X-ray Photoelectron Peak Research Articles

Interpretation of complex x-ray photoelectron peak shapes. II. Case study of Fe 2p3/2 fitting applied to austenitic stainless steels 316 and 304

In this paper, a review of the analysis of Fe 2p3/2 peak and other transition metals in the austenitic stainless steel literature is presented. It reveals the significant shortcomings of the most widely used approaches, based on the principle of “chemistry fitting,” where single symmetric peaks are used to represent either individual oxidation states or specific compounds. No meaningful conclusions can be drawn from these commonly employed two- or three-component peak fitting (2C and 3C) approaches; the implication being that a large portion of the literature that relies on this approach is flawed. As a significantly more accurate and reliable alternative to “chemistry fitting,” we also assess “envelope fitting” (using empirical multiplet structures) and examine its limitations when applying the approach to austenitic stainless steel data. A detailed comparison of these two fitting approaches is described in Part I. For other elements such as Cr 2p, the problems associated with using single components to represent oxidation states or compounds are not as severe. It was found that it does not impact binding energy measurements, but does influence relative intensities, which will have a flow-on effect for oxide thickness calculations and obtaining a correct understanding of the surface more broadly.

Interpretation of complex x-ray photoelectron peak shapes. I. Case study of Fe 2p3/2 spectra

Analyzing transition metal XPS peaks is widely used to determine surface composition and chemistry. However, these peaks have a complex structure, which is still the subject of investigation. Fe 2p analysis is a case in point where the multiplet structure and many-electron-effects lead to peak shapes that cannot be analyzed using standard approaches. Examination of the literature reveals that one of the most widely used approaches to data reduction when processing Fe 2p3/2 spectra involves using symmetric two- or three-component peak fitting with each peak effectively acting to capture a single chemical species (chemistry fit) in the complex spectra. Herein, this approach is compared to an envelope fit approach using Biesinger multiplet components of known iron oxides to determine how effective these methods are in reproducing iron oxide composition. Mixed oxide and metal XPS Fe 2p spectra were synthesized using reference spectra collected experimentally. For the first time, the accuracy and differences between the two approaches are reported. It is demonstrated that no meaningful conclusions can be drawn using single symmetric peaks to analyze complex Fe 2p3/2 spectra, implying that a large portion of the literature is flawed. The envelope fit approach, however, is shown to provide useful information regarding oxide ratios in mixed iron oxide materials, though limitations do exist. A methodology for evaluating the quality of XPS analysis of Fe 2p3/2 spectra is proposed for benchmarking new submissions so that reviewers, authors, and editors can assess these submissions.

Hydrogen reduction of iron ore pellets: A surface study using ambient pressure X-ray photoelectron spectroscopy

Using Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS), this study investigates the reduction behavior of iron oxides in direct reduction (DRI) and blast furnace (BF) pellets. H2, CO, and a H2–CO mixture are used as reducing agents at 650 °C. The investigation aimed to elucidate variations in the rate of reduction over time and under different conditions. Additionally, contour plots are generated to visualize the X-ray photoelectron peak intensity variations as a function of time. Furthermore, phase stability diagrams based on Fe–O–C and Fe–O–H systems are employed to enhance the understanding of reduction behavior. Results revealed that an increased gas flow rate significantly accelerated the reduction rate due to enhanced gas diffusion, while elevated pressure facilitated the reduction of wüstite to metallic iron. Notably, the DRI pellet achieves around 90% metallization degree reduction with hydrogen, but the introduction of carbon monoxide into the reducing gas prevented the reduction of the DRI pellet. In the case of BF pellet reduction, approximately 20% metallization degree is observed using H2–CO (50:50), yet subsequent reoxidation of the reduced iron to wüstite and magnetite occurred. Further investigation identified a significant increase in the partial pressure of H2O and CO2, particularly within the surface porosities, as the underlying cause of this reoxidation phenomenon.

