Research Powerhouse:X-ray Photoelectron Spectroscopy (XPS) Spectral Analysis

X-ray Photoelectron Spectroscopy (XPS) is an advanced analytical technique used in the microscopic analysis of electronic materials and components. It is often employed in conjunction with Auger Electron Spectroscopy (AES). XPS can measure the binding energy of core electrons and their chemical shifts more accurately than AES, providing valuable information for both chemical research and electronic material studies. Specifically, XPS offers insights into molecular structures and atomic valence states for chemical analysis, while also providing details on elemental composition, chemical states, molecular structures, and chemical bonds of various compounds for electronic materials research.

When analyzing electronic materials, XPS not only delivers comprehensive chemical information but also provides surface, localized region, and depth distribution data. Additionally, since the X-ray beam incident on the sample surface is a photon beam, it causes minimal damage to the sample. This feature is particularly advantageous for analyzing organic and polymer materials.

X-ray Photoelectron Spectroscopy (XPS) is an advanced analytical technique used in the microscopic analysis of electronic materials and components. It is often employed in conjunction with Auger Electron Spectroscopy (AES). XPS can measure the binding energy of core electrons and their chemical shifts more accurately than AES, providing valuable information for both chemical research and electronic material studies. Specifically, XPS offers insights into molecular structures and atomic valence states for chemical analysis, while also providing details on elemental composition, chemical states, molecular structures, and chemical bonds of various compounds for electronic materials research.

Basic Principles

The basic principle of X-ray photoelectron spectroscopy involves irradiating the sample surface with X-rays of a specific energy. This interaction causes electrons in the atoms of the sample to escape from their atomic orbitals, becoming free electrons. The process can be described by the following equation:

ℎ_𝜈 = 𝐸_𝑘 + 𝐸_𝑏 + 𝐸_𝑟 (1)

Where: h_ν: Energy of the X-ray photon, 𝐸_𝑘: Kinetic energy of the photoelectron, E_b: Binding energy of the electron, E_r: Recoil energy of the atom

Figure 1:Diagram Illustrating the Basic Principles of XPS

In this equation, 𝐸_𝑟 is typically very small and can be neglected.

For solid samples, the reference point for calculating the binding energy is not the stationary electron in a vacuum but the Fermi level. The energy consumed by an inner electron transitioning to the Fermi level is the binding energy ((E_b)). The energy required for an electron to escape from the Fermi level into a vacuum as a free electron is the work function ((\Phi)). The remaining energy becomes the kinetic energy of the free electron ((E_k)). Equation (1) can thus be rewritten as:

[ h\nu = E_k + E_b + \Phi \tag2 ]

[ E_b = h\nu - E_k - \Phi \tag3 ]

The work function (\Phi) of the instrument material is a constant, approximately 4 eV. Since the incident X-ray photon energy ((h\nu)) is known, measuring the kinetic energy ((E_k)) of the emitted electron allows the binding energy ((E_b)) of electrons in the solid sample to be determined.

The binding energies of orbital electrons in various atoms and molecules are specific and fixed. Therefore, by measuring the energy of photons emitted from the sample, the elemental composition of the sample can be identified.

The chemical environment of an element can cause slight variations in its binding energy, known as chemical shifts. The magnitude of these chemical shifts can be used to determine the state of the element. For example, when an element loses electrons to become a cation, its binding energy increases. Conversely, if the element gains electrons to form an anion, its binding energy decreases. By analyzing chemical shift values, the valence state and form of the element can be determined.

Characteristics of Electron Spectroscopy

Comprehensive Elemental Analysis:
Electron spectroscopy can analyze all elements except hydrogen (H) and helium (He). It directly measures the energy distribution of photoemitted electrons from individual energy levels of the sample, providing direct information about the electronic energy level structure.
Atomic-Level Precision:
In terms of energy range, if the information provided by infrared spectroscopy is referred to as "molecular fingerprints," the information obtained from electron spectroscopy can be described as "atomic fingerprints." It provides details about chemical bonding by directly measuring the energy levels of valence and core electrons. Spectral lines of the same energy levels in neighboring elements are well-separated, minimizing interference and offering strong specificity for elemental identification.
Non-Destructive Analysis:
Electron spectroscopy is a non-destructive analytical method that preserves the integrity of the sample during analysis.
High Sensitivity Surface Analysis:
This technique is highly sensitive and capable of ultra-trace surface analysis. It requires only about (10^-8) grams of sample for examination, with an absolute sensitivity reaching as low as (10^-18) grams. The analysis depth is approximately 2 nanometers, making it highly effective for surface and near-surface studies.

