Multi-scale modeling of early-stage morphology in solution-processed polycrystalline thin films.

A model is introduced for treating early-stage nucleation, growth kinetics, and mesoscale domain structure in submonolayer polycrystalline films prepared by solution-phase processing methods such as spin casting, dip coating, liquid-based printing, and related techniques. The model combines a stochastic treatment of nucleation derived from classical nucleation theory with deterministic computation of the spatiotemporal dynamics of the monomer concentration landscape by numerical solution of the two-dimensional diffusion equation, treating nuclei as monomer sinks. Results are compared to experimental measurements of solution-processed submonolayer tetracene films prepared using a vapor-liquid-solid deposition technique. Excellent agreement is observed with most major kinetic and structural film characteristics, including the existence of distinct induction, nucleation, and growth regimes, the onset time for nucleation, the number of domains formed per unit area, and the micron- to millimeter-scale spacing statistics of those domains. The model also provides a detailed description the dynamically-evolving monomer concentration landscape during film formation as well as quantities derived from it, such as time- and position-dependent domain nucleation and growth rates.

Correction: A simple model of burst nucleation.

Correction for 'A simple model of burst nucleation' by Alexandr Baronov et al., Phys. Chem. Chem. Phys., 2015, 17, 20846-20852.

A simple model of burst nucleation.

We introduce a comprehensive quantitative treatment for burst nucleation (BN)-a kinetic pathway toward self-assembly or crystallization defined by an extended post-supersaturation induction period, followed by a burst of nucleation, and finally the growth of existing stable assemblages absent the formation of new ones-based on a hybrid mean field rate equation model incorporating thermodynamic treatment of the saturated solvent from classical nucleation theory. A key element is the inclusion of a concentration-dependent critical nucleus size, determined self-consistently along with the subcritical cluster population density. The model is applied to an example experimental study of crystallization in tetracene films prepared by organic vapor-liquid-solid deposition, where good agreement is observed with several aspects of the experiment using a single, physically well-defined adjustable parameter. The model predicts many important features of the experiment, and can be generalized to describe other self-organizing systems exhibiting BN kinetics.

Zero-reabsorption doped-nanocrystal luminescent solar concentrators.

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.

Stamping oriented molecular monolayers using liquid crystal inks.

Thermotropic liquid crystal (LC)-based inks are combined with patterned anchoring stamps to deposit organic monolayer films with simultaneous control over positional and molecular orientational order in a single step.

Cooperative ordering at liquid crystal interfaces and its role in orientational memory.

Orientational memory in interfacial liquid crystal films occurs when cells heated above the isotropic transition temperature return to their initial ordered texture upon cooling. First observed over 80 years ago, the origins of orientational memory, which is sometimes called the surface memory effect, remain poorly understood. In this study, films of the thermotropic liquid crystal 4'-octyl-4-cyanobiphenyl on graphite were studied by scanning tunneling and polarizing optical microscopy. Strong orientational memory was observed despite relatively weak molecule-surface interactions of the kind previously thought to be responsible for this effect. By preparing cells in a uniformly oriented initial reference state and separately measuring bulk and surface order parameters as systems were thermally disordered, cooperative interactions were found to play an important role, leading to the recovery of long-range order that neither the bulk nor surface layers alone retained. When the surface and bulk layers were partially decoupled using a magnetic field, orientational memory in the surface layer almost disappeared. The findings provide a new interpretation of the origins of orientational memory in liquid crystal films and underscore the potentially important role of cooperativity in bulk <--> interfacial liquid crystal interactions.

Engineered growth of organic crystalline films using liquid crystal solvents.

Thin films of organic molecular crystals have drawn widespread attention for their scientifically interesting and potentially useful electronic, photonic, and chemical properties. However, because their properties are extremely sensitive to structural imperfections, domain size, and crystallographic orientation, preparation of high-quality thin films with controlled microstructural organization under technologically favorable conditions has long been a bottleneck toward practical applications and better controlled fundamental studies. Here a technique is introduced combining atmospheric pressure vapor-phase deposition with solution-phase growth in a thin layer of thermotropic liquid crystal solvent. The method produces relatively large crystals, enables control over crystallographic orientation and growth habit, and involves mild processing conditions compatible with a variety of substrates and organic materials. Results are presented for the organic semiconductor tetracene, along with a discussion of film growth and alignment mechanisms.

Atomistic molecular dynamics simulations of chemical force microscopy.

Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.