AN ELECTRON ENERGY LOSS SPECTRAL LIBRARY AND ITS APPLICATION TO MATERIALS SCIENCE

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AN ELECTRON ENERGY LOSS SPECTRAL

LIBRARY AND ITS APPLICATION TO MATERIALS SCIENCE

N. Zaluzec

To cite this version:

N. Zaluzec. AN ELECTRON ENERGY LOSS SPECTRAL LIBRARY AND ITS APPLICATION TO MATERIALS SCIENCE. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-429-C2-432.

�10.1051/jphyscol:1984297�. �jpa-00224012�

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JOURNAL DE PHYSIQUE

Colloque C2, supplgment a u n02, Tome 45, fgvrier 1984 page C2-429

AN ELECTRON ENERGY LOSS SPECTRAL LIBRARY AND ITS APPLICATION TO M A T E R I A L S sc I ENCE*

N . J . Zaluzec

Materials Science and Technology Division, Argonne NationaZ Luboratory, Argonne, I2 Zinois 60439, U.S.A.

Rdsumd - On p r d s e n t e un c a t a l o g u e de donndes s p e c t r a l e s en p e r t e s d ' d n e r g i e d e s E l e c t r o n s pour d e s niveaux profonds. Des a p p l i c a t i o n s 1 des p r o b l h e s t y p i q u e s en s c i e n c e d e s matdriaux sont montrdes.

A b s t r a c t - The format a s w e l l a s t h e m o t i v a t i o n behind t h e development of an EELS l i b r a r y of inner-shell edge p r o f i l e s i s o u t l i n e d and i t s applica- t i o n t o t y p i c a l problems i n m a t e r i a l s r e s e a r c h d i s c u s s e d .

I - INTRODUCTION

The use of e l e c t r o n energy l o s s spectroscopy (EELS) i n c o n j u n c t i o n w i t h a t r a n s m i s s i o n e l e c t r o n microscope (TEM) i s becoming an important technique f o r m a t e r i a l s c h a r a c t e r i z a t i o n . Unlike i t s x-ray energy d i s p e r s i v e spectroscopy c o u n t e r p a r t (XEDS), t h e measured i n t e n s i t y p r o f i l e s can be extremely complex and t h u s t a b l e s of edge e n e r g i e s a r e n o t always s u f f i c i e n t t o uniquely i d e n t i f y a l l e l e m e n t a l s p e c i e s . A s such i t becomes r e l e v a n t t o know t h e s y s t e m a t i c changes i n t h e p r o f i l e s f o r core-loss edges a c r o s s t h e p e r i o d i c t a b l e a s w e l l a s i n v a r i o u s phases o r compounds. It has now been two y e a r s s i n c e t h e f i r s t experimental l i b r a r y of t h i s s o r t has been d i s t r i b u t e d /1,2/ and i t has proven i n v a l u a b l e t o t h e r o u t i n e use of EELS i n m a t e r i a l s c i e n c e . I n complex multi-element systems a simple a t l a s of edge shapes /3/ i s f r e q u e n t l y i n s u f f i c i e n t , s i n c e i n e v i t a b l y , s p e c t r a l o v e r l a p occurs obscuring o r complicating t h e d e s i r e d i n f o r m a t i o n . I n t h e s e s i t u a t i o n s , t h e use of l i b r a r y s p e c t r a f o r t h e purpose of i d e n t i f i c a t i o n and removal of s p e c t r a l o v e r l a p s can provide t h e a n a l y s t w i t h s e m i - q u a n t i t a t i v e r e s u l t s i n s i t u a t i o n s which would have been p r e v i o u s l y considered i n s o l u b l e . I1 - FORMAT OF THE LIBRARY

D i s t r i b u t i o n of t h i s l i b r a r y i s c u r r e n t l y i n two forms. The f i r s t a s i n t e n s i t y p r o f i l e s and t h e second a s ASCII d a t a f i l e s on a p p r o p r i a t e mass s t o r a g e d e v i c e s

( u s u a l l y floppy d i s k ) f o r use w i t h compatible computer systems, both of which a r e b r i e f l y o u t l i n e d below.

