Tag: UFRGS

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

Posted on Saturday 30 de November de 2019

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.]

People from our community: interview with João Alziro Herz da Jornada.

Posted on Friday 30 de June de 2017

joaojornada (1)João Alziro Herz da Jornada was born on June 1, 1949 in São Borja (Rio Grande do Sul State, Brazil). Between 1968 and 1971, he studied Physics at the Federal University of Rio Grande do Sul (UFRGS), in the city of Porto Alegre. Shortly after receiving his bachelor’s degree, he started his master’s degree in Physics, also at UFRGS, which he completed in 1973. His master’s dissertation focused on one of the subjects which he would pursue throughout his scientific career, the effect of high pressures on materials.

In August of 1974, he assumed the position of assistant professor of the Physics Institute of UFRGS. From 1977 to 1979 he did a PhD in Science at UFRGS, where he developed new research on the effects of high pressures on materials, guided by Professor Fernando Claudio Zawislak. His doctoral thesis received praise from UFRGS. In 1983 and 1984, he carried out his postdoctoral studies at the National Institute of Standards and Technology (NIST), an institute dedicated to promoting innovation and industrial competitiveness through metrology, science and technology in the United States. In April 1985, he became a full professor at the Physics Institute of UFRGS, a position he held until his retirement in February 2016. Since then, he has been a guest contributor at this institution. Throughout his academic career at UFRGS, he held several management positions, including president of the university’s research chamber and coordinator of post-graduate programs at the Institute of Physics. Professor Jornada also created and coordinated the Laboratory of High Pressures and Advanced Materials of IF-UFRGS.

From 1993 to 2000, Jornada was the coordinator of the executive committee of the Rio Grande do Sul Metrology Network Association (RS Metrology Network), an entity created in 1992, acting in qualified metrology.

From 2000 to 2004, Jornada was director of scientific and industrial metrology at the National Institute of Metrology, Quality and Technology (Inmetro), a federal agency linked to the Ministry of Industry, created in 1973, whose mission is to strengthen national companies, increasing their productivity by adopting mechanisms aimed at improving the quality of products and services.

In December 2004, Professor Jornada assumed the presidency of Inmetro, remaining in the position for 11 years, until December 2015. During his mandate, Jornada promoted changes in the strategy, training, infrastructure and management of Inmetro, which led the institution to increase its national and international scientific recognition and to develop interactions with academia, companies and government.

Jornada received a series of honors, mainly from the Rio Grande do Sul Research Foundation (FAPERGS), from the Presidency of the Republic, Brazilian Air Force, Ministry of Foreign Affairs and Brazilian Navy. He has been a member of the Brazilian Academy of Sciences since 2001, and a fellow of TWAS (The World Academy of Sciences for the advancement of science in developing countries) since 2008. Since 2016, he has been a distinguished fellow of the Global Federation of Competitiveness Councils, a network of individuals and organizations involved in competitiveness strategies, based in Washington (USA).

The scientist is the author of about 100 papers published in scientific journals, including Science and Nature.

SBPMat Bulletin: Tell us what led you to become a scientist and, in particular, to work in the area of Condensed Matter Physics.

