Featured paper: Disclosing structural disorder in nanomaterials.


[Paper: Decreasing Nanocrystal Structural Disorder by Ligand Exchange: An Experimental and Theoretical Analysis. Gabriel R. Schleder, Gustavo M. Azevedo, Içamira C. Nogueira, Querem H. F. Rebelo, Jefferson Bettini, Adalberto Fazzio, Edson R. Leite. J. Phys. Chem. Lett. 2019 10 1471-1476. https://doi.org/10.1021/acs.jpclett.9b00439]

Disclosing structural disorder in nanomaterials

It is known that it is very important to know and control the structure of a material (how its atoms are arranged in three-dimensional space) as it is largely responsible for the properties of the material and therefore for its applications. For example: regions of disorder in crystalline materials (whose atoms, ideally, are ordered in regular patterns) change some expected behaviors for these materials. Unfortunately, knowing the structure of some materials in detail can be a difficult task – particularly when it comes to nanomaterials.

Concentrating various skills and experimental and theoretical resources, a Brazilian team developed a method to establish the degree and location of disorder in the structure of crystalline and non-crystalline nanomaterials, interfaces and surfaces. The method, which is based on the combination of an experimental technique (transmission electron microscopy), a data analysis method (pair distribution function) and computational simulations, is already available to the scientific community at the Brazilian National Nanotechnology Laboratory (LNNano), and should help develop better performing materials.

In addition to developing the technique, the team applied it in the study of structural disorder in nanocrystals, which are basic elements of nanotechnology and are used for example, in solar cells and electronic devices. Although by definition they have ordered structures, these crystals of nanometric dimension can exhibit, in practice, regions with structural disorder.

In order to carry out the study, the scientists produced faceted nanocrystals of about 3.2 nm in diameter, formed by a core of zirconium dioxide (ZrO2), inorganic material, and a shell made up of organic substances known as ligands, whose atoms form chemical bonds with atoms that are on the surface of the inorganic nucleus. Ligands have the important role of stabilizing the nanocrystals, thus preventing them from aggregating.

The team produced a first series of nanoparticles with ligands containing an aromatic ring and analyzed it using the developed method. The samples were then subjected to a process known as ligand exchange in which chemical reactions occur in the material in the presence of a solvent at a temperature above its boiling point. In these reactions, some connections break down and new connections occur. As a result of the ligand exchange, the team was able to produce nanoparticles with shells containing oleic acid, which were also analyzed using the developed method.

This figure refers to a nanocrystal of ZrO2 before and after the ligand exchange. The figure includes high-resolution images of transmission electron microscopy, structural models and PDF patterns obtained by the developed method.
This figure refers to a nanocrystal of ZrO2 before and after the ligand exchange. The figure includes high-resolution images of transmission electron microscopy, structural models and PDF patterns obtained by the developed method.

The scientists concluded that, unlike the ideal nanocrystal of zirconium dioxide, the two types of nanocrystals analyzed had a degree of structural disorder located on the surface of the nucleus.  In addition, in the second group of nanoparticles, the disorder was significantly lower. The researchers interpreted this reduction as a result of the high temperature of the ligand exchange process, which altered the tensions of the network of atoms.

“In our work, we were able to directly assess the degree and location of disorder in the nanocrystals, which until then was not technically feasible,” says Gabriel Schleder, PhD candidate in the Graduate Program in Nanosciences and Advanced Materials of the Brazilian Federal University of the ABC (UFABC).

By better understanding structural disorder and its causes, the researchers were able to point out a way to control it. “Any property that significantly depends on surface-located structural disorder could be in principle controlled by this kind of ligand exchange process,” says Schleder. “Mechanical properties, photoluminescence, electronic transport and catalytic properties are some of them,” he adds.

The research was reported in a recently published article in The Journal of Physical Chemistry Letters (impact factor = 8,709).

Overcoming the challenge through collaborations

The initial idea of the study appeared in a meeting held at the end of 2017 at the National Center for Research in Energy and Materials (CNPEM), located in the city of Campinas, São Paulo. At the meeting, a group of reserachers discussed the implementation in Sirius (the next Brazilian synchrotron light source) of a technique that allows locally analyzing structural issues such as disorder and defects, called pair distribution function (PDF). The technique describes the distances between pairs of atoms by means of a mathematical function. To apply it, the specialist generally uses the results of X-ray diffraction measurements – an experimental technique that provides information about the structure of materials. However, in order to implement the analysis by PDF, the X-ray beam focused on the sample must be of very high energy – higher than that provided by the current Brazilian synchrotron light source.

During the meeting at CNPEM, Professor Gustavo de Medeiros Azevedo, researcher at the National Laboratory of Synchrotron Light (LNLS), and Professor Edson Leite, LNNano’s scientific director, decided to begin applying PDF using electron diffraction results, a specialty of LNNano’s researcher Jefferson Bettini. The electron beams would be generated by the transmission electron microscope (TEM) of LNNano. In fact, this instrument allows the control of the electron beam so that it focuses a small area of the sample, allowing the desired local analysis of the structure. Besides that, when switching from the “diffraction mode” to the “image mode”, the microscope would made possible to choose precisely the area of the sample to be analyzed.

