Structure-dependent mechanical properties of ultrathin zinc oxide nanowires
© Lee et al; licensee Springer. 2011
Received: 8 December 2010
Accepted: 20 April 2011
Published: 20 April 2011
Mechanical properties of ultrathin zinc oxide (ZnO) nanowires of about 0.7-1.1 nm width and in the unbuckled wurtzite (WZ) phase have been carried out by molecular dynamics simulation. As the width of the nanowire decreases, Young's modulus, stress-strain behavior, and yielding stress all increase. In addition, the yielding strength and Young's modulus of Type III are much lower than the other two types, because Type I and II have prominent edges on the cross-section of the nanowire. Due to the flexibility of the Zn-O bond, the phase transformation from an unbuckled WZ phase to a buckled WZ is observed under the tensile process, and this behavior is reversible. Moreover, one- and two-atom-wide chains can be observed before the ZnO nanowires rupture. These results indicate that the ultrathin nanowire possesses very high malleability.
Wide band gap semiconductor materials, such as AlN, GaN, and ZnO, have attracted a lot of attention in the past because of their excellent performance in electronic, optoelectronic, and piezoelectric properties [1, 2]. As for the II-VI semiconductor material compound ZnO, it possesses a wide direct band gap (3.37 eV) and a strong excitation binding energy (60 MeV), such that it can be used in solar cells [3–7], optical sensitizers , and quantum devices. In 2001, Feick et al. identified one-dimensional ZnO nanorods , and since then many experts have successively identified and synthesized various kinds of ZnO nanostructures by experiment [1, 2, 10–18]. Manoharan  synthesized ZnO nanowires with diameters of 200-750 nm by using the vapor-liquid-solid (VLS) technique.
In 2006, Wang et al. [1, 2] found that the ZnO nanowire has piezoelectric property which can convert nanoscale energies such as mechanical, vibrational, or hydraulic energy into electrical energy in bending deformation. This performance indicates that the ZnO nanowire has both semiconducting and piezoelectric properties. This result allows the ZnO nanowire to have some applications in energy output by material deformation. He et al.  have performed nanomanipulation to measure the in situ I-V characteristics of a single ZnO nanowire. It has been demonstrated that a single ZnO nanowire can be a rectifier simply by mechanically bending it, similar to a p-n junction-based diode. Utilizing the coupled piezoelectric and semiconducting dual properties of ZnO, ZnO nanowire was used to compose piezoelectric field-effect transistors (PE-FET), which was then demonstrated as a force sensor in the nanonewton range . Fei et al.  report that the bent ZnO PFW cantilever can create a piezoelectric potential distribution across its width at its root and simultaneously produce a local reverse depletion layer with a much higher donor concentration than normal, dramatically changing the current flowing from the source electrode to drain electrode when the device is under a fixed voltage bias.
Because of the excellent properties and diversified application of the ZnO nanowire, it is necessary to develop a precise understanding of the mechanical property of ZnO nanowire. Experimentally, Manoharan  measured the Young's modulus of the ZnO nanowires with diameters of 200-750 nm by performing cantilever bending experiments and found that the Young's modulus was estimated to be about 40 GPa, which is smaller than that in the bulk scale (140 GPa). However, Chen et al.  and Agrawal et al.  found that the Young's moduli increased as the diameter decreased, and the values of these Young's moduli were larger than the bulk value. On the theoretical side, Kulkarni et al.  used molecular dynamics (MD) and first principles calculations to investigate the phase transformation of ZnO nanowires from wurtzite (WZ) to a graphite-like hexagonal (HX) structure under uniaxial tensile strain. Their results show that the WZ and HX structures of ZnO nanowires had different properties. The stress-induced phase transformation significantly alters the modulation of piezoelectric constant and thermal conductivity of the nanowire. Wang et al.  also reported structural transformation from WZ to HX structure for ultrathin ZnO nanowires under uniaxial elongation and compression. Moreover, the band gap, Young's modulus, and Milliken charges were calculated by density functional theory (DFT), and it was found that the band gaps of ZnO nanowires depended on the size and geometry of the nanowire, i.e., the WZ phase of ZnO nanowires had a larger band gap than the HX phase. However, for Young's modulus of the ZnO nanowires, HX phase was higher than the WZ phase, increasing with a decrease in the size of ZnO naowires (HX). Wu et al.  found a structure transformation by DFT calculation from the HX phase to WZ phase as the diameter and length of the AlN, GaN and ZnO nanowire increased. They also justified that the phase transformation is caused by competition between the bond energy, the Coulomb energy, and