Brief interviews with scientists: Bernhard Keimer (Max Planck Institute for Solid State Research, Germany).

Posted on Thursday 28 de June de 2018

Superconductivity and giant magnetoresistance are examples of phenomena that can occur in some materials or systems driven by the so-called electronic correlation, in which the behavior of an electron is strongly influenced by the behavior of other electrons of the same system.

At one of the Max Plank Institutes, located in Stuttgart, Germany, a group of researchers led by Professor Bernhard Keimer works hard to understand and control the behavior of correlated electrons. For this, the team produces heterostructures (structures composed of several materials with differentiated characteristics) of metallic oxides, and characterizes them using a series of experimental techniques, mainly of spectroscopy.

Professor Keimer will be at the XVII B-MRS Meeting in September talking about this research program in the lecture “Spectroscopy of collective excitations in oxide heterostructures”. In his plenary talk, Keimer will present methods and results, including some possibilities of controlling correlated-electrons phenomena.

Bernhard Keimer has been director of the Max Planck Institute for Solid State Research as well as honorary professor at the University of Stuttgart since 1998. From 1992 to 1998, he was Professor of Physics at Princeton University. He graduated in Physics from the Technical University of Munich in 1985 and, in 1991, obtained his PhD in Physics from the Massachusetts Institute of Technology (MIT), where he remained for one year as a postdoc. According to Google Scholar, Keimer has an H index of 86 and his scientific production has more than 24,500 citations.

See our mini interview with this German scientist.

B-MRS Newsletter: – One of the goals of the research you perform with your team at the Max Plank Institute is to control the behavior of strongly correlated electrons, right? In your opinion, what could be the most promising applications emerging from this control? Comment shortly, please.

Bernhard Keimer: – Quantum correlations between electrons generate a large variety of electronic ordering phenomena with vastly different macroscopic properties. Understanding and controlling the collective behavior of electrons in “quantum materials” is a grand intellectual challenge for fundamental research. In the long term, research on quantum materials might enable the design of a new generation of devices based on electrons flow with minimal – or even zero – dissipation.

B-MRS Newsletter: – We want to know more about your work. Please choose a paper of your own (your favorite one) related to the subject of the plenary lecture and briefly describe it, as well as share the reference.

Bernhard Keimer: – As a general introduction to the physics of quantum materials, I recommend a recent review article (B. Keimer & J.E. Moore, Nature Physics 13, 1045 (2017)) A particularly fascinating topic is high-temperature superconductivity. My group uses heterostructures and superlattices to investigate novel collective phenomena emerging at the interface between high-temperature superconductors and other quantum materials. As an example, the figure below shows a kaleidoscope of quantum phases in a 50 nm thin layer of a copper oxide superconductor sandwiched between two layers of an oxide ferromagnet (A. Frano et al., Nature Materials 15, 831 (2016)). My group is developing spectroscopic methods that allow visualization of these phases in a depth-resolved manner.

This schematic figure above shows electronic ordering phenomena in a layer of the high-temperature superconductor YBa2Cu3O7 (YBCO) between two ferromagnetic manganese-oxide layers as a function of temperature (T) and distance across the layer. FM = ferromagnetism, SC = superconductivity, AFI = antiferromagnetic insulator, SDW = spin density wave, CDW = charge density wave. The graph below shows the density of mobile charge carriers, p, as a function of distance. (A. Frano et al., Nature Materials 15, 831 (2016)).

For more information on this speaker and the plenary talk he will deliver at the XVII B-MRS Meeting, click on the speaker’s photo and the title of the speech here https://www.sbpmat.org.br/17encontro/home/

Featured scientist: interview with Prof. Oscar Manoel Loureiro Malta.

Posted on Wednesday 29 de November de 2017

Brazil, besides having one of the world’s largest reserves of ores with lanthanide elements, also occupies a prominent place in the research of these elements and their compounds, which have significant applicability in strategic areas such as energy, health and catalysis, as well as in many other areas.

One of the most prominent Brazilian scientists in this research field is Oscar Manoel Loureiro Malta, born in the city of Recife (state of Pernambuco) 63 years ago. Malta is Professor at the Department of Fundamental Chemistry of the Federal University of Pernambuco (UFPE). Over the course of four decades, he has made important contributions to the research on lanthanides, both in the fundamental and applied fields.

