Mechanically-Reconfigurable Molecular Junctions with Sub-Nanometer Tunability

Diverse and unique functionalities of molecules have extended their applications beyond electronics to fields including optics, plasmonics, mechanics, spintronics, and quantum technologies. Molecular devices with structurally-tunable, nanometer-thin critical dimensions are highly desirable for their extended sensitivity, energy efficiency and speed. Building such molecular platforms reliably, however, has been a challenge due to the fabrication limitations posed by top-down techniques.<br/><br/>Here, we present nanoparticle contact printing as a technique for scalable fabrication of molecular junctions in the form of mechanically active plasmonic resonators where the molecules serve as nanoscale reconfigurable springs with sub-nm control. The plasmonic resonator is configured as a nanoparticle-on-mirror structure, where a compressible dielectric molecular spacer is sandwiched between a metallic nanoparticle and a metallic film. To achieve scalable patterning of these active resonators, we developed nanoparticle contact printing, where &gt;2000 colloidal plasmonic gold nanoparticles are deterministically patterned onto template-stripped gold with sub-50 nm control over positional accuracy. The resulting nanoparticle-on-mirror constructs are highly sensitive to sub-nm structural changes in the molecular junction. We demonstrate the dynamic mechanical tunability of the molecular junction and its limits by testing the resonators under thermal cycling, where the molecular layer undergoes thermal expansion and recovery. The ~1.2 nm spectral tunability observed upon heating reflects a mere ~30 pm change in the molecular gap, highlighting the sensitivity and precision of this platform. Such tunable structures, whose response can be modified through choice of the molecular layer, can form building blocks for mechanically active molecular devices, enabling high-throughput statistical and correlative characterization of the molecular mechanics, and integration into emerging nanoscale sensors, actuators, reconfigurable optical devices, and tunable metasurfaces.