Photon, Electron, and Ion Probes of Polymer Structure and Properties

20 Metal-Polymer Interfaces: Studies with X-Ray Photoemission

Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch020

J. M. BURKSTRAND Physics Department, General Motors Research Laboratories, Warren, MI 48090

Polymer substrates are often coated with metals for a wide variety of reasons. In many cases, the chemical condition of the surface has been found (1) to alter the adhesion of the metal film to the substrate. In particular, oxygen plasma treatment of polymer surfaces before metal deposition has been found (2, 3, 4) to increase the adhesion of the metal. We have been able to identify (5) hydroxyl, carbonyl and ester groups which were created on the polymer surfaces during oxygen plasma treatment. We have also identified (6) the formation of metal-oxygen-polymer complexes at an oxygen treated polymer surface and correlated their presence with an increase in the adhesion of the metal film. In complimentary experiments, a number of other workers (7) (14) have studied with photoemission the electronic structure of thin metal overlayers on inorganic substrates. In particular, Tibbetts and Egelhoff found for small metal clusters on a clean, amorphous carbon substrate that a l l the metal electronic binding energies increased by about 0.6eV with respect to the bulk values. They attributed this to either decreased extra-atomic relaxation energies or to an atomic renormalization (expansion) of the valance orbitals. We have attributed (15) analogous changes in the core binding energies of copper atoms on polystyrene to changes in both extra-atomic and intra-atomic relaxation energies. We briefly describe here the results obtained from studies of copper, nickel and chromium overlayers deposited on polystyrene, polyvinyl alcohol, polyvinyl methyl ether, polyethylene oxide, polyvinyl acetate and polymethyl methacrylate. Using Xray photoemission spectroscopy we measured significant variations in the core binding energies and lineshapes as we varied both the metal and the substrate atoms. These changes can be related to both differences between the intrinsic properties of the metal atoms as well as to differences in the interactions with the substrates. In the following sections we describe the details of 0097-6156/81/0162-0339$05.00/0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

P H O T O N , E L E C T R O N , A N D ION PROBES

340

the e x p e r i m e n t a l p r e p a r a t i o n , the data a c q u i s i t i o n , the e x p e r i m e n t a l r e s u l t s , and t h e s i g n i f i c a n c e o f t h e m e a s u r e m e n t s . Experimental The i n t e r a c t i o n s t a k i n g p l a c e on t h e s u r f a c e were m o n i t o r e d w i t h X - r a y p h o t o e m i s s i o n s p e c t r o s c o p y ( X P S ) u s i n g t h e same s y s t e m as d e s c r i b e d p r e v i o u s l y (_5, 6). This c o n s i s t e d o f a commercial (16) d o u b l e p a s s c y l i n d r i c a l m i r r o r a n a l y z e r and a Mg K X - r a y s o u r c e i n an u l t r a h i g h v a c u u m . ^ y s t e m whose o p e r a t i n g p r e s s u r e was a b o u t 2 . 6 x 10~ P a (2 x 10 Torr). The c y l i n d r i c a l m i r r o r a n a l y z e r was u s e d w i t h a c o n s t a n t p a s s e n e r g y o f 50 eV o r .08 eV energy r e s o l u t i o n . The p o l y m e r f i l m s were s o l v e n t c a s t on s t a i n l e s s steel s u b s t r a t e s and a i r d r i e d a t 2 2 C ; t h e i r f i n a l t h i c k n e s s was a b o u t 0 . 0 0 1 mm. A f t e r i n s e r t i o n i n t o t h e u l t r a - h i g h vacuum chamber t h r o u g h a l o a d - l o c k chamber, t h e p o l y m e r s were warmed t o t e m p e r a t u r e s above t h e i r r e s p e c t i v e g l a s s t r a n s i t i o n t e m p e r a t u r e s f o r t h e t i m e needed t o remove t h e r e m a i n i n g s o l v e n t from t h e b u l k o f the f i l m . Copper and n i c k e l were d e p o s i t e d from m e t a l f o i l wrapped around a hot t u n g s t e n f i l a m e n t . Chromium was e v a p o r a t e d from a chrome p l a t e d t u n g s t e n w i r e . XPS measurements were made t o d e t e r m i n e t h e m e t a l c o v e r a g e as w e l l as t h e e l e c t r o n i c s t r u c t u r e a t the i n t e r f a c e . The m e t a l c o v e r a g e was d e t e r m i n e d by s u b s t i t u t i n g t h e e x p e r i m e n t a l l y m e a s u r e d a r e a s u n d e r the XPS c u r v e s , c o r e h o l e c r o s s s e c t i o n s (17), and e l e c t r o n mean f r e e p a t h i n b o t h t h e m e t a l (18) and the p o l y m e r (19) and an i n s t r u m e n t response f u n c t i o n i n t o the e q u a t i o n s f o r e m i t t e d e l e c t r o n i n t e n s i t y ( 2 0 ) . A t c o v e r a g e s n e a r one m o n o l a y e r , t h e s e v a l u e s were c h e c k e d w i t h r e s u l t s from a q u a r t z c r y s t a l t h i n f i l m m o n i t o r . During p h o t o e m i s s i o n , a small b u i l d u p o f p o s i t i v e charge o c c u r r e d on t h e s u r f a c e , r e s u l t i n g i n a s h i f t i n g o f t h e e n e r g y s c a l e by 1.0 - 2 . 5 e V . T h i s c h a r g i n g was a c c o u n t e d f o r by s e t t i n g the b i n d i n g e n e r g y r e l a t i v e to t h e f e r m i l e v e l o f t h e C - l s e l e c t r o n s t o 285.OeV f o r c a r b o n atoms i n v o l v e d i n CEL bonds and r e f e r e n c i n g the o t h e r l e v e l s t o t h a t v a l u e , a m e t h o d w h i c h has been s u c c e s s f u l l y used i n the e x a m i n a t i o n o f o t h e r polymer s u r f a c e s . ( 5 , 6, 12, 2 1 ) .


