Optical properties of dye molecules adsorbed on evaporated metal surfaces

Journal of Ekctron Spectroscopy and Related Phenomena, 45 (1987) 123-132 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

OPTICAL

PROPERTIES

H. UEBA,

OF DYE MOLECULES

C. TATSUYAMA

Department

ADSORBED

ON EVAPORATED

123

METAL

SURFACES

and T. MORIGUCHI

of Electronics,

Toyama

University,

Gofuku,

Toyama,

930 Japan

SUMMARY The optical absorption, fluorescence (FL) and Raman scattering (RS) have been observed for dye molecules rhodamine 66 (R6G) and fuchsin basic (FB) adsorbed on evaporated Ag films. The absorption peak, due to the plasma resonance and is characteristic to the Ag island film, shift to shorter wavelengths The substantially damped with- a broader structure at longer wavelength region. enhanced RS and FL of the dves. observed on the Aq films of different thickness, reach their maxima at about d(=125 A for R6G and-80 A for FB, where the the dye coated Ag films exhibit the absorption peak near the incident laser light. The excitation profiles of RS for different dAg also exhibit the peaks correspnding to the plasma resonance mediated absorption maxima of a composite dye/Ag system, while that of FL show a continuous increase toward resonant FL, thereby demonMoreover, staring a hierarchy of the EM resonance between RS and FL processes. comparison of the FL intensities of R6G and FB on the Ag films relative to a glass substrate reveals an importance of their effective quantum efficiencies at the metal surfaces.

INTRODUCTION Since a discovery of molecules have

of the so-called

adsorbed

at metal surfaces

been

made

to

elucidate

not

molecules,

but

also

of the

metal

surface

enhanced

optical

tion

of

the

molecules

effect

the

(EM)

of charge

optical

surface

itself,

take place.

mechanisms

electromagnetic

and

such as pyridine, of dye

resonant

molecules

Raman

and Burstein

of rhodamine

Scattering"

field charge

properties on which

the

a wide

is now widely

efforts adsorbed

variety

accepted

of that

namely

the strong

amplifica-

with

macroscopic

roughness

associated transfer

of

(SERS)

interaction

between

adsorbed

an increasing adsorbed

scattering

(RRS)

et al (ref. 1).

works on SERS for non-luminescence

interest

on SERS by

dye

has been focused

active

metals.

molecules

were

The first

on optical

luminescence observed

They found that the luminescence

on a rough Ag island film.

been made to investigate scattering

036%2048/87/$03.50

the optical

Since that, extensive

absorption

efforts

(ref. 2), luminescence

(RS) (ref. 4) of dyes on Ag island

0 1987 Elsevier Science Publishers B.V.

films.

by

quenching

66 (R6G) at a smooth Ag surface is offset by the EM enhancement

they are adsorbed

and Raman

Raman

and substrates.

properties

Ritchie

It

in nature,

In addition to a large body of experimental molecules

Enhanced

such as Ag, Au and Cu, extensive

only

processes

SERS is due to two different

and the chemical

"Surface

when have

(ref. 3)

In particular,

124 Weitz

et al

Raman,

(ref. 5) studied

a hierarchy

RRS and fluorescence

found that FL intensity but is crucially

observed

et al

They

the energy

dissipation

conductivity between

also

of Ag and

the adsorbed

Recently,

we observed

results

dyes

Raman

They RS,

intensity

coincide dyes

are

depending

of an overlapping

with

in is

that of

stabilized

by

on the

thermal

of the absorption

maxima

of R6G, FB and NB molecules

The differences

films. films

Because

of R6G/Ag

be published

dyes

to Ag,

with

without

by the excitation

(Au) plasma

of these

resonance

in

source of Ar' laser, provide rich

optical

processes

of.dye

the present

of space limitations,

and Au, FB/Ag

elsewhere

adsorbed

in the optical properties

(Ag) and

insight into the surface enhanced

surfaces.

normal

films.

(QE) of the molecules

the adsorbed

RS and FL spectra

and the metal

information

that

the excited

the spectral region accessible

on metal

of the adsorbed

degree

between

dyes and Ag films.

on Ag and Au evaporated molecules

maxima

the

efficiency

6) reported that the maximum

emphasized

from

ratios

on Ag island

in the same sense as RS or resonant

on the quantum

(ref.

when the absorption

the Ag films.

of enhancement

dye molecules

is not enhanced

dependent

Yamada

solution.