Malta defined his interest in science during his high school years. In 1974, he started the Chemical Engineering course at UFPE and the Physics course at the Catholic University of Pernambuco. After completing his degree in Physics, he left the Chemistry course to join the Master’s degree in Physics at UFPE. There he carried out research work on spectroscopy of lanthanide compounds, mentored by Professor Gilberto Fernandes de Sá. In December of 1977, he obtained the master’s degree. He continued his studies on lanthanide spectroscopy in his doctorate at the University of Paris VI (France), also known as Pierre et Marie Curie Université, guided by Professor Yves Jeannin. He obtained his doctorate in March 1981. He then returned to Recife, where that same year he became professor at UFPE. In 1986, he returned to France for one year as a visiting researcher in the group of Paul Caro, a world-renowned scientist in the lanthanide area, linked to the French National Center for Scientific Research (CNRS).

Oscar Malta was visiting professor in several international institutions: University of Wroclaw (Poland) in 2015; University of Aveiro (Portugal) in 2005; Industrial University of Santander (Colombia) in 2000; University of São Paulo, USP, in 1995, 1996 and 1999, and Paulista State University Júlio de Mesquita Neto, UNESP, in 1994-95 and 1998.

At UFPE, he participated in the creation and consolidation of the Department of Fundamental Chemistry, where he served as department head (1987-89) and postgraduate coordinator (1991-93 and 1999-2001). He was also the coordinator of two national research networks: the National Network of Molecular Nanotechnology and Interfaces, RENAMI (2001 – 2009), and the National Institute of Science and Technology for Integrated Markers, INAMI (2009-2015).

Malta has received a number of acknowledgments for his scientific trajectory. On November 15, 2017, he received an honorary doctorate from the University of Wroctaw, an important institution in Poland where nine Nobel laureates have emerged. In 2016, a special edition of the Journal of Luminescence (publisher Elsevier) on lanthanide spectroscopy was dedicated to this researcher from Pernambuco (https://doi.org/10.1016/j.jlumin.2015.11.024). In 2015, Malta received the Ricardo Ferreira Award for Scientific Merit, recently created by the Foundation for Science and Technology of Pernambuco, Facepe. In 2014, he received the Professor Paulo José Duarte Medal from the Brazilian Chemistry Association. In 2003, he became a full member of the Brazilian Academy of Sciences, ABC.

In this year, Malta was chairman of the International Conference on Luminescence (ICL), which, after seventeen editions in the northern hemisphere, was held in the Brazilian city of João Pessoa.

With a productivity research grant 1A of CNPq, Oscar Malta is the author of approximately 180 papers published in international journals, with about 7,000 citations in the Web of Science. The scientist has a 42 H index.

Here is our interview with Oscar Manoel Loureiro Malta.

SBPMat newsletter: What do you believe are your main contributions to the Materials area and why do you consider them more relevant?

Oscar Malta: Since my master’s degree, which I started in 1977, my work has been in the areas of theoretical chemistry, binding field theory, 4f-4f spectral intensities, non-radioactive energy transfer, in particular intramolecular energy transfer in coordination compounds with lanthanide ions whose theory I developed between 1996 and 1998 and which until today I continue working on, as well as several groups in Brazil and abroad. Over the last three decades, in a work that involves great and extraordinary synergy between theory and experiment, we have been able to construct a very successful scheme for the modeling of highly functional luminescent lanthanide ion coordination compounds with the potential for diverse applications such as luminescent markers in bioassays. Many of these results were obtained during the time I coordinated two national nanotechnology networks. The first, National Network of Molecular Nanotechnology and Interfaces (RENAMI), was in force from 2001 to 2009, the second, the National Institute of Science and Technology for Integrated Markers (inct-INAMI), was in force from 2009 to 2015. Coupled to these results two important themes were also developed: the effect of metal nanoparticle plasmas on the luminescence of compounds with lanthanide ions, a subject that is currently linked to the so-called plasmon, and the concept of polarizability of the coating region in the chemical bond as a way to quantify covalence, which I introduced between 2002 and 2005 in order to better understand the chemical bond involving 4f orbitals. This concept was subsequently generalized to any chemical bonding, from single molecules to complex materials. In all these results it is important to emphasize the students’ participation, from scientific initiation to the doctorate.

SBPMat Bulletin: You started researching in the field of lanthanide ion compounds spectroscopy in your master’s degree, 40 years ago, and you’re still working in the area. What most appeals to you in this research topic? Is it still a promising area? What has changed in the research in this area in Brazil since the 1970s so far?

