Charge density, optical properties of multicomponent crystals

 

APIs in the design of multi-component functional solids are shown. Credit: Marlena Gryl Optical materials serve a major role in modern sciences and technology. Many of the devices we use feature technology resulting from material discoveries in this fast moving area of research. Nowadays, the need for more efficient devices and minimisation in optoelectronics requires a novel approach towards crystal engineering of functional solids. A solution can be multicomponent materials built from either organic or mixed organic and inorganic components selected in a specific way, to combine molecular and structural properties to form a 3D architecture. Optical properties of a crystal strongly depend on two factors, i.e. the spatial distribution of molecules in the crystal structure and the electronic properties of molecular building blocks. The latter are easy to predict whereas the former are not. Crystal symmetry is often a key to obtaining a desired property. Noncentrosymmetric crystal structure (chiral/polar) is a necessary (limiting) condition for such properties as nonlinear properties of even order and linear properties like optical activity, piezoelectricity, pyroelectricity and ferroelectricity. However, fulfilling symmetry rules does not guarantee the existence of a physical effect. The choice of building blocks is crucial; in ideal cases, push-pull molecules should be linked with constituents enabling synthon formation flexibility. Active pharmaceutical ingredients (APIs), through their favourable donor/acceptor spatial distribution and synthon formation flexibility, are attractive building blocks in modern materials crystallography. An API is a substance or a mixture of substances used in the manufacture of a drug product and which becomes an active ingredient in the drug product itself. Here, a Polish scientist (working in Professor Katarzyna Stadnicka's group at the Jagiellonian University in Kraków) presents design strategies for optical materials based on selected pharmaceutical molecules. Gryl successfully presents the factors that contribute to molecular recognition in the four selected polar/chiral crystal phases. Theoretically predicted optical properties of the molecular/ionic building blocks as well as bulk effects were all confirmed experimentally. This work shows that quantitative crystal engineering techniques combining structural analysis, charge density studies, prediction of properties and their measurements enable the full analysis of the obtained functional materials in terms of their usefulness in practical applications. The study is just a first step in the design of novel optical materials based on push-pull molecules and APIs. This work presents an alternative application for pharmaceutical solids that are of major interest in the pharmaceutical industry. Dr Gryl's journey with optical materials based on API started with three polymorphs of urea and barbituric acid adduct [Gryl, Krawczuk & Stadnicka (2008). Acta Cryst. B64, 623-632; doi:10.1107/S0108768108026645]. The co-crystals display synthon polymorphism (a possibility to use the same donor and acceptor sites in many ways) and hence enable the manipulation of the outcome of the engineering process. Why not use the same "flexible" molecules and incorporate them in a lattice containing components with high molecular (hyper)polarizability? This is a next step in Dr Gryl's research. First, of course, as much as possible needs to be known about the selected building blocks and there is no better way than to study crystal structures containing those building blocks. Story Source: The above post is reprinted from materials provided by International Union of Crystallography. Note: Materials may be edited for content and length. Journal Reference: Marlena Gryl, Anna Krawczuk, Katarzyna Stadnicka. Polymorphism of urea–barbituric acid co-crystals. Acta Crystallographica Section B Structural Science, 2008; 64 (5): 623 DOI: 10.1107/S0108768108026645

New hyper-efficient light-emitting crystal created

 

A glowing quantum dot seamlessly integrated into a perovskite crystal matrix. Credit: Sargent Group/ U of T Engineering It's snack time: you have a plain oatmeal cookie, and a pile of chocolate chips. Both are delicious on their own, but if you can find a way to combine them smoothly, you get the best of both worlds. Researchers in The Edward S. Rogers Sr. Department of Electrical & Computer Engineering used this insight to invent something totally new: they've combined two promising solar cell materials together for the first time, creating a new platform for LED technology. The team designed a way to embed strongly luminescent nanoparticles called colloidal quantum dots (the chocolate chips) into perovskite (the oatmeal cookie). Perovskites are a family of materials that can be easily manufactured from solution, and that allow electrons to move swiftly through them with minimal loss or capture by defects. The work is published in the international journal Nature on July 15, 2015. "It's a pretty novel idea to blend together these two optoelectronic materials, both of which are gaining a lot of traction," says Xiwen Gong, one of the study's lead authors and a PhD candidate working with Professor Ted Sargent. "We wanted to take advantage of the benefits of both by combining them seamlessly in a solid-state matrix." The result is a black crystal that relies on the perovskite matrix to 'funnel' electrons into the quantum dots, which are extremely efficient at converting electricity to light. Hyper-efficient LED technologies could enable applications from the visible-light LED bulbs in every home, to new displays, to gesture recognition using near-infrared wavelengths. "When you try to jam two different crystals together, they often form separate phases without blending smoothly into each other," says Dr. Riccardo Comin, a post-doctoral fellow in the Sargent Group. "We had to design a new strategy to = convince these two components to forget about their differences and to rather intermix into forming a unique crystalline entity." The main challenge was making the orientation of the two crystal structures line up, called heteroexpitaxy. To achieve heteroepitaxy, Gong, Comin and their team engineered a way to connect the atomic 'ends' of the two crystalline structures so that they aligned smoothly, without defects forming at the seams. "We started by building a nano-scale scaffolding 'shell' around the quantum dots in solution, then grew the perovskite crystal around that shell so the two faces aligned," explained coauthor Dr. Zhijun Ning, who contributed to the work while a post-doctoral fellow at UofT and is now a faculty member at ShanghaiTech. The resulting heterogeneous material is the basis for a new family of highly energy-efficient near-infrared LEDs. Infrared LEDs can be harnessed for improved night-vision technology, to better biomedical imaging, to high-speed telecommunications. Combining the two materials in this way also solves the problem of self-absorption, which occurs when a substance partly re-absorbs the same spectrum of energy that it emits, with a net efficiency loss. "These dots in perovskite don't suffer reabsorption, because the emission of the dots doesn't overlap with the absorption spectrum of the perovskite," explains Comin. Gong, Comin and the team deliberately designed their material to be compatible with solution-processing, so it could be readily integrated with the most inexpensive and commercially practical ways of manufacturing solar film and devices. Their next step is to build and test the hardware to capitalize on the concept they have proven with this work. "We're going to build the LED device and try to beat the record power efficiency reported in the literature," says Gong. This work was supported by the Ontario Research Fund Research Excellence Program, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the King Abdullah University of Science & Technology (KAUST).   University of Toronto Faculty of Applied Science & Engineering. "Engineered hybrid crystal opens new frontiers for high-efficiency lighting: New hyper-efficient light-emitting crystal created." ScienceDaily. ScienceDaily, 15 July 2015. www.sciencedaily.com/releases/2015/07/150715133508.htm.