Synthesis of germanium nanocrystals in high temperature supercritical fluid solvents

Crystalline germanium (Ge) nanocrystals were synthesized by arrested precipitation in supercritical hexane and octanol at 400similar to550 degreesC and 20.7 MPa in a continuous flow reactor. Two Ge precursors were explored, diphenylgermane (DPG) and tetraethyl germane (TEG), which undergo thermolysis to crystalline Ge under these conditions. Octanol is added to control particle growth, which appears to serve as a capping ligand that binds to the particle surface through an alkoxide linkage. The nanocrystals were characterized by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), FTIR spectroscopy, and X-ray diffraction (XRD). The average nanocrystal diameter could be changed over a wide range, from similar to2 nm to similar to70 nm, by varying the reaction temperature and precursor concentration. Relatively size-monodisperse nanocrystals could be produced, with standard deviations about the mean diameter as low as similar to10%. UV-visible absorbance and photoluminescence (PL) spectra of Ge nanocrystals in the 3 to 4 nm diameter size range exhibit optical absorbance and PL spectra blue-shifted by approximately 1.7 eV relative to the band gap of bulk Ge, with quantum yields up to 6.6%.