Real-Time In-Situ Investigation of the Neutron Irradiation Resistance Ability of Nd3+-Doped Gd3Sc2Al3O12 Laser Crystal

In optical crystals, photodarkening will occur after they were irradiated with high-energy particles, and such induced optical loss generally results in significant performance degradation whether they are used as passive or active optical elements. In the present study, the effects of neutron irradiation on the optical response of the Nd3+-doped Gd3Sc2Al3O12 (Nd:GSAG) single crystal has been revealed in real-time and in-situ. Transient and permanent transmittance reduction in the crystal induced by neutron radiation has been observed and the reduction mechanisms have been analyzed. The XRD characterization method demonstrated that the crystal structure remained constant both before and after neutron irradiation. Importantly, the X-ray photoelectron peak of the O 1s core level shifts to high binding energy, indicating that oxygen vacancies were produced in the crystal after irradiation with neutrons. Thus, the permanent reduction in the transmittance of the crystal after irradiation with neutrons can be attributed to the generation of oxygen vacancies in the crystal. To the best of our knowledge, it is the first time the damage types in rare earth oxide laser crystals caused by neutron irradiation were revealed.

Predicting X-ray Photoelectron Peak Shapes: the Effect of Electronic Structure

X-ray photoelectron spectroscopy (XPS) is a widely used tool for quantitative analysis of surfaces, providing critical information about elemental composition and the chemical state(s) of each elem...

Important roles of native-oxides on the electronic band offsets at Ge-oxide/Ge(0 0 1) heterojunction in ambient environment

Metal/semiconductor-oxide interface is a major focus in condensed matter physics due to its rich fundamental properties and potential novel electronic applications. Our understanding of the impact of native oxide on metal/semiconductor-oxide interface (e.g. Ge) remains nonetheless elusive. Herein, we address this by purposefully exposing a clean Ge(001) surface to ambient environment (up to 600 days). The extend of Ge oxidation and binding energy (BE) shifts in the core-levels of Ge X-ray photoelectron peaks (Ge3d and Ge2p3/2) are found to correlate directly with the GeOx/Ge (x ≤ 2) atomic composition. A universal scaling in the band offsets at the GeOx/Ge heterojunction with increasing Ge-oxide content is established independent of oxidation time. A maximum upward band bending of ~0.6 eV is observed in the electronic band structure, which can be attributed to the presence of Ge valence states (Ge1+, Ge2+, Ge3+, Ge4+) at the GeOx/Ge junction formed during oxidation. Our results exemplify the importance of native oxide on semiconductor in determining the fundamental properties of semiconductor-oxide interfaces.

Effect of hydroxyl groups in NiO x on the open circuit voltage of lead iodide perovskite solar cells

The effect of NiO x (0 < x) hole transport layer prepared by a radio-frequency sputtering method on the photovoltaic properties of planer-type CH3NH3PbI3 perovskite solar cells (PVSCs) was investigated. The open circuit voltage of PVSC decreases with increasing applied power of the sputtering machine. The lack of hydroxyl groups on the surface of the metal oxides shifts the work function (WF) to higher energy levels. The X-ray photoelectron peaks of Ni 2p3/2 at 855.6 eV and O 1s at 531.3 eV assigned to ONi(OH) decrease with the increasing power. Therefore, the decrease in the number of hydroxyl groups must have shifted the WF to higher energy levels. The shunt resistance of current–voltage curve and the internal quantum efficiency of the PVSCs is independent of NiO x prepared at various powers. Assuming that the recombination effect can be neglected, the open circuit voltage (V OC) decrease with increasing power is due to the shifted WF to higher energy levels.

