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RAMAN SPECTROSCOPY Scattering mechanisms

RAMAN SPECTROSCOPY Scattering mechanisms. Rayleigh Mie Raman - local modes, vibrations, rotations Brillouin - collective modes (sound). Elastic. Random motions Vibrations Rotations. Raman scattering. Detects normal modes Vibrations or rotations in gases or liquids

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RAMAN SPECTROSCOPY Scattering mechanisms

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  1. RAMAN SPECTROSCOPYScattering mechanisms Rayleigh Mie Raman - local modes, vibrations, rotations Brillouin - collective modes (sound) Elastic Random motions Vibrations Rotations

  2. Raman scattering • Detects normal modes • Vibrations or rotations in gases or liquids • Phonon modes in solids • Fingerprint of bonds (elements) • Sensitive to • State of matter, crystalline or amorphous • Defects • Particle size • Temperature • …. • Experimental: narrow laser line + good spectrometer

  3. Raman lines of semiconductors

  4. Raman scatteringInteraction between applied field and normal modes Applied optical field: Induces polarization Polarizability Vibrations: Displacement Raman active modes: Small amplitudes -e +e

  5. Raman Lines Polarization Momentum selection rule: k₀ - k q +G=0 Only transitions at q=0

  6. Selection rules – Raman active modes: Polarizability ellipsoids of molecule. is Raman active: the polarizability is different at the two extremes. On the other hand and are not Raman active.

  7. Raman scattering from Si nanocrystalsBonds in Si (Diamond structure) S1: Vibrational frequencies (0.1 eV) S2: Optical frequencies (3.4 eV)

  8. Raman spectrum of Si

  9. Phonons in bulk Si Experiments: Neutron scattering

  10. Size effects in phonon modes • Well-known for thin films • 0-D systems: • No band gap in amorphous matrix - reduce confinement effects • Fluctuations in size, shape, and orientation • Effect on Raman spectrum: • Shift of peak • Broadening of line • selection rule lifted -

  11. Raman spectrum Faraci et al. PRB 73, 033307 (2006)

  12. Confinement function Decays towards edge of nanocrystal

  13. Calculating spectrum

  14. Calculated spectra Large shift with size Asymmetric shape of spectrum

  15. Comparison to experiments

  16. Bond charge model

  17. Bond charge model

  18. Transition from amorphous to nano crystalline Si film Yue, Appl. Phys. Lett., 75, 492 (1999) PECVD deposition at 230˚C onglass PL spectra: a-Si at 1.3 eV c-Si at 0.9 eV

  19. Temperature dependence Si nc’s on graphite. Shift of Stokes and Anti Stokes lines. Ratio between Stokes and Anti Stokes determine temperature Faraci et al. PRB 80 193410 (2009)

  20. Raman spectroscopy on carbon nanotubes Jung, Bork, Holmgaard, Kortbek 8th semester report (n,m) tube

  21. Metallic and semiconducting tubes

  22. Radial and transverse modes

  23. Radial breadingmodes

  24. ConclusionsRaman spectroscopy • Elemental specific optical technique • Fast and reliable • Distinguish crystalline and amorphous phases • Size sensitive for nc’s ~1-10 nm

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