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Advanced Infrared Spectroscopy Techniques for Analyzing Thin Films and Liquid Crystals

This presentation explores cutting-edge infrared spectroscopy methodologies utilized in the analysis of thin films and liquid crystal elastomers. Key topics include transmission and reflection techniques, analysis of out-of-plane dipole moments, transition moment orientational analysis, and the application of the Lambert-Beer law. Techniques such as spectroscopic ellipsometry and complex refractive index measurement will be highlighted, offering insights on absorbance and reflectivity properties of ultrathin films. This research aims to enhance the understanding of molecular dynamics and interactions in advanced materials.

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Advanced Infrared Spectroscopy Techniques for Analyzing Thin Films and Liquid Crystals

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  1. papadopoulos@physik.uni-leipzig.de 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 Transmission - absorption Specular reflection • Simplified: no interference, etc. Absorbance Reflectivity Absorption coefficient α Molar absorption coefficient ε=α/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

  7. IR spectral range 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) salol Vibrational Spectroscopy in Life Science, F. Siebert, P. Hildebrandt J. Hanuza et al. / Vib. Spectrosc. 34 (2004) 253–268

  8. IR spectral range 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 Reference axis Molecular segment Transition dipole “parallel” vibration || “perpendicular” vibration 

  9. Quantitative IR spectroscopy 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

  10. Setup Arbitrary direction of electric field – 3D 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)

  11. Setup Experimental setup • Simultaneous IR and mechanical measurements • Temperature variation (RT – 45 °C) Detector W. Cossack et al. Macromolecules 43, 7532 (2010)

  12. Theory 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): θ d W. Cossack et al. Macromolecules 43, 7532 (2010)

  13. Boundary conditions of Maxwell equations are taken into account E//, k// and D are the same at both sides of reflecting surface Theory Propagation in biaxial lossy medium θ • 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)

  14. Analysis of spectra 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 C-O stretch Absorbance tensor Not diagonal!

  15. Applications PEDOT:PSS spin-coated on Ge • Spin coated sample ~ 20 nm thick • Molecular chains lie on the xy-plane • 2D study would be inadequate z y x

  16. Applications Smectic C* elastomer: vibrations Repeating unit of main chain • Main chain is LC • Sample is too thick for MIR • In NIR the combination bands and overtones are observed • C=O • C-O Doping with chiral group Crosslinker W. Cossack et al. Macromolecules 43, 7532 (2010)

  17. Applications z y x Smectic C* elastomer: biaxiality • Stretching parallel to director • No effect on biaxiality • Biaxiality at 25 °C (smectic X) comparable with 40 °C (smectic C) Carbonyl C=O Aliphatic C-H Ester C-O

  18. Applications z y x Smectic C* elastomer: director reorientation • Shear • After small threshold, reorientation starts Rotation angles Biaxiality Reorientation on xy-plane

  19. Applications Smectic C* elastomer: model • Unlike NLCE, the director is strongly coupled to the network

  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. Applications 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

  22. Applications Nematic elastomer: vibrations • C-H out-of-plane bending: • Si-O- stretching (overtone): Si O Si O

  23. Applications 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 z z y z y x y x x Main chain (Si-O) Side chain (mesogen)

  24. Applications z y x Nematic elastomer: biaxiality C-C mesogen • Strething parallel to the director: • Small change of biaxiality • No reorientation • Stretching perpendicular: • No reorientation either! stretch // stretch 

  25. Applications Nematic elastomer: model • Only the polymer network is deformed • Different from previous studies on NLCE Macromol. Chem. Phys. 206, 709 (2005)

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