A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 1 2 LIGHT TRANSFORMS MATERIALS Physics professor David Hsieh of Caltech, Pasadena, Calif., and his team recently used lasers to dramatically alter the properties of materials without producing any excess damaging heat. The researchers found an ideal material to demonstrate their method— a semiconductor called manganese phosphorus trisulphide, which naturally absorbs only a small amount of light over a broad range of infrared frequencies. The team used intense infrared laser pulses, each lasting about 10-13 seconds, to rapidly change the en- ergy of electrons inside the material. As a result, the material shifted from a highly opaque state to a highly transparent one for certain colors of light. The process also was found to be reversible. When the laser turns off, the material instantly goes back to its original state unscathed. The heat-free manipulation used in the new process is known as coherent optical engineering. The method works because the light alters the differences between the energy levels of electrons in the semiconductor without kicking the electrons themselves into different energy levels, which is what generates heat. The findings, Hsieh says,mean that other researchers can now potentially use light to artificially create materials, such as exotic quantummagnets, which are difficult or impossible to create naturally. “In principle, this method can change optical, magnetic, and many other properties of materials,” say the researchers. “This is an alternative way of doing materials science. Rather than making new materials to realize different properties, we can take just one material and ultimately give it a broad range of useful properties.” caltech.edu. MINERAL-BASED SEMICONDUCTORS A research team from Missouri University of Science and Technology introduced new potential for creating advanced semiconductor devices using a naturally occurring mineral. They demonstrated a new 2D material heterostructure that has many applications in compact sensors and detectors, optical communication, optical integrated circuits, and quantum computers. The team found that flakes of len- EMERGING TECHNOLOGY Researchers from Paragraf, U.K., and Queen Mary University of London successfully fabricated organic lightemitting diodes (OLEDs) with a monolayer graphene anode instead of using indium tin oxide (ITO). The team says the graphene OLEDs achieve identical performance to ITO OLEDs, which are widely used in mobile phone touchscreens and require the rare earth element indium. www.qmul.ac.uk. BRIEF genbachite, a mineral discovered a century ago in Switzerland, have strong anisotropic properties, meaning the flakes vary along axis lines depending on the orientation. The researchers say the characteristic could have implications for directional light-emitting devices, encrypted data transfer and signal processing, and polarization-sensitive photodetectors. They obtain ultrathin lengenbachite flakes—around 30 nanometers thick—by mechanically exfoliating the bulk mineral using Nitto PVC tape. Lengenbachite is composed of stacks of alternating, weakly bonded layers of four-atom-thick lead sulfide and five-atom-thick arsenic trisulfide. Notably, the researchers observed out-of-plane one-dimensional rippling structures along the lengenbachite flake surface. The ripples are caused by the periodic mechanical strain generated between the alternating atomic layers. With the help of several optical spectroscopic techniques, the researchers also found strong anisotropic optical properties in the flakes. mst.edu. Figure is a zoomed-in view of one crystal cluster of lengenbachite mineral rock with several blade-like crystal plates. A strong laser is seen illuminating a material in a low-temperature chamber. Courtesy of Caltech/David Hsieh Lab.
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