Tag: unicamp

Featured article: Probing electrons of actinide compounds.

Posted on Wednesday 29 de November de 2017

box englishA team led by researchers from Brazil was able to unveil details of the distribution of electrons in materials based on actinide elements (the 15 chemical radioactive elements, with atomic numbers ranging from 89 to 103).

The group of scientists developed an experimental method that allowed a unique probing of the 5f and 6d orbitals and their hybridization in materials based on uranium (one of the most abundant actinide elements in the earth’s crust). This allowed the team to demonstrate, for example, that 5f-6d hybridization determines the magnetic properties of the studied materials. The work left as a legacy an experimental system for research on various magnetic materials (3d metals, rare earths, actinides and others), available to be used by the international scientific community at the Brazilian Synchrotron Light Laboratory (LNLS).

The study was reported in a paper that was recently published in Nature Communications (Impact Factor 12,124). “In this paper, we demonstrate the use of magnetic circular dichroism (XMCD) on the L-border of uranium to directly probe the 6d and 5f orbitals and also their degree of hybridization, rather than just probing the 5f orbitals as for instance the actinides M absorption edges,” details the corresponding author of the paper, Narcizo Marques de Souza Neto, professor at UNICAMP and researcher at LNLS.

In order to probe the orbitals of the uranium compounds, especially UCu2Si2 and UMn2Si2, the scientists had to overcome the difficulties of manipulating the materials due to their toxicity. They also had to make a series of adjustments in the high-energy XMCD technique to improve its sensitivity (to extend its detection limits).

These developments were initially performed at the LNLS DXAS line, dedicated to X-ray absorption techniques. Currently, the XMCD instrumentation is part of the XDS line of LNLS which is dedicated to X-ray diffraction and spectroscopy, where it is being used and improved. In the future the technique will be available in Sirius (the latest generation of synchrotron light source which is being built in Campinas), more precisely in the EMA line, which will be dedicated to X-ray techniques under extreme conditions of pressure and temperature. According to Souza-Neto, who coordinates both the XDS line and the EMA project, the conditions for studying actinides and similar materials by XMCD will be unparalleled in Sirius.

In addition to advancing the knowledge on actinides, the research demonstrated the potential of the XMCD technique improved by the Brazilian team to continue unveiling the characteristics of these still experimentally understudied elements. A deeper understanding of actinides, says Souza-Neto, is necessary to propose new uses for these elements, and also to be able to use them more efficiently in existing applications, such as, for example, power generation, diagnosis and treatment of diseases and the production of special glasses.

Ricardo dos Reis (left) and Narcizo Souza-Neto (right), main authors of the paper. Between them, a screen with the representation of EMA beamline where XMCD experiments will be available in Sirius fourth-generation synchrotron source.
Ricardo dos Reis (left) and Narcizo Souza-Neto (right), main authors of the paper. Between them, a screen with the representation of EMA beamline where XMCD experiments will be available in Sirius fourth-generation synchrotron source.

The history behind this work

The origin of this work dates back to 2009, when Souza-Neto was studying rare earth electronic structure and magnetism during his postdoctoral fellowship at the Argonne National Laboratory in the United States. “I had the idea of expanding the study of rare earths to actinide compounds (Souza-Neto et al., Phys. Rev. Lett., 102, 057206 (2009)) using XMCD to probe a charge transfer in the 4f and 5d orbitals”, the researcher reports. Looking for materials with similar characteristics, he came across uranium compounds. “We first tried to start this study in Argonne, but the conditions there to carry this out were not as we had hoped,” he adds. He returned to Brazil in 2010 as a researcher of CNPEM, with the desire to continue this initiative. Thus, in 2011, Souza-Neto began to guide the doctoral research of Ricardo Donizeth dos Reis on this subject together with the co-supervisor Flávio César Guimarães Gandra, a professor at Unicamp, with whom he had previously collaborated.

