Showing 1–50 of 52 results for author: Miwa, R H

Understanding the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Metal Intercalated Biphenylene Bilayers

Authors: Henri G. Mendonça, Pedro H. Souza, Walter Orellana, Roberto H. Miwa

Abstract: We conducted an {\it ab initio} study of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-encapsulated biphenylene bilayers, B/M/B, with M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, W, Os and Pt. In most systems, the intercalated metal sits at the square carbon sites (C$^{468}$) of the biphenylene lattice. Using a computational hydrogen electrode approach, we evaluate… ▽ More We conducted an {\it ab initio} study of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-encapsulated biphenylene bilayers, B/M/B, with M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, W, Os and Pt. In most systems, the intercalated metal sits at the square carbon sites (C$^{468}$) of the biphenylene lattice. Using a computational hydrogen electrode approach, we evaluated the reaction energetics at these active sites. Several B/M/B systems show competitive ORR and OER performance. Among the investigated systems, Cu, Pt, Ru, and Mn exhibit the lowest ORR overpotentials of 0.42, 0.44, 0.50, and 0.56 V, respectively, while Fe is identified as the most active catalyst for OER with an overpotential of 0.44 V. To understand the catalytic trends, we looked at the electronic structure through the metal $d-$band centers, the C$^{468}$ $p_z-$band centers, and the corresponding orbital charge populations. The band centers did not give a simple polynomial dependence on the overpotentials, though they did point to favorable electronic ranges for the best catalysts. The $d-$orbital charge population of the encapsulated metal, however, correlated most clearly with activity-especially for OER-yielding volcano-type plots. From these, B/Fe/B emerges as the best OER catalyst, while B/Mn/B lies closest to the ORR optimum. The $p-$orbital population at the active carbon site also captures the main trends, albeit less strongly. Overall, these results show that straightforward electronic descriptors can predict catalytic behavior in metal-encapsulated biphenylene bilayers and guide the search for efficient catalysts where the carbon framework itself drives the reactivity. △ Less

Submitted 7 August, 2026; originally announced August 2026.

Comments: 15 pages, 8 figures

Beyond Hexagonal Boron Nitride: First-Principles Study of Pentaoctite-BN and Pop-BN Monolayers

Authors: Victor M. S. da Conceição, Erika N. Lima, Roberto H. Miwa, Igor S. S. de Oliveira

Abstract: We investigate two novel non-hexagonal boron nitride monolayers, pentaoctite-BN (PO-BN) and pop-BN (PP-BN), using first-principles calculations. Their structural, electronic, mechanical, vibrational, thermal, and optical properties are systematically analyzed to assess their stability and potential applications. Despite being metastable with respect to hexagonal BN, both polymorphs satisfy the cri… ▽ More We investigate two novel non-hexagonal boron nitride monolayers, pentaoctite-BN (PO-BN) and pop-BN (PP-BN), using first-principles calculations. Their structural, electronic, mechanical, vibrational, thermal, and optical properties are systematically analyzed to assess their stability and potential applications. Despite being metastable with respect to hexagonal BN, both polymorphs satisfy the criteria for dynamical, mechanical, and thermal stability, indicating that they are viable two-dimensional materials. Both systems are indirect-gap semiconductors whose electronic states near the band edges are dominated by out-of-plane pz orbitals. Their distinct pentagon-octagon ring networks also give rise to different in-plane elastic anisotropies. Many-body optical calculations reveal strong excitonic effects and pronounced polarization-dependent optical absorption, with lattice engineering shifting the optical response from the ultraviolet toward the visible and infrared regions. These findings demonstrate that engineering non-hexagonal lattice architectures provides an effective strategy for tuning the electronic and optical properties of two-dimensional BN, highlighting PO-BN and PP-BN as promising candidates for future optoelectronic and photonic applications. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Tunable Electronic and Transport Properties of Biphenylene via Fluorination and Disorder

Authors: Lucas Soares Sousa, Felipe Crasto de Lima, Roberto Hiroki Miwa

Abstract: Biphenylene (BPN) network is a newly synthesized 2D carbon allotrope hosting anisotropic Dirac electronic states. Here, we investigate how fluorination and correlated chemical disorder modify the electronic structure and charge transport of fluorinated biphenylene (F/BPN) using density functional theory, Wannier-based tight-biding Hamiltonian, and quantum transport simulations. We show that fluori… ▽ More Biphenylene (BPN) network is a newly synthesized 2D carbon allotrope hosting anisotropic Dirac electronic states. Here, we investigate how fluorination and correlated chemical disorder modify the electronic structure and charge transport of fluorinated biphenylene (F/BPN) using density functional theory, Wannier-based tight-biding Hamiltonian, and quantum transport simulations. We show that fluorination reshapes the transport response of BPN, producing concentration-dependent anisotropic conduction regimes. For pristine and ordered fluorinated systems, we identified the emergence of negative differential resistance (NDR) and a bias-induced inversion of the preferred transport direction, from armchair to zigzag and vice versa. In contrast, disorder suppresses the NDR, driving the system toward an approximately Ohmic transport regime. At high fluorine coverage, we further observed a nonmonotonic dependence of the armchair current on adatom concentration, which we attribute to the formation of correlated quasi-linear fluor conformation that promote armchair-oriented C-$π$ transport channels while simultaneously suppressing transport along the zigzag direction. Our results demonstrate that correlated fluorination can be used as an active mechanism to engineer electronic transport. △ Less

Submitted 16 June, 2026; originally announced June 2026.

Intrinsic Instabilities and Mechanical Anisotropy in Halide Perovskite Monolayers

Authors: Gabriel X. Pereira, Lucas M. Farigliano, Roberto H. Miwa, Gustavo M. Dalpian

Abstract: Halide perovskites have been extensively studied owing to their excellent optoelectronic properties and their unique lattice characteristics, that are very soft and anharmonic. Recent studies indicate the importance of a deep understanding of their surfaces and, in the limit, the properties of low-dimensional structures based on these materials. To investigate the structural and electronic propert… ▽ More Halide perovskites have been extensively studied owing to their excellent optoelectronic properties and their unique lattice characteristics, that are very soft and anharmonic. Recent studies indicate the importance of a deep understanding of their surfaces and, in the limit, the properties of low-dimensional structures based on these materials. To investigate the structural and electronic properties of halide perovskite monolayers (i.e., perovskenes), this work uses first-principles simulations. We have studied three different stoichiometries (ABX3, ABX4, and A2BX4) and structural phases for iodide, bromide, and chloride perovskite monolayers. Their thermodynamic behavior was evaluated through the construction of phase diagrams, highlighting the instability of the ABX4 stoichiometry, which was further supported by its mechanical instability. Structurally, the covalent characteristics of the Pb--X bond, in contrast to the Cs--X bonds, induce a strong anisotropy in the Young's modulus and Poisson's ratio along different crystallographic directions, and also account for the lower stiffness observed in the phases where the octahedra are not aligned. The electronic properties are somewhat similar to those of their 3D counterparts, but with a slightly larger band gap; in the monolayers, the band gap increases with halogen electronegativity (I, Br, Cl) and octahedral tilting. Moreover, the non-symmetric ABX3 stoichiometry exhibited a spin splitting due to the internal dipole moment in these layers. Overall, our work lays the groundwork for a deeper understanding of low-dimensional structures based on halide perovskites. △ Less

Submitted 25 February, 2026; originally announced February 2026.

Unveiling the Electronic Origin of Anomalous Contact Conductance in Twisted Bilayer Graphene

Authors: Kevin J. U. Vidarte, Caio Lewenkopf, F. Crasto de Lima, R. Hiroki Miwa, Felipe Pérez Riffo, Eric Suárez Morell

Abstract: This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $θ$, with a peak at… ▽ More This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $θ$, with a peak at $θ\approx 5^\circ$, a finding that markedly departs from the well-known magic angle TBG behavior. To elucidate this phenomenon, we develop a comprehensive theoretical and computational framework. Our calculations, performed on both relaxed and rigid TBG structures, simulate contact conductance by analyzing the local density of states across a range of biases and rotational angles. Contrary to the current interpretation, our results demonstrate that the maximum conductance at $θ\approx 5^{\rm o}$ is not caused by structural relaxation or AA stacking zone changes. Instead, we attribute this peak to the evolution of the electronic band structure, specifically the shifting of van Hove singularities (vHs) to the Fermi level as the twist angle decreases. We further show that the precise location of this conductance maximum is dependent on the applied bias voltage. This interplay between twist angle, bias, and vHs energy provides a robust explanation for the experimental findings. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 9 pages, 6 figures

TCNQ self-assembly driven by molecular coverage over borophene monolayers

Authors: G. H. Silvestre, R. H. Miwa

Abstract: Boron monolayers, also known as borophene, have recently attracted interest due to their electronic properties, e.g. the facility to form various allotropes with interesting properties. In this work, we investigate the adsorption process of the tetracyanoquinodimethane (TCNQ) on the borophene $β_{12}$ and $χ_3$ using the density functional theory (DFT). We observed that molecules bond to the borop… ▽ More Boron monolayers, also known as borophene, have recently attracted interest due to their electronic properties, e.g. the facility to form various allotropes with interesting properties. In this work, we investigate the adsorption process of the tetracyanoquinodimethane (TCNQ) on the borophene $β_{12}$ and $χ_3$ using the density functional theory (DFT). We observed that molecules bond to the borophene layer through the van der Waals interaction, where, at the low coverage limit, the binding strength of TCNQ / borophene is comparable to that of TCNQ / WSe$_2$. By increasing the molecular coverage, $10^{13} \rightarrow 10^{14}$ molecules/cm$^{2}$, we found the (exothermic) formation of self-assembled (SA) structures of TCNQ on borophene, where the molecule-molecule interactions rule the SA process. The structural stability of the SA-TCNQ molecule on borophene was verified via ab initio molecular dynamics simulations. Finally, we show that the formation of the vdW interface leads to the tunability of the hole-doping of the borophene layer by an external electric field. We believe that our results bring an important contribution to the atomic-scale understanding of a powerful electron acceptor molecule, TCNQ, adsorbed on a promising 2D material, borophene. △ Less

Submitted 1 April, 2025; originally announced April 2025.

