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s u r f a c e s c i e n c e

E L S E V I E R S u r f a c e S c i en c e 3 8 6 ( 1 9 9 7 ) 1 15 1 23 ,

F u n d a m e n t a l c o n s i d e r a t io n s in t h e m a n i p u l a t i o n o f a s in g le C 6 0m o l e c u l e o n a s u r fa c e w i th a n S T M

H . T a n g a M . T . C u b e r e s b , C . J o a c h i m a . . , J . K . G i m z e w s k i ba CEMES- CNRS, 29, Rue J. Marvig, 31055 Toulouse, Cedex, France

b IBM Research Division, Science and Technology, Zurich Research Laboratory, CH-8803 Raschlikon, SwitzerlandR e c e i v e d 5 N o v e m b e r 1 9 96 ; a c c e p t e d f o r p u b l i c a t i o n 4 F e b r u a r y 1 9 97

Ab s t r a c tW e d i s c u s s t h e c o n d i t i o n s n e e d e d t o p e r f o r m m e c h a n i c s o n t h e n a n o s c a l e . V a r i o u s m o d e s f o r a d s o r b a t e s m a n i p u l a t i o n u s i n g a

s c a n n i n g t u n n e l i n g m i c r o s c o p e ( S T M ) a r e p r e s e n t ed . A m o n g s t t h o s e , w h i c h i n c l u d e s li d in g , p u l li n g a n d p u s h i n g , w e f i n d t h a tm o l e c u l e s s u c h a s f u l l e r e n e C 6o a r e r e p o s i t i o n a b l e b y p u s h i n g . T h i s m e c h a n i s m i s m o d e l e d u s i n g m o l e c u l a r m e c h a n i c s c a l c u l a t i o n s ,w h i c h p r o v i d e t h e t h r e s h o l d S T M t u n n e l j u n c t i o n r e s i s t a n c e n e ce s s a r y to t r a n s l a t e C 6 o p a r a l l e l a n d p e r p e n d i c u l a r t o a s t e p e d g e o na C u ( 1 0 0 ) s u r f a c e . P e r p e n d i c u l a r t r a n s l a t i o n p r o v i d e s t h e o p p o r t u n i t y f o r m o r e c o m p l e x m e c h a n i c s o n t h e n a n o s c a l e s u c h a s t h econ t ro l o f the k ine t ic ene rgy o f a s ing le adso rba te . 1997 E lsev ie r Sc ience B.V.Keywords." D i f f u s i o n a n d m i g r a t i o n ; F u l l e r e n e s ; S c a n n i n g t u n n e l i n g m i c r o s c o p y ; S e m i - e m p i r i c a l m o d e l s a n d m o d e l c a l c u l a t i o n s ;Sur face d i f fus ion ; T unne l ing

1. IntroductionIn the macroscopic world, the law of inertia, the

equations of motion and the law of action andreaction constitute the basis of Newtonian mechan-ics [1 ]. The ability offered by the scanning tunnel-ing microscope (STM) to manipulate moleculeson a individual basis [2,3] opens the way toperform mechanics on the nanoscale and to com-pare their differences with macroscopic scalemechanics. On the nanoscale, mechanics differarising from the ultra low masses of the objects(10 -21 g), from the very small momentum transferand from the large number of internal degrees offreedom in a molecule. However, the internal

* Co rresp ond ing au th o r . T e l : (+ 33 ) 5 62257800; fax : (+ 33 ) 562257999; e -ma i l : joach im @ c eme s . f r

0039-6028 /97 /$17 .00 1997 E lsev ie r Sc ience B.V. Al l r igh ts re se rved .Pl l S 0 0 3 9 - 6 0 2 8 ( 9 7 ) 0 0 3 0 8 - 7

behavior of recently synthesized molecular shuttles[4] or turnstile molecules [5] are usually des-cribed in terms of the macroscopic mechanics.Furthermore, chemistry at the nanoscale may beviewed as a particular branch of mechanics: tworeactants adsorbed on a surface can actually beforced to react if controlled scattering occursbetween them [6].

