|The IUCr is an International Scientific Union. Its objectives are to promote international cooperation in crystallography and to contribute to all aspects of crystallography, to promote international publication of crystallographic research, to facilitate standardization of methods, units, nomenclatures and symbols, and to form a focus for the relations of crystallography to other sciences.|
Symmetry is ubiquitous in the natural world. It occurs in gemstones and snowflakes and even in biology, an area typically associated with complexity and diversity. There are striking examples: the shapes of virus particles, such as those causing the common cold, are highly symmetrical and look like tiny footballs.
A research programme led by Reidun Twarock at the University of York, UK has developed new mathematical tools to better understand the implications of this high degree of symmetry in these systems. The group pioneered a mathematical theory that reveals unprecedented insights into how different components of a virus, the protein container encapsulating the viral genome and the packaged genome within, mutually constrain each other's structures [Acta Cryst. (2013). A69, 140–150; doi:10.1107/S0108767312047150].
A paper recently published in Acta Crystallographica Section A: Foundations and Advances [Acta Cryst. (2014). A70, 162–167; doi: 10.1107/S2053273313034220] in collaboration with Pierre-Philippe Dechant from the University of Durham, UK shows that these mathematical tools apply more widely in the natural world and interestingly also account for the structures of Russian-doll-like arrangements of carbon cages known as carbon onions. It was known previously that individual shells could be modeled using symmetry techniques, but the fact that the entire structure is collectively constrained by a single symmetry principle is a surprising new result.
Such insights are crucial for understanding how different components contribute collectively to function. In the case of viruses this work has resulted in a new understanding of the interplay of the viral genome and protein capsid in virus formation, which in turn has opened up novel opportunities for anti-viral intervention that are actively being explored. Similarly, we expect that the work on carbon onions will provide a basis for a better understanding of the structural constraints on their overall organisation and formation, which in the future can be exploited in nanotechnology applications.P.-P. Dechant and R. Twarock
The development of solar panel materials that are both non-toxic and made from readily available elements rather than rare and precious metals is a priority in developing a sustainable technology. Sulfide materials containing the relatively common metals copper, tin and zinc, so-called kesterites, have been proposed as solar cell absorber materials because they comply with these two demands. Experimental solar cells using Cu2ZnSnS4 (CZTS) have demonstrated energy conversion efficiencies of 8.4% and 12% for a seleno-sulfide analogue. New structural information is crucial to improving on these figures still further.
Unfortunately, kesterites are not amenable to conventional X-ray diffraction because copper and zinc ions are indistinguishable. Now, Alain Lafond and his colleagues at Nantes University and Pierre Fertey from Soleil synchrotron have demonstrated that it is possible to carry out resonant diffraction of a single crystal of the semiconductor CZTS.
The powdered precursor was prepared using a ceramic synthesis at a high temperature (1023 K) from the corresponding elements Cu, Zn, Sn and S. The product is heated for a further 96 hours to anneal it before it is plunged into ice-water to lock in the chemical structure present at that elevated temperature, a process known as quenching. Tiny single crystals of sufficient quality for X-ray diffraction were picked out of the powder. The researchers used laboratory powder X-ray diffraction and energy-dispersive X-ray spectroscopy analyses to test the purity of their product. They then carried out high-performance resonant diffraction on the CRISTAL beamline at the Soleil French synchrotron, which gives them the possibility to adjust the radiation wavelength in order to enhance the contrast between copper and zinc.
The data they obtained showed the annealing process generates a disordered structure that can be distinguished from the order kesterite structure despite the otherwise similar X-ray scattering pattern that would be generated by the copper and zinc ions in the ordered form. The team points out that the fabrication process for making a thin absorber film from CZTS in a solar panel is carried out at an elevated temperature and the disordered form is likely to be the active form produced which probably precludes high photovoltaic performance.
The findings offer important clues for the development of CZTS and related materials that avoid expensive and rare materials such as indium and tellurium in solar cells.
"The next step in this research is to determine the relationship between the synthesis conditions (quenching or slow cooling) and the actual Cu/Zn distribution in the kesterite structure," Lafond told us. He revealed that a new proposal to the Soleil French Synchrotron Facility has been deposited for the next experimental period and in the meantime structural disorder in kesterite materials can be investigated by solid-state NMR and Raman spectroscopy.
Researchers in China [J. Appl. Cryst. (2014). 47, 527–531; DOI: 10.1107/S1600576713034535] have found a convenient way to selectively prepare germanium sulfide nanostructures, including nanosheets and nanowires, that are more active than their bulk counterparts and could open the way to lower cost and safer optoelectronics, solar energy conversion and faster computer circuitry.
