MIT - Page 7

A collaboration between the A*STAR Singapore Institute of Manufacturing Technology (SIMTech) and the Massachusetts Institute of Technology (MIT) in the United States has proposed a versatile, directional graphene-based X-ray source that potentially could fit on a laboratory bench.

An X-ray source that is both small and powerful is a highly desirable concept. The researchers wanted to create something that is compact and also capable of producing very intense X-rays, essentially implementing the concept behind the enormous free-electron-laser sources on a scale small enough to fit on a laboratory table or even a microchip. For this purpose, the team utilized graphene's ability to support plasmons — collections of electronic oscillations that can be used to confine and manipulate light on scales of around ten nanometers. The scientists explain that Graphene plasmons are a natural option because they are capable of confining electromagnetic radiation to very small scales.

Scientists at MIT have designed a way to manipulate electrons in graphene within the first few femtoseconds of photo-excitation. With this technique, the researchers can redirect high-energy electrons before they interact with other electrons in the material.

Researchers at MIT conducted simulations and reached a new theory, according to which a sheet of graphene could be used to make X-rays. In this method, the graphene is used to generate surface waves called plasmons when the sheet is struck by photons from a laser beam. These plasmons in turn could be triggered to generate a sharp pulse of radiation, tuned to wavelengths anywhere from infrared light to X-rays.

The radiation produced by the system would be of a uniform wavelength and tightly aligned, similar to that from a laser beam. The researchers say this could potentially enable lower-dose X-ray systems in the future, making them safer. The new system could, in principle, create ultraviolet light sources on a chip and table-top X-ray devices that could produce the sorts of beams that currently require huge, multimillion-dollar particle accelerators.

Researchers from Massachusetts Institute of Technology, Harvard, University of California Riveriside and US Army Research Laboratory have integrated graphene with silicon microelectromechanical systems (MEMS) to make a flexible, transparent, and low-cost device for the mid-infrared range. 

Tests showed it could be used to detect a person’s heat signature at room temperature (300 K or 27 degrees C/80 degrees F) without cryogenic cooling, which is normally required  to filter out background radiation, or noise, to create a reliable image (which complicates the design and adds to the cost and the unit’s bulkiness and rigidity).

Researchers at MIT and Lawrence Berkeley National Laboratory have developed a novel kind of tunable catalyst that could potentially replace costly rare metals that are currently used in fuel cells. The catalyst is made of graphite, with additional compounds attached to the edges of the 2D sheets of graphene that the material is composed of. The catalyst’s characteristics can be tuned to promote specific chemical reactions by altering the composition and the quantities of the additional compounds.

The scientists aimed at merging the attributes of two different electrocatalysts - Molecular electrocatalysts, that can be tuned through chemical treatment so their selectivity and reactivity can be customized, and heterogeneous electrocatalysts that can't be tuned precisely but are known for their durability and ease of processing into a device. The team worked towards chemically modifying graphite’s structure to provide the desired tunability.

A team of researchers at MIT developed a method of coating condensers used in power plants with graphene, to make them durable and transfer heat rapidly. Condensers are the equipment that collects steam and condense them back to water in electricity-producing plants. Improving their efficiency could greatly contribute to the overall power plant efficiency.

The scientists coated the condenser surfaces with a layer of graphene, and found that this can improve the rate of heat transfer by a factor of four. The improvement in condenser heat transfer could lead to an overall improvement in power plant efficiency of 2 to 3 percent, enough to make a significant change in global carbon emissions.

Researchers at MIT and the University of Michigan developed a new roll-to-roll manufacturing method, that promises to enable continuous production using a thin metal foil as a substrate, in an industrial process where the material is deposited onto the foil as it moves from one spool to another. The resulting size of the sheets would be limited only by the width of the rolls of foil and the size of the chamber where the deposition would take place.

The new process is an adaptation of a CVD method already used at MIT (and additional places) to make graphene. The new system uses a similar vapor chemistry, but the chamber is in the form of two concentric tubes, one inside the other, and the substrate is a thin ribbon of copper that slides smoothly over the inner tube. Gases flow into the tubes and are released through precisely placed holes, allowing for the substrate to be exposed to two mixtures of gases sequentially. The first region is called an annealing region, used to prepare the surface of the substrate; the second region is the growth zone, where the graphene is formed on the ribbon. The chamber is heated to approximately 1,000 degrees Celsius to perform the reaction.

MIT researchers managed to use graphene, deposited on top of a similar 2D material called hexagonal boron nitride (hBN), to couple the properties of the different 2D materials to provide a high degree of control over light waves. They state this has the potential to lead to new kinds of light detection, thermal-management systems, and high-resolution imaging devices. 

Both materials are structurally alike (in that they're both composed of hexagonal arrays of atoms that form 2D sheets), but they react to light differently. These different reactions, though, were found by the researchers to be complementary, and assist in gaining control over the behavior of light. The hybrid material blocks light upon applying a particular voltage to the graphene, while allowing a special kind of emission and propagation, called hyperbolicity, when a different voltage is applied. This means that an extremely thin sheet of material can interact strongly with light, allowing beams to be guided, funneled, and controlled by voltages applied to the sheet. This poses a phenomenon previously unobserved in optical systems. 

Researchers at MIT, NIST, University of Maryland, Imperial College London, and the National Institute for Materials Science (NIMS) in Japan have created a "whispering gallery" effect for electrons in a sheet of graphene, making it possible to precisely control a region that reflects electrons within the material. This accomplishment could help in heralding new kinds of electronic lenses, as well as quantum-based devices that combine electronics and optics.

The process uses a probe (the same as in STM - Scanning Tunneling Miscroscopy) that allows control of both the location and the size of the reflecting region within graphene. When the sharp tip is positioned over a sheet of graphene, it produces a circular barrier on the sheet that "acts as a perfect curved mirror" for electrons, according to the scientists, reflecting them back toward the center of the circle. This controllable reflectivity is similar to so-called "whispering gallery" confinement modes that have been used in optical and acoustic systems - but these have not been tunable or adjustable.

Researchers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have devised a process to repair leaks, cracks and holes that are formed in graphene in the process of creating membranes for water filtration and desalination.

The process relies on a combination of chemical deposition and polymerization techniques. The team also used a process it developed previously to create tiny, uniform pores in the material, small enough to allow only water to pass through. These two techniques combined yielded a relatively large defect-free graphene membrane, about the size of a penny.