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SURFACE ENHANCED RAMAN SCATTERING : AN OVERVIEW OF EXPERIMENTS
R. Chang
To cite this version:
R. Chang. SURFACE ENHANCED RAMAN SCATTERING : AN OVERVIEW OF
EXPERIMENTS. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-283-C10-294.
�10.1051/jphyscol:19831059�. �jpa-00223517�
JOURNAL DE PHYSIQUE
Colloque CIO, supplément au n°12, Tome 44, décembre 1983 page C10-283
SURFACE ENHANCED RAMAN SCATTERING : AN OVERVIEW OF EXPERIMENTS
R.K. Chang
Yale University, Section of Applied Physics and Center for Laser Diagnostics, New Haven, Connecticut 06520, U.S.A.
Résumé - Les aspects expérimentaux de l'effet Raman exalté de surface sont brièvement passés en revue avec des expériences démontrant les méca- nismes d'excitation électromagnétique et de transfert de charge, en insistant sur les mécanismes d'exaltation. Le manque de données quanti- tatives sur le recouvrement moléculaire sur des sites de surface
spécifiques est discuté, ainsi que la nécessité de corréler les résultats de l'effet Raman exalté de surface avec d'autres données de physique des surfaces.
Abstract - Experimental aspects of surface enhanced Raman scattering (SERS) are briefly reviewed with experiments demonstrating electromagnetic and charge-transfer excitation enhancement mechanisms being specifically noted. The lack of quantitative data on molecular coverage at specific surface sites is discussed, as well as the need to correlate SERS results with other surface physics sensitive data.
I - INTRODUCTION
An overview of the experimental status of surface enhanced Raman scattering (SERS) is simplified because of several in-depth review articles published prior to 1983 [1-4] or soon to be published [5-8]. Summarized here are some of the well accepted experimental observations, remaining controversies which still challenge experimentalists, and finally several results obtained by other surface physics techniques which relate closely to the overall understanding of SERS mechanisms.
An overview of the current theoretical status of SERS can be found in Ref. 9.
The most accepted experimental fact is that some form of surface roughness increases the detected Raman signal. This roughness could be created by various types of processes, such as: (1) electrochemical - oxidation and reduction of the elec- trode [1-3,7,8] or plating of the metallic ions in the electrolyte [10]; (2) photo- chemical - photoreduction of adsorbates on electrodes [11], single crystals [12], or powder surfaces [13]; (3) evaporation - vacuum deposition of metallic films on smooth insulating or metallic substrates [14], or on already roughened insulating or metallic island substrates [15,16]; (4) lithography - grating structures on Ag surfaces [17] or uniformly shaped Ag microstructures on an array of Si0£ posts [5,18]; (5) chemical reduction - metallic colloids produced in aqueous solution [2,4,19], (6) vapor deposition - clusters of a few metal atoms surrounded by a matrix [20]; and (7) mechanical polishing - use of different sized abrasives [21], Many of the currently existing controversies stem from the experimentalists' ignor- ance of the complete surface topology. Conventional scanning electron microscopy
(SEM), having ^200 A resolution, can easily be used to characterize the topology of roughened surfaces without much sample preparation. Therefore, much of the existing literature describes attempts to correlate 2200 A topology to SERS results.
Along with these £200 A structures, submicroscopic (e.g., in the size range 20 - 200 A) and even atomic scale roughness (e.g., from a single atom to 20 A) can also exist. Isolated submicroscopic structures (e.g., spheres and spheroids) are known classically to produce large electromagnetic intensity on the surface when
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831059
C 10-284 JOURNAL
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PHYSIQUEt h e i n c i d e n t o r s c a t t e r e d photon e n e r g y i s i n r e s o n a n c e w i t h t h e l o c a l i z e d s u r f a c e plasmon r e s o n a n c e 1221. The a t o m i c s c a l e r o u g h n e s s ( e . g . , a s i n g l e adatom o r c l u s t e r s of a few adatoms) i s known quantum m e c h a n i c a l l y t o enhance t h e Raman p o l a r i z a b i l i t y i n two ways [ 7 ] ; (1) t o i n c r e a s e t h e p h o t o n - f r e e e l e c t r o n c o u p l i n g b e c a u s e o f r e l a x a t i o n of t h e momentum s e l e c t i o n r u l e ; and ( 2 ) t o i n d u c e charge- t r a n s f e r s u r f a c e e n e r g y l e v e l s i f c h e m i s o r p t i o n e x i s t s between t h e a d s o r b e d mole- c u l e and t h e m e t a l a t o m i c s c a l e roughness; r e s o n a n c e Raman e f f e c t c a n t h e r e f o r e be a c h i e v e d when t h e i n c i d e n t o r s c a t t e r e d photon e n e r g y i s e q u a l t o t h e e n e r g y d i f - f e r e n c e between t h g c h a r g e - t r a n s f e r and Fermi l e v e l s . Thus, a c c u r a t e c h a r a c t e r i z a - t i o n of t h e 2200 A s u r f a c e t o p o l o g y i s e s s e n t i a l t o u n r a v e l i n g t h e q u a n t i t a t i v e importance o f t h e " c l a s s i c a l " v s " c h e ~ i c a l " mechanisms. Conventional t r a n s m i s s i c n e l e c t r o n microscopy (TEM), h a v i n g 1 0 A r e s o l u t i o n , can be r e a d i l y used t o c h a r a c - t e r i z e c o l l o i d s b u t i s n o t u s a b l e f o r s o l i d s u r f a c e s o r f i l m s c o a t e d o n t h i c k sub- s t r a t e s . Only r e c e n t l y , Ag i s l a n d f i l m s ( w i t h o r w i t h o u t Au o r N i d e c o r a t i o n s ) c o a t e d on e s p e c i a l l y p r e p a r e d t h i n s u b s t r a t e s have been c h a r a c t e r i z e d by TEM w i t h b e t t e r t h a n 1 0 A r e s o l u t i o n [ 2 3 ] . However, some doubt e x i s t s a s t o whether t h e a t o m i c s c a l e roughness w i l l remain s t a b l e when t h e e n v i r o n m e n t a l c o n d i t i o n s a r e changed, e . g . , i n t h e t r a n s f e r from t h e UHV chamber t o t h e ambient o r when t h e a d s o r b a t e coverage i s a l t e r e d . Consequently, u n l e s s i n - s i t u TEM c h a r a c t e r i z a t i o n c a n b e made, t h e c o r r e l a t i o n of SERS t o a t o m i c s c a l e r o u g h n e s s must r e l y o n more i n d i r e c t means. The f u t u r e e x p e r i m e n t a l c h a l l e n g e is t o d e v i s e o t h e r t e c h n i q u e s s e n s i t i v e t o t h e d i s t r i b u t i o n s i n t h e s u r f a c e morphology which can be employed w h i l e SERS measurements a r e b e i n g t a k e n . Some r e c e n t l y developed t e c h n i q u e s p a r t i c u l a r l y s e n s i t i v e t o a t o m i c s c a l e r o u g h n e s s w i l l be n o t e d h e r e . A s SERS i s s e n s i t i v e t o s u r f a c e morphology c h a r a c t e r i s t i c s and t o a l e s s e r e x t e n t t o s p e c i f i c t y p e s o f a d s o r b e d m o l e c u l e s , r e s u l t s o b t a i n e d u s i n g d i f f e r e n t t y p e s o f s u r f a c e roughness w i l l b e p r e s e n t e d . I n a d d i t i o n , r e c e n t advances i n Raman i n s t r u m e n t a - t i o n w i l l be summarized.
