X-ray Excited Auger Electron Spectroscopy Research Articles

Achieving superlubricity using selected tribo-pairs lubricated by castor oil and unsaturated fatty acids

Systematic friction tests have been performed to unveil the key factors for reaching superlubricity (CoF < 0.01) in the presence of unsaturated fatty acid-based lubricants under boundary lubrication regime. The impact of different tribo-pairs and lubricants has been investigated. When the counterpart is steel, results show that a hydrogen-free DLC coating is indispensable to achieve superlubricity in castor oil. Among pure unsaturated fatty acids as lubricants, oleic and ricinoleic acid gave superlow friction but linoleic acid is not recommended to lubricate steel/a-C contact due to the occurrence of high friction and wear. X-ray photoelectron spectroscopy (XPS) and X-Ray-Excited Auger Electron Spectroscopy (XAES) analyses reveal that hydrogen-free carbon can activate two pathways to passivate surface and to achieve superlubricity with fatty acid-based lubricants (i) carbon re-hybridization by forming a high-density sp2 carbon network and (ii) –(CH2-CH2)n– oligomers formation on top surface. At the opposite, linoleic acid hinders the formation of –(CH2-CH2)n– oligomers and oxidizes carbon surfaces.

Sp-based thin films synthesized by magnetron sputtering of dehydrohalogenated Polyvinylidenchloride

We present a modified approach to the synthesis of sp-based films. This technique includes two stages: the dehydrohalogenation of polyvinylidene chloride powder and its subsequent sputtering. The synthesized films were characterized by Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray excited Auger electron spectroscopy. The films were shown to consist of sp-hybridized carbon chains and nanocrystalline graphite. Sp-carbon evaporation and sp-hybridized clusters sputtering mechanisms leading to sp-clusters transfer from an sp-rich target to the growing film surface were discussed.

Fe2O3-WO3 and Fe2O3-CuO nanoheterostructures by XPS

The detrimental effects of gaseous nitrogen oxides (NOx, with x = 1 and 2) on both human health and the environment have triggered efforts aimed at the development of solar-activated photocatalysts for their efficient removal. In this regard, Fe2O3-WO3 and Fe2O3-CuO nanoheterostructures were prepared by a two-step vapor phase route. In particular, chemical vapor deposition (CVD) of β-Fe2O3, a scarcely investigated iron(III) oxide polymorph, was followed by radio frequency-sputtering of WO3 or CuO under mild conditions. The adopted strategy enabled the obtainment of nanoheterostructures with a peculiar pyramidal morphology and a uniform dispersion of CuO or WO3 onto the pristine iron(III) oxide. In this work, the chemical composition of the target systems was investigated by means of x-ray photoelectron and x-ray excited-Auger electron spectroscopies. In addition to the identification of elemental chemical states, the reported results confirmed the formation of pure and O-deficient systems, in which the direct interplay between the single components opens the door to air purification using the developed systems as photocatalysts.

(Invited) Chalcopyrites for Solar Water Splitting: Soft X-Ray and Electron Spectroscopy of the Chemical and Electronic Surface Structure

In this presentation, it will be demonstrated that the combination of soft x-ray and electron spectroscopies (using lab-based surface characterization and high-brilliance synchrotron radiation) can be used to gain insights into the electronic and chemical surface properties of Cu(In,Ga)(S,Se)2-based chalcopyrite devices for solar water splitting. It will be shown that x-ray and UV photoelectron spectroscopy (XPS and UPS), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band edge positions, band gaps, work functions, and electronic level alignments. Furthermore, insights into chemical stability can be gained, both with experiments in ultra-high vacuum, as well as under in situ and operando conditions.

Utilization of a dicopper(II) complex of tetrapyridyl ligand as the precursor for the synthesis of copper-based composites and their catalysis

Herein reported are the catalytic properties of the composites derived from complex [Cu2(tpbn)(CH3OH)4(ClO4)2](ClO4)2 (1) of tetrapyridyl ligand N,N,N′,N′-tetrakis(2-pyridyl-methyl)-1,4-butanediamine (tpbn). Complex 1 was a precursor of the composites catalytically active for water oxidation and CO2 reduction reactions. Under the anodic potential of +1.20 V vs. Ag/AgCl, composites deposited on tin-doped indium oxide (ITO) electrodes from the solutions of complex 1 at pH 9.0–13.5 are highly active for water oxidation reaction (WOR). In contrast, under the cathodic potential of −1.20 V vs. Ag/AgCl, the species deposited on the copper surface from the solutions of complex 1 at pH 6.7 can efficiently catalyze CO2 reduction to CO. Scanning electron microscopy (SEM) showed that the composites on ITO and Cu surfaces were particulate materials. X-ray photoelectron spectroscopy (XPS) and x-ray-excited Auger electron spectroscopy (XAES) analysis revealed that the composites mainly contain cuprous oxide (Cu2O) and copper oxide (CuO) in different ratios, which showed different catalytic activity to water oxidation and CO2 reduction reactions in copper-free aqueous solutions under low overpotentials. This work demonstrated that metal complexes are good candidates for preparing robust, noble-metal-free catalysts for reactions relevant to renewable energy sources.

