10.1007-bf02872196 propiedades electricas

7/29/2019 10.1007-BF02872196 propiedades electricas

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rot. 43 No. t l S C I E N C E IN C H I N A (Ser ies A) ~owml~20o0P HYSI CSS i n g l e w a l l c a r b o n n a n o t u b e s a n d t h e i r e l e c t r i c a l p r o p e r t i e sX U E Z e n g q u a n ( i ~ > ' ~ ~ r ' , L I U W e i m i n ( ~ ] r H O U S h i m i n ( ~ . ~ . ~ ) 1 ,S H I Z u j i n ( $ ( ~ . , ~ - I t . - { ~ ) 2 , G U Z h e n n a n ( E ~ J ~ -, r ~ ) 2 , L I U H o n g w e n ( Y ~ ] ~ r _ ~ ) ' ,Z H A O X i n g y u (~2~.Y-~-, ,~ ) ~ Z H A N G Z h a o x i a n g ( ~ E ~ ) ~ ,W U M i a n l e i ( ~ . ~ @ ) ' , P E N G L i a n m a o ( . ~ ~ $ ~ ) ' & W U Q u a n d e ( . ~ ) ~ -~ ,~ ) x1. Dep artmen t of E lectronics, Peking University, Bei j ing 100871, Ch ina;2. Co l lege of C hem istry and M olecular Eng ineering, Peking University, Be i jing 100871, ChinaCorrespondence should be addressed to Xue Zengquan (em ai l :zexue @263. net )Received July 21, 1999Abstract S i n g le - w a l l c a r b o n n a n o t u b e s ( S W C N T s ) w e r e s y n t h e s i z e d a n d p u r i fi e d . A w a t e r c o ll o ido f S W C N T s w a s p r e p a r e d a n d u s e d t o a s s e m b l e S W C N T s o n t o a g o ld fi lm s u r f a c e . 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 T M ) i m a g e s s h o w e d t h a t s h o r t S W C N T s s t o o d o n g o ld fi lm s u r f a c e s . U s i n g S T Mt ips made o f SW CN Ts , a c ry s t a l g ra i n ima ge o f a go ld t h i n f ilm an d an a t om ic res o lu t ion image o f h i gh -l y o r i en t ed py ro l y t i c g raph i t e were s uc c es s f u l l y ob t a i ned . The e lec t r i c a l p roper t i es o f s ho r t SWCNTs ,w h i c h s t o o d o n t h e s u r fa c e o f g o ld fi lm , w e r e m e a s u r e d u s i n g S T M . T h a t S W C N T s s t a n d o n g o ld t h inf ilms is a p rom is i ng t ec hn iqu e f o r s t udy ing s t ruc t u res and p rop er t ies o f c a rbon nan o t ube s , as we l l asas s em b l i ng and f ab r i ca t i ng h i gh - i n t ens i t y c ohere n t e l ec t ron s o urc e s , f ie l d em is s ion fl a t pane l d i s p lay ,t ips f o r s c ann ing p robe m ic ros c op es , new na noe lec t ron i c dev i c es , e t c .K e y w o r d s : c a r b o n n a n o t u b e s , t i p s f o r s c a n n in g p r o b e m i c r o s c o p e s , f i e ld e m i s s i o n p a t t e r n , e l e ct r i ca l p r o p e r t ie sp a r a l l e l t o t h e a x i s o f S W C N T s .

Carbon nanotubes, a new one among isomers of carbon discovered first by Iijima in 1991 Ell ,may be the most typical and fundamental nanometer-scale material. A carbon nanotube can bedescribed as graphite sheets rolled into seamless cylindrical tubes. There are muhiwall carbonnanotubes (MWCNTs) and single-wall ones; both of them can be different in diameter and ehi-rality. Carbon nanotubes have aroused great excitement, and more and more researchers focustheir attention on carbon nanotubes. Now, the emphasis are laid on the preparation and purifica-tion methodE2'33 , structural characteristics ~4'51 , electrical and optoelectric properties [6-9] , me-chanical properties ~1~ , etc. Because of their simple and well-defined structure , single-wall ear-bon nanotubes (SWCNTs) are preferable in the basic research.

