ELLIPSOMETRY IN ABSORBING SOLUTIONS

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ELLIPSOMETRY IN ABSORBING SOLUTIONS

R. Greef, P. Pearson

To cite this version:

R. Greef, P. Pearson. ELLIPSOMETRY IN ABSORBING SOLUTIONS. Journal de Physique Collo-

ques, 1983, 44 (C10), pp.C10-505-C10-508. �10.1051/jphyscol:198310103�. �jpa-00223463�

ELLIPSOMETRY IN ABSORBING SOLUTIONS R. Greef and P.J. Pearson

Chemistry Dept., Southampton University, U.K.

Résume - Les résultats d'expériences ellipsométriques sur des miroirs métalliques immergés dans des solutions de colorant sont présentes sous forme de spectres de dispersion delta et psi pour une gamme de longueurs d'onde variant de 300 à 650 nm. Ces spectres sont compares aux

prédictions d'un modèle consistant en une couche mince adsorbée compacte de colorant optiquement isotrope .

Abstract - The results of ellipsometric experiments on metal mirrors immersed in dye solutions are presented in the form of delta, psi dispersion spectra in the wavelength range 300-650 nm. These spectra are compared to the predictions of a model consisting of a thin, compact adsorbed dye layer which is optically isotropic.

Introduction

Ellipsometry is generally carried out in a transparent immersion medium. There is however no reason in principle why the immersion medium should not be absorbing, although practical difficulties can be forseen in achieving high enough reflected light intensity. The incentive for doing such experiments is particularly strong in the field of electrochemistry, where the spectroscopy of adsorbed layers is currently of importance, and where in situ methods are necessary in order

properly to explore changes in surface structures with variation of the electrode potential. The expected advantage of ellipsometry is the usual one, i.e. that the technique is so sensitive that it should not be necessary to apply pulses of potential to the electrode, as in differential reflectivity measurements (1), which can have various serious side-effects.

This paper describes ellipsometric measurements using very dilute water solutions of a dye. No background electrolyte was used and the potential was not varied. The experiments were designed to determine the practicality of the method preparatory to electrochemical investigations.

Practice

In a wavelength scan, A and ty are likely to change greatly because of the variation of the refractive index of the metal. The perturbation in A and 4* due to an adsorbed layer will be small compared to this, so the ellipsometer has to track over a wide range but still be very sensitive. The construction and optics of the ellipsometer used have been described by A.C. Lowe (2). This instrument has been computerised by us.

Theory

In moving to an absorbing medium, the usual form of the Fresnel equations has to be modified, substituting the real refractive index of the medium with a complex index. As long as the beam enters the medium through an immersed window

perpendicular to it, however, the angle of incidence is still a real rather than a complex quantity.

Determination of the refractive index of an absorbing medium is difficult. In

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:198310103

C 10-506 JOURNAL DE PHYSIQUE

p r i n c i p l e

,

e l l i p s o m e t r y from t h e s u r f a c e s h o u l d provide a good method, and such e x p e e m e n t s have been c a r r i e d o u t a s p a r t of this study. The s u r f a c e o f t h e s o l u t i o n however i s n o t t y p i c a l of t h e b u l k , a s t h e dye may be s u r f a c e a c t i v e , s o t h e

r e s u l t s a r e n o t a r e l i a b l e i n d i c a t i o n of t h e bulk p r o p e r t i e s . The r e f r a c t i v e i n d i c e s o f t h e s o l u t i o n s were t h e r e f o r e e s t i m a t e d from t h e bulk o p t i c a l c o n s t a n t s of t h e dye and t h e s o l v e n t as follows. The o p t i c a l c o n s t a n t s of t h e s o l i d dye, d e p o s i t e d from a c e t o n e s o l u t i o n o n t o g l a s s , were measured by e l l i p s o m e t r y . The band shape of t h e s o l i d , m i c r o c r y s t a l l i n e dye spectrum was found, a s expected

( 3 ) ,

t o be much broader t h a n t h a t of t h e s o l u t i o n . Because of t h i s broadening and o t h e r e f f e c t s such a s s o l v a t i o n i n t h i s l i q u i d phase, only a q u a l i t a t i v e guide can be ex- pected from t h i s comparison. The measured r e f r a c t i v e i n d i c e s were t h e n converted t o complex d i e l e c t r i c c o n s t a n t s and used i n t h e Maxwell-Garnett e x p r e s s i o n , which r e l a - t e s t h e e f f e c t i v e d i e l e c t r i c c o n s t a n t ( E ) of a two-phase system t o t h e b u l k cons- t a n t s ( E ~and E ~ and t h e volume f r a c t i o n s of t h e c o n s t i t u e n t s ) :

V2 i s t h e volume f r a c t i o n of t h e second p h a s e , which i n t h i s case i s t h e dye.

The v lume f r a c t i o n V2 i n t h e s t r o n g e s t dye s o l u t i o n used h e r e (10%) i s about

~xIO-'. A t such low volume f r a c t i o n s . Naiorell-Garnett t h e o r y shows t h a t t h e r e f r a c t i v e index of t h e s o l v e n t i s perturbed n l y i n t h e f i f t h p l a c e o f decimals, while t h e e x t i n c t i o n c o e f f i c i e n t i s a b o u t

lo-'.