f l

Results Copper, n i c k e l , o r chromium was d e p o s i t e d on the clean polymers at coverages i n j m 0.002 to 10.0 m o n o l a y e r s . A t each c o v e r a g e , t h e m e t a l 2p c o r e e l e c t r o n b i n d i n g e n e r g y was measured with XPS. The peak p o s i t i o n s o f 2p lineshapes for a number o f c o v e r a g e s o f c o p p e r , n i c k e l and chromium on p o l y s t y r e n e and on p o l y v i n y l a l c o h o l a r e p l o t t e d i n F i g s . 1-3 r e s p e c t i v e l y . The d a t a f o r the o t h e r m e t a l / o x y g e n c o n t a i n i n g p o l y m e r s y s t e m s a r e s i m i l a r t o t h a t shown f o r p o l y v i n y l a l c o h o l ( i f a l l t h e m e t a l b i n d i n g e n e r g i e s a r e p l o t t e d as a d i f f e r e n c e from t h e b u l k v a l u e ) .

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

BURKSTRAND

341

Metal-Polymer Interfaces


3 934 -

E 933

CD

932 0.001

0.01

—1

I

I I I I I L

J

I

I

0.1

I 1.0

1 I I I I I

COPPER COVERAGE (monolayers) 3/2

Figure 1. Variation in the Cu-2p electron core binding energy as a function of coverage for vapor-deposited Cu on PS and PVA. The individual data points for PS are not shown.

3/2

Figure 2. Variation in the Ni-2p electron core binding energy as a function of coverage for vapor-deposited Ni on PS and PVA



342

PHOTON, E L E C T R O N , AND

0.01

0.1

1.0

ION

PROBES

10.0

CHROMIUM COVERAGE (monolayers)

Figure 3.

3/2

Variation in the Cr-2p electron core binding energy as a function of coverage for vapor-deposited Cr on PS and PVA

Gaussian Parameters 533.0 4200 2.25 531.7 700 2.25

Binding Energy (eV)

Figure 4. The 0-7 s XPS spectrum ( ) from PVA following deposition of 0.3 monolayer of Ni. The inner curves (-\-), having the parameters as shown (peak center, height, FWHM), were fit to the data as described in the text. The outer curve (+) is the sum of the inner curves and shows the accuracy of the fit to the experimental data.



20.

BURKSTRAND

343


I n c o m p a r i n g the d i f f e r e n t s e t s o f d a t a , a number o f o v e r a l l d i f f e r e n c e s are s t r i k i n g . F i r s t , t h e d a t a f o r C u , N i and C r on p o l y s t y r e n e were a l l q u i t e r e p r o d u c a b l e , w i t h i n 0 . 2 e V . On the o t h e r h a n d , when t h e s e same m e t a l s were d e p o s i t e d on t h e o x y g e n c o n t a i n i n g p o l y m e r s , t h e s c a t t e r was o f t e n as l a r g e as l . O e V , w i t h the data forming a b r o a d band. S e c o n d , when n i c k e l and c h r o m i u m were d e p o s i t e d on p o l y s t y r e n e , t h e b u l k v a l u e o f t h e 2p core e n e r g y was r e a c h e d when t h e m e t a l c o v e r a g e was w e l l l e s s t h a n one monolayer. H o w e v e r , when t h e s e same m e t a l s were d e p o s i t e d on p o l y v i n y l a l c o h o l , t h e m e t a l c o v e r a g e needed t o be a b o u t one m o n o l a y e r b e f o r e t h e £ u J . k v a l u e was o b t a i n e d . T h i r d , t h e l a r g e s t d i f f e r e n c e i n t h e 2p c o r e b i n d i n g energy from the b u l k v a l u e f o r C r and N i on PS i s a b o u t 0 . 5 e V w h i l e f o r t h e s e m e t a l s on p o l y v i n y l a l c o h o l i t i s as l a r g e as 2 . 2 e V . I n a d d i t i o n t o t h e i n f o r m a t i o n o b t a i n e d from t h e c o r e l i n e shapes o f t h e o v e r l a y e r a t o m s , t h e r e i s much i n f o r m a t i o n t o be d e r i v e d from t h e l i n e s h a p e s o f s u b s t r a t e p o l y m e r a t o m s . For Cu, N i and Cr d e p o s i t e d on p o l y s t y r e n e , t h e r e were no m e a s u r a b l e changes i n the C - l s core l e v e l e x c e p t f o r a d e c r e a s e i n i n t e n s i t y c a u s e d by o v e r l a y e r a t t e n u a t i o n . However, the a d d i t i o n o f these m e t a l atoms to t h e s u r f a c e o f the o x y g e n c o n t a i n i n g p o l y m e r s m o d i f i e d the l i n e s h a p e s o f the c o r e l e v e l s o f the s u r f a c e c a r b o n and o x y g e n a t o m s . On the c l e a n p o l y v i n y l a l c o h o l s u r f a c e , t h e C - l s c o r e l i n e shape c o n s i s t s o f a s i n g l e b r o a d peak c e n t e r e d a t 2 8 5 . 8 e V and t h e 0 - 1 s l i n e s h a p e o f a s i n g l e n a r r o w peak a t 533.OeV b i n d i n g e n e r g y . The c a r b o n l i n e c a n be decomposed i n t o two components ( 2 2 ) , one w i t h a peak a t 285.OeV a s s o c i a t e d w i t h a C H g r o u p and one w i t h a peak a t 2 8 6 . 6 e V a s s o c i a t e d w i t h a HCOH g r o u p . A f t e r C u , N i , o r C r atoms were d e p o s i t e d , t h e o v e r a l l i n t e n s i t y d e c r e a s e d and the lineshape a l t e r e d . T h i s a l t e r a t i o n c a n b e s t be d e s c r i b e d as a d e c r e a s e i n t h e 2 8 6 . 6 e V component r e l a t i v e t o t h a t a t 2 8 5 . O e V . The o v e r a l l i n t e n s i t y d e c r e a s e o f b o t h t h e c a r b o n and o x y g e n l i n e s c a n be a c c o u n t e d f o r by s i m p l e o v e r l a y e r a t t e n u a t i o n . The r e l a t i v e d e c r e a s e o f t h e c a r b o n 2 8 6 . 6 eV component i m p l i e s a m o d i f i c a t i o n o f the o r i g i n a l c a r b o n - o x y g e n b o n d . F o l l o w i n g d e p o s i t i o n o f C u , N i o r C r on p o l y v i n y l a l c o h o l , t h e o x y g e n I s c o r e l e v e l s p e c t r u m was b r o a d e n e d on t h e l o w b i n d i n g e n e r g y s i d e by the a d d i t i o n o f a new p e a k . F i g u r e 4 shows an example o f t h i s s t r u c t u r e f o r coverage o f about 0.3 monolayer o f Ni. I n a l l c a s e s , t h e c u r v e d e c o m p o s i t i o n was made by a n o n l i n e a r l e a s t s q u a r e s f i t t i n g o f t h e d a t a by l i n e s h a p e s e a c h composed o f a G a u s s i a n and a L o r e n t i a n , t h e r e l a t i v e w e i g h t i n g b e i n g 0 . 7 and 0 . 3 respectively. The i n s t r u m e n t a l f u n c t i o n was n o t d e c o n v o l v e d o u t , and the new a d d i t i o n a l l i n e s h a p e was assumed t o h a v e t h e same shape as t h e o r i g i n a l o x y g e n I s s p e c t r u m . W i t h t h e s e a s s u m p t i o n s , t h e m a t h e m a t i c a l u n c e r t a i n t y o f the v a r i o u s component e n e r g y p o s i t i o n s i s about + 0 . 1 eV. I f t h e r e s t r i c t i o n s on l i n e s h a p e s a r e r e d u c e d , t h i s u n c e r t a i n t y becomes somewhat l a r g e r . 2