Self-catalytic growth of AlN microrods on sapphire substrate

One-dimensional AlN microrods have been grown on sapphire substrate by oxide-assisted vapor utilizing chemical vapor deposition method via vapor-solid mechanism. The structure and morphology of as-grown samples were analyzed by HRXRD, Raman, FT-IR, SEM and TEM. AlN microrods possess single crystalline hexagonal structure with average diameter ~0.7µm and length ~5µm. Chemical compositions have been estimated from EDX analysis. X-ray photoelectron peaks of Al, N, O and C were observed on the surface of AlN microstructures. Photoluminescence spectrum reveals broad ultraviolet, blue emission bands corresponding to deep-level defects. From the optical absorption spectrum direct bandgap was found to be 5.66eV.

Surface analysis and cathodoluminescence degradation of undoped ZnO and ZnO:Zn phosphors

Commercial ZnO:Zn and undoped ZnO with a hexagonal wurtzite structure (JCPDS 80-0075) were studied. The direct optical bandgaps were 3.25 and 3.28 eV for the undoped and ZnO:Zn, respectively. Undoped ZnO exhibited strong ultraviolet band to band emission as well as weak deep level defect emission around the green, orange, and red regions. ZnO:Zn emits only green light centered at 514 nm which was attributed to oxygen vacancies. Electron irradiated degradation of the ZnO:Zn sample showed a rapid initial decrease in cathodoluminescence intensity and then a subsequent recovery and increase to up to double the initial value during prolonged electron irradiation. Deconvoluted peaks of the O1s x-ray photoelectron peaks confirmed the presence of oxygen-related defects in both samples. The ZnO:Zn sample was found to be very stable under electron bombardment, which makes it suitable for the use in field emission displays.

Sample-morphology effects on x-ray photoelectron peak intensities. III. Simulated spectra of model core–shell nanoparticles

The National Institute of Standards and Technology database for the simulation of electron spectra for surface analysis has been used to simulate Cu 2p photoelectron spectra for four types of spherical copper–gold nanoparticles (NPs). These simulations were made to extend the work of Tougaard [J. Vac. Sci. Technol. A 14, 1415 (1996)] and of Powell et al. [J. Vac. Sci. Technol. A 31, 021402 (2013)] who performed similar simulations for four types of planar copper–gold films. The Cu 2p spectra for the NPs were compared and contrasted with analogous results for the planar films and the effects of elastic scattering were investigated. The new simulations were made for a monolayer of three types of Cu/Au core–shell NPs on a Si substrate: (1) an Au shell of variable thickness on a Cu core with diameters of 0.5, 1.0, 2.0, 5.0, and 10.0 nm; (2) a Cu shell of variable thickness on an Au core with diameters of 0.5, 1.0, 2.0, 5.0, and 10.0 nm; and (3) an Au shell of variable thickness on a 1 nm Cu shell on an Au core with diameters of 0.5, 1.0, 2.0, 5.0, and 10.0 nm. For these three morphologies, the outer-shell thickness was varied until the Cu 2p3/2 peak intensity was the same (within 2%) as that found in our previous work with planar Cu/Au morphologies. The authors also performed similar simulations for a monolayer of spherical NPs consisting of a CuAux alloy (also on a Si substrate) with diameters of 0.5, 1.0, 2.0, 5.0, and 10.0 nm. In the latter simulations, the relative Au concentration (x) was varied to give the same Cu 2p3/2 peak intensity (within 2%) as that found previously. For each morphology, the authors performed simulations with elastic scattering switched on and off. The authors found that elastic-scattering effects were generally strong for the Cu-core/Au-shell and weak for the Au-core/Cu-shell NPs; intermediate elastic-scattering effects were found for the Au-core/Cu-shell/Au-shell NPs. The shell thicknesses required to give the selected Cu 2p3/2 peak intensity for the three types of core–shell NPs were less than the corresponding film thicknesses of planar samples since Cu 2p photoelectrons can be detected from the sides and, for the smaller NPs, bottoms of the NPs. Elastic-scattering effects were also observed on the Au atomic fractions found for the CuAux NP alloys with different diameters.

