PHYSICAL REVIEW B 66,075426 (2002) Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation C.V.Falub Interfaculty Reactor Institute,Delft University of Technology,Mekelweg 15,NL-2629 JB Delft,The Netherlands P.E.Mijnarends Interfaculty Reactor Institute,Delft University of Technology,Mekelweg 15,NL-2629 JB Delft,The Netherlands and Department of Physics,Northeastern University,Boston,Massachusetts 02115 S.W.H.Eijt,M.A.van Huis,A.van Veen,and H.Schut Interfaculty Reactor Institute,Delft University of Technology,Mekelweg 15,NL-2629 JB Delfi,The Netherlands (Received 26 March 2002;published 30 August 2002)
A 2D-ACAR setup coupled to the intense (8*107e+/s) monochromatic slow e+beam POSH (POSitrons from the HOR reactor)is used for depth-selective research. The Anger cameras,used to detect the 511-keV y radiation stemming from the annihilation of positrons in the sample, consist of 41.8-cm-diameter 1.25-cm thick NaI crystal scintillators,optically coupled to a close packed honeycomb array of 61 photomultipliers.A detector- detector distance of ~23 m provides an angular view of ~51*51 mrad2 (1 mrad is equivalent to 0.137 momentum a.u.)in a 256*256 pixel matrix
A 2 D - A C A R s e t u p c o u p l e d t o t h e i n t e n s e ( 8 * 1 0 7 e + / s ) m o n o c h r o m a t i c s l o w e + b e a m P O S H ( P O S i t r o n s fr o m t h e H O R r e a c t o r ) i s u s e d fo r d e p t h - s e l e c t i v e r e s e a r c h. T h e A n g e r c a m e r a s, u s e d t o d e t e c t t h e 5 1 1 - k e V γ r a d i a t i o n s t e m m i n g fr o m t h e a n n i h i l a t i o n o f p o s i t r o n s i n t h e s a m p l e, c o n s i s t o f 4 1.8 - c m - d i a m e t e r 1.2 5 - c m t h i c k N a I c r y s t a l s c i n t i l l a t o r s, o p t i c a l l y c o u p l e d t o a c l o s e p a c k e d h o n e y c o mb a r r a y o f 6 1 p h o t o m u l t i p l i e r s. A d e t e c t o r - d e t e c t o r d i s t a n c e o f ~ 2 3 m p r o v i d e s a n a n g u l a r v i e w o f ~ 5 1 * 5 1 mr a d 2 ( 1 mr a d i s e q u i v a l e n t t o 0.1 3 7 m o m e n t u m a.u.) i n a 2 5 6 * 2 5 6 p i x e l m a t r i x
depth(nm) 50100 300 500 1000 0.66 0 0一 MgO(100):Li -VEP 86 一。一 MgO(110):Li -VEP --MgO(110):Li-POSH 0-58 0.54 0.52 0.50 0.48 0.46 2D-ACAR 2D-ACAR 2D-ACAR 0.44 5 10 15 20 25 positron energy (keV) FIG.2.S parameter vs positron implantation energy for Li im- planted MgO(100)and MgO(110).Peak 1 corresponds to the Al layer;peak 2 to the Li implantation range. VEP (a 22Na based slow positron beam)
VEP (a 22Na based slow positron beam)
a A detailed analysis by means of the fitting code VEPFIT has shown that the S parameter curves depicted in Fig.2 can be fitted very well with a model consisting of five layers: ■ An Al top layer, An intermediate MgO layer, A Li implantation layer (MgO:Li), A layer with the Li implantation tail, ■the MgO bulk
A detailed analysis by means of the fitting code VEPFIT has shown that the S parameter curves depicted in Fig. 2 can be fitted very well with a model consisting of five layers: An Al top layer, An intermediate MgO layer, A Li implantation layer (MgO:Li), A layer with the Li implantation tail, the MgO bulk
a) b) 0.0010 0.0010 0.0008 0.0008 0.0006 0.0006 0.0004 0.0004 0.0002 0.0002 P.(,c)[0o 9.oooo P.(to'm,c)[001] o.000o p,(10me)(100] p,(10'mge)[100] c) d) 0.o010 0.0010 o.o006 0.0008 0.0006 0,0006 0.0004 0,0004 0.0002 0.0002 B(10me)[00 P.(10'me)[100] P.(10im,e) o.0000 P,(10'mE)[100] 2D-ACAR distributions for Li implanted MgO(100)at positron energies of(a)4 keV,(b)7.5 keV,and(c)11.5 keV;(d) 2D-ACAR spectrum for bulk MgO(100).The distributions are normalizedto equal total numbers of counts
2 D - A C A R d i s t r i b u t i o n s fo r L i i mp l a n t e d M g O ( 1 0 0 ) a t p o s i t r o n e n e r g i e s o f( a ) 4 k e V, ( b ) 7.5 k e V, a n d ( c ) 1 1.5 k e V; ( d ) 2 D - A C A R s p e c t r u m fo r b u l k M g O ( 1 0 0 ). T h e d i s t r i b u t i o n s a r e n o r m a l i z e d t o e q u a l t o t a l n u mb e r s o f c o u n t s