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

2. Raman scattering Detects normal modes Fingerprint of bonds (elements)
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 scattering Interaction 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 sele ction 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 nanocrystals Bonds 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, (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 on glass
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 (2009)

20. Raman spectroscopy on carbon nanotubes
Jung, Bork, Holmgaard, Kortbek
8th semester report
𝐶 ℎ =𝑛 𝑎 1 +𝑚 𝑎 2
(n,m) tube

21. Metallic and semiconducting tubes

22. Radial and transverse modes

23. Radial breadingmodes

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