They were able to measure a strong photoluminescence signal from the silicon germanium nanowires. Photoluminescence is a good proxy for how efficiently it will light up - carriers are generated optically with a light source more energetic than the bandgap of the material. This is much easier than fabricating a full device with electrical contacts.

Silicon is typically a really lousy photon emitter because it’s an indirect bandgap material. Turning an electron/hole pair into a photon requires an interaction with a phonon. It seems by getting the silicon to grow in a hexagonal orientation, it becomes a direct bandgap material leading to much higher emission efficiency.