surface science unit 6 study guides

Key Concepts and Principles

• X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), and Ultraviolet Photoelectron Spectroscopy (UPS) are surface-sensitive techniques that provide information about the elemental composition, chemical state, and electronic structure of surfaces
• XPS and UPS are based on the photoelectric effect, where incident photons cause the emission of electrons from the sample surface
  • XPS uses X-ray photons, typically from an Al Kα or Mg Kα source, with energies of 1486.6 eV and 1253.6 eV, respectively
  • UPS employs ultraviolet photons, usually from a He discharge lamp, with energies of 21.2 eV (He I) or 40.8 eV (He II)
• AES relies on the Auger effect, where an electron from a higher energy level fills a core hole created by the initial ionization, resulting in the emission of an Auger electron
• The kinetic energy of the emitted electrons depends on the binding energy of the atomic orbitals and the work function of the material, providing element-specific information
• The escape depth of the emitted electrons is limited to a few nanometers, making these techniques highly surface-sensitive
• Chemical shifts in the binding energies of the emitted electrons provide information about the chemical state and bonding environment of the elements present on the surface
• The intensity of the detected photoelectron or Auger electron peaks is related to the concentration of the corresponding elements on the surface

Instrumentation and Equipment

• XPS, AES, and UPS instruments consist of three main components: a photon source, an electron energy analyzer, and a detection system
• The photon source generates the incident photons that interact with the sample surface
  • For XPS, X-ray sources such as Al Kα or Mg Kα anodes are commonly used
  • UPS employs He discharge lamps that emit ultraviolet photons
  • AES often uses an electron gun to create the initial core hole
• The electron energy analyzer measures the kinetic energy of the emitted electrons
  • Hemispherical analyzers are widely used, consisting of two concentric hemispheres with a potential difference applied between them
  • The electrons are deflected by the electric field in the analyzer, and only electrons with a specific energy range can pass through the exit slit and reach the detector
• The detection system typically consists of an electron multiplier or a multichannel detector that amplifies and counts the electrons reaching the detector
• Ultra-high vacuum (UHV) conditions ($10^{-9}$ to $10^{-10}$ mbar) are necessary to minimize surface contamination and ensure the unimpeded travel of electrons from the sample to the detector
• Sample manipulation systems, such as precision stages and sample holders, allow for the positioning and orientation of the sample relative to the photon source and electron analyzer

Sample Preparation and Handling

• Proper sample preparation is crucial for obtaining reliable and reproducible results in XPS, AES, and UPS measurements
• Samples must be clean and free of contaminants to ensure that the measured signals originate from the surface of interest
• Common sample preparation techniques include:
  • Mechanical cleaning: Abrasion with sandpaper or polishing with diamond paste to remove surface layers and expose a fresh surface
  • Chemical cleaning: Rinsing with solvents (acetone, ethanol) or etching with acids to remove organic contaminants and surface oxides
  • Ion sputtering: Bombardment with inert gas ions (Ar$^+$) to remove surface layers and contaminants, often followed by annealing to restore surface order
• Samples must be compatible with the UHV environment, meaning they should have low vapor pressure and be stable under the measurement conditions
• Conducting samples are preferred to avoid surface charging effects, which can shift the measured electron energies
  • Non-conducting samples can be measured using charge compensation techniques, such as electron flood guns or conductive coatings (gold, carbon)
• Sample mounting should ensure good electrical contact between the sample and the spectrometer to prevent charging and provide a reliable energy reference
• Samples should be handled with clean tools and stored in a clean environment to minimize contamination before and after the measurements