Samples of uranium compounds were prepared and characterized in the Laboratory of Metals and Alloys of Unicamp, coordinated by Professor Gandra, where there was already research experience on actinide and rare earth materials. The X-ray absorption spectroscopy experiments were performed at Argonne’s Advanced Photon Source and at LNLS. “All experiments on the L edges of uranium, which make up the main innovative contribution of this work, were carried out at LNLS,” Souza-Neto details. “At Argonne the experiments were carried out on the M edge of uranium to probe the contribution of the 5f orbitals separately and corroborate our interpretation of the results,” he adds. Furthermore, the Brazilian group had the participation of a researcher from France in the theoretical simulations performed for interpreting the data.

The research was carried out with financial resources from the São Paulo Research Foundation; from the Brazilian federal agency Capes; from the Ministry of Science, Technology and Innovation of Brazil, and from the Office of Science of the United States Department of Energy.

Scientific paper:

“Unraveling 5f-6dhybridization in uraniumcompounds via spin-resolved L-edge spectroscopy”. R. D. dos Reis, L. S. I. Veiga, C. A. Escanhoela Jr., J. C. Lang, Y. Joly, F. G. Gandra, D. Haskel & N. M. Souza-Neto. Nature Communications 8:1203 (2017). DOI: 10.1038/s41467-017-01524-1. Link: https://www.nature.com/articles/s41467-017-01524-1

Featured paper: Nanosheets and nanoparticles interconnected for wearable electronics.

Posted on Tuesday 30 de May de 2017

[Paper: Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode. Rajesh Kumar, Rajesh K. Singh, Alfredo R. Vaz, Raluca Savu, Stanislav A Moshkalev. ACS APPLIED MATERIALS & INTERFACES. 2017, 9, 8880 – 8890. DOI: 10.1021/acsami.6b14704].

Nanosheets and nanoparticles interconnected for wearable electronics

A team of researchers from the State University of Campinas (Unicamp), in Brazil, and a researcher from the Central University of Himachal Pradesh (CUHP), in India, have developed a flexible and tiny high-performance supercapacitor with a hybrid material made of graphene oxide (GO) nanosheets and iron oxide (Fe3O4) nanoparticles. The work was recently reported in the journal Applied Materials & Interfaces (impact factor 7.145), of the American Chemical Society.

“The main contribution of this work is for the new and really promising research area of flexible electronics”, says PhD Rajesh Kumar, researcher at Unicamp’s Center for Semiconductor Components (CSC) and corresponding author of the article. “Since capacitors are among the main components of electronic devices, these performant and flexible graphene oxide-based microsupercapacitors can be used in the near future as components in wearable and flexible electronic devices (mobile phones, smart watches, health monitoring devices, energy storage devices etc.)”, adds the Indian born researcher.

The genesis of the study goes back to 2015, when Rajesh Kumar, who had been working with graphene microsupercapacitors in other countries, applied for a postdoctoral fellowship to work in the group of Professor Stanislav Moshkalev, director of CSC at Unicamp. “I saw a great opportunity in this group, as their main research line is nanofabrication and nanoelectronics based on nanostructured carbon,” reports Kumar. The Indian PhD obtained a grant from CNPq, the Brazilian federal research agency, as a visiting specialist, to carry out a project in CSC – Unicamp. Initially, he made fine sheets of graphene oxide called “buckypapers”. Then, working in interaction with a group of five other people of CSC – Unicamp, he searched for new strategies to improve the properties of the material.

The CSC- Unicamp team thus faced the challenge of making a hybrid material of graphene and iron oxide with controlled structure using a simple process, and it was successful in do so by simply exposing graphite oxide and ferric chloride (FeCl3) to microwave radiation.

SEM image of the 3D hybrid material Fe3O4/rGO (left), and a representative scheme of the material´s morphology (right).
SEM image of the 3D hybrid material Fe3O4/rGO (left), and a representative scheme of the material´s morphology (right).

The obtained material presented an interesting morphology: a three-dimensional network in which interconnected graphene nanosheets form “tunnels” that harbor crystalline and multifaceted iron oxide nanoparticles of 50 – 200 nm, strongly attached to the nanosheets, as shown in the figure beside.

The morphology, structure, composition, thermal stability and other properties were analyzed using several techniques available at CSC – Unicamp and at the Indian university.