João A. Herz da Jornada: I had a great interest in science from a very early age. The environment in the late 1950s and early 1960s, during my childhood and adolescence, was especially stimulating for the scientific career, especially Physics.  There was so much emphasis in the press on topics that fascinated me, such as rockets, sputnik, space race, nuclear power, transistor, computers… It was a time when the world saw Science with extreme optimism and confidence, truly the “endless frontier”, in the words of Vannevar Bush. Science represented certainties, providing the sure way to answer all questions, large and small, a true, complete and unified worldview – perhaps the apex of the Enlightenment ideology. All this fascinated me. I have always enjoyed reading, learning, experimenting and building things involving Physics, Chemistry and Electronics, enjoying the pleasure of discovery and accomplishment. Therefore, following a scientific career was very natural. I graduated in Physics and did a masters and PhD in Experimental Physics, applying techniques of Nuclear Physics to problems of Condensed Matter Physics, under the guidance of Fernando Zawislak. At that time Condensed Matter Physics was emerging dynamically, there were plenty of interesting problems to tackle and also relevant demands for applications in various areas. My PhD work involved designing and building very high pressure chambers, requiring deeper knowledge about some materials properties; so I began to take interest beyond Condensed Matter Physics, entering into Materials Science. Moreover, I was enthusiastic about the potentialities of the Condensed Matter Physics technique, because it allows considerable and controllable variations of interatomic distances, determinants of properties of solids, besides generating phase transformations. As there was no expertise at all in high pressure in Brazil, I decided to create a Laboratory to develop the technique, implement good experimental infrastructure and explore its possibilities as a new research instrument in our surroundings. In fact, we set up a good laboratory, with different types of systems for producing high pressure, designed and built right here, enabling high-temperature and in-situ measurements using various probing techniques such as optical spectroscopy and x-ray diffraction. We were then able to develop several lines of research in Condensed Matter Physics. I am using the plural to emphasize teamwork with a fantastic team of students and collaborators. The mastery of this technique further increased my interest in Materials Science because it offered a new window of opportunity for the production of new materials, especially superhard materials such as diamond and its composites. The production of synthetic diamonds in our Laboratory undeniably led us to Materials Science, with some very representative research lines, such as diamond synthesis by high pressures and by CVD, production of compacts and composites of high hardness materials, production of diamond cutting tools and cBN, etc. Subsequently we started work on ceramic materials, involving both basic research and applied research, in association with companies to produce structural ceramics.

But there is also a factor I believe has influenced my career choice: both Condensed Matter Physics and Materials Science offer tremendous possibilities for innovations and wealth generation for society, our society that despite the difficulties, supports and pays for our work. I have a sense of duty, shared by many of my generation, in order to effectively help our Country’s development.

SBPMat Bulletin: What do you believe are your main contributions to the Materials area? We would like to ask you to go beyond listing the results and briefly describe the contributions you consider as the most relevant or most outstanding. In your response, we ask that you consider all aspects of scientific activity. 

João A. Herz da Jornada: The answer is not easy, given the multiple dimensions of the question and the natural difficulty of speaking about one’s own deeds. I will comment briefly on some points. Firstly, the formation of people, in a varied spectrum of levels within the area of Materials: Doctors, Masters, undergraduate students and scientific initiation fellows. In fact, I believe the formation of quality human resources is the greatest contribution of basic research in a country like Brazil, still under development. I am very proud to have contributed to the scientific development of many people, in particular the many doctors I have helped who are now in important leadership positions. Another aspect that I consider relevant is with regard to the construction, together with dedicated students and collaborators, of the unique laboratory infrastructure in the area of high pressures and associated techniques, enabling many research works and also some that support the Industry. We implemented the high pressure technique in Brazil, building various types of equipment, and applied it in a wide range of scientific and technological works, including synthesizing diamonds and other advanced materials for the first time in the Country.

Like all Brazilian researchers, my scientific contributions, especially publications, are detailed in the Lattes Curriculum, but from a personal point of view I have been very pleased with some of the publications in high impact journals, such as Science, Nature, PRL and PR, which were the results of works entirely carried out in our Laboratory, with own ideas and with equipments largely constructed by us, often using scrap from old equipments. Another contribution to the Science of Materials was the creation of the Materials Laboratory at Inmetro, during my term as president of that institution. In addition to being an interesting scientific program and a very high-level team, the largest electronic microscopy infrastructure in the Southern Hemisphere was implemented, accessible to the entire scientific and technological community in the country. At UFRGS, I was one of the founders of the Postgraduate Program in Materials Science and of the Center for Microscopy and Microanalysis. I also highlight the construction of a network of international partnerships involving materials and high pressure studies.

SBPMat Bulletin: You will be honored at the XVI SBPMat/B-MRS Meeting with the “Joaquim da Costa Ribeiro” Memorial Lecture. Could you briefly comment on what you will discuss in this talk and/or leave an invitation to our readers.