In this paper, the mechanical operations, whichcan be achieved experimentally using an STM,with a single C6o adsorbed on a metal surface arediscussed in terms of the laws of inertia and action-and-reaction. In Section2, the important roleplayed by the surface diffusion barrier as opposedto the adsorbate mass on the nanometer scale isanalyzed. In Section 3, different STM manipula-tion modes of an adsorbate are discussed in rela-tion to the experimental repositioning of a C6o

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11 6 H. Tang e t al . / Sur face Sc ience 386 (1997 ) 115-12 3mole c u l e w i th a n S TM t i p . Th i s r e pos i t i on ing i sc a l c u l a t e d a long a s t e p e dge w i th a nd w i thou t t hepresenc e o f a k ink s i te . In Sec t ion 4 , the push ingof a C6o mole c u l e pe rpe nd i c u l a r t o t he s t e p e dgei s c ons ide re d a nd t he a na lys i s c ompa re d w i thexperimenta l da ta . F ina l ly , based on these resul t s ,w e d i s c us s fu tu re d i re c t i ons a nd ope n q ue s t i onsre l a t i ng t o i n t ra mole c u l a r me c ha n i c s on t hena nosc a l e .

2 . W h a t i s t h e r o l e o f i n e r ti a o n t h e n a n o s c a l e ?

t

F i g . 1 . U H V - S T M c o n s t a n t c u r r e n t i m a g e o f a s i n g l e C 6o m o l e -c u le s t a b il i z e d a t r o o m t e m p e r a t u r e b y a m o n o - a t o m i c s t e p e d g e2.54 ,~, in h eigh t o f a C u( 111 ) surfa ce.

The c o r ru ga t i on o f the po t e n t i a l e ne rgy su r fa c ec re a t e s c ond i t i ons t o s t a b i l i z e a n a dso rba t e pos i -t ion on a g iven surface s i te . Be low a g iven thresh-o ld su r fa c e t e mpe ra tu re a nd w i th no ne t fo rc eappl ied to the adsorba te , i t w i l l not exper iencea c c e l e ra t i on . H ow e ve r , t he i ne r t i a l a w doe s no texac t ly hold here . This i s because i t i s the d i f fus ionba r r i e r he igh t r a t he r t ha n t he a dso rba t e ma ssw hic h c on t ro l s t he r e l a t i on be tw e e n t he ne t fo rc ea pp l i e d t o t he a dso rba t e a nd i t s r e su l t i ng a c c -e l e ra t i on . The a s soc i a t e d t h re sho ld t e mpe ra tu red e p e n d s b o t h o n t h e c h e m i s o r b e d o r p h y s i s o r b e dc h a r a c te r o f a d s o r p t i o n p r o c e s s a n d o n s u r f ac ee l e c tron i c c o r ruga t i on [ 7 ]. F o r e xa mple , t he t h re sh -o ld va lue i s be low a t e m pe ra tu re o f 10 K fo r ara re ga s a t om suc h a s Xe phy s i so rbe d on a me t a ll i c(110) su r fa c e [ 8 ] . In c on t ra s t , c ond i t i ons c a n befou nd fo r l a rge mole c u l e s t o be re a d i l y imm obi l i z e don m e ta ll i c su r fa c e s e ve n a t room t e mp e ra tu re [ 3 ].T o p e r f o r m m e c h a n i c s w i t h a n S T M t i p o n amo lecule su ch as the fu l le rene C6o, i t is e ssent ia lto f i rs t ly s tabi l ize i t s la te ra l po s i t ion a t T = 300 K.Expe r ime n ta l l y , t he a d so rp t i o n o f C6o mole c u l e son Cu ( 111 ) w a s ima ge d w i th a u l t r a -h igh va c uu m( U H V ) S T M ( F i g . 1 ) . A t r o o m t e m p e r a t u r e , t h em o l e c u le s w e re f o u n d t o b o t h b e i m m o b i l iz e d a n dnuc l e a t e d a long a tomic s t e p e dge s o r on de fe c ts i tes as i l lus t ra ted in F ig . 1 . A s tep edge c rea tes ane ne rgy po t e n t i a l gu ide o r " g roove " fo r r e pos i t i on -ing t o be fe a s ib l e a t room t e mpe ra tu re .T o g a i n a n u n d e r s t a n d i n g o n t h i s " g r o o v e " , w eha ve c a l c u l a t e d t he e ne rgy ba r r i e r fo r su r fa c ed i f fus ion o f a C6o mo le c u l e on a C u(1 00) su r fa c emode l e d w i th a s i ng l e mono-a tomic s t e p (F ig . 2 ) .In t he mode l , C6o w a s c ons ide re d phy s i so rbe d o n

