Featured paper: Defect-free doped graphene for use in electronic devices.


Graphene-based products are already being used by manufacturers, from heat-dissipating helmets to antistatic packaging. However, this wonderful material, as it is often called, still has much to deliver to society. As it is two-dimensional, flexible and excellent conductor of electricity, among other properties, graphene can be the basis of a series of high-performance miniaturized electronic and optoelectronic devices. However, this requires producing, at an industrial scale, a graphene whose network of atoms is free of unwanted impurities, but which contains, besides the carbon inherent in the graphene, small amounts of other elements (doping) in order to control its electronic properties.

In a work totally carried out in Brazil, a scientific team has proposed a process that can help produce large-scale graphene that is suitable for electronic devices. “The process developed in our group allows us to improve and adjust the graphene properties, as well as the removal of contaminants from its surface,” said Professor Claudio Radtke (UFRGS), corresponding author of an article reporting the study, recently published in The Journal of Physical Chemistry C.

The authors of the paper, from the left: Henri Boudinov, Cláudio Radtke Gabriel Vieira Soares (all UFRGS Professors) and Guilherme Koszeniewski Rolim (postdoc at the graduate program on microelectronics at UFRGS).
The authors of the paper, from the left: Henri Boudinov, Cláudio Radtke Gabriel Vieira Soares (all UFRGS Professors) and Guilherme Koszeniewski Rolim (postdoc at the graduate program on microelectronics at UFRGS).

The team acquired graphene samples produced by chemical vapor deposition (CVD) and transferred to silicon substrates. This technique is currently one of the most suitable for large-scale production of relatively large area graphene sheets, but it leaves residual impurities and generates defects in the graphene. To remove impurities, it is common to apply a heat treatment in an atmosphere of carbon dioxide (CO2), which is efficient in removing contaminants, but ends up generating new defects in the graphene sheet. The good news is that these defects can be neutralized (passivated).

While looking for strategies to passivate these defects, then PhD student Guilherme Koszeniewski Rolim found a scientific paper from 2011, which pointed to, through theoretical calculations, the possibility of using nitric oxide (NO) to passivate graphene defects with nitrogen atoms, while doping it to modulate its electronic properties (mainly transforming it into a semiconductor material, an essential condition for using graphene in electronic devices).

The team then decided to experimentally verify the theoretical prediction and, after performing the traditional treatment with CO2 at 500 °C, they applied a second heat treatment to the samples, this one in nitric oxide atmosphere and at different temperatures, from room temperature to 600 °C.

After the process, the researchers used various characterization techniques to check the results and gladly confirmed that nitrogen doping had taken place and that it had passivated the defects, thus improving the material’s electronic properties. However, the researchers also noted an unwanted effect of nitric oxide treatment: etching of graphene sheets at some points. After much scientific work, the team was able to determine the cause. During heating, there was a conversion of NO to NO2, which, as it is a much more reactive compound than the former, eventually oxidized the graphene.

However, the Brazilian team found a solution to this problem. The “eureka” moment occurred as the researchers were trying to determine the amount of nitrogen atoms that had been incorporated into graphene using a technique based on the analysis of nuclear reactions triggered by the effect of an ion beam on the graphene samples. In order to apply this technique, the team had to use an isotopically enriched nitric oxide in the heat treatment, which has a purity of 99.9999% instead of 99.9% of the gas previously used.

Illustrative scheme of the parameters to be controlled in the process proposed by the Brazilian team. Balancing gas purity and temperature ensures better graphene sheets for use in electronic devices.
Illustrative scheme of the parameters to be controlled in the process proposed by the Brazilian team. Balancing gas purity and temperature ensures better graphene sheets for use in electronic devices.

The analysis did not yield the expected results as it failed to quantify nitrogen, which was below the detection limit. However, the use of the enriched gas eventually brought great satisfaction to the team. Indeed, when the researchers compared the electronic properties of both sample types, they found that graphene treated with enriched gas always had superior properties. “Initially, such a result created much confusion in the interpretation of the results,” says Professor Radtke. “But after a few more experiments, it became one of the most important points of the article, highlighting the importance of gas purity during processing,” he adds. Specifically, the conclusion was that by properly controlling the temperature and purity of the gas during the treatment one can eliminate the problem of oxidative graphene degradation.