João A. Herz da Jornada: I am honored by this recognition and invite the readers to the lecture; I will be very pleased to have the meaningful participation of our community. The theme will be the connection between Materials Science and Innovation, from a perspective not often discussed in Brazil, more specifically the complex mechanisms that generate economic and social impact from basic research.  I believe this theme is currently very relevant at a time of severe budgetary restrictions for Science in Brazil. It is important to have an in-depth understanding of the subject, using the same scientific approach we work with, based on evidence, good logic, rigor, critical thinking, open-mindedness and broad discussion. We will discuss the need to work with new concepts, such as the capacity for absorption, capacity for appropriation of knowledge and connectivity, to better understand the problem. We will see that Materials Science is a particularly important area, not only because the specific associated knowledge is very close to applications, but also because its multidisciplinary nature unavoidably involves a wide range of connections – one of the important factors of an innovative “ecosystem”.

SBPMat Bulletin: Please leave a message for the readers who are starting their scientific careers.

João A. Herz da Jornada: As a message to those who are beginning their career, I would like to suggest reflecting on a famous idea of the great Enlightenment philosopher, David Hume, who wrote this famous quote: “reason is, and ought only to be the slave of the passions”. What does it mean in the present context? Science is an essentially rational undertaking of the human spirit. It requires logic, intelligence, disciplined and rigorous work. But it also requires creativity, imagination, connection with people, dreams, and a lot of will power – primarily passion. Passion inspires us and mobilizes us for work, however, it is also nourished by the challenges and results of a beautiful work, and also nourished by the highly social and stimulating nature of the scientific environment. These two dimensions must also be recognized and properly cared for. Materials Science provides us with a huge range of beautiful challenges, constantly renewed by their own dynamics and by the demands for applications, which are always connecting us with society. It provides good chances of rewarding results, both scientific and technological. Its multidisciplinary nature, always requiring much interaction, gives us a rich and stimulating human experience.

Featured paper. Super efficient nanoparticles to catalyze production of hydrogen, an alternative fuel.

Posted on Friday 29 de July de 2016

[Paper: Hybrid tantalum oxide nanoparticles from the hydrolysis of imidazolium tantalate ionic liquids: efficient catalysts for hydrogen generation from ethanol/water solutions. Virgínia S. Souza, Jackson D. Scholten, Daniel E. Weibel, Dario Eberhardt, Daniel L. Baptista, Sérgio R. Teixeira and Jairton Dupont. J. Mater. Chem. A, 2016, 4, 7469-7475. DOI: 10.1039/C6TA02114J.]

Super efficient nanoparticles to catalyze production of hydrogen, an alternative fuel.

While some automobiles which use hydrogen fuel are entering the market, scientists from around the world are still trying to find cleaner, more sustainable, safer and cost-effective ways to generate and store hydrogen. In fact, even though it is the most abundant element in the universe and found in the water and in numerous other compounds, hydrogen cannot actually be found in its pure form on our planet. It must therefore be obtained from other chemical compounds.

One of the best methods to produce hydrogen, from ecological and economical points of view, is water splitting. This technique consists of separating water molecules into its two primary elements, generating hydrogen (H2) and oxygen (O2) gases. This separation can be achieved through the use of the abundant solar energy, at room temperature. However, in practice, for sunlight to split one water molecule, it requires nanoparticles made of semiconducting materials to act as catalysts, or more specifically, as photocatalysts.

In a study fully carried out in Brazil, a team of scientists developed a new simple and efficient method to produce tantalum oxide nanoparticles (Ta2O5) with outstanding performance catalysts for hydrogen generation. The research was reported in a paper recently published in the Journal of Materials Chemistry A (impact factor: 8.262).

Fotos dos autores principais do artigo. Começando pela esquerda do leitor: a doutora Virgínia Souza, o professor Jackson Scholten e o professor Jairton Dupont.
Picture of the main authors of the paper. From left to right: PhD Virgínia Souza, Prof. Jackson Scholten and Prof. Jairton Dupont.

This study was funded by the Brazilian research agencies CAPES and CNPq, as the doctoral research of Virgínia Serra Souza at the Chemistry Institute of the Federal University of Rio Grande do Sul (IQ-UFRGS), under the guidance of Professor Jairton Dupont.

