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Structure-dependent mechanical properties of ultrathin zinc oxide nanowires.

Lee WJ, Chang JG, Ju SP, Weng MH, Lee CH - Nanoscale Res Lett (2011)

Bottom Line: 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.These results indicate that the ultrathin nanowire possesses very high malleability.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-sen University Kaohsiung, 804, Taiwan. jushin-pon@mail.nsysu.edu.tw.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus

Cross-section views of the structurally optimized ZnO. (a)-(c) are the snapshots of ZnO nanowires for Type I-III.
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Figure 2: Cross-section views of the structurally optimized ZnO. (a)-(c) are the snapshots of ZnO nanowires for Type I-III.

Mentions: This study addresses the tensile test of single crystalline HX phase ZnO nanowires of different wire width along the [0001] direction. Figure 1 shows the minimized procedure of potential energy per ZnO monomer as a function of minimization iteration, and the insets show the corresponding structure at different minimization steps. It is found that the energy gradually descends to -38.9 eV. The width of the nanowires becomes thicker and the WZ phase transforms to a HX phase after the energy minimization. The energy of the final equilibrium is slightly lower than that in the crystal phases (39.5-39.7 eV) [36,39] and is similar to that of ZnO in WZ, rocksalt, and blended structures confined within silica nanopores [39] and carbon nanotubes [40]. Note that the HX phase is an unbuckled WZ phase. This transformation phenomenon has been observed by the DFT study [25,26]. To understand the width effect on the mechanical property and the deformation mechanism in this work, three different widths of ZnO nanowires are optimized by energy minimization. Those optimized nanowire structures are denoted as Type I, II, and III. The cross-sectional structures of the ultrathin ZnO nanowires for Type I-III with diameters of approximately 0.7-1.1 nm are three-, two-, and sixfold axis symmetry structures, respectively. The cross-sectional views of the three optimized ZnO nanowires are presented in Figure 2, which are the most typical growth morphologies for ZnO nanowires found in experiments [41,42]. Compared to the average bond length of bulk ZnO with WZ structure, the value of Type I, II, and III are somewhat lower and are close to the DFT calculation results of 1.978, 1.989, and 1.999 Å [25], as shown in Table 2. Generally speaking, the ratio of the numbers of surface atoms to the total number of atoms increases as the width of the nanowire decreases. The relaxation of the surface atoms increases with the pre-compressive stress inside the nanowire [43]. Therefore, a nanowire width of smaller than critical size leads to an increase in the fraction of surface atoms significant enough to allow for the phase transformation to occur.


Structure-dependent mechanical properties of ultrathin zinc oxide nanowires.

Lee WJ, Chang JG, Ju SP, Weng MH, Lee CH - Nanoscale Res Lett (2011)

Cross-section views of the structurally optimized ZnO. (a)-(c) are the snapshots of ZnO nanowires for Type I-III.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211442&req=5

Figure 2: Cross-section views of the structurally optimized ZnO. (a)-(c) are the snapshots of ZnO nanowires for Type I-III.
Mentions: This study addresses the tensile test of single crystalline HX phase ZnO nanowires of different wire width along the [0001] direction. Figure 1 shows the minimized procedure of potential energy per ZnO monomer as a function of minimization iteration, and the insets show the corresponding structure at different minimization steps. It is found that the energy gradually descends to -38.9 eV. The width of the nanowires becomes thicker and the WZ phase transforms to a HX phase after the energy minimization. The energy of the final equilibrium is slightly lower than that in the crystal phases (39.5-39.7 eV) [36,39] and is similar to that of ZnO in WZ, rocksalt, and blended structures confined within silica nanopores [39] and carbon nanotubes [40]. Note that the HX phase is an unbuckled WZ phase. This transformation phenomenon has been observed by the DFT study [25,26]. To understand the width effect on the mechanical property and the deformation mechanism in this work, three different widths of ZnO nanowires are optimized by energy minimization. Those optimized nanowire structures are denoted as Type I, II, and III. The cross-sectional structures of the ultrathin ZnO nanowires for Type I-III with diameters of approximately 0.7-1.1 nm are three-, two-, and sixfold axis symmetry structures, respectively. The cross-sectional views of the three optimized ZnO nanowires are presented in Figure 2, which are the most typical growth morphologies for ZnO nanowires found in experiments [41,42]. Compared to the average bond length of bulk ZnO with WZ structure, the value of Type I, II, and III are somewhat lower and are close to the DFT calculation results of 1.978, 1.989, and 1.999 Å [25], as shown in Table 2. Generally speaking, the ratio of the numbers of surface atoms to the total number of atoms increases as the width of the nanowire decreases. The relaxation of the surface atoms increases with the pre-compressive stress inside the nanowire [43]. Therefore, a nanowire width of smaller than critical size leads to an increase in the fraction of surface atoms significant enough to allow for the phase transformation to occur.

Bottom Line: 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.These results indicate that the ultrathin nanowire possesses very high malleability.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-sen University Kaohsiung, 804, Taiwan. jushin-pon@mail.nsysu.edu.tw.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus