X-ray Photoelectron Spectroscopy Depth Profiling Research Articles

Understanding the Mechanisms of Methylammonium-Induced Thermal Instability in Mixed-FAMA Perovskites.

Despite a recent shift toward methylammonium (MA)-free lead-halide perovskites for perovskite solar cells, high-efficiency formamidinium lead iodide (FAPbI3) devices still often require methylammonium chloride (MACl) as an additive, which evaporates away during the annealing process. In this article, it is shown that the residual MA+, however, triggers thermal instability. To investigate the possibility of an optimal concentration of MA+ that may improve thermal stability, the intrinsic thermal stability of pure FA, FA-rich, MA-rich, and pure MA perovskite films (FA1-xMAxPbI3, FAMA) is studied. The results show that the thermal stability of FAMA perovskites decreases with more MA+, under degradation conditions that isolate the intrinsic thermal stability of the material (i.e., without moisture and oxygen effects). X-ray diffraction (XRD), proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS), photoluminescence (PL) and UV-visible spectroscopy, and depth-profiling X-ray Photoelectron Spectroscopy (XPS) are employed to show that the observed trend is mainly due to the decomposition of the MA+ cation, as opposed to other effects such as the precursor solvent and film morphologies. It is also found that the surfaces of these FAMA films are MA+ rich, although this phenomenon does not appear to affect thermal stability. Finally, it is demonstrated that this trend is unaffected by the presence of Spiro-OMeTAD atop the film, and thus solar cell devices should preserve this trend.

Practical guides for x-ray photoelectron spectroscopy: Use of argon ion beams for sputter depth profiling and cleaning

Ion beams are used in x-ray photoelectron spectroscopy (XPS) to clean samples and perform compositional sputter depth profiles. The purpose of this article is to compile good practice, recommendations, and useful information related to the use of argon ion sources for inexperienced users of XPS instrumentation. The most used type of ion source generates monoatomic argon ions at a range of energies from a fixed direction relative to the instrument. The angle and direction of the ion beam with respect to the surface are normally altered by manipulating the sample, and this may involve tilting the sample to change the angle of incidence or rotating the sample to change the azimuthal incidence angle. Atomic argon ion beams cause damage to the structure of the material surface, which may exhibit itself as a change in stoichiometry or topography as well as the implantation of argon atoms. Therefore, caution is required in the interpretation of XPS depth profiles. Gas cluster ion sources offer new possibilities and choices to XPS users. Gas cluster sources enable the sputtering of organic materials with high yield in comparison to inorganic materials and offer the potential for nearly damage-free depth profiling of delicate organic materials as well as low damage cleaning of inorganic materials. It may be possible to use argon clusters to reduce damage during the depth profiling of inorganic materials, but there is currently insufficient evidence to make any general recommendations.

XPS Depth-Profiling Studies of Chlorophyll Binding to Poly(cysteine methacrylate) Scaffolds in Pigment-Polymer Antenna Complexes Using a Gas Cluster Ion Source.

X-ray photoelectron spectroscopy (XPS) depth-profiling with an argon gas cluster ion source (GCIS) was used to characterize the spatial distribution of chlorophyll a (Chl) within a poly(cysteine methacrylate) (PCysMA) brush grown by surface-initiated atom-transfer radical polymerization (ATRP) from a planar surface. The organization of Chl is controlled by adjusting the brush grafting density and polymerization time. For dense brushes, the C, N, S elemental composition remains constant throughout the 36 nm brush layer until the underlying gold substrate is approached. However, for either reduced density brushes (mean thickness ∼20 nm) or mushrooms grown with reduced grafting densities (mean thickness 6-9 nm), elemental intensities decrease continuously throughout the brush layer, because photoelectrons are less strongly attenuated for such systems. For all brushes, the fraction of positively charged nitrogen atoms (N+/N0) decreases with increasing depth. Chl binding causes a marked reduction in N+/N0 within the brushes and produces a new feature at 398.1 eV in the N1s core-line spectrum assigned to tetrapyrrole ring nitrogen atoms coordinated to Zn2+. For all grafting densities, the N/S atomic ratio remains approximately constant as a function of brush depth, which indicates a uniform distribution of Chl throughout the brush layer. However, a larger fraction of repeat units bound to Chl is observed at lower grafting densities, reflecting a progressive reduction in steric congestion that enables more uniform distribution of the bulky Chl units throughout the brush layer. In summary, XPS depth-profiling using a GCIS is a powerful tool for characterization of these complex materials.

