Abstract
1. Introduction
Modern technological pressure to develop miniature electronic devices with improved performance has led to the rapid expansion of research into low-dimensional metal oxide semiconductors (MOSs), which have been extensively applied in thin film transistors[
Generally, due to the limitations of size and the quantity of defects, nanosized MOSs (e.g., ZnO, CuO, Nb2O5, and WO3, etc.) can sustain a larger strain of ~3%−10% compared with their bulk counterparts[
Nonetheless, most of the experimental researches have focused on the macroscopic mechanical properties within the elastic range; while knowledge regarding the microscopic deformation mechanisms, especially the defect dynamics is far less developed. Due to the experimental limitation in manipulating the nanosized materials, studies regarding defect evolutions under mechanical stress heavily rely on the theoretical predictions, whereas the interatomic potentials, idealized crystal structure, and ultra-high strain rate may affect the simulated results[
This review focuses on the recent progresses regarding the microscopic deformation mechanisms in CuO and ZnO NWs. Based on the direct evidence of atomistic phase transition processes induced by the mechanical stresses, the deformation mechanisms are unveiled by applying both first-principles calculations and molecular dynamics (MD) simulations. The e-beam irradiation effects on low-dimensional MOS are then discussed. Finally, some future perspectives on in-situ nanomechanical researches in low-dimensional MOS are given.
2. In-situ nano-mechanics in low-dimensional MOS
Mechanical deformations in crystalline materials are contributed by the recoverable elastic strain (i.e., the lattice expansion/contraction), followed by the irrecoverable plastic strain (i.e., the lattice distortion), which is represented by the nucleation and movement of different types of defects[
There are three major kinds of defect structures reported in the MOSs. The first are planar defects, such as stacking faults (SFs) or twinning. SF or twinning deformations have been widely applied to enhance the strength and plasticity of metals and alloy[
Figure 1.(Color online) Anelasticity in CuO induced by the point defects migration[
The phase boundaries between CuO0.67 and CuO represent the possible point defect diffusion pathways (Fig. 2(a)). Accordingly, three possible pathways (
Figure 2.(Color online) Oxygen vacancy diffusion pathways in CuO[
Apart from the defect behaviors, phase transition could be another deformation mechanism related with the plasticity in MOS. For instance, there are three kinds of structures in bulk ZnO—wurtzite (WZ), zinc blende (ZB), and rock salt (RS) structures[
Figure 3.(Color online) The phase transition of ZnO NW with
To clarify the underlying phase transition mechanisms, the surface and bulk energies of the NW with WZ, h-MgO, and BCT structures (Fig. 4(a)) under different tensile strains were calculated. A phase diagram related to the width and thickness of the NW, under 0% and 7% strain is presented in Fig. 4(b). The results elucidate that both the sample morphology and strain play important roles in mediating the phase stability of ZnO, which is consistent with the previous reports[
Figure 4.(Color online) The phase stability of WZ, h-MgO, and BCT structures in ZnO[
3. Influence of e-beam irradiation
Although the high-energy electrons inside TEM can provide ultra-high spatial (e.g. sub-Å) and energy resolution for imaging and elemental analysis[
Here, by taking CuO and ZnO as examples, we show our attempt to minimize the possible irradiation effects on the deformation behaviors. The irradiation effects on the structural changes in CuO and ZnO nanomaterials have been investigated[
Figure 5.(Color online) The effects of e-beam irradiation on the anelasticity in CuO NWs[
Figure 6.The phase transition of ZnO NW by applying 200 kV e-beam[
4. Conclusion and perspectives
With the continuous development of ultra-high resolution and NEMS systems, in-situ TEM plays a key role in the study of the structure-property relationship of MOSs. In this review, the mechanical deformation mechanisms of low-dimensional MOS have been studied and discussed at the atomic scale. These findings could provide first-hand experimental basis for rational and optimized design of MOS-based nanodevices from the smallest bottom. However, the basic material physics and chemistry of MOS has remained mysterious considering the complex defect structures. The interaction of different types of defects subjected to various kinds of external mechanical stress (e.g., tension, compression, and bending, etc.) remains largely unexplored. The effects of size, morphology, and mechanical loading directions on the mechanical behaviors have to be clarified. In addition, the atomistic view of the defect-induced electronic structure changes is far from well-known, which could be resolved by the newly developed segmented annular all field (SAAF) technique[
Acknowledgements
This work was supported by the National Natural Science Foundation of China (52071237, 12074290, 51871169, 51671148, 11674251, 51601132, 52101021, and 12104345), the Natural Science Foundation of Jiangsu Province (BK20191187), the Fundamental Research Funds for the Central Universities (2042019kf0190), the Science and Technology Program of Shenzhen (JCYJ20190808150407522) and the China Postdoctoral Science Foundation (2019M652685).
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