the energy originating from the dipole field of the WZ structure. Hu et al.  and Wang et al.  presented the mechanical properties of ZnO nanowires with WZ structure and nanotubes as a function of diameter by using MD simulation. Their results show that Young's modulus of ZnO nanowire is inversely proportional to the diameter of nanowire, which is a result in agreement with Wang et al. . They demonstrate that the size-dependent elastic properties of nanowires principally arise from the stress-induced surface stiffening. Wang et al.  found a novel stress-strain relationship with two stages of linear elastic deformation in -oriented ZnO nanorods under tensile loading . This phenomenon is caused by a phase transformation from WZ to a body-centered tetragonal structure with four-atom rings (BCT-4). In addition, they show that the two stages of linear elastic deformation still exist at a high temperature of 1500 K. Dai et al.  found the single atom chain structure during the tensile process. They explain that the growth of the single atom chain results from the bond breakage at the junction of the chain and the amorphous bulk. Moreover, they also propose a mechanics-based criterion for neck propagation. However, the Young's modulus and yielding stress of ZnO nanowires with thickness less than 1.6 nm is much lower than the other larger cases, which could be due to the significant deformation in the initial structure.
Until now, the research of the mechanical properties of ZnO nanowires with HX structure has concentrated almost solely on the elastic property. There is still no research discussing the deformation mechanism in detail. As a result, the present work uses MD simulation for detailed discussions of the mechanical properties (yield stress and Young's modulus) and the deformation behavior of ZnO nanowires under uniaxial tension.
In the present study, the mechanical property and deformation behavior of ZnO nanowires in the HX phase are investigated by MD simulations [30, 31]. To understand the lateral size effect of HX phase ZnO nanowires, three ZnO nanowires were chosen as the initial systems. These nanowires were initially in the WZ phase, and had a diameter of about 0.7-1.1 nm and length of 7.2 nm, because only ZnO nanowires with diameters less than 1.3 nm can transform to the HX phase [25, 26]. After the structural optimization by Genetic Algorithms software module, the ZnO nanowires transform to the HX phase. For the tensile test, canonical ensemble (NVT ensemble) [30, 31] is employed in the MD simulation. We intercept in the middle region of the optimized nanowires, because the structure of both ends is somewhat nucleated, which could affect the intrinsic property of the nanowire. The details will be discussed in the first paragraph of "Results and discussion" section. The lengths of three nanowires are all set 5.5 nm. The atom numbers for the three nanowires are 364, 448, and 512. Prior to elongation, the Zn atoms and O atoms consist of two atomic layers at both ends of the ZnO nanowire, which are kept fixed, whereas the remaining layers are the thermal control portion. This relaxation process was used to eliminate the internal stresses. For the thermal control portion, the Nosè-Hoover method is adopted to ensure a constant system temperature at 1 K throughout the elongation procedure and the Velocity Verlet algorithm [30, 31] is also employed to calculate the trajectories of the atoms. A time step of 1 fs was set for the time integration. In the axial tensile process, a tension with strain rate 0.02% ps-1 is applied to the nanowire by applying a constant velocity to the two fixed layers in the axial direction. To measure the stress of the ZnO nanowire under elongation, the formulation of atomic level stress  is employed, which includes the kinetic and potential effects.
Parameters of Buckingham and shell model potentials used in simulation
Results and discussion
Mechanical properties and bond length of ZnO nanowire with different type of structure under elongation test
σ y (GPa)
ε y (%)
Nanowire (HX) 
Nanowire (WZ) 
Nanobelt (WZ) 
We note that two stages of linear elastic deformation were observed in the tensile test of -oriented ZnO nanorods at a temperature higher than 300 K . However, the simulation result in this work shows a three stage stress-strain curve, which could result from the very low temperature, leading to the slow growth of phase transformation in stage II. The super ductility of the single atom chain and two atom row structures of ZnO have been observed in  ZnO nanowire under tensile loading by Dai et al. , as well as the carbon nanotube , and other metal nanowires . In addition, Horlait and Coasne et al. [39, 40] present the diversified atomic structure and morphology of ZnO nanostructure confined in carbon nanotube and porous silicas, discussing the effect of pore size and degree of pore filling on the self-assembly structure. The single atom chain, tubular structure, and both a four-atom ring and a six-atom ring are observed. These works verify the possible structural formation in this work.
Molecular dynamics simulations of tensile tests of ultrathin ZnO nanowires have been employed to study intrinsic behavior. Three different types of ultrathin ZnO nanowires, with diameter from 0.7 to 1.1 nm, are simulated, with maximum tensile strength, yielding strain, and Young's modulus calculated. Simulated nanowires were of three different cross-sectional shapes, Type I, II, and III. As the width of the nanowire decreases, the yielding strength, yielding strain, and Young's modulus increase, while the bond length decreases. The yielding strength and Young's modulus of Type III is much lower than the other two types, because Type I and II have the prominent edges on the cross-section structure of the nanowire, which leads a stronger surface tension. Observation of the deformation mechanism shows that the HX structure of the ultrathin nanowire under uniaxial tensile loading transforms to a buckled structure to relax the tensile stress until the structure is buckled throughout the nanowire. This phase transformation process is reversible, which implies that the process is an elastic stretching process. In addition, we found that a one-atom-wide and a two-atom-wide chain appear before the nanowires are broken for Type I and II, respectively.
density functional theory
piezoelectric field-effect transistors
The authors would like to thank the National Science Council of Taiwan, under Grant No. NSC98-2221-E-110-022-MY3, National Center for High-performance Computing, Taiwan, and National Center for Theoretical Sciences, Taiwan, for supporting this study.
- Qin Y, Wang XD, Wang ZL: Microfibre-nanowire hybrid structure for energy scavenging. Nature 809-U5.
- Wang ZL, Song JH: Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312: 242–246. 10.1126/science.1124005View ArticleGoogle Scholar
- Keis K, Magnusson E, Lindstrom H, Lindquist SE, Hagfeldt A: A 5% efficient photo electrochemical solar cell based on nanostructured ZnO electrodes. Sol Energy Mater Sol Cells 2002, 73: 51–58. 10.1016/S0927-0248(01)00110-6View ArticleGoogle Scholar
- Touskova J, Kindl D, Tousek J: Preparation and characterization of CdS/CdTe thin film solar cells. Thin Solid Films 1997, 293: 272–276. 10.1016/S0040-6090(96)09113-4View ArticleGoogle Scholar
- Chou HC, Rohatgi A, Jokerst NM, Kamra S, Stock SR, Lowrie SL, Ahrenkiel RK, Levi DH: Approach toward high efficiency CdTe/CdS heterojunction solar cells. Mater Chem Phys 1996, 43: 178–182. 10.1016/0254-0584(95)01626-6View ArticleGoogle Scholar
- Ferekides C, Britt J: CdTe solar cells with efficiencies over 15%. Sol Energy Mater Sol Cells 1994, 35: 255–262.View ArticleGoogle Scholar
- Niemegeers A, Burgelman M: Effects of the Au/CdTe back contact on IV and CV characteristics of Au/CdTe/CdS/TCO solar cells. J Appl Phys 1997, 81: 2881–2886. 10.1063/1.363946View ArticleGoogle Scholar
- Sebastian PJ, Ocampo M: A photodetector based on ZnCdS nanoparticles in a CdS matrix formed by screen printing and sintering of CdS and ZnCl2. Sol Energy Mater Sol Cells 1996, 44: 1–10. 10.1016/0927-0248(95)00168-9View ArticleGoogle Scholar
- Huang MH, Mao S, Feick H, Yan HQ, Wu YY, Kind H, Weber E, Russo R, Yang PD: Room-temperature ultraviolet nanowire nanolasers. Science 2001, 292: 1897–1899. 10.1126/science.1060367View ArticleGoogle Scholar
- Kong T, Chen Y, Ye YP, Zhang K, Wang ZX, Wang XP: An amperometric glucose biosensor based on the immobilization of glucose oxidase on the ZnO nanotubes. Sens Actuators B Chem 2009, 138: 344–350. 10.1016/j.snb.2009.01.002View ArticleGoogle Scholar
