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XPS

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a surface-sensitive quantitative spectroscopic technique that measures the kinetic energy of photoelectrons emitted from a material's surface when irradiated by X-rays, enabling determination of the elemental composition, chemical state, and electronic state of elements within the outermost ~10 nm (~30 atomic layers) of a sample.^[1]^[2] This non-destructive method, based on the photoelectric effect, detects all elements except hydrogen and helium, providing binding energy data that reveals empirical formulas and bonding environments with a typical detection limit of 0.1–1 at.%.^[3]^[4] The fundamental principle of XPS relies on the interaction of monochromatic X-ray photons (commonly Al Kα at 1486.6 eV or Mg Kα at 1253.6 eV) with core-level electrons in atoms, ejecting them as photoelectrons whose kinetic energy (KE) relates to the photon energy (hν) and binding energy (BE) via the equation BE = hν - KE - φ, where φ accounts for spectrometer work function and sample charging effects.^[3]^[4] The short inelastic mean free path of photoelectrons (~0.5–3 nm in solids) confines analysis to the surface, making XPS ideal for studying thin films, interfaces, and contamination layers, though it requires ultra-high vacuum conditions (~10^{-9} Torr) to minimize scattering.^[1]^[5] Quantitative analysis is achieved through peak intensity measurements, often calibrated with sensitivity factors, yielding compositional data accurate to within a few percent for homogeneous samples.^[6] Historically, the photoelectric effect was first observed by Heinrich Hertz in 1887 and theoretically explained by Albert Einstein in 1905, laying the groundwork for XPS; however, its development as an analytical tool began in the 1950s with Kai Siegbahn's work at Uppsala University, leading to commercial instruments in the 1960s and Siegbahn's 1981 Nobel Prize in Physics for contributions to high-resolution electron spectroscopy.^[3]^[7] Early applications focused on core-level shifts for chemical state identification, evolving with advancements in instrumentation, such as synchrotron sources for higher resolution and angle-resolved XPS for depth profiling.^[8] Over the past three decades, XPS has become the most widely used surface analysis method due to its versatility and user-friendly commercial systems.^[6] XPS finds broad applications in materials science, including characterization of semiconductors for doping profiles and oxidation states, analysis of catalysts for active site identification, and evaluation of biomaterials for surface biocompatibility.^[9] In polymer and thin-film studies, it assesses adhesion, wettability, and corrosion resistance, while in nanotechnology, it probes nanoparticle compositions and interfaces.^[3]^[10] Limitations include sensitivity to surface contamination, challenges with insulating samples due to charging, and inability to distinguish isotopes, often necessitating complementary techniques like Auger electron spectroscopy for validation.^[1]^[2]

Overview

Definition and Principles

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a surface-sensitive analytical technique that determines the elemental composition and chemical states of materials by measuring the kinetic energies of photoelectrons emitted from the surface upon irradiation with X-rays.^[11]^[1] In XPS, monochromatic X-rays excite core-level electrons in atoms, ejecting them as photoelectrons whose energies provide information about the atomic binding energies, enabling identification of elements and their bonding environments.^[12] This method, first demonstrated in the 1950s, relies on the photoelectric effect as its underlying quantum mechanism.^[13] The core principle of XPS involves the calculation of binding energy (BE) for each photoelectron, given by the equation:

\text{BE} = h\nu - \text{KE} - \phi

where h\nu is the energy of the incident X-ray photon, KE is the measured kinetic energy of the emitted photoelectron, and \phi is the work function of the spectrometer.^[3]^[11]^[10] Core-level electrons from inner shells (such as 1s, 2p) are targeted because their binding energies are characteristic of the element and sensitive to the local chemical environment.^[12] XPS exhibits high surface sensitivity, probing only the top 5–10 nm of a sample, as photoelectrons are attenuated by inelastic scattering within the material; the inelastic mean free path (IMFP) of these electrons typically ranges from 0.5 to 3 nm, depending on the electron energy and material composition.^[3]^[1]^[11] This limited escape depth ensures that XPS primarily analyzes the outermost atomic layers, making it ideal for studying surface phenomena such as adsorption, corrosion, or thin films.^[10] Key outputs from XPS experiments include survey spectra, which provide broad scans to identify elements present based on their characteristic photoelectron peaks, and high-resolution spectra, which resolve fine details such as chemical shifts of 0.1–1 eV that indicate variations in oxidation states or bonding configurations—for instance, shifts in the binding energy of carbon 1s electrons distinguish between C–C and C–O bonds.^[3]^[1]^[11] These shifts arise from differences in the effective nuclear charge experienced by core electrons due to surrounding atoms.^[10]

Historical Context

The foundations of X-ray photoelectron spectroscopy (XPS) trace back to the photoelectric effect, first observed by Heinrich Hertz in 1887 and theoretically explained by Albert Einstein in 1905, which laid the groundwork for understanding electron emission from surfaces under irradiation.^[14]^[15] In the mid-20th century, Swedish physicist Kai Siegbahn pioneered the application of these principles to high-resolution electron spectroscopy at Uppsala University, focusing on electron spectroscopy for chemical analysis (ESCA) to probe chemical bonding and elemental composition.^[12]^[16] Siegbahn's innovations in the 1950s, including the development of an iron-free double-focusing magnetic spectrometer, enabled precise measurements of photoelectron energies, earning him the 1981 Nobel Prize in Physics for contributions to high-resolution electron spectroscopy.^[16]^[17] Key milestones in XPS's evolution occurred in the 1950s and 1960s, with Siegbahn's group at Uppsala recording the first well-defined XPS spectrum in 1954 using non-monochromatized X-ray excitation.^[18]^[19] During the 1960s, the group advanced ESCA through systematic studies of core-level chemical shifts, publishing seminal works that demonstrated its utility for surface chemical analysis, as detailed in their 1967 monograph.^[13]^[20] These efforts transformed electron spectroscopy from a basic research tool into a method capable of distinguishing chemical states, with early applications to gases, solids, and surfaces.^[21] The 1970s marked the technique's transition to widespread use, with commercialization by companies such as PerkinElmer, which introduced reliable XPS instruments integrated with vacuum systems for practical laboratory applications.^[22]^[23] During this period, the nomenclature shifted from ESCA—emphasizing chemical analysis—to XPS, highlighting the specificity of X-ray excitation and aligning with growing adoption in surface science.^[24]^[25] By the 1980s and 1990s, XPS became a standard analytical tool through integration with ultrahigh vacuum (UHV) systems, which minimized surface contamination and enabled cleaner measurements, and the development of angle-resolved techniques for depth profiling without sputtering.^[12]^[26] These advancements, building on Siegbahn's foundational work, solidified XPS's role in materials characterization across physics, chemistry, and engineering disciplines.^[27]

