2014 CaSTL Summer School Linear and Nonlinear Raman Spectroscopy in the Single Molecule Limit Dr. Syed Mubeen, Dr. Alessia Pallaoro, Dr. Chrysafis Andreou, Dr. Gary Braun, Dr. Seungjoon Lee, Dr. Sylvia Lee, Dr. Namhoon Kim Carl Meinhart, Galen Stucky, Ted Laurence (LLNL), HongXing Xu (CAS) Martin Moskovits Historical Foundation of SERS
Plasma resonance and plasma waves Irving Langmuir
Can such resonances be excited with light? Yes, but you need to arrange the geometry such that you conserve energy AND appropriately match the wavelengths in the two media, i.e. conserve momentum.
p s Absorbing medium Non absorbing medium
This technique continues to be used both as a surface spectroscopic approach and in sensing.
k grating =2 π /(repeat length)
Deng et al., Nano Lett. 2010, 10 , 1780–1786
Quick summary ω 2 ε − 1 ε = ε + − p α = 3 1 R ω + ωγ ε + b 2 2 i ε ω − ω + ωγε 3 2 2 R ( ) i α = b p b ε + ω − ω + ωγ ε + 2 2 i (( 3 ) ) ( 3 ) b p b ω ω = p All else being equal, this term predicts ε + R 3 which metals will show the most intense b SERS and their rank as enhancers. Localized Surface-plasmon resonance G. Mie, Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen, Annalen der Physik , 25 , 377-445 (1908)
SERS Jeanmaire and Van Duyne, J. Electroanal. Chem., 84 , 1977, 1-20
Who discovered SERS? “ In order to study the behaviour of species adsorbed at about monolayer coverage with Raman techniques it has been found necessary therefore to prepare solid metal electrodes with high surface areas…..The formation and reduction of silver chloride during each cycle of this treatment resulted in considerable etching of the silver surface which at the end of the pre-treatment appeared pale cream in colour. Some preliminary electrochemical experiments have shown that the surface area increases by at least a factor of 10 during this treatment .” ( Fleischmann et al. CPL 26, 163 1974 ) “Given that the experimentally observed intensities of NR scattering from adsorbed pyridine in our laboratory are 5-6 orders of magnitude greater than expected , we felt that some property of the electrode surface or the electrode/solution interface is acting to enhance the effective Raman scattering cross section for these adsorbed amines. The mechanism for this signal enhancement has not been conclusively determined in the present work; however, a plausible hypothesis will be presented below” ( Jeanmaire and Van Duyne , J. Electroanal. Chem., 84 ( 1977 ) 1-20)
“ … only colloids containing Group 1b metals (Cu, Ag, Au) (leaving the alkalis aside) possess optical conduction resonance in or near the visible region of the spectrum. This result can be applied equally well to the pseudo-two- dimensional colloid formed by the surface microroughness…This then explains the reluctance of other metals to produce enhanced Raman scattering… The above model also predicts that similar enhancements would be observed in true silver and copper colloids covered with adsorbate … ” Moskovits , JCP , ( 1978 ) 69, 4159-4161
Molecular orientation determination: For a symmetric molecule that adsorbs on the surface in an oriented fashion so that one of its axes of symmetry corresponds to the surface normal (call it z), those modes that belong to the same irreducible representation as z 2 does will be preferentially enhanced as one proceeds towards the red.
Käll and Xu Basic electromagnetic theory of SERS: the em field enhancement, β , enters as β 4 although the enhanced Raman intensity is 2 , that is, to the incident intensity. SERS is, proportional to E o therefore, a linear effect despite the β 4 dependence. The enhancement both of the incident and Raman-scattered field comes out of the redistribution of the incident em energy surrounding the particle. Enhancement, G= I SERS /I Raman ~ β 4 for small Stokes shift and ~ β 2-4 for very large Stokes shifts.
An optical antenna collects light from a large volume and concentrates much of it in a “hot spot”. The local electric near-field excited in molecules resident in the hot spot undergoing transitions then cause the entire antenna to radiate resonantly. SERS is actually scattering by the metal. That’s the reason why the enhancement is so great.
T. Shegai, Z. Li, T. Dadosh, Z. Zhang, Hongxing Xu, and G. AS , 2008, 105, 16448–16453 Haran, PNAS However, optical processes at “hot spots” dominate most SERS spectra.
