Completely understand X-ray photoelectron spectroscopy (XPS)

1.Introduction to XPS

(1) XPS, short for X-ray Photoelectron Spectroscopy, is a method that collects and utilizes the energy distribution of photoelectrons and Auger electrons when X-ray photons irradiate the surface of a sample.

XPS can be used for qualitative and semi-quantitative analysis. Information such as elemental composition, chemical states, and molecular structures of the sample's surface can be obtained from the peak position and peak shape in the XPS spectrum. The elemental content or concentration on the sample's surface can be derived from the peak intensity, although this is less commonly used.

(2) XPS is a typical surface analysis technique. The reason lies in the fact that although X-rays can penetrate deep into a sample, only photoelectrons emitted from a thin layer near the surface can escape.

The probing depth (d) of a sample is determined by the electron escape depth (λ), which is influenced by factors such as the wavelength of the X-rays and the state of the sample. Typically, the sampling depth is defined as d = 3λ. For metals, λ is typically 0.5-3 nm; for inorganic non-metallic materials, it is 2-4 nm; and for organic materials and polymers, it ranges from 4 to 10 nm.

2.Basic Principles of XPS

XPS can be used for qualitative analysis to determine the following:The elemental composition of the sample surface;The chemical states and molecular structure of the surface elements.

(1) XPS for Qualitative Analysis of Elemental Composition

The basic principle—involves photoionization: When a beam of photons irradiates the surface of a sample, the photons can be absorbed by electrons in the atomic orbitals of a particular element. This absorption causes the electron to break free from the atomic nucleus's binding force and be emitted from the atom with a certain amount of kinetic energy, turning into a free photoelectron. The atom itself becomes an excited ion. According to Einstein's photoelectric emission law, the kinetic energy of the emitted photoelectron can be described as:𝐸𝑘=ℎ𝜈−𝐸𝐵 where:𝐸𝑘 is the kinetic energy of the emitted photoelectron;hν is the energy of the X-ray source photons;𝐸𝐵is the binding energy of a specific atomic orbital (which varies for different orbitals). This implies that for a specific monochromatic source and a specific atomic orbital, the energy of the photoelectron is characteristic. When the energy of the excitation source is fixed, the energy of the photoelectron depends solely on the type of element and the specific atomic orbital from which it was ionized. Therefore, the binding energy of the photoelectron can be used to qualitatively analyze the type of element.

(2) XPS for Qualitative Analysis of Chemical States and Molecular Structure

Basic Principle: The binding energy of core electrons in atoms can vary depending on the chemical environment, which is reflected in the spectrum as peak shifts (chemical shifts). This variation in the chemical environment can result from different types or numbers of elements bonded to the atom, or the atom having a different chemical valence state. Notes: General rules:1)Oxidation typically increases the binding energy of core electrons; the more electrons lost due to oxidation, the larger the increase.2)Reduction generally decreases the binding energy; the more electrons gained during reduction, the greater the decrease in binding energy.3)For atoms with a given valence shell structure, the shift in binding energy is nearly identical across all core electron levels. This can be understood as follows: with oxidation, when electrons are lost, the remaining electrons experience a stronger attraction from the atomic nucleus, resulting in increased binding energy. Conversely, in reduction, the gain of electrons weakens the attraction experienced by all electrons, leading to reduced binding energy.

3.Detection Range of Elements with XPS

XPS typically uses Al K_α or Mg K_α X-rays as the excitation source and can detect all elements in the periodic table except for hydrogen (H) and helium (He). Generally, the detection limit is 0.1% (atomic percent).

XPS cannot detect H and He due to the following reasons: 1)Low Photoionization Cross-Section: The photoionization cross-section of H and He is low, resulting in weak signals.

2)High Mobility of H: The 1s electron in hydrogen tends to shift easily and, in most cases, transfers to nearby atoms, making it challenging to detect.

3)Lack of Core Electrons in H and He: H and He do not have core electrons; their outer electrons are used for bonding, and hydrogen exists as a nucleus. Therefore, when X-rays are used to excite, no photoelectrons are available for detection.

