The researchers… discovered that gallium arsenide can be tuned with a small strain to function efficiently as a single light-emitting diode or a photodetector because of a hexagonal crystal structure, referred to as wurtzite.
In wurtzite semiconductors, the atoms are located in very specific positions along the nanowire. That leads to electron and hole wave-functions overlapping strongly but optical transitions between these states being impaired by symmetry. If you change that symmetry with a strain, you can switch between direct or pseudo-direct bandgaps.
“When you pull the nanowire along its length, the nanowire is in a state that we call “direct bandgap” and it can emit light very efficiently; when instead you compress the length of the wire, its electronic properties change and the material stops emitting light,” said IBM scientist Giorgio Signorello in an IBM release. “We call this state “pseudo-direct”: the III-V material behaves similarly to silicon or germanium and becomes a good light detector.”