11. INSTRUMENTATION ADVANCEMENT
The replacement of t h e c o n v e n t i o n a l d o u b l e monochromator and a p h o t o m u l t i p l i e r w i t h a t r i p l e - s t a g e s p e c t r o g r a p h and a m u l t i - e l e m e n t quantum-noise-limited d e t e c t o r i s now p o s s i b l e w i t h commercially a v a i l a b l e i n s t r u m e n t a t i o n [ 2 4 ] . R e p l a c i n g t h e former s i n g l e - c h a n n e l a n a l y z e r (SCA) w i t h t h e o p t i c a l m u l t i c h a n n e l a n a l y z e r (OMA) h a s e n a b l e d s i m u l t a n e o u s measurement of t h e t e m p o r a l e v o l u t i o n o f many SERS peaks and t h e b r o a d background w i t h ".I s t i m e r e s o l u t i o n a s t h e p o t e n t i a l of t h e e l e c - t r o c h e m i c a l c e l l i s v a r i e d [25,26]. I f t h e SERS s i g n a l i s l a r g e enough, p r e s e n t l y a v a i l a b l e m u l t i - e l e m e n t d e t e c t o r s (SIT o r l i n e a r - a r r a y p h o t o d i o d e s ) c a n complete a t o t a l s p e c t r a l s c a n e v e r y 1 / 3 0 s o r even f a s t e r . I n a d d i t i o n t o t e m p o r a l measure- m e n t s , t h e OMA equipped w i t h a two-dimensional d e t e c t o r ( e . g . , an SIT t u b e and o p t i c a l m i c r o s c o p e ) h a s been used t o map o u t a c t i v e and i n a c t i v e SERS a r e a s on a Ag e l e c t r o d e ( w i t h s p a t i a l r e s o l u t i o n b e t t e r t h a n 4 pm) [27]. I n f a c t , by u s i n g a two-dimensional d e t e c t o r and t r i p l e - s t a g e s p e c t r o g r a p h , which c a n p r e s e r v e t h e s p a t i a l i n t e g r i t y of a l i n e image formed a t i t s e n t r a n c e s l i t , t h e e x p e r i m e n t a l i s t c a n m o n i t o r t h e t e m p o r a l e v o l u t i o n o f t h e SERS s p e c t r a ( e . g . , d i s p e r s e d a l o n g t h e x - a x i s ) from many p o i n t s a l o n g a l i n e i l l u m i n a t e d on t h e s u r f a c e ( e - g . , s p a t i a l l y p r e s e r v e d a l o n g t h e y - a x i s ) .
Another c o n t r i b u t i o n of t h e OMA t o t h e SERS f i e l d i s i t s g r e a t l y improved s i g n a l - t o - n o i s e r a t i o when compared w i t h t h a t of t h e SCA. By a c c u m u l a t i n g t h e weak Raman s i g n a l on t h e e s s e n t i a l l y n o i s e - f r e e , s i n g l e - p h o t o e l e c t r o n s e n s i t i v e t a r g e t u n t i l t h e t o t a l c h a r g e p e r s p e c t r a l c h a n n e l i s l a r g e enough t o overcome t h e e l e c t r o n i c n o i s e i n t r o d u c e d by t h e r e a d mode, t h e O M . i s c a p a b l e of measuring a Raman spec- trum below t h e l i m i t s o f t h e SCA ( a s much a s
l o 3 - l o 4
t i m e s [ 2 4 ] ) . Consequently, e x p e r i m e n t a l i s t s need n o t j u s t measure t h e " t i p o f t h e i c e b e r g " ( i . e . , t h e l a s t 10' enhancement o f t h e t o t a l 1 0 5 - 1 0 7 enhancement) b u t , i n p r i n c i p l e , c a n m o n i t o r t h e e n t i r e enhancement from v a r i o u s mechanisms, b e g i n n i n g w i t h no enhancement on a f l a t m e t a l l i c s u r f a c e and f o l l o w i n g t h e enhancement a s t h e d i f f e r e n t s i z e s o f roughness a r e produced. I n f a c t , Raman s c a t t e r i n g from monolayers of m o l e c u l e s p l a c e d o n s u r f a c e s w i t h o u t SERS c o n d i t i o n s h a s a l r e a d y been r e p o r t e d ( e . g . , on low-index f a c e s of Ag c r y s t a l s [ 2 8 ] and on smooth 1000 A t h i c k A1 s u r f a c ef i l m s [ 2 9 ] ) . The improved s i g n a l - t o - n o i s e r a t i o and f a s t e r d a t a - g a t h e r i n g speed a r e a p p a r e n t when comparing OMA s p e c t r a [ 2 8 , 2 9 ] w i t h SCA s p e c t r a from "smooth"
b u t n o t n e c e s s a r i l y p e r f e c t l y smooth Ag s u r f a c e s [ 2 1 ( b ) , 3 0 3 . 111 - SERS RESULTS
( a ) C o l l o i d s
I n p r i n c i p l e , c o l l o i d s i n aqueous s o l u t i o n a r e i d e a l f o r t e s t i n g t h e v a l i d i t y o f t h e c l a s s i c a l e l e c t r o m a g n e t i c enhancement mechanism s i n c e , a c c o r d i n g o t o TEM
m i c r o g r a p h s , t h e c o l l o i d s a r e n e a r l y s p h e r i c a l i n s h a p e , s m a l l (400 A i n d i a m e t e r ) , and nonaggregated. (Only 3% form a g g r e g a t e s of 2 o r more c o l l o i d s [ 3 1 ] . ) Such a c o l l e c t i o n of Ag o r Au c o l l o i d s i s t r a c t a b l e f o r c l a s s i c a l e l e c t r o m a g n e t i c c a l c u l a t i o n s . The e x t i n c t i o n , e l a s t i c s c a t t e r i n g , s u r f a c e - a v e r a g e d e l e c t r i c f i e l d i n t e n s i t y , and r a d i a l component of t h e e l e c t r i c f i e l d i n t e n s i t y c a n be r e a d i l y c a l c u l a t e d by t h e LorenzlMie formalism a s a f u n c t i o n o f s p h e r e d i a m ~ t e r and r a d i a - t i o n w a v e l e n g t h [ 3 2 ] . For Ag s p h e r e s w i t h d i a m e t e r s l e s s t h a n 400 A, a l l t h e s e c a l c u l a b l e q u a n t i t i e s a r e p r e d i c t e d t o e x h i b i t a s i n g l e s h a r p peak a t t h e l o c a l i z e d s u r f a c e plasmon wavelength (%400 nm f o r Ag i n w a t e r [ 3 2 ] ) . For p r e - dominantly nonaggregated c o l l o i d s , t h e measured e x t i n c t i o n and e l a s t i c s c a t - t e r i n g s p e c t r a a g r e e d w i t h t h e c a l c u l a t i o n s . S u r p r i s i n g l y , t h e measured SERS e x c i t a t i o n spectrum on t h e same c o l l o i d s d i d n o t a g r e e w i t h t h e c a l c u l a t e d w a v e l e n g t h dependence of t h e s u r f a c e - a v e r a g e d e l e c t r i c f i e l d i n t e n s i t y o r i t s r a d i a l component [ 3 1 , 3 3 ] . T h i s d i s c r e p a n c y seems t o imply t h a t t h e observed SERS e x c i t a t i o n c u r v e may n o t be t o t a l l y a s s o c i a t e d w i t h t h e e l e c t r o m a g n e t i c mechanism.
However, one group r e p o r t s t h a t t h e measured SERS e x c i t a t i o n c u r v e f o l l o w s t h e measured e x t i n c t i o n s p e c t r a , b o t h p e a k i n g i n t h e b l u e and f a l l i n g i n t h e r e d [ 3 4 ] . For more a g g r e g a t e d c o l l o i d s , t h e measured e x t i n c t i o n and e l a s t i c s c a t t e r i n g e x h i - b i t e d a s e c o n d a r y peak i n t h e r e d and t h e measured SERS e x c i t a t i o n c u r v e , which s t i l l peaked i n t h e r e d , was e x p l a i n e d a s b e i n g commensurate w i t h t h e l o n g i t u d i n a l s u r f a c e plasmon r e s o n a n c e r e l a t e d t o a c h a i n formed by 2-4 Ag s p h e r e s [2,19].
There i s agreement t h a t a g g r e g a t i o n d o e s i n c r e a s e t h e SERS s i g n a l b u t c o n t r o v e r s y e x i s t s a s t o w h e t h e r a g g r e g a t e d s t r u c t u r e i s e s s e n t i a l f o r t h e o b s e r v a t i o n o f SERS.
A t a f i x e d wavelength (514.5 nm), t h e o v e r a l l SERS i n t e n s i t y from f i x e d numbers of Ag c o l l o i d s was found t o i n c r e a s e 5 t i m e s when t h e amount of a g g r e g a t i o n was d e l i b e r a t e l y i n c r e a s e d from 3% t o more t h a n 90% by a d d i n g ~ a + i o n s i n t h e s o l u - t i o n [ 3 1 ] .