Theory

Photoelectric Effect

The photoelectric effect is a quantum mechanical phenomenon in which incident photons of sufficient energy interact with a material, ejecting electrons from its atoms or molecules. This process requires the photon frequency f to exceed a material-specific threshold frequency f_0, defined by the relation h f_0 = \phi, where h is Planck's constant and \phi is the work function representing the minimum energy needed to remove an electron from the surface. Below this threshold, no electrons are emitted regardless of light intensity, highlighting the particle-like nature of light.^[28] In 1905, Albert Einstein provided a theoretical explanation for the effect by extending Max Planck's quantum hypothesis to light, treating photons as discrete energy packets. He derived the equation for the maximum kinetic energy of the ejected photoelectrons:

KE_{\max} = h f - \phi,

where the excess photon energy h f - \phi is converted into the electron's kinetic energy after overcoming the work function. This model resolved discrepancies between classical wave theory and experimental observations, such as the linear dependence of KE_{\max} on frequency. For core-level electrons in atoms, the equation extends by replacing \phi with the orbital binding energy BE, allowing ejection from inner shells when h f > BE.^[28]^[29] The likelihood of photoionization is governed by the photoionization cross-section, which quantifies the effective interaction area for photon-electron ejection and varies with photon energy and orbital symmetry. Cross-sections decrease with increasing photon energy above the threshold but exhibit orbital-specific behavior due to differences in radial wavefunction overlap with the continuum state.^[29] Under the electric dipole approximation, valid for photon energies much less than the electron's rest energy, the angular distribution of emitted photoelectrons follows a \cos^2 \theta pattern relative to the polarization vector of the incident light, arising from the l = 1 selection rule in the dipole transition. This distribution reflects the photon's electric field vector aligning with the transition dipole moment, leading to preferential emission along the polarization direction for certain partial waves.

Electron Spectroscopy Fundamentals

In X-ray photoelectron spectroscopy (XPS), the detection of photoelectrons involves measuring the distribution of their kinetic energies to determine the binding energies of electrons in the sample, where the binding energy is calculated as the difference between the incident X-ray photon energy and the measured kinetic energy, adjusted for instrumental work function. This process begins with the photoelectric effect, in which monochromatic X-ray photons eject core electrons from atoms, and the resulting photoelectrons are collected and analyzed for their energy distribution to produce a spectrum that reveals elemental composition and chemical states. Typical energy resolution in modern XPS systems ranges from 0.5 to 1 eV, enabling the distinction of closely spaced binding energy levels.^[4]^[10] Selection rules in XPS dictate that only electrons from filled orbitals, particularly core-level electrons, produce detectable signals due to the requirement for a final state with a core hole that can be ionized by the incident photons. Valence band electrons, while present near the Fermi level, exhibit low photoionization cross-sections for typical X-ray energies (e.g., Al Kα at 1486.6 eV), resulting in weak signals that limit detailed valence band analysis in standard XPS setups. These rules arise from the dipole selection criteria in the photoemission process, favoring transitions from core orbitals with well-defined angular momentum.^[4]^[30] Inelastic scattering of photoelectrons occurs as they travel through the sample material, where collisions with atoms cause energy loss and attenuate the signal, thereby limiting the technique to surface analysis with an information depth of approximately 5–10 nm. The inelastic mean free path (IMFP), which quantifies the average distance an electron travels before an inelastic collision, is typically on the order of 0.5–3 nm for kinetic energies relevant to XPS (50–1500 eV) and is modeled by the TPP-2M empirical equation, which predicts λ based on electron kinetic energy E, material density of valence electrons N_v, bulk density ρ, and bandgap energy E_g through fitted parameters including the plasmon energy E_p = 28.8 (N_v ρ / M)^{0.5} (with M the molecular weight) and others (β, γ, C, D). This scattering process broadens peaks and reduces intensity for deeper origins, emphasizing the surface sensitivity of XPS.^[31]^[32]^[33] Chemical shifts in XPS manifest as variations in core-level binding energies due to changes in the local electrostatic potential around the atom, influenced by its chemical environment, oxidation state, or coordination. For instance, in transition metals, the shift from metallic to oxide states typically increases the binding energy by 3–5 eV, as seen in the 2p levels of nickel (e.g., Ni metal at ~852.6 eV vs. NiO at ~856 eV), reflecting the higher effective nuclear charge and reduced screening in the oxidized form. These shifts arise primarily from initial-state effects (changes in orbital energy) and final-state relaxation (screening of the core hole), providing a direct probe of chemical bonding without requiring theoretical modeling for basic identification.^[34]^[35]^[36]

Instrumentation

X-ray Sources

In X-ray photoelectron spectroscopy (XPS), X-ray sources provide the incident photons necessary to eject core-level electrons from the sample surface, enabling analysis of elemental composition and chemical states within the top few nanometers. These sources are typically laboratory-based or facility-scale, with photon energies selected to probe binding energies up to approximately 1500 eV for standard applications. The choice of source influences spectral resolution, background noise, and suitability for different sample types, such as insulating materials prone to charging. Non-monochromatic X-ray sources commonly utilize twin anode systems, where aluminum (Al Kα line at 1486.6 eV) and magnesium (Mg Kα line at 1253.6 eV) targets are alternately bombarded by electrons from a heated filament to generate characteristic X-rays. These dual-anode configurations allow flexibility in selecting photon energies for different binding energy ranges, with Al Kα preferred for higher-energy core levels and Mg Kα for lower ones. However, such sources produce a continuous bremsstrahlung background from decelerated electrons and satellite peaks arising from multiple electron transitions, which broaden peaks and increase noise in spectra.^[37]^[27] Monochromatic sources address these limitations by incorporating a bent quartz crystal monochromator, often in a Rowland circle geometry, to diffract and select a single X-ray wavelength—typically the Al Kα1 line at 1486.7 eV—while suppressing the bremsstrahlung continuum and satellite structure. This results in narrower linewidths (as low as 0.4 eV full width at half maximum) and reduced sample charging due to lower overall photon flux and energy spread, enhancing resolution for fine chemical shifts. Commercial systems operate at powers of 100–600 W, delivering typical fluxes of 10^9 to 10^11 photons per second onto the sample, sufficient for high signal-to-noise ratios in routine surface analysis.^[27]^[37]^[38] Synchrotron radiation sources offer superior tunability, with photon energies adjustable from 50 eV to over 10,000 eV via monochromators on insertion device beamlines, facilitating depth-profiling by varying inelastic mean free paths (from ~0.5 nm at low energies to ~10 nm at high energies) and resonant excitation for enhanced core-level intensities or site-specific probing. These advantages stem from the high brilliance (up to 10^12 photons/s/mm²/mrad²/0.1% bandwidth) and polarization of synchrotron light, enabling experiments inaccessible in labs, such as buried interface studies or time-resolved measurements. Prominent facilities include the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory and Diamond Light Source in the UK, where dedicated XPS endstations support such applications.^[39]^[40]^[41] Routine maintenance of laboratory X-ray sources is essential for longevity and safety, including water cooling systems to manage anode temperatures exceeding 1000°C and prevent thermal damage, with flow rates monitored to avoid overheating. Filament replacement is required every 100–500 hours of operation, depending on emission current, to maintain stable electron beams for consistent X-ray output. Safety measures focus on minimizing X-ray leakage through beryllium windows and shielding, with interlock systems, radiation surveys, and compliance with standards like those from the International Electrotechnical Commission to protect operators from unintended exposure.^[42]^[43]^[12]

Analyzers and Detectors

In X-ray photoelectron spectroscopy (XPS), the core of electron energy analysis lies in hemispherical analyzers, which are the predominant devices for resolving the kinetic energies of photoelectrons emitted from the sample. These analyzers consist of two concentric conductive hemispheres that function as electrodes, creating an electrostatic field to filter electrons based on their energy. Electrons enter through a narrow entrance slit and follow curved trajectories determined by the applied voltage difference between the inner and outer hemispheres; only those with a specific pass energy reach the exit slit. This design, pioneered in early XPS instrumentation, enables precise energy dispersion while maintaining high transmission efficiency.^[44]^[27] Operation of hemispherical analyzers typically employs fixed pass energy modes, where the pass energy—the kinetic energy of electrons allowed to traverse the analyzer—is held constant while scanning voltages on retarding lenses adjust the electron input. Two common variants include fixed analyzer transmission (FAT), which maintains constant absolute energy resolution across the spectrum, and fixed retarding ratio (FRR), which provides constant relative resolution but is less common in modern systems due to varying throughput. Electrostatic lens systems, positioned between the sample and analyzer, play a crucial role by focusing the divergent photoelectrons and transporting them efficiently to the entrance slit, often retarding their energy to match the selected pass energy for optimal performance. These lenses, composed of multiple electrode stages, enhance collection efficiency and enable spatial selectivity in analysis.^[44]^[3] Detection of the filtered electrons relies on high-gain multipliers to amplify the inherently low photoelectron signals. Channeltron detectors, which are single-channel electron multipliers utilizing a continuous dynode structure, provide gains of $10^6 to $10^8, enabling sensitive pulse counting for sequential energy scans. For parallel detection, microchannel plate (MCP) detectors—arrays of millions of microscopic channels acting as parallel multipliers—offer position-sensitive imaging with similar gains, allowing simultaneous collection of electrons across multiple energy bins at the analyzer's focal plane and improving signal-to-noise ratios in low-intensity measurements.^[27]^[45]^[46] Energy resolution in these systems, quantified as the full width at half maximum (FWHM) of a narrow peak like Ag 3d_{5/2}, is primarily governed by entrance and exit slit widths and the selected pass energy, with lower pass energies yielding better resolution at the cost of intensity. In monochromated XPS setups, typical resolutions achieve 0.4 eV FWHM or better, limited by factors such as analyzer geometry and source linewidth, enabling discrimination of subtle chemical shifts.^[44]^[47]^[27]

Vacuum Systems and Sample Stages

X-ray photoelectron spectroscopy (XPS) demands ultrahigh vacuum (UHV) conditions to preserve the integrity of the sample surface, as even trace residual gases can adsorb onto the surface or scatter emitted photoelectrons, compromising the technique's surface sensitivity.^[1] Typical operating pressures in the analysis chamber range from 10^{-9} to 10^{-10} Torr, ensuring minimal contamination during measurements that probe the top 5–10 nm of the sample.^[48]^[10] Achieving and maintaining UHV involves a combination of pumping systems, including turbomolecular pumps for initial roughing and high-speed evacuation, ion pumps for sustained low-pressure operation, and cryopumps or titanium sublimation pumps to capture residual gases like water vapor.^[49]^[50]^[51] Bakeout procedures are essential to reach base pressures, where the chamber and components are heated to 150–250°C for 12–48 hours to desorb adsorbed species, followed by controlled cooling under continuous pumping.^[52]^[44]^[53] Vacuum chambers for XPS are typically constructed from low-outgassing 304 or 316 stainless steel to minimize gas evolution, with inner linings of mu-metal (a nickel-iron alloy) providing magnetic shielding against Earth's field and stray fields that could deflect charged particles.^[54]^[55] The mu-metal, often 5 mm thick, is annealed post-assembly to optimize permeability, ensuring residual fields below 10 nT in the analysis region for precise electron trajectory control.^[54]^[56] Sample introduction occurs via load-lock systems, which are preliminary chambers pumped separately to allow air-exposed samples to reach intermediate vacuum (around 10^{-6} Torr) before transfer to the main UHV chamber, preventing full system venting and contamination.^[57] Transfer mechanisms include magnetically coupled rods or wobble sticks, sometimes supplemented by glove ports for in-vacuum manipulation, enabling safe handling without pressure compromise.^[58] Sample stages within the analysis chamber provide precise positioning and environmental control to facilitate varied XPS experiments. These stages typically offer XYZ translation for alignment, azimuthal rotation, and polar tilt (up to ±60°) to enable angle-resolved XPS (ARXPS), which probes depth-dependent composition by varying takeoff angles.^[59] Integrated heating and cooling capabilities extend from cryogenic temperatures (down to 20 K using liquid helium cryostats) to over 1000 K via resistive or electron bombardment heaters, allowing in-situ studies of thermal effects on surface chemistry.^[45]^[60]

Experimental Methods

Sample Preparation

Sample preparation for X-ray photoelectron spectroscopy (XPS) is crucial to ensure that the surface under analysis accurately represents the material's chemistry without introducing artifacts or contaminants, as XPS probes only the top 5-10 nm of the surface.^[6] Proper preparation minimizes adventitious contamination and preserves native surface states, enabling reliable quantitative and qualitative analysis.^[44] Ex-situ cleaning methods are commonly employed to remove gross contaminants prior to introducing the sample into the vacuum chamber. Solvent rinsing with isopropyl alcohol (IPA) or acetone effectively dissolves organic residues and particulates, often followed by drying under nitrogen flow to prevent recontamination.^[61] Ultrasonic baths in these solvents enhance cleaning efficiency by dislodging stubborn particles through cavitation, typically performed for 10-15 minutes at room temperature, though care must be taken to avoid damaging delicate samples.^[61] For inorganic contaminants like oxides, ion sputtering with Ar⁺ ions is widely used, employing energies of 0.5-5 keV and current densities of 1-10 μA/cm² to etch surface layers; however, this technique risks preferential sputtering, where elements with lower sputter yields are depleted, altering the surface stoichiometry.^[62]^[63] In-situ preparation techniques, performed within the vacuum environment, are preferred for generating pristine surfaces and avoiding atmospheric re-exposure. Fracturing or cleaving the sample in vacuum exposes fresh, uncontaminated interfaces, ideal for studying bulk materials or interfaces without oxide formation.^[6] Annealing under ultra-high vacuum desorbs volatile contaminants and reorganizes surface atoms, often at temperatures up to several hundred degrees Celsius depending on the material's stability.^[6] Mounting the sample securely on the holder is essential for stable analysis and to mitigate charging effects. For conductive samples, metal clips or direct grounding ensures electrical contact with the spectrometer, aligning Fermi levels and preventing peak shifts.^[44] Insulating samples are mounted using conductive indium foil, which provides both mechanical support and electrical conductivity when pressed into the foil, or double-sided carbon tape for powders and fragile materials, though the latter may contribute minor carbon signals.^[64] Contamination control begins with handling protocols to avoid introducing impurities, such as using powder-free gloves, clean tweezers, and minimizing fingerprints or skin oils on the analysis area. Atmospheric exposure should be limited, as even brief air contact leads to adsorption of hydrocarbons; vacuum transfer systems are thus important for sensitive samples to preserve surface integrity.^[65] A common indicator of such contamination is the adventitious carbon peak at 284.8 eV binding energy in the C 1s spectrum, serving as a marker for unavoidable background hydrocarbons.^[66] Charging during analysis, particularly for insulators, can be briefly addressed by ensuring proper mounting, though dedicated flood guns may be required for neutralization.^[44]

Measurement Techniques

X-ray photoelectron spectroscopy (XPS) measurements begin with the acquisition of survey scans to identify the elemental composition of the sample surface. These scans cover a wide binding energy range, typically from 0 to 1400 eV, using low energy resolution with step sizes of 1-2 eV to efficiently survey all detectable elements.^[67]^[44] Dwell times per step are generally set between 50 and 200 ms to balance acquisition speed and signal intensity, allowing for rapid initial characterization of the outermost few nanometers.^[68]^[3] Following survey scans, high-resolution scans are performed on specific regions of interest to resolve chemical states and fine spectral features. These scans focus on narrow energy windows, such as 20-50 eV centered on core-level peaks, with finer step sizes of 0.05-0.1 eV to achieve the necessary resolution for distinguishing binding energy shifts associated with different chemical environments.^[44]^[69] Multiple sweeps, often 10-50 passes, are accumulated to improve the signal-to-noise ratio without excessively prolonging acquisition times.^[70]^[3] Angle-resolved XPS (ARXPS) extends standard measurements by varying the photoelectron takeoff angle to non-destructively probe depth-dependent information from layered structures. Takeoff angles are typically varied from 15° to 90° relative to the surface normal, with surface sensitivity enhanced at grazing angles due to the probing depth scaling approximately with \cos \theta, where \theta is the takeoff angle and the effective depth is limited by the inelastic mean free path of the electrons.^[71]^[72] This technique provides compositional gradients over depths of 1-10 nm without material removal, relying on the angular dependence of signal intensity from subsurface layers.^[44] For deeper analysis beyond the surface, depth profiling combines XPS with sequential ion beam etching to reveal subsurface composition. Ion sources, such as Ar⁺ or cluster ions like C₆₀, are used for sputtering, with C₆₀ clusters preferred for organic and delicate materials due to their lower damage rates and reduced chemical alteration compared to monatomic ions.^[73]^[74] XPS spectra are acquired after each etch cycle, typically forming a sputter crater whose central region is analyzed to minimize edge effects, enabling profiles over hundreds of nanometers to microns in depth.^[44]^[3]

Data Analysis

Qualitative Interpretation

Qualitative interpretation of X-ray photoelectron spectroscopy (XPS) spectra begins with peak identification, where characteristic binding energies of core-level photoelectrons are matched against established databases to determine elemental composition. The NIST X-ray Photoelectron Spectroscopy Database serves as a primary reference, compiling over 22,000 line positions from peer-reviewed literature for elements across the periodic table.^[75] For example, the O 1s peak typically appears at approximately 532 eV binding energy for oxygen in metal oxides, while the C 1s peak from adventitious carbon or hydrocarbons is observed around 285 eV.^[76] These values provide initial elemental fingerprints, though calibration to a known reference, such as adventitious carbon, is often necessary to account for charging or instrumental shifts.^[77] Chemical shift analysis extends identification to reveal the local chemical environment, as binding energies vary by 1–5 eV depending on factors like electronegativity and oxidation state. Higher binding energies generally correspond to more positive oxidation states due to increased effective nuclear charge on core electrons, as seen in transition metal oxides where Fe^{3+} shifts to higher energy than Fe^{2+}.^[12] Additionally, spin-orbit coupling causes p, d, and f orbitals to split into doublets, such as the 2p_{3/2} and 2p_{1/2} components, with characteristic energy separations (e.g., ~20 eV for many 2p levels) and intensity ratios of 2:1 reflecting orbital degeneracies.^[44] These features enable differentiation of chemical states, like distinguishing metal from oxidized forms in alloys.^[78] Auger peaks, arising from Auger electron emission following core-hole decay, appear in XPS spectra and must be distinguished from photoelectron peaks for accurate interpretation. For oxygen, the KVV Auger transition exhibits a kinetic energy of approximately 510 eV, manifesting as a broad feature in the spectrum.^[79] Unlike photoelectron peaks, which shift in kinetic energy when the X-ray source energy changes (e.g., from Al Kα to Mg Kα), Auger peaks remain fixed because their kinetic energy is independent of the incident photon energy, relying instead on the differences in core-level binding energies involved.^[80] This property aids identification, particularly for light elements like oxygen where the Auger signal can overlap with photoelectron regions. Prior to peak assignment, background subtraction is essential to isolate true spectral features from inelastic scattering tails. The Shirley method iteratively constructs a background by assuming the inelastic tail is proportional to the integrated peak intensity above it, effectively removing the sloped background under core-level peaks.^[81] For more physically grounded modeling, the Tougaard approach accounts for electron transport and inelastic mean free paths using parameterized functions, such as those incorporating universal loss spectra, to subtract depth-dependent backgrounds more accurately.^[81] These techniques enhance the reliability of qualitative assignments by clarifying peak positions and shapes without altering intrinsic line widths.

Quantitative Analysis

Quantitative analysis in X-ray photoelectron spectroscopy (XPS) involves deriving atomic concentrations and layer thicknesses from measured peak intensities, enabling compositional and structural characterization of surfaces. This process relies on established models that account for photoionization probabilities, electron attenuation, and instrumental factors, assuming homogeneous samples within the analysis depth of approximately 5–10 nm.^[82] The atomic fraction C_i of element i in a homogeneous sample is calculated using the formula:

C_i = \frac{I_i / S_i}{\sum_j (I_j / S_j)}

where I_i is the background-subtracted peak intensity (typically the integrated area) for element i, and S_i is the relative sensitivity factor for that element and transition.^[82] The sensitivity factors S_i are derived from theoretical photoionization cross-sections, such as those calculated by Scofield using Hartree-Slater methods for Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) X-rays, combined with inelastic mean free path (IMFP) values and analyzer transmission functions.^[83] For example, Scofield cross-sections provide the subshell photoionization probabilities σ, with S_i \propto \sigma_i \lambda_i T(E_i), where λ_i is the IMFP at kinetic energy E_i and T(E_i) is the analyzer transmission.^[82] This approach yields atomic percentages when multiplied by 100, with typical accuracies of ±5–10% for major elements in well-characterized systems.^[82] For determining overlayer thicknesses on substrates, the attenuation of the substrate signal through the overlayer is commonly used. For a uniform overlayer of thickness t, the substrate peak intensity I_s is given by I_s = I_s^\infty \exp(-t / (\lambda_s \sin \theta)), where I_s^\infty is the intensity from the pure substrate, λ_s is the IMFP of substrate photoelectrons in the overlayer material, and θ is the electron take-off angle from the surface normal.^[82] Solving for t yields:

t = -\lambda_s \sin \theta \ln \left( \frac{I_s}{I_s^\infty} \right)

This simplified equation assumes flat geometry, negligible surface roughness, and similar IMFP values; for overlayers where film and substrate photoelectrons have differing kinetic energies, iterative corrections incorporating atomic densities N and cross-sections are applied.^[82] Thicknesses are typically accurate to ±10–20% for t < 10 nm, with angle-resolved XPS (ARXPS) enhancing depth resolution by varying θ.^[82] Instrumental corrections are essential for accuracy, particularly the analyzer transmission function T(E), which describes the energy-dependent efficiency of electron detection and is often empirically calibrated using standards like polycrystalline gold.^[84] For modern hemispherical analyzers, T(E) scales approximately as E^{-1} to E^{0.5}, depending on pass energy mode, and must be included in S_i to avoid systematic errors up to 50% at low kinetic energies.^[82] In insulating samples, matrix effects such as differential charging can distort peak shapes and intensities, requiring flood gun neutralization or ex-situ referencing to maintain quantitative reliability.^[82] Key error sources in quantitative XPS include assumptions of sample homogeneity and uniform overlayer coverage, which can lead to inaccuracies if lateral variations or island growth occur, as confirmed by imaging modes revealing heterogeneity.^[77] Validation typically involves comparing results to known standards, such as sputter-cleaned Au foil where the Au 4f_{7/2} peak is set at 84.0 eV binding energy to calibrate the energy scale and verify sensitivity factors.^[85] Such practices ensure traceable quantification per ISO 18118:2024 standards.^[86]

Applications

Surface and Interface Studies

X-ray photoelectron spectroscopy (XPS) is extensively employed in surface and interface studies due to its ability to probe the top 5–10 nm of a material, providing elemental and chemical state information essential for understanding atomic-scale interactions at boundaries. This technique excels in characterizing adsorption processes, where adsorbate coverage and bonding are quantified through peak intensity analysis, often revealing monolayer formations on substrates like metals or oxides. For instance, in studies of self-assembled monolayers, XPS data fitted to the Langmuir adsorption isotherm model demonstrate saturation coverage at approximately one molecular layer, as observed in alkanethiol films on gold surfaces. Additionally, valence band XPS measurements detect shifts in the Fermi edge position, correlating with work function modifications induced by adsorbate-induced dipoles, typically on the order of 0.5–1 eV for submonolayer coverages on clean metal surfaces. In interface chemistry, angle-resolved XPS (ARXPS) enables non-destructive depth profiling to assess interdiffusion and layering at buried interfaces, such as those between metals and polymers. By varying the electron takeoff angle, ARXPS distinguishes surface enrichment from subsurface diffusion; for example, in copper evaporated onto Teflon AF1600, the technique reveals a diffusion profile extending several nanometers into the polymer, with copper concentration decaying exponentially from the interface. Similarly, ARXPS quantifies native oxide thicknesses at semiconductor interfaces, such as the SiO₂/Si system, where layers of 1–2 nm are routinely measured through the attenuation of substrate silicon signals relative to the oxide component in Si 2p spectra. For catalysis research, in-situ XPS under near-ambient pressure conditions (up to 1 mbar) tracks active site speciation by monitoring oxidation state changes in metal catalysts. In palladium-based systems for fuel cell applications, operando XPS identifies transitions between metallic Pd(0) and oxidized Pd(II)/Pd(IV) states during oxygen reduction or CO oxidation reactions, with Pd 3d peak shifts of 1–2 eV indicating oxide formation that influences catalytic activity. These measurements, often performed at pressures relevant to practical devices, highlight how surface palladium oxide layers form and reduce dynamically, optimizing turnover frequencies in electrochemical environments. In corrosion studies, XPS monitors oxide layer growth and composition on metals like aluminum and iron, providing insights into passivation mechanisms. On aluminum, sequential XPS analysis after exposure to humid environments shows the evolution of Al₂O₃ layers from 2–5 nm thick, with O 1s and Al 2p spectra confirming hydroxyl incorporation at early stages of growth. For iron, XPS depth profiles of passive films on Fe-Al alloys reveal a bilayer structure, with an outer Fe-enriched oxide overlying an Al₂O₃-rich inner layer approximately 3 nm thick, correlating with enhanced corrosion resistance due to aluminum segregation.

