VNU. JOURNAL OF SCIENCE, Mathematics - Physics, T.xx. N03AP, 2004
P R E P A R A TIO N AND P R O P E R T IE S OF SnOz N AN O W IRES
N g u y e n T h a n h B i n h , L e T h i T h a n h B i n h , L e D u y K h a n h , N g u y e n N g o e L o n g D epartm ent o f Physics, College o f Science, V N U
Abstract: Tin oxide (SnOj) nanowires have been synthesized in bulk quantities a t 1000°c b y th e rm a l e v a p o ra tio n o f g ra n u la r m e ta llic tin (S n). T h e X ray d iffra c tio n (X R D ) p a tte rn a n d sca n n in g e le ctro n m ic ro sco p e (S E M ) im a g e s sh o w th a t th e n a n o w ire s ha ve th e w id th in th e ran g e o f 3 0 -2 0 0 n m . T he p h o to lu m in e s c e n c e (P L ) an d p h o to lu m in e sce n ce e xc ita tio n (P L E ) s p e c tra w e re investigated. The peaks of P L spectra appeared at ~ 415 nm, 437 nm, 580 nm, 6 5 2 nm an d th a t o f e x c ita tio n s pe ctra a t 3 6 9 nm,
1. I n t r o d u c t i o n
S em iconductor o ne-dim ensional n a n o stru ctu res such as nanowirep, n anorods and n anoribbons, h a ve s tim u la te d a great in te re st due to th e ir im p orta n ce m basic scie n tific research. T h e y also h a ve attra cte d a g reat a tte n tio n fo r t h e ir p ote n tia l a p p lica tio n s in device and in tercon nect in te g ra tio n in nano e le ctro n ics a n d m ole cula r e lectron ics [1]. Am ong them, S n 02 is the m ost im p o rta n t m etal-oxide sem iconductor. I t is w e ll kn o w n fo r its potential applications in gas sensors, transparent conducting coating o f glass and sola r cells [2].
T h e na no stru ctu res S n 02 h a ve been fa b rica te d by d iffe re n t m ethods such as m agnetron s p u tte rin g [3], p re cu rsor th e rm a l decom position [4], sol-gel [5] a n d the rm a l vacuum evaporation [6, 7]. In th is p ap er th e p re pa ra tio n a n d o p tic a l p ro pe rties o f SnOọ n ano stru ctu res b y s im p le the rm a l evaporation are reported.
2. E x p e r im e n t
T h e e x p e rim e n ta l a p p a ra tu s used fo r th e p re p a ra tio n con sists o f a h o riz o n ta l tube furnace, a q u a rtz tube w it h one sealed e nd an d some cera m ic crucib le s. T h e g ra n u la r m eta llic t in w a s u se d as the source m a te ria l th a t wa s p laced in a cera m ic crucible. Th e cera m ic c ru cib le w a s th e n located a t the sealed end o f the q u a rtz tube. S evera l S i pla te s (5 mm X 5 mm) w ere p laced h o riz o n ta l on the cera m ic crucible.
T h e tem p era tu re o f the fu rn a ce w a s in creased from room tem p era tu re to 1000°c w ith the rate o f 10 K .m in -1 a n d th e ke p t at 1000°c fo r 4h. A fte r th e fu rn a ce w a s cooled to room tem perature, the cera m ic cru cib le is c a re fu lly rem oved o u t o f th e q u a rtz tube. It was observed th a t a t h ic k ivory-ye llow cotton-wool-like product w a s fo rm ed in a h igh yie ld on the s urface o f th e c ru cib le a n d on the S i plates. T h e p roducts can b e taken o ff from crucible in a form o f a b u lk sa m ple o r pow der sam ple. T h e s tru c tu ra l p ro pe rties o f the deposited product w a s ch a ra cteriz ed by S I M E N S D 5005 X -R a y d iffractom eter. T h e surface m orphology o f the pro du ct wa s analyzed by m eans o f sca n n ing e lectron m icroscopy using J E O L 5 4 1 0 V L m icroscope. T h e P L a n d P L E spectra o f the p ro du ct w ere recorded by spectro-fluorm eter FL3-2 2.
14
Preparation and properties of Sn02.
153. R e s u lt s a n d d is c u s s io n
T h e typ ical X R D p a tte rn o f the sam ple is show n in Fig. 1. The chemical composition o f the nanostructures waa determ ined to be S n 0 2. It. can be seen th a t a ll d iffra c tio n peaks are indexed to a tetragonal ru tile stru c tu re o f SnO., w ith la ttice c onstants o f a =4.7334 A and c = 3.1845 A , w h ic h are consistent w ith tho se o f p a tte rn 41-1445. N o ch a ra cteris tic peaks o f im p u ritie s such a s elem ental Sn o r o th er tin oxides were observed. T h e average g rain size estim ated by the S h c rre r’s fo rm ula is 377 nm.
