SURFACE-GAS INTERACTIONS / SURFACE ENERGY How many atoms are typically present at the surface? Why do many surface characterization techniques work in ultra-high vacuum? What is the relationship between the surface energy and the Gibbs free energy? Why have we not considered surface energy when we learned the Gibbs free energy in UG thermodynamics class? Can we relate the surface energy to the bond energy of a material? Why is a liquid droplet in free space always in the spherical shape? What determines the shape of solid nanoparticles? How to measure the surface energy? Will the surface energy affect the bulk property? Or it will not matter because the bulk structure is always determined by the chemical bonds in the bulk phase? If the bulk property is affected by the surface contribution, when will it happen? X-RAY PHOTOELECTRON SPECTROSCOPY - ELEMENTAL COMPOSITION ANALYSIS Basic principles: What properties of a material does XPS probe? How can we identify what elements are present in the surface of a sample using XPS? How does the binding energy of electron orbital changes with atomic number? What orbitals can be probed using an Al K X-ray source? How is the binding energy of the photoelectron measured? Why are photoelectrons detected in the energy regions between two ''quantum mechanically allowed'' binding energies? What relaxation processes can occur upon / during the photoelectron emission process? Can similar relaxation process occur when the sample is bombarded with a high energy electron beam? What other characterization techniques rely on such relaxation processes? (Auger electron spectroscopy / Energy dispersive X-ray spectroscopy) How many peaks are expected for photoemission from a given electron orbital? If there is more than one peak, what are their relative intensities? What determines the ''surface sensitivity'' in XPS analysis? How deep can XPS probe for a given material? Why can''t we see hydrogen in XPS? Instrumentation: How are X-rays generated? How can we get monochromatic X-rays? Why is it important to use Monochromatic X-rays? How is the kinetic energy of photoelectron measured? What is the reference used to measure the kinetic energy of photoelectron? Is the Einstein''s energy conservation equation sufficient to calculate the binding energy from the measured kinetic energy? Or should something else be considered? If the sample is not conducting, then it can be charged during the XPS analysis. What problem does it cause? How can we resolve it? Can XPS be operated in near-ambient pressure? Why would one want to do XPS analysis in near-ambient pressure conditions? How can we detect the amount of the energy-screened photoelectrons? How can we increase the signal-to-noise ratio in the photoelectron detection? How can we do chemical imaging with XPS? Qualitative Analysis: Is the binding energy of a core electron fixed or changed depending on the chemical status of the element? - initial state effects In the case of the C 1s signal of organic compounds, what affect the binding energy of the photoelectron? How can we distinguish the photoelectron emission from carbon in ether group vs. alcohol group? How can we expand the chemical identification capacity of C 1s XPS for various organic groups? What governs the energy level of the valence band? What can be learned by analyzing the valence band with XPS? In classical mechanics concept, the "final" state "after" the photoelectron emission would not affect the kinetic energy of the photoelectron. But, the photoelectron emission is a quantum mechanical process whose probability depends on the initial and final state wavefunctions. What final state effects are to be considered in analysis of transition metals? In the case of transition metals, measuring the binding energy of the most representative peak does not always give the sufficient information to determine the oxidation state of the element. Then, what else can we do? Quantitative Analysis How to remove / subtract the background originating from the inelastic scattering? What governs the peak shape and width? When should we consider asymmetric peak shape? Why is it helpful or not helpful to consider the asymmetry in XPS peak fitting? What are the critical factors affecting the peak intensity (=area) in XPS? Is the relative sensitivity factor (RSP) constant for a given element? Depth Profiling How can we use the photoelectron escape probability for depth profiling? What is the max depth this method can be used? For deep depth profiling, we need to use an ion sputter gun. What governs the depth resolution in sputter depth profile