Arranged a s a f u n c t i o n of atomic number, a t y p i c a l e n t r y from hardcopy format of t h e second e d i t i o n of t h i s l i b r a r y c o n t a i n s t h e following i n f o r m a t i o n . At t h e t o p of each page i s p s h o r t d e s c r i p t i o n of t h e specimen, method of p r e p a r a t i o n , and experimental c o n d i t i o n s employed d u r i n g t h e measurement ( i . e . a c c e l e r a t i n g v o l t a g e , r e s o l u t i o n , beam convergence, s c a t t e r i n g a n g l e , r e l a t i v e specimen t h i c k n e s s , e t c . ) , followed by f o u r s p e c t r a . The f i r s t spectrum i s t h e low l o s s p r o f i l e , while t h e second i s t h e high l o s s region surrounding t h e c h a r a c t e r i s t i c edge; both t h e s e a r e unprocessed experimental d a t a . The t h i r d spectrum p r e s e n t s t h e edge p r o f i l e a f t e r background s u b t r a c t i o n . Here e i t h e r a conventional A . E - ~ . polynominal, o r log-polynominal background model (which e v e r i s most a p p r o p r i a t e ) i s used t o remove t h e continuous background under t h e edge, while t h e l a s t p r o f i l e shows t h e edge a f t e r removal of m u l t i p l e s c a t t e r i n g by deconvolution.

*work supported by t h e U.S. Department of Energy

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984297

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C2-430 JOURNAL DE PHYSIQUE

T h i s f i n a l spectrum s h o u l d , t h e r e f o r e , r e p r e s e n t t h e s i n g l e s c a t t e r i n g p r o f i l e w i t h t h i c k n e s s e f f e c t s m a t h e m a t i c a l l y removed. When s u f f i c i e n t d a t a e x i s t s about t h e specimen ( i . e . i t s a c t u a l composition and t h i c k n e s s ) t h e p r o f i l e s a r e

c o n v e r t e d i n t o a b s o l u t e c r o s s - s e c t i o n measurements, o t h e r w i s e t h e d a t a i s simply p r e s e n t e d a s c o u n t s / c h a n n e l . Each edge, when f e a s i b l e , i s measured under a t l e a s t two s c a t t e r i n g a n g l e s . The f i r s t a v a l u e c o r r e s p o n d i n g t o a n a n g l e 0 2

E0/(2 EC) which y i e l d s a n o p t i m a l peaklbackground r a t i o , and i s u s u a l l y w i t h i n t h e d i p o l e approximation. Here E0 i s t h e r e l a t i v i s t i c e l e c t r o n energy

[ 1 / 2 ( m c ~ ) ~ ] and EC t h e c r i t i c a l e x c i t a t i o n energy of t h e r e s p e c t i v e s h e l l . The second i s a t a very l a r g e s c a t t e r i n g a n g l e -50-100 m r t o a l l o w non-dipole t r a n s i t i o n s t o be v i s i b l e . In a d d i t i o n , a l l i n t e n s i t y p r o f i l e s a r e d i s t r i b u t e d i n an unbound ( l o o s e - l e a f ) format s o t h a t t h e u s e r s may simply update t h e i r c o p i e s by t h e a d d i t i o n of i n d i v i d u a l pages r a t h e r t h a n having m u l t i p l e bound c o p i e s of u n c o r r e l a t e d d a t a . T h i s was deemed t h e most a p p r o p r i a t e format s i n c e such a l i b r a r y can i n r e a l i t y never be complete and o t h e r e x p e r i m e n t a l i s t s may wish t o c o n t r i b u t e t o t h e scope of t h i s work a s a b e n e f i t t o t h e g e n e r a l s c i e n t i f i c community. The E l e c t r o n Microscopy C e n t e r of t h e M a t e r i a l s Science and Technology D i v i s i o n a t Argonne N a t i o n a l Laboratory has e s t a b l i s h e d an EELS r e f e r e n c e d a t a base f o r t h i s purpose and a l l t h o s e i n t e r e s t e d i n c o n t r i b u t i n g t o t h i s l i b r a r y a r e welcome. Copies of t h e complete EELS l i b r a r y a r e a v a i l a b l e w i t h o u t charge by submission of a w r i t t e n r e q u e s t . F i g u r e 1 is a b r i e f s e l e c t i o n of edge p r o f i l e s ( a f t e r background s u b t r a c t i o n ) f o r t h e most f r e q u e n t l y observed edges i n m a t e r i a l s a n a l y s i s from Lithium (2=3) t h r o u g h S i l v e r (Z=47).

