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In research carried out in a number of Brazilian laboratories, a multidisciplinary scientific team developed a magnetic, luminescent nanomaterial capable of chemically binding to molecules of interest, such as drugs or proteins. This nanomaterial also showed low toxicity in tests with living organisms. With this set of characteristics, the new material can be seen as a multifunctional nanoplatform that is promising for the development of various applications, especially in the areas of biotechnology, health and environment. The study was reported in an article published in ACS Applied Nano Materials (American Chemical Society journal released in 2018), and featured on the cover of the June issue of the journal.
The properties of this nanoplatform derive from the presence of several compounds and elements with distinct properties: iron oxide (Fe3O4, known as magnetite) nanoparticles responsible for magnetism; lanthanide element ions (Gd3 +, Ce3 + and Tb3 +, known as rare earths) responsible for luminescence or light emission, and chitosan (biopolymer obtained from the crustacean exoskeleton), essential for providing chemical bonds of the nanoplatform surface to the external molecules of interest.
The nanoplatform was developed at the Brazilian National Nanotechnology Laboratory of the National Center for Energy and Materials Research (LNNano – CNPEM). The process used for its synthesis comprises a series of steps. Initially, the iron oxide nanoparticles that form the core of the nanoplatforms are synthesized and coated with silicon dioxide (SiO2). Then the luminescent elements and chitosan are incorporated into the nanoparticles forming an outer layer. The result is nanoplatforms of approximately 170 nm in diameter (on average), called Fe3O4@SiO2/GdOF:xCe3+,yTb3+.
To study the magnetic and luminescent properties of the nanoplatform and to characterize its structure and morphology, research groups from the State University of Campinas (Unicamp) and the University of São Paulo (USP) participated in the study.
In addition, the main authors of the paper decided to evaluate the toxicity of nanoplatforms with relation to living organisms – a key step when thinking about health or environmental applications, and they decided to conduct a well-established in vivo test, in which zebrafish embryos (scientific name Danio rerio) are exposed to the material whose toxicity is to be evaluated. These freshwater fish, in fact, has a high genetic similarity to humans (about 70%) and at the same time is cheaper and easier to study than mice or rats, among other advantages.
In the toxicity test, a few dozen freshly fertilized zebrafish eggs were placed in aqueous medium containing the nanoplatforms at various concentrations. The embryos were examined at different development stages using an optical microscope to check for mortality, malformation, edema or changes in size. Tests included embryos with and without chorion (membrane that protects the embryo in the early stages of development). The test results carried out at LNNano showed that nanoplatforms, even at high concentrations (100 mg/L), have low toxicity for all embryo groups.
“This work brings an unprecedented contribution that involves evaluating the toxicity of hybrid nanomaterials using the zebrafish model, a promising alternative method in nanotoxicology, and the influence of the chorion,” says Diego Stéfani Teodoro Martinez, CNPEM researcher at LNNano and one of the corresponding authors of the article.
The embryos were also analyzed at the Brazilian National Synchrotron Light Laboratory (LNLS – CNPEM) to verify the distribution and concentration of nanoplatforms in the organism of the embryos. To do this, the scientists used the synchrotron light X-ray fluorescence microscopy (SXRF) technique, which can accurately map certain chemical elements in biological systems. This technique is available at one of the LNLS experimental stations, coordinated by the researcher Carlos Alberto Pérez, who is one of the corresponding authors of the article.
SXRF analysis showed that nanoplatforms had accumulated in the embryos as a function of exposure time, with higher concentrations in the gastrointestinal tract in the case of already developed mouth embryos – a result that may be significant, for example in the context of healthcare applications involving oral nanoplatform ingestion.
The study was carried out in the context of a postdoctoral project by fellow Latif Ullah Khan, also corresponding author of the article. The completion of the project, says Martinez, was made possible by the availability of skills and facilities at CNPEM’s multi-user laboratories. However, partnerships with other laboratories were also crucial, adds the CNPEM researcher. Professor Marcelo Knobel’s group performed the magnetometry studies at Unicamp. The groups of professors Hermi Felinto Brito and Magnus Gidlund carried out the luminescence and functionalization studies at USP. Finally, Professor Diego Muraca (Unicamp) and researcher Jefferson Bettini (CNPEM) contributed to the structural and morphological characterization using transmission electron microscopy techniques.