- Wang RM, Xing YJ, Xu J, Yu DP: Fabrication and microstructure analysis on zinc oxide nanotubes. New J Phys 2003, 5: 115.View ArticleGoogle Scholar
- Rao BB: Zinc oxide ceramic semi-conductor gas sensor for ethanol vapour. Mater Chem Phys 2000, 64: 62–65. 10.1016/S0254-0584(99)00267-9View ArticleGoogle Scholar
- Ozgur U, Alivov YI, Liu C, Teke A, Reshchikov MA, Dogan S, Avrutin V, Cho SJ, Morkoc H: A comprehensive review of ZnO materials and devices. J Appl Phys 2005, 98: 041301. 10.1063/1.1992666View ArticleGoogle Scholar
- Gao PX, Wang ZL: Nanoarchitectures of semiconducting and piezoelectric zinc oxide. J Appl Phys 2005, 97: 044304. 10.1063/1.1847701View ArticleGoogle Scholar
- He JH, Hsin CL, Liu J, Chen LJ, Wang ZL: Piezoelectric gated diode of a single ZnO nanowire. Adv Mater 2007, 19: 781. 10.1002/adma.200601908View ArticleGoogle Scholar
- Rodriguez JA, Jirsak T, Dvorak J, Sambasivan S, Fischer D: Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and density functional studies on the formation of NO3. J Phys Chem B 2000, 104: 319–328. 10.1021/jp993224gView ArticleGoogle Scholar
- Tien LC, Sadik PW, Norton DP, Voss LF, Pearton SJ, Wang HT, Kang BS, Ren F, Jun J, Lin J: Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods. Appl Phys Lett 2005, 87: 3.Google Scholar
- Sberveglieri G, Groppelli S, Nelli P, Tintinelli A, Giunta G: A novel method for the preparation of NH3 sensors based on ZnO-In thin films. Sensors and Actuators B: Chemical 1995, 25: 588–590. 10.1016/0925-4005(95)85128-3View ArticleGoogle Scholar
- Manoharan MP, Desai AV, Neely G, Haque MA: Synthesis and elastic characterization of zinc oxide nanowires. Journal of Nanomaterials 2008, 2008: 849745.View ArticleGoogle Scholar
- Wang XD, Zhou J, Song JH, Liu J, Xu NS, Wang ZL: Piezoelectric field effect transistor and nanoforce sensor based on a single ZnO nanowire. Nano Lett 2006, 6: 2768–2772. 10.1021/nl061802gView ArticleGoogle Scholar
- Fei P, Yeh PH, Zhou J, Xu S, Gao YF, Song JH, Gu YD, Huang YY, Wang ZL: Piezoelectric Potential Gated Field-Effect Transistor Based on a Free-Standing ZnO Wire. Nano Lett 2009, 9: 3435–3439. 10.1021/nl901606bView ArticleGoogle Scholar
- Chen CQ, Shi Y, Zhang YS, Zhu J, Yan YJ: Size dependence of Young's modulus in ZnO nanowires. Phys Rev Lett 2006, 96: 075505.View ArticleGoogle Scholar
- Agrawal R, Peng B, Gdoutos EE, Espinosa HD: Elasticity Size Effects in ZnO Nanowires-A Combined Experimental-Computational Approach. Nano Lett 2008, 8: 3668–3674. 10.1021/nl801724bView ArticleGoogle Scholar
- Kulkarni AJ, Zhou M, Sarasamak K, Limpijumnong S: Novel phase transformation in ZnO nanowires under tensile loading. Phys Rev Lett 2006, 97: 105502.View ArticleGoogle Scholar
- Wang BL, Zhao JJ, Jia JM, Shi DN, Wan JG, Wang GG: Structural, mechanical, and electronic properties of ultrathin ZnO nanowires. Appl Phys Lett 2008, 93: 021918. 10.1063/1.2951617View ArticleGoogle Scholar
- Wu YL, Chen GD, Ye HG, Zhu YZ, Wei SH: Origin of the phase transition of AlN, GaN, and ZnO nanowires. Appl Phys Lett 2009, 94: 253101. 10.1063/1.3159816View ArticleGoogle Scholar
- Hu J, Liu XW, Pan BC: A study of the size-dependent elastic properties of ZnO nanowires and nanotubes. Nanotechnology 2008, 19: 285710. 10.1088/0957-4484/19/28/285710View ArticleGoogle Scholar
- Wang J, Kulkarni AJ, Ke FJ, Bai YL, Zhou M: Novel mechanical behavior of ZnO nanorods. Computer Methods in Applied Mechanics and Engineering 2007, 197: 3182.View ArticleGoogle Scholar
