B-MRS Newsletter. Year 5, issue 12.


 

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Newsletter of the
Brazilian Materials
Research Society

Year 5, issue 12. December 21, 2018.

boas festas sbpmat

New Year Message

With this New Year Message, I greet the entire B-MRS community in Brazil and abroad.

There is great expectation that in 2019 Brazil will resume its investments in science, technology and innovation, which we believe to be minimally necessary for the continuation of our system, developed with tremendous effort over the last decades. For the meeting that the academic community had with the future minister of science and technology in early December 2018, the main suggestion B-MRS sent was about reinstating the budget in the area. It is known that Brazil needs to invest much more in the generation of knowledge to solve its serious economic and social problems.

Reinstating the budget is merely the first and most urgent step given the situation of universities and research centers in the country. Also important and part of B-MRS ‘s suggestions is the emphasis on partnerships between ministries to address the problems of society at large, which always entail knowledge, technology and innovation, and the search for synergy with the private sector to transform knowledge into wealth and social well-being.

The materials area in particular has proved to be essential for any and all development. From fundamental discoveries of basic science for the understanding of nature and the conquest of space, to applications in medicine and agriculture, areas that today advance significantly from new materials and methodologies created to study them. In Brazil, for example, the inauguration of the first phase of Sirius, a new source of synchrotron light, at the National Center for Research in Energy and Materials, revealed a masterpiece of materials science and engineering. Even more relevant is that much of this technology is Brazilian.

B-MRS has continuously strived to provide space for researchers from Brazil and abroad to communicate and discuss their scientific and technological contributions and to seek partnerships for new challenges in a highly multidisciplinary field. Our last meeting in Natal (RN), in September 2018, repeated the success of previous editions with this perspective. Some of the many advances in materials science and technology have been highlighted in the B-MRS Newsletters, as a sample of the quality work being done in Brazil. B-MRS also considers scientific dissemination as strategic. In 2018 it created a new section of the Newsletter, entitled “From idea to innovation,” with special reports on the effect of technologies that reached society.

I end by thanking the B-MRS community, wishing all much health and success in 2019, and hoping to meet many of you in our next Meeting at Balneário Camboriú (SC), September 22-26.

Professor Osvaldo Novais de Oliveira Junior
President of B-MRS

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XVIII B-MRS Meeting/ Encontro da SBPMat
(Balneário Camboriú, SC, Brazil, September 22 – 26, 2019)

The event website is up and running! www.sbpmat.org.br/18encontro/

Symposia. 23 symposia proposed by the international scientific community were approved by the organization and comprise this edition of the event. See the symposia list, here.

Abstract submission. The submission of abstracts is open until April 15. Approval, modification, or rejection notifications will be sent by May 31. Final notices for abstracts needing modification will be sent by June 21.

Student awards. To participate in the Bernhard Gross Award, authors must submit an extended abstract by July 11 in addition to the conventional abstract. Learn more, here.

Registrations. Registration is now open. More information, here.

Venue. The meeting will be held in the delightful Balneário Camboriú (State of Santa Catarina, Brazil), at the Hotel Sibara Flat & Conventions, located in the center of the city, close to many hotels, restaurants and shops, and only 100 meters from the sea. More information, here.

Memorial lecture. The traditional Memorial Lecture Joaquim da Costa Ribeiro will be given by Professor Yvonne Primerano Mascarenhas (IFSC – USP).

Plenary lectures. Leading scientists from institutions in Germany, Italy, Spain and the United States will deliver plenary talks on border issues at the event. There will also be a plenary session by the Brazilian scientist Antônio José Roque da Silva, director of CNPEM and the Sirius project (new Synchrotron Light Lab). Learn more about the plenary sessions, here.

Organization. The chair of the event is Professor Ivan Helmuth Bechtold (Physics Department of UFSC) and the co-chair is Professor Hugo Gallardo (Department of Chemistry of UFSC). The program committee is formed by professors Iêda dos Santos (UFPB), José Antônio Eiras (UFSCar), Marta Rosso Dotto (UFSC) and Mônica Cotta (Unicamp). Get to know all the organizers, here.

