Cathodoluminescence-Activated Nanoimaging: Noninvasive Near- Field Optical Microscopy in an Electron Microscope Connor G. Bischak, † Craig L. Hetherington, †,§ Zhe Wang, ⊥ Jake T. Precht, † David M. Kaz, †,§,○ Darrell G. Schlom, ⊥,¶ and Naomi S. Ginsberg* ,†,‡,§,∥,∇ † Department of Chemistry and ‡ Department of Physics, University of California, Berkeley, California 94720, United States § Physical Biosciences and ∥ Materials Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States ¶ Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States ∇ Kavli Energy NanoSciences Institute, University of California, Berkeley, California 94720, United States * S Supporting Information ABSTRACT: We demonstrate a new nanoimaging platform in which optical excitations generated by a low-energy electron beam in an ultrathin scintillator are used as a noninvasive, near-field optical scanning probe of an underlying sample. We obtain optical images of Al nanostructures with 46 nm resolution and validate the noninvasiveness of this approach by imaging a conjugated polymer film otherwise incompatible with electron microscopy due to electron-induced damage. The high resolution, speed, and noninvasiveness of this “cathodoluminescence-activated” platform also show promise for super-resolution bioimaging. KEYWORDS: cathodoluminescence, nanoimaging, nanostructures, soft materials, super-resolution imaging, resonant near-field coupling T he emergence of far-field super-resolution techniques, such as stochastic localization, 1−4 stimulated emission depletion (STED), 5 and structured illumination 6 microscopies has revolutionized the fluorescence imaging of labeled bio- logical structures. Yet, capturing nanoscale biological dynamics and imaging systems using their endogenous chromophores both remain challenging for these methods. Near-field optical probes 7−17 have proved valuable for the characterization of complex structures and of processes in solid state materials, soft matter, and biological samples that occur over length scales smaller than the wavelength of light. In most variants of near- field scanning optical microscopy (NSOM), an optical probe is integrated with a scanning tip and rastered over a sample to form an image. Yet, images acquired with NSOM require mechanical scanning and can contain artifacts from tip−sample interactions. On the other hand, in scanning electron microscopy (SEM), a focused electron beam is electronically scanned over a sample to obtain nanoscale images by correlating the detected scattered electrons with the position of the beam, recently achieving the resolution to image single atoms. 18 Traditional electron microscopy is incapable of spectrally specific excitation and damages soft materials such as biological samples. One can, however, detect light generated in the sample by the electron beam in a process called cathodoluminescence (CL), which has been used historically to investigate the nanoscale properties of solid luminescent materials 19 and more recently to characterize a variety of metallic nanostructures. 20−24 New CL approaches have enabled both mapping directional emission with angular-resolved detection, 25 spatially resolving carrier transport through integration of electron and near-field optics, 26,27 and hyper- spectral imaging on the nanometer scale. 28 CL has been used to image biological samples. Yet, direct CL of stained dehydrated samples 29,30 did not hold up well to electron damage, and although inorganic cathodoluminescent nanoparticle labels 31−33 are more robust, imaging with nanoparticle labels remains invasive because the electron beam must penetrate into the sample, precluding repeated measurements or observations of dynamics. By contrast, to take advantage of the tight focus of an electron beam for spectrally specific and noninvasive imaging, our aim is to use optical excitations generated by a nanoscale electron beam in a cathodoluminescent material above the sample as a noninvasive, near-field optical scanning probe. Although some efforts have been exploring using quantum dot films 34 or moderately cathodoluminescent materials 35 to generate hybrid electron and optical scanning probes, we recently proposed to combine the nanoscale focus of electron Received: February 20, 2015 Revised: April 2, 2015 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.5b00716 Nano Lett. XXXX, XXX, XXX−XXX