Doping colloidal bcc crystals - interstitial solids and meta-stable clusters

Effect of fluorine tin oxide substrate on the deposited SnO2: Ni thin films properties for gas sensing

This study explores the deposition of Tin Oxide and Ni-doped SnO2 thin films (NSO) via spray pyrolysis from aqueous solutions. The deposition process was conducted under uniform conditions on two substrates, namely glass and fluorine tin oxide (FTO), with varying Ni percentages. The aim was to evaluate their potential for gas sensing applications. The as-deposited thin films exhibit diverse properties influenced by both Ni content and substrate type. X-Ray Diffraction (XRD) measurements reveal polycrystalline structures characterized by broad SnO2 diffraction lines, with the emergence of a NiO phase, particularly evident at higher Ni content. Notably, thin films deposited on FTO show the appearance of a secondary phase of SnO and enhanced crystallinity. Furthermore, lattice parameters and crystallite size decrease with increasing Ni percentage. The Field Emission Scanning Electron Microscopy (FE-SEM) analysis highlights significant alterations in surface nanostructures based on nickel content and substrate type. Higher nickel concentrations result in the formation of cauliflower-like structures, varying in size and density. This structural divergence significantly impacts the sensitivity of NSO-based NO2 gas sensors. Particularly, thin films with 20 % Ni, especially those deposited on FTO, exhibit optimal configurations for gas sensor applications, display sensitivity of 501 % at 100 ppm for nanocrystalline NSO/FTO compared to 436 % for glass-deposited samples. Our findings highlight the crucial role of Ni content and substrate type in modifying the structural and sensing properties of NSO thin films, for enhanced gas sensing applications.

Porous crystals in charged sphere suspensions by aggregate-driven phase separation

The kinetics of phase transition processes often governs the resulting material microstructure. Using optical microscopy, we here investigate the formation and stabilization of a porous crystalline microstructure forming in low-salt suspensions of charged colloidal spheres containing aggregates comprising some 5-10 of these colloids. We observe the transformation of an initially crystalline colloidal solid with homogeneously incorporated aggregates to individual, compositionally refined crystallites of perforated morphology coexisting with an aggregate-enriched fluid phase filling the holes and separating individual crystallites. A preliminary kinetic characterization suggests that the involved processes follow power laws. We show that this route to porous materials is neither restricted to nominally single component systems nor to a particular microstructure to start from. However, it necessitates an early rapid solidification stage during which the aggregates become trapped in the bulk of the host-crystals. The thermodynamic stability of the reconstructed crystalline scaffold against melting under increased salinity was found comparable to that of pure phase crystallites grown very slowly from a melt. Future implications of this novel route to porous colloidal crystals are discussed.

Salt-concentration-dependent nucleation rates in low-metastability colloidal charged sphere melts containing small amounts of doublets

We determined bulk crystal nucleation rates in aqueous suspensions of charged spheres at low metastability. Experiments were performed in dependence on electrolyte concentration and for two different particle number densities. The time-dependent nucleation rate shows a pronounced initial peak, while postsolidification crystal size distributions are skewed towards larger crystallite sizes. At each concentration, the nucleation rate density initially drops exponentially with increasing salt concentration. The full data set, however, shows an unexpected scaling of the nucleation rate densities with metastability times the number density of particles. Parameterization of our results in terms of classical nucleation theory reveals unusually low interfacial free energies of the nucleus surfaces and nucleation barriers well below the thermal energy. We tentatively attribute our observations to the presence of doublets introduced by the employed conditioning technique. We discuss the conditions under which such small seeds may induce nucleation.

High antisite defect concentrations in hard-sphere colloidal Laves phases

Binary mixtures of hard spheres can spontaneously self-assemble into binary crystals. Computer simulations have been especially useful in mapping out the phase behaviour of these mixtures, under the assumption that the stoichiometry of the binary crystal is ideal. Here we show that for a size ratio of q = 0.82 this assumption is not valid near the coexistence region between the fluid and the stable binary crystal, the MgZn2 Laves phase. Instead we find a surprisingly high number of antisite defects: up to 2% of the large spheres are replaced by small spheres in equilibrium. We demonstrate that the defect concentration can be estimated using simple approximations, providing an easy way to identify systems where antisite defects play an important role. Our results shed new light on the self-assembly of colloidal Laves phases, and demonstrate the importance of antisite defects in binary crystals.