New publication in Scientific Reports – Thermal conductivity of graphene-hBN superlattice ribbons

Our third paper of 2018 “Thermal conductivity of graphene-hBN superlattice ribbons”  has just been published in Scientific Reports, a publication of the Nature Publishing Group.

In this work we investigate coherent (wave-like) and incoherent (particle-like) phonon thermal transport in superlattices graphene and hexagonal boron nitride, which have been produced recently with sharp edges and controlled domain sizes. We employ non-equilibrium molecular dynamics simulations to investigate the thermal conductivity of superlattice nanoribbons with equal-sized domains of graphene and BN. We analyze the dependence of the conductivity with the domain sizes and with the total length of the ribbons, and determine a minimum thermal conductivity of 89 W m−1K−1 for ribbons with a superlattice period of 3.43 nm. The effective phonon mean free path is also determined and shows a minimum value of 32 nm for the same superlattice period. Our results reveal a crossover from coherent to incoherent phonon transport at room temperature as the superlattice period becomes comparable to the phonon coherence length. Analyzing phonon populations relative to the smallest superlattice period, we attribute the minimum thermal conductivity to a reduction in the population of flexural phonons when the superlattice period equals 3.43 nm. The ability to manipulate thermal conductivity using superlattice-based two-dimensional materials, such as graphene-hBN nanoribbons, opens up opportunities for application in future nanostructured thermoelectric devices.

We are particularly proud of this work for two reasons. First, because it is a product of Isaac’s Master’s thesis, my first graduate student supervision ever. Second, because it has been completely developed and executed at UFRN, including the computational support provided by our supercomputing center NPAD. We expect to publish further developments of this work in 2018.

The open access publication is available here.

Coverage by UFRN news.

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Paper accepted in Nanoscale – Borophene hydride: a stiff 2D material with high thermal conductivity and attractive optical and electronic properties

Our second paper of 2018 “Borophene hydride: a stiff 2D material with high thermal conductivity and attractive optical and electronic properties”  has just been accepted for publication in Nanoscale, published by the Royal Society of Chemistry.

In this work we investigate two-dimensional structures of hydrogenated borophene, called borophene hydride, which have been produced in recent experiments. We conducted extensive first-principles calculations to explore mechanical, electronic, optical and heat conduction properties of this novel material. The mechanical response of borophene hydride was found to be anisotropic with an elastic modulus of 131 N/m and a tensile strength of 19.9 N/m along the armchair direction. We also show that by applying mechanical loading the electronic character of borophene hydride can be altered from metallic to direct band-gap semiconductor, very appealing for application in nanoelectronics. The absorption edge of the imaginary part of the dielectric function is predicted to be in the visible range for parallel polarized light. Finally, we estimate room temperature thermal conductivities of 335 W/m-K and 293 W/m-K along zigzag and armchair directions, respectively. Our study confirms that borophene hydride presents an outstanding combination of mechanical, electronic, optical and thermal conduction properties, promising for the design of novel nanodevices.

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

The accepted paper is available here.

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Bachelor’s thesis defense: Fabrizio Giovanni Oliviero

Undergraduate student Fabrizio Giovanni Oliviero will present his Bachelor’s thesis today (01.12.2017) at 10:50 am, in the seminar room #1 of the Physics Department (DFTE).

The thesis, entitled “Electronic structure of carbon nanotubes”, employed analytical and numerical approaches to investigate the electronic structure of nanotubes within the tight-binding approximation. The recursive Green’s function method was implemented to obtain the electronic density of states, reproducing results from the 1990’s.

Everybody is welcome to the public defense.

UPDATE: Fabrizio’s thesis was approved by the examining committee. He will now pursue a Master’s degree under the supervision of Prof. Rodrigo G. Pereira at UFRN. Congratulations!

Prof. Rodrigo Pereira, Fabrizio, Prof. Dory Helio, Prof. Luiz Felipe Pereira

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Encontro Potiguar de Física 2017

The event “Encontro Potiguar de Física 2017” will take place on 05 and 06 of December, in the city of Caicó. This is a regional meeting  aiming to gather physicist working in Rio Grande do Norte and neighboring states, organized in collaboration with the Brazilian Physical Society.

I am on the organizing committee for this years edition, with my colleagues from UFRN, IFRN and UERN. We expect around 100 participants during this two-day event.

Although I am not presenting any works this year, our group will be represented by Dr. R. M. Tromer and Mr. I. M. Felix.

