Simulations help to tailor the properties of 2D materials

Cover image: The illustration shows multiple layers of lithium ions intercalated into two atom-thin sheets of graphene.

Be it for more efficient energy harvesting and storage or for better superconductors — Arkady Krasheninnikov’s simulations carried out with the help of PRACE resources, provide a better understanding of promising 2D materials and a basis to create new materials with tailored properties. His work has made him one of the most cited researchers in his field worldwide.

Emerging 2D materials are probably about to blow the lid off the electronic industry. That’s not quite how Arkady Krasheninnikov, a group leader at Helmholtz-Zentrum Dresden-Rossendorf and visiting professor at Aalto University in Helsinki, puts it, but the expert in the investigation of 2D materials points to their great potential for the next generation of potent, miniaturised and pliable electronics. These materials consist of a single layer of atoms forming an atomically thin sheet, and such a geometry gives rise to unusual electronic properties. Krasheninnikov is focused on understanding the nature of these materials and how to specifically tailor their traits on an atomic level.

One of the materials the physicist and his group are studying is graphene, the 2D-layer version of the bulk carbon mineral graphite. It consists of carbon atoms arranged in a flat honeycomb lattice. A bilayer built out of two such graphene sheets is a promising candidate for improving the efficiency of batteries. Like its 3D counterpart graphite, bilayer graphene can be used as a host for lithium ions in battery electrodes. But, compared to graphite, it has a distinct advantage, as has been shown by the first-principles simulations carried out by Krasheninnikov’s group on PRACE supercomputers.

The power of multiple layers

The only input these first-principles simulations need, is the information on the chemical elements that make up the material. Based on the principles of quantum mechanics, the simulation calculates the forces acting on the atoms. It predicts the energetically favoured structure, its properties and the behaviour of the electrons within the structure. These kinds of simulations are run on supercomputers and can deal with structures composed of several thousand atoms.

For the analysis of graphene, Krasheninnikov teamed up with specialists in experimental investigation of 2D materials. The scientists had previously conducted measurements using transmission electron microscopy — a method that allows one to observe how certain ions, lithium for example, move in and out of a 2D sample in atomic resolution. It was also well known that lithium ions intercalate as a single layer in a conventional 3D-graphite host. That’s how lithium-ion batteries work today: Lithium ions move back and forth between two graphite electrodes and intercalate into the electrode material, thereby transferring free electrons to create an electrical current. Surprisingly though, while testing and analysing the bilayer graphene, the experimental results pointed to a much denser structure and a higher lithium uptake.

The illustration shows multiple layers of lithium ions intercalated into two atom-thin sheets of graphene.

Not one, but multiple layers of lithium ions (green) can be placed between two atom-thin sheets of graphene, according to Krasheninnikov’s simulations. This makes the 2D material a promising candidate to boost the efficiency of future batteries.
Image credit: M. Ghorbani-Asl, HZDR

Krasheninnikov’s simulations helped identify the responsible lithium-graphene structure and revealed that two stacked graphene sheets can host not one, but multiple layers of lithium ions between them. This geometry is energetically favoured according to the calculations and, at the same time, matched the results of the experimental data. “Simply put, while the available space in 3D material is essentially fixed, the 2D-sheets can accommodate more than one ion layer by moving apart and yielding space to the lithium ions”, explains Krasheninnikov.

Sodium as a candidate to replace lithium

The work published in Nature has strong implications in battery technology, as the efficiency of today’s batteries is limited by the amount of lithium ions that the electrode material can store. In contrast, the intercalation of multiple ion layers in a 2D material adds up to a higher energy density and, consequently, to more efficient batteries.

Krasheninnikov’s team went on to analyse the intercalation of other, similar ions into two graphene sheets and found that sodium also possesses the ability to intercalate in multiple layers. This is significant, because it supports the idea that sodium could one day replace the currently omnipresent lithium. Even though sodium possesses a lower energy density than its rival, it provides significant advantages over the rare and expensive lithium: It is readily available in vast amounts in the environment and can be harvested in a cost-effective manner.

Investigating possible superconductors

Krasheninnikov’s team is also investigating potential superconductors. A superconductor’s electrical resistance disappears at temperatures near the absolute zero — a valuable property that among else is used in the most powerful existing electromagnets. In 2018, in a publication in Nature from scientists at the MIT reported that, while rotating two stacked graphene sheets in respect to one another, a sort of magical angle was found where the material suddenly became superconductive.

