Extinction spectra of colloidal germanium nanocrystals suspended in benzonitrile without the use of ligands were measured, and absolute absorption cross sections are reported. Comparison to cross sections calculated using the Mie solution to Maxwell's equations reveals that, as the mean nanocrystal size is reduced from 11 to 4 nm, the absorption features below 3.5 eV blueshift because of quantum confinement effects. The shifts are not, however, sufficiently large for the nanocrystal cores to produce the blue photoluminescence commonly observed from germanium nanocrystals. At energies greater than 3.5 eV the Mie and measured cross sections overlap, indicating a bulk-like band structure.
Bibliographical noteFunding Information:
This work was supported by NSF under Grant CBET-0756326, IGERT Grant DGE-0114372, and MRSEC Grant DMR-0819885. Partial support was also provided by the DOE Energy Frontier Research Center for Advanced Solar Photophysics and by the UMN Center for Nanostructure Applications.