Monday, February 24th, 2020

Time	Event
11:29a	Mirrored chip could enable handheld dark-field microscopes Do a Google search for dark-field images, and you’ll discover a beautifully detailed world of microscopic organisms set in bright contrast to their midnight-black backdrops. Dark-field microscopy can reveal intricate details of translucent cells and aquatic organisms, as well as faceted diamonds and other precious stones that would otherwise appear very faint or even invisible under a typical bright-field microscope. Scientists generate dark-field images by fitting standard microscopes with often costly components to illumate the sample stage with a hollow, highly angled cone of light. When a translucent sample is placed under a dark-field microscope, the cone of light scatters off the sample’s features to create an image of the sample on the microscope’s camera, in bright contrast to the dark background. Now, engineers at MIT have developed a small, mirrored chip that helps to produce dark-field images, without dedicated expensive components. The chip is slightly larger than a postage stamp and as thin as a credit card. When placed on a microscope’s stage, the chip emits a hollow cone of light that can be used to generate detailed dark-field images of algae, bacteria, and similarly translucent tiny objects. The new optical chip can be added to standard microscopes as an affordable, downsized alternative to conventional dark-field components. The chip may also be fitted into hand-held microscopes to produce images of microorganisms in the field. “Imagine you’re a marine biologist,” says Cecile Chazot, a graduate student in MIT’s Department of Materials Science and Engineering. “You normally have to bring a big bucket of water into the lab to analyze. If the sample is bad, you have to go back out to collect more samples. If you have a hand-held, dark-field microscope, you can check a drop in your bucket while you’re out at sea, to see if you can go home or if you need a new bucket.” Chazot is the lead author of a paper detailing the team’s new design, published today in the journal Nature Photonics. Her co-authors are Sara Nagelberg, Igor Coropceanu, Kurt Broderick, Yunjo Kim, Moungi Bawendi, Peter So, and Mathias Kolle of MIT, along with Christopher Rowlands at Imperial College London and Maik Scherer of Papierfabrik Louisenthal GmbH in Germany. Forever fluorescent In an ongoing effort, members of Kolle’s lab are designing materials and devices that exhibit long-lasting “structural colors” that do not rely on dyes or pigmentation. Instead, they employ nano- and microscale structures that reflect and scatter light much like tiny prisms or soap bubbles. They can therefore appear to change colors depending on how their structures are arranged or manipulated. Structural color can be seen in the iridescent wings of beetles and butterflies, the feathers of birds, as well as fish scales and some flower petals. Inspired by examples of structural color in nature, Kolle has been investigating various ways to manipulate light from a microscopic, structural perspective. As part of this effort, he and Chazot designed a small, three-layered chip that they originally intended to use as a miniature laser. The middle layer functions as the chip’s light source, made from a polymer infused with quantum dots — tiny nanoparticles that emit light when excited with fluorescent light. Chazot likens this layer to a glowstick bracelet, where the reaction of two chemicals creates the light; except here no chemical reaction is needed — just a bit of blue light will make the quantum dots shine in bright orange and red colors. “In glowsticks, eventually these chemicals stop emitting light,” Chazot says. “But quantum dots are stable. If you were to make a bracelet with quantum dots, they would be fluorescent for a very long time.” Over this light-generating layer, the researchers placed a Bragg mirror — a structure made from alternating nanoscale layers of transparent materials, with distinctly different refractive indices, meaning the degrees to which the layers reflect incoming light. The Bragg mirror, Kolle says, acts as a sort of “gatekeeper” for the photons that are emitted by the quantum dots. The arrangement and thicknesses of the mirror’s layers is such that it lets photons escape up and out of the chip, but only if the light arrives at the mirror at high angles. Light arriving at lower angles is bounced back down into the chip. The researchers added a third feature below the light-generating layer to recycle the photons initially rejected by the Bragg mirror. This third layer is molded out of solid, transparent epoxy coated with a reflective gold film and resembles a miniature egg crate, pocked with small wells, each measuring about 4 microns in diameter. Chazot lined this surface with a thin layer of highly reflective gold — an optical arrangement that acts to catch any light that reflects back down from the Bragg mirror, and ping-pong that light back up, likely at a new angle that the mirror would let through. The design for this third layer was inspired by the microscopic scale structure in the wings of the Papilio butterfly. “The butterfly’s wing scales feature really intriguing egg crate-like structures with a Bragg mirror lining, which gives them their iridescent color,” Chazot says. An optical shift The researchers originally designed the chip as an array of miniature laser sources, thinking that its three layers could work together to create tailored laser emission patterns. “The initial project was to build an assembly of individually switchable coupled microscale lasing cavities,” says Kolle, associate professor of mechanical engineering at MIT. “But when Cecile made the first surfaces we realized that they had a very interesting emission profile, even without the lasing.” When Chazot had looked at the chip under a microscope, she noticed something curious: The chip emitted photons only at high angles forming a hollow cone of light. Turns out, the Bragg mirror had just the right layer thicknesses to  only let photons pass through when they came at the mirror with a certain (high) angle. “Once we saw this hollow cone of light, we wondered: ‘Could this device be useful for something?’” Chazot says. “And the answer was: Yes!” As it turns out, they had incorporated the capabilities of multiple expensive, bulky dark-field microscope components into a single small chip. Chazot and her colleagues used well-established theoretical optical concepts to model the chip’s optical properties to optimize its performance for this newly found task. They fabricated multiple chips, each producing a hollow cone of light with a tailored angular profile.   “Regardless of the microscope you’re using, among all these tiny little chips, one will work with your objective,” Chazot says. To test the chips, the team collected samples of seawater as well as nonpathogenic strains of the bacteria E. coli, and placed each sample on a chip that they set on the platform of a standard bright-field microscope. With this simple setup, they were able to produce clear and detailed dark-field images of individual bacterial cells, as well as microorganisms in seawater, which were close to invisible under bright-field illumination. In the near future these dark-field illumination chips could be mass-produced and tailored for even simple, high school-grade microscopes, to enable imaging of low-contrast, translucent biological samples. In combination with other work in Kolle’s lab, the chips may also be incorporated into miniaturized dark-field imaging devices for point-of-care diagnostics and bioanalytical applications in the field.   “This is a wonderful story of discovery based innovation that has the potential for widespread impact in science and education through outfitting garden-variety microscopes with this technology,” says James Burgess, program manager for the Institute for Soldier Nanotechnologies, Army Research Office. “Additionally, the ability to obtain superior contrast in imaging of biological and inorganic materials under optical magnification could be incorporated into systems for identification of new biological threats and toxins in Army Medical Center laboratories and on the battlefield.” This research was supported, in part, by the National Science Foundation, the U.S. Army Research Office, and the National Institutes of Health. (Comment on this)
1:10p	MIT Framework for Publisher Contracts yields new open-access model The MIT Libraries has negotiated an innovative open-access agreement with the Association for Computing Machinery (ACM) that allows MIT authors to make ACM articles freely available at no cost to them. It is the libraries’ first publisher contract completed under the principles for open scholarship in the MIT Framework for Publisher Contracts, released in October 2019, and the agreement aligns with all elements in the framework. The libraries negotiations team developed the agreement in collaboration with colleagues at the University of California, Carnegie Mellon University, and Iowa State University, who each signed a three-year contract with ACM for an open-access pilot.   “This is a great example of scholarly societies and research libraries working together on an equitable and sustainable model that supports open science and the important work of societies,” says Chris Bourg, director of MIT Libraries.   This first agreement under the MIT framework meets its goals, including protecting scholars’ and their communities’ control over their own intellectual output. The libraries have been using the framework as a foundation for negotiations with publishers since the fall, when it was released with more than 100 libraries and consortia in North America endorsing it.  Under the ACM contract, MIT faculty and students will continue to receive unrestricted access to articles in the ACM Digital Library. In addition, as of Jan. 1, research articles in ACM’s journals and conference proceedings with corresponding authors from MIT will be made open access at the time of publication. For these articles, authors will not be charged open-access publication fees, authors will have the option to retain copyright, and articles will be published under a license permitting open sharing, with a default to a Creative Commons Attribution license. In addition, ACM will automatically deposit the manuscripts of all MIT co-authored articles into MIT’s institutional repository, DSpace. The agreement also includes rights for computational (text or data mining) access to the ACM Digital Library. The agreement is the result of an unusual collaboration among multiple libraries and a scholarly society, yielding a new open-access business model that ACM is now using around the world.  “This collaboration demonstrates the power and potential of libraries and scholarly societies as partners in shaping the future of scholarly communication in productive ways,” says Ellen Finnie, head of Scholarly Communications and Collections Strategy and convener of the MIT Libraries’ negotiations team. “We will be able to learn from this agreement as we continue to explore sustainable ways to realize the potential of the digital age for openly sharing the results of research.”  MIT authors who are corresponding authors of ACM articles and who would like to make the articles freely available at no cost to them can find more information on the MIT Libraries’ page for open access publishing support (under Association for Computing Machinery).  (Comment on this)
2:50p	A material’s insulating properties can be tuned at will Materials whose electronic and magnetic properties can be significantly changed by applying electrical inputs form the backbone of all of modern electronics. But achieving the same kind of tunable control over the thermal conductivity of any material has been an elusive quest. Now, a team of researchers at MIT have made a major leap forward. They have designed a long-sought device, which they refer to as an “electrical heat valve,” that can vary the thermal conductivity on demand. They demonstrated that the material’s ability to conduct heat can be “tuned” by a factor of 10 at room temperature. This technique could potentially open the door to new technologies for controllable insulation in smart windows, smart walls, smart clothing, or even new ways of harvesting the energy of waste heat.  