The week before Easter is usually a busy one. However, Dr. Natalia Alexeeva (who had returned briefly from Finland where she lives) and Dr. Irmantas Kašalynas found the time to eat a celebration cake.
This time, the reason for the joy was not birthday.
On 29 March 2023, the European Patent Office granted a patent for "Method of polymethylmethacrylate (PMMA) removal from a graphene surface by photoexposure".
This means that the invention by these two scientists (which we will tell you about shortly) is the first of its kind in Europe and even worldwide, as there is already a proposal to extend the patent to the US, Japanese and Chinese markets.
The patent also means that in Europe, Center for Physical Sciences and Technology (FTMC) has the right to use, license and sell the technology first, as long as it pays the annual patent fee. Therefore, FTMC's Innovation and Technology Office is currently looking for ways to get businesses interested in the new method - to monetise the invention.
(The ideal crystalline structure of graphene is a hexagonal grid. Illustration: AlexanderAlUS / Wikipedia.org)
Simple ways to solve a complex problem
When Russian scientists Andre Geim and Konstantin Novoselov discovered a new form of carbon, graphene, in 2004, it caused a real explosion: the material, which is a million times thinner than a human hair, is also the world's strongest (200 times stronger than steel). In addition, graphene, a "relative" of graphite (which we find in pencils), is an excellent electrical conductor.
So expectations are high - but it is already finding applications in electronics, semiconductor manufacturing, energy storage, sensors and more. For example, a team at the University of Texas at Austin has successfully tested a graphene sensor that detects whether a person has the flu or COVID-19 within 10 seconds.
And Geim and Novoselov won the 2010 Nobel Prize for their discovery of graphene.
Dr. Irmantas Kašalynas, Chief Scientist of the Department of Optoelectronics at FTMC, has yet to win this prize (hopefully), but he and his colleague Dr. Natalia Alexeeva, now a former FTMC scientist, have already discovered something that no one in the world has ever managed to do before. And it involves something seemingly simple - cleaning graphene.
"Graphene is a 'sheet' of a single atomic layer. It has many unique physical and chemical properties, but it is graphene's optical transparency and the high electron mobility of such sheets that we find interesting," says Kašalynas.
(A lump of graphite, a graphene transistor, and a tape dispenser. Donated to the Nobel Museum in Stockholm by Andre Geim and Konstantin Novoselov in 2010. Photo: Wikipedia.org)
The scientist works at the Terahertz Photonics Laboratory, where graphene is being studied among other interesting and important materials. Nobel laureates discovered the material when they unwrapped and transported it from graphite with a sticky tape. However, such flakes are not well suited to industry because it is difficult to control their removal (the tape will pick up a different 'amount' of graphene each time, which is not convenient). Therefore, for use in the semiconductor industry, scientists make (synthesize) graphene from vapor.
"It then forms on a film of nickel or copper. Immediately we have a problem: how do we get the graphene off the film and onto a device? Graphene is transparent and very thin (we are talking about angstroms, fractions of nanometres. A nanometre is a millionth of a millimetre). We will not be able to pick it up with our bare hands or with tweezers. It will tear immediately," he says.
There is a solution: polymethylmethacrylate, a supporting plastic, is used for this purpose. Perhaps because a mortal cannot pronounce the name, this plastic is more simply called PMMA. In the laboratory, a PMMA coating of a few tens (or up to a hundred) nanometres thick is produced, which is then 'bonded' to graphene - and this compound can then be placed on the desired device.
(Environmentally friendly removal of PMMA plastic from graphene: UV irradiation on the left, immersion in a water/alcohol mixture on the right. Illustration: Nanomaterials magazine)
But here we encounter a new problem: how to remove the PMMA itself, so that only the graphene remains on the device, with as little change as possible during transfer? According to Kašalynas, these issues complicate the application of graphene in the semiconductor industry:
"People routinely use chemicals to remove PMMA, which are aggressive: they change the properties of the graphene itself, and they can also leave a lot of plastic pieces or dry cleaning products on it. All of this affects the single-atom layer of graphene, which can become less electrically conductive and slow down its response to external influences. So the challenge is how to clean the graphene from the supporting plastic after it has been transferred?
The essence of our approach is this: we irradiate the PMMA before wiping. We irradiate with ultraviolet light, which changes the mechanical and chemical properties of that plastic, but does not damage the graphene layer because of its too low quantum energy. We have an ultraviolet chamber with lamps for disinfecting rooms (a device often used in medicine).
And the second step, which we used from another study, is to immerse the irradiated PMMA in a properly prepared mixture of water and alcohol. It turns out that, when mixed in the right proportions, these two common liquids act as a cleaner on such PMMA, and when you pull the tray out, only the graphene remains on the surface.
The method presented by FTMC scientists is comparable in efficiency to chemical solvents used in industry. And, importantly, it is environmentally friendly.
