DSpace at VNU: Synthesis of ZnO whiskers by a simple thermal evaporation

6 Ngac An Bang, Nguyen Ngoc Long26 degree Figure 1.. at 650 °c on polished Si substrate... Synthesis of ZnO whiskers by a simple thermal evaporation 7Figure 5.

S Y N T H E S IS OF ZnO W H IS K E R S BY A S IM P L E T H E R M A L

EV APO RA TIO N

N g a c A n B a n g , N g u y e n N g o c L o n g

D e p a rtm en t o f Physics, College o f Science, V N U

Abstract Single-crystalline zinc oxide (ZnO) whiskers were succe ssfu lly

s y n th e s iz e d o n th e Si/Si02 substrates by th e rm a lly eva p o ra tin g m etal z inc

p o w d e rs a t te m p e ra tu re s o f 6 0 0 a n d 6 5 0 ° c E D S m ic ro a n a ly s is a n d X R D

m e a s u re m e n t s h o w e d th a t th e p ro d u ct w a s s in g le -c ry s ta llin e Z n O w ith a

h e xa g o n a l w u rtz ite structure S E M a n a lysis re v e a le d th a t th e syn th e s iz e d Z n O

w h is k e rs h a v e an ave ra g e dia m e te r o f 2 0 0 n m a n d ty p ica l le n g th s o f up to 1 àm

T h e P L s p e c tru m o f t h e s y n th e size d sa m p le excite d a t a w a v e le n g th o f 3 2 5 nm

c o n s is te d o f a u ltra -vio le t e m issio n at a b o u t 3 7 8 nm a n d a m u ch b ro a d e r gre en

th e d e e p -le v e l e m issio n s, resp ective ly

1 I n t r o d u c t i o n

H ig h ly o r ie n te d a n d w e ll p a tte rn e d a rra y s o f z in c o xid e (ZnO) n a n o s tru c tu re s

h a v e been d e m o n s tra te d to h a v e p o te n tia l a p p lic a tio n s in n a n o sca le la s e rs [1], fie ld

e ffect t r a n s is to r s [2], T h e r e a re v a r io u s te c h n iq u e s th a t h a ve been su ccessfu lly

e m p lo ye d to s y n th e s iz e w e ll a lig n e d Z n O n a n o ro d s on s o lid su b stra te s A m o n g them ,

th e s im p le th e r m a l e v a p o r a tio n te c h n iq u e h a s been w id e ly u se d d ue to its s im p lic ity

a n d r e la tiv e ly h ig h - q u a lity pro du cts In t h is re p ort, w e d e m o n s tra te a s im p le ro u te o f

fa b r ic a tin g f a ừ ly a lig n e d Z n O w h is k e rs in a ữ a t r e la tiv e ly lo w te m p e r a tu re w ith o u t the

p re se n ce o f e ith e r c a r r ie r gas o r m e ta llic ca ta ly st T h e c r y s ta l s tr u c tu r e a n d m orphology

as w e ll as o p tic a l p ro p e rtie s o f th e sy n th e s iz e d sa m ple s w e re in ve stig a te d

2 E x p e r im e n t s

T h e s y n th e s is o f Z n O w h is k e r s w a s c a r r ie d o u t in a h o r iz o n t a l tu b e furnace

S tr ip s o f p o lis h e d a n d u n p o lis h e d (100) S i w e re u se d as sub stra te s T h e y w e re faced

d o w n w a rd on th e top o f a n a lu m in a boa t lo a d e d w it h m e ta l z in c p ow d ers a t a v e r tic a l

d is ta n c e o f a b o u t 5 mm T h e r m a l e va p o ra tio n o f th e z in c p o w d e r sou rce w a s then

c a r r ie d o u t a t 6 0 0 a n d 650 ° c fo r 45 m in u te s in a ứ A fte r c o o lin g dow n, a w h ite la y e r o f

m a te r ia ls w a s fo u n d ove r th e S i sub stra te T h e c r y s ta l s tr u c tu r e a n d m orph o lo g y o f the

s y n th e s iz e d s a m p le s w e re c h a ra c te r iz e d b y a S iem e n s D 5 0 0 5 X R D d iffra c to m e te r a n d a

J E O L - J S M 5 4 1 0 L V s c a n n in g e lectron m icroscop y (S E M ), re sp e ctive ly T h e S E M

eq u ip p e d w it h th e O X F O R D - I S I S 300 ene rg y d isp e rs iv e X -ra y s p ectrom e te r (E D S ) was also e m p lo ye d to c o n d u c t th e E D S sp ectrosco p y a n d e lem e nt m apping

P h o to lu m in e s ce n ce (P L ) sp e c tr a w e re m ea su re d a t room te m p e ra tu re u s in g a F lu o ro lo g

F L 3 - 2 2 S p e c tr o flu o ro m e te r w it h a X en o n la m p o f 450 w as a n e x c ita tio n source

6 Ngac An Bang, Nguyen Ngoc Long

26 (degree)

Figure 1. X R D pattern o f Z nO whiskers

Energy (keV)

