The miniaturization of electronic and optoelectronic devices at the nanoscale presents unprecedented opportunities across various technological domains. In these miniature devices, materials are confined to spaces smaller than a micrometer, which raises a fundamental question: do the properties of these materials remain identical to their bulk counterparts? While the quantum confinement effect typically dominates at scales below 10 nm, certain materials and phenomena are influenced by long-range interactions and global propagations, extending the pronounced confinement effects to lengths greater than 100 nm. Moreover, confined materials are inherently more susceptible to their boundaries due to their substantial surface-to-volume ratio, including interactions with substrates and surrounding structures.
However, understanding the properties of these highly confined nanomaterials, influenced by their local carrier and exciton dynamics, presents a major challenge due to the limited applicability of conventional optical spectroscopy techniques at the nanoscale. In this presentation, we demonstrate the use of infrared scattering scanning near-field optical microscopy (IR s-SNOM) and its time-resolved variant, ultrafast IR s-SNOM, to investigate the impact of spatial confinement on nanomaterials. By integrating an atomic force microscope and infrared pulsed excitation, IR s-SNOM offers versatility and a spatial resolution below 50 nm.
Leveraging the Drude response associated with free carriers in the infrared frequency range, IR s-SNOM can distinguish between metals and insulators. We apply this technique to study vanadium dioxide nanoparticles (VO2 NPs), enabling the resolution of their insulator-to-metal transitions at the individual particle level (see Fig 1a). Our observations reveal that smaller NPs exhibit transitions at higher temperatures, possibly due to the presence of fewer nucleation sites within their confined volumes. Additionally, we identify stochastic behavior, where identical NPs exhibit phase transitions at different temperatures during repeated heating cycles. Both findings underscore the metastable nature of superheated VO2 NPs, and IR s-SNOM provides a unique opportunity to observe the signatures of metastability in individual NPs.
Moreover, the infrared frequency range encompasses the distinctive response of photoinduced excitons, including hydrogen-like 1s-2p transitions. We investigate these exciton transitions in carbon nanotubes on a quartz substrate using interferometric ultrafast IR s-SNOM (Fig 1b). This method, employing a lock-in sideband detection scheme, enhances sensitivity to detect photoinduced near-field scattering changes down to ~0.01%. Simultaneously, interferometric detection ensures the extraction of pure near-field contrasts. With ultrafast IR s-SNOM, we not only demonstrate distinct exciton relaxation dynamics between different nanotubes but also reveal significant heterogeneity within each nanotube. Specifically, our research uncovers substantial heterogeneity within bundles of carbon nanotubes, arising from interactions both among tubes and with the substrate.
Our findings establish tip-based infrared nano-spectroscopy as a valuable tool for investigating the effects of nanoconfinement on materials. This understanding holds significant promise in advancing the fields of nanomaterials and nanodevices.