Materials Characterization

X-ray photoelectron spectroscopy (XPS) plays a crucial role in characterizing the electronic and structural properties of advanced materials by providing surface-sensitive information on elemental composition, chemical states, and electronic structure. In semiconductors, XPS enables the analysis of doping profiles and surface band bending, which are essential for understanding charge carrier behavior and device performance. For thin films, it assesses stoichiometry and valence band features, aiding in the optimization of material functionality. Applications extend to nanomaterials and polymers, where XPS reveals compositional gradients and surface modifications critical for tailored properties. In semiconductors, XPS detects dopant incorporation through core-level shifts and peak intensities. For instance, nitrogen doping in n-type silicon is quantified via the N 1s peak at approximately 399 eV, allowing evaluation of dopant concentration and distribution near the surface.^[87] This technique is particularly useful for nanoscale doping, where self-assembled monolayers facilitate controlled nitrogen introduction, as confirmed by XPS alongside secondary ion mass spectrometry. Band bending at semiconductor surfaces, arising from charge accumulation or depletion, is measured by shifts in the Fermi edge position relative to core levels. In n-type GaN, for example, upward band bending of several electron volts is observed through extrapolation of the valence band onset, with Fermi edge shifts indicating surface state pinning.^[88] These measurements provide insights into interface potentials without invasive probes.^[89] For thin films, XPS determines stoichiometry by quantifying peak area ratios of constituent elements. In titanium dioxide (TiO₂) films, the Ti 2p/O 1s intensity ratio, typically around 0.5 for stoichiometric TiO₂, reveals oxygen deficiencies or excesses, influencing photocatalytic and dielectric properties.^[90] Valence band XPS spectra map the density of states (DOS), correlating peak features with electronic structure. In metal oxide thin films like In₂O₃, the valence band spectrum shows O 2p-dominated states near the Fermi level, with DOS profiles matching density functional theory calculations to assess conductivity mechanisms.^[91] This approach highlights hybridization effects without requiring ultra-high vacuum conditions beyond standard XPS setups. In nanomaterials, angle-resolved XPS (ARXPS) elucidates core-shell architectures by varying emission angles to probe depth-dependent compositions. For Au@Ag nanoparticles, ARXPS intensity ratios of Ag 3d to Au 4f peaks at different takeoff angles confirm silver shell thicknesses on the order of 1-5 nm, distinguishing core-shell from alloy structures.^[92] This non-destructive method aligns with transmission electron microscopy for validation. For carbon nanotubes, XPS identifies functionalization through changes in C 1s spectra, such as the emergence of π* shake-up peaks or shifts indicating sp³ hybridization. Functionalization with oxygen-containing groups, like carboxyls, increases the C-O/C-C ratio from near zero in pristine tubes to 0.1-0.3, enhancing dispersibility and reactivity.^[93] In polymers, XPS detects surface segregation of additives, which alters interfacial properties. In polystyrene blends, low-surface-energy additives like deuterated polystyrene migrate to the surface, enriching it relative to the bulk as evidenced by enhanced C 1s and D signals. For polystyrene incorporating oxygen-containing groups, such as via plasma treatment, XPS shows surface oxygen concentrations up to 20 at.%, with O 1s peaks at 532 eV indicating hydroxyl or carbonyl segregation driven by thermodynamic minimization of interfacial energy.^[94] This segregation impacts adhesion and wettability, quantifiable through quantitative XPS models.^[95]

Biological and Environmental Uses

X-ray photoelectron spectroscopy (XPS) has emerged as a valuable tool for investigating protein adsorption on biomaterial surfaces, where the nitrogen 1s (N 1s) peak at approximately 400.4 eV serves as a marker for amide groups in the protein backbone. For instance, studies on bovine serum albumin (BSA) adsorption onto stainless steel substrates have utilized high-resolution N 1s spectra to quantify protein coverage and orientation, revealing how surface chemistry influences biofouling processes critical for implant design and biocompatibility. This elemental and chemical specificity allows XPS to differentiate adsorbed proteins from contaminants, providing insights into interfacial interactions without invasive labeling.^[96]^[97] To preserve the native structure of delicate biomolecules, cryogenic XPS enables analysis of frozen-hydrated samples, minimizing dehydration artifacts that plague traditional vacuum-based methods. By rapidly freezing biological specimens at liquid nitrogen temperatures (around 77 K), cryo-XPS maintains the hydration shell around proteins and cells, allowing direct examination of surface composition in near-native states. Applications include bacterial cell envelopes, where cryo-XPS has identified key elemental distributions such as phosphorus and nitrogen in phospholipids, aiding understanding of membrane integrity and antibiotic resistance mechanisms. This approach contrasts with ambient drying, which can alter surface chemistry, and has been validated through comparative studies showing preserved spectral features in hydrated versus dehydrated samples.^[98]^[99] In environmental science, XPS facilitates speciation of heavy metal contaminants in soil and water matrices, distinguishing oxidation states that dictate toxicity and mobility. For chromium, XPS binding energy shifts in the Cr 2p region differentiate Cr(VI) (around 580 eV) from less toxic Cr(III) (around 577 eV), enabling assessment of remediation efficacy in polluted sites. Analysis of long-term contaminated soils has shown Cr(VI) comprising up to 20% of total chromium near industrial sources, correlating with leaching risks into groundwater. Similarly, for atmospheric aerosols, ambient pressure XPS coupled with aerodynamic lenses probes particle surfaces in real-time, revealing organic coatings and inorganic salts that influence cloud formation and air quality. These techniques handle heterogeneous environmental samples by focusing on the top 5-10 nm, where contaminant partitioning occurs.^[100]^[101] Beam-induced damage poses a significant challenge for organic and biological samples in XPS, often manifesting as decarboxylation or bond breakage under standard X-ray fluxes. Mitigation strategies include operating at low photon fluxes below 10^{12} photons/cm²/s to reduce secondary electron cascades that exacerbate damage, alongside using soft X-rays with photon energies under 1000 eV to limit penetration and photoabsorption in sensitive organics. These protocols have extended analysis times for amino acids and peptides from minutes to hours without spectral distortion, preserving quantitative accuracy for carbon, oxygen, and nitrogen functionalities.^[102]^[103] Hybrid techniques integrating XPS with infrared (IR) spectroscopy or atomic force microscopy (AFM) enhance biofilm studies by combining chemical speciation with morphological and mechanical data. For example, XPS identifies elemental shifts in extracellular polymeric substances during Pseudomonas biofilm development, while correlative FTIR maps vibrational modes of polysaccharides and AFM quantifies adhesion forces on substrates. Such multimodal approaches have elucidated antifouling mechanisms in marine environments, showing reduced nitrogen signals in treated biofilms indicative of disrupted protein matrices. This synergy addresses XPS's limitations in spatial resolution, providing a holistic view of biofilm dynamics on engineered surfaces.^[104]^[105]

