X-ray photoelectron spectroscopy (XPS) is a powerful surface analysis technique that reveals the elemental composition and chemical states of materials. By bombarding a sample with X-rays and measuring the ejected electrons, XPS provides crucial insights into surface properties and reactions.

Understanding XPS principles is key to grasping surface spectroscopy techniques. From photoemission basics to instrument components, XPS knowledge enables researchers to probe the top layers of materials, making it invaluable for fields like materials science, chemistry, and engineering.

XPS Fundamentals

Principles and Applications

• XPS is a surface-sensitive spectroscopic technique that measures the elemental composition, chemical state, and electronic state of the elements within a material
• XPS spectra are obtained by irradiating a solid surface with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material being analyzed
• XPS requires ultra-high vacuum (UHV) conditions (typically 10^-9 to 10^-10 mbar) to ensure that the emitted photoelectrons reach the detector without interference from gas phase collisions
• XPS is widely used for surface analysis in various fields, including materials science (studying surface properties of novel materials), chemistry (investigating chemical reactions and catalysis), physics (exploring electronic structure and band gaps), and engineering (characterizing thin films and coatings), to study the properties and behavior of surfaces and interfaces
• XPS can provide quantitative information about the surface composition, including the identification of elements present, their relative abundance, and their chemical state such as oxidation state (e.g., distinguishing between Fe^2+ and Fe^3+) and bonding environment (e.g., differentiating between C-C, C-O, and C=O bonds)

Quantitative Analysis

• XPS is a quantitative technique, allowing for the determination of the relative atomic concentrations of elements present on the surface
• The atomic concentration of an element is calculated by measuring the area under its corresponding photoelectron peak and applying sensitivity factors that account for differences in photoionization cross-sections and instrument response
• Quantitative analysis in XPS requires careful consideration of factors such as peak overlaps, background subtraction, and sample charging, which can affect the accuracy of the results
• XPS can detect elements with atomic numbers greater than 2 (He), with a typical detection limit of 0.1 to 1 atomic percent, depending on the element and the instrument configuration
• Depth profiling can be achieved in XPS by combining the technique with ion sputtering, allowing for the analysis of the elemental composition and chemical state as a function of depth below the surface (e.g., studying the composition of layered structures or the diffusion of elements across interfaces)

Photoemission in XPS

Photoemission Process

• Photoemission is the process by which electrons are emitted from a material when it absorbs electromagnetic radiation, such as X-rays, with sufficient energy to overcome the work function of the material
• In XPS, the incident X-rays have a fixed energy (typically Al Kα at 1486.6 eV or Mg Kα at 1253.6 eV), which is greater than the binding energy of the core-level electrons in the atoms of the sample
• When an X-ray photon is absorbed by an atom, it can cause the ejection of a core-level electron (photoelectron) from the atom if the photon energy is greater than the electron's binding energy
• The kinetic energy of the emitted photoelectron depends on the incident X-ray energy and the binding energy of the electron in the atom, as described by the photoelectric effect equation: $KE = hν - BE - φ$, where $KE$ is the kinetic energy of the photoelectron, $hν$ is the incident X-ray energy, $BE$ is the binding energy of the electron, and $φ$ is the work function of the material
• The photoemission process in XPS is governed by the conservation of energy, with the incident X-ray energy being partitioned between the binding energy of the electron and the kinetic energy of the emitted photoelectron

Photoelectron Generation

• Photoelectrons are generated when the incident X-rays interact with the core-level electrons in the atoms of the sample
• The probability of photoelectron generation depends on the photoionization cross-section, which is a measure of the likelihood of an electron being ejected from an atom by a photon of a given energy
• Photoionization cross-sections vary for different elements and subshells (e.g., 1s, 2s, 2p), and they generally decrease with increasing atomic number and binding energy
• The angular distribution of the emitted photoelectrons is anisotropic, with a higher probability of emission in the direction perpendicular to the sample surface, which is exploited in angle-resolved XPS (ARXPS) to study the depth distribution of elements and chemical states
• The escape depth of photoelectrons is limited by inelastic scattering events, such as electron-electron and electron-phonon interactions, which cause the photoelectrons to lose energy and change direction, limiting the probing depth of XPS to the top few nanometers of the sample

Binding Energy vs Kinetic Energy

Binding Energy

• The binding energy (BE) of an electron is the energy required to remove the electron from its atomic orbital to the vacuum level, which is the energy level at which an electron can escape from the material's surface
• Binding energies are characteristic of specific elements and their chemical states, allowing for the identification of elements and their chemical environment in the sample
• Core-level binding energies are sensitive to the local chemical environment of an atom, with changes in the oxidation state, bonding, and coordination leading to shifts in the binding energy, known as chemical shifts
• Chemical shifts in binding energy can be used to differentiate between different chemical states of an element, such as distinguishing between metallic and oxidized forms of an element (e.g., Cu^0 vs Cu^2+) or identifying the presence of different functional groups (e.g., C-C, C-O, C=O) in organic compounds
• Binding energy reference levels are typically chosen based on well-defined peaks in the XPS spectrum, such as the Au 4f7/2 peak at 84.0 eV or the C 1s peak of adventitious carbon at 284.8 eV, to account for sample charging and to enable comparisons between different instruments and laboratories

Kinetic Energy and Work Function

• The kinetic energy (KE) of a photoelectron is the energy it possesses after being ejected from the atom, and it depends on the incident X-ray energy and the binding energy of the electron
• The work function (φ) is the minimum energy required to remove an electron from the Fermi level of a material to the vacuum level, and it is a characteristic property of the material
• The relationship between binding energy, kinetic energy, and work function is described by the photoelectric effect equation: $KE = hν - BE - φ$, where $hν$ is the incident X-ray energy
• In XPS, the binding energy of an electron is determined by measuring the kinetic energy of the emitted photoelectron and using the known values of the incident X-ray energy and the work function of the material
• The work function of a material can vary depending on factors such as surface composition, crystallographic orientation, and the presence of adsorbates or contaminants, which can lead to shifts in the measured kinetic energies of photoelectrons
• Calibration of the binding energy scale in XPS is typically achieved by measuring the Fermi edge of a clean metallic reference sample (e.g., gold or silver) and setting the Fermi level to zero binding energy, allowing for the accurate determination of binding energies across different samples and instruments

XPS Instrument Components

X-ray Source and Monochromator

• X-ray source: Generates the incident X-rays used to excite the sample, typically using an Al Kα (1486.6 eV) or Mg Kα (1253.6 eV) anode
• The X-ray source consists of a cathode (electron emitter) and an anode (target material) housed in a high-vacuum chamber
• Electrons emitted from the cathode are accelerated towards the anode by a high voltage (typically 10-15 kV), causing the emission of characteristic X-rays from the anode material
• Monochromator (optional): Filters the X-ray beam to improve energy resolution and reduce satellite peaks, resulting in higher-quality spectra
• Monochromators are typically based on Bragg diffraction from a single crystal (e.g., quartz or germanium) or a multilayer mirror, which selects a narrow energy range around the desired X-ray line (e.g., Al Kα or Mg Kα)
• The use of a monochromator reduces the background signal and improves the signal-to-noise ratio in XPS spectra, allowing for better resolution of closely spaced peaks and the detection of subtle chemical shifts

Sample Stage and Electron Energy Analyzer

• Sample stage: Holds the sample in place and allows for precise positioning and manipulation of the sample during analysis
• The sample stage is typically equipped with a sample holder that can accommodate a variety of sample sizes and shapes, and it may include features such as sample heating, cooling, or electrical biasing
• The sample stage is mounted on a manipulator that allows for the adjustment of the sample position in three dimensions (x, y, and z) and the rotation of the sample around one or more axes, enabling the optimization of the sample orientation for analysis
• Electron energy analyzer: Measures the kinetic energy of the emitted photoelectrons, typically using a hemispherical or cylindrical sector analyzer
• The electron energy analyzer consists of two concentric hemispheres or cylinders with a potential difference applied between them, creating an electric field that disperses the photoelectrons based on their kinetic energy
• Photoelectrons entering the analyzer are deflected by the electric field, with higher kinetic energy electrons following a longer path and lower kinetic energy electrons following a shorter path, allowing for the separation of photoelectrons based on their kinetic energy
• The energy-dispersed photoelectrons are then focused onto a detector, which measures the intensity of the photoelectron signal as a function of kinetic energy, generating the XPS spectrum

Detector and Vacuum System

• Detector: Counts the number of photoelectrons at each kinetic energy, usually using a channeltron or multichannel plate detector
• Channeltron detectors are single-channel electron multipliers that amplify the photoelectron signal by secondary electron emission, providing high sensitivity and fast response times
• Multichannel plate detectors consist of an array of miniature electron multipliers that allow for the simultaneous detection of photoelectrons over a wide energy range, improving the data acquisition speed and the signal-to-noise ratio
• The detector is typically positioned at the focal plane of the electron energy analyzer, where the energy-dispersed photoelectrons are focused, and it may be equipped with a position-sensitive anode (e.g., resistive anode or CCD) to enable parallel detection of photoelectrons at different energies
• Ultra-high vacuum (UHV) system: Maintains the necessary low-pressure environment (typically 10^-9 to 10^-10 mbar) to ensure that the emitted photoelectrons can travel from the sample to the detector without interference from gas phase collisions
• The UHV system consists of a series of pumps (e.g., turbomolecular, ion, and titanium sublimation pumps) and vacuum gauges that work together to achieve and maintain the required vacuum level
• The UHV environment is essential for XPS measurements, as it prevents the scattering and energy loss of photoelectrons by collisions with gas molecules, ensures the cleanliness of the sample surface, and extends the lifetime of the X-ray source and detector components

Data Acquisition and Processing

• Data acquisition and processing system: Collects and analyzes the XPS data, generating spectra and allowing for the identification and quantification of elements and their chemical states
• The data acquisition system typically consists of a computer interface that controls the electron energy analyzer, detector, and sample stage, and it records the photoelectron intensity as a function of kinetic energy or binding energy
• Data processing software is used to analyze the XPS spectra, including background subtraction, peak fitting, and quantification of elemental composition and chemical states
• Background subtraction is performed to remove the contribution of inelastically scattered electrons and secondary electrons from the XPS spectrum, using methods such as linear, Shirley, or Tougaard background subtraction
• Peak fitting is used to deconvolute overlapping peaks and to extract information about the chemical states of elements, by fitting the experimental data with a series of model functions (e.g., Gaussian, Lorentzian, or Voigt) that represent the individual photoelectron peaks
• Quantification of elemental composition and chemical states is achieved by integrating the area under the fitted peaks and applying sensitivity factors that account for differences in photoionization cross-sections and instrument response, yielding the relative atomic concentrations of elements and their chemical states on the sample surface