344


The d a t a c u r v e shown i n F i g . 4 i s c o m p r i s e d o f a l a r g e peak a t 5 3 3 . O e V and one a t 5 3 1 . 7 e V . The h i g h e r b i n d i n g e n e r g y compone n t i s t h a t from t h e o x y g e n atoms i n p o l y v i n y l a l c o h o l w h i c h h a v e n o t b e e n a f f e c t e d by the p r e s e n c e o f N i a t o m s . The new o x y g e n s i g n a t u r e i s s h i f t e d 1.3eV t o l o w e r b i n d i n g e n e r g y . For t h i s c a s e , and i n g e n e r a l , t h e s i z e o f t h i s s h i f t i n b i n d i n g e n e r g y was independent o f metal coverage. The i n t e n s i t y o f t h e new p e a k s s c a l e approximately w i t h coverage for metal coverages l e s s than one m o n o l a y e r . These same r e s u l t s a r e t r u e f o r p o l y v i n y l m e t h y l e t h e r and p o l y e t h y l e n e o x i d e — b o t h p o l y m e r s w i t h s i n g l e b o n d e d oxygen. The a n a l y s i s o f the d a t a from p o l y v i n y l a c e t a t e and p o l y m e t h y l m e t h a c r y l a t e i s n o t as s t r a i g h t f o r w a r d . F i g u r e 5 shows t h e o x y g e n I s s p e c t r u m from c l e a n p o l y m e t h y l m e t h a c r y l a t e w h i c h i s e a s i l y decomposed i n t o two c o m p o n e n t s : a peak a t h i g h e r b i n d i n g e n e r g y f r o m t h e s i n g l e bonded o x y g e n and a peak a t l o w e r b i n d i n g e n e r g y f r o m t h e d o u b l e bonded o x y g e n . When 0 . 6 m o n o l a y e r o f N i i s d e p o s i t e d on t h i s s u r f a c e , t h e o x y g e n I s s p e c t r u m changes t o t h a t shown by t h e s o l i d c u r v e i n F i g . 6 . T h i s c a n be d e c o n v o l v e d i n two w a y s . F i r s t , we c a n assume t h a t t h e o r i g i n a l components a r e f i x e d i n e n e r g y and t h a t o n l y a t h i r d peak need be a d d e d , as i s i l l u s t r a t e d i n F i g . 6. W h i l e t h i s i s a s t r a i g h t f o r w a r d d e c o m p o s i t i o n , we b e l i e v e t h a t b o t h t h e s i n g l e and d o u b l e bonded o x y g e n atoms s h o u l d i n t e r a c t w i t h t h e a d s o r b e d N i a t o m s . Thus t h e r e s h o u l d be a n o t h e r peak " h i d d e n " b e n e a t h t h a t a t 5 3 2 . 2 e V . This f o u r t h peak w o u l d be t h e XPS s i g n a t u r e o f t h e s i n g l e bonded o x y g e n atoms w h i c h i n t e r a c t e d w i t h t h e N i . I f we assume t h a t t h i s peak i s s h i f t e d i n b i n d i n g e n e r g y t h e same as t h o s e s i n g l e bonded o x y g e n atoms i n p o l y v i n y l a l c o h o l and p o l y v i n y l m e t h y l e t h e r ( 1 . 3 e V ) , t h e n t h e r e s u l t i n g c u r v e d e c o m p o s i t i o n i s shown i n F i g . 7 . S i m i l a r p r o c e d u r e s were c a r r i e d out on t h e o t h e r m e t a l / p o l y m e r systems. The changes i n the o x y g e n I s c o r e b i n d i n g e n e r g i e s are summarized b e l o w . F o r p o l y v i n y l a c e t a t e and p o l y m e t h y l m e t h a c r y l a t e , t h e f i r s t v a l u e i s f o r t h e s i n g l e bonded o x y g e n , t h e s e c o n d f o r the d o u b l e bonded o x y g e n .

Polyvinyl

alcohol

P o l y v i n y l methyl

ether

Cu

Ni

Cr

-2.7

-1.3

-1.7

-1.7

-1.3

-1.3

-1.2

-1.3

Polyethylene oxide Polyvinyl Polymethyl

acetate

1.3,

- 1.4

methacrylate

-1.3,

-0.9

-1.3,

-1.0

-1.3,

- 1.5

-1.3,

-1.6

Discussion After

a careful

analysis

o f the

data,

it

is

clear

that


the

20.

BURKSTRAND


345

Gaussian Parameters 533.75 1150 2.25 532.25 1150 2.25


1800

|

538

1

1

536

1

1

534

1

1

532

Binding Energy

Figure 5.

1

1

530

r

528

(eV)

The O-ls XPS spectrum ( ) from clean polymethyl methacrylate. The curves comprised of(-\-) are as described in Figure 4.

