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Papers by Eduardo Estevez Girao
Physical Review B, 2014
We develop a graph theoretical formalism to account for the fact that sp 2 carbon can become spin... more We develop a graph theoretical formalism to account for the fact that sp 2 carbon can become spin ordered or generate free radicals for purely topological reasons. While this phenomenon has been previously considered a binary operator, we here show a quantification in discrete units of frustrations. The graph theory method is combined with open density functional theory calculations to establish the existence of an energy of frustration that is shown to greatly improve the description of carbon nanostructure energetics using classical force fields. The methodology is illustrated for a number of systems and, owing to the small computational overhead associated with its evaluation, is expected to be easily integrable into any modeling approach based on a structure's adjacency matrix.
Advances in Atom and Single Molecule Machines, 2012
This chapter showcases selected illustrations of various manifestations of nanoscale and molecula... more This chapter showcases selected illustrations of various manifestations of nanoscale and molecular electronic effects as investigated by quantum mechanical methods. The examples include results demonstrating (1) how graphitic nanoribbons can be assembled into multiterminal networks and the influence on electron transport; (2) how the position of a single embedded molecule can be modified to change the overall conduction state of a nanowire; (3) how carbon nanotubes can be assembled into complex covalent arrays and how these can be obtained experimentally; (4) how quantum interference can be understood as emerging from the presence of multiple levels of confinements in carbon nanorings; (5) how new functionality emerges at the nanoscale due to the interplay of magnetic, electronic, and structural properties of individual graphitic nanoribbons assembled into wiggle-like structures; and (6) how quantum chemical modeling can lead to the design of electrodes with enhanced interfaces for molecular coupling.
Journal of Computational Electronics, 2013
ABSTRACT The efficient calculation of the Green's function is a central issue for assessi... more ABSTRACT The efficient calculation of the Green's function is a central issue for assessing electronic transport at the nanoscale. In a near-to-equilibrium description, it can be obtained from a matrix inversion, combined with iterative algorithms developed in the 80s. However, this procedure becomes computationally challenging when dealing with very large systems. A set of algorithms (known as knitting and sewing) based on the recursive application of Dyson's equation were recently proposed, where the Green's function elements are obtained in a selective way and without the need of explicit matrix inversion, by including one matrix element at a time. Here we propose a variation of these algorithms adapted to parallel computing. The approach is based on the division of the system in a set of domains whose individual Green's functions are computed independently. The domains are then merged to yield the necessary elements of the Green's function for subsequent evaluation of the electronic transport properties. Promising scaling behavior is found, depending on the details of the domain decomposition.
Carbon, 2014
ABSTRACT We model the atomistic restructuring of different types of carbon nanoribbons as they ar... more ABSTRACT We model the atomistic restructuring of different types of carbon nanoribbons as they are irradiated and subjected to uniaxial stress. Time scales relevant to realistic experimental conditions are achieved with an original Monte-Carlo algorithm that enacts rare events in a stochastic manner using the structure adjacency information. We use a Hubbard model Hamiltonian to analyze the appearance of magnetic domains emerging from the concerted rearrangement of armchair edged sections into zigzag configurations. The self-consistent Hamiltonian is also used to compute electronic transport properties to establish the presence of spin-polarized current flowing across the initially non-magnetic nanoribbons.
Applied Sciences, 2014
We propose a graphene nanoribbon-based heterojunction, where a defect-free interface separates tw... more We propose a graphene nanoribbon-based heterojunction, where a defect-free interface separates two zigzag graphene nanoribbons prepared in opposite antiferromagnetic spin configurations. This heterospin junction is found to allow the redirecting of low-energy electrons from one edge to the other. The basic scattering mechanisms and their relation to the system's geometry are investigated through a combination of Landauer-Green's function and the S-matrix and eigen-channel methods within a tight-binding + Hubbard model validated with density functional theory. The findings demonstrate the possibility of using zigzag-edged graphene nanoribbons (zGNRs) in complex networks where current can be transmitted across the entire system, instead of following the shortest paths along connected edges belonging to the same sub-lattice.
Physical Review B, 2014
A combination of theoretical and computational methods is used to study the electronic properties... more A combination of theoretical and computational methods is used to study the electronic properties of threeterminal graphitic nanowiggles (3TGNWs). 3TGNWs consisting of three GNWs connected at 120 • to each other have been recently synthesized. Theory predicts that individual GNWs possess a broad set of electronic and magnetic properties. It follows that the presence of different magnetic states in single GNWs yields a rich set of possibilities in assembled GNWs. Here, we analyze twelve distinct magnetic states in one 3TGNW structure and determine that each state corresponds to unique electronic transport properties. We also show that these nanostructures possess attractive characteristics amenable to being exploited as basic building blocks for operative electronic devices.
Theoretical Perspectives, 2013
ABSTRACT This chapter reviews recent progress in describing electron transport in graphitic nanor... more ABSTRACT This chapter reviews recent progress in describing electron transport in graphitic nanoribbons (GNRs) from a theoretical and computational perspective and how works reported in the literature provide unique physicochemical insight applicable to the development of novel GNR-based materials and devices. The chapter first analyses the general quantum mechanical framework needed to evaluate electronic transport in nanostructured materials. The chapter then goes on describing the general properties of GNRs, including their transport and thermoelectric properties. Those properties are reviewed and discussed within the context of experimental feasibility.