Subsequently, at Unicamp, the team tested the efficiency of the material to act as electricity storage. The tests proved the high performance of the material as a supercapacitor electrode, and the scientific team concluded that this efficiency was favored by the special morphology of the 3D hybrid material. Particularly, by the faceted nanoparticles strongly attached to the nanosheets, the separation among the nanosheets, the “tunnels” that shelter individual nanoparticles avoiding agglomerations, and the large surface area of the network of nanosheets.

“These microsupercapacitors can and for sure will, in the near future, replace the traditional capacitors in electronic devices,” says Kumar. According to the researcher, their main advantages are high performance, mechanical strength, reduced size and, most important, flexibility – an essential property for wearable electronics.

In addition, the method developed by the Unicamp and CUHP team can become a good alternative to fabricate other hybrid materials based on carbon and metal oxides.

The work was carried out with financial support from CNPq and FAPESP (the São Paulo State research foundation).

Pictures of the authors of the paper. From the readers´ left, Rajesh Kumar (Unicamp), Rajesh Kumar Singh (CUHP), Alfredo Vaz (Unicamp), Raluca Savu (Unicamp), and Stanislav Moshkalev (Unicamp).
Pictures of the authors of the paper. From the readers´ left, Rajesh Kumar (Unicamp), Rajesh Kumar Singh (CUHP), Alfredo Vaz (Unicamp), Raluca Savu (Unicamp), and Stanislav Moshkalev (Unicamp).

Featured paper: Advanced material for ultra-capacity supercapacitors.

Posted on Friday 31 de March de 2017

[Paper: One-step electrodeposited 3D-ternary composite of zirconia nanoparticles, rGO and polypyrrole with enhanced supercapacitor performance. Alves, Ana Paula P.; Koizumi, Ryota; Samanta, Atanu; Machado, Leonardo D.; Singh, Abhisek K.; Galvao, Douglas S.; Silva, Glaura G.; Tiwary, Chandra S.; Ajayan, Pulickel M. NANO ENERGY, volume 31, January 2017, 225–232. DOI: 10.1016/j.nanoen.2016.11.018.]

Advanced material for ultra-capacity supercapacitors.

Supercapacitors are electrical storage devices with a particular feature of releasing large amounts of energy in a short time interval. They are already used, for example, in electric or hybrid vehicles, camera flashes and elevators, but they can still be improved – largely with the contribution of Materials Science and Technology – for current and potential applications. Putting it simply, a supercapacitor consists of two electrodes, positive and negative, separated by a substance containing positive and negative ions (the electrolyte).

An article recently published in the scientific journal Nano Energy (Impact Factor 11,553) reports on a contribution from an international and interdisciplinary scientific team to develop materials that improve the performance of supercapacitors. Using a simple and easily scalable process, the team of researchers from Brazil, the United States and India produced electrodes made of a composite material composed of polypyrrole (PPi), reduced graphene oxide (rGO) and zirconium oxide (ZrO2) nanoparticles. By combining the three materials, the scientists were able to generate a large surface area and high porosity electrode – basic characteristics to promote the interaction of the electrolyte ions with the surface of the electrodes and therefore enhance the performance of the supercapacitor.

“Our unique contribution was the synthesis, in a single and simple stage of electrodeposition, of a hybrid containing graphene, zirconium oxide and polypyrrole, and the experimental demonstration of considerable gains in electrochemical properties, parallel to the theoretical modeling in order to understand the role of the components of the material”, states Glaura Goulart Silva, professor in the Department of Chemistry at the Federal University of Minas Gerais (UFMG) and a corresponding author of the paper.

In addition to preparing samples of the ternary (i.e., composed of three elements) composite PPi/rGO/ZrO2, using the same method for comparison purposes, the team prepared samples of the PPi/rGO binary composite, and pure polypyrrole samples. The three materials were analyzed using XPS (spectroscopy of X-ray excited photoelectrons), SEM (Scanning Electron Microscopy), Raman spectroscopy and transmission electron microscopy to determine their composition, structure and morphology.

As seen in the SEM images of the figure below, the scientists noted that the addition of graphene oxide and zirconia nanoparticles significantly changed the morphology of the material. While the pure polypyrrole had formed a cracked, wire-like film, the graphene composite had a granular morphology, with no cracks, and the zirconium oxide material had a leaf-like appearance.

