MXenes are revolutionizing nanotechnology with the help of Raman Spectroscopy

Yury Gogotsi, Ph.D., is a co-inventor of MXenes (pronounced ‘maxines’). It’s touted as the next evolutionary leap in two-dimensional nanomaterials and materials science in general, and could provide countless benefits like high-power energy storage (think of a car battery charging in a few minutes), metallic electrical conductivity, flexibility and transparency required for wearable technology.

Gogotsi is an expert in the fields of Materials Science and Engineering and Nanotechnology and a Distinguished University and Bach professor in Drexel University’s College of Engineering.

Yet excitement over the past 15 years surrounded the discovery of two-dimensional layers of carbon called graphene. These are monoatomic layers of carbon atoms [1], with have extreme physical properties, including mechanical strength and heat conductivity, which enable applications of graphene in energy storage, protective coatings, sensors, and electronics [2].

After billions of dollars in investment, companies are beginning to find practical applications for graphene, but it is just one material: there are many others two-dimensional nanomaterials. There is boron nitride, which has a chicken-wire hexagonal network of atoms like graphene, but with alternating carbon and nitrogen atoms. Boron nitride, molybdenum disulfide and some other two-dimensional materials have been used for many years as lubricants, but they are driving advances in electronics, communications, catalysis and energy storage nowadays.

MXenes are nanoscaled, two-dimensional transition metal carbides and nitrides with carbon or nitrogen atoms sandwiched between metal layers. Their metallic conductivity and hydrophilic nature give them a unique combination of properties. Consider them to be the thinnest possible, water-soluble metal sheets or conductive clay. MXenes can be used as building blocks and combined with other two-dimensional sheets for building any type of structure with the desired and computer-programmed properties. They can enhance the strength and conductivity of polymers, and since they are hydrophilic, they can be made into water-based paints or dyes.

About 50 MXenes with various combinations of metal, carbon, and nitrogen atoms, as well as surface functional groups, such as oxygen or halogens, have been reported. More than 100 of simple compositions have been predicted. However, since one can arrange and mix atoms in many different ways, a virtually infinite number of materials can be made in this system, each with unique properties. This opens new horizons in the materials world.

In all nanomaterials, confinement in subtle layers does not allow electromagnetic waves, light, electrons, or phonons (vibrations in crystal lattice) propagate in a normal direction to the surface. That produces interesting effects. Plain materials simply get different and very useful properties.

“We just need to learn better how to assemble devices and artificial materials, in the future, by combining these various two-dimensional sheets with various properties,” Gogotsi said.

Gogotsi’s initial plan wasn’t to make new two-dimensional carbides. “We started at one point, trying to make a material for anodes of lithium-ion batteries,” he said. “And we ended up to creating an entire family of two-dimensional materials. This is how research sometimes goes.”

The process begins with fundamental research, and is often met with many disappointments, but it can eventually lead to success if one is persistent. “We work from basic science, from fundamental scientific discovery, to practical application,” Gogotsi said. “And sometimes we fail. In many cases we fail because we do something no one has done before. You're supposed to fail. But sometimes we do succeed, and then we succeed big time.”

Among those discoveries are new materials. Researchers determine properties of new materials, and learn how to control materials at the nanoscale to achieve control over their properties. Then they determine how those properties and those materials can be useful by developing applications.

For example, there have been several interesting discoveries of new carbon morphologist materials, like the development of a method of making porous carbide-derived carbons, which are used in electrical chemical energy storage supercapacitors, and scientists have used those materials to achieve progress in a variety of areas, including electric chemical capacitors.

MXenes were first established in a paper published in 2011. But Gogotsi said the timing was unfortunate, since graphene garnered the bulk of scientific interest in that period. So researchers initially ignored MXenes. But as soon as Gogotsi and other researchers started to make more and more different MXenes, and show properties that were exceeding properties of many as a materials in many fields, the interest increased dramatically.

But developing MXenes and other two-dimensional materials took a change in the scientific philosophy.

