{111} {100} Seed crystal a) Ni b) Co c) Fe d) Transmission 200 μ m Fig. 1. Schematic drawing and X-ray images of a synthetic diamond grown at high pressure and high temperature utilizing a Fe 55 Ni 29 Co 16 solvent. a) Ni, b) Co, c) Fe and d) transmission images. Diamond is a unique substance possessing a h i g h t h e r m a l c o n d u c t i v i t y , h a r d n e s s a n d e l e c t r i c re si st iv it y. Th e ap pl ic at io ns of di am on d, va lu ab le a s h a r d m a t e r i a l s , a r e e x p a n d i n g i n t o e l e c t r o n i c s a n d o p t i c s . D i a m o n d c a n b e s y n t h e s i z e d u s i n g either high pressure techniques or chemical vapor deposition ( CVD ) techniques. To produce a single l a r g e d i a m o n d c r y s t a l , h o w e v e r , c r y s t a l l i z a t i o n s h o u l d o c c u r i n a m e t a l l i c s o l v e n t a t a h i g h t e m p e r a t u r e a n d h i g h p r e s s u r e . T h e m o s t co mm on ly us ed me ta ll ic so lv en ts ar e Cr , Mn , Fe , Co, Ni, and Cu or a combination. Although many studies have examined the phase diagrams of Fe, N i a n d C o w i t h c a r b o n a t h i g h p r e s s u r e s , t h e mechanism of diamond synthesis and the nature of the resulting synthesized diamond remain unclear. We utilized synchrotron radiation-excited X-ray f l u o r e s c e n c e ( X R F ) a n a l y s i s t o a n a l y z e t r a c e impurities in synthetic diamonds. XRF analysis is MICRO XAFS CHARACTERIZATION OF TRACE NICKEL DISSOLVED INTO SYNTHETIC DIAMONDS the most suitable of various analytical methods to c h a r a c t e r i z e m e t a l l i c i m p u r i t i e s d u e t o t h e s t r o n g e l e m e n t a l s e l e c t i v i t y , h i g h s e n s i t i v i t y a n d l a r g e c a p a c i t y f o r s p a t i a l r e s o l u t i o n , a s c o m p a r e d t o o p t i c a l a b s o r p t i o n o r e l e c t r o n s p i n r e s o n a n c e ( E S R ) . T o i n v e s t i g a t e t h e d i s t r i b u t i o n o f t r a c e i m p u r i t i e s i n d i a m o n d s g r o w n i n a F e 5 5 N i 2 9 C o 1 6 solvent, we utilized an X-ray microprobe system at beamline BL39XU [1] with a spatial resolution of 10 μ m . F i g u r e 1 d i s p l a y s t h e N i , C o a n d F e X R F i m a g e s a n d t h e t r a n s m i t t e d X - r a y i m a g e o f t h e {11 0} dia mon d waf er of a 278 mm thi ckn ess . As p r e v i o u s l y r e p o r t e d [ 2 ] , N i a n d C o p r e f e r t o b e d i s s o l v e d i n t o { 1 1 1 } g r o w t h s e c t o r s ; t h e c o n c e n t r a t i o n s of these metal s are below detec tion l i m i t s i n { 1 0 0 } g r o w t h s e c t o r s . F e c a n n o t b e d e t e c t e d in either the { 1 1 1 } o r t h e { 1 0 0 } g r o w t h s e c t o r s . T h e N i c o n c e n t r a t i o n is approximately 30 ppm. The Co concentration is an order of magnitude lower than that of Ni. Fig. 3. EXAFS oscillations χ (k) of the dissolved Ni within the diamond. Wavenumber (k) -10 -5 0 5 10 10 8 6 4 χ χ (k) k 3 F i g . 2 . N i K X A F S s p e c t r u m o b t a i n e d f r o m d i s s o l v e d N i w i t h i n t h e d i a m o n d u s i n g t h e X R F y i e l d m e t h o d . T h e N i c o n c e n t r a t i o n i s a p p r o x i m a t e l y 3 0 w e i g h t p p m ( 6 a t o m i c p p m ) in the {111} growth sector. 0.20 0.15 0.10 0.05 8800 8600 8400 8200 Photon energy ( eV ) Intensity (a. u.) T h e c o m b i n a t i o n o f t h e b r i l l i a n t u n d u l a t o r radiation from SPring-8 and the gap tuning method h a s m a d e p o s s i b l e a d i s c u s s i o n o f t h e l o c a l structure of trace Ni and Co from X-ray absorption f i n e s t r u c t u r e ( X A F S ) s p e c t r a . T h e n e a r e d g e spectra of both Co and Ni display a characteristic pre-edge feature and these are completely different from the spectra of metallic Ni and Co which can be o b s e r v e d w h e r e i n c l u s i o n s e x i s t [ 3 ] . F i g u r e 2 shows the XAFS spectrum of Ni in the {111} growth sector. To obtain greater sensitivity, we utilized a b e a m s i z e o f 1 5 0 μ m f o r t h e s e m e a s u r e m e n t s . Ut il iz in g th is be am si ze an d Ni co nc en tr at io n, on ly 4 × 10 12 atoms of Ni in the diamond contributed to t h e X A F S s p e c t r u m . F i g u r e 3 d e m o n s t r a t e s t h e extracted EXAFS (Extended X- ray Absorption Fine S t r u c t u r e ) . F o u r i e r t r a n s f o r m a t i o n o f t h e E X A F S oscillation possesses two peaks resulting from the dissolved Ni ( Fig. 4 ). One peak, consistent with the m o d e l d e r i v e d f r o m t h e p r e - e d g e p e a k , c o r r e s p o n d s t o N i i n t h e t e t r a h e d r a l s i t e o f t h e d i a m o n d l a t t i c e . T h e o t h e r p e a k i n d i c a t e s t h e existence of Ni-Ni bond. This result suggests the presence of an extremely small metallic cluster of N i w i t h i n t h e d i a m o n d ; f u r t h e r i n v e s t i g a t i o n s a r e now in progress. Radial distance (Å) Intensity ( arb. units) 0 2 4 6 8 0 2 4 6 8 F i g . 4 . F o u r i e r t r a n s f o r m a t i o n o f E X A F S oscillations around Ni atoms within the diamond. Shinjiro Hayakawa a and Masao Wakatsuki b ( a ) H i r o s h i m a U n i v e r s i t y ( b ) N a t i o n a l I n s t i t u t e o f M a t e r i a l s a n d C h e m i c a l R e s e a r c h E-mail: hayakawa @ hiroshiam-u.ac.jp References [1 ] S. Ha ya ka wa et al . , Sp ec tr oc he m. Ac ta B 54 (1999) 171. [2] X. Jia et al. , Proc. of 4 th ICNDST (1994) 525. [ 3 ] S . H a y a k a w a , X . - P . J i a , M . W a k a t s u k i , Y . Gohshi and T. Hirokawa, J. Crystal. Growth 210 (2000) 388.
home
- JP
- EN