14.2 Surface spectroscopy techniques

Surface spectroscopy techniques are powerful tools for analyzing material surfaces at the atomic and molecular level. These methods, including XPS, AES, and LEED, provide crucial information about surface composition, chemical states, and structure.

Vibrational spectroscopy and mass spectrometry techniques like SERS, ATR, RAIRS, and SIMS offer complementary insights into surface chemistry and molecular interactions. These advanced methods enable researchers to probe surfaces with unprecedented sensitivity and resolution.

Electron-based Techniques

X-ray Photoelectron Spectroscopy (XPS)

• Analyzes surface composition and chemical states of elements
• Utilizes X-ray irradiation to eject core electrons from atoms
• Measures kinetic energy of emitted photoelectrons
• Determines binding energy of electrons using Einstein's photoelectric equation
• Provides information on elemental composition, oxidation states, and chemical environments
• Typical X-ray sources include Al Kα (1486.6 eV) and Mg Kα (1253.6 eV)
• Sampling depth ranges from 1-10 nm, making it highly surface-sensitive
• Requires ultra-high vacuum conditions to minimize surface contamination
• Applications include materials science, catalysis, and semiconductor research

Auger Electron Spectroscopy (AES)

• Probes chemical composition of surfaces with high spatial resolution
• Involves excitation of core electrons and subsequent Auger electron emission
• Auger process occurs when outer shell electron fills core hole, releasing energy
• Energy released ejects another outer shell electron (Auger electron)
• Kinetic energy of Auger electrons characteristic of specific elements
• Offers excellent spatial resolution (nanometer scale) for surface mapping
• Commonly used in conjunction with scanning electron microscopy (SEM)
• Effective for light elements (Z < 20) due to higher Auger yield
• Applications include thin film analysis, corrosion studies, and quality control in electronics

Low-Energy Electron Diffraction (LEED)

• Investigates surface structure and crystallography of ordered surfaces
• Employs low-energy electrons (20-200 eV) as probes
• Electrons interact with surface atoms, producing diffraction patterns
• Diffraction pattern reflects surface periodicity and symmetry
• Provides information on surface reconstruction and adsorbate ordering
• Requires ultra-high vacuum conditions for clean surfaces
• Pattern analysis yields information on atomic positions and bond lengths
• Complements other surface techniques like scanning tunneling microscopy (STM)
• Applications include studying surface phase transitions and epitaxial growth

Vibrational Spectroscopy

Surface-Enhanced Raman Spectroscopy (SERS)

• Amplifies Raman scattering signals from molecules adsorbed on rough metal surfaces
• Enhancement factors can reach 10^10 - 10^11, enabling single-molecule detection
• Utilizes localized surface plasmon resonance (LSPR) of metal nanostructures
• Electromagnetic enhancement mechanism dominates signal amplification
• Chemical enhancement also contributes through charge transfer processes
• Common SERS substrates include silver, gold, and copper nanoparticles
• Offers high sensitivity and molecular specificity for surface analysis
• Applications include biosensing, trace analysis, and art conservation
• Enables in situ monitoring of surface reactions and interfacial processes

Attenuated Total Reflectance (ATR) Spectroscopy

• Non-destructive sampling technique for infrared spectroscopy
• Utilizes total internal reflection of IR radiation in a high refractive index crystal
• Evanescent wave penetrates sample in contact with crystal surface
• Typical penetration depth ranges from 0.5 to 2 μm
• Suitable for analyzing liquids, solids, and thin films
• Requires minimal sample preparation compared to transmission IR
• Common ATR crystals include diamond, germanium, and zinc selenide
• Provides surface-sensitive information on molecular structure and composition
• Applications include polymer analysis, quality control, and environmental monitoring

Reflection-Absorption Infrared Spectroscopy (RAIRS)

• Analyzes thin films and adsorbates on reflective surfaces (metal substrates)
• Utilizes grazing incidence geometry to maximize surface sensitivity
• Exploits surface selection rule for enhanced molecular orientation information
• Only vibrations with dipole moment perpendicular to surface are IR-active
• Requires polarized IR radiation (p-polarized) for optimal signal
• Typical incidence angles range from 80° to 88° from surface normal
• Provides information on adsorbate-substrate interactions and molecular orientation
• Often combined with ultra-high vacuum systems for in situ surface studies
• Applications include catalysis research, self-assembled monolayer characterization, and corrosion studies

Mass Spectrometry

Secondary Ion Mass Spectrometry (SIMS)

• Analyzes surface composition and depth profiles of solid materials
• Bombards sample surface with primary ion beam (Cs+, O2+, Ar+)
• Sputters secondary ions from surface for mass spectrometric analysis
• Offers high sensitivity (ppm to ppb range) and excellent depth resolution
• Two main operational modes: static SIMS and dynamic SIMS
• Static SIMS provides surface-specific molecular information
• Dynamic SIMS enables depth profiling and 3D chemical imaging
• Time-of-flight (ToF) analyzers commonly used for high mass resolution
• Applications include semiconductor analysis, forensics, and materials characterization
• Challenges include matrix effects and quantification of secondary ion yields