Chapter 4 Other Techniques: Microscopy, Spectroscopy, Thermal

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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)
- 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
SEM
scanning electron microscopy
Photos of SEM
EDS
Energy Dispersive Spectroscopy
An attachment of EM
Transmission Electron Microscopy
TEM
Wavelength of electrons
 = h(2meV)-1/2
At 90 kV accelerating voltage,  ~ 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.
Basic components of a TEM
HREM of an intergrowth tungsten bronze, Rb0.1WO3
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.
Atomic force microscope (AFM)
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.
Spectroscopic techniques
Vibrational spectroscopy : IR and Raman
• IR
Raman
Visible and ultraviolet spectroscopy
UV/visible
Nuclear magnetic resonance (NMR) spectroscopy
Magic 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.
Example 1
Example 2
Electron spin resonance (ESR) spectroscopy
: detect unpaired electrons
X-ray spectroscopy : XRF, AEFS, EXAFS
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.
EXAFS
EXAFS
XANES
AEFS (or XANES)
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
Core, valence and virtual levels
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 of hv = Ek + ef + Eb
Ek
ef
hv
Eb
XPS
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 and AES
XPS spectra of Na2S2O3 and Na2SO4
XPS spectrum of KCr3O8
XPS spectrum of NaWO3
Band Structures can be seen by XPS
The relative amount of electrons
filled in a band can be seen.
XPS of Co and its oxide
Binding Energy Table
Band Structures can be seen by XPS
An example of UPS
Franck-Condon Principle and
Changes of Vibration Frequencies
UPS of O2
antibonding
bonding
Thermal analysis
• Thermogravimetry (TGA)
• Differential thermal analysis (DTA)
• Differential scanning calorimetry (DSC)
• Thermomechanical analysis (TMA)
Setup of Thermogravimetric Analysis
Thermogravimetric Analysis
(TGA) measures weight
changes in a material as a
function of temperature (or
time) under a controlled
atmosphere.
TGA curve
Heating rate dependence of TGA curve
CuSO4.5H2O heated in air in a Pt crucible
TGA of CaCO3
Possibility of Mixtures
The DTA method
Differential Thermal Analysis (DTA) measures the difference in
temperature between a sample and a thermally inert reference as the
temperature is raised. The plot of this differential provides information
on exothermic and endothermic reactions taking place in the sample.
Variation of peak temperature of kolin with
rate of temperature increase
TGA and DTA curves of kaolin minerals
Application of DTA 1 : melting/solidification
Application of DTA 2 : melting behavior of crystal
Application of DTA 3 : phase diagram determination
Application of TGA : stepwise decomposition of Ca(COO)2·H2O
Conversion of TGA to DTG
Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) is a
thermal analysis technique which is used to
measure the temperatures and heat flows
associated with transitions in materials as a
function of time and temperature.
Thermomechanical analysis (TMA)
Thermomechanical analysis (TMA) is used to determine the
deformation of a sample (changes in length or thickness) as a
function of temperature.
Linear Variable
Displacement Transducer
Mössbauer spectroscopy
• Oxidation state, coordination numbers, bond character
Mössbauer (MB) Spectroscopy
Nuclear transition: absorption of g-rays by sample.
The condition for absorption depend on the electron density
about the nucleus and the number of peaks obtained is related to
the symmetry of the compound.
The energy of emitted Eg = Er + D – R
Er : Eexcited state – Eground state of the source nucleus
D: the Doppler shift due to the transitional motion of the
nucleus
R : the recoil energy of the nucleus
The energy of the g-ray absorbed Eg = Er + D + R
Distribution of energy of emitted and absorbed g-rays
Emitted Eg = Er + D - R = Er + D + R = Eg absorbed
The main cause for non matching of g-rays energies
is the recoil energy.
R  10-1 eV for a gaseous molecule
Doppler effect D  2  104 cm sec-1
R can be reduced by increasing mass by placing the
nucleus of the sample and source in a solid.
PA = mvR = -Eg/c
R = mvR2/2 = PA2/2m = Eg2/2mc2
P = momentum
2
R=
Eg
2mc 2
Three main types of interaction
of the nuclei with the chemical
environment:
1. Resonance line shifts form changes in electron
environment
2. Quardrupole interactions
3. Magnetic interactions
isomer shifts, center shifts, chemical shifts
MB of Fe3+FeIII(CN)6
Fe3+: weak field
0.53 mm sec-1
FeIII: strong field
0.03 mm sec-1
The isomer shift results from the electrostatic
interaction of the charge distribution in the nucleus
with the electron density that has a finite probability of
exiting at the nucleus.
Only s electrons have a finite probability of
overlapping in the nuclear density.
p, d and other electron densities  screening effect
Dr
2
IS 
D s (0)
r
Dr = re  rg
Dr/r is positive and any factor which increases in the total s
electron density increases the isomer shift (IS, CS)
1. Increase in the covalent bonds
2. Decrease the p or d-populations.
Increase in oxidation state of a transition metal.
3. Decrease in coordination number
Tetrahedral (sp3) more s character higher IS
Octahedral (d2sp3)
Dr/r is negative for Fe
Fe2+ (d6) has an appreciably larger center shift than Fe3+ (d5)
Isomer Shift for High Spin Iron Compounds
+1
Oxidation state
~+2.2
Isomer shift
+2
~+1.4
+3
+4
~+0.7 ~+0.2
+6
~-0.6
Quadrupole interactions
Quadrupole splitting arises from the presence of an electron
filed gradient (EFG) at the nucleus.
The magnitude of the QS represents the asymmetry of the
electron cloud around the nucleus.
QS
IS
IS
QS
57Fe, 119Sn
197Au
The I = 3/2 state is split into two sublevels by the presence of
an electron-field gradient.
The magnitude of the QS is one-half of the quadrupole
coupling constant (QCC) for the I = 3/2 state.
Strong field case
t2g6
Low spin case Fe(II)
NO quadrupole
weak field case t2g4eg2
high spin case Fe(II)
large quadrupole
In octahedral geometry only d3, d8, d10 , high spin d5 and low
spin d6 configurations make no contribution to the QS.
Mössbauer of KFeS2
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