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Trions are what scientists call “quasiparticles,” bundles of energy, electric charge and spin that zoom around inside semiconductors.
Trions were first observed in quantum wells in 1993 and shortly thereafter in GaAs-AlGaAs quantum wells. Trions were were predicted and found in the photoluminescence and absorption spectra of various optically excited semiconductors, especially in quantum dots , quantum wells and carbon nanotubes.
“Trion” may soon play a key role in electronic devices. Duke researchers have for the first time pinned down some of the behaviors of these one-of-a-kind particles, a first step towards putting them to work in electronics.
Trions could also revolutionize information transmission. by University of California – Riverside
Trions
During the optical excitation of carriers in a semiconductor, the minimum energy re‐ quired to form free carriers is called the band gap. The energy below that value cannot excite free carriers. However, low-temperature absorption studies of semiconductors have shown excitation just below the band gap. This excitation is associated with the formation of an electron and an electron hole bound to each other, otherwise called an exciton.
It is an electrically neutral quasiparticle like in a hydrogenic state. At low temperatures, the bound states are formed and the Coulomb interaction between the electron and the hole becomes prominent. The negative trion (X-) is created due to the additional electron bound to a pre-existing exciton and if a hole is bound to an exciton, a positive trion (X+ ) is created. Both the negative and positive trions are complex electronic excited states of the semiconductors and therefore, the 3-body problem is raised. Although Lampert in 1958 originally and theoretically predicted the negative trion in semiconductors, K.Kheng et al. experimentally achieved a negative trion in Cd Te/Cd Zn Te quantum well.
The rapid progress of semiconductor technology in the recent years has allowed the fabrication of low dimension electronic nanostructures. Such nanostructures confine charged particles in all three space dimensions. In low dimensional, especially in quantum dots (three dimension confinement), the picture is different because it is below a nanometer wide, a few nanometers thick, and in various shapes.
The quantum confinement increases highly, and this quantum confinement leads to more stability of the excitons and trions by increasing their binding energy. The stability of such particles remains up to room temperature. A proper identification of the (X-) was not achieved until the early 1990’s in remotely doped, high-quality quantum-well (QW) structures . Since then, extensive work has been carried out on (X-) inside the two-dimensional (2D) [wide quantum wells and quantum dots, which the first observations of the QD-confined charged excitons (trions) were performed on ensembles of the QDs
Excitonic effects in semiconductors are determined by the exciton binding energy and electron-hole interaction and play a critical role in optoelectronic devices. Charged exciton complexes such as negative (T −) and positive (T +) trions are formed when a single exciton is correlated with an additional electron in a conduction band or hole in a valence band, respectively, has been proposed by Lampert. In the meantime T − and T + trions have been the subject of intense studies in the last two decades, both experimentally and theoretically. Their observation in bulk semiconductors has been hampered due to their rather small binding energies and became a challenging task.
Future electronics may ride on new three-in-one particle
“Trions display unique properties that you won’t be able to find in conventional particles like electrons, holes (positive charges) and excitons (electron-hole pairs that are formed when light interacts with certain materials),” said Yusong Bai, a postdoctoral scholar in the chemistry department at Duke. “Because of their unique properties, trions could be used in new electronics such as photovoltaics, photodetectors, or in spintronics.”
Usually these properties – energy, charge and spin – are carried by separate particles. For example, excitons carry the light energy that powers solar cells, and electrons or holes carry the electric charge that drives electronic devices. But trions are essentially three-in-one particles, combining these elements together into a single entity – hence the “tri” in trion.
“A trion is this hybrid that involves a charge marrying an exciton to become a uniquely distinct particle,” said Michael Therien, the William R. Kenan, Jr. Professor of Chemistry at Duke. “And the reason why people are excited about trions is because they are a new way to manipulate spin, charge, and the energy of absorbed light, all simultaneously.”
Until recently, scientists hadn’t given trions much attention because they could only be found in semiconductors at extremely low temperatures – around 2 Kelvin, or -271 Celcius. A few years ago, researchers observed trions in carbon nanotubes at room temperature, opening up the potential to use them in real electronic devices.
Bai used a laser probing technique to study how trions behave in carefully engineered and highly uniform carbon nanotubes. He examined basic properties including how they are formed, how fast they move and how long they live.
He was surprised to find that under certain conditions, these unusual particles were actually quite easy to create and control.
“We found these particles are very stable in materials like carbon nanotubes, which can be used in a new generation of electronics,” Bai said. “This study is the first step in understanding how we might take advantage of their unique properties.”
Physicists’ finding could revolutionize information transmission by University of California – Riverside
A research team led by physicists at the University of California, Riverside, has observed, characterized, and controlled dark trions in a semiconductor—ultraclean single-layer tungsten diselenide (WSe2)—a feat that could increase the capacity and alter the form of information transmission.
In a semiconductor, such as WSe2, a trion is a quantum bound state of three charged particles. A negative trion contains two electrons and one hole; a positive trion contains two holes and one electron. A hole is the vacancy of an electron in a semiconductor, which behaves like a positively charged particle. Because a trion contains three interacting particles, it can carry much more information than a single electron.
Most electronics today use individual electrons to conduct electricity and transmit information. As trions carry net electric charge, their motion can be controlled by an electric field. Trions can, therefore, also be used as information carriers. Compared to individual electrons, trions have controllable spin and momentum indices and a rich internal structure, which can be used to encode information.
Trions can be categorized into bright and dark trions with distinct spin configurations. A bright trion contains an electron and a hole with opposite spins. A dark trion contains an electron and a hole with the same spin. Bright trions couple strongly to light and emit light efficiently, meaning they decay quickly. Dark trions, however, couple weakly to light, meaning they decay much more slowly than bright trions.
The researchers measured the lifetime of dark trions and found they last more than 100 times longer than the more common bright trions. The long lifetime enables information transmission by trions over a much longer distance.
“Our work allows the writing and reading of trion information by light,” said Chun Hung (Joshua) Lui, an assistant professor of physics and astronomy at UC Riverside, who led the research. “We can generate two types of trions—dark and bright trions—and control how information is encoded in them.”
“Our results could enable new ways of information transmission,” said Erfu Liu, the first author of the research paper, and a postdoctoral researcher in Lui’s lab. “Dark trions, with their long lifetime, can help us realize information transmission by trions. Just like increasing your Wi-Fi bandwidth at home, trion transmission allows more information to come through than individual electrons.”
The researchers used a single layer of WSe2 atoms, resembling a graphene sheet, because the dark trion energy level in WSe2 lies below the bright trion energy level. The dark trions can therefore accumulate a large population, enabling their detection.
Lui explained that most trion research today focuses on bright trions because they emit so much light and can be easily measured.
“But we focus on dark trions and their detailed behavior under different charge densities in single-layer WSe2 devices,” Lui said. “We were able to demonstrate a continuous tuning from positive dark trions to negative dark trions by simply adjusting an external voltage. We were also able to confirm dark trions’ distinct spin configuration from bright trions.
“If we can use trions to transmit information, our information technology will be greatly enriched,” he added. “The major obstacle in such a development has been the short lifetime of bright trions. Now the long-lived dark trions can help us overcome this obstacle.”
Next, his team plans to demonstrate the actual transport of information by dark trions.
“We intend to demonstrate the first working device that uses dark trions to transport information,” Lui said. “If such a prototype trion device works, dark trions can then be used to transport quantum information.”