Gaussian Parameters 533.75 700 2.25 532.20 900 2.25 531.10 220 2.20

Binding Energy

(eV)

Figure 6. The O-ls XPS spectrum ( ) from polymethyl methacrylate following deposition of 0.6 monolayer of Ni. The curves comprised of (-{-) are as described in Figure 4.



346

PHOTON,

ELECTRON,

AND

ION

PROBES

Gaussian Paramete rs 2.25 680 533.75 2.25 220 532.45 2.25 680 532.20 2.25 250 531.10

538

536 534 532 Binding Energy (eV)

530

528

Figure 7. The O-ls XPS spectrum ( ) from polymethyl methacrylate following deposition of 0.6 monolayer of Ni. The curves comprised of (-{-) are as described in Figure 4.



20.

BURKSTRAND


347

s u b s t r a t e p l a y s a s i g n i f i c a n t r o l e i n determining the p r o p e r t i e s of the metal o v e r l a y e r . This i s not e n t i r e l y s u r p r i s i n g i n l i g h t o f p r e v i o u s work u s i n g m e t a l and s e m i c o n d u c t o r s u b s t r a t e s . When a m e t a l atom f i r s t bonds t o an o x y g e n - c o n t a i n i n g p o l y m e r s u r f a c e , i t i s a b l e t o bond r a t h e r s t r o n g l y w i t h an o x y g e n atom a l r e a d y present, forming a metal-oxygen-polymer c o m p l e x (6). This changes b o t h t h e i n i t i a l e l e c t r o n i c s t a t e o f t h e m e t a l atom as w e l l as t h e a t o m i c and e x t r a - a t o m i c s c r e e n i n g c o n t r i b u t i o n s t o t h e measured p h o t o e m i s s i o n e n e r g y . I t h a s p r e v i o u s l y been p o i n t e d o u t (8, j ) , 11_, _12^ 1 5 , 23) t h a t changes i n s c r e e n i n g c a n a c c o u n t f o r t h e g e n e r a l changes i n b i n d i n g e n e r g y as a f u n c t i o n o f c o v e r a g e s u c h as a r e o b s e r v e d i n F i g u r e s 1-3. A s t h e s i z e o f t h e m e t a l c l u s t e r i n c r e a s e s , t h e amount o f extra-atomic screening o f the core hole l e f t a f t e r photoemission i n c r e a s e s u n t i l the s c r e e n i n g equals that o f the b u l k . The d i f f e r e n c e s b e t w e e n C u , N i a n d C r on p o l y s t y r e n e c a n b e a t t r i b u t e d (23) t o d i f f e r e n c e s i n the m o b i l i t i e s o f the m e t a l atoms. Both t h e N i and C r 2p c o r e l e v e l s r e a c h t h e b u l k v a l u e b e f o r e one m o n o l a y e r , i n d i c a t i n g t h a t they form l a r g e r c l u s t e r s w i t h b u l k s c r e e n i n g v a l u e s more r a p i d l y t h a n does C u . B u t when t h e s e m e t a l s a r e d e p o s i t e d on p o l y v i n y l a l c o h o l , t h e shapes o f t h e c u r v e s change d r a m a t i c a l l y . I n a d d i t i o n , the FWHM f o r N i and C r a l s o c h a n g e . This suggests a r a t h e r d i f f e r e n t s t a t e f o r t h e m e t a l atoms on t h e s u r f a c e i n w h i c h t h e i n i t i a l s t a t e a n d / o r a t o m i c and e x t r a - a t o m i c s c r e e n i n g i s d i f f e r e n t . We c a n o b t a i n a n o t h e r c l u e t o t h e makeup o f t h i s new s t a t e by e x a m i n i n g t h e changes i n t h e s u b s t r a t e c o r e l i n e s h a p e s , F i g u r e 4 . The changes i n t h e c a r b o n ( n o t shown) and t h e o x y g e n l i n e s h a p e s i n d i c a t e t h a t t h e m e t a l atoms have p e r t u r b e d t h e o r i g i n a l C - 0 bonds and formed ( p a r t i a l ) bonds w i t h t h e o x y g e n a t o m s . The r e l a t i v e i n t e n s i t y o f t h e change i s a measure o f t h e s t r e n g t h o f t h e new b o n d s . The c o p p e r , n o t shown, p r o d u c e s t h e s m a l l e s t change i n b o t h t h e c a r b o n and o x y g e n s p e c t r a — t h e Cu c o v e r a g e i s a l m o s t t w i c e as l a r g e as t h a t f o r N i and C r . This i s not u n e x p e c t e d , as c o p p e r does n o t r e a c t as s t r o n g l y w i t h o x y g e n as does n i c k e l and c h r o m i u m ( 2 4 , 2 5 , 2 6 , 2 7 ) . The i n i t i a l s t i c k i n g c o e f f i c i e n t o f o x y g e n o n Cu i s l o w e r t h a n i t i s o n C r o r N i . I n a d d i t i o n , t h e d i s s o c i a t i o n e n e r g y o f c o p p e r o x i d e i s much l o w e r t h a n t h a t o f chromium o r n i c k e l o x i d e . F o r a l l t h r e e m e t a l s , t h e a d d i t i o n a l s t r u c t u r e i n t h e O - l s s p e c t r a l i e s between the b i n d i n g e n e r g y o f t h e o x y g e n i n t h e c l e a n p o l y m e r and t h e a p p r o p r i a t e b u l k m e t a l - o x i d e b i n d i n g energy ( 2 4 , 25) o r the b i n d i n g energy f o r c h e m i s o r b e d o x y g e n (23, 2 6 , 2 7 ) . T h i s i m p l i e s t h a t t h e o x y g e n i s s t i l l p a r t i a l l y bonded t o t h e p o l y m e r , and i n d e e d a m e t a l - p o l y m e r c o m p l e x h a s b e e n formed a t t h e i n t e r f a c e . The e x i s t e n c e o f t h e s e m e t a 1 - o x y g e n - c a r b o n c o m p l e x e s c a n a c c o u n t f o r many o f t h e o b s e r v e d changes a l r e a d y d i s c u s s e d . We e x p e c t t h e m e t a l t o b e more t i g h t l y bonded t o t h e p o l y m e r ( p o l y v i n y l a l c o h o l ) t h r o u g h s u c h a c o m p l e x t h a n i f one was n o t p r e s e n t ,

