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Ag Nanocrystals Field Enhancement Optical Forces and Photochemical Growth Alex Hallock Peter Redmond Ken Bosnick Mathieu Maillard Jiang Jiang Amy Michaels Lou Brus Chemistry Department Columbia University Surface Enhanced Raman Spectroscopy SERS Raman cross sections of molecules adsorbed on rough surfaces or particles are enhanced many orders of magnitude x10 N N Jeanmaire and Van Duyne J Electroanal Chem 1977 84 1 Normally Raman spectra are extremely weak Raman cross section 10 29 cm2 Fleischmann Hendra and McQuillan Chem Phys Lett 1974 26 163 Albrecht and Creighton J Am Chem Soc 1977 99 5215 Single Particle Near Field Enhancement P COHERENT ac oscillating POLARIZATION Eint P 4 Eint Vacuum a E 3 2 E 1 2 0 0 Esurf E0 3 int P Eint and Esurf show resonance behavior when 2 minimum for spherical shape Ag 380 nm 2 i0 2 Elongated shapes show resonances at longer wavelengths ISERS IL E E E E s L L 2 s R R 2 Predicted 105 to 106 enhancemen Dark Field Microscopy Rayleigh scattering intensity measures the magnitude of scattering dipole which also creates the enhanced local field White Light Source Center Stop Paraboloid Mirror Sample plane Scattered Light Only scattered light is collected The background appears dark while particles appear bright Can obtain scattering spectra for individual particles White Light Rayleigh Scattering Image and Spectra of Ag Nanocrystals 10 um Intensity True color image on 35mm slide film 4000 3000 2000 1000 0 450 500 550 600 650 700 750 800 Wavelength nm Scattering spectra are directly dependent upon the geometry of the particle 1997 Unexpected Single Molecule SERS Kneipp et al 1 Nie et al 2 Molecule Crystal Violet Rhodamine 6G Metal Substrate Ag colloid Ag colloid Excitation 830 nm 514 5 nm SERS 10 17 to 10 16 cm2 10 15 cm2 1 Kneipp et al Phys Rev Lett 1997 78 1667 2 Nie and Emory Science 1997 275 1102 AFM Raman and Rayleigh Single particle single molecule experiment AFM or Tungsten Lamp Betzig and Trautman Single Molecule Fluorescence

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