Microscopy and Surface Analysis 1 doc

Microscopy and Surface Analysis Lecture Date: March 11th, 2008 Reading Assignments for Microscopy and Surface Analysis  Skoog, Holler and Nieman, Chapter 21, “Surface Characterization by Spectroscopy and Microscopy”  Hand-out Review Article: R J Hamers, “Scanned Probe Microscopies in Chemistry,” J Phys Chem., 1996, 100, 13103-13120 Microscopy and Surface Analysis  Microscopic and imaging techniques: – – – – Optical microscopy Confocal microscopy Electron microscopy (SEM and TEM, related methods) Scanning probe microscopy (STM and AFM, related methods)  Surface spectrometric techniques: – – – – X-ray fluorescence (from electron microscopy) Auger electron spectrometry X-ray photoelectron spectrometry (XPS/UPS/ESCA) Other techniques:  Secondary-ion mass spectrometry (SIMS)  Ion-scattering spectrometry (ISS)  IR/Raman methods Why Study Surfaces?  Surface – the interface between two of matter’s common phases: – Solid-gas (we will primarily focus on this) – – – – Solid-liquid Solid-solid Liquid-gas Liquid-liquid  The majority of present studies are applied to this type of system, and the techniques available are extremely powerful  The properties of surfaces often control chemical reactions Microscopy  Why is microscopy useful? What can it tell the analytical chemist? – – – – Sample topography Structural stress/strain Electromagnetic properties Chemical composition  Plus - a range of spectroscopic techniques, from IR to Xray wavelengths/energies, have been combined with microscopy to create some of the most powerful analytical tools available… Imaging Resolution and Magnification  Some typical values for microscopic methods: Magnification Method Resolution Human Eye 0.1-0.2 mm - Optical Microscopy 0.1-0.2 um ~1200 Electron 30-50 Å 10-75,000 500,000 (x) Microscopy Probe Microscopy Optical Microscopy - History  An ancient technique – the lens has been around for     thousands of years Chinese tapestries dating from 1000 B.C depict eyeglasses In 1000 A.D., an Arabian mathematician (Al Hasan) made the first theoretical study of the lens Copernicus (1542 A.D.) made the first definitive use of a telescope As glass polishing skills developed, microscopes became possible John and Zaccharias Jannsen (Holland) made the first commercial and first compound microscopes Then came lens grinding, Galileo, the biologists, and many great discoveries… Modern Optical Microscopy in Chemistry  As optical microscopy developed, the compound microscope was applied to the study of chemical crystals  The polarizing microscope (1880): can see boundaries between materials with different refractive indices, while also detecting isotropic and anisotropic materials http://www.microscopyu.com/articles/polarized/polarizedintro.html Optical Microscope Design  Microscope design has not changed much in 300 years – But the lenses are more perfect – free of abberations  Objective lenses are Diagram from Wikipedia (public domain) characterized NA (numerical aperature) – The numerical aperture of a microscope objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance – Large NA = finer detail = better light gathering http://www.microscopyu.com/articles/polarized/polarizedintro.html The Diffraction Limit  The image of an infinitely small point of light is not a point – it is an “Airy” disk with concentric bright/dark rings Airy disk 0.61  d NA NA  n sin  rairy   The minimum distance between resolved point objects of equal intensity is the “Airy” disk radius (rairy), since resolution of a conventional optical microscope is limited by Fraunhofer diffraction at the entrance aperture of the objective lens Resolved Not resolved http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm, http://www.olympusmicro.com/primer/java/mtf/airydisksize/ See Y Garini, Current Opinion in Biotechnology 2005, 16:3–12 The Diffraction Limit  Traditional optical microscopy is known as “far-field” microscopy Its lateral resolution is limited to ~200 nm – The need for the light-gathering objective lens and its aperture in a conventional microscope leads to a diffraction limit  Newer techniques make use of “near-field” methods to overcome the diffraction limit A fiber tip with an aperture 10000x more detail than visible light! Electron Microscopy: Resolution  What about relativistic corrections?  The electrons in an EM can in some cases be moving pretty close to the speed of light Example – what is the wavelength for a 100 kV potential? Using the relativistically corrected form of the previous equation:   h m h  m 2eV 2meV (1  2eV ) mc 6.63 10 34 J s -19 (1 6010 2(9.11 10-31 kg)(1.60  10-19 C)(104 V)(1  (9.11.10-31 kg)C)(10 V )/ s )2 ) ( 310 m   3.7 103 nm At high potentials, EM can see atomic dimensions 13 Electron Microscopy: Sample-Beam Interactions  Sample-beam interactions control how both SEM and TEM (i.e STEM) operate: – Formation of images – Spectroscopic/diffractometric analysis  There are lots (actually eight) types of sample-beam interactions (which can be confusing and hard to remember!)  It helps to classify these types into two classes of sample-beam interactions: – bulk specimen interactions (bounce off sample – “reflected”) – thin specimen interactions (travel through sample- “transmitted”) SEM: Sample-Beam Interactions Backscattered Electrons (~30 keV)  Caused by an incident electron colliding with an atom in the specimen which is almost normal to the incident electron’s path The electron is then scattered "backward" 180 degrees  Backscattered electron intensity varies directly with the specimen's atomic number This differing production rates causes higher atomic number elements to appear “brighter” than lower atomic number elements This creates contrast in the image of the specimen based on different average atomic numbers  Backscattered electrons can come from a wide area around the beam impact point (see pg 552 of Skoog) – this also limits the resolution of a SEM (along with abberations in the EM lenses) 14 SEM: Sample-Beam Interactions Secondary Electrons (~5 eV)  Caused by an incident electron passing "near" an atom in the specimen, close enough to impart some of its energy to a lower energy electron (usually in the K-shell) This causes a slight energy loss, a change in the path of the incident electron and ionization of the electron in the specimen atom The ionized electron then leaves the atom with a very small kinetic energy (~5 eV) One incident electron can produce several secondary electrons  Production of secondary electrons is closely linked to sample topography Their low energy (~5 eV) means that only electrons very near to the surface (