Comments: 7 pages, 6 figures

Catenary-like rippled biphenylene/graphene lateral heterojunction

Authors: Victor M. S. da C. Dias, Danilo de P. Kuritza, Igor S. S. de Oliveira, Jose E. Padilha, Roberto H. Miwa

Abstract: In this study, we conduct a first-principles analysis to explore the structural and electronic properties of curved biphenylene/graphene lateral junctions (BPN/G). We start our investigation focusing on the energetic stability of BPN/G by varying the width of the graphene region, BPN/Gn. The electronic structure of BPN/Gn reveals (i) the formation of metallic channels mostly localized along the BP… ▽ More In this study, we conduct a first-principles analysis to explore the structural and electronic properties of curved biphenylene/graphene lateral junctions (BPN/G). We start our investigation focusing on the energetic stability of BPN/G by varying the width of the graphene region, BPN/Gn. The electronic structure of BPN/Gn reveals (i) the formation of metallic channels mostly localized along the BPN stripes, where (ii) the features of the energy bands near the Fermi level are ruled by the width (n) of the graphene regions, Gn. In the sequence, we find that the hydrogenation of BPN/Gn results in a semiconductor system with a catenary-like rippled geometry. The electronic states of the hydrogenated system are mainly confined in the curved Gn regions, and the dependence of the bandgap on the width of Gn is similar to that of hydrogenated armchair graphene nanoribbons. The effects of curvature on the electronic structure, analyzed in terms of external mechanical strain, revealed that the increase/decrease of the band gap is also dictated by the width of the Gn region. Further electronic transport calculations reveal a combination of strong transmission anisotropy and the emergence of negative differential resistance. Based on these findings, we believe that rippled biphenylene/graphene systems can be useful for the design of two-dimensional nanodevices. △ Less

Submitted 10 March, 2025; originally announced March 2025.

Comments: For supplementary material please contact

Interacting Virtual Topological Phases in Defect-Rich 2D Materials

Authors: Felipe Crasto de Lima, Roberto H. Miwa, Caio Lewenkopf, Adalberto Fazzio

Abstract: We investigate the robustness of {\it virtual} topological states -- topological phases away from the Fermi energy -- against the electron-electron interaction and band filling. As a case study, we employ a realistic model to investigate the properties of vacancy-driven topological phases in transition metal dichalcogenides (TMDs) and establish a connection between the degree of localization of to… ▽ More We investigate the robustness of {\it virtual} topological states -- topological phases away from the Fermi energy -- against the electron-electron interaction and band filling. As a case study, we employ a realistic model to investigate the properties of vacancy-driven topological phases in transition metal dichalcogenides (TMDs) and establish a connection between the degree of localization of topological wave functions, the vacancy density, and the electron-electron interaction strength with the topological phase robustness. We demonstrate that electron-electron interactions play a crucial role in degrading topological phases thereby determining the validity of single-particle approximations for topological insulator phases. Our findings can be naturally extended to {\it virtual} topological phases of a wide range of materials. △ Less

Submitted 8 May, 2025; v1 submitted 11 December, 2024; originally announced December 2024.

Journal ref: Phys. Rev. B 111, 195135 (2025)

Exploring Topological Transport in Pt$_2$HgSe$_3$ Nanoribbons: Insights for Spintronic Device Integration

Authors: Rafael L. H. Freire, F. Crasto de Lima, Roberto H. Miwa, A. Fazzio

Abstract: The discovery of the quantum spin Hall effect led to the exploration of the electronic transport for spintronic devices. Here, we theoretically investigated the electronic conductance in large-gap realistic quantum spin Hall system, Pt$_2$HgSe$_3$ nanoribbons. By an ab initio approach, we found that the edge states present a penetration depth of about $0.9$\,{nm}, which is much smaller than those… ▽ More The discovery of the quantum spin Hall effect led to the exploration of the electronic transport for spintronic devices. Here, we theoretically investigated the electronic conductance in large-gap realistic quantum spin Hall system, Pt$_2$HgSe$_3$ nanoribbons. By an ab initio approach, we found that the edge states present a penetration depth of about $0.9$\,{nm}, which is much smaller than those predicted in other 2D topological systems. Thus, suggesting that Pt$_2$HgSe$_3$ allows the exploitation of topological transport properties in narrow ribbons. Using non-equilibrium Green's functions calculations, we have examined the electron conductivity upon the presence of Se\,$\leftrightarrow$\,Hg antistructure defects randomly distributed in the Pt$_2$HgSe$_3$ scattering region. By considering scattering lengths up to $109$\,nm, we found localization lengths that can surpass $μ$m sizes for narrow nanoribbons ($<9$\,nm). These findings can contribute to further understanding the behavior of topological insulators under realistic conditions and their integration within electronic, spintronic devices. △ Less

Submitted 10 May, 2024; originally announced May 2024.

Journal ref: Phys. Rev. B 110, 035111 (2024)

Spin-polarized nearly-free electron channels on the Ca$_{2}$N electrenes

Authors: P. H. Souza, J. E. Padilha, R. H. Miwa

Abstract: Two-dimensional (2D) materials combined with the presence of surface nearly-free electrons (NFE) have been considered quite interesting platforms to be exploited for the development of 2D electronic devices. Further incorporation of foreign elements adds a new degree of freedom to engineer the electronic as well as the magnetic properties of 2D materials. Here we have performed an ab-initio study… ▽ More Two-dimensional (2D) materials combined with the presence of surface nearly-free electrons (NFE) have been considered quite interesting platforms to be exploited for the development of 2D electronic devices. Further incorporation of foreign elements adds a new degree of freedom to engineer the electronic as well as the magnetic properties of 2D materials. Here we have performed an ab-initio study of Ca$_{2}$N, electrenes fully (i.e., both sides) adsorbed by hydrogen (H/Ca$_{2}$N/H) and fluorine (F/Ca$_{2}$N/F) atoms. The NFE states are suppressed in these systems, followed by the appearance of a net magnetic moment localized in the nitrogen atoms intercalated by the hydrogenated or fluorinated calcium layers. In the sequence, we have proposed lateral heterostructures combining the H/Ca$_{2}$N/H or F/Ca$_{2}$N/F regions with pristine Ca$_{2}$N, electrenes [(Ca$_{2}$N)(X/Ca$_{2}$N/X), with X=H or F]. We found that the magnetic moment of the hydrogenated or fluorinated regions promotes the emergence of spin-polarized NFE states confined along the pristine (Ca$_{2}$N) stripes. Further electronic transport calculations reveal that the (X/Ca$_{2}$N/X) regions act as spin-dependent scattering centers, spin-filters. We believe that these findings make an important contribution to the development of spintronic devices based on 2D electrides. △ Less

Submitted 12 December, 2023; v1 submitted 11 December, 2023; originally announced December 2023.

Comments: 6 pages, 5 figures

Unveiling the electronic properties of BiP$_3$ triphosphide from bulk to graphene-based heterostructure by first-principles calculations

Authors: Dominike P. de Andrade Deus, Igor S. S. de Oliveira, Roberto Hiroki Miwa, Erika N. Lima

Abstract: Triphosphides, with a chemical formula of XP$_3$ (X is a group IIIA, IVA, or VA element), have recently attracted much attention due to their great potential in several applications. Here, using density functional theory calculations, we describe for the first time the structural and electronic properties of the bulk bismuth triphosphide (BiP$_3$). Phonon spectra and molecular dynamics simulations… ▽ More Triphosphides, with a chemical formula of XP$_3$ (X is a group IIIA, IVA, or VA element), have recently attracted much attention due to their great potential in several applications. Here, using density functional theory calculations, we describe for the first time the structural and electronic properties of the bulk bismuth triphosphide (BiP$_3$). Phonon spectra and molecular dynamics simulations confirm that the 3D crystal of BiP$_3$ is a metal thermodynamically stable with no bandgap. Unlike the bulk, the mono-, bi-, tri-, and tetra-layers of BiP$_3$ are semiconductors with a bandgap ranging from 1.4 to 0.06 eV. However, stackings with more than five layers exhibit metallic behavior equal to the bulk. The results show that quantum confinement is a powerful tool for tuning the electronic properties of BiP$_3$ triphosphide, making it suitable for technological applications. Building on this, the electronic properties of van der Waals heterostructure constructed by graphene (G) and the BiP$_3$ monolayer (m-BiP$_3$) were investigated. Our results show that the Dirac cone in graphene remains intact in this heterostructure. At the equilibrium interlayer distance, the G/m-BiP$_3$ forms an n-type contact with a Schottky barrier height of 0.5 eV. It is worth noting that the SHB in the G/m-BiP$_3$ heterostructure can be adjusted by changing the interlayer distance or applying a transverse electric field. Thus, we show that few-layers BiP$_3$ is an interesting material for realizing nanoelectronic and optoelectronic devices and is an excellent option for designing Schottky nanoelectronic devices. △ Less

Submitted 8 September, 2023; v1 submitted 5 September, 2023; originally announced September 2023.

