NT/ Super-resolution microscopy provides a nano-scale look at viruses in action

Paradigm

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Paradigm

22 min readJun 10, 2024

Nanotechnology & nanomaterials biweekly vol.57, 27th May — 10th June

TL;DR

A new, nano-scale look at how the SARS-CoV-2 virus replicates in cells may offer greater precision in drug development, a Stanford University team reports in Nature Communications. Using advanced microscopy techniques, the researchers produced what might be some of the most crisp images available of the virus’s RNA and replication structures, which they witnessed form spherical shapes around the nucleus of the infected cell.
Purdue University engineers have developed a patent-pending method to synthesize high-quality, layered perovskite nanowires with large aspect ratios and tunable organic-inorganic chemical compositions.
Researchers have developed a safer, cheaper, better performing, and more flexible battery option for wearable devices. A paper describing the “recipe” for their new battery type was published in the journal Nano Research Energy.
Chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and their collaborators have uncovered new details of the reversible assembly and disassembly of a platinum catalyst. The new understanding may offer clues to the catalyst’s stability and recyclability.
Scientists at the Terasaki Institute for Biomedical Innovation (TIBI), have employed artificial intelligence techniques to improve the design and production of nanofibers used in wearable nanofiber acoustic energy harvesters (NAEH). These acoustic devices capture sound energy from the environment and convert it into electrical energy, which can then be applied to useful devices, such as hearing aids.
Researchers at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences, in collaboration with research groups from the University of Electronic Science and Technology of China and Fudan University, have developed a fatigue-free ferroelectric material based on sliding ferroelectricity. The study is published in Science.
Skoltech scientists have proposed a fast, scalable, wasteless chemical treatment technique for endowing carbon nanotube films with all the right properties to improve the performance of solar panels, touchscreens, and more.

Nanotech Market

Nanotechnology deals with the ability to see, understand, measure, predict, produce or control matter at the nanoscale (below 100 nanometers). The realm of nanotechnology lies between 0.1 and 100 nanometers, wherein a nanometer is defined as one-thousandth of a micron. As a versatile technology with widespread applications in a wide range of end-use sectors, nanotechnology is currently facing a mixed bag of challenges and opportunities as the COVID-19 pandemic continues to spread across the globe. With the world fighting its biggest public health crisis in history, nanotechnology healthcare applications are storming into the spotlight led by the focus on nano intervention in terms of designing effective ways to identify, diagnose, treat and eliminate the spread of COVID-19 infections. Their role as nanocarriers has the potential to design risk-free and effective immunization strategies. In the post-COVID-19 period, the use of nanotechnology solutions in the production of a multitude of devices & products will continue to grow.

Amid the COVID-19 crisis, the global market for Nanotechnology estimated at US$42.2 Billion in the year 2020, is projected to reach a revised size of US$70.7 Billion by 2026, growing at a CAGR of 9.2% over the analysis period. Nanocomposites, one of the segments analyzed in the report, is projected to record an 8.7% CAGR and reach US$35.4 Billion by the end of the analysis period. After a thorough analysis of the business implications of the pandemic and its induced economic crisis, growth in the Nanomaterials segment is readjusted to a revised 10.1% CAGR for the next 7-year period.

Global nanotechnology market to reach US $126.8 billion by the year 2027. Amid the COVID-19 crisis, the global market for Nanotechnology is estimated at US $54.2 billion in the year 2020 and is projected to reach a revised size of US $126 billion.

Latest News & Research

Nanoscale cellular organization of viral RNA and proteins in SARS-CoV-2 replication organelles

by Leonid Andronov et al in Nature Communications

A new, nano-scale look at how the SARS-CoV-2 virus replicates in cells may offer greater precision in drug development, a Stanford University team reports in Nature Communications. Using advanced microscopy techniques, the researchers produced what might be some of the most crisp images available of the virus’s RNA and replication structures, which they witnessed form spherical shapes around the nucleus of the infected cell.