Data Collection Techniques

• XPS, AES, and UPS data are collected by measuring the kinetic energy and intensity of the emitted electrons
• Survey scans are performed over a wide energy range to identify the elements present on the sample surface
  • For XPS, survey scans typically cover a binding energy range of 0-1000 eV
  • UPS survey scans focus on the valence band region, usually within 0-20 eV binding energy
• High-resolution scans are acquired for selected energy regions to obtain detailed information about the chemical state and bonding environment of specific elements
  • Narrow energy windows (20-50 eV) are used to resolve the core-level peaks and their chemical shifts
• The electron energy analyzer is set to a specific pass energy to control the energy resolution and signal intensity
  • Lower pass energies provide better energy resolution but lower signal intensity
  • Higher pass energies are used for survey scans to increase the signal intensity and reduce the acquisition time
• Data acquisition parameters, such as step size, dwell time, and number of scans, are optimized to achieve the desired signal-to-noise ratio and energy resolution
• Angle-resolved measurements can be performed by varying the angle between the sample surface and the electron analyzer
  • Changing the emission angle provides depth-sensitive information, as the escape depth of the electrons depends on their kinetic energy and the material properties
• In situ measurements can be conducted to study surface processes under controlled conditions, such as adsorption, desorption, or chemical reactions
  • Specialized sample environments (reaction cells, gas dosing systems) are used to introduce gases or liquids while maintaining UHV conditions

Data Analysis and Interpretation

• XPS, AES, and UPS data are analyzed to extract quantitative and qualitative information about the sample surface
• Peak identification is performed by comparing the measured electron energies with reference values for the elements and their chemical states
  • XPS and AES databases provide binding energies and kinetic energies for various elements and compounds
  • UPS data are compared with theoretical calculations or reference spectra for the valence band structure
• Background subtraction is necessary to remove the contribution of inelastically scattered electrons and secondary electrons from the measured spectra
  • Common background subtraction methods include linear, Shirley, and Tougaard backgrounds
• Peak fitting is used to deconvolute overlapping peaks and determine the relative contributions of different chemical states or bonding environments
  • Gaussian-Lorentzian (GL) functions are commonly used to model the peak shapes
  • Constraints on peak positions, widths, and area ratios can be applied based on the known chemical properties of the elements
• Quantitative analysis is performed by calculating the atomic concentrations of the elements present on the surface
  • The peak areas are divided by the corresponding sensitivity factors, which account for the differences in photoionization cross-sections and instrument response
  • Matrix effects, such as electron attenuation and elastic scattering, should be considered for accurate quantification in heterogeneous samples
• Depth profiling can be achieved by combining XPS or AES with ion sputtering to remove surface layers and analyze the composition as a function of depth
  • Sputtering rates and ion beam parameters must be carefully controlled to minimize surface damage and maintain depth resolution
• Data interpretation should consider the surface sensitivity of the techniques, the possibility of surface contamination or artifacts, and the limitations of the quantification methods

Applications in Surface Science

• XPS, AES, and UPS are widely used in surface science to study the chemical composition, electronic structure, and reactivity of surfaces
• Catalysis: Investigating the surface composition and chemical state of catalysts to understand their activity and selectivity
  • Identifying active sites, such as metal nanoparticles or surface defects
  • Studying the adsorption and desorption of reactants and products on catalyst surfaces
• Thin films and coatings: Characterizing the composition, thickness, and interface properties of thin films and coatings
  • Monitoring the growth and deposition processes (physical vapor deposition, chemical vapor deposition)
  • Evaluating the adhesion, stability, and performance of protective or functional coatings
• Corrosion and oxidation: Examining the surface chemistry and oxide layer formation on metals and alloys exposed to corrosive environments
  • Identifying the composition and thickness of passive oxide layers
  • Studying the kinetics and mechanisms of corrosion reactions
• Semiconductor devices: Analyzing the surface and interface properties of semiconductor materials and devices
  • Investigating the band alignment and Fermi level position at semiconductor heterojunctions
  • Studying the effects of surface treatments (cleaning, passivation) on device performance
• Biomaterials and biosensors: Characterizing the surface chemistry and biocompatibility of materials used in medical implants and biosensors
  • Detecting the adsorption and conformation of proteins or other biomolecules on surfaces
  • Evaluating the effectiveness of surface modifications for improved biocompatibility or specific bio-recognition
• Environmental and atmospheric chemistry: Studying the surface reactions and adsorption processes of pollutants and atmospheric species on solid surfaces
  • Investigating the role of mineral dust or aerosol particles in atmospheric chemistry
  • Assessing the effectiveness of catalytic converters or air filtration materials