th i s su r fa c e w h ic h i s a j u s t i f i e d a pprox ima t ions ince the meta l Fermi leve l l i e s wi th in the C6oH i g h e s t O c c u p i e d M o l e c u l a r O r b i t a l - L o w e s tU n o c c u p i e d M o l e c u l a r O r b i t a l ( H O M O - L U M O )ga p on Cu (10 0) [ 9] . W ha t i s c r i ti c a l i n ou r c a l c u la -t i ons , how e ve r , i s t he Cu-C in t e ra c t i on po t e n t i a l .H e re , w e ha ve c hose n a Buc k ingha m po t e n t i a lw i t h i n t h e m o l e c u l a r m e c h a n i c ( M M 2 ) r o u t i n e[ 10] t o de sc r i be e a c h o f t he C u- C in t e ra c ti onsw hi l e p re se rv ing t he s t a nda rd MM2 de sc r i p t i on o ft he C6o i n t e rna l me c ha n i c s . Th e a dv a n t a ge o f t heM M 2 r o u t i n e w a s t o e n a b l e t h e c a l c u l a t i o n o fC6o repo s i t ioning wi th in rea l i s tica l ly ach ievablec o m p u t a t i o n t i m e l i m i t a t i o n s c o m p a r e d t o t h euse o f f i rst pr inc iple ca lcula t ion s [ 11] or even anA t o m S u p e r p o s i t i o n a n d E l e c t r o n D e l o c a l i z a t i o n( A S E D ) a p p r o a c h [1 2]. F u r t h e r m o r e , t h e p a r a m e -t r i z a t i on use d he re c a n be op t imiz e d f i r s t by a bin it io c a l c u l a t i ons be fo re i nc lus ion i n MM 2. The rea r e 5 7 2 a to m s o n t h e C u ( 1 0 0 ) s u r f ac e ( F i g . 2 )w i th no c yc l ic bo un da r y c ond i t i ons i nc lude d t oa vo id spu r ious e f fe ct s l ike t i p - t i p d i re c t i n t er -a c t i ons be tw e e n t he un i t c e l l s . The t i p i s ma de o f2 3 6 a t o m s a r r a n g e d i n a s q u a r e d b a s e p y r a m i d( 1 - 4 - 9 - 1 6 . . . ) w i t h ( 1 1 1 ) f a c e t s .

The potent ia l energy charac te r i s t ic s for a C6omole c u l e d i f fus ing bo th a long a nd pe rpe nd i c u l a rt o a m o n o - a t o m i c s t e p e d g e o n C u ( 1 0 0 )( i l lus t ra ted in F ig . 2) a re presented in F ig . 3 . Thesec u rve s w e re c a lc u l a t e d by m in imiz ing t he po t e n t i a le ne rgy o f t he Cro -su r fa c e sys t e m a t e a c h pos i t i ona long one t r a j e c to ry o f t he C6o mole c u l e . Th i sleads to a ve ry smal l d i f fus ion barr ie r on a de fec tf re e su r fa c e a nd a l so a long t he s t e p e dge . Inc on t ra s t , pe rpe nd i c u l a r t o t he s t e p e dge , C6o e xpe r i -

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1t. Tan g et al. / Surface Science 386 (1997) 115-12 3 117

