Gap-Plasmon Tip-Enhanced Raman Scattering of Semiconductor Nanostructures

本项目旨在研究空间分辨率为纳米级的金属纳米结构附近的单个半导体纳米结构在间隙等离子体激元诱导下对拉曼散射的增强效应。金属纳米结构中的局域表面等离子体（LSP）的倏逝电磁场在相邻的半导体纳米晶体（NCs）中引起光学共振现象，包括尖端增强拉曼散射（TERS）和表面增强拉曼散射（SERS）。在金属纳米结构和金属纳米结构之间激发的间隙等离子体激元可以进一步大幅增强局部电磁场，从而增强放置在间隙中的半导体纳米结构的TERS信号。利用Langmuir-Blodgett技术合成由CdSe、CdS、cu或PbS组成的可变尺寸纳米结构，并将其沉积在Au纳米团簇阵列上是研究的重点。将放置在阵列上的二硫化钼单层作为模型结构来研究间隙模式TERS现象的性质，包括共振效应，近场和远场的影响以及退极化特性。金属纳米结构的制备，即用于进一步NC沉积的金属（Au）纳米团簇阵列，将得到改进。对于TERS实验，在尖端放置金属（Au, Ag）纳米团簇的悬臂梁将产生特定的LSP共振（LSPR）能量。利用ANSYS HFSS™进行三维全波模拟，选择具有目标等离子体特性的纳米结构的形貌。通过透射电子、扫描电子、原子力显微镜以及x射线光电子能谱，可以控制纳米结构和金属纳米结构的尺寸、形态和表面。共振SERS和TERS在激发光、NCs中的激子和金属纳米结构（悬臂和纳米簇）中的LSPR能量一致的条件下，可以将拉曼强度提高几个数量级，从而研究单金属半导体纳米结构的振动和电子能谱。在半导体纳米结构中，由于初等激发（第一轮是受限光学声子和表面光学声子）散射而产生的拉曼模式的能量位置、它们的强度和偏振依赖性将为单个半导体纳米结构中的约束效应、机械应变、结构转变和表面条件提供有价值的信息。由于半导体和金属纳米结构的尺寸、形状和结的性质控制，大大提高了光学和电子特性的可调性，因此，位于悬臂金属化顶点和金属纳米结构之间的半导体纳米结构中的初等激发对间隙等离子体激元的前所未有的增强将与纳米空间分辨率一起实现。

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.