“The idea for this research came when we were looking for an alternative and efficient route for the synthesis of Ta2O5 nanoparticles, and after some experiments we decided to test the possibility of using ionic liquids as stabilizing sources and agents of the nanomaterials”, says Professor Jackson Damiani Scholten, who is one of the corresponding authors of the paper and member of the research group of IQ-UFRGS. This group has extensive experience in the study and development of ionic liquids (salts which are in liquid state at room temperature). Due to their physicochemical properties, ionic liquids can be used in the preparation of nanoparticles as stabilizers to keep the particles in the nanometric range.

Souza, Scholten and Dupont prepared two types of ionic liquids containing tantalum and create the conditions for the hydrolysis reaction (breaking the chemical bonds of a compound by the addition of water). The elements resulting from the hydrolysis, from the water and the ionic liquid, recombine to form tantalum oxide nanoparticles.

The team realized it had produced tantalum oxide nanoparticles ranging between 1.5 and 22 nm, the smaller ones had been generated from one of the ionic liquids and the larger ones from the other. With the assistance of Professor Daniel E. Weibel, also from IQ-UFRGS, they studied the surface composition of the nanoparticles. These scientists proposed that the nanoparticles obtained were hybrid: remains of ionic liquid were observed around the tantalum oxide.

To see how the nanoparticles behaved as catalysts in the separation of water molecules to generate hydrogen, the team conducted photocatalytic tests at the facilities of the Institute of Physics – UFRGS, provided by Professor Sérgio R. Teixeira. The tests were carried out in a solution that besides water contained ethanol – a compound that helps to increase the hydrogen production rate.

“We were delighted that the Ta2O5 nanoparticles showed one of the best results ever published for the production of H2 from a water/ethanol solution”, recalls Professor Scholten. In the article, this exceptional result was attributed to the presence of ionic liquid in the nanoparticles. “We believe that the residual ionic liquid enhances the formation of a hydrophilic regions on the surface of Ta2O5, favoring the approximation of polar molecules (water and ethanol)”, explains Scholten. To be certain about this, the scientists removed the ionic liquid from the nanoparticles by heat treatment and confirmed their very low photocatalytic activity.

In another stage of the research, Dario Eberhardt, then professor at the University of Caxias do Sul (UCS), collaborated with the team in the deposition of roughly 1 nm platinum nanoparticles on the surface of the hybrid tantalum oxide nanoparticles by the sputtering technique, carried out at IF-UFRGS. Professor Daniel L. Baptista, of IF-UFRGS, helped to characterize the material. In the tests, the performance of the tantalum oxide nanoparticles with photocatalytic ionic liquid was even better with the addition of platinum.

This work, carried out in southern Brazil, presented a new method to produce super-efficient catalysts for hydrogen production, a promising alternative fuel from water and ethanol, two renewable and abundant resources.

This image provided by the authors of the paper exhibits the process to produce tantalum oxide nanoparticles from the hydrolysis of ionic liquids, followed by the deposition of platinum nanoparticles on the first material, and the application of the second material to obtain hydrogen gas in the “water splitting” process.
This image provided by the authors of the paper exhibits the process to produce tantalum oxide nanoparticles from the hydrolysis of ionic liquids, followed by the deposition of platinum nanoparticles on the first material, and the application of the second material to obtain hydrogen gas in the “water splitting” process.

 

Featured paper: Taming the reactivity of nanoalloys.

Posted on Monday 29 de February de 2016

[Paper: Charge transfer effects on the chemical reactivity of PdxCu1−x nanoalloys. M. V. Castegnaro, A. Gorgeski, B. Balke, M. C. M. Alves and J. Morais. Nanoscale, 2016,8, 641-647. DOI: 10.1039/C5NR06685A]

Taming the reactivity of nanoalloys

When, in 2009, the Electron Spectroscopy Laboratory (LEe-) group of the Federal University of Rio Grande do Sul (UFRGS) decided to start developing in-house metal nanoparticles required for their studies, they came across some issues. Many synthesis methods reported in the literature did not provide the expected results when made in the Brazilian laboratory.