“Let’s go back to the stone age,” Gogotsi said. “What did ancient humans do? They will take a piece of stone or a bone or wood and carve something out of it, like a knife. This is how initially people used to make materials, take available material, and carve something out.”

Then the world moved to Bronze Age, and after that Iron Age. Humans learned learn how to melt material and make a piece of metal. And again, carve something out of the material. The same continued with polymers and plastics. People made the bulk of plastic or polymer and shaped something out of it.

However, you have a limited number of materials. You're always losing a very large amount of material when making components. In the new age, instead of carving or removing materials to create a component or a device, we are assembling them from nanometer-thin two-dimensional sheets.

“The world is moving to the era where we use nanometer-thin, angstrom thin building blocks, two-dimensional materials, and one-dimensional nanotubes like nanowires, one-dimensional dots, and we can start assembling them,” he said.

“So instead of taking a big chunk of material and carving something out it, like etching in silicon technology, we are combining nanoflakes and nanoparticles to produce materials with properties which don't exist in nature."

It's really taking us to a new era where we can build materials from atomically thin building blocks, two-dimensional materials, one-dimension nanotubes and nanowires, zero-dimensional nanodots, and scientists can start assembling these in new ways.

Two-dimensional materials are important nowadays, just as the world is moving towards wearable electronics, printable electronics, and the internet of things. When you want to make things wearable, flexible, printable, transparent, you need extremely thin layers of materials. You need materials that you can put in an inkjet printer and print or spray like a paint on a wall from solution to make a device. This is exactly what all these nanomaterials bring.

It's really next-generation technology, which is already finding numerous applications in conducting dyes, all smart labels, and thin-film coatings on cell phone screens.

Nanomaterials are breaking into all fields of human activity. Cell phones are so small because of components made of nanomaterials. Hidden inside are carbon nanotubes, which contains nanoparticles. There are various types of thin-film electronic devices which contain nanomaterials.

Your sweater is built of fibers which have a natural nanoscale component and made by humans or by nature and modified by humans.

MXenes can go farther. For example, as we move to a 5G communication world, we’ll require antennas everywhere. Conventionally, they're made of copper foil, with a 10 or 20 micron thickness. Yet we cannot print copper foil with current materials. Yet MXenes can be mixed with water, fabricated into an ink, and used to print antennas. These printed antennas perform almost like copper antennas, but about 10 times thinner and lighter.

As mentioned, MXenes retain their flexibility, strength, and conductivity and offer other benefits in communication applications.

In 2016, U.S. and Canadian diplomats in Cuba began experiencing unusual illnesses. Symptoms included dizziness, loss of balance, hearing loss, anxiety, and something they described as "cognitive fog." It became referred to as "Havana Syndrome." A similar outbreak occurred in China leading up to 2018. [3]

U.S. government investigations blamed directed microwave radiation, or pulsed radio frequency energy as the cause for the illnesses.

MXene’s properties can provide electromagnetic interference shielding to protect against similar attacks in the future. Due to their flexibility and high conductivity, MXenes can incorporated into textiles. [4] In fact, MXenes can provide better electromagnetic interference shielding than solid metal, Gogotsi said.

From communication to energy storage to medicine, MXenes have the potential to change the way we live. They have been explored everywhere, from lasers to medical electrodes, to brain electrodes; to electronics and optoelectronics to sensors; and to transparent windows that can produce and store electrical energy.

Imagine a cell phone that charges in a minute. Or a Tesla that doesn’t have to be plugged in overnight. We strive to have a battery that will store lots of energy, but can be charged quickly. Electrically conductive MXenes, where no solid state diffusion, unlike in the currently used battery materials, is needed may help us to achieve this.

In biomedical applications, for example, MXenes have been found to absorb urea. Gogotsi is working with nephrologists to develop materials for dialysis. In the future, scientists may use MXenes to make wearable kidneys for those whose life depends on dialysis.

