Size and emission color tuning in the solution phase synthesis of highly luminescent germanium nanocrystals

Highly luminescent germanium nanocrystals (NCs) are synthesized at room temperature by hydride reduction of germanium tetrachloride (GeCl4) within inverse micelles. Regulation of the Ge NC size is achieved by varying the alkyl chain length of the cationic quaternary ammonium surfactants used to form the inverse micelles. The Ge NCs are chemically passivated with allylamine ligands using a Pt-catalyzed hydrogermylation reaction, minimizing surface oxidation while rendering the NCs dispersible in a range of polar solvents. Transmission electron microscopy shows that the NCs are highly crystalline with well-defined core diameters tuned from 3 to 5 nm. UV-Vis absorbance and photoluminescence (PL) spectroscopy show significant quantum confinement effects, with moderate absorption in the UV spectral range, and a strong, narrow luminescence in the visible with a marked dependency on excitation wavelength. A maximum quantum yield of 20% is shown for the nanocrystals, and a transition from primarily blue to green emission is observed as the NC diameter increases to 4.5 nm.