Physical Review Letters, 2011
Graphitic nanowiggles are periodic repetitions of non-aligned finite-sized graphitic nanoribbon d... more Graphitic nanowiggles are periodic repetitions of non-aligned finite-sized graphitic nanoribbon domains seamlessly stitched together without structural defects. These complex nanostructures have been recently fabricated (Cai et al, Nature 466, 470 (2010)) and are here predicted to possess unusual properties, such as tunable bandgaps and versatile magnetic behaviors. We used first-principles theory to highlight the microscopic origin of the emerging electronic and magnetic properties of the main subclasses of GNWs. Our study establishes a road-map for guiding the design and synthesis of specific GNWs for nanoelectronic, optoelectronic, and spintronic applications.
Physical Review B, 2013
Periodic repetitions of nonaligned and finite-sized graphene nanoribbon domains, known as graphen... more Periodic repetitions of nonaligned and finite-sized graphene nanoribbon domains, known as graphene nanowiggles, can be synthesized using a surface-assisted bottom-up approach. They have been predicted to possess unusual properties such as tunable band gaps and versatile magnetic behaviors. Here, a first-principles many-body Green's function approach within the GW approximation is used to accurately compute their band gaps, which are in the range 0.00-3.65 eV depending on geometry and magnetism. We also perform spin-polarized density functional theory calculations to demonstrate that the previously predicted complex spin states for free-standing nanowiggles are not significantly altered by the presence of the gold substrate on which they are synthesized.
Nanotechnology, 2014
The electronic transport properties of three-terminal graphene-based triangular patches are inves... more The electronic transport properties of three-terminal graphene-based triangular patches are investigated using a combination of semi-empirical tight-binding calculations and Green's function-based transport theory within Landauer's framework. The junctions are composed of a triangular structure based on armchair edged graphene nanoribbons. We show how details of the central region influence the resonant electronic transport across the triangular patches and highlight the unique features of the current flow as a function of geometry. These properties indicate an array of functionalities for the development of carbon-based complex nanocircuits and operational devices at the nanoscale.
Journal of Hazardous Materials, 2010
This work reports a theoretical study of nicotine molecules interacting with single wall carbon n... more This work reports a theoretical study of nicotine molecules interacting with single wall carbon nanotubes (SWCNTs) through ab initio calculations within the framework of density functional theory (DFT). Different adsorption sites for nicotine on the surface of pristine and defective (8,0) SWCNTs were analyzed and the total energy curves, as a function of molecular position relative to the SWCNT surface, were evaluated. The nicotine adsorption process is found to be energetically favorable and the molecule-nanotube interaction is intermediated by the tri-coordinated nitrogen atom from the nicotine. It is also predicted the possibility of a chemical bonding between nicotine and SWCNT through the di-coordinated nitrogen.
The Journal of Chemical Physics, 2008
We theoretically investigate the electronic charge transport in a molecular system composed of a ... more We theoretically investigate the electronic charge transport in a molecular system composed of a donor group (dinitrobenzene) coupled to an acceptor group (dihydrophenazine) via a polyenic chain (unsaturated carbon bridge). Ab initio calculations based on the Hartree-Fock approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. For small bridge lengths (n=0-3) we find a homogeneous distribution of the frontier molecular orbitals, while for n>3 a strong localization of the lowest unoccupied molecular orbital is found. The localized orbitals in between the donor and acceptor groups act as conduction channels when an external electric field is applied. We also calculate the rectification behavior of this system by evaluating the charge accumulated in the donor and acceptor groups as a function of the external electric field. Finally, we propose a phenomenological model based on nonequilibrium Green's function to rationalize the ab initio findings.
International Journal of Quantum Chemistry, 2006
A systematic study of the structural and electronic properties of a 1,2-dichlorobenzene (DCB) mol... more A systematic study of the structural and electronic properties of a 1,2-dichlorobenzene (DCB) molecule interacting with metallic single-wall carbon nanotubes is reported. The calculations were performed through ab initio methods using the SIESTA code. The interaction between DCB and nanotube is observed to depend on the diameter and it is larger for metallic nanotubes when compared with semiconducting. The binding
Applied Physics Letters, 2011
ABSTRACT
ACS Nano, 2012
Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of vary... more Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of varying chiralities. They are characterized by the presence of a number of possible different spin distributions along their edges and by electronic band-gaps that are highly sensitive to the details of their geometry. These two properties promote these experimentally observed carbon nanostructures as some of the most promising candidates for developing high-performance nanodevices. Here, we highlight this potential with a detailed understanding of the electronic processes leading to their unique spin-state dependent electronic quantum transport properties. The three classes of GNWs containing at least one zigzag edge (necessary to the observation of multiple-magnetic states) are considered in two distinct geometries: a perfectly periodic system and in a one-GNW-cell system sandwiched between two semi-infinite terminals made up of straight GNRs. The present calculations establish a number of elementary rules to relate fundamental electronic transport functionality, electronic energy, the system geometry, and spin state.
Physical Review B, 2014
We develop a graph theoretical formalism to account for the fact that sp 2 carbon can become spin... more We develop a graph theoretical formalism to account for the fact that sp 2 carbon can become spin ordered or generate free radicals for purely topological reasons. While this phenomenon has been previously considered a binary operator, we here show a quantification in discrete units of frustrations. The graph theory method is combined with open density functional theory calculations to establish the existence of an energy of frustration that is shown to greatly improve the description of carbon nanostructure energetics using classical force fields. The methