ON THE "FIRST LAYER" ENHANCEMENT IN RAMAN SCATTERING OF PYRIDINE ON SILVER

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ON THE ”FIRST LAYER” ENHANCEMENT IN RAMAN SCATTERING OF PYRIDINE ON SILVER

C. Pettenkofer, A. Otto

To cite this version:

C. Pettenkofer, A. Otto. ON THE ”FIRST LAYER” ENHANCEMENT IN RAMAN SCATTERING OF PYRIDINE ON SILVER. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-337-C10-340.

�10.1051/jphyscol:19831067�. �jpa-00223525�

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JOURNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome **, décembre 1983 page C10-337

ON T H E "FIRST L A Y E R " E N H A N C E M E N T IN RAMAN SCATTERING OF P Y R I D I N E ON SILVER

C. Pettenkofer and A. Otto

Physikalisches Institut, Lehrstuhl III, Universität Düsseldorf, D-4000 Düsseldorf 1, F.R.G.

Résumé - Dans le cas de pyridine adsorbée sur Ag, la section efficace de la diffusion Raman est amplifiée pour la première monocouche sur une face vicinale (110), mais pas sur une face (110). On attribue cette différence S un couplage électron-photon plus important sur des marches particulières.

Abstract - We found no short range Raman enhancement for pyridine adsorbed to a Ag(110) face, but a "first layer enhancement" on a Ag face vicinal to Ag(110). We tentatively assign this to increased electron-photon coupling at special steps.

I. INTRODUCTION

The discovery of surface enhanced Raman scattering (SERS) from adsorbates on silver surfaces /l/ has led to two classes of models for the enhancement: Electrodynamic resonances in supraatomic surface roughness and short range ("first layer") enhancement based on the electronic interaction between metal and adsorbate, including charge transfer / 2 / .

The second class of models has been tested on relatively well defined surfaces of silver single crystals. Sanda et al. / 3 / investigated an essentially (111) oriented Ag surface, with an inscribed grating structure, approximately sinusoidal with wavelength ~ 10000 A and ~ 1000 A height, the modulation wave vector oriented along the (110) direction. The Raman spectrum of pyridine adsorbed to this surface dis- played only one line at 990 cm--1 for coverages between one and 36 monolayers. The Raman enhancement for the first two monolayers of pyridine was found to be about 20 times stronger than for the subsequent pyridine layers /3/.

Udagawa et al. / 4 / reported Raman spectra of pyridine on "smooth" Ag(100). After 10 L exposure to pyridine at 150 K sample temperature (equivalent to about 2 monolayers of pyridine), Raman lines at 1004 and 1032 cm"-'- were observed. When the exposure was increased to 100 L an additional peak at 991 cm"1 appeared. The Raman bands (peak intensity about 2 counts/sec) were superimposed on a background of the order of 20 counts/sec. Udagawa et al. compared the cross section per molecule of pyridine adsorbed to the Ag(100) surface to that of free gaseous species. They assumed, that all the molecules adsorbed after 10 L exposure contributed to the 1004 cm- 1 band and concluded that the enhancement factor for the adsorbed species was 4.4 • 10^ (± 20%) over the gaseous species. Udagawa et al. considered that the observed intensity at 1004 cm"1 might arise from molecules bound to defect sites

"which may (owing to the rough surface in the neighbourhood of the defect) exhibit enhancement factors of 1 04 to 106". This was ruled out because no signal could be observed after 1 L exposure, and because Udagawa et al. expected a preferential adsorption to defects at low exposures.

As the 1004 cm"1 peak was approximately the same for 10 and 100 L exposure, the 1004 c m- 1 peak was assigned to pyridine in close contact with the silver surface, whereas the signal at 991 cm"1 was assigned to molecules in the third and further layers. Therefore, from the work of Udagawa et al. / 4 / one had to conclude that there was a "first layer enhancement" of about 440. Quite the opposite, namely no

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831067

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C 10-338 JOURNAL DE PHYSIQUE

" f i r s t l a y e r enhancement" f o r p y r i d i n e on A g ( l l l ) , (110), and (100) faces was found by Campion and M u l l i n s /5/. Bands a t about 993 and 1034 cm-l grew l i n e a r w i t h exposure from 1 t o 20 L (uncorrected), no band was observed a t 1004 and 1006 cm-l.

These and f u r t h e r r e s u l t s were i n t e r p r e t e d as normal Raman s c a t t e r i n g w i t h o u t enhancement. Campion and M u l l i n s concluded, t h a t t h e i r r e s u l t supported t h e hypothesis /6/ t h a t s p e c i a l adsorption s i t e s a r e r e s p o n s i b l e f o r a s u b s t a n t i a l f r a c t i o n o f t h e t o t a l enhancement o f t h e Raman cross s e c t i o n o f t h e p y r i d i n e s i l v e r system.

I n view o f t h e a p p a r e n t l y c o n t r a d i c t o r y r e s u l t s of Refs. /3,4,5/, we performed s i m i - l a r experiments f o r p y r i d i n e adsorted t o a A g ( l l 0 ) surface and a v i c i n a l f a c e c l o s e t o (110).

