Gilad HaranChemical Physics DepartmentWeizmann Institute of Science

Charge-transfer effects in Raman Scattering of Individual

Molecules

FRISNO, EIN-BOKEK, February 2004

Surface-Enhanced Raman Scattering

on a nanosphere

0surface EEm

2

3

Electromagnetic Enhancement

300 350 400 450 500-15

-10

-5

0

5

Re(

Wavelength (nm)

mmetal dielectric function

medium dielectric function

A new charge transfer band is formed when a molecule is adsorbed on a metal surface

The ‘Chemical’ (Charge Transfer) Mechanism

Molecular levels

Vacuum level

EF

HOMO

LUMO

Metal levels

Avouris and Demuth., 1981

Substrates supportingSingle-molecule SERS

0 100 200

100

200

0

nm

Colloids Silver islands

Electromagnetic Enhancement

The local field can be huge!

From Xu et al., PRE 2000

4

4

)(

)(

I

L

E

EG ILE , -local, incident field

G=1012

G=1011

S

S

S

S

lycedod

dodecyl

SH

HS

Oligo-thiophene

Exploring smSERS in dimers

POSTER BY TALI DADOSH, Tuesday

10 –

50 nm

Frequency (cm-1)

SERS of Rhodamine 6G

Hildebrandt and Stockburger, 1984

•Very large cross-section

•Involvement of halide ions

Weiss & Haran, JPC B (2001) 105, 12348

Single-molecule Raman spectrometer

532 nm laser

Spectrograph+CCD camera

microscope

scanning stage

SERS spectrum of a single molecule

Frequency (cm-1)

Time (seconds)

Intensity scale

Fluctuations in total intensity of a series of molecules

Fluctuations in total intensity

SERS spectrum of a single molecule

Frequency (cm-1)

Raman shift (cm-1)

Time (seconds)

Intensity scale

Spectral fluctuations in one molecule

Similar behavior seen in crystal violet molecules

The EM selection rule

E>>E

How many equilibrium orientations? ~1-2

But in R6G- semi-continuous fluctuations!

Also – no correlation between different parts of spectrum

E

EHO N CH2CH3

CH3CH3

NHCH2CH3

C

O

O CH2CH3

Cl+

Resonance Raman-Charge Transfer

Pyridine on electrodes, Arenas et al., 1996

Resonance Raman transition within this band is responsible for surface enhancement (RR-CT).

s0

s1

614 cm-1

774 cm-1

1650 cm-1

Frequency (cm-1)

C-C stretches (A term Raman scattering?)

Bend vibrations

Polarized Raman measurements

polarizing prism

Raman scattered light

parallel

perpendicular

larperpendicuparallel

larperpendicuparallel

II

II

x

POSTER BY TIMUR SHEGAI, Monday

Probing the Raman Scattering TensorIn resonance-enhanced scattering involving a

non-degenerate electronic excitation – a single-element tensor

00

011

)2cos(

larperpendicuparallel

larperpendicuparallel

II

II

0 400 800 1200 1600 2000

0

150

300

450

600

750

inte

nsity

, a.u

.

Raman shift, cm-1

vertical horizontal614 773 1650

0 20 40 60 80 100

-0.4

-0.2

0.0

0.2

, p

ola

riza

tion

func

tion

time, s

614 cm-1

773 cm-1

1650 cm-1

total intensity

0 20 40 60 80 100 120

0.00

0.25

0.50

0.75

,

pol

ariz

atio

n fu

nctio

n

, degree

614 cm-1

773 cm-1

1650 cm-1

0 = 100

0 20 40 60 80 100

-0.4

0.0

0.4

0.8

, p

olar

izat

ion

func

tion

, degree

614 cm-1

773 cm-1

1650 cm-1

0 = 400

Angular dependence of

100

400

Distribution of 0

The low-frequency bands have a different tensor than that of high-frequency bands

Hildebrandt & Stockburger, 1984

773 cm-1

A CT band in R6G?

Molecular levels

Vacuum level

EF

HOMO

LUMO

Metal levels

vacLUMOCT EEE

laserCT EE

On resonance:

Smoluchowski’s smoothing effect

The local work function can vary along the surface.

Methods to measure:•Photoemission of adsorbed xenon (PAX)

•STM

Wandelt, 1987

Possible causes for local work function changes at an adsorbed molecule

•Motion of silver adatoms / surface features

•Diffusion of the adsorbed molecule

0 10 20 30 40 500.2

0.4

0.6

0.8

1

in water in glycerol

C(t

)

Time (sec)

Slowing down of fluctuations in glycerol-a viscosity effect

2)()()()( tItItIC

Haran, Israel J. Chem. 2004

Laser power effect on whole-spectrum correlation functions

2)()()()( tItItIC

Dependence of correlation times on laser power

Are we heating the system (colloid + molecule)?

Rwater

KRc

QT 1.0

4~

Q - amount of heat/unit time - density of silverc – specific heat of silver - heat diffusivity in water

Assuming: illumination intensity 100W/cm2 absorption cross section 10-10 cm2

From Hirai et al., Appl. Surf. Sci. 1998

• Ds~10 Å2/sec

• Depends exponentially on electrode potential

• A linear dependence expected for oscillating fields

Possible effect of EM field on the adatom diffusion constant?

Possible role for surface roughness relaxation?

1 Sg D

The relaxation time depends on surface tension and surface diffusion

Can or DS can depend on the electromagnetic field?

PROBABLY NOT!

- surface tension

DS- diffusion coefficient

Lukatsky, Haran & Safran, PRE (2003) 67, 062402

Photodissociation can lead to sampling of different surface areas

local local'

CT!

Quantifying fluctuations by using ratios between Raman band intensities

I614cm-1/ I1650cm-1

0 200 400 600 800 10000

1

2

Ra

tio

Time (Seconds)

Distribution of ratio values R=I614cm-1/I1650cm-1

0.00 0.25 0.50 0.75 1.000.00

0.02

0.04

0.06P

roba

bilit

y

R/Rmax

)exp()2(

)exp()2()(

2

20

2

20

2

)(2/12

2

)(2/12

EEEP

Probability function for local work function fluctuations

)()( RPEP

Distribution of ratio values R=I614cm-1/I1650cm-1

01.042.00

E

Assuming eVE 15.00

ev06.0

0.00 0.25 0.50 0.75 1.000.00

0.02

0.04

0.06

Pro

babi

lity

R/Rmax Haran, Israel J. Chem. 2004

Conclusions

• SERS fluctuations are due to modulation of charge transfer.

• This modulation is due to lateral motion of molecules and sampling of different local work functions.

• Lateral diffusion is facilitated by light.

• Analysis of spectral fluctuations leads to better understanding of molecule-surface interactions involved in SERS.

Amir Weiss

Timur Shegai

Yamit Sharaabi

Thanks to:

Dima LukatskySam Safran

Tali DadoshPaulina PłochockaIsrael Bar-Joseph

Timur

Yamit