(608a) Germanium Nanocrystals : Colloidal Synthesis and Optical Characterization

Interest in fabricating solar cells comprised of semiconductor nanocrystals has grown in the past few years. Because of solubility of colloidal nanocrystals, the process involving deposition of active layers can be cheaper and more efficient than current thin-film technology. Colloidal inorganic nanocrystals offer an added advantage over organic polymers: nanocrystals passivated with capping ligands present better stability in air. Germanium is a particularly intriguing candidate for the use in photovoltaic cells, because of its low toxicity compared to II-VI semiconductors, the relative abundance, and its compatibility with current microelectronics technology. Moreover, the lifetime of excitons in germanium is longer than many other II-VI semiconductors suggested as the alternative to silicon technology. However, the use of germanium in solar cells has been limited due in part to the indirect nature of its band gap. Also, germanium is susceptible to oxidation, yielding electronically and chemically unstable germanium oxides. Nanocrystals with the size below exciton Bohr radius exhibit quantum confinement, which in the case of germanium could solve the indirect band gap problem. By growing nanocrystals with a stable surface coating, one could prevent the growth of oxide layers on the surface. Yet, the capability of synthesizing colloidal germanium nanocrystals is lacking, mainly due to the strong covalent bond of germanium and the need for high temperatures to break chemical bonds in precursors. The quest for facile syntheses that hints the growth of germanium nanocrystals with direct band-gap has thus intensified. We synthesized germanium nanocrystals via wet chemistry in a Schlenk setup. Germanium diiodide was used as the germanium precursor, and variables such as solvents, reducing agents, injection temperature, reaction temperature, reaction time and capping ligands were changed to monitor the best synthetic conditions. Capping ligands appear to affect the oxidation of germanium: surface passivation with covalently-bonded capping ligands allows the nanocrystals to be more resistive to even a severe oxidative environment. Colloidal germanium nanocrystals were characterized with transmission electron microscopy, x-ray diffraction, fluorometers operated at near-infrared or visible energy range, and ultrafast laser spectroscopy. We have synthesized the first germanium nanocrystals with strong infrared emission, with quantum yields as high as 10%.