Excitations of Quasi-Particles in Nanostructured Systems

The excitation of quasiparticles, like the investigated excitons and plasmons here, are the optically most prominent responses of materials. In nanostructured system, the sample quality is crucial for quantitative investigations of these optical excitations. We used electron beam evaporation, nano-second laser dewetting, and electron metalorganic chemical vapor deposition techniques to prepare well-defined and “clean” transmission electron microscopy (TEM) samples. Electron energy-loss microscopy (EELS) performed in STEM mode was employed to investigate the structural and electro-optical properties. Quantifit software was used to analyze the EELS spectra quantitatively in terms of inelastic scattering probability, energy and lifetime.

We found that the ferroplasmon originates from induced excitation by the Ag’s intrinsic dipole mode at low energy, and it has a redshift with increasing particle size. Because the bimetallic system is associated with one dipole mode only, the ferroplasmons is strongly dependent on geometry. Disc-skirt AgCo nanostructures also show ferroplasmons because plasmon excitation mode of Ag disc is similar in geometry to Ag spherical, while the nanotriangles and nanobowties did not show a ferroplasmon. The bulk plasmon (BP) did not have a significate change from the pure metals to the metals in the bimetallic systems, indicating that the electron density did not change through the contact of the metals.

In semiconductors, high binding energy excitons were detected universally at room temperature by EELS for the first time. The states associated with these excitons were identified as molecular states. The singlet S₀ state can be directly excited to the triplet T₁ state by electrons, even though the transition is forbidden optically. The conclusion on molecular states was based on the fact that this excitation can be bleached with time, and recovered in minutes. Bandbending was observed when the semiconductor is in contacting with Au nanoparticles. This exciton has a signal reduction and blue shift introduced by the band bending. The higher energy exciton can be excited from the S₀ state to the singlet S₁ state when the band bending is large enough. The distribution of the point defects can be mapped with high precision through mapping the intensity of the exciton.