Authors of the paper. From the top left: Marcus Vinicius Castegnaro, Andreia Gorgeski, PhD Benjamin Balke, Prof. Maria do Carmo Martins Alves and Prof. Jonder Morais.

Strongly motivated by curiosity, as usual, says professor Jonder Morais, LEe- researcher, the group members were able, after much dedication, to develop new routes of synthesis that, in addition to being reproducible, are environment-friendly, efficient and cost-effective. “The first articles were published in international journals in 2013, initially with palladium (Pd), platinum (Pt) and silver (Ag) nanoparticles applied to the catalytic decomposition of nitric oxide. Subsequently, we published some works focused in “in situ” studies aimed at determining the mechanisms of formation and growth of monometallic nanoparticles. We have recently started reporting the results obtained with more complex systems, such as palladium and copper (Pd-Cu) nanoalloys,” states Professor Morais.

The latter group includes the results recently reported in an article published in the journal Nanoscale, whose main authors are Professor Jonder Morais and Marcus Vinicius Castegnaro, a physics doctoral student at UFRGS, advised by Morais. The research covered the entire process from the production of nanomaterials to the survey of their applications. “It was important to have dedicated students, willing to face the challenge of preparing accurately their own samples, and correlating the electronic and structural properties to understand the final properties in terms of chemical reactivity,” says Morais.

In the article published in Nanoscale, nanoparticles composed of palladium and copper alloys were produced by applying a simple method developed by the LEe- team. This process is carried out under mild conditions to the environment and health (aqueous, ambient temperature and pressure, and use of cheap and innocuous substances, such as ascorbic acid and sodium citrate). Several samples were synthesized by this route, containing three different amounts of palladium and copper atoms.

The synthesized nanoparticles have undergone a series of analyses conducted at UFRGS, in Porto Alegre (Rio Grande do Sul State), they traveled to Campinas (São Paulo State) for another series of analyses on equipment of the National Center for Research in Energy and Materials (CNPEM) and crossed the ocean to Johannes Gutenberg University, in Germany, for some additional measures. From characterization, the authors concluded that the nanoparticles were approximately 4 nm in size and were highly crystalline, among other characteristics. In addition, through experiments conducted by the XANES in situ technique, the team of scientists exposed the nanoparticles to carbon monoxide (CO) at 450 ° C and surveyed the reactivity of the nanoalloys, i.e., their ability to react chemically.

After studying the results of the characterization, the authors of the article were able to conclude that the alloy composition affects the ability of nanoalloys to reduce (gain electrons) and to oxidize (lose electrons). In fact, the greater the amount of palladium, the easier the reduction, and the harder the oxidation.

Representative scheme of the correlation between the partial charge transfer between the Pd and Cu atoms (observed by XPS), and the reactivity after exposure to CO (surveyed by XANES in situ) for Pdx¬Cu1-x nanoalloys. It was observed that the higher is the amount of Pd present in nanoalloys, the greater is the reactivity of the sample after CO reduction, and the greater is the oxidation resistance of the atoms comprising it.

“The published results, obtained by the association of several experimental techniques are relevant to an understanding of the origin of high catalytic reactivity of palladium and copper (Pd-Cu) nanoalloys, as well as to elucidating similar behavior of other bimetallic systems”, highlights Jonder Morais. “Mostly, these results can be used in the “design “of new nanomaterials more efficient for various applications, such as in the petrochemical industry, in fuel cells or in the control of greenhouse gas emissions,” he concludes.

 

Featured paper: United atoms, adhered films.

Posted on Thursday 3 de September de 2015

[Paper: Identification of the Chemical Bonding Prompting Adhesion of a-C:H Thin Films on Ferrous Alloy Intermediated by a SiCx:H Buffer Layer. F. Cemin, L. T. Bim, L. M. Leidens, M. Morales, I. J. R. Baumvol, F. Alvarez, and C. A. Figueroa. ACS Appl. Mater. Interfaces, 2015, 7 (29), pp 15909–15917. DOI: 10.1021/acsami.5b03554]

United atoms, adhered films

With an innovative approach on an academic and industrial problem, a study wholly conducted in Brazil has brought significant advances in the understanding of the adhesion of DLC (diamond-like carbon) films on steels. The results of the work, which were recently published in the journal Applied Materials and Interfaces of the American Chemical Society (ACS), can help optimize such adhesion, thus prolonging the life of DLC films and expanding their use in the industry.

