Normalized PL spectra of GaAs/GaInAs MQW shell on GaAs NW with different quantum-well widths. (a) 8.0 nm, (b) 11 nm, and (c) 16 nm. The red line represents the PL spectra of the planar GaInAs MQWs grown on GaAs substrate. Inset in (a) shows the increase of PL intensity of GaAs NW due to the presence of MQW shells. PL peak corresponding to GaInAs shell shifted to lower energy with the increase of GaInAs shell width.