Antibacterial and antibiofilm surfaces through polydopamine-assisted immobilization of lysostaphin as an antibacterial enzyme.

Antibiotic resistance and the colonization of bacteria on surfaces, often as biofilms, prolong hospitalization periods, increase mortality, and are thus major concerns for health care providers. There is an urgent need for antimicrobial and antibiofilm surface treatments that are permanent, can eradicate both biofilms and planktonic pathogens over long periods of time, and do not select for resistant strains. In this study, we have demonstrated a simple, robust, and biocompatible method that utilizes the adhesive property of polydopamine (PDA) to covalently attach the antimicrobial enzyme lysostaphin (Lst) to a variety of surfaces to generate antibacterial and antibiofilm interfaces. The immobilization of the recombinant Lst onto PDA-coated surfaces was carried out under physiological conditions, most probably through the C-terminal His6-tag fragment of the enzyme, minimizing the losses of bioagent activity. The modified surfaces were extensively characterized by X-ray photoelectron spectroscopy and peak force quantitative nanomechanical mapping (PeakForce QNM) AFM-based method, and the presence of Lst on the surfaces was further confirmed immunochemically using anti-Lst antibody. We also found that, in contrast to the physically adsorbed Lst, the covalently attached Lst does not leach from the surfaces and maintains its endopeptidase activity to degrade the staphylococcal cell wall, avoiding most intracellular bacterial resistance mechanisms. Moreover, the Lst-coated surfaces kill hospital strains of Staphylococcus aureus in less than 15 min and prevent biofilm formation. This immobilization method should be applicable also to other proteins and enzymes that are recombinantly expressed to include the His6-tag fragment.

Sample-morphology effects on x-ray photoelectron peak intensities. II. Estimation of detection limits for thin-film materials

The authors show that the National Institute of Standards and Technology database for the simulation of electron spectra for surface analysis (SESSA) can be used to determine detection limits for thin-film materials such as a thin film on a substrate or buried at varying depths in another material for common x-ray photoelectron spectroscopy (XPS) measurement conditions. Illustrative simulations were made for a W film on or in a Ru matrix and for a Ru film on or in a W matrix. In the former case, the thickness of a W film at a given depth in the Ru matrix was varied so that the intensity of the W 4d5/2 peak was essentially the same as that for a homogeneous RuW0.001 alloy. Similarly, the thickness of a Ru film at a selected depth in the W matrix was varied so that the intensity of the Ru 3p3/2 peak matched that from a homogeneous WRu0.01 alloy. These film thicknesses correspond to the detection limits of each minor component for measurement conditions where the detection limits for a homogeneous sample varied between 0.1 at. % (for the RuW0.001 alloy) and 1 at. % (for the WRu0.01 alloy). SESSA can be similarly used to convert estimates of XPS detection limits for a minor species in a homogeneous solid to the corresponding XPS detection limits for that species as a thin film on or buried in the chosen solid.

Magnetic properties of ultrathin Ni81Fe19 films with Ta and Ru capping layers

Magnetic properties of Ni81Fe19 (permalloy) ultrathin films with Ru and Ta capping layers (CLs) were investigated for applications to magnetic random access memory units (MRAM). The sample structure, which simulated an MRAM free layer, is Si- sub./SiO2/Ni81Fe19/Ru(Ta). The Ni81Fe19 thin films less than 3 nm thick with Ru CL show low coercive fields compared with the Ta capping layer. Both systems showed loss of momentum equivalent to magnetically dead layers of thickness (δ) ∼0.6 nm for Ru cap layer and ∼1.4 nm for Ta cap layer, respectively. Moreover, after annealing the thicknesses are slightly increased to an equivalent magnetic dead layer thickness of δ ∼ 0.84 nm and ∼1.80 nm for Ru and Ta CL, respectively. Our calculations showed that the presence of only 11% Ta concentration at the interface reduced the Ni momentum to zero, with the Ni–Ta coupling being anti-ferromagnetic; while 50% Ru intermixing at the interface reduced the Ni momentum to zero with the coupling between Ru and Ni being ferromagnetic. To find out more about the intermixing at the interface, the composition and chemical states were characterized by the x-ray photoelectron spectroscopy and peak decomposition technique. The result showed that the peak positions were different from the pure metallic case at the interface region, mainly because of the intermixing between two layers. In conclusion, the Ru capping layer might be important for MRAM use in terms of low coercive field and small δ layer thickness if compared with the Ta capping layer.

