Featured paper: Isolating nanoribbons with conducting regions.


[Paper: Topologically Protected Metallic States Induced by a One-Dimensional Extended Defect in the Bulk of a 2D Topological Insulator. Erika N. Lima, Tome M. Schmidt, and Ricardo W. Nunes. Nano Lett., 2016, 16 (7), pp 4025–4031. DOI: 10.1021/acs.nanolett.6b00521]

Isolating nanoribbons with conducting regions

A research carried out in Brazil made an important contribution to the study of topological insulators, a class of materials that was theoretically predicted in 2005 and experimentally confirmed in 2007. The study was reported in an article recently published  in Nano Letters (impact factor: 13.779).

A unique property of Topological insulators is that they behave as insulators on the inside and as conductors on its surface or edge. According to Ricardo Wagner Nunes, professor at the Federal University of Minas Gerais (UFMG) and corresponding author of the article, “non-topological insulators may also have conductive surfaces, but in the case of topological insulators, conduction of charge and spin on the surface is robust, as it is “protected” by time reversal symmetry”.

In the article in Nano Letters, Professor Nunes and colleagues, Erika Lima, of the Federal University of Mato Grosso (UFMT) – Rondonópolis campus, and Tome Schmidt, of the Federal University of Uberlândia (UFU), reported their work on a two-dimensional topological insulator, a bismuth nanoribbon of only two layers of bismuth atoms (one-atom thick), superimposed and bonded. Using computational methods, the scientists showed that the interior of the bismuth nanoribbon, instead of being fully insulating, may have conductive states (also called metallic states) generated from a particular type of irregularity in the atomic structure of the material, known as 558 extended defect.

Representation of bismuth bilayer nanoribbon with the defect 558, top view (left) and side view (right). The green balls represent the atoms of the top layer of the material and the blue balls, the atoms of the lower layer. In the center of the left figure, the defect is clearly seen: pentagons and an octagon stop the repetition of the hexagons.

 

“In our work, we show that a linear defect within a two-dimensional topological insulator can generate one-dimensional electronic quantum states that conduct spin and charge within the material”, say the authors.

This conclusion was supported through calculations performed on supercomputers, simulating what would happen to the electrons in quantum states, in the material, in the presence of defects. “We used first-principles Density Functional Theory calculations”, specify the authors, who relate that the computer simulation of defects in bismuth nanostructures required approximately 400 hours of computer simulations on supercomputers in the Department of Physics – UFMG and at the National Center for High Performance Computing in São Paulo (Cenapad) – UNICAMP.

A figura mostra a curva de dispersão dos estados topológicos metálicos, localizados no defeito 558, marcados em azul e vermelho.
The figure shows, marked in blue and red, the dispersion curve of the metal topological states located in the defect 558.

In the article, the authors also propose the existence of pentaoctite, a new two-dimensional topological insulator. This material, which has not been synthesized yet, is a bismuth bilayer with a crystal lattice formed by atoms arranged in pentagons and octagons. As stated by the authors, “In our calculations we show that this new “phase” of the two-dimensional bismuth has low formation energy, which opens the possibility to be synthesized in the laboratory”.

According to the authors, the work reported in Nano Letters raises several issues in the scope of fundamental research, such as the influence of magnetic and non-magnetic impurities on the spin and charge transport in the proposed topological states, and the connection between the network symmetries and nature of the topological edge states on pentaoctite. “From the point of view of applications, it would be interesting if our work could motivate experimental studies of two-dimensional topological insulators based on bismuth and other materials, enabling theoretical and experimental collaboration on this issue”, comment the authors, leaving an open invitation to experimental research groups.

The origin of this research work

“The work originated by combining my interest in extended topological defects in two-dimensional and three-dimensional materials, with the experience of Professor Tome Mauro Schmidt (UFU) and Erika Lima, his doctoral student in the subject of topological insulators”, states Nunes.