(FL) from

and a brief remark

on NB/Ag,

paper

molecules

reports

the

and the rest will

(ref. 7).

EXPERIMENTALS Metal glass

substrates

substrates

at a base estimated

pressure from

evaporated

VP-CZA

thin films

were

equipped

Thicknesses

then coated

with

behavior

up to the benchmark

to decrease

dipole-dipole benchmark

that the FL intensity

due to increasing

coupling

on Ag films,

(ref. 8).

and luminescence controlled

spectra

adsorbed

were

by minicomputer

measured

MELCOM

Ag

thin

concentration,

also The

of 10v4 M ethanol

beyond

the

dyes

with

otherwise

the intensity

and nonradiative lo-" M below

The absorption

spectra

by Hitachi-330.

Jasco CT-80DC

If

of

(10‘7-10‘2

shows a linear

which

of aggregates

dyes were measured

70/10.

were

(ref. 1).

of R6G in ethanol solution

population

on

oscillator,

films

spectra

on a glass and Ag films

double

stated,

the

of the Raman

monochrometer

the unpolarized

164) was used as the excitation.

The beam

lens were used to reduce the incident power down to about

0.1 mW, which causes negligible

photochemical

of both Raman and luminescence

was observed

457.9-514.5nm

evaporation

a quartz

dyes by spinning

In this work,

488 nm line of At-'laser (Spectra Phys. expander and cylindrical

of the

were used dye adsorption.

metal films with and without

vacuum

with

of the absorption

By varying the molar concentration

solution.

increase

of 10e5 Torr.

(- 8 A/min)

by a slow

a Sinku-Kiko

the the peak positions

M), we have observed

starts

were prepared

using

of Ar' laser.

of the source, monochrometer,

The

data were

and optics.

reaction. by using corrected

The excitation the several

spectrum

lines between

for the spectral

response

125 RESULTS

and

DISCUSSION

R6G on Ag and Au Having

confirmed

the shift of the plasma

island

films,

with

an increase

coated

by R6G

were

observed.

the Ag film The peak

and R6G having

intensity

using the

THe

of the

absorption

of the Raman

334 A.

A tail of the FL peaked

however,

the

plotted A.

Raman RS

spectra

is shown

at near

488

maximum

at about

spectrum

to the Ag

of the Ag films

not a simple

superposition

composite

and the peak Excitation

therefore,

R6G/Ag

system.

Raman

become

lines exhibit

film

quenched dA9=334 A,

extremely

weak.

As

the peaks at about dnp=125

not Ag alone, at dAg=125 A showed

of the R6G/Ag,

incident

in

dA9=125 and

on glass is substantially On thicker

falls

A typical

in Fig. 2 for R6G on glass,

on dng=125 A.

of

530 nm in solution. damped

as shown in Fig. 1.

IRa.and FL intensity

nm of the

spectra

is substantially

at 18300 cm“

in Fig. 3, IRa. of several

Since the absorption

a peak

of the

lines were observed

intensities

characteristic

for RS and FL measurements,

band

example

and enhanced

Ag film

were

upon dye adsorption,

488 nm line of Ar' laser

range

spectra

the absorption

of the virgin

shifts to shorter wavelengths

resonance

of drs, the absorption

source,

as

indicated

by

the

arrows

in

1200 cps R6G

I 400' 0

1 100 Ag MASS

KS'MonGlass

onAgfilm(334Al

1 200 THICKNESS

300 (ii)

RAMAN

SHIFT

(cm”)

Fig. 1. (LEFT) Absorption peak position of the evaporated Ag films (open circles) and composite R6G/Ag system (solid circles) at various Ag mass thickness. The allow indicates the excitation wavelength used in the Raman Scattering and fluorescence experiments. Fig. 2. (RIGHT) Raman spectra of R6G obseryed c&=125 and 334 A, with 488 nm excitation

on a glass,

Ag film

of thickness

126 Fig. 1, the change of the RS intensity a significance changes

of the

plasma

of the FL intensity

vs. Ag film thickness

resonance

mediated

IFt as a function

seem to demonstrate

resonance

RS of R6G/Ag.