"Alcohol and water are the greenest, most accessible materials. So we offer a clean and sustainable (green) way to remove plastic from graphene," says Irmantas.
His team has carried out extensive research on this topic, the results of which have been published in the prestigious journal Nanomaterials
in November 2022. According to him, this achievement would not have been possible without the financial support of the Research Council of Lithuania and the European Union for the T-HP project. T-HP is a simple acronym for the complex title "Hybrid Plasmonic Components for the Terahertz band".
(Dr. Natalia Alexeeva. Photo from personal archive)
The scientist remembers the day Natalia Alexeeva came back from the so-called Cleanrooms - the ultra-clean laboratories on the basement floor of FTMC where graphene production and research is carried out: "She says: 'There are results that show that our method works!' Then we realized we could prepare a patent application."
He wonders: perhaps this eco-friendly way of removing PMMA plastics could help tackle the problem of microplastics in nature? Of course, plastics come in all sorts of forms, and FTMC patented method would not work for everything. But the question is open and exciting.
What is the significance of microscopic grooves?
This is the second European invention to which Dr. Irmantas Kašalynas has contributed. The first one was patented together with Dr. Simonas Indrišiūnas from the Department of Laser Technologies at FTMC.
First, they registered a Lithuanian patent and then (with a team of scientists from Poland) they obtained an international patent entitled "Method for fabrication of recessed electrical elements", which was published by the European Patent Office at the end of November 2022.
(Dr. Simonas Indrišiūnas. Photo from personal archive / FTMC)
The FTMC scientist explains what it is all about:
"We use lasers to produce gallium nitride semiconductor devices - diodes, transistors, resistors. The laser will be used not only for material cutting or tuning, but also simply for the production of electronic components and integrated circuits. So, with the laser, we make recesses, grooves in the surface of the nitride structure, which we then fill in. These grooves are tiny, just 100 nanometres deep.
Depending on how deep we make the recesses and what we fill them with, we will be able to give different functions to each of these laser-created dots. It can be a diode, a transistor, an insulator, etc. This way, individual components are created, connected electrically, and whole integrated circuits are formed. Using laser micromachining, integrated semiconductor fabrication is more flexible and faster, without the need for photolithography, photo templates or other processes.
Semiconductor chips are used extensively in electronics - in smartphones, computers, cars and more - and today we could not imagine our daily lives without them. That's why the work of the Kašalynas group contributes to moving the field forward.
(Electronic components. Photo: Hannes Grobe / „Wikimedia Commons“)
The patent is the result of years of collaboration between the team at the Terahertz Photonics Laboratory, led by I. Kašalynas, and scientists at the Institute of High Pressure Physics of the Polish Academy of Sciences, the expert says. They are world leaders in their field and are able to grow high quality gallium nitride crystals and to produce blue or shorter-wavelength light bulbs and lasers from them. Scientists in both countries are developing gallium nitride-based semiconductor materials that are capable of controlling electric currents down to the terahertz range, and which are today on the verge of dominating electronics.
And what is the material used for the laser method? It is the so-called gallium nitride heterostructures, which produce a two-dimensional electron gas with high mobility and electrical conductivity. Interestingly, when cutting a gallium nitride crystal, the laser does not damage this gas, which remains where it was. And when the metal of choice is placed in the laser's "burnt" recess, it comes into contact with the gas - with the desired results. This opens up the possibility of making electronic components, combining them into electrical circuits or providing other additional functions.
"Nobody has thought of this and tried to do it without us. We had to work hard to convince our Polish colleagues to do this research. Of course, the commercial success and development of this patent (as well as graphene cleaning) will depend on a lot of help from FTMC Innovation and Technology Office, whose work here is very important and necessary.
But what we have developed is very innovative. Because people still use lithography processes, various chemicals or charged particles, ions, to make recesses in chips. There are machines that use special materials; a vacuum is needed, where ionised atoms are used to form grooves in the material. And we simply focus laser light onto the surface of the chip: these are photons - quanta of energy that are not material, no need for a particle accelerator or other similar devices."
(Institute of High Pressure Physics of the Polish Academy of Sciences. Photo: Panek / Wikipedia.org)
The method has been developed in two departments at FTMC: Optoelectronics, which consists of five laboratories (and one of them is the Terahertz Photonics Laboratory), and the Departments of Laser Technologies (of course, we're talking about lasers!). And the specialists in this unit have a lot of expertise and the necessary technology.
According to him, there is a lot of work in this field, and Irmantas has submitted several more invention applications to the European Patent Office, which are awaiting approval. What is certain is that Lithuania, which is famous for its semiconductors and lasers, will surprise the world of science once again:
"Consider the laser printer. People wondered at first how it would be used to print text on paper... But today, we are already making semiconductor electronic circuits using lasers."
Written by Simonas Bendžius
(Top right: Dr. Irmantas Kašalynas. Photo: Remigijus Juškėnas / FTMC)