Synthesis of germanium nanocrystals in high temperature supercritical CO(2)

Synthesis and structure of free-standing germanium quantum dots and their application in live cell imaging

Colloidally synthesized free-standing Ge qdots with a unique core–shell structure were demonstrated to be a viable bio-imaging probe.

Synthesis of nanoamorphous germanium and its transformation to nanocrystalline germanium

A simple reaction between a mild reducing agent such as a trialkoxysilane and Ge(IV) species such as germanium tetraalkoxides in a room-temperature water/alcohol solution produces silica-coated ultrasmall (2-3 nm) amorphous germanium nanoparticles (na-Ge/SiO₂). The initial reaction involves the straightforward hydrolysis and condensation of the precursors, Ge(OCH₂ CH₃)₄ and (CH₃CH₂O)₃ SiH, where the reaction rate depends on the water concentration in the reaction medium. These processes can be further accelerated by adding acid to the reaction medium or carrying out the reaction at higher temperatures. At low water contents (up to 50% water/ethanol) and low acid concentrations, the reaction proceeds as a clear solution, and no precipitation is observed. The initially colorless clear solution progressively changes to pale yellow, yellow, orange, red, and finally dark red as the na-Ge particles grow. Evaporation of the solvent yields a reddish-brown powder/monolith consisting of na-Ge, embedded in an encapsulating amorphous silica matrix, na-Ge/SiO₂. The formation of na-Ge proceeds extremely slowly and follows a first-order dependence on both water concentration and diameter of the na-Ge particles under the reaction conditions used. Annealing of the na-Ge/SiO₂ powder under an inert atmosphere at 600 °C produces ultrasmall germanium nanocrystals (nc-Ge) embedded in amorphous silica (nc-Ge/SiO₂). Freestanding, colloidally stable nc-Ge is obtained by chemical etching of the encapsulating silica matrix.

Size and bandgap control in the solution-phase synthesis of near-infrared-emitting germanium nanocrystals

We present a novel colloidal synthesis of alkyl-terminated Ge nanocrystals based on the reduction of GeI(4)/GeI(2) mixtures. The size of the nanocrystals (2.3-11.3 nm) was controlled by adjusting both the Ge(IV)/Ge(II) ratio and the temperature ramp rate following reductant injection. The near-infrared absorption (1.6-0.70 eV) and corresponding band-edge emission demonstrate the highly tunable quantum confinement effects in Ge nanocrystals prepared using this mixed-valence precursor method. A mechanism is proposed for the observed size control, which relies upon the difference in reduction temperatures for Ge(II) versus Ge(IV).

Chemical reactions on surface molecules attached to silicon quantum dots

This Article describes research on chemical reactions on molecules attached to the surface of silicon quantum dots that have been performed to produce quantum dots with reactive surface functionalities such as diols and epoxides. Characterization of the surface reactions includes NMR and FT-IR studies, and the quantum dots were characterized by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Cytotoxicity and cell viability assay conducted on silicon dots capped with polar molecules indicated low toxicity with quantum dots with more reactive functionalities found to be more toxic. The silicon quantum dots photoluminesce and have been used as a blue chromophore for the biological imaging of cells.

The evolution of pseudo-spherical silicon nanocrystals to tetrahedra, mediated by phosphonic acid surfactants

Silicon nanocrystals were synthesized at high temperatures and high pressures by the thermolysis of diphenylsilane using a combination of supercritical carbon dioxide and phosphonic acid surfactants. Size and shape evolution from pseudo-spherical silicon nanocrystals to well-faceted tetrahedral-shaped silicon crystals with edge lengths in the range of 30-400 nm were observed with sequentially decreasing surfactant chain lengths. The silicon nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering spectroscopy.

Solution-phase synthesis of germanium nanoclusters using sulfur

Sulfur is used to promote the formation of germanium nanocrystals from the robust organometallic precursor, triphenylgermanium chloride, at elevated temperatures (300 °C) in the surfactant/solvent hexadecylamine. Transmission electron microscopy shows that 8 nm germanium nanocrystals are produced that self-assemble into uniform-sized 60 nm germanium nanoclusters after purification. Electron diffraction studies show that the germanium nanoclusters have a diamond germanium crystal structure and energy dispersive x-ray spectroscopy shows that the nanoclusters are primarily composed of pure germanium.