of dns. also

depicted

exhibits a peak at almost the same position as 1~~~. It is remarked on the Ag films the arrow

were weaker

), except

at about

is due to the increased

compared

dns=125 A.

nonradiative

This can be indicative

Ag films.

and FL quenching

on observed

to that on glass The

decrease

FL intensity.

by Weitz et al (ref. 5).

makes no contribution

to the enhancement

QE molecules decay

of adsorbed

dyes

between

portion

is mitigated

excited

of FL from high

by the additional

states.

notice that the peak values of IRan and IFI at resonance as large as compared

the EM enhancement

of Fig. 3 is consistent

in the emission

in their

In Fig. 3, we also

dAg are only several times

to the values at the off-resonance

dns.

In order to clarify

whether the peaks of IRan and 1~~ are solely due to the EM resonance experiments

were

spectra

the

of

5dg6s2-5d'06s present

performed bare

Au

electronic

spectral

region

for

films

R6G

transition, and

*LLI

on

showed

the

Au the

but spectra

evaporated minimum

no plasma remains

films. at

about

resonance almost

or not, similar The 500

R6G/Ag

absorption nm

structure

unchanged

!5

WG/Ag

Ag films

R6G on the coalalesced

They showed that the EM resonance

such as R6G, since the EM resonance

processes

of IFL on thick

The result

with a model developed

in Fig. 3,

here that IFL's

(, which is indicated by

decay of the adsorbed

of the competition

The

due

to

in the

upon

R6G

RSG/AU I

?O-

15-

IO-

5-

0Ag THICKNESS (A,

100

200

300

Ag THICKNESS (A)

Fig. 3. Raman scattering (LEFT) and fluorescence (RIGHT) intensities of R6G/Ag as a function of the Ag thickness, with 488 nm excitation, where the fluorescence intensity on a glass is indicated by the arrow, Also inserted is the change of the fluorescence intensity of R6G/Au vs. Au thickness.

127 adsorption,

thereby

suggesting

films and the molecule. IFt

of R6G on Au evaporated

peak at about dru=30 A. IFL

to solely

their FL intensities

explained

that the

broad

peak

band width

than

observed

that

of adsorbed

of pyridine/Ag on the Ag island

thickness

until

effect may also be responsible, for R6G/Ag, increase

Au systems,

in adsorbed

and height with the increase to

the

increase

coalescence partly

in

occurs.

the

in the average

adsorbed

We therefore

excitation

to gain spectra

amount

falls

in

the u4 frequency

the

toward

in Fig. 1, and grows

R6G

530 nm of the absorption

effect

the

wavelength, the plasma hand,

the

depends

excitation

on the

in intensity

resonant

the

of

FL

at

Fig. 4(a)

factor

shoulder

for dA,=20 and 36 A.

With

increase

A comparison

for R6G on

with

dns.

475

Ag film.

between

resonance

of adsorbed

increasing

toward

of dA,.

gradual increase

observed

at about

at the

RS process.

that the number

of R6G coated

a continuous

along

due to the ordinary

associated

R6G/Ag.

a small

absorption

exhibits

appeared

resonance

of

dAa, where

with the increase

an another

the

range

430-55Onm,

maximum

as previously

to suggest

from

island

processes, in the

appeared

mediated

profile exhibits

roughness

exhibits

mediated

FL intensity

530 nm, except

until

shift to longer wavelength

peak of R6G,

in size

in IRa. with dns is

of R6G in solution.

resonance

dAg also seems

profile

FL intensity resonance

grow

This then leads

measured

resonance

The absence of this feature

for thinner

molecules

shows

of the plasma

for dag=117 A, the excitaion

colloidal Ag (ref. 11). Raman

the

were

The excitation

a small

with

roughness

is due to the

islands

molecules

of

maximum

factor was not considered.

shown

a

profile of the Raman band at 1650 cm-' for several

This is a clear evidence However,

plasma

effective

increases

how much

The

that larger

R6G on the islands.

role

FL of R6G/Ag

about 465 nm for dAs=20 A exhibits peak position

the

be a

of both IRan and IFL

that the increase

the

on dAg and near the absorption

shows the excitation

of

of adsorbed

into

RS and

films.

times

Such

Ag mass thickness.

postulate

insight

of the

which

459.9-514.5nm, depending

an

Ag

may

The

adsorption occurs.

it is hard to estimate

caused by that in the number

In order

coalescence

They plasma

to estimate

surface.

films for molecule

on the rough

origin

are 20-60

more or less, for the peaks

although

amount

films

smooth

greater

Yamada et al (ref. 10)

in UV region

on the

island

by

possible

Recently

spectrum

adsorption

than NB on Ag films.

is caused

Another

island films.