Exhibitors and sponsors. 9 companies have already confirmed their participation in the event. Those interested in booking booths or participating with other forms of sponsorship can contact Alexandre at the e-mail comercial@sbpmat.org.br.

News from B-MRS Members

– B-MRS member Prof. Sidney José Lima Ribeiro (UNESP Araraquara) was elected Fellow of the European Academy of Sciences. Know more.

– B-MRS member Prof. Luciana Reyes Pires Kassab (Fatec São Paulo) is co-editor of Elsevier’s new book on photonic nanostructures. Know more.

– B-MRS members Prof. Sérgio Ricardo de Lázaro and Luis Henrique da Silveira Lacerda (UEPG) are authors of a new book on Density Functional Theory. Know more.

Reading Tips

– A Brazilian scientific team obtained the largest sheet of graphene ever reported with a simple and low-cost chemical method. The work is the cover article of the flagship journal of the Royal Society of Chemistry (RSC) and was published in the RSC release (Chemical Science). Know more.

– Scientists are able to film microparticles colliding with metals at high speed and, based on the data obtained, they can predict what can occur with the particle and the material. The study can benefit industrial processes based on the impact of microparticles on surfaces (Nature Communications). Know more.

– When conducting experiments with gold nanoparticles, scientists discover an unexpected way of light – matter interaction, which can help develop optical nanoantennas and nanocomponents (ACS Nano). Know more.

Opportunities

Postdoctoral positions for glass research at CeRTEV (São Carlos, Brazil). Know more.

Events

Escola de Fabricação e Caracterização de Nanomateriais e Nanodispositivos. Duque de Caxias, RJ (Brazil). February 18 – 22, 2019. Site.

International Workshop on Advanced Magnetic Oxides (IWAMO 2019). Aveiro (Portugal). April 15 – 17, 2019. Site.

2019 E-MRS Spring Meeting e IUMRS – ICAM. Nice (France). May 27 – 31, 2019. Site.

XVIII B-MRS Meeting. Balneário Camboriú, SC (Brazil). September 22 – 26, 2019. Site.

Follow us on social media

You can suggest news, opportunities, events or reading tips in the materials field to be covered by B-MRS Newsletter. Write to comunicacao@sbpmat.org.br.

 

 

 

New Year Message from B-MRS.


With this New Year Message, I greet the entire B-MRS community in Brazil and abroad.

There is great expectation that in 2019 Brazil will resume its investments in science, technology and innovation, which we believe to be minimally necessary for the continuation of our system, developed with tremendous effort over the last decades. For the meeting that the academic community had with the future minister of science and technology in early December 2018, the main suggestion B-MRS sent was about reinstating the budget in the area. It is known that Brazil needs to invest much more in the generation of knowledge to solve its serious economic and social problems. Reinstating the budget is merely the first and most urgent step given the situation of universities and research centers in the country. Also important and part of B-MRS ‘s suggestions is the emphasis on partnerships between ministries to address the problems of society at large, which always entail knowledge, technology and innovation, and the search for synergy with the private sector to transform knowledge into wealth and social well-being.

The materials area in particular has proved to be essential for any and all development. From fundamental discoveries of basic science for the understanding of nature and the conquest of space, to applications in medicine and agriculture, areas that today advance significantly from new materials and methodologies created to study them. In Brazil, for example, the inauguration of the first phase of Sirius, a new source of synchrotron light, at the National Center for Research in Energy and Materials, revealed a masterpiece of materials science and engineering. Even more relevant is that much of this technology is Brazilian.

B-MRS has continuously strived to provide space for researchers from Brazil and abroad to communicate and discuss their scientific and technological contributions and to seek partnerships for new challenges in a highly multidisciplinary field. Our last meeting in Natal (RN), in September 2018, repeated the success of previous editions with this perspective. Some of the many advances in materials science and technology have been highlighted in the B-MRS Newsletters, as a sample of the quality work being done in Brazil. B-MRS also considers scientific dissemination as strategic. In 2018 it created a new section of the Newsletter, entitled “From idea to innovation,” with special reports on the effect of technologies that reached society.