Dynamic recycling behavior of Cu/Zn-based electrodes in electrochemical CO2 reduction

Recycling and electrode stability are crucial in electrochemical CO2 reduction (EC CO2R), especially due to changes in electrode behavior under negative reduction potentials. This study investigates the EC CO2R products over multiple recycling steps using Cu, Zn, and CuZn metals, along with their oxides. Our findings reveal significant variations in reduction products and their Faradaic efficiencies (FEs), dependent on the electrode type. Initially, Cu and its oxide, as well as the CuZn alloy and its oxide, displayed varying FEs, but their performance normalized in later recycles. Conversely, Zn and its oxide showed consistent differences throughout the recycles. Depth-profiling X-ray photoelectron spectroscopy was used to analyze the electrodes’ composition and oxidation states post-reaction. The analyses highlighted substantial changes in oxidation states and the distribution of metallic and oxidized states, particularly in the Zn/Cu ratio. These insights are essential for developing stable electrodes for energy and environmental applications.

Unraveling the Intrinsic Origin of the Superior Sodium-Ion Storage Performance of Metal Selenides Anode in Ether-Based Electrolytes.

Metal selenides show outstanding sodium-ion storage performance when matched with an ether-based electrolyte. However, the intrinsic origin of improvement and deterministic interface characteristics have not been systematically elucidated. Herein, employing FeSe2 anode as the model system, the electrochemical kinetics of metal selenides in ether and ester-based electrolytes and associated solid electrolyte interphase (SEI) are investigated in detail. Based on the galvanostatic intermittent titration technique and in situ electrochemical impedance spectroscopy, it is found that the ether-based electrolyte can ensure fast Na+ transfer and low interface impedance. Additionally, the ether-derived thin and smooth double-layer SEI, which is critical in facilitating ion transport, maintaining structural stability, and inhibiting electrolyte overdecomposition, is concretely visualized by transmission electron microscopy, atomic force microscopy, and depth-profiling X-ray photoelectron spectroscopy. This work provides a deep understanding of the optimization mechanism of electrolytes, which can guide available inspiration for the design of practical electrode materials.

Exploring the influence of tensile load on the nature of passive film during stress corrosion cracking in a TRIP Fe39Mn20Co20Cr15Si5Al1 (at.%) high entropy alloy

A passive film serves as a first defense of any material against the attack of a corrosive environment for any material. So, it is essential to understand the parameters affecting the characteristics of the passive film. The present work focused on investigating the effect of tensile load during stress corrosion cracking (SCC) condition on the characteristics of the passive film developed on a TRIP Fe39Mn20Co20Cr15Si5Al1 high entropy alloy (HEA) in a 3.5 wt.% NaCl solution at room temperature. A constant load of ∼25% of the yield strength was applied to a flat dog-bone-shaped tensile specimen for 24 h in a 3.5 wt.% NaCl salt environment. For comparison purposes, a specimen with no-load was kept in a salt solution under similar conditions. Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) techniques were used for passive film characterization. EIS results revealed that the passive film deposited on the no-load and load conditions exhibited a dense and stable structure. However, the passive film deposited on the no-load specimen showed relatively better resistance to corrosion ions. XPS results indicated that the passive film deposited on the no-load and load specimens consisted of Cr rich oxide and hydroxide, forming a monolayer type of passivation film. Nonetheless, the passive film on the no-load specimen exhibited higher concentrations of Cr than the passive film on the loaded specimen. XPS depth profile results showed that the film thickness was relatively greater for the no-load specimen than for the loaded specimen. This study underscores the critical influence of tensile load on the degradation of passive film properties of the HEA.

Material screening for future diffusion barriers in Cu interconnects: Modeling of binary and ternary metal alloys and detailed analysis of their barrier performance

One of the challenges in the semiconductor industry is to find new barrier materials and copper (Cu) alternative solutions in interconnects. In this work, we focused to find alternative diffusion barrier materials. Different binary (CoMo, CoRu, CoTa, CoW, MoRu, RuTa, RuW) and ternary (CoMoTa, CoRuTa, MoRuTa) metal alloys were evaluated theoretically with the Miedema model to find the amorphous phase composition range. Afterward, thin films of the alloys with various compositions were deposited by magnetron sputtering and theoretical values were compared to the experimental results. From the experimental measurements, which included grazing incidence x-ray diffraction analysis and resistivity measurements, suitable binary and ternary alloys were chosen for diffusion analysis. By annealing thin film stacks at temperatures ranging from 500 to 675°C, diffusion was induced and detected by x-ray photoelectron spectroscopy depth profiles. Seventeen alloys were evaluated by their diffusion barrier effectiveness, and five of those, which include Ru60Ta40, Ru45W55, Mo47Ru53, Mo36Ru50Ta14, and Co40Mo35Ta25, showed excellent barrier properties against copper diffusion. Furthermore, all of the stated materials have a lower resistivity than TaN. Last, the adhesion of the best performing alloys to SiCOH and Cu was evaluated by the modified edge lift-off test. Only Ru45W55 had reasonable adhesion at both interfaces. The other materials showed low adhesion strength to Cu, which would make an adhesion promoter (liner), such as cobalt, necessary for the integration.

Oxidative corrosion resistance of a CrxFeyNi1-x-y composition spread alloy film (CSAF) in dry air

Composition Spread Alloy Films (CSAFs) were used to study the resistance to oxidative corrosion of CrxFeyNi1-x-y alloys in dry air and determine the critical Cr composition, xCr∗, necessary to prevent bulk oxidation. CrxFeyNi1-x-y CSAFs were made using physical vapor deposition (PVD) and then oxidized in dry air between 100 – 500 °C for periods of 1 h. A sharp passivation boundary was observed at 300 °C separating alloys showing evidence of heavy oxidation from Cr-rich alloys that were passivated. X-ray photoelectron spectroscopy (XPS) depth profiling was used to measure the extent of oxidation and penetration depth of the oxide layer on either side of the passivation boundary. XPS measurements confirmed the presence of a critical Cr composition, xCr∗, ranging from 18 to 26 at.%. For alloys with xCr≥xCr∗, a sufficiently thick passivating layer of oxidized Cr, Cr(ox), had formed on the top surface preventing the oxidative corrosion of either Fe or Ni. On the other hand, when xCr<xCr∗, bulk oxidation was observed, with appreciable Fe(ox) and/or Ni(ox) measured on the surface and often penetrating through the entire alloy film thickness. The trajectory of xCr∗ plotted within CrxFeyNi1-x-y composition space supports the trend towards using higher Cr-content stainless steel grades in harsh oxidative environments.

In situ polymerized polydioxolane interlayer enabled dendrite-free argyrodite-based solid-state batteries

Solid-state batteries (SSBs) involving argyrodite-type sulfide electrolytes are considered as promising candidates for the state-of-art lithium-ion batteries (LIBs) due to their superior safety, energy and power density. However, the incompatibility between the argyrodite-type sulfide and lithium anode has plagued the development of the sulfide-based SSBs. Herein, we introduced a poly-dioxolane-based (PDOL) interlayer at the lithium anode, which is ring-opening polymerized via poly(acrylamide-2-methyl-1-propane-sulfonate) (PAMPS) loaded on the separator. The in-situ polymerized PDOL interlayer could match with the solvent-sensitive lithium argyrodite while enabling a stable and tight lithium interface. The symmetric lithium cell with the PDOL interlayer exhibits over-1200-hour dendrite-free lithium deposition and stripping at 0.5 mA cm-2, and the critical current density reaches 2.7 mA cm-2. The as-cycled lithium interface is further investigated via the depth-profiling X-ray photoelectron spectroscopy, and solid electrolyte interphase (SEI) is determined to be composed of ether polymer as well as inorganic compounds like LiC, Li2O, LiF, and Li2S. The Li|LiCoO2 SSB with PDOL interlayer presents a high discharge capacity of 120.7 mAh g-1 at 0.1 C and a capacity retention rate of 95.4% after 200 cycles. The rational design of the PDOL interlayer at the lithium anode opens a new alternative for high-performance SSBs.

