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Scanning transmission electron microscopy at near-atomic resolution: The present state of the art
sCANNING
TRANSMISSION
ELECTRON
MICROSCOPY
SCANNING TRANSMISSION ELECTRON MICROSCOPY AT NEAR-ATOMIC RESOLUTION: THE P R E S E N T STATE OF THE A R T (Distinguished Guest M.
IMAGING OF M O L E C U L E S , (Invited Paper) A.
PAST AND P R E S E N T
J. M e l m e d Surface Science Division, National Bureau of Standards, Washington, D.C. 2 0 2 3 4 ,
Lecture)
Isaacson Department of Physics, University Chicago, Chicago, Illinois
Several types of m i c r o s c o p y have been and are b e i n g a p p l i e d to the quest for d e t a i l e d i n f o r m a t i o n a b o u t the m o r p h o l ogy and atomic s t r u c t u r e of molecules, w i t h special i n t e r e s t in i m p o r t a n t b i o l o g i c a l molecules. These will be r e v i e w e d w i t h e m p h a s i s on the results thus far o b t a i n e d and the a p p a r e n t obstacles to i m p r o v e d imaging capability. P a r t i c u l a r a t t e n t i o n will be given to the field e m i s s i o n (FEEM and FIM) efforts.
of
Over the past decade there has been an i n c r e a s e d c o u p l i n g of m i c r o s c o p y (which gives us spatial r e s o l u t i o n information) at spatial r e s o l u t i o n levels s m a l l e r than the w a v e l e n g t h of light. The c o u p l i n g is best e x e m p l i f i e d w i t h the t r a n s m i s s i o n e l e c t r o n microscope. It has been p u s h e d to the extreme such that very fine beams of e l e c t r o n s can be p r o d u c e d w h i c h are as small as 2.5 in diameter. We are t h e r e f o r e in a situ a t i o n w h e r e we may expect to be able to p e r f o r m c h e m i c a l l o c a l i z a t i o n at near atomic spatial r e s o l u t i o n from a w i d e V a r i e t y of objects. One can p r e s e n t l y p e r f o r m s p e c t r o s c o p y of fast e l e c t r o n s t r a n s m i t t e d t h r o u g h thin samples from areas a p p r o a c h i n g 20 R in size. In addition, i n d i v i d u a l h e a v y atoms can be v i s u a l i z e d on light e l e m e n t substrates using such atomic d i m e n s i o n e l e c t r o n beams. We w i l l try to r e v i e w the stateo f - t h e - a r t of the c o u p l i n g of m i c r o s c o p y and s p e c t r o s c o p y using fine e l e c t r o n beams and s p e c u l a t e on some of the potential a p p l i c a t i o n s in the b i o l o g i c a l and m a t e r i a l s sciences.
BIOLOGICAL
F I E L D ION IMAGING ORGANIC MOLECULES Franklin
Hutchinson
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520
Large b i o l o g i c a l m o l e c u l e s have been i m a g e d on the surface of a field ion tip by s u r r o u n d i n g them w i t h layers of metal atoms from an e v a p o r a t i o n source. The e v a p o r a t e d atoms s t r i k i n g the m o l e c u l e s to be imaged e i t h e r do not stick, or are f i e l d - d e s o r b e d as the e l e c t r i c field is raised to v i s u a l i z e the surface by field ion m i c r o s c o p y . The image of the m o l e c u l e is formed by the edges of the cavity in the a d s o r b e d m e t a l layer, and r e s o l u t i o n is i m p r o v e d as the ads o r b e d layers b ~ c o m e more ordered. U n m i s t a k a b l e images have been o b t a i n e d of D N A and of t r a n s f e r RNA m o l e c u l e s to a r e s o l u t i o n of about i0 ~. Images of p h t h a l o c y a n i n e Could not be u n e q u i v o cally d i s t i n g u i s h e d from defects in e v a p o r a t e d m e t a l layers d e p o s i t e d in the absence of o r g a n i c molecules. The m e t h o d is similar in p r i n c i p l e to that used by M u e l l e r and Rendulic, w h o e m b e d d e d o r g a n i c m o l e c u l e s in a metal layer e l e c t r o p l a t e d on a field ion tip. The m e t h o d has s i m i l a r i t i e s to that in w h i c h an image is formed by field d e s o r p t i o n of e a s i l y ionized m o l e c u l e s c o n d e n s e d on a tip b e a r i n g the object to be imaged. F i e l d ion m i c r o s c o p y can scan a much s m a l l e r area for s u i t a b l e images than is p o s s i b l e w i t h t r a n s m i s s i o n m i c r o s c o p y , and this imposes a r e l a t i v e l y serious limitation.
IMAGING
P R O T E I N ON M E T A L S U R F A C E S (Dis t i n g u i s h e d G u e s t Lecture) Ivar G i a e v e r General Electric Schenectady, New
OF DNA AND O T H E R (Invited Paper)
Company, York
All the enzymes and many of the hormones in the human body are g l o b u l a r p r o t e i n molecules. The s t r u c t u r e of a p r o t e i n m o l e c u l e is closely related to its function. Some of these features will be d i s c u s s e d in an e l e m e n t a r y way. I l l u s t r a t i o n s will be given of how some of the p r o p e r t i e s of p r o t e i n m o l e c u l e s can be s t u d i e d by a d s o r b i n g the m o l e c u l e on m e t a l surfaces. Finally, some problems and d i f f i c u l t i e s a s s o c i a t e d w i t h the a d s o r p t i o n of p r o t e i n onto field e m i t t e r tips w i l l be mentioned. 365
TRANSMISSION
ELECTRON
MICROSCOPY
SCANNING TRANSMISSION ELECTRON MICROSCOPY AT NEAR-ATOMIC RESOLUTION: THE P R E S E N T STATE OF THE A R T (Distinguished Guest M.