At the end of the experimental stage of the study, the scientists performed a series of tests to measure the performance of the three materials as supercapacitors. The results showed that the capacity to store electrical charges (capacitance) had increased up to 100% in the ternary composite with respect to the polypyrrole. Moreover, instead of decreasing this performance due to the use of the electrode, it increased by 5% after 1,000 recharges in the binary and ternary composites.

This was the first paper that presented the introduction of zirconium oxide nanoparticles in polypyrrole and graphene electrodes for supercapacitors. Therefore, the team performed computational modeling to analyze the role of zirconium oxide in the performance of the composite. The simulations confirmed the beneficial effects of the nanoparticles on the stability of the material, directly related to extending the life of the electrodes.

Illustrative diagram of charge storage and interaction of ions near the surface of pure polypyrrole electrodes (PPi), reduced graphene oxide (PPi/rGO) and polypyrrole PPi/rGO/ZrO2 (above), based on the morphology associated with the SEM images of the surface of the electrodes with the respective materials under carbon fiber substrate (below). Image by Ana Paula Pereira Alves for her PhD thesis.
Above, illustrative diagram of charge storage and interaction of ions near the surface of pure PPi electrodes, PPi/rGO electrodes, and PPi/rGO/ZrO2 electrodes, based on the morphology associated with the SEM images of the surface of the electrodes with the respective materials under carbon fiber substrate (below). Image by Ana Paula Pereira Alves for her PhD thesis.

“There is great potential in the application of these new composites in supercapacitors due to the need to increase the energy density provided by the device, in parallel with its miniaturization,”declares Professor Goulart Silva. “The alternative developed in the work in question allows better performance in terms of cycling stability with gains in the safety of the supercapacitor. The use of supercapacitors and batteries in electric and hybrid cars is one of the technological fronts where these materials can be applied,” she adds.

From the reader's left: Professor Glaura Goulart Silva (UFMG), Professor Pulickel Ajayan (Rice University) and Ana Paula Pereira Alves, a recently graduated doctor from UFMG.
From the reader’s left: Professor Glaura Goulart Silva (UFMG), Professor Pulickel Ajayan (Rice University) and Ana Paula Pereira Alves, a recently graduated doctor from UFMG.

The work is part of the doctorate in Chemistry of Ana Paula Pereira Alves, conducted with the guidance of Professor Goulart Silva and defended in February of this year at UFMG with a thesis about synthesis and characterization of advanced materials for supercapacitors. During her doctoral work at the University of Minas Gerais, Pereira Alves carried out intensive training in synthesis techniques and physical-chemical analysis of conjugated polymers and graphene and in the characterization of supercapacitors. In 2015, she went to the United States for a one-year “sandwich” internship, with the support of the National Council for Scientific and Technological Development (CNPq), in the Department of Materials Science and Nanoengineering at Rice University, in the research group of Professor Pulickel Ajayan (researcher with h=139 index according to Google Scholar), who has collaborated with Professor Goulart Silva’s group since 2010. “Professor Ajayan has systematically proposed radical innovations in synthesis and design of batteries and supercapacitors, with significant international impact in the area,” she adds.

The experimental work reported in the paper was carried out at Rice University, with the presence of all authors, including those from Brazil and India, and also Professor Goulart Silva, who was there in February 2016, with the support of Minas Gerais Research Foundation (Fapemig). “The highly interdisciplinary environment of the Department of Materials Science and NanoEngineering at Rice made possible for the engineers, physicists, and chemists to come together to work on a current major problem.”, says Goulart Silva.

The computational modeling was carried out by Brazilian researchers from the State University of Campinas (Unicamp) and the Federal University of Rio Grande do Norte (UFRN) –among them Professor Douglas Galvão (Unicamp), who has maintained a scientific collaboration with Professor Ajayan since before the beginning of this research.

“I consider this work to be an excellent example of success, where the competence of the Brazilian groups joined that of a highly productive and impactful group in the international scenario and complement each other,” declares Goulart Silva. “The stability and increase of investments in research and development in Brazil are essential for endeavors as this to be widespread. Research is an investment that needs to be done over the long term, without setbacks, to enable a high rate of return in terms of materials, technologies and highly qualified people. Ana Paula Alves is now a young doctor in search of the opportunity to put together her research group and hence train new students and hence contribute to face the challenges of our country,” reaffirms Goulart Silva.