American Chemical Society Library 1155 16th St. N. w. Washington, D. Structure C. 20038 In Photon, Electron, and Ion Probes of Polymer and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


348


as on c l e a n p o l y s t y r e n e . Indeed, q u a n t i t a t i v e adhesion r e s u l t s c o n f i r m t h i s — c o p p e r , n i c k e l and chromium a d h e r e b e t t e r to p o l y v i n y l a l c o h o l t h a n t o c l e a n p o l y s t y r e n e and b e t t e r t o o x y g e n t r e a t e d p o l y s t y r e n e t h a n t o c l e a n p o l y s t y r e n e (6, 2 8 ) . A s t r o n g e r bond b e t w e e n t h e m e t a l and t h e p o l y m e r w o u l d r e d u c e t h e m i g r a t i o n o f m e t a l atoms on t h e s u r f a c e . T h i s means s m a l l e r c l u s t e r s w o u l d be formed a t any g i v e n c o v e r a g e , i m p l y i n g s m a l l e r r e l a x a t i o n e n e r g i e s and l a r g e r c h e m i c a l s h i f t s w i t h a r e s u l t i n g l a r g e r s h i f t i n c o r e b i n d i n g energy from the b u l k v a l u e , as e v i d e n c e d i n F i g s . 2 - 3 . T h i s w o u l d a l s o a c c o u n t f o r t h e l a r g e r s c a t t e r i n t h e d a t a on p o l y v i n y l a l c o h o l , f o r t h e m e t a l atoms w o u l d t e n d t o s t a y n e a r where t h e y f i r s t c o n t a c t e d t h e s u r f a c e . Thus a s e r i e s o f d i f f e r e n t d e p o s i t i o n s on p o l y v i n y l a l c o h o l c o u l d easily give a series of different r e s u l t s . H o w e v e r , on p o l y s t y r e n e , where t h e b i n d i n g o f t h e m e t a l atoms i s r e l a t i v e l y weak, t h e memory o f t h e a r r i v a l s i t e t e n d s t o be e r a s e d . U n f o r t u n a t e l y , the exact c o n f i g u r a t i o n o f t h i s complex c a n n o t be deduced from t h e e n e r g y v a l u e s o f t h e XPS s t r u c t u r e . There i s s i m p l y n o t enough i n f o r m a t i o n . H o w e v e r , i t i s p o s s i b l e t h a t a v i b r a t i o n a l s p e c t r o s c o p y s u c h as i n f r a r e d , Raman or e l e c t r o n e n e r g y l o s s w o u l d y i e l d the m o l e c u l a r c o n f i g u r a t i o n o f s u c h a polymer-metal-complex. On t h e o t h e r h a n d , t h e XPS d a t a we do have g i v e s u s a c l u e t o t h e a t o m i c makeup o f t h e s e c o m p l e x e s . A t the l o w e s t c o v e r a g e s , t h e 2p c o r e b i n d i n g e n e r g i e s o f N i and C r on p o l y v i n y l a l c o h o l a r e a b o u t t h e same as t h o s e f o r b u l k N i O and C r ^ O ^ , 8 5 4 . 6 e V ( 2 5 ) and 5 7 6 . 6 e V ( 2 6 ) r e s p e c t i v e l y . This suggests that these polymer m e t a l c o m p l e x e s c o n t a i n one o r two m e t a l atoms p e r o x y g e n a t o m . I n a d d i t i o n , t h e v a l u e o f the o x y g e n I s c o r e b i n d i n g e n e r g y i s a l s o a p p r o a c h i n g t h a t f o r t h e s e m e t a l o x i d e s . The o x y g e n I s c o r e s h i f t o b t a i n e d a f t e r d e p o s i t i n g Cu i s , h o w e v e r , l a r g e r t h a n one f i r s t expects. The l o w e r e l e c t r o n e g a t i v i t y o f Cu i m p l i e s t h a t a Cu atom w o u l d t r a n s f e r a s m a l l e r e l e c t r o n i c c h a r g e t h a n N i o r C r , and t h i s w o u l d p r o d u c e a s m a l l e r c o r e s h i f t i n t h e o x y g e n . This i m p l i e s t h a t more Cu atoms a r e i n v o l v e d i n t h e c o m p l e x e s t h a n N i or Cr atoms. T h i s i s c o n f i r m e d by the r e l a t i v e c o v e r a g e d a t a — i t t a k e s a b o u t t w i c e as much Cu t h a n N i o r C r t o p r o d u c e an e q u a l number o f new m e t a l - o x y g e n c o m p l e x e s . I f t h e N i and C r c o m p l e x e s c o n t a i n one m e t a l a t o m , t h e n t h i s s u g g e s t s t h a t t h e Cu c o m p l e x e s on p o l y v i n y l a l c o h o l c o n t a i n two Cu a t o m s . The d a t a f o r the o t h e r o x y g e n - c o n t a i n i n g p o l y m e r s i s a l i t t l e less clear. The s i z e s o f the o x y g e n I s c o r e shifts f o l l o w i n g d e p o s i t i o n o f N i s u g g e s t s t h a t t h e N i atoms interact w i t h the s i n g l e bonded o x y g e n s t o f o r m c o m p l e x e s w h i c h a r e s i m i l a r t o one a n o t h e r and a p p r o x i m a t e l y i n d e p e n d e n t o f the k i n d o f polymer. T h i s i s p r o b a b l y t h e c a s e w i t h Cr a l s o . However, the oxygen I s core s h i f t f o l l o w i n g Cr d e p o s i t i o n i s s m a l l e r f o r p o l y v i n y l m e t h y l e t h e r and p o l y e t h y l e n e o x i d e t h a n i t i s f o r t h e other polymers. T h i s i s n o t t o t a l l y u n r e a s o n a b l e , as t h e o x y g e n i n t h e s e two p o l y m e r s i s bonded b e t w e e n two c a r b o n a t o m s , and one m i g h t e x p e c t t h a t the C r c o u l d n o t i n t e r a c t as s t r o n g l y w i t h i t .


