From the excitation of surface plasmon polaritons ... - Andreas Otto

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 „Ottoconfiguration“
2) Roughness spectrum of a smooth silver film obtained by SPP-SPP
scattering.
3) Collecting light from „SPP-cone“ with the „Weierstraß-prism“ (WP)
4) Proof of the excitation of SPP‘s by „hot electrons“ in metal-insulatormetal
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 1955
Proc.Phys.Soc.A68(1955)525
ε(ω) = 0 at
ca.3.75eV

Surface plasmon polariton of silver
bulk
plasmon
(Fröhlich,
Pelzer 1955)
hω
( eV )
4
3
2
1
radiative
SPP-dispersion relation
ω/k = c
nonradiative
Surface plasmon-polariton of plane silver with
phase velocity parallel surface < c
Dielectric theory with retardation: „plasmaradiation“
by Ritchie & Eldridge 1962, SPP
dispersion in thin film by Otto 1965 (first?)
k parallel
ω εω ( )
( ω)
= ( )
c εω ( ) + 1
Surface Plasmon: Ritchie,
Phys.Rev.106 (1957) 874,
Ferrell and Stern 1958, 1960
unretarded
1/2
k parallel
surface

c
α
silver
v phase,parallel surface = c/n sin α
c
Problem and idea
phase velocity length surface
v phase,parallel surface = c/sin α >c
c c
α ≥ α
total reflection: n sin α > 1 evanescent field with v phase,parallel surface < c
c/n
total reflection
c/n
PUT the silver sample in about a wavelength
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
measuring D by intererence
fringes of white light
gap width d ~λ
Want to know more about
history, priority, who
invented and introduced the
names „X-configuration“?
Look for
www.fkphy.
uni-duesseldorf.de/
lecturexiamen/LectureI
SPP resonance only observed for p-polarized light (equivalent to TH polarization)

Experimental dispersion
ω
k( hω)=
sinα
c
minimum of reflectivity

In 1968, the sensitivity of the ATR – SPP resonance was clear,
but real surface diagnostic needs were not known, at least to me.
error bar
+2nm AgS
+1nm AgS
calculated
experiment

2) Roughness spectrum of a smooth
silver film obtained by SPP-SPP
scattering

k = kSPP
out
Fluid prism (refractive
index of BK7 glass)
in
ω
k = n
c
k
=
ω
c
k x
Scattered
intensity
Laser
in
Quantitative measurement of the
roughness spectrum of silver films,
J. Bodesheim, A. Otto Surf. Sci. 45 (1974) 441
SPP(in) – SPP(out) –
scattering
with „fluid-prism“

oughness spectrum of a „smooth“ silver film
without ATR-prism
with ATR-prism,
perpendicular
incidence
from SPP – SPP
scattering

3) Collecting light from „SPP-cone“
with the
„Weierstraß-prism“ (WP)

Invention of the „Weierstraß-prism“ : Integration over all emitted SPP‘s
from an emitting point
k = kSPP
out
out
ω
k = n
c
k
ω
=
c
k x
W. Wittke, A. Hatta, A. Otto
Efficient use of the surface plasmon polariton resonance in light scattering from adsorbates.
Applied Physics A 48 (1989) 289-294

Adjusting the gap sample-Weierstraßprism
R=1 d=2λ L R=0.94 d=3/2 λ L
gap modes
R=0.71 d=5/4 λ L R=0.35 d=λ L
Peter Borthen, Diplomarbeit Düsseldorf 1988
SPP
R=0.45 d=3/4 λ L
R=0.81 d< 1/2 λ L
R=0.61 d=1/2 λ L

4) Proof of the excitation of SPP‘s by
„hot electrons“ in metal-insulator-
metal junctions, using the WP

Ag
hot
electrons
excitation of
slow modes?
excitation
of SPP‘s ?
Reversed bias:
No excitation
of SPP‘s ?
Light emission from
Al/AlOx/Ag tunnelling
junctions
D. 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

Inset: integrated normalized emission
as function of d
200nm
400 nm
1000nm
2000nm
Diesing, G. Kritzler, A. Otto
D. Surface reactions of hot electrons at metal-liquid interfaces
Topic in Applied Physics 85, p.365-421 Springer 2003
Reversed bias: No light,
no excitation of SPP‘s !
Proof of hot electron – SPP
mechanism

5) Using the Weierstraß-prism in
surface Raman spectroscopy from
single crystal copper electrodes

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
Raman
light
0.01 M pyridine +
0.1M KClO 4
aqueous electrolyte
Laser beam
Hg/Hg 2 SO 4
reference
ring breathing Raman band of
pyridine at optimal electrode
potential

Reversible potential dependance of SERS of pyridine at Cu electrodes of different
crystallographic orientation
Cu-electrode potential
electrolyte: 0.01 M pyridine in 0.1
M KClO 4, (Hg/Hg 2 SO 4 ) reference electrode
Capacity measurements: pyridine stays
adsorbed at all electrode potentials E.
The dependence of intensity on potential
reflects tuning in and out of the transient
electron transfer resonance.
(metal-adsobate) (metal + -adsorbate - )
(Socalled „first layer chemical effect“ in
SERS)

SERS of pyridine at Cu electrodes originates at defects (SERS active sites)
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 Cu
films deposited at T substrate
120K
240K
300K
pyr. at defects pyridine at Cu(111) facets
Electromagnetic
enhancement by
SPP resonance is
not enough to
observe pyridine at
atomic smooth
facets

Concentration of surface defects is unknown in this experiment
83
40
19
3
Average enhancement of SERS of pyridine
at Cu electrodes of different
crystallographic orientation
electrolyte: 0.01 M pyridine in 0.1
M KClO 4, (Hg/Hg 2 SO 4 ) reference electrode
Average enhancement with respect to
pyridine in the liquid gap (assuming
SERS originates from all adsorbed
pyridine molecules, rather than only
from species adsorbed at defects)

6) SERS active sites

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 about
40 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) for
the indicated average thickness
d cold of additional silver deposited
on top at about 40 K. 1 W,
integration times 800,400, 400,400
s.
SERS active sites
normal sites

SERS intensity and
thermo-desorptionspectroscopy
of CO
on cold-deposited Ag
exposures
desorption from defects
CO-stretch
SERSintensity
W. Akemann, A. Otto
Vibrational modes of CO adsorbed to disordered copper
films investigated with Raman spectroscopy.
J. Raman Spectrosc. 22 (1991) 797-803
Thermal
desorption
desorption temperature (K)
SERS only from
a minority of
sites, where CO
is most tighly
bound and settles
first
multilayer desorption

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 preferentially
to kink sites

Clean Cu(211), CO settles first at kink sites
A. Otto,
M. Lust,
A. Pucci,
G. Meyer
Proceedings
of SERRS
2006, in press
in
Canadian
Journal of
Analytical
Sciences and
Spectroscopy
“SERS active
sites“, facts and
open questions”

A.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
No CT-SERS at smooth surfaces
resonant Raman effect of
the complex by internal
charge transfer

SERS active sites = sites, where electrons are trapped for some fs
A. Otto, The „chemical“ (electronic) contribution to SERS, J. Raman Spectr. 36 (2005)497
acts for a short time of about 5fs as an isolated metalmolecule
complex („hole does not run away“). Internal
resonance Raman effect by charge transfer for molecules
with π* orbital becomes possible. There is no „first layer
SERS-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.

Merci á
Alain Dereux