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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 m medium dielectric function 5 0 Re( Esurface 3 E0 2 m -5 -10 -15 300 350 400 450 Wavelength (nm) 500 The ‘Chemical’ (Charge Transfer) Mechanism A new charge transfer band is formed when a molecule is adsorbed on a metal surface Metal levels Vacuum level Molecular levels LUMO EF HOMO Avouris and Demuth., 1981 Substrates supporting Single-molecule SERS 200 100 0 0 100 200 nm Colloids Silver islands Electromagnetic Enhancement The local field can be huge! G=1011 G=1012 From Xu et al., PRE 2000 EL ( ) 4 G EI ( ) 4 E L, I -local, incident field Exploring smSERS in dimers ly cedod HS S S S S SH Oligo-thiophene dodecy l POSTER BY TALI DADOSH, Tuesday 10 – 50 nm 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 Single-molecule Raman spectrometer 532 nm laser Spectrograph+CCD camera microscope scanning stage SERS spectrum of a single molecule Frequency (cm-1) Fluctuations in total intensity of a series of molecules Time (seconds) Fluctuations in total intensity SERS spectrum of a single molecule Frequency (cm-1) Time (seconds) Spectral fluctuations in one molecule Raman shift (cm-1) Similar behavior seen in crystal violet molecules The EM selection rule E>>E E O C O CH2CH3 CH3 CH3 CH3CH2NH O NH CH2CH3 + Cl E How many equilibrium orientations? ~1-2 But in R6G- semi-continuous fluctuations! Also – no correlation between different parts of spectrum Resonance Raman-Charge Transfer Resonance Raman transition within this band is responsible for surface enhancement (RRCT). s1 s0 Pyridine on electrodes, Arenas et al., 1996 774 cm-1 C-C stretches (A term Raman scattering?) 614 cm-1 1650 cm-1 Bend vibrations Frequency (cm-1) Polarized Raman measurements Raman scattered light parallel polarizing prism perpendicular x I parallel I perpendicular I parallel I perpendicular POSTER BY TIMUR SHEGAI, Monday Probing the Raman Scattering Tensor In resonance-enhanced scattering involving a non-degenerate electronic excitation – a single-element tensor 11 0 0 0 I parallel I perpendicular cos(2 ) intensity, a.u. I parallel I perpendicular 750 vertical horizontal 614 773 600 1650 450 300 150 0 0 400 800 1200 1600 2000 Raman shift, cm -1 , polarization function 614 cm-1 773 cm-1 0.2 1650 cm-1 total intensity 0.0 -0.2 -0.4 0 20 40 60 time, s 80 100 Angular dependence of 0.75 0.50 0.25 0 40 0 0 = 10 0.00 0.8 0 20 40 60 80 , degree 0 10 100 120 , polarization function , polarization function -1 614 cm -1 773 cm -1 1650 cm -1 614 cm -1 773 cm -1 1650 cm 0.4 0.0 -0.4 0 0 = 40 0 20 40 60 , degree 80 100 number of molecules Distribution of 0 3.6 2.7 1.8 0.9 0.0 0 10 20 30 40 0 The low-frequency bands have a different tensor than that of high-frequency bands A CT band in R6G? 773 cm-1 Hildebrandt & Stockburger, 1984 Metal levels Vacuum level Molecular levels LUMO ECT ELUMO Evac HOMO On resonance: EF ECT Elaser Smoluchowski’s smoothing effect Wandelt, 1987 The local work function can vary along the surface. Methods to measure: •Photoemission of adsorbed xenon (PAX) •STM Possible causes for local work function changes at an adsorbed molecule •Motion of silver adatoms / surface features •Diffusion of the adsorbed molecule Slowing down of fluctuations in glycerola viscosity effect C( ) I (t ) I (t ) I (t ) 2 1 in water in glycerol 0.8 C(t) 0.6 0.4 0.2 0 10 20 30 40 50 Time (sec) Haran, Israel J. Chem. 2004 Laser power effect on whole-spectrum correlation functions C( ) I (t ) I (t ) I (t ) 2 2 0 W/cm 6 18 48 100 1.00 Correlation 0.75 0.50 0.25 0.00 0 10 20 30 40 Time (seconds) 50 Dependence of correlation times on laser power 0.3 -1 Rate (sec ) 0.4 0.2 0.1 0.0 0 25 50 75 2 Power (W/cm ) 100 Are we heating the system (colloid + molecule)? water Q T ~ 0.1K 4cR R 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 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 Possible role for surface roughness relaxation? The relaxation time depends on surface tension and surface diffusion g DS 1 - surface tension DS- diffusion coefficient Can or DS can depend on the electromagnetic field? PROBABLY NOT! Lukatsky, Haran & Safran, PRE (2003) 67, 062402 Photodissociation can lead to sampling of different surface areas CT! local local ' Quantifying fluctuations by using ratios between Raman band intensities I614cm-1/ I1650cm-1 Ratio 2 1 0 0 200 400 600 Time (Seconds) 800 1000 Distribution of ratio values R=I614cm-1/I1650cm-1 Probability 0.06 0.04 0.02 0.00 0.00 0.25 0.50 R/Rmax 0.75 1.00 Probability function for local work function fluctuations P (E ) (2 ) 2 1 / 2 exp( (2 ) 2 1 / 2 P(E ) P( R) ( E E0 ) 2 exp( 2 2 ( 0 ) 2 2 2 ) ) Distribution of ratio values R=I614cm-1/I1650cm-1 Probability 0.06 E0 0.04 Assuming E0 0.15eV 0.06ev 0.02 0.00 0.00 0.42 0.01 0.25 0.50 R/Rmax 0.75 1.00 Haran, Israel J. Chem. 2004 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. Thanks to: Amir Weiss Timur Shegai Dima Lukatsky Sam Safran Yamit Sharaabi Tali Dadosh Paulina Płochocka Israel Bar-Joseph Timur Yamit