Featured paper: How to make more stable perovskite nanocrystals for more efficient LEDs.

Posted on Monday 27 de February de 2017

Paper: Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes. Emre Yassitepe, Zhenyu Yang, Oleksandr Voznyy, Younghoon Kim, Grant Walters, Juan Andres Castañeda, Pongsakorn Kanjanaboos, Mingjian Yuan, Xiwen Gong, Fengjia Fan, Jun Pan, Sjoerd Hoogland, Riccardo Comin, Osman M. Bakr, Lazaro A. Padilha, Ana F. Nogueira, and Edward H. Sargent. Adv. Funct. Mater. 2016. DOI: 10.1002/adfm.201604580.

How to make more stable perovskite nanocrystals for more efficient LEDs.

Nesta imagem ilustrativa, enviada por Emre Yassitepe, pontos quânticos azuis, verdes e vermelhos excitados por radiação ultravioleta exibem uma brilhante luminescência.
In this illustrative image, sent by Emre Yassitepe, blue, green and red quantum dots excited by ultraviolet radiation exhibit a brilliant luminescence.

Perovskite quantum dots have been seen as great candidates to compose a next generation of displays and lighting devices. In fact, these luminescent nanoparticles are able to emit high brightness light and very vivid and pure colors when receiving external energy. But the technological use of perovskite quantum dots still runs into some limitations, mainly linked to their instability, for these tiny particles can quickly react with the medium, agglomerate or increase in size, for example.

A team of scientists from institutions in Canada, Brazil and Saudi Arabia has found a solution to one of the problems limiting the advance of research and development in the field, the degradation of perovskite quantum dots during their synthesis. The study was reported in an article recently published in the journal Advanced Functional Materials (impact factor: 11.38).

The manufacture of perovskite quantum dots is traditionally carried out by placing in a flask a solution with a series of compounds which react among them and generate perovskite nanoparticles coated (passivated) with oleic acid (C18H34O2) and oleylamine (C18H35NH2).

The team performed experiments and computational simulations to understand how the formation of perovskite quantum dots occurred step by step and thus formulate a manufacturing method that would avoid the problem of degradation. The scientists realized that the key to the solution was to reformulate the “ingredients” of the process in order to remove the oleylamine that eventually created the conditions for the degradation of the quantum dots, which precipitated to the bottom of the flask.

“We focused on developing the new synthetic technique to passivate perovskite quantum dots with oleic acid,” says Emre Yassitepe, postdoc at the Nanotechnology and Solar Energy Laboratory of the Institute of Chemistry of Unicamp, who signs the article as the first author. “Oleic acid is one of the most common ligands to date to stabilize the quantum dots and we wanted to see the impact on stabilization and LED performance between different ligands.”

Following the new “recipe”, the team was able to produce quantum dots of about 8 nm, coated only with oleic acid, made of cesium, lead and elements of the group of halogens and having perovskite structure (which is a certain organization of the atoms). Green quantum dots (CsPbBr3), blue (CsPb (Br, Cl) 3) and red (CsPb (Br, I) 3) were produced and characterized.

One of the main gains achieved with the new method was the colloidal stability of the quantum dots: they remained intact more than oleylamine capped perovskite quantum dots after the purification step, which removes from the nanocrystals the residual compounds that usually remain from the manufacturing process.

The team went beyond the manufacturing and experimental analysis of quantum dots and built with them LED devices (light-emitting diodes, now widely used in lamps and displays) emitting green, blue and red light, in order to check their efficiency. They made thin films with the obtained perovskite quantum dots and placed a layer of this material “sandwiched” between a layer of titanium dioxide, in charge of transporting electrons (carriers of negative charge) and a polymer layer, destined to transport the so-called “holes” (positive charge carriers). In this LED, when applying an electric field, electrons and holes move to the quantum dots layer and excite them, causing them to emit photons and thus generate the desired light.