20.

BURKSTRAND


349

I n g e n e r a l t h e n , i t a p p e a r s as i f t h e s e m e t a l - p o l y m e r c o m p l e x e s have a s i m i l a r n a t u r e . The d a t a i n d i c a t e s t h a t t h e m e t a l atoms bond t h r o u g h t h e o x y g e n atoms t o t h e p o l y m e r c h a i n s . The r a t i o o f N i o r C r atoms t o i n t e r a c t e d o x y g e n atoms ( u s i n g peak a r e a s ) i s about 1:2. We s u g g e s t , t h e n , t h a t t h e s i m p l e s t m o d e l f o r t h e s e c o m p l e x e s w h i c h i s c o n s i s t e n t w i t h t h e d a t a and w i t h b u l k m e t a l - p o l y m e r c h e m i s t r y i s one i n w h i c h a one o r two atom c l u s t e r o f m e t a l atoms forms a c h e l a t e (2^9) s t r u c t u r e w i t h t h e polymer. I n p o l y v i n y l a l c o h o l t h i s w o u l d have t h e f o r m o f a m e t a l atom o r c l u s t e r " c r o s s l i n k i n g adjacent polymer chains through f o u r oxygen atoms. On t h e o t h e r p o l y m e r s , t h e a r r a n g e m e n t o f t h e atoms i n t h e c o m p l e x e s w o u l d be s i m i l a r , b u t n o t as s i m p l e ; e s p e c i a l l y i n t h e c a s e s where b o t h s i n g l e and d o u b l e bonded o x y g e n atoms a r e p r e s e n t i n t h e p e n d a n t g r o u p s .


