Figure 7: A schematic of a gradient bandgap structure of a ZnO fiber.

The strain gradient creates a continuously varying bandgap and a transverse piezoelectric field. (a) Under a large bandgap gradient, free excitons (FX) drift towards the tensile side and emit photons via a radiative recombination in the narrow bandgap, while bound excitons (BX) are ionized at irradiation sites of the e-beam by the piezoelectric field. (b) Low bending strain induces a small bandgap gradient and a low piezoelectric field. The FX and BX recombine at the excited site. (c) High bending strain induces a large bandgap gradient and a high piezoelectric field, in which more free excitons aggregate and recombine in the narrow bandgap, and BX convert into FX.