Photoemission Spectroscopy Investigation Research Articles

Core-shells particles grown in a tubular reactor: Influence of the seeds nature and MPCVD conditions on boron-doped diamond crystalline quality

A trend in the synthesis of Boron Doped Diamond (BDD) materials resides in increasing surface areas to exalt reactivity at the BDD/electrolyte interface. This can be achieved either by surface structuration of BDD films or conformal growth on 3D substrates. Another strategy considers isolated BDD particles, mainly obtained by milling of BDD layers that can be further deposited on surfaces or embedded in a conductive matrix. Here, we propose an alternative method to obtain boron-doped diamond particles using a core-shell approach, which consists in growing diamond coatings on spherical silica templates seeded with nanodiamonds. We investigated the effects of the nature and the density of diamond seeds as well as growth parameters on the crystalline quality of the coatings. The microstructure and the boron incorporation in the diamond shell was investigated by combining Scanning Electron Microscopy (SEM), UV Raman and X-ray Photoemission Spectroscopy (XPS) investigations. Such characterizations were compared to Scanning TEM (STEM) imaging, Energy Dispersive Spectroscopy (EDS) and Electron energy loss spectroscopy (EELS) performed on thin cross-sections of core-shells prepared by focused ion beam (FIB) milling.

Engineering lanthanum into Pt doped CeO2 for Intermediate temperature solid oxide fuel cells

Platinum doped oxides with extremely low utilization of noble metal and excellent catalytic activity have gained attention in high-temperature electrochemical cells. However, platinum ions tend to be reduced in reducing atmospheres, resulting in the catalyst deactivation. In this work, lanthanum is introduced into Pt-CeO2, which aims to deal with the above problem and provides an approach to promote Pt-CeO2 based catalysts for intermediate temperature solid oxide fuel cells. In specific, lanthanum doped Pt-CeO2 catalysts are prepared and infiltrated into the anode. The results show the highest peak power density with 10 mol% lanthanum doped Pt-CeO2 among all samples. In-situ ambient pressure X-ray photoemission spectroscopy investigation reveals that the reducibility of CeO2 can be tailored by lanthanum dopants, which leads to exceptional stability of active Pt2+ in reducing atmospheres. Our results foster the development of more stable and inexpensive Pt-CeO2 for intermediate temperature solid oxide fuel cells.

Chromium-Doped Nickel Cobaltite Nanoneedles as a Cathodic Material for Li–O2 Cells: An X-ray Photoemission and Photoabsorption Spectroscopy Investigation

Chromium-Doped Nickel Cobaltite Nanoneedles as a Cathodic Material for Li–O₂ Cells: An X-ray Photoemission and Photoabsorption Spectroscopy Investigation

Exploring the Charge Density Wave Phase of 1T-TaSe_{2}: Mott or Charge-Transfer Gap?

1T-TaSe_{2} is widely believed to host a Mott metal-insulator transition in the charge density wave (CDW) phase according to the spectroscopic observation of a band gap that extends across all momentum space. Previous investigations inferred that the occurrence of the Mott phase is limited to the surface only of bulk specimens, but recent analysis on thin samples revealed that the Mott-like behavior, observed in the monolayer, is rapidly suppressed with increasing thickness. Here, we report combined time- and angle-resolved photoemission spectroscopy and theoretical investigations of the electronic structure of 1T-TaSe_{2}. Our experimental results confirm the existence of a state above E_{F}, previously ascribed to the upper Hubbard band, and an overall band gap of ∼0.7 eV at Γ[over ¯]. However, supported by density functional theory calculations, we demonstrate that the origin of this state and the gap rests on band structure modifications induced by the CDW phase alone, without the need for Mott correlation effects.

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Evidence of a topological edge state in a superconducting nonsymmorphic nodal-line semimetal

Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators (2DTIs) and their stacked three-dimensional (3D) equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose layered superconducting material CaSn as a WTI with nontrivial Z2 as well as nodal line semimetal protected by crystalline non-symmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunnelling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement and corresponding ab initio calculation strongly support the existence of TES in the non-symmorphic nodal line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.

