Synthesis and optical properties of colloidal germanium nanocrystals

Highly crystalline germanium (Ge) nanocrystals in the size range 2-10 nm were grown in inverse micelles and purified and size separated by high-pressure liquid chromatography with on-line optical and electrical diagnostics. The nanocrystals retain the diamond structure of bulk Ge down to at least 2.0 nm (containing about 150 Ge atoms). One of the objectives of the work was to demonstrate visible light emission from these nanocrystals, and, as it turned out, this dictated emphasis on small (equal to or less than5 nm) sizes. The background- and impurity-free extinction and photoluminescence (PL) spectra of 2 and 4 nm nanocrystals revealed rich structure which could be interpreted in terms of the band structure of Ge shifted to higher energies by quantum confinement. The sifts ranged from similar to0.1 eV to over 1 eV for the various transitions. PL in the range 350-700 nm was observed from nanocrystals 2-5 nm in size. The 2.0-nm nanocrystals yielded the most intense PL (at 420 nm) which is believed to be intrinsic and attributed to direct recombination at Gamma. Excitation at high energy (250 nm) populates most of the conduction bands resulting in competing recombination channels and the observed broad PL spectra.