Limitations and Advances

Technical Challenges

One of the primary technical challenges in X-ray photoelectron spectroscopy (XPS) is surface contamination, particularly the rapid adsorption of hydrocarbons from residual gases in the vacuum environment. At pressures around 10^{-6} Torr, a monolayer of contaminants can form in as little as 2 seconds, assuming a sticking coefficient of 1, necessitating ultra-high vacuum (UHV) conditions (typically 10^{-9} to 10^{-10} Torr) to minimize accumulation during analysis.^[3] Even in UHV, hydrocarbons adsorb continuously through competitive adsorption-desorption processes, leading to carbon overlayers that can obscure true surface composition unless spectra are acquired rapidly, often within minutes of sample introduction. Charging effects pose another significant issue, especially for insulating samples, where photoelectron emission creates positive charge buildup on the surface, shifting binding energy peaks to higher values—typically up to 10 eV depending on sample conductivity and charging uniformity.^[106] This distortion complicates accurate chemical state identification and peak assignment. Mitigation strategies include low-energy electron flood guns, which neutralize charge by supplying electrons (e.g., at 0–5 V bias), and referencing to the adventitious carbon C 1s peak at approximately 284.8 eV, though the latter can introduce errors if the carbon overlayer is influenced by the substrate.^[106] XPS is inherently limited to surface and near-surface analysis, with an information depth of only 5–10 nm due to the short inelastic mean free path of photoelectrons (typically 1–4 nm), precluding direct bulk material characterization.^[107] For deeper profiling beyond this range, complementary techniques like secondary ion mass spectrometry (SIMS) are required, as XPS depth profiling via ion sputtering introduces artifacts such as atomic mixing and preferential sputtering, limiting resolution for bulk-like compositions.^[107] Additionally, XPS exhibits insensitivity to hydrogen, as it lacks core electron levels suitable for detection, with the 1s electron being a valence orbital that produces negligible signal in standard core-level spectra.^[108] Indirect inference of hydrogen presence is possible through valence band analysis or chemical shifts in associated elements (e.g., excess oxygen indicating hydroxides), but these methods lack the directness and quantification of core-level measurements for other elements.^[108]

Modern Developments

One significant advancement in XPS since the early 2000s is ambient pressure X-ray photoelectron spectroscopy (APXPS), which enables measurements at pressures up to several bar (e.g., 1–2.5 bar as of 2025), bridging the gap between ultrahigh vacuum conditions and realistic operating environments.^[109] This technique uses differentially pumped electron analyzers and synchrotron sources to maintain signal quality while introducing gases or vapors, allowing in situ studies of dynamic processes such as catalytic reactions on surfaces under near-ambient conditions. For instance, APXPS has revealed real-time changes in oxide formation and adsorbate binding during heterogeneous catalysis, where traditional XPS is limited by vacuum requirements. Membrane inlets, often made of silicon nitride, facilitate precise gas dosing without compromising pressure differentials, enhancing operando analysis of electrochemical interfaces. Hard X-ray photoelectron spectroscopy (HAXPES), utilizing photon energies exceeding 2000 eV, has emerged as a complementary modality to conventional XPS, providing bulk-sensitive probing depths of 20-40 nm due to increased inelastic mean free paths of photoelectrons. Developed primarily at synchrotron facilities like SPring-8 starting in the early 2000s, HAXPES addresses the surface dominance of standard XPS by accessing buried interfaces in multilayer devices, such as those in semiconductors and photovoltaics. This depth resolution has proven essential for characterizing dopant distributions and oxidation states in complex nanostructures, with laboratory-based implementations now enabling routine non-destructive analysis of device architectures.^[110] Spin-resolved XPS, enhanced by magnetic dichroism effects, has advanced the study of magnetic properties in ferromagnetic surfaces since the 2010s, particularly through integration with hard X-ray sources for deeper penetration. By measuring spin polarization in photoelectrons excited with circularly polarized light, this technique detects magnetic asymmetries in core-level spectra, revealing spin-orbit coupling and exchange interactions at interfaces. For example, angular-resolved spin-resolved HAXPES has quantified magnetic dichroism in buried ferromagnetic thin films, correlating surface magnetism with bulk ordering. Such measurements are often combined with magneto-optic Kerr effect (MOKE) spectroscopy to validate in-plane magnetization, providing a non-destructive probe for spintronics materials.^[111] Multimodal XPS setups have proliferated in the 2010s and 2020s, integrating XPS with ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) in single ultrahigh vacuum chambers for holistic surface characterization. These systems enable correlated measurements of chemical composition (XPS), valence electronic structure (UPS), atomic ordering (LEED), and topography (STM), facilitating comprehensive insights into surface science phenomena like epitaxial growth and reconstruction. Commercial platforms, such as those from Scienta Omicron, support large samples up to 2-inch wafers and in situ preparation via heating or evaporation, while facilities like Brookhaven National Laboratory's Center for Functional Nanomaterials offer variable-temperature capabilities for dynamic studies.^[112]^[113] In parallel, artificial intelligence has transformed XPS data analysis since the early 2020s, with machine learning algorithms automating peak fitting and reducing subjectivity in interpreting complex spectra. The Expectation-Conditional Maximization (ECM) algorithm, for instance, performs high-throughput background subtraction and pseudo-Voigt profile decomposition, processing thousands of spectra to map chemical gradients in materials like SnS films with a mean processing time of approximately 33 seconds per dataset (median 7 seconds).^[114] Large language models, such as GPT-4, assist in real-time curve fitting and parameter interpretation at synchrotron beamlines, enhancing accessibility for non-specialists while improving accuracy in noisy or overlapping peak scenarios. These AI tools prioritize efficiency in large-scale experiments, such as ambient pressure studies, without replacing fundamental spectroscopic principles.^[115] Recent advances as of 2025 include cryo-XPS, which enables in situ analysis of frozen samples like lithium metal batteries to probe unstable interfaces and chemical compounds under cryogenic conditions, providing more reliable performance insights. Additionally, machine learning applications have expanded to quantitative XPS analysis of lattice distortions in alloyed materials, improving accuracy in composition determination for advanced semiconductors.^[116]^[117]