Sample-morphology effects on x-ray photoelectron peak intensities

The authors have used the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis to simulate photoelectron spectra from the four sample morphologies considered by Tougaard [J. Vac. Sci. Technol. A 14, 1415 (1996)]. These simulations were performed for two classes of materials, two instrument configurations, and two conditions, one in which elastic scattering is neglected (corresponding to the Tougaard results) and the other in which it is included. The authors considered the Cu/Au morphologies analyzed by Tougaard and similar SiO2/Si morphologies since elastic-scattering effects are expected to be smaller in the latter materials than the former materials. Film thicknesses in the simulations were adjusted in each case to give essentially the same chosen Cu 2p3/2 or O 1s peak intensity. Film thicknesses with elastic scattering switched on were systematically less than those with elastic scattering switched off by up to about 25% for the Cu/Au morphologies and up to about 14% for the SiO2/Si morphologies. For the two morphologies in which the Cu 2p3/2 or O 1s peak intensity was attenuated by an overlayer, the ratios of film thicknesses with elastic scattering switched on to those with elastic scattering switched off varied approximately linearly with the single-scattering albedo, a convenient measure of the strength of elastic scattering. This variation was similar to that of the ratio of the effective attenuation length to the inelastic mean free path for the photoelectrons in the overlayer film. For the two morphologies in which the Cu 2p3/2 or O 1s photoelectrons originated from an overlayer film, the ratios of film thicknesses with elastic scattering switched on to those with elastic scattering switched off varied more weakly with the single-scattering albedo. This weaker variation was attributed to the weaker effects of elastic scattering for photoelectrons originating predominantly from near-surface atoms than for photoelectrons that travel through an overlayer film.

Chemical composition, thermal stability and hydrogen plasma treatment of laser-cut single-crystal diamond surface studied by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

X-ray photoelectron spectroscopy examination shows that after laser cutting under ambient condition, the upper surface of diamond consists of a heavy oxidized layer consisting of a variety of carbon–oxygen chemical states comprising –C═O, –C–O–C– and –C–O–H species. The thickness of the oxide layer was estimated to be ∼22 Ǻ. Upon vacuum annealing to 700 °C the thickness of the oxide layer decreases to ∼10 A and the upper surface layer becomes more diamond-like through desorption of C–O species. Exposure of the laser cut diamond surface to a microwave hydrogen (MW-H) plasma results in removal of the oxide layer and exposure of the diamond phase. This is evidenced by the appearance of characteristic diamond surface and bulk plasmons which accompanied the C (1s) X-ray photoelectron peak. Our studies show that the surface chemical composition and thermal stability of the laser cut and polished surfaces both after MW-H exposure are nearly similar. The morphology of the laser cut surface shows an ill-defined laminar structure without any characteristic features which is not significantly affected by MW-H plasma exposure. This is in contrast to the polished surfaces for which exposure to the MW-H may result in its planarization.

