One efficient method to obtain disordered colloidal packing is to reduce the stability of colloidal particles by adding electrolytes to the colloidal dispersions. But the correct amount of additional electrolytes must be found empirically. Here, the effect of CaCl on polystyrene colloidal dispersions is studied, and a link between the amount of CaCl and the corresponding glassy colloidal structure is quantitatively built. A threshold concentration of CaCl is found by dynamic light scattering. When exceeding this threshold, different nanoparticle oligomers are observed in the dispersions by analytical ultracentrifugation. The second objective is to achieve free-standing samples, which is required for many optical measurements. A universal method is established, using a centrifugal field to produce robust samples by polymerizing coassembled hydrophilic monomers to form a network, which traps the glassy colloidal structures. Photon time of flight measurements shows that the CaCl concentration threshold should not be exceeded. Otherwise an optical shortcut may take place. Thus, the work provides a feasible universal route to prepare macroscopic free-standing photonic glasses from electrostatically stabilized nanoparticles, suitable for further optical investigation.
The process of symmetry breaking that leads to the formation of anisotropic, colloidal semiconductor nanocrystals is an important issue for understanding the physical mechanism of nucleation and growth. One-dimensional growth of nanorods is assumed to occur under reaction-limited control at a high concentration of monomer, which preferentially reacts at the crystal facet that has the highest energy and least passivation by surface ligands. In this study, it is shown that instead of assuming a homogeneous distribution of monomer in the reaction solution, the seeded growth of CdS nanorods on CdSe particles is driven by the formation of one-dimensional reaction intermediates that act as local monomer reservoirs. These result in heterogeneously distributed hotspots of the nucleating species that guarantees fast deposition and one-dimensional growth of the nanorod exclusively in one direction. Thus, performing anisotropic particle growth reactions under conditions that favor formation of transient, metastable intermediates lead to particles with a higher aspect ratio and better mechanistic reaction control.
Complex, anisotropic nanocrystals made from two or more components are extremely interesting functional materials that can drive directional, light-activated processes like charge separation and photocatalysis. However, while some synthetic protocols exist, little is known about the reaction mechanism for regioselective, heterogeneous nucleation of a second semiconductor material onto nanocrystal seeds. This paper presents the mechanism that leads to growth of a single tip at one end of CdS nanorods with yields between 50 and 80%. It is shown that the growth of only one tip is a result of tight control of the available, nucleating monomer in the reaction solution by working at a large chalcogenide excess. Conditions that facilitate this reaction pathway are characterized by a kinetic barrier to homogeneous growth. These match those for the formation of metastable magic-size clusters. Through this boundary condition, it can be understood why the formation of telluride tips is favored in comparison to selenides and sulfides, for which the regimes for cluster formation and nucleation on surfaces do not overlap.
Transition-metal-doped semiconductor nanocrystals have garnered interest in the field of spintronics because of their capability to couple an applied magnetic field to a photonic response, and vice versa. These structures are a synthetic challenge due to difficulties in controlling the distribution of dopants as well as introducing the impurity atoms into the host material in the first place. In this paper we present a route toward anisotropically Co 2+ -doped CdSe/CdS dot-in-rods, in which the dopants are localized at the nanorod tips, at a defined distance from the CdSe seed particle. The localized doping of seeded nanorods allows to gain a deeper understanding of the interaction of the dopant with an exciton through competitive photoluminescence quenching, highlighting the fact that the dopant locale is more important than dopant concentration. It also yields information about the kinetics that govern impurity diffusion, with an activation energy for Co 2+ in nanocrystalline CdS of E a = 61.11 kJ mol −1 .
In article number https://doi.org/10.1002/smll.201701392, Helmut Cölfen and co‐workers, as a first prepare freestanding photonic glasses with controllable disorder/structure by sedimentation in a centrifugal field. A proof of principle photon time of flight experiment demonstrates the good quality and promise of the obtained colloidal glasses. Until now, light localization phenomena had been predicted but never observed in 3D materials, largely due to very few satisfactory materials available for study.
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