The team of scientists was particularly interested in the DLC potential to increase the energy efficiency of internal combustion engines. In fact, if all car engine components were coated with DLC films, the owner of that car would spend 5-10% less fuel and save the environment a good deal of greenhouse gas emissions and other pollutants, among other advantages. The reason for such saving lies in the ultra-low friction of DLC, since friction is the force responsible for wasting fuel while providing resistance to the motion that the parts of the engine make among themselves.

However, DLC has a drawback: it does not adhere to steel, causing quick delamination of the films from the substrate. To work around this problem, both in the laboratory and in industry, it is customary to deposit a layer containing silicon, known as interlayer, over the steel. The DLC film is then deposited on top of it. The result is a “sandwich”, which does not come undone easily.

In the paper published in the ACS journal, the authors experimentally analyzed a “sandwich” consisting of a steel substrate, an interlayer of silicon carbide (SiC) and a DLC film. Both the interlayer and the film were deposited by a quick process that generated thin layers of a few nanometers (up to 10). Mainly, two issues differentiated this study from other similar studies in the scientific literature. Firstly, the team focused in analyzing what happened in two regions corresponding to the interfaces of the interlayer with the film (upper) and with the steel (lower). Secondly, the scientists made a chemical approach on the matter of adhesion.

“This work has identified the chemical structure that generates adhesion in lower (SiCx: H/steel) and upper (a-C:H/SiCx:H) interfaces, which make up the a-C:H/SiCx:H/steel sandwich structure”, said Carlos A. Figueroa, professor at the University of Caxias do Sul (UCS) and corresponding author of the article. “The mechanisms found in the bibliography raised physical or mechanical aspects, but not chemical ones,” said Figueroa, who graduated in chemical sciences from the University of Buenos Aires (UBA) and has a doctorate degree in physics from the State University of Campinas (Unicamp). “However, adhesion is generated by the sum of all individual chemical bonds existing among DLC, the interlayer and steel,” he adds.

Scientists kept a constant film deposition temperature, but varied the interlayer deposition temperature, generating a group of samples deposited at 100° C and another one at over 300° C. After analyzing them by a variety of techniques, especially, X-ray photoelectron spectroscopy (XPS), researchers found that the lower interface of the interlayer, regardless of the deposition temperature, was largely composed of silicon atoms (from the interlayer) bonded to iron atoms (from the substrate). At the upper interface of the interlayer, the team found differences according to the deposition temperature of the interlayer. In the samples deposited at 100° C, oxygen atoms bonded many of the silicon and carbon atoms, preventing the carbon of the film to strongly bond to the silicon of the interlayer, and resulting in a film without good adhesion. In turn, scientists did not find oxygen in the interface of the samples deposited at over 300° C, but bonds between carbon and silicon atoms, which caused the film adhere well to the interlayer.

Schematic illustration of the chemical bonds present in the upper and lower interfaces of the interlayer deposited at 100° C (left) and over 300° C (right). In the center, a real engine cylinder displays, on the left side, a DLC film (in black) delaminated on the interlayer deposited at 100° C and, in the right side, the same film well adhered on the interlayer deposited over 300° C.

Besides Figueroa and students of the research group he leads in UCS, also authored the paper researchers from the Institute of Physics at Unicamp, where the XPS measures were made, as well as a scientist from the Federal University of Rio Grande do Sul (UFRGS) that, together with the other authors, participated in the discussion of results.

The work received the support from Brazilian Science funding bodies (Capes, CNPq through INCT National Institute of Surface Engineering, Fapergs), of Petrobras, UCS, the European Commission (Marie Skłodowska – Curie Actions) and Plasmar Tecnologia (a small company that is developing, through a TECNOVA RS project, an industrial equipment to deposit DLC on steel aiming to increase the energy efficiency of car engines).