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


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.


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.


[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.


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.

Featured paper: United atoms, adhered films.


[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).

SBPMat´s community people: interview with Marcelo Knobel.


Marcelo Knobel. Credits: Antonio Scarpinetti – Ascom – Unicamp.

Scientific research, magnetic materials, scientific dissemination and higher education would be perhaps the biggest expressions in a cloud of tags to represent Professor Marcelo Knobel.

Born in Buenos Aires (Argentina) in 1968, Marcelo Knobel came to live in Brazil, more specifically in Campinas (SP), by the age of 8 years-old, following his parents, the psychologist Clara Freud de Knobel and the psychiatrist Maurício Knobel. The family was escaping from the coup d’etat that had just established in Argentina a military dictatorship that fired Maurício from the University of Buenos Aires (UBA). In Brazil, which was also governed by a military dictatorship, Maurício had been contracted for the State University of Campinas (Unicamp).

Ten years after the arrival in Campinas, Marcelo Knobel joined Unicamp to a graduation in Physics. In parallel to the studies, he started to do research on magnetic properties of materials. After he obtained the bachelor´s degree, Knobel remained in Unicamp for the doctorate in the same area, receiving the diploma of doctor in Physics when he defended his dissertation on magnetism and structure of nanocrystalline materials in 1992. After that, he went to Europe, where he conducted two postdoctoral internships; one at Istituto Elettrotecnico Nazionale Galileo Ferraris, Italy, and the other at the Instituto de Magnetismo Aplicado, Spain.

Returning to Brazil and to Unicamp, in 1995, Marcelo Knobel started his career of professor and researcher of the Institute of Physics Gleb Wataghin (IFGW). From 1999 to 2009 he was the coordinator of the Laboratory of Materials and Low Temperatures, where he acts as researcher until the present moment, always investigating magnetism and magnetic materials. Together with his collaborators of the laboratory, Knobel have carried pioneering works in the study of the giant magnetoresistance and magnetoimpedance in certain materials – two different concepts that are related to the opposition that a material offers to the passage of the electricity in consequence of the application of an external magnetic field. In 2008, Knobel became Full Professor of the Department of Condensed Matter Physics at IFGW.

In the area of scientific dissemination, Marcelo Knobel started in the year 2000 to perform education and research activities at the Laboratory of Advanced Studies in Journalism (LABJOR) of Unicamp. Moreover, Knobel was one of the creators of NanoAventura, an interactive and itinerary exposition on nanotechnology that started in 2005 and was visited by more than 50 thousand people, mainly children, until now. NanoAventura received honorable mentions at Scientific Cine and Video Festival of Mercosur (2006) and at Mercosur Science and Technology Award (2015), as well as an award in 2009, from the Latin American and Caribbean Network for the Popularization of Science and Technology (RedPOP). From 2006 to 2008, Knobel was the first director of the Exploratory Museum of Science, linked to Unicamp. In 2008, he became editor in chief of the magazine Ciência & Cultura of the Brazilian Society for the Progress of Science (SBPC), position that he occupies until now. In the publishing field, Knobel coordinates a collection of science dissemination books of the Unicamp Publisher, called Meio de Cultura, released in 2008.

In 2007 Marcelo Knobel received the Young Scientist Prize from the TWAS-ROLAC (office of Latin America and the Caribbean of the The World Academy of Sciences for the advancement of science in developing countries), an awars for young scientists of the region. In the same year, he was selected, together with about 50 people of different professional areas and several countries of the world, to participate of the program Eisenhower Fellowships, which aims to strengthen the leadership potential of its fellows. The group travelled for the United States during 7 weeks complying with a schedule of meetings and seminars. In 2009, he was chosen as a fellow of John Simon Guggenheim Memorial Foundation, receiving resources for research.

From 2009 to 2013, he was Vice-President for Undergraduate Programs of Unicamp. In this position, he was responsible for the implantation of the Interdisciplinary Program of Higher Education (ProFIS). ProFIS is a higher education course of 4 semesters that provides a general, multidisciplinary and critical formation, and makes possible to its alumni (former students of public schools chosen by their good grades in the Brazilian National Exam of the Secondary School (ENEM) that they enter graduation courses at Unicamp without passing for Brazilian admission university exam). The program was distinguished in 2013 with the Prize Péter Murányi – Education, destined to actions that increase well-being of populations of the south hemisphere.