IMAGING OF M O L E C U L E S , (Invited Paper) A.
PAST AND P R E S E N T
J. M e l m e d Surface Science Division, National Bureau of Standards, Washington, D.C. 2 0 2 3 4 ,
Lecture)
Isaacson Department of Physics, University Chicago, Chicago, Illinois
Several types of m i c r o s c o p y have been and are b e i n g a p p l i e d to the quest for d e t a i l e d i n f o r m a t i o n a b o u t the m o r p h o l ogy and atomic s t r u c t u r e of molecules, w i t h special i n t e r e s t in i m p o r t a n t b i o l o g i c a l molecules. These will be r e v i e w e d w i t h e m p h a s i s on the results thus far o b t a i n e d and the a p p a r e n t obstacles to i m p r o v e d imaging capability. P a r t i c u l a r a t t e n t i o n will be given to the field e m i s s i o n (FEEM and FIM) efforts.
of
Over the past decade there has been an i n c r e a s e d c o u p l i n g of m i c r o s c o p y (which gives us spatial r e s o l u t i o n information) at spatial r e s o l u t i o n levels s m a l l e r than the w a v e l e n g t h of light. The c o u p l i n g is best e x e m p l i f i e d w i t h the t r a n s m i s s i o n e l e c t r o n microscope. It has been p u s h e d to the extreme such that very fine beams of e l e c t r o n s can be p r o d u c e d w h i c h are as small as 2.5 in diameter. We are t h e r e f o r e in a situ a t i o n w h e r e we may expect to be able to p e r f o r m c h e m i c a l l o c a l i z a t i o n at near atomic spatial r e s o l u t i o n from a w i d e V a r i e t y of objects. One can p r e s e n t l y p e r f o r m s p e c t r o s c o p y of fast e l e c t r o n s t r a n s m i t t e d t h r o u g h thin samples from areas a p p r o a c h i n g 20 R in size. In addition, i n d i v i d u a l h e a v y atoms can be v i s u a l i z e d on light e l e m e n t substrates using such atomic d i m e n s i o n e l e c t r o n beams. We w i l l try to r e v i e w the stateo f - t h e - a r t of the c o u p l i n g of m i c r o s c o p y and s p e c t r o s c o p y using fine e l e c t r o n beams and s p e c u l a t e on some of the potential a p p l i c a t i o n s in the b i o l o g i c a l and m a t e r i a l s sciences.
BIOLOGICAL
F I E L D ION IMAGING ORGANIC MOLECULES Franklin
Hutchinson
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520
Large b i o l o g i c a l m o l e c u l e s have been i m a g e d on the surface of a field ion tip by s u r r o u n d i n g them w i t h layers of metal atoms from an e v a p o r a t i o n source. The e v a p o r a t e d atoms s t r i k i n g the m o l e c u l e s to be imaged e i t h e r do not stick, or are f i e l d - d e s o r b e d as the e l e c t r i c field is raised to v i s u a l i z e the surface by field ion m i c r o s c o p y . The image of the m o l e c u l e is formed by the edges of the cavity in the a d s o r b e d m e t a l layer, and r e s o l u t i o n is i m p r o v e d as the ads o r b e d layers b ~ c o m e more ordered. U n m i s t a k a b l e images have been o b t a i n e d of D N A and of t r a n s f e r RNA m o l e c u l e s to a r e s o l u t i o n of about i0 ~. Images of p h t h a l o c y a n i n e Could not be u n e q u i v o cally d i s t i n g u i s h e d from defects in e v a p o r a t e d m e t a l layers d e p o s i t e d in the absence of o r g a n i c molecules. The m e t h o d is similar in p r i n c i p l e to that used by M u e l l e r and Rendulic, w h o e m b e d d e d o r g a n i c m o l e c u l e s in a metal layer e l e c t r o p l a t e d on a field ion tip. The m e t h o d has s i m i l a r i t i e s to that in w h i c h an image is formed by field d e s o r p t i o n of e a s i l y ionized m o l e c u l e s c o n d e n s e d on a tip b e a r i n g the object to be imaged. F i e l d ion m i c r o s c o p y can scan a much s m a l l e r area for s u i t a b l e images than is p o s s i b l e w i t h t r a n s m i s s i o n m i c r o s c o p y , and this imposes a r e l a t i v e l y serious limitation.
IMAGING
P R O T E I N ON M E T A L S U R F A C E S (Dis t i n g u i s h e d G u e s t Lecture) Ivar G i a e v e r General Electric Schenectady, New
OF DNA AND O T H E R (Invited Paper)
Company, York
All the enzymes and many of the hormones in the human body are g l o b u l a r p r o t e i n molecules. The s t r u c t u r e of a p r o t e i n m o l e c u l e is closely related to its function. Some of these features will be d i s c u s s e d in an e l e m e n t a r y way. I l l u s t r a t i o n s will be given of how some of the p r o p e r t i e s of p r o t e i n m o l e c u l e s can be s t u d i e d by a d s o r b i n g the m o l e c u l e on m e t a l surfaces. Finally, some problems and d i f f i c u l t i e s a s s o c i a t e d w i t h the a d s o r p t i o n of p r o t e i n onto field e m i t t e r tips w i l l be mentioned. 365