- Dai L, Cheong WCD, Sow CH, Lim CT, Tan VBC: Molecular Dynamics Simulation of ZnO Nanowires: Size Effects, Defects, and Super Ductility. Langmuir 2009, 26: 1165–1171.View ArticleGoogle Scholar
- Hoover WG: Canonical dynamics: Equilibrium phase-space distributions. Phys Rev A 1985, 31: 1695–1697. 10.1103/PhysRevA.31.1695View ArticleGoogle Scholar
- Nosé S: A unified formulation of the constant temperature molecular dynamics methods. J Chem Phys 1984, 81: 511. 10.1063/1.447334View ArticleGoogle Scholar
- Chandra N, Namilae S, Shet C: Local elastic properties of carbon nanotubes in the presence of Stone-Wales defects. Phys Rev B 2004, 69: 094101.View ArticleGoogle Scholar
- Sun XW, Chu YD, Song T, Liu ZJ, Zhang L, Wang XG, Liu YX, Chen QF: Application of a shell model in molecular dynamics simulation to ZnO with zinc-blende cubic structure. Solid State Commun 2007, 142: 15–19. 10.1016/j.ssc.2007.01.035View ArticleGoogle Scholar
- Raymand D, van Duin ACT, Baudin M, Hermannson K: A reactive force field (ReaxFF) for zinc oxide. Surf Sci 2008, 602: 1020–1031. 10.1016/j.susc.2007.12.023View ArticleGoogle Scholar
- Dick BG, Overhauser AW: Theory of the Dielectric Constants of Alkali Halide Crystals. Phys Rev 1958, 112: 90. 10.1103/PhysRev.112.90View ArticleGoogle Scholar
- Oba F, Tanaka I, Nishitani SR, Adachi H, Slater B, Gay DH: Geometry and electronic structure of /(1230) Σ = 7 symmetric tilt boundary in ZnO. Philos Mag 2000, 80: 1567.View ArticleGoogle Scholar
- Zaoui A, Sekkal W: Pressure-induced softening of shear modes in wurtzite ZnO: A theoretical study. Phys Rev B 2002, 66: 174106.View ArticleGoogle Scholar
- Sun XW, Liu ZJ, Chen QF, Lu HW, Song T, Wang CW: Heat capacity of ZnO with cubic structure at high temperatures. Solid State Commun 2006, 140: 219–224. 10.1016/j.ssc.2006.08.024View ArticleGoogle Scholar
- Coasne B, Mezy A, Pellenq RJM, Ravot D, Tedenac JC: Zinc Oxide Nanostructures Confined in Porous Silicas. J Am Chem Soc 2009, 131: 2185–2198. 10.1021/ja806666nView ArticleGoogle Scholar
- Horlait D, Coasne B, Mezy A, Ravot D, Tedenac J-C: Molecular simulation of zinc oxide nanostructures confined in carbon nanotubes. Mol Simul 2010, 36: 1045–1058. 10.1080/08927022.2010.501798View ArticleGoogle Scholar
- Limpijumnong S, Jungthawan S: First-principles study of the wurtzite-to-rocksalt homogeneous transformation in ZnO: A case of a low-transformation barrier. Phys Rev B 2004, 70: 054104.View ArticleGoogle Scholar
- Yin M, Gu Y, Kuskovsky IL, Andelman T, Zhu Y, Neumark GF, O'Brien S: Zinc oxide quantum rods. J Am Chem Soc 2004, 126: 6206–6207. 10.1021/ja031696+View ArticleGoogle Scholar
- Ju SP, Lin JS, Lee WJ: A molecular dynamics study of the tensile behaviour of ultrathin gold nanowires. Nanotechnology 2004, 15: 1221–1225. 10.1088/0957-4484/15/9/019View ArticleGoogle Scholar
- Kulkarni AJ, Zhou M, Ke FJ: Orientation and size dependence of the elastic properties of zinc oxide nanobelts. Nanotechnology 2005, 16: 2749–2756. 10.1088/0957-4484/16/12/001View ArticleGoogle Scholar
- Weng M-H, Ju S-P, Wu W-S: The collective motion of carbon atoms in a (10,10) single wall carbon nanotube under axial tensile strain. J Appl Phys 2009, 106: 063504. 10.1063/1.3181056View ArticleGoogle Scholar
- Wen B, Sader JE, Boland JJ: Mechanical Properties of ZnO Nanowires. Phys Rev Lett 2008, 101: 175502.View ArticleGoogle Scholar
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