In Situ Characterization of Photothermal Nanoenergetic Combustion on a Plasmonic Microchip

Abstract

Plasmonic gratings facilitate a robust in situ diagnostic platform for photothermal combustion of nanoenergetic composite thin films using an optical microscope and a high-speed camera. Aluminum nanoparticles (Al NPs) embedded in a fluoropolymer oxidizer are cast onto a plasmonic grating microchip and ignited using a low-power laser. The plasmonic grating enhances both spatial resolution and sufficient photothermal coupling to combust small Al NP clusters, initiating localized flames as small as 600 nm in size. Two-color pyrometry obtained from a high-speed color camera indicates an average flame temperature of 3900 K. Scattering measurements using polarized light microscopy enabled precise identification of individual Al NPs over a large field of view, leading to 3D reconstruction of combustion events.

Surface Plasmon Enhanced Fluorescence Temperature Mapping of Aluminum Nanoparticle Heated by Laser

“…Thus, the only remaining high intensity light to reach the camera is the scattered, depolarized light from the Al NP. Analyzing such changes in scattering intensity has been used extensively for in situ observation of laser-ignited Al NP and THV combustion [9,10]. Al NPs heated in the current experiment do not show the characteristic changes in scattering intensity brought about by restructuring and oxidation of the aluminum during laser-induced combustion.…”

“…A relatively low (0.1% w/w) ratio of Al NP to THV resulted in sparsely distributed particles within a relatively thick (~1 µm) THV matrix. The film was cured in a saturated acetone vapor environment for 4 h to improve film uniformity and smoothness [10], then dried in a vacuum oven at 60 • C for 1 h to remove any remaining solvent. The final cured nanocomposite thin film retains the low ratio of Al NP:THV, but dye concentration increases to~4 mM due to volume reduction from solvent evaporation ( Figure 1A).…”

“…The imaging objective lens was an Olympus UAPON 100× oil-immersion lens with a numerical aperture of 1.49. The combination of the high-power objective lens and grating coupling effect provided subwavelength resolution (36 nm per pixel), facilitating the visualization of an individual nanoparticle [10]. ImageJ software was used to process experimental images and to convert fluorescence intensity into temperature maps.…”

“…The reaction properties and mechanisms of energetic fluoropolymer films have been widely studied by measuring the reaction temperature evolution to understand thermal characteristics and heating kinetics [6][7][8]. Recently, we reported an in situ study of isolated fuel nanoparticles reactions by photothermal heating on a plasmonic grating platform to understand differences in the behavior of Al NP reactions with fluoropolymer and metal oxide constructs [9,10]. Despite numerous recent advances, many fundamental aspects of energy flow and reaction mechanisms of nanoenergetic materials are poorly understood, in part, due to experimental limitations in high-resolution thermal sensing.…”

“…Recently, excitation of fluorophores involving surface plasmon resonance (SPR) coupling of the incident excitation energy at a metal-dielectric interface (e.g., Ag-Al 2 O 3 ) was shown to enhance the intensity of excited dyes by a factor of 100 compared to glass [47][48][49][50][51]. The free-space SPR effect provided by a dielectric-coated metal grating may also enhance the photothermal heating rate experienced by a nanoparticle near the grating surface [9,10].…”