The f o l l o w i n g q u e s t i o n s a r e b e i n g asked r e g a r d i n g t h e i n i t i a l assumptions aasumed t o b e v a l i d f o r c o l l o i d s : (1) Is t h e b u l k d i e l e c t r i c c o n s t a n t ~ ( w ) a p p l i c a b l e f o r s u c h s m a l l d i a m e t e r c o l l o i d s , s i n c e Landau damping r e s u l t i n g from t h e more f r e q u e n t s u r f a c e s c a t t e r i n g w i l l d e f i n i t e l y i n c r e a s e t h e imaginary p a r t o f E ( W ) and t h e r e b y d e c r e a s e e l e c t r o m a g n e t i c enhancement [ 3 5 ] ? (2) Is t h e s u r - f a c e of t h e c o l l o i d s t o t a l l y devoid of a t o m i c - s c a l e r o u g h n e s s s i n c e upon c l o s e TEM e x a m i n a t i o n , t h e Ag c o l l o i d s c o n s i s t o f c r y s t a l l i n e f a c e t s j o i n e d by s h a r p b o r d e r s and c o r n e r s [31]? (3) Are t h e adsorbed m o l e c u l e s u n i f o r m l y d i s t r i b u t e d o v e r t h e c o l l o i d s p h e r e s s i n c e t h e r e c a n be a d i f f e r e n c e i n c h e m i c a l bonding o f m o l e c u l e s w i t h f l a t f a c e t s and w i t h d e f e c t s i t e s ( i n c l u d i n g k i n k s , s t e p s , d i s - l o c a t i o n s , and bumps) [ 3 , 7 ] ? (4) Does t h e adsorbed m o l e c u l e r e a l l y s e n s e t h e e x t e r n a l f i e l d of a n i d e a l i z e d a b r u p t boundary i n t e r f a c e (between t h e Ag m e t a l and w a t e r ) s i n c e o n t h e a t o m i c s c a l e t h e normal component of t h e e l e c t r o m a g n e t i c f i e l d c a n v a r y r a p i d l y and d i f f e r s c o n s i d e r a b l y from t h e c l a s s i c a l c a l c u l a - t i o n [ 3 6 ] ? (5) When t h e m o l e c u l e i s l o c a t e d o n t h e s u r f a c e , i s t h e r e a s h o r t r a n g e i n t e r a c t i o n ( e . g . , i n v o l v i n g t i g h t - b i n d i n g m a t r i x e l e m e n t s ) between t h e m o l e c u l e and t h e m e t a l s i n c e t h e a p p r o x i m a t i o n w i t h a c l a s s i c a l image d i p o l e
( r e n o r m a l i z a t i o n f a c t o r f o r t h e Raman p o l a r i z a b i l i t y ) and w i t h a l o c a l &(a) b c r e e n i n g by t h e e l e c t r o n s i n t h e m e t a l ) i s no l o n g e r j u s t i f i e d [ 3 6 , 3 7 ] ? I n f a c t , most of t h e s e q u e r i e s a r e a p p l i c a b l e t o o t h e r m e t a l roughness found on e l e c t r o d e s , l i t h o g r a p h i c a l l y p r e p a r e d s t r u c t u r e s , and f i l m s . Furthermore, q u e r i e s (4) and (5) a r e even p e r t i n e n t t o a t o m i c a l l y smooth s i n g l e - c r y s t a l s u r f a c e s . A l l t h e s e ques- t i o n s need t o be answered q u a n t i t a t i v e l y b e f o r e t h e d i f f e r e n t enhancement mechanisms c a n b e d i s t i n c t l y s e g r e g a t e d .
ClO-286 J O U R N A L DE PHYSIQUE
Even though some s k e p t i c i s m e x i s t s about c o l l o i d s a s a n i d e a l t e s t c a s e f o r t h e c l a s s i c a l e l e c t r o m a g n e t i c enhancement, t h e f a c t t h a t t h e morphology of t h e c o l l o i d can be more e a s i l y c h a r a c t e r i z e d by TEM t h a n o t h e r commonly used SERS media i s a n a p p e a l i n g f a c t o r . A t l e a s t t h e e x p e r i m e n t a l i s t s can know t h e a v e r a g e submicro- s c o p i c s h a p e , s i z e , and s e p a r a t i o n and c a n v a r y t h e s i z e by p r e p a r a t i o n t e c h n i q u e s and t h e s e p a r a t i o n by changing t h e i n i t i a l m e t a l i o n c o n c e n t r a t i o n i n t h e s o l u t i o n and/or by a d d i n g c a t i o n c o n c e n t r a t i o n t o i n d u c e a g g r e g a t i o n . Furthermore, m e t a l s o t h e r t h a n Ag, Au, and Cu c a n be p r e p a r e d . For example, al;t enhancement f a c t o r of
%7 was r e c e n t l y r e p o r t e d f o r P t (cN):- i o n s adsorbed o n 1 0 A s p h e r i c a l P t c o l - l o i d s [ 3 8 ] . The e x c i t a t i o n p r o f i l e peaked a t %400 nm i n f a i r agreement w i t h t h a t of benze2e m o l e c u l e s adsorbed on i r r e g u l a r l y shaped P t p a r t i c l e s w i t h s i z e s i n t h e 100 A r a n g e [ 3 9 ] and w i t h t h e Lorenz/Mie c a l c u l a t i o n of t h e s u r f a c e - a v e r a g e d i n t e n s i t y [ 381.
The a b i l i t y of Ag c o l l o i d s t o coadsorb o p t i c a l l y nonabsorbing m o l e c u l e s ( p y r i d i n e o r c i t r a t e ) and dye m o l e c u l e s opened up new o p p o r t u n i t i e s t o i n v e s t i g a t e t h e p o s s i - b i l i t y of s u r f a c e enhancement of m o l e c u l e s a l r e a d y e x h i b i t i n g r e s o n a n c e Raman e f f e c t 1401. SERS was observed f o r t h e o p t i c a l l y nonadsorbing m o l e c u l e s (%5 x
l o 5
enhancement f a c t o r f o r p y r i d i n e o r c i t r a t e ) b u t n o t f o r t h e o p t i c a l l y a b s o r b i n g dye molecules (methyl o r a n g e , methyl r e d , o r c r y s t a l v i o l e t ) . The l a c k of enhancement f o r t h e dye m o l e c u l e s i s i n d i r e c t c o n t r a d i c t i o n t o p r e v i o u s r e p o r t s of enhancement f o r s i m i l a r o r i d e n t i c a l dyes on c o l l o i d s 1411, e l e c t r o d e s [ 4 2 ] , wedged Ag
f i l m s [ 4 3 ] , and i s l a n d Ag f i l m s [44-461. It s h o u l d b e emphasized t h a t i n i n t e r - p r e t i n g t h e enhancement f a c t o r of t h e d i s c r e t e i n e l a s t i c spectrum from o p t i c a l l y a b s o r b i n g m o l e c u l e s t h e d i s t i n c t i o n needs t o be c a r e f u l l y made among r e s o n a n t Raman e f f e c t , r e s o n a n t f l u o r e s c e n c e , and f l u o r e s c e n c e [ 4 5 ] . Furthermore, i n a n a l y z i n g t h e p o l a r i z a t i o n dependence of t h e i n c i d e n t and s c a t t e r e d r a d i a t i o n , Franck-Condon m a t r i x e l e m e n t s need t o be c o n s i d e r e d f o r o p t i c a l l y a b s o r b i n g mole- c u l e s i n a d d i t i o n t o t h e u s u a l c l a s s i c a l e l e c t r o m a g n e t i c f i e l d d i s t r i b u t i o n on t h e s p h e r e [ 1 9 ] .
V a p o r i z a t i o n of Ag i n a n argon atmosphere c a u s e s s m a l l a t o m i c c l u s t e r s t o grow t h r o u g h t h e thermomolecular p r o c e s s e s . These below s u b m i c r o s c o p i c dimension c l u s t e r s a r e swept i n t o a s e p a r a t e chamber where t h e a r g o n g a s i s removed and t h e p a r t i c l e s a r e condensed w i t h CO, e t h y l e n e , a c e t y l e n e , o r e t h a n e 120,471. The e x c i - t a t i o n s p e c t r a f o r t h e SERS i n t e n s i t y a s s o c i a t e d w i t h t h e former t h r e e t y p e s of m o l e c u l e s i n c r e a s e d toward t h e v i o l e t a s a f u n c t i o n of photon energy ( i n accor- dance w i t h t h e e x t i n c t i o n s p e c t r a ) w h i l e no SERS was found f o r e t h a n e 1471. The l a c k of SERS f o r s p e c i f i c m o l e c u l e s i m p l i e s t h a t f o r t h e s e m o l e c u l e s t h e "chemical"
mechanisms a r e n o t c o n t r i b u t i n g and t h u s t h e o v e r a l l enhancement i s below t h e d e t e c t i o n t h r e s h o l d used i n t h e i n v e s t i g a t i o n s . However, t h e e l e c t r o m a g n e t i c mechanism c a n s t i l l c o n t r i b u t e t o t h e o v e r a l l enhancement.
