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