Thus, based on solid knowledge and scientific method, as well as some serendipity, the UFRGS team was able to develop a process of waste removal, defect neutralization and graphene doping, which improved the electronic properties of the material without producing deleterious side effects. Because it is a heat treatment in a gas atmosphere, a step that is now part of the industrial production of graphene, the process proposed by the Brazilian team could be easily applied in the production of graphene sheets for devices.

“The insertion of heteroatoms (such as nitrogen) into the graphene network without the degradation of its properties is especially important in the production of optoelectronic devices, high speed transistors, low power electronics and photovoltaic cells,” says Radtke, noting that manufacturing these graphene-based devices may be a reality in years to come. “The Graphene Flagship (European consortium of industries, universities and institutes) has announced the implementation of a pilot plant to integrate graphene at different production stages of devices as early as 2020,” comments the professor from UFRGS.

The study, which was funded by the Brazilian agencies CNPQ (mainly through INCTsNamitec and INES), Capes and Fapergs, was developed within the PhD in Microelectronics by Guilherme Koszeniewski Rolim, held at the UFRGS Graduate Program in Microelectronics and defended in 2018. The experimental work was carried out at the UFRGS Solid Surface and Interfaces Laboratory and the Brazilian National Synchrotron Light Laboratory.

[Paper: Chemical Doping and Etching of Graphene: Tuning the Effects of NO Annealing. G. K. Rolim, G. V. Soares, H. I. Boudinov, and C. Radtke. J. Phys. Chem. C,  2019, 123, 43, 26577-26582. https://doi.org/10.1021/acs.jpcc.9b02214.]

Featured paper: Nanorods to develop new anti-inflammatory drugs.


[Paper: Characterization of the structural, optical, photocatalytic and in vitro and in vivo anti-inflammatory properties of Mn2+ doped Zn2GeO4 nanorods. Suzuki, V. Y.; Amorin, L. H. C; Lima, N. M; Machado, E. G; Carvalho, P. E.; Castro, S. B. R.; Souza Alves, C. C.; Carli, A. P.; Li, Maximo Siu; Longo, Elson; Felipe La Porta. J. Mater. Chem. C, 2019, 7, 8216. DOI: 10.1039/c9tc01189g]

nanobastoesA team of researchers from Brazilian universities found, in cylindrical nanostructures known as nanorods, an anti-inflammatory effect equivalent to that achieved by commercial drugs. Researchers have also demonstrated the effectiveness of these nanorods as catalysts (accelerators) in the degradation of a pollutant. These applications are even more relevant considering that the scientific team was able to produce large quantities of the material through a simple and fast process. The work carried out shows the potential of these nanorods for the development of new medicines and for the treatment of effluents.

The work originated about three years ago when Professor Felipe de Almeida La Porta, who had recently joined the faculty of the Federal Technological University of Paraná (UTFPR), Londrina campus, was implementing a research group on nanotechnology and computational chemistry at this university. “Our laboratory was investigating some classes of emerging materials, with the perspective of aligning theory and practice, thus driving new discoveries and applications,” says La Porta. One of the materials studied by the group was zinc germanate (Zn2GeO4), a versatile semiconductor with well-known applications in sensors, catalysts, batteries and other devices.

Together with undergraduate researcher Victor Yuudi Suzuki, the professor started a project in which he synthesized pure Zn2GeO4 nanorods at the UTFPR laboratory with very small percentages of manganese ions. To produce this series of nanorods, they used “microwave assisted hydrothermal synthesis.” The method consists, in broad lines, of mixing aqueous solutions containing certain compounds, heating the final solution in a microwave oven and allowing the compounds to react for a certain period of time at controlled pressure and temperature. In this study, the manganese ion-doped Zn2GeO4 was prepared, and the reactions were performed at 140 °C for 10 minutes. The resulting material from these reactions was collected at room temperature, then washed and dried, which generate the nanorods.

Professor La Porta and his research group were able to optimize one of the process steps, the crystallization of materials, thus reducing the synthesis time from hours to a few minutes, but maintaining the quality of the material and the possibility to control its shape.

