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 - absorptionSpecular reflection Absorbance Absorption coefficient α Molar absorption coefficient ε=α/c Lambert-Beer law: Reflectivity Normal incidence in air

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 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. Hildebrandt J. 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 ||  “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

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 n eff 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 d θ 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k 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 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 Absorbance tensor Not diagonal! 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 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 Applications Repeating unit of main chain Doping with chiral group Crosslinker 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=OAliphatic C-HEster 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 anglesBiaxiality

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): Applications SiO 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 y y 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)