Historical Foundation of SERS Plasma resonance and plasma waves - - PowerPoint PPT Presentation
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
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
2014 CaSTL Summer School
Linear and Nonlinear Raman Spectroscopy in the Single Molecule Limit
Irving Langmuir
Plasma resonance and plasma waves
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.
kgrating=2π/(repeat length)
Deng et al., Nano Lett. 2010, 10, 1780–1786
2 1
3
+ − = ε ε α R
ωγ ω ω ε ε i
p b
+ − + =
2 2
1
) 3 ( ) ) 3 (( ) (
2 2 2 2 3
+ + − + + − =
b p b b p b
i i R ε ωγ ω ω ε ωγε ω ω ε α
3 + =
b p R
ε ω ω
Localized Surface-plasmon resonance
All else being equal, this term predicts which metals will show the most intense SERS and their rank as enhancers.
Quick summary
Annalen der Physik, 25, 377-445 (1908)
Jeanmaire and Van Duyne, J.
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
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.
“… 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
corresponds to the surface normal (call it z), those modes that belong to the same irreducible representation as z2 does will be preferentially enhanced as
red.
Basic electromagnetic theory of SERS: the em field enhancement,
β, enters as β4 although the enhanced Raman intensity is
proportional to Eo
2, that is, to the incident intensity. SERS is,
therefore, a linear effect despite the β4 dependence. The enhancement both
Raman-scattered field comes out of the redistribution of the incident em energy surrounding the particle. Enhancement, G= I SERS/I Raman ~ β4 for small Stokes shift and ~ β2-4 for very large Stokes shifts.
Käll and Xu
SERS is actually scattering by the metal. That’s the reason why the enhancement is so great.
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.
Haran, PNAS AS, 2008, 105, 16448–16453
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.
nanoparticle cluster Dimers and other small clusters of nanoparticles Rough surfaces possessing small, closely coupled features, e.g. cold- deposited films (superposition of gratings) Island Film One or two-dimensional gratings
FON Van Duyne et al. Aroca et al
Au nanofingers Li and Williams, HP
Tay and Hulse
But even with a simple nanoparticle dimer, the enhancement depends on:
SJ Lee, Z Guan, H Xu, and M Moskovits, J. P . C. C 111, 17985-17988 (2007)
A SERS substrate with an engineered structure
Kasemo, and M. Käll, Appl.
Varying exclusively the geometrical parameters of the substrate can alter the enhancement by over 104.
and M. Moskovits, J. Phys.
(2007)
When these results are combined with the prior results obtained
lithographically fabricated substrates
Syed Mubeen (UCSB), Shunping Zhang, Hongxing Xu (Chinese Academy of Sciences, Beijing) Nano Lett. 2012
Particle on Mirror
Why do we see any signal with s-polarization?
SERS intensity Dependence on spacer thickness
0.00 0.20 0.40 0.60 0.80 1.00 450 550 650 750 850 Wavelength (nm) Reflectance 30 60 90 120 150 Phase Φ (nm)
Rs Rp Φ
SERS is all about producing SERS “hot-spots” and placing the analyte in the hot spots. Originally that meant creating nanoparticle
appropriately designed or “engineered” substrates.
Chrysafis Andreou
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
still “up for grabs”.
In 1996-97 Shuming Nie and Katrin Kneipp independently reported single molecule
SERS, riveting the field of
revitalized interest in SERS. It also lent evidence in the mind
SERS was, as believed by the Chemical Enhancement devotées, indeed mysterious.
Shuming Nie & Emory, Science, 275, 1102 (1997), Katrin Kneipp, et al., Phys. Rev. Lett.
76, 2444 (1996)) Ag NPs incubated with NaCl
With a staggering 1014 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
mysterious and plasmons don’t seem to have much to do with it. The 1014 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:
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?
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.
This approach is based on the fact that the probability distribution of picking NA
two molecules is given by
How sensitive is SERS as a single or few molecule sensor?
Because one or two molecules of the target analyte (adenosine) produce SERS intensity changes originating from hundreds of Raman reporter molecules resident on the 20 nm NP , the system works like a “molecular amplifier”.
80 nm Au NP 20 nm Au NP
Nam Hoon Kim, Adv. Mater. 2011
1,2-dithiane polyethylenimine (PEI) makes amide bond
Single-stranded stem-loop structure for reversibility
Nam Hoon Kim
Adenosine concentration
Ted Laurence (LLNL) et al. J. Am. Chem. Soc., 2009, 131 (1), 162-169
The dimer’s anisotropic polarization depedence can be used to tease out single particle signals
Monomers show no polarization anisotropy so they would not show pronounced rotational autocorrelation effects. Ensembles of (rotationally uncorrelated) dimers would show much smaller rotational autocorrelation. Trimers and higher clusters are in low abundance and only the linear versions will show strong rotational autocorrelations.
The autocorrelation function for rotation diffusion implies we’re tracking individual dimers
2.E-01 3.E-01 4.E-01 5.E-01 6.E-01 7.E-01 8.E-01 9.E-01 1.E+00 1.E+00 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02
Ted A. Laurence, et al. Robust SERS Enhancement Factor Statistics Using Rotational Correlation Spectroscopy, Nano Lett., 12, 2912−2917 (2012) recalculated
Ted A. Laurence, et al. Robust SERS Enhancement Factor Statistics Using Rotational Correlation Spectroscopy, Nano Lett., 12, 2912−2917 (2012)
Average dimer’s enhancement is at least 4 orders of magnitude greater than average monomer’s
How close are we to a fully quantum theory of SERS? Some preliminary quantum effects