Student Name: Mr. Renu Raman Sahu
Date/Time: on 02/06/2025 at 11AM
Research Supervisor : Dr. Tapajyoti Das Gupta
Venue: IAP Auditorium.
Title: On Fluidic Plasmonics: Harnessing Liquid Metals for Tunable Nanophotonics
Abstract: Low melting point metals, in particular, Ga and its alloys, have been used in electronics applications extensively, while their photonic applications in the visible region (2-3eV photon energy) of the electromagnetic spectrum have been limited owing to the difficulties in obtaining a homogenously dispersed nanoparticles[1]. Ga has a bulk plasma frequency of 13.7eV [2], whereas the Frolich condition[3] for nanoparticles determines the Localised Surface Plasmon Resonance (LSPR) to be at 9eV in an air medium. However, with high refractive index material as a media enclosing Ga nanodroplets, the LSPR resonance can be reduced further. For example, Ga embedded in a transparent media of refractive index 1.4 has an LSPR at 7eV, which is still deep in the ultraviolet region, unsuitable for application in visible frequencies[4]. To overcome this problem, plasmon hybridization[5] can be employed, which predicts the splitting of the single particle plasmon mode energies as two nanoparticles come close sufficiently enough for coulombic interactions between the charge densities excited to localized surface plasmons. The lower energy modes, also known as the bonding mode, can have the plasmon frequencies in the visible range. To obtain the hybridized modes, a non-coalescent assembly of the nanoparticles with static relative positions with respect to each other is required. However, most of the metal nanoparticles are obtained in colloidal media where the surrounding media is a liquid at non-zero temperature, resulting in random thermal motion of the nanoparticles and thus inhibiting the hybridization of LSPR modes. Furthermore, owing to the surface tension of Ga, it is energetically difficult to obtain homogenously dispersed Ga nanoparticles[6].
This thesis talks about overcoming the above challenges by employing single-step thermal evaporation of Ga with a smart choice of substrate. Polydimethylsiloxane (PDMS) is an optically transparent elastomer with a refractive index of 1.4. PDMS is thermally cured after mixing its base with a cross-linking agent. However, even after thermal curing, PDMS retains uncured oligomers, which are in a liquid state. As soon as Ga nanodroplets start forming on the substrate, the oligomers engulf them to reduce the system’s overall surface energies. Owing to the liquid state and high surface tension of liquid Ga, the nanoparticle remains spherical in shape. Furthermore, this phenomenon of engulfing keeps Ga nanodroplets from coalescing into bigger particles. The assembly of Ga nanodroplets, engulfed with the oligomers, thus remain on the substrate with their relative positions static to each other. Macroscopically, the optical images of the sample exhibit vibrant, visible colors, owing to the nanostructure characterized by the distribution of Ga nanosphere sizes and the gaps between them, which depend on the amount of oligomers and the cross-linking of the PDMS substrate. These structural colors can find applications in reflective displays, as they are shown to possess the properties of non-fading, longevity, and robustness.
By varying the proportion of the cross-linking agent of PDMS, the amount of oligomers in the substrate can be tuned. Thus, with a single physical vapor deposition process, multiple structural colors can be fabricated. Moreover, since the substrate is an elastomer, the inter-droplet gaps can be tuned by mechanical deformations like linear strain, twists, and bending of the sample, enabling a controlled manipulation of plasmon hybridization. On applying a uniaxial strain, the sample exhibits a blue shift of the optical spectra, resulting in a color change. The strain causes the inter-droplet gap to increase, resulting in a lowering of the strength of plasmon hybridization, leading to an increase in the energy of bonding mode energies and, hence, a blue shift. This property of mechanoresponsiveness of the samples is shown to be repeatable and reversible, and hence, it is robust and reliable for thousands of strain cycles. They can have potential applications in the fabrication of soft-robotics sensors; strain gauges with visual calibration scales and prosthetic components.
The fabricated samples allow for probing precise temperature dependence of the optical properties of the liquid metal, thanks to the liquid metal morphology featuring sub10nm spatial proximity of nanodroplets. With an increase in temperature, the samples exhibit a blue shift of optical spectra, owing to a temperature-dependent shift of the permittivity of the liquid Ga, as corroborated by in-situ thermal Electron Energy Loss Spectroscopy (EELS).
In the framework of the Drude-Lorentz model, which best works for usual plasmonic materials like gold and silver, increasing temperature results in a higher electron collisional rate owing to the decrease of electron-phonon scattering time, an optical blue-shift is not expected [7]. To explain the observed non-intuitive phenomena when we use liquid metal as the plasmonic material, we propose a modified Drude-Lorentz model, introducing a temperature-dependent shift term in the framework of Ziman’s theory of liquid metal. The introduced shift term can be traced back to the structure factor of the liquid metal. Our results have the potential to determine the temperature dependence of the structure factor of liquid Gallium, which, to date, is determined by the inelastic scattering of slow neutrons.
References
[1] M. D. Dickey, “Stretchable and Soft Electronics using Liquid Metals,” 2017. doi: 10.1002/adma.201606425.
[2] M. Horák, V. Čalkovský, J. Mach, V. Křápek, and T. Šikola, “Plasmonic Properties of Individual Gallium Nanoparticles,” Journal of Physical Chemistry Letters, vol. 14, no. 8, 2023, doi: 10.1021/acs.jpclett.3c00094.
[3] S. A. Maier, Plasmonics: Fundamentals and applications. 2007. doi: 10.1007/0-387-37825-1.
[4] R. Raman Sahu and T. Das Gupta, “Real-time tuneable bright bonding plasmonic modes in Ga nanostructures.”
[5] P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. I. Stockman, “Plasmon hybridization in nanoparticle dimers,” Nano Lett, vol. 4, no. 5, 2004, doi: 10.1021/nl049681c.
[6] R. R. Sahu et al., “Single-step fabrication of liquid gallium nanoparticles via capillary interaction for dynamic structural colours,” Nat Nanotechnol, vol. 19, no. 6, 2024, doi: 10.1038/s41565-024-01625-1.
[7] A. Alabastri et al., “Molding of plasmonic resonances in metallic nanostructures: Dependence of the non-linear electric permittivity on system size and temperature,” Materials, vol. 6, no. 11, 2013, doi: 10.3390/ma6114879.