Featured paper: Controlling the electronic properties of graphene on silicon carbide.


[Paper: Graphene on the oxidized SiC surface and the impact of the metal intercalation. J.E.Padilha, R.B.Pontes, F. Crasto de Lima, R. Kagimura, R. H. Miwa. Carbon, Volume 145, April 2019, Pages 603-613.]

Controlling the electronic properties of graphene on silicon carbide

A Brazilian scientific team performed a study based on supercomputer simulations that reveals a way to overcome the challenge of controlling the electronic properties of graphene. Solutions to this challenge can make a difference in the development of two-dimensional electronic devices – a dimension in which graphene, the one-atom thick network of carbon atoms, stands out for its properties.

In fact, graphene is an extremely tough, lightweight, flexible and transparent material. It is also an excellent conductor of heat and electricity. However, it is still difficult to control the concentration and flow of electric charges in graphene, which limits its use in electronics.

Solutions have been proposed to overcome this technological limitation of graphene. Some of them are based on the insertion of small amounts of metallic atoms that modulate the electronic properties of the material without impairing the other characteristics. The method is similar to the doping of silicon, practiced routinely in the manufacture of semiconductors for the electronics industry.

Representation of the studied system: graphene sheet on substrate of oxidized silicon carbide with layer of intercalated metallic atoms (in this case, gold).
Representation of the studied system: graphene sheet on substrate of oxidized silicon carbide with layer of intercalated metallic atoms (in this case, gold).

In the study that was recently reported in the scientific journal Carbon (impact factor 7,466), the Brazilian team investigated the structure and electronic properties of a graphene sheet on a silicon carbide (SiC) substrate – material often used to deposit or grow graphene. In this system, graphene remains attached to the substrate without chemical bonds, by means of distance-dependent weak attraction forces, called Van der Waals forces.

Given that in the production of graphene the presence of oxygen usually oxidizes the surface of the silicon carbide, the Brazilian scientists included in the simulations a layer of silicon oxide between the graphene and the substrate. Finally, in order to understand in detail the effect of the insertion of metallic atoms into this type of materials, the scientists added to the simulated system a layer of gold or aluminum atoms embedded in the oxide layer (in this case, Si2O5) in the interface region with graphene.

The researchers verified that the presence of the metallic layer modulates the concentration of the positive (so-called holes) and negative (the electrons) charge carriers in both the graphene sheet and Si2O5. In addition, the gold and aluminum atoms embedded in the Si2O5, which is semiconductor, induce the formation of conducting regions on the surface of this layer, in which the excess of electrons or holes is concentrated, induced by the presence of gold or aluminum, respectively. As a result, conductive channels are formed on the surface of the Si2O5, through which the charges flow.

This two-dimensional map series shows the concentrations of electrons and holes in the graphene sheet in the two graphene systems on the silicon carbide surface finished in Si [(a) and (c)] and terminated in C [(b) and ( d)]; in the presence of an aluminum monolayer [(a) and (b)] and the other containing a gold layer [(c) and (d)].
This two-dimensional map series shows the concentrations of electrons and holes in the graphene sheet in the two graphene systems on the silicon carbide surface finished in Si [(a) and (c)] and terminated in C [(b) and ( d)]; in the presence of an aluminum monolayer [(a) and (b)] and the other containing a gold layer [(c) and (d)].
Finally, the team verified that the “doping” effect (the change in the concentration of electrons and holes) can be enhanced by the application of an external electric field, perpendicular to the interface between the graphene and the substrate.

Based on these evidences, which were obtained mainly through computational simulations based on the Density Functional Theory, the paper suggests a way to control the concentration and flow of electrical charges on graphene sheets on silicon carbide substrates. The study also shows that the system studied (graphene sheet on oxidized silicon carbide with intercalated metal layer) can be a good platform for engineering electronic properties.

“The main contribution of the study is to show an efficient way of controlling the electronic properties of graphene on a solid surface covered with a metallic layer, by applying an external electric field,” says Professor Roberto Hiroki Miwa (Federal University of Uberlândia, UFU ), corresponding author of the paper. “We show that in addition to controlling the doping level of graphene, which is fundamental for the development of electronic devices in two-dimensional (2D) systems, the presence of the metallic monolayer allows the formation of conducting channels on the surface of the silicon carbide,” he adds. According to Miwa, the study may contribute to the development of faster, more accurate sensors, transistors and other electronic devices for charge transport and signal delivery.

At the beginning the work was motivated by the interest of UFU professors Roberto Hiroki Miwa and Ricardo Kagimura in understanding the graphene/oxide interfaces at the atomic level. The focus of the study matured as the authors delved into the scientific literature. As the volume and complexity of calculations increased, the researchers included new collaborators: a physics doctoral student at UFU (Felipe David Crasto de Lima) and professors from other institutions (José Eduardo Padilha de Sousa, from the Federal University of Paraná – Jandaia do Sul campus, and Renato Borges Pontes, Federal University of Goiás).

