Photoelectron Spectromicroscopy Research Articles

Catalytically Active Materials Visualized by Scanning Photoelectron Spectro-Microscopy

Catalytically Active Materials Visualized by Scanning Photoelectron Spectro-Microscopy

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.

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.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

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.

(Invited) Operando Scanning Electron and Photoelectron Spectromicroscopy to Study the Nucleation and Growth Phenomena in Liquid and Reactive Solid Electrolytes: The Technique Development

Nucleation and growth (NG) phenomena play a central role in a variety of applications spanning from the back end of line (BEOL) microfabrication to environmental remediation and energy devices performance. Therefore, the development of analytical metrologies, capable to access these phenomena with the high chemical, spatial and temporal resolution is amongst the key tasks of the research community. Probing nucleation and growth phenomena in situ, using traditional high analytical power tools such as SEM, XPS, AES, SPEM, and PEEM, under realistic sample environments, is a great aim but also an experimental challenge due to the apparent sample-to-detector “pressure gap”. Implementation of the electron transparent membranes that separate the reactive, liquid, or dense gaseous sample environments from the high vacuum instrumentation, in principle, resolves this impediment thus the standard analytical surface science equipment can be used. The sample and or device, therefore, need to be covered or encapsulated with a mechanically robust electron transparent impermeable membrane. The thickness of such a membrane defines the energy range and broadening of the transmitted electrons and, therefore, the thinnest membranes such as those made of 2D materials are desirable. Here, using variable pressure SEM, we exemplify the sensitivity of electrochemically driven NG processes to the composition of the ambient environment. Using model liquid electrolytes, we review the capabilities, advantages, and limitations of the graphene membrane-based liquid cells to probe NG processes with electron spectromicroscopy. In particular, we discuss graphene-cupped single orifice and microchannel arrays (MCA), and their applications for electrochemical polarization, nucleation, and growth studies using SEM, AES, SPEM, and PEEM. In addition to local spectromicroscopy, MCA-like designs enable application of high throughput combinatorial data mining algorithms to collect multi-dimensional/ hyperspectral datasets. The experimental artifacts and major limitation of the technique related to radiation damage of the sample will be discussed.

Nanoscopic correlations from curve fitting of photoelectron spectromicroscopy data cubes of lead zirconate titanate films

Photoelectron spectromicroscopy data cubes with sub-micrometer spatial resolution from a lead zirconate titanate thin film are fitted in each spatial point for several areas investigated, and correlations between the results of the fitting parameters are established, enabling one to suggest that surface band bending manifests also in the case of samples with multiple polarization states. These surface band bendings may be used for preliminary assessment of polarization states by using the contrast in binding energy. However, the individual curve fitting of each spectrum and the analysis of correlations between the derived parameters (binding energies and amplitudes) yields results of deeper significance in deriving the distribution of band bending. This allows in some cases discriminating between the origin of the individual components, assessing charging effects and compensation mechanisms, also enabling one to establish correlations between areas with different polarization. It is found that the major part of the investigated areas consist in regions with outwards polarization and without out-of-plane polarization, with the presence of some isolated metal Pb clusters. On a single area, components attributed to inwards polarization are detected, and their formation occurs at the expense of areas without out-of-plane polarization, mostly for a given proportion of the outwards polarization.

Nanoscopic Correlations from Curve Fitting of Photoelectron Spectromicroscopy Data Cubes of Lead Zirconate Titanate Films

Nanoscopic Correlations from Curve Fitting of Photoelectron Spectromicroscopy Data Cubes of Lead Zirconate Titanate Films

Nanoscopic Correlations from Curve Fitting of Photoelectron Spectromicroscopy Data Cubes of Lead Zirconate Titanate Films

Nanoscopic Correlations from Curve Fitting of Photoelectron Spectromicroscopy Data Cubes of Lead Zirconate Titanate Films

On the Promotion of Catalytic Reactions by Surface Acoustic Waves.

