Periklis Papadopoulos

1
Infrared Spectroscopy in thin films

• Periklis Papadopoulos
• Universität Leipzig, Fakultät für Physik und
  Geowissenschaften
• Institut für Experimentelle Physik I, Abteilung
  "Molekülphysik

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
Reflectivity
Absorption coefficient a
Molar absorption coefficient ea/c
Normal incidence in air
Lambert-Beer law
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
Fourier Transform Infrared Spectrometry, P. R.
Griffiths, J.A. de Haseth, Wiley
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Polarization dependence
IR spectral range

• 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)

salol
Vibrational Spectroscopy in Life Science, F.
Siebert, P. Hildebrandt J. Hanuza et al. / Vib.
Spectrosc. 34 (2004) 253268
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Order parameter
IR spectral range

• 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

Reference axis
Molecular segment
Transition dipole
parallel vibration

perpendicular vibration
?
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Limitations of polarization-dependent
measurements in 2D
Quantitative IR spectroscopy

• 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 nn-i n ?
• Tensor of refractive index ?
• Arbitrary principal axes

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Arbitrary direction of electric field 3D
Setup
z

• 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

x
y
W. Cossack et al. Macromolecules 43, 7532 (2010)
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Experimental setup
Setup
Detector

• Simultaneous IR and mechanical measurements
• Temperature variation (RT 45 C)

W. Cossack et al. Macromolecules 43, 7532 (2010)
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Propagation in biaxial lossy medium complicated!
Theory

• Wave equation from Maxwells 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 (Snells law)

?
d
W. Cossack et al. Macromolecules 43, 7532 (2010)
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Propagation in biaxial lossy medium
Theory

• Boundary conditions of Maxwell equations are
  taken into account
• E//, k// and D? are the same at both sides of
  reflecting surface

?

• 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

k?
k//
W. Cossack et al. Macromolecules 43, 7532 (2010)
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Analysis
Analysis of spectra

• 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

C-O stretch
Absorbance tensor
Not diagonal!
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PEDOTPSS spin-coated on Ge
Applications

• Spin coated sample 20 nm thick
• Molecular chains lie on the xy-plane
• 2D study would be inadequate

z
y
x
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Smectic C elastomer vibrations
Applications
Repeating unit of main chain

• Main chain is LC
• Sample is too thick for MIR
• In NIR the combination bands and overtones are
  observed
• CO
• C-O

Doping with chiral group
Crosslinker
W. Cossack et al. Macromolecules 43, 7532 (2010)
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Smectic C elastomer biaxiality
Applications

• Stretching parallel to director
• No effect on biaxiality
• Biaxiality at 25 C (smectic X) comparable with
  40 C (smectic C)

Carbonyl CO
Aliphatic C-H
Ester C-O
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Smectic C elastomer director reorientation
Applications

• Shear
• After small threshold, reorientation starts

Rotation angles
Biaxiality
Reorientation on xy-plane
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Smectic C elastomer model
Applications

• Unlike NLCE, the director is strongly coupled to
  the network

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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

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Liquid crystalline elastomersNematic
Applications

• The elastomer has LC side chains
• Nematic phase
• With TMOA it is possible to find the order of the
  backbone and the mesogen

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Nematic elastomer vibrations
Applications

• C-H out-of-plane bending
• Si-O- stretching (overtone)

Si
O
Si
O
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Nematic elastomer biaxiality
Applications

• 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

z
z
y
z
y
x
y
x
x
Main chain (Si-O)
Side chain (mesogen)
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Nematic elastomer biaxiality
Applications
C-C mesogen

• Strething parallel to the director
• Small change of biaxiality
• No reorientation
• Stretching perpendicular
• No reorientation either!

stretch //
stretch ?
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Nematic elastomer model
Applications

• Only the polymer network is deformed
• Different from previous studies on NLCE

Macromol. Chem. Phys. 206, 709 (2005)