Controlled topological transitions in thin film phase separation

In this paper the evolution of a binary mixture in a thin-film geometry with a wall at the top and bottom is considered. By bringing the mixture into its miscibility gap so that no spinodal decomposition occurs in the bulk, a slight energetic bias of the walls towards each one of the constituents ensures the nucleation of thin boundary layers that grow until the constituents have moved into one of the two layers. These layers are separated by an interfacial region where the composition changes rapidly. Conditions that ensure the separation into two layers with a thin interfacial region are investigated based on a phase-field model. Using matched asymptotic expansions a corresponding sharp-interface problem for the location of the interface is established. It is then argued that this newly created two-layer system is not at its energetic minimum but destabilizes into a controlled self-replicating pattern of trapezoidal vertical stripes by minimizing the interfacial energy between the phases while conserving their area. A quantitative analysis of this mechanism is carried out via a thin-film model for the free interfaces, which is derived asymptotically from the sharp-interface model.Comment: Submitted 23/12/201

Propagating topological transformations in thin immiscible bilayer films

A physical mechanism for the topological transformation of a two-layer system confined by two substrates is proposed. Initially the two horizontal layers, A and B, are on top of each other, but upon a sufficiently large disturbance, they can rearrange themselves through a spontaneously propagating sectioning to create a sequence of vertical alternating domains ABABAB. This generic topological transformation could be used to control the morphology of fabricated nanocomposites by first creating metastable layered structures and then triggering their transformation. The generality is underscored by formulating conditions for this topological transformation in terms of the interface energies between phases and substrates. The theoretical estimate for the width of the domains is confirmed by simulations of a phase-field model and its thin-film/sharp-interface approximation

Propagating topological transformations in thin immiscible bilayer films

A physical mechanism for the topological transformation of a two-layer system confined by two substrates is proposed. Initially the two horizontal layers, A and B, are on top of each other, but upon a sufficiently large disturbance, they can rearrange themselves through a spontaneously propagating sectioning to create a sequence of vertical alternating domains ABABAB. This generic topological transformation could be used to control the morphology of fabricated nanocomposites by first creating metastable layered structures and then triggering their transformation. The generality is underscored by formulating conditions for this topological transformation in terms of the interface energies between phases and substrates. The theoretical estimate for the width of the domains is confirmed by simulations of a phase-field model and its thin-film/sharp-interface approximation

Photo-induced halide redistribution in organic-inorganic perovskite films.

Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance

Reverse Coarsening and the Control of Particle Size Distribution through Surfactant

The minimization of surface area, as a result of the minimization of (positive) surface energy, is a well-known driving force behind the spontaneous broadening of (nano) particle size distribution. We show that surfactant molecules binding to particle surfaces effectively decrease the surface energy and may change its sign. In this case, contrary to the expected broadening behavior, a minimum of free energy is achieved at the maximum surface area for all particles, i.e., when the particles are identical. Numerical simulations based on the classical Lifshitz–Slyozov–Wagner theory with surfactant-induced surface energy renormalization confirm the collapse of the particle size distribution. As the particle size evolution is much slower than particle nucleation and growth, the manipulation of surface energy with in-situ replacement of surfactant molecules provides a method for controlling particle size distribution with great potential for creating mono-disperse nanoparticles, a key goal of nanotechnology

Ligand-Assisted Growth of Nanowires from Solution

We consider the development of ligand-assisted growth processes for generating shape-anisotropic nanomaterials. Using statistical mechanics, we analyze the conditions under which ligand-assisted growth of shape-anisotropic crystalline nanomaterials from solution can take place. Depending on ligand-facet interaction energy and crystal facet area, molecular ligands can form compact layers on some facets leaving other facets free. The growth process is then restricted to free facets and may result in significant anisotropy in crystal shape. Our study uncovers the conditions for ligand-assisted growth of nanoplatelets and nanowires from isotropic or anisotropic seed nanocrystals of cuboid shape. We show that in contrast to nanoplatelets, ligand-assisted growth of nanowires requires certain anisotropy in the ligand-facet interaction energy

Properties of Spatially Indirect Excitons in Nanowire Arrays

This paper deals with the excitons formed by electrons and holes located in different, closely placed semiconducting nanowires (spatially indirect excitons). We calculated the charge densities and the binding energies of the excitons for different nanowire diameters D and distances h between the nanowires. Together with the estimated exciton lifetimes, these results suggest that at certain h and D, the spatially indirect excitons in the nanowire arrays may have the potential to serve as information-processing units. Possible ways of exciton generation in the nanowire arrays are discussed

Properties of Spatially Indirect Excitons in Nanowire Arrays

This paper deals with the excitons formed by electrons and holes located in different, closely placed semiconducting nanowires (spatially indirect excitons). We calculated the charge densities and the binding energies of the excitons for different nanowire diameters D and distances h between the nanowires. Together with the estimated exciton lifetimes, these results suggest that at certain h and D, the spatially indirect excitons in the nanowire arrays may have the potential to serve as information-processing units. Possible ways of exciton generation in the nanowire arrays are discussed