In 2010, with 42 years-old, Knobel was honored with the title of Commendatore of the Order of the Scientific Merit by the Brazilian Presidency of the Republic.

Holder of a productivity scholarship 1A (the highest) at CNPq, Marcelo Knobel has published about 300 scientific articles in peer-review international journals and 15 chapters of books on magnetic materials and properties, popularization of science, public perception of science and higher education. Also he is the author of articles about science and education published in diverse medias. He has 6.370 citations, according to Google Scholar.

Marcelo Knobel had just assumed, in August 3rd, the position of director of the Brazilian National Laboratory of Nanotechnology (LNNano), of the National Center of Research in Energy and Materiais (CNPEM).

Here follows a brief interview with the scientist.

SBPMat newsletter: – Tell us what made you become a researcher and work in the field of Materials.

Marcelo Knobel: – I chose the area of Physics because of the curiosity, without knowing exactly what this meant. But already in the first semester I realized that it was what I wanted for my life, to try to understand the nature. Early in the beginning of the graduation, I had a laboratory class with professor Reiko Sato, who later invited me to do scientific initiation in her laboratory. She worked with magnetic properties of amorphous metals, and that was the subject of my research. Later, I went to the doctorate also with her, already working with nanocrystals, and later I followed the postdoctoral in the same area.

SBPMat newsletter: – Which are, in your own evaluation, your main contributions to the field of Materials?

Marcelo Knobel: – I am acting in nanoscopic magnetic systems, mainly investigating dipole interactions in magnetic nanosystems, using several experimental techniques, theoretical models and computational simulations. These systems, beyond the interest in basic research, have many possible applications, mainly in systems of magnetic record and nanomedicine. The research group that I helped to consolidate develops new nanocrystalline materials and carries studies through the development of new magnetic, structural and transport techniques. In the scope of these researches, we were pioneer in the study of giant magnetoresistance in granular systems and in the research of giant magnetoimpedance in amorphous and nanocrystalline wires and ribbons. But I have also been dedicating myself to scientific dissemination, being one of the responsible for the creation of the Exploratory Museum of Sciences of Unicamp.  I was the coordinator of NanoAventura project, which is an interactive and itinerary exhibit on nanoscience and nanotechnology for children and adolescents. I still work in research in the area of public perception of science, I coordinate the series “Meio de Cultura” of Unicamp Publisher and I act as an editor in chief of the magazine Ciência & Cultura, of SBPC. Recently, I was Vice-President for Undergraduate Programs of Unicamp, where I highlight the implantation of the Interdisciplinary Program of Higher Education (ProFIS). Currently, I am initiating a new challenge, as Director of the National Laboratory of Nanotechnology (LNNano).

SBPMat newsletter: – You have an especially strong performance in dissemination of science and scientific culture. Comment with our readers, students and researchers, which is, for you, the importance to carry through this type of activity.

Marcelo Knobel: – I became a scientist after reading books and magazines of dissemination and visiting museums of sciences. I believe that we must stimulate the new generations to think critically, to have curiosity, to search to unmask the mysteries that surround them. For Brazil it is basic to stimulate young talents for science. Without them we will not have a future… Moreover, it is our obligation to give account to the society, which is the financer of the scientific research in public universities and research institutes. It is important to show the science that is done in our country, and the importance to follow investing, more and more, in science and technology.

SBPMat newsletter: – If you want, you can leave a message for the readers that are initiating their scientific careers.

Marcelo Knobel: – I do not have doubts that it is the passion that must guide the careers of everybody, and mainly of the scientists. But beyond the passion, a solid formation is necessary, not only in the specific content, but also in personal abilities, as work in team, communication (including Portuguese and English, scientific writing) and general knowledge. The scientific activity demands effort and devotion, but we are rewarded, I guess, with a life full of new challenges and opportunities.