Halogen-Induced Crystallinity and Size Tuning of Microwave Synthesized Germanium Nanocrystals

Abstract

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–xSnx 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 (I2) and bromine (Br2) 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 °C for 30 min. I2 and Br2 are shown to oxidize GeI2 to GeI4 in situ, providing good control over size and crystallinity. The kinetics of Br2 oxidation of GeI2 is slightly different, but both I2 and Br2 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 I2 and Br2 improve the crystallinity of the Ge NCs, I2 provides overall higher crystallinity in the NCs compared to Br2. Absorption (UV–vis–NIR) spectroscopy is consistent with quantum confinement for Ge NCs. The solutions of I2, GeI2, and colloidal Ge NCs were investigated with Fourier transform infrared and 1H 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 I2, 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