At p r e s e n t a l i m i t e d d i s t r i b u t i o n of t h e l i b r a r y i s a l s o being explored u s i n g magnetic mass s t o r a g e . The major l i m i t i n g f a c t o r h e r e , i s t h e wide v a r i e t y of d a t a f o r m a t s p r e s e n t on t h e computer d a t a s t o r a g e d i s k e t t e s being used by t h e XEDSIEELS M u l t i c h a n n e l Analyzer m a n u f a c t u r e r s . These c o p i e s are being d i s t r i b u t e d t o g e t h e r w i t h a g e n e r i c t r a n s l a t i o n program which allows t h e d a t a f i l e s t o be converted i n t o t h e d i f f e r e n t f o r m a t s used on t h e i n d i v i d u a l microcomputers. When t h e d e t a i l s of t h i s mode of d i s t r i b u t i o n have been

f i n a l i z e d a l l t h o s e on t h e m a i l i n g l i s t w i l l be n o t i f i e d . I n t h i s mode, however, i t w i l l be n e c e s s a r y f o r t h e i n d i v i d u a l u s e r t o s u p p l y t h e i r own mass s t o r a g e media, c o p i e s w i l l s t i l l be provided g r a t i s .

I11 - APPLICATION TO MATERIALS

Aside from t h e u s e of a n EELS l i b r a r y a s a s i m p l e a t l a s of s p e c t r a l shapes, t h e most i m p o r t a n t a s p e c t of t h i s s t u d y i s t h e a b i l i t y t o provide a d i r e c t measure- ment of p a r t i a l i o n i z a t i o n c r o s s - s e c t i o n s f o r t h e v a r i o u s e l e m e n t s s t u d i e d . T h i s i s p a r t i c u l a r l y important f o r q u a n t i t a t i v e measurements s i n c e t h e hydrogenic models developed by Egerton /4,5/ a l l o w s o n l y a f i n i t e range of e l e m e n t s t o be a n a l y z e d . P o t e n t i a l l y t h e d a t a b a s e of c r o s s - s e c t i o n v a l u e s being developed can be used i n t h e refinement of p a r a m e t r i c e q u a t i o n s used f o r q u a n t i f i c a t i o n . T h i s i s p a r t i c u l a r l y t r u e f o r t h e c a s e of h i g h e r Z m a t e r i a l s where v i r t u a l l y no s i m p l e f o r m u l a t i o n s e x i s t f o r L, M, o r N s h e l l t r a n s i t i o n s 161.

A second and e q u a l l y i m p o r t a n t a p p l i c a t i o n of t h i s l i b r a r y i n v o l v e s t h e d i r e c t a p p l i c a t i o n of r e f e r e n c e s p e c t r a f o r d a t a r e d u c t i o n purposes, which presupposes a c c e s s of d a t a f i l e s through an a p p r o p r i a t e computer based d a t a a n a l y s i s system. The p r e s e n t system i n u s e a t ANL c o n s i s t s of a DEC LSI 1112 u s i n g t h e Version 4.0 RT-11 o p e r a t i n g system, a T e k t r o n i c s 4010 g r a p h i c s t e r m i n a l and RX02 f l o p p y d i s k mass s t o r a g e . I n a d d i t i o n , an EDAX SW9100170 o r a n ORTEC EEDSII m u l t i c h a n n e l a n a l y z e r s b o t h having DEC hardware a r e u t i l i z e d a s a p p r o p r i a t e .

The s t u d y of l i g h t element d i s t r i b u t i o n s i n p r e c i p i t a t e s of s t e e l i s a good example of t h e problems encountered d a r i n g a " r e a l " a p p l i c a t i o n of EELS t o m a t e r i a l s r e s e a r c h . Shown i n f i g u r e 2 a r e p a r t i a l s p e c t r a from p r e c i p i t a t e s formed upon thermal a g i n g of t y p e 316 S t a i n l e s s S t e e l 171. These s p e c t r a were o b t a i n e d from p r e c i p i t a t e s i d e n t i f i e d a s MC, M23C6, and Laves phases. XEDS a n a l y s i s of each r o u t i n e l y y i e l d s t h e r e l a t i v e composition of a l l s p e c i e s w i t h

