How To Read A Photoelectron Spectrum

Photoelectron spectroscopy (PES) is a powerful and versatile technique used to probe the electronic structure of matter. By measuring the kinetic energies and intensities of emitted photoelectrons, PES provides valuable insights into the elemental composition, chemical states, and electronic band structure of various materials. Mastering the interpretation of photoelectron spectra is crucial for researchers across diverse fields, including materials science, chemistry, and physics. This article provides a comprehensive guide to understanding and interpreting photoelectron spectra, covering the fundamental principles, spectral features, and practical considerations necessary for extracting meaningful information from experimental data.

The Fundamentals of Photoelectron Spectroscopy

At its core, PES relies on the photoelectric effect, where photons of known energy interact with a sample, causing electrons to be ejected. The kinetic energy of these photoelectrons is then measured using an electron analyzer. The energy conservation principle dictates the relationship between the photon energy (hν), the binding energy (E_B) of the electron, and its kinetic energy (E_K):

hν = E_B + E_K + Φ

Where Φ is the work function of the spectrometer, representing the minimum energy required to remove an electron from the sample to the vacuum level. Since hν and Φ are known, measuring E_K allows for the determination of E_B, which is a characteristic property of the element and its chemical state.

Types of Photoelectron Spectroscopy:

• X-ray Photoelectron Spectroscopy (XPS): Uses X-ray photons (typically Mg Kα at 1253.6 eV or Al Kα at 1486.6 eV) to probe core-level electrons. XPS is surface-sensitive, with a typical probing depth of a few nanometers.
• Ultraviolet Photoelectron Spectroscopy (UPS): Employs ultraviolet photons (typically He I at 21.2 eV or He II at 40.8 eV) to investigate valence band electrons. UPS is even more surface-sensitive than XPS and provides information about the electronic structure near the Fermi level.
• Angle-Resolved Photoelectron Spectroscopy (ARPES): Measures the kinetic energy and emission angle of photoelectrons, allowing for the determination of the electronic band structure as a function of momentum (k). ARPES is particularly useful for studying crystalline materials.

Key Components of a Photoelectron Spectrum

A photoelectron spectrum is a plot of the number of detected photoelectrons as a function of their binding energy. Several key features contribute to the overall shape and information content of the spectrum.

• Core-Level Peaks: These peaks arise from electrons ejected from core levels, which are atomic orbitals with well-defined binding energies. The position, intensity, and shape of core-level peaks provide information about the elemental composition and chemical state of the sample.
• Valence Band Spectrum: This region of the spectrum corresponds to electrons ejected from the valence band, which is formed by the overlap of atomic orbitals in a solid. The valence band spectrum provides insights into the electronic structure, chemical bonding, and density of states near the Fermi level.
• Auger Peaks: These peaks are due to Auger electron emission, a process where an excited atom relaxes by filling a core-level vacancy with an electron from a higher energy level, simultaneously ejecting another electron (the Auger electron). Auger peaks can be used for elemental identification and to complement information obtained from core-level peaks.
• Loss Features: These features appear as satellite peaks or shoulders on the high binding energy side of core-level peaks. They arise from energy loss processes that photoelectrons undergo as they travel through the material, such as plasmon excitation or electron-hole pair creation.
• Background Signal: The background signal in a photoelectron spectrum is primarily due to inelastically scattered electrons that have lost some of their initial kinetic energy. The background intensity generally increases with increasing binding energy.

Interpreting Core-Level Spectra

Core-level spectra are a treasure trove of information about the elemental composition and chemical states of a sample. Here's a step-by-step guide to interpreting core-level spectra:

1. Identifying Elements:

• Each element has a unique set of core-level binding energies. By comparing the observed peak positions with reference databases (e.g., the NIST XPS Database), you can identify the elements present in the sample.
• Consider the relative intensities of different core-level peaks for the same element. These intensities are related to the photoionization cross-sections and the number of atoms present.

2. Determining Chemical States:

• The chemical environment of an atom influences its core-level binding energies. Changes in oxidation state, bonding environment, and coordination number can lead to shifts in the peak positions. These shifts are known as chemical shifts.
• A positive chemical shift (higher binding energy) typically indicates a more oxidized state, while a negative chemical shift (lower binding energy) suggests a more reduced state.
• Compare the observed chemical shifts with reference data for known compounds to identify the chemical states present in the sample.