The p r e s e n c e of monomeric, d i m e r i c , and even t r i m e r i c Ag m o l e c u l e s h a s been deduced from o p t i c a l a b s o r p t i o n measurements [ 4 8 ] , a l o n g w i t h t h e Raman s p e c t r a of Ag2 and Ag3 c l u s t e r s which have v i b r a t i o n a l f r e q u e n c i e s i n t h e 900 - 1100 cm-' range [ 4 9 ] . Furthermore, i n f r a r e d s p e c t r a of pyridine-Ag atoms and pyridine-Ag c l u s t e r s have been o b s e r v e d and t h u s p r o v i d e a u s e f u l comparison f o r t h e SERS s t u d i e s [ 5 0 ] . The i n f r a r e d s t u d i e s i n d i c a t e d t h a t t h e 992 cm-I b r e a t h i n g mode of f r e e p y r i d i n e m o l e c u l e s i s i n c r e a s e d t o 1000.2 cm-l when low c o n c e n t r a t i o n s of Ag atoms a r e d e p o s i t e d . F u r t h e r i n c r e a s e s t o 1009.8 cm-l a r e observed when t h e con- c e n t r a t i o n of Ag atoms i s r a i s e d . The 1000.2 and 1009.8 cm-1 bands were a s s i g n e d t o p y r i d i n e complexes i n v o l v i n g a s i n g l e Ag atom and a s m a l l Ag c l u s t e r (most l i k e l y Ag2), r e s p e c t i v e l y 1501. These i n f r a r e d r e s u l t s a r e i n agreement w i t h numerous e x p e r i m e n t a l SERS i n v e s t i g a t i o n s which showed t h a t t h e b r e a t h i n g mode i n c r e a s e s from 992 cm-I f o r p h y s i s o r b e d p y r i d i n e t o 1005 cm-l f o r chemisorbed p y r i - d i n e w i t h Ag s u r f a c e s . T h i s agreement l e n d s s u p p o r t t o t h e importance oE atomic s c a l e roughness i n enhancing p a r t of t h e SERS s i g n a l and i n i n f l u e n c i n g t h e p y r i - d i n e v i b r a t i o n a l f r e q u e n c y , The v a p o r i z a t i o n t e c h n i q u e of p r o d u c i n g a t o m i c c l u s - t e r s condensed i n a m a t r i x of Raman a c t i v e m a t e r i a l i s b e i n g extended t o o t h e r m e t a l s such a s K [ 5 1 ] .
(b) R e g u l a r l y a r r a y e d Ag m i c r o s t r u c t u r e s
Ag m i c r o s t r u c t u r e s ( r e s e m b l i n g s p $ e r o i d s w i t h 3 : l and 2 : l a s p e c t 5 a t i o ) e v a p o r a t e d on r e g u l a r S i 0 2 p o s t s w i t h ".I000 A d i a m e t e r , 5000
A
h i g h b a ''~500 A r a d i u s o f cur- v a t u r e a t t h e t o p s of t h e p o s t s , and s e p a r a t i o n o f 3200 A i n a r e g u l a r s q u a r e a r r a y [ 1 8 ] have b e e n used i n o r d e r t o s t u d y t h e e l e c t r o m a g n e t i c enhancement mecha- nism. U n l i k e t h e c o l l o i d a l system, e a c h s p h e r o i d h a s n e a r l y uniform shape, s i z e , and s e p a r a t i o n between t h e n$crostructures. H o w e ~ e r , ~ t h e dimensions of each of t h e s p h e r o i d s ( e . g . , 2a = 3000 A o r 2000A
and 2b = 1000 A) a r e much l a r g e r t h a n t h o s e of t h e c o l l o i d s d i s c u s s e d i n t h e p r e v i o u s s e c t i o n . The e l e c t r o s t a t i c R a y l e i g h a p p r o x i m a t i o n ( i . e . , w i t h i n t h e e l e c t r i c - d i p o l e l i m i t ) c a n no l o n g e r be u s e d . E x p e r i m e n t a l l y , t h e peak of t h e e x c i t a t i o n s p e c t r a o f t h e SERS from Ag(CN)2 adsorbed o n t h e s e m i c r o s t r u c t u r e s a f t e r t h e e n t i r e a r r a y was exposed t o HCN v a p o r was found t o s h i f t t o t h e red under t h e f o l l o w i n g c o n d i t i o n s : (1) w i t h i n c r e a s e of a s p e c t r a t i o from 2 : 1 t o 3 : l ; (2) w i t h i n c r e a s e of t h e d i e l e c t r i c c o n s t a n t of t h e s u r r o u n d i n g medium, e . g . , from a i r E, = 1, t o H20 E~ = 1 . 7 7 , and t o cyclohexane E, = 2.04. Although t h e s e two r e s u l t s a g r e e d w i t h t h e g e n e r a l p r e d i c t i o n s of t h e e l e c t r o s t a t i c R a y l e i g h a p p r o x i m a t i o n , t h e measured l i n e w i d t h of t h e e x c i t a t i o n peaks was much t o o broad ( W . 4 eV). I n o r d e r t o match t h e e x p e r i m e n t a l r e s u l t s w i t h t h e R a y l e i g h r e s u l t s , size-dependent r a d i a t i o n damping e f f e c t s , i n t e r a c t i o namong t h e Ag m i c r o s t r u c t u r e s , l a r g e r imaginary p a r t s o f ~ ( w ) , and f l u c t u a t i o n i n t h e s p h e r o i d a : b r a t i o have been i n t r o d u c e d t o improve t h e i n a p p r o p r i a t e R a y l e i g h c a l c u l a t i o n s [ 5 , 1 8 , 5 2 ] . Recent e x a c t c a l c u l a t i o n s , u s i n g t h e e l e c t r o d y n a m i c L o r e n z l ~ i e formalism, were made on t h e s u r f a c e - a v e r a g e d i n t e n s i t y and t h e o i n t e n s i t y a t t h e t i p 5 ( l i g h t n i n g - r o d e f f e c t ) f o r a n i s o l a t e d s p h e r o i d o f 2a = 2000 A and 2b = 1000 A and found t o be i n r e a s o n a b l e agreement w i t h t h e measured SERS e x c i t a - t i o n p r o f i l e , t h e broad l i n e w i d t h , and t h e o b s e r v e d s h i f t t o t h e r e d w i t h i n c r e a s i n g a:b r a t i o and E,.
The most s i g n i f i c a n t r e s u l t of t h i s computer i n t e n s i v e e l e c t r o d y n a m i c c a l c u l a t i o n i s t h a t t h e e l e c t r o m a g n e t i c enhancement f a c t o r i s more t h a n two o r d e r s of magnitude lower f o r Ag m i c r o s t r u c t u r e s w i t h l a r g e dimensions and b u l k d i e l e c t r i c c o n s t a n t s . Consequently, t h e e x p e r i m e n t a l l y deduced
".lo
enhancement f a c t o r ( f o r t h e 2144 cm-l CN s t r e t c h i n g mode) a t t h e peak of t h e e x c i t a t i o n spectrum i s e r r o n e o u s , o r o t h e r SERS mechanisms, i n a d d i t i o n t o t h e e l e c t r o m a g n e t i c mechanism, c o n t r i b u t e ".lo3 t o t h e o v e r a l l enhancement f a c t o r . Assuming t h a t t h e l a t t e r i s t r u e , i t can t h e n be deduced from t h e q u a l i t a t i v e agreement between t h e wavelength dependence o f t h e e l e c t r o m a g n e t i c i n t e n s i t y and t h e measured SERS e x c i t a t i o n spectrum t h a t t h e o t h e r SERS mechanisms have l i t t l e o r no wavelength dependence t h r o u g h o u t t h e v i s i b l e r a n g e . Furthermore, t h e o t h e r SERS mechanisms cannot be i n f l u e n c e d by immersion i n Hz0 and cyclohexane. Many of t h e q u e r i e s r a i s e d a b o u t c o l l o i d s a l s o a p p l y h e r e t o t h e s e r e g u l a r l y a r r a y e d m i c r o s t r u c t u r e s . Furthermore, i t c a n be s p e c u l a t e d t h a t d u r i n g t h e HCN e x p o s u r e some a t o m i c roughness i s formed o n t h e Ag micro- s t r u c t u r e (even i f none e x i s t e d p r i o r t o HCN e x p o s u r e ) . For example, m i c r o s c o p i c bumps on t h e f l a t c r y s t a l l i n e f a c e t s o f Ag g r a n u l e s were produced when t h e Ag powder (presumably h a v i n g a Ag20 f i r s t l a y e r ) was exposed t o NO2 g a s . P e r h a p s a t o m i c s c a l e r o u g h n e s s c a n e x i s t w i t h t h e s e newly induced m i c r o s c o p i c bumps [ 1 3 ] . However, when s i m i l a r Ag powder was exposed t o HCN, t h e s e m i c r o s c o p i c bumps were n o t observed by SEM [ 5 3 ] .Another i m p o r t a n t i m p l i c a t i o n from t h e e l e c t r o d y n a m i c c a l c u l a t i o n i s t h a t t h e s u r f a c e - a v e r a g e d i n t e n s i t y enhancement f a c t o r s h o u l d be s e n s i t i v e t o t h e i n c i d e n t p o l a r i z a t i o n , w h e t h e r i t i s a l o n g t h e major o r minor a x i s . E x p e r i m e n t a l l y , sample o r i e n t a t i o n r e l a t i v e t o t h e i n c i d e n t p o l a r i z a t i o n was n o t found t o be a s e n s i t i v e p a r a m e t e r 1181. Consequently, t h i s o b s e r v a t i o n h a s c a s t some doubt on t h e importance of t h e e l e c t r o m a g n e t i c mechanism o n even t h e s e s p h e r o i d s .