Oscar Malta: Lanthanides and their compounds are fascinating. They took me into the world of theoretical chemistry, in the world of angular momentum algebra, in the world of the interaction of radiation with matter, and into the world of spectroscopy. When I finished my master’s degree, everything was in place for me to go on to do a doctorate in England to work in atomic physics. At that time he was in Recife, at the invitation of Gilberto Sá and Ricardo Ferreira, Paul Caro, one of the most renowned researchers in lanthanide spectroscopy. He presented a seminar that really impressed me. I gave up on going to England and went to work for Paul Caro’s group at CNRS in Meudon-Bellevue in France. At first the plan was to develop an experimental thesis. However, I wanted to work on the theory. Paul Caro accepted this without problems, and a very fruitful theory/experiment interaction emerged that extended to other groups and continues to this day, always with much to do from a fundamental point of view and from the point of view of applications. Brazil is one of the world leaders in this field, with extremely active and internationally recognized research groups in the country. In fact there is again a discussion about the production of lanthanides since Brazil is a country rich with the minerals of these elements, so important for today’s technology and undoubtedly for the future. We cannot overlook this.

SBPMat Bulletin: Now we invite you to leave a message for the readers who are starting their scientific careers.

Oscar Malta: There is now a strong tendency of young researchers (I am referring to the scientific area under consideration here) to exacerbate the value of applied science in a short-sighted manner. As a result they forget the theoretical foundations and they often do not know the history of the subject, even the experimental history, that they work with or intend to work with. It is exhausting (a fact) to notice this in scientific meetings and I usually am amazed. This is like a linear inflationary process in which money is thrown into the market without having a stabilizer. Sooner or later it ends up in trouble, problems whose creative solutions (an assumption that must accompany a scientist) could be found if greater investment had been deposited in the theoretical foundation and greater attention given to the history of the situation at hand. Therefore, with respect to this question, my message is: do not neglect good theoretical formation and the knowledge of the origin of the subject with which you intend to work. Countries that are now developing and exporting good technology realize how important this is.

SBPMat Bulletin: Feel free to share other comments with our community.

Oscar Malta: Science and technology are more than ever a social activity that requires creativity (as always), training, and therefore education, dedication and strong interdisciplinary cooperation. And it requires investments. Without these ingredients, coupled with sound and sensible ethics committees, we will not be able to create intelligent and reliable science and technology policies that will ensure the continuation of human civilization. The great astronomer Carl Sagan said that not taking these ingredients seriously and the notion that five billion years from now our solar system will have been burned (by our red giant), we will have no chance of getting out of here. This sounds like science fiction, but it’s not. Hopefully the next generations, especially our leaders, will realize this. But I am optimistic in this regard, like a great neuroscientist (Miguel Nicolelis) who wrote “Beyond Boundaries”, which I recommend to my colleagues in Materials Science – especially with respect to emerging properties.

Featured paper: Towards two-dimensional diamond.

Posted on Tuesday 31 de October de 2017

Two-dimensional materials, those whose thickness goes from an atom to a few nanometers, have unique properties related to their dimensionality and are protagonists in the development of nanotechnology and nanoengineering.

A team of scientists from five Brazilian institutions and one American institution took an important step in the development of the two-dimensional diamond version. This work on 2D diamond was reported in a paper published in Nature Communications (impact factor 12,124) with open access.

“Our work presented spectroscopic evidence of the formation of a two-dimensional diamond, which we named diamondene”, says Luiz Gustavo de Oliveira Lopes Cançado, professor at the Brazilian Federal University of Minas Gerais (UFMG) and corresponding author of the paper. In choosing the name of the new material, the scientists followed the tradition of using the suffix “ene” for two-dimensional materials, as with graphene, 2D version of the graphite.

In fact, it was from the compression of graphene sheets that the diamondene was obtained by the team led by Professor Cançado. Initially, the team deposited two layers of graphene one on top of the other and transferred the graphene bilayer to a Teflon substrate, chosen for being chemically inert, preventing the formation of bonds with the graphene.

