X-Ray Photoelectron Spectroscopy of Interfaces

1. X-Ray Photoelectron Spectroscopy of Interfaces ChE5535 ALEXANDER COUZIS

2. What Is XPS (ESCA)? • XPS is an abbreviation for X-ray Photoelectron Spectroscopy • ESCA is an acronym for Electron Spectroscopy for Chemical Analysis

3. What is XPS ? Photoelectrons When light strikes an atom an electron may be ejected if the energy of the light is high enough. The energy in the light is determined by its wavelength or frequency (short wavelength = high energy and high frequency = high energy) X-rays have high energy. When X-rays strike a solid electrons are always ejected from the near-surface region of the solid. • An XPS instrument has two main components: • An X-ray source, preferably monochromatic(The exciting photon is a characteristic soft x-ray from a suitable metal, Al (1486.6eV) & Mg K (1253.6eV) being the most common.) • An electron energy analyser, usually a spherical sector analyzer

4. XPS Principles • If we consider a single atom with just one x-ray photon on the way, the total energy is hv+Ei, where hv is the photon energy and Ei the energy of the atom in its initial state. • Following the absorption of the photon and the emission of the photoelectron, the total energy is now KE+Ef, where KE is the electron kinetic energy and Ef the final state energy of the atom (now an ion). Because total energy is conserved hv+Ei = KE+Ef or hv-KE = Ef-Ei = BE where we call the difference between the photon energy (which we know) and the electron energy (which we measure), the binding energy of the orbital from which the electron was expelled. We can see that the binding energy is determined by the difference between the total energies of the initial-state atom and the final-state ion. • It is roughly equal to the Hartree-Fock energy of the electron orbital and so peaks in the photoelectron spectrum can be identified with specific atoms and hence, a surface compositional analysis performed.

5. What is XPS ? If we measure the energy of the ejected photoelectrons we can calculate its Binding Energy which is the energy required to remove the electron from its atom. From the binding energy we can learn some important facts about the sample under investigation: • The elements from which it is made • The relative quantity of each element • The chemical state of the elements present • Modern XPS instruments can also produce images or maps showing the distribution of the elements or their chemical states over the surface. A good instrument would have a spatial resolution of a few microns.

6. Components of an XPS Instrument The measurements must be made in ultra-high vacuum (uhv), for two reasons: • To allow the photoelectrons to travel from the surface of the sample to the detector without striking a gas atom • If a clean surface is prepared for analysis, it would become contaminated if it were not under uhv. Other, optional, items may also be present on an XPS spectrometer: • A low energy electron flood gun which must be used to prevent insulating samples from becoming charged during analysis • An ion source which is used both to clean a surface prior to analysis and to erode the surface of the sample so that concentration depth profiles can be measured.

7. XPS Spectra

8. XPS Spectra Chemical bonding will clearly have an effect on both the initial state energy of the atom and the final state energy of the ion created by emission of the photoelectron. The changes brought about in the initial state energy by bond formation are well-understood and can, in principle, be calculated by quantum chemical methods. They are basically due to the redistribution of electrons as the constituent atoms of a molecule or crystal come together in the solid state and will depend principally upon the electro-negativities of the atoms involved. The creation of the ion by photoemission will cause a further redistribution of the electrons surrounding the target atom and this will have an impact on the final-state energy. This process, called electronic relaxation, has both an intra-atomic and an extra-atomic component, and will be dominated by the polarizabilities of the atoms involved. So the presence of chemical bonding (and hence, neighboring atoms) will cause binding energy shifts, that can be used to extract information of a chemical nature (such as atomic oxidation state) from the sample surface. For this reason, XPS is also known as Electron Spectroscopy for Chemical Analysis (ESCA).

9. XPS Spectra

10. XPS Spectra

11. Depth Profiling An important characteristic of the XP experiment is its surface dependence. Although X-rays penetrate to a depth of several micrometers, ejected photoelectrons generally come from only the first several nanometers of material. Thus, XPS is very much a surface technique, much more so than X-ray fluorescence. This aspect of XPS necessitates great care in experimental design, as the surface may be contaminated, non-uniform, or unrepresentative. At the same time, surface phenomena may be addressed explicitly. Composition also may be studied as a function of distance from the surface through the use of ion sputtering or etching, whereby a stream of ions, usually Ar+, is used to remove a defined surface layer.

12. Depth Profiling

13. Thiol SAMs Chemisorption is Epitaxial. Long Alkyl Chain Dialkyldisulfides Long Alkyl Chain Thiol

16. Structure of Thiol SAMs Au-S distance is 1.905Å Hollow sites are 4.99 Å apart Gold atoms are 2.884Å apart Thiol SAM on Au (111)

17. Tilt Structure of Thiols 110o 110o ODD # of Cs Even # of Cs

18. Thiols on Gold • Short alkyl chain thiols are adsorbed onto gold at a higher rate than long alkyl chain thiols in a diffusion-controlled processes. Furthermore, we also studied the effect of the surface conditions of gold before 1-octadecanethiol (ODT)-SAM formation using the XPS

19. Thiol Monolayers On Gold

20. Thiol Monolayers On Gold