Transport & Nanostructures Group

Our fifth paper of 2017 “Light propagation in quasiperiodic dielectric multilayers separated by graphene” has just been accepted for publication in Physical Review B. This is officially our most productive year so far.

The manuscript is my first work in graphene photonics, and is a product of a collaboration with departmental colleague Prof. Claudionor Bezerra and his former postdoc Dr. Carlos Humberto Costa, now a faculty member at the Federal University of Ceará.

The accepted manuscript is available here. A free-access preprint version is available here.

Today we kick-off the ninth edition of the Brazilian Meeting on Simulational Physics. This year we celebrate 20 years of this event, which started small and keeps growing in every edition. It has now become a traditional event in the Brazilian physics community. Although there is “Brazilian” in the title, this year we welcome more than 40 participants from a dozen different countries.

We will have two tutorial sections with 3 x 50 minute lectures, presented by David P. Landau, Ying-Wai Li and Werner Krauth. We will also have 25 speakers among invited and contributed, presenting 50 min talks each. All the talks and lectures will be available on the IIP youtube channel.

We hope that everybody enjoys the meeting as much as we enjoyed organizing it.

Here is the group photo of the meeting.

Our second graduate student, Mr. Louis Gustavo da Costa Sobral e Sá, will present his Master’s thesis on Monday (24.07.2017) at 10:00 am, in the auditorium of the Physics Department (DFTE).

The thesis, entitled “Thermal and mechanical properties of  NHG under strain”, employed non-equilibrium molecular dynamics simulations to calculate the thermal conductivity and elastic moduli of nitrogenated holey graphene.

Everybody is welcome to the public defense.

UPDATE: Louis Gustavo’s thesis was approved by the examining committee. We wish him the best of luck in his future endeavors. Congratulations!

Our fourth paper of 2017 “ Electronic, optical and thermal properties of highly stretchable 2D carbon Ene-yne graphyne” has just been accepted for publication in Carbon.

A new carbon-based two-dimensional (2D) material “carbon Ene-yne” (CEY), was successfully synthesized recently. In this work, we examine electronic, optical and thermal properties of this novel material. We studied the stretchability of CEY via density functional theory (DFT) calculations. Using the PBE and HSE06 functionals, as well as the G0W0 method and the Bethe-Salpeter equation, we systematically explored electronic and optical properties of 2D CEY. We investigated the change of band-gap and optical properties under uniaxial and biaxial strain. Ab-initio molecular dynamics simulations confirm that CEY is stable at temperatures as high as 1500 K. Using non-equilibrium molecular dynamics simulations, the lattice thermal conductivity of CEY was predicted to be anisotropic and three orders of magnitude smaller than that of graphene. We found that in the visible range, the optical conductivity under high strain levels is larger than that of graphene. This enhancement in optical conductivity may allow CEY to be used in photovoltaic cells. Moreover, CEY shows anisotropic optical responses for x- and y- polarized light, which may be suitable as an optical linear polarizer. We hope that the comprehensive insight provided by the present investigation should serve as a guide for possible applications of CEY in nanodevices.

The manuscript is another product of our ongoing collaboration with Dr. Bohayra Mortazavi and Prof. Timon Rabczuk at Bauhaus-Universität Weimar.

The paper is available here. Free access link until October 26, 2017. A preprint version is available here.

Our third paper of 2017 “Anomalous strain effect on the thermal conductivity of borophene: a reactive molecular dynamics study” has just been accepted for publication in Physica E: Low-dimensional systems and nanostructures.

Borophene, an atomically thin, corrugated, crystalline two-dimensional boron sheet, is one of the latest additions to the 2D materials family. In this work we investigate mechanical properties and lattice thermal conductivity of borophene using reactive molecular dynamics simulations. We found an anisotropy in 2D elastic moduli along perpendicular directions, dubbed zigzag and armchair directions, in analogy with the direction in graphene. We attribute the anisotropy to the buckling of the borophene structure along the zigzag direction. We performed non-equilibrium molecular dynamics simulations to calculate the lattice thermal conductivity, and found an anisotropy along the in-plane directions, also in accordance with our estimate for the effective phonon mean free paths.  In this case, the anisotropy is attributed to differences in the density of states of low-frequency phonons, with lower group velocities and possibly shorten phonon lifetimes along the zigzag direction. Finally, we found that when borophene is strained along the armchair direction there is a significant increase in thermal conductivity along that direction. Meanwhile, when the sample is strained along the zigzag direction there is a much smaller increase in thermal conductivity along that direction. Our predictions are in agreement with recent first principles results, at a fraction of the computational cost.

