Probing electronics as a function of size and surface of colloidal germanium nanocrystals

Inorganic semiconductor nanoparticles are of significant interest for applications that benefit from their size-dependent properties. The work presented here focuses on the characterization of solution-based microwave synthesized Ge nanocrystals (NCs). Three differently capped Ge NCs were investigated: oleylamine (OAM), dodecanethiol (DDT), and a functionalized N4,N4,N4′,N4′-tetraphenylbiphenyl-4,4′-diamine (TPD) ligand, which is commonly used as hole-transporting units. The optical gaps followed the expected trend for quantum confinement; however, the absolute value depended upon the ligand. We found that the DDT-capped Ge NCs feature consistently larger bandgaps than OAM-capped Ge NCs of a similar size. Cyclic voltammetry (CV) was used to determine the valence band energy for OAM-capped Ge NCs, and the conduction band energy was estimated from the optical gap. By contrast, DDT-capped Ge NCs and the OAM/DDT-capped Ge NCs did not exhibit an oxidative signal in the cyclic voltammetry. This was attributed to the removal of surface defects of OAM-capped Ge NCs through stronger Ge-S surface bonds. TPD-capped Ge NCs were investigated and showed a shift to slightly higher oxidation potential compared with the free ligand and bandgap values in between that of the OAM-capped and DDT-capped Ge NCs. The higher oxidation potential is attributed to TPD orientation, and the bandgap value reflects the lower number of Ge-S bonds on the surface due to ligand sterics.