Development of a CL-SEM Platform for Nanoscale Optical Characterization

Cathodoluminescence (CL) refers to the phenomenon of light emission when a luminescent material is bombarded by electrons [1]. When integrated with a scanning electron microscope (CL-SEM), it enables correlative imaging that directly links optical features with morphology and composition (Fig. 1(a)) [2]. Compared with conventional confocal photoluminescence (PL) microscopy, CL-SEM offers spatial resolution more than two orders of magnitude higher, providing nanoscale insight into optical and electronic properties [3]. One key operational mode is hyperspectral CL imaging, which maps spatially resolved emission spectra (Fig. 1(b)). Another mode, based on the g(2) function (second-order autocorrelation function), can pinpoint single-photon emitters in quantum materials (Fig. 1(c)). Our team has developed a high-spatial-resolution CL-SEM platform spanning the ultraviolet (UV) to infrared (IR) spectral range. This system was realized in collaboration with the Developmental and Analytical Microscopy (DAM) group at the National Research Council (NRC), and Hitachi High-Tech Canada. The CL module is integrated into NRC’s high-resolution Hitachi S5500 SEM, equipped with a cold field-emission source and in-lens technique, enabling a nominal spatial resolution of 0.4 nm at an accelerating voltage of 30 kV. The relatively low accelerating voltage operation is essential for minimizing beam-induced damage in beam-sensitive materials, such as certain quantum materials [4]. In addition, multiple detection modes, including secondary electron (SE), bright field (BF), dark field (DF), and electron energy-loss spectroscopy (EELS), are available for simultaneous multimodal characterization. The CL-SEM platform consists of three main subsystems: (i) a light collection system featuring a custom-designed CL holder incorporating a pair of compact ellipsoidal mirrors coupled to a light guide rod; (ii) a detection module comprising either a dispersive spectrometer or a photomultiplier tube (PMT) with optical filters for hyperspectral CL imaging; and (iii) a Hanbury Brown and Twiss (HBT) interferometer integrated with a time-correlated single-photon counting (TCSPC) unit for g(2) measurements. Moreover, custom-designed CL chips with ultrathin SiNx membrane windows have been developed to accommodate a variety of sample requirements. In the initial implementation of the hyperspectral mode using the prototype CL holder (Fig. 1(d–e)), we demonstrated preliminary results from various cathode ray tube (CRT) phosphor powders (Fig. 2(a-d)). The corresponding SE images are shown on the left (Fig. 2(a,c)), with correlated CL colormaps displayed on the right (Fig. 2(b,d)). For each scanned pixel, a full emission spectrum is recorded, with a representative example shown in Fig. 2(d). Phosphor species can be readily distinguished by the colormaps based on their characteristic emission bands (Fig. 2(b)). Notably, the CL maps also reveal additional structural features that are not observed in the SE images, underscoring the complementary optical information. For samples exhibiting lower emission, a PMT detector equipped with interchangeable band-pass filters is employed. As shown in Fig. 2(e), Ce:LuAG nanophosphors with significantly smaller interaction volumes were investigated. The SE image is presented, with the corresponding CL images acquired using the PMT (top) and full spectroscopic measurements (bottom) shown alongside. Within the selected region of interest, the band-pass filters provide coarse yet reliable spectral discrimination consistent with the spectrometer data, enabling rapid identification of emission features. An additional example of gold nanorods is presented in Fig. 2(f–g). The emission observed for gold nanorods arises from the radiative decay of different plasmon modes [5]. In this case, the CL image provides spatially resolved optical information that directly correlates with nanoscale structural features. In conclusion, we have realized a CL-SEM platform that integrates seamlessly with a high-resolution SEM, providing robust performance and straightforward operation. Ongoing efforts focus on improving light collection and detection efficiency through the integration of an EMCCD camera and the implementation of g(2) CL measurements. These upgrades will expand the system’s functionality and enhance its utility for investigating low-dimensional light-emitting materials. (a) Schematic illustration of the CL-SEM system, including bright-field (BF), dark-field (DF), and electron energy-loss spectroscopy (EELS) detection modes. The CL emission can be analyzed using two principal configurations: (b) hyperspectral CL and (c) second-order correlation, g(2) CL. (b) In hyperspectral mode, the CL signal is directed into a spectrometer composed of (1) an entrance slit, (2) a collimating mirror, (3) a diffraction grating, (4) a focusing mirror, and (5) a linear-array detector. (c) For g(2) CL measurements, the emission is routed to a Hanbury Brown and Twiss (HBT) interferometer consisting of a 50/50 beam splitter (BS) and two avalanche photodiodes (start and stop detectors) connected to a time-correlated single-photon counting (TCSPC) module. (d–e) Photographs of the prototype CL holder, showing the mirror assembly at the tip (d) on the table and (e) installed inside the SEM chamber. Secondary-electron (SE) images (a, c) and corresponding cathodoluminescence (CL) hyperspectral images (b, d) of cathode-ray-tube (CRT) phosphors. The CL intensity maps are averaged over selected spectral intervals, with colors indicating different wavelength ranges. A representative CL spectrum from a randomly selected pixel is shown in (d). (e) SE image of Ce:LuAG nanophosphors (left) and corresponding CL images (right). Top: CL image obtained using a band-pass-filtered photomultiplier tube (PMT), representing the average intensity over the full spectral range, with the corresponding spectrum shown for the selected area. Bottom: CL image obtained using a spectrometer, corresponding to the integrated intensity within the shaded wavelength region, with the corresponding spectrum shown for the similar selected area. (f) Dark-field (DF) image and (g) corresponding CL image of gold nanorods.