Infrared Spectroscopy in thin films

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)