Evidence of direct electronic band gap in two-dimensional van der Waals indium selenide crystals

Metal monochalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photoresponse. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy, we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the \ensuremath{\Gamma} point of the Brillouin zone. STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction-band minimum. This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle-resolved photoemission spectroscopy (ARPES) investigation. In fact, a hole effective mass of about ${m}^{*}/{m}_{0}=\ensuremath{-}0.95$ ($\overline{\mathrm{\ensuremath{\Gamma}}K}$ direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic technologies.

Metal-Organic Framework Co-MOF-74-Based Host-Guest Composites for Resistive Gas Sensing.

Increasing demands in the field of sensing, especially for gas detection applications, require new approaches to chemical sensors. Metal–organic frameworks (MOFs) can play a decisive role owing to their outstanding performances regarding gas selectivity and sensitivity. The tetrathiafulvalene (TTF)-infiltrated MOF, Co-MOF-74, has been prepared following the host–guest concept and evaluated in resistive gas sensing. The Co-MOF-74-TTF crystal morphology has been characterized via X-ray diffraction and scanning electron microscopy, while the successful incorporation of TTF into the MOF has been validated via X-ray photoemission spectroscopy, thermogravimetric analysis, UV/vis, infrared (IR), and Raman investigations. We demonstrate a reduced yet ample uptake of CO2 in the pores of the new material by IR imaging and adsorption isotherms. The nanocomposite Co-MOF-74-TTF exhibits an increased electrical conductivity in comparison to Co-MOF-74 which can be influenced by gas adsorption from a surrounding atmosphere. This effect could be used for gas sensing.

Deep levels in the MBE ZnO:As/n-GaN diodes – Photoluminescence, electrical properties and deep level transient spectroscopy

In this paper the current understanding of defects in acceptor doped ZnO is briefly reviewed. The results of investigations of ZnO:As/n-GaN heterojunctions which have been successfully fabricated by plasma assisted molecular beam epitaxy method are presented. The electrical properties of the junctions, as well as deep levels, have been studied by means of current-voltage (I-V), capacitance-voltage (C-V) characteristics and deep level transient spectroscopy (DLTS). Electrical measurements were supplemented by photoluminescence- (PL) along with secondary ion mass spectrometry (SIMS) and X-ray photoemission spectroscopy (XPS) investigations. The I-V measurements allowed for detailed analysis of current mechanisms, through which the presence of trap states in the investigated junction was confirmed. The C-V results proved that the depletion region of the diode is located within the ZnO:As layer. The DLTS measurements revealed the presence of three hole trap related signals. The activation energies and capture cross sections of these traps were determined and their possible origin has been ascribed. Obtained results allowed for better identification of defects found in the case of the analyzed heterostructure, as well as the studies of its electronic properties.

Chemical characterization of fluorinated/hydrogenated mixed monolayers grafted on gold nanoparticles

Perfluorocarbons are omniphobic compounds as a consequence of their unique properties such as, e.g., high electronegativity, low polarizability, high ionization energy, and low surface tension. The possibility of combining such properties with the features of nanoscale materials promoted a recent upsurge in the production of fluorinated nanoparticles for application in different fields. Attainment of mixed fluorinated/hydrogenated stabilizing layers is also desired in some cases, but often the composition of the ligand mixture used in the exchange reaction is not reflected in the resulting monolayer. Thus, methodologies for determining the resulting chemical composition of the final mixed fluorinated/hydrogenated monolayers grafted on the nanoparticle (NP) surface are needed. Here we propose a combined Fourier Transform Infrared (FTIR) and X-ray Photoemission Spectroscopy (XPS) investigation for assessing the final chemical composition of a mixed monolayer of hydrogenated and fluorinated alkanethiols grafted on the surface of gold NPs obtained through two different synthetic routes. This analysis allowed the evaluation of the exchange efficiency of the two synthetic routes, and also, coupled with UV–vis studies, highlighted that the colloidal stability of the final NPs was not only dependent on the fluorinated/hydrogenated ratio but also on the chosen exchange route, which seems to influence the ligand organization on the mixed monolayer.