3. Peak Fitting and Quantification:

• Core-level spectra often contain overlapping peaks from different chemical states. Peak fitting is a process of decomposing the spectrum into its constituent peaks, allowing for the determination of the relative amounts of each chemical state.
• Common peak fitting procedures involve using Gaussian, Lorentzian, or Voigt functions to model the peak shapes.
• The area under each peak is proportional to the concentration of the corresponding element and chemical state.
• Careful consideration of peak shapes, widths, and positions is crucial for accurate peak fitting and quantification.

4. Analyzing Peak Shapes and Widths:

• The shape and width of a core-level peak can provide additional information about the sample.
• Asymmetric peak shapes can indicate the presence of unresolved chemical states or the influence of solid-state effects.
• Peak broadening can be caused by factors such as instrumental resolution, vibrational broadening, or the presence of defects and disorder.

Example: Analyzing a Carbon 1s Spectrum:

A typical carbon 1s (C 1s) spectrum might exhibit several peaks corresponding to different carbon bonding environments.

• C-C/C-H: A peak at around 284.5 eV typically corresponds to sp³ hybridized carbon in aliphatic chains or adventitious carbon contamination.
• C-O: A peak at around 286.5 eV indicates carbon bonded to a single oxygen atom, such as in alcohols or ethers.
• C=O: A peak at around 287.8 eV corresponds to carbon doubly bonded to oxygen, as in carbonyl groups (ketones, aldehydes).
• O-C=O: A peak at around 289.0 eV indicates carbon in a carboxyl group or ester.
• Carbonate (CO₃^2-): A peak at around 290.0 eV suggests the presence of carbonate species.

By analyzing the positions and intensities of these peaks, you can gain insights into the carbon-containing functional groups present on the sample surface.

Interpreting Valence Band Spectra

Valence band spectra provide information about the electronic structure of a material, including the density of states (DOS) and the nature of chemical bonding.

1. Identifying Band Features:

• The valence band spectrum is composed of a series of peaks and features that correspond to different electronic states.
• The shape and position of these features are related to the atomic orbitals that contribute to the valence band and the interactions between atoms.
• Comparing the experimental valence band spectrum with theoretical calculations (e.g., density functional theory, DFT) can aid in the identification of specific electronic states.

2. Determining the Fermi Level:

• The Fermi level (E_F) is the energy level at which there is a 50% probability of finding an electron.
• In a photoelectron spectrum, the Fermi level is typically located at the binding energy where the valence band spectrum crosses the baseline.
• Accurate determination of the Fermi level is crucial for understanding the electronic properties of materials, such as their conductivity and work function.

3. Analyzing the Density of States:

• The shape of the valence band spectrum reflects the density of states (DOS), which is the number of electronic states per unit energy.
• Features in the DOS can be related to specific chemical bonds or electronic transitions.
• For example, in a metal, the DOS near the Fermi level is high, reflecting the presence of free electrons. In a semiconductor or insulator, the DOS near the Fermi level is low, indicating the presence of a band gap.

4. Investigating Chemical Bonding:

• The valence band spectrum can provide insights into the nature of chemical bonding in a material.
• For example, the presence of sharp, well-defined peaks in the valence band can indicate strong covalent bonding, while broad, featureless bands may suggest metallic bonding.
• Changes in the valence band spectrum upon chemical modification or doping can reveal changes in the electronic structure and bonding environment.

Interpreting Auger Spectra

Auger electron spectroscopy (AES) is often performed in conjunction with XPS to provide complementary information about the elemental composition and chemical states of a sample. Auger peaks arise from a three-step process:

1. Core-Level Ionization: An incident X-ray photon or electron beam creates a core-level vacancy.
2. Electron Relaxation: An electron from a higher energy level fills the core-level vacancy.
3. Auger Emission: The energy released during electron relaxation is transferred to another electron, which is then ejected from the atom as an Auger electron.

1. Identifying Elements:

• Like core-level peaks, Auger peaks have characteristic kinetic energies that can be used to identify elements.
• Auger transitions are typically labeled using a three-letter notation, such as KL₁L_2,3, where K represents the initial core level vacancy, L₁ is the level from which the electron fills the vacancy, and L_2,3 is the level from which the Auger electron is emitted.
• Reference databases can be used to identify Auger peaks based on their kinetic energies.

