Chapter 4 Other Techniques: Microscopy, Spectroscopy, Thermal Analysis

Chapter 4Chapter 4

Other Techniques:Other Techniques:

Microscopy, Spectroscopy, Thermal AnalysisMicroscopy, Spectroscopy, Thermal Analysis

Microscopic techniquesMicroscopic 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

ApplicationsApplications

• 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 SEM scanning electron microscopyscanning electron microscopy

Photos of SEMPhotos of SEM

EDSEDS Energy Dispersive Spectroscopy

An attachment of EM

TEMTEMTransmission Electron Microscopy

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 TEMBasic components of a TEM

HREM of an intergrowth tungsten bronze, RbHREM of an intergrowth tungsten bronze, Rb0.10.1WOWO33

Scanning tunneling microscope (STM)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)Atomic force microscope (AFM)

Field Emission SEM (FESEM)Traditional SEM:Traditional SEM: Thermionic Emitters use electrical current to heat up a filament

FESEM: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.

FESEMFESEM uses Field Emission Gun producing a cleaner image, less uses Field Emission Gun producing a cleaner image, less electrostatic distortions and spatial resolution < 2nm.electrostatic distortions and spatial resolution < 2nm.

Spectroscopic techniquesSpectroscopic techniques

Vibrational spectroscopy : Vibrational spectroscopy : IR and RamanIR and Raman

• IR

Raman

Visible and ultraviolet spectroscopyVisible and ultraviolet spectroscopy

UV/visible

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.

Example 1

Example 2

Electron spin resonance (ESR) spectroscopy

: detect unpaired electrons

X-ray spectroscopy : XRF, AEFS, EXAFSX-ray spectroscopy : XRF, AEFS, EXAFS

X-ray fluorescence (XRF)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.

EXAFSEXAFS

EXAFSEXAFS

XANESXANES

AEFS (or XANES)AEFS (or XANES)

Electron spectroscopiesElectron spectroscopies

• ESCAESCA

• XPSXPS

• UPSUPS

• AES AES

• EELSEELS

Origins of ESCA and Auger spectraOrigins of ESCA and Auger spectra

Electron Spectroscopy for Chemical Analysis: XPS, UPS

Auger electrons are secondary electrons

Core, valence and virtual levelsCore, valence and virtual levels

X-ray photoelectron spectroscopy XPSXPS is a surface chemical analysis technique

XPS is used to measure: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 + e + Eb

Ek = kinetic energy of escaped electronsework function (energy from Fermi level to continuous states)Ebbinding energyEk is measured experimentallyEb contains information of electronic structure

hv Eb

e

Ek

Schematic representation of hv = Ek + e + 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 + e + Eb +Evib

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 electronsfilled 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

bonding

antibonding

Thermal analysisThermal analysis

• Thermogravimetry (TGA)

• Differential thermal analysis (DTA)

• Differential scanning calorimetry (DSC)

• Thermomechanical analysis (TMA)

Setup of Thermogravimetric AnalysisSetup 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 methodDifferential 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)

Linear Variable Displacement Transducer

Thermomechanical analysis (TMA) is used to determine the deformation of a sample (changes in length or thickness) as a function of temperature.

MMöössbauer spectroscopyssbauer spectroscopy

• Oxidation state, coordination numbers, bond character

MMössbauer (MB) Spectroscopyössbauer (MB) Spectroscopy

Nuclear transition: absorption of -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.

Distribution of energy of emitted and absorbed-rays

The energy of the -ray absorbed E = Er + D + R

The energy of emitted E = Er + D – R Er : Eexcited state – Eground state of the source nucleus D: the Doppler shift due to the transitional motion of the nucleusR : the recoil energy of the nucleus

Emitted E = Er + D - R = Er + D + R = E absorbed

The main cause for non matching of -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.

2

2

2mc

ER

PA = mvR = -E/cR = mvR

2/2 = PA2/2m = E

/2mc2

P = momentum

1.1. Resonance line shifts form changes in electron Resonance line shifts form changes in electron environmentenvironment

2.2. Quardrupole interactionsQuardrupole interactions3.3. Magnetic interactionsMagnetic interactions

Three main types of interaction Three main types of interaction of the nuclei with the chemical of the nuclei with the chemical environment:environment:

MB of MB of FeFe3+3+FeFeIIIIII(CN)(CN)66

FeFe3+3+:: weak field weak field

0.53 mm sec0.53 mm sec-1-1

FeFeIIIIII: strong field: strong field

0.03 mm sec0.03 mm sec-1-1

isomer shifts, center shifts, chemical shifts

The isomer shift results from the The isomer shift results from the electrostaticelectrostatic interaction of the charge distribution in the nucleus interaction of the charge distribution in the nucleus with the with the electron densityelectron density that has a finite probability of that has a finite probability of exiting at the nucleus.exiting at the nucleus.

Only Only s electronss electrons have a finite probability of have a finite probability of overlapping in the nuclear density.overlapping in the nuclear density.

p, dp, d and other electron densities and other electron densities screeningscreening effect effect

2)0(sr

rIS

ge rrr

r/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)

r/r is negative for Fe

Fe2+ (d6) has an appreciably larger center shift than Fe3+ (d5)

Isomer Shift for High Spin Iron Compounds

Oxidation state +1 +2 +3 +4 +6

Isomer shift ~+2.2 ~+1.4 ~+0.7 ~+0.2 ~-0.6

Quadrupole interactions

The magnitude of the QS represents the asymmetry of the electron cloud around the nucleus.

Quadrupole splitting arises from the presence of an electron filed gradient (EFG) at the nucleus.

QS

QS

IS

IS

57Fe, 119Sn197Au

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 weak field case t2g

4eg2

Low spin case Fe(II) high spin case Fe(II)

NO quadrupole 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