ZnO-based nanocomposites prepared by a vapor phase route, investigated by XPS

Nanostructured materials based on ZnO, eventually functionalized with titanium oxide (TiO2) or tungsten oxide (WO3), were fabricated on fluorine-doped tin oxide-coated glass substrates by a combined chemical vapor deposition/radio frequency-sputtering route. In particular, the present work focuses on the use of x-ray photoelectron and x-ray excited Auger electron spectroscopies for a detailed investigation of the system O 1s, Zn 2p3/2, Zn 3p, and Zn LMM core levels, as well as Ti 2p and W 4f photoelectron peaks. In a nutshell, the results of these analyses highlight the obtainment of pure ZnO nanodeposits, as well as of ZnO-TiO2 and ZnO-WO3 composites, in which the identity of each component is preserved, and the occurrence of an electronic interplay between ZnO and WO3 phases in the latter system.

(Invited) Chemical and Electronic Surface Structure of Compound Semiconductors for Solar Water Splitting

The purpose of this talk is to utilize a tool chest of soft x-ray and electron spectroscopies (in particular using high-brilliance synchrotron radiation) to gain insights into the electronic and chemical surface properties of candidate materials for solar water splitting. Focus will be placed on GaInP2- and Cu(In,Ga)(S,Se)2-based material systems, and how ambient conditions and deliberate tailoring can modify and impact the surface of such materials. It will be shown that photoelectron spectroscopy (PES), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band edge positions, band gaps, electronic level alignment, and insights into chemical stability, both with experiments in ultra-high vacuum, as well as under in situ and operando conditions.

The Effect of Stress in the Density of States of Amorphous Carbon Films Determined by X-Ray Excited Auger Electron Spectroscopy

Amorphous carbon films can be prepared with a large variety of structure and have been used in a number of technological applications. Many of their properties have been determined, but very little is known concerning the effect of pressure on their properties. In this work we investigate the influence of pressure of graphite-like amorphous carbon films on the density of states (DOS) using X-ray Excited Auger Electron Spectroscopy (XAES) and the second derivate method of the XAES. The films were deposited by ion beam deposition and simultaneously bombarded with argon, which is responsible for the variation of the film stress, reaching extremely high values (4.5 GPa). Marked variations of the density of states of the pπ, pσ, sp, and s components were observed with increasing stress.

Temperature dependence of L3M45M45 Auger transition in Fe1−xNix alloys

This paper reports the X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) measurements on Fe1−xNix (x=0.4, 0.5, 0.6, 0.7, 0.9) alloys prepared by arc melting method. Valence band spectrum of Fe0.6Ni0.4 alloy exhibits a shift of about 0.15eV to higher binding energy at 10K compared to that at 300K suggesting the precipitation of antiferromagnetic phase below 20K. Ni L3M45M45 Auger spectra exhibit composition induced changes. The Ni L3M45M45 Auger spectrum of Fe0.6Ni0.4 alloy exhibits an anomalous increase of intensity in the kinetic energy range from 841 to 845.8eV at 10K compared to that of 300K. Such an increase in intensity of the Auger signal also supports the precipitation of new fcc antiferromagnetic phase below 20K. Fe L3M45M45 Auger overlaps with Ni L3M23M23 Auger transition and thus becomes complicated to analyze and obtain temperature or composition induced changes.

The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy--A chemical inert interface with significant band discontinuities.

ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n–type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices.

TiO2-Fe2O3 and Co3O4-Fe2O3 nanocomposites analyzed by X-ray Photoelectron Spectroscopy

The present work is focused on the characterization of TiO2-Fe2O3 and Co3O4-Fe2O3 nanocomposites, of potential interest as photoanodes for photoelectrochemical (PEC) water splitting triggered by solar light. In particular, Fe2O3 nanostructures were deposited onto fluorine-doped tin oxide (FTO)-coated glass substrates by plasma enhanced-chemical vapor deposition (PE-CVD), and functionalized by either TiO2 or Co3O4, obtained via atomic layer deposition (ALD) or radio frequency (RF)-sputtering, respectively. The resulting systems were investigated by complementary techniques in order to obtain detailed information on their structure and morphological organization. In particular, their chemical composition was analyzed through the use of X-ray Photoelectron and X-ray Excited-Auger Electron Spectroscopies (XPS and XE-AES). To this regard, detailed spectra for C 1s, O 1s, Fe 2p and Ti 2p (or Co 2p and Co LMM) regions are reported and discussed.

