From the excitation of surface plasmon polaritons (SPP’s) by evanescent waves to „SERS active sites”
Andreas Otto
1) History of plasmons, surface plasmon polaritons (SPP) and „Otto-configuration“
2) Roughness spectrum of a smooth silver film obtained by SPP-SPPscattering.
3) Collecting light from „SPP-cone“ with the „Weierstraß-prism“ (WP)
4) Proof of the excitation of SPP‘s by „hot electrons“ in metal-insulator-metal junctions, using the WP
5) Using the WP in surface Raman spectroscopy from single crystal copper electrodes
6) SERS active sites
Bulk-plasma oscillations (plasmons)
1) Oscillations in ionized gases: Tonks & Langmuir 1929
2) Bulk plasma oscillations in free electron metal:Pines & Bohm, Phys.Rev.85(1952)338
3) Bulk plasmon in Silver, Fröhlich & Pelzer 1955ε(ω) = 0 at ca.3.75eV
Proc.Phys.Soc.A68(1955)525
Surface plasmon polariton of silver( )eVωh
SPP-dispersion relation 1/ 2( )( ) ( )( ) 1parallelk
cω ε ωω
ε ω=
+
Surface plasmon-polariton of plane silver with phase velocity parallel surface < cDielectric theory with retardation: „plasma-radiation“ by Ritchie & Eldridge 1962, SPPdispersion in thin film by Otto 1965 (first?)
ω/k = c
4
3
2
1
k parallelsurface
nonradiativeradiative Surface Plasmon: Ritchie, Phys.Rev.106 (1957) 874,Ferrell and Stern 1958, 1960unretarded
bulk plasmon
(Fröhlich, Pelzer 1955)
α
silver
c
vphase,parallel surface = c/n sin α
Problem and ideacphase velocity length surface vphase,parallel surface = c/sin α >c
c c
c/n
c/n
total reflection: n sin α > 1 evanescent field with vphase,parallel surface < c
total reflectionα α≥
PUT the silver sample in about a wave-length distance BELOW the prism!
The realization (1968)A. Otto, Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection, Z. Physik 216 (1968) 398, download from http://fkphy.uni-duesseldorf.de
gap width d ~λmeasuring D by intererence
fringes of white light
SPP resonance only observed for p-polarized light (equivalent to TH polarization)
Want to know more about history, priority, who invented and introduced the names „X-configuration“?Look forwww.fkphy. uni-duesseldorf.de/lecturexiamen/LectureI
In 1968, the sensitivity of the ATR – SPP resonance was clear,but real surface diagnostic needs were not known, at least to me.
experiment
calculated
+1nm AgS
+2nm AgS
error bar
SPP(in) – SPP(out) –scattering
with „fluid-prism“
kx
k ncω
=
kcω
=
SPPk k=
inout
Quantitative measurement of the roughness spectrum of silver films, J. Bodesheim, A. Otto Surf. Sci. 45 (1974) 441
Laser in
Scattered intensity
Fluid prism (refractive index of BK7 glass)
roughness spectrum of a „smooth“ silver film
without ATR-prism
with ATR-prism,perpendicular incidence
from SPP – SPPscattering
Invention of the „Weierstraß-prism“ : Integration over all emitted SPP‘sfrom an emitting point
kx
k ncω
=
kcω
=
SPPk k=
outout
W. Wittke, A. Hatta, A. OttoEfficient use of the surface plasmon polariton resonance in light scattering from adsorbates. Applied Physics A 48 (1989) 289-294
Adjusting the gap sample-WeierstraßprismPeter Borthen, Diplomarbeit Düsseldorf 1988
R=1 d=2λL R=0.94 d=3/2 λL
R=0.71 d=5/4 λL R=0.35 d=λL
R=0.45 d=3/4 λL R=0.61 d=1/2 λL
R=0.81 d< 1/2 λL
SPPgap modes
4) Proof of the excitation of SPP‘s by„hot electrons“ in metal-insulator-
metal junctions, using the WP
Light emission fromAl/AlOx/Ag tunnelling
junctionsD. Diesing, G. Kritzler, A. Otto, Surface reactions of hot electrons at metal-liquid interfaces, in Solid Liquid Interfaces, Macroscopic Phenomena and Microscopic Understanding, eds. S. Thurgate and K. Wandelt,
Topic in Applied Physics 85, p.365-421 Springer 2003
d
excitation ofslow modes?
hot
electrons
Ag
excitationof SPP‘s ?
Reversed bias: No excitation of SPP‘s ?
Inset: integrated normalized emission as function of d
D. Diesing, G. Kritzler, A. OttoSurface reactions of hot electrons at metal-liquid interfaces Topic in Applied Physics 85, p.365-421 Springer 2003
Proof of hot electron – SPPmechanism
200nm
400 nm
Reversed bias: No light, no excitation of SPP‘s !