Deterioration of Li-doped phenanthroline-based charge generation layer for tandem organic light-emitting diodes

Understanding the deterioration of the charge generation layer (CGL) is an uprising issue for efficient and stable tandem organic light-emitting diodes. Here, we comprehensively investigated the change in the electrical characteristics of Li-doped CGLs according to different stress conditions and compared the stability of Li diffusion in two phenanthroline-based electron transport materials. Through current density–voltage and capacitance-frequency measurements, it was revealed that electrical stress and thermal stress affect CGL’s electrical properties differently. Moreover, the exponential trap distribution model analysis informed that the Li diffusion is expedited toward narrowing depletion width in the CGL. Diffusion of Li and thermal effect to the diffusion was experimentally observed by X-ray photoelectron spectroscopy (XPS) depth profiling. We also evaluated variations in the operating voltage of tandem devices that differed only in nCGL. Overall, Li diffusion occurs more favorably in sparse Bphen film with weak binding energy rather than closely packed BPPB film with strong binding energy, which highlights that atomic geometry and morphological characteristics are crucial for the stability of tandem devices.

Effect of Bias Potential on the Interface of a Solid Electrolyte and Electrode during XPS Depth Profiling Analysis.

Depth profiling is an essential method to investigate the physical and chemical properties of a solid electrolyte and electrolyte/electrode interface. In conventional depth profiling, various spectroscopic tools such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) are utilized to monitor the chemical states along with ion bombardment to etch a sample. Nevertheless, the ion bombardment during depth profiling results in an inevitable systematic error, i.e., the accumulation of mobile ions at the electrolyte/electrode interface, known as the ion pile-up phenomenon. Here, we propose a novel method using bias potential, the substrate-bias method, to prevent the ion pile-up phenomena during depth profiling of a solid electrolyte. When the positive bias potential is applied on the substrate (electrode), the number of accumulating ions at the electrolyte/electrode interface is significantly reduced. The in-depth XPS analysis with the biased electrode reveals not only the suppression of the ion pile-up phenomena but also the altered chemical states at the interfacial region between the electrolyte and electrode depending on the bias. The proposed substrate-bias method can be a good alternative scheme for an efficient yet precise depth profiling technique for a solid electrolyte.

Depth profiling of microwave nitrogen-terminated polycrystalline diamond surfaces by energy-dependent X-ray photoelectron spectroscopy

Nitrogen-terminated diamonds hold promise for stabilizing near-surface NV− centers, which is essential for reliable quantum sensing. Among various surface preparation methods, microwave (MW) nitrogen plasma, known for its minimal surface damage, appears as the most effective choice. In this investigation, we explore the nature of nitrogen bonding of polycrystalline diamond (PCD) surfaces exposed to MW nitrogen plasma using X-ray Photoelectron Spectroscopy (XPS) depth profiling and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the C K- and N K-edges. XPS depth profiling with atomic resolution, achieved by varying the probing photon energy using synchrotron radiation and supported by a physical model considering the associated inelastic mean free path, suggests a surface of almost fully saturated nitrogen in two main bonding configurations of similar contribution and a low coverage of ∼5 % of graphene-like islands residing atop the nitrogen-terminated diamond surface. The thermal stability of these surface groups is monitored by in situ annealing up to 700 °C. The depth profiles reveal that nitrogen atoms do not diffuse in the diamond crystal, resulting in excellent diamond crystallinity in the first atomic planes below the surface. C K-edge NEXAFS analysis reveals the position of unoccupied surface states within the diamond bandgap, opening new perspective on the stabilization of near-surface NV− centers.

