In a pump-probe experiment, the pump laser pulse first excites the 2D material, and later, at controllable time-delays, the probe laser pulse returns to the energy-pumped site to provide information about the evolution of the pump’s effect on the material. In the ORNL experiment, absorption of pumped energy first generated two excitons, X1 and X2. Dissociation of these excitons through hole trapping at the substrate freed their electrons. Then the arriving probe pulse generated new electron–hole pairs, which joined the remaining free electrons to form trions T1 and T2. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy
Quasiparticles – excitations that behave collectively like particles – can be difficult to detect. Researchers have seen evidence of quasiparticles called negative trions forming and fading in an ultrathin layer of semiconducting material 100,000X thinner than a human hair. They used ultrafast laser spectroscopy at the Center for Nanophase Materials Sciences (CNMS) and explored the behavior of the charged quasiparticle in a 2D semiconductor, an excellent absorber of sunlight. It may prove important for advancing technologies for solar energy and quantum computing.
“We observed negative trions in a 2D tungsten disulfide monolayer excited by a laser beam,” said ultrafast laser spectroscopist Abdelaziz Boulesbaa. “This discovery may open new opportunities to optoelectronic applications, including information technology, as well as fundamental research in the physics of low-dimensional materials.”
When a semiconductor absorbs light, electrons can be knocked loose and can participate in an electrical current. However, typically 2 charges form — 1 negative (an electron) and 1 positive (a hole) – and are bound to each other for a short time, traveling through the crystal as a quasiparticle called an “exciton.” When an exciton binds to an additional electron, the complex formed is a negative trion, or if it binds to an additional hole, the resulting quasiparticle is a positive trion.
Getting electrons and holes together is the basis for everyday LEDs. When an electron and hole recombine in an LED, a photon is emitted. That’s the light we see in applications from traffic lights and electronic signage to camera flashes and vehicle headlights. Whereas LEDs emit light, solar cells absorb light and convert its energy into electricity. To make solar cells work, scientists try to separate the electrons from the holes and collect those charges before they have a chance to recombine. Future materials may make use of negative trions to improve charge collection in solar cells, according to Boulesbaa.
To harness negative trions for improving solar cells and other optoelectronic technologies, scientists need answers to basic questions: How do negative trions form? How long do they live? Why do they form so efficiently in an ultrathin semiconductor?
Method: They fired laser pulses of 40 femtoseconds to excite an ultrathin crystal of tungsten disulfide. Then, for their super slow-motion movie, they fashioned a strobe using the other half of the laser beam – an ultrafast flash of white light – and passed it through the crystal at different delayed times. By measuring the photon energy wavelengths (colors) the crystals absorbed at each time, the scientists built, frame by frame, a slow-motion “movie” of how trions form and fade in 1 nanosecond.
RESULTS: Their movie revealed trions form only after electron-hole pairs form. Then the holes get trapped, most likely by the substrate in contact with the crystal, leaving extra electrons >> allow the crystal to absorb another photon to form a negative trion. As ultrathin crystals are all “surface,” they have a lot of opportunity to interact with surroundings and to separate charges that are created, making them great trion generators.
Because the researchers used white light, a mixture of all frequencies of light in the visible spectrum, their observation of light of different colors revealed that 2 different trions had formed, which had not been seen previously. Next the scientists will study the role of the substrate in defining optical and electrical properties of 2D semiconducting materials. Some substrates trap electrons, leaving excess holes to carry charges, whereas others trap holes, leaving excess electrons to carry charges. Furthermore, they will isolate the 2D semiconductor from the substrate by introducing, in between, an insulator to prevent holes and electrons from reaching the substrate, allowing excitons to live longer and emit light for a greater duration. https://www.ornl.gov/news/laser-spectroscopy-ultrathin-semiconductor-reveals-rise-%E2%80%98trion%E2%80%99-quasiparticles
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