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Spin Orbit Splitting
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[81]
Introductory guide to backgrounds in XPS spectra and their impact ...
Sep 23, 2020 · This introductory guide provides some basic information about backgrounds in the XPS analysis, describes the types of background models that are commonly used,
[82]
Practical guides for x-ray photoelectron spectroscopy: Quantitative ...
Jul 8, 2020 · In the most general form of XPS analysis, the peak intensities are extracted from spectra and divided by sensitivity factors to provide numbers ...
[83]
https://doi.org/10.1016/0368-2048(76)80015-1
[84]
[PDF] Transmission Correction - CasaXPS
Transmission correction is central to quantitative XPS. Without proper transmission correction, a comparison of results measured from the same sample using ...
[85]
Gold | XPS Periodic Table | Thermo Fisher Scientific - HK
The Au4f7/2 peak can be a useful binding energy reference (Au4f7/2 at 84.00eV). Binding energy shifts may be observed with Au nanoparticles, however. Casaletto ...Missing: 4f7/ validation
[86]
https://www.iso.org/standard/81742.html
[87]
Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers
Jul 31, 2015 · This report presents a nitrogen-doping method by chemically forming self-assembled monolayers on silicon.
[88]
Accurate surface band bending determination on Ga-polar n-type ...
Nov 14, 2019 · In order to determine the band bending at the surface, the method of linear extrapolation on the valence band leading edge has widely been used ...
[89]
Extracting band edge profiles at semiconductor heterostructures ...
Aug 3, 2020 · In multi-layer structures, band-edge profiles comprise band bending within each layer and band offsets at the junctions of these layers. For ...
[90]
TiO2 Thin Films by Chemical Vapor Deposition: An XPS ...
Mar 23, 2009 · The present contribution is devoted to the XPS analysis of a TiO2 thin film obtained at 450 °C. Besides the wide scan spectrum, detailed spectra ...<|separator|>
[91]
Valence-band density of states and surface electron accumulation in ...
Oct 8, 2014 · The XPS results were correlated with density functional theory calculation of the partial density of states in the valence-band and semicore ...Article Text · INTRODUCTION · RESULTS AND DISCUSSION · SUMMARY AND...
[92]
How to Determine the Core-Shell Nature in Bimetallic Catalyst ...
Preparation of Au–Ag, Ag–Au core–shell bimetallic nanoparticles for surface-enhanced Raman scattering. Scripta Mater. 2008, 58, 862–865. [Google Scholar] ...
[93]
Controlled Functionalization of Carbon Nanotubes by a Solvent-free ...
XPS was a key technique for the detailed chemical analysis of the CNT surface. The formation of two major reaction products was observed, namely a cyclic benzyl ...
[94]
Surface Segregation in Blends of Hydrogenous Polystyrene and ...
Combined XPS and ToF-SIMS study of miscible polymer blend surfaces: polystyrene/poly(2,6-dimethyl-1,4-phenylene oxide) (PS/PDMPO). Surface and Interface ...
[95]
Surface Characterization of Polymer Blends by XPS and ToF-SIMS
This review provides a brief introduction to the principles of XPS and ToF-SIMS and their application to the study of the surface physical and chemical ...
[96]
XPS characterisation of BSA adsorption on stainless steel
The N1s XPS spectrum exhibits a signal at 400.4 eV which is characteristic of the amine or amide groups of the protein. The XPS data reveal that the protein is ...
[97]
Surface-protein interactions on different stainless steel grades - NIH
Protein adsorption was described by the relative atomic ratio between N (N 1s) and the oxidized carbon C 1s peaks (C2 + C3) as described in Sect. 3.1.1.
[98]
Applying Cryo-X-ray Photoelectron Spectroscopy to Study ... - PubMed
May 28, 2021 · This technique allows studies of intact fast-frozen hydrated samples without the need for pre-treatment procedures that may cause the cell ...
[99]
Comparative Study of NAP-XPS and Cryo-XPS for the Investigation ...
Apr 29, 2021 · However, recent developments in XPS allow for analysis of biological samples at near ambient pressure (NAP-XPS) or as frozen hydrated specimens ...
[100]
Chromium Distribution, Leachability and Speciation in a Chrome ...
Jan 10, 2022 · Moreover, X-ray photoelectron spectroscopy (XPS) results revealed the speciation of Cr in the long-term contaminated soils. A large amount of Cr ...Chromium Distribution... · 2. Materials And Methods · 3. Results And Discussion
[101]
A combined aerodynamic lens/ambient pressure x-ray photoelectron ...
Jan 20, 2010 · We discuss a new approach for the measurement of the surfaces of free aerosol particles with diameters from 50 to 1000 nm.
[102]
XPS insights: Sample degradation in X‐ray photoelectron ...
This XPS Insights paper gives a brief overview to this phenomenon, supported by specific examples and experimental advice to assess and minimise damage during ...
[103]
Soft-X-ray damage to biological samples - PubMed
These X-ray fluxes are lower than the flux required for the X-ray microscope observation of biological samples at a resolution higher than that of light ...Missing: XPS beam mitigation
[104]
[PDF] Combining AFM, XPS and chemical hydrolysis to understand the ...
To apprehend this complexity, an experimental approach combining AFM, XPS, and chemical hydrolysis followed by. HPAEC-PAD was developed to understand the cell ...
[105]
(PDF) Characterization of the Cell Surface and Cell Wall Chemistry ...
This work shows how the combination of potentiometric titrations, modeling, XPS, and FTIR spectroscopy allows a more comprehensive characterization of bacterial ...<|control11|><|separator|>
[106]
XPS guide: Charge neutralization and binding energy referencing ...
This guide deals with methods to control surface charging during XPS analysis of insulating samples and approaches to extracting useful binding energy ...
[107]
Evaluation methods for XPS depth profiling; A review - ScienceDirect
Oct 10, 2025 · In this review we present the advantages and disadvantages of the techniques available for tackling these issues focusing especially on ion ...
[108]
Hydrogen | XPS Periodic Table | Thermo Fisher Scientific - US
Hydrogen in any compound does not have a core electron so it will not produce a core photoelectron peak. Any signal would only appear in the valence band, ...
[109]
Recent applications of hard x-ray photoelectron spectroscopy
May 5, 2016 · This review will discuss several capabilities of HAXPES, focusing on recent scientific achievements. While interest in HAXPES has expanded ...Missing: post- | Show results with:post-<|separator|>
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[111]
BNL | CFN Equipment - STM/XPS/UPS/ISS/LEED/TPD
X-ray photoelectron spectroscopy (XPS) for analysis of surface elemental compositions, chemical states, relative atomic concentrations (1–10 nm from top surface) ...
[112]
Home - Scienta Omicron
### Summary of MULTIPROBE System with XPS, UPS, and Large Sample SPM
[113]
Full article: High-throughput XPS spectrum modeling with ...
We propose a fitting model that automatically conducts the background subtraction during high-throughput peak fitting.
[114]
Large Language Model-Informed X-ray Photoelectron Spectroscopy ...
Mar 27, 2024 · This study uses LLMs, specifically GPT-3.5/4 Turbo, to aid in XPS data analysis, enhancing curve-fitting and interpreting fitted parameters.