I end by thanking the B-MRS community, wishing all much health and success in 2019, and hoping to meet many of you in our next Meeting at Balneário Camboriú (SC), September 22-26.

Professor Osvaldo Novais de Oliveira Junior

President of B-MRS

ano novo sbpmat

B-MRS Member is elected Fellow of the European Academy of Sciences.


Prof. Sidney Ribeiro
Prof. Sidney Ribeiro

Professor Sidney José Lima Ribeiro (UNESP Araraquara), a member of B-MRS, was elected Fellow of the European Academy of Sciences (EurASc) in December.

EurASc Fellows are elected through a process that consists of a nomination made by a member of the society, who must present curriculum information of the nominee and nomination and support letters, followed by acceptance of the application by the Society’s General Board. After that, the candidate is invited to be a Fellow.

Members of B-MRS are authors of a new book on Density Functional Theory.


Prof Sérgio Ricardo de Lázaro (left) and Luis Henrique da Silveira Lacerda.
Prof Sérgio Ricardo de Lázaro (left) and Luis Henrique da Silveira Lacerda.

The members of B-MRS Sérgio Ricardo de Lázaro (UEPG professor) and Luis Henrique da Silveira Lacerda (PhD student at UEL/UEPG/UNICENTRO) are the authors of the book “Teoria do Funcional da Densidade e Propriedades dos Materiais“, published by the Brazilian publishing house CRV. The book is co-authored by Renan Augusto Pontes Ribeiro, also a doctoral student of the program. The Density Functional Theory (DFT) is based on Quantum Mechanics and was applied in the area of Materials Chemistry.

More information about the book: https://editoracrv.com.br/produtos/detalhes/33536-crv

B-MRS Member is editor of Elsevier’s new book on photonic nanostructures.


Prof. Luciana R. Pires Kassab
Prof. Luciana R. Pires Kassab

Professor Luciana Reyes Pires Kassab (School of Technology of São Paulo, Fatec São Paulo), member of B-MRS, is co-editor of the book “Metal Nanostructures for Photonics”. Published by the Elsevier publishing house, the book discusses the photonic properties and applications of metallic nanostructures, as well as the preparation and characterization techniques of these materials. Professor Luciana R. Pires Kassab edited the book with Professor Cid Bartolomeu de Araujo (UFPE). More than 40 authors from different countries sign the book’s 12 chapters.

More information about the book: https://www.elsevier.com/books/metal-nanostructures-for-photonics/pires-kassab/978-0-08-102378-5

B-MRS Newsletter. Year 5, issue 11.


 

If you cannot see this newsletter properly, see it here

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Newsletter of the
Brazilian Materials
Research Society

Year 5, issue 11. December 7, 2018.
Featured Paper

Experiments carried out with quantum dots by Brazilian scientists revealed a situation in the energy band structure of these nanoparticles that had never before been observed in any material. The study was reported in Nano Letters. Know more.

2PA QD.jpeg

Featured Scientist

We interviewed Heinz von Seggern, Professor at TU Darmstadt and former researcher at Bell Labs and Siemens. This distinguished German scientist, whose contributions range from fundamental research to invention of devices, has a history of interactions with the Brazilian materials community, including the pioneer Bernhard Gross. Know more.

seggern

News from B-MRS Members

  • Prof. Valmor Roberto Mastelaro (IFSC-USP) took over as editor of the Journal of Alloys and Compounds. Know more.

  • Founding member Prof. Fernando Galembeck (Unicamp) wins national award in the area of Nanotechnology. Know more.

XVIII B-MRS Meeting/ Encontro da SBPMat
(Balneário Camboriú, SC, Brazil, September 22 – 26, 2019)

Symposia. A large number of proposals was received. The list of approved symposia will be announced shortly.

Abstract submission. The important dates will be announced shortly.

Plenary speakers and Memorial Lecture. Find out what the plenary talks of the event will be and who will deliver the traditional Joaquim da Costa Ribeiro Memorial Lecture. See here.

Organization. The meeting chair is Prof. Ivan Helmuth Bechtold (Department of Physics, UFSC) and the co-chair is Prof. Hugo Gallardo (Department of Chemistry, UFSC).

Exhibitors and sponsors. Companies interested in participating in the event with booths or sponsoring can contact Alexandre at comercial@sbpmat.org.br.

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Reading tips

  • After understanding the relationship between microstructure and properties in superalloy, scientists apply treatment that leaves this material even stronger and resistant at high temperatures, allowing its use in electric generators and nuclear reactors (Science Advances). Know more.

  • Scientists find experimental evidence that two-dimensional CrI3 material can behave as a magnetic topological insulator without having an external magnetic field, and point to the possibility of using the material in spintronics (Physical Review X). Know more.

  • Inspired by plant leaves, a polymer with a micro-channel network through which fluids circulate regulates its own temperature and generates thermoelectric energy, promising applications in health, civil construction, aerospace industry and more (Scientific Reports). Know more.

  • Scientists develop a transmission electron microscopy tool to study the interaction of light and matter in individual particles at the nano scale in real time and at very high resolution (Scientific Reports). Know more.

Opportunities

  • Postdoctoral fellowship in Condensed Matter Physics at the Federal University of ABC (SP, Brazil). Know more.

  • Postdoctoral fellowship at USP (SP, Brazil). Know more.

Events

  • VII Curso de Análise de Minerais/Minérios pelas Técnicas de DRX e FRX. Fortaleza, CE (Brazil). December 10 – 14, 2018. Site.

  • International Workshop on Advanced Magnetic Oxides (IWAMO 2019). Aveiro (Portugal). April 15 – 17, 2019. Site.

  • 2019 E-MRS Spring Meeting e IUMRS – ICAM. Nice (France). May 27 – 31, 2019. Site.

Follow us on social media

You can suggest news, opportunities, events or reading tips in the Materials field to be covered by B-MRS Newsletter. Write to comunicacao@sbpmat.org.br.
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Featured scientist: Prof. Heinz von Seggern (TU Darmstadt).


Prof. Heinz von Seggern.
Prof. Heinz von Seggern.

In the late 1970s, when he was a doctoral student, German scientist Heinz von Seggern came to Brazil for the first time and met some local researchers with whom he still collaborates. His host was Bernhard Gross, another German scientist who settled in Brazil and is considered a pioneer of materials research in the country. This was the beginning of a series of scientific visits of Heinz von Seggern to Brazil, which included the participation in five B-MRS Meetings, the annual events of the Brazilian Materials Research Society. In the last edition of the event, Prof. Heinz von Seggern gave a plenary lecture on ferroelectrets.

Heinz von Seggern graduated in physics from the University of Hannover (Germany) in 1976. He received his PhD degree in electrical engineering from the Technical University of Darmstadt (Germany) in 1979. His thesis advisor was Prof. Gerhard Sessler, one of the inventors of the electret microphone, the most common type of microphone in use today. After that, Heinz von Seggern became a postdoc and then a principal investigator at AT&T Bell Laboratories (USA). From 1985 to 1997 he worked at Siemens research center in Erlangen (Germany), starting as a principal investigator and then being promoted to department head. Since 1997, he is Full Professor at the Technical University of Darmstadt, where he leads the Electronic Materials Group.

Throughout four decades of scientific research, Heinz von Seggern has made important contributions to the field of materials in understanding fundamental phenomena, developing analysis techniques and applications, and inventing devices. He has published over 280 scientific papers in peer-reviewed journals with more than 7.800 citations, and his h-index is 46 (Google Scholar).

See our interview with this scientist.

B-MRS Newsletter: – What motivated you to become a scientist and, particularly, a materials scientist?