Germanium monosulfide, GeS, is emerging as one of the most important "IV–VI" semiconductor materials with potential in opto-electronics applications for telecommunications and computing, and as an absorber of light for use in solar energy conversion. One important property is its much lower toxicity and environmental impact when compared with other semiconductors made with cadmium, lead and mercury. It is less costly than other materials made with rare and noble metal elements. Indeed, glassy GeS has been used in lasers, fibre optic devices and infrared lenses as well as rewritable optical discs and non-volatile memory devices for several years. It is also used extensively as a solid electrolyte in conductive bridging random access memory (RAM) devices.
The repertoire of this material might be extended much further with the extra control that its use as nanostructured systems might allow. Liang Shi and Yumei Dai of the University of Science and Technology of China, in Hefei, point out that research in this area has lagged behind that with other IV–VI semiconductors. They hope to change that and have focused on how nanosheets and nanowires of GeS might be readily formed. They have used X-ray powder diffraction, transmission electron microscopy, energy-dispersive X-ray spectrometry and scanning electron microscopy to investigate the structure, morphology, composition and optical absorption properties of their samples.
The team used simple "wet" chemistry to synthesize their products using germanium dichloride–dioxane complex, thiourea and oleylamine (OLA) as starting materials. The ingredients were mixed in a sealed reaction flask, blasted with ultrasound to exclude air and then stirred and heated. The team was able to make nanosheets of GeS this way if the process was carried out for several hours at 593 K. At higher temperature, 613 K, they found that the sheets wind up into nanowires. Indeed, the precise heating time and temperature allowed them to control the structure of the final product. The team suggests that the rolling up of the nanosheets into nanowires is driven by the surface tension between the sheet and the OLA molecules during the heating.
Having proven the structural integrity of their GeS nanowires and nanosheets, the team built several test devices – a photoresponsive unit – which they used to evaluate the optical and electronic properties of the products. The team says that they have demonstrated "outstanding photoresponsive behaviour". This "indicates the potential use of as-synthesized GeS nanosheets and nanowires in solar energy conversion systems, such as the fabrication of photovoltaic devices".
Business development manager, IUCr
The International Year of Crystallography in 2014 is a perfect opportunity for the International Union of Crystallography to refurbish all of its journals and publishing activities. Not least, significant changes are under way at Acta Crystallographica Section C. The subtitle of the journal has been changed to Structural Chemistry and the scope widened to reflect the fact that small-molecule crystallography undeniably plays a crucial role in most aspects of the chemical sciences. Section C now specializes in the rapid publication of articles that highlight interesting science and research enabled by the determination, calculation or analysis of small-molecule crystal and molecular structures, with a view to how the structural observations help the understanding of a chemical, physical or structural question being investigated.
In order to inaugurate the new scope of the journal and demonstrate to readers and potential authors the types of papers that the journal is keen to attract, the journal has published four special issues over the last few months and we are very excited about the quality and breadth of papers in these issues.
The first issue was on Scorpionates (Guest Editor: Dr Glenn P. A. Yap, Department of Chemistry & Biochemistry, University of Delaware, USA; September 2013) and included contributions from some of the best authors in the field of this chemically important family of prolific and work-horse ligands wherein the structure has a palpable effect on reactivity.
The next special issue was titled Pharmaceuticals, drug discovery and natural products (Guest Editor: Professor Christopher S. Frampton, Pharmorphix, Cambridge, England; November 2013) and addressed and discussed the timeless questions of polymorphism, stereochemistry and H-atom position. The structural data presented originates not just from X-ray analyses, but also from NMR and computational studies, thus demonstrating the widened scope of the journal.
The effort on refocusing the journal in 2013 culminated in the third special issue, on Interplay of crystallography, spectroscopy and theoretical methods for solving chemical problems (Guest Editors: Professor Larry Falvello, Universidad de Zaragoza, Spain, and Professor Alberto Albinati, Universita degli Studi di Milano, Italy; December 2013). This issue presented research articles and scientific comments demonstrating that the vast utility of structure determination is manifest in other techniques in addition to single-crystal diffraction analysis, and that a description of structure can derive immense leverage from physical measurements and computational modelling.
The turn of the year saw the move to full electronic publication for Section C and the journal launch fully into its science-facing mission as an outlet for research in structural chemistry in its broadest sense. The most recent special issue, on Computational materials discovery (Guest Editor: Professor Artem Oganov, Center for Materials by Design, State University of New York, USA; February 2014), highlights the two most important breakthroughs of this field, i.e. crystal structure prediction and electronic structure calculations, and their applications to specific problems of materials science.
These four exciting special issues contain excellent papers covering a variety of topics in structural chemistry and herald a new identity for Acta Crystallographica Section C, one underpinned by structural work but with interest in and eyes on all chemical fields where this work plays a significant role. The papers are expected to be highly cited and you are now invited to continue the surge of Section C to the forefront of structural chemistry by submitting your own papers in the style of those represented in these special issues.Anthony Linden