The changes i n d and Y, p r e d i c t e d by such s m a l l changes i n t h e medium r e f r a c t i v e i n d e x a r e far t o o small t o be d e t e c t e d by t h e p r e s e n t a p p a r a t u s . The r e f r a c t i v e index of pure w a t e r t h e r e f o r e was used as t h e medium r e f r a c t i v e index.

R e s u l t s

The r e s u l t s p r e s e n t e d a r e f o r C r y s t a l V i o l e t dye i n w a t e r w i t h platinum a s t h e s u b s t r a t e metal. F i g s l a and l b g i v e r e s p e c t i v e l y t h e $A and

d q

d i s p e r s i o n s , which a r e t h e v a l u e s recorded w i t h dye s o l u t i o n minus t h e v a l u e s w i t h w a t e r a s t h e medium. The s p e c t r a p r e s e n t e d a r e as-recorded, which shows t h e n o i s e l e v e l t o be expected w i t h this a p p a r a t u s i n wavelength-scanning mode. S t r o n g s t r u c t u r e i s d i s p l a y e d i n t h e v i c i n i t y of t h e absorbance band of t h e dye s o l u t i o n , which because it cannot o r i g i n a t e from t h e bulk of t h e s o l u t i o n , must a r i s e from a c o n c e n t r a t e d dye l a y e r a t t h e m e t a l s u r f a c e . The s t r u c t u r e becomes more i t e n s e a s t h e c o n c e n t r a t i o n o f t h e dye i s i n c r e a s e d i n t h e range 1.7 t o 1 0 . 2 ~ 1 0 - % .

A simple t h r e e - l a y e r model w a s a p p l i e d , and u s i n g a computer program (4) t h e o p t i c a l c o n s t a n t s of t h e l a y e r were found assuming v a r i o u s t h i c k n e s s e s f o r t h e o p t i c a l l y homogeneous a b s o r b i n g l a y e r . S o l u t i o n s were found f o r o n l y a r e s t r i c t e d range of t h i c k n e s s e s from 0 . 6 t o 1.8 nm, and a t y p i c a l s e t of r e s u l t s i s shown i n F i g 2 ( a ) and 2 (b)

.

The shape of t h e n and k d i s p e r s i o n s found i s s i m i l a r t o t h o s e o f t h e s o l i d dye. There i s no s t r o n g shift of t h e k maximum, b u t t h e curve f o r n i s d i s p l a c e d toward t h e r e d a s compared w i t h t h e s o l i d dye curve. The absorbance of t h e most c o n c e n t r a t e d dye s o l u t i o n i s shown i n F i g 2(b) f o r comparison. These r e s u l t s a r e s t r o n g l y i n d i c a t i v e of t h e e x i s t e n c e o f a t h i n , h i g h l y compact dye l a y e r a t t h e m e t a l s u r f a c e .

Experiments on o t h e r dyes have been c a r r i e d o u t . F l u o r e s c e i n behaves s i m i l a r l y t o C r y s t a l V i o l e t , b u t i n t h e c a s e of Rhoaamine B a s t r o n g r e d shift i n t h e peak of k of a b o u t 60nm was a p p a r e n t .

P u r e l y e l e c t r o c h e m i c a l measurements on adsorbed o r g a n i c l a y e r s i n d i c a t e a high degree of o r d e r i n g , and a s t h e &ye molecule i s s t r o n g l y o p t i c a l l y a n i s o t r o p i c , it i s n o t c o r r e c t t o t r e a t t h e l a y e r a s homogeneous i n t h e simple t h r e e - l a y e r model t h a t has been used s o far.

Conclusions

The r e s u l t s show that e l l i p s o m e t r y can be s u c c e s s f u l l y c a r r i e d o u t i n d i l u t e dye s o l u t i o n s . I n t h e example shown t h e r e s u l t s a r e i n f a i r agreement w i t h a model c o n s i s t i n g o f a homogeneous compact adsorbed dye l a y e r . More d e t a i l e d i n t e r - p r e t a t i o n however must a w a i t a more complete model which t a k e s account o f t h e o p t i c a l a n i s o t r o p y of t h e dye molecules.

F i g s 2(a) and 2(b)

-

Calculated n and k values f o r the 6 . 8 ~ 1 o - k s o l u t i o n and a 1.8nm t h i c k l a y e r , w i t h t h e s o l i d dye v a l u e s and t h e s o l u t i o n absorbance f o r cornpaz%son.

C10-508 JOURNAL DE PHYSIQUE

Acknowledgement

We thank IBM (U.K.) Ltd. for making the ellipsometer available, and Carol M. Jeffs who carried out some of the measurements.

References

(1) W.J. Plieth, P.Gruschinske, H.J. Hensel, Ber. Bunsensges. Phys. Chem,, 82 (1978) 625

-

(3)

.J.W. Weigl, J. Chem. Phys.,

2

(1956)

364.

(4) B.D. Cahan, Surf. Sci., (1976)

3 9