KEYWORDS: Nanorods, Lithography, Nanolithography, Polymethylmethacrylate, Gold, Super resolution, Color imaging, Nanostructuring, Particles, Control systems
Earlier, our group proposed a lens made of metallic nanorods, stacked in 3D arrays tapered in a conical shape. This nanolens could theoretically realize super-resolution color imaging in the visible range. The image could be magnified and transferred through metallic nanorods array. Lithography or self-assembly are common ways to fabricate such nanostructured devices. However, to precisely arrange nanorods is challenging due to the limitations to scale down components, and to increase accuracy of assembling particles in large area.
Here we experimentally demonstrated 2D nanolens with long chains of metallic nanorods placed at tapered angles in a fan-like shape to magnify images. In the fabrication, we chemically synthesized gold nanorods coated with CTAB surfactant to ensure a 10 nm gap between the rods for the resonance control of nanolens. And we prepared trenches patterned by FIB lithography on a PMMA coated glass substrate. The different hydrophobicity of PMMA and CTAB coats enabled to optimize capillary force in gold nanorod solution and selectively assemble nanorods into hydrophilic trenches. Finally, we obtained 2D nanolens after lift-off of the PMMA layer.
We numerically estimated the resonance property of nanorods chain and found a broad peak in the visible range located at a wavelength of 727 nm. The broadness of this peak (~178 nm) confirms that a broad range of wavelength can be resonant with this structure. This phenomenon was also confirmed experimentally by optical measurements. These results show that the combination of lithography and self-assembly has the potential to realize plasmonic nanolens.
Modern farming relies highly on pesticides to protect agricultural food items from insects for high yield and better
quality. Increasing use of pesticide has raised concern about its harmful effects on human health and hence it has become
very important to detect even small amount of pesticide residues. Raman spectroscopy is a suitable nondestructive
method for pesticide detection, however, it is not very effective for low concentration of pesticide molecules. Here, we
report an approach based on plasmonic enhancement, namely, particle enhanced Raman spectroscopy (PERS), which is
rapid, nondestructive and sensitive. In this technique, Raman signals are enhanced via the resonance excitation of
localized plasmons in metallic nanoparticles. Gold nanostructures are promising materials that have ability to tune
surface plasmon resonance frequency in visible to near-IR, which depends on shape and size of nanostructures. We
synthesized gold nanorods (GNRs) with desired shape and size by seed mediated growth method, and successfully
detected very tiny amount of pesticide present on food items. We also conformed that the detection of pesticide was not
possible by usual Raman spectroscopy.
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