2024年4月11日发(作者：)

Nanostructures

of zinc oxide

Zinc oxide (ZnO) is a unique material that exhibits

semiconducting, piezoelectric, and pyroelectric

multiple properties. Using a solid-vapor phase

thermal sublimation technique, nanocombs,

nanorings, nanohelixes/nanosprings, nanobows,

nanobelts, nanowires, and nanocages of ZnO have

been synthesized under specific growth conditions.

These unique nanostructures unambiguously

demonstrate that ZnO is probably the richest family

of nanostructures among all materials, both in

structures and properties. The nanostructures could

have novel applications in optoelectronics, sensors,

transducers, and biomedical science because it is

bio-safe.

by Zhong Lin Wang

Since the discovery of oxide nanobelts of

semiconducting oxides in 2001

1

, research into

functional oxide-based, one-dimensional

nanostructures has rapidly expanded because of

their unique and novel applications in optics,

optoelectronics, catalysis, and piezoelectricity.

Semiconducting oxide nanobelts are a unique group

of quasi-one-dimensional nanomaterials, which have

been systematically studied for a wide range of

materials with distinct chemical compositions and

crystallographic structures.

Belt-like, quasi-one-dimensional nanostructures (called

nanobelts) have been synthesized for semiconducting oxides

of Zn, Sn, In, Cd, and Ga, by simply evaporating the desired

commercial metal oxide powders at high temperatures. The

as-synthesized oxide nanobelts are pure, structurally uniform,

single-crystalline, and mostly free from dislocations; they

have a rectangular-like cross-section with constant

dimensions. The belt-like morphology appears to be a unique

and common structural characteristic of this family of

semiconducting oxides with cations of different valence

states and materials of distinct crystallographic structures.

Field-effect transistors

2

, ultrasensitive nano-sized gas

sensors

3

, nanoresonators

4

, and nanocantilevers

5

have been

fabricated based on individual nanobelts. Thermal transport

along the nanobelt has also been measured

6

. Very recently,

nanobelts, nanosprings

7

, and nanorings

8

that exhibit

piezoelectric properties have been synthesized, which could

be candidates for nanoscale transducers, actuators, and

sensors.

School of Materials Science and Engineering,

Georgia Institute of Technology,

Atlanta, GA 30332-0245 USA

E-mail: @

26June 2004

ISSN:1369 7021 © Elsevier Ltd 2004

REVIEWFEATURE

Among the functional oxides with perovskite, rutile, CaF

2

,

materials for applications in optoelectronics, lasing, and

spinel, and wurtzite structures

9

, ZnO is unique because it

piezoelectricity. The two important characteristics of the

exhibits dual semiconducting and piezoelectric properties.

wurtzite structure are the noncentral symmetry and polar

ZnO is a material that has diverse structures, whose

surfaces. The structure of ZnO, for example, can be described

configurations are much richer than any known

as a number of alternating planes composed of tetrahedrally

nanomaterials including carbon nanotubes. Using a solid-

coordinated O

2-

and Zn

2+

ions, stacked alternately along the

state thermal sublimation process and controlling the growth

c

-axis (Fig. 2a). The oppositely charged ions produce

kinetics, local growth temperature, and the chemical

positively charged (0001)-Zn and negatively charged

composition of the source materials, a wide range of

(0001)-O polar surfaces, resulting in a normal dipole moment

nanostructures of ZnO have been synthesized (Fig. 1). This

and spontaneous polarization along the

c

-axis, as well as a

review focuses on the formation of nanohelixes, nanobows,

divergence in surface energy.

nanopropellers, nanowires, and nanocages of ZnO.

By adjusting the raw materials with the introduction of

impurities, such as In, we have synthesized a nanoring

Nanohelixes/nanosprings and

structure of ZnO (Fig. 2)

8

. High-magnification scanning

seamless nanorings

electron microscopy (SEM) images clearly show the perfect

The wurtzite structure family has a few important members,circular shape of the complete ring, with uniform shape and

such as ZnO, GaN, AlN, ZnS, and CdSe, which are importantflat surfaces. Transmission electron microscopy (TEM) images

Fig. 1 A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders. Most of the structures presented can be produced with 100%

purity.

June 2004

27