Fig. 2. Representation of the Cu(100) surface used for C6o manipulation calculations. The kink introduced along the mono-atomicstep stabilizes the C6o position.

e n c es a d e e p m i n i m u m d o w n t h e s t ep a n d a l a r gec h a n g e o n t h e t o p e d g e o f t h i s s t ep . C o n s e q u e n t l y ,a t r o o m t e m p e r a t u r e , C 6 o m o l e c u l e s d i f fu s e o n af r e e o f d e f e c t C u ( 1 0 0 ) s u r f a c e u n t i l t h e y r e a c hd e f e c t s s u c h a s a s t e p e d g e w h e r e t h e y a r e s t a b i l i z e df o r e x a m p l e b y a k i n k o r a d e f e c t a l o n g t h e e d g ea s sh ow n in F ig . 1 . The se s t a b le s i t e s a r e thes t a r ti n g p o i n t o f m e c h a n i c s e x p e ri m e n t s o n as ing le C 6o m o le c u le .

3. M echan ics of C6o mo tion induced by an ST M tipW h e n a n a d s o r b a t e i s s t a b i l i z e d o n a s u r f a c e ,

t h e r e a r e a t l e a s t t h r e e w a y s t o r e m o v e i t f r o m i t sa d s o r p t i o n s it e: a n i n c r e a s e o f t h e s u r f a c e t e m p e r -a t u r e, a p h o t o n a b s o r p t i o n b y t h e a d s o r b a t e a n dm o r e r e c en t l y b y t h e m e c h a n i c a l a c t i o n o f a n S T Mt ip a pe x .

I n c r e a s i n g t h e t e m p e r a t u r e i s a v e r y e f f i c i e n tm e t h o d t o i n d u c e m o t i o n o f a n a d s o r b a t e . I t c ane i t h e r o s c i l l a t e a b o u t i t s e q u i l i b r i u m p o s i t i o n o rd i f f u s e . F o r e x a m p l e , a n a d - a t o m w i l l r e m a i n o n l y10 - 7 s i n i t s e q u i l ib r ium s i t e a t 77 K f o r a 0 . 1 e Vd i f f us io n ba r r i e r [ 13 ]. N e ve r the l e s s , t h i s i s a ve r yi n e ff i ci e n t w a y t o c o n d u c t d i r e c t i o n a U y c o n t r o l l e dm e c h a n i c s s i n c e i m p u l s e t r a n s f e r f r o m a h o t s u r -f a ce t o a n a d s o r b a t e i s a r a n d o m p r o ce s s w i t h n od i r e c t i o n a l i t y u n l e s s t h e s u r f a c e i s h i g h l y a n i s o -

t r o p ic s u c h a s t h e A u ( l l 0 ) - ( 2 x 1 ) r e c o n s t r u c te ds u r f ac e . F u r t h e r m o r e , t h e a v e r a g e s p e e d o f t h ec e n t e r o f m a s s o f a m o l e c u l e d o e s n o t v a r y i n t i m ea f t e r t h e r m a l i z a t i o n .

A b s o r p t i o n o f p h o t o n s b y t h e a d s o r b a t e t ot r a n s f e r m o m e n t u m t o t h e a d s o r b a t e v i a m o m e n -t u m c o n s e r v a t i o n r u l e s i s a n o t h e r p o s s i b i l i t y .To the f i r s t o r de r , t h i s i nvo lve s no se l e c t ived i r e c t i o n a l i t y .