Smalley, who won the Nobel Prize in 1996 for his discovery of C6o, indicated that carbonnanotubes may be an important material for nanoelectronic devices In] . We have explored newfunctional materials different from silicon and tried to measure single electron phenomena at roomtemperature. Scanning probe microscopes (SPMs) are important tools for atomic resolution obser-vation and nano-fabrication. Many methods have been adopted to prepare carbon nanotubes, butusually a mixture of many different structures is obtained and most of them are MWCNTs. Synthesismethods for SWCNTs are also developed by some research groups. The structure and electricalproperties of a single SWCNT or MWCNT lying on a substrate have also been studiedN2-143 .


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N o . 1 1 S I N G L E W A L L C A R B O N N A N O T U B E S & T H E I R E L E C T R I C A L P R O P E R T I E S 1 1 8 3

1 P r e p a ra ti on o f S W C N T sThe SWCNTs were produced by DC are discharge method. The anode was an extremely pure

graphite rod with 6 mm diameter, on which a hole 3 mm in diameter was drilled and filled withgraphite and YNi2 powder with molar ratio 1 : 1. The cathode, which was a graphite rod 10 mm indiameter, was shaped into a sharp tip so that the evaporation of the cathode could be minimized.The are was generated by a current of 40 -- 100 A in a helium atmosphere at a pressure of 1 x104pa--7 x 10# Pa. During the discharge process , the distance between the anode and the cath-ode was kept about 5 mm. The obtained cloth-like soot contained about 40 % SWCNTs. The sootwas extracted with CS2 and HC1 of molar ratio 1 : 1, and then it was baked at 100~ to removefullerenes and catalysts. As measured by the high resolution transmission electron microscopy,the purity of SWCNTs was 90% after MWCNTs were filtrated out. A water colloid of SWCNTswas prepared after SWCNTs was cut short by oxidation. The length of SWCNTs was about 10--40n l n .2 SWCNTs assembled perpendicul arly on the surface of crystal Au thin film

We studied the behavior of SWCNTs on a gold thin film. The gold thin film was depositedon a freshly cleaved mica substrate retained at 300 ~ in a chamber of a vacuum system under apressure of 10 -4 Pa. The deposition rate of gold was about 1 nm ' s -1 . The gold thin film wasabout 500 nm thick. Atomic resolution images of Au thin films were obtained with TEM and scan-ning tunnel ing microscopy (STM). A water colloid of SWCNTs described above was dropped ontothe gold thin film after it was taken out from the vacuum chamber. This sample was observed bySTM after the water evaporated. The STM was P47 SPM made by the NT-MDT Company of Rus-sia. A mechanically formed tip was made of an 80 ~m diameter platinum wire welded on a 0.3mm diameter tungsten wire. STM was operated at a constant current mode with a bias voltage of0.94 V and a setpoint current of 1.10 nA. The section analysis showed that the apparent heightof SWCNTs was about 10 nm and the diameter was about 1.4 nm (f ig . 1 ) . Fig. 2 gives an STMcurrent image of SWCNTs on the gold thin film. The STM worked at the condition of a constantbias voltage, with the tunnel ing current as the image signal. Known from the section analysis , thediameter of the electron emission area was about 1. 4 nm, and the current of an SWCNT couldreach about 30 hA. These results indicated that SWCNTs all stood discretely on the gold thin filmsurface, and that SWCNTs were able to transport electrons better than that of the gold thin film.

