Metal nanoparticles (MNPs) are submicron scale entities made of pure metals or their compounds. The optical properties of these type of metals have been extensively studied for decades. So far, it is well understood that the change of the electromagnetic field around MNPs is dominated by the collection of induced oscillation of conduction electrons in metal nanoparticles, which is also known as localized surface plasmon resonance (LSPR). LSPR can generate strong scattering and absorption, resulting in an optical cross-section that is tens of times larger than the geometric cross-section of the particles. If the MNPs are placed on the interface between two dielectric materials, most of the scattered light will be prone to traveling toward the material with higher refractive index, offering a promising light trapping strategy for many dielectric materials. Consequently, Gold (Au) and Silver (Ag) nanoparticles have been widely used due to their good scattering efficiency. Au demonstrates good stability while Ag has better scattering efficiency. On this account, Ag is preferred.
Researchers from the University of Iowa: Bingtao Gao, Wenqi Duan, Aaron Silva, Alexander Walhof, Weitao Dai and led by Professor Fatima Toor developed a novel time- and cost-effective method of fabricating a light management structure based on Ag nanoparticles on the surface of planar Si substrates.. The researchers utilized a hydrofluoric acid (HF) and silver nitrate (AgNO3) based vacuum-free electroless plating method to deposit Ag NPs on the Si surface in a controlled manner. Typically vacuum-based, high-cost, and unscalable techniques, such as sputtering or e-beam evaporation are utilized to deposit Ag NPs. The researchers focused on utilizing localized surface plasmon resonance of Ag NPs in their endeavor to trap light into Si. Their work is currently published in the research journal, Optical Materials Express.
In their approach, a thin layer of dense Ag nanoparticles was deposited on the surface of Si substrate by electroless plating, followed by a water bath at 90 ˚C to form Ag nanoparticles of different sizes. With the help of a MATLAB-based analytical model on Mie theory, the size distribution of Ag nanoparticles for desired optical properties was determined. In the end, the fabricated Ag nanoparticles were exposed in laboratory environment for 2 weeks so as to study the effect of Ag degradation in atmosphere.
The authors reported that the reflection of the best performance sample decreased by up to 24.8% at a wavelength of 371 nm. In addition, characterization of samples exposed to the environment revealed that the LSPR response of unprotected Ag nanoparticles was markedly impaired after 14 days, while the LSPR response of aluminum oxide protected Ag nanoparticles was unchanged even after 90 days.
In summary, the study reported on a time- and cost- effective electroless plating method to fabricate randomly distributed Ag nanoparticles on the surface of Si which could reduce the reflection of Si in the visible spectrum. The authors highlighted that the aluminum oxide coated sample demonstrated a strong reflection reduction, exhibiting a reflection of as low as 7.6% at a wavelength of 662 nm and a weighted average spectral reflectance (Rave) of 12.2%. In an interview with Advances in Engineering, Professor Fatima Toor pointed out that their newly presented approach could be utilized in fabricating surface plasmonic materials with selective light absorption and in realizing light confinement on Si surfaces.
Bingtao Gao, Wenqi Duan, Aaron D. Silva, Alexander C. Walhof, Weitao Dai, Fatima Toor. Light management on silicon utilizing localized surface plasmon resonance of electroless plated silver nanoparticles. Optical Materials Express, Volume 9, No. 9 / 1 September 2019.