“We have not seen COVID infecting cells at this high resolution and known what we are looking at before,” said Stanley Qi, Stanford associate professor of bioengineering in the Schools of Engineering and of Medicine and co-senior author of the paper. “Being able to know what you are looking at with this high resolution over time is fundamentally helpful to virology and future virus research, including antiviral drug development.”

The work illuminates molecular-scale details of the virus’ activity inside host cells. In order to spread, viruses essentially take over cells and transform them into virus-producing factories, complete with special replication organelles. Within this factory, the viral RNA needs to duplicate itself over and over until enough genetic material is gathered up to move out and infect new cells and start the process over again.

The Stanford scientists sought to reveal this replication step in the sharpest detail to date. To do so, they first labeled the viral RNA and replication-associated proteins with fluorescent molecules of different colors. But imaging glowing RNA alone would result in fuzzy blobs in a conventional microscope. So they added a chemical that temporarily suppresses the fluorescence. The molecules would then blink back on at random times, and only a few lit up at a time. That made it easier to pinpoint the flashes, revealing the locations of the individual molecules.

Using a setup that included lasers, powerful microscopes, and a camera snapping photos every 10 milliseconds, the researchers gathered snapshots of the blinking molecules. When they combined sets of these images, they were able to create finely detailed photos showing the viral RNA and replication structures in the cells.

“We have highly sensitive and specific methods and also high resolution,” said Leonid Andronov, co-lead author and Stanford chemistry postdoctoral scholar. “You can see one viral molecule inside the cell.”

The resulting images, with a resolution of 10 nanometers, reveal what might be the most detailed view yet of how the virus replicates itself inside of a cell. The images show magenta RNA forming clumps around the nucleus of the cell, which accumulate into a large repeating pattern.

“We are the first to find that viral genomic RNA forms distinct globular structures at high resolution,” said Mengting Han, co-lead author and Stanford bioengineering postdoctoral scholar.

The clusters help show how the virus evades the cell’s defenses, said W. E. Moerner, the paper’s co-senior author and Harry S. Mosher Professor of Chemistry in the School of Humanities and Sciences.

“They’re collected together inside a membrane that sequesters them from the rest of the cell, so that they’re not attacked by the rest of the cell.”

Compared to using an electron microscope, the new imaging technique can allow researchers to know with greater certainty where virus components are in a cell thanks to the blinking fluorescent labels. It can also provide nanoscale details of cell processes that are invisible in medical research conducted through biochemical assays.

The conventional techniques “are completely different from these spatial recordings of where the objects actually are in the cell, down to this much higher resolution,” said Moerner. “We have an advantage based on the fluorescent labeling because we know where our light is coming from.”

Seeing exactly how the virus stages its infection holds promise for medicine. Observing how different viruses take over cells may help answer questions such as why some pathogens produce mild symptoms while others are life-threatening. The super-resolution microscopy can also benefit drug development.

“This nanoscale structure of the replication organelles can provide some new therapeutic targets for us,” said Han. “We can use this method to screen different drugs and see its influence on the nanoscale structure.”

Indeed, that’s what the team plans to do. They will repeat the experiment and see how the viral structures shift in the presence of drugs like Paxlovid or remdesivir. If a candidate drug can suppress the viral replication step, that suggests the drug is effective at inhibiting the pathogen and making it easier for the host to fight the infection.

The researchers also plan to map all 29 proteins that make up SARS-CoV-2 and see what those proteins do across the span of an infection.

“We hope that we will be prepared to really use these methods for the next challenge to quickly see what’s going on inside and better understand it,” said Qi.

Molecular templating of layered halide perovskite nanowires

by Wenhao Shao et al in Science

Purdue University engineers have developed a patent-pending method to synthesize high-quality, layered perovskite nanowires with large aspect ratios and tunable organic-inorganic chemical compositions.

Letian Dou, the Charles Davidson Associate Professor of Chemical Engineering in the College of Engineering and associate professor of chemistry, by courtesy, leads an international team that includes postdoctoral research assistant Wenhao Shao and graduate research assistant Jeong Hui Kim of the Davidson School of Chemical Engineering.

