Infrared Spectroscopy in thin films Periklis Papadopoulos Universitt

Infrared Spectroscopy in thin films Periklis Papadopoulos Universität Leipzig, Fakultät für Physik und Geowissenschaften Institut für Experimentelle Physik I, Abteilung "Molekülphysik“ papadopoulos@physik. uni-leipzig. de

Outline Techniques Transmission Reflection Out-of-plane dipole moments Transition Moment Orientational Analysis Example: Liquid crystal elastomers 2

Transmission – reflection modes Simplified: no interference, etc. Transmission - absorption Absorbance Absorption coefficient α Molar absorption coefficient ε=α/c Specular reflection Reflectivity Normal incidence in air Lambert-Beer law: 3

Thin films – coatings Absorption is too low Reflection might be more important incident (Spectroscopic) Ellipsometry: reflected intensity for s and p polarizations Attenuated total reflection reflected transmitted 4

Ultrathin polystyrene films Spin-coated polystyrene Measured in transflection geometry Possible to measure thin samples, below 5 nm 5

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 6

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 7

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 Reference axis Molecular segment Molecular order parameter Transition dipole “parallel” vibration “perpendicular” vibration || 8

Quantitative IR spectroscopy Limitations of polarization-dependent measurements in 2 D Lambert-Beer law No correction for reflection Direct application may be problematic 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 9

Setup Arbitrary direction of electric field – 3 D 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 x y Transmission mode: better than ellipsometry for the absorption coefficient W. Cossack et al. Macromolecules 43, 7532 (2010) 10

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

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 θ Effective optical path (Snell’s law): d W. Cossack et al. Macromolecules 43, 7532 (2010) 12

Theory 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 θ Two values of the refractive index are allowed Birefringence k// k 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) 13

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! 14

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

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

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

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

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

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 20

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 21

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

Applications Nematic elastomer: biaxiality 3 D 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 y z x y x x Main chain (Si-O) Side chain (mesogen) 23

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

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