Self-Assembled Active Deformable Liquid-Metal Based Photonic Nanostructures via Fluid Instabilities

The use of liquid metals, particularly Gallium, and their alloys allow for the fabrication of flexible, bendable, and mechanically reconfigurable devices when deposited on an elastomeric substrate[1]. They have been the choice materials for electronic applications, particularly in the domain of soft and stretchable electronics[2]. Although an excellent optical material due to low comparative loss and UV-based plasma frequency, their applications in the photonic domain have still been limited to the THz region[3]. This is because of its inability to be scaled down to nanoscale owing to their intrinsic large surface tension, thus hindering practical use in visible, dynamic photonic systems with sub 100 nm feature dimensions. However, the possibility of exploiting fluid instabilities for nanofabrication has been well established[4].<br/><br/>In this talk, for the first time we will show a single-step procedure that exploits the fluid instabilities of liquid metal, tending to minimize surface energy by forming spherical droplets of sizes in sub-100nm length scales. By tuning the softness and the temperature of the substrate, the rate of deposition, and the amount of the liquid metal being deposited, we demonstrate a fabrication technique of gallium-based plasmonic thin film with tuneable structural colors for the first time, paving the way toward a novel paradigm in soft photonics and large area reflective displays. By tuning and optimizing a range of experimental parameters, and the choice of substrate material we obtain vibrant coloration encompassing a wide range of chromaticity coordinates as a result of the controlled manipulation of size distribution of Ga nanodroplets. Analysis of scanning electron microscope images for structural characterization of the samples fabricated by our methodology emphasizes the importance of liquid oligomers in the heat-cured Poly-dimethyl siloxane in the formation of non-overlapping Gallium nanodroplets, which is not the case when the oligomers are deliberately removed by toluene treatment. In the latter case, the absence of oligomers results in the formation of nanodroplets without penetration into the substrate, forming a film of nanodrops akin to a thin film over a substrate.<br/><br/>As a proof-of-principle, we show pallets of structural colors, whose advantages can be leveraged in in reflective display technologies. The application of the thus-fabricated device also exhibits mechanochromic sensing, which has been characterized to be of high fidelity and reliability, functionalized by exploiting the fluidic properties of Gallium. For at least 1000 cycles of uniaxial stretching and relaxing, the reflectivity spectrum of the sample for a given strain was unchanged, thus proving the reliable reconfiguration of sample response to the application and removal of mechanical strain. The reversible change in the reflectivity spectrum results in the chromaticity coordinates of the sample, hence a visible change in the sample color.<br/>Our results have the potential to offer opportunities to dynamically reconfigure thin-film-based functional nanodevices in situ as well as process technology for high throughput fabrication to achieve the same in a single-step method.<br/><br/>References<br/>[1] Martin Monier, L., Gupta, T. Das Gupta et al. <i>Adv. Funct. Mater.</i> <b>31</b>, 2021<br/>[2] Dickey, M. <i>Advanced Materials</i> <b>29</b>, 2017<br/>[3] Reineck, P. et al. <i>Sci. Rep.</i> <b>9</b>, 2019<br/>[4] Das Gupta, T. et al. <i>Nat. Nanotechnol.</i> <b>14</b>, 2019