Dou said the Purdue method creates layered perovskite nanowires with exceptionally well-defined and flexible cavities that exhibit a wide range of unusual optical properties beyond conventional perovskites.

“We observed anisotropic emission polarization, low-loss waveguiding below 3 decibels per millimeter and efficient low-threshold light amplification below 20 microjoules per square centimeter,” he said. “This is due to the unique 2D quantum confinement inside the 1D nanowire as well as the greatly improved crystal quality.”

The research has been published in the journal Science. Dou and his team disclosed their innovation to the Purdue Innovates Office of Technology Commercialization, which has applied for a patent from the U.S. Patent and Trademark Office to protect the intellectual property.

Shao said layered metal halide perovskites, commonly called 2D perovskites, can be synthesized in solution and their optical and electronic properties tuned by changing their composition. They easily grow into large, thin sheets, but growth of one-dimensional forms of the materials is limited.

“Traditional methods like vapor-phase growth or lithographically templated solution phase growth have high processing complexity and cost,” he said. “They also have limited scalability and design flexibility.”

Kim said the Purdue method uses organic templating molecules that break the in-plane symmetry of layered perovskites and induce one-dimensional growth through secondary bonding interactions.

“Specifically, these molecules introduce in-plane hydrogen bonding that is compatible with both the ionic nature and octahedron spacing of halide perovskites,” she said. “Nanowires of layered perovskites could be readily assembled with tailorable lengths and high-quality cavities to provide an ideal platform to study lasing, light propagation and anisotropic excitonic behaviors in layered perovskites.”
Dou said, “Our approach highlights the structural tunability of organic-inorganic hybrid semiconductors, which also brings unprecedented morphological control to layered materials. This work really breaks the boundary between the traditional 1D and 2D nanomaterials, combining different features into one material system and opening many new possibilities.”
“This is just a start of an exciting new direction,” Dou said. “We are currently developing new compositions and structures to further improve the lasing performance and stability. We are also looking into large-scale patterning of these 1D nanostructures to build integrated photonic circuits. We are also interested in partnering with industry to scale up the chemistry and device applications.”

A self-healing aqueous ammonium-ion micro batteries based on PVA-NH 4Cl hydrogel electrolyte and MXene-integrated perylene anode

by Ke Niu et al in Nano Research Energy

Researchers have developed a safer, cheaper, better performing and more flexible battery option for wearable devices. A paper describing the “recipe” for their new battery type was published in the journal Nano Research Energy.

Fitness trackers. Smart watches. Virtual-reality headsets. Even smart clothing and implants. Wearable smart devices are everywhere these days. But for greater comfort, reliability and longevity, these devices will require greater levels of flexibility and miniaturization of their energy storage mechanisms, which are often frustratingly bulky, heavy and fragile. On top of this, any improvements cannot come at the expense of safety.

As a result, in recent years, a great deal of battery research has focused on the development of “micro” flexible energy storage devices, or MFESDs. A range of different structures and electrochemical foundations have been explored, and among them, aqueous micro batteries offer many distinct advantages.

Aqueous batteries — those that use a water-based solution as an electrolyte (the medium that allows transport of ions in the battery and thus creating an electric circuit) are nothing new. They have been around since the late 19th century.

However, their energy density — or the amount of energy contained in the battery per unit of volume — is too low for use in things like electric vehicles as they would take up too much space. Lithium-ion batteries are far more appropriate for such uses.

At the same time, aqueous batteries are much less flammable, and thus safer, than lithium-ion batteries. They are also much cheaper. As a result of this more robust safety and low cost, aqueous options have increasingly been explored as one of the better options for MFESDs. These are termed aqueous micro batteries, or just AMBs.

“Up till now, sadly, AMBs have not lived up to their potential,” said Ke Niu, a materials scientist with the Guangxi Key Laboratory of Optical and Electronic Materials and Devices at the Guilin University of Technology — one of the lead researchers on the team. “To be able to be used in a wearable device, they need to withstand a certain degree of real-world bending and twisting. But most of those explored so far fail in the face of such stress.”