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified. Using stroboscopic X‐ray photoelectron spectro‐microscopy, we were able to evidence a sub‐nanosecond work function change during propagation of 500 MHz SAWs on a 9 nm thick platinum film. We quantify the work function change to 455 μeV. Such a small variation rules out that electronic effects due to elastic deformation (strain) play a major role in the SAW‐induced promotion of catalysis. In a second set of experiments, SAW‐induced intermixing of a five monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be a major cause for catalytic promotion.

Photoelectron Momentum Microscope at BL6U of UVSOR-III synchrotron

Photoelectron spectroscopy resolved in three-dimensional momentum space with a microscopic field of view is realized by combining a so-called Momentum Microscope with a soft X-ray synchrotron radiation source. A new experimental station for momentum-resolved photoelectron micro-spectroscopy and spectro-microscopy has been built at BL6U, an undulator-based soft X-ray beamline of the UVSOR synchrotron facility. This experimental station specializes in characterizing the electronic structure of surface atomic sites, thin films, molecular adsorbates, and bulk crystals. The instrument details are described along with possible measurement techniques.

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.

Highlighting the Dynamics of Graphene Protection toward the Oxidation of Copper Under Operando Conditions.

We performed spatially resolved near-ambient-pressure photoemission spectromicroscopy on graphene-coated copper in operando under oxidation conditions in an oxygen atmosphere (0.1 mbar). We investigated regions with bare copper and areas covered with mono- and bi-layer graphene flakes, in isobaric and isothermal experiments. The key method in this work is the combination of spatial and chemical resolution of the scanning photoemission microscope operating in a near-ambient-pressure environment, thus allowing us to overcome both the material and pressure gap typical of standard ultrahigh-vacuum X-ray photoelectron spectroscopy (XPS) and to observe in operando the protection mechanism of graphene toward copper oxidation. The ability to perform spatially resolved XPS and imaging at high pressure allows for the first time a unique characterization of the oxidation phenomenon by means of photoelectron spectromicroscopy, pushing the limits of this technique from fundamental studies to real materials under working conditions. Although bare Cu oxidizes naturally at room temperature, our results demonstrate that such a graphene coating acts as an effective barrier to prevent copper oxidation at high temperatures (over 300 °C), until oxygen intercalation beneath graphene starts from boundaries and defects. We also show that bilayer flakes can protect at even higher temperatures. The protected metallic substrate, therefore, does not suffer corrosion, preserving its metallic characteristic, making this coating appealing for any application in an aggressive atmospheric environment at high temperatures.

Influence of interface dipole layers on the performance of graphene field effect transistors

The linear band dispersion of graphene's bands near the Fermi level gives rise to its unique electronic properties, such as a giant carrier mobility, and this has triggered extensive research in applications, such as graphene field-effect transistors (GFETs). However, GFETs generally exhibit a device performance much inferior compared to the expected one. This has been attributed to a strong dependence of the electronic properties of graphene on the surrounding interfaces. Here we study the interface between a graphene channel and SiO2, and by means of photoelectron spectromicroscopy achieve a detailed determination of the course of band alignment at the interface. Our results show that the electronic properties of graphene are modulated by a hydrophilic SiO2 surface, but not by a hydrophobic one. By combining photoelectron spectromicroscopy with GFET transport property characterization, we demonstrate that the presence of electrical dipoles in the interface, which reflects the SiO2 surface electrochemistry, determines the GFET device performance. A hysteresis in the resistance vs. gate voltage as a function of polarity is ascribed to a reversal of the dipole layer by the gate voltage. These data pave the way for GFET device optimization.