The d i s t a n c e dependence of t h e e l e c t r o m a g n e t i c i n t e n s i t y away from a l a r g e Ag s p h e r o i d a l s u r f a c e h a s n o t been c a l c u l a t e d , a l t h o u g h i t i s p r e d i c t e d t o be d i f f e r e n t from t h a t o f a s p h e r e i n t h e R a y l e i g h l i m i t ( i . e . , d i f f e r e n t from t h e r e s u l t of [ a ~ ( a + d ) ~ I ~ , where a i s t h e s p h e r e r a d i u s and d i s t h e d i s t a n c e from t h e
s u r f a c e ) . Nor have s p a c e r e x p e r i m e n t s been made o n t h e s e r e g u l a r l y a r r a y e d Ag m i c r o s t r u c t u r e s . Such distance-dependence c a l c u l a t i o n s and s p a c e r e x p e r i m e n t s w i l l
CIO-288 JOURNAL DE PHYSIQUE
no doubt b e c r i t i c a l t o q u a n t i f y i n g t h e magnitude of t h e c l a s s i c a l e l e c t r o m a g n e t i c enhancement f o r s u c h l a r g e m i c r o s t r u c t u r e s .
I n a d d i t i o n t o Ag, o t h e r m e t a l l i c m i c r o s t r u c t u r e s (Au and Al) have been e v a p o r a t e d o n s i m i l a r r e g u l a r l y a r r a y e d S i 0 2 p o s t s [ 5 4 ] . For Au m i c r o s t r u c t u r e s , t h e CN mode t e m p o r a l l y evolved from a narrow 2144 cm-l peak t o a b r o a d e r peak a t 2191 cm-l
(implying c h e m i c a l r e a c t i o n w i t h t h e a m b i e n t ) , and t h e e x c i t a t i o n spectrum f o r t h e 3 : l s p h e r o i d s e x h i b i t e d a s i n g l e peak a t %640 nm w i t h n a r r o w e r l i n e w i d t h t h a n t h e c o r r e s p o n d i n g peak a t %540 nm f o r 3 : l Ag s p h e r o i d s . The observed enhancement f a c t o r f o r Au s p h e r o i d s was %5 x
l o 6 .
For A 1 m i c r o s t r u c t u r e s , SERS from a mono- l a y e r o f p - n i t r o b e n z o i c a c i d was observed w i t h 488 nm e x c i t a t i o n and was found t o be o n l y s 5 0 0 t i m e s weaker t h a n t h e SERS from a s i m i l a r l a y e r p l a c e d d i r e c t l y on t h e Ag m i c r o s t r u c t u r e s . Between 564 and 459 nm, t h e SERS i n t e n s i t y i n c r e a s e d by a f a c t o r o f 3 . However, t h e A 1 r e s u l t s a r e c o m p l i c a t e d by two f a c t s : (1) SEM c h a r a c t e r i z a t i o n showed t h a t t h e s h a p e o f t h e A1 p a r t i c l e s i s d i f f e r e n t from t h a t of t h e Ag o r Au p a r t i c l e s . ( 2 ) I n t h e l a b o r a t o r y atmosphere, a n o x i d e c o a t i n g(Q3nrn) e x i s t e d o n t h e s u r f a c e o f e a c h A 1 p a r t i c l e . A t p r e s e n t , no e l e c t r o d y n a m i c c a l c u l a t i o n e x i s t s f o r Au and A 1 s p h e r o i d s . T h e r e f o r e , no comparison between c l a s s i c a l e l e c t r o m a g n e t i c t h e o r y and e x p e r i m e n t s c a n be made. The f a c t t h a t SERS was o b s e r v e d on A1 1541, e s p e c i a l l y w i t h a b u i l t - i n 3 nm s p a c e r , i m p l i e s t h a t i n t h e f u t u r e SERS measurements a s a f u n c t i o n of s p a c e r t h i c k n e s s may b e p o s s i b l e from s u c h r e g u l a r l y a r r a y e d m i c r o s t r u c t u r e s . Aluminum o x i d e [ 5 5 ] and SiOx [ 4 6 ] s p a c e r l a y e r s h a v e p r e v i o u s l y been used i n Ag i s l a n d f i l m s t u d i e s . S i n c e t h e n o n l i n e a r o p t i c a l s i g n a l h a s a h i g h e r o r d e r dependence on t h e i n c i d e n t f i e l d i n t e n s i t y t h a n d o e s SERS 1581, s i m u l t a n e o u s SERS and s u r f a c e enhanced n o n l i n e a r o p t i c a l m e a s u r e - ments ( e . g . , second-harmonic g e n e r a t i o n [ 5 6 ] o r hyper-Raman s c a t t e r i n g [ 5 7 ] ) on Ag, Au, and A 1 m i c r o s t r u c t u r e s c a n i n t h e f u t u r e a i d i n q u a n t i f y i n g t h e v a r i o u s enhancement p r o c e s s e s .
( c ) E l e c t r o d e s
Most SERS e x p e r i m e n t s a r e c a r r i e d o u t i n a n e l e c t r o c h e m i c a l c e l l , and s e v e r a l r e v i e w p a p e r s a r e a v a i l a b l e o n t h e e l e c t r o c h e m i s t r y a s s o c i a t e d w i t h t h e o x i d a t i o n and r e d u c t i o n c y c l e (ORC), SERS, and s u r f a c e enhanced n o n l i n e a r o p t i c a l r e s u l t s [ 8 , 5 9 , 601. There i s g e n e r a l agreement a b o u t t h e f o l l o w i n g p r o c e s s e s i n v o l v e d w i t h Ag e l e c t r o d e s immersed i n s i m p l e e l e c t r o l y t e s ( e . g . , i n a l k a l i - h a l i d e s o r pseudo- h a l i d e s ) : (1) During t h e o x i d a t i o n h a l f c y c l e , o x i d a t i o n o f t h e Ag e l e c t r o d e o c c u r s , Ago + ~ g +
+
e- which c a n combine w i t h t h e a n i o n s w i t h i n t h e s o l i d - l i q u i d i n t e r f a c e (double l a y e r ) t o form a n a d s o r b a t e l a y e r which, depending o n i t s s o l u - b i l i t y and a d h e s i v e n e s s , may o r may n o t a d h e r e t o t h e e l e c t r o d e ; t h e Ag s u r f a c e i s roughened and t h e amount of Ag o x i d a t i o n c a n be monitored by t h e t o t a l c h a r g e p a s s e d t h r o u g h t h e e l e c t r o d e . (2) During t h e r e d u c t i o n h a l f c y c l e , t h e a d s o r b a t e l a y e r and some of t h e A ~ + i n t h e d o u b l e l a y e r a r e r e d u c e d , A ~ ++
e- -t Ago. Theseadatoms c a n d i f f u s e ( d i f f u s i o n r a t e depends o n t h e a d s o r b a t e complex o n t h e e l e c - t r o d e ) a l o n g t h e s u r f a c e and be l o s t upon i n c o r p o r a t i o n i n t o d e f e c t s i t e s . (3) The c o v e r a g e of v a r i o u s a d s o r b a t e s i s a f u n c t i o n o f p o t e n t i a l . (4) The v i b r a t i o n a l f r e q u e n c y o f some a d s o r b a t e s v a r i e s w i t h p o t e n t i a l a s a r e s u l t of i n c r e a s e d c o v a l e n c y between t h e Ago-molecular bonding w i t h i n c r e a s e d e x c e s s p o s i t i v e c h a r g e o n t h e e l e c t r o d e . (5) Both t h e SERS i n t e n s i t y and t h e continuum background i n c r e a s e a f t e r one o r s e v e r a l ORCs; t h e amount of i n c r e a s e i s i n i t i a l l y monotonic w i t h t h e amount o f A ~ + reduced o n t h e e l e c t r o d e [ 6 0 ] . ( 6 ) F u r t h e r enhancement c a n o c c u r w i t h p h o t o r e d u c t i o n o f t h e a d h e r e n t l a y e r which, a c c o r d i n g t o SEM p h o t o g r a p h s , a f f e c t s t h e t o p o l o g y o f t h e r e c o n s t r u c t e d Ag s u r f a c e [ I l l . ( 7 ) F u r t h e r enhancement can o c c u r d u r i n g t h e ORC f o l l o w i n g r a p i d hydrogen g a s e v o l u t i o n a t a n e g a t i v e p o t e n t i a l ; i t i s s p e c u l a t e d t h a t t h e H2 b u b b l e s a d h e r e n t on t h e e l e c t r o d e a f f e c t t h e t o p o l o g y o f t h e Ag s u r f a c e d u r i n g t h e ORC [ 6 1 ] . (8) I r r e v e r s i b l e l o s s of t h e SERS s i g n a l o c c u r s when t h e p o t e n t i a l a p p r o a c h e s t h e p o t e n t i a l of z e r o c h a r g e
(PZC), where t h e Ag adatom-adsorbate complexes a r e d i s s o c i a t e d , a l l o w i n g t h e Ag adatoms t o d i f f u s e and b e l o s t a t d e f e c t s i t e s [ 2 5 , 2 6 ] ; s u c h i r r e v e r s i b l e l o s s i s n o t e d t o o c c u r a l s o when t h e t e m p e r a t u r e i s c y c l e d a t a f i x e d p o t e n t i a l and i s a t t r i b u t e d t o t h e r m a l d i s s o c i a t i o n o f t h e Ag adatom-adsorbate complexes [ 6 2 ] .