R e p o s i t i o n i n g w i t h a n S T M t i p a p e x i s a r e c e n ta p p r o a c h . W i t h a v e r a g e t ip s c a n n i n g s p e e d s o f c a1 0 - ~ m s - ~, a n d w i t h a d s o r b a t e s m a s s t y p i c al l y< 1 0 -2 ~ g , m o m e n t u m t r a n s f e r f r o m t h e t i p a p e xt o t h e a d s o r b a t e m a y b e c o n s i d e r e d n e g l i g i b l e .T h e r e f o re , " k i c k i n g " a n a d s o r b a t e o u t o f it s eq u i -l i b r i u m s i te b y a l a t e ra l i m p u l s e u s i n g a n S T M t i pi s n o t r e a l i z a b l e . T h e t i p a p e x - a d s o r b a t e m e c h a n -i c a l i n t e r a c t i o n s w i l l o n l y p e r m i t d r a g g i n g o r p u s h -i n g t h e a d s o r b a t e . D e p e n d i n g o n t h e p o s i t i o n o ft h e a d s o r b a t e u n d e r t h e t i p d u r i n g i t s i n t e r a c t i o nw i t h t h e t i p a p e x , t h r e e m e c h a n i c a l m a n i p u l a t i o nm o de s h a ve be e n ide n t i f i e d : a s li d ing p r oc e s s [ 8 ] ;a pu l l i ng p r oc e s s [ 14 ] ; a nd a push ing p r oc e s s [ 3 ] .I d e n t if i c at i o n o f t h e r e p o s i t io n i n g m o d e c a n b eo b t a i n e d b y r e c o r d i n g t h e S T M " t u n n e l c u r r e n ti n t e n s i t y - t ip a p e x d i s p l a c e m e n t " I (x ) c ha r a c t e r i s t i cf o r a c o n s t a n t h e i g h t m a n i p u l a t i o n o r t h e S T M" z - p i e z o f e e d b a c k A z s i g n a l -t i p a p e x d i s p l a c e m e n t "A z ( x ) fo r a c o n s t a n t c u r r e n t m o d e o f o p e r a t io n

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t l . Tang et al. / Surface Science 386 (1997) 115 -123 119t h e a d s o r b a t e w i l l r e m a i n a t t r a c t e d b y t h e r e a rf a c e t o f ti p a p e x . I f t h e a d s o r b a t e d i f f u s i o n b a r r i e ri s l o w e r t h a n t h i s a t t r a c t i v e f o r c e , t h e a d s o r b a t ew i l l b e p u l l e d b y t h e t i p a p e x . T h e c o r r e s p o n d i n gA z ( x ) s i g n a l h a s s a w t o o t h f o r m ( F i g . 4 b ) a n d t h ep r o n o u n c e d s t e p s a re a s s o c i a t e d w i t h t h e d i s c o n t in -u o u s a t t r a c t io n o f t he a d s o r b a t e b y t h e t i p a p e x .

D u r i n g a p u s h i n g p r o c e s s [ 3 ] , t h e a d s o r b a t e i si n f r o n t o f t h e t i p . T h e a p e x i s f i r s t a p p r o a c h e dl a t e r a l l y t o w a r d s t h e a d s o r b a t e i n t h e c o n s t a n tc u r r e n t m o d e . C o n t r a r y t o t h e p u l l i n g m o d e , z i sn o w s e l e c t e d s u c h t h a t i t i s l e s s t h a n t h e o v e r a l lh e i g h t o f t h e a d s o r b a t e . I n t h a t c a s e , t h e a d s o r b a t ei s p u s h e d b y t h e a d v a n c e m e n t o f t h e t i p a p e x [ 3 ]d u e t o c o r e - c o r e r e p u l s i o n . T h i s p r o c e s s w o r k s f o ra m o l e c u l e b e t t e r t h a n f o r a n a t o m b e c a u s e a sm a l ld i s p l a c e m e n t o f t h e t i p a p e x p a r a l l e l to t h e s u r f a c ew i l l r e s u l t in t h e l a t e r a l e s c a p e o f t h e a t o m . T h ea s s o c i a t e d A z ( x ) S T M s i g n a l h a s a l s o a s a w t o o t hf o r m , b u t w i t h t h e r e v er se a s y m m e t r y o f i t s fo r mc o m p a r e d t o t h e p u l l i n g m o d e ( F i g . 4 c ).