“This article was the result of integrating the experience of different Brazilian groups; an interdisciplinary study on the frontier of knowledge in nanobiotechnology and nanotoxicology,” says Martinez, adding that one of the main challenges of the work was integrating knowledge and techniques from different areas, such as Materials, Biology and Toxicology, a task that was coordinated by Martinez and Pérez.
The study received financial support from Brazilian agencies CAPES (including through the CAPES-CNPEM agreement), FAPESP and CNPq (including through INCT-Inomat); from the Brazilian Ministry of Science, Technology, Innovations and Communications (MCTIC) through SisNANO, and The World Academy of Sciences for advancement of science in developing countries (TWAS). The study also received financial support from the Brazil-China Nanotechnology Research and Innovation Center (CBC-Nano).
Applications: biotechnology, health and the environment
According to Martínez, the nanoplatform developed opens perspectives for applications in biotechnology, health and the environment, such as biological tissue and cell imaging systems, medical diagnostic kits, and environmental systems for pollutant detection and remediation
The applications would take advantage of the interesting set of nanoplatform properties. Because they are magnetic, using an external magnet, nanoplatforms could be directed and retained in a particular biological tissue or isolated from, for example, contaminated blood or water. In addition, the luminescence of the nanomaterial would allow visualizing the nanoplatforms within the biological tissues and cells of interest. Finally, the presence of chitosan would enable the chemical binding of drugs and other molecules that would serve for the diagnosis and/or treatment of diseases. “However, much study is still needed for real applications and commercialization of this nanoplatform, as it is a new material and needs to be tested on different models in the future,” says Martinez Martinez.
[Paper: Fe3O4@SiO2 Nanoparticles Concurrently Coated with Chitosan and GdOF:Ce3+,Tb3+ Luminophore for Bioimaging: Toxicity Evaluation in the Zebrafish Model. Latif U. Khan, Gabriela H. da Silva, Aline M. Z. de Medeiros, Zahid U. Khan, Magnus Gidlund, Hermi F. Brito, Oscar Moscoso-Londoño, Diego Muraca, Marcelo Knobel, Carlos A. Pérez, Diego Stéfani T. Martinez. ACS Appl. Nano Mater. 2019, 2,6, 3414-3425. https://doi.org/10.1021/acsanm.9b00339.]
Professor Oswaldo Luiz Alves (IQ – UNICAMP), a member of B-MRS, was awarded the title of Professor Honoris Causa of the Federal University of Ceará (UFC). The title was granted by the University Council of the institution on December 17, 2018. In addition to being a full professor of UNICAMP, Alves has been a collaborating professor of the Graduate Program in Physics of UFC for more than 30 years.
In October 2018, Professor Alves received another important distinction, the admission to the Brazilian National Order of Scientific Merit in the Grand Cross class.
[Paper: Evidence of Band-Edge Hole Levels Inversion in Spherical CuInS2 Quantum Dots. Gabriel Nagamine, Henrique B. Nunciaroni, Hunter McDaniel, Alexander L. Efros, Carlos H. de Brito Cruz, and Lazaro A. Padilha. Nano Lett., 2018, 18 (10), pp 6353–6359. DOI: 10.1021/acs.nanolett.8b02707]
Quantum dots with unique rules
A paper led by Brazilian researchers has revealed surprising news about the rules that determine the energy levels of electrons in quantum dots of copper and indium disulphide (CuInS2,) which stand out in the family of quantum dots for being non-toxic. The work was recently reported in Nano Letters (impact factor of 12.08).
The results of the study, confirmed by experimental and theoretical methods, showed a situation in the structure of energy bands that had never before been observed in other materials.
The band structure is a well-established scientific model that shows which energy states or levels the electrons can occupy in a given material. These states of energy are represented as allowed bands (those that the electrons can reach) and forbidden bands (those in which the electrons cannot be found).
In semiconductors, the energy bands that are allowed for an electron and that determine the properties of a material are the valence band and the conduction band. Both are separated by a band gap. For electrons to “jump” from the valence band to the conduction band, transposing the forbidden band in a process called transition, it is necessary they receive extra energy, which can occur when the material absorbs photons. When they lose energy, these electrons can once again occupy their places in the conduction band, and surplus energy can be emitted as photons (light). This light emission from the absorption of photons is known as photoluminescence.
Researchers at the Gleb Wataghin Institute of Physics at UNICAMP (Campinas, SP, Brazil) discovered that the quantum dots they were studying did not follow the same transition rules as other semiconductor materials and nanomaterials. “Generally, in semiconductors, bulk or nanostructured, the states that form the top of the valence band and the bottom of the conduction band are such that a transition between these states by absorption of a photon is allowed,” contextualizes Lázaro Aurélio Padilha Junior, professor at UNICAMP and corresponding author of the article. “What we showed was that in the studied material (CuInS2 quantum dots), this transition is forbidden by absorbing one photon. The interaction with two photons is required for this transition to occur. As far as we know, this is the first semiconductor system that presents this inversion of states,” says Padilha.
The discovery, besides showing that the norms that govern the electron states in semiconductors are not valid for all the materials, can influence the applications of the quantum dots studied. According to Padilha, the conditions discovered favor the simultaneous emission of two photons in the material when the electrons return to the conduction band. “This could be attractive to lasers systems that emit light in two distinct colors at the same time, and with color adjustment over a wide spectral range,” says the professor. In addition, adds Gabriel Nagamine, the first author of the article, understanding the structure of material bands can improve the performance of existing applications such as luminescent solar concentrators – a technology that can be used both to generate electricity from sunlight and to increase the production of food in greenhouses. “All these applications come from the unique characteristics of the electronic bands of these materials,” says Nagamine.
History of a theoretically announced experimental result
The history of this discovery goes back to 2015, when Professor Padilha, who has worked with quantum dots since 2010, his master’s student Gabriel Nagamine and other members of the research group decided to devote their efforts in studying the quantum dots of CuInS2. “This material caught our attention because it did not have heavy metals in its composition, which made it interesting for applications in biology and medicine, such as fluorescent biological markers,” says Padilha. In fact, quantum dots, which were discovered in the 1980s and are now present in products such as TV screens, present very interesting properties to be used in the detection of diseases and other applications in the health area, but almost all of them are toxic due to their chemical composition.
The UNICAMP team then collaborated with the company UbiQD, located in Los Álamos (USA) and specialized in the production of quantum dots, and which provided samples of spherical and pyramidal quantum dots. The characterization of the samples was performed partly in the company and also in the National Nanotechnology Laboratory (LNNano) of CNPEM, in the city of Campinas (SP, Brazil).
Initially, Padilha and his team set out to investigate how strong the absorption of two photons was in the chosen material, since this optical process allows to make three-dimensional images of the material, which can be very useful in its characterization and also in its application in several areas. To do this, in early 2016, the team performed the main experiments of the work at UNICAMP using a spectroscopy technique that allows detecting light emission from the absorption of two photons. “The first measurements revealed an absorption peak of two photons at smaller energies than those of linear absorption – a fact never previously observed experimentally,” Padilha says. “We believed it was a problem in our laser source and we repeated the experiment, achieving the same results,” he adds. These results, which are shown in the figure to the side, have arisen from the experiments performed with spherical quantum dots. In the pyramid-shaped quantum dot samples, the predominance of of two-photon absorption was not observed.
In May of the same year Padilha met with Dr. Alexander Efros (Naval Research Laboratory, USA) at a conference in South Korea. “He, who is one of the most respected theorists working on the electronic structure of semiconductor quantum dots, mentioned that he had made calculations that predicted a reversal in parity of states in these nanomaterials. We immediately noticed that I had proven his theory, “says Prof. Padilha. After that, they began working together and trying to understand other aspects of the problem, until they submitted the article to Nano Letters. The paper was accepted in less than two months.
The research that originated the paper is part of Gabriel Nagamine’s master’s thesis, defended in 2017 at UNICAMP, and received financial support from Brazilian research agencies (FAPESP and federal CNPq), the student support service (SAE) of UNICAMP and the Office of Naval Research (USA).
Fernando Galembeck’s interest in research began in adolescence, when he realized the economic value of scientific knowledge while working in his father’s company in the pharmaceutical segment. Today, at age 75, Fernando Galembeck can look back at his own scientific trajectory and tell many stories about the generation and application of knowledge.