The use of polymer transport layers processed from solution instead of layers processed from evaporation to make perovskite LEDs was also an innovation made possible by the new “recipe”, which made quantum dots more robust against this type of processing.

As a final result, the team of scientists achieved bright and efficient blue and green LEDs. Perovskite LEDs made with oleylamine-free quantum dots demonstrated better performance in some respects than conventional perovskite LEDs produced with oleylamine coated quantum dots.

Pictures from the authors of the paper from Brazilian institutions. From the left: Ana Flávia Nogueira and Emre Yassitepe (Institute of Chemistry, Unicamp), Juan Andrés Castañeda and Lázaro Padilha (Institute of Physics, Unicamp).
Pictures from the authors of the paper from Brazilian institutions. From the left: Ana Flávia Nogueira and Emre Yassitepe (Institute of Chemistry, Unicamp), Juan Andrés Castañeda and Lázaro Padilha (Institute of Physics, Unicamp).

“We have demonstrated a new synthetic method that enhances the colloidal stability of perovskite quantum dots by capping them solely by oleic acid”, summarizes Yassitepe. “The enhancement of stability of oleic acid capped perovskite quantum dots allows us to remove excess organic content in thin films. The excess inorganic content acts as an insulator between quantum dots reducing performance. By reducing the excessive ligands we are able to make more efficient and solution-processed perovskite quantum dot light emitting diodes” concludes the postdoc.

The work was funded by Canadian agencies, FAPESP (São Paulo State Research Foundation) and King Abdullah University of Science and Technology (Saudi Arabia). The experiments of transient ultra-fast absorption and analysis by transmission electron microscopy were carried out at Unicamp to characterize the quantum dots. The synthesis of the nanocrystals and the manufacture of LEDs were carried out at the University of Toronto in the group of Professor Edward H. Sargent, where Yassitepe performed a one-year internship during within his post-doc at Unicamp. “I am grateful to FAPESP- Bolsa Estágio de Pesquisa no Exterior project for giving me this opportunity,” says Yassitepe.

Inscrições abertas para concurso docente no IQ/Unicamp, na área de Química Orgânica.

Posted on Monday 19 de December de 2016

Estão abertas as inscrições para o concurso público de provas e títulos, para provimento de 01 (um) cargo de Professor Doutor na área de Química Orgânica, do Departamento de Química Orgânica do IQ/Unicamp.

Inscrições até 21 de março de 2017

Edital: http://www.sg.unicamp.br/dca/concursos/abertos/concursos-para-professor-doutor/instituto-de-quimica

Featured paper: Isolating nanoribbons with conducting regions.

Posted on Wednesday 31 de August de 2016

[Paper: Topologically Protected Metallic States Induced by a One-Dimensional Extended Defect in the Bulk of a 2D Topological Insulator. Erika N. Lima, Tome M. Schmidt, and Ricardo W. Nunes. Nano Lett., 2016, 16 (7), pp 4025–4031. DOI: 10.1021/acs.nanolett.6b00521]

Isolating nanoribbons with conducting regions

A research carried out in Brazil made an important contribution to the study of topological insulators, a class of materials that was theoretically predicted in 2005 and experimentally confirmed in 2007. The study was reported in an article recently published  in Nano Letters (impact factor: 13.779).

A unique property of Topological insulators is that they behave as insulators on the inside and as conductors on its surface or edge. According to Ricardo Wagner Nunes, professor at the Federal University of Minas Gerais (UFMG) and corresponding author of the article, “non-topological insulators may also have conductive surfaces, but in the case of topological insulators, conduction of charge and spin on the surface is robust, as it is “protected” by time reversal symmetry”.

In the article in Nano Letters, Professor Nunes and colleagues, Erika Lima, of the Federal University of Mato Grosso (UFMT) – Rondonópolis campus, and Tome Schmidt, of the Federal University of Uberlândia (UFU), reported their work on a two-dimensional topological insulator, a bismuth nanoribbon of only two layers of bismuth atoms (one-atom thick), superimposed and bonded. Using computational methods, the scientists showed that the interior of the bismuth nanoribbon, instead of being fully insulating, may have conductive states (also called metallic states) generated from a particular type of irregularity in the atomic structure of the material, known as 558 extended defect.