The chemical structure of the ZnO/SiC heterointerface as revealed by electron spectroscopies

ZnO layers were deposited on 6H-SiC single crystalline wafers by radio frequency magnetron sputtering. The chemical structure of the ZnO/SiC interface was studied by x-ray photoelectron and x-ray excited Auger electron spectroscopy. A complex chemical structure, involving not only silicon–carbon and zinc–oxygen bonds but also silicon–oxygen and zinc–silicon–oxygen bonds was revealed to form at the ZnO/SiC interface. Based on the comparison with the presumably inert (i.e. chemically abrupt) ZnO/Mo interface, it was concluded that a willemite-like zinc silicate (i.e. Zn2SiO4) interface species develops between ZnO and SiC. The presence of this species at the ZnO/SiC interface will affect the electronic structure of the heterojunction and thus needs to be considered for device optimization.

Using Soft X-Rays and Electrons to Determine the Chemical and Electronic Structure of Semiconductors for Solar Water Splitting

In order to maximize the contribution of solar energy to the future global energy portfolio, it is important to optimize the performance, stability, and cost of the devices involved in solar energy conversion (in particular solar water splitting). For that purpose, novel material classes are being exploited, invariably leading to the investigation and use of complex, often ternary or even multinary compounds. The purpose of this talk is to demonstrate how a tool chest of soft x-ray and electron spectroscopies (in particular using high-brilliance synchrotron radiation) is uniquely suited to unravel the electronic and chemical properties of surfaces and interfaces of such materials, and how an understanding on such fundamental level can lead to increased performance. It will be shown how photoelectron spectroscopy (PES), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band gaps, study local chemical bonding and electronic level alignment, and derive insights into chemical stability. As examples, WO3- and GaInP2-based water-splitting devices will be discussed, and Cu(In,Ga)(S,Se)2 solar materials will be used to further illustrate pertinent aspects of the experimental tool chest.

Composition dependence of MNN Auger Transition in AgPd alloys

We investigated Ag and Pd MNN Auger transitions in AgPdx alloys with low Pd concentrations using high-resolution x-ray excited Auger electron spectroscopy. The Ag MNN Auger profile exhibits composition-dependent kinetic energy shifts and also broadening. The Auger kinetic energy shift varies as the square root of the Pd concentration in the alloy, and the highest value of the shift observed for AgPd is about 0.27 eV. The fine structure observed in the Ag MNN Auger transition is independent of composition, but the Pd MNN Auger transition exhibits dramatic changes in intensities of fine features. The Ag MNN Auger transition exhibits a predominantly atomic character, whereas the Pd MNN Auger profile changes from quasi-atomic to band behavior with an increase in Pd concentration.

LMM Auger primary excitation spectra of copper

The shape and intensity of measured Auger peaks are strongly affected by extrinsic excitations due to electron transport out of the surface and to intrinsic excitations induced by the sudden creation of the two static core holes. Following a method developed for XPS in a previous work [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17], we have calculated the effective energy-differential inelastic electron scattering cross-sections, including the effects of the surface and of the two core holes, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The Auger spectra are then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial Auger process, i.e. L–S coupling and vacancy satellite effects. The shape of this primary excitation spectrum is fitted to get close agreement between the theoretical and the experimental spectra obtained from X-ray excited Auger electron spectroscopy (XAES). We have performed these calculations of XAES spectra for various LMM Auger transitions of pure Cu (L3M45M45, L3M23M45, L3M23M23 and L2M45M45 transitions). We compare the resulting primary excitation spectra with theoretical results published in the literature and obtain reasonable quantitative agreement. In particular, we extract from experimental spectra quantitative intensities due to Coster–Kronig, shake-off and shake-up processes relative to the intensity from the “normal” Auger process.

CoPc and CoPcF16 on gold: Site-specific charge-transfer processes.