The sample of bi-layered graphene on Teflon was then subjected to high pressures and simultaneously analyzed by Raman spectroscopy at the Laboratory of Vibrational Spectroscopy and High Pressure of the Department of Physics of the Brazilian Federal University of Ceará (UFC). The experimental system used was a diamond anvil cell with a coupled Raman spectrometer. This equipment allows high pressure to be applied to small samples that are immersed in a pressure transmitting medium (in this case, water). The pressure is applied through two pieces of diamond (material chosen for being one of the hardest and resistant to compression), which compress the transmitting medium, which passes the pressure to the sample. At the same time, the spectrometer allows to monitor the changes that occur in the structure of the sample material against the different pressures applied. “In Raman spectroscopy, light behaves like a probe that measures vibrational states of the material,” explains Cançado. As a result of the probing, the spectrometer generates graphs (spectra), through which it is possible to identify the structure of the material being studied.

By analyzing the spectra, the team of scientists observed changes in the two-dimensional material that indicated the transition from a graphene structure to a diamond structure. The researchers were able to conclude that the diamondene was obtained at a pressure of 7 gigapascals (GPa), tens of thousands of times higher than the atmospheric pressure. “The evidence we present in this work is a signature in the vibrational spectrum obtained from a two-dimensional carbon material that indicates the presence of sp3 bonds, typical of the structure of the diamond,” says Professor Cançado.

To explain the formation of diamondene, the team used first principles calculations following the Density Functional Theory and Molecular Dynamics simulations. “These theoretical results guided the experiments and allowed us understanding the experimental results,” says Cançado.

Scheme of the diamondene formation mechanism from two layers of graphene submitted to high pressures (blue arrows) in water as pressure transmitting medium. The gray colored balls represent the carbon atoms; the red ones, the oxygen atoms, and the blue ones, the hydrogen atoms.

According to the theoretical results, when the bilayer graphene system on inert substrate with water as pressure transmitting medium is subjected to high pressures, the distances between the elements of the system decrease and new connections occur among them. “When applying this level of pressure on graphene, connections can change, going from the sp2 configuration to the sp3 configuration,” explains Professor Cançado. The carbon atoms in the upper graphene layer then establish covalent bonds with four neighboring atoms: the atoms of the lower layer and the chemical groups offered by water (OH- and H). The latter are fundamental to stabilize the structure. In the lower layer, in contact with the inert substrate, half of the carbon atoms are bound to only three neighboring atoms. “The pending connections give rise to a gap opening in the electronic structure, as well as polarized spin bands,” adds Cançado.

This feature makes diamondene a promising material for the development of spintronics (the emerging strain of electronics at the nanoscale in spin-bases electronics). According to Cançado, diamondene could also be used in quantum computing, microelectromechanical systems (MEMS), superconductivity, electrodes for electrochemistry-related technologies, DNA engineering substrates and biosensors – applications in which thin diamond films have already proven to have good performance.

However, there is still a long way to go before demonstrating the diamondene applications. Firstly, because the diamondene shown in the article dismantles under normal pressure conditions. To overcome this limitation, the group of Professor Cançado at UFMG is setting up an experimental system that will allow the application of much higher pressures to the samples in the order of 50 GPa and analyze them using Raman spectroscopy. “With this we intend to produce stable diamondene samples, which remain in this form even after having the pressure reduced to the level of ambient pressure,” says Cançado.

In addition, since Raman spectroscopy provides indirect evidence of the structure of the material, it will be necessary to perform direct measurements of the diamondene to know its structure in detail. “The most promising techniques in this case would be X-ray diffraction in synchrotron light sources or electron diffraction,” suggests Cançado. “The complicating factor in this experiment is the need to have the sample subjected to high pressures,” he adds.

The Brazilian history of diamondene

The idea of the 2D diamond formation originated in the doctoral research of Ana Paula Barboza, conducted under the guidance of Professor Bernardo Ruegger Almeida Neves and defended in 2012 in the Department of Physics of UFMG. In this work, Cançado says, atomic force microscopy (AFM) tips were used to apply high pressures on one, two and several layers of graphene. Indirect evidence of the formation of a two-dimensional diamond was obtained by means of electric force microscopy (EFM). The work showed the importance of the presence of two layers of graphene and water for the formation of the sp3 two-dimensional structure. The main results of the research were reported in the article Room-temperature compression induced diamondization of a few-layer graphene [Advanced Materials 23, 3014-3017 (2011)].

Main article authors. On the left, Luiz Gustavo Pimenta Martins (MSc from UFMG and doctoral student at MIT). On the right, Professor Luiz Gustavo Cançado (UFMG).