Simulation of an ideal ZrO2 nanocrystal.
Simulation of an ideal ZrO2 nanocrystal.

The development team also involved professors Içamira Costa Nogueira, from the Federal University of Amazonas (UFAM) and Querem Hapuque Felix Rebelo, from the Federal University of the West of Pará (UFOPA), who contributed with the synthesis of nanocrystals that would be studied and with the development of the analysis methodology.

During the development of the technique, another challenge had to be faced. To interpret the PDF results, it would be necessary to have a simulation of an ideal nanocrystal – a nanocrystal model without structural disorganization that could be used as a reference.

New skills were then incorporated into the team, which was then joined by Professor Adalberto Fazzio, director general of LNNano and leader of a UFABC research group dedicated to computational techniques applied to materials, and his doctoral student Gabriel Schleder. Based on the Density Functional Theory (DFT), a computational modeling method in the field of Quantum Physics, the researchers were able to simulate the ideal nanocrystal that served as the analysis model.

“Something very positive we perceived is that the main results arose from the process of interaction, discussion and exchange of information mainly between theory/computational simulation and experiments. Without this, we certainly would not have good final conclusions,” says Schleder.

The authors of the paper. From the left: Gabriel R. Schleder, Gustavo M. Azevedo, Içamira C. Nogueira, Querem H. F. Rebelo, Jefferson Bettini, Adalberto Fazzio and Edson R. Leite.
The authors of the paper. From the left: Gabriel R. Schleder, Gustavo M. Azevedo, Içamira C. Nogueira, Querem H. F. Rebelo, Jefferson Bettini, Adalberto Fazzio and Edson R. Leite.

B-MRS newsletter. Year 4, issue 2.


 


The newsletter of the Brazilian Materials Research Society 

News update from Brazil for the Materials community

English edition. Year 4, issue 2.

B-MRS (SBPMat) news
Young Researcher Award. SBPMat launches an award for postdocs in partnership with E-MRS. Application submission is open. Here.
Registration exemption for the E-MRS Meeting. Meet the selected students who will attend the 2017 E-MRS Spring Meeting exempt from registration fee. Here.
SBPMat Membership. The discount period for the 2017 annuity is still open. See the reasons and advantages of being a member of SBPMat and how to pay the annuity. Here.
Institutional members. Companies and organizations of all types are also welcome to SBPMat’s membership community. Meet the new institutional partners of SBPMat: Altmann and Interprise. Here.
XVI B-MRS Meeting (Gramado, Brazil, September 10-14)
  • Symposia. The list of approved symposia will be on the event’s website soon. 
  • Organization. Meet the organizing committee. Here.
  • Exhibitors. The event website shows the 14 companies that have already confirmed their participation. Companies interested in participating in the event with booths and other means of dissemination should contact Alexandre via this e-mail: comercial@sbpmat.org.br.   
Featured paper

A team of researchers from Unicamp has developed an innovative “formula” to manufacture luminescent perovskite nanocrystals (quantum dots) that can be purified without degradation. With the robust nanocrystals, the team prepared brilliant and efficient LEDs with innovative architecture. The work was reported in a paper published in Advanced Functional Materials. See our news article.

People from our community

We interviewed Aloísio Nelmo Klein, professor at UFSC and one of the architects of Materials research and teaching at that university. With more than 60 patents and a history of interaction with companies, Klein defines himself as a researcher who is convinced that science is one of the main driving forces for a nation’s development. Learn more about this researcher and his background, starting with his childhood in a village of German descendants in Rio Grande do Sul, and see the message he left for the younger audience and readers. See the interview.

Two new laboratories of the Federal University of Pelotas are named after former presidents of SBPMat Elson Longo (UNESP, UFSCar) and José Arana Varela (in memoriam). Know more. 

Reading tips
  • Technological innovations made in Brazil in steels used in electric motors and transformers improve energy efficiency.  Here.
  • Collaboration of LNNano (CNPEM) with alcohol plant generates technology for the transformation of sugarcane bagasse into active coal, which can be used in the purification of water and air. Here.
  • Striving for aerospace applications, a team with Brazilian participation studies what happens with nanotubes during high-speed impacts and improves the material.
    Here.
Events
  • Pan-American Polymer Science Conference (PanPoly). Guarujá (Brazil). March 22 – 24, 2017. Site.
  • 9th International Conference on Materials for Advanced Technologies. Suntec (Singapore). June 18 – 23, 2017. Site. 
  • XXXVIII Congresso Brasileiro de Aplicações de Vácuo na Indústria e na Ciência (CBRAVIC) + III Workshop de Tratamento e Modificação de Superfícies (WTMS). São José dos Campos (Brazil). August, 21 – 25, 2017. Facebook.
  • IUMRS-ICAM 2017. Kyoto (Japan). August 27 – Setember 1, 2017. Site.
  • International Conference on Luminescence (ICL-2017).  João Pessoa (Brazil). August 27 – September 1, 2017.
    Site.
  • XVI Encontro da SBPMat/ XVI B-MRS Meeting. Gramado (Brazil). September 10 – 14, 2017. Site.


Submit your suggestion for any section of our newsletter: comunicacao@sbpmat.org.br

 
 

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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