“It's really simply a new world of materials,” Gogotsi said. “And that's why we believe it is truly revolutionary. We're not talking about one material changing one application or improving something in one application. We're talking about a new way of making materials, assembling them into devices and a way to make things that could not be made before.”

Yet it will probably take four or five years before the first MXene products are commercialized. Many companies are exploring applications now.

“It used to take 15 to 25 years from discovering a material to introduction into technology,” Gogotsi said. “Now, the cycle is shorter, but it will still be a few years and keep in mind, billions of dollars have been invested into development of graphene and applications. Since the Nobel Prize in graphene in 2010, there has been $100 million a year in funding for Graphene Flagship project in Europe and enormous amount of funding elsewhere. MXenes are really getting pennies compared to this.”

But research into MXenes is accelerating.

“The last time I checked, it showed that research is going everywhere on all continents,” he said. “And what does it mean? That breakthroughs are going to come and commercial applications are on the way.”

New atomically thin materials cannot be seen with the naked eye - you need tools to characterize them. That is, no one can really do much with this material or benefit from its nanoscale features, from 2D confinement and properties without knowing its structure.

“Optical spectroscopy techniques are largely the fastest and actually the least expensive methods of characterization of materials here, particularly Raman spectroscopy,” Gogotsi said. “It has become the workhorse in the world of characterization of two-dimensional materials.”

For example, for bulk materials, X-ray diffraction used to be the main tool. But if you look at carbon nanomaterials, for example, nanotubes and graphene, researchers have turned to Raman spectroscopy, because one can take a spectrum from a single particle in a certain location. “If we want to count number of layers in a graphene flake or determine the carbon nanotube diameter, we use Raman spectroscopy, not microscopy. So, and this is exactly where spectroscopy is absolutely enabling tool for nanomaterials.”

“You can record a spectrum in seconds or less,” Gogotsi said. “You can do research in a lab, or you can do quality control on a production line through a fiber optic probe. This is where spectroscopy really became the first tool for characterization. If we want to know the quality of a material, we use Raman spectroscopy. If we want to find the degradation of our materials, we use spectroscopy. If we want to see the composition of particles, again, we use Raman spectroscopy.”

Raman spectroscopy provides a useful fingerprint of chemistry and structure of nanomaterials. Gogotsi’s Ph.D. student Asia Sarycheva created a library of Raman spectra of MXenes. As a result, for quality control, you can look at the Raman spectrum of the material and tell if it is what you were expecting. Secondly, you can look at the spectrum and say, for example, it has stronger peaks, hoping to see how good the quality of the material is or how many monolayers on the flakes there are.

“We can do characterization of single flakes on a substrate, under the microscope,” he said. “We can do characterization of a film and get the average value for entire film or map the surface, but we can also probe with Raman spectroscopy flakes in colloidal solution, because we usually start with processing the solution. The tool is truly versatile.”

A paper Gogotsi published with a colleague in 2008 on materials for electrochemical capacitors became the second most cited materials science paper in the 21^st century, he said, next to graphene. Now his new discovery has the promise to match or exceed that achievement because of the world-wide interest to MXenes.

As research accelerates into the properties and applications of MXenes, we get closer to harnessing the potential of these unique materials. Although it may take until the middle of the 2020s to begin realizing this potential, the myriad of research points to a revolution in technological advances in areas from energy to electronics and medicine, first of all in flexible and wearable technology.

[1] D’Angelo, M., Matsuda, I.  Monatomic Two-Dimensional Layers, Chapter 1 - Basics and Families of Monatomic Layers: Single-Layer 2D Materials, 2019, Pages 3-22.

[2] Graphene-info, 2020.

[3] 'Havana syndrome' likely caused by directed microwaves - US report. BBC. Dec. 6, 2020.

[4] Shahzad, F, Alhabeb, M.,Hatter, C, Anasor, B, Hong, S.M., Koo, C.M., Gogotsi, Y. Electromagnetic interference shielding with 2D transition metal carbides (MXenes). Science. Sept. 9, 2016. Vol. 353, Issue 6304, pp. 1137-1140.

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