Faulted ZnS:Mn,Cu Nanorods Provide Nanoscale Mechanoluminescence Excitation in a Biologically Relevant Force Range

Mechanoluminescent materials emit photons when subjected to mechanical stress, with the emission intensity proportional to the applied force. This property enables their use as force sensors with a direct optical readout. However, spatial resolution of the force sensing is limited by the crystal size, with thresholds near 100 μm. Seeking to overcome this limitation, we introduce stacking fault-rich, highly crystalline, monodisperse ZnS nanorods codoped with Mn and Cu (ZnS:Mn,Cu) approximately 60 × 20 nm in size. The design of these nanorods leveraged insights from the nanoscale mechanism for elastic mechanoluminescence at stacking faults that was known for micrometer-scale ZnS crystals doped with Mn. Here, ensemble impact tests confirm that the faulted ZnS:Mn,Cu nanorods indeed exhibit mechanoluminescence, where the intensity is dependent on the concentrations of both Mn and Cu. The mechanoluminescence intensity peaks at 0.15 wt % of Mn. Furthermore, a proportional increase in intensity is observed within the range of the tested Cu concentrations. The mechanoluminescence of individual nanorods with optimized dopant concentrations was investigated by using correlated atomic force and optical microscopy, with multiple force cycles delivered to individual nanorods to track force-dependent changes in the intensity of the optical emission on a single-particle level. Mechanoluminescence was detected at force amplitudes ranging between 13.9 and 100 nN, with no observable change in the nanorod morphology. Our results confirm that the introduction of stacking faults enables excitation of repeatable elastic mechanoluminescence in single nanometer crystals, which are not embedded in any matrix. This approach enables high-resolution force sensing in three dimensions in the low-nanometer range relevant to biological applications.