On the other hand, the external forces such as compressing, stretching and stimuli, allow an additional way to tailor the interparticle distance. As discussed in section 2.2, compression of LB films at the air-water interface will force the nanoparticles close to each other, leading to a decreasing interparticle separation. Experimental results indicated that as the surface press increased, the nanoparticles experienced changes from non-coupling, to strong coupling, and finally delocalization of surface plasmons (Figure 2.9c).27 During this process, both the frequency and the strength of the plasmonic resonance peak were tuned with increasing compression. Such compression induced coupling control incorporation with the shape diversity allowed for tunability of optical properties of 2D assemblies with a wide range.71
Stretch-induced plasmonic property study further supports the investigation of gap plasmon mode of 2D plasmonic nanosheet system.64 The NB nanosheet was lifted up with a PDMS sheet and spin coated with another layer of PDMS on top of sheet surface before conducting strain induced study. Trends of the blue shift were clearly observed on the plasmon resonance peak with increasing strain up to 35% due to the reduction of dipole-dipole coupling. The increase in the strain also led to the decrease of plasmon resonance intensity and broadening of the resonance peak. Such optical behavior could be explained by the decay of the near-field coupling strength between adjacent nanoparticles that is owing to the larger gap between particles under the strain and less interacting particles over the beam spot area. Remarkably, such responses can be fully reverted after releasing the applied strain. Furthermore, the stretch-induced plasmonic properties were found to be particles shape dependent. For example, under 35 ± 1.5% strain, ~36 ± 5.0 nm blue-shift was found for nanobrick nanosheet while only ~12 ± 1.0 nm blue shift could be observed for nanocube nanosheet.