Effects of Co for Mn substitution on the electronic properties of Mn2-xCoxVAl as probed by XPS

Detailed theoretical and experimental investigations on the electronic and magnetic properties of the Heusler compounds Mn2-xCoxVAl with L21 structure are presented. The samples have been examined by X-ray diffraction, magnetization measurements and X-ray photoemission. The magnetic measurements show that the Curie temperatures decrease with Co content, ranging between 771 K (x = 0) and 289 K (x = 1), while the magnetization of the samples follows the Slater-Pauling rule.Additionally, electronic band structure calculations using the Korringa-Kohn-Rostoker (KKR) Greens function method have been performed. The evolution of the half-metallic character with the Co content x in the Mn2-xCoxVAl alloys is discussed based on the density of states (DOS) calculations. The influence of the Co doping on the individual magnetic moments of the Mn and V has been evidenced. Also, the small individual spin moments of Mn and V determined by theoretical calculations in MnCoVAl (x = 1) are confirmed by the Mn 3s and V 3s core level analysis of the X-ray photoemission spectroscopy investigations.

Effects of Octahedral Ni2+ on Structural and Transport Properties of NixFe3-xO₄ Thin Films

Polycrystalline NixFe<SUB>3-x</SUB>O₄ thin films prepared by using a sol-gel process exhibited phase-pure spinel (Fd3m) structure for the Ni composition up to x = 1.0. X-ray photoemission spectroscopy (XPS) and Raman spectroscopy investigations revealed that Ni ions mostly have valence of +2 and occupy the octahedral sites of the cationic sublattice. The NixFe<SUB>3-x</SUB>O₄ films exhibited higher electrical resistivity than that of Fe₃O₄ below 300 K. The increase in the resistivity is primarily ascribed to decrease in Fe<SUP>2+</SUP>-Fe<SUP>3+</SUP> polaronic hopping rate in the octahedral sites due to the increase in the octahedral Ni<SUP>2+</SUP> population.

Advanced Photoemission Spectroscopy Investigations Correlated with DFT Calculations on the Self-Assembly of 2D Metal Organic Frameworks Nano Thin Films.

Metal-organic frameworks (MOFs) deposited from solution have the potential to form 2-dimensional supramolecular thin films suitable for molecular electronic applications. However, the main challenges lie in achieving selective attachment to the substrate surface, and the integration of organic conductive ligands into the MOF structure to achieve conductivity. The presented results demonstrate that photoemission spectroscopy combined with preparation in a system-attached glovebox can be used to characterize the electronic structure of such systems. The presented results demonstrate that porphyrin-based 2D MOF structures can be produced and that they exhibit similar electronic structure to that of corresponding conventional porphyrin thin films. Porphyrin MOF multilayer thin films were grown on Au substrates prefunctionalized with 4-mercaptopyridine (MP) via incubation in a glovebox, which was connected to an ultrahigh vacuum system outfitted with photoelectron spectroscopy. The thin film growth process was carried out in several sequential steps. In between individual steps the surface was characterized by photoemission spectroscopy to determine the valence bands and evaluate the growth mode of the film. A comprehensive evaluation of X-ray photoemission spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and inverse photoemission spectroscopy (IPES) data was performed and correlated with density functional theory (DFT) calculations of the density of states (DOS) of the films involved to yield the molecular-level insights into the growth and the electronic properties of MOF-based 2D thin films.

X-ray photoemission spectroscopy investigation of CaTiO3:Eu for luminescence property: effect of Eu3+ ion

Charge compensation of on-site Eu 4f–5d transition that determines the luminescent performance was confirmed with valance band spectrum. Influence of photoelectrons from CaTiO3: Eu to the corresponding luminescent performance was discussed based on the crystal structure, doping level and the relations to their luminescent property. This paper is important to further optimize the luminescent performance for improving the efficiency and reducing the cost in light emitting diode industry.

Intercorrelation of Electronic, Structural, and Morphological Properties in Nanorods of 2,3,9,10-Tetrafluoropentacene.

We evidence the intercorrelation of electronic, structural, and morphological properties in nanorods of a substituted fluorine-based pentacene, 2,3,9,10-tetrafluoropentacene, deposited on gold single crystals by using photoemission and X-ray absorption spectroscopy investigations. Our investigations show changes in the XPS spectroscopy lines, and NEXAFS features correlate with the specific structure of the assemblies and their morphology. Consequently, the chemical structure affects not only the molecular electronic structure and the way the molecules assemble in a film but also the film morphology leading to specific thin film electronic properties.