Ag film.

for NB on Au and Cu films,

NB/Au

in Au films.

surface area of the thin evaporated film

for

a broad

the peak of

of the dye-coated

results

pyridine

for monomolecular

increasing

the metal

Nevertheless,

be hard to attribute

less intense

observed

factor of the evaporated the absorption

between

at any dA,.

resonance

similar

were much

resonance

the numbers

it might

the plasma

roughness measured

interaction

films, shown in the insert of Fig. 3, exhibited

et al (ref. 9) also observed

although

of any

no RS for R6G/Au

Consequently,

and 1~~~ for R6G/Au

Glass

an absence

We observed

Fig. 4(b) excitation

nm caused

by

On the other

resonance

Figs. 4(a)

FL at

and 5(b)

128 reveals

that the the effect

of the plasma

coated Ag films much more pronounced

resonance

Let A(o,) and A(w,) be the amplification incident

o1 and scattered

EM enhancement, the Raman

intensity

is the usual However,

is enhanced

resonance

is strongly

suppressed

responsible

is essentially

resonance

for

the

exhibits

damped

the

leading

life times

shifts

to short

of the molecular

overall

on a Ag

results colloid,

that

no

For R6G, the when

the

The EM enhancement

is expected

net

independent.

wavelengths

molecules.

in spite

which

which has no optical

absorption

EM enhancement

so that

resonance

This may

enhancement

of the fact

on

to be weaker be partly

of

that

RS

from

coadsorbed

strong SERS (ref. 12).

resonance

significantly,

and

molecule

molecules

range of the dye absorption.

experimental

In the case of FL process, the plasma

of the peak

IA(ol

of the island Ag films.

than for non-fluorescence

red is observed

thereby

and

by the broadening

Consequently,

for the dye molecules

pyridine

factor

falls in the spectral

the metal.

methyl

by a factor of IA(w~)lZIA(w~)lZ N

enhancement

of R6G

of the EM field at the

with oi and ar is small for the RS process

band near the plasma

resonance is also

coefficients

this is only the case for non-fluorescence

absorption plasma

SERS

absorption

RS than the FL process.

When GI, is near the frequency

tir lights.

the variation

mediated

for the resonance

to

of the the

on the other composite

weaker

the FL intensity

of the molecular

EM

enhancement

is determined

excited

hand,

R6G/Ag

states

ar becomes

system for

so that FL

surfaces

for

IA(0112>>IA(o,)12,

compared

by the radiative on metal

off resonance

to

RS.

More

and nonradiative (ref. 13).

On a

6

1

450

'.2QA 500 E)(CITATIONWAVE LENGTHU%-n)

EXCITATION WAVELENGTH

(nm)

Fig. 4. Excitation profiles of (a) the Raman band at 1650 cm-', (b) fluorescence, of R6G/Ag for different Ag thickness.

129 smooth and

Ag metal

Eagen

surface,

(ref. 14)

to

an attenuated observe

transfer

from R6G to surface

of R6G.

On a Ag rough

the

plasmon

surface

total reflection surface

mode,

plasmon

which

fabiricated

technique emission

enabled

via

the

Weber energy

then leads to the FL quenching

by holographic

grating,

Tsang

et al

(ref. 15) also observed a broad peak centered at 18000 cm-', which was tentatively assigned

to the emission

the dye molecule and

plasma

system

on the rough

emissions.

is an evidence

damping

from the surface Ag metal

Nevertheless, of the

plasmon

polariton.

are possibly the

reduced

weaker

emission

of R6G, in spite of the increased

RS

The FL spectra

composed from

yield

the

caused

of

of both molecular composite by

an

R6G/Ag

additional

excitation'rate.

FB on Ag

In the resonant additional

optical

nonradiative

films.