A founding member of B-MRS, Galembeck was chosen this year to deliver the Memorial Lecture “Joaquim da Costa Ribeiro” – a distinction awarded annually by B-MRS to the trajectory of a distinguished researcher in the Materials area. The honor is also a tribute to Joaquim da Costa Ribeiro, pioneer of experimental research in Materials in Brazil. The lecture, titled “Materials for a better future,” will take place at the opening of the XVII B-MRS Meeting on September 16 of this year, and will address issues such as needs, shortages and promises in the Materials area.
Galembeck graduated in Chemistry in 1964 from the University of São Paulo (USP). After getting his degree, he remained at USP working as an instructor (1965-1980) while doing his Ph.D. in Chemistry (1965-1970), in which he developed research on dissociation of a metal-metal bond. After his Ph.D., he completed post-doctoral internships in the United States, at the universities of Colorado, in the city of Denver (1972-1973) and California, in the city of Davis (1974), working in the field of Physical-Chemistry of biological systems. In 1976, back at USP, he had the opportunity to create a laboratory of colloids and surfaces at the Institute of Chemistry, in an agreement that involved the Institute, Unilever, the Brazilian Academy of Sciences and the Royal Society. From that moment on, Galembeck became more and more involved with the development of new materials, especially polymeric materials, and their manufacturing processes.
In 1980, he joined the State University of Campinas (Unicamp), after which he became a full professor in 1988, where he remained until his retirement in 2011. Since then, he has been a contributing professor at the institution. At Unicamp, he held management positions, notably vice-rector of the university, as well as director of the Institute of Chemistry and coordinator of its post-graduate program. In July 2011, he took over the newly created Brazilian National Nanotechnology Laboratory (LNNano), at the National Center for Energy and Materials Research (CNPEM), remaining in this post until 2015.
Throughout his career, he has held direction or coordination positions at the Brazilian Academy of Sciences (ABC), the Ministry of Science, Technology and Innovation (MCT), the National Council for Scientific and Technological Development (CNPq), Sao Paulo Research Foundation (FAPESP), Brazilian Chemical Society, (SBQ), Brazilian Society for the Advancement of Science (SBPC) and the Brazilian Society of Microscopy and Microanalysis (SBMM), among other entities.
Prof. Galembeck is the author of roughly 279 scientific papers published in peer-reviewed journals, with over 3,700 citations, 35 patents and more than 20 books and book chapters. He has supervised nearly 80 master’s and doctoral degrees.
Fernando Galembeck received numerous awards and distinctions, among them the Anísio Teixeira Award, from CAPES, in 2011; the Telesio-Galilei Gold Metal 2011, from the Telesio-Galilei Academy of Science (TGAS), the Almirante Álvaro Alberto Award for Science and Technology 2006, from CNPq and the Conrado Wessel Foundation; the José Pelúcio Ferreira Trophy, from Finep, in 2006; the Grand Cross of the National Order of Scientific Merit, in 2000, and the National Commendation of Scientific Merit, in 1995, both from the Presidency of the Republic of Brazil. He also received a series of acknowledgments from companies and associations, such as CPFL, Petrobrás, Union Carbide do Brasil, the Brazilian Association of Paint Manufacturers, the Brazilian Chemical Industry Association, the Union of Chemical Industry for Industrial Purposes of the State of Rio de Janeiro, Brazilian Polymer Association, Brazilian Chemical Society (which created the Fernando Galembeck Technological Innovation Award), the Union of Engineers in the State of São Paulo and the Electrostatic Society of America.
This scientist has been a fellow of TWAS (The World Academy of Sciences) since 2010 and from the Royal Society of Chemistry since 2014.
In this interview, you will be able to know a little more about this Brazilian researcher and his work.
SBPMat Newsletter: – Tell us what led you to become a scientist and work on issues in the field of Materials.
Fernando Galembeck: – My interest in research work started during my adolescence, when I comprehended the importance of new knowledge, of discovery. I found this when I was working, after school, at my father’s pharmaceutical laboratory, as I could see how the newest, latest products, were important. I also saw how costly it was, for the lab, to depend on imported raw materials, which were not produced in Brazil, and that in the country there was no competence to manufacture them. Then I realized the value of new knowledge, as well as the importance and the economic and strategic significance of such breakthroughs.
This feeling was increased when I took my major in Chemistry. I enrolled into the Chemistry course because one of my school teachers had suggested that I should seek a career related to research. He must have seen some inclination, some tendency of mine. So I attended the Chemistry course provided by the Philosophy School, in an environment where the research activity was very vivid. Because of that, I decided to conduct my Doctoral studies at USP. At that time, there were no regular graduate studies in Brazil yet. The advisor with whom I defended my dissertation, Professor Pawel Krumholz, was a great researcher, who also had built a very important career working on a company. He was the industrial director of Orquima, a major company by that time. That boosted my interest in research.
I worked with Chemistry for some years and my interest in materials came from a curious occurring. I was almost graduating, in my last vacations during the undergraduate studies. I was at an apartment, resting after lunch. I remember looking at the walls of this apartment and noticing that, with all I had learned in the Chemistry course, I did not have much to say about the things I could see: the paint, the coverings etc. That was Chemistry, but also Materials, and there was not much interest in Materials in the Chemistry course. Actually, Materials became very important in Chemistry mainly because of plastic and rubber, which, at the time, did not have the importance they have today. I’m talking about 1964, when petrochemicals were practically non-existent, in Brazil.
Well, then I started to work with Physical Chemistry, to later work a little in a field that is more oriented to Biochemistry, that is Biological Physical Chemistry and, in 1976, I received a task from the USP Department, which was to build a colloids and surfaces laboratory. One of our first projects was to modify plastic surfaces, in that case, Teflon. Then I realized that a major part of the colloids and surfaces Chemistry existed due to Materials, because the subject lends itself to create and develop new materials. From that moment on, I was getting increasingly involved with Materials, mainly polymers, a little less with ceramics, and even less with metals.
SBPMat Newsletter: – What are, in your own opinion, your main contributions to the field of Materials? Consider, in your answer, all aspects of your professional activity, including cases of knowledge transfer to the industry.
Fernando Galembeck: – I will tell the story in order, more or less. I think that the first important result in the field of Materials was exactly a technique intended to modify the surface of Teflon, that material in which it is very difficult to stick something. There is even that expression, “Teflon politicians”, the ones for which does not matter what you throw at them, they do not stick to anything. But, in certain situations, we want the Teflon to have adhesion. So, by a somewhat complicated path, I managed to see that I already knew how to modify Teflon, but I had never realized that is was important. I knew the phenomenon; I had observed it during my PhD defense. I knew that there was a change happening in Teflon. But it was during a visit to a Unilever laboratory in 1976, when I was talking to a researcher, that I saw that there were people striving to modify the surface of Teflon and achieve adhesion. Then, bringing the problem and the solution together, as soon as I returned to Brazil, I tried to see if I what I had previously observed was really useful, and it worked. That led to the first paper I wrote by myself and my first patent application, at a time when almost nobody talked about patents in Brazil, especially in the university environment. I was very enthusiastic about this: I was approached by companies that were interested in applying what I had done; one the modification in Teflon itself, the other in a different polymer. So I felt great, because I had made a discovery, I had a patent, and there were companies which, at least, would like to know what it was to see if there was a way to use it. One more thing: soon after the paper I wrote was published, I was invited to attend a conference in the United States, which addressed exactly the issue of modifying surfaces. Polymers, plastic and rubber surfaces, a subject with which I was involved for pretty much the rest of my life, up until now.
I will mention a second fact that did not have the same effects, so far. I discovered a method that enables the characterization and separation of very small particles. That was a very interesting paper. It was released, also produced a patent, but had no practical consequences. Recently, there have been some issues related to nanoparticles, which is a very important subject in Materials now, offering a chance to apply what I did over 30 years ago. The name of the technique is osmosedimentation.
Next there was some work that I did by collaborating in projects with Pirelli Cabos. With all this story of surfaces and polymers, I think I had become more or less known and was approached by Pirelli, which contracted me as a consultant and commissioned projects I had at Unicamp. An outcome of these projects, that I think is the most important, was the development of an insulator for very high voltages. This work was not only mine, but rather of a very large team, in which I took part. There were several people from Pirelli, and several from Unicamp. The result of this project was that the Brazilian Pirelli managed to be hired to provide high voltage cables for the Eurotunnel, back in the ‘80s. I think this was a very important case, as it led to a product and brought substantial economic results. I would like to stress that this was done in Brazil, by a Brazilian team. They were not a Brazilian company, but the team was based here.