Heinz von Seggern: – Already as a high school student my main interest was directed towards natural science. This passion was strongly motivated by one of my teachers who really understood to ignite my love for this field. So it was a natural thing for me to study physics, however, I always kept in touch with mathematics and chemistry. After finishing my diploma in physics at the Technical University of Hannover, I started my PhD work at the Technical University of Darmstadt in the electrical engineering department and two and a half years later I received my doctoral degree in electrical engineering. The following four out of five years I spend at Bell Laboratories in Murray Hill, New Jersey, USA whose fabulous working conditions made my bonds to science even stronger. My path to materials science started with my return to the Corporate Research Laboratories of Siemens AG in Erlangen where I spend 12 years before being appointed as full professor at the Technical University of Darmstadt, Germany. In the time at Siemens my work was focused on more practical aspects of science which I started to be increasingly interested in. The ability to change and adapt material properties to practical demands, which is the basic task of materials science, fascinated me more and more.

B-MRS Newsletter: – In your opinion, what are your main contributions to the materials field? Please, select a couple of discoveries/developments, describe them briefly, describe the context in which they were developed, and share the papers or patents references.  

Heinz von Seggern: – Since I have been working on different subjects it is not so easy to point out my or our major contributions to the materials field. I will concentrate on one discovery or development in each field. Let’s start with my PhD study. My task was to understand the charge transport and electronic trap structure of Teflon FEP thin films to back up the lifetime expectation of electret microphones. I discovered by means of thermally stimulated discharge measurements that Teflon FEP contains two different types of energetically deep electron traps which are located near the surface and in the bulk of the films, respectively. This discovery was made possible by comparing TSD results of corona and electron beam charged samples whereby corona charging leads to filling of traps close to the surface and electron-beam charging allowed for deposition of charge into surface and bulk traps dependent on the utilized electron energy. From this finding a charge transport model was developed depending on the initial location of the electrons after charging. The model is based on trapping and thermally induced release. In case of corona charging the transport is initiated by a thermally induced release of electrons from surface traps and a subsequent capture and release by deeper bulk traps [Ref: H. von Seggern, J. Appl. Phys. 50, 7039 (1979), Heinz von Seggern, J. Appl. Phys. 50, 2817 (1979)]. The morphological reason for the different trap depth of surface and bulk can be seen in the film production process where different cooling rates apply to the surface and the bulk of the films.

After finishing my PhD degree I continued this research at Bell Laboratories investigating the transport of positive charges. In contrast to the electron traps, hole traps are relatively shallow and are distributed through the complete film. Once filled they empty relatively fast already at room temperature. On the other hand the number to energetically deep traps was found to be rather small resulting in a low capture rate. This implies that holes have a high probability to penetrate the film via hopping through shallow traps without being captured by deep traps which implies a rather low charge stability of Teflon FEP for holes. We were able to show that this problem can be circumvented by charging at high temperatures filling only deep traps [H. von Seggern, J. West, J. Appl. Phys. 55, 2754 (1984)]. This charge stabilization for positive charges recently became important with respect to so called piezoelectrets where by symmetry breaking a novel piezoelectric material was generated utilizing only nonpolar components. For these devices the stability of both charge types is essential.

During my time at the Corporate Research Labs of Siemens AG in Erlangen the field of interest changed to x-ray storage phosphors which are currently applied in so called image plates used commercially in x-ray diagnostics. The image plate thereby combines the classically utilized silver halide film and the intensifying screen where latter was applied to convert incoming x-rays to visible photons which are then detected by the photographic silver halide film. The working principle of the image plate is that by x-ray exposure electrons and holes are generated and trapped as F-centers and Vk-centers, respectively. Readout occurs by photostimulation of the electron and radiative recombination with the Vk-center. The released energy is then converted to a rare earth ion which emits light at its characteristic wavelength. Thereby the intensity of the emitted photons is indicative for the locally absorbed x-ray dose. My major contribution to this field was the discovery of the basic working principle of these photostimulable phosphors and the existence of spatially correlated and uncorrelated PSL centers which allowed for a deeper insight into the physics of storage phosphors [H. von Seggern et al., J. Appl. Phys. 64, 1405 (1988)]. Another contribution was the invention of neutron image plates fabricated by mixing an effective neutron absorber to the granular storage phosphor particles [T. Bücherl, H. von Seggern et al., Nucl. Instr. Meth. A333, 502 (1993)]. This technique is still widely used in neutron image detection.

