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Gilad Haran Chemical Physics Department Weizmann Institute of Science

Charge-transfer effects in Raman Scattering of Individual Molecules. Gilad Haran Chemical Physics Department Weizmann Institute of Science. FRISNO, EIN-BOKEK, February 2004. Surface-Enhanced Raman Scattering. Electromagnetic Enhancement. on a nanosphere. metal dielectric function.

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Gilad Haran Chemical Physics Department Weizmann Institute of Science

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  1. Charge-transfer effects in Raman Scattering of Individual Molecules Gilad Haran Chemical Physics Department Weizmann Institute of Science FRISNO, EIN-BOKEK, February 2004

  2. Surface-Enhanced Raman Scattering Electromagnetic Enhancement on a nanosphere metal dielectric function medium dielectric function

  3. HOMO Metal levels Molecular levels LUMO Vacuum level  EF Avouris and Demuth., 1981 The ‘Chemical’ (Charge Transfer) Mechanism A new charge transfer band is formed when a molecule is adsorbed on a metal surface

  4. 200 100 0 0 100 200 nm Substrates supporting Single-molecule SERS Colloids Silver islands

  5. G=1011 G=1012 From Xu et al., PRE 2000 -local, incident field Electromagnetic Enhancement The local field can be huge!

  6. 10 – 50 nm Exploring smSERS in dimers Oligo-thiophene POSTER BY TALI DADOSH, Tuesday

  7. SERS of Rhodamine 6G • Hildebrandt and Stockburger, 1984 • Very large cross-section • Involvement of halide ions Frequency (cm-1) Weiss & Haran, JPC B (2001) 105, 12348

  8. 532 nm laser Spectrograph+CCD camera microscope scanning stage Single-molecule Raman spectrometer

  9. SERS spectrum of a single molecule Frequency (cm-1)

  10. Fluctuations in total intensity of a series of molecules Intensity scale Time (seconds)

  11. Fluctuations in total intensity

  12. SERS spectrum of a single molecule Frequency (cm-1)

  13. Spectral fluctuations in one molecule Time (seconds) Intensity scale Raman shift (cm-1)

  14. Similar behavior seen in crystal violet molecules

  15. E E The EM selection rule E>>E How many equilibrium orientations? ~1-2 But in R6G- semi-continuous fluctuations! Also – no correlation between different parts of spectrum

  16. s1 s0 Resonance Raman-Charge Transfer Resonance Raman transition within this band is responsible for surface enhancement (RR-CT). Pyridine on electrodes, Arenas et al., 1996

  17. 774 cm-1 614 cm-1 1650 cm-1 Frequency (cm-1) C-C stretches (A term Raman scattering?) Bend vibrations

  18. x Polarized Raman measurements Raman scattered light parallel polarizing prism perpendicular POSTER BY TIMUR SHEGAI, Monday

  19. Probing the Raman Scattering Tensor In resonance-enhanced scattering involving a non-degenerate electronic excitation – a single-element tensor

  20. Angular dependence of 

  21. Distribution of 0 The low-frequency bands have a different tensor than that of high-frequency bands

  22. A CT band in R6G? 773 cm-1 Hildebrandt & Stockburger, 1984

  23. HOMO On resonance: Metal levels Molecular levels LUMO Vacuum level  EF

  24. Wandelt, 1987 Smoluchowski’s smoothing effect The local work function can vary along the surface. • Methods to measure: • Photoemission of adsorbed xenon (PAX) • STM

  25. Possible causes for local work function changes at an adsorbed molecule • Motion of silver adatoms / surface features • Diffusion of the adsorbed molecule

  26. Slowing down of fluctuations in glycerol- a viscosity effect Haran, Israel J. Chem. 2004

  27. Laser power effect on whole-spectrum correlation functions

  28. Dependence of correlation times on laser power

  29. R water • Q - amount of heat/unit time • - density of silver c – specific heat of silver  - heat diffusivity in water Assuming: illumination intensity 100W/cm2 absorption cross section 10-10 cm2 Are we heating the system (colloid + molecule)?

  30. Possible effect of EM field on the adatom diffusion constant? • Ds~10 Å2/sec • Depends exponentially on electrode potential • A linear dependence expected for oscillating fields From Hirai et al., Appl. Surf. Sci. 1998

  31. Possible role for surface roughness relaxation? The relaxation time depends on surface tension and surface diffusion Can  or DS can depend on the electromagnetic field? PROBABLY NOT! - surface tension DS- diffusion coefficient Lukatsky, Haran & Safran, PRE (2003) 67, 062402

  32. CT! Photodissociation can lead to sampling of different surface areas

  33. Quantifying fluctuations by using ratios between Raman band intensities I614cm-1/ I1650cm-1

  34. Distribution of ratio values R=I614cm-1/I1650cm-1

  35. Probability function for local work function fluctuations

  36. Assuming Distribution of ratio values R=I614cm-1/I1650cm-1 Haran, Israel J. Chem. 2004

  37. Conclusions • SERS fluctuations are due to modulation of charge transfer. • This modulation is due to lateral motion of molecules and sampling of different local work functions. • Lateral diffusion is facilitated by light. • Analysis of spectral fluctuations leads to better understanding of molecule-surface interactions involved in SERS.

  38. Thanks to: Amir Weiss Timur Shegai Yamit Sharaabi Dima Lukatsky Sam Safran Tali Dadosh Paulina Płochocka Israel Bar-Joseph

  39. Timur Yamit

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