research highlights OPTICAL CLOAKING layer that increases the overall reflectivity of rather than by the Stark effect. It is, in A phase compensation trick their metasurface. This design, which can in fact, known that in charged nanocrystals, Science 349, 1310–1314 (2015) principle be scaled up, works for incoming the recombination of electrons and holes light within 30° angles with respect to the happens primarily through a non-radiative perpendicular direction. AM phenomenon known as the Auger effect. Delehanty and colleagues also simulated NANOCRYSTALS a realistic neuron firing event by applying Getting on your nerves to the device an electric voltage pulse Nano Lett. 15, 6848–6854 (2015) recorded independently from a murine cortical neuron. The intensity of the Semiconductor nanocrystals could luminescence followed the voltage profile potentially be used to study the electrical accurately, with a total variation of up to 5% activity of neurons by monitoring their in luminescence. FP luminescence during a firing event. The basic principle is that an electric field affects CONTRAST AGENTS AAAS the spatial separation of electrons and holes Nanobubbles that don’t pop in nanocrystals (a phenomenon known as Angew. Chem. Int. Ed. http://doi.org/f3ggns (2015) A cloak is a device that makes objects the quantum confined Stark effect) and, invisible to an external observer. The first therefore, affects the time it takes for these Microbubbles, which are typically made cloaks that could work with visible light charges to recombine and ultimately the up of a lipid or protein shell and contain air used bulky containers with special optical intensity of the luminescence emitted. To or perfluorocarbon in the core, are useful properties, but, although they concealed the put this principle into practice, however, contrast agents in ultrasound imaging object, the containers could still be visible to it is essential to understand the exact because they reflect ultrasound waves an observer. Xiang Zhang and colleagues at mechanism of luminescence variation and, differently from soft tissues in the body. the University of California, Berkeley have above all, whether these variations occur on Recently, nanobubbles have gained interest now reported a proof-of-principle cloaking a timescale comparable to the firing events. because they can traverse smaller capillary device made of an 80-nm-thick surface that James Delehanty and colleagues at the Naval networks of organs and potentially be can be wrapped around objects of any shape, Research Laboratory in the US have now more widely applicable. However, stable making them invisible to 730-nm light. explored these issues by experimentally nanobubbles are difficult to make. Now, This ultrathin cloak reroutes light scattered simulating the action of neurons Guojun Liu and colleagues at Queen’s from an object so that it seems as though on nanocrystals. University in Canada show that stable and the light has been reflected by a mirror. The researchers fabricated a device in echogenic air nanobubbles can be prepared To obtain such an effect, the researchers which an electric field can be applied in a by lining the inside of the shell with highly patterned a gold surface with six differently controlled way to a film of nanocrystals. hydrophobic fluorinated polymers. sized gold nanoantennas with subwavelength Increasing the applied electric field Inspired by the water-repelling properties dimensions that could compensate for leads to a reduction of the luminescence of perfluorinated coatings on cotton the phase shift acquired by the scattered intensity. However, the time variation of the fabrics, the researchers made nanobubbles radiation. Because phase compensation luminescence, and hence the electron–hole using a triblock polymer consisting of occurs locally and covers the full spectrum recombination speeds, is barely affected. poly(acrylic acid), poly(2-cinnamoyloxyethyl (from 0 to 360°), objects with sharp edges can According to the team, this means that the methacrylate) and poly(2-perfluorooctylethyl potentially be cloaked. To retain most of the luminescence is reduced by the electric- methacrylate). As a control, a light intensity, the team added a dielectric field-induced charging of the nanocrystals, similar polymer with poly(glyceryl monomethacrylate) and poly(tert-butyl acrylate) replacing poly(acrylic acid) and ORGANIC–INORGANIC PEROVSKITES poly(2-cinnamoyloxyethyl methacrylate), Now in two dimensions Science 349, 1518–1521 (2015) respectively, was made. When injected into a tube filled with water or human blood, the Hybrid organic–inorganic perovskites, such as CH3NH3PbI3, are a class of semiconducting fluorinated nanobubbles could be detected material that have attracted considerable research attention in recent years due to their by ultrasound. Further experiments show potential in photovoltaic applications. The materials can be used to create solution- that their echogenicity is due to the air processed devices and rapid improvements in their capabilities mean that perovskite trapped in the cavity. Nanobubbles that solar cells currently offer power conversion efficiencies of over 20%. Peidong Yang and are allowed to age for 4 hours or 3 weeks colleagues at the University of California, Berkeley, Lawrence Berkeley National Laboratory remain strongly echogenic. Furthermore, and ShanghaiTech University have now shown that these layered materials can be grown as their average lifetime before signal decay atomically thin two-dimensional sheets. was about 100 times longer than commercial The researchers synthesized sheets of the hybrid perovskite (C4H9NH3)2PbBr4 by microbubbles. Transmission electron dropping a dilute precursor solution on a silica surface and allowing it to dry under mild microscopy showed that the ultrasonicated heating. With the approach, micrometre-sized crystals were created that had well-defined samples retained their structural integrity, square shapes and were a single or a few unit cells thick. Detailed analysis of the materials confirming that no air was displaced revealed that they exhibit a structural relaxation that shifts their bandgap compared with by water. ALC the bulk material. The two-dimensional crystals also exhibit a blue photoluminescence that could be tuned by varying the thickness of the sheets or their composition. OV Written by Ai Lin Chun, Alberto Moscatelli, Fabio Pulizzi and Owain Vaughan. 912 NATURE NANOTECHNOLOGY | VOL 10 | NOVEMBER 2015 | www.nature.com/naturenanotechnology © 2015 Macmillan Publishers Limited. All rights reserved.