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2022: Scientists have found a way to change the electronic properties of carbon nanotubes and adjust them for use in electronic devices
On February 10, 2022, representatives MIPT said that together with scientists Skoltech they had found a way to change the electronic properties of carbon nanotubes and adjust them for use electronic in devices. The work is published in the journal Carbon.
As reported, carbon nanomaterials represent a wide class of compounds: graphene, fullerenes, nanotubes, nanofibers and others. The physical properties of many of them have already been included in textbooks, but scientists continue to create structures and find practical applications for them. One such macrostructure is films composed of carbon nanotubes, which are oriented in random order. Externally, the films are somewhat similar to the thinnest spiders: their length and width can reach several tens of centimeters, and thickness - millions of times less, several nanometers.
Carbon nanotube films have a combination of physical and chemical properties. They are mechanically stable, flexible and extensible, characterized by proper adhesion to various substrates, chemical inertia and exceptional electrical and optical properties.
Having proper conductivity, films have advantages over metal films: ease and flexibility. They can be used in various electric devices: screens, modulators, antennas, bolometers, etc.
In order to make the most effective use in practice of the electrical and electrodynamic properties of films, it is necessary to study what physical principles such properties determine. Of greatest interest are the terahertz and far infrared ranges (radiation wavelength - from 2 mm to 500 nm), in which films exhibit properties characteristic of metal conductors.
Scientists from MIPT and Skoltech studied the conductivity of films in the terahertz and infrared frequency ranges. To work, the authors took films synthesized by gas phase deposition. Some of the films were prepared from nanotubes of varying lengths, which ranged from 0.3 to 13 microns. Another group of films was exposed to oxygen plasma for 100-400 seconds. This effect changed the electrodynamic properties of the films.
Earlier, the authors in their work proved that the conductivity of films is well described by the model of conductivity fair for metals. In such films, the energy of the free electrons is sufficient to overcome potential barriers between the individual contacting nanotubes. Such electrons move almost "freely" throughout the film, which leads to proper conductivity.
But with a decrease in the length of the tubes (up to 0.3 μm) or long-term exposure to plasma (> 100 s), conductivity at terahertz frequencies (<0,3 ТГц) значительно уменьшается. Оказалось, что изменения проводимости пленок при воздействии на них плазмой или при уменьшении длины нанотрубок аналогичны. Это связано с тем, что облучение плазмой приводит к увеличению числа дефектов в нанотрубках, а следовательно, росту числа потенциальных барьеров на пути электронов. С уменьшением длины нанотрубок число потенциальных барьеров на единицу площади также увеличивается. Эти барьеры существенно влияют на проводимость нанотрубок (а, следовательно, и пленок) на постоянном токе и на достаточно низких частотах. Эффект объясняется тем, что при низких температурах кинетическая энергия электронов слишком мала, чтобы электроны могли преодолеть потенциальный барьер. На достаточно высоких частотах, как показано авторами, электроны перестают чувствовать присутствие барьеров и ведут себя как свободные. Так что в пленках, составленных из коротких трубок или из трубок, обработанных плазмой, на достаточно низких частотах и на постоянном токе будет наблюдаться возрастание температурного коэффициента сопротивления, который показывает, как меняется сопротивление с изменением температуры.
If the irradiation duration is more than 100 seconds or if the length of the nanotubes is less than 0.3 μm, the temperature coefficient of resistance goes to saturation: with these parameters, the original structure of the film is violated, and along with this, the original properties disappear.
Scientists at MIPT and Skoltech plan to continue the study of modified films, for example, stretched in one or more directions.
 | If nanotubes have been studied in detail and have long been studied, then macro objects from these tubes - films - began to be studied relatively recently. Compared to metal films, they are much lighter, chemically and mechanically stable, making them attractive for electronics applications. Knowing the fundamental physics that determines the electrical properties of films, we can purposefully adjust these properties for specific practical applications. Particularly relevant are research on terahertz frequencies, which will be working in the means of telecommunications of the near future. said Boris Gorshunov, head of the laboratory of terahertz spectroscopy MIPT |  |
| | It has been found that controlled destruction of this material by treatment of the films with microwave plasma results in unexpected properties. In particular, we observe an increase in the temperature coefficient of resistance in films of single-layer carbon nanotubes. This is due to the fact that competing contributions to conductivity from metal and semiconductor tubes cease to play an important role, and the conductivity of the film is mainly determined by the formed defects. This is of great interest for the creation of next-generation devices, for example, high-speed bolometers operating at room temperature. said Albert Nasibulin, head of the laboratory of nanomaterials at the Skolkovsky Institute of Science and Technology | |
2019: Physicists of the Russian Academy of Sciences and MIPT determined the type of quasi-particles in semiconductor carbon nanotubes
On December 5, 2019, the MIPT reported that scientists from the A. M. Prokhorov Institute of General Physics of the Russian Academy of Sciences and MIPT, together with colleagues, investigated the effect of "traps" on the optical properties of carbon nanotubes. When treated with hydrochloric acid, separate hydrogen atoms remain on the surface of the tubes. They do not form chemical bonds with the surface, and therefore do not introduce defects into the structure of the nanotube. These atoms serve as "traps" - a quasiparticle that has fallen into their zone of influence cannot "escape" (becomes localized). Based on the data obtained by spectroscopy methods, physicists came to the conclusion that an exciton (consisting of an electron and a "hole") and a trion (an exciton to which another hole or electron joined) were trapped. The results are published in the journal Scientific reports.