Tuning electronic properties by oxidation-reduction reactions at graphene-ruthenium interfaces

Mass production of graphene is associated with the growth on catalysts used also in other chemical reactions. We exploit the oxidation-reduction to tailor the properties of single layer graphene domains with incorporated bi-layer patches on ruthenium. Using photoelectron spectromicroscopy techniques, we find that oxygen, intercalating under single layer and making it p-doped by the formation of Ru-Ox, does not intercalate under the bilayer patches with n-doped upper layer, but decorates them under single layer surrounding creating lateral p-n junctions with chemical potential difference of 1.2 eV. O-reduction by thermal treatment in vacuum results in C-vacancy defects enhancing electronic coupling of remained graphene to Ru, whereas in H2, vacancy formation is suppressed. For the domains below 15–25 μm size, after O-reduction in H2, graphene/Ru coupling is restored, while wrinkle pattern produced by O-intercalation is irreversible and can trap reaction products between the wrinkles and Ru surface step edges. In fact, in certain regions of bigger domains, the products, containing H2O and/or its fragments, remain at the interface, making graphene decoupled and undoped.

Unraveling the Structural and Electronic Properties at the WSe2–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe2 thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe2 crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe2 islands nucleate from defective WSex seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe2 islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe2 and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe2–gr...

In situ chemical and morphological characterization of copper under near ambient reduction and oxidation conditions

By using spatially resolved photoelectron spectromicroscopy, we have investigated the formation of Cu(I) oxide phases when a bulk Cu foil is exposed to a near ambient pressure of molecular oxygen. The experiment was performed by using a novel cell for in operando measurements capable to overcome the so called ‘pressure gap’. We have found that the oxidation of Cu proceeds through the formation of Cu(I) oxide domains in the micron and submicron range size, the evolution of which affects the overall morphology of the Cu surface as well. Chemical and morphological changes have been simultaneously detected. It was, surprisingly, found that during the oxidation process in many areas domains having +1 valence state do not spread on the metallic regions which homogenously oxidize; no chemical oxidizing wave was detected by the available lateral resolution and time frame.

Fluorine and sulfur simultaneously co-doped suspended graphene

Suspended graphene flakes are exposed simultaneously to fluorine and sulfur ions produced by the μ-wave plasma discharge of the SF6 precursor gas. The microscopic and spectroscopic analyses, performed by Raman spectroscopy, scanning electron microscopy and photoelectron spectromicroscopy, show the homogeneity in functionalization yield over the graphene flakes with F and S atoms covalently bonded to the carbon lattice. This promising surface shows potential for several applications ranging from biomolecule immobilization to lithium battery and hydrogen storage devices. The present co-doping process is an optimal strategy to engineer the graphene surface with a concurrent hydrophobic character, thanks to the fluorine atoms, and a high affinity with metal nanoparticles due to the presence of sulfur atoms.

Electronic structure of self-doped layeredEu3F4Bi2S4material revealed by x-ray absorption spectroscopy and photoelectron spectromicroscopy

We have studied the electronic structure of ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$ using a combination of Eu ${L}_{3}$-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu ${L}_{3}$-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$. The bulk charge doping was estimated to be $\ensuremath{\sim}0.3$ per Bi site in ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$, which corresponds to the nominal $x$ in a typical RE${\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$ system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the $X$ point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

Polarization landscape effects in soft X-ray-induced surface chemical decomposition of lead zirco-titanate, evidenced by photoelectron spectromicroscopy.

The stability of thin films of lead zirco-titanate (PZT) under intense soft X-ray beams is investigated by time-resolved photoelectron spectromicroscopy with a lateral resolution below 1 micrometer. Surface dissociation is observed when samples are irradiated with intense (5 × 1023 photons per s per m2) soft X-rays, with promotion of reduced lead on the surface. On areas exhibiting outwards polarization (P(+)), the reduced lead is formed at the expense of P(+)-PZT. On areas presenting co-existing P(+) states with areas without out-of-plane polarization (P(0)), the reduced lead is formed at the expense of the P(0)-PZT component, while the P(+)-PZT remains constant. The main dissociation mechanism was found to be triggered by 'hot' electrons in the conduction band, with energies exceeding the surface dissociation energies. Dissociation occurs basically when the electron affinity is larger than the dissociation energy of PbO (for P(+) areas) or PbO- (for P(0) areas). Such mechanisms may be adapted for dissociation of other molecules on surfaces of ferroelectric thin films or for quantifying the stability of ferroelectric surfaces interacting with other radiation, with applications in the fields of photocatalysis or photovoltaic devices.