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

Experimental dispersion

minimum of reflectivityωk( ω)= sinαc

h

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

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

scattering

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

3) Collecting light from „SPP-cone“with the

„Weierstraß-prism“ (WP)

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

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

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)

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 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.

Merci áAlain Dereux