The findings are reported today in the journal Nature Materials, in a paper by MIT professors Bilge Yildiz and Gang Chen, recent graduates Qiyang Lu PhD ’18 and Samuel Huberman PhD ’18, and six others at MIT and at Brookhaven National Laboratory. Thermal conductivity describes how well heat can transfer through a material. For example, it’s the reason you can easily pick up a hot frying pan with a wooden handle, because of wood’s low thermal conductivity, but you might get burned picking up a similar frying pan with a metal handle, which has high thermal conductivity. The researchers used a material called strontium cobalt oxide (SCO), which can be made in the form of thin films. By adding oxygen to SCO in a crystalline form called brownmillerite, thermal conductivity increased. Adding hydrogen to it caused conductivity to decrease. The process of adding or removing oxygen and hydrogen can be controlled simply by varying a voltage applied to the material. In essence, the process is electrochemically driven. Overall, at room temperature, the researchers found this process provided a tenfold variation in the material’s heat conduction. Such an order-of-magnitude range of electrically controllable variation has never been seen in any material before, the researchers say. In most known materials, thermal conductivity is invariable — wood never conducts heat well, and metals never conduct heat poorly. As such, when the researchers found that adding certain atoms into the molecular structure of a material could actually increase its thermal conductivity, it was an unexpected result. If anything, adding the extra atoms — or, more specifically, ions, atoms stripped of some electrons, or with excess electrons, to give them a net charge — should make conductivity worse (which, it turned out, was the case when adding hydrogen, but not oxygen). “It was a surprise to me when I saw the result,” Chen says. But after further studies of the system, he says, “now we have a better understanding” of why this unexpected phenomenon happens. It turns out that inserting oxygen ions into the structure of the brownmillerite SCO transforms it into what’s known as a perovskite structure — one that has an even more highly ordered structure than the original. “It goes from a low-symmetry structure to a high-symmetry one. It also reduces the amount of so-called oxygen vacancy defect sites. These together lead to its higher heat conduction,” Yildiz says. Heat is conducted readily through such highly ordered structures, while it tends to be scattered and dissipated by highly irregular atomic structures. Introducing hydrogen ions, by contrast, causes a more disordered structure. “We can introduce more order, which increases thermal conductivity, or we can introduce more disorder, which gives rise to lower conductivity. We could figure this out by performing computational modeling, in addition to our experiments,” Yildiz explains. While the thermal conductivity can be varied by about a factor of 10 at room temperature, at lower temperatures the variation is even greater, she adds. The new method makes it possible to continuously vary that degree of order, in both directions, simply by varying a voltage applied to the thin-film material. The material is either immersed in an ionic liquid (essentially a liquid salt) or in contact with a solid electrolyte, that supplies either negative oxygen ions or positive hydrogen ions (protons) into the material when the voltage is turned on. In the liquid electrolyte case, the source of oxygen and hydrogen is hydrolysis of water from the surrounding air. “What we have shown here is really a demonstration of the concept,” Yildiz explains. The fact that they require the use of a liquid electrolyte medium for the full range of hydrogenation and oxygenation makes this version of the system “not easily applicable to an all-solid-state device,” which would be the ultimate goal, she says. Further research will be needed to produce a more practical version. “We know there are solid-state electrolyte materials” that could theoretically be substituted for the liquids, she says. The team is continuing to explore these possibilities, and have demonstrated working devices with solid electrolytes as well. Chen says “there are many applications where you want to regulate heat flow.” For example, for energy storage in the form of heat, such as from a solar-thermal installation, it would be useful to have a container that could be highly insulating to retain the heat until it’s needed, but which then could be switched to be highly conductive when it comes time to retrieve that heat. “The holy grail would be something we could use for energy storage,” he says. “That’s the dream, but we’re not there yet.” But this finding is so new that there may also be a variety of other potential uses. This approach, Yildiz says, “could open up new applications we didn’t think of before.” And while the work was initially confined to the SCO material, “the concept is applicable to other materials, because we know we can oxygenate or hydrogenate a range of materials electrically, electrochemically” she says. In addition, although this research focused on changing the thermal properties, the same process actually has other effects as well, Chen says: “It not only changes thermal conductivity, but it also changes optical properties.” “This is a truly innovative and novel way for using ion insertion and extraction in solids to tune or switch thermal conductivity,” says Juergen Fleig, a professor of chemical technology and analytics at the University of Vienna, Austria, who was not involved in this work. “The measured effects (caused by two phase transitions) are not only quite large but also bi-directional, which is exiting. I’m also impressed that the processes work so well at room temperature, since such oxide materials are usually operated at much higher temperatures.” Yongjie Hu, an associate professor of mechanical and aerospace ngineering at the University of California at Los Angeles, who also was not involved in this work, says “Active control over thermal transport is fundamentally challenging. This is a very exciting study and represents an important step to achieve the goal. It is the first report that has looked in detail at the structures and thermal properties of tri-state phases, and may open up new venues for thermal management and energy applications.” The research team also included Hantao Zhang, Qichen Song, Jayue Wang and Gulin Vardar at MIT, and Adrian Hunt and Iradwikanari Waluyo at Brookhaven National Laboratory in Upton, New York. The work was supported by the National Science Foundation and the U.S. Department of Energy. (Comment on this)

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