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RESEARCH NEWS NOVEMBER 2011 | VOLUME 14 | NUMBER 11 517 A novel and facile approach to detecting ultraviolet radiation in the hazardous 320 – 400 nm (UV-A) part of the spectrum has been developed by researchers in China (Fudan University) and Japan (NIMS). Their high performance photodetector comprises niobium(V) oxide nanobelts that are 100 – 500 nm wide and 2 – 10 micrometres long. The team synthesized these quasi-aligned nanobelts using hydrothermal treatment of niobium foil in potassium hydroxide solution and subsequent proton exchange and calcinations [Fang et al., Adv Funct Mater (2011) doi:10.1002/ adfm.201100743]. The same detectors might be useful in optoelectronics circuitry operating in the UV-A band. Photodetectors are becoming increasingly important in many applications. Fundamentally, they convert an optical signal into an electrical one and so can be used, as the name would suggest, simply as a light detector, as well as acting as binary optical switches for imaging, optical communications, and optoelectronic circuits. Given the growing interest in nanotechnology it requires no great stretch of the imagination to see that integrating photodetectors at the nanoscale is the inevitable next step. Indeed, such nano devices are inherently more effective than their “bulk” semiconductor counterparts because of their higher surface to volume ratio, even in bridging the gap between micro and nano. Researchers across the globe have thus focused on creating one-dimensional nanowire based photodetectors and efforts have been made to nudge the sensitivity of these devices into the ultraviolet part of the electromagnetic spectrum. Unfortunately, most efforts have led to only poorly efficient UV-A detectors. Fang and colleagues hoped to fill the gap by turning to niobium(V) oxide, a material transparent to visible light that has a bandgap of 3.4 eV, which they suggest, makes it an ideal candidate for a “visible- blind” UV-A photodetector. The visible transparency means that the detection process essentially ignores incident visible light. Tests on their nanobelt UV-A photodetector reveal it to live up to expectations with high sensitivity and high external quantum-efficiency of well over 6000 %. The prototype nanobelt detector also has a photocurrent stability of more than 40 minutes. The team suggests that optimization of the annealing process used in the final stage of preparation of the nanobelts, could be further optimized to improve the active life time of the materials. There is also a need to eliminate defects and so improve efficiency and sensitivity still further. David Bradley Belting up ultraviolet visibility NANOTECHNOLOGY Nanobelt arrays. Courtesy Xiaosheng Fang. Click chemistry describes the process of quickly and easily joining smaller molecules together to form larger ones. However, in spite of a name which implies a kind if chemical Lego, while moving forward is easy, reversing the reactions can be rather difficult. Such is the case for the highly stable 1,2,3-triazole moiety, which strongly resists being reverted into its azide and alkyne precursors, rebuffing attempts to reverse the reaction using simple chemical and thermal techniques. Tackling the problem will instead require a different approach, and thanks to a team from the University of Texas at Austin, it looks as though we may have a solution to “unclick the click” [Brantley et al., Science (2011) 333, 1606]. Prof Christopher Bielawski and colleagues implanted the stable triazole inside a polymer chain, and then managed to break the chain at the triazole site using ultrasonic sound waves. Speaking to Materials Today , Bielawski exaplained how such a mechanochemical approach works: “The polymer chains function as handles that respond to the forces generated under ultrasonication. In an acoustic field, solvent cavitation generates small bubbles that rapidly expand and implode. Solvated polymer chains near these growing cavities essentially are pulled toward the void volume. If this happens to a polymer chain attached to one side of the triazole, but not to the polymer chain attached to the other side of the triazole, tensile forces are generated in the center of the chain, right where the triazole is located. It is believed that this mechanical force destabilizes the molecule through bond distortion, which ultimately lowers the energy needed for the cycloreversion to occur.” As the length of the polymer chain is dependent on sonochemical reactions, as well as the position of the mechanically sensitive molecule within the chain, the team was also able to demonstrate that the reversion was purely the result of the mechanical action, rather than the effect of any induced heating. These control experiments suggest that the triazole must be located near the center of the chain in order to experience the required force. The researchers believe that such an ability to “selectively deconstruct tiazoles with high fidelity” could find use in mechanoresponsive materials. Bielawski revealed, “An interesting application of our work could be the development of systems or sensors that use mechanical forces to reversibly label biomolecules (e.g., proteins) with a variety of small molecules.” The team is “currently undertaking a theoretical study to understand the role that mechanical forces play in the reactivity we have observed. We are also exploring new areas, such as the application of mechanical forces in a biological context.” Stewart Bland Mechanical chemistry TOOLS AND TECHNIQUES Bielawski and colleagues make reversing the reaction look easy. Courtesy Christopher Bielawski.