To overcome this, any fractures or failure points in an AMB would need to be self-healing following such stress. Unfortunately, the self-healing AMBs that have been developed so far have tended to depend on metallic compounds as the carriers of charge in the battery’s electric circuit.

This has the undesirable side-effect of strong reaction between the metal’s ions and the materials that the electrodes (the battery’s positive and negative electrical conductors) are made out of. This in turn reduces the battery’s reaction rate (the speed at which the electrochemical reactions at the heart of any battery take place), drastically limiting performance.

“So we started investigating the possibility of non-metallic charge carriers, as these would not suffer from the same difficulties from interaction with the electrodes,” added Junjie Shi, another leading member of the team and a researcher with the School of Physics and Center for Nanoscale Characterization & Devices (CNCD) at the Huazhong University of Science and Technology in Wuhan.

The research team alighted upon ammonium ions, derived from abundantly available ammonium salts, as the optimal charge carriers. They are far less corrosive than other options and have a wide electrochemical stability window.

“But ammonium ions are not the only ingredient in the recipe needed to make our batteries self-healing,” said Long Zhang, the third leading member of the research team, also at CNCD.

For that, the team incorporated the ammonium salts into a hydrogel — a polymer material that can absorb and retain a large amount of water without disturbing its structure. This gives hydrogels impressive flexibility — delivering precisely the sort of self-healing character needed. Gelatin is probably the most well-known hydrogel, although the researchers in this case opted for a polyvinyl alcohol hydrogel (PVA) for its great strength and low cost.

To optimize compatibility with the ammonium electrolyte, titanium carbide — a “2D” nanomaterial with only a single layer of atoms — was chosen for the anode (the negative electrode) material for its excellent conductivity. Meanwhile, manganese dioxide, already commonly used in dry cell batteries, was woven into a carbon nanotube matrix (again to improve conductivity) for the cathode (the positive electrode).

Testing of the prototype self-healing battery showed it exhibited excellent energy density, power density, cycle life, flexibility, and self-healing even after ten self-healing cycles.

The team now aims to further develop and optimize their prototype in preparation for commercial production.

Unravelling the origin of reaction-driven aggregation and fragmentation of atomically dispersed Pt catalyst on ceria support

by Haodong Wang et al in Nanoscale

Chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and their collaborators have uncovered new details of the reversible assembly and disassembly of a platinum catalyst. The new understanding may offer clues to the catalyst’s stability and recyclability.

The work, described in a paper just published in the journal Nanoscale, reveals how single platinum atoms on a cerium oxide support aggregate under reaction conditions to form active catalytic nanoparticles — and then, surprisingly, fragment once the reaction is stopped.

Fragmentation may sound shattering, but the scientists say it could be a plus.

“Such reversible fragmentation of a platinum nanocatalyst on cerium oxide could be potentially useful for controlling the catalyst’s long-term stability,” said Anatoly Frenkel, a chemist at Brookhaven Lab and professor at SBU who led the research.

When the platinum atoms return to their starting positions, they can be used again to remake active catalytic particles. Plus, the post-reaction fragmentation makes those active particles much less likely to fuse together irreversibly, which is a common mechanism that ultimately deactivates many nanoparticle catalysts.

“Part of the definition of a catalyst is that it helps disassemble and reassemble reacting molecules to form new products,” Frenkel noted. “But it was shocking to see a catalyst that also assembles and disassembles itself in the process.”

The paper describes how the scientists observed the nanoparticles forming as single platinum atoms aggregated on the cerium oxide surface at 572 degrees Fahrenheit (300 degrees Celsius) — the temperature of the reaction they were studying.

“After the reaction, we expected that these nanoparticles would stabilize once back at room temperature in whatever particle size they reached when they were activated,” Frenkel said. “But what we observed was a reverse process. The particles began fragmenting into single atoms again.”

