Microscopy and Surface Analysis 2 pptx

Microscopy and Surface Analysis Lecture Date: March 17th, 2008 Reading Assignments for Microscopy and Surface Analysis  Skoog, Holler and Nieman, Chapter 21, “Surface Characterization by Spectroscopy and Microscopy”  Hand-out Review Article: 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 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 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 5 eV for transition metals Material Crystal State Workfunction (eV) Na polycrystalline 2.4 Cu polycrystalline 4.4 Ag polycrystalline 4.3 Au polycrystalline 4.3 Pt polycrystalline 5.3 W 4.5 single crystal 4.39 W(100) single crystal 4.56 W(110) single crystal 4.68 W(112)  polycrystalline W(111) single crystal 4.69 The workfunction is the ‘barrier” to electron emission – like the wall in the particle-in-a-box concept Data from CEM 924 Lectures presented at MSU (2001) Basic Considerations for Surface Spectroscopy  Common sampling “modes” – Spot sampling – Raster scanning – Depth profiling  Surface contamination: – The obvious contamination/alteration of surfaces that can be the result of less-than careful sample preparation – Solid surfaces can adsorb gases:  At 10-6 torr, a complete monolayer of a gas (e.g CO) takes just seconds to form  At 10-8 torr, monolayer formation takes 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 (from electron 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? 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 Secondary Name of Technique photon (X-ray/UV) electron XPS (ESCA) and UPS photon (X-ray) or electron electron Auger electron spec (AES) ion ion SIMS (secondary ion MS) photon ion LMMS (laser microprobe MS) electron Photon (X-ray) 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 WDS and EDS detectors are used (as in XRF), elemental mapping Not particularly surface sensitive! Electron Microprobes: X-ray Emission Electron Spectroscopy   Electron spectroscopy – measuring the energy of electrons Major forms: – Auger electron spectroscopy – X-ray/UV photoelectron spectroscopy – Electron energy loss spectroscopy (EELS) Electron Spectroscopy: Surface Sensitivity  Electrons can only escape from shallow depths in the surface of a sample, because they will undergo collisions and lose energy XPS/AES region, electrons that have not been inelastically scattered from shallow regions (mostly excitation of conduction-band electrons) Deep electrons that undergo inelastic collisions but lose energy (exciting e.g phonons) Auger Electron Spectrometry (AES)  The Auger process can also be a source of spectral information Auger electrons are expelled from atomic/molecular orbitals and their kinetic energy is characteristic of atoms/molecules  However, since it is an electron process, analysis of electron energy is necessary! – This is unlike the other techniques we have discussed, most of which measure photon wavelengths or energy  Auger electron emission is a three-step (three electron) process, that leaves an atom doubly-ionized AES: Basic Mechanism See Figure 21-7 in Skoog, et al for a related figure AES: Basic Mechanism  Auger electrons are created from outer energy levels (i.e less-tightly bound electrons, possibly valence levels) This example would be called a LMM Auger electron Other Common types are denoted KLL and MNN AES: Efficiency of Auger Electron Production  Two competing processes: – X-ray fluorescence – Auger electron emission  Auger electrons  predominate at lower atomic number (Z) Photoelectron emission does not compete! K  number of K photons produced number of K shell vacancies created  Auger    K Top Figure from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989 AES: Spectrometer Design  AES instruments are designed like  an SEM – often they are integrated with an SEM/EDXA system Unlike an SEM, AES instruments are designed to reach higher vacuum (10-8 torr) Electron detector Electron Gun Energy analyzer – Helps keep surfaces clean and free from adsorbed gases, etc… Auger electrons  Basic components: – – – – – Electron source/gun Electron energy analyzer Electron detector Control system/computer Ion gun (for depth profiling) Sample AES (and XPS): Electron Energy Analyzers  Two types of electron energy analyzers (also used in XPS): Electrons only