Rational design of ligand-free Rh-OsNanotrees for plasmonic tip localization and advanced third-order nonlinear...

Ligand-free Rh-Osnanotree nanostructures were designed to show superior third-order nonlinear optical properties and stable tip-localized plasmonic responses. The branched structures were synthesized by a surfactant-free co-reduction approach, and they showed ultrasharp tips (6-10 nm radius), high aspect ratios, and well-distributed Rh/Os, which was verified by HR-TEM, EDS mapping, and XRD analysis. The specific surface area was as high as 127.4 m²/g, promoting strong light-matter interactions. UV-Vis-NIR absorption spectra showed dual plasmonic peaks at ~ 432 nm and ~ 691 nm corresponding to dipolar and multipolar localized surface plasmon resonances (LSPRs). FDTD simulations and electron energy-loss spectroscopy revealed localized field enhancements of over 180 × in tip sites. Z-scan measurements with 800 nm femtosecond pulses indicated a large nonlinear refractive index (n₂) of 3.1 × 10⁻¹³ cm²/W and third-order susceptibility (χ³) of 8.4 × 10⁻¹⁰ esu-almost an order of magnitude larger than that of conventional gold nanorodsThe optical nonlinearity and stability were seen by a two-photon absorption coefficient of 4.6 cm/GW and optical damage threshold of 2.3 J/cm². These Rhosnabruk- Osnanotrees performed better in the thin-film optical modulators because they confirmed that they can be used in ultrasonic photonic, optoelectronic, and nonlinear sensing.