“The idea of measuring the Raman spectrum of graphene under high pressure conditions (using anvil diamond cells) came after Luiz Gustavo Pimenta Martins, an undergraduate student at the time, developed a very efficient method of transferring graphene to different substrates,” says Professor Cançado. This development was carried out during a visit to the laboratory of Professor Jing Kong at the Massachusetts Institute of Technology (MIT), after having won a grant for international mobility of the Formula Santander Award. During his master’s degree at the Physics Department of UFMG, carried out under the guidance of Professor Cançado and defended in 2015, Pimenta Martins carried out an extensive and systematic work to obtain Raman spectra of graphene samples subjected to high pressures. “There were many visits to UFC and much study until understanding the diamondene formation mechanisms,” explains Cançado.

The research reported in the Nature Communications paper was made possible by the collaborative work of several Brazilian research groups with recognized expertise in various subjects, as well as the participation of the MIT researcher in the sample preparations. Scientists from the physics departments of UFMG and UFC have contributed their recognized expertise in Raman spectroscopy applied to carbon nanomaterials and, in the case of UFC, in experiments under high pressure. Also participating in these experiments were researchers from the Brazilian Federal Institute of Education, Science and Technology of Ceará and the Brazilian Federal University of Piauí (UFPI). In addition, theoretical physicists from the Brazilian Federal University of Ouro Preto (UFOP) and UFMG performed calculations and computational simulations.

The research was funded by Brazilian federal agency CNPq, state agencies FAPEMIG and FUNCAP, Formula Santander Program and UFOP.

[Paper: Raman evidence for pressure-induced formation of diamondene. Luiz Gustavo Pimenta Martins, Matheus J. S. Matos, Alexandre R. Paschoal, Paulo T. C. Freire, Nadia F. Andrade, Acrísio L. Aguiar, Jing Kong, Bernardo R. A. Neves, Alan B. de Oliveira, Mário S.C. Mazzoni, Antonio G. Souza Filho, Luiz Gustavo Cançado. Nature Communications 8, Article number: 96 (2017). DOI:10.1038/s41467-017-00149-8. Disponível em: https://www.nature.com/articles/s41467-017-00149-8]

Postdoctoral Fellowship in Bioelectrochemistry.

Posted on Monday 15 de August de 2016

The Laboratory of Bioelectrochemistry and Interfaces at São Paulo University (USP), campus at São Carlos, São Paulo state, Brazil, invites applications for a postdoctoral research fellowship in bioelectrochemistry under supervision of Professor Frank Crespilho and funded by Foundation for Research Support of the State of São Paulo (FAPESP) contract. The successful candidate will conduct research on the molecular interaction between biomolecules and nanostructures, including:

developing and testing enzymes bioelectrodes;
transferring high activity/selectivity biocatalyst to a surface-confined environment;
studying the interplay between charge transport, mass transport, and molecular conformation of enzymes;
developing new tools for FTIR Chemical Imaging coupled with electrochemistry.

Fellowship for postdoctoral researchers allows you to carry out a long-term research project (12-24 months), starting at 10/2016.

Applicants must have a Ph.D. in chemistry, chemical engineering, electrochemical engineering, or a related field. Experience in enzymes immobilization and basic electrochemistry is required. Experience with UV-Vis spectroscopy, and FTIR spectroscopy, is preferred. The successful candidate must have excellent communication skills and excel in a highly collaborative research environment. In addition to the timely publication of research results in peer-reviewed journals, the responsibilities of the postdoc include drafting progress reports. Interested individuals should send a (i)cover letter, (II) CV and list of publications, and have (III) two letters of recommendation sent to bioelectrochemistry@iqsc.usp.br. The deadline for application is 09/11/2016.

People in SBPMat community: interview with Sidney Ribeiro.

Posted on Wednesday 29 de June de 2016

Sidney José Lima Ribeiro was born in Sao Paulo (Brazil) in 1959. In high school he signed up for a technical course in Chemistry in the seaside city of Santos. He later moved to Araraquara also in the state of São Paulo where he graduated with a bachelor`s (1982), master`s (1987) and then a doctorate degree (1992) in Chemistry at the University of the State of São Paulo Júlio de Mesquita Filho (UNESP). He began his teaching career in the Chemistry Institute – UNESP in 1986. From 2001 to 2003 he was the head of the General and Inorganic Chemistry Department. In 2008, he became a full professor. His postdoctoral fellowship was in France, at the École Centrale Paris (1994) and at the Centre National d’ Etudes des Telecomunications, CNET (1995).

