VNU- JOURNAL OF SCIENCE, Mathematics - Physics, T.xx, Nq3AP, 2004
F A B R IC A T IO N AND C H A R A C TE R IZ A T IO N O F ZnO NANORODS
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 D epartm ent o f Physics, College o f Science, V N U
A b s tr a c t: Z n O n a n o ro d s w e re s u cce ssfu lly syn th e s iz e d o n S i s u b s tra te s b y th e th e rm a l e va p o ra tio n o f z in c po w de r a t 1 0 00 o c . T h e m ic ro s tru c tu re o f sa m p le s w a s s tu d ie d b y X ra y d iffra c tio n (XR D ) a n d S E M im a g e m e a s u re m e n ts . T h e as- s y n th e s iz e d Z n O ro d s ha ve th e d ia m e te r in th e ra n g e o f 8 0 - 2 0 0 nm . T h e p h o to lu m in e s c e n c e (P L ) s p e c tra o f th e s a m p le s w e re in ve s tig a te d a t room te m p e ra tu re . F o r all th e sam p le s, th e lu m in e s c e n c e s p e c tra c o n s is t o f th e ne ar b a n d e d g e e m issio n (~ 3 8 0 n m ) and a bro a d g re e n b a n d ( - 5 0 0 nm ).
1. I n t r o d u c t i o n
In recen t year, o n e -d im en s io n a l (ID ) n a n o s tru c tu res have a ttra cte d a g reat deal o f a tte ntion d ue to t h e ir u n iq u e properties a n d p o te n tia l fo r fu tu re technology. T h e re have been m any reports on the synthesis o f I D nanostructure, such as: S n 02 [1], ln203 [2], Z nO [3]...
T h ese m a te ria ls h a ve a p p lica tio n s in c on stru ctin g n anoscale electron ic a n d optoelectronic devices.
Z in c o xid e (ZnO), w h ich has a dire ct b an d gap o f 3.37 e V a n d la rg e exciton b in d in g energy o f 60 m eV a t room tem perature, can e nsu re an efficie n t exciton u ltra v io le t (UV) em ission a t room tem p era tu re u n d e r a lo w e x c ita tio n energy. S evera l m ethods have been reported fo r the s y n th esis o f one - d im en sion a l Z n O n a n o stru ctu res (nanow ires, nanorods, nanobelts) such as: a rc discharge, la ser va p o riza tio n , p yrolysis, electro deposition, chem ical va p o r d eposition a n d th e rm a l evaporation...
In th is w ork, w e have used therm al eva p ora tio n m ethod fo r syn th esis o f zin c oxide (ZnO) n anorods b y a n n e a lin g z in c (Zn) pow der in a ir at 1000°c.
2. E x p e r im e n t
Z n O n a no ro ds w e re prepa red by th e rm a lly e vap oratin g z in c pow ders as source m aterials. Z inc p ow d ers w ere p laced a t the b ottom o f a q u a rtz tube. S ev era l s trip - lik e S i wa fers w ere p laced in sid e th e tube to act as th e substra tes fo r g ro w in g m aterials. Th e d istan ce from source m a te ria ls to the substra tes w a s k e p t a t 2 cm. T h is q u a rtz tube was r a p id ly heated to 1000°c fro m room tem perature. S ubsequently, th e fu rn a ce w a s kept at 1000° c fo r a n h o u r a n d it w a s cooled to room tem perature.
T h e c ry s ta l stru c tu re s w e re determ ined w it h a Siem ens D 5005 X ray d iffra ctio n (X R D ) th a t used a C u K a ra d ia tion . Th e m orphologies w ere observed by u sin g J S M 5410 L V sca n n in g electron m icroscopy (SEM). T h e p hotolum inescence spectra w ere investigated a t room tem p era tu re b y s p ectrofluorm eter F L 3 — 22.