The team had a hypothesis to explain what they were seeing, which was confirmed by thermodynamic calculations performed by colleagues at Chungnam National University in Korea. Carbon monoxide, one of the products of the reaction — often considered a “poison” for catalysts — was actively tearing the nanoparticles apart.

“Carbon monoxide molecules have a very strong repulsive interaction when they are next to each other,” Frenkel explained. During the “reverse water gas shift” reaction, which converts carbon dioxide (CO2) and hydrogen (H2) into carbon monoxide (CO) and water (H2O) at high temperatures, the CO typically leaves the catalyst surface as a gas. But once the heat is turned off, the CO molecules bind strongly to the platinum atoms of the catalyst. This brings the CO molecules closer to each other as the system cools down and their numbers rise.
“That is a perfect storm,” said Frenkel.
“When the CO molecules find themselves very close together on the surface of the nanoparticles, they repel. And, when they repel, because they are strongly bound to the platinum atoms, they sort of pull the least-tightly bound platinum atoms from the perimeter of the nanoparticle and drag them onto cerium oxide support,” Frenkel said.

The scientists used a combination of atomic-level spectroscopic and imaging techniques to make these observations.

One technique used bright X-rays at the Quick X-ray Absorption and Scattering beamline of the National Synchrotron Light Source-II (NSLS-II) to produce a spectrum of the energy absorbed by the atoms that make up the catalyst. The scientists used this technique to study the catalyst at different temperatures and stages of the reaction. These X-ray absorption spectra are strongly influenced by the electronic states of the atoms and can be used to decipher which atoms are nearby.

“This technique can tell us that the platinum atoms have oxygen neighbors from the cerium oxide particles of the catalyst support, carbon monoxide neighbors from the reaction products, or other metal neighbors — more platinum atoms,” Frenkel said. But it “lumps together information from many platinum atoms and only gives average information,” he noted.
“It can’t tell us whether all platinum atoms have the same environment or whether we have different groups of atoms — some dispersed on the support and some within the nanoparticles. We needed additional tools to unravel the possibilities,” he said.

Infrared spectroscopy, performed in Frenkel’s Structure and Dynamics of Applied Nanomaterials (SDAN) laboratory in the Brookhaven Lab Chemistry Division, revealed the presence of two distinct groups — single atoms with no metal neighbors and nanoparticles made only of platinum. The scientists used the technique to track the relative abundance of each group as the reaction progressed.

“This technique tells us how molecules such as CO interact with our platinum atoms. Do they show features of single atoms only or nanoparticles only or both?” Frenkel said. “During the cooling down after the reaction, we observed that CO was interacting with single atoms again.”

Electron microscopy, performed by Lihua Zhang of Brookhaven’s Center for Functional Nanomaterials (CFN), produced nanoscale images of both species — single atoms and nanoparticles. These images show that, at room temperature before the catalyst is activated, there are no nanoparticles, and after the reaction, “we saw both nanoparticles and single atoms,” Frenkel said.

“These techniques together tell us that, once the reaction stops and the temperature drops, the nanoparticles have started to fragment into single atoms,” Frenkel said. “Each measurement independently would not have given us enough data to understand what we are dealing with. We couldn’t have done this work without our collaborators at NSLS-II and CFN and without the capabilities at these DOE Office of Science user facilities.”

Understanding these differences at stages of the reaction is critical to understanding how the catalyst works, Frenkel said.

“In our experiment, we deliberately went from one extreme to the other. We went from only single atoms to only nanoparticles. In the process, we had them coexist at different fractions so we could systematically investigate how the catalytic activity changes, how the structure changes,” he said.

Frenkel noted that the nanoparticles don’t assemble perfectly. They have more defects — irregular atomic sites — compared to nanoparticles synthesized by commonly used methods. These defects could turn out to be another feature that improves catalytic performance.

That’s because disorder, or strain, can contribute to the alignment of the electronic levels of chemical reactants and metal atoms in the catalyst so they can interact more easily, he explained.