Buried junction and efficient carrier separation in CdSexTe1−x/CdTe solar cells

Alloying CdTe absorbers with Se is a critical advancement for the fabrication of highly efficient CdTe thin-film solar cells. Herein, the role of the Se concentration gradient in CdSexTe1−x/CdTe solar cells is stressed in addition to the decreased bandgap and passivation effect of Se. The buried graded CdSexTe1−x at the front interface of the device was stripped and analyzed by quasi in situ x-ray photoelectron spectroscopy depth profiling. A serial shift of Fermi level toward the valence band was probed as the Se concentration decreased in the graded CdSexTe1−x absorber, revealing the presence of n-type CdSexTe1−x region near the front contact. Kelvin probe force microscopy characterizations and voltage-dependent photocurrent collection analysis further confirmed the existence of a buried junction in the graded CdSexTe1−x absorber. The CdS-free CdTe solar cell with a graded CdSexTe1−x/CdTe absorber fabricated in this study showed an efficiency of 17.6%. These results indicated that the non-uniform distribution of Se introduced a built-in field in the graded CdSexTe1−x absorber and enabled efficient separation of photogenerated carriers, yielding high-performance CdTe solar cells in the absence of a conventional n-type CdS window layer. This study deepened the understanding of the performance enhancement in Se-containing CdTe solar cells and provided new ideas for further performance optimization.

Inhibited Surface Diffusion in Nanoporous Multi-Principal Element Alloy Thin Films Prepared by Vacuum Thermal Dealloying

Nanoporous structures with 3D interconnected networks are traditionally made by dealloying a binary precursor. Certain approaches for fabricating these materials have been applied to refractory multi-principal element alloys (RMPEAs), which can be suitable candidates for high-temperature applications. In this study, nanoporous refractory multi-principal element alloys (np-RMPEAs) were fabricated from magnesium-based thin films (VMoNbTaMg) that had been prepared by magnetron sputtering. Vacuum thermal dealloying (VTD), which involves sublimation of a higher vapor pressure element, is a novel technique for synthesizing nanoporous refractory elements that are prone to oxidation. When VMoNbTaMg was heated under vacuum, a nanoporous structure was created by the sublimation of the highest vapor pressure element (Mg). X-ray photoelectron spectroscopy depth profiling indicated significantly less ligament oxidation during VTD as compared to traditional dealloying methods. Furthermore, np-RMPEAs exhibited outstanding stability against coarsening, retaining smaller ligaments (~25 nm) at elevated temperature (700 °C) for a prolonged period (48 h).

Solid–electrolyte interphase formation during Li metal deposition in LiN(SO2F)2-based solvate ionic liquids

AbstractSolvate ionic liquids (SILs) are promising electrolytes for Li metal batteries. In this study, Li plating-stripping reactions in SILs were investigated using electrochemical quartz crystal microbalance measurements and X-ray photoelectron spectroscopy (XPS). SILs were prepared by mixing Li salt and glyme (triglyme or tetraglyme) in a 1:1 molar ratio. During the Li plating-stripping reaction, a reversible mass change was observed in LiN(SO2F)2 (LiFSA) based SILs. In contrast, a considerably higher mass change than the theoretical value calculated from the Faraday current was observed in the LiN(SO2CF3)2 (LiTFSA) based SILs, owing to the accumulation of decomposition products of electrolytes on the electrode surface. XPS depth profiling for the deposited Li suggested the formation of thin solid-electrolyte interphase (SEI) in the LiFSA-based SILs while considerably thicker SEI was formed in the LiTFSA-based SILs. The SEI formed in LiFSA-based SILs effectively suppressed the decomposition of electrolytes. Thus, LiFSA-based SILs are favorable for achieving highly reversible charge–discharge of Li electrodes in Li metal batteries.

External Voltage-Induced Restructuring of the Solid-State Electrode/Electrolyte Interface Revealed by X-ray Photoelectron Spectroscopy Depth Profiling Analysis.

Solid/solid interfaces between electrodes and electrolytes play an important role in all-solid-state energy devices, while microscopic investigations of the buried interfaces remain challenging. Here, we construct metal|yttria-stabilized zirconia (YSZ)|Au model cells consisting of a metal film cathode (metal (M) = Au, Ni, and Ag), a single crystalline YSZ electrolyte, and a Au film anode, and use quasi in situ X-ray photoelectron spectroscopy depth profiling analysis to investigate the restructuring of buried interfaces between metal cathodes and YSZ. After applying 2.9 V at 500 °C, interfacial Zr4+ ions in the electrolyte are reduced and then interdiffuse with metal cathode overlayers, forming a miscible ZrM alloy interlayer. The interface restructuring degree follows the sequence of Au|YSZ|Au > Ni|YSZ|Au > Ag|YSZ|Au. Meanwhile, surface segregation of Zr on the cathode surface is also observed, whose degree follows the sequence of Ag|YSZ|Au > Ni|YSZ|Au > Au|YSZ|Au. Notably, the strong ZrM alloy formation enhances the interface restructuring but suppresses the Zr surface segregation. This work provides a fundamental understanding of the interfacial reaction at the buried electrode/electrolyte interface.

