Gap-plasmonic tuneable structural colors by capillary interactions

Capillary interactions of oligomers in PDMS and Ga enable the single-step formation of noncoalescent Ga nanodroplets, enabling tunable gap-plasmon resonances, and eliminating the need for complex processes, thus allowing the fabrication of multiple mechanoresponsive structural colors.
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Gap-plasmonic tuneable structural colors by capillary interactions

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Colors rendered via nanostructures, particularly on elastomers, have fascinated scientists due to their numerous advantages, including anti-fading, colour-brilliance, eco-friendliness, low power consumption, and functionalities in many biological systems. With the advent of nanotechnology, various structures could be fabricated to generate structural colors. However, most relied on precise lithographic techniques with low throughput or self-assembly methods, where the focus was to optimize the precursor that would be deposited onto a desired substrate. In both approaches, multiple chemical reagents and procedures were employed. Our lab (Laboratory of Advanced Nanostructures for Photonics and Electronics (LANSPE)) is dedicated to developing novel processing techniques for the fabrication of advanced nanostructure for potential applications. In this work, with a meticulous choice of substrate and active material, capillary interactions between them were employed to obtain mechanoresponsive structural colors.

Figure 1| Single-step fabrication of multiple mechanoresponsive structural color via capillary interactions. a, Thermal evaporation of Ga onto PDMS. b, Oligomers in PDMS determine the nanostructures. c, In the presence of oligomers, PDMS 5 and PDMS 10 show different nanostructures and, hence, different colors. On removing oligomers from the PDMS before thermal evaporation, the nanostructures in both PDMS become almost similar, and the resulting colour obtained is dull bluish-white. d, The gap-plasmon resonances excited in the nanostructure (top). Increasing the inter-droplet gap results in blue-shift of the reflectivity spectra (bottom).  e, Change of color of the sample  (left) from pink to green due to blue shift on stretching (right). f, Applications in displays and mechanochromic sensing.

In our work, we use a bio-compatible elastomer called Polydimethylsiloxane as a substrate. It contains uncured oligomers, which we showed to be in the fluidic state. We deposit Ga, a low-melting-point metal with excellent plasmonic properties, onto PDMS substrates by single-step thermal evaporation (Fig. 1a). The capillary interactions between the oligomers and liquid Ga take over the task of determining Ga nanostructures (Fig. 1b). Since it is the oligomer content which can be experimentally controlled one can obtain multiple nanostructures and hence the structural color on a single deposition by putting in PDMS with different oligomer content into the evaporation chamber. This allows the simultaneous fabrication of multiple structural colors.

 The size trends of Ga nanodroplets in PDMS were explained by it's capillary interaction with the liquid oligomers in PDMS (Fig. 1c). Further, the fluidity of oligomers makes the size distribution of Ga narrow and sub-100nm scale (Fig 1c, left), a feat that was difficult to achieve by a physical method.  Removal of liquid oligomers from the substrate results in a poly-dispersed distribution of Ga, with a bluish-white color of the sample, irrespective of the initial oligomer content (Fig. 1c, right). Thus, the oligomer in the substrate dictates the nanostructure of Ga, enabling the substrate to play an active role in determining the resultant structural colour.

 Electron microscopy reveals the Ga nanodroplets being engulfed within the substrate, with a strong dependence of the nanostructure on the oligomer content of PDMS (Fig 1c left). It further reveals the non-coalescence of Ga nanodroplets wherein gap-plasmon resonances can be excited (Fig. 1d top). Hence, their plasmonic resonances are tuneable owing to the elastomeric property of PDMS.  Thus, on mechanical stretching, the increasing of inter-droplet gaps results in a blue shift of the spectra in the visible region, changing the colour of the sample (Fig. 1d bottom, Fig. 1e).

 With this approach, a single-step physical vapour deposition method could be employed to fabricate multiple mechanoresponsive structural colours, which can be employed in prototypes of reflective displays and sensors for monitoring body parts, smart bandages, and real-time force-mapping devices can be fabricated. This illustrates the technology’s broad applicability in large-area displays, devices for human-computer interactions, healthcare, and visual communication (Fig. 1f).


To conclude, this process allows non-coalescent Ga nanodroplets to form, allowing gap-plasmon resonances and exploiting the plasmonic properties of Ga. The inter-droplet gap acts as an electromagnetic energy storehouse, which can be used with quantum dot systems for applications in lasers and LEDs. The samples provide a rich collection of sites with nm-scale gaps between metals in which quantum plasmonic effects might be observed and studied. Hence, this work opens up the scope for fundamental research as well as applications in potentially numerous fields.

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    Reference: Renu Raman Sahu, Alwar Samy Ramasamy et al. Single-step fabrication of liquid gallium nanoparticles via capillary interaction for dynamic structural colours. Nat. Nanotechnol. (2024).

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