After accepting the position as full professor in Materials Science at the Technische Universität Darmstadt I concentrated my efforts on the field of Organic Electronics which I already started at Siemens some years before. In the first years we focused on the energetic trap distribution of organic semiconductors. We were the first to experimentally prove the existence of a Gaussian trap distribution predicted earlier by Bässler et al. through Monte Carlo simulation. The experimental method used a refined thermally stimulated discharge technique known as fractional discharge, where a stepwise increase in temperature combined with the corresponding thermal release of charge allows one to determine the trap distribution which up to now is the only technique known to directly determine the distribution of traps  [N. von Malm et al., J. Appl. Phys. 89, 5559 (2001); R. Schmechel et al., Phys. Stat. Sol. (a) 201, 1215 (2004)].The largest scientific attention we received, however, for the invention of the organic light emitting transistor (OFET) based on tetracene and a polyfluorene derivative. In such OFETs it was shown to be possible to obtain ambipolar transport by injection of electrons and holes from source and drain, respectively [A. Hepp, H.von Seggern et al., Phys. Rev. Let. 157406, 1 (2003); M. Ahles, H. von Seggern et al., Appl. Phys. Let. 84, 428 (2004)].  It was also shown that the same ambipolar transport can be used to construct colour tunable OFETs [E. J. Feldmeier, H. von Seggern et al., Adv. Mater 22, 3568 (2010)] where the motion of the emissive recombination zone through the transistor channel is used to excite different overlaying organic semiconductors with different emission wavelength.

In the last years at TU Darmstadt I have revisited charge storage in organic polymers known as ferro- or piezoelectrets. The cellular polymer polypropylene has shown by Finnish scientists to exhibit large piezoelectric d33 coefficients after poling by high electric fields with the only disadvantage that the trapped charge turned out to be thermally unstable. Therefore structures changed quickly to Teflon based sandwiches of solid FEP /ePTFE/ solid FEP, where ePTFE is a highly porous PTFE consisting of up to 98% air, and later to completely air filled structures. My contribution to that field is the physics explaining the hysteresis and thereof the deduction of the maximal stable polarization which then allows for the theoretical deduction of the piezoelectric d33 coefficient for plane-parallel structures. This knowledge allows for the optimization of the piezoelectric effect and therewith increases the potential for future applications [S. Zhukov, H. von Seggern et al., J. Appl. Phys. 102, 044109 (2007); S. Zhukov, H. von Seggern et al., Scientific reports 8, 4597 (2018)].

B-MRS Newsletter: – Please make a brief story of your interaction with Brazil and with Prof Bernhard Gross.

Heinz von Seggern: – During my PhD work at TU Darmstadt Prof. Gross was a frequent guest of Prof. G.M. Sessler, my thesis adviser. Before my final PhD defense he invited me to visit the Institute of Physics of the University of Sao Paulo (USP) in Sao Carlos. Here I met all the people with whom I am still in contact and friendship with, namely Roberto M. Faria and Jose A. Giacometti who in the meantime have become established professors at USP. After finishing my PhD study I went to Bell Laboratories, Murray Hill, NJ. where Profs. Gross and Sessler were welcomed guest almost every year. The collaboration was extremely fruitful and resulted in a number of joint publications. In 1984 I then left Bell Labs and started to work at Siemens Corporate Research on different topics for the next 12 years and naturally the collaboration was at rest. But as soon as I became appointed Professor at TU Darmstadt I revitalized my connection to the Institute of Physics of Sao Carlos, whose polymer group is now called Grupo de Polimeros “Prof. Bernhard Gross”. From that year on I visited initially Prof. Giacometti and later Prof. Faria almost yearly for up to two month financed generously by FAPESP through various programs. These stays were always very enjoyable and busy, and quite a few publications have resulted.