4.Specific Methods for Qualitative Analysis in XPS

(1) Survey Spectrum Analysis: Analysis of Element Types in Compounds

For a sample with unknown chemical composition, a full-spectrum scan should be performed to preliminarily determine the surface's chemical composition. The typical energy range for a survey scan is 0-1200 eV, as nearly all elements' strongest peaks fall within this range. Given the unique characteristic energy values of photoelectron lines and Auger lines for various elements, their binding energies can be compared to those in XPS reference manuals or databases to identify the presence of specific elements. Identification Sequence: 1)Identify lines for elements that are always present, such as carbon (C) and oxygen (O). 2)Identify the major strong lines for main elements and any related secondary strong lines in the sample. 3)Identify remaining weak lines by assuming they are the strongest lines for unknown elements. The XPS reference handbook commonly used is:Chastain, Jill, and Roger C. King, eds. Handbook of X-ray photoelectron spectroscopy: A reference book of standard spectra for identification and interpretation of XPS data. Eden Prairie, MN: Physical Electronics, 1995. For XPS databases, the following are commonly used: 1)srdata.nist.gov/xps/ElmSpectralSrch.aspx%3FselEnergy%3DPE 2)thermofisher.cn/cn/zh/home/materials-science/learning-center/periodic-table/other-metal/tin.html

(2)Narrow Region Scan (also called High-Resolution Spectrum): Analysis of Specific Element Chemical States and Structures

Charge Correction When measuring insulators or semiconductors with XPS, continuous emission of photoelectrons can result in a lack of electron replenishment, leading to electron depletion on the sample's surface. This phenomenon is called the "charge effect." It creates a stable surface potential (𝑉𝑠), which exerts some restrictive force on escaping electrons. As a result, the charge effect causes energy shifts, making the measured binding energy deviate from the actual value, leading to inaccurate test results. To correct for this deviation, which is called "charge correction," adjustments are made to compensate for the charge effect. Note: Charge correction is generally required for samples to accurately determine peak shifts and electron transfer.

Charge Correction Methods A common approach is to use the C1s peak of external contamination carbon as the reference peak. The difference between the measured value and the reference value (284.8 eV) serves as the charge correction value (Δ), which is used to correct the binding energy of other elements in the spectrum. Procedure: 1)Calculate the charge correction value: Standard peak position for carbon (usually 284.8 eV) minus the measured peak position for carbon = charge correction value (Δ). 2)Apply the charge correction to other spectra: Add Δ to the binding energy of the XPS spectrum of the element you want to analyze to get the corrected peak position (the intensity of the XPS spectrum remains constant throughout the process). The corrected XPS spectrum is obtained by plotting the corrected peak positions and intensities.

Determination of Element Valence States in High-Resolution Spectrum

High-resolution spectrum qualitative analysis for element valence states primarily focuses on:1)Comparing peak positions with standard values (from the NIST database or literature) to determine the chemical state. 2)For p, d, f orbitals with double peaks (due to spin-orbit splitting), the distance between the double peaks is also an important indicator for assessing the chemical state.

Other Common Uses of High-Resolution Spectrum

In many cases, the interest lies not just in the oxidation state of a specific surface element, but also in comparing chemical shifts before and after processing to indicate changes in surface chemical states or electron interactions between surface elements. Generally, when an element loses electrons, its binding energy shifts toward a higher field, indicating oxidation. Conversely, when an element gains electrons, the binding energy shifts toward a lower field, indicating reduction. For atoms with a given valence shell structure, the shift in binding energy for all core electrons is nearly identical. This tendency in electron movement can indicate the nature of electronic interactions between different elements. 5.XPS Quantitative Analysis Methods and Principles (Rarely Used)

A. Basic Principle

After X-ray irradiation, the intensity of emitted photoelectrons from a sample's surface (measured by the peak area of a characteristic peak) has a linear relationship with the concentration of the corresponding atom in the sample. This relationship can be used for semi-quantitative elemental analysis. A simple representation is: 𝐼=𝑛×𝑆 I=n×S, where :S is the sensitivity factor (based on empirical standard constants, sometimes requiring calibration). For two elements 𝑖and 𝑗 in a given solid sample, if their sensitivity factors 𝑆𝑖 and Sj are known, and their respective specific spectral intensities Ii and Ijare measured, then the ratio of their atomic concentrations is: 𝑛𝑖:𝑛𝑗=(I_i/S_i):(I-j/S_j) Thus, you can determine the relative concentration.