XPS depth profiling of functional materials: applications of ion beam etching techniques

X-ray photoelectron spectroscopy depth profiling combined with ion beam etching methods is a primary tool to study the chemical composition of functional materials at different scales from the surface to the bulk.

A sustainable CVD approach for ZrN as a potential catalyst for nitrogen reduction reaction.

In pursuit of developing alternatives for the highly polluting Haber-Bosch process for ammonia synthesis, the electrocatalytic nitrogen reduction reaction (NRR) using transition metal nitrides such as zirconium mononitride (ZrN) has been identified as a potential pathway for ammonia synthesis. In particular, specific facets of ZrN have been theoretically described as potentially active and selective for NRR. Major obstacles that need to be addressed include the synthesis of tailored catalyst materials that can activate the inert dinitrogen bond while suppressing hydrogen evolution reaction (HER) and not degrading during electrocatalysis. To tackle these challenges, a comprehensive understanding of the influence of the catalyst's structure, composition, and morphology on the NRR activity is required. This motivates the use of metal-organic chemical vapor deposition (MOCVD) as the material synthesis route as it enables catalyst nanoengineering by tailoring the process parameters. Herein, we report the fabrication of oriented and facetted crystalline ZrN thin films employing a single source precursor (SSP) MOCVD approach on silicon and glassy carbon (GC) substrates. First principles density functional theory (DFT) simulations elucidated the preferred decomposition pathway of SSP, whereas ab initio molecular dynamics simulations show that ZrN at room temperature undergoes surface oxidation with ambient O2, yielding a Zr-O-N film, which is consistent with compositional analysis using Rutherford backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS) depth profiling. Proof-of-principle electrochemical experiments demonstrated the applicability of the developed ZrN films on GC for NRR and qualitatively hint towards a possible activity for the electrochemical NRR in the sulfuric acid electrolyte.

Growth mechanism and field emission of B doped AlN films

This paper highlights the effect of boron dopant on the field emission and growth mechanism of AlN films. The films have been grown by reactive RF magnetron sputtering method. For undoped films, columnar AlN structure was obtained. In the case of boron doped AlN films, the structure growth proceeded gradually through four layers of different thicknesses and structures. It was found that boron favors the formation of an amorphous layer within which oxygen and boron based precipitates develops, retarding the growth of columnar morphology. Depth profiling X-ray photoelectron spectroscopy analyses confirm the presence of B-O, B-N, Al-B and Al-O groups.The study on the field emission (FE) properties of undoped and B doped AlN films on Si substrates indicates that the addition of boron allow to improve FE properties with low turn on field of 2.6 V/μm and low threshold filed of 3.07 V/μm. Such good field emission properties can be attributed to the formation of densely nanograined surface and the appearance of boron based groups.

The effect of ramp heating on the microstructure and surface chemistry of APMT FeCrAl alloy

The APMT FeCrAl alloy is a cladding candidate for the accident-tolerant fuel (ATF) systems in light water reactors (LWRs) due to its ability to form a protective Al2O3 film on the surface at temperatures greater than 1000 °C. In order to understand the alumina evolution at the early stages of high-temperature exposure in short periods, an APMT tube specimen was heated up rapidly to melting temperature in a flowing 100 % water and steam environment. Using transmission electron microscopy (TEM), the presence of a ∼ 90 nm thick oxide layer was confirmed. The surface chemistry of APMT was characterized layer by layer using X-ray photoelectron spectroscopy (XPS) depth profiling. The results indicated a thin layer composed of oxides and hydroxides of Al, Cr, and Fe with varying proportions at different depths in the oxide layer. The microstructure of the oxide layer was characterized using X-ray diffraction and showed the presence of α-Al2O3 and γ-Fe2O3/Fe3O4 after ramp heating. The results depict the ability of the APMT FeCrAl alloy to form a protective oxide layer in less than a second on exposure to steam under high ramp rates.