The field emission properties of SWCNTs, which stood discretely on the surface of Au thinfilm, could be studied using field emission microscopy (FEM). Short SWCNTs were assembledon a W tip and the whole tip was installed in an ultrahigh vacuum system, with the base vacuumabove 3 x 10- 7 Pa. The distance between the tip and the phosphor screen was 5 cm. Field emis-sion patterns could be observed when a 3 000V voltage was applied on the screen (the tip wasgrounded). Fig. 3( a) gives a field emission pattern of a single SWCNT, and fig. 3 (b) was atheoretically calculated electron cloud distribution on the open end of a (9 ,9) "armchair" SWC-NT; they were consistent with each other. Fig. 3 (a ) was a nearly atomic resolution pattern,which was astonishing because it was theoretically impossible to get an atomic resolution imageusing FEM with metal tips . These results indicated that these SWCNTs with open ends weremetallic and the measure field emission current could reach I.LA. The electron beam emitted fromSWCNTs was very thin and coherent probably, which nearly could be used as a coherent electron


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84 SCIENCE IN CHINA (Series A) Vol. 43

|t Y i i ) -

ti m

10 i . . , .

(c):5 10 15 n m

Fig. 1 STM topography image of SWCNTs standing on the surface of gold thin film. (a ) Two-dimension image ;(b ) three-dimension image ; (c) section analysis.

3O20-

nA30

20

10J

a~

( b)

. . . . . . . . . . . . . . . . .~ . . . . . . . 2~L ....

0 10 20 30 nmFig. 2. STM current image of SWCNTs standing on the surface of gold thin film. (a ) Two-dimension ima ge; (b )section analysis.


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No. 11 SINGLE WALL CARBON NANOTUBES & THEIR ELECTRICAL PROPERTIES 1185

source with high lumina nce. Details of the field emission properties of SWCNTs will be publishedelsewhere .

(hiFig. 3. Field emission properties of SWCNTs. ( a) Field emission pattern; (b ) theoretically calculated elec-tron cloud distribution on the open end of a ( 9, 9) "armchair" SWCNT.

3 S W C N Ts u se d a s t ip s f or S TMThe properties of SWCNTs standing on the surface of gold thin fi lm can be exploited in nu-

merous areas. One interesting ex ample is the use of SWCNTs as t ips for SPM s. First tips for STMwere successfully made of SWCNTs. To make sure whether the SWCNT did p lay an importantrole, we made a 1 .0 mm diameter small ball at an end of a gold wire and assem bled an SWCNTon the top of the gold ball as an STM tip, the schematic diagram was shown in fig. 4 ( a ) . Thequali ty of such t ips was examined by measuring the I ( z ) function, where I was the tunnelingcurrent and z was the distance between the tip and the sample . A typical I-z curve was shown infig. 4 ( b ) . Usually a t ip is good if the current is dropped to half of the start value in 10angstro ms; a tip is excellen t enough to reach atomic resolution if the current is drop ped to half of

(b)

Fig. 4. (a ) Schematic diagram of an STM tip made of SWCNT on the top of a gold ball; (b ) l - z curve.


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1186 SCIENCE IN CHINA (Series A) Vol. 43

the s tar t value in 3 . This is a character is t ic of our P47 SP M. The t ip of an SWC NT was verygood known ( f ig . 4 (b ) ) . F ig . 5 ( a ) was an STM image of a go ld th in f ilm conta in ing many c rys -ta l gra ins and had a very good dept h of f ie ld on the topo grap hy, which dem onstra te d that theSWCNT t ip cou ld r each in to deep t r ench es . STM was opera ted wi th a b ias vo l tage of 0 .9 4 V andcur r en t s e tpo in t o f 1 .0 8 nA . An a tomic r eso lu t ion image of h igh ly orien ted pyro ly t ic g r aphi te(H O PG ) i s shown in f ig . 4 (b ) , the b ias vo l tage was 49 mV and the cur r en t s e tpo in t was 0 .5 5nA . Such t ips were ve ry s tab le and r e l i ab le . The end of an SWCN T is open or has a hemispherecap ; i t s a tomic s t ruc ture and e lec t ron ic s ta te s can be de te rmined by ca lcu la t ions o r exper imenta lmeasu remen ts . I t i s more convenien t to do deconvolu t ions and thus the pe r formance of STM canbe improved when such t ips a r e used to s tudy samples .

nm50 =

0 500 1000 nm 0 O.S I0 1:5 20, 25 3:0 nm

nm12~l o

o

4f f20'e,

Fig. 5. STM image obtained with an STM tip made of SWCNT. (a) A crystal grain image of gold film; (b) anatomic resolution image of HOPG.

