Structural Insights on Microwave-Synthesized Antimony-Doped Germanium Nanocrystals.

Doped and alloyed germanium nanocrystals (Ge NCs) are potential candidates for a variety of applications such as photovoltaics and near IR detectors. Recently, bismuth (Bi) as an n-type group 15 element was shown to be successfully and kinetically doped into Ge NCs through a microwave-assisted solution-based synthesis, although Bi is thermodynamically insoluble in bulk crystalline Ge. To expand the composition manipulation of Ge NCs, another more common n-type group 15 element for semiconductors, antimony (Sb), is investigated. Oleylamine (OAm)- and OAm/trioctylphosphine (TOP)-capped Sb-doped Ge NCs have been synthesized by the microwave-assisted solution reaction of GeI2 with SbI3. Passivating the Ge surface with a binary ligand system of OAm/TOP results in formation of consistently larger NCs compared to OAm alone. The TOP coordination on the Ge surface is confirmed by 31P NMR and SEM-EDS. The lattice parameter of Ge NCs increases with increasing Sb concentration (0.00-2.0 mol %), consistent with incorporation of Sb. An increase in the NC diameter with higher content of SbI3 in the reaction is shown by TEM. XPS and EDS confirm the presence of Sb before and after removal of surface ligands with hydrazine and recapping the Ge NC surface with dodecanethiol (DDT). EXAFS analysis suggests that Sb resides within the NCs on highly distorted sites next to a Ge vacancy as well as on the crystallite surface. High Urbach energies obtained from photothermal deflection spectroscopy (PDS) of the films prepared from pristine Ge NC and Sb-doped Ge NCs indicate high levels of disorder, in agreement with EXAFS data. Electrical measurements on TiO2-NC electron- and hole-only devices show an increase in hole conduction, suggesting that the Sb-vacancy defects are behaving as a p-type dopant in the Ge NCs, consistent with the vacancy model derived from the EXAFS results.

Surface coordination chemistry of germanium nanocrystals synthesized by microwave-assisted reduction in oleylamine.

As surface ligands play a critical role in the colloidal stability and optoelectronic properties of semiconductor nanocrystals, we used solution NMR experiments to investigate the surface coordination chemistry of Ge nanocrystals synthesized by a microwave-assisted reduction of GeI2 in oleylamine. The as-synthesized Ge nanocrystals are coordinated to a fraction of strongly bound oleylamide ligands (with covalent X-type Ge-NHR bonds) and a fraction of more weakly bound (or physisorbed) oleylamine, which readily exchanges with free oleylamine in solution. The fraction of strongly bound oleylamide ligands increases with increasing synthesis temperature, which also correlates with better colloidal stability. Thiol and carboxylic acid ligands bind to the Ge nanocrystal surface only upon heating, suggesting a high kinetic barrier to surface binding. These incoming ligands do not displace native oleylamide ligands but instead appear to coordinate to open surface sites, confirming that the as-prepared nanocrystals are not fully passivated. These findings will allow for a better understanding of the surface chemistry of main group nanocrystals and the conditions necessary for ligand exchange to ultimately maximize their functionality.

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).