Formation of single-crystalline CuS at the organic-aqueous interface.

We report here the results of a study to understand the formation mechanism of single crystals of the transition metal chalcogenide, CuS, at the water-toluene interface through an interfacial reaction. Systematic measurements carried out using synchrotron x-ray scattering, electron microscopy, atomic force microscopy and calorimetric techniques clearly show that nano-crystallites of CuS form within a few minutes at the interface as the reagents are brought from the organic (upper) and aqueous (lower) layers to the interface, then crystallization of CuS proceeds over a few hours only by reorganization, despite the large excess available in both upper and lower liquid phases. The interface confinement and passivation by organics is critical here in the formation of single crystals having sizes of 6 and 200 nm along the normal and in-plane directions of the liquid-liquid interface.

Growth kinetics of platinum nanocrystals prepared by two different methods: role of the surface.

In order to examine the applicability of the diffusion-limited Ostwald ripening model to the growth kinetics of nanocrystals, platinum nanocrystals prepared by two different methods have been investigated by a combined use of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). One of the methods of synthesis involved the reduction of chloroplatinic acid by sodium citrate while in the other method reduction was carried out in the presence of polyvinylpyrrolidone (PVP) as a capping agent. The growth of platinum nanocrystals prepared by citrate reduction in the absence of any capping agent follows a Ostwald ripening growth with a D(3) dependence. In the presence of PVP, the growth of platinum nanocrystals does not completely follow the Ostwald ripening model, making it necessary to include a surface reaction term in the growth equation. Thus, the growth of platinum nanocrystals in the presence of PVP has contributions both from diffusion and surface reaction, exhibiting a D(3)+D(2) type behavior.

Binding of DNA nucleobases and nucleosides with graphene.

Interaction of two different samples of graphene with DNA nucleobases and nucleosides is investigated by isothermal titration calorimetry. The relative interaction energies of the nucleobases decrease in the order guanine (G)>adenine (A)>cytosine (C)>thymine (T) in aqueous solutions, although the positions of C and T seem to be interchangeable. The same trend is found with the nucleosides. Interaction energies of the A-T and G-C pairs are somewhere between those of the constituent bases. Theoretical calculations including van der Waals interaction and solvation energies give the trend G>A approximately T>C. The magnitudes of the interaction energies of the nucleobases with graphene are similar to those found with single-walled carbon nanotubes.

Investigations of the growth kinetics of capped CdSe and CdS nanocrystals by a combined use of small angle X-ray scattering and other techniques.

The growth of capped CdSe and CdS nanocrystals formed by the reaction of selenium or sulfur with cadmium stearate in toluene solution in the presence of dodecanethiol or trioctylphosphine oxide and tetralin, has been investigated by a variety of techniques to obtain reliable data. Whereas small angle X-ray scattering has provided statistically satisfactory data on the variation of size distribution with time, TEM has been used as a direct probe of the particle size, although with a limited sampling size. UV/Vis and photoluminescence spectroscopies have also provided information about the time evolution of the average diameter (D) of the nanocrystals. By employing all these techniques, we have obtained the D(t) data and fitted then to various growth models. Although certain qualitative observations suggest growth of the nanocrystals to be controlled by diffusion-limited Ostwald ripening, we have found it is necessary to include the surface reaction term in the growth equation. Thus, the growth of CdSe and CdS nanocrystals has contributions from both diffusion and surface reaction, with a D(3)+D(2) type behavior, independent of the capping agent.

Growth kinetics of gold nanocrystals: a combined small-angle X-ray scattering and calorimetric study.

The growth of gold nanocrystals prepared by the reduction of tetrachloroauric acid by tetrakis(hydroxymethyl)phosphonium chloride, which allows slow reduction, is investigated by small-angle X-ray scattering and isothermal titration calorimetry in combination with transmission electron microscopy. The growth of the nanocrystals does not follow the diffusion-limited Ostwald ripening but instead follows a sigmoidal rate curve. The activation energy obtained from the temperature-dependent growth study is very small. The heat change associated with the growth is determined for the first time as approximately 10 kcal mol(-1) per 1 nm increase in the nanocrystals' diameter.

Effects of changes in the interparticle separation induced by alkanethiols on the surface plasmon band and other properties of nanocrystalline gold films.

Effects of changing the interparticle separation on the surface plasmon bands of ultrathin films of gold nanoparticles have been investigated by examining the interaction of alkanethiols of varying chain length on nanocrystalline gold films generated at the organic-aqueous interface. Adsorption of alkanethiols causes blue-shifts of the surface plasmon adsorption band, the magnitude of the shift being proportional to the chain length. The disordered nanocrystals thus created (lambdamax, 530 m) are in equilibrium with the ordered nanocrystals in the film (lambdamax, 700 m) as indicated by an isosbestic point around 600 nm. Long chain thiols disintegrate or disorder the gold films more effectively, as demonstrated by the increased population of the thiol-capped gold nanocrystals in solution. The rate of interaction of the thiols with the film decreases with the decreasing chain length. The effect of an alkanethiol on the spectrum of the gold film is specific, in that the effects with long and short chains are reversible. The changes in the plasmon band of gold due to interparticle separation can be satisfactorily modeled on the basis of the Maxwell-Garnett formalism. Spectroscopic studies, augmented by calorimetric measurements, suggest that the interaction of alkanethiols involves two steps, the first step being the exothermic gold film-thiol interaction and the second step includes the endothermic disordering process followed by further thiol capping of isolated gold particles.