In 2012, Nunes and collaborators published an article in Nano Letters on magnetic states (non topological) generated by linear extended defects in a monolayer of graphene. Later, in a conversation with Schmidt, a collaboration was decided in order to investigate if an extended defect with the same morphology would lead to the formation of topological states in a bidimensional topological insulator made of bismuth.

In her post-doctorate in the group of Professor Nunes, in 2015, Erika Lima performed all computer calculations. The three researchers, who are the authors of the article, interpreted the results and wrote the paper.

The research that led to the article received funding from Brazilian agencies CAPES, CNPq, FAPEMIG and from the National Institute of Science and Technology on Carbon Nanomaterials.

autores
Photos of the authors. From left to right, Erika Lima, currently a professor at UFMT, Tome Schmidt, professor at UFU, and Ricardo Nunes, professor at UFMG.

Featured paper: ultrasmall crystals with modulated shell thickness.


The scientific paper by members of the Brazilian community on Materials research featured this month is:

Anielle Christine A. Silva, Sebastião W. da Silva,  Paulo C. Morais, and Noelio O. Dantas. Shell thickness modulation in ultrasmall CdSe/CdS(x)Se(1-x)/CdS core/shell quantum dots via 1-thioglycerol. ACS Nano, 2014 Feb 25; 8(2):1913-22. DOI: 10.1021/nn406478f.

Ultrasmall crystals with modulated shell thickness.

When Professor Noelio Oliveira Dantas, at the Brazilian Federal University of Uberlândia (UFU), had the idea of a new chemical method for the synthesis of quantum dots (semiconductor crystals with only a few nanometers of size), he was studying ways to synthesize the tiny crystals aiming to biotechnological applications.

Representation of a quantum dot obtained with new method reported in the article.

However, the results exceeded the initial expectations. In addition to produce ultrasmall quantum dots, composed, basically, of a cadmium selenide (CdSe) core and a cadmium sulfide (CdS) shell, the new method, which is cheap and highly reproducible, was a surprise because of its capacity to modulate the quantum dots’ shell thickness, an innovation compared to other known routes.

The study was developed during the PhD research that Anielle Christine Almeida Silva develops at the Physics Institute at UFU, under Professor Dantas supervision. Some results of the research were published this year by the journal ACS Nano, in an article signed by the PhD student and her advisor, jointly with two collaborators from University of Brasília, who contributed with the characterization of the quantum dots using Raman spectroscopy, and took part discussing the results.

The paper

“The main scientific contribution of this article is the proposal of a new synthesis route, via an aqueous solution, which allowed the modulation of the CdS shells’ thickness in CdSe ultrasmall quantum dots, depending on the concentration of 1-thioglycerol”, summarized Anielle. More precisely, the researchers discovered that, while smaller concentrations of 1-thioglycerol limit the growth of the cores of the quantum dots, larger amounts of the compound promote the modulated growth of their shells.

The produced ultrasmall crystals had less than 2 nm in the core, and from 0.50 to 1.25 nm in the shell. In order to calculate these dimensions from Raman spectra, the authors of the article used a model of phonon confinement with modifications proposed by themselves and another author in an article published in 2013 (Anielle Christine Almeida Silva; Ernesto Soares de Freitas Neto; Sebastião William da Silva; Paulo Cesar de Morais; Noelio Oliveira Dantas. Modified Phonon Confinement Model and its Application to CdSe/CdS Core-Shell Magic-Sized Quantum Dots Synthesized in Aqueous Solution by a New Route. Journal of Physical Chemistry. C, v. 117, p. 1904-1914, 2013.).

Regarding the biotechnological application initially expected, the authors explain that such quantum dots obtained with the new route are promising due their capacity to be easily dispersed in aqueous environments. According to the scientists, the structure of the produced ultrasmall crystals, which is similar to a sandwich with two slices of bread and a fine filling, may contribute to a higher quantum efficiency and stability in biological environments.