(9) A r e v e r s i b l e component of t h e SERS s i g n a l e x i s t s w i t h p o t e n t i a l c y c l i n g o r s t e p p i n g , p r o v i d e d t h a t t h e most n e g a t i v e p o t e n t i a l e x c u r s i o n d o e s n o t approach
t h e PZC, t h e adatom d e n s i t y i s n o t d e c r e a s e d w i t h i n a p o t e n t i a l r a n g e , and t h e d i f - f e r e n t a d s o r b a t e c o v e r a g e s a r e s i n g l e - v a l u e d f u n c t i o n s o f t h e p o t e n t i a l ( i . e . , no h y s t e r e s i s ) [ 2 5 , 2 6 3 . (10) Changes i n t h e i n c i d e n t photon e n e r g y c a u s e t h e maximum o f t h e SERS i n t e n s i t y v s p o t e n t i a l t o s h i f t l i n e a r l y i n t h e c a s e of p y r i d i n e i n v a r i o u s e l e c t r o l y t e s [ 6 3 , 6 4 ] ; t h e s e i m p o r t a n t r e s u l t s p r o v i d e r a t h e r i n d i s p u t a b l e e v i d e n c e t h a t a c h a r g e - t r a n s f e r (CT) e x c i t a t i o n mechanism i s i m p o r t a n t and t h a t t h e r e s o n a n c e between t h e i n c i d e n t photon e n e r g y and t h e energy l e v e l s o f t h e A ~ O -
a d s o r b a t e complexes c a n be t u n e d by t h e p o t e n t i a l . (11) The Ago-adsorbate com- p l e x e s c a n b e c o m p l i c a t e d o r i n f l u e n c e d by c a t i o n s (e.g., H+ o r ~ a + ) n o t neces-
s a r i l y i n d i r e c t c o n t a c t w i t h t h e e l e c t r o d e ; f u r t h e r m o r e , t h e p y r i d i n e and s l i g h t l y c o n t a c t a d s o r b i n g a n i o n s ( e . g . , F-, and ClOZ) c a n form complexes c a p a b l e of s t a b i - l i z i n g t h e Ago adatoms o n t h e s u r f a c e .
The major weakness i n a l l t h e e l e c t r o d e - r e l a t e d work and, f o r t h a t m a t t e r , a l l t h e SERS r e s u l t s o b t a i n e d from d i f f e r e n t t y p e s o f s u b s t r a t e s i s t h e l a c k o f q u a n t i t a - t i v e i n f o r m a t i o n o n t h e enhancement f a c t o r a s s o c i a t e d w i t h s e v e r a l p r e s e n t l y pro- posed mechanisms. The problem i s even more c o m p l i c a t e d b e c a u s e t h e d e t e c t e d s i g n a l i s dependent o n t h e m o l e c u l a r c o v e r a g e on d i f f e r e n t t y p e s o f s u r f a c e s i t e s (e.g.. adatoms, s u b m i c r o s c o p i c , and m i c r o s c o p i c ) of y e t u n s p e c i f i e d s u r f a c e d e n s i t y . Much of t h e e x i s t i n g e l e c t r o c h e m i c a l i n v e s t i g a t i o n i n v o l v e s smooth c r y s t a l l i n e s u r f a c e s [ 6 5 ] w h i l e , i n o r d e r t o i n c r e a s e t h e Raman s i g n a l s t r e n g t h , t h e ORC c a u s e s t h e s u r f a c e s t o d i s o r d e r , t h e r e b y making t h e c o r r e l a t i o n between e l e c t r o c h e m i s t r y and SERS s t u d i e s d i f f i c u l t . The s u r f a c e c o v e r a g e o f C l - , B r - , I-, N?, and NCS- a s a f u n c t i o n o f p o t e n t i a l h a s r e c e n t l y been s t u d i e d f o r a n e l e c t r o c h e m i c a l l y roughened p o l y c r y s t a l l i n e Ag e l e c t r o d e [ 6 6 ] , s i m i l a r t o t h a t used f o r SERS e x p e r i m e n t s . From t h e s e d i f f e r e n t i a l c a p a c i t a n c e and k i n e t i c p r o b e measurements, t h e coverage v s p o t e n t i a l c u r v e s showed a monotonic d e c r e a s e a s t h e p o t e n t i a l was v a r i e d from t h e most p o s i t i v e v a l u e toward t h e PZC. The c o v e r a g e v s p o t e n t i a l h a s n o t been d e t e r m i n e d f o r p y r i d i n e adsorbed o n a p o l y c r y s t a l l i n e roughened Ag e l e c t r o d e . The r o u g h n e s s f a c t o r ( t h e e l e c t r o d e s u r f a c e a r e a d i v i d e d by t h e g e o m e t r i c a r e a ) h a s been d e t e r m i n e d and was found t o be %2.5-5 when t h e s e e l e c t r o d e s e x h i b i t i n t e n s e SERS s p e c t r a , i m p l y i n g a g a i n t h a t t h e observed SERS i s n o t d u e t o a
l o 4
-l o 6
i n c r e a s e i n t h e s u r f a c e a r e a 1661. The s u r f a c e c o v e r a g e o f adsorbed dye m o l e c u l e s (2-amino 5 n i t r o p y r i d i n e , ANP) d u r i n g t h e SERS measurement h a s been d e t e r m i n e d u s i n g a n o p t i c a l a b s o r p t i o n t e c h n i q u e [ 6 7 ] . However, none o f t h e above t e c h n i q u e s a r e s e n s i t i v e t o s e l e c t i v e a d s o r p t i o n o n d i f f e r e n t s u r f a c e t o p o l o g i e s . These s u r f a c e - a v e r a g e d coverage measurements c a n t h e r e f o r e p r o v i d e o n l y p a r t i a l i n f o r m a t i o n needed f o r q u a l i t a t i v e SERS e x p e r i m e n t s .I n SERS s t u d i e s w i t h Ag e l e c t r o d e s , t h e u s e of dye m o l e c u l e s e x h i b i t i n g r e s o n a n c e Raman e f f e c t i n t h e v i s i b l e r e g i o n h a s s t a r t e d 1681. Energy l e v e l s h i f t s a s a r e s u l t of p h y s i s o r p t i o n w i t h t h e Ag s u r f a c e a r e deduced by n o t i n g t h e wavelength d i f f e r e n c e between t h e a b s o r p t i o n maximum and t h e Raman e x c i t a t i o n maximum. The r e s u l t s f o r o t h e r n o b l e m e t a l e l e c t r o d e s (Au and Cu) a r e s i m i l a r t o t h o s e f o r Ag e l e c t r o d e s [ 8 ] and w i l l n o t b e r e p e a t e d h e r e . The u s e o f f o r e i g n m e t a l atoms
( e . g . , Cu and Pb) d e p o s i t e d o n t h e Ag e l e c t r o d e (by t h e u n d e r - p o t e n t i a l d e p o s i - t i o n t e c h n i q u e , UPD) i s p r e s e n t l y b e i n g pursued and i s e x t e n d i n g o u r under- s t a n d i n g of adatom-molecular complexes and SERS s t u d i e s beyond t h e c o i n a g e m e t a l s [ l o ] . M e t a l a l l o y (Agl-,PdX) s t u d i e s have concluded t h a t SERS i s s i g n i f i - c a n t l y d e c r e a s e d by t h e l a r g e r f r e e e l e c t r o n damping i n t r o d u c e d by Pd [ 6 9 ] . SERS s p e c t r a of p y r i d i n e and CO adsorbed on e l e c t r o c h e m i c a l l y roughened Pdl-,Hx e l e c - t r o d e s have a l s o been r e p o r t e d , w i t h emphasis o n t h e c r i t i c a l chemical p r e p a r a - t i o n needed t o d e p o s i t t h e Pd i o n s o n t h e e l e c t r o d e [ 7 0 ] . Very weak Raman spec- t r a of CO, 1 2 , and p h e n y l h y d r a z i n e o n P t e l e c t r o d e s have been r e p o r t e d [ 7 1 ] . Research i n v o l v i n g nonaqueous e l e c t r o l y t e s i s c u r r e n t l y under way [ 7 2 ] . Pre- l i m i n a r y r e s u l t s i n d i c a t e t h a t t h e e x c e s s n e g a t i v e c h a r g e on t h e e l e c t r o d e c a n be e s t a b l i s h e d w i t h o u t H2 g a s e v o l u t i o n [ 7 2 ] , t h e r e b y a l l o w i n g e x p e r i m e n t a l i s t s t o i n v e s t i g a t e t h e s p e c i f i c a d s o r p t i o n c h a r a c t e r i s t i c s o f c a t i o n s .
