Gap-Plasmon Tip-Enhanced Raman Scattering of Semiconductor Nanostructures

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The proposed project aims at studying the effect of gap-plasmon induced enhancement of Raman scattering by single semiconductor nanostructures in the vicinity of metallic nanostructures with nanometer spatial resolution. The evanescent electromagnetic (EM) field of a localized surface plasmon (LSP) in metal nanostructures induces optical resonance phenomena in adjacent semiconductor nanocrystals (NCs) including tip-enhanced Raman scattering (TERS) and surface-enhanced Raman scattering (SERS). Gap-plasmons excited between a metallic TERS tip and metallic nanostructures can further enhance drastically the local EM field and hence the TERS signal of semiconductor nanostructures placed in the gap. Hybrid nanostructures composed of CdSe, CdS, CuS, or PbS NCs with variable size synthesized by the Langmuir-Blodgett technique and deposited on arrays of Au nanoclusters are the key objects. A monolayer of MoS2 placed on the arrays will be used as a model structure to study the nature of the gap-mode TERS phenomenon including resonance effects, influence of near and far fields and depolarization properties. The preparation of metallic nanostructures, i.e. arrays of metal (Au) nanoclusters for further NC deposition, will be improved. For TERS experiments, the cantilevers with metal (Au, Ag) nanoclusters placed at the tip apex providing specific LSP resonance (LSPR) energies will be fabricated. The morphology of the nanostructures with the targeted plasmonic properties will be chosen on the base of the 3D full-wave simulations using ANSYS HFSS™. The sizes, morphologies, and surfaces of the NCs as well as the metal nanostructures will be controlled by transmission electron, scanning electron, and atomic force microscopies as well as by X-ray photoelectron spectroscopy.Resonant SERS and TERS in the condition of coinciding energies of the exciting light, exciton in NCs, and LSPR in metal nanostructures (cantilevers and nanoclusters) will allow enhancing the Raman intensity by several orders of magnitude and thus studying the vibrational and electronic spectra of single metal-semiconductor nanostructures. The energy positions of the Raman modes due to scattering by elementary excitations (in the first turn, confined optical and surface optical phonons) in the semiconductor nanostructures, their intensities, and polarization dependences will provide valuable information on the confinement effect, mechanical strain, structural transformations, and the surface conditions in single semiconductor nanostructures. The unprecedented enhancement of gap-plasmon TERS by elementary excitations in semiconductor nanostructures located in the gap between the metallized apex of a cantilever and a metal nanostructure will be achieved together with nanometer spatial resolution due to the vastly improved tunability in optical and electronic properties of both semiconductor and metal nanostructures controlled by their size, shape, and nature of junction.