Nanocrystal and Nanowire Synthesis and Dispersibility in Supercritical Fluids

Abstract

Here we review major breakthroughs in the rapidly developing research area of nanomaterial synthesis in supercritical fluids focusing on both the fundamental science and processing advantages in the application areas of separations, assembly and scale-up of synthesis. Density-tunable control of the solvation strength, along with favorable interfacial tensions, wetting, and transport properties make supercritical fluids (SCFs) unique and potentially superior solvent media for the synthesis and processing of nanoscale metals and semiconductors. The steric repulsion between particle cores may be tuned by adjusting the solvent density to control the nanocrystal size and polydispersity during single-phase synthesis, and to achieve size-selective nanocrystal precipitation. Compressed solvents at the vapor pressure can also be used to deposit nanocrystal monolayers at controlled evaporation rates without dewetting at higher rates than conventional solvents. The high synthetic temperatures accessible in SCFs can be used to promote the crystallization of covalent nanomaterials, while maintaining a reaction environment capable of solvating reactants and capping ligands. This allows semiconductor nanocrystals exhibiting size-dependent photoluminescence to be synthesized at temperatures well above the boiling point of conventional solvents. Finally, gold nanocrystal seed particles can be used to synthesize bulk quantities of defect-free silicon and germanium nanowires, several micrometers in length.

Self-assembled templates for the generation of arrays of 1-dimensional nanostructures: From molecules to devices

“…[157][158][160][161]. SCF deposition of semiconductor nanowires into porous templates can also be highly effective due to the low viscosity, negligible surface tension and good wetability of the fluid phase [162]. We have reported the Au seeded growth of Ge nanostructures, with controlled crystallinities, within the channels of PAA membranes [147].…”

Embedding colloidal nanoparticles inside mesoporous silica using gas expanded liquids for high loading recyclable catalysts

“…14 Despite the use of SCFs for NP synthesis and processing, SCFs are not commonly used for impregnation pre-synthesized colloidal NPs, which is mainly attributed to the weak solvating power of many SCFs. 15 In this study we show that a mixture of hexane and sc-CO2 can be used to effectively impregnate organically capped NPs into SBA-15 support without the need for surface pre-treatment. Importantly, SCFs are effective for embedding NPs with diameters comparable to that of the support pore diameter.…”

“…Dodecanethiol capped NPs have been reported to be soluble in supercritical alkanes and octanol, therefore the hexane was chosen as the solvent media. 15 The NPs could be dispersed in hexane pressurised with sc-CO2. Figure 2(b) shows the SBA-15 after impregnation with AgPd NPs.…”

Unique diffusion behavior observed in supercritical ethanol

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Their physicochemical properties such as viscosity ͑͒, dielectric constant ͑͒, and ion product exhibit dramatic changes with respect to temperature ͑T͒ and pressure ͑P͒. SCFs are considered to be inhomogeneous and perturbed due to the intense density fluctuations and temporary clustering of the molecules both in space and in time.…”