11. EXPERIMENT AND RESULTS

A s i l v e r s i n g l e c r y s t a l w i t h a (110) s u r f a c e and a (110) v i c i n a l plane o f t h e <001>

zone i n c l i n e d about 6O w i t h r e s p e c t t o t h e (110) face (see i n s e r t o f F i g . 1) was prepared i n the f o l l o w i n g way: The c r y s t a l was mechanically p o l i s h e d w i t h diamond pastes ( s m a l l e s t g r a i n s i z e 0,25p) and t h a n t r a n s f e r r e d i n t o a s p e c i a l l y designed u l t r a h i g h vacuum apparatus f o r Raman s c a t t e r i n g i n v e s t i g a t i o n s /7/. Here t h e c r y s t a l was several times s p u t t e r e d by Ar" bombardment and annealed. AES o n l y revealed a very low r e s i d u a l carbon contamination. The (110) f a c e showed sharp LEED p a t t e r n s w i t h low background. A (110) LEED p a t t e r n was a l s o observed from t h e v i c i n a l face, b u t t h e spots were more extended, changing i n s i z e w i t h e l e c t r o n beam energy. However, t h e i d e a l s p l i t t i n g o f every "(110)-spot" i n t o 4 neighbouring separate spots c o u l d n o t be observed. The plane o f incidence o f t h e 488 nm, 200

mK

l a s e r beam was p a r a l l e l t o t h e <001> d i r e c t i o n (see F i g . 1 i n s e r t ) , t h e angle o f incidence was about 70 degrees, t h e p o l a r i z a t i o n was p a r a l l e l t o t h e plane o f i n c i - dence. The p o l a r i z a t i o n o f t h e s c a t t e r e d l i g h t was n o t i n v e s t i g a t e d f o r reasons

1 0 3 ~ 993 Ramon shift cm-I

Fig. 1

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r i g h t : Raman spectrum o f t h e A g ( l l 0 ) surface cooled t o 120 K and exposed subsequently t o 10, 30, and 100 L (uncorrected) o f p y r i d i n e . l e f t : t h e same f o r t h e (110) v i c i n a l f a c e (see i n s e r t ) .

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o f low i n t e n s i t y . Raman spectra were recorded w i t h a conventional double spectrometer, a cooled p h o t o m u l t i p l i e r w i t h S 11 c h a r a c t e r i s t i c (dark count r a t e < 1 c/sec) and photon p u l s e d i s c r i m i n a t i o n .

The s l i t w i d t h o f 750 p corresponds t o a r e s o l u t i o n o f 7 cm-l. I n most cases, t h e spectra were recorded w i t h a ratemeter ( t i m e c n s t a n t s e t t o 30 sec), a r e c o r d e r

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and a scan speed o f t h e spectrometer o f 0.037 /sec ( e q u i v a l e n t t o 0.014 cm-l/sec), i n some cases by adding several f a s t e r scans w i t h t h e h e l p o f a multichannel ana- l y z e r . The s p e c t r a i n F i g . l f o r p y r i d i n e on t h e (110) and t h e v i c i n a l face were obtained i n d i f f e r e n t s e r i e s o f measurements. The f r e s h l y cleaned and annealed s i l v e r c r y s t a l was cooled t o 120 K, and exposed subsequently t o 10, 30, and 100 L of p y r i d i n e (uncorrected gauge reading). A monolayer of adsorbed p y r i d i n e corresponds t o about 11 L /8/ t h e p y r i d i n e ( m u l t i l a y e r - ) coverage i s assumed t o be l i n e a r t o t h e exposure /9/ (see a l s o s e c t i o n 111).

For 10 L p y r i d i n e exposure o f t h e (110) face, t h e s i g n a l t o n o i s e r a t i o i s n o t b e t t e r than one, whereas i n a l l o t h e r s p e c t r a two l i n e s centered approximately a t 993 and 1034 cm-1 a r e c l e a r l y discernable. The l i n e s a r e superimposed on a background o f about 6

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⁸cts/sec f o r t h e (110) face and about 20 cts/sec f o r t h e v i c i n a l face. The peak i n t e n s i t i e s ( u n c e r t a i n t i e s

+

0,5 c t s / s e c ) o f t h e two l i n e s versus coverage a r e given i n F i g . 2. I n one case we i n v e s t i g a t e d t h e Raman scat- t e r i n g o f an area on t h e (110) face ( d i s p l a y i n g good LEED spots), b u t from which t h e Rayleigh s c a t t e r i n g was increased i n comparison t o t h e r e s t o f t h e (110) face.

I n t h i s case, t h e p y r i d i n e l i n e s were superimposed on a background of about

100 c t s / s e c (which i s n o t caused by s t r a y l i g h t i n t h e spectrometer). A p l o t s i m i l a r t o t h e one i n F i g . 2 showed an i n t e n s i t y versus exposure r e l a t i o n i n t e r m e d i a t e between those o f F i g . 2.

Fig. 2

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Peak i n t e n s i t y above background f o r t h e p y r i d i n e bands near 993 cm-I ( t r i a n g l e s ) and 1034 cm-1 ( c i r c l e s ) f o r t h e A g ( l l 0 ) s u r f a c e (open symbols) and t h e A g ( l l 0 ) v i c i n a l f a c e ( f u l l symbols). One monolayer of p y r i d i n e corresponds t o about 11 L (uncorrected) exposure /8/.

111. DISCUSSION

Our d i s c u s s i o n i s based on t h e assumption t h a t t h e s t i c k i n g c o e f f i c i e n t o f p y r i d i n e on s i l v e r and s o l i d p y r i d i n e a t about 120 K i s one and t h e r e f o r e does n o t depend on t h e c r y s t a l l o g r a p h i c s t r u c t u r e o f t h e surface. Therefore t h e coverage grows l i n e a r w i t h exposure i n t h e same way on any surface. Our r e s u l t s from t h e (110) f a c e corroborate those o f Campion and M u l l i n s /5/. The maximum f i r s t l a y e r enhancement compatible w i t h o u r data i s f o u r . The l i n e a r r e l a t i o n between s i g n a l and coverage of t h e (110) surface f i t s w e l l t o t h e l i n e a r r e l a t i o n found f o r covera es above 3 monolayers o f p y r i d i n e on A g ( l l 0 ) /7,9/. We t h i n k , t h a t t h e 1004 cm-

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l i n e f o r p y r i d i n e on a Ag(100) s u r f a c e prepared by Udagawa e t a l . /4/ does n o t o r i g i n a t e from p y r i d i n e adsorbed on a t o m i c a l l y smooth (100) t e r r a c e s , b u t from p y r i d i n e adsorbed a t atomic s c a l e d e f e c t s : I n a r e c e n t SERS study o f p y r i d i e on c o l d l y de- p o s i t e d copper f i l m s E r t i i r k e t a l .

/ l o /

showed, t h a t t h e 993 band o f p y r i d i n e

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C10-340 JOURNAL D E PHYSIQUE

i s due t o p y r i d i n e adsorbed on a t o m i c a l l y smooth regions o f t h e s u r f a c e (low index faces) whereas t h e 1004

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1006 cm-1 band o r i g i n a t e s from p y r i d i n e adsorbed a t defects, where they a r e s u b j e c t t o an a d d i t i o n a l enhancement mechanism (besides electromagnetic enhancement) o f a t l e a s t two orders o f magnitude. P r e l i m i n a r y analogous experiments on s i l v e r show t h e same r e s u l t . The average enhancement o f 440 estimated by Udagawa e t a l . /4/ i s t h e r e f o r e a t t r i b u t e d t o t h e a d d i t i o n a l enhancement a t defects. Our d a t a i n d i c a t e a " f i r s t l a y e r enhancement" o f about 10 on t h e (110) v i c i n a l face. One should note, t h a t t h e " f i r s t l a y e r enhancement" r e p o r t e d by Sanda e t a l . /3/ o r i g i n a t e d from (111) v i c i n a l faces. I n b o t h cases, l i t t l e i s known on t h e exact nature o f the steps s e p a r a t i n g t h e (110) o r (111) t e r r a c e s . I n b o t h cases, t h e p y r i d i n e b r e a t h i n g v i b r a t i o n i s n o t found a t 1004

-

1006 cm-1 b u t a t 990

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993 cm-1. (However, i t i s n o t understood, why Sanda e t a l . /3/ d i d n o t observe a v i b r a t i o n a t about 1034 cm-1.) E r t u r k e t a l .

/ l o /

have speculated on a s o - c a l l e d

" p r o x i m i t y e f f e c t " : A " h o t " e l e c t r o n i s created by photon a n n i h i l a t i o n a t a d e f e c t s i t e and i s t e m p o r a r i l y trapped i n t h e l o w e s t unoccupied o r b i t a l o f a p y r i d i n e molecule adsorbed a t a nearby t e r r a c e . I n t h i s sense one m i g h t e x p l a i n t h e apparent

" f i r s t l a y e r e f f e c t " a t special v i c i n a l faces by increased electron-photon c o u p l i n g a t s p e c i a l steps. An i n d i c a t i o n f o r t h i s p o s s i b i l i t y i s t h e increased i n e l a s t i c background from t h e (110) v i c i n a l f a c e compared t o t h e (110) f a c e (see Fig. 1 ) . The background i s caused by electron-photon coupling, p r e f e r e n t i a l l y a t surface and b u l k d e f e c t s /11,12,13,14/. I n summary, o u r r e s u l t s c o n f i r m t h e d i s c u s s i o n o f SERS i n two r e c e n t reviews /15,16/.

ACKNOWLEDGEMENT

We thank J. Eicknians f o r h e l p i n t h e p r e p a r a t i o n of t h e Ag s i n g l e c r y s t a l surface and U. E r t i i r k f o r h e l p d u r i n g t h e Raman measurements.

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