Thursday, October 8, 2015

Laser spectroscopy of ultrathin semiconductor reveals rise of ‘trion’ quasiparticles

Insight into dynamics may spur improved materials for solar energy, quantum computing

Quasiparticles—excitations that behave collectively like particles—are central to energy applications but can be difficult to detect. Recently, however, researchers have seen evidence of quasiparticles called negative trions forming and fading in a layer of semiconducting material that is 100,000 times thinner than a human hair. Scientists at the Department of Energy’s Oak Ridge National Laboratory used ultrafast laser spectroscopy at the Center for Nanophase Materials Sciences (CNMS) to demystify the dynamics of the negative trions. They explored the behavior of the charged quasiparticle in a two-dimensional (2D) semiconductor that is an excellent absorber of sunlight.

Their insights, published in the journal Physical Review B, may prove important for advancing technologies for solar energy and quantum computing.

“We observed negative trions in a two-dimensional tungsten disulfide monolayer excited by a laser beam,” said ultrafast laser spectroscopist Abdelaziz Boulesbaa, who co-led the study with theorist Bing Huang and consulted with laser spectroscopy expert Alex Puretzky. “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 two charges form—one negative (an electron) and one 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.

Quasiparticles like excitons may sound exotic, but getting electrons and holes together is the basis for everyday light-emitting diodes (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.

Pump–probe experiment

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?

To answer these questions, the ORNL scientists needed a “camera” of sorts that could make a super-slow-motion movie to reveal quasiparticle dynamics, akin to the camera techniques photographers employ to capture speeding bullets obliterating apples—only a billion times faster. A split laser beam created that camera.

Employing half the laser beam, they fired laser pulses lasting a mere 40 femtoseconds (million-billionths of a second) 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.  They probably skipped the popcorn, as their movie lasted only a nanosecond (one billionth of a second).

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.  

These extra electrons allow the crystal to absorb another photon to form a negative trion.  Because the 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 two 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, the researchers 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.

The title of the paper is “Observation of two distinct negative trions in tungsten disulfide monolayers.” This research was conducted at the Center for Nanophase Materials Sciences at ORNL. Computations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. Both are DOE Office of Science User Facilities.