Optical Absorption Imaging by Photothermal Expansion with 4 nm Resolution

Abstract

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.

Ion‐Induced Defects in Graphite: A Combined Kelvin Probe and Raman Microscopy Investigation

“…Despite continuous progress in TERS instrumentation, confocal Raman spectroscopy is still the most common and straightforward method to analyze carbon nanomaterials. Recently, we demonstrated a versatile but powerful combination of AFM with optical absorption excitation that provided optical information from a sample with the same spatial resolution as in AFM . We were even able to visualize a bilayer defect on graphite.…”

Advanced Characterization Methods for Electrical and Sensoric Components and Devices at the Micro and Nano Scales

“…This can be explained by the high absorption of IR radiation, the mechanical properties, and the thermal conductivity of proteins. However, the laser does not necessarily need to be pulsed at high frequencies, as our earlier study shows . Herein, we utilized nano‐Vis to demonstrate a spatial resolution of 4 nm using a chopper to modulate the laser at the resonance frequency of the cantilever.…”

“…Optical absorption at the nanoscale can be measured and mapped indirectly using the photothermal expansion that a material experiences after illumination and light absorption. The detection of the tiny photothermal expansion, estimated for a particular example to be in the range of ≈3 pm, and its 2D mapping are possible by the amplification and detection capabilities of AFM cantilevers . When the laser source is spectrally tuned to the wavelength corresponding to absorption, there is a rapid thermal expansion of the absorbing part of the sample.…”

“…For nano‐IR and PTIR, the sample's photothermal expansion detection is very similar but a pulsed laser is used instead of a chopper wheel . Adapted with permission . Copyright 2018, American Chemical Society.…”

“…The most typical configuration for nano‐IR is a thin film on an IR transparent ZnSe prism, which is illuminated from the bottom, though top‐side illumination allows the investigation of opaque samples …”