Comments: 12 pages, 14 figures, 1 table

Bridging Borophene and Metal Surfaces: Structural, Electronic, and Electron Transport Properties

Authors: Wanderlã L. Scopel, F. Crasto de Lima, Pedro H. Souza, José E. Padilha, Roberto H. Miwa

Abstract: Currently, solid interfaces composed of two-dimensional materials (2D) in contact with metal surfaces (m-surf) have been the subject of intense research, where the borophene bilayer (BBL) has been considered a prominent material for the development of electronic devices based on 2D platforms. In this work, we present a theoretical study of the energetic, structural, and electronic properties of th… ▽ More Currently, solid interfaces composed of two-dimensional materials (2D) in contact with metal surfaces (m-surf) have been the subject of intense research, where the borophene bilayer (BBL) has been considered a prominent material for the development of electronic devices based on 2D platforms. In this work, we present a theoretical study of the energetic, structural, and electronic properties of the BBL/m-surf interface, with m-surf = Ag, Au, and Al (111) surfaces, and the electronic transport properties of BBL channels connected to the BBL/m-surf top contacts. We find that the bottom-most BBL layer becomes metalized, due to the orbital hybridization with the metal surface states, resulting in BBL/m-surf ohmic contacts, meanwhile, the inner and top-most boron layers kept their semiconducting character. The net charge transfers reveal that BBL has become $n$-type ($p$-type) doped for m-surf = Ag, and Al (= Au). A thorough structural characterization of the BBL/m-surf interface, using a series of simulations of the X-ray photoelectron spectra, shows that the formation of BBL/m-surf interface is characterized by a redshift of the B-$1s$ spectra. Further electronic transport results revealed the emergence of a Schottky barrier between 0.1 and 0.2\,eV between the BBL/m-surf contact and the BBL channels. We believe that our findings are timely, bringing important contributions to the applicability of borophene bilayers for developing 2D electronic devices. △ Less

Submitted 10 May, 2023; originally announced May 2023.

Journal ref: J. Phys. Chem. C 127, 17556 (2023)

Noncentrosymmetric two-dimensional Weyl semimetals in porous Si/Ge structures

Authors: Emmanuel V. C. Lopes, Rogerio J. Baierle, Roberto H. Miwa, Tome M. Schmidt

Abstract: In this work we predict a family of noncentrosymmetric two-dimensional (2D) Weyl semimetals composed by porous Ge and SiGe structures. These systems are energetically stable graphenylene-like structures with a buckling, spontaneously breaking the inversion symmetry. The nontrivial topological phase for these 2D systems occurs just below the Fermi level, resulting in nonvanishing Berry curvature ar… ▽ More In this work we predict a family of noncentrosymmetric two-dimensional (2D) Weyl semimetals composed by porous Ge and SiGe structures. These systems are energetically stable graphenylene-like structures with a buckling, spontaneously breaking the inversion symmetry. The nontrivial topological phase for these 2D systems occurs just below the Fermi level, resulting in nonvanishing Berry curvature around the Weyl nodes. The emerged Weyl semimetals are protected by $C_3$ symmetry, presenting one-dimensional edge Fermi-arcs connecting Weyl points with opposite chiralities. Our findings complete the family of Weyl in condensed-matter physics, by predicting the first noncentrosymmetric class of 2D Weyl semimetals. △ Less

Submitted 15 January, 2024; v1 submitted 9 May, 2023; originally announced May 2023.

Journal ref: Journal of Physics: Condensed Matter 36, 185701 (2024)

Stacking order effects on the energetic stability and electronic properties of $n$-doped graphene/h-BN van der Waals heterostructures on SiC(0001)

Authors: D. P. de Andrade Deus, J. M. J. Lopes, Roberto H. Miwa

Abstract: Heterostructures made of stacked 2D materials with different electronic properties are studied for their potential in creating multifunctional devices. Graphene (G) and hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively researched, including recent studies on synthesizing h-BN on graphene/SiC(0001) templates. These studies suggest that h-BN encapsulation could occur i… ▽ More Heterostructures made of stacked 2D materials with different electronic properties are studied for their potential in creating multifunctional devices. Graphene (G) and hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively researched, including recent studies on synthesizing h-BN on graphene/SiC(0001) templates. These studies suggest that h-BN encapsulation could occur in addition to vdW epitaxy. This work presents theoretical research on G/h-BN heterostructures on SiC(0001) with a carbon buffer layer. The results show an energetic preference for h-BN encapsulation below a single layer of graphene: G/h-BN/SiC in bilayer systems and G/h-BN/G/SiC in trilayer systems. Electronic structure calculations reveal that the linear energy band dispersion of graphene is maintained in bilayer systems but with Dirac points at different energy positions due to the varied electron doping level of graphene. In trilayer systems, the doping level of graphene also depends on the stacking order. The electronic band structure of G/h-BN/G/SiC features two Dirac points below the Fermi level but with different energies. Less stable systems like G/G/h-BN/ and h-BN/G/G/SiC display parabolic bands near the Fermi level. Additional structural characterizations were performed based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near-edge spectroscopy (XANES) to aid future experimental spectroscopy in these graphene/h-BN vdW systems. △ Less

Submitted 14 March, 2023; originally announced March 2023.

Comments: 11 pages, 10 figures

Topological insulating phase arising in transition metal dichalcogenide alloy

Authors: F. Crasto de Lima, B. Focassio, R. H. Miwa, A. Fazzio

Abstract: Transition metal dichalcogenides have been the subject of numerous studies addressing technological applications and fundamental issues. Single-layer PtSe2 is a semiconductor with a trivial bandgap, in contrast, its counterpart with 25% of Se atoms substituted by Hg, Pt2HgSe3 (jacutingaite, a naturally occurring mineral), is a 2D topological insulator with a large bandgap. Based on ab-initio calcu… ▽ More Transition metal dichalcogenides have been the subject of numerous studies addressing technological applications and fundamental issues. Single-layer PtSe2 is a semiconductor with a trivial bandgap, in contrast, its counterpart with 25% of Se atoms substituted by Hg, Pt2HgSe3 (jacutingaite, a naturally occurring mineral), is a 2D topological insulator with a large bandgap. Based on ab-initio calculations, we investigate the energetic stability, and the topological transition in Pt(HgxSe1-x)2 as a function of alloy concentration, and the distribution of Hg atoms embedded in the PtSe2 host. Our findings reveal the dependence of the topological phase with respect to the alloy concentration and robustness with respect distribution of Hg. Through a combination of our ab-initio results and a defect wave function percolation model, we estimate the random alloy concentration threshold for the topological transition to be only 9%. Our results expand the possible search for non-trivial topological phases in random alloy systems. △ Less

Submitted 12 December, 2022; originally announced December 2022.

Journal ref: 2023 2D Mater. 10 035001

Nanoscale Structural and Electronic Properties of Cellulose/Graphene Interfaces

Authors: Gustavo H. Silvestre, Felipe Crasto de Lima, Juliana S. Bernardes, Adalberto Fazzio, Roberto H. Miwa

Abstract: The development of electronic devices based on the functionalization of (nano)cellulose platforms relies upon an atomistic understanding of the structural, and electronic properties of the combined system, cellulose/functional element. In this work, we present a theoretical study of the nanocellulose/graphene interface (nCL/G) based on first-principles calculations. We find that the binding energi… ▽ More The development of electronic devices based on the functionalization of (nano)cellulose platforms relies upon an atomistic understanding of the structural, and electronic properties of the combined system, cellulose/functional element. In this work, we present a theoretical study of the nanocellulose/graphene interface (nCL/G) based on first-principles calculations. We find that the binding energies of both hydrophobic/G (nCL$^{\rm phob}$/G) and hydrophilic/G (nCL$^{\rm phil}$/G) interfaces are primarily dictated by the van der Waals interactions, and are comparable with that of their 2D interface counterparts. We verify that the energetic preference of nCL$^{\rm phob}$/G has been reinforced by the inclusion of an aqueous media via the implicit solvation model. Further structural characterization was carried out using a set of simulations of Carbon K-edge X-ray absorption spectra to identify and distinguish the key absorption features of the nCL$^{\rm phob}$/G and nCL$^{\rm phil}$/G interfaces. The electronic structure calculations reveal that the linear energy bands of graphene lie in the band gap of the nCL, sheet, while depletion/accumulation charge density regions are observed. We show that external agents, i.e. electric field and mechanical strain, allow for tunability of the Dirac cone and the charge density at the interface. The control/maintenance of the Dirac cone states in nCL/G is an important feature for the development of electronic devices based on cellulosic platforms. △ Less

Submitted 24 August, 2022; originally announced August 2022.

Journal ref: Phys. Chem. Chem. Phys., 2023,25, 1161-1168

Magnetic anisotropy energies and metal-insulator transitions in monolayers of $α$-RuCl$_3$ and OsCl$_3$ on graphene

Authors: P. H. Souza, D. P. de Andrade Deus, W. H. Brito, R. H. Miwa

Abstract: Transition metal thriclorides, with $4d$ or $5d$ electrons, are materials at the forefront of recent studies about the interplay of spin-orbit coupling and strong Coulomb interactions. Within our first-principles calculations (DFT+$U$+SOC) we study the effects of graphene on the electronic and magnetic properties of the monolayers of $α$-RuCl$_3$ and OsCl$_3$. Despite the spatially inhomogeneous… ▽ More Transition metal thriclorides, with $4d$ or $5d$ electrons, are materials at the forefront of recent studies about the interplay of spin-orbit coupling and strong Coulomb interactions. Within our first-principles calculations (DFT+$U$+SOC) we study the effects of graphene on the electronic and magnetic properties of the monolayers of $α$-RuCl$_3$ and OsCl$_3$. Despite the spatially inhomogeneous $n$-type doping induced by graphene, we show that the occupancy of the upper Hubbard bands of MLs of \rucl and OsCl$_3$ can be tuned through external electric fields, and allows the control of (i) metal-insulator transitions, and (ii) the magnetic easy-axis and anisotropy energies. Our findings point towards the tunning of electronic and magnetic properties of transition metal thriclorides monolayers by using graphene and external electronic fields. △ Less

Submitted 11 March, 2022; originally announced March 2022.