Z > 11, however t h e low Z compositions were s t i l l unknown. EELS i n t h e c a s e of

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t h e MC and t h e M23C6 phases c l e a r l y i d e n t i f i e s t h e p r e s e n c e of a C a r b i d e , s i n c e i t i s e a s y t o show, t h a t t h e Carbon measured h e r e i s n o t amorphous by t h e p r e s e n c e of t h e near-edge f i n e s t r u c t u r e c l e a r l y v i s i b l e a t t h e Carbon K s h e l l o n s e t (-283 eV). Amorphous carbon i s n e a r l y f e a t u r e l e s s / 2 , 3 / w i t h t h e e x c e p t i o n of a weak t r a n s i t i o n a t t h e edge o n s e t . The d i f f i c u l t y i n t h e s e a n a l y s e s a r i s e s due t o t h e p r e s e n c e of h i g h atomic number a l l o y i n g e l e m e n t s , i n t h i s c a s e Molybdenum. U n f o r t u n a t e l y f o r t h e a n a l y s t , MO has an M-shell edge i n t h e energy l o s s r e g i o n between 200 and 800 eV l o s s . As a n added c o m p l i c a t i o n , t h e shape of t h i s edge i s v e r y extended having t h e c l a s s i c a l d e l a y e d maximum / 2 , 6 / and a few r e l a t i v e l y s h a r p b u t small M32 t r a n s i t i o n s i n t h e v i c i n i t y of 400 eV l o s s . A p a r t i a l EELS spectrum from nominally p u r e MO i s shown a t t h e bottom of f i g u r e 2. Although one can e a s i l y i d e n t i f y t h e f i r s t two s p e c t r a as Ti-based and Cr- based C a r b i d e s , q u a n t i f i c a t i o n r e q u i r e s measurement of t h e n e t i n t e g r a l s of e a c h e l e m e n t ' s edge p r o f i l e over a s u b s t a n t i a l energy window (-100 eV). C l e a r l y , f o r b o t h t h e s e c a s e s t h e s e v e r e o v e r l a p between t h e MO M-shell and t h e C K-shell would p r e c l u d e a n a l y s i s . By r e c a l l i n g , from a l i b r a r y f i l e , t h e edge p r o f i l e from pure MO, s u b t r a c t i n g i t s background t h e n s c a l i n g t h e r e s u l t a n t t o t h e M s h e l l o n s e t j u s t v i s i b l e b e f o r e t h e C K-shell and f i n a l l y s u b t r a c t i n g t h i s s c a l e d MO M-shell p r o f i l e from t h e c a r b i d e s p e c t r a a l l o w s t h e a n a l y s t t o o b t a i n t h e n e t carbon i n t e g r a l . It t h e n becomes p o s s i b l e t o proceed w i t h t h e a n a l y s i s a t l e a s t on a s e m i - q u a n t i t a t i v e b a s i s . This procedure presumes t h a t t h e e f f e c t s of chemical s h i f t s between t h e d i f f e r e n t bonding s t a t e s of MO a r e n e g l i g i b l e , t h a t b o t h s p e c t r a a r e o b t a i n e d from s u f f i c i e n t l y t h i n r e g i o n s t h a t m u l t i p l e s c a t t e r i n g e f f e c t s a r e minimal, and t h a t t h e nominal e x p e r i m e n t a l c o n d i t i o n s a r e s i m i l a r . The d e t a i l s of t h e s e assumptions a r e c u r r e n t l y under i n v e s t i g a t i o n ; however, even i n l i g h t of t h e s e approximations it s t i l l a l l o w s t h e a n a l y s t t o proceed w i t h a t l e a s t a s e m i - q u a n t i t a t i v e a n a l y s i s . An even more complex s i t u a t i o n e x i s t s f o r t h e MO-rich Laves phase. One would c e r t a i n l y be remiss i n a t t e m p t i n g t o q u a n t i f y t h e carbon c o n t e n t of t h i s phase a t t h i s time due t o t h e n e c e s s i t y of s u b t r a c t i n g two l a r g e q u a n t i t i e s and a n a l y z i n g t h e s m a l l r e s i d u a l . It i s p e r t i n e n t , however, t o p o i n t o u t t h a t i n t h i s s i t u a t i o n , t h e w e l l r e s o l v e d edges n e a r 400 eV c a n n o t be simply a t t r i b u t e d t o t h e Mg2 t r a n s i t i o n s of Molybdenum, and one concludes t h a t a n a d d i t i o n a l element, i n t h i s c a s e N i t r o g e n , i s p r e s e n t . Such a r e s u l t would have been i m p o s s i b l e u s i n g a XEDS system, even of t h e Windowless v a r i e t y , due t o t h e i r r e l a t i v e l y poor r e s o l u t i o n i n t h i s energy regime.