T h e A z ( x ) s i g n a l c a n b e m e a s u r e d e x p e r i m e n -t a l l y i n a l l t h e s l i d i n g , p u l l i n g a n d p u s h i n g m o d e s[ 1 5 ] w h i c h m a k e s t h i s s i g n a l v e r y i m p o r t a n t t or e c o r d i n a n y a d s o r b a t e m a n i p u l a t i o n . U s u a l l y ,t h e r e s u l t o f a m a n i p u l a t i o n i s k n o w n a f t e r , b yi m a g i n g t h e s u r f a c e t o p r o v e t h a t t h e a d s o r b a t eh a d b e e n d i s p l a c e d . W i t h A z ( x ) , t h e a d s o r b a t ep o s i t io n a n d c o n f o r m a t i o n u n d e r t h e t i p a p e x c a nb e f o l l o w e d d u r i n g a m a n i p u l a t i o n .

E x p e r i m e n t a l l y , i t h a s b e e n d e m o n s t r a t e d t h a tC 6 o m o l e c u l e s c a n b e i n d i v i d u a l l y r e p o s i t i o n e da l o n g a C u ( 11 1 ) m o n o - a t o m i c s t e p e d g e i n a w e l ld e f in e d m a n n e r [ 17 ] u s i n g a n S T M m a n i p u l a t i o njunc t ion r e s i s t a nc e s c a 1 . 4 M f ~ ( F ig . 5 ) . The r e , t het i p a p e x is p o s i t i o n e d i n t h e c o n s t a n t c u r r e n t S T Mi m a g i n g m o d e t o t h e c l o s e p r o x i m i t y o f a s i n g l eC 6 o. T h e S T M t i p b i a s v o l t a g e is t h e n r e d u c e d t ol o w e r t h e t i p a p e x t o w a r d s t h e s u r f a c e . A s t h e t i pa p e x i s s u b s e q u e n t l y s c a n n e d a l o n g t h e s t e p e d g e ,r e p o s i t i o n i n g o f t h e C 6 o t o t h e d e s i r e d l o c a t i o no c c u r s a l o n g t h e s t e p [ 1 7 ] .

W e h a v e c a l c u l a t e d w h i c h o f t h e p o s s i b l e t h r e ep r o c e s se s l e a d s t o t h i s r e p o s i t i o n i n g . T o r e p r o d u c ea s l id i n g m o d e , t h e t i p a p e x w a s a p p r o a c h e d o nt h e t o p o f a C 6 o m o l e c u l e s t a b i l i z e d a l o n g a s t e pe d g e . F o r d i f f e r e n t v a l u e s o f z , t h e c o n f o r m a t i o no f t h e C 6o c a g e w a s o p t i m i z e d u n d e r t h e t i p a p e x

Fig. 5. Experimental STM image of the pushing process of asingle C6o molecule along a C u (l ll ) su rface step edge. (a) AnSTM im age before the pu shing, (b) an STM image at an inter-mediate C6o position and (c) a n STM imag e at the end of thepushing process.

c o n s t r a i n t u s i n g t h e M M 2 ( M o l e c u l a r M e c h a n i c s)r o u t i n e [ 1 0 ] . K e e p i n g z c o n s t a n t , t h e t i p a p e x w a ss c a n n e d u n d e r c o n d i t i o n s w h e r e w e e x p e c t a sl i d in gm o t i o n o f C 6 o. S u r p r i s in g l y , n o e v i d e n c e o f m o l e c -u l a r r e p o s i ti o n i n g w a s f o u n d . A n a l y s i s o f th e d a t ai n d i c a t e s t h a t d e f o r m a t i o n s o f C6 o b y t h e a p e xd o e s n o t c h a n g e t h e C 6 o s t a b i l i z a t i o n e n e r g y o nthe su r f a c e . H e r e , t he C 60 c a ge a c t s a s a t i pc o n s t r a in d a m p e r .

