The emerging field of valleytronics, which takes advantage of the momentum bias of excited electrons, or excitons, in a variety of optoelectronic devices, is closely associated with the fabrication of novel 2D materials only atoms thick. This month, a group of Valleytronics researchers from Central South University in Changsha, China, developed such a 2D material that greatly improves the usability of these exciting particles.
The details of manufacture and an explanation of its properties are described in the magazine Nano-research†
In the field of materials science, the term 2D materials refers to solids that are only one layer of atoms thick. These are interesting not only because they are very small, but also because new physical properties arise when a material is thinned to just this one atomic layer. Perhaps the most well-known 2D material is graphene, a single layer of carbon atoms, which has some amazing properties that are quite different from other forms that carbon takes when delivered in bulk (or more formally, “bulk crystal”), including so 200 times stronger than steel.
But there are hundreds of other types of 2D materials, which offer very different properties than their bulk crystal form. Such a 2D material, transition metal dichalcogenide, or TMD, is of particular interest in the world of optoelectronics, the science and technology of luminescent and light-detecting devices. The basis of all optoelectronic devices is the photovoltaic effect, or the generation of electric current in a material when struck by a beam of light, such as in a photovoltaic cell in a solar panel, and its inverted form, the production of light from electrical signals.
Such technology relies on materials that are semiconductors. To use the PV cell example again, when light hits a semiconductor, this energy is enough to excite electrons to jump a “band gap” from an atom’s valence level to its conduction level — where it’s excited. electrons, or more simply excitons, can now flow freely in an electric current. In fact, this special band gap property of semiconductors converts light into electrical energy. This same band gap property allows transistors — made of semiconductor material such as silicon — to act as on/off switches used to store data in the form of ones and zeros, or “bits” in computers.
The 2D material graphene, a semi-metal, has no band gap. It is a conductor, not a semiconductor. However, single layers (“monolayers”) of TMD — made of a transition metal atom such as molybdenum or tungsten bonded to an atom from the same column on the periodic table as oxygen (the chalcogens), such as sulfur, selenium, or tellurium — cause this to have a bandgap. This makes TMDs very interesting for the fabrication of transistors and other optoelectronic devices.
Just as the monolayer of a material has different properties than the same material in bulk crystal form, 2D materials that are two or three layers (double layer or three layer) thick can have different properties than the same material in monolayer form. And a multi-layered 2D material composed of layers of two or more different materials is called a heterostructure, which will have even more differences in properties.
Strictly speaking, the term exciton refers to both the electron and the empty space or ‘hole’ it leaves behind but to which it remains attracted and thus bound: an electron-hole pair. Since the electron has a negative charge, the electron hole can be said to have a positive charge. Combined, the electron-hole pair, or exciton, is an electrically neutral “quasiparticle”.
Excitons in 2D materials also prefer one of two states of momentum, depending on the polarization of the light that generated them. These favored moments are often known as “descending” because it takes a lot of energy to move an exciton up from one favored momentum state to another.
This on/off, binary nature of such exciton valleys may provide a new way to store a bit and perform logic operations. The emerging field of “valleytronics,” investigating this phenomenon, has exploded in recent years due to its range of potential applications, including incredibly fast logic operations and, perhaps one day, small room-temperature quantum computers.
Usually, excitons exist in a layer of 2D material – an intralayer exciton. But there is also an exotic type of interlayer exciton, one that exists between two monolayers, where the electron and hole are in different layers. These interlayer excitons themselves have several novel and excitatory properties, including significantly longer lifetimes than their intralayer counterparts, expanding applications in long-life exciton devices.
TMD bilayers have become especially attractive to optoelectronics researchers in recent years because they are particularly good at hosting these interlayer excitons.
But the researchers at Central South University thought they could go one layer better.
“Most TMD exciton studies are obsessed with heterostructures composed of two different monolayer TMDs,” said Yanping Liu, a physicist and engineer specializing in valleytronics and the corresponding author of the paper. “But our interest was in designing a three-layer heterostructure with type II band alignment.”
Compared to double-layer TMD heterostructures with type II band alignment, the three-layer type II band alignment basically offers a series of efficiencies, and the interlayer excitons should have an even longer lifetime, increasing the application potential of TMDs in devices such as photo detectors. . , light-emitting diodes, lasers and photovoltaics. But until now, the interlayer excitons had only been observed in double-layered TMD heterostructures.
The team was able to fabricate a three-layer TMD heterostructure (composed of molybdenum and sulfur, molybdenum and selenium, and tungsten and selenium), which they then observed using photoluminescence spectroscopy. They confirmed the presence of excitons between the layers and described different properties and requirements of the phenomenon.
After fabricating the new TMD heterostructure, confirming the existence of the long-lived excitons between the layers, and extensively cataloging properties and requirements, the team must now more closely examine the range of potential applications for their TMD in optoelectronic devices. .
Atomic thin semiconductors for nanophotonics
Biao Wu et al, Observation of interlayer excitons in three-layer type II transition metal dichalcogenide heterostructures, Nano-research (2022). DOI: 10.1007/s12274-022-4580-3
Provided by Tsinghua University Press
Quote: Valleytronics researchers fabricate new 2D material enjoying long-lived excitons (2022, June 28) retrieved June 28, 2022 from https://phys.org/news/2022-06-valleytronics-fabricate-2d-material -long-life.html
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