Limitations and Challenges

• XPS, AES, and UPS have certain limitations and challenges that should be considered when applying these techniques
• Surface sensitivity: While the surface sensitivity is an advantage for studying surface properties, it also means that the techniques are not suitable for probing the bulk composition or structure of materials
  • The information depth is limited to a few nanometers, depending on the electron kinetic energy and material properties
  • Surface contamination or oxidation can significantly affect the measured spectra and lead to misinterpretation of the results
• Insulating samples: Analyzing insulating samples can be challenging due to surface charging effects
  • Charging can shift the measured electron energies and distort the peak shapes, making data interpretation difficult
  • Charge compensation techniques, such as electron flood guns or conductive coatings, can help mitigate charging effects but may introduce additional artifacts or complexity
• Sample damage: The incident photons or electrons can cause damage to sensitive samples, such as organic materials or biological specimens
  • X-rays can induce chemical bond breaking or desorption of surface species
  • Electron beams in AES can cause surface decomposition or structural changes
  • Careful control of the measurement conditions and minimizing the exposure time can help reduce sample damage
• Quantification accuracy: Quantitative analysis in XPS and AES relies on the use of sensitivity factors and assumes a homogeneous distribution of elements in the sampling depth
  • Deviations from these assumptions, such as surface enrichment or depletion, can lead to errors in the calculated atomic concentrations
  • Accurate quantification may require additional calibration standards or advanced data processing techniques
• Peak overlaps: In some cases, the photoelectron or Auger electron peaks of different elements or chemical states may overlap in the measured spectra
  • Overlapping peaks can complicate the data interpretation and require careful peak fitting and deconvolution procedures
  • The use of high-resolution spectra and advanced fitting algorithms can help resolve overlapping peaks
• Detection limits: XPS and AES have detection limits of typically 0.1-1 atomic percent, depending on the element and measurement conditions
  • Trace elements or low-concentration surface species may be difficult to detect or quantify reliably
  • Longer acquisition times or signal averaging can improve the detection limits but may also increase the risk of sample damage or contamination

Comparison of XPS, AES, and UPS

• XPS, AES, and UPS are complementary techniques that provide different types of information about the surface composition and electronic structure
• Excitation source:
  • XPS uses X-ray photons (Al Kα or Mg Kα) with energies of 1486.6 eV or 1253.6 eV
  • AES employs an electron beam with energies typically in the range of 1-10 keV
  • UPS uses ultraviolet photons from a He discharge lamp with energies of 21.2 eV (He I) or 40.8 eV (He II)
• Information depth:
  • XPS probes the top 1-10 nm of the surface, depending on the electron kinetic energy and material properties
  • AES has a similar information depth to XPS, typically 1-5 nm
  • UPS is more surface-sensitive, with an information depth of 0.5-2 nm due to the lower kinetic energy of the emitted electrons
• Chemical information:
  • XPS provides detailed information about the elemental composition and chemical state of the surface
  • AES is more sensitive to light elements (Z < 3) and can detect elements with concentrations down to 0.1 atomic percent
  • UPS focuses on the valence band region and provides information about the electronic structure, work function, and molecular orbitals
• Spatial resolution:
  • XPS has a typical spatial resolution of 10-100 μm, limited by the X-ray spot size
  • AES can achieve higher spatial resolution (down to 10 nm) using focused electron beams, making it suitable for surface mapping and imaging
  • UPS has a lower spatial resolution (100-500 μm) due to the larger spot size of the UV source
• Depth profiling:
  • XPS and AES can be combined with ion sputtering to perform depth profiling and analyze the composition as a function of depth
  • UPS is not commonly used for depth profiling due to its high surface sensitivity and the potential for surface damage during sputtering
• Sample requirements:
  • XPS and UPS require conductive samples or charge compensation methods for insulating materials
  • AES is less affected by surface charging and can analyze both conductive and insulating samples
  • All three techniques require clean and UHV-compatible samples to minimize surface contamination and ensure reliable measurements