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Periklis Papadopoulos
Universität Leipzig, Fakultät für Physik und Geowissenschaften
Institut für Experimentelle Physik I, Abteilung "Molekülphysik“
Infrared Spectroscopy in thin films
2
Outline
Techniques Transmission Reflection
Out-of-plane dipole moments Transition Moment Orientational Analysis
Example: Liquid crystal elastomers
3
Transmission – reflection modes
Simplified: no interference, etc.
Transmission - absorption Specular reflection
Absorbance
Absorption coefficient αMolar absorption coefficient ε=α/c
Lambert-Beer law:
1
0
logI
AI
1 0 0e el clI I I
ln10 ln10
l clA
Reflectivityref
0
IR
I
Normal incidence in air2
1
1
nR
n
4
Thin films – coatings
Absorption is too low Reflection might be more important (Spectroscopic) Ellipsometry: reflected intensity for s and
p polarizations Attenuated total reflection
incident reflected
transmitted
5
Ultrathin polystyrene films
Spin-coated polystyrene Measured in transflection geometry Possible to measure thin samples, below 5 nm
6
Complex refractive index
The imaginary part is proportional to the absorption coefficient
Dielectric function Real and imaginary parts are related through Kramers-
Kronig relations
Example:polycarbonate
n n in
0
0
exp 2
exp 4 exp 4
4
t
t
E x E i n x
I I i n x n x
n
2n
Fourier Transform Infrared Spectrometry,P. R. Griffiths, J.A. de Haseth, Wiley
7
Polarization dependence
Example: salol crystal All transition dipoles (for a certain transition) are perfectly
aligned Intensity of absorption bands depends greatly on crystal
orientation
Dichroism: difference of absorption coefficient between two axes
Biaxiality (all three axes different)
IR spectral range
salol
Vibrational Spectroscopy in Life Science, F. Siebert, P. HildebrandtJ. Hanuza et al. / Vib. Spectrosc. 34 (2004) 253–268
8
Order parameter
Non-crystalline solids: molecules (and transition dipole moments) are not (perfectly) aligned
Rotational symmetry is common Different absorbance A|| and A
Dichroic ratio R= A|| / A
Molecular order parameter
IR spectral range
Reference axis
Molecular segment
Transition dipole
||
2
2
3 cos 1
2molS P
1
0 :2
mol RS
R
1
: 22 2
mol RS
R
“parallel” vibration
“perpendicular” vibration
9
Limitations of polarization-dependent measurements in 2D
Lambert-Beer law Direct application may be problematic
No correction for reflection Problem near strong absorption bands
IR ellipsometry? Needs model, unsuitable for thick samples in NIR Too many free parameters
Biaxiality ? Complex n*=n’-i n” ? Tensor of refractive index ?
Arbitrary principal axes
Quantitative IR spectroscopy
0 expln10
CxI I Cx A
10
Arbitrary direction of electric field – 3D
By tilting the sample (0 ... ±70°) the E-field can have almost any direction (x,y,z)
The complex refractive index for every wavelength can be measured
Transmission mode: better than ellipsometry for the absorption coefficient
Setup
x y
z
W. Cossack et al. Macromolecules 43, 7532 (2010)
11
Experimental setup
Setup
Detector
Simultaneous IR and mechanical measurements
Temperature variation (RT – 45 °C)
W. Cossack et al. Macromolecules 43, 7532 (2010)
12
Propagation in biaxial lossy medium – complicated!
Wave equation from Maxwell‘s equations: The wavevector depends on the orientation Effective refractive index neff
When reflection is negligible, or can be removed (e.g. baseline correction in NIR) the tensor of absorption coefficient can be easily obtained
Effective optical path (Snell’s law):
Theory
2 20 0
1 1
eff effn n k E k D εE
12
0
1T
effn ε I kk E E
d eff
eff2 2
effRe sin
nd d
n
θ
20ε ε n
W. Cossack et al. Macromolecules 43, 7532 (2010)
13
Propagation in biaxial lossy medium
Boundary conditions of Maxwell equations are taken into account
E//, k// and D are the same at both sides of reflecting surface
Theory
k//k
θ
22
20 2
2
2
0
0 0 0
0
k
kc
k k
k k
k
k
εE
Two values of the refractive index are allowed Birefringence
The polarization eigenstates are not necessarily s and p
The values can be used in the Fresnel equations
W. Cossack et al. Macromolecules 43, 7532 (2010)
14
Analysis
The absorption coefficient (or absorbance) as a function of polarization and tilt angles can be fitted with 6 parameters
3 eigenvalues and 3 Euler angles No assumption for the orientation of the principal axes is
necessary
Analysis of spectra
030
6090
120
150
180
0
1
2
3
-60-40
-200
2040
60
Abs
orba
nce
Tilt an
gle
Polarization angle
3.52 0.44 0.15
0.44 0.14 0.07
0.15 0.07 0.04
A
Absorbance tensor
Not diagonal!
1A QΛQ
C-O stretch
15
PEDOT:PSS spin-coated on Ge
Spin coated sample ~ 20 nm thick
Molecular chains lie on the xy-plane
2D study would be inadequate
1300 1200 1100 1000 900
0.00
0.01
0.02
Abs
orba
nce
wavenumber [cm-1]
x y z
Applications
z
x
y
16
Smectic C* elastomer: vibrations
Main chain is LC Sample is too thick for MIR
In NIR the combination bands and overtones are observed
C=O C-O
-13430 cm
-13330 cm
Applications
Repeating unit of main chain
Doping with chiral group Crosslinker
7000 6500 6000 5500 5000 4500 4000 3500
0.0
0.2
0.4
0.6
Abs
orba
nce
wavenumber [cm-1]
x y z
W. Cossack et al. Macromolecules 43, 7532 (2010)
17
Smectic C* elastomer: biaxiality
Stretching parallel to director No effect on biaxiality Biaxiality at 25 °C (smectic X)
comparable with 40 °C (smectic C)
Applications
Carbonyl C=O Aliphatic C-H Ester C-O
y
z
x
18
Smectic C* elastomer: director reorientation
Shear After small threshold,
reorientation starts
Applications
y
z
x
Reorientation on xy-plane Rotation angles Biaxiality
19
Smectic C* elastomer: model
Unlike NLCE, the director is strongly coupled to the network
Applications
20
Summary
Absorbance from thin films is low, reflection must be taken into account
Ellipsometry is commonly applied New technique: TMOA
Applied to thick biaxial films Promising for thin films as well
21
Liquid crystalline elastomers:Nematic
The elastomer has LC side chains
Nematic phase
With TMOA it is possible to find the order of the backbone and the mesogen
Applications
22
Nematic elastomer: vibrations
C-H out-of-plane bending:
Si-O- stretching (overtone):
-1844 cm
-12110 cm
2200 2000 1800 1600 1400 1200 1000 800 6000
1
2
x y z
Ab
sorb
an
ce
wavenumber [cm-1]
Applications
Si O Si O
23
Nematic elastomer: biaxiality
3D polar plot of absorbance The main chains are oriented along the stretching direction The mesogen is perpendicular to the main chain No perfect rotational symmetry
Main chain (Si-O) Side chain (mesogen)
Applications
x
z
yy
x
z
x
y
z
24
Nematic elastomer: biaxiality
Applications
Strething parallel to the director: Small change of biaxiality No reorientation
Stretching perpendicular: No reorientation either!
C-C mesogen
stretch //
stretch
y
z
x
25
Nematic elastomer: model
Only the polymer network is deformed Different from previous studies on NLCE
Applications
Macromol. Chem. Phys. 206, 709 (2005)