Featured article: New natural insulator for ultrathin devices.



A team of researchers from Brazilian institutions took the first steps towards the use of phlogopite, a mineral abundant in Brazil, as a natural source of ultrathin insulating layers that could be used in optoelectronic devices of the future. “Our work presents the insertion of this natural mineral in materials research and its possible use in different areas of nanotechnology”, says scientist Alisson R. Cadore, corresponding author of the article recently published in the journal 2D Materials.

The work is part of the search for two-dimensional materials (one or a few atoms thick) at low cost. In this search, the scientific community has investigated the so-called “lamellar materials”, which are formed by stacked ultrathin layers, joined together by the so-called “van der Waals forces”. Because they are relatively weak, these physical forces allow the lamellae separation to be performed using several methods. One of the best-known examples of this group is graphite, which is the source of graphene. Another example is that of micas, a family of minerals that includes phlogopite, which had never been studied in its two-dimensional form before the work of the Brazilian team.

In this work, the researchers used macroscopic phlogopite crystals mined in Itabira (state of Minas Gerais, Brazil) to generate two-dimensional layers by mechanical exfoliation. This extremely simple technique gained fame for having led to the first successful production of graphene, which earned Andre Geim and Konstantin Novoselov the Nobel Prize in Physics in 2010. In this method, a common adhesive tape is used to separate flakes from the material, by sticking them to the tape. The procedure is repeated until reaching the thinnest possible layer, the monolayer.

Using this method, the authors of the paper obtained monolayers and few-layers flakes of phlogopite. With them, they carried out a robust characterization that involved several experimental techniques in laboratories at Universidade Presbiteriana Mackenzie (UPM), at the National Laboratory of Synchrotron Light (LNLS) and at the Federal University of Minas Gerais (UFMG). Some results were confirmed with computer simulations carried out by the authors of the Federal University of Lavras (UFLA).

Natural macroscopic crystal of phlogopite; optical microscopy image of phlogopite flakes exfoliated with different thicknesses; representation of the crystal structure of a phlogopite monolayer calculated by computer simulations.
Natural macroscopic crystal of phlogopite; optical microscopy image of phlogopite flakes exfoliated with different thicknesses; representation of the crystal structure of a phlogopite monolayer calculated by computer simulations.

“In this work, we theoretically and experimentally identified the chemical and structural composition of phlogopite and demonstrated that this natural and abundant insulator can be exfoliated to the limit of a single layer, maintaining its physical characteristics”, summarizes Cadore, who is currently a researcher at the National Laboratory of Nanotechnology (LNNano), but carried out the work at UPM, where he was a Professor. One of the main findings of the study was the fact that the ultrathin phlogopite is stable when subjected to thermal processing, as well as when exposed to the environment (the samples were left for 13 months in ambient conditions without showing degradation).

In addition, the work compared the characteristics of phlogopite with those of the synthetic insulator most used as a substrate in nanodevices, hexagonal boron nitride. For this, the Brazilian team developed a collaboration with the National Institute of Materials Science (NIMS, Japan), where the material with the best physical properties is produced. The Japanese group supplied the synthetic crystals.

According to the authors, given that two-dimensional phlogopite is an excellent electrical and thermal insulator, in addition to being naturally abundant and easy to extract, it can be used as a low-cost material in devices that are still little explored at the nano scale, such as transistors, capacitors and photodetectors. These applications become even more promising given the possibility of using two-dimensional phlogopite in structures with unique properties called “van der Waals heterostructures”, which are formed by stacking ultrathin layers of different materials, joined by van der Waals forces. Therefore, the authors of the article assembled phlogopite and tungsten disulfide heterostructures and studied some of their properties. “We emphasize that two-dimensional phlogopite is a stable insulator under ambient conditions and can be easily combined with other 2D materials, creating ultrathin hybrid heterostructures, which expands the application of this nanomaterial in new future optoelectronic devices”, says Cadore.

Optical microscopy image of a van der Waals heterostructure containing a monolayer of tungsten disulfide over a few layers of phlogopite. Analysis of optical properties of 2D heterostructures.
Optical microscopy image of a van der Waals heterostructure containing a monolayer of tungsten disulfide over a few layers of phlogopite. Analysis of optical properties of 2D heterostructures.

Carried out as part of Raphaela de Oliveira’s doctorate, the work on the two-dimensional phlogopite is part of a research line begun in the Physics Department at UFMG. “Our studies have always involved different national and international researchers in obtaining and characterizing different natural 2D materials and their application in nanodevices and nanophotonics”, says Cadore, whose PhD in Physics at UFMG was on two-dimensional graphene heterostructures. The objective of these works is to find materials with the ideal characteristics for these applications, in order to replace the synthetic materials, which are expensive.

The work developed with phlogopite was inspired by studies carried out with soapstone, started at UFMG in 2015. Studies with this mineral continued at LNLS – CNPEM conducted by researcher Ingrid Barcelos, who also completed her PhD in Physics at UFMG with research on van der Waals heterostructures. In 2021, Ingrid won, for her work with soapstone, one of the For Women in Science awards, granted by L’Oréal Brasil in partnership with UNESCO and the Brazilian Academy of Sciences (ABC).

Some of the main authors of the article. From left: Alisson R. Cadore, Raphaela de Oliveira, Raphael Longuinhos, Ingrid D. Barcelos and Christiano J. S. de Matos.
Some of the main authors of the article. From left: Alisson R. Cadore, Raphaela de Oliveira, Raphael Longuinhos, Ingrid D. Barcelos and Christiano J. S. de Matos.

The research on two-dimensional phlogopite was funded by CAPES, CNPq, Mackenzie Research and Innovation Fund, FAPESP, FAPEMIG and the L’OREAL-UNESCO-ABC Award for Women in Science.


Paper reference: Exploring the structural and optoelectronic properties of natural insulating phlogopite in van der Waals heterostructures. Alisson R Cadore, Raphaela de Oliveira, Raphael Longuinhos, Verônica de C Teixeira, Danilo A Nagaoka, Vinicius T Alvarenga, Jenaina Ribeiro-Soares, Kenji Watanabe, Takashi Taniguchi, Roberto M Paniago, Angelo Malachias, Klaus Krambrock, Ingrid D Barcelos and Christiano JS de Matos. 2022 2D Mater. 9 035007. https://doi.org/10.1088/2053-1583/ac6cf4.

Corresponding author contact: alissoncadore@gmail.com


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