Applications of X-ray Photoelectron Spectroscopy (XPS)

Qualitative Elemental Analysis
Each element has characteristic electron binding energies, producing unique spectral lines in the energy spectrum. By analyzing the positions of these lines in the spectrum, all elements except hydrogen (H) and helium (He) can be identified. A full scan of the sample allows the detection of most or all elements in a single measurement.
Quantitative Elemental Analysis
The quantitative analysis in XPS is based on the intensity of the photoelectron peaks (proportional to the peak area), which reflects the relative concentration or content of atoms. In practice, quantitative analysis is performed by comparing the sample with standard references, achieving an analytical accuracy of 1%–2%.
Solid Surface Analysis
The solid surface refers to the outermost 1–10 atomic layers, with a thickness of approximately 0.1–1 nm. It has long been recognized that the composition and properties of the surface differ from the bulk material. Surface studies include analyzing the elemental and chemical composition, atomic valence states, surface energy state distribution, electron cloud distribution, and energy level structure of surface atoms. XPS is the most commonly used tool for such analyses and is applied in areas like surface adsorption, catalysis, metal oxidation and corrosion, semiconductors, electrode passivation, and thin film materials.
Compound Structure Identification
XPS precisely measures the chemical shifts of core electron binding energies, providing valuable information about chemical bonds and charge distribution in compounds.

X-ray Photoelectron Spectra

Main Peaks: Photoelectron lines that identify elements.
Accompanying Peaks: Auger lines, X-ray satellite lines, vibrational excitation lines, shake-up and shake-off lines, multiplet splitting lines, energy loss lines, and ghost lines, which aid in interpreting the spectrum and studying electronic structures.

Typical XPS Spectrum

Horizontal Axis: Electron binding energy or kinetic energy, directly reflecting electron shell/energy level structures.
Vertical Axis: Counts per second (cps), representing relative photoelectron intensity.

The spectral peaks directly correspond to the binding energies of atomic orbitals.

Background Radiation: Due to bremsstrahlung (inelastic scattering of primary and secondary electrons). At higher binding energies, the background increases gradually.

XPS Spectrum Analysis – Primary Peaks

The strongest photoelectron peak, typically the most intense, narrowest, and most symmetrical in the spectrum, is referred to as the primary peak of the XPS spectrum.
Each element (excluding H and He) has its strongest and most characteristic photoelectron line, serving as the primary basis for qualitative elemental analysis.
Generally:
- As (n) (principal quantum number) decreases, the peak intensity increases.
- For equal (n), as (l) (angular momentum quantum number) increases, the peak intensity also increases.
Common strong photoelectron lines include 1s, 2p₃/₂, 3d₅/₂, and 4f₇/₂.
In addition to the strong photoelectron lines, there are weaker photoelectron lines originating from other atomic shells, some of which are extremely weak.
The width of a photoelectron line is influenced by four factors:
- The intrinsic natural width of the element's signal.
- The natural width of the X-ray source.
- Instrumental broadening.
- The condition of the sample itself.

Figure 3:XPS Spectrum of Al Thin Film (Surface F Contamination)

Figure 4:Positions of the Strongest Characteristic Peaks for Selected Elements

XPS Spectrum Analysis – Peak Shifts

Chemical Shifts:

Factors causing chemical shifts include:

Different oxidation states.
Compound formation.
Variations in coordination number or lattice site occupancy.
Differences in crystal structures.

Physical Shifts:

Factors causing physical shifts include:

Surface charging effects.
Pressure effects in free molecules.
Solid-state thermal effects.

Chemical Shifts in XPS Spectra:

Chemical shifts manifest as peak shifts relative to the pure elemental peaks in the XPS spectrum.