Comments: 10 pages and 8 figures

Electronic properties of the Weyl semimetals Co$_2$MnX (X=Si, Ge, Sn)

Authors: Abhishek Sharan, Felipe Crasto de Lima, Shoaib Khalid, Roberto H. Miwa, Anderson Janotti

Abstract: Using first-principles electronic structure calculations, we show that ferromagnetic Heusler compounds Co$_2$MnX (X= Si, Ge, Sn) present non-trivial topological characteristics and belong to the category of Weyl semimetals. These materials exhibit two topologically interesting band crossings near the Fermi level. These band crossings have complex 3D geometries in the Brillouin zone and are charact… ▽ More Using first-principles electronic structure calculations, we show that ferromagnetic Heusler compounds Co$_2$MnX (X= Si, Ge, Sn) present non-trivial topological characteristics and belong to the category of Weyl semimetals. These materials exhibit two topologically interesting band crossings near the Fermi level. These band crossings have complex 3D geometries in the Brillouin zone and are characterized by non-trivial topology as Hopf links and chain-like nodal lines, that are protected by the perpendicular mirror planes. The spin-orbit interaction split these nodal lines into several zero-dimensional Weyl band crossings. Unlike previously known topologically non-trivial Heusler materials, these majority-spin band crossings lie in the band gap of minority spin bands, potentially facilitating its experimental realization. △ Less

Submitted 28 November, 2021; originally announced November 2021.

Machine learning of microscopic ingredients for graphene oxide/cellulose interaction

Authors: Romana Petry, Gustavo Silvestre, Bruno Focassio, F. Crasto de Lima, Roberto H. Miwa, Adalberto Fazzio

Abstract: Understanding the role of microscopic attributes in nanocomposites allows for a controlled and, therefore, acceleration in experimental system designs. In this work, we extracted the relevant parameters controlling the graphene oxide binding strength to cellulose by combining first-principles calculations and machine learning algorithms. We were able to classify the systems among two classes with… ▽ More Understanding the role of microscopic attributes in nanocomposites allows for a controlled and, therefore, acceleration in experimental system designs. In this work, we extracted the relevant parameters controlling the graphene oxide binding strength to cellulose by combining first-principles calculations and machine learning algorithms. We were able to classify the systems among two classes with higher and lower binding energies, which are well defined based on the isolated graphene oxide features. By a theoretical X-ray photoelectron spectroscopy analysis, we show the extraction of these relevant features. Additionally, we demonstrate the possibilities of a refined control within a machine learning regression between the binding energy values and the system's characteristics. Our work presents a guiding map to the control graphene oxide/cellulose interaction. △ Less

Submitted 2 July, 2021; originally announced July 2021.

Journal ref: Langmuir 38, 1124 (2022)

Oxidation of 2D electrenes: structural transition and the formation of half-metallic channels protected by oxide layers

Authors: Pedro H. Souza, Danilo Kuritza, José E. Padilha, Roberto H. Miwa

Abstract: Based on first-principles calculations we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized $A_{2}B$, electrenes, with the following combinations, (i) $A$=Ca, Sr, and Ba for $B$=N; (ii) $A$=Sr and Ba for $B$=P; and Y$_{2}$C, and Ba$_{2}$As. We have considered one side oxidation of single layer electrenes… ▽ More Based on first-principles calculations we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized $A_{2}B$, electrenes, with the following combinations, (i) $A$=Ca, Sr, and Ba for $B$=N; (ii) $A$=Sr and Ba for $B$=P; and Y$_{2}$C, and Ba$_{2}$As. We have considered one side oxidation of single layer electrenes $(O/A_{2}B)$, and two side oxidation of bilayer electrenes $(O/(A_{2}B)_{2}/O)$. We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the oxidized systems. Our total energy results reveal an exothermic structural transition from hexagonal to tetragonal (h$\rightarrow$t) geometry, resulting in layered tetragonal structures [$(AOAB)^{t}$, and $(AO(AB)_{2}AO)^{t}$]. Phonon spectra calculations and molecular dynamic simulations show that the $O/A_{2}B$, and $O/(A_{2}B)_{2}/O$ systems, with $A$=Ba, Ca, Sr, and $B$=N, become dynamically and structurally stable upon such a h$\rightarrow$t transition. Further structural characterizations were performed based on simulations of the near edge X-ray absorption spectroscopy at the nitrogen K-edge. Finally, the electronic structure calculations and transport calculations reveal the formation of half-metallic bands spreading out through the $A$N layers, which are shielded by oxide $A$O sheets. These findings indicate that $(AOAB)^{t}$, and $(AO(AB)_{2}AO)^{t}$, (with $B$=N) are quite interesting platforms for application in spintronics; since the half-metallic channels along the $A$N or $(A\text{N})_2$ layers (core) are protected against the environment conditions by the oxidized $A\text{O}$ sheets (cover shells). △ Less

Submitted 21 April, 2022; v1 submitted 16 June, 2021; originally announced June 2021.

Comments: 13 pages, 13 figures

Unveiling the dopant segregation effect at hematite interfaces

Authors: Felipe Crasto de Lima, Gabriel R. Schleder, João B. Souza Junior, Flavio L. Souza, Fabrício B. Destro, Roberto H. Miwa, Edson R. Leite, Adalberto Fazzio

Abstract: Understanding the effects of atomic structure modification in hematite photoanodes is essential for the rational design of high-efficiency functionalizations. Recently it was found that interface modification with Sn/Sb segregates considerably increases hematite photocatalytic efficiency. However, the understanding of the different electronic effects of these modifications at the atomic level is s… ▽ More Understanding the effects of atomic structure modification in hematite photoanodes is essential for the rational design of high-efficiency functionalizations. Recently it was found that interface modification with Sn/Sb segregates considerably increases hematite photocatalytic efficiency. However, the understanding of the different electronic effects of these modifications at the atomic level is still lacking. This letter describes the segregation effects of two different dopants-Sn and Sb-on both the solid-solid (grain-boundaries) and solid-liquid interfaces (surfaces) of hematite. Within an ab-initio approach, we quantitatively extract the potential barrier reduction on polycrystalline interfaces due to the dopant, which causes an increase in the inter-grain electron transport. Concomitantly, the dopants' segregation on hematite surfaces results in a decrease of the oxygen vacancy formation energy. Such vacancies lead to the experimentally observed rise of the flat-band potential. The comprehension of the electronic effects of dopants on both types of interfaces explains the experimental peak efficiency of interface-modified hematite with dopant segregates, also enabling the control and design of interfaces for different higher-efficiency applications. △ Less

Submitted 3 May, 2021; originally announced May 2021.

Journal ref: Appl. Phys. Lett. 118, 201602 (2021)

Role of the rare-earth doping on the multiferroic properties of BaTiO$_3$: First-principles calculation

Authors: A. P. Aslla-Quispe, R. H. Miwa, J. D. S. Guerra

Abstract: Ab-initio spin-polarized Density Functional Theory plus U is used to study the electronic and magnetic properties of tetragonal doped barium titanate (Ba$_{1-x}$Eu$_x$O$_3$) system for different europium (Eu$^{3+}$) concentrations. For this study, the Projector Augmented Wave (PAW) method and a Perdew-Zunger (LSDA) approximation, which has been used for the exchange correlation energy, have been c… ▽ More Ab-initio spin-polarized Density Functional Theory plus U is used to study the electronic and magnetic properties of tetragonal doped barium titanate (Ba$_{1-x}$Eu$_x$O$_3$) system for different europium (Eu$^{3+}$) concentrations. For this study, the Projector Augmented Wave (PAW) method and a Perdew-Zunger (LSDA) approximation, which has been used for the exchange correlation energy, have been considered taking into account a supercell model. In this model, the spin polarization as well as the Hubbard's potential have been used for the correction of the electron-electron Coulomb interactions in the rare-earth ions partially filled f-orbitals. The electronic bands-structure reveals that the band-gap energy as well as the dielectric properties decreases with the increase of the doping concentration. On the other hand, the modern theory of polarization also shows that the spontaneous electric polarization increases with the increase of the europium content, whereas the states-density reveals ferromagnetic characteristics (with non-zero total magnetization), without an applied magnetic field, for the Ba$_{1-x}$Eu$_x$O$_3$ system. The magnetic properties also reveal to be strongly dependent on the exchange interaction of the strong localized Eu 4f-states in the crystal lattice. △ Less

Submitted 4 January, 2021; v1 submitted 1 January, 2021; originally announced January 2021.

Comments: 19 pages, 3 tables, 9 figures

Journal ref: Physica B 615 (2021) 413107

Jacutingaite-family: a class of topological materials

Authors: F. Crasto de Lima, R. H. Miwa, A. Fazzio

Abstract: Jacutingate, a recently discovered Brazilian naturally occurring mineral, has shown to be the first experimental realization of the Kane-Mele topological model. In this letter we have unveiled a class of materials $M_2NX_3$ ($M$=Ni, Pt, Pd; $N$=Zn, Cd, Hg; and $X$=S, Se, Te), sharing jacutingaite's key features, i.e., high stability, and topological phase. By employing first-principles calculation… ▽ More Jacutingate, a recently discovered Brazilian naturally occurring mineral, has shown to be the first experimental realization of the Kane-Mele topological model. In this letter we have unveiled a class of materials $M_2NX_3$ ($M$=Ni, Pt, Pd; $N$=Zn, Cd, Hg; and $X$=S, Se, Te), sharing jacutingaite's key features, i.e., high stability, and topological phase. By employing first-principles calculations we extensively characterize the energetic stability of this class while showing a common occurrence of the Kane-Mele topological phase. Here we found Pt-based materials surpassing jacutingaite's impressive topological gap and lower exfoliation barrier while retaining its stability. △ Less

Submitted 29 October, 2020; originally announced October 2020.