I V - CONCLUDING REMARKS

An e l e c t r o n energy l o s s s p e c t r a l l i b r a r y can be an i n v a l u a b l e t o o l i n m a t e r i a l s r e s e a r c h from a fundamental as w e l l a s a p r a c t i c a l s t a n d p o i n t . Although i t w i l l n o t a l l e v i a t e a l l t h e c o m p l i c a t i o n s a s s o c i a t e d w i t h q u a n t i f i c a t i o n , t h i s t y p e of l i b r a r y can h e l p t o e l u c i d a t e d e t a i l s of s p e c t r a l p r o f i l e s p r e v i o u s l y found i n t r a c t a b l e . T h i s work was s u p p o r t e d by t h e U. S. Department of Energy. The a u t h o r a l s o wishes t o e x p r e s s h i s g r a t i t u d e t o t h e o r g a n i z i n g committee f o r p a r t i a l f i n a n c i a l s u p p o r t provided t o a t t e n d t h i s meeting.

V - REFERENCES

1. ZALUZEC N. J., A n a l y t i c a l E l e c t r o n Micrsocopy, 1981, ed. R. H. G e i s s , San F r a n c i s c o P r e s s , Proc. of Workshop on AEM, V a i l Colorado, U.S.A. ( J u l y 1981) 193.

2. ZALUZEC N. J., Ultramicroscopy 9 (1982) 319.

3. AHN C. C. and KKIVANEK 0. L., Proc. of t h e 4 0 t h EMSA, Washington, D.C., ed. G * W. B a i l e y , C l a i t o r s P u b l i s h i n g , (August 1982) 738.

4. EGERTON R. F., Ultramicroscopy 4 (1979) 169.

5. EGERTON R. F., Proc. of 3 9 t h EMSA Meeting, A t l a n t a , GA, e d . G. W. B a i l e y , C l a i t o r s P u b l i s h i n g , Baton Rouge, LA (August 1981) 198.

6. REZ P. and LEAPMAN R.D., A n a l y t i c a l E l e c t r o n Microscopy 1981, e d . R. H. G e i s s , proc. of t h e AEM Workshop, V a i l , Colorado (July 1981) 1981.

7. MAZIASZ P. J. and ZALUZEC N. J . , Proc. of 4 0 t h EMSA, o p - c i t . , 498, a d d i t i o n a l

r e s e a r c h i n p r o g r e s s .

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JOURNAL DE PHYSIQUE

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F i g u r e 1. A s e l e c t i o n of EELS edge p r o f i l e s a f t e r background s u b r a c t i o n f o r t h e e l e m e n t s L i t h r o u g h P (K s h e l l ) ; Sc t h r o u g h Ge (L s h e l l ) ; and Y t h r o u g h Ag (M s h e l l ) . Energy s c a l e s of 5, 10, and 20 e v l d i v i s i o n a s i n d i - c a t e d . S p e c t r a a r e a r b i t r a r i l y d i s p l a c e d f o r d i s p l a y purposes.

F i g u r e 2. P a r t i a l EELS s p e c t r a a f t e r back- ground s u b t r a c t i o n f o r a Ti-based (MC) c a r b i d e , Cr-based (M23~6) p r e c i p i t a t e and a Mo-rich Laves phase i n t h e r m a l l y aged t y p e 316 S t a i n l e s s S t e e l . For comparison, a p a r t i a l spectrum from pure Molybdenum showing t h e M-shell t r a n s i t i o n which o c c u r s over t h e same energy regime i s a l s o shown. Note t h a t t h e p r e s e n c e of t h e MO M-shell i n t h e MC, M23C6, and Laves phases p r e c l u d e s q u a n t i - f i c a t i o n w i t h o u t f i r s t removing t h e MO s p e c t r a l o v e r l a p .