Bulk Sensitive Methods Research Articles

Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy.

The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.

Effect of Three-in-One Surface Modification of Spherical, Co-Free Li-Rich Cathode Material for Li-Ion Batteries (Li1.2Mn0.6Ni0.2O2) with Citric Acid

The electrochemical activation of Li2MnO3 domains in Li- and Mn-rich layered oxides (LRLO) is highly important, and can be tuned by surface modification of the active materials to improve their cycling performance. In this study, citric acid was employed as a combined organic acid, reducing agent, and carbon precursor in order to remove surface residues from the calcination process, implement an oxygen deficient layer on the surface of the primary LRLO particles, and cover their surface with a carbon-containing coating after a final annealing step. A broad selection of bulk and surface sensitive characterization methods was used to characterize the post-treated spherical particles, providing the evidence for successful creation of an oxygen deficient near-surface region, covered by carbon-containing deposits. Post-treated materials show enhanced electrochemical discharge capacities after progressive Li2MnO3 activation, reaching maximum capacities of 247 mAh g−1. Gassing measurements reveal the suppression of oxygen release during the first cycle, concomitant with an increased CO2 formation for the carbon-coated materials. The voltage profile analysis in combination with post-mortem characterization after 300 cycles provide insights into the aging of the treated materials, which underlines the importance of the relationship between structural changes during scalable post-treatment and the electrochemical performance of the powders.

Magneto-optical detection of topological contributions to the anomalous Hall effect in a kagome ferromagnet

A single ferromagnetic kagome layer is predicted to realize a Chern insulator with quantized Hall conductance, which upon stacking can become a Weyl-semimetal with large anomalous Hall effect (AHE) and magneto-optical activity. Indeed, in the kagome bilayer material Fe$_3$Sn$_2$, a large AHE was detected, however, it still awaits the direct probing of the responsible band structure features by bulk sensitive methods. We measure the optical, both diagonal and Hall, conductivity spectra over a broad spectral range and identify the origin of the intrinsic AHE with the help of momentum- and band-decomposed first-principles calculations. We find that low-energy transitions, tracing "helical volumes" in momentum space reminiscent of the formerly predicted helical nodal lines, substantially contribute to the AHE, which is further increased by contributions from multiple higher-energy interband transitions. Our study also reveals that local Coulomb interactions lead to band reconstructions near the Fermi level.

Depth-resolved analysis of the effect of RbF post deposition treatment on CIGSe with two different Cu concentrations

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGSe) absorbers are among the most attractive non-Si alternatives. The key to their steadily increasing efficiency is a post-deposition treatment (PDT) with alkali salts. For co-evaporated CIGSe, a RbF-PDT was demonstrated as the most efficient, however, the mechanism of the RbF influence on the CIGSe absorber is not completely understood. Here, we focus on the impact of RbF on the surface of co-evaporated CIGSe absorbers in dependence on their bulk composition. Surface-as well as bulk-sensitive methods with overlapping information depths are used to examine an overall depth profile of RbF-free and RbF-treated CIGSe samples with different Cu contents. We show a gradual depletion of copper towards the surface for the as-deposited CIGSe absorber layers. The following RbF-PDT sharpens this effect especially for the sample grown with the overall lower Cu content. Under the Cu-depleted surface layer, the composition of the CIGSe gradually changes until it reaches the respective bulk composition. As a result of the RbF-PDT, Ga diffuses towards the surface and Rb gets incorporated at the surface where GaF3 and RbInSe2 secondary phases are formed, respectively. A higher Cu content leads to less surface oriented Ga diffusion, less Rb incorporation and to a thinner RbInSe2 layer. A thinner RbInSe2 barrier layer, in turn, maintains the gain in open-circuit voltage and prevents the fill factor loss of the CIGSe device.

Reversible oxidation–reduction of epitaxial iron oxide films on Pt(111): Magnetite–hematite interconversion

We studied magnetite-hematite-magnetite transformations in an ultrathin epitaxial film on Pt(111) using surface sensitive and bulk sensitive methods. At initial oxidation stages at elevated temperature (810 K), a 5-nm thick magnetite Fe3O4(111) film became non-stoichiometric toward maghemite and then showed the first signs of hematite phase formation under an oxygen exposure of 3 × 103 L. Finally, under 2 × 104 L, the film fully transformed to hematite α-Fe2O3(0001), maintaining a high single-crystal quality. A comparison of the conversion electron Mössbauer spectra and low energy electron diffraction pattern showed that at intermediate oxidation stages, hematite dominated at the surface, whereas a spinel phase was still observed in the deeper layers. The magnetite-hematite conversion was fully reversed by annealing under ultra-high vacuum at temperatures exceeding 600 K, and despite a change in morphology, the magnetite film preserved the original crystal structure and orientation.

