Atomic Level Understanding of Surface Enhanced Raman Spectroscopy Mediated by Noble Metal Nanoparticles on Silicon Nanostructures for Its Optimal Utilization

Description

Noble metal nanoparticles are powerful optical nano-antennas which significantly enhance the light that is incident on them and scattered. These unique optical properties of noble metal nanoparticles find useful applications in the non-destructive, noninvasive, non-toxic and ultrasensitive characterization of biological samples. Conventional Raman scattering typically provides a weak spectroscopic signal of molecules, rendering it useless for most practical purposes. However, when the molecules are introduced on the rough surfaces of noble metals or their nanoparticles, the amplified field provides an enormous enhancement of the Raman signal, thereby making the detection of a single molecule possible. Among the two field enhancement factors of Surface Enhanced Raman Spectroscopy (SERS), the electromagnetic interactions are conceptually well understood and quantitatively accounted for. By contrast, the other component contributed by the chemical interactions is still only qualitatively explored and not quite clearly understood. An atomic level understanding of these two factors and their interplay in SERS is imperative for optimizing the enhancement effect in bio-sensory applications. We propose to develop a theoretical approach which will adequately account for both the components of SERS.The development of computational procedures that couple light with both nanoparticles and molecules at sub-nanometer scale is challenging. Appropriate theoretical models addressing such a broad length scale is needed for a quantitative extraction of the chemical contribution to SERS. We will implement an effective computational model for SERS into the versatile and efficient density functional tight binding method (DFTB) at the quantum mechanical level. The effects of shape, size and distribution of noble metal (such as Ag and Cu) nanoparticles on the silicon nanostructures will be thoroughly examined to maximize the dielectric sensitivity of our SERS substrate, which provides a vast surface area available to adsorbates. We will address the response of the dielectric-sensitive nano-antennas to the introduction of molecules, starting with simple molecules for standardization and gradually switching to complex bio-molecules pertaining to sensory applications. We plan to develop our approaches for practical bio-sensory applications, say, for immunoassay readout. Our many years of experience in computational approach development and study of nanostructures and surface adsorption will ensure a successful fulfillment of the proposed research.