Previously,

signals

of dyes

processes,

decay

channels

the

it has been conceived

on Ag,

due to the cooperative

which

are

effect

EM enhancement

of

dye

is mitigated on

the

that the the appearance

submerged

of both

molecules

in the

strong

the EM enhancement

by

island

the

metal

of the Raman

FL in solution,

are

and the fluorescence

1.0 FBh%)/Ag

FBIO-•M

RBGIO-‘M

on Ad36fi)

R6G IO-‘M

on Gloss

on Ag(5d) ,

3 WAVENUMBER

(IO’cd)

EXCITATION

500 WAVE LENGTH (nm)

Fig. 5. (LEFT) Fluorescence spectra of FB on a glass and on Ag of thickness 52 A, with 514.5 nm excitation. The spectra of R6G/Ag with 488 nm excitation are also shown for comparison. Fig. 6. (RIGHT) Excitation profiles of the Raman band at 1590 cm“ of FB/Ag, whose absorption spectra for different Ag thicknesses are also inserted.

130 quenching

on

understood

metal

so that the weaker

FL is observed

in QE of molecules and Ag

for R6G/Ag

results

resonance

on Ag.

(52 A) by 514.5

at about

by the enhanced

depicted

profiles

of the Raman

in Fig. 6.

For all

the wavelength, exhibits

at which

peak of the uncoated by FB and display the increase

a growing

d,,,,

and FL are peaked

in Fig. 3 for R6G/Ag.

intensities

increase

coated

with

toward

FB(10e4 M)

It is noted here that the absorption

structure

wavelengths at longer

when they are coated

wavelength

to Fig. 6, the excitation

FB/Ag is a replica of the absorption

By varying

signals

of the Ag films

in Fig. 6.

broader

FL emission

band at 1590 cm-' for dAs=40, 80 and 175 A

dRg's, the Raman

In contrast

from a thin

enhanced

as shown

Ag films shifts to shorter

in dAg.

the FL spectra

on the Ag films. Raman

of

of FB (10m4 M) on

For comparison,

manner,

the absorbance

broad peaks, as inserted

to the theory

with the'decrease

to R6G, the FL emission

signals

certainly

increases

the FL spectra

of the enhanced

at d,,=BO-100 A, in a similar

are

in the FL process

according

but a remarkably

weak,

Raman

that the intensities

The excitation are

However,

In contrast

system

is quenched

nm excitation.

layer of FB on a glass is extremely is accompanied

a R6G/Ag

of FL intensity

Fig. 5 shows

in solutions.

(36 A) are also shown.

we observed

for

enhancement

5), the enhancement

Weitz et al (ref.

a glass

The

surfaces.

that the plasma

region,

profile

tail of FB and no structure

with

of FL from

was observed

near

460 nm excitation.

In separate 488

experiments,

nm excitation,

The RS spectrum of ethanol

RS.

Having the

were adsorbed

film

Interestingly

into

enough, we observed

Raman

line

superimposed results

pure

that

on Ag", not in solution,

the plasma resonance. is an absolute

requisite

coadsorbed

for SERS or not.

have led to a wide acceptance

SERS,

molecular

the

evidence dependence

which

These

Raman

a

in this

quartz

cell.

of the intense

is the strongest lines

due to the

were

again

mounting

NB

in spite

the EM field of

the plasma resonance experimental

results

by the EM enhancement, to the chemical

surface

These

"adsorbed

for ethanol,

whether

is pertained of the

the enhanced

on Ag and in solution.

on Ag and experience

Although

RS lines

molecules in

is composed

RS is observed

of the role played

that ample

solution

RS is certainly

It has been under controversy

also been recognized including

solution.

no enhanced

of the fact that both molecules

of NB and ethanol ethanol

of NB adsorbed

the observed while

NB

with

630 nm.

due to NB in solution.

weak one at 884 cm“,

ethanol

on a weak FL background

demonstrate

FL background

that the RS spectrum

lines of NB and a relatively for

solution

at about

cell shows the well assigned

on the

frequencies

immersed

Raman

observed

peak

on the Ag island film, they exhibited

the Raman

was

RS of NB (10m5 M) in ethanol the NB absorption

in a quartz

superimposed

confirmed

Ag

we observed is far from

of the sample

molecules

When NB molecules

way,

which

enhanced

it has

aspect

optical

of

pro-

131 cesses on the plasma

resonance

active

rough metal surfaces.