Multi-particle effects overwhelm single-particle signals. This is why single- particle excitation spectra have not been often reported. Dimers and other small clusters of nanoparticles Au nanofingers FON Li and Williams, HP Van Duyne et al. Tay and Hulse Island Film Aroca et al Rough surfaces possessing small, closely coupled nanoparticle features, e.g. cold- cluster deposited films (superposition of gratings) One or two-dimensional gratings
But even with a simple nanoparticle dimer, the enhancement depends on : the gap size The particle size Particle shape The wavelength Secondary roughness (i.e. the actual geometry of the nanoparticles) The polarization of the light The dielectric properties of the ambient
A SERS substrate with an engineered structure . C . C 111 , 17985-17988 (2007) SJ Lee, Z Guan, H Xu, and M Moskovits, J. P
When these results are combined with the S. J. Lee , Z. Guan, H. Xu, prior results obtained and M. Moskovits, J. Phys. on e-beam Chem. C 111 , 17985-17988 (2007) lithographically fabricated substrates of Kasemo and Kall L. Gunnarsson, E. J. Bjerneld, H. Xu, S. Petronis, B. Kasemo, and M. Käll, Appl. Phys. Lett. 78, 802 (2001) Varying exclusively the geometrical parameters of the substrate can alter the enhancement by over 10 4 .
Particle on Mirror Syed Mubeen (UCSB), Shunping Zhang, Hongxing Xu (Chinese Academy of Sciences, Beijing) Nano Lett . 2012
Why do we see any signal with s-polarization?
SERS intensity Dependence on spacer thickness
1.00 150 0.80 120 Phase Φ (nm) Reflectance 0.60 90 0.40 60 Rs 0.20 30 Rp Φ 0.00 0 450 550 650 750 850 Wavelength (nm)
SERS is all about producing SERS “hot-spots” and placing the analyte in the hot spots. Originally that meant creating nanoparticle aggregates. More generally it means creating appropriately designed or “engineered” substrates.
Chrysafis Andreou
B. N. J. Persson , On the Theory of Surface-Enhanced Raman Scattering, Chem. Phys. Lett ., 1981, 82 , 561-565
Regrettably the chemical enhancement concept, and especially the idea, put forward by some, that transient charge transfer could produce enormous enhancements, took SERS for a stroll through an impenetrable forest for some 25 years, during which time many held fast to the notion that SERS was a phenomenon bereft of a guiding model that could account for the observations and make robust predictions. I.e. that it was a problem still “up for grabs”.
In 1996-97 Shuming Nie and Katrin Kneipp independently reported single molecule SERS , riveting the field of SERS. This event enormously revitalized interest in SERS. It also lent evidence in the mind of some that the origin of SERS was, as believed by the Chemical Enhancement devotées, indeed mysterious.
With a staggering 10 14 enhancement, Nie claimed that his effect arose from individual particles, some of which were “hot” others not, an attribute that did not correlate with the particle’s geometry. Conclusion : SERS is mysterious and plasmons don’t seem to have much to do with it. Shuming Nie & Emory, Science , 275 , 1102 (1997), Katrin Kneipp, et al., Phys. Rev. Lett . 76 , 2444 (1996)) Ag NPs incubated with NaCl The 10 14 enhancement has since been shown to be spurious. (Le Ru and Etchegoin ( J. Phys. Chem. C 111 , 13794 (2007 )). And all “hot particles” are actually aggregates (Michaels and Brus (JACS, 2000).
SERS Enhancement Factors For a comprehensive discussion see: E. C. Le Ru, E. Blackie, M. Meyer, and P . G. Etchegoin, Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study, J. Phys. Chem. C 2007, 111 , 13794-13803
How do we know that we’re measuring a Raman spectrum from a single molecule? 1. Measurements as a function of reduced concentration. (Shown by Le Ru and Etchegoin to lead to poor statistics on account of the non-uniformity of enhancement by most SERS-active (plasmonic) systems: most of the enhancement in most systems arise from hot spots.) See review in Annu. Rev. Phys. Chem . 2012, 63 , 65-87. 2. The Bianalyte method This approach is based on the fact that the probability distribution of picking N A of molecules A and N B of molecules B from an equally probably reservoir of the two molecules is given by
How sensitive is SERS as a single or few molecule sensor?
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