Journal ref: Phys. Rev. B 102, 235153 (2020)

Disassembling of TEMPO-oxidized cellulose fibers: intersheet and interchain interactions in the isolation of nanofibers and unitary chains

Authors: Gustavo H. Silvestre, Lidiane O. Pinto, Juliana S. Bernardes, Roberto H. Miwa, Adalberto Fazzio

Abstract: Cellulose disassembly is an important issue in designing nanostructures using cellulose-based materials. In this work, we present a joint of experimental and theoretical study addressing the disassembly of cellulose nanofibrils. Through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation processes, combined with atomic force microscopy results, we find the formation of nanofibers with… ▽ More Cellulose disassembly is an important issue in designing nanostructures using cellulose-based materials. In this work, we present a joint of experimental and theoretical study addressing the disassembly of cellulose nanofibrils. Through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation processes, combined with atomic force microscopy results, we find the formation of nanofibers with diameters corresponding to a single cellulose polymer chain. The formation of these polymer chains is ruled by repulsive electrostatic interactions between the oxidized chains. Further first-principles calculations have been done in order to provide an atomistic understanding the cellulose disassembling processes, focusing on the balance of the interchain and intersheet interactions upon oxidation. Firstly we analyse these interaction in pristine systems, where we found the intersheet interaction stronger than the interchain one. In the oxidized systems, we have considered the formation of (charged) carboxylate groups along the inner sites of elementary fibrils. We show a net charge concentration on the carboxylate groups, supporting the emergence of repulsive electrostatic interactions between the cellulose nanofibers. Indeed, our total energy results show that the weakening of the binding strength between the fibrils is proportional to the concentration and the net charge density of the carboxylate group. Moreover, by comparing interchain and intersheet binding energies, we found that most of the disassembly processes should take place by breaking the interchain O--H$\cdots$O hydrogen bond interactions, and thus supporting the experimental observation of single and double cellulose polymeric chains. △ Less

Submitted 16 March, 2021; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: 9 pages, 5 figures

Magnetic and Electronic Switch in Metal Intercalated Two-Dimensional GeP$_3$

Authors: D. P. de A. Deus, I. S. S. de Oliveira, J. B. Oliveira, W. L. Scopel, R. H. Miwa

Abstract: Intercalation of foreign atoms in two dimensional hosts has been considered a quite promising route in order to engineer the electronic, and magnetic properties in 2D plataforms. In the present study, we performed a first-principles theoretical investigation of the energetic stability, and the magnetic/electronic properties of 2D GeP$_3$ doped by Cr atoms. Our total energy results reveal the forma… ▽ More Intercalation of foreign atoms in two dimensional hosts has been considered a quite promising route in order to engineer the electronic, and magnetic properties in 2D plataforms. In the present study, we performed a first-principles theoretical investigation of the energetic stability, and the magnetic/electronic properties of 2D GeP$_3$ doped by Cr atoms. Our total energy results reveal the formation of thermodynamically stable Cr doped GeP$_3$ bilayer [(GeP$_3$)$_{BL}$], characterized by interstitial Cr atoms lying in the van der Waals (vdW) gap between (GeP$_3$)$_{BL}$ [(GeP$_3$)$_{BL}^{Cr}$]. We show that the ground state row-wise antiferromagnetic (RW-AFM) phase of (GeP$_3$)$_{BL}^{Cr}$ can be tuned to a ferromagnetic (FM) configuration upon compressive mechanical strain ($\varepsilon$), Cr$^{\uparrow \downarrow} \xrightarrow{\varepsilon}$Cr$^{\uparrow \uparrow}$. By considering its stacked counterparts, (GeP$_3$)$_{BL}^{Cr}$/(GeP$_3$)$_{BL}^{Cr}$, and (GeP$_3$)$_{BL}^{Cr}$/Cr/(GeP$_3$)$_{BL}^{Cr}$, we found that such a magnetic tuning is dictated by a combination of intralayer and interlayer couplings, where the RW-AFM phase change to layer-by-layer FM (Cr$^{\uparrow \uparrow}$//Cr$^{\uparrow \uparrow}$) and AFM (Cr$^{\uparrow \uparrow}$/Cr$^{\downarrow \downarrow}$/Cr$^{\uparrow \uparrow}$) phases, respectively. Further electronic band structure calculations show that these Cr doped systems are metallic, characterized by the emergence of strain induced spin polarized channels at the Fermi level. These findings reveal that the atomic intercalation, indeed, offers a new set of degree of freedom for the design and control the magnetic/electronic properties in 2D systems. △ Less

Submitted 7 October, 2020; originally announced October 2020.

Comments: 8 pages and 6 figures

Journal ref: Phys. Rev. Materials 5, 054002 (2021)

Simulation of XANES spectroscopy and the calculation of total energies for N-heterocyclic carbenes on Au(111)

Authors: Felipe Crasto de Lima, Adalberto Fazzio, Alastair B. McLean, Roberto H. Miwa

Abstract: It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed {\it first-principles} total energy calculations for NHCs on Au(111) and simulations of… ▽ More It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed {\it first-principles} total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene ($^B$NHC$^X$, with X=Me, Et, and iPr, respectively) and the bis-$^B$NHC$^X$ complexes with Au derived from these molecules. We present a comprehensive analysis of the energetic stability of both the $^B$NHC$^X$ and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the $^B$NHC$^X$/Au geometry and the N($1s$) $\rightarrow$ $π^\ast/σ^\ast$, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. Our simulations are compared with recent experimental results. △ Less

Submitted 20 July, 2020; originally announced July 2020.

Structural Transition in Oxidized Ca$_2$N Electrenes: CaO/CaN 2D heterostructures

Authors: Pedro H. Souza, José E. Padilha, Roberto H. Miwa

Abstract: Based on first-principles calculations we show that the oxidation of ultrathin films of Ca$_2$N electrides, electrenes, drives a hexagonal$\rightarrow$tetragonal structural transition. The ground state configuration of the oxidized monolayer (ML) and bilayer (BL) systems can be viewed as CaO/CaN and CaO/(CaN)$_2$/CaO two dimensional (2D) heterostructures. In both systems, we found nearly free elec… ▽ More Based on first-principles calculations we show that the oxidation of ultrathin films of Ca$_2$N electrides, electrenes, drives a hexagonal$\rightarrow$tetragonal structural transition. The ground state configuration of the oxidized monolayer (ML) and bilayer (BL) systems can be viewed as CaO/CaN and CaO/(CaN)$_2$/CaO two dimensional (2D) heterostructures. In both systems, we found nearly free electron (NFE) states lying near the vacuum level, and the spatial projection reveals that they are localized above the oxidized CaO surface. Focusing on the magnetic properties, we find that the nitrogen atoms of the oxidized Ca$_2$N becomes spin-polarized ($\sim$1 $μ_{\rm B}$/N-atom); where (i) the ferromagnetic and the anti-ferromagnetic phases are nearly degenerated in the ML system, CaO/CaN, while (ii) there is an energetic preference for the ferromagnetic phase in CaO/(CaN)$_2$/CaO. We show that such a FM preference can be strengthened upon mechanical compression. Further electronic structure calculations reveal that the FM CaO/Ca$_2$N/CaO presents half-metallicity, where the metallic channels project (predominantly) on the N-$2p_{x,y}$ orbitals. In addition to the total energy results, molecular dynamic and phonon spectra calculations have been done in order to verify its thermal and structural stabilities. Those findings suggest that CaO/Ca$_2$N/CaO is a quite interesting, and structurally stable, 2D FM heterostructure characterized half-metallic bands sandwiched by NFE states lying on the oxidized surfaces. △ Less

Submitted 4 May, 2020; v1 submitted 7 April, 2020; originally announced April 2020.

Comments: 7 pages, 6 figures

Engineering metal-$sp_{xy}$ Dirac bands on the oxidized SiC surface

Authors: F. Crasto de Lima, R. H. Miwa

Abstract: The ability to construct 2D systems, beyond materials natural formation, enriches the search and control capability of new phenomena. For instance, the synthesis of topological lattices of vacancies on metal surfaces through scanning tunneling microscopy. In the present study we demonstrate that metal atoms encaged in silicate adlayer on silicon carbide is an interesting platform for lattices desi… ▽ More The ability to construct 2D systems, beyond materials natural formation, enriches the search and control capability of new phenomena. For instance, the synthesis of topological lattices of vacancies on metal surfaces through scanning tunneling microscopy. In the present study we demonstrate that metal atoms encaged in silicate adlayer on silicon carbide is an interesting platform for lattices design, providing a ground to experimentally construct tight-binding models on an insulating substrate. Based on the density functional theory, we have characterized the energetic and the electronic properties of 2D metal lattices embedded in the silica adlayer. We show that the characteristic band structures of those lattices are ruled by surface states induced by the metal-$s$ orbitals coupled by the host-$p_{xy}$ states; giving rise to $sp_{xy}$ Dirac bands neatly lying within the energy gap of the semiconductor substrate. △ Less

Submitted 29 January, 2020; originally announced January 2020.

Journal ref: Nano Letters (2020) 10.1021/acs.nanolett.0c01111

Topological flat band, Dirac fermions and quantum spin Hall phase in 2D Archimedean lattices

Authors: F. Crasto de Lima, Gerson J. Ferreira, R. H. Miwa

Abstract: Materials with designed properties arises in a synergy between theoretical and experimental approaches. In this study we explore the set of Archimedean lattices forming a guidance to its electronic properties and topological phases. Within these lattices, rich electronic structure emerge forming type-I and II Dirac fermions, topological flat bands and high-degeneracy points with linear and flat di… ▽ More Materials with designed properties arises in a synergy between theoretical and experimental approaches. In this study we explore the set of Archimedean lattices forming a guidance to its electronic properties and topological phases. Within these lattices, rich electronic structure emerge forming type-I and II Dirac fermions, topological flat bands and high-degeneracy points with linear and flat dispersions. Employing a tight-binding model, with spin-orbit coupling, we characterize a quantum spin Hall (QSH) phase in all Archimedean lattices. Our discussion is validated within density functional theory calculations, where we show the characteristic bands of the studied lattices arising in 2D carbon allotropes. △ Less

Submitted 14 August, 2019; originally announced August 2019.