F ig .2 show s the S E M im age o f SnOo b u lk sam ple. A large q u a n tity o f n anow ires a n d p a rticle s und er the n a no w ire s a re c le a rly observed, Fig . 3 shows a typ ical high m ag nifica tion S E M im age o f the as-prepared sam ple. T h e w ir e lik e shape is fu rth e r v e rifie d by the S E M im age. T h e typ ical d ia m eter o f the n a no w ire s are in the range o f 30-200 nm. A lo t o f na no w ire s h as a u nifo rm w id th alo ng its e n tire length.
T h e lengths o f the n a no w ire s are up to one h un d re d m icrom eter.
In the g ro w th o f S n 02 nanow ires, the m e ltin g o f m eta llic tin takes place a t 231.9°c. T h e liq u id t in c ould react w ith oxygen to y ie ld t in oxides. A s the tem perature fu rth e r increases the liq u id tin is o xid ized ra p id ly a t 700-800°c. In general, S nO form s a t the in it ia l stage o f the o xid a tio n o f tin . Because o f absence o f a c a rry in g gas the S n O va p o r can be deposited over the source m aterial. S n O is m etastable and w ill decompose to S n O-2 a n d S n by the fo llow ing reaction:
2S nO -» S nO , + Sn.
u
) iF ig .l: X R D pattern of the as- prepared SnO., nanoribbons.
F ig . 3. H igh magnification S EM image of the as-prepared SnO., In general, there are two possible m odels fo r the
g rowth o f nanow ires, na m e ly the va p o r-liqu id-solid (V L S ) an d vapor-solid (VS). V L S m echanism often takes place in the case o f n anow ires grow n by catalytic-assisted technique. It is w e ll kn ow n th a t in V L S grow th process, the d ro ple t is located a t the growth front o f the n a no w ire s and acts as the c a ta ly tic activ e site. F ro m S E M im age in Fig. 2, no solid ifie d sp he rica l d roplet w a s observed a t the end o f th e n anow ires. So the growth m echanism o f SnOo n a no w ire s can be ascribed to th e v s m echanism .
F ig . 2. S E M image of the as- prepared SnO., nanoribbons
T h e room tem peratu re excitation a n d em ission spectra o f the as-prepared b ulk sam ple are shown in Fig. 4. T h e excitation spectrum shows a broad band a t 369 nm (3.36 eV).
Th e e m ission spectrum present fo u r bands a t 415nm (2.99 eV), 437 nm (2.84 eV), 580 nm
16
Nguyen Thanh Binh, Le Thi Thanh Binh,
(2.14 eV) a n d 652 nm (1.90 eV), respectively. S ince lum inescence p ro perties o f S nO , stron gly depend on g ro w th m eth od a n d p re p a ra tio n conditions, the em ission peaks observed in d iffe re nt researches a re q uite different. C om p ared to e a rlie r P L an d P L E re su lts [5, 8, 9, 10], we suppose that
the electron tra n s itio n from va le n t band to oxygen vacancy V,,’ state corresponds to the peak a t 3.36 e V in e xcita tion spectrum , T h e P L peaks a t 2.99 e V a n d 2.84 e V m ig ht be attrib u te d to donor-acceptor p a ir recom bination, in w h ich vacancy v*„ p lays the ro le of donor. T h e peaks a t 2.14 e V an d 1.90 e V m ight origin a te from the lum inescence centers such as tin in te rs titia ls in the present S n O , n anow ires o r re sid u a l stra in s w ith in th in n anowires, b ut th a t is not ye t clear.
4. C o n c lu s io n
In sum m ary, SnO-> n anow iress w ere fabricated s im p ly through the rm a l evaporation u sin g g ra n u la r m e ta llic Sn as source m aterial. T h e g row th m echanism o f S n O2 n anow ires can be ascribed to the v s m echanism . F ro m X -ra y m easurem ents, the c hem ical com position o f the na no stru ctu res w a s determ ined to be S n O j w ith la ttice con stan ts a = 4.7334 A, c = 3.1845 A . S E M im ages o f as-prepared S n O , n anow ires shows th a t the sam ples consist of a large q u a n tity o f n a no w ire s w ith ty p ica l d ia m eter in the range o f 30 -200 nm. Th e P L peaks in the b lue range are a ttrib u te d to donor-acceptor recom bination. T h e o th er P L peaks are related w ith c ry s ta l defects crea ted d u rin g the g rowth o f the SnOg n anowires.
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(a) (b)
Fig.4: Room temperature P L E (a) and P L (b) spectra of as-prepared S n02 bulk sample
