Diorganyl Dichalcogenides as Surface Capping Ligands for Germanium Nanocrystals

Indirect ligand exchange methods have been demonstrated to replace the oleylamine capping with dodecanethiol for germanium nanocrystals (Ge NCs). In these methods, hydrazine is employed to effectively remove the oleylamine ligand before the NCs are passivated by microwave-assisted heating with dodecanethiol. In this work, octadecanethiol passivation is accomplished by the in situ reaction of dioctadecyl disulfide with diphenylphosphine. 1H NMR and FTIR are used to characterize the surface ligand capping and the effectiveness of ligand exchange. Thiol passivation is achieved either by an exchange reaction at room temperature or by using microwave-assisted heating. Indirect and direct ligand exchange methods are demonstrated to be effective. The microwave-assisted reaction of GeI2 with dioctadecyl disulfide also achieves thiol passivation without interfering in the formation of Ge NCs. Additional experiments study the effects of nanoparticle synthesis temperature, solvent, and ligand concentration on the exchange of oleylamine for octadecanethiol ligands

Solvent Effects on Growth, Crystallinity, and Surface Bonding of Ge Nanoparticles

Solvent effects on the microwave-assisted synthesis of germanium nanoparticles are presented. A mixture of oleylamine and 1-dodecene was used as the reaction solvent. Oleylamine serves as a reducing agent in the synthesis while both molecules act as binding ligands. Increased concentrations of 1-dodecene in the solvent mixture were found to increase the size of the formed nanoparticles. Crystallinity was also dependent on the solvent mixture. Amorphous nanoparticles were obtained at lower 1-dodecene concentrations, whereas, at higher concentrations, particles contained crystalline and amorphous domains. 11-Methoxyundec-1-ene was synthesized to replace 1-dodecene in the reaction mixture for nuclear magnetic resonance (NMR) studies. ¹H NMR of the reaction products shows that both solvent molecules in the system act as binding ligands on the nanoparticle surface. Nanoparticles were characterized using powder X-ray diffraction, scanning transmission electron microscopy, and spectroscopy techniques (Raman, UV–vis, FT-IR, and NMR)

Solvent Effects on Growth, Crystallinity, and Surface Bonding of Ge Nanoparticles

Solvent effects on the microwave-assisted synthesis of germanium nanoparticles are presented. A mixture of oleylamine and 1-dodecene was used as the reaction solvent. Oleylamine serves as a reducing agent in the synthesis while both molecules act as binding ligands. Increased concentrations of 1-dodecene in the solvent mixture were found to increase the size of the formed nanoparticles. Crystallinity was also dependent on the solvent mixture. Amorphous nanoparticles were obtained at lower 1-dodecene concentrations, whereas, at higher concentrations, particles contained crystalline and amorphous domains. 11-Methoxyundec-1-ene was synthesized to replace 1-dodecene in the reaction mixture for nuclear magnetic resonance (NMR) studies. ¹H NMR of the reaction products shows that both solvent molecules in the system act as binding ligands on the nanoparticle surface. Nanoparticles were characterized using powder X-ray diffraction, scanning transmission electron microscopy, and spectroscopy techniques (Raman, UV–vis, FT-IR, and NMR)

Halogen-Induced Crystallinity and Size Tuning of Microwave Synthesized 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–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