Then I worked on several studies with nanoparticles, at a time when we did not even call them nanoparticles; we called them fine particles or simply small colloidal particles. The first work I published on nanoparticles was in 1978. There were other things I did next, which ultimately turned into a work on aluminum phosphate, which gave rise to several theses carried out in the laboratory and publications, and was also licensed by a company of the Bunge group, which basically exploits phosphates. The subject started in my laboratory, stayed in the laboratory for several years, then a company from the Bunge group here in Brazil became interested, started to participate, we collaborated. This became a rather large development project. Bunge later found the project unfeasible in Brazil and today it is the United States. I think it’s a pity that it is there, but there were other issues involved, including disagreements with Unicamp, which owns the patents. Recently, the company of the group that worked with these phosphates was Amorphic Solutions, which offered the product on the Internet, for various applications. From what I understand, they are currently emphasizing its use as an anticorrosive material for steel protection. I have recent information that Bunge has negotiated the rights to these products with a large chemical company, but I do not know the details.
About the same time, in another project on nanoparticles, clay/natural rubber nanocomposites were developed. This was licensed by a Brazilian company called Orbys, which released a product called Imbrik, that showed to be good for rubber rolls for paper manufacturing.
Another case with a product. I had done a project with Oxiteno, which manufactures raw materials for latex, the surfactants. They wanted to get an ideia of how much you can change the latex changing the surfactant. I conducted a project with them that I consider one of the most interesting among those in which I have been involved. In the end, we realized that, by changing the surfactant a bit, we changed the latex a lot. These are used in paints, adhesives, resins. So we realized we had a great versatility. This work was published and promoted. It did not result in a patent because it was a comprehension project. So, another company, Indústrias Químicas Taubaté (IQT) approached me to produce cationic latex, but using a new path. Cationic latex in general is made of quaternary ammonium salts, which have some environmental restrictions. The company wanted an alternative that did not have those restrictions. By the end of the project, we produced cationic latex without environmental restrictions, and the IQT put the product on the market.
My participation in a Navy project of developing carbon fibers was a great challenge that gave me big satisfaction. My group participated by synthesizing copolymers of acrylonitrile, up to the scale of ten liters. The results were transferred to a company that produced pilot scale production at the old Rhodia-Ster and Radicci plant in São José dos Campos. The selected copolymer was spun and then pyrolyzed, at the Technological Center of Marinha, in São Paulo. It resulted in a high performance carbon fiber, which was used in the manufacture of a centrifuge, used in Aramar. The challenge was to find the copolymer that showed good performance in the later stages of fiber production, which was achieved.
There was another case that was also very interesting, even though it was canceled. Here in Brazil, there was a large manufacturer of polyethylene terephthalate, PET, which is used for many things, including bottles. They knew about the work I had done with nanocomposites, the one with Orbys I mentioned before, so they approached me wanting to produce PET nanocomposites. We had to find out how to escape from what was already patented abroad and discovered a whole new path. The company was called Rhodia-Ster, and today it is part of another Italian company, called Mossi e Ghisolfi. The company was enthusiastic and ended up patenting it in Brazil, and then later abroad. At a certain point, they decided that they would conduct the work internally, and so they did for some years. One day, my contact within the company called me to tell this: “look, we were working with two technologies; the one held by Unicamp and another one, in another country. Both are working, but the company has reached a point where it has chosen to complete the development of only one”. When coming to the final stage in developing materials, the projects costs are too high. One have to use large amounts of materials, run many tests with customers. So, the company decided to take one project further, and, unfortunately, it was not the one in which I had worked. At the end, it was a little frustrating, but I think that it was interesting, because, during this whole time, the company invested a lot in the path we had started here. Not only that, each project brings resources for the laboratory, jobs at the university and the company etc. So, these projects result in many benefits, even when they are not concluded.