An increase in valence electrons enhances shielding effects, lowering binding energy.
Conversely, a decrease in valence electrons increases the effective positive charge, raising binding energy.
For example, as the oxidation state of tungsten (W) increases, more valence electrons transfer to oxygen ions, causing the binding energy of 4f electrons to shift to higher energy.

XPS Spectrum Analysis – Auger Lines

Auger lines exhibit two key characteristics:

They are independent of the X-ray source; changing the X-ray source does not affect Auger lines.
Auger lines appear as groups of spectral lines.

In XPS, four series of Auger lines can be observed: KLL, LMM, MNN, and NOO.

KLL:
- The first letter represents the electron shell of the initial vacancy.
- The second letter denotes the electron shell of the electron filling the vacancy.
- The third letter indicates the shell of the electron emitting the Auger electron.

XPS Spectrum Analysis – Satellite Lines

X-ray Satellite Lines:

The monochromatic X-rays used to irradiate the sample are not perfectly monochromatic. For instance, conventional Al/Mg Kα₁,₂ radiation contains small amounts of higher-energy Kα₃,₄,₅,₆ and Kβ lines.

These lines arise from fluorescence effects, as electrons in the L₂, L₃, and M energy levels transition to the K level within the anode material.
These lines are collectively referred to as XPS satellite lines.

Thus, in addition to the main peaks excited by Kα₁,₂, XPS spectra also display smaller accompanying peaks.

XPS Spectrum Analysis – Multiplet Splitting Lines

When the valence shell of an atom contains unpaired spin electrons (e.g., transition elements in the d-block, lanthanide elements in the f-block, most noble gas atoms, and a few molecules like NO and O₂), the core-level vacancy formed during photoionization can couple with these unpaired electrons. This interaction can produce multiple final states, which appear as split lines in the XPS spectrum.

In XPS spectra, the most prominent splitting lines typically arise from spin-orbit coupling. These split lines are commonly seen in:

p orbitals: ( p_1.5, p_0.5 )
d orbitals: ( d_1.5, d_2.5 )
f orbitals: ( f_2.5, f_3.5 )

The energy difference between the split lines varies by element. However, not all elements exhibit significant spin-orbit coupling splitting, and the energy gap between splits can also vary depending on the chemical state.

XPS Spectrum – Shake-Up and Shake-Off Lines

During photoemission, the sudden formation of a core-level vacancy can cause a change in the central potential of the atom, triggering transitions in outer-shell electrons. This results in two phenomena:

Shake-Up: Outer-shell electrons are excited to higher energy levels.
Shake-Off: Outer-shell electrons are excited to a non-bound continuum, becoming free electrons.

Both shake-up and shake-off processes consume energy, reducing the kinetic energy of the initial photoelectron, which is reflected as reduced-energy peaks in the XPS spectrum.

XPS Spectrum Analysis – Energy Loss Lines

Photoelectron energy loss lines arise when photoelectrons undergo inelastic collisions while traversing the sample surface, losing energy. These appear as secondary peaks in the spectrum.

The magnitude of characteristic energy loss is sample-dependent.
The intensity of energy loss peaks depends on the sample’s properties and the kinetic energy of the electrons passing through it.

XPS Spectrum Analysis – Ghost Lines

Unexplained photoelectron lines in XPS, often called ghost lines, may originate from:

Impurities or contamination in the anode material.
X-rays generated from trace elements in these impurities.

XPS Spectrum Analysis – Line Identification Procedure

Identify C and O Lines:
Since carbon and oxygen are commonly present, first identify their photoelectron lines, Auger lines, and other related spectral lines.
Locate Strong Peaks:
Using peak position tables from X-ray photoelectron spectroscopy handbooks, determine other strong peaks and label their corresponding secondary peaks. Note that some elemental peaks may overlap or interfere with one another.
Identify Remaining Weak Peaks:
Assume that remaining weak peaks correspond to major peaks of elements present in low concentrations. For unresolved small peaks, verify if they could be ghost peaks associated with already-identified elements.
Validate Identification Results:
For ( p ), ( d ), and ( f ) doublet lines, the spacing and intensity ratios are typically constant:
- p peaks: Intensity ratio of ( 1:2 )
- d peaks: Intensity ratio of ( 2:3 )
- f peaks: Intensity ratio of ( 3:4 )