The manuscript is a product of our ongoing collaboration with Dr. Bohayra Mortazavi and Prof. Timon Rabczuk at Bauhaus-Universität Weimar. The thermal transport simulations were performed at the High Performance Computing Center (NPAD) at UFRN.

The paper is available here.  Free access here (until 11/Aug/2017). A preprint version is available here.

May has been an exciting month for the transport in nanostructures  group. Doctorate student Isaac de Macedo Felix has passed the written qualifying exam on his first try. Congratulations to him! We also welcome a new group member: José Roberto da Silva will pursue his doctorate under my supervision.

Our second paper of 2017 “Thermal conductivity decomposition in two-dimensional materials: Application to graphene” has just been accepted for publication in Physical Review B.

In this work we propose a decomposition of the microscopic heat current in atomistic molecular dynamics simulations into in-plane and out-of-plane components, corresponding to the in-plane and out-of-plane phonon dynamics, respectively. This decomposition allows a direct computation of the corresponding thermal conductivity components in two-dimensional materials. We apply this decomposition to study heat transport in suspended graphene, using both equilibrium and non-equilibrium molecular dynamics simulations. We show that the flexural component is responsible for about two thirds of the total thermal conductivity in unstrained graphene, and the acoustic flexural component is responsible for the logarithmic divergence of the conductivity when a sufficiently large tensile strain is applied, in accordance with my PRB Editors’ Suggestion paper with Prof. Davide Donadio in 2013.

The manuscript is a product of our ongoing collaboration with Dr. Zheyong Fan and Dr. Ari Harju at Aalto University, and a follow-up to our PRB paper in 2015.

The paper is available here.  A preprint version is available here.

Our first paper of 2017 “Atomic Adsorption on Nitrogenated Holey Graphene” has just been accepted for publication in The Journal of Physical Chemistry C.

In this work we employ first principles calculations and a hybrid Quantum Mechanics:Molecular Mechanics method (ONIOM) to investigate the adsorption of H, B, and O on nitrogenated holey graphene (NHG). It is a novel 2D material with C2N stoichiometry which has been recently synthesized in the group of Prof. Jong-Beom Baek at the Ulsan National Institute of Science and Technology in Korea. We find that H and B prefer to adsorb on top of a nitrogen atom, while O prefers to adsorb on top of a carbon-carbon bond. We show that the electronic structure of NHG changes due to the presence of adatoms, with the appearance of midgap states, close to the Fermi level. In the case of NHG+H and NHG+B we observe the appearance of a finite magnetic moment, related to the midgap states, which could give rise to a magnetoresistance effect.

The manuscript is a product of my first postdoc supervision, developed by Dr. Raphael M. Tromer at UFRN since early 2016. It also includes collaboration with Prof. M.G.E. da Luz (UFPR) and Prof. Mauro S. Ferreira (Trinity College Dublin). Computational resources for this project were provided by the High Performance Computing Center (NPAD) at UFRN and Laboratorio Central de Processamento de Alto Desempenho (LCPAD) at UFPR.

This is another chemistry first for me: it is my first paper as senior author in a physical chemistry journal. Can I call myself a chemical physicist now? Really?

The paper is available here.  Direct link to PDF here.

Today we kick-off our event Thermal and Electronic Transport in Nanostructures, which takes place at the International Institute of Physics – Universidade Federal do Rio Grande do Norte from 31/10/2016 to 11/11/2016. It is a two-week event focusing on the conduction of heat and electricity in the nanoscale. The event includes a series of lectures aimed at students and several scientific talks. We have around 60 participants from three continents and more than 12 different countries.

The idea for this workshop came from a post-seminar discussion with visiting Prof. Giulio Casati, back in 2014. We submitted the proposal to the international advisory board of the IIP early 2015, when it was approved. Since than we had full support from the IIP staff, who guided us through all the bureaucracy, including the request of additional funds from the Brazilian government. We are grateful for the financial support provided by CAPES to this event.

I hope the participants will enjoy this event as much as we enjoyed organizing the whole thing.