Journal ref: Phys. Chem. Chem. Phys., 2019

Double flat bands in kagome twisted bilayers

Authors: F. Crasto de Lima, R. H. Miwa, E. Suárez Morell

Abstract: We have studied how a generic bilayer kagome lattice behave upon layer rotation. We employed a Tight Binding model with one orbital per site and found (i) for low rotational angles, and at low energies, the same flat bands structure like in twisted bilayer graphene; though, for a larger value of the magic angle. Moreover, (ii) at high energies, due to the superstructure symmetry regions, we found… ▽ More We have studied how a generic bilayer kagome lattice behave upon layer rotation. We employed a Tight Binding model with one orbital per site and found (i) for low rotational angles, and at low energies, the same flat bands structure like in twisted bilayer graphene; though, for a larger value of the magic angle. Moreover, (ii) at high energies, due to the superstructure symmetry regions, we found the characteristics three band dispersion of the kagome lattice. In the latter, its band width decreases for lower angles confining them within a few meV. Therefore, we found in twisted kagome lattice the coexistence of two sets of flat bands in different energies and lying in different spatial regions of the bilayer system. △ Less

Submitted 17 July, 2019; originally announced July 2019.

Journal ref: Phys. Rev. B 100, 155421 (2019)

Electronic Stripes and Transport Properties in Borophene Heterostructures

Authors: G. H. Silvestre, R. H. Miwa, Wanderlã L. Scopel

Abstract: We performed a theoretical investigation of the structural and electronic properties of (i) pristine, and (ii) superlattice structures of borophene. In (i), by combining first-principles calculations, based on the density functional theory (DFT), and simulations of the X-ray Absorption Near-Edge Structure (XANES) we present a comprehensive picture connecting the atomic arrangement of borophene and… ▽ More We performed a theoretical investigation of the structural and electronic properties of (i) pristine, and (ii) superlattice structures of borophene. In (i), by combining first-principles calculations, based on the density functional theory (DFT), and simulations of the X-ray Absorption Near-Edge Structure (XANES) we present a comprehensive picture connecting the atomic arrangement of borophene and the X-ray absorption spectra. Once we have characterized the electronic properties of the pristine systems, we next examined the electronic confinement effects in 2D borophene superlattices (BSLs) [(ii)]. Here, the BSL structures were made by attaching laterally two different structural phases of borophene. The energetic stability, and the electronic properties of those BSLs were examined based on total energy DFT calculations. We find a highly anisotropic electronic structure, characterized by the electronic confinement effects, and the formation of metallic channels along the superlattices. Combining DFT and the Landauer-Büttiker formalism, we investigate the electronic transport properties in the BSLs. Our results of the transmission probability reveal that the electronic transport is ruled by π or a combination of π and σ transmission channels, depending on the atomic arrangement and periodicity of the superlattices. Finally we show that there is huge magnification on the directional dependence of the electronic transport properties in BSLs, in comparision with the pristine borophene phase. Those findings indicate that BSLs are quite interesting systems in order to design conductive nanoribbons in a 2D platform. △ Less

Submitted 13 November, 2019; v1 submitted 12 July, 2019; originally announced July 2019.

Comments: Nanoscale 2019

Journal ref: Nanoscale 2019

Orbital pseudospin-momentum locking in two-dimensional chiral borophene

Authors: Felipe Crasto de Lima, Gerson J. Ferreira, Roberto H. Miwa

Abstract: Recently, orbital-textures have been found in Rashba and topological insulator (TI) surface states as a result of the spin-orbit coupling (SOC). Here, we predict a $p_x/p_y$ orbital texture, in linear dispersive Dirac bands, arising at the K/K' points of $χ$-$h_0$ borophene chiral monolayer. Combining first-principles calculations with effective hamiltonians, we show that the orbital pseudospin ha… ▽ More Recently, orbital-textures have been found in Rashba and topological insulator (TI) surface states as a result of the spin-orbit coupling (SOC). Here, we predict a $p_x/p_y$ orbital texture, in linear dispersive Dirac bands, arising at the K/K' points of $χ$-$h_0$ borophene chiral monolayer. Combining first-principles calculations with effective hamiltonians, we show that the orbital pseudospin has its direction locked with the momentum in a similar way as TIs' spin-textures. Additionally, considering a layer pseudospin degree of freedom, this lattice allows stackings of layers with equivalent or opposite chiralities. In turn, we show a control of the orbital textures and layer localization through the designed stacking and external electric field. For instance, for the opposite chirality stacking, the electric field allows for an on/off switch of the orbital-textured Dirac cone. △ Less

Submitted 7 August, 2019; v1 submitted 9 July, 2019; originally announced July 2019.

Journal ref: Nano Letters 2019

Layertronic control of topological states in multilayer metal-organic frameworks

Authors: F. Crasto de Lima, G. J. Ferreira, R. H. Miwa

Abstract: We investigate the layer localization control of two-dimensional states in multilayer metal-organic frameworks (MOFs). For finite stackings of (NiC4S4)3 MOFs, the weak van der Waals coupling between adjacent layers leads to a Fermi level dependent distribution of the electronic states in the monolayers. Such distribution is reflected in the topological edge states of multilayer nanoribbons. Moreov… ▽ More We investigate the layer localization control of two-dimensional states in multilayer metal-organic frameworks (MOFs). For finite stackings of (NiC4S4)3 MOFs, the weak van der Waals coupling between adjacent layers leads to a Fermi level dependent distribution of the electronic states in the monolayers. Such distribution is reflected in the topological edge states of multilayer nanoribbons. Moreover, by applying an external electric field, parallel to the stacking direction, the spacial localization of the electronic states can be controlled for a chosen Fermi energy. This localization behavior is studied comparing density functional theory calculations with a kagome lattice tight-binding model. Furthermore, for infinite stacked nanoribbons, a new V-gutter Dirac state is found in the side surfaces, which allows anisotropic current control by tuning the Fermi energy. Our results can be immediately extended to other kagome MOFs with eclipsed stackings, introducing a new degree of freedom (layer localization) to materials design. △ Less

Submitted 12 June, 2019; v1 submitted 8 March, 2019; originally announced March 2019.

Control of magnetism in bilayer CrI$_{3}$ by an external electric field

Authors: E. Suarez Morell, Andrea León, R. Hiroki Miwa, P. Vargas

Abstract: Recently intrinsic ferromagnetism in two-dimensional(2D) van der Waals materials was discovered [1, 2, 3]. A monolayer of Chromiun triiodide(CrI3) is ferromagnetic while a bilayer structure was reported to be anti-ferro magnetic, moreover an external electric field changes its magnetic phase [4]. We have studied the two found in nature stackings of CrI3 bilayers and found that indeed the magnetic… ▽ More Recently intrinsic ferromagnetism in two-dimensional(2D) van der Waals materials was discovered [1, 2, 3]. A monolayer of Chromiun triiodide(CrI3) is ferromagnetic while a bilayer structure was reported to be anti-ferro magnetic, moreover an external electric field changes its magnetic phase [4]. We have studied the two found in nature stackings of CrI3 bilayers and found that indeed the magnetic phase of one of them can be tuned by an external electric field while the other remains ferromagnetic. We simulate those results with ab initio calculations and explain them with a simple model based on a rigid shift of the bands associated with different spins. The model can be applied to similar van der Waal stacked insulating bilayer anti-ferromagnets. △ Less

Submitted 28 November, 2018; originally announced November 2018.

Comments: 11

Large Disparity Between Optical and Fundamental Band Gaps in Layered In2Se3

Authors: Wei Li, Fernando P. Sabino, Felipe Crasto de Lima, Tianshi Wang, Roberto H. Miwa, Anderson Janotti

Abstract: In$_2$Se$_3$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density func… ▽ More In$_2$Se$_3$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density functional theory and the HSE06 hybrid functional, we investigated the structural and electronic properties of four layered phases of In$_2$Se$_3$, addressing their relative stability and the nature of their fundamental band gaps, i.e., direct {\em versus} indirect. Our results show large disparities between fundamental and optical gaps. The absorption coefficients are found to be as high as that in direct-gap III-V semiconductors. The band alignment with respect to conventional semiconductors indicate a tendency to $n$-type conductivity, explaining recent experimental observations. △ Less

Submitted 4 June, 2018; originally announced June 2018.

Journal ref: Phys. Rev. B 98, 165134 (2018)

Quantum anomalous Hall effect in metal-bis(dithiolene), magnetic properties, doping and interfacing graphene

Authors: F. Crasto de Lima, G. J. Ferreira, R. H. Miwa

Abstract: The realization of the Quantum anomalous Hall effect (QAHE) in two dimensional (2D) metal organic frameworks (MOFs), (MC$_4$S$_4$)$_3$ with M = Mn, Fe, Co, Ru and Rh, has been investigated based on a combination of first-principles calculations and tight binding models. Our results for the magnetic anisotropy energy (MAE) reveal that the out-of-plane (in-plane) magnetization is favored for M = Mn,… ▽ More The realization of the Quantum anomalous Hall effect (QAHE) in two dimensional (2D) metal organic frameworks (MOFs), (MC$_4$S$_4$)$_3$ with M = Mn, Fe, Co, Ru and Rh, has been investigated based on a combination of first-principles calculations and tight binding models. Our results for the magnetic anisotropy energy (MAE) reveal that the out-of-plane (in-plane) magnetization is favored for M = Mn, Fe, and Ru (Co, and Rh). Given the structural symmetry of (MC$_4$S$_4$)$_3$, the QAHE takes place only for M = Mn, Fe and Ru. Such a quantum anomalous Hall phase has been confirmed through the calculation of the Chern number, and examining the formation of topologically protected (metallic) edge states. Further electron ($n$-type) doping of the MOFs has been done in order to place the Fermi level within the non-trivial energy gap; where we find that in (RuC$_4$S$_4$)$_3$, in addition to the up-shift of the Fermi level, the MAE energy increases by 40\%. Finally, we show that in MOF/graphene (vdW) interfaces, the Fermi level tunning can be done with an external electric field, which controls the charge transfer at the MOF/graphene interface, giving rise to switchable topologically protected edge currents in MOFs. △ Less

Submitted 30 May, 2018; originally announced May 2018.