FINAL REMARKS We have observed The maximum

RS and FL from

RS enhancement

the excitation

wavelength

peak Of the dye-coated This then manifests feature

a kind

optical

the result dyelAg

dA9 including

or on a glass. contribution

nonradiative

QE

is due

vibrational molecules

R6G

emission

large

in ethanol

indicate

the longer

at

emission

in the effective

by TR/(TR+pR).

system

invokes

themselves

damljing pn

It is remarked we

observed

average

caused

energy

degrees of freedom

by

of the enhanced

that the high emission

yield

FB/Ag system.

ori gin of the FL enhancement, is highlighted

and rough surfaces

beside

of the the low

between

neighboring quenching

here that, for a given concentration that

the

spacing

onset

of

Instead,

concentration This

on the island

seems

Ag films

on the islands.

may be favorable

than

The loss to reduce

However,

system,

to

this can

otherwise

the

Weitz et al (ref. 5) claimed

governs

the FL intensity

study is required

due to the increased

by the RS and FL of a crystal of Ag and Au (ref. 16).

the

substrate.

FL of the FB/Ag

of the Ag islands

More elaborate

the

intramolecular

(coverage)

, and the resultant rise of QE of the adsorbed molecules. origin

in

of

solution, the

transfer

on a surface

should also be enhanced.

increase

decrease

the increase

in dilute

than on a glass

intermolecular

The

the

and/or

For molecules

IA(o

on the Ag films

of the intramolecular

smooth

except

light makes no significant

to the increase

on a glass, or the decrease in the degrees of aggregation

view point

absorption.

This was not the case

below the onset of concentration

solution,

is higher

composite

dye

as a result of the reduced

is defined dye/Ag

The intermolecular

(on substrate).

not be a primary

the

while it is only

of the

than on a glass,

of the scattered

nonradiative

relaxations.

FL of the R6G/Ag

of

of the plasma

FL, in spite of its low QE in solution

I-"Rof the molecules

TR through

quenching

P

channels

gateway,

region

FL is attributed

composite

can be neglected

in solution of

damping

to the

Since the excitation

that the excitation

are weaker

factor,

band of the dyes.

effective

is a dominant

absorption

FB/Ag system.

QE of the

damping

profile.

probes

spectral

the enhanced

Since the EM resonance

The QE for a molecular effective

mediated

On the other hand, such

caused by the island film.

to FL, the enhanced

QE of the composite

radiative

system in the

a roughness

damping

for the FB/Ag, which exhibited

the

demonstrates

of R6G on the Ag films

by nonradiative

profiles.

spectroscopy,

to the dye molecule

the optimum

with the plasma resonance

in the FL excitation

of modulation

The FL intensities

on the Ag films.

Ag film, and also falls in the absorption

of the composite

transferred

adsorbed

at dnp=125 A for R6G and 80 A for FB, where

itself in the RS excitation

transitions,

resonance

yield

coincides

was less substantial

profile,

the dye molecules

was observed

to clarify excitation violet

of the

the physical rate.

This

(CV) adsorbed

The FL intensity

On

by CV showed

132 rough Ag > rough Au > glass > smooth on the smooth mechanism,

surfaces

thereby

suggesting

on rough island films, strong Raman from

the metal

changes

intensity enhancemet numbers

with

the

average

and

the

mass

thickness

not only

nonradiative

of dye molecules

in nature.

glass

of the adsorbed

determined

We also note that,

with the metal

contact

are mainly

molecules

show the

responsible

of the evaporated

but

also

by

the

distance

at the metal

by the competition

quenching,

in direct

results

from the EM enhancement

FL, while those having the largest

at the supporting

Since the number

is crucially

effect

These

Ag in sequence.

dyes being in contact

and quenched

or adsorbed

FL.

Au > smooth

to what is expected

the chemical

only those

scattering

the observed film

are contrary

for

island

films,

the

FL

beteween

the EM

difference

in the

to and apart from the metal.

ACKNOWLEDGEMENTS This

work

Ministry would

like

Pettinger

was

supported

of Education to express

for valuable

by a Grant-in-Aid

of Japan, his

Science

sincere

for Scientific

and Culture.

thanks

to

Research

from

One of the authors

Professor

H. Yamada

and

the

(H.U)

Dr.

B.

discussion.

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