pass if their KE is: KEelectron  ke V k R1 R2 R2  R12 Concentric hemispherical analyzer (higher resolution) – better resolution, mostly for XPS/UPS Cylindrical mirror analyzer (higher efficiency) More common for AES (Right) Diagram from http://www.cea.com/cai/auginst/caiainst.htm (Left) Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989 AES: Detectors  More sophisticated detectors are needed to detect low numbers of Auger electrons Two types of electronmultiplier detectors: Discrete dynode Continuous dynode  Both types of detector are also used in XPS/UPS!!! 10 AES: Surface Analysis  AES is very surface sensitive (10-50 Ǻ) and its reliance on an electron beam results in excellent lateral resolution  Electron beam does not have to be monochromatic – Note: an X-ray beam can also be used for AES, but is less desirable b/c it cannot currently be focused as tightly (as is the case in XPS)  Auger electrons typically have energies of < 1000 eV, so they are only emitted from surface layers Diagram from http://www.cea.com/cai/auginst/caiainst.htm AES: Spectral Interpretation  AES Electron Kinetic Energies* versus Atomic Number (Most intense peaks only Valid for CMA-type analyzers.) *Data is from J.C Vickerman (Ed.), "Surface Analysis: The Principal Techniques“, John Wiley and Sons, Chichester, UK, 1997 Image from http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html (accessed 12-Nov-2004) 11 AES: Typical Spectra AES: Elemental Surface Analysis    Very common application of AES - elemental surface analysis For true surface analysis, AES is better than SEM/X-ray emission (electron microprobe) because it is much more surface sensitive AES can be easily made quantitative using standards Image from http://www.cem.msu.edu/~cem924sg/ (accessed 12-Nov-2004) 12 AES: Chemical Shifts  Chemical information (i.e on bonding, oxidation states) should be found in Auger spectra because the electron energy levels are sensitive to the chemical environment  In practice, it is not (usually) there because too many electron energy levels are involved – it is difficult to calculate and simulate Auger spectra X-ray Photoelectron Spectrometry (XPS)  Photoelectron spectroscopy is used for solids, liquids and gases, but has achieved prominence as an analytical technique for solid surfaces  XPS: “soft” x-ray photon energies of 200-2000 eV for analysis of core levels  UPS: vacuum UV energies of 10-45 eV for analysis of valence and bonding electrons  Photoelectric effect: Proposed by A Einstein (1905), harnessed by K Siegbahn (1950-1970) to develop XPS 13 XPS: Basic Concepts  Like in AES, photoelectrons can not escape from depths  greater than 10-50 A inside a material Schematically, the photoelectron process is: A  h  A  *  e  atom or molecule  cation Like in AES, the kinetic energy of the emitted electron is measured in a spectrometer XPS: Review of X-ray Processes 14 XPS: Photoelectron Emission and Binding Energy  The kinetic energy of the emitted electron can be related to the “binding energy”, or the energy required to remove an electron from its orbital – Higher binding energies mean tighter binding – e.g as atomic number goes up, binding energies get tighter because of increasing number of protons Ebinding  h  IP (gas) Ebinding  h  BE  w (solid) http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm XPS: Binding Energy  The workfunction w is usually linked to the spectrometer  (if the sample is electrically connected) In gases, the BE is directly related to IP – Ionization potential – the energy required to take an electron out of its orbital all the way to the “vacuum” (i.e far away!) – PE spectroscopy on gases is used to check the accuracy of modern quantum chemical calculations  In conducting solids the workfunction is involved  Koopman’s Theorem: binding energy = -(orbital energy)  – Orbital energies can be calculated from Hartree-Fock Another definition for XPS binding energy: the minimum energy required to move an inner electron from its orbital to a region away from the nuclear charge Absorption edges result from this same effect 15 XPS: Sources  Monochromatic sources using electrons fired at elemental targets that emit x-rays – Can be coupled with separate post-source monochromators containing crystals, for high resolution (x-ray bandwidth of