F o r a p u s h i n g , t h e t i p w a s s c a n n e d o n c e a g a i na t c o n s t a n t a l t i t u d e t o w a r d s a C 6o m o l e c u l e s t a b i -l i z e d a l o n g t h e s t e p e d g e o n t h e C u ( 1 0 0 ) s u r f a c e .T h i s c a l c u l a t i o n w a s d o n e f o r a r a n g e o f z v a lu e su s i n g o u r m o l e c u l a r m e c h a n i c a p p r o a c h [ 3 ] .F o l l o w i n g th e E h r e n f e s t th e o r e m , a m o l e c u l a rd y n a m i c a p p r o a c h is n o t r e q u i r e d . T h e C 6 o m a s si s s o s m a l l t h a t t h e a c c e l e r a t i o n t e r m i n t h e s e m i -c l as s i ca l E h r e n f e s t e q u a t i o n o f m o t i o n d e s c r i b i n gt h e d y n a m i c s o f t h e C 6o c e n t e r o f m a s s i s z e r o.F u r t h e r m o r e , t h e t i m e i n t e r v a l b e t w e e n t w o e l e -m e n t a r y t i p s c a n i n c r em e n t s i s o rd e r s o f m a g n i t u d el o n g e r t h a n t h e C 6o v i b r a t i o n r e l a x a t i o n t i m e , e v e nf o r t h e c o h e r e n t l o w f r e q u e n c y v i b r a t i o n o r r o t a -t i o n m o d e s . T a b l e 1 s u m m a r i z e s o u r d i f f e re n tr e su l ts a n d t h e c o r r e s p o n d i n g t h r e s h o l d S T M j u n c -t i o n r e s i s t a n c e w h e r e m o l e c u l a r m o t i o n o c c u r s .

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12 0 H. Tang et al. / Surface Science 386 (1997) 115-123T a b l e 1C o n d i t i o n s f o r p u s h i n g a s i n g l e C ~o m o l e c u l e a l o n g a s t e p o n aC u ( 1 0 0 ) s u r f a c eA l t i tu d e T u n n e l i n g j u n c t i o n T r a n s l a t i o n E x p e r i m e n t a l( A ) r e s i st a n c e ( M f ~ ) b y p u s h i n g r e s is t a n c e ( M ~ )13 536 .7 N o 50012 14.4 N o11 1.05 Ye s 1.410 - Yes

9 - YesT h e a l t i t u d e i s t h e t i p a p e x t o s u r f a c e d i s t a n c e , t h e S T M t u n n e lj u n c t i o n r e s i s t a n c e i s b e f o r e t h e p u s h i n g . T h e c o r r e s p o n d i n ge x p e r i m e n t a l t h r e s h o l d r e s i s t a n c e f o r t r a n s l a t i n g C 6 o b y p u s h i n gi s a l so g iven .

Tra ns l a t i on o f the C6o mole c u l e a l ong t he s t e pe dge s e t s i n w i th a S TM junc t i on re s i s t a nc e< 1 0 M ~ .K n o w i n g t h e c o n d i t i o n s f o r C 6o m a n i p u l a t io n ,th e I ( x ) S TM s igna l w a s c a l c u l a t e d fo r c ons t a n th e i g h t s c a n s u s i n g b o t h t h e S T M - E S Q C ( E l a s t i c

S c a t t e r i ng Qua n tum Che mis t ry ) t e c hn iq ue t o c a l -c u l a t e t he t unne l i ng c u r re n t [ 18 ] a nd t he MM2rou t i ne fo r t he de t e rmina t i on o f the push ingme c ha n i c s . Th i s c a l c u l a t i on w a s pe r fo rme d a t asu r fa c e t e mp e ra tu re o f 0 K . A s e v ide n t f rom F ig . 6 ,I ( x ) ha s t he s a w too th s i gnal c ha ra c te r i s ti c o f push -ing, but i t i s not as regula r as the idea l ized s igna lp re se n t e d i n F ig . 4 . W e a sc r i be t he c om ple x s t ruc -

