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Chapter 4 Other Techniques: Microscopy, Spectroscopy, Thermal Analysis. Microscopic techniques. Optical microscopy - polarizing microscope - reflected light microscope Electron microscopy - scanning electron microscopy (SEM) - transmission electron microscopy (TEM)
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Microscopic techniques • Optical microscopy - polarizing microscope - reflected light microscope • Electron microscopy - scanning electron microscopy (SEM) - transmission electron microscopy (TEM) - high resolution electron microscopy (HREM) EDS: Energy Dispersive Spectroscopy
Applications • Optical microscopy - phase identification, purity, and homogeneity - crystal defects : grain boundaries and dislocation - refractive index determination • Electron microscopy - particle size and shape, texture, surface detail - crystal defects - precipitation and phase transitions - chemical analysis - structure determination
Energy Dispersive Spectroscopy EDS An attachment of EM
Wavelength of electrons • = h(2meV)-1/2 At 90 kV accelerating voltage, l ~ 0.04 Å Consequently, the Bragg angles for diffraction are small and the diffracted beams are concentrated into a narrow cone centered on the undiffracted beam.
Scanning tunneling microscope (STM) The STM can obtain images of conductive surfaces at an atomic scale of 0.2 nm, and also can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electron from atoms or molecules.
Field Emission SEM (FESEM) Traditional SEM:Thermionic Emitters use electrical current to heat up a filament FESEM:A Field Emission Gun (FEG); also called a cold cathode field emitter, does not heat the filament. The emission is reached by placing the filament in a huge electrical potential gradient. FESEM uses Field Emission Gun producing a cleaner image, less electrostatic distortions and spatial resolution < 2nm.
Nuclear magnetic resonance (NMR) spectroscopyMagic Angle Spinning NMR (MAS-NMR) If a solid-state sample is allowed to spin at an angle of θ=54.7° to a strong external magnetic field, dipolar coupling (D) will be zero.
X-ray fluorescence (XRF)-coordination number -bond distance -oxidation state
X-ray absorption techniques • Absorption edge fine structure (AEFS) or X-ray absorption near edge structure (XANES) Information can be obtained - oxidation state, site symmetry, surrounding ligands, the nature of the bonding • Extended X-ray absorption fine structure (EXAFS) Information can be obtained - bonding distance, coordination number
Extended X-Ray Absorption Fine Structure This introduction to the theory of EXAFS is divided into basic, relatively simple and complicated parts. EXAFS spectra are a plot of the value of the absorption coefficient of a material against energy over a 500 - 1000 eV range (including an absorption edge near the start of the spectrum). Through careful analysis of the oscillating part of the spectrum after the edge, information relating to the coordination environment of a central excited atom can be obtained. The theory as to what information is contained in the oscillations is described here.
Electron spectroscopies • ESCA • XPS • UPS • AES • EELS
Origins of ESCA and Auger spectra Electron Spectroscopy for Chemical Analysis: XPS, UPS Auger electrons are secondary electrons
X-ray photoelectron spectroscopy XPS is a surface chemical analysis technique
XPS is used to measure: • 1) elemental composition of the surface (1–10 nm usually) • 2) empirical formula of pure materials • 3) elements that contaminate a surface • 4) chemical or electronic state of each element in the surface • 5) uniformity of elemental composition across the top of the surface (line profiling or mapping) • 6) uniformity of elemental composition as a function of ion beam etching (depth profiling)
XPS and UPS • XPS: core-level photoelectron spectroscopy • UPS: valence-level photoelectron spectroscopy hv = Ek + ef + Eb Ek = kinetic energy of escaped electrons ef = work function (energy from Fermi level to continuous states) Eb = binding energy Ek is measured experimentally Eb contains information of electronic structure
Schematic representation ofhv = Ek + ef + Eb Ek ef Eb hv
Resolution of XPS and UPS • XPS conventionally has lower resolution (0.2 ~1.2 eV). Cannot see vibration (< 0.5 eV or 4000 cm-1) • UPS has better resolution ( < 0.01 eV). Can see vibration frequency. • For UPS, hv = Ek + ef + Eb +DEvib Synchrotron-based light source can enhance resolution
Different oxidation states determined by XPS Example of "High Energy Resolution XPS Spectrum" also called High Res spectrum. This is used to decide what chemical states exist for the element being analyzed. In this example the Si (2p) signal reveals pure Silicon at 99.69 eV, a Si2O3 species at 102.72 eV and a small SiO2 peak at 103.67 eV. The amount of Si2O at 100.64 eV is very small.
XPS spectrum of NaWO3 Band Structures can be seen by XPS The relative amount of electrons filled in a band can be seen.