After preparing the samples, they traveled from Londrina (state of Paraná) to São Carlos (São Paulo state) to characterize the materials at the Center for Functional Materials Development (CDMF) at the Federal University of São Carlos (UFSCar) and at the Institute of Physics at the University of São Paulo (USP). Together with the local researchers, they were able to analyze the shape, structure and luminescence of the four types of nanorod compositions produced: manganese-free and with 1, 2 and 4% of this element incorporated into the structure of Zn2GeO4.

Finally, knowing that compounds containing zinc, germanium or manganese exhibit considerable effects on living things, the team contacted some collaborators to investigate these properties in the nanorods. Thus, several experiments were performed at the Departments of Chemistry and Pharmacy of the Federal University of Juiz de Fora and at the Federal University of Vales do Jequitinhonha and Mucuri, both in the state of Minas Gerais.

The authors of the paper. From the left: Victor Suzuki, Luís Amorin, Felipe La Porta, Maximo Si Li, Elson Longo, Sandra de Castro, Paloma de Carvalho, Alessandra Carli, Emanuelle Machado, Caio Alvez, Nerilson Lima.
The authors of the paper. From the left: Victor Suzuki, Luís Amorin, Felipe La Porta, Maximo Si Li, Elson Longo, Sandra de Castro, Paloma de Carvalho, Alessandra Carli, Emanuelle Machado, Caio Alvez, Nerilson Lima.

To study the anti-inflammatory action, the team performed in vitro tests (in contact with cells in laboratory containers) and also in vivo tests (using rats with paw edema, within the norms of the Brazilian code for laboratory animal use). Both types of experiments revealed that nanorods with about 4% manganese were the most effective in controlling inflammation. The in vitro tests showed these nanostructures were able to modulate molecules that regulate inflammation without causing cell death (without cytotoxicity). In the in vivo experiments, the nanorods reduced the induced rat paw edema with results similar to that of the application of dexamethasone, a well-known drug of the corticoid group.

“At first, we thought that combining these elements to form a ternary oxide could somehow potentiate these effects. But we had no idea the results would be so significant. Given that the drugs currently available in therapy are proving to be less effective every day, these results may encourage the use of these nanorods, for example in the production of a new pharmaceutical formulation, especially for cases of inflammation,” says Felipe La Porta, who is the corresponding author of the paper that was recently published by the research team in the Journal of Materials Chemistry C (impact factor 6,641).

In addition to proving the potential of the material for this application in the health area, the authors of the paper have experimentally verified the ability of nanorods to degrade a chemical dye widely found in industrial effluents, known as methylene blue. For this application, 2% manganese nanostructures were the most efficient, completely decomposing the dye in 10 minutes. “Due to the manufacture simplicity of this system, coupled with its excellent properties, this material is also promising for cleaning various environmental pollutants, and can be easily recovered at the end of this process,” adds Prof La Porta.

In the center, a cluster of 4% manganese zinc germanate nanorods. Clockwise: photoluminescence measurements of the samples; representation of the structure of manganese-doped zinc germanate; pollutant degradation mechanism and methylene blue degradation measures; anti-inflammatory action of nanorods and other treatments in induced-edema rat paw.
In the center, a cluster of 4% manganese zinc germanate nanorods. Clockwise: photoluminescence measurements of the samples; representation of the structure of manganese-doped zinc germanate; pollutant degradation mechanism and methylene blue degradation measures; anti-inflammatory action of nanorods and other treatments in induced-edema rat paw.

The superior properties that the Brazilian scientific team found in the nanorods with manganese can be related to the structural defects observed in these samples. In fact, the three-dimensional network of atoms that forms zinc germanate is crystalline, that is, organized in regular patterns. The introduction of manganese generates irregularities, and new properties emerge.

The scientific paper that reports this work was selected to be part of the Materials and Nano Research in Brazil collection, prepared by the Royal Society of Chemistry in celebration of the 18th B-MRS Meeting, and can therefore be accessed free of charge until October 15 of this year, here.

The work was carried out with funding from Brazilian research support agencies: the federal CNPq and Capes, and the state Araucaria Foundation, Fapesp and Fapemig.