As noted in MIPT, carbon nanotubes are a light and durable material, promising from many points of view. Films of carbon nanotubes with semiconductor conductivity in the future are able to replace indium-tin oxide - a solid transparent material that has been used for 60 years to create transparent electrodes. Without rare earth indium, displays and touch screens will become cheaper, and in addition, they can be bent and folded without harm.
Thin film transistors are responsible for switching pixels on a flexible screen. The faster the charge is able to move in the material, the faster the transistors react and the faster the screen response. To describe the processes of charge transfer in semiconductors, physicists introduced the concept of "quasiparticle." An example is the "hole" - the free space left after the electron broke off into the orbit of the atom. The quasi-particle exciton (from the Latin "excitation") is a pair of "electron-hole," which moves as if the particles are "tied" to each other. If another particle is attached to the exciton, a trion is obtained.
To investigate the quasi-particles, scientists added hydrochloric acid to an aqueous suspension of carbon nanotubes with semiconductor conductivity. The authors then examined the absorption spectra of suspensions with different amounts of hydrochloric acid. The higher the acid concentration, the more "traps" were formed - hydrogen atoms settled on the surface of the tubes - and the more excitons and trions fell into them.
Nanotube energy can only take on certain values. The energy levels are similar to the shelves of the cabinet - you can put the book on the second or tenth, but you can not on 9 the. Physicists obtain an absorption spectrum by exposing matter to radiation: if the energy that a photon can transmit to a particle in a collision coincides with the "distance between the shelves," the particle absorbs it and goes to a higher level. By changing the wavelength of the incident radiation, it is possible to determine when it is absorbed by the substance more strongly, and determine the location of the "shelves."
In addition, scientists studied the spectra of photoluminescence. In this method, the particles go into an excited state under the influence of radiation, and then return to the original, emitting a photon (following the analogy, we push the books to the upper shelves, and then record the noise from their fall to the lower shelves). Scientists noted that with an increase in the number of hydrogen atoms settled on the tube, the number of excitons decreases. But a new energy transition appears, designated as the X-band. This transition is also visible on absorption spectra. Researchers have suggested that it corresponds to particles caught in "traps."
The above methods do not allow you to separately consider energy transitions separated by very small periods of time (on the order of 10-12 seconds) - they merge, and in the end it is not clear which particles are in the "trap." Therefore, the spectra were further examined using the pump-probe spectroscopy method.
The processing of information obtained by this method made it possible to isolate energy levels that form at different times after the pulse. The first two corresponded to the formation of an exciton, free and caught in a proton trap. The third was formed after a noticeable time (about one picosecond) after exciton levels, on the basis of which the authors linked it to the formation of a quasiparticle, a trion, in a "trap."
| | "Doped single-walled carbon nanotubes have demonstrated previously their unique properties as conductive transparent electrodes. In this work, we identified multi-particle optical excitations in such nanotubes and elucidated the mechanisms of energy migration. The development of this direction opens up wide prospects for non-linear optics, " noted' Timofey Yeremin, Associate Researcher, Laboratory of Nanocarbon Materials, MIPT, one of the authors of the work | |
The findings contribute to a better understanding of the energy structure of impurity-added carbon nanotubes, which is important not only from a fundamental but also from a practical point of view. In the future, scientists plan to investigate the energy levels of carbon nanotubes with various types of "traps."
The work was carried out by a team of scientists from the Moscow Institute of Physics, the Institute of General Physics named after A.M. Prokhorov RAS, Moscow State University, MEPhI, FTI named after Ioffe, as well as the University of Eastern Finland. The work is supported by the Russian Scientific Foundation.
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