Changing the shape of ZnO nanostructures by controlling Zn vapor release

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

CHANGING THE SHAPE OF ZnO NANOSTRUCTURES

BY CONTROLLING Zn VAPOR RELEASE

T a D in h C a n h , T ra n T hi Q uyn h H oa, D ao Q u a n g D u y ,
N g u y e n V ie t T u y e n , N g u y en Q u a n g H oa, N g u y e n N g o e L ong

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

Abstract. Our fabrication of ZnO nanostructures was based on a thermal
evaporation process of ZnO powder without the presence of a catalyst. The
source material was pure ZnO powder mixed with different contents of graphite
(molar ratio 1:1.2: 1:1.1: 1:1; 1:0.8, respectively). Temperature is the critical
experimental parameter for the formation of different morphologies of ZnO
nanostructures, Zn or Zn suboxide plays a crucial role for the nucléation of ZnO
nanostructures. The structure of the deposited materials was investigated by
X-ray diffraction, scanning electron microscopy. Photoluminescence measurements
were performed.

1. In tr o d u c tio n

Z n O n a n o s tru c lu re s are very p rom isin g fo r ap p lica tion s in fie ld e m ission displays
and photonic devices operating in blue and u v spectral ranges due to its wide band gap 
(3.37 eV) a n d large exciton b in d in g energy (60 m eV) [1], Th erefore, fa b rica tio n and
properties o f Z n O n a n o stru ctu res have attracted considerable a tte ntion re ce n tly [2,3],

In th is w ork, we fa b rica te d Z n O nanow ires from a m ixtu re o f Z n O a n d carbon in
order to in ve stig a te the change in the shape o f obtained stru ctu re s by v a r y in g the release
rate of Zn vapor.

2. E x p e rim e n ts

ZnO nanowires array was fabricated by a thermal evaporation process of the ZnO - c 
pow der mixture. The mixture of zinc oxide and graphite powders was loaded in a quartz
boat and placed in the cen te r o f a h orizo nta l q u a rtz tube furnace, w h e re the tem perature,
pressure and evaporation time were controlled. The thermal evaporation of the ZnO + c 
powder source was performed at 1000 UC for 1 h under an argon flow with the rate of 500 
stan d ard cub ic cen tim e te rs per m inu te (seem) m a ita in in g a con stan t pressure o f 0.2 atm in
the fu rn a ce cham ber. S evera l cleaned s ilico n substra tes w ere placed in sequence a t the
dow nstream in the q u a rtz tube fo r collection o f the products.

T h e as-synthesized products were chara cterized by X -ray d iffra c tio n (X R D ) (Siemens
D505 with Cu K„ radiation), Scanning electron microscope (SEM) (JSM 5410 LV) for the 
ana lysis o f the m icrostru ctu re . Photolum inescence m easurem ents w e re perform ed by using
a FL 3 - 22 spectrometer with a Xenon lamp as the excitation source at room temperature.

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3 0

Ta Dinh Canh, Tran Thi Quynh Hoa...

3. R e su lts an d d is c u s s io n

?ĩ beta-Scale

Fig.l. XRD patterns of the as - made ZnO 
nanowires with different ratio of ZnO
(a ) X = 0 .8 : (b) X = 1.0: (c) X = 1.1; (d) X = 1.2

F ig . l show s th e X R D p a tte rn s o f as
synthesized Z nO n a n o w ire s w ith th e source
m aterial w ith d iffe re n t con te n ts o f g raph ite
(m olar ra tio s o f p u re Z n O a n d g ra p h ite are
x = 0.8. 1.0, 1.1 a n d 1.2, respectively).

The crystallinity of the ZnO nanowires, 
the existence of zinc flakes in the as-made 
sample as X = 0.8 and 1.0 w e re studied by X -
ray diffraction. From these spectra, it is found 
that both samples with X = 0.8 and 1.0 show a 
typical wurtzite hexagonal structure like bulk 
ZnO with unitcell constants of a = 3.248 Â and 
c = 5.206 Â.

A re p resentative energy dispersive X-ray
(ED S) spectrum o f the n a no w ire s is d epicted in F ig .2.
O n ly the peaks associated w ith Zn and o atoms are
seen in the E D S spectrum , le a d in g to the obvious fact
th a t the n anow ires a re in de e d Z n O m aterial.

T h e S E M im ages in Fig .3 e x h ib it the side view
o f d iffe re nt shapes o f Z n O n a no w ire s grown on a Si
substrate in the tem p era tu re range (650-500°C).
T h ese im ages show th a t th e n a n o w ire s are grown not
from the Si substra te b ut from a th ick layer o f Z nO
p oly crystallin e m a te ria l on the top o f the Si

substrates, in d ic a tin g that the th ick la ve r is formed before th e fo rm a tio n o f the nanowires.
Fig.2. The EDS spectrum of the ZnO

Fig.3. The side view images of the material grown on Si substrate: 
temperature range (650 - 500 oC) with different ratio of ZnO and carbon powder

(a) X = 1.2; (b) X = 1.1 ; (c) X = 1.0; <d) X = 0 .8

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Changing the shape of ZnO nanostructures by...

Wave length (nm)
Fig.4. The P L E spectra o f ZnO nanowires
on Si subtrates with different contents of c 
(a) X = 0.8; (b) X = 1.0; (c) X = 1.1; (d) X = 1.2

ZnO + c -» Zn + CO,

ZnO + CO —> Zn + CO2 .
Zn vapor should condense on the iner 
wall on the quartz tube forming liquid
droplets, which are ideal nuclei o f ZnO
nanowires for the vapor-liquid-solid (VLS) 
reaction [1]. (Fig.3, a, b).

Figure 4 presents photoluminescence 
excitation (PLE) spectra o f nanowires with 
different ratio of ZnO and c powder on Si 
su bstrates for th e green em ission monitored 
at 500nm. The peak of excitation spectra appears at 371nm (= 3.34 eV) approximated to 
the energy band edge.

Photolum inescence spectra o f ZnO 2E*007,,— 
-nanowires a t room tem perature were 16E*007- ig
measured and shown in F ig .5. A weak u v - *
band a t 380 nm and a strong green band at

500 nm w ere detected from all ZnO products.

The u v band em ission o f ZnO, maybe, has 
been understood to be related to the exciton 
em ission, w h ile th e m echanism of green
em ission has been su g g ested mainly due to 
th e present o f various point defects, either 
extrinsic or intrinsic, which can easily form 
recombination centers.

Wavelength (nm)
Fig .5. Room temperature P L spectra of ZnO
nanowires with different contents of carbon
( a ) X = 0.8 ; (b) X = 1.0; (c) X = 1.1; (d) X = 1.2

4. C o n clu sio n s

U sin g zinc oxide and graphite powders a s source m aterials, ZnO nanowires were 
farbicated on Si su b stra tes at low tem peratures. The growth process w as attributed to a 
VLS m echanism. The ZnO nanowires exhibit an em ission at u v and green region.

A cknow ledgem ents. T his work is supported by Natural Science Council o f VN under

code No 811304. The authors would like to thank th e Center for Material Science, 
Faculty of Physics, U n iversity of Science • VNU for help us in experim ents.
R e fer en ces

1. Y.u.Leung, A.B.Djurisic, J.Gao. M.H.Xie, W.K.Chan, Chermical Physics leters, 
385(2004). 155-159.

2. J.S.Lee, K.s.Park, M.I.Kang, I.w.Park, S.W..Kim, W.K.Cho, H.S.Han, s.Kim , Journal

o f crystal grow th, 254(2003), 423-431.

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