Important t o SERS d e l i b e r a t i o n s a r e t h e r e c e n t e x p e r i m e n t a l r e s u l t s of e l e c t r o - r e f l e c t a n c e o n A g ( l l l ) , (110), and (100) i n 0 . 5 M NaF e l e c t r o l y t e s (F- i s n e a r l y n o n c o n t a c t a d s o r b i n g due t o h y d r a t i o n w i t h H20 m o l e c u l e s ) [ 7 3 ] . The e l e c t r o - r e f l e c t a n c e s t r u c t u r e s a r e i n t e r p r e t e d a s e l e c t r o n i c t r a n s i t i o n s from o c c u p i e d b u l k
CIO-290 JOURNAL DE PHYSIQUE
Ag e l e c t r o n i c l e v e l s t o v a c a n t s u r f a c e Ag e l e c t r o n i c l e v e l s 1731. Such s u r f a c e l e v e l s have been c a l c u l a t e d t o be w i t h i n t h e g a p s o f t h e p r o j e c t e d b u l k d e n s i t y o f s t a t e s and t o be above t h e Fermi energy 1741. The e n e r g y d i f f e r e n c e between t h e s e d i s c r e t e s u r f a c e l e v e l s and t h e broad b u l k bands c a n be tuned by c h a r g i n g t h e e l e c - t r o d e n e g a t i v e l y o r p o s i t i v e l y ( i . e . , v a r y i n g t h e p o t e n t i a l n e g a t i v e o r p o s i t i v e o f t h e PZC). Whether s i m i l a r e l e c t r o r e f l e c t a n c e s t r u c t u r e s e x i s t f o r Ago a d s o r - b a t e complexes o n roughened SERS a c t i v e e l e c t r o d e s r e m a i n s t o be i n v e s t i g a t e d .
(d) Films
SERS s t u d i e s o n Ag f i l m s a r e a l s o b e i n g a c t i v e l y p u r s u e d . Two d e t a i l e d and up-to- d a t e r e v i e w s o n t h e s e i n v e s t i g a t i o n s have j u s t been completed [ 6 , 7 ] . The a p p e a l i n g f e a t u r e o f f i l m s i s t h e a b i l i t y o f e x p e r i m e n t a l i s t s t o p e r f o r m many f u n c t i o n s i n o n e UHV chamber, e . g . , t o e v a p o r a t e t h e f i l m o n a s u b s t r a t e (of any k i n d and h e a t e d o r c o o l e d t o any t e m p e r a t u r e ) , t o expose t h e f i l m t o any one o r s e v e r a l g a s mole- c u l e s a n d , i n p r i n c i p l e , t o s i m u l t a n e o u s l y u s e o t h e r s u r f a c e p h y s i c s t e c h n i q u e s , s u c h a s UPS and EELS. S e v e r a l r e s e a r c h l a b o r a t o r i e s a r e a l r e a d y c o n s t r u c t i n g UHV chambers f o r s u c h m u l t i - p u r p o s e s u r f a c e a n a l y s e s . A t p r e s e n t , however, r e s u l t s o n s e p a r a t e l y performed e x p e r i m e n t s i n v o l v i n g SERS, EELS, and UPS a r e s t i l l b e i n g r e p o r t e d .
The SERS e x p e r i m e n t s [ 5 , 7 ] h a v e been conducted on t h i c k Ag f i l m s , f i l m s c o a t e d on roughened CaF2 1151, t h i n i s l a n d f i l m s ( I F ) on smooth s u b s t r a t e s a t 300°K o r h i g h e r [ 2 0 , 4 4 ] , t h i c k c o l d l y e v a p o r a t e d f i l m s (CEF, 10°K o r 150°K) on smooth sub- s t r a t e s [ 5 , 7 , 1 4 ] , o r o n I F [ 1 6 ] , and f i l m s w i t h a g r a d a t i o n o f t h i c k n e s s [ 7 5 ] . There i s a c o n s e n s u s a b o u t many SERS r e s u l t s , b u t c o n t r o v e r s i e s e x i s t on t h e i r
i n t e r p r e t a t i o n , p a r r i c u l a r l y on t h e r e l a t i v e i m p o r t a n c e o f e l e c t r o m a g n e t i c v s CT models, s i n c e s u b m i c r o s c o p i c and a t o m i c s c a l e r o u g h n e s s h a s n o t been q u a n t i f i e d on t h e s e v a r i o u s f i l m s .
There i s g e n e r a l agreement c o n c e r n i n g t h e f o l l o w i n g : (1) Spacer e x p e r i m e n t s w i t h PMMA [ 1 5 ] , o x i d e s of A 1 1551, and SiOx [ 4 6 ] i n d i c a t e a l o n g range component t o SERS, which i s d i f f e r e n t from t h e enhancement f a c t o r f o r t h e f i x s t l a y e r . (2) Cov- e r a g e dependence [ 7 6 ] e x p e r i m e n t s have shown molecular-molecular i n t e r a c t i o n , c a u s i n g t h e e x c i t a t i o n p r o f i l e t o s h i f t 161 and some of t h e SERS i n t e n s i t y and v i b r a t i o n a l modes t o change ( e - g . , when CO i s coadsorbed w i t h p y r i d i n e ) [ 1 6 ] .
( 3 ) No enhancement o f t h e s u b s t r a t e ( e - g . , S i ) h a s y e t been d e t e c t e d even though SERS o f PNBA c o a t i n g on t o p o f I F h a s been r e a d i l y observed [ 7 7 ] . (4) Temperature quenching o f t h e SERS s i g n a l i s pronounced i n CEF b u t much l e s s s o i n I F e v a p o r a t e d a t room t e m p e r a t u r e o r above [ 6 , 7 , 1 4 , 1 6 ] . (5) Temperature enhancement i s observed
( e . g . , when c o o l e d t o 1 O 0 K , I F and CEF covered w i t h p y r i d i n e gave a n u n d e t e c t a b l e s i g n a l b u t SERS was i n t e n s e upon warming t h e s e f i l m s t o 100°K [ 1 6 1 ) . (6) Molecu- l a r s p e c i f i c i t y i s n o t e d ( e . g . , many o r g a n i c m o l e c u l e s e x h i b i t s t r o n g SERS, b u t methane, e t h a n e , and propane do n o t e x h i b i t d e t e c t a b l e Raman s i g n a l s ; t h e SERS of N 2 i s a t l e a s t 100 t i m e s weaker t h a n CO [6,20]. (7) M e t a l s u r f a c e s p e c i f i c i t y i s a l s o noted ( e . g . , a t 90°K, SERS of CO was o b s e r v e d on CEF b u t n o t observed on I F whereas b o t h s u r f a c e s e x h i b i t e d SERS f o r p y r i d i n e [ 1 6 ] ; i n a n a n a l o g o u s e x p e r i m e n t , SERS o f benzene was a b s e n t f o r t e r r a c e d A g ( l l 1 ) b u t was p r e s e n t f o r CEF).