Professor Sidney Ribeiro is a member of the editorial board of the Journal of Sol-Gel Science and Technology (Springer) and the Journal of Non-Crystalline Solids (Elsevier) and editor of the Eclectic Chemistry journal (Chemistry Institute of UNESP).

He is the author of over 300 peer reviewed articles published in international journals, 7 books or book chapters and 19 patent applications. His scientific production has approximately 5,000 citations. He has mentored or supervised a hundred research works, including doctoral theses, master`s dissertations, postdoctoral research and scientific initiation projects.

He was a visiting researcher at the National Institute for Research in Inorganic Materials (Japan) and visiting professor at the University of Trento (Italy), at the Universities of Angers and Toulouse (France), the University of Aveiro (Portugal) and the Federal University of Juiz de Fora (Brazil).

He has been a member of the São Paulo State Academy of Sciences since 2012 and full member of the Brazilian Academy of Sciences (ABC) since 2015.

Here is a brief interview with the researcher.

SBPMat newsletter: – Tell us what led you to become a scientist and work in the Materials area.

Sidney Ribeiro: – I am a chemist. I studied a technical Chemistry course at the Carmelite High School in Santos. Afterward, by now truly enjoying chemistry, I got my Bachelor`s Degree in Chemistry here in Araraquara. I graduated in 1982. I completed my Master`s in Spectroscopy of Lanthanide here at UNESP under the guidance of Professor Ana Maria G. Massabni which included a national doctoral “sandwich” program, with a part of the work done here in Araraquara and another part at the Federal University of Pernambuco under the guidance of Prof. Gilberto Sá. In my doctoral research I started to work in the interface between Chemistry – Physics – Materials Science, in which we participate until today. My post-doctoral research was at the École Centrale Paris and CNET France Telecom from 1994-95.

SBPMat newsletter: – In your opinion, what are your main contributions to the Materials area, considering all aspects of scientific activity?

Sidney Ribeiro: – We have worked with materials containing rare earth ions with applications in photonics and biomedicine. We have two very well-cited review papers that can serve as an example for those interested in learning more about our work:

1-Carlos, LD et al, Lanthanide-Containing light-emitting organic-inorganic hybrids: a bet on the future, Advanced Materials (2009) 21(5) 509-534.

2-Correia SFH et al, Luminescent solar concentrators: challenges for lanthanide-based organic-inorganic hybrid materials, J. of Materials Chemistry A (2014) 2 (16) 5580-5596.

Our postgraduate program is classified by Capes as level 7 (the highest) and our undergraduate courses are among the best in Latin America. This basic science work has resulted in the training of skilled labor (27 master’s degrees, 20 doctoral degrees and 23 postdoctoral supervisions and dozens of undergraduate students), the deposit of 19 patent applications, and spin-offs or cooperation with a dozen small businesses that now manufacture products developed in our laboratories. The trinomial research-education-extension is definitely well explored at IQ-UNESP.

SBPMat newsletter: – Please leave a message to the readers who are beginning their scientific careers.

Sidney Ribeiro: – We are all born liking science. Who, as a child, in a moment of scientific inspiration, didn’t mix our mother’s perfume with insecticide and some olive oil just to “see what came out of it”? This taste for science has to be preserved in our educational system. And for those who are starting out I say: go ahead. The country needs you. Someone said that when you do what you love you will never “have to work”. Work becomes your pastime and it’s really awesome.

Artigo científico em destaque: Estrutura molecular e eletrônica de cromóforos desvendada.

Posted on Thursday 29 de August de 2013

O artigo científico de membros da comunidade brasileira de pesquisa em Materiais em destaque neste mês é:
Marcelo G. Vivas, Daniel L. Silva, Leonardo De Boni, Yann Bretonniere, Chantal Andraud, Florence Laibe-Darbour, J.-C. Mulatier, Robert Zalesny, Wojciech Bartkowiak, Sylvio Canuto, Cleber Mendonca. Revealing the Electronic and Molecular Structure of Randomly Oriented Molecules by Polarized Two-Photon Spectroscopy. Journal of Physical Chemistry Letters, 2013, 4, 1753-1759. DOI: 10.1021/jz4007004.

