Throughout this action, the researcher has led the design and optimization of a cutting-edge microscopy system, which combines the capability of probing single nanoparticles with two different probes – and hence modalities – simultaneously. This advanced technique allows for the correlative interrogation of both optical and mechanical properties, at the sub-nanoparticle scale. This ultra-stable, ultra-high-resolution, capability, marries off an atomic force microscope with an inverted optical microscope – to enable the application of mechanical forces on the nanoscale from above with the simultaneous measurement of optical emission (and optical excitation) from below – allowing for elucidation of mechano-optical properties of nanoscale materials with unprecedented mechanical and optical resolutions.
Using this capability, the researcher has studied a novel class of lanthanide-doped upconverting nanoparticles – discovered in the host laboratory at the start of the action (Lee, et al., Nature 2021), and termed “photon-avalanching” nanoparticles. Through a chain reaction, photon-avalanching nanoparticles translate small changes in input to exceptionally large changes in output, and possess the steepest nonlinear response of any nanoscale material. The researcher isolated a single photon-avalanching nanoparticle and pressed upon it with the microscope tip (a needle 10,000 times thinner than a human hair) while optically imaging its emission from below – and discovered that the avalanche is extremely sensitive to miniscule physical forces. Tiny mechanical forces (equivalent to the weight of a grain of salt, divided by 1,000,000) lead to giant changes in the optical signal observed.
Photon avalanching excitation wavelengths and (upconverted) emission wavelengths are both in the near-infrared – benign wavelengths that can penetrate deeply into devices, or, alternately, into human tissue – without causing harm or damage to either. This exceptional characteristic of complete addressability with near-infrared allows for remote interrogation of nanoscale forces without the need for an electrical or mechanical extension; input and output are controlled via deeply-penetrating near-infrared. Hence, photon-avalanching nanoparticles are ideally suited for remote nanoscale sensing of mechanical force, especially from subsurface or interfacial sites.
Throughout the course of the action, the researcher has leveraged this extreme responsiveness of the avalanche to mechanical force to design different nanoparticles that react differently to different scales of force. Exploiting new nanoparticle schemes with precisely engineered energy transfer between the lanthanides within and photon avalanching mechanisms, the researcher has demonstrated novel force-dependent avalanche modalities, from emission intensity changes to emission wavelength changes, with unprecedented mechanical sensitivities.
The results of this action have been disseminated and presented on various platforms around the globe: from international conferences ranging from multidisciplinary to specialized, to outreach events targeting general audiences of various ages and backgrounds.
