Nanoparticle superlattices produced with controllable
interparticle
gap distances down to the subnanometer range are of superior significance
for applications in electronic and plasmonic devices as well as in
optical metasurfaces. In this work, a method to fabricate large-area
(∼1 cm2) gold nanoparticle (GNP) superlattices with
a typical size of single domains at several micrometers and high-density
nanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variable
constituents (from organothiols to inorganic S2–) is demonstrated. Our approach is based on the combination of interfacial
nanoparticle self-assembly, subphase exchange, and free-floating ligand
exchange. Electrical transport measurements on our GNP superlattices
reveal variations in the nanogap conductance of more than 6 orders
of magnitude. Meanwhile, nanoscopic modifications in the surface potential
landscape of active GNP devices have been observed following engineered
nanogaps.In situoptical reflectance measurements
during free-floating ligand exchange show a gradual enhancement of
plasmonic capacitive coupling with a diminishing average interparticle
gap distance down to 0.1 nm, as continuously red-shifted localized
surface plasmon resonances with increasing intensity have been observed.
Optical metasurfaces consisting of such GNP superlattices exhibit
tunable effective refractive index over a broad wavelength range.
Maximal real part of the effective refractive index,nmax, reaching 5.4 is obtained as a result of the extreme
field confinement in the high-density subnanometer plasmonic gaps.