4 Electrical properties of short SW CN TsThe mos t p romis ing appl ica t ion d i r ec t ion of ca rbon nanotubes i s the exp lora t ion of nanoe lec -

t ron ics . Carbon nanotubes can be func tiona l ma te ria l s fo r nanoe lec t ron ic dev ices . They can a l sose rve as conduc t ing nanomete r - sca le wi r es . At p r esen t , mos t measurements on ca rbon nanotubeslying on s i l icon subs tra tes were car r ied out a t l iquid hel ium or even lower temperatures due to verylarge environ men tal param eter ef fects El3 'lS l . The e lectr ica l p roper t ies of SW CN Ts, which s tood onthe sur f ace of go ld th in f i lm , cou ld be measured a t h igh tempera ture bec ause envi ronmenta l e f -fects were minimized dra s t ica l ly . Af ter an STM image of SWCN Ts was obt a in ed, the t ip was pos i-t ioned on an SWCNT. Whe n the t ip touched the SWC NT, the f eedback of the ampl i f i e r c i r cu i twas swi tched off and cur r en t ve r sus vo ltage ( I - V ) curves o f the junc t ion be tween the P t t ip andthe SWCNT were measured a t r oom tempera ture in a i r . Whe n I-V curves were r eproduc ib le sev-e r a l t imes , the l a s t da ta was r ecord ed . I f the P t t ip d id no t touch an SWCN T, the scanning tun-ne l ing mic roscopy curves were d i f f e r en t and unsymmetr ica l . Four typ ica l I -V da ta were se lec ted .Shown in f ig . 6 ( a ) was an I -V curve . F ig . 6 (b ) was i ts d i f f er en t ial condu c tance ve rsus vo l tage(dI /d V-V) curve . The n ormal ized d if f e ren t ia l condu c tance i s a measure o f the dens i ty o f s ta te s .There was a por t ion of eve ry I -V cu rve in which the cur r en t was ze ro , cor r esponding to the smal l-es t conduc tance in the dI/dV-V curve . Accord ing to the theore t ica l ca lcu la t ions and the exper i -mental resul ts of SWCNTs lying on the Si subs tra te Es'14] , SWCN Ts can be metal l ic or sem icon-duc t ing . The gap for the semiconduc t ing SWCNTs is in the r ange be tween 0 .5 eV to 0 .7 eV .The lowes t conduc tance wid th for the meta l li c SWCNTs r anges from 1 .7 to 1 .9 eV . Mos t o f our


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No , 1 1 S I NGLE W ALL CARBON NANOTUBES & THEI R ELECTRI CAL P ROP ERTIES 1 1 8 7

6 04 02 0

- 2 0- 4 0 i- 6 0 ~

- 3

(a )

- 2 - 1 0 1 2 3

101 0 0 58 0 ~ . 06o _,4 0 - 1 02 0 - 1 50 -20

(b )

