcogenides (TMDC) flakes (from semiconducting 2H phase to metallic 1T phase) can be achieved by thermal annealing of a TMDC/Au/mica system. The local dewetting of Au particles and resulting tensile strain produced on the TMDC flakes, strongly bound to the Au surface through effective S-Au bonds, can induce a local structural phase transition. An important role is also played by the defects induced by the thermal annealing: when vacancies are present, the threshold strain needed to trigger the phase transition is significantly reduced. Scanning photoelectron microscopy (SPEM) was revealed to be the perfect tool to monitor the described phenomena.

Origin of Intergranular Li Metal Propagation in Garnet-Based Solid Electrolyte By Direct Electronic Structure Analysis and Performance Improvement By Bandgap Engineering

Garnet-structured oxide electrolyte (Li7La3Zr2O12, LLZO) has the significant advantage of being chemically and electrochemically stable against Li metal, which allows the implementation of Li metal batteries. However, a short-circuit failure by Li penetration through the LLZO electrolyte has remained a crucial issue for safety and is a major hurdle for Li-based batteries to overcome. Here, we investigate the mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (LLZTO) by examining their energy band structures and defect states using reflection electron energy loss spectroscopy (REELS), scanning photoelectron microscopy (SPEM), and nanoscale charge-based deep level transient spectroscopy (Nano Q-DLTS) techniques. The experimental results reveal that the Schottky barrier height (SBH) was lowered by 0.5 eV due to the defect states localized in grain boundaries and the metallic Li propagation along the grain boundaries is caused by the SBH reduction. Based on the analytical results, the laser annealing of LLZTO was performed as a bandgap engineering method to suppress the Li dendrite formation by forming a mixed surface layer of amorphous LLZTO and Li2O, which has a wide bandgap to block the electron injection into the grain boundaries. The electrochemical measurements for the laser treated LLZTO demonstrate that the stability and cycling performance are significantly improved. This study sheds light on the importance of electronic structure, in particular, defect states to develop high performance oxide solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.

Interfacial studies in CNT fibre/TiO2 photoelectrodes for efficient H2 production

An attractive class of materials for photo(electro)chemical reactions are hybrids based on semiconducting metal oxides and nanocarbons (e.g. carbon nanotubes (CNT), graphene), where the nanocarbon acts as a highly-stable conductive scaffold onto which the nanostructured inorganic phase can be immobilised; an architecture that maximises surface area and minimises charge transport/transfer resistance. TiO2/CNT photoanodes produced by atomic layer deposition on CNT fabrics are shown to be efficient for H2 production (0.07 μmol/min H2 at 0.2 V vs Ag/AgCl), nearly doubling the performance of TiO2 deposited on planar substrates, with 100% Faradaic efficiency. The results are rationalised based on electrochemical impedance spectroscopy measurements showing a large reduction in photoelectron transport resistance compared to control samples and a higher surface area. The low TiO2/CNT interfacial charge transfer resistance (10 Ω) is consistent with the presence of an interfacial Ti-O-C bond and corresponding electronic hybridisation determined by spatially-resolved Scanning Photoelectron Microscopy (SPEM) using synchrotron radiation.