1 H-1,2,3-triazole, a promising precursor for chemical vapor deposition of hydrogenated carbon nitride

Well-crystallized hydrogenated carbon nitride thin films have been prepared by microwave plasma enhanced chemical vapor deposition (MWPECVD). 1 H-1,2,3-triazole+N 2 and Si (1 0 0) were used as precursor and substrate, respectively. Substrate temperature during the deposition was recorded to be 850 °C. The synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photo-electron spectroscopy (XPS) analyses. The plasma compositions were checked by optical emission spectroscopy (OES). XRD observation strongly suggests that the films contain polycrystalline carbon nitride with graphitic structure of (1 0 0), (0 0 2), (2 0 0) and (0 0 4). XPS peak quantification reveals that the atomic ratio of the materials C:N:O:Si is 32:41:18:9. X-ray photo-electron peak deconvolution shows that the most dominant peak of C (1s) and N (1s) narrow scans correspond to sp 2 hybrid structure of C 3N 4. These observations indicate that 1 H-1,2,3-triazole favors the formation of hydrogenated carbon nitride with graphitic phase by CVD method and thus is in good agreement with XRD results. SEM of surface and OES of plasma also support the formation of polycrystalline carbon nitride films from 1 H-1,2,3-triazole+N 2 by CVD.

Evidence for FeO formation at the Fe/MgO interface in epitaxial TMR structure by X-ray photoelectron spectroscopy

Epitaxial Fe/MgO/Fe tunnelling magnetoresistance (TMR) structure was grown on the MgO substrate by molecular beam epitaxy, characterized in-situ by reflection high-energy electron diffraction and ex-situ by high-resolution transmission electron microscopy. The chemical states at the Fe/MgO interface have been investigated by X-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there is FeO formation at interface of Fe/MgO, clearly identified by the existence of Fe 2+ and Fe 3+ peaks at 708.2 and 710.4 eV, respectively, which locate at the high-energy side of the main metallic Fe peak. The presence of FeO layer at the Fe/MgO interface proved by XPS is likely to have considerable influence on its MR value and hence on its application in spintronics.

ATOMIC ENVIRONMENT IN LEAD-BISMUTHATE GLASSES CONTAINING MANGANESE

The atomic environment of 20MnO · 80(x Bi 2 O 3 · y PbO ) glass samples having the ratio of bismuth to lead atoms of 8:1 and 3:1 was investigated with respect to the thermal induced structural changes, cationic field strength and electronic structure of the compound. The position and full width at half maximum of X-ray photoelectron peaks were estimated using spectra simulation based on the summation of Lorentzian and Gaussian functions. Data obtained from XPS core-level spectra Bi 4f, Pb 4f, Mn 2p, and O 1s indicate a reduction of glass disorder both by heat treatment and by increasing the PbO content in the samples. The cations behave essentially as network formers that could be correlated with an intermediate range structure.

SiO2/Ta interface reaction in magnetic multilayers and its influence on Ta buffer layers

Ta is often used as a buffer layer in magnetic multilayers. In this study, Ta/Ni81Fe19/Ta multilayers were deposited by magnetron sputtering on sing-crystal Si with a 300-nm-thick SiO2 film. The composition and chemical states at the interface region of SiO2/Ta were studied using the X-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there is an 'inter-mixing layer" at the SiO2/Ta interface due to a thermodynamically favorable reaction: 15 SiO2 + 37 Ta = 6 Ta2O5 + 5 Ta5Si3. Therefore, the Ta buffer layer thickness used to induce NiFe (111) texture increases.

Formation of a Surface-Mediated Donor−Acceptor Complex: Coadsorption of Trimethylamine and Boron Trifluoride on the Silicon (001) Surface

The interaction of trimethylamine and boron trifluoride on the Si(001) surface has been investigated using X-ray photoelectron (XP) spectroscopy. XP spectra show the TMA forms a dative bond with the clean Si(001) surface, while on the clean surface BF3 bonds dissociatively, yielding Si−BF2 and Si−F species. If the surface is first “saturated” with TMA, however, BF3 will continue to adsorb, giving rise to new F(1s) XP peaks. Based on an analysis of the energies and thermal behavior of these peaks, we propose that the adsorption of TMA and BF3 on the Si(001) surface leads to the formation of a novel surface-mediated donor−acceptor complex of the structure TMA−Si−Si−BF3.