i . . . . .- - 1 0 1 2 3

141 2 710 >8

42

V / V V / V

8 ~ 2 100 >16 < # 8 04 ~ -20- ~ 4 0

- 4 0 2 2 0- 1 ~ ;2 - o i i - 6 o _ 3 - 2 - ' 1 ; 't 2 ' , o

V/V VN

F i g . 6 . I-V curves and d l / dV -V c u r v e s o f S W C N T s .r e s u lt s w e r e in t h is r a n g e , w i t h o n l y fe w e x c e p t i o n s . E s d e n o t e s t h e l o w e s t c o n d u c t a n c e w i d t h .F o u r r e s u lt s w e r e s h o w n i n t a b l e 1 . T h e d i f fe r e n c e b e t w e e n o u r ex p e r i m e n t a l r e s u lt s a n d t h o s e o fo th e r g r o u p s w e r e l a r g e , f o r w h i c h t h e e x a c t r e a s o n w a s n o t c l e a r y e t . T h e d i a m e t e r o f S W C N T ss ta n d in g o n th e su r f ac e o f g o l d t h in fi lm w a s 1 . 4 n m , a n d t h e l e n g th w a s a b o ut 1 0 - - 1 5 n m . T h ed if fe r en c e in le n g t h c o u ld n o t r e s u l t in s o l a r g e a d i ff e r e n c e a m o n g t h e e le c t r i c a l p r o p e r t i e s . Es p e -c ia l l y , p e a k s a p p e a r e d o n t h e d I / d V - V c u rv e s o f f i g s . 6 ( b ) a n d 6 ( c ) b e tw e e n w h i c h t h e d i s -t a n c e wa s n e a r ly t h e s a m e . Ac c o r d in g t o th e t i g h t b in d in g c a l c u la t i o n s b y Ca r r o l e t a l . ~ 5 3 , t h e r ee x i st e d r e s o n a n t s t a te s i n t h e v a l e n c e b a n d o f t h e ca p o f S W C N T s , s o p e a k s a p p e a r e d i n t h e d e n -s it y o f s t a t e s , w h i c h w a s t h e e l e c tr o n s t a t e d i s tr i b u t io n p e r p e n d i c u la r t o t h e a x i s o f S W C N T s . B u tt h e el e c tr i c al p r o p e r ti e s m e a s u r e d i n o u r ex p e r i m e n t s w e r e p a r a ll e l t o t h e a x i s o f S W C N T s . T h e r em u s t b e m o r e r e s o n a n t s ta t e s i f t h e re w e r e r e s o n a n t s t a te s i n t h e v a l e n c e b a n d , a n d t h e s y m m e t r ym a y b e b e t te r u n d e r s o m e c o n d i t i o n s . F u r t h er s t u d i e s o n t h e e l e c tr i c a l p r o p e r t ie s a re i n p r o g r e s s .

T a b le 1 E g o f S W C N T s m e a s u r e d p a r a l l e l l y t o t h e a x i s o f SW C N T sNo. (a) (b) (c) (d)

E s / e V 0 . 4 0 1 . 2 5 1 . 8 0 3 . 8 0

5 C o n c l u s i o nS W C N T s w e r e sy n t h e s i z e d b y D C a rc d i s ch a r g e m e t h o d , a w a t e r co l l o id o f s h o r t S W C N T s

w a s p r e p a r ed a ft er S W C N T s w e r e s e g r e g a t e d , p u r i fi e d a n d c u t s h or t b y o x i d a t i o n . T h e w a t e r c o l -l o id o f S W C N T s w a s u s e d t o a s se m b l e S W C N T s o n t o a g o l d f il m s u r f a c e . U s i n g S T M t ip s m a d e o fS W C N T s , a c r y st a l g ra in i m a g e o f a g o ld t h i n f il m a n d a n a t o m i c r e s o l u t io n i m a g e o f h i g h l y o r i-e n t e d p y r o ly t ic g r a p h it e w e r e s u c c e s s f u l l y o b t a i n e d . T h a t S W C N T s s t a n d o n g o l d t h in f i lm s i s ap r o m i s i n g t e c h n i q u e f o r a s s e m b l i n g a n d f a b r i c a t i n g h i g h - i n t e n s i t y c o h e r e n t e l e c t r o n s o u r c e s , f i e l de m i s s i o n f la t p a n e l d i s p l a y , t i p s f o r s c a n n i n g p r o b e m i c r o s c o p e s , n e w n a n o e l e c t ro n i c d e v i c e s ,e t c .