t u re i n F ig . 6 t o t he c onfo rm a t ion o f the C6ocageunde r p re s su re w h ic h i s s e q ue n t i a l l y pa s s ing ove rtop a nd ho l l ow s i t e s i n a more o r l e s s r a ndomm a n n e r . T h e s e sm a l l c o n f o r m a t i o n c h a n g e s m o d i f ythe t unne l i ng c ha nne l s t h ro ugh C6o a nd t he i rmixing, induc ing the i r regula r i t ie s in the I ( x ) signalp re se n t e d i n F ig . 6 . H a v ing p re v ious ly foun d noe v ide nc e o f pu l l i ng fo r t h i s l a rge a dso rb a t e , t h isresul t s impl ies tha t the la te ra l t ip apex a t t rac t ionof C6o i s da m pe d by C6o c a ge de fo rm a t ions . A troo m t e mpe ra tu re , ro t a t i on o f the C6o mole c u l e i ni t s e q u i l i b r i um pos i t i on c a n oc c u r a nd t h i s " s a w -too th" fo rm i s t he n t ime a ve ra ge d .The i n t e rna l C6o me c ha n i c s du r ing push ing(F ig . 7 ) fo l l ow s t he su r fa c e c o r ruga t i on a long t hes t e p e dge w i th on ly sma l l de fo rma t ions o f the c a ge .H e re , i t i s no t pos s ib l e t o i nduc e a c ohe re n tro t a t i on o f t he C6o c a ge a long a r e gu l a r s t e p e dge .This i s a d i rec t consequ ence o f the negl ig ibleine r ti a l mo me n tum o f a C6o mole c u l e .To und e rs t a nd t he ro l e o f me c ha n i c s i n a mo rec omple x a nd re a l i s t i c s i t ua t i on , w e ha ve i nc lude da k ink a long t he s t e p e dge (F ig . 2 ) . W he n t heC6o mole c u l e i s pushe d by t he t i p a s i t a pp roa c he sthe k ink e dge , C6o ro t a t e s t o a vo id t he k ink e dgeas i l lus t ra ted in F ig . 8 . Ho we ver , th i s ro ta t ions tops a f t e r i t pa s se s t he k ink e dge a nd t he no rma lpush ing s e q ue nc e re sume s .

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- ~ . o i p p ~ ( A } " - 3 . 0 1.0F i g . 6 . T h e c a l c u l a t e d I (x ) s i g n a l f o r p u s h i n g w i t h a c o n s t a n tt i p a p e x - t o - s u r f a c e d i s t a n c e h e i g h t ; - - - f o r z = 1 2 , ~ , a n d - -f o r z = 7 A ,. T h e r e i s n o r e p o s i t i o n i n g o f t h e m o l e c u l e f o r z =1 2 P t ( s e e T a b l e 1 ) .

4 . A c t i o n a n d r e a c t io n o n t h e m o l e c u l a r s c a l e

Kine t i c e ne rgy i s no t supp l i e d t o a n a dso rba t ed u r i n g a n a p e x m a n i p u l a t i o n p r o c e s s w i t h a n S T Mt ip . W he n t he s c a nn ing i s s t oppe d a nd t he t i pre t ra c t e d , t he a dso rba t e r e sume s a n immobi l i z e ds t a t e . Conse q ue n t l y , me c ha n i c s w i thou t k ine t i ce ne rgy i s r a t he r l imi t e d . One a pproa c h w e sugge s tt o p rov ide k ine t i c e ne rgy t o a n a dso rba t e i s t oincrease f i rs t i t s in te rna l potent ia l energy. Par t oft h i s e ne rgy ma y be c onve r t e d i n k ine t i c e ne rgy ,t he o the r pa r t be ing ra ndomiz e d w i th in t he a dso r -ba t e v ib ra t i on ma n i fo ld .A s ign if ic a n t i nc rea se o f t he a d so rb a t e po t e n t i a le n e r g y m i g h t b e a c h i ev e d b y p u s h i n g t h e a d s o r b a t ea ga ins t a " ha rd w a l l " suc h a s a s t e p e dge . W i th as l id ing or a pul l ing process , no la te ra l cons t ra insa re e xe r t ed on t he a dso rba t e . Th e c om pre s s ion w i ll

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