Band edge modulated SnO₂ Quantum dot for light harvesting and non expensive plasmonic composite for highly improved SERS applications: Experimental and Theoretical insights

Semiconductor metal oxide quantum dots and their composites with plasmon active nanostructures are of great interest for catalysis, light harvesting and SERS applications. These nanomaterials offer large absorption cross-section, high stability and conformance to the desirable tuneable band structure, stoichiometry, and surface modifications. We demonstrate that the CBM and VBM of SnO₂ could be tuned beyond -0.33 eV and +3.4 eV respectively by modulating size and defects. In consequence, the forbidden superoxide radical generation by the bulk SnO₂ was then enabled by the band edge engineering in the QDs which allowed a degradation of 10 ppm methylene blue dye within 3 minutes for the first time [1]. Mott schottky plots, EPR, and scavenger studied along with structural investigations helped explaining the unique features of QDs. Further by using an inverse morphology of decoration of the quantum sized SnO₂ on the plasmonic surfaces of Ag nanoparticle revealed charge transfer based novel SERS applications in contrast to the traditional metal semiconductor heterojunctions. The inverse morphology with minute amount of Ag( <5%) enabled a detection of Rhodamine 6G dye down to 10^-13 M with a limit of detection of 0.8×10¹⁰ [2]. The electromagnetic enhancement by the propagation of evanescent near field from Ag to SnO₂ was verified by the COMSOL simulation. Whereas the DFT studies unveiled the chemical enhancement due to an efficient charge transfer through a suitable band alignment between the Ag - SnO₂ QDs and the R6G analyte.