Single-Crystalline Germanium Nanocrystals via a Two-Step Microwave-Assisted Colloidal Synthesis from GeI₄

Colloidal germanium (Ge) nanocrystals (NCs) are of great interest with possible applications for photovoltaics and near-IR detectors. In many examples of colloidal reactions, Ge(II) precursors are employed, and NCs of diameter ∼3-10 nm have been prepared. Herein, we employed a two-step microwave-assisted reduction of GeI₄ in oleylamine (OAm) to prepare monodispersed Ge NCs with a size of 18.9 ± 1.84 nm. More importantly, the as-synthesized Ge NCs showed high crystallinity with single-crystal nature as indicated by powder X-ray diffraction, selected area electron diffraction, and high-resolution transmission electron microscopy. The Tauc plot derived from photothermal deflection spectroscopy measurement on Ge NCs thin films shows a decreased bandgap of the Ge NCs obtained from GeI₄ compared with that of the Ge NCs from GeI₂ with a similar particle size, indicating a higher crystallinity of the samples prepared with the two-step reaction from GeI₄. The calculated Urbach energy indicates less disorder in the larger NCs. This disorder might correlate with the fraction of surface states associated with decreased particle size or with the increased molar ratio of ligands to germanium. Solutions involved in this two-step reaction were investigated with ¹H NMR spectroscopy and high-resolution mass spectrometry (MS). One possible reaction pathway is proposed to unveil the details of the reaction involving GeI₄ and OAm. Overall, this two-step synthesis produces high-quality Ge NCs and provides new insight on nanoparticle synthesis of covalently bonding semiconductors.