Size and Bandgap Control in the Solution-Phase Synthesis of Near-Infrared-Emitting Germanium Nanocrystals

We present a novel colloidal synthesis of alkyl-terminated Ge nanocrystals based on the reduction of GeI4/GeI2 mixtures. The size of the nanocrystals (2.3−11.3 nm) was controlled by adjusting both the Ge(IV)/Ge(II) ratio and the temperature ramp rate following reductant injection. The near-infrared absorption (1.6−0.70 eV) and corresponding band-edge emission demonstrate the highly tunable quantum confinement effects in Ge nanocrystals prepared using this mixed-valence precursor method. A mechanism is proposed for the observed size control, which relies upon the difference in reduction temperatures for Ge(II) versus Ge(IV).