Indexed on: 08 Jun '18Published on: 25 May '18Published in: ACS Photonics
For quite a long time, people thought of the diffraction limit of light as a fundamental unbreakable barrier that prevents seeing objects with sizes smaller than half the wavelength of light. Super-resolution optical methods and near-field optics enabled overcoming this limitation. Here we report an alternative approach based on tracking the photothermal expansion that a nano-object experiences upon visible light absorption, applied successfully in the characterization of samples with a spatial/lateral resolution down to 4 nm. Our device consists of an atomic force microscope coupled with a solid-state laser and a mechanical chopper synchronized with the natural oscillation mode of an in-house-made gold tip cantilever system. This configuration enhances the detection of nanostructures due to the intermittent light excitation and the consequent intermittent thermal expansion of the sample under investigation. The sensitivity and spatial resolution are further improved by the electric field enhancement due to the excitation of localized surface plasmons at the tip apex. Our concept is demonstrated by the analysis of a two-dimensional material (GaSe) on crystalline sp2 carbon (graphite) and by an array of multiwalled carbon nanotubes lithographically designed in a SiO2 matrix. The photothermal expansion originating from light absorption leads to an unprecedented spatial resolution for an optical absorption event imaged below 10 nm.