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1188 SCIENCE IN CHINA (Series A) Vol . 43

A ckn ow led gem ent s This work is partia lly suppo rted by the National Natura l Scien ce Foundation of China (Gr ant N os .69890221 , 69701001) .

Refer ence s1 . l i j ima , S . , He l i ca l mie ro tubes o f g laph i ti e ca rbon , N a tu re , 1991 , 358 : 56 .2 . T hes s , A . , L ee , R . , N ika laev , P . e t a l . , C rys ta ll ine ropes o f me ta l l i c ca rbon nano tube s , Sc ien ce , 1996 , 273 : 483 .3 . J ourne t , C . , Mas e r , W . , Be rn ie r , P . e t a l . , L a rge -s ca le p roduc t ion o f s ing le-wa l l ed ca rbon nano tubes by the e l ec t r ic -a re

techn ique , Na tu re , 1997 , 388 : 756 .4 . Odom, T . W . , Huang , J . L . , K im, P . e t a l . , A tomic s t ruc tu re and e lec t ron ic o f s ing le -wa l l ed ca rbon nano tubes , N a tu re ,

1998 , 391 : 62 .5 . Ca r ro l l , D . L . , Red l i eh , P . , A jayan , P . M. e t a l . , E lec t ron ic s t ructu re and loca li zed s t a t es a t c a rbon nano tube t ips , Phys .

R e v . L e t t . , 1 9 9 7 , 7 8 ( 1 4 ) : 2 8 1 1 .6 . Meun ie r , V . , L ambin , P . , T igh t -b ind ing computat ion o f the ST M image o f ca rbon nano tub es , Phys . Rev . L e f t . , 1998 , 81

( 2 5 ) : 5 5 8 8 .7 . T ans , S . J . , Ve rs chue ren , A . R . M. , Dekke r , C . , Room-tempera tu re t rans i s to r ba s ed on a sing le ca rbon nano tube , Na -tu re , 1998 , 393 : 49 .

8 . Kas umov , A . , Deb loek , R . , Koc iak , M. e t a l . , Supe reur ren ts through s ing le -wa l l ed ca rbon nano tubes , Sc ience , 1999 ,284 : 1508.

9 . Bene rd , J . , S toe ld i , T . , Ma ie r , F . e t 8 .1 ., F ie ld -emis s ion- induced lumines cence f rom ca rbon nano tubes , Phys . Rev . L e f t . ,1 9 9 8 , 8 1 ( 7 ) : 1 4 4 1 .

10 . Grag , A . , Han , J . , S inno t t , S . B . , In te rac t ions o f ea rbon-nano tube le p rox ima l p robe t ips wi th d iamond and g raphene ,P h y s . R e v . L e t t . , 1 9 9 8 , 8 1 ( 1 1 ) : 2 2 6 0 .

11 . Se rv ice , R . F . , Superst rong nano tubes s how they a re s mar t , t oo , S c ience , 19 98 , 281 : 940 .12 . Co l l ins , P . G . , Z e td , A . , Bando , H . e t a l . , Nanotuhe nanodev iee , Sc ience , 1997 , 278 : 100 .13 . j i ang tao , H . , Ouyang , M . , Yang , P . e t a l . , Con tro ll ed g rowth and e lec t r i ca l p rope r t ie s o f he te ro june tions o f ca rbonnanotubes and s i l icon nanow ires , Natu re , 1999 , 39 9 : 48 .14 . T ans , S . J . , Devore t , M. H . , Da l , H . J . e t a l . , Gee rl igs quan tum wi re s , Na tu re , 1997 , 386 : 474 .15 . Wi lodoe r , J . W . G . , Vene ma , L . C . , R inz le r , A . G . e t a l . , E lec t ron ic s t ruc tu re o f a tomica lly re s o lved ca rbon nano tubes ,

Na tu re , 1998 , 391 : 62 .

10.1007-bf02872196 propiedades electricas

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