Krasheninnikov’s team is also studying another 2D-material class for their potential as superconductors, the so-called chalcogenides. “Even more possibilities arise when combining different 2D materials like graphene and a chalcogenide like molybdenum disulfide (MoS2) into a bilayer structure and then dote this material with different intercalating ions”, says Krasheninnikov. “This opens up the possibility to create a multitude of new materials with tailored properties.”

Creating smart defects

In particular though, Krasheninnikov is interested in material defects. Such defects can consist of a vacancy — a missing atom in otherwise flawless structure — or an impurity, where an atom is replaced with an atom of another element. Impurities can deliberately be introduced into a semiconductor structure and they lie at the heart of present-day semiconductor technology. They govern many crucial properties such as how well electrons can travel through a material. That’s why Krasheninnikov and his co-workers investigate the creation and behaviour of such defects in 2D materials and the respective differences compared to 3D materials.

The illustation shows the evolvement of the excitation energy from a passing electron in the molecular structure of molybdenum disulfide.

A peak into the molecular structure of a 2D molybdenum disulfide (MoS2) sheet reveals the effect of a passing electron: From the initial stage of the excitation after 0.02 femtoseconds (a millionth of a billionth of a second), the electronic excitation shown in red and blue spreads through the structure extremely swiftly, within just 1.6 femtoseconds.
Image credit: S. Kretschmer et al. Nano Lett. (2020)

Again using PRACE resources and collaborating with experimentalists, the team recently simulated the irradiation of 2D materials with electron beams, which is a way to introduce defects. Their first-principles simulations of the chalcogenide MoS2 helped the scientists understand the mechanisms of defect creation. Through the process, they saw how the excitation energy from an incoming high-energy electron evolves spatially within the 2D material and how quickly it spreads over the whole system. The simulations also showed how these electronic excitations assist a mechanism of defect creation: the so-called knock-on damage mechanism, in which an electron acts as a sort of high-energy bullet shooting an atom out of the structure.

Most cited researcher

Whether it’s about storage of ions in 2D materials or the investigation of defects, Krasheninnikov’s work has been widely recognised: He was named on the list of the most highly cited researchers in physics 2020 by the analytics company Clarivate. Even so, when asked about his enormous success, Krasheninnikov remained humble.

“The trick is that I am a physicist who closely works together with chemists and material scientists. Their communities are large, and this helps to be cited a lot”, he says, adding, “I don’t feel that I am a great scientist, to be honest.”

For once, he is probably wrong.

Project title:
17th call: Supported two-dimensional transition metal dichalcogenides under ion irradiation
14th call: Two-dimensional inorganic materials under electron beam: insights from advanced first-principles calculations

Resources awarded:
17th call: 37.1 million core hours on Hazel Hen hosted by GCS at HLRS, Germany
14th call: 16 million core hours on Hazel Hen hosted by GCS at HLRS, Germany

Research field: Chemical Sciences & Materials

Selcted References:

Kretschmer S., Lehnert T., Kaiser U. and Krasheninnikov A.V. Formation of Defects in Two-Dimensional MoS2 in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations, Nano Lett. (2020)
https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c00670

Chepkasov I.V., Ghorbani-Asl M., Popov Z.I., Smet J.H. and Krasheninnikov A.V. Alkali metals inside bi-layer graphene and MoS2: Insights from first-principles calculations, Nano Energy (2020)
https://www.sciencedirect.com/science/article/pii/S2211285520304845

Joseph T., Ghorbani-Asl M., Kvashnin A.G., Larionov K.V., Popov Z.I., Sorokin P.B. and Krasheninnikov A.V. Nonstoichiometric Phases of Two-Dimensional Transition-Metal Dichalcogenides: From Chalcogen Vacancies to Pure Metal Membranes, J. Phys. Chem. Lett. (2019)
https://pubs.acs.org/doi/10.1021/acs.jpclett.9b02529

Kühne M., Börrnert F., Fecher S., Ghorbani-Asl M., Biskupek J., Samuelis D., Krasheninnikov A.V., Kaiser U. and Smet J.H. Reversible superdense ordering of lithium between two graphene sheets, Nature (2018)
https://www.nature.com/articles/s41586-018-0754-2

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