A machine learning-guided design and manufacturing of wearable nanofibrous acoustic energy harvesters

by Negar Hosseinzadeh Kouchehbaghi et al in Nano Research

Scientists at the Terasaki Institute for Biomedical Innovation (TIBI), have employed artificial intelligence techniques to improve the design and production of nanofibers used in wearable nanofiber acoustic energy harvesters (NAEH). These acoustic devices capture sound energy from the environment and convert it into electrical energy, which can then be applied in useful devices, such as hearing aids.

Many efforts have been made to capture naturally occurring and abundant energy sources from our surrounding environment. Relatively recent advances such as solar panels and wind turbines allow us to efficiently harvest energy from the sun and wind, convert it into electrical energy, and store it for various applications. Similarly, conversions of acoustic energy can be seen in amplifying devices such as microphones, as well as in wearable, flexible electronic devices for personalized health care.

Currently, there has been much interest in using piezoelectric nanogenerators — devices that convert mechanical vibrations, stress, or strain into electrical power — as acoustic energy harvesters. These nanogenerators can convert mechanical energy from sound waves to generate electricity; however, this conversion of sound waves is inefficient, as it occurs mainly in the high frequency sound range, and most environmental sound waves are in the low frequency range. Additionally, choosing optimal materials, structural design, and fabrication parameters make the production of piezoelectric nanogenerators challenging.

As described in their paper, the TIBI scientists’ approach to these challenges was two-fold: first, they chose their materials strategically and elected to fabricate nanofibers using polyvinylfluoride (PVDF), which are known for their ability to capture acoustic energy efficiently. When making the nanofiber mixture, polyurethane (PU) was added to the PVDF solution to impart flexibility, and electrospinning (a technique for generating ultrathin fibers) was used to produce the composite PVDF/PU nanofibers.

Secondly, the team applied artificial intelligence (AI) techniques to determine the best fabrication parameters involved in electrospinning the PVDF/polyurethane nanofibers; these parameters included the applied voltage, electrospinning time, and drum rotation speed. Employing these techniques allowed the team to tune the parameter values to obtain maximum power generation from their PVDF/PU nanofibers.

To make their nanoacoustic energy harvester, the TIBI scientists fashioned their PVDF/PU nanofibers into a nanofibrous mat and sandwiched it between aluminum mesh layers that functioned as electrodes. The entire assembly was then encased by two flexible frames.

In tests against conventionally fabricated NAEHs, the resultant AI-generated PVDF/PU NAEHs were found to have better overall performance, yielding a power density level more than 2.5 times higher and a significantly higher energy conversion efficiency (66% vs. 42%).

Furthermore, the AI-generated PVDF/PU NAEHs were able to obtain these results when tested with a wide range of low-frequency sound — well within the levels found in ambient background noise. This allows for excellent sound recognition and the ability to distinguish words with high resolution.

“Models using artificial intelligence optimization, such as the one described here, minimize time spent on trial and error and maximize the effectiveness of the finished product,” said Ali Khademhosseini, Ph.D., TIBI’s director and CEO. “This can have far-reaching effects on the fabrication of medical devices with significant practicability.”

Developing fatigue-resistant ferroelectrics using interlayer sliding switching

by Renji Bian et al in Science

Researchers at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences, in collaboration with research groups from the University of Electronic Science and Technology of China and Fudan University, have developed a fatigue-free ferroelectric material based on sliding ferroelectricity. The study is published in Science.

Ferroelectric materials have switchable spontaneous polarization that can be reversed by an external electric field, which has been widely applied to non-volatile memory, sensing, and energy conversion devices.

Due to the inherited ionic motion of ferroelectric switching, ferroelectric polarization fatigue inevitably occurs in conventional ferroelectric materials as the number of polarization reversal cycles increases. This can lead to performance degradation and device failure, thus limiting the practical applications of ferroelectric materials.

To solve this fatigue problem, the researchers developed a fatigue-free ferroelectric system based on sliding ferroelectricity. A bilayer 3R-MoS2 dual-gate device was fabricated using the chemical vapor transport method.