B. Why is XPS Considered a Semi-Quantitative Analysis Technique?

The intensity of photoelectrons is influenced not only by atomic concentration but also by factors like the mean free path of photoelectrons, surface smoothness, the chemical state of the elements, the intensity of the X-ray source, and the state of the instrument. Consequently, XPS usually cannot provide absolute elemental concentrations, only relative concentrations. Note: Sensitivity factors vary not only by element type but also by their state in the substance and the condition of the instrument. Therefore, uncalibrated measurements can yield significant errors in relative concentrations. In practical analysis, calibrated standard samples can be used to correct and measure the relative concentrations of elements.

6.XPS Spectrum

(1) Key Structures in the XPS Spectrum

An XPS spectrum generally includes the following structures: photoelectron lines, satellite peaks (also known as side bands), Auger electron lines, and spin-orbit splitting (SOS). 1)Photoelectron Lines: Each element has its characteristic photoelectron line, which is the primary basis for qualitative analysis. The peak with the highest intensity, the narrowest width, and the best symmetry is called the main peak in XPS. 2)Satellite Peaks (Side Bands): Standard X-ray sources (Al/Mg K_α1,2) are not monochromatic; they contain some higher energy minor lines (e.g., K_α3,4,5 and K_β), leading to satellite peaks in addition to the main peaks excited by K_α1,2. 3)Auger Electron Lines (KLL): After electron ionization, core-level vacancies occur. During relaxation, if another electron is excited to become a free electron, it becomes an Auger electron. Auger electron lines often accompany XPS but are broader and more complex, often appearing as groups of lines. A characteristic of Auger electron lines is that their kinetic energy does not depend on the incident photon energy ℎ𝜈. 4)Spin-Orbit Splitting (SOS): Due to coupling between orbital and spin motions, the orbital energy levels split. For inner shells with 𝑙>0, the spin-orbit splitting is represented by the quantum number 𝑗(where 𝑗=∣𝑙±𝑚_𝑠∣ ∣. If 𝑙=0,𝑗=1/2. If 𝑙=1,then 𝑗=1/2 or 𝑗=3/2. All other subshells (except s subshells) will split into two peaks. For a specific valence state of an element, the double-peak distance and peak height ratio for p, d, and f orbitals are generally constant. For p-orbitals, the intensity ratio is 1:2; for d-orbitals, it's 2:3; for f-orbitals, it's 3:4. Notably, the 4p line may have an intensity ratio lower than 1:2. The double-peak distance is also an important indicator of the chemical state of elements. 5)Ghost Peaks: Sometimes, if the X-ray source's anode is impure or contaminated, it produces impure X-rays. Photoelectron lines excited by non-anode materials are called "ghost peaks."

(2) Similarities and Differences Between XPS and EDS Full Spectrum Analysis

Both Energy Dispersive X-ray Spectroscopy (EDS) and XPS can be used for qualitative and quantitative elemental detection. Differences between EDS and XPS: Basic Principle: XPS uses X-rays to eject electrons, detecting electrons, whereas EDS uses electrons to generate X-rays, detecting the X-rays. Sensitivity and Information Obtained: EDS detects elemental composition and concentration but cannot determine the oxidation state of elements, with a higher detection limit (concentration > 2%), indicating lower sensitivity. XPS can determine both surface elements, their concentration, and their oxidation state, with a higher sensitivity (minimum detection concentration > 0.1%). Usage: EDS is often used with Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) for point scanning, line scanning, and area scanning, allowing for a straightforward analysis of surface (with SEM) or bulk (with TEM) elemental distribution. XPS is typically used independently to detect surface information, allowing for the determination of elemental composition, chemical states, and molecular structure. Note: EDS or EDX stands for Energy Dispersive X-ray Spectroscopy.