Chemical Composition and Micro Morphology of Golden Laminae in the Wall Painting “La Maestà” by Simone Martini: A Study by Optical Microscopy, XRD, FESEM-EDS and ToF-SIMS

The chemical characterization of gilding decorations in works of art is fundamental in order to elucidate the techniques and materials used by the artists. In the present work we investigated by a combination of bulk and surface sensitive methods the composition and micro stratigraphy of the gilding laminae in the wall painting of the 14th century “La Maestà”, the masterpiece of Simone Martini. The aim of this study was to determine the composition of the gilding leaves and of the adhesive organic materials used to glue the metallic decorations to the wall painting. Due to the altered state of the samples we could not univocally identify the nature of the adhesive materials. Time of flight secondary ion mass spectrometry measurements showed that the gilding layers consisted of a gold leaf which was laid either directly on a preparation layer or on a tin lamina. The high sensitivity of ToF-SIMS and its spatial resolution allowed us to find traces of silver in the gold leaves and in the tin laminae which were not revealed by energy dispersive X-ray analysis.

Probing ligand removal and ordering at quantum dot surfaces using vibrational sum frequency generation spectroscopy

HypothesisControlling nanomaterial interfaces for emerging technologies has driven the need to understand the molecular species located there; however, challenges arise using traditional analytical techniques to directly characterize the molecular structure and local environments of these interfacial species due to their low relative populations. We hypothesized that vibrational sum frequency generation (vSFG) spectroscopy would be uniquely sensitive to the chemical modification of nanoparticle surfaces that is obscured using traditional bulk sensitive methods. ExperimentsOctadecylamine ligands were removed from model CdSe quantum dot surfaces using a common precipitation-resuspension procedure with polar protic and aprotic nonsolvents. Vibrational spectra of the ligands at the surface were collected with vSFG to directly probe the ligand ordering and coverage. Photoluminescence (PL), optical absorption, NMR, and mass spectrometry measurements were conducted for comparison. FindingsvSFG was found to be sensitive to subtle changes in ligand disorder over multiple precipitation-resuspension washes, and a limit to the number of ligand molecules removed from the surface and subsequent amount of disorder introduced to their packing was clearly observed. We also find that nonsolvents do not remain associated with the surface after washing.

A Study of Ziegler-Natta Propylene Polymerization Catalysts by Spectroscopic Methods.

Ziegler–Natta polymerization catalysts were characterized by a complex of surface- and bulk-sensitive methods (DRIFTS, XPS, ESR, and XAS = XANES + EXAFS). A diffuse-reflectance Fourier-transform IR spectroscopy (DRIFTS) study showed the presence of strong Lewis acid sites in different concentrations and absence of strong basic sites in the polymerization catalysts. X-ray photoelectron spectroscopy (XPS), electron-spin resonance (ESR), and (X-ray absorption near-edge structure (XANES) analysis revealed the presence of Ti4+, Ti3+, Ti2+, and Ti1+ species in the surface layers and in the bulk of catalysts. The samples under study differ drastically in terms of the number of ESR-visible paramagnetic sites. The EXAFS study shows the presence of a Cl atom as a nearest neighbor of the absorbing Ti atom.

Effect of Cu and In content in precursor solution on the structural and optical properties of CuInS2 in CH ordered nanostructured films

CuInS2 nanostructured films were prepared by Ultrasonic spray pyrolysis (USP) method. Variation of structural and optical properties of the CuInS2 films including Cu and In in the precursor solution were investigated. The films were found to be p-type by using hot probe method. Raman Spectroscopy and X-ray diffraction (XRD) methods have been chosen to study the structural properties of the films as Raman spectroscopy allows the investigation of the near surface region, while XRD is a bulk-sensitive method. Optical properties were obtained from UV–vis absorption, transmittance and reflectance spectra of the nanostructured CuInS2 films. The films have polycrystalline structure with a preferential growth along the (112) plane. The grain size of all the films was found to be lower than 15nm. The Raman peak at 296cm−1 has been assigned to the A1 mode of the CuInS2 in CH-ordered. Absorption coefficient of the films have increased by increasing photon energy and decreasing wavelength. The optical constants of the films such as refractive index, extinction coefficient as well as the real and imaginary parts of dielectric constant have changed by increasing the In content of the precursor solution.