Confinement and Fermion Doubling Problem in Dirac-like Hamiltonians

Authors: B. Messias de Resende, F. Crasto de Lima, R. H. Miwa, E. Vernek, G. J. Ferreira

Abstract: We investigate the interplay between confinement and the fermion doubling problem in Dirac-like Hamiltonians. Individually, both features are well known. First, simple electrostatic gates do not confine electrons due to the Klein tunneling. Second, a typical lattice discretization of the first-order derivative $k \rightarrow -i\partial_x$ skips the central point and allow spurious low-energy, high… ▽ More We investigate the interplay between confinement and the fermion doubling problem in Dirac-like Hamiltonians. Individually, both features are well known. First, simple electrostatic gates do not confine electrons due to the Klein tunneling. Second, a typical lattice discretization of the first-order derivative $k \rightarrow -i\partial_x$ skips the central point and allow spurious low-energy, highly oscillating solutions known as fermion doublers. While a no-go theorem states that the doublers cannot be eliminated without artificially breaking a symmetry, here we show that the symmetry broken by the Wilson's mass approach is equivalent to the enforcement of hard-wall boundary conditions, thus making the no-go theorem irrelevant when confinement is foreseen. We illustrate our arguments by calculating the following: (i) the band structure and transport properties across thin films of the topological insulator Bi$_2$Se$_3$, for which we use ab-initio density functional theory calculations to justify the model; and (ii) the band structure of zigzag graphene nanoribbons. △ Less

Submitted 25 October, 2017; v1 submitted 11 August, 2017; originally announced August 2017.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 96, 161113 (2017)

Tuning the topological states in metal-organic bilayers

Authors: F. Crasto de Lima, Gerson J. Ferreira, R. H. Miwa

Abstract: We have investigated the energetic stability and the electronic properties of metal-organic topological insulators bilayers (BLs), $(MC_4S_4)_3$-BL, with M=Ni and Pt, using first-principles calculations and tight-binding model. Our findings show that $(MC_4S_4)_3$-BL is an appealing platform to perform electronic band structure engineering, based on the topologically protected chiral edge states.… ▽ More We have investigated the energetic stability and the electronic properties of metal-organic topological insulators bilayers (BLs), $(MC_4S_4)_3$-BL, with M=Ni and Pt, using first-principles calculations and tight-binding model. Our findings show that $(MC_4S_4)_3$-BL is an appealing platform to perform electronic band structure engineering, based on the topologically protected chiral edge states. The energetic stability of the BLs is ruled by van der Waals interactions; being the AA stacking the energetically most stable one. The electronic band structure is characterized by a combination of bonding and anti-bonding kagome band sets (KBSs), revealing that $(NiC_4S_4)_3$-BL presents a Z$_2$-metallic phase, whereas $(PtC_4S_4)_3$-BL may present both Z$_2$-metallic phase or quantum spin Hall phase. Those non-trivial topological states were confirmed by the formation of chiral edge states in $(MC_4S_4)_3$-BL nanoribbons. We show that the localization of the edge states can be controlled with a normal external electric field, breaking the mirror symmetry. Hence, the sign of electric field selects in which layer each set of edge states are located. Such a control on the (layer) localization, of the topological edge states, bring us an additional and interesting degree of freedom to control the transport properties in layered metal-organic topological insulator. △ Less

Submitted 30 September, 2017; v1 submitted 25 May, 2017; originally announced May 2017.

Journal ref: Phys. Rev. B 96, 115426 (2017)

H2 O incorporation in the phosphorene/a-SiO2 interface: A first-principles study

Authors: Wanderla L. Scopel, Everson S. Souza, R. H. Miwa

Abstract: Based on first-principles calculations, we investigate the energetic stability and the electronic properties of (i) a single layer phosphorene (SLP) adsorbed on the amorphous sio2 surface (SLP/a-sio2), and (ii) the further incorporation of water molecules at the phosphorene/a-sio2 interface. In (i), we find that the phosphorene sheet bonds to a-sio2 through van der Waals interactions, even upon th… ▽ More Based on first-principles calculations, we investigate the energetic stability and the electronic properties of (i) a single layer phosphorene (SLP) adsorbed on the amorphous sio2 surface (SLP/a-sio2), and (ii) the further incorporation of water molecules at the phosphorene/a-sio2 interface. In (i), we find that the phosphorene sheet bonds to a-sio2 through van der Waals interactions, even upon the presence of oxygen vacancy on the surface. The \slp/a-\sio\ system presents a type-I band alignment, with the valence (conduction) band maximum (minimum) of the phosphorene lying within the energy gap of the a-\sio\ substrate. The structural, and the surface-potential corrugations promote the formation of electron-rich and -poor regions on the phosphorene sheet and at the SLP/a-sio2 interface. Such charge density puddles have been strengthened by the presence of oxygen vacancies in a-sio2. In (ii), due to the amorphous structure of the surface, we have considered a number of plausible geometries of water embedded in the SLP/a-sio2 interface. There is an energetic preference to the formation of hydroxyl (OH) groups on the a-sio2 surface. Meanwhile, upon the presence of oxygenated water or interstitial oxygen in the phosphorene sheet, we find the formation of metastable OH bonded to the phosphorene, and the formation of energetically stable P--O--Si chemical bonds at the SLP/a-sio2 interface. Further x-ray absorption spectra (XAS) simulations have been done, aiming to provide additional structural/electronic informations of the oxygen atoms forming hydroxyl groups or P--O--Si chemical bonds at the interface region. △ Less

Submitted 30 August, 2016; originally announced August 2016.

An ab initio investigation of Bi$_2$Se$_3$ topological insulator deposited on amorphous SiO$_2$

Authors: I. S. S. de Oliveira, W. L. Scopel, R. H. Miwa

Abstract: We use first-principles simulations to investigate the topological properties of Bi$_2$Se$_3$ thin films deposited on amorphous SiO2, Bi$_2$Se$_3$/a-SiO$_2$, which is a promising substrate for topological insulator (TI) based device applications. The Bi$_2$Se$_3$ films are bonded to a-SiO$_2$ mediated by van der Waals interactions. Upon interaction with the substrate, the Bi$_2$Se$_3$ topological… ▽ More We use first-principles simulations to investigate the topological properties of Bi$_2$Se$_3$ thin films deposited on amorphous SiO2, Bi$_2$Se$_3$/a-SiO$_2$, which is a promising substrate for topological insulator (TI) based device applications. The Bi$_2$Se$_3$ films are bonded to a-SiO$_2$ mediated by van der Waals interactions. Upon interaction with the substrate, the Bi$_2$Se$_3$ topological surface and interface states remain present, however the degeneracy between the Dirac-like cones is broken. The energy separation between the two Dirac-like cones increases with the number of Bi$_2$Se$_3$ quintuple layers (QLs) deposited on the substrate. Such a degeneracy breaking is caused by (i) charge transfer from the TI to the substrate and charge redistribution along the Bi$_2$Se$_3$ QLs, and (ii) by deformation of the QL in contact with the a-SiO$_2$ substrate. We also investigate the role played by oxygen vacancies (V$_O$) on the a-SiO$_2$, which increases the energy splitting between the two Dirac-like cones. Finally, by mapping the electronic structure of Bi$_2$Se$_3$/a-SiO$_2$, we found that the a-SiO$_2$ surface states, even upon the presence of V$_O$, play a minor role on gating the electronic transport properties of Bi$_2$Se$_3$. △ Less

Submitted 29 August, 2016; originally announced August 2016.

Metallic nanolines ruled by grain boundaries in graphene: an ab initio study

Authors: F. D. C. de Lima, R. H. Miwa

Abstract: We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene upon the presence of grain boundaries (GBs). Our results reveal an energetic preference for the TMs lying along the GB sites (TM/GB). Such an energetic preference has been strengthened by increasing the concentration of the TM a… ▽ More We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene upon the presence of grain boundaries (GBs). Our results reveal an energetic preference for the TMs lying along the GB sites (TM/GB). Such an energetic preference has been strengthened by increasing the concentration of the TM adatoms; giving rise to TM nanolines on graphene ruled by GBs. Further diffusion barrier calculations for Fe adatoms support the formation of those TM nanolines. We find that the energy barriers parallel to the GBs are sligthly lower in comparision with those obtained for the defect free graphene; whereas, perpendicularly to the GBs the Fe adatoms face higher energy barriers. Fe and Co (Mn) nanolines are ferromagnetic (ferrimagnetic), in contrast the magnetic state of Ru nanolines is sensitive to the Ru/GB adsorption geometry. The electronic properties of those TM nanolines were characterized through extensive electronic band structure calculations. The formation of metallic nanolines is mediated by a strong hybridization between the TM and the graphene ($π$) orbitals along the GB sites. Due to the net magnetization of the TM nanolines, our band structure results indicate an anisotropic (spin-polarized) electronic current for some TM/GB systems. △ Less

Submitted 7 October, 2015; originally announced October 2015.

Pyridine intercalated Bi$_2$Se$_3$ heterostructures: controlling the topologically protected states

Authors: I. S. S. de Oliveira, R. H. Miwa

Abstract: We use ab initio simulations to investigate the incorporation of pyridine molecules (C$_5$H$_5$N) in the van der Waals gaps of Bi$_2$Se$_3$. The intercalated pyridine molecules increase the separation distance between the Bi$_2$Se$_3$ quintuple layers (QLs), suppressing the parity inversion of the electronic states at the $Γ$-point. We find that the intercalated region becomes a trivial insulator.… ▽ More We use ab initio simulations to investigate the incorporation of pyridine molecules (C$_5$H$_5$N) in the van der Waals gaps of Bi$_2$Se$_3$. The intercalated pyridine molecules increase the separation distance between the Bi$_2$Se$_3$ quintuple layers (QLs), suppressing the parity inversion of the electronic states at the $Γ$-point. We find that the intercalated region becomes a trivial insulator. By combining the pristine Bi$_2$Se$_3$ region with the one intercalated by the molecules, we have a non-trivial/trivial heterojunction characterized by the presence of (topologically protected) metallic states at the interfacial region. Next we apply an external compressive pressure to the system, and the results are (i) a decrease on the separation distance between the QLs intercalated by pyridine molecules, and (ii) the metallic states are shifted toward the bulk region, turning the system back to insulator. That is, through a suitable tuning of the external pressure in Bi$_2$Se$_3$, intercalated by pyridine molecules, we can control its topological properties; turning-on and -off the topologically protected metallic states lying at the non-trivial/trivial interface. △ Less

Submitted 24 September, 2015; originally announced September 2015.