Now, fast forwarding, I will arrive at a more recent result of my work at CNPEM, where I was until 2015. A goal of CNPEM is the use of renewable source materials to make advanced materials. It has a whole philosophy behind it, related to the depletion of natural resources, to sustainability… The goal was to do new things with materials derived from biomass, and the main interest is in cellulose. It is the most abundant polymer in the world, but it is a very difficult polymer to work with. You cannot process pulp as you process polyethylene, for example. One of the goals is to plasticize cellulose; that is, to work the cellulose as closely as possible to the one we use to work with synthetic polymers. An initial result within this idea was the creation of cellulose adhesives in which the only polymer is cellulose itself. Then, by then no longer at CNPEM, we obtained graphite exfoliation, which generated a family of paints, pastes and conductive adhesives, which are the object of a PIPE project recently approved by Fapesp.
This is the latest case. In the middle of the way, many other projects were conducted with companies, for issues of their interest. Coating something, gluing another, modifying a polymer to achieve a certain result. But these were answers to demands from companies, instead of researches started at the laboratory.
SBPMat Newsletter: – Leave a message for our readers who are starting their careers as scientists.
Fernando Galembeck: – First of all, in any chosen career, there must be a dose of passion. It does not matter if you are going to work in the Stock Market, Healthcare or whatever you may do; above all, your taste must decide. If a person chooses a career because it will give them money or status… I think it is a bad choice. If you do things with pleasure, with interest, the money, prestige and status will come from other paths. The goal is to do what makes you happy, what makes you feel good when you do it, what makes you feel accomplished. It is true not only for the scientific career, but also to any other career. In science, it is crucial.
Another point is that you must be prepared to work hard. There is no easy way. I know some young people who are constantly seeking the great idea that will bring them success with relatively little work. Well, I’d better not count on it. It may even happen, but waiting for it is almost the same as wait to win the Lottery and get rich.
I’m over 75, therefore I have met many people and seen many things happen. Something that strikes me is how young people who seemed very promising end up not working very well. Frankly, I think it is bad for youngsters to achieve success too early, because I have the impression they get used to this idea that things will always work out fine. And the problem is that there isn’t anything, anyone, any company that will always work. There will always be the moment of failure, the moment of frustration. If the person is prepared for that, when the times come, he or she will overcome it, while others are crushed – they cannot move one. That is why we must be careful not to be deceived by our success and think that, because it worked once, it will always work. You must be prepared to fight.
When I was in college, thinking about doing research seemed a very strange thing to do, crazy talk. People did not know very well what it was, or why would someone choose to do it. Some people said that research was something like priesthood. I have always worked with research, associated with teaching, consulting and, without having ever sought to become rich, I managed to have an economic status that I deem very comfortable. But I insist, my goal was to enable the development, to produce material, not the money I would receive. Money came, as it does. So, I suggest you to focus on your work, on the results and the contribution that said work may give to other people, to the environment, to the community, to the country, to knowledge. The rest comes as a bonus.
In short, my message is: work seriously, earnestly and passionately.
Finally, I would like to point out that I think the research work, the development work, really helps you to grow as a person. It will depart you from ideas that are not very fruitful and guide you towards attitudes that are really important and helpful. A student asked Galileo once: “Master, what is the method?”, and Galileo’s answer was: “The method is the doubt”. I think it is very important in the research activity, which, for Materials in particular, is especially interesting because the final product is something you can hold in your hands. In the research activity you have to always wonder, “I’m thinking like this, but is this right?”, or “This guy wrote this, but what are his bases to write it?”. This attitude is very different from the dogmatic one, which is common in the realms of politics and religion, and very different from the attitude of someone who has to deceive, as the lawyer who works for a corrupt or drug dealer. The researchers have to commit themselves to the truth. Of course there are also people who call themselves researchers and spread disinformation. Some years ago, people were talking about something called “Bush science”, an expression referring to President Bush. This Bush science was the arguments fabricated by people who gained money as scientists, but who produced arguments to sustain Bush’s policies. In other words, the problem exists in science as well, but then we get back to what I said earlier. You cannot become a scientist because of money, or to achieve prestige, or to be invited to have dinner with the president; you must enter this field because of your interest in the subject itself.
[Exploring Au Droplet Motion in Nanowire Growth: A Simple Route toward Asymmetric GaP Morphologies. Bruno C. da Silva*, Douglas S. Oliveira, Fernando Iikawa, Odilon D. D. Couto Jr., Jefferson Bettini, Luiz F. Zagonel, and Mônica A. Cotta*. Nano Lett., 2017, 17 (12), pp 7274–7282. DOI: 10.1021/acs.nanolett.7b02770]
Moving nanoparticles for asymmetric nanowires.
