Figure 3.

Demonstration of the direct band gap of wurtzite GaP nanowires. (a) Photoluminescence spectra of a ZB (100) GaP substrate (red spectrum) and of WZ GaP/Al_0.4Ga_0.6P nanowires (blue spectrum). For both spectra, an excitation wavelength of 405 nm at a power of 0.6 W/cm² is used. (b) Integrated PL emission of the main ZB substrate peak at 2.317 eV (red line) is compared to that of the main WZ wire peak at 2.09 eV for different nanowire core diameters (blue data points). (c) Time-resolved measurements of the samples in (a), showing a fitted lifetime of 254 ns for the ZB substrate and 0.78 ns for the WZ wires, demonstrating the direct nature of the WZ GaP semiconductor material. The same data is shown within a larger time window in the Supporting Information S3. (d) Integrated PL intensity and lifetime of the WZ wire peak at 2.09 eV as a function of the Al_0.4Ga_0.6P shell thickness. (a–d) All measurements are performed at 4 K.