Liquids confined down to the atomic scale can show radically new properties. However, only indirect and ensemble measurements operate in such extreme confinement, calling for novel optical approaches that enable direct imaging at the molecular level. Here we harness fluorescence originating from single-photon emitters at the surface of hexagonal boron nitride for molecular imaging and sensing in nanometrically confined liquids. The emission originates from the chemisorption of organic solvent molecules onto native surface defects, revealing single-molecule dynamics at the interface through the spatially correlated activation of neighbouring defects. Emitter spectra further offer a direct readout of the local dielectric properties, unveiling increasing dielectric order under nanometre-scale confinement. Liquid-activated native hexagonal boron nitride defects bridge the gap between solid-state nanophotonics and nanofluidics, opening new avenues for nanoscale sensing and optofluidics.
Details
Title
Liquid-activated quantum emission from native hBN defects for nanofluidic sensing
Note
New version of a preprint published 2022-04-13 with a different abstract: Nanostructures made of two-dimensional (2D) materials have become the flagship of nanofluidic discoveries in recent years. By confining liquids down to a few atomic layers, anomalies in molecular transport and structure have been revealed. Currently, only indirect and ensemble averaged techniques have been able to operate in such extreme confinements, as even the smallest molecular fluorophores are too bulky to penetrate state-of-the-art single-digit nanofluidic systems. This strong limitation calls for the development of novel optical approaches allowing for the direct molecular imaging of liquids confined at the nanoscale. Here, we show that native defects present at the surface of hexagonal boron nitride (hBN) - a widely used 2D material - can serve as probes for molecular sensing in liquid, without compromising the atomic smoothness of their host material. We first demonstrate that native surface defects are readily activated through interactions with organic solvents and confirm their quantum emission properties. Vibrational spectra of the emitters suggest that their activation occurs through the chemisorption of carbon-bearing liquid molecules onto native hBN defects. The correlated activation of neighboring defects reveals single-molecule dynamics at the interface, while defect emission spectra offer a direct readout of the local dielectric properties of the liquid medium. We then harvest these effects in atomically smooth slit-shaped van der Waals channels, revealing molecular dynamics and increasing dielectric order under nanometre-scale confinement. Liquid-activated native defects in pristine hBN bridge the gap between solid-state nanophotonics and nanofluidics and open up new avenues for nanoscale sensing and optofluidics.