1 1

Abstract The interfaces formed by evaporating copper, nickel and chromium layers on polystyrene, polyvinyl alcohol, polyethylene oxide, polyvinyl methyl ether, polyvinyl acetate and polymethyl methacrylate have been studied with X-ray photoemission spectroscopy (XPS). At submonolayer coverages of the metals, the peak positions and widths of the metallic electron core levels vary significantly from one polymer substrate to another. Most of these variations can be accounted for in terms of changes in the atomic and extra-atomic relaxation energies during the photoemission process. Much of this change is brought about when the metal atom deposited on an oxygen containing polymer interacts with the substrate oxygen and forms a metal-oxygen-polymer complex. The presence of this complex is verified by changes in the photoemission lineshapes of the substrate carbon and oxygen atoms. The XPS signature of these various complexes are quite similar and suggest that they are chelate-like complexes. Literature Cited 1.

Mittal, K . L . , J . Vac. Sci. Technol., 1976, 13, 19 and references therein.

2.

Thayer, D.W. and Wedel, R., (unpublished).

3.

Hall, J.R., Westerduhl, C.A., Devine, A.T. and Bodnar, M.J., J . Appl. Polym. S c i . , 1969, 13, 2085.

4.

Hall, J.R., Westerduhl, C.A., Devine, A.T. and Bodnar, M.J., J . Appl. Polym. S c i . , 1972, 16, 1465.

5.

Burkstrand, J.M., J. Vac. Sci. Technol., 1978, 15 223.



350

PHOTON, ELECTRON, AND ION PROBES

6.

Burkstrand, J.M., Appl. Phys. Lett., 1978, 33, 387.

7.

Kiang, K.S., Salaneck, W.R. and Aksay, I.A., Solid State Commun., 1976, 19, 329.

8.

Ascarelli, P., Cini, M., Missoni, G. and Nistico, N . , J . Phys., (Paris) Colluque, 1977, 38, 125.

9.

Takasu, Y . , Unwin, R., Tesche, B., Bradshaw, A. and Grunze, M., Surface Science, 1978, 77, 219.

10.

Tibbetts, G.G. and Egelhoff, W.F., J r . , Phys. Rev. Lett., 1978, 41, 188.

11.

Tibbetts, G.G. and Egelhoff, W.F., J r . , (a) J . Vac. Sci. Technol., (in press); (b) Solid State Commun., (in press); (c) Phys. Rev., (to be published).

12. Mason, M.G., Gerenser, L.G. and Lee, S.T., Phys. Rev. 1977, 39, 288.

Lett.,

13.

Kim, K.S. and Winograd, N . , Chem. Phys. Lett., 1975, 30, 91.

14.

Mason, M.G., Baetzold, R.C., J. Chem. Phys., 1976, 64, 271.

15.

Burkstrand, J.M., J . Vac. Sci. Technol., 1978, 15, 658; Surface Science, 1978, 78 513.

16.

Physical Electronics Industries, Eden Prairie, Minn.

17.

Scofield, J . H . , Lawrence Livermore Laboratory Report No. UCRL 51326.

18.

Tracy, J.C. and Burkstrand, J.M., CRC Crit. Review, Solid State S c i . , 1974, 4, 381.

19. Evans, S., J . Phys. C., Solid State Phys., 1977, 10 2483; Clark, D.T., Thomas, H.R. and Shuttleworth, D., J. Polym. Sci. Polym. Letters Ed., 1978, 16, 465. 20.

See for example: Joshi, A., Davis, L.E. and Palmberg, P.W., Czanderna, A.W. Ed. "Methods of Surface Analysis," Elsevier Scientific Publishing Company, Amsterdam, 1975, p. 159.

21.

Clark, D.T. and Thomas, H.R. J. Polym. Sci. Chem. Ed., 1976, 14, 1671.

22.

Clark, D.T. and Thomas, H.R., J. Polym. Sci. Chem. Ed., 1978, 16, 791.


20. BURKSTRAND 23.


351

Burkstrand, J.M., J. Appl. Phys., 1979, 50, 1152.

24. Haber, J., Machej, T., Ungier, L. and Ziolkowski, J., Journal of Solid State Chemistry, 1978, 25, 207. 25.

Fleisch, T., Winograd, N. and Delgass, W., Surf. S c i . , 1978, 78, 141.


26. Allen, G.C., Tucker, P.M. and Wild, R.K., Faraday Trans. II., 1978, 74, 1126. 27.

Tibbetts, G.G., Burkstrand, J.M. and Tracy, J . C . , Phys. Rev. B . , 1977, 15, 3652.

28.

Burkstrand, J.M., to be published.

29.

For the bulk analog, see Tsuchida, E. and Nishide, H . , "Advances in Polymer Sciences," Vol. 24, Springer-Verlag, New York, 1977, p. 1.

RECEIVED

January 21, 1981.


Photon, Electron, and Ion Probes of Polymer Structure and Properties

Recommend Documents