The authors of the paper. From the left: J. E. Padilha, R. B. Pontes, F. Crasto de Lima, R. Kagimura, R. H. Miwa.
The authors of the paper. From the left: J. E. Padilha, R. B. Pontes, F. Crasto de Lima, R. Kagimura, R. H. Miwa.

In order to perform the calculations that support the simulations, the authors used computational resources from the Brazilian National Center for High Performance Processing (CENAPD) and the SDumont supercomputer of the Brazilian National Laboratory of Scientific Computation (LNCC). The work was funded by federal agencies CNPq and CAPES and the state agency FAPEMIG (Minas Gerais).

Featured paper: Virtues of a bad metal.


[Paper: Electronic localization and bad-metallicity in pure and electron-doped troilite: A local-density-approximation plus dynamical-mean-field-theory study of FeS for lithium-ion batteries. Craco, L; Faria, JLB. J. Appl. Phys. 119, 085107 (2016); http://dx.doi.org/10.1063/1.4942843]

Virtues of a bad metal

Computer image of the troilite crystal structure (FeS) with lithium ion insertion. The image, produced by Professor Jorge Faria, began with the modeling of pure troilite. Subsequently, numerical analyses were carried out by local density approximation (LDA) using methods based on the functional density theory (DFT) to obtain the network parameters with different concentrations of lithium and by observing its most stable position in the unit cell.

Because of their numerous advantages, rechargeable lithium-ion batteries are the most commonly used to power portable electronic devices (smartphones, tablets, laptops…). In addition, these batteries have great potential to be used in electric cars and in other applications.

Motivated by the potential application of iron sulfides (FeS) to be used as electrodes for next generation lithium-ion rechargeable batteries, Luis Craco, professor at the Institute of Physics of the Brazilian Federal University of Mato Grosso (IF-UFMT), undertook, along with his colleague, Jorge Luiz Brito de Faria, a theoretical study on the behavior of troilite (a phase of iron sulfide that is an insulator at ambient pressure and temperature) doped with lithium ions.

In the study, Craco and Faria sought to understand what happened in troilite after the electronic doping – a procedure that can transform an insulator into semiconductor or bad metal by inserting atoms (lithium ions) causing a structural reorganization of the material, by introducing electrons into it.

High-Performance Computing Cluster of IF-UFMT: the calculation time can be shortened using parallel processing.

The work began with a series of calculations by first-principles based on the density functional theory (DFT) performed by Jorge Faria. These calculations use crystal structure data obtained experimentally. Next, Luis Craco carried out a detailed study using calculations based on dynamical mean-field theory (DMFT), to study the effect of electronic correlations between electrons in different orbitals (regions around the nucleus of an atom in which an electron has some probability of being found). In these correlations, a change experienced by an electron in an orbital causes a related change in another electron from another orbital. Correlated electrons act coordinately, although they are spatially separated. According to Luis Craco, “We should bear in mind that the theoretical description introduced in this work is entirely new in the context of troilite and its derivatives, as well as in other compounds containing iron and sulfur as constituent elements”.

In a recently published paper in the Journal of Applied Physics, the professors from UFMT reported a description of the electronic and transport properties of the doped troilite and showed that the material exhibits unconventional behavior. In fact, although the iron sulfide is an insulator even with high concentrations of lithium, their computer simulations showed the emergence of metallic states after high electron doping. Near this insulator-metal transition state, the material can be classified as a Mott insulator. Furthermore, the authors found that the metal states emerged only in certain atomic orbitals, which is the behavior of a bad metal; in other words, a different behavior from that expected from a metal within consolidated theories in Physics.

Being a bad metal, however, does not imply being banned from the overall applications. On the contrary, according to the article, the inconsistent behavior of electrons in the doped iron sulfide can be used to achieve unconventional optical and transport effects within room temperature and pressure.

“This work is a continuing effort involving many researchers in Brazil and abroad, which aims to clearly demonstrate that systems with correlated electrons represent an important class of materials for various technological applications”, said Professor Craco.

“Now we hope the scientific community, related to the physics of correlated electron systems and / or materials physics, for example, becomes aware of our study and results, and that in the near future corroborates our theoretical description of the electronic properties and unconventional transport in electron-doped troilite, thereby consolidating the relevance of our study for future applications of troilite and its derivatives in renewable energy storage or to generate new unconventional non-Fermi liquid electronic phases, with great contemporary scientific and technological appeal”, concludes Craco.

The research was funded by the Brazilian National Council for Scientific and Technological Development (CNPq).