Figure 2. E D S spectrum o f ZnO whiskers

Figure 3. S E M image o f Z nO whiskers grown Figure 4. S E M image of Z nO whiskers grown

at 600 °c on rough Si substrate at 650 °c on polished Si substrate

3 R e s u lt s a n d d i s c u s s io n

F ig u r e 1 sh o w s a t y p ic a l X R D p a tte rn o f th e s y n th e s iz e d s am ple A l l the

d iffr a c tio n p e a ks can be w e ll in de xe d to a h ex a g on a l w u r t z ite s tr u c tu r e o f Z nO N o

o th e r p ha se s s u c h as Zn a re detected A s tro n ge st a n d s h a rp e s t (002) p e a k in the

sp ectrum in d ic a te s t h a t th e Z n O w h is k e rs grew p r e fe r e n tia lly a lo n g th e c -a xis (<0001 >

d ire ction ) o rie n te d n o rm a l to th e S i sub stra te I t sho uld , h ow ev er, be n o tice d the presence o f th e r e la tiv e ly stro n g (101) a n d (103) d iffra c tio n p e a k s w h ic h im p lie s th a t

th e re is a c e rta in degree o f is o tro p ic a n d ra n d o m o r ie n ta tio n in th e sa m ple T h e E D S

sp ectrum is d e p ic te d in F ig 2 I t is c le a r th a t, b e sid e the tra ce o f S i su b s tra te , a l l the

E D S p e a k s a re a t tr ib u te d to th e b in d in g e nergy o f th e Zn a n d 0 elem e nts

F ig u r e s 3 a n d 4 sho w S E M im ag es o f the Z n O sa m p le s s y n th e s iz e d a t 600 ° c on

u n p o lish e d s id e a n d a t 650 ° c o n w e ll p o lis h e d s id e o f S i su b s tra te s I n b oth cases, high

d e n s ity a r r a y o f f a ir ly w e ll-a lig n e d Z n O w h is k e rs is observed T h e w h is k e r s are

o rie n te d n e a rly p e r p e n d ic u la r to the substr'ate surfa ce H ig h m a g n ific a tio n S E M im ag es sho w n as in s e ts i n the se fig u r e s re v e a l th a t the w h is k e r s a re s tr a ig h t a n d a b o u t 1 pm in

le n g th T h e h e x a g o n a l fa c e ts on the tip s can be c le a r ly id e n t ifie d c o n fir m in g t h a t the

w h is k e rs a c tu a lly g re w a lo n g the <0001> d ire ctio n T h e ave ra g e d ia m e te r s o f Z n O

w h is k e rs g ro w n a t 600 “C a n d 650 ° c a re a b ou t 150 n m a n d 200 nm , re sp e ctive ly

Synthesis of ZnO whiskers by a simple thermal evaporation 7

Figure 5. Room temperature P L spectrum o f the ZnO whiskers synthesized at 600 °c

T h e P L sp e c tr u m o f th e s y n th e s iz e d Z n O w h is k e rs , e x c ite d a t a w a v e le n g th o f 325

n m , c o n s is ts o f a u ltr a - v io le t (Ư V ) e m is s io n a t a b ou t 378 n m (3.28 eV) a n d a m uch

b ro a d e r g re en e m is s io n b a n d c e n te re d a t a b ou t 495 n m (2.5 eV) as sho w n in F ig 5 T h e

u v e m is s io n b a n d can be a ttr ib u te d to th e n e a r ban d-ed g e (N B E ) tr a n s itio n , nam ely,

g reen e m is s io n b a n d r e s u lt s fro m the r a d ia l re co m b in a tio n o f electron s w it h holes tra p p e d in s in g ly io n iz e d oxygen v a ca n cie s (4] a n d i t is c om m on ly seen in Z n O m a te ria ls

s y n th e s iz e d u n d e r o x y g e n -d e fic ie n t con d itio n s T h e s tro n g e r green e m is s io n in

c o m p a ris o n to th e N B E e m is s io n in d ic a te s th a t th e s y n th e s iz e d Z n O w h is k e rs a re ric h

in a to m ic defects

4 C o n c lu s io n s

A r r a y s o f f a ừ ly a lig n e d Z n O w h is k e rs w ere s u c c e s s fu lly sy n th e s iz e d on the

S i/ S i02 s u b s tra te s v ia s im p le th e r m a l e v a p o r a tio n m ethod T h e w h is k e rs grew

p r e fe r e n tia lly a lo n g th e c -a x is (<0001> d ire ction ) o rie n te d n o rm a l to th e S i substra te

T h e ave ra g e d ia m e te r is in th e ra n g e o f 150 n m to 200 n m d e p e n d in g on th e g ro w n tem p era tu re T h e P L s p e c tru m is d o m in a te d by a stro n g g re en em ission , a lth o u g h u v

e m is s io n is also observed

A c k n o w le d g m e n t s W e w is h to t h a n k T h e N a t u r a l S cie n ce C o u n c il o f V ie tn a m fo r fin a n c ia l s u p p o rt (u n d e r C od e N° 811304) T h a n k s m ust a ls o go to th e C e n te r fo r

M a t e r ia ls S cie n ce , H a n o i U n iv e r s ity o f S cience, a n d th e I n s t it u te o f M a t e r ia ls Science,

V A S T fo r m a k in g som e e x p e rim e n ta l f a c ilitie s a v a ila b le to us

R e f e r e n c e s

1 H u a n g M , M a o s., F e ick H., Y a n H., W u Y , K in d H., W ebw er E-, R usso R., Y a n g p.,

Science 292(2001), 1897

2 B a i X D., W ang E G , G ao p X and W ang z L-, Appl Phys L ett. 82(2003), 4806

3 K ong Y c „ Y u D p., Z han g B-, F a n g w „ Fe ng Q „ Appl Phys Lett. 78(2001), 407

4 V anh eusd en K., W a rre n w L , T a lla n t D R., V o ig h t J A a n d G nade B E., J Appl