Ultra-Stable Plasmon-Enhanced Optical Nanospectroscopy for Defect Analysis of 2D Materials

Plasmon-enhanced Raman and optical nanospectroscopy, which achieves true nanometric spatial resolution in optical microscopy, has been employed for chemical characterization of materials in various research field, such as biology and 2D materials. While plasmon-enhanced optical nanospectroscopy has found its innovative multidimensional nanoscale applications in the recent past, it has a vital glitch in imaging large-sized samples, even for samples with the size of a few micrometers. This is because it is challenging to maintain stable optical signal of a sample during long-time measurement, especially when using an atomic force microscopy (AFM)–based plasmon-enhanced optical nanospectroscopy system. The instability of the scattered signal originates from thermal and vibrational drift of the metallic nanotip with respect to the focus spot of the incident light as well as the focus position in the optical axis. Because of these uncontrollable drifts, conventional plasmon-enhanced optical nano-imaging must be completed typically within 30 min; otherwise, optical signal deteriorates beyond the acceptable level. The restriction of measurement time indeed limits the use of plasmon-enhanced optical nanospectroscopy for advanced analysis, such as quality evaluation of optoelectronic devices, high-resolution imaging of biological cells, which requires large field of view.<br/>In this work, we present ultrastable plasmon-enhanced optical nanospectroscopy setup that has a home-built feedback system to compensate possible drift in all three dimensions [R. Kato et al., Science Advances (2022)]. This technical development we achieved overcomes the long-standing issue of the system drift, and thus the imaging time is no longer limited by the mechanical drift. Our ultrastable optical nanoimaging system enables characterization of nanoscale defects in micrometer-sized WS₂ layers at a high pixel resolution down to 10 nm without losing substantial optical signal. Owing to the ultrastable nanospectroscopy system, we could reveal that the defect density on the surface of WS₂ layers in large area equivalent to the device scale was indeed higher than the previously reported defect density of the smaller area. Furthermore, such long-duration plasmon-enhanced optical imaging led us to find rare properties of the materials, such as unique defects of WS₂, which one can easily miss with conventional systems. The present work paves the way for nanoscale optical spectroscopy and imaging of large-sized samples not only for optoelectronic devices but also biological cells, heterogeneous catalysis.