Interface properties of cobalt(II) phthalocyanine (CoPc) and cobalt(II) hexadecafluoro-phthalocyanine (CoPcF16) to gold are investigated by photo-excited electron spectroscopies (X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and X-ray excited Auger electron spectroscopy (XAES)). It is shown that a bidirectional charge transfer determines the interface energetics for CoPc and CoPcF16 on Au. Combined XPS and XAES measurements allow for the separation of chemical shifts based on different local charges at the considered atom caused by polarization effects. This facilitates a detailed discussion of energetic shifts of core level spectra. The data allow the discussion of site-specific charge-transfer processes.

Fe2O3-CuO Nanocomposites Prepared by a Two-step Vapor Phase Strategy and Analyzed by XPS

β-Fe2O3-CuO nanosystems were developed by using a two-step vapor-phase strategy. β-Fe2O3 matrices (hosts) were initially deposited by Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) on Indium Tin Oxide (ITO) substrates. Subsequently, CuO nanoparticles (NPs, guests) were over-deposited by means of Radio Frequency (RF)-sputtering under mild preparation conditions. A thorough characterization highlighted the dispersion of CuO NPs inside the host iron oxide. To this regard, X-ray Photoelectron and X-ray Excited Auger Electron Spectroscopies (XPS and XE-AES) analyses provided valuable information on the system chemical composition. In particular, attention has been devoted to the analysis of the O 1s, Fe 2p, Cu 2p core levels and Cu LMM Auger peak, employing a non-monochromated MgKα source. The investigation confirmed the presence of Fe(III) and Cu(II) oxides, highlighting the formation of nanocomposites in which the host and guest species maintained their chemical identity.

Investigation on Hydrogenation of Metal–Organic Frameworks HKUST-1, MIL-53, and ZIF-8 by Hydrogen Spillover

The stabilities of three moisture-stable MOFs containing different metal clusters, i.e., HKUST-1 (Cu), MIL-53(Al), and ZIF-8 (Zn), were investigated in dihydrogen and dissociated hydrogen (caused by doped Pt nanoparticles) environments. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray-excited Auger electron spectroscopy (XAES) results showed that all three MOFs were stable in dihydrogen environment. However, the structure of Pt-doped HKUST-1 collapsed in the presence of dissociated hydrogen due to the higher reduction potential of Cu compared with H, and the degree of reduction that occurred to the divalent copper in HKUST-1 increased with temperature. Unlike HKUST-1, MIL-53 and ZIF-8 maintained their structures in both dihydrogen and dissociated hydrogen environments at temperatures up to 150 °C. Moreover, comparison of Pt-doped HKUST-1 samples synthesized by chemical vapor deposition (CVD) and incipient wetness impregnation showed that the contact between the doped Pt particles...

Epitaxial-like Growth of Co3O4/ZnO Quasi-1D Nanocomposites

The development of quasi-1D Co3O4/ZnO nanocomposites by a two-step plasma enhanced-chemical vapor deposition (PE-CVD) process is presented. Arrays of ⟨001⟩ oriented ZnO nanorods were first grown on Si(100) and subsequently used as templates for the PE-CVD of Co3O4, whose amount was tailored as a function of deposition time. The obtained composites were thoroughly characterized by means of a multitechnique approach, involving field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), micro-Raman and Fourier-transform infrared (FT-IR) spectroscopies, X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), glancing incidence X-ray diffraction (GIXRD), and reflection high energy electron diffraction (RHEED). The use of moderate deposition temperatures (≤300 °C), together with the unique activation provided by nonequilibrium plasmas, prevented undesired solid-state reactions between the two oxides and promoted Co3O4 growth on the tips of vertic...

Ag and Pt Particles Sputtered on β-Fe2O3: An XPS Investigation

Supported β-Fe2O3 systems were grown and functionalized with Ag or Pt nanoparticles (NPs) adopting a two-step synthetic strategy. Iron oxide matrices (host) were initially deposited by thermal Chemical Vapor Deposition (CVD) using Fe(hfa)2⋅TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N',N'-tetramethylethylenediamine) as precursor, at a growth temperature of 400 °C and total pressure of 3.0 mbar. Subsequently, Ag or Pt (guest) were deposited on β-Fe2O3 by means of Radio Frequency (RF)-sputtering under mild conditions (Ar plasmas; total pressure = 0.3 mbar; substrate temperature = 60 °C; RF-power = 5 W). A thorough structural and morphological characterization performed on these systems highlighted the dispersion of NPs over the host oxide. To this regard, x-ray Photoelectron and x-ray Excited Auger Electron Spectroscopies (XPS and XE-AES) analyses provided valuable information on the system chemical composition, with special focus on NPs surface interaction with iron(III) oxide matrices. At variance with platinum, which retained its metallic state, an appreciable Ag surface oxidation was observed, an important issue in view of possible functional applications in photocatalysis and gas sensing.