Electronics with excitons

Fig.1: Envision circuits in which excitons-electron-hole pairs (red and blue balls)-act as carriers[Image: LANES EPFL]

Excitonics

With electronics governing the present lifestyle, search for different modes of electronics other than electron e.g. electron spins (spintronics), photons (photonics) and now excitons (excitonics), are always catching up with the scientists and engineers in order to design electronic devices which consume less energy and are smaller and faster than current electronic devices and also make use of semiconductors rather than metals. By manipulating excitons, researchers are expecting upon a whole new approach to electronics. Excitons (Fig.1) are the quasi-particles which describe the interaction between the particles that comprise a given substance, rather than the substance itself. The 

exciton is considered as an elementary excitation of condensed matter which can transport energy without transporting net electric charge. When a photon of the right frequency (and, thus, energy) is absorbed by a semiconducting material, it kicks an electron up to the semiconductor’s higher-energy conduction band-leaving behind a positively charged “hole” in the valence band. The electron and hole, still bound by Coulomb attraction, form a “quasiparticle” called an exciton that exists in insulators, semiconductors and in some liquids. However, the exciton ultimately disappears when the electron and hole recombine, thus, releasing another photon.

Excitonic systems

Recently, the interest has increased inexcitonic systems which rely on the manipulation of these quasiparticles (excitons), similar to the way that electronic systems rely on the manipulation of electrons. Such systems, it is thought, could offer an efficient way to convert between photonic and electronic systems in communications networks and other settings, since excitons are, in a sense, natural intermediates between photons and electrons. Nowadays, researchers have begun looking at the properties of excitons in the context of electronic circuits. The energy in excitons had always been considered too fragile and the excitons’ life span too short to be of any real interest in this domain. In addition, excitons could only be produced and controlled in circuits at extremely low temperatures (around -173 ºC). Recently, scientists discovered how to control the life span of the excitons and how to move them around. The excitons in special materials exhibit a particularly strong electrostatic bond and, even more importantly, they are not quickly destroyed at room temperature. Creating a special type of exciton, where the two sides are farther apart than in the conventional particle can delay the process in which the electron returns to the hole and light is produced. It’s at this point, when the excitons remain in dipole form for slightly longer, that they can be controlled and moved around using an electric field. Practical excitonics will require devices, such as excitonic transistors, that allow “currents” of excitons to be controlled.

Developmental status

Exciton-the bound electron-hole pairs formed when photons excite electrons in a semiconductor. A nagging efficiency bottleneck in today’s communications networks is the need to convert between the optical signals that transmit data over long distances, and the electrical signals used in data processing. One potential solution lies in devices that manipulate not electrons or photons, but excitons. But thus far, the “excitonic” devices demonstrated using bulk semiconductor materials have had to operate at frigid temperatures, a disadvantage that has held back practical applications.Now, a research team has used an ingenious stack of 2-D materials to develop a key component for practical excitonics: an excitonic transistor that can operate at room temperature. The prototype device could, the researchers believe, open the door to a generation of excitonic devices using 2-D materials. Those devices, in turn, could allow compact, energy-efficient optical-electronic interconnects, not only in communications but for a variety of other applications.


 Fig. 1: Schematic of the excitonic transistor[Image: LANES EPFL]
The uses involving the energy of excitons had previously been considered too fragile and short-lived to be of use to electronic circuits – in addition, it could only be produced and controlled in circuits at temperatures around -173° C. But now the researchers added 2D materials into the mix – tungsten diselenide (WSe2) and molybdenum disulfide (MoS2)- which exhibit a particularly strong electrostatic bond

and are not quickly destroyed at room temperature. A new type of transistor (Fig.2) that uses excitons instead of electrons has been developed by researchers at EcolePolytechniqueFédérale de Lausanne (EPFL), setting the stage for optoelectronic devices. Fragments of transparent graphene (1, 2, and 3) act as gating electrodes that control the movement of excitons (electron-hole pairs) through a heterostructure consisting of tungsten diselenide and molybdenum disulfide. A hexagonal boron nitride shell encapsulates the device. The exciton-based transistor uses 2D materials as semiconductors, enabling it to function effectively at room temperature – a challenge that had not been met by previous research. The electrons always found their way to the MoS2 while the holes always ended up in the WSe2, delaying the process by which the electron returns to the hole. This allowed the excitons to be controlled and moved around using an electron field. A further refinement of the research protected the 2D materials with boron nitride, which kept the excitons going even longer. The result is a system in which the exciton “lives” not in a single 2-D material layer, but between the two layers. Such an interlayer exciton, it turns out, has a spatial separation between the electron and the hole that’s large enough to allow the exciton to survive 100 times longer than it would in a single 2-D material layer. Yet the exciton can still exist and thrive at room temperature. Further, the two-layer structure means that the exciton has a built-in out-of-plane dipole moment. That means it can be manipulated and controlled by an electric field and voltage bias in ways that would be impossible with excitons in a single 2-D layer.

Future applications

Further, the study is also being carried on the behavior of excitons trapped in quantum wells made of crystalline, halide-based perovskite compounds. As a result, this will able to create a scale by which labs can determine the binding energy of excitons, and thus the band gap structures, in perovskite quantum wells of any thickness. This could in turn aid in the fundamental design of next-generation semiconductor materials. Conventional photonic or optoelectronic devices are difficult to manufacture and require complex and costly growth techniques. Hybrid perovskites, colloidal quantum dots or low-dimensionality semiconducting nanoparticles pushed the door of the solution-processable materials for optoelectronics family. These new materials not only share some advantages of a technological point of view. They also target the same applications, concentrated around the generation and detection of light. They also share many common physical properties with organic semiconductors, such as tunable absorption and emission spectra in the visible spectrum. More fundamentally however, in all these novel materials, the question of the nature and properties of excitons is central. In confined systems, strongly-bound excitons guarantee that optical properties are for most part immune to the macroscopic ordering of the environment; while the low binding energies of excitons in perovskites lifestyle are important to explain their remarkable performance. Solar cells that turn light into electricity are optoelectronic devices. So are devices that turn electricity into light, including light-emitting diodes (LEDs) and the ubiquitous semiconductor lasers that power barcode readers, laser printers, disc players and other technologies. Excitonic materials are at the heart of green photonics because their development is directly oriented towards the production of solar energy and low-consumption solid-state light sources; and also because these devices offer perspectives for using resources and methods that are more sustainable and have less impact on the environment than currently established technologies.

Conclusion

The researchers argue that their results make a strong case for integrating two-dimensional materials in future excitonic devices to enable operation at room temperature. Such devices, they believe, could prove more energy efficient and compact than previously demonstrated fast optical switches, the comparatively large size of which (approximately 10 microns) limits their on-chip packing density. The team concludes that excitons could revolutionize the way engineers approach electronics. The prototypes demonstrated could open the way for wider studies and applications of excitonic devices in the academic and industrial sectors. This breakthrough sets the stage for optoelectronic devices that consume less energy and are both smaller and faster than current devices. With this, it will be possible to integrate optical transmission and electronic data-processing systems into the same device. Further, it will reduce the number of operations needed and make the systems more efficient.

Acknowledgment: The use of information retrieved through various references/sources of the internet in this article is highly acknowledged.

By: Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106.

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