analysis? How to improve the depth resolution in the sputter profiling? the same is applied to secondary ion mass spectrometry (SIMS) What are possible artifacts encountered during the sputter depth profiling? Is XPS analysis truly non-destructive if ion sputtering is not used? X-RAY ABSORPTION SPECTROSCOPY (XAS) What electronic transition is probed in XAS? What causes fine structures in X-ray absorption process? Is XAS a surface-sensitive or bulk analysis? What is measured during the XAS analysis? How can the orientation of molecules be determined in NEXAFS / XANES? How can the local structure around a specific atom be determined in EXAFS? LIGHT PROPAGATION THROUGH A MATTER & REFLECTION AT AN INTERFACE What parameter governs the speed of light propagating in a medium? How is that parameter related to dielectric constant or permittivity? What electronic transition is responsible for absorption in the infrared regime of the light spectrum? How is it described in quantum mechanics? What determines the line shape of the absorption band? What classical mechanics model can describe the absorptivity of a molecule? Why is the absorption band, in general, sharp for gas phase molecules, broad for crystalline solid materials, and even broader for amorphous glasses? What governs the reflection and refraction of light at an interface of two non-absorbing media? What are fundamental equations that describe the reflection and transmission of light at an interface? When does total reflection occur? How does the light field penetrate the second medium when total reflection occurs? How is it described mathematically? How is the refractive index change when we deal with a light-absorbing medium? How is the total reflection altered when the second medium is absorbing a light at specific wavelength? REFLECTION-IR ANALYSIS (ATR-IR, SR-IR, PARIS, PM-RAIRS, IR IMAGING) - MOLECULAR SPECIATION What is the probe depth in ATR-IR? Penetration depth? Effective path length? Will the ATR-IR spectrum look similar to the transmission-IR spectrum? Anomalous dispersion effect? What governs the intensity of SR-IR? Can you assume the intensity plotted in the log(1/R) in ATR-IR and SR-IR to be the same as the intensity plotted in absorbance, A = -log(Tout/Tin), in transmission-IR because they are dimensionally the same? Why is the reflectance of metal high? What is the metal selection rule for RAIRS analysis of molecules on metal surfaces? How can the orientation of molecules on a metal surface be determined in RAIRS? What is the main advantage of PM-RAIRS over RAIRS? And disadvantage? What kind of objective lens is used in IR microscopy? What are new methods to achieve spatial resolution in IR imaging beyond the diffraction limit? ELLIPSOMETRY - FILM THICKNESS MEASUREMENT What principle is used in a simple reflectometry to measure thickness of a film? What is X-ray reflectometry (XRR)? Why can XRR give higher resolution than reflectometry using visible light? Why does the linearly polarized light become elliptically polarized when the light is reflected from a thin film interface? How is the ellipticity of the light defined and analyzed? How can the ellipticity of the reflected light (when the incident light is linearly polarized) be related to the thickness of thin film at the interface? What other properties of the interface can be analyzed or determined using ellipsometry? NON-LINEAR OPTICAL PROCESS (SUM FREQUENCY GENERATION, SFG) What are the main differences between linear and non-linear optical processes? Why is a high-power pulsed laser needed to observe non-linear optical effects? What are the energy and momentum conservations in SFG? What is the molecular hyperpolarizability? What is the symmetry rule for SFG? Why can SFG selectively detect interfacial species buried in the medium containing the same species? What governs the peak intensity and shape in SFG? Is SFG sensitive to the interface only? or something else? MULTIVARIATE ANALYSIS (PRINCIPAL COMPONENT ANALYSIS/REGRESSION, PCA/PCR) - DATA PROCESSING What are the main benefits of using multiple peaks (variates) in calibration & regression? What are the covariances of the data? What is the factor (= principal component) of the data? How can we find it? What is the score? How is it be related to physical quantity (such as absorbance)? How be the factors and scores be used for regression, i.e.

predicting concentrations of unknowns from measured absorbances? SURFACE TOPOGRAPHY (OPTICAL PROFILOMETRY & ATOMIC FORCE MICROSCOPY) How the distance and tilt angle between two interfaces can be determined from light interference? What is the.