The crystallization and physical properties of Al-doped ZnO nanoparticles

The crystallization and physical properties of Al-doped ZnO nanoparticles

K.J. Chen

, T.H. Fang

, F.Y. Hung

*, L.W. Ji

_{, S.J. Chang}

_{, S.J. Young}

_{, Y.J. Hsiao}

a_{Institute of Microelectronics & Department of Electrical Engineering, Center for Micro/Nano Science and Engineering,}

National Cheng Kung University, Tainan 701, Taiwan

b

Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan

c

Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan

d

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan

1. Introduction

The applications of ZnO have attracted much attention in recent

years. With a wide direct bandgap energy (3.37 eV) and a larger

binding energy (60 meV), ZnO is potentially useful in various

optoelectronic applications such as optical sensors and light

emitters

[1,2]

, etc. In addition, ZnO is also potentially useful in

surface acoustic wave (SAW) devices, gas sensing devices and

piezoelectric devices

[3–7]

. In fact, devices containing bulk ZnO,

ZnO films, ZnO nanowires and ZnO nanoparticles have all been

demonstrated

[1–7]

.

Recently, ZnO have been prepared by sputtering

[8]

, chemical

vapor deposition (CVD)

[9]

, molecular beam epitaxy (MBE)

[10]

,

spry pyrolysis

[11]

, pulse laser deposition

[12]

and the sol–gel

process

[13,14]

. Among these methods, the sol–gel process is

particularly attractive because of its simplicity and acceptable

costs, however the crystalline quality of the ZnO prepared by the

sol–gel process might be inferior to the other methods. Notably,

the sol–gel processes with an annealing treatment are intimately

affect the crystallization and physical properties. Previous

literature, Kuo et al.

[14]

have investigated the optical and

electrical properties of sol–gel derived ZnO thin films with a low

annealing temperature. Also, Zhou et al.

[15]

have studied the

effect of annealing temperature on the microstructure, electrical

and optical properties of Al-doped ZnO films. Notably, a low

annealing temperature cannot improve the crystallization, and the

effect of Al-doped concentration is worthy of further investigation.

In addition, relevant reports for ZnO doped with metals (Al

_,

In

_{, Ga}

_{, etc.) indicate that the doping effect increased the optical}

and electrical properties of the ZnO

[16–18]

. However, the

crystallization at high temperatures has still not been investigated.

Furthermore, the sols concentration also affects the crystalline,

optical and electrical properties of ZnO. Schuler and Aegerter

[19]

have investigated the effects of sols concentration on the optical,

electrical and structural properties of ZnO: Al coatings. However,

higher concentration of sols has not been studied.

To understand the effect of high concentration sols with metal

dopant and heat treatment on the structural characteristics and

physical properties of ZnO nanoparticles, this study doped Al (0–

9 at.%) into ZnO (2 M) nanoparticles by the sol–gel process to

investigate the microstructural variations and used different heat

treatment conditions to analyze the physical properties of AZO

nanoparticles.

2. Experiments

In addition to zinc acetate, it is possible to fabricate sol–gel ZnO

samples using zinc nitrate

[20]

. ZnO prepared with zinc acetate

A R T I C L E I N F O Article history:

Received 4 January 2008

Received in revised form 25 February 2008 Accepted 14 March 2008

Available online 26 March 2008 Keywords:

Sol–gel ZnO Crystallization

A B S T R A C T

Un-doped Al (0–9 at.%) nanoparticles and doped ZnO powders were prepared by the sol–gel method. The nanoparticles were heated at 700–800 8C for 1 h in air and then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectra and photoluminescence (PL). The results of un-doped (ZnO) and Al-un-doped ZnO (AZO) nanoparticles were also compared to investigate the structural characteristics and physical properties. XRD patterns of AZO powders were similar to those of ZnO powders, indicating that micro-Al ions were substituted for Zn atoms and there were no variations in the structure of the ZnO nanoparticles. From the XRD and SEM data, the grain size of the AZO nanoparticles increased from 34.41 to 40.14 nm when the annealing temperature was increased. The Raman intensity of the AZO nanoparticles (Al = 5 at.%) increased when the annealing temperature was increased. Increasing the degree of crystalline not only reduced the residual stress, but also improved the physical properties of the nanoparticles.

ß2008 Elsevier B.V. All rights reserved.

* Corresponding author.

E-mail address:[email protected](F.Y. Hung).

C o n t e n t s l i s t s a v a i l a b l e a t

S c i e n c e D i r e c t

Applied Surface Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.080

Al concentration, the crystalline quality of ZnO degenerated, which

is associated with the stress generated which resulted in lattice

disorder. For Raman spectroscopy, the 438 and 582 cm

_modes

decreased with increasing Al concentration, which is attributed to

Al ions bonding with oxygen or Al ions substituting for oxygen

deficiency. In addition, the two peaks at 543 and 658 cm

_were

assigned as Zn–C and Zn–CH

modes, respectively. The PL

characteristics show that the optical quality degenerated gradually

with increasing the Al concentration. 5 at.% Al-doped ZnO powders

had the best characteristics and optical properties in this study.

Acknowledgement

The authors are grateful to the Chinese National Science Council

for its financial support (Contract: NSC 96–2221-E-006–103-MY2;

NSC 97–2218-E-006–011).

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