Surface- and Strain-Mediated Reversible Phase Transformation in Quantum-Confined ZnO Nanowires

The phase stability of ZnO in a quantum-confinement size regime (sub-2-nm) remains fiercely debated. Applying in situ (scanning) transmission electron microscopy, we present the atomistic view of the phase transitions from the original wurtzite structure to an intermediate body-centered tetragonal and h-MgO structure under tensile strain in quantum-confined ZnO nanowires. Strikingly, such structural transitions are reversible after releasing the stress. Further theoretical calculations mirror the transition pathway and provide basic insight into the overall landscape regarding surface- and strain-dependent phase transition behavior. Our results provide the critical piece to solve the puzzle in phase stability of ZnO, which may prove essential for advancing a variety of nanotechnologies, e.g., quantum-dot light-emitting devices.

Figure 1

Time-resolved HAADF images (a)–(c) along the ${[0001]}_{\mathrm{WZ}}$ zone axis showing structural changes of ZnO NWs with ${[1\overline{2}10]}_{\mathrm{WZ}}$ axial direction. (d)–(f) The magnified view of the regions pointed out by arrowheads in (a), (b), and (c), respectively. (g)–(i) Structural models showing the phase transition process corresponding to (a)–(c). Gray and red spheres denote Zn and O atoms, respectively.

Figure 2

The reversible phase transitions from WZ structure (a) to an intermediate bct (b) and final h-MgO structure (c), which recovered back to the original WZ structure (d)–(e). The time-lapsed HRTEM images along the ${[10\overline{1}0]}_{\mathrm{WZ}}$ zone axis showing reversible structural changes of ZnO NWs with ${[1\overline{2}10]}_{\mathrm{WZ}}$ axial direction. (f)–(j) Structural models showing the reversible phase transition process corresponding to (a)–(e), respectively.

Figure 3

Estimation of the tensile strain or stress on the ZnO NWs with axial direction of ${[1\overline{2}10]}_{\mathrm{WZ}}$ showing the local strain of 3.8% (a)–(b), 3.5% (c)–(d), and 2.4% (e)–(f) of three individual ZnO NWs. (g) Strain or stress evolution of ZnO NWs.

Figure 4

The simulation results showing the phase transition among WZ (a), bct (b), and h-MgO (c) structures in NWs with axial direction of ${[1\overline{2}10]}_{\mathrm{WZ}}$. (d)–(f) The cross-section view of the NW during the phase transition. (g)–(h) The phase diagrams of ZnO under 0% and 7% strains, respectively.