Download Optical Measurements

Optical
Measurements
MSN506 notes
note: Most of the information in this presentation is collected off the
web for educational purposes.
Overview
• Remember some relevant parts of EM
theory
• Survey of some optical techniques
– Large number of variants and different
techniques are present
– It is not possible to cover all of them
– Those that may be related to nanomaterial
characterization are highlighted
Why optical measurements
• Optical properties of materials can be
naturally measured with optical
measurements (i.e. measurements that
involve light generation or scattering)
• Optical properties can be used to
determine structural or other physical
properties
• Generally non-destructive
Interaction of light with matter
• Light is an electromagnetic wave and electric (or
magnetic fields) interact with charge
• For light to interact with matter, generally
carriers must be present (which they generally
are)
• Light can interact with bound or free carriers
• Interaction can be non-resonant or resonant
(i.e. frequency or wavelength of light can
coincide with a characteristic oscillation
frequency of the sample)
The dipole moment
• Emission (and absorption or scattering) of light
by the presence of a carrier is strongly affected
by the dipole moment and its properties
Oscillating dipole
Proportional to the dipole moment
Power radiated by a classical dipole
Dipole moment (Quantum Mech.)
• The dipole moment can be calculated
for classical charge distributions
1D potential well
pd
G
= er
Time dependent dipole during a
transition between energy levels
• During a transition,
dipole oscillates
with a frequency
=ω = ΔE
A
B
++
A+B
A-B
Light can be generated
Power radiated by a classical dipole
-
-
+
Absorption can also be resonant
(this can be understood via time reversal
symmetry of electromagnetic waves)
Dipoles and refractive indices
Refractive index as a function of frequency
is determined by the AC permitivity
Dipoles and refractive indices
Refractive index as a function of frequency
is determined by the AC permitivity
Sometimes negative, depends on convention
Generally does not depend on
frequency at optical frequencies
n* = n + i · k
(n + ik)2 = ε' + i · ε''
⎛
2 ⎜
⎝
⎛
⎝
⎛
2 ⎜
⎝
⎛
⎝
1
n2
=
1
κ2
=
ε' 2 + ε'' 2
ε' 2 + ε'' 2
⎞
⎠
⎞
⎠
+ ε' 2
– ε' 2
⎞
⎟
⎠
⎞
⎟
⎠
Oscillator model
Damping factor
Classical charge oscillator
Solution
(can be used to calculate
polarization)
Lorentzian lineshape
Absorbed Energy
Oscillator strength can
be calculated through
the transition matrix
element for two given
levels
Oscillator strength
Example
Single oscillator
Multiple oscillators
Silicon
Density of oscillators
Strength of oscillators
Absorption due to an oscillator
Single oscillator
Absorption coefficient
Density of oscillators
Strength of oscillators
Classical oscillator model is intuitive for absorption measurements
Example: Atomic spectra
Absorption lines characteristic for each atom
The Monochromator
Can be used to select a certain
wavelength in a beam of white light
Diffraction grating
Multiple orders can be observed
depending on the period and
wavelength
Orders will repeat especially if the
grating period is large
Blazed at an angle for higher efficiency
Summary of optical measurements
• Thin films: absorption, reflection, transmission of
thin films, Ellisometry, refractive index models,
applications, FTIR
• Atomic absorption measurements
• Light scattering measurements: Raman
Scattering, Dynamic light scattering
• Nonlinear property measurements
• Photoluminescence
• Pump/Probe experiments
• Diffraction, X-ray Diffraction
Thin Films
• A material is deposited (or coated) uniformly on
a substrate.
• Measuring optical propeties of the film we can
learn a lot about the material
• Semiconductors
• Band gap, absorption, refractive index,
impurities, defects etc.
• Nanoparticles
• Size distribution, crystallinity etc.
• Organic layers
• Molecules
• Requires modelling of the measurement scheme
Thin film measurements
incident
substrate
transmitted
reflected
absorbed
Film of material to be characterized
Support substrate of
known optical properties
Incident Power = Reflected + Absorbed + Transmitted Powers
Fresnel Reflection
Normal incidence
Thin Film Reflection
Transmittance
If the substrate is
transparent to some
degree in the wavelength
range of interest,
transmission
measurements can be
used to determine the
optical constants. Simple
formulas are available for
restricted cases
1
T
Transmittance
Reflection at an angle
The Ellipsometer
• Absolute quantities are always hard to measure,
and most of the time inaccurate
• Making reflection measurements at two different
polarizations quickly one after another can yield
better results
The Ellipsometer
Less sensitive to
intensity fluctuations
The Ellipsometer
The Ellipsometer
Porosity as well
Modelling for Ellipsometry
• Refractive index models (semiempirical or
empirical)
• Surface and interface roughness
• Composite material models, porosity
• Layered materials and gradients
• A lot of complexity…
• Multiple models can produce similar results
• One solution is Variable Angle Spec. Ellips.
• Experience helps a lot in modelling
Refractive index models
•
•
•
•
Cauchy
Sellmeier
Other Models
Effective medium models (particularly
important for composite materials)
Example software: NKDGen
http://www.fen.bilkent.edu.tr/~aykutlu/elips.html
Good illustration of thin film measurements
Example: Material Characterization using
transmission data
Bulk Absorption Measurements
Example: Atomic Absorption Spectrometer
Specialized elemental lamp for different atoms
Burn the sample and analyze absorption of the flame!
Scattering Measurements
• Light (generaly a laser) is incident upon
the sample
• Interaction of sample and light generates
modulation of light frequency, spatial
distribution and/or time
Raman Scattering
• observed by C.V. Raman 1928
• Received Nobel price in 1930
Electronic vibration excited by external
monochromatic light
Mechanical vibration excited by phonons
• If these are coupled by a nonlinear interaction, sum and
difference frequencies of light can be generated
• Light then carries information about the phonon density
• Scattered light intensity is very weak compared to original
Energy Level Picture
• Selection Rules: Polariation
• Raman Active modes
• Requires a sharp clean light source and a high
resolution monochromator
Example applications
Chemistry: Molecular vibrations
Example applications: Bulk Crystals
Different structures all at once!
SiOx: GeSiO 120 sccm Silicon
1
o
1000 C
o
900 C
o
800 C
o
700 C
o
600 C
Intensity (a.u)
2
Type VII
ta= 2 hours
1
5 SiGe alloy formation
0
300
350
400
450
-1
Frequency shift (cm )
500
Example:Low-resolution Raman Spectroscopy
• Still good for material recognition
Low resolution Raman spectrum of 1:1:1 mixture of ethanol, 2propanol, and 2-methyl.2-propanol.
Variants
• Surface Enhanced Raman Spec.
– Plasmonic effects enhance the Raman signal
– Single molecule sensitivity!
• Waveguide Raman
– Multiple (large number) of interactions of laser
with sample film enhances the signal
– Submonolayer sensitivity
• Micro Raman and Tip Enhanced Raman
can be used for high spatial resolution
FTIR: Fourier Transform Infra Red
Spectroscopy
• Interferometer has periodic resonance condition
• This is used as an advantage in FTIR
FTIR principle
FTIR Operation
FTIR Operation
FTIR Operation
FTIR bands
FTIR summary
• Characteristic Frequencies for certain
bonds
• Dipole moment different for each bond
type, intensity varies
• Density of bonds affect intensity
• Frequency of vibration affected by film
stress
• Overtones (harmonics) possible
• … to get accurate quantitative information
meticulous analysis is needed
Dynamic Light Scattering
•
1.
2.
3.
4.
5.
6.
7.
8.
Theory of Operation
A beam of monocromatic light is directed through a sample and the fluctuation of the intensity of
scattered light by the molecules is analyzed by an Avalanche Photo Diode.
The Avalanche Photo Diode then sends electrical pulses to the Digital Signal Processor which
counts the number of photons detected in each successive time sample.
The frequency spectrum of this signal is determined by the mathematical technique known as
autocorrelation. The similarity between the signal wave form and a slightly time delayed copy of
itself is determined by multiplying the two wave forms together and then summing to give the
autocorrelation function.
From this the Translation Diffusion Coefficient, DT can be calculated by performing a nonlinear
least-squares fit of the autocorrelation coefficients to an exponential decay.
Under the assumption of Brownian Motion and that the molecules in solution are spheres, the
Hydrodynamic Radius, RH can be calculated by using Stokes’ Equation.
RH = kbT / 6πηDT
kb = Boltzman’s Constant
T = Absolute Temperature in Kelvin
η = Solvent Viscosity
Can also estimate the Molecular Weight from the RH and the sample temperature using a standard
curve of Molecular Weight vs. RH of globular proteins.
The instruments assumes the sample fits a standard monomodal gaussian type distribution
(monodisperse) and through a monomodal curve fit, determines the uniformity of the sizes of the
species in solution.
If the sample is found to be polydisperse or nonmonomodal, the user can use the software to
resolve a bimodal size distribution (bimodal analysis).
Dynamic Light Scattering
Measures size distributions of colloidal
(nano) particles in a fluid medium
Dynamic Light Scattering
20 nm beads
64 nm beads
Nonlinear optical property
measurements
• Z-Scan
Small spot size
means large
energy density
Large spotsize means low energy density
Photoluminescence
• Measures radiative decay properties of
optically generated carriers
Photoluminescence
Photoluminescence
•
•
•
•
•
Temperature dependent
Time resolved
Resonant excitation
Pump power dependent
Polarization dependent
• By modelling carrier
absorption, relaxation
and emission we can
estimate sample
properties
Streak Camera for fast time-resolved
measurements
Streak Camera
Pump-Probe experiments
• Ultrafast or fast pulses are used to excite (pump) and
probe sample
• Extremely fast carrier dynamics of samples can be
measured
Example setup
1 meter delay gives
3 nsec time delay
Pump-Probe experiments
X-Ray diffraction
• X-rays have extreme small wavelengths
compared to visible light (angstroms)
• They can diffract off atomic planes!
• X-Ray diffraction can be used to get
information about crystals, nano and micro
structure of materials etc.
X-Rays
• How are they produced?
High energy electrons knock out core electrons
XRD
• Angle of diffraction
• Width of peak gives information about
crystal domain size
XRD
• Angle of diffraction tells lattice parameters
• Width of peak gives information about
crystal domain size
• Molecular substances can be crystallized
and analyzed by XRD