Periodic arrays of intercalated atoms in twisted bilayer graphene: an \it{ ab initio} investigation

Authors: R. H. Miwa, P. Venezuela, E. Suárez Morell

Abstract: We have performed an \it {ab initio} investigation of transition metals (TMs = Mo, Ru, Co, and Pt) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasi-periodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for th… ▽ More We have performed an \it {ab initio} investigation of transition metals (TMs = Mo, Ru, Co, and Pt) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasi-periodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, Co and Pt atoms. The net magnetic moment (m) of Mo and Ru atoms may be quenched upon intercalation, depending on the stacking region (AA or AB). For instance, we find a magnetic moment of 0.3 $μ_{\rm B}$ (1.8 $μ_{\rm B}$) for Ru atoms intercalated between the AA (AB) regions of the stacked twisted layers. Through simulated scanning tunneling microscopy (STM) images, we verify that the presence of intercalated TMs can be identified by the formation of bright (hexagonal) spots lying on the graphene surface. △ Less

Submitted 24 August, 2015; originally announced August 2015.

Comments: 8 pages 6 Figures. To appear on PRB

Topological Phases in Triangular Lattices of Ru Adsorbed on Graphene: ab-initio calculations

Authors: C. Mera Acosta, Matheus P. Lima, R. H. Miwa, Antonio J. R. da Silva, A. Fazzio

Abstract: We have performed an ab initio investigation of the electronic properties of the graphene sheet adsorbed by Ru adatoms (Ru/graphene). For a particular set of triangular arrays of Ru adatoms, we find the formation of four (spin-polarized) Dirac cones attributed to a suitable overlap between two hexagonal lattices: one composed by the C sites of the graphene sheet, and the other formed by the surfac… ▽ More We have performed an ab initio investigation of the electronic properties of the graphene sheet adsorbed by Ru adatoms (Ru/graphene). For a particular set of triangular arrays of Ru adatoms, we find the formation of four (spin-polarized) Dirac cones attributed to a suitable overlap between two hexagonal lattices: one composed by the C sites of the graphene sheet, and the other formed by the surface potential induced by the Ru adatoms. Upon the presence of spin-orbit coupling (SOC) nontrivial band gaps take place at the Dirac cones promoting several topological phases. Depending on the Ru concentration, the system can be topologically characterized among the phases i) Quantum Spin Hall (QSH), ii) Quantum Anomalous Hall (QAH), iii) metal iv) or trivial insulator. For each concentration, the topological phase is characterized by the ab-initio calculation of the Chern number. △ Less

Submitted 30 April, 2014; v1 submitted 22 January, 2014; originally announced January 2014.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 89, 155438 (2014)

Lithium incorporation at the MoS2/graphene interface: an ab initio investigation

Authors: Roberto H. Miwa, Wanderla L. Scopel

Abstract: Based on ab initio calculations, we examine the incorporation of Li atoms in the MoS2/graphene interface. We find that the intercalated Li atoms are energetically more stable than Li atoms adsorbed on the MoS2 surface. The intercalated atoms interact with both graphene sheet and MoS2 layer, increasing the Li binding energies. However, the equilibrium geometries are ruled by the MoS2 layer, where t… ▽ More Based on ab initio calculations, we examine the incorporation of Li atoms in the MoS2/graphene interface. We find that the intercalated Li atoms are energetically more stable than Li atoms adsorbed on the MoS2 surface. The intercalated atoms interact with both graphene sheet and MoS2 layer, increasing the Li binding energies. However, the equilibrium geometries are ruled by the MoS2 layer, where the intercalated Li atoms lie on the top (Li_T) and hollow (Li_H) sites of the MoS2 layer. We calculate the Li diffusion barriers, along the Li_T -> Li_H diffusion path, where we find similar energy barriers compared with that obtained for Li adatoms on the MoS2 surface. Our results allow us to infer that the Li storage capacity increases at MoS2/G interfaces, in comparison with Li adatoms on the MoS2 surface, however, with no reduction on the mobility of the intercalated Li atoms. Those properties are interesting/useful to the development of Li batteries based on MoS2. △ Less

Submitted 5 August, 2013; originally announced August 2013.

Spin-texture and magnetic anisotropy of Co adsorbed Bi$_2$Se$_3$ topological insulator surfaces

Authors: Tome M. Schmidt, R. H. Miwa, A. Fazzio

Abstract: Based upon first-principles methods, we investigate the magnetic anisotropy and the spin-texture of Co adatoms embedded in the topmost Se network of the topological insulator Bi$_2$Se$_3$ surface. We find the formation of energetically stable magnetic moment perpendicular to the surface plane, S$_z$. Our results for the pristine Bi$_2$Se$_3$ surface indicate the presence of helical spin-texture no… ▽ More Based upon first-principles methods, we investigate the magnetic anisotropy and the spin-texture of Co adatoms embedded in the topmost Se network of the topological insulator Bi$_2$Se$_3$ surface. We find the formation of energetically stable magnetic moment perpendicular to the surface plane, S$_z$. Our results for the pristine Bi$_2$Se$_3$ surface indicate the presence of helical spin-texture not only in the massless surface Dirac states, but also surface states resonant within the valence band present spin-texture. On the other hand, upon the presence of Co adatoms we find that the out-of-plane surface magnetism represents the dominant spin state (S$_z$), while the planar spin components, S$_x$ and S$_y$, are almost suppressed. △ Less

Submitted 19 July, 2011; originally announced July 2011.

Doping of graphene adsorbed on the a-SiO$_2$ surface

Authors: R. H. Miwa, T. M. Schmidt, A. Fazzio

Abstract: We have performed an {\it ab initio} theoretical investigation of graphene sheet adsorbed on amorphous SiO$_2$ surface (G/a-SiO$_2$). We find that graphene adsorbs on the a-SiO$_2$ surface through van der Waals interactions. The inhomogeneous topology of the a-SiO$_2$ clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as… ▽ More We have performed an {\it ab initio} theoretical investigation of graphene sheet adsorbed on amorphous SiO$_2$ surface (G/a-SiO$_2$). We find that graphene adsorbs on the a-SiO$_2$ surface through van der Waals interactions. The inhomogeneous topology of the a-SiO$_2$ clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as hole-rich regions on the graphene. Such anisotropic distribution of the charge density may contribute to the reduction of the electronic mobility in G/a-SiO$_2$ systems. Furthermore, the adsorbed graphene sheet exhibits a net total charge density gain. In this case, the graphene sheet becomes n-type doped, however, with no formation of chemical bonds at the graphene--SiO$_2$ interface. The electronic charge transfer from a-SiO$_2$ to the graphene sheet occurs upon the formation of a partially occupied level lying above the Dirac point. We find that such partially occupied level comes from the three-fold coordinated oxygen atoms in the a-SiO$_2$ substrate. △ Less

Submitted 4 June, 2011; originally announced June 2011.

Hydrogenated grain boundaries in graphene

Authors: W. H. Brito, R. Kagimura, R. H. Miwa

Abstract: We have investigated by means of first principles calculations the structural and electronic properties of hydrogenated graphene structures with distinct grain boundary defects. Our total energy results reveal that the adsorption of a single H is more stable at grain boundary defect. The electronic structure of the grains boundaries upon hydrogen adsorption have been examined. Further total energy… ▽ More We have investigated by means of first principles calculations the structural and electronic properties of hydrogenated graphene structures with distinct grain boundary defects. Our total energy results reveal that the adsorption of a single H is more stable at grain boundary defect. The electronic structure of the grains boundaries upon hydrogen adsorption have been examined. Further total energy calculations indicate that the adsorption of two H on two neighbor carbons, forming a basic unit of graphane, is more stable at the defect region. Therefore, we expect that these extended defects would work as a nucleation region for the formation of a narrow graphane strip embedded in graphene region. △ Less

Submitted 22 March, 2011; originally announced March 2011.

Theoretical Study of Carbon Clusters in Silicon Carbide Nanowires

Authors: J. M. Morbec, R. H. Miwa

Abstract: Using first-principles methods we performed a theoretical study of carbon clusters in silicon carbide nanowires. We examined small clusters with carbon interstitials and antisites in hydrogen-passivated SiC nanowires growth along the [100] and [111] directions. The formation energies of these clusters were calculated as a function of the carbon concentration. We verified that the energetic stabili… ▽ More Using first-principles methods we performed a theoretical study of carbon clusters in silicon carbide nanowires. We examined small clusters with carbon interstitials and antisites in hydrogen-passivated SiC nanowires growth along the [100] and [111] directions. The formation energies of these clusters were calculated as a function of the carbon concentration. We verified that the energetic stability of the carbon defects in SiC nanowires depends strongly on the composition of the nanowire surface: the energetically most favorable configuration in carbon-coated [100] SiC nanowire is not expected to occur in silicon-coated [100] SiC nanowire. The binding energies of some aggregates were also obtained, and they indicate that the formation of carbon clusters in SiC nanowires is energetically favored. △ Less

Submitted 20 July, 2011; v1 submitted 5 October, 2010; originally announced October 2010.

Comments: 6 pages, 5 figures; 8 pages, http://www.hindawi.com/journals/jnt/2011/203423/

Journal ref: Journal of Nanotechnology, vol. 2011, Article ID 203423, 2011

Hydrogen adsorption on boron doped graphene: an {\it ab initio} study

Authors: R. H. Miwa, T. B. Martins, A. Fazzio

Abstract: The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\it ab initio} total energy calculations. The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\it ab initio} total energy calculations. △ Less

Submitted 26 May, 2007; originally announced May 2007.

Comments: 7 pages, 4 figures