B-MRS Newsletter: – How many times did you attend the B-MRS Meetings? Do you remember when was the first time?

Heinz von Seggern: – In total I have attended five Brazilian MRS meetings starting in Natal 2007, Florianopolis 2012, Joan Pessoa 2014, Rio de Janeiro 2015 and again Natal in 2017. My first stay in Natal 2007 I remember especially since I was allowed to present our work in front of a great audience on the recently discovered light emitting organic field effect transistor and, on a more personal note, I also remember the wonderful Caipirinha my wife and me were enjoying every evening during sunset at the ocean side.

B-MRS Newsletter: – You have about 40 years of strong experience as a researcher. Please leave a short message with some advice for the students and junior scientists of our community. 

Heinz von Seggern: – In the context of scientific education Prof. Bernhard Gross once said to me: “For someone who knows nothing, everything is possible.” There is a lot of truth in these few words. We all tend to sometimes talk about things that seem to be obvious to us but in reality they are not. My advice therefore is, especially to young scientists, to always ask yourself whether you understand the physical and/or chemical grounds of your current research. If not I suggest to you to acquire the missing basics, which then allows you to select from “everything is possible” the physical meaningful trials. This will definitely help you to make the right decisions to continue your research in a meaningful way.

Featured paper: Quantum dots with unique rules.


[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

box englishA 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.

Diagrama simplicado da estrutura de bandas de um semiconductor. https://en.wikipedia.org/wiki/Valence_and_conduction_bands#/media/File:Semiconductor_band_structure_(lots_of_bands_2).svg
Simple diagram of semiconductor band structure. Credits

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.

Foto dos estudantes Gabriel Nagamine (na frente) e Henrique Nunciaroni, os dois primeiros autores do paper, trabalhando no laboratório.
Photo of the students Gabriel Nagamine (front) and Henrique Nunciaroni, the first authors of the paper, working in the laboratory.

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.

Esta figura mostra o espectro de absorção de dois fótons (pontos amarelos) e o espectro de absorção de um fóton (línea azul) em pontos quânticos de CuInS2 esféricos. As setas indicam os picos de absorção de dois fótons (setas amarelas) e de um fóton (seta azul). No canto superior esquerdo, há uma imagem de microscopia eletrônica de transmissão mostrando um dos pontos quânticos.
Two-photon (yellow dots) and one-photon (blue line) absortion spectra in spherical CuInS2 quantum dots. Arrows: absortion peaks of two photons (yellow) and one photon (blue). In the upper left corner, TEM image showing one of the quantum dots.

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).

XVIII B-MRS Meeting: plenary lectures and memorial lecture.


XVIII B-MRS Meeting (Balneário Camboriú, SC, Brazil, September 22 – 26, 2019).

logo médioMemorial Lecture

Prof. Yvonne Primerano Mascarenhas

Instituto de Física de São Carlos – USP

 

Plenary Lectures

Prof. Julia Greer

California Institute of Technology – USA

Title: Materials by Design: Three-Dimensional Nano-Architected Meta-Materials

Prof. Stefano Baroni

Scuola Internazionale Superiore di Studi Avanzati – Italy

Title: Multi scale simulation of the color optical properties of natural dyes in solution

Prof. Alan Taub

University of Michigan – USA

Title: Challenges in Processing of Materials to Reduce Weight of Structural Components

Prof. Norbert Koch

Humboldt-Universität zu Berlin – Germany

Title: Hybrid inorganic/organic semiconductor structures for opto-electronics

Prof. Mingzhong Wu

Colorado State University – USA

Title: Spin Transfer in Topological Insulator/Magnetic Insulator Bi-Layered Structures

Prof. Maurizio Prato

Università degli Studi di Trieste – Italy

Title: Multifunctional Hybrid Carbon Interfaces