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.Comment: 16 pages, 5 figure