05/12/2017 – Comunicações Orais (16:00-18:00) – Física Estatística e Computacional / Física da Matéria Condensada e de Materiais / Óptica e Fotônica
16:00-16:30 A simple approach to obtain the relaxation time via Boltzmann transport theory
Raphael M. Tromer e Luiz Felipe C. Pereira
16:30 – 16:45 Controle da condutividade térmica em nanofitas de grafeno-hBN
Isaac de Macêdo Félix e Luiz Felipe C. Pereira

06/12/2017 – Comunicações Orais (15:00-17:00) – Física Estatística e Computacional / Física da Matéria Condensada e de Materiais / Óptica e Fotônica
15:30-15:45 Transporte de fônons em super-redes quase-periódicas de grafeno-hBN
Isaac de Macêdo Félix e Luiz Felipe C. Pereira

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New paper accepted in Physica E – Tuning the Fano factor of graphene via Fermi velocity modulation

Our latest work “Tuning the Fano factor of graphene via Fermi velocity modulation” has just been accepted for publication in Physica E: Low-dimensional systems and nanostructures.

In this work we investigate the influence of a Fermi velocity modulation on the Fano factor of periodic and quasi-periodic graphene superlattices. We use the transfer matrix method to solve the Dirac-like equation for graphene where the electrostatic potential, energy gap and Fermi velocity are piecewise constant functions of the position. We found that in the presence of an energy gap, it is possible to tune the energy of the Fano factor peak and consequently the location of the Dirac point, by a modulations in the Fermi velocity. Hence, the peak of the Fano factor can be used to identify the Dirac point in experiments. We show that for higher values of the Fermi velocity the Fano factor goes below 1/3 in the Dirac point. Furthermore, we show that in periodic superlattices the location of Fano factor peaks is symmetry when the Fermi velocity of the regions is swapped, however by introducing quasi-periodicity this  symmetry is lost. The Fano factor usually holds a universal value for a specific transport regime, therefore the possibility of controlling it in graphene is a notable result.

The manuscript is a product of our ongoing collaboration with Prof. Jonas R. F. Lima and Prof. Anderson L. R. Barbosa at Universidade Federal Rural de Pernambuco and my departmental colleague Prof. Claudionor G. Bezerra.

The accepted paper is available hereFree access link until December 28, 2017. A free preprint version is available here.

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New group member: Dr. Ana Claudia Kipper

This week we welcome a new member of the Transport in Nanostructures Group: Dr. Ana Claudia Kipper will work with us a postdoctoral associate, telecommuting from Recife. Dr. Kipper completed her Doctorate at Universidade Federal de Santa Maria, where she investigated thermal transport in carbon nanostructures via molecular dynamics simulations. Here, she will continue on that line of work, investigating heat transport in nanostructured superlattices. Welcome, Ana Claudia!

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New manuscript accepted in Nano Letters – Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations

Our sixth publication of 2017 “Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations” has just been accepted in Nano Letters.

In this work we investigate the effect of grain boundaries in the thermal conductivity of graphene, which are inherent defects in wafer-scale samples prepared by chemical vapor deposition. They can strongly influence the mechanical properties and electronic and heat transport in graphene. Here, we employ extensive molecular dynamics simulations to study thermal transport in large suspended polycrystalline graphene samples. Samples of different controlled grain sizes are prepared by a recently developed efficient multiscale approach based on the phase field crystal model. In contrast to previous works, our results show that the scaling of the thermal conductivity with the grain size shows a typical bimodal behaviour with two effective Kapitza lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a Kapitza length one order of magnitude larger than that of the in-plane phonons. We also show that in order to get quantitative agreement with the most recent experiments, quantum corrections need to be applied to both the Kapitza conductance of grain boundaries and the thermal conductivity of pristine graphene and the corresponding Kapitza lengths must be renormalized accordingly.

The manuscript is a product of our ongoing collaboration with Dr. Zheyong Fan, Dr. Ari Harju and Prof. Tapio Ala-Nissila at Aalto University, as well as Prof. Davide Donadio at UC Davis. It is also related to our PRB papers in 2015 and 2017.

The accepted manuscript is available here. An open-access preprint version is available here.

Coverage by UFRN news.

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New paper accepted in Physical Review B – Light propagation in quasiperiodic dielectric multilayers separated by graphene

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.

In this publication we investigate photonic crystals, artificial materials whose dielectric properties can be tailored according to the stacking of its constituents. We employed a transfer matrix treatment to study the propagation of light waves in Fibonacci quasiperiodic dieletric multilayers with graphene embedded. We calculated their dispersion and transmission spectra in order to investigate the effects of the graphene monolayers and quasiperiodic disorder on the physical behavior of the system. Our numerical results show that the presence of graphene monolayers reduces the transmissivity on the whole range of frequency and induces a transmission gap in the low frequency region. This Graphene Induced Photonic Band Gap is omnidirectional, therefore light cannot propagate in this structures for frequencies lower than a certain value, whatever the incidence angle.

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.

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IX Brazilian Meeting on Simulational Physics

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.

 

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Master’s Thesis Defense: Louis Gustavo da Costa S. e Sá

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!

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