Texto de divulgação:
Estrutura molecular e eletrônica de cromóforos desvendada

Em um artigo publicado no Journal of Physical Chemistry Letters (JPCL), pesquisadores dos institutos de Física da USP de São Carlos e São Paulo, em colaboração com químicos da França e da Polônia, aplicaram a espectroscopia baseada no fenômeno de absorção de dois fótons em cromóforos (partes de moléculas responsáveis por sua cor) orgânicos quirais (que não podem ser sobrepostos à sua imagem no espelho). A técnica revelou informações muito relevantes de sua estrutura eletrônica e molecular.

“A principal contribuição deste trabalho foi mostrar, através de uma prova de conceito, que a espectroscopia de absorção de dois fótons com controle de polarização é capaz de fornecer informações preciosas e adicionais a respeito da estrutura molecular e eletrônica de cromóforos randomicamente orientados”, resume Marcelo Vivas, primeiro autor do artigo e pesquisador de pós-doutorado no grupo de Fotônica do Instituto de Física da USP São Carlos.

O grupo de Fotônica realiza há mais de quinze anos estudos experimentais em óptica não linear de materiais orgânicos e inorgânicos. Em particular, os estudos sobre espectroscopia óptica de absorção de dois fótons com controle de polarização iniciaram no grupo há cerca de cinco anos. “Uma vez que os integrantes do grupo são majoritariamente físicos de formação e, portanto, não realizam a síntese desses materiais, há sempre a necessidade de colaboração com grupos do Brasil e exterior para aquisição de amostras”, comenta Vivas. Uma dessas colaborações ocorreu com o grupo da professora Chantal Andraud da École Normale Supérieure de Lyon (França), especialista na síntese de materiais orgânicos quirais. Quanto aos pesquisadores da Polônia, químicos teóricos, a colaboração surgiu uns seis anos atrás quando um dos autores do trabalho foi realizar um estágio de um ano na universidade polonesa.

Espectroscopia de absorção de dois fótons
A espectroscopia em questão se baseia no fenômeno óptico não linear da absorção de dois fótons, no qual dois fótons, não necessariamente da mesma frequência, são absorvidos por átomos ou moléculas em um mesmo evento quântico. Os fótons, sobrepostos espaço-temporalmente, promovem uma transição eletrônica para um nível de energia real, correspondente à soma em energia dos fótons individuais. O efeito foi proposto teoricamente por Maria Goppert-Mayer (Goppert-Mayer, M. On Elementary acts with two quantum jumps. Annalen der Physik 8, 273-294) durante sua tese de doutorado, defendida em 1930, mas só foi verificado experimentalmente em 1961, após o advento do laser (W. Kaiser and C.G.B. Garrett, Two-photon excitation in CaF2:Eu2+, Physical Review Letters 7, 229–232 ).

Na espectroscopia, explica Vivas, devido às diferentes regras de seleção da mecânica quântica, o efeito de absorção de dois fótons permite obter acesso e identificar estados eletrônicos que são inacessíveis por técnicas convencionais que utilizam efeitos ópticos lineares.

A pesquisa publicada no JPCL
Os autores do artigo escolheram para o trabalho duas moléculas quirais, uma com estrutura linear e outra com estrutura em forma de V, que, apesar de serem constituídas pelo mesmo grupo químico (fenil-acetileno), apresentam estruturas eletrônicas distintas.

“Moléculas π-conjugadas, como aquelas estudadas no trabalho da JPCL, têm atraído considerável atenção de físicos, químicos, engenheiros e biólogos, uma vez que elas permitem explorar distintos efeitos da interação radiação-matéria e, além disso, possuem aplicações latentes em novas tecnologias”, contextualiza Vivas.

Para estudar sua estrutura eletrônica e molecular com base na absorção de dois fótons, os cientistas utilizaram a técnica de Varredura-Z fazendo uso de um amplificador óptico paramétrico bombeado por um sistema laser amplificado de femtossegundos, e cálculos teóricos baseados na Teoria do Funcional da Densidade.

Um dos resultados mais significativos reportados no artigo do JPCL é a determinação da direção dos momentos de dipolo elétricos, obtidos por espectroscopia de absorção de dois fótons e corroborados por resultados de química quântica. Neste diagrama ilustrativo da geometria molecular das moléculas quirais estudadas, as direções dos dipolos indicam a estrutura molecular mais provável para as duas moléculas dissolvidas em solução de clorofórmio.