1000nm
2000nm
Advantage of „Otto –configuration“, when using flat single crystals of different orientations: Variation of gap width, no excessive heating of the sample
A. Bruckbauer, A. Otto,, J. Raman Spectrosc., 29 (1998) 665-672
0.01 M pyridine + 0.1M KClO4aqueous electrolyte
Hg/Hg2SO4reference
Laser beam
ring breathing Raman band ofpyridine at optimal electrodepotential
Raman light
Capacity measurements: pyridine stays adsorbed at all electrode potentials E.
The dependence of intensity on potentialreflects tuning in and out of the transient
electron transfer resonance. (metal-adsobate) (metal+-adsorbate-)
(Socalled „first layer chemical effect“ in SERS)
electrolyte: 0.01 M pyridine in 0.1 M KClO4, (Hg/Hg2SO4) reference electrode
Reversible potential dependance of SERS of pyridine at Cu electrodes of differentcrystallographic orientation
Cu-electrode potential
Cu(110) vicinal at E = -1000mV
Cu(110) vicinal at E=-500 mV
Cu(110) vicinal at E=-1700 mV
Cu(111) at E=-1100 mV
SERS of pyridine on Cufilms deposited at Tsubstrate
pyr. at defects pyridine at Cu(111) facets
120K
240K
300K
SERS of pyridine at Cu electrodes originates at defects (SERS active sites)
Electromagnetic enhancement by SPP resonance is not enough to observe pyridine at atomic smooth facets
Concentration of surface defects is unknown in this experiment
Average enhancement of SERS of pyridineat Cu electrodes of differentcrystallographic orientation
electrolyte: 0.01 M pyridine in 0.1 M KClO4, (Hg/Hg2SO4) reference electrode
3
19
40
83
Average enhancement with respect topyridine in the liquid gap (assumingSERS originates from all adsorbed pyridine molecules, rather than only from species adsorbed at defects)
pyridine on silver in Ultra-High-Vacuum,
influence of atomic scale roughness
Ü.Ertürk,D.Gherban, A.Otto, Surf.Sci.203, 554(1988)
(a) Top: Raman spectrum of a silver film, deposited at room temperature, exposed at about40 K to 1 L of pyridine in the range of the C-C breathing mode, 1 W, integration time 2000 s.Bottom: Raman spectrum of liquid pyridine. (b) Raman spectra of the sample described in (a) forthe indicated average thickness dcold of additional silver deposited on top at about 40 K. 1 W, integration times 800,400, 400,400 s.
SERS active sites
normal sites
W. Akemann, A. OttoVibrational modes of CO adsorbed to disordered copper films investigated with Raman spectroscopy.J. Raman Spectrosc. 22 (1991) 797-803
SERS only froma minority ofsites, where COis most tighly bound and settles first
CO-stretchSERS-intensity
Thermaldesorption
multilayer desorptiondesorption from defects
SERS intensity and thermo-desorption-spectroscopy of CO
on cold-deposited Ag
desorption temperature (K)
exposures
STM on Cu(211)
Ball model of the Cu(211) surface. The distance between intrinsic steps is 0.625nm. Horizontal chains from left to right are along the (0, -1, 1) direction. Copper atoms in the step edge are labeled A, at the kink site: B
CO binds preferentiallyto kink sites
Clean Cu(211), CO settles first at kink sites
A. Otto, M. Lust, A. Pucci, G. MeyerProceedingsof SERRS
2006, in press
inCanadianJournal ofAnalytical
Sciences and Spectroscopy
“SERS active sites“, facts and open questions”
No CT-SERS at smooth surfacesA.Otto, M.Futamata, Electronic Mechanisms of SERS, in Surface enhanced Raman scattering, physics and applications,
eds K.Kneipp, M.Moskovits, H.Kneipp, Topics inApplied Physics 103 (2006)147-182
resonant Raman effect ofthe complex by internal
charge transfer
SERS active sites = sites, where electrons are trapped for some fsA. Otto, The „chemical“ (electronic) contribution to SERS, J. Raman Spectr. 36 (2005)497
acts for a short time of about 5fs as an isolated metal-molecule complex („hole does not run away“). Internal resonance Raman effect by charge transfer for molecules with π* orbital becomes possible. There is no „first layerSERS-effect“ at a smooth surface.
Acknowledgments to my collaborators 1977-2002
visitors, now at SPP3: Lopez-Rios, Futamata
to Prof. Annemarie Pucci and her people (e.g. Lust, Sinther, Priebe), M.Futamata (Tsukuba) and G.Meyer (IBM Zürich) and my first„SPP-students“ in München (1970-1974): Sohler, Bodesheim, Huber.
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