Excitation spectra of surface enhanced Raman scattering on sols.

Journal of Molecular

Structure, 143 (1986) 143-146 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

EXCITATION SPECTRA OF SURFACE ENHANCED F. MARTIN' and R. AROCA* 1 Fisica de Estado Solido, 2 Department of Chemistry,

Universidad

RAMAN SCATTERING

de Valladolid,

University

143

of Windsor,

ON SOLS.

Valladolid,

Windsor,

Spain

Ont., Canada N9B 3P4

ABSTRACT A phenomenological approach is described for practical computations of the excitation spectra in surface enhanced Ramn scattering (SERS) obtained with aggregated metal colloids.

RESULTS AND DISCUSSIONS SERS on colloidal Silver hydrosols uniform

spherical

absorption diameters general

particles

maximum between

prepared

approximately

at about 400 nm.

has been intensively

studiedlm4.

have been shown to consist of fairly 20 nm in diameter

that show a strong

Gold sols contain metal spheres with

12 and 18 nm with an absorption

maximum

at 523 nm.

The

theory

(Mie theory) of light scattering by a sphere with complex index is well documented 536 . A typical computation for silver spheres

refractive

with diameter maximum

silver and gold particles

when freshly

of 20 nm produces

at 400 nm.

The experimental

than the theoretical solution,

curve probably

of a narrow distribution

20 nm that increases Creighton profiles

on colloidal

spectrum

curves with a strong

of colloidal

due to the presence, of particle

silver is broader

in the colloidal

sizes about the central

value of

band.

, have carried out extensive studies of SERS excitation

silver and gold2'5.

profiles

They have shown that the maxima

occur with a wavelength

peak in the extinction

been associated

and scattering

the width of the spectral

et. al.2s7

SERS excitation secondary

absorption

with colloidal

spectrum

value similar

of hydrosols.

aggregation,

particularly

in

to that of a

This secondary chainlike

peak has

aggregation.

In a recent report Kerker et. al.3 have simulated the chainlike aggregation using prolate Enhancements fbrmalism

spheroids

having double and triple the volume of a single sphere.

of Raman intensities

described

by these spheroids

by Wang and Kerker8.

signal may be observed

were computed within

They concluded

even if only a small fraction

the

that a strong SERS

of aggregates

is present

in sols.

In the present report an empirical approach is explored that reproduces the experimentally

0022-2860/86/$03.50

found SERS excitation

0

profiles.

In the spirit of the electro-

1986 Elsevier Science Publishers B.V.

144 magnetic

interpretation,

colloidal

aggregates,

the aggregate

it is assumed

and the curve fitting

are maintained

This assumption

structure.

the aggregation)

here that SERS intensity

while metal spheres

of the secondary

could be achieved

preserves

the geometry

peak in the extinction

by changing

is mainly

as an essential

the optical

due to part of

of the problem

spectrum

properties

(due to of the

metal. In the particle-plasmon plasma frequency

model

in resonance

For a sphere,

enhancement.

E(W) + 2 = 0; that requires the sphere,

denoted

the resonance

for the interpretation

the surface

plasmom

condition

the real part of the relative

by the complex number

condition

of SERS intensities,

for an spheroid

a

light would provide maximum

with the exciting

is simply given by dielectric

E(W), to be equal

is written

to -2.

constant

of

Similarly,

as

E(W) + P = 0 where

to p=_-

Qi

‘1,

Q-associated

(!i’J) ; 50

(501

Legendre

Both expressions

-spheroidal

show the dependence

real part of the dielectric of the secondary

secondary

of the plasma

Therefore,

constant.

frequency

the secondary

on

extinction

in silver and gold sols can be fitted using the computational

scheme of the Mie theory and introducing

adjustable

and

functions.

explicitly

the real part of the dielectric peak observed

coordinate,

constant

an adjustable

parameter

takes the value 9.5.

curve fitting

at 558 nm is achieved

A similar

peak in gold sols at 684 nm produces

"n" to the

For instance,

of the metal.

peak of silver sols observed7

parameter

calculation

when the

for the

a value of 11.7 for the empirical

parameter. A clear computational

algorithm

in view of its applications colloidal

also used an empirical

plasmom model

parameter

of surface

SERS excitation

in the interpretation

For instance,

aggregates.

the discussion

of this adjustable

values of the dielectric

computed

by Gersten

constant

metal

profile

has to help

colloids.

using the particle-

and Nitzan'

(details are given the adjusted

have been used; i.e. E(W) has been replaced

by E (w) + n. excitation

of the metal

to the study of metal island films lo) where

in a recent application

Computed

constant

with

Moskovits4

rules for SERS on aggregated

were subsequently

in the form presented

seems important

of SERS data obtained

in a recent communication

in the dielectric

selection

profiles

parameter

for the 1038 cm

-1

Raman band of pyridine

on

silver sols is shown in figure 1. The circles correspond to experimental points 2 reported by Creighton et al. The bandwith of the calculated excitation profiles is somewhat

narrower

then the corresponding

bandwidth

of the experimental

points.

145 0

FIGURE

1.

SERS EXCITATION FOR SILVER

0

PROFILE

SOLS. CIRCLES 0

ARE MEASUREMENTS REFERENCE

FROM

2.

cl

:

0 0

/I’ YAVELEN.GTH

FIGURE

\

0

.(n3

2.

SERS EXCITATION

PROFILE

FOR GOLD SOLS. CIRCLES ARE MEASUREMENTS REFERENCE

FROM

2.

0

500

600

VAVELENCTH

700

cnn)

GO0

146 This deviation

is most probably

for metal particles

due to the fact that calculated

of one specific

result of contributions

distribution

would have the net effect of broadening Computed

excitation

also in good agreement

profiles

Experimental

size.

from a narrow

observations

of particles

the excitation

profiles

are

are the

sizes that

spectra.

for the 1014 cm-' Raman band on gold sols are

with the published

experimental

points and are presented

in figure 2. All calculations for Mie calculations 8087 coprocessor

have been performed

on an IBM/XT microcomputer.

and SER factor were written

for high speed mathematical

Programs

in BASIC and compiled

with the

computations.

REFERENCES

1 2 : 5 6 7 8 9 10

R.K. Chang, T.E. Furtak, Eds. "Surface Enhanced Raman Scattering"; Plenum Press:New York, 1982. C.G. Blatchford, J.R. Campbell, J.A. Creighton, Surf. Sci., 120(1982)-435. M. Kerker, 0. Siiman, D.S. Wang, J. Phys. Chem., 88(1984) 3168. M. Moskovits, J.S. Suh, J. Phys. Chem., 88(1984) 5526. G. Mie Annalen der Physik 25(1908) 377. M. Kerker "The Scattering of Light and other Electromagnetic Radiation"; Academic Press:New York, 1969. J.A. Creighton, C.G. Blatchford, M.G. Albrecht, J. Chem. Sot. Faraday 11 75 (1979) 790. D.S. Wang, M. Kerker, Phy. Rev. B 24 (1981) 1777. J. Gersten, A. Nitzan, J. Chem. Phys., 73(1980) 3023. R. Aroca, F. Martin, J. Raman Spectrosc., 16(1985)156.