Halogen-Induced Crystallinity and Size Tuning of MicrowaveSynthesized Germanium Nanocrystals

The reduction of Ge halides in oleylamine (OAm) provides a simple, yet effective high-yield synthetic route to germanium nanocrystals (NCs). Significant advances based on this approach include size control of Ge NCs, Bi doping of Ge NCs, and synthesis of Ge1- xSrx alloys. It has been shown that the size of Ge NCs can be controlled by the ratio of Ge2+/Ge4+ in the reaction. Here, we show that finer control of absolute size and crystallinity can be achieved by the addition of molecular iodine (I-2) and bromine (Br-2) to germanium(II) iodide (GeI2). We also show the presence of a Ge-amine-iodide complex and production of hydrogen and ammonia gases as side products of the reduction reaction. All reactions were carried out by microwave-assisted heating at 250 degrees C for 30 min. I-2 and Br-2 are shown to oxidize GeI2 to GeI4 in situ, providing good control over size and crystallinity. The kinetics of Br-2 oxidation of GeI2 is slightly different, but both I-2 and Br-2 provide size control of the Ge NCs. The samples are highly crystalline as indicated by powder X-ray diffraction, selected area electron diffraction, transmission electron microscopy and Raman spectroscopy. Although both I-2 and Br-2 improve the crystallinity of the Ge NCs, I-2 provides overall higher crystallinity in the NCs compared to Br-2. Absorption (UV-vis-NIR) spectroscopy is consistent with quantum confinement for Ge NCs. The solutions of I-2, GeI2, and colloidal Ge NCs were investigated with Fourier transform infrared and H-1 NMR spectroscopies and showed no evidence for imine or nitrile formation. The hydrogen on the amine in OAm is shifted downfield with increasing amounts of I-2, consistent with a more acidic ammonium species. Hydrogen and ammonia gases were detected after the reaction by gas chromatography and high-resolution mass spectrometry. The presence of a Ge-amine-iodide complex was also confirmed with no evidence for a hydrazine-like species. These results provide an efficient fine-tuning of size and crystallinity of Ge NCs using halogens in addition to the mixed-valence precursor synthetic protocol previously reported and demonstrate the formation of hydrogen as a reducing agent in OAm.