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

VNU. JOURNAL OF SCIENCE, Mathematics - Physics, T.xx, N03AP. 2004

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 ac A n Bang, N g uy en N goc Lon g

D epartm ent o f Physics, College o f Science, V N U

Abstract. Single-crystalline zinc oxide (ZnO) whiskers were succe ssfu lly
s yn th e size d o n th e Si/Si0 2 substrates by th e rm a lly eva p o ra tin g m etal z inc
po w de rs a t te m p e ra tu re s o f 6 0 0 an d 65 0 ° c . E D S m ic ro a n a ly s is a n d X R D
m e a su re m e n t s ho w ed th a t the p ro du ct w as s in g le-c ry s tallin e Z nO w ith a
h e xag on al w u rtz ite structure. SE M a n alysis re v e aled th at th e syn th e s iz e d Z nO
w h iske rs ha ve an ave rag e dia m e te r o f 20 0 nm a n d ty p ica l len gth s o f up to 1 àm .
T he P L s p e ctru m o f th e s y nthe size d sam p le excite d a t a w a v e le n g th o f 32 5 nm
c o n siste d o f a u ltra -vio le t e m ission at a b o u t 3 7 8 nm a n d a m uch bro ad er green
e m ission b a n d cen te re d at ab ou t 4 9 5 nm . T hey w ere attrib uted to th e N B E and
th e d e e p -le ve l e m ission s, respectively.

1.

I n t r o d u c tio n
H ig h ly o rie 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 ra n s is to rs

[2], T h e r e

a re

v a rio u s

te c h n iq u e s

th a t

h ave

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 an 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 rm a l e v a p o ra 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 ra 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 tru c tu re 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 sam ple s w e re in ve stig ate 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 rs w a s c a rrie d o u t in a h o riz o n t a l tu b e furnace.

S trip 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 an a lu m in a boat lo a d e d w it h m e ta l z in c p ow d ers a t a v e rtic a l
d is ta n c e o f a b o u t 5 mm. T h e rm 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 rrie 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 ria ls w as fo u n d ove r th e S i sub stra te . T h e c ry s ta l s tru c tu re 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 riz 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
JE O L-JS M 5 4 1 0 LV

s c a n n in g

e lectron

m icroscop y

(S E M ),

re sp e ctive ly.

The

SEM

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

conduct

th e

EDS

sp ectrosco p y

and

e lem e nt

m apping.

P h o to lu m in e s ce n ce (P L ) sp e c tra 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 tro flu o ro m e te r w it h a X en o n la m p o f 450 w as an e x c ita tio n source.

5


NgacAnBang,NguyenNgocLong

6

26 (degree)

Figure 1. X R D

Energy (keV)

Figure 2.

pattern o f Z nO whiskers.

Figure 3.

S E M image o f Z nO whiskers grown
at 600 °c on rough Si substrate.

3.

E D S spectrum o f ZnO whiskers.

Figure 4.

S E M image of Z nO whiskers grown
at 650 °c on polished Si substrate.

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 ll

the

d iffra c tio n p e aks can be w e ll in de xe d to a h ex ag on al w u r t z ite s tru c tu re 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 an d s h a rp e s t (002) p e ak 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 re 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) an 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 rie n ta tio n in th e sam 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 trib u te d to th e b in d in g e nergy o f th e Zn an 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 sam 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 rra y o f fa ir ly

w e ll-a lig n e d

ZnO

w h is k e rs

is observed.

The

w h is k e rs

are

o rie n te d n e a rly p e rp e n d ic u la r to the substr'ate surface . 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 in the se fig u re s re v e a l th a t the w h is k e rs 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 rly id e n t ifie d c o n firm 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 rag e d ia m e te rs 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 ab ou t 150 n m a n d 200 nm , re sp e ctive ly.
H ig h e r g ro w n te m p e ra tu re re s u lts in th e fo rm a tio n o f la rg e r d ia m e te r w h is k e rs .


SynthesisofZnOwhiskersbyasimplethermalevaporation

Figure 5.

7

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

T h e P L sp e c tru 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 ltra - v io le t (Ư V ) e m is s io n a t ab 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 ab 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 ttrib u te d to th e n e a r ban d-ed g e (N B E ) tra n s itio n , nam ely,
th e re c o m b in a tio n o f fre e e xcito n s th ro u g h an e xcito n -ex citon c o llis io n process [3]. T h e

g reen e m is s io n b a n d r e s u lt s fro m

the ra 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 acan cie s (4] a n d it 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

under

o x y g e n -d e fic ie n t

con d itio n s .

The

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 fa ừ ly

S i/ S i0 2 s u b s tra te s

a lig n e d Z n O

v ia

s im p le

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

th e rm a l

e v a p o ra tio n

m ethod.

The

w h is k e rs

grew

p re 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 substrate.
T h e ave rag 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 fe 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-,

3.

K ong Y . c „ Y u D. p., Z han g B-, F a n g w „ Fe ng Q „

4.

V anh eusd en K., W a rre n

Phys.

79(1996) 7983.

Appl. Phys. Lett.

82(2003), 4806.

Appl. Phys. Lett.

78(2001), 407.

w. L ., T a lla n t D. R., V o ig h t J . A . an d G nade B. E., J.

Appl.