Gap-Plasmon-Enhanced High-Spatial-Resolution Imaging by Photothermal-Induced Resonance in the Visible Range

Abstract

Chemical characterization at the nanoscale is of significant importance for many applications in physics, analytical chemistry, material science, and biology. Despite the intensive studies in the infrared range, high-spatial-resolution and high-sensitivity imaging for compositional identification in the visible range is rarely exploited. In this work, we present a gap-plasmon-enhanced imaging approach based on photothermal-induced resonance (PTIR) for nanoscale chemical identification. With this approach, we experimentally obtained a high spatial resolution of ∼5 nm for rhodamine nanohill characterization and achieved monolayer sensitivity for mapping the single-layer chlorophyll-a islands with the thickness of only 1.9 nm. We also successfully characterized amyloid fibrils stained with methylene blue dye, indicating that this methodology can be also utilized for identification of the radiation-insensitive macromolecules. We believe that our proposed high-performance visible PTIR system can be used to broaden the applications of nanoscale chemical identification ranging from nanomaterial to life science areas.

A guide to nanoscale IR spectroscopy: resonance enhanced transduction in contact and tapping mode AFM-IR

“…Proof-of-principle measurements that extend RE-AFM-IR to the visible spectrum were made on a chlorophyll-a monolayer and methylene-blue-stained amyloid fibrils using a laser with fixed wavelength (671 nm) and tunable repetition rate (1 kHz to 200 kHz). 59 Although these experiments have achieved monolayer sensitivities the current lack of wavelength tunability limits their spectroscopic utility. By comparison, less sensitive measurements using the ringdown modality and OPOs with fixed (1 kHz) repetition rate enable experiments over a wide wavelength range (500 nm to 2300 nm) 8,17 which is critical to determine the bandgap and optically active defects.…”

Visible to Mid-IR Spectromicroscopy with Top-Down Illumination and Nanoscale (≈10 nm) Resolution

“…37 Although gold-coated cantilevers work well in the mid-IR, the extinction coefficient of gold is a non monotonic function in the visible range which may distort the shape of PTIR absorption spectra in the visible range. Proof-of-principle PTIR measurements with top illumination using a gold-coated AFM probe and a laser with a fixed wavelength (671 nm) have been reported previously 38 but lacked spectroscopic capabilities to assess such effects.…”

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films

“…In a different approach, Dietler and Sekatskii and co-workers developed a gap-plasmon enhanced photothermal-induced resonance (PTIR) technique which uses visible light instead of the typical IR radiation to improve spatial resolution to 5 nm. 40 In this approach, Au tip−Au substrate gap plasmon resonances are efficiently induced by visible wavelength radiation which, when resonant with an electronic absorption process of a thin film sample, induces thermal expansion that results in mechanical deflection of the AFM cantilever. Small changes in the thickness of a sample can lead to large changes in the electromagnetic field amplification in the Au tip− substrate gap region, thereby improving resolution and contrast of the images.…”