Microscopy and Surface Analysis 2

Microscopy and Surface Analysis 2 Lecture Date: March 17th, 2008 Reading Assignments for Microscopy and Surface Analysis  Skoog, Holler and Nieman, Chapter 21, “Surface Characterizationby Spectroscopy and Microscopy”  Hand-out ReviewArticle: C. R. Brundle, J. F. Watts,and J. Wolstenholme,“X-ray Photoelectron and Auger Electron Spectroscopy”, in Ewing’s Analytical Instrumentation Handbook, 3rd Ed. (J. Cazes, Ed.), Marcel-Dekker 2005. 1 Introduction to the Solid State  In solids, atomic and molecular energy levels broaden into bands that in principle contain as many states as there are atoms/molecules in the solid.  Bands may be separated by a band gap with energy Eg P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987. C.Kittel, Solid-state Physics, 7th Ed, Wiley, 1999. W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989. Energy Bands in the Solid State  Bands are continuous and delocalized over the material  Band “widths” are determined by size of orbital overlap  The highest-energy filled band (which may be only partially filled) is called the valence band  The lowest-energy empty band is called the conduction band P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987. C.Kittel, Solid-state Physics, 7th Ed, Wiley, 1999. W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989. 2 The Workfunction: A Barrier to Electron Emission  How does the electronic arrangement in solids affect surfaces? In particular,how can an electron be removed? Free electron!  For some electron being removed, its energy just as it gets free is EV  The energy required to remove the electron is the workfunction  (typically several eV) P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987. C.Kittel, Solid-state Physics, 7th Ed, Wiley, 1999. W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989. The Workfunction: A Barrier to Electron Emission  Workfunctions vary from <2 eV for alkali metals to >5 eV for transition metals. Material Na Cu Ag Au Pt W W(111) W(100) W(110) W(112) Crystal State polycrystalline polycrystalline polycrystalline polycrystalline polycrystalline polycrystalline single crystal single crystal single crystal single crystal Workfunction (eV) 2.4 4.4 4.3 4.3 5.3 4.5 4.39 4.56 4.68 4.69  The workfunction is the ‘barrier” to electron emission – like the wall in the particle-in-a-boxconcept. Data from CEM 924Lectures presented at MSU (2001). 3 Basic Considerations for Surface Spectroscopy  Common sampling “modes” – Spot sampling – Raster scanning – Depth profiling  Surface contamination: – The obvious contamination/alterationof surfaces that can be the result of less-than careful sample preparation – Solidsurfaces can adsorb gases:  At 10-6 torr, a completemonolayerof a gas (e.g. CO) takes just 3 seconds to form.  At 10-8 torr, monolayerformationtakes 1 hour. – Most studies are conducted under vaccuum– although there are newer methods that don’t require this. D. M. Hercules and S. H. Hercules, J. Chem. Educ.,1984,61, 403. Surface Spectrometric Analysis  Surface spectrometric techniques: – X-ray fluorescence (fromelectron microscopy) – Auger electron spectrometry – X-ray photoelectron spectrometry (XPS/UPS) – Secondary-ion mass spectrometry (SIMS)  Depth profiling – if you are going to study surfaces with high lateral resolution (e.g. using microscopy), then wouldn’t it be nice to obtain information from various depths within the sample? 4 The Basic Idea Behind Surface Spectrometry Photons, electrons, ions: they can go in and/or out!!! Leads to lots of techniques, and lots of acronyms! Primary photon electron ion Secondary photon electron ion Surface Primary photon (X-ray/UV) photon (X-ray) or electron ion photon electron Secondary electron electron ion ion Photon (X-ray) Name of Technique XPS (ESCA) and UPS Auger electron spec. (AES) SIMS (secondary ion MS) LMMS (laser microprobe MS) SEM “electron microprobe” Electron Microprobes and X-ray Emission  Electron microscopy (usually SEM) can also be used to perform X-ray emission analysis in a manner similar to X-ray fluorescence analysis – see the X-ray spectrometry lecture for details on the spectra  The electron microprobe (EM) is the commonly used name for this type of X-ray spectrometry  Both WDSand EDS detectors are used (as in XRF), elemental mapping  Not particularly surface sensitive! 5 ... - tailieumienphi.vn