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface

Although physisorption is a widely occurring mechanism of bonding at the organic/metal interface, contradictory interpretations of this phenomenon are often reported. Photoemission and X-ray absorption spectroscopy investigations of nanorods of a substituted pentacene, 2,3,9,10-tetrafluoropentacene, deposited on gold single crystals reveal to be fundamental to identify the bonding mechanisms. We find fingerprints of a fractional charge transfer from the clean metal substrate to the physisorbed molecules. This phenomenon is unambiguously recognizable by a nonrigid shift of the core-level main lines while the occupied states at the interface stay mostly unperturbed, and the unoccupied states experience pronounced changes. The experimental results are corroborated by first-principles calculations.

Effect of temperature on the initial stages of oxidation of γ-Al4Cu9(110)

Combined X-ray photoemission spectroscopy and low energy electron diffraction investigations have been performed to study the influence of temperature on the first stages of oxidation of the (110) surface of γ-Al4Cu9 complex metallic alloy in the pressure range 2–6×10−7mbar. The oxidation mechanism is driven by aluminium segregation. At 115K, a pure aluminium oxide film is formed, which does not present any long range order. CuO bonds are never observed. The lower oxidation kinetics compared to that previously observed at room temperature is due to blocking of the oxygen dissociation and/or a decrease in the aluminium segregation. However, aluminium segregation is still observed, even for this very low temperature, due to the low energy value of the activation energy for aluminium diffusion in γ-Al4Cu9 (0.65±0.12eV) which is related to the presence of two vacancies in the crystal structure. At 925K, fast aluminium segregation and possible oxygen diffusion into the bulk leads to the formation of domains or clusters of γ-like alumina either on top of or, more likely, below the surface.

Fabrication and Characterization of Anodic Films onto the Type-304 Stainless Steel in Glycerol Electrolyte

Porous films without periodic structuration were formed by direct current anodizing the surface of type-304 stainless steel in glycerol solution containing NH4F and H2O at elevated temperatures. Formed in the solutions containing both NH4F and H2O at the constant voltage mode of up to 50 V, these films are thin (∼1.0–1.2 μm) and yellow, whereas those formed at higher voltages, ca. 60–70 V, are thicker (up to 15 μm) and greenish. Note that these porous films are composed of both amorphous and crystalline species. X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and X-ray photoemission spectroscopy (XPS) investigations and the detailed chemical analysis of incorporated elements revealed the complex nature of these films varied depending on the anodizing solution composition and voltage value applied. On the basis of the obtained data, the possible mechanism of formation of the glycerol anodic films on the SS304 substrate is presented. Compositional and structural transitions of as-grown films to mate...

X-ray Photoemission Spectroscopy Investigation of the Interaction between 4-Mercaptopyridine and the Anatase TiO2 Surface

In polymer-metal oxide hybrid solar cells, an extremely careful engineering of the interface is required to ensure good device performances. Recently, very promising results have been obtained by functionalizing titanium dioxide (TiO2) by means of 4-mercaptopyridine (4-MPy) molecules, showing the beneficial effect of these molecules on the interface morphology. This study investigates the nature of the interaction of 4-MPy molecules with the TiO2 surface by means of X-ray photoemission spectroscopy. In order to mimic the device processing conditions, our analysis is carried out on molecules adsorbed from solution on a nanocrystalline surface. According to our analysis, 4-MPy molecules (C5H5NS) are likely bound with the oxide through the nitrogen atom. The bonding precedes either via a covalent interaction with Lewis surface sites, or via hydrogen mediation, possibly in the form of hydrogen bonds. Interestingly, in the latter case, we also observe strong changes in the spectroscopic features attributed to the thiol group.

Spectroscopic Evidence for Neutral and Anionic Adsorption of (S)-Glutamic Acid on Ag(111)

We report here on a combined photoemission and vibrational spectroscopy investigation of (S)-glutamic acid adsorption on Ag(111). We show that, in the temperature range 250 K ≤ T ≤ 400 K, non-zwitterionic adsorption takes place and the anionic form prevails at nonvanishing coverage. Significant conformational changes of the self-assembled layer must occur above 300 K, corresponding to a substantial reduction of the sticking probability and a modification of the vibrational spectrum. The similarity of behavior with respect to glutamic acid adsorption on the previously investigated Ag(100) and Ag(110) surfaces is also discussed.