Stabilization, nanocrystal

Both sc-ethane and SC-CO2 provide density tunable dispersibility for nanocrystals. Partially fluorinated ligands enabled the first example of a sterically stabilized nanocrystal dispersion in pure CO2. The nanocrystals show LCST phase behavior with increased dispersibility at higher solvent densities. Additionally, arrested precipitation to synthesize nanocrystals in SC-CO2 has been developed. The technique yields chemically robust nanocrystals that are fully passivated with fluorinated ligands allowing for collection and redispersion of the particles without any change in size or polydispersity. The nanocrystal size produced depends on both the solvent density and length of the ligand, with smaller less polydisperse particles formed at conditions of adequate steric stabilization. 

Surfactants are crucial in the colloidal chemical synthesis of nanocrystals. Popularly used surfactants include tri-n-octylphosphine (TOP), tri-n-octylphosphine oxide (TOPO 6, 14), hexadecylamine (17), fatty acids, and phosphonic acids (18-20). In general, the polar head groups of the surfactants bind to the surface of the nanocrystals, as illustrated in Figure 6.10 (21). The surfactants play crucial roles in the nanocrystal synthesis (14-16). First, the surfactant capping prevents the agglomeration of the nanocrystals, endowing them with a good colloidal stability. Nanocrystals have a... 

Flarfenist S A and Wang Z L 1999 Fligh-temperature stability of passivated silver nanocrystal superlattices J. Phys. Chem. B 103 4342... 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

TBP and injected into a hot ( 350 °C) solution of TOPO (12 g). The injection of CdSe precursors into the hot solution ofTOPO resulted in spontaneous nucleation of CdSe nanocrystals and a decrease in temperature. Once the temperature was stabilized, an additional amount (0.4 mL) of the precursor solution was added for the growth of the nucleated nanocrystals. Here, Ostwald ripening was avoided by separating the nucleation and growth processes. All the reagents and the reaction were kept under an Ar atmosphere to avoid fire hazard and surface oxidation of the nanocrystals. 

Rogach, A. L., Kornowski, A., Gao, M. Y, Eychmuller, A. and Weller, H. (1999) Synthesis and characterization of a size series of extremely small fhiol-stabilized CdSe nanocrystals. /. Phys. Chem. B, 103, 3065-3069. 

A similar route has been followed most recently by Weiss et al. who have fabricated a multi-island SE device from self-assembled ID gold nanocrystal chains [28]. They assembled thiol-stabilized 50 nm Au particles in a chainlike structure and used subsequently electron-beam lithography for electrode fabrication. Here it has to be... 

Shah, P.S., Husain, S., Johnston, K.P. and Korgel, B.A. (2002) Role of steric stabilization on the arrested growth of silver nanocrystals in supercritical carbon dioxide. Journal of Physical Chemistry B, 106 (47), 12178-12185. 

The reactions used for synthesis of II-VI (CdSe, CdTe), III-V (InP, InAs), and IV-VI (PbS, PbSe, PbTe) semiconductor nanocrystals are outlined by Schemes 3-5.4,17,30-32 The syntheses are carried out at high temperatures, and in the presence of long-chain alkylphosphines (trioctylphosphine,TOP), alkyl-phosphine oxides (trioctylphosphine oxide, TOPO), alkylamines (hexadecyl-amine, HD A), and alkylphosphonic acids as the stabilizing agents. 

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