After 106 switching cycles with different pulse widths ranging from 1 ms to 100 ms, the ferroelectric polarization dipoles showed no loss, indicating that the device still retained its memory performance.

Compared with commercial ferroelectric devices, this device exhibits a superior total stress time of 105 s in an electric field, demonstrating its excellent endurance.

By means of a novel machine-learning potential model, theoretical calculations revealed that the fatigue-free property of sliding ferroelectricity can be attributed to its immobile charged defects.

This work provides an innovative solution to the problematic performance degradation of conventional ferroelectric.

High-temperature adsorption of nitrogen dioxide for stable, efficient, and scalable doping of carbon nanotubes

by Eldar M. Khabushev et al in Carbon

Skoltech scientists have proposed a fast, scalable, wasteless chemical treatment technique for endowing carbon nanotube films with all the right properties to improve the performance of solar panels, touchscreens, and more.

Reported in Carbon, the team’s experiments show that exposure to even small amounts of nitrogen dioxide gas at elevated temperatures modifies carbon nanotube films in a way that promotes both transparency and electrical conductivity, and this modification resists degradation.

Carbon nanotube films conduct electricity and let light through, making them an excellent material for transparent electrodes. These are essential for solar cells and touchscreens, which used to rely on brittle and unsustainable indium tin oxide films and other conventional materials.

As of today, carbon nanotubes doped with additional atoms of other elements provide better conductivity and transparency, as well as bending capacity for flexible devices.

“Doping is very crucial here. Unfortunately, current technology does not permit the manufacture of carbon nanotubes that would have the necessary characteristics in their pure form. That said, there are a range of doping agents that alter nanotube properties. Depending on which chemical is used, one can make the film either highly conductive or transparent or stable. With some luck, you could have two of these three attributes. We managed to combine all three,” said the study’s principal investigator Professor Albert Nasibulin from the Photonics Center.

One common doping agent is hydrogen tetrachloroaurate, for example. It offers peak performance in terms of nanotube electrical conductivity, along with reasonably good transparency. However, this modification is rather unstable, so the effect wears off quickly.

Copper bromide and other metal halides provide a decent combination of stability and conductivity, but the transparency is poor. Similar trade-offs are involved with any chemical used for carbon nanotube doping today.

“We found a solution that does good on all counts. Our doping agent is a gas called nitrogen dioxide, occasionally referred to as ‘fox tail’ for its bright orange color. In fact, we were investigating another, rather unstable modification this gas causes when nanotubes are exposed to it at much lower temperatures,” study co-author Assistant Professor Dmitry Krasnikov added.
“Sort of by accident, we stumbled upon a different range of temperatures where the resulting modifications are highly stable. A further benefit of working with a gas-phase agent is that it makes the doping technology fast, scalable, and wasteless. Indeed, nitrogen dioxide will be easy to integrate into existing technological processes, and it is easy to remove from the reactor, because it turns to liquid once cooled to 20 degrees Celsius.”

According to the study, over the course of one year, the effect of the new doping suffers a mere 1.5 times degradation within a short period of time, followed by a stable plateau, compared with three times drawn out over an extended period for the current champion tetrachloroaurate.

The new agent’s effect on conductivity is comparable to tetrachloroaurate’s, and better than that of any other agent. The transparency is good, too: Being a gas, nitrogen dioxide seems to avoid multilayer adsorption to carbon nanotube films providing an exclusively molecular-thick layer. Unlike solid agents — including tetrachloroaurate — no additional particles settle on top of those previously deposited.

The team hopes that transparent carbon nanotube-based electrodes doped with nitrogen dioxide will soon make their way into photovoltaic elements, touchscreens, and other interactive surfaces in homes, cars, or public spaces. Such electrodes would also be biologically compatible, so they could find applications in implantable devices.

Optical components, such as the varifocal Fresnel zone plate for harnessing terahertz radiation in 6G communication and X-ray-free medical imaging and security scans, would also benefit from the improved characteristics of carbon nanotube films, achieved with the new doping agent.