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Nguyen Thanh Binh, Le Thi Thanh Binh, Le Duy Khanh
3. R e s u lt s a n d d is c u s s io n s F ig u r e 1 show s the X R D pattern o f the synthesized sam ple. A l l the d iffra ctio n peaks show good agreem ent w ith those o f the hexagonal Z nO w ith lattice constants a=0.3245nm, c = 0.5208 nm. N o d iffra c tio n peaks o f oth er im p u ritie s w e re fo un d in a n y of o u r samples.
A typ ica l S E M im ag e fo r tetrapod-like Z n O n anorods is shown in Fig. 2. T h e crysta ls con sist o f fo ur
needles—shaped tetra h ed ra l-a rra ng e d F ig .l: X RD pattern o f tetrapod-like ZnO nanorods legs connected at the center, fo rm in g a
tetra pod stru ctu re. T h e se rods have the diam eter in the range o f 80-200nm . T h e le n g th c the legs is in the range o f l-4nm .
F ig . 2: S EM micrograph of tetrapod-like ZnO nanorods on the Si substrate
In general, there e xist severa l m echanism s proposed to a cco u nt fo r the g row th o f ID structure, in c lu d in g v a p o r-s o lid (VS), va p o r-liq u id-so lid (V LS ), s o lid - liq u id - s o lid (SLS) and so on [5, 6], T h e V L S m echanism is im possible because no m eta l c a ta lv st w a s used and there w ere no n a n o p a rticle observed on any tip s o f the legs in o u r sam ples. T h e S L S m echanism s could also be im possible because no solu tio n phase w a s used in the experim ent. Th erefore, it is lik e ly th a t the g row th o f tetra p od -lik e Z n O n anorods is governed by the v a p o r-s o lid (VS). H e re in , a t the h ig h tem p eratu re, th e Zn va p o r generated from Zn pow ders com bines w ith oxygen to form Z nO va p o r an d w h ic h deposit on the substrate.
Ph oto lu m inescen ce spectrum o f the sam ple a t room te m p e r a tu re w a s m ea sured and shown in Fig. 3. F o r a ll the sam ples, the lum inescence spectra c on sist o f the n ea r band edge em ission (~ 380 nm) a n d a broad green band (~ 500 nm). T h e P L in te n s ity depends on pre pa ra tio n conditions. T h e peak a t u ltra v iole t (U V ) region w h ic h is a ro u n d 3S0nm, is
F a b r ic a tio n a n d c h a r a c te r i z a t i o n o f Z n O n ạ n o r o d s
observed by o th er authors- [3, 4, 6, 7], T h is peak can be in te rp re te d as exciton recom bination [7], In t h is paper, we only deal w ith a broad green b a n d centered a t 496 nm. T h is green photolu m in escent em ission may be assig n ed to the presence o f oxygen vaca n cie s in the Z nO film [8], In o rd e r to in ve stig a te the o rig in s o f these peaks, w e w ill m easure the P L sp ectra a t low tem p era tu re and rep ort n ex t tim e in the o th e r w ork.
4. C o n c lu s io n s
F ig . 3: The P L spectra at room temperature k„ = 300 nm In sum m ary, tetra p od -lik e Z nO
n anorods have been fa b rica te d by therm al e vaporation o f Z n pow der at
1000°c v ia the v s g ro w th m echanism s w ith la ttice constants a = 0.3245nm , c = 0.5208nm.
Th ese ro d s h ave the d ia m e te r in the range o f 80 -200nm.
R oom tem p era tu re P L spectra o f the sam ples showed a u v e m ission a t 380nm and a broad green e m ission at 496 nm, w h ich w a s assigned to re com bination o f exciton a n d the donor-acceptor p airs, respectively.
Acknow ledgem ent
T h is w o rk is s u p p o rte d by the Project T N 04 - 04
T h e a u th ors w ou ld lik e to th a n k M r. N g uy en Q u a n g H o a fo r X -ra y m easurem ents a n d M r. H o a n g D ue A n h fo r ta k in g the S E M m icrographs.
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