Nanoscale mechanical characteristics of vertical ZnO nanowires grown on ZnO:Ga/glass templates

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 225603 (5pp) doi:10.1088/0957-4484/18/22/225603

Nanoscale mechanical characteristics of

vertical ZnO nanowires grown on

ZnO:Ga/glass templates

S J Young

1

_{, L W Ji}

2

_{, S J Chang}

1

_{, T H Fang}

3

_{, T J Hsueh}

1

_,

T H Meen

2

and I C Chen

4

1_{Institute of Microelectronics and Department of Electrical Engineering, National Cheng}

Kung University, Tainan 701, Taiwan

2_{Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632,}

Taiwan

3_{Institute of Mechanical and Electromechanical Engineering, National Formosa University,}

Yunlin 632, Taiwan

4_{Micro Systems Technology Center, Industrial Technology Research Institute South,}

Tainan 709, Taiwan

E-mail:[email protected]

Received 13 December 2006, in final form 8 March 2007

Published 4 May 2007

Online at

stacks.iop.org/Nano/18/225603

Abstract

The mechanical properties of vertical single-crystal ZnO nanowires on

ZnO:Ga/glass templates were characterized by nanoindentation experiments

in this work. The results from x-ray diffraction and Raman spectra show

good crystal quality for the ZnO nanowires. The buckling loads were found

to be 1465 and 215

μN for ZnO nanowires of 100 and 30 nm diameters,

respectively. When the fixed–fixed column mode was used, it was found that

the Young’s modulus values of the ZnO nanowires of 100 and 30 nm

diameters were 117 and 232 GPa, while the critical buckling strains were

0.62% and 0.35%, respectively. On the other hand, when we employed the

fixed–pinned column mode, it can be seen that the Young’s modulus values

were 229 and 454 GPa, while the critical buckling strains were 0.32% and

0.18%, respectively. Buckling behaviour of the ZnO nanowires was

significantly predicted by the Euler buckling model in this work.

1. Introduction

One-dimensional (1D) materials such as nanowires, nanobelts and nanorods have attracted considerable interest in recent years. They present the utmost challenge to semiconductor technology, making fascinating novel devices possible. It has been demonstrated that these 1D materials exhibit superior electrical, optical, mechanical and thermal properties. It has also been shown that these materials are potentially useful for nanoscale interconnects, active components of optical electronic devices and nanoelectromechanical systems (NEMS). However, it is important to understand the mechanical characteristics of these nanowires prior to any feasible applications [1–3]. For example, mechanical properties of carbon nanotubes (CNTs) have been studied extensively by tensile loading, bending and buckling [4–10].

To our knowledge, mechanical properties of various Si and III–V semiconductor-based nanowires have also been reported [11–13].

1D oxide systems such as SnO2 [14], tungsten oxide

(

W18O49

)

[15], GeO2 [16], indium tin oxide (ITO) [17],

Al2O3 [18] and ZnO [1–3, 19, 20] nanowires have also

attracted much attention in recent years. Among them, ZnO is an n-type direct-gap semiconductor with a large exciton binding energy of 60 meV and wide bandgap energy of 3.37 eV at room temperature. Hence, ZnO is regarded as a promising photonic material [21,22]. However, only a few reports on the mechanical properties of ZnO nanowires can be found in the literature [1,2,23–25]. Previously, Song et al [1] used atomic force microscopy (AFM) to measure the elastic properties of vertically aligned ZnO nanowires. By scanning AFM tips

Nanotechnology 18 (2007) 225603 S J Young et al

catastrophe is immediate. If ideal loading is wholly plastic on the other hand, then the material’s yield strength limits the load that can be applied. Failure in practice is governed by elastic–plastic interaction, such as the dashed locus of the sketch. Thus, the load-carrying capacity of a wire decreases significantly as the slenderness ratio

L/r

increases, especially in the middle region of

(L/r)

cvalues.

4. Conclusion

In summary, we report the experimental observations of buckling instabilities in vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates by a self-catalyzed vapour– liquid–solid process without any metal catalyst. X-ray diffraction and Raman spectra all show good crystal quality of our ZnO nanowires. The mechanical properties of vertical ZnO nanowires under uniaxial compression were performed by using the nanoindentation technique. The critical buckling loads of the ZnO NWs are found to be 1465 and 215

μ

N for samples A (100 nm diameter) and B (30 nm diameter), respectively. Furthermore, the buckling energies were 3

.

62

×

10−10J and 3

.

69

×

10−11J for samples A and B, respectively. The Euler buckling model can be employed in evaluating the Young’s modulus

(E)

and the critical buckling strain

(ε

cr

)

of

individual ZnO nanowires. A size dependence of Young’s modulus in [0001]-oriented ZnO nanowires has been found in this work.

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