(8) S u r f a c e t o p o l o g y of t h e s u b s t r a t e c a n i n f l u e n c e t h e enhancement f a c t o r ( e - g . , SERS from CEF e v a p o r a t e d on smooth q u a r t z was 1 0 t i m e s weaker t h a n from CEF evapo- r a t e d on I F [ 1 6 ] ) .
There a r e c l o s e a n a l o g i e s between e v a p o r a t i o n o f f i l m s and e l e c t r o c h e m i c a l reduc- t i o n o f e l e c t r o d e s , a s w e l l a s between t h e r m a l d e s o r p t i o n and p o t e n t i a l - c o n t r o l l e d d e s o r p t i o n . The a r r i v a l r a t e s and s u r f a c e d i f f u s i o n o f Ag atoms a r e b o t h much s l o w e r f o r t h e e l e c t r o c h e m i c a l c a s e . For b o t h e l e c t r o d e s and f i l m s , t h e adatom d i f f u s i o n r a t e s a r e s t r o n g l y a f f e c t e d by t h e t y p e o f a d s o r b a t e , a d s o r b a t e c o v e r a g e , and, o f c o u r s e , t e m p e r a t u r e . S t a b i l i z a t i o n o f Ag adatoms i s c o n s t a n t l y p r e s e n t o n t h e e l e c t r o d e i n c o n t a c t - a d s o r b i n g e l e c t r o l y t e s d u r i n g t h e s u r f a c e r e c o n s t r u c t i o n phase b u t n o t p r e s e n t on t h e f i l m d u r i n g t h e e v a p o r a t i o n p h a s e . A c l o s e p a r a l l e l e x i s t s between thermal-induced quenching o r enhancement observed o n f i l m s and p o t e n t i a l - i n d u c e d quenching o r enhancement on e l e c t r o d e s .
UHV chamber e v a p o r a t e d and exposed f i l m s a r e more amenable t o a n a l y s i s by o t h e r w e l l e s t a b l i s h e d s u r f a c e p h y s i c s t e c h n i q u e s . EELS i n v e s t i g a t i o n on CO, 02, C2H4, and p y r i d i n e exposed CEF was r e p o r t e d t o e x h i b i t s t r o n g CT peaks w h i l e N2-exposed CEF f a i l e d t o e x h i b i t any CT peaks [ 7 8 ] . S i n c e t h e s e EELS peaks were a b s e n t on smooth Ag s u r f a c e s , i t was concluded t h a t t h e CT e x c i t a t i o n s a r e a s s o c i a t e d w i t h Ag adatom-molecular complexes which a r e n e c e s s a r y i n g r e d i e n t s f o r t h e "chemical"
mechanism i n SERS. UPS i n v e s t i g a t i o n on CEF and on room t e m p e r a t u r e e v a p o r a t e d f i l m ( w i t h and w i t h o u t p r o l o n g e d RT a n n e a l i n g ) i n d i c a t e d t h a t CEF h a s a g r e a t e r d e n s i t y of s t a t e s (due t o s u r f a c e d e f e c t s ) n e a r t h e t o p o f t h e d-band, some 4 e x below t h e b u l k Fermi l e v e l [ 7 9 ] . S i m i l a r r e s u l t s were observed f o r Ag f i l m ( 3 A t h i c k ) e v a p o r a t e d o n a much t h i c k e r , room t e m p e r a t u r e annealed Ag f i l m cooled t o 12OoK [SO]. The observed n a r r o w e r d e n s i t y of s t a t e s f o r t h e s e s u r f a c e d e f e c t s p r o p o r t i o n a t e l y d e c r e a s e s t h e r e s o n a n t e n e r g y denominator a s s o c i a t e d w i t h CT from t h e s u r f a c e d e f e c t s t a t e s a t t h e t o p o f t h e d-band t o t h e l o w e s t unoccupied o r b i t a l s o f t h e Ago-adsorbate complex, a n i m p o r t a n t r e q u i r e m e n t t o i n c r e a s e Raman p o l a r i z a b i l i t y . LEED and Auger s p e c t r o s c o p y have been used t o i n s u r e c l e a n and w e l l c h a r a c t e r i z e d smooth s i n g l e - c r y s t a l Ag s u r f a c e s , ( I l l ) , (110), and ( l o o ) , b e f o r e p y r i d i n e m o l e c u l e s a r e adsorbed a t 120°K. Unlike SERS o b s e r v a t i o n s from roughened s u r f a c e s , smooth s i n g l e - c r y s t a l s y i e l d e d Raman s i g n a l s w i t h t h e f o l - lowing c h a r a c t e r i s t i c s [ 2 8 ] : (1) e x t r e m e l y weak, comparable t o t h e f r e e molecu- l a r Raman c r o s s s e c t i o n ; ( 2 ) u n s h i f t e d symmetric b r e a t h i n g mode a t 993 c m - l , i n s t e a d of a t %I005 cm-I, t h e u s u a l l y o b s e r v e d f r e q u e n c y f o r t h e f i r s t p y r i d i n e l a y e r (most l i k e l y chemisorbed) on roughened Ag s u r f a c e s e x h i b i t i n g SERS;
(3) comparable i n t e n s i t y f o r t h e C-H mode a t 3050 cm-l, a s i n t h e c a s e of p y r i - d i n e l i q u i d b u t d i s s i m i l a r t o t h e u s u a l SERS s p e c t r a of p y r i d i n e ; and (4) p o l a r i z a - t i o n s e n s i t i v e f o r b o t h t h e i n c i d e n t (obeying F r e s n e l e q u a t i o n s ) and Raman r a d i a - t i o n , u n l i k e t h e t o t a l l y d e p o l a r i z e d s i t u a t i o n f o r SERS s p e c t r a .
F u t u r e q u a n t i t a t i v e i n v e s t i g a t i o n s o n c l e a n smooth s i n g l e - c r y s t a l s u r f a c e s a r e e s s e n - t i a l t o o u r u n d e r s t a n d i n g o f roughened SERS a c t i v e s u r f a c e s . We must know t h e pre- c i s e e f f e c t a smooth s i n g l e - c r y s t a l s u r f a c e h a s o n t h e Raman p o l a r i z a b i l i t y , t h e symmetry of t h e s c a t t e r i n g t e n s o r , and t h e e l e c t r o n i c l e v e l s f o r b o t h p h y s i s o r b e d and chemisorbed m o l e c u l e s .
I V - CONCLUSION
The r e a l i z a t i o n t h a t SERS c a n p r o v i d e m o l e c u l e and s u r f a c e s i t e s p e c i f i c informa- t i o n and h a s t h e p o t e n t i a l o f p r o v i d i n g a unique i n - s i t u d i a g n o s t i c probe f o r s u r - f a c e p h y s i c s a n a l y s e s under h i g h p r e s s u r e g a s - s o l i d and l i q u i d - s o l i d environments h a s s p u r r e d a r a t h e r s u s t a i n e d h i g h l e v e l o f a c t i v i t y i n t h i s f i e l d . Some groups a r e now a p p l y i n g SERS a s a d i a g n o s t i c p r o b e i n i n d u s t r i a l l y i m p o r t a n t a r e a s w h i l e o t h e r s c o n t i n u e t o i n v e s t i g a t e t h e fundamental problems r e l a t e d t o t h e c a u s e s f o r s u c h l a r g e enhancement f a c t o r s . A l l t h e s e SERS a c t i v i t i e s may e v e n t u a l l y p r o v i d e a d e t a i l e d u n d e r s t a n d i n g of m o l e c u l a r i n t e r a c t i o n s a t s u r f a c e s and i n t e r f a c e s i n environments i n a c c e s s i b l e by o t h e r s u r f a c e d i a g n o s t i c t e c h n i q u e s .
V - ACKNOWLEDGMENTS
We g r a t e f u l l y acknowledge t h e p a r t i a l s u p p o r t of t h i s work by t h e Army Research O f f i c e (Grant No. DAAG29-82-K-0040).
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