Representation of bismuth bilayer nanoribbon with the defect 558, top view (left) and side view (right). The green balls represent the atoms of the top layer of the material and the blue balls, the atoms of the lower layer. In the center of the left figure, the defect is clearly seen: pentagons and an octagon stop the repetition of the hexagons.

 

“In our work, we show that a linear defect within a two-dimensional topological insulator can generate one-dimensional electronic quantum states that conduct spin and charge within the material”, say the authors.

This conclusion was supported through calculations performed on supercomputers, simulating what would happen to the electrons in quantum states, in the material, in the presence of defects. “We used first-principles Density Functional Theory calculations”, specify the authors, who relate that the computer simulation of defects in bismuth nanostructures required approximately 400 hours of computer simulations on supercomputers in the Department of Physics – UFMG and at the National Center for High Performance Computing in São Paulo (Cenapad) – UNICAMP.

A figura mostra a curva de dispersão dos estados topológicos metálicos, localizados no defeito 558, marcados em azul e vermelho.
The figure shows, marked in blue and red, the dispersion curve of the metal topological states located in the defect 558.

In the article, the authors also propose the existence of pentaoctite, a new two-dimensional topological insulator. This material, which has not been synthesized yet, is a bismuth bilayer with a crystal lattice formed by atoms arranged in pentagons and octagons. As stated by the authors, “In our calculations we show that this new “phase” of the two-dimensional bismuth has low formation energy, which opens the possibility to be synthesized in the laboratory”.

According to the authors, the work reported in Nano Letters raises several issues in the scope of fundamental research, such as the influence of magnetic and non-magnetic impurities on the spin and charge transport in the proposed topological states, and the connection between the network symmetries and nature of the topological edge states on pentaoctite. “From the point of view of applications, it would be interesting if our work could motivate experimental studies of two-dimensional topological insulators based on bismuth and other materials, enabling theoretical and experimental collaboration on this issue”, comment the authors, leaving an open invitation to experimental research groups.

The origin of this research work

“The work originated by combining my interest in extended topological defects in two-dimensional and three-dimensional materials, with the experience of Professor Tome Mauro Schmidt (UFU) and Erika Lima, his doctoral student in the subject of topological insulators”, states Nunes.

In 2012, Nunes and collaborators published an article in Nano Letters on magnetic states (non topological) generated by linear extended defects in a monolayer of graphene. Later, in a conversation with Schmidt, a collaboration was decided in order to investigate if an extended defect with the same morphology would lead to the formation of topological states in a bidimensional topological insulator made of bismuth.

In her post-doctorate in the group of Professor Nunes, in 2015, Erika Lima performed all computer calculations. The three researchers, who are the authors of the article, interpreted the results and wrote the paper.

The research that led to the article received funding from Brazilian agencies CAPES, CNPq, FAPEMIG and from the National Institute of Science and Technology on Carbon Nanomaterials.

autores
Photos of the authors. From left to right, Erika Lima, currently a professor at UFMT, Tome Schmidt, professor at UFU, and Ricardo Nunes, professor at UFMG.

IUMRS-ICAM Best Poster Award to Brazilian work.

Posted on Monday 30 de November de 2015

Among the nearly 1,300 papers accepted for presentation at the International Conference on Advanced Materials, IUMRS-ICAM 2015, held on the beautiful island of Jeju (Korea) in late October, eight studies were conducted in Brazil and one of them was awarded by the organization with the Best Poster Award.

The work, entitled “Flame Aerosol nanostructured titanium dioxide for coating: the control of crystallite size and phase by oxy-hydrogen flame” was presented as a poster by Mirella Nagib de Oliveira Boery, professor at the Federal Institute of Education, Science and Technology of Bahia (IFBA). Mirella developed the research along with collaborators of the Federal University of Bahia (UFBA) and the State University of Campinas (Unicamp). “The idea of developing this research emerged during my master’s degree at Unicamp, in light of my concerns regarding the widespread use of TiO2, from paint to sunscreen,” she said. Mirella is currently continuing her studies at Unicamp, in the doctoral course of Mechanical Engineering.

Mirella at the conference, and the certificate of the award.