Quasiparticles: Negative trions lasered

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  • Published: Oct 15, 2015
  • Author: David Bradley
  • Channels: Atomic
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Ultrafast, ultrathin

Ultrafast laser spectroscopy has offered evidence of the formation of quasiparticles, negative trions, in a layer of semiconductor. The research could lead to new routes to energy conversion and quantum computing applications.

Ultrafast laser spectroscopy has offered evidence of the formation of quasiparticles, negative trions, in a layer of semiconductor. The research could lead to new routes to energy conversion and quantum computing applications.

Quasiparticles are excitations in a material that behave collectively as if they were particles, their existence underpins energy applications but they can be rather elusive. However, one particular class of quasiparticles, so-called negative trions have been observed forming and fading in an ultrathin layer of semiconducting material, according to researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL). They used ultrafast laser spectroscopy at the Center for Nanophase Materials Sciences (CNMS) to investigate these negative trions in a two-dimensional monolayer of tungsten disulfide an excellent photon-absorbing material. The team reports details of their work in the journal Physical Review B.

"We observed negative trions in a two-dimensional tungsten disulfide monolayer excited by a laser beam," explains Abdelaziz Boulesbaa, who worked with theorist Bing Huang and 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."

Holes apart

The absorption of photons by a semiconductor commonly leads to electron excitation, which can be tapped as an electric current, in familiar photovoltaic panels. The process typically generates two charges a negative (electron) and a positive (hole) which are bound together for a short time and travel through the material in the form of a quasiparticle, an "exciton." The reverse process - the recombination of electron and hole and emission of a photon - is the basis of how the light-emitting diode (LED) works. However, it is the binding of an exciton to an additional electron that forms a negative trion. Conversely, bonding an exciton to an extra hole, produces a positive trion.

Future solar cells that separate electrons from holes more effectively and collect the charges to tap a current before they recombine and re-emit the photons or lose energy as heat might exploit trions to improve photo-efficiency. Before scientists can harness negative trions for improved solar cells or other optoelectronic applications, there are several fundamental questions that need to be answered in detail: How do negative trions form? How long do they persist? Why do they form so efficiently in an ultrathin semiconductor?

Hold on for longer

The team used femtosecond laser spectroscopy to examine their ultrathin crystal of tungsten disulfide. One laser beam excites the crystal, and another beam probes it at different times. They could thus build up frame by frame, a slow-motion "movie" lasting just one nanosecond of trion formation and breakdown. The work revealed that trions form only after electron–hole pairs form. The holes are then trapped by the substrate in contact with the crystal, leaving extra electrons behind. 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. The use of white light in the study revealed the existence of two distinct trions.

The next step will be to investigate what part the substrate plays in defining the optical and electrical properties in 2D semiconducting materials. Some substrates trap electrons, leaving excess holes to carry charges, whereas others trap holes, leaving excess electrons to carry charges. The team also plans to isolate the 2D semiconductor from the substrate by sandwich an insulator in between the two layers to preclude holes and electrons from reaching the substrate; that should allow excitons to persist and emit light for longer.

"What we hope to achieve ultimately is to be able design a system based on these 2D semiconductors where we have full control over what we generate," Boulesebaa told SpectroscopyNOW. "If we want something to emit light for longer (like LEDs), we prevent the substrate (the support of the 2D semiconductor) from trapping holes (or electrons), and thus the exciton quasiparticle does not dissociate and emits light.  If we want more quantum states (for information storage devices), we use a substrate that traps the hole to generate negative trion quasiparticle (which is a quantum state) or a substrate that traps electrons and thus generate positive trions (which is another quantum state)."

Related Links

Phys Rev B 2015, 92, 115443: "Observation of two distinct negative trions in tungsten disulfide monolayers"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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