A team of scientists presented a route to grow semiconductor nanowires having an asymmetric morphology, different from the traditional cylindrical one. The possibility of producing, in a controlled manner, nanowires with particular formats and without defects, can be exploited in several applications, including the production of more efficient solar cells.
The researchers discovered the process while studying the production of nanowires of gallium phosphide (GaP) for optoelectronic applications. The team chose to produce the nanowires by chemical beam epitaxy (CBE), preceded by a thermal treatment (annealing). In this technique, a substrate of a suitable material – in this case gallium arsenide (GaAs) – is placed inside a chamber. Then, chemical compounds in the form of vapor are introduced into the chamber. Some elements of the vapored material deposit over the substrate, layer upon layer, generating films. To promote the growth of nanowires instead of films, metallic nanoparticles (in this case, gold nanoparticles) are placed on the substrate before being exposed to vapor. During exposure, these catalytic nanoparticles cause the material to deposit preferentially underneath them, causing wire-like structures to grow.
While the researchers were analyzing the nanowires they had obtained in the first few months of the work, they found a significant amount of asymmetric nanostructures. “Besides having a particular morphology, we saw that these nanowires had an hexagonal crystal structure (wurtzite) and a very low density of crystallographic defects, which motivated us to study in detail the causes for the formation of this unusual structure,” says Bruno da Silva, PhD student at UNICAMP and corresponding author of the paper.
Da Silva and his supervisor Prof. Mônica Cotta then began to raise and test hypotheses for the cause behind the formation of the peculiar structures. After several experiments and analyses, they focused on a phenomenon that caught their attention: in the early stages of the process, the gold nanoparticles spontaneously moved over the substrate. Hence, the duo undertook a systematic work on heating substrates with nanoparticle catalysts, growing nanowires under various conditions, and analyzing the resulting samples through scanning and transmission electron microscopes and atomic force microscopy.
Da Silva, Prof. Cotta and their collaborators from UNICAMP and LNNano were able to find out why the growth process they used resulted in asymmetric nanowires. The main reason was the movement of the gold nanoparticles, which was thermally activated with the initial annealing. Based on that discovery, the team established a recipe for producing asymmetric semiconductor nanowires in a controlled manner. “Our work was the first to show that the mechanical instability of the nanoparticle catalyst can be used to modify the growth of semiconductor nanowires, in our case, particularly affecting their morphology,” says Bruno da Silva.
The mechanism of the asymmetric nanowires growth presented in the NanoLetters paper can be described as follow. When heated together with the substrate, the nanoparticles begin to crawl and advance through the substrate while consuming the oxide layer that naturally covers the gallium arsenide. Thus, the nanoparticles form asymmetric grooves a few nanometers deep and a few hundred nanometers long. These trails become fertile ground for the growth of the nanowires, since the deposition rate of the vapored material is greater there than in the rest of the substrate, which is covered by the oxide. A pedestal then forms along the grooves and the nanowire grows on top of the pedestal with an asymmetrical format. “We showed that the movement of the particle generates a zone of preferential deposition, and that the combination of this phenomenon with the axial growth “vapor – liquid – solid” leads to the asymmetry in the nanowire,” summarizes da Silva.
Besides describing the formation mechanism of asymmetric nanowires, the work of the Brazilian team generated detailed knowledge about the movement of heated metallic nanoparticles. “We have shown that in addition to temperature, vacuum conditions and surface quality of the substrate are crucial for nanoparticle stability, and that the motion direction is related to the asymmetry of gold dissolution on semiconductor surfaces III-V,” details the doctorate student.
Concerning possible applications, the asymmetry of these nanowires can be explored, for example, in the construction of antireflective layers that reduce the amount of light lost by reflection in solar cells. Another possibility would be to exploit the green emission of these wurtzite gallium phosphide nanowire in lighting devices. Or, why not, to develop an alternative process to electronic litography taking advantage of the gold nanoparticles movement and the trails it forms on the substrate.
The work was funded by Unicamp, the Brazilian federal agencies CNPq and CAPES and the São Paulo Research Foundation (FAPESP).
A 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.
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.
“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
[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.
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).
[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.
“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.
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.
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.
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.
“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.