2, Indian Institute of Technology Delhi, New Delhi, NCR, India
3, Indian Institute of Technology Delhi, New Delhi, NCR, India
Strong electric field enhancement in localised surface plasmon resonance (LSPR) modes provides a useful way to channel energy transfer between light and matter in nanostructured metals. Shaped metallic nanoparticles exhibit controllable red shifts of absorption potentially useable in solar cells and near infrared detectors, though usually with spread out absorption response. Several methods of fabricating shaped metallic nanostructures exist, including electron beam lithography, and nanosphere lithography, besides solution-processed techniques. Large area solar cells require low cost absorbing materials accessible through solution-phase deposition using environmentally safe inks. We report a refinement of solution-processed methods using green synthesis under controlled temperature, and illumination conditions, involving two-generation seeded growth and evolution of seeded spherical silver nanoparticles into triangular nanoparticles of controlled shape and size. Initial synthesis of silver nanoparticles using a standard citrate method was unremarkable. Measured absorbance at 392 nm was correlated with spherical silver particles with a size distribution centered at 8 nm measured using transmission electron microscopy (TEM). Addition of Ag(I) ions under 590 nm illumination evolved the aqueous ink over a period of 72 hours to triangular nanoparticles with edge lengths centered at 65 nm and a redshift of the absorption to the 600-800 nm band. Longer illuminated period synthesis is found to extend the band tail of absorption to 1200 nm, thereby degrading the sharpness of the absorption. A second generation seeded growth using these triangular nanoparticles under dark at 25°C results in an absorption peak at 720 nm with full width at half maximum (FWHM) of 170 nm. This absorption response correlates well with numerical first-principles finite difference time domain (FDTD) modelling (absorption peak ~815 nm) based on a uniform distribution with concentration levels and particle dimensions matched with the experimental TEM data. Dynamic light scattering (DLS) measurements for these triangular nanoparticles yield a hydrodynamic diameter estimate of 86±8 nm, in excellent agreement with TEM measurements with edge lengths of 72±16 nm. This synthetic pathway demonstrates the possibility of narrow tuning of the absorption under controlled illumination and temperature conditions for large volume ink production at industrial scale for spectrally selective solar absorbers.