2. Chemical State Information:

• The kinetic energies of Auger peaks can also be influenced by the chemical environment of the atom.
• However, chemical shifts in Auger spectra are often more complex than those in core-level spectra due to the involvement of multiple electronic levels in the Auger process.
• The modified Auger parameter, which is the sum of the core-level binding energy and the Auger kinetic energy, is often used to characterize chemical states. The Auger parameter is less sensitive to charging effects than the individual core-level and Auger energies.

3. Peak Shape Analysis:

• The shape of an Auger peak can provide information about the electronic structure and bonding environment of the atom.
• For example, the shape of the carbon KLL Auger peak can be used to distinguish between different forms of carbon, such as graphite, diamond, and amorphous carbon.

Practical Considerations for Spectrum Interpretation

Obtaining accurate and reliable photoelectron spectra requires careful attention to experimental parameters and data processing procedures.

1. Sample Preparation:

• The surface sensitivity of PES makes sample preparation critical. The sample surface must be clean and representative of the bulk material.
• Common sample preparation techniques include cleaving, sputtering, and chemical etching.
• Care must be taken to avoid introducing contaminants or altering the chemical state of the sample during preparation.

2. Charge Compensation:

• Non-conducting samples can accumulate charge during PES measurements, leading to shifts in the peak positions and distortions of the spectrum.
• Charge compensation techniques, such as using a flood gun to provide low-energy electrons to neutralize the surface charge, are often necessary.

3. Energy Calibration:

• Accurate energy calibration is essential for determining the binding energies of core-level and valence band features.
• The spectrometer work function must be carefully determined and accounted for in the energy calibration.
• Reference materials with well-known core-level binding energies are often used for energy calibration.

4. Data Processing:

• Raw photoelectron spectra typically require several processing steps before they can be interpreted.
• Common data processing procedures include:
  • Background Subtraction: Removing the background signal due to inelastically scattered electrons. Shirley or Tougaard background subtraction methods are commonly used.
  • Smoothing: Reducing noise in the spectrum using smoothing algorithms, such as Savitzky-Golay smoothing.
  • Normalization: Normalizing the spectrum to a constant intensity to facilitate comparison between different samples or experimental conditions.
• Peak fitting and quantification require specialized software and careful attention to peak shapes, widths, and positions.

5. Artifacts and Limitations:

• Photoelectron spectra can be affected by various artifacts, such as:
  • Charging Effects: As mentioned earlier, charging can lead to shifts in peak positions and distortions of the spectrum.
  • Satellite Peaks: Satellite peaks can arise from various processes, such as shake-up and shake-off excitations, and can complicate the interpretation of core-level spectra.
  • Diffraction Effects: In crystalline materials, diffraction effects can lead to variations in the photoelectron intensity as a function of emission angle.
• It's important to be aware of these potential artifacts and to take them into account when interpreting photoelectron spectra.
• PES is a surface-sensitive technique, meaning that it only probes the outermost few nanometers of a material. The results may not be representative of the bulk material.

Examples of Photoelectron Spectroscopy Applications

PES is employed in a wide range of research and industrial applications. Here are a few examples:

• Materials Science: Characterizing the composition, chemical states, and electronic structure of thin films, catalysts, and nanomaterials.
• Surface Chemistry: Studying adsorption, oxidation, and corrosion processes on surfaces.
• Polymer Science: Analyzing the surface composition and chemical modifications of polymers.
• Semiconductor Industry: Monitoring the composition and quality of semiconductor devices.
• Environmental Science: Investigating the chemical composition of environmental samples, such as soil and water.

Conclusion

Interpreting photoelectron spectra requires a thorough understanding of the fundamental principles of PES, the key features of the spectrum, and the practical considerations for data acquisition and processing. By carefully analyzing core-level, valence band, and Auger spectra, researchers can gain valuable insights into the elemental composition, chemical states, electronic structure, and bonding environment of a wide range of materials. With its versatility and surface sensitivity, PES remains a powerful and essential tool for materials characterization and surface science. Continuous advancements in instrumentation and data analysis techniques are further expanding the capabilities and applications of photoelectron spectroscopy.