Exam 2 - Materials Characterization - Part 2 (spectroscopy)

Important parts of a UV-VIS spectrometer

Source, entrance slit, monochromator, exit slit, sample, dispersion device, detector

What do optical spectrophotometers measure?

Optical Density (OD)

Absorbance (A)

Transmittance (T) 

What law is UV-VIS based on?

Beer Lambert’s Law

How does UV-Vis work?

Passes UV/VIS light through sample and see how much is absorbed - molecules absorb certain wavelengths of light

Applications for UV-Vis spectroscopy

Measure various concentrations of a material and their absorption

Quantified metal speciation (ppb)

Looking at conjugated systems

Comparing sugars/salts/organics with nucleic acids, proteins (size difference)

First derivative UV-Vis

Shows slope, emphasizes changes 

Second derivative UV-Vis

Emohasizes changes more strongly, shows curvature

Which derivative for Uv-Vis gives the best data in terms of peak sharpness

2nd derivative

What region in wavelength is vibrational spectroscopy?

10^-4 nm to 10^-6 nm

What region in wavenumber is vibrational spectroscopy?

10²-10⁴ cm^-1

For stretching vibrations, what is the maximum number of bonds?

1

For bending vibrations (in-plane), what is the maximum number of bonds?

2

For bending vibrations (out of plane), what is the maximum number of bonds?

3

For torsion vibrations, what is the maximum number of bonds?

3

What model determines molecular vibrations by quantum mechanics?

Harmonic Oscillator

Rate the size of energy gaps of each degree of freedom from smallest to largest

Translation, Rotation, Vibration, Electronic 

General FTIR schematic

Incident beam split into 2 by beam splitter, interferogram can be interpreted based on its Fourier Component

What kind of molecules are good to look at with FTIR?

Molecules with changing dipole moments

Look at bond changes (such as polymers, liquids, NOT SALTS)

Stokes Scattering 

Go from v=0 to virtual state back down to state right above v=0

Rayleigh Scattering

Go from v=0 to virtual state back down to v=0

Ant-stokes scattering

Go from state right above v=0 to virtual state and then down to v=0

What does Raman measure?

Stokes, Anti-Stokes, Rayleigh from E0 to E1

Raman and IR do/do not see the same features

Can see the same but can also not

Raman - polarizability

Molecule must undergo change in polarizability during vibration, meaning that is it easy or difficult to distort electrons from their original positions?

Polarizability of a molecule decreases with:

• increasing e- density

• increasing bond strength

• decreasing bond length

Symmetrical stretching, Raman/IR active? CO2

Raman - Yes

IR - no

Asymmetrical stretching, Raman/IR active? CO2

Raman - no

IR - Yes

Bending, Raman/IR active? CO2

Raman - no

IR - yes

Raman Microscopy Applications in Materials Science

• Composition analysis

• Strain/stretch of materials will shift their peaks and intensities

• Examine materials with microscopy - geological samples

Pros of Using Raman (over FTIR)

• Low frequency modes

• Easy sampling

• Resonance and surface enhancements possible

• Water compatible

FTIR Advantages

• Good fingerprint libraries abailable

• Fundamental vibrations

FTIR and Raman both have _______ linewidths

narrow

Electron spectroscopy general technique

Analytical technique that detects emitted electrons from atoms and solids, can provide chemical and electronic information from surface

Electron spectroscopy is complementary to these two materials characterization methods 

XRD 

Vibrational Spectroscopy

X-Ray Photoelectron Spectroscopy (XPS) does what?

Measures binding energies of core electrons ejected by X-rays

What can XPS measure

Element ID (Z>3), near-surface composition

Peaks on an XPS occur when

Excitation occurs (energy excited core level)A

Auger lines

Outer shell electron fills inner hole vacancy, energy from that can result in emission of Auger electron

XPS is surface-specific because

inelastic mean free path can differ based on the material, so the sampling depth can change between 1-10 nm

How can XPS perform elemental composition

Wide scan at modest energy and label all peaks 

Chemical shifts XPS

Oxidation/bonding modifies local potential

Peak width XPS

Reflects state distribution, inhomogeneity, relaxation effects

Splutter XPS 

Analyze chemical composition of material at different depths 

Auger Process

1. create core hole by electron beam or x ray

2. upper electron drops to fill hole and transfer energy to another electron

3. second electron emitted as auger electron

KLL

LMM

MNN

KLL - initial

LMM - final hole

MNN - final hole 

Auger Electron Spectroscopy

Surface analytical technique detecting element specific Auger electrons after core-level ionization and non-radiative relaxation

AES used for

• extreme surface sensitivity

• elemental mapping (Z >= 3)

• depth profiling with sputtering

AES does not measure _______ composition well

bulk

KLL 

Electron in K shell removed (hole)

Electron in L shell drops to K to fill hole

Energy released from dropping used to eject electron from L shell

LMM

Hole in L shell

M to L drop

Energy to M electron

Direct mode AES

Auger peaks on large secondary-electron background

Derivative mode AES

Peaks become zero crossings, improve visibility/Auger structure

Each element has distinct these three lines AES

KLL

LMM

MNN

Auger yield is _______ at low Z 

high

X-ray yield is ________- at high Z

high

AES excellent for _______ elements, Xray good for __________ elements

light;heavy

Depth profiling AES

Sputter thin layer with inert ions, measure Auger windows sequentially 

“Fake” Auger peaks

• Plasmon loss peaks

• Ionization loss peaks

• Secondary electrons

• Checks

Scanning Auger Microscopy (SAM)

Scan a focused e- beam, at each pixel measure elements Auger intensity (higher intensity = brighter pixel)

Limitations of AES

• charging (contamination)

• beam damage

• topography

• sputtering/mixing

• interferences 

Size of AES samples

Wafers, cut carefully to avoid contamination

PES

Photoemission from solids/molecules

Measures binding energy distribution of occupied states

Photoemission model

1. optical excitation of electron

2. transport electron to surface

3. electron can escape across surface barrier into vacuum —> analyzer

UPS 

UV-Photoelectron Spectroscopy

Why is UPS surface-sensitive?

Valence electrons sample first top players, sensitive to overlayers

UPS Method

• Apply small negative sample bias to separate cutoff

Electron Energy Loss Spectroscopy 

Measures energy lost by fast electrons transmitted through thin specimens 

What kind of samples does Electron Energy Loss Spectroscopy work for?

Light elements, bonding

Singal from EELS

Incident electrons undergo inelastic scattering, lose energy, can track and sort electrons by energy on detector

Low-Loss EELS 

1-50 eV 

Plasmons, interband, bandgap

Core loss EELS

>50 eV, element-specific (KLM)

X-ray Photons

Can penetrate deeper and closer to the nuclei

EDS detector 

Specific number of electron-hole pairs generated, dependent on X-ray energy

EDS or WDS have higher StN ratio

WDS

EDS lightest element detected

Oxygen (Z=8)

XRF analysis goal

Understand elemental composition, can find weight fraction and compare against intensity 

X-ray absorption spectroscopy source

• table top X ray emitter

• Synchrotron