Quantum confinement of volume plasmons and interband transitions in germanium nanocrystals

Abstract

The plasmonic properties of individual quantum-sized Ge nanocrystals (NCs) were observed and systematically analyzed by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). For this purpose, Ge NCs embedded in an SiO${}_{2}$ matrix with controllable size, density, and structure were fabricated using magnetron sputtering. The size dependence of the Ge plasmon energies in the size range of 5--9 nm is shown to be well depicted by the so-called medium quantum confinement (QC) model, with an effective mass of 0.57${m}_{0}$ (contrary to expectations of a stronger quantum effect). In the very low-loss region of the EEL spectra, an apparent blue shift of the ${E}_{2}$ interband transition peak up to 2 eV and a strong reduction in the oscillator strength were measured for the NCs in the size range of 4--6 nm. It indicates for this smaller size range a transition to a QC regime where the band structure and the density of states are modified dramatically. These trends are explained by a combination of low-loss and core-loss EELS results, which show that the Ge NCs are surrounded uniformly by nearly stoichiometric SiO${}_{2}$. This local chemistry is shown to provide an infinite potential barrier and to confine electrons and holes in the spherically shaped Ge NCs. In addition to pure QC effects in the Ge NCs, the SiO${}_{2}$ matrix thus plays an important role in the strength of the observed QC and interband transitions.