Nano Lett. http://doi.org/f9s7dk (2017)

Nanowires have emerged as promising probes in biological cells owing to their small size and optoelectronic properties. Now, Joanna Oracz and co-workers from Germany, Poland and Sweden have used ground state depletion (GSD) nanoscopy to resolve photoluminescent, heterostructure semiconductor nanowires at a resolution below the diffraction limit of light at room temperature. The nanowires consist of alternating non-luminescent gallium phosphide (GaP) and luminescent gallium indium phosphide (GaInP) segments, forming a barcode-like pattern, with an emission lifetime of 10–100 ps. The approach works by controlling the transfer of electrons out of the valence band ('off' state) into the conduction band of GaInP ('on' state), in a coordinate-targeted manner. The researchers show that the GSD nanoscopy technique can resolve nanowires with diameters down to 20 nm — a fivefold resolution enhancement over confocal imaging. The resolution of the GSD system is believed to be limited by the material properties of the nanowires and the residual intensity in the minimum of the doughnut-shaped beam. Because of the far-red excitation wavelength (∼700 nm) and low excitation power (∼3 mW), GSD nanoscopy is expected to be suitable for biological applications.