Role and Optimization of the Active Oxide Layer in TiO2‐Based RRAM

TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X‐ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post‐deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X‐ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.

Significance of xylan on the stability and water interactions of cellulosic nanofibrils

In this paper, the significance of xylan on the behaviour of kraft birch pulp based nanofibrillated cellulose (CNF) is discussed. The influence of CNF xylan content on fibril morphology, charge and stability as well as on the film formation ability was investigated, and the features detected on nanoscale and on macroscale are compared. In addition to this, the ability of fibrils to uptake water molecules were investigated by bulk and surface sensitive methods which are dynamic water sorption analysis (DVS) and quartz crystal microbalance with dissipation monitoring (QCM-D) equipped with the humidity module, respectively. Surface xylan plays a significant role as an electrosteric stabilizer in dilute CNF dispersions when the surface forces are dominant whereas the removal of xylan drastically changes the CNF dispersion properties. The settling of the unstable CNF dispersions displays behaviour which is typical for hindered sedimentation. When considering thin nanoscale layers of CNF, nanofibrillated cellulosic materials with high content of surface xylan has somewhat higher ability to bind water molecules. However, it seems that in more concentrated CNF dispersions where the fibrillar network itself plays also a decisive role, especially with respect to film formation ability, the impact of xylan diminishes. Solvent cast macroscale CNF films are still enough dense to maintain good oxygen barrier performance at higher humid conditions although agglomeration tendency of fibrils is higher due to the excessive xylan removal. These findings are of high relevance when considering nanocellulosic materials, especially in the form of gels and films, as templates for high added value material solutions.

Direct observation of half-metallicity in the Heusler compound Co2MnSi

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

Influence of the surface potassium species in Fe–K/Al2O3 catalysts on the soot oxidation activity in the presence of NOx

Cordierite monolith-supported and powder Fe–K/Al2O3 catalysts were prepared and thoroughly characterized by bulk (XRD, Raman spectroscopy, XRF, ICP–OES, TPR) and surface (XPS, IR, N2-BET, NOx-TPD, K-TPD) sensitive methods. The catalytic activity was tested in TPO (temperature programmed oxidation) of model soot. The influence of the calcination temperature, i.e. 450 and 650°C, on the catalysts physicochemical properties and reactivity, type of surface states of K promoter and their role in the soot oxidation mechanism in the presence of NOx was evaluated. In the catalysts calcined at 450°C potassium was found to be mostly as free KNO3 whereas calcination at 650°C successfully transformed KNO3 into K2O, which during the soot oxidation yielded K2CO3 species. Such carbonates underwent decomposition in the presence of the oxygen surface groups of the support, leading to the formation of active basic O2− groups. NOx species were found to adsorb strongly on these O2− sites. The increased stability of thus formed NOx-species resulted in lower soot oxidation activity. In the same time, the catalysts calcined at 650°C evidenced higher ability towards the reduction of NOx, occurring simultaneously with the soot oxidation process. Additionally, the higher calcination temperature led to significantly higher stability of K promoter species as evidenced by potassium thermodesorption experiments. An overall schematic model of the catalyst morphology and relative distribution of the active components (K, Fe) over the Al2O3 support is proposed.

Charging and Discharging Behavior of Solvothermal LiFePO4 Cathode Material Investigated by Combined EELS/NEXAFS Study

LiFePO4 cathode material obtained via solvothermal synthesis was investigated at several charging and discharging stages by a combination of bulk (EELS) and surface sensitive (NEXAFS) methods in order to get insight into the distribution of Li in the surface and bulk region of the particles. Different behavior was observed as a function of the particles morphology. The turbostratic stacking of layers in nanosized crystals induces higher dimensional defects responsible for sluggish Li insertion and extraction within the structure. In contrast, microstructured crystals without higher dimensional defects do not exhibit sluggish Li insertion/extraction, and achieve a higher degree of reversibility during electrochemical cycling.

Dynamic Self-Organization of Supported Pd/Au Catalysts during Vinyl Acetate Synthesis

It is known that vinyl acetate monomer synthesis over bimetallic Pd1Au1 catalysts is highly structurally sensitive and that these structures are dynamically formed and disintegrated in reactant gas atmosphere. Here we show via a combination of bulk and surface sensitive methods that independent of their nominal composition, Pd/Au bimetallic particles undergo marked reconstruction and phase partition during vinyl acetate synthesis. While temperature-induced reorganization of SiO2-supported PdxAuy particles leads to all three thermodynamically metastable phases, i.e., Pd3Au, Pd1Au1, PdAu3, with mainly Au-enriched surface compositions, the reactive atmosphere induces selectively the formation of the Pd1Au1 phase and separated Pd particles. The Pd in the Pd1Au1 has higher specific activity and selectivity than the sole Pd nanoparticles, exemplifying the importance of changing its electronic nature. As the Pd1Au1 phase dominates the catalytic activity, novel tailored catalysts could be produced, limiting the presence of dispersed Pd.

X-ray-Raman-scattering-based EXAFS beyond the dipole limit

X-ray Raman scattering (XRS) provides a bulk-sensitive method of measuring the extended X-ray absorption fine structure (EXAFS) of soft X-ray absorption edges. Accurate measurements and data analysis procedures for the determination of XRS-EXAFS of polycrystalline diamond are described. The contributions of various angular-momentum components beyond the dipole limit to the atomic background and the EXAFS oscillations are incorporated using self-consistent real-space multiple-scattering calculations. The properly extracted XRS-EXAFS oscillations are in good agreement with calculations and earlier soft X-ray EXAFS results. It is shown, however, that under certain conditions multiple-scattering contributions to XRS-EXAFS deviate from those in standard EXAFS, leading to noticeable changes in the real-space signal at higher momentum transfers owing to non-dipole contributions. These results pave the way for the accurate application of XRS-EXAFS to previously inaccessible light-element systems.

Major and trace elements in mouse bone measured by surface and bulk sensitive methods

We present experimental investigations of the mouse bone in its original form and after calcination. We used various techniques, like scanning electron microscopy, secondary neutral mass spectroscopy, particle induced X-ray emission, and X-ray photoelectron spectroscopy. Among the major element’s concentration profiles we also determined the observable trace elements of the mouse bone.

Properties of hybrid organic-inorganic systems: Au nanoparticles embedded into an organic CuPc matrix

The evolution of the morphology and the electronic structure of the hybrid organic-inorganic system composed of gold nanoparticles (NPs) distributed in an organic matrix—copper phthalocyanine (CuPc)—as a function of nominal gold content was studied by transmission electron microscopy and by surface and bulk sensitive spectroscopic methods. The gold atoms deposited onto the CuPc surface diffuse into the organic matrix and self-assemble to NPs. There is no formation of a continuous metallic Au film on top of the CuPc film up to large nominal coverage of about 130 Å considered in the present study. The gold is assembled in well defined NPs with metallic properties.

Growth of epitaxial iron nitride ultrathin film on zinc-blende gallium nitride

The authors report the growth of iron nitride on zinc-blende gallium nitride using molecular beam epitaxy. First, zinc-blende GaN is grown on a magnesium oxide substrate having (001) orientation; second, an ultrathin layer of FeN is grown on top of the GaN layer. In situ reflection high-energy electron diffraction is used to monitor the surface during growth, and a well-defined epitaxial relationship is observed. Cross-sectional transmission electron microscopy is used to reveal the epitaxial continuity at the gallium nitride-iron nitride interface. Surface morphology of the iron nitride, similar to yet different from that of the GaN substrate, can be described as plateau valley. The FeN chemical stoichiometry is probed using both bulk and surface sensitive methods, and the magnetic properties of the sample are revealed.

Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods

A single crystal monolith of La 0.9Sr 0.1FeO 3 and thin pulsed laser deposited film of La 0.8Sr 0.2Fe 0.8Ni 0.2O 3 were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimens were not prepared in situ. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variation in the more surface representative valence band spectra near the Fermi energy. X-ray photoelectron diffraction images reflect the symmetry as expected from bulk iron perovskites. The correlation of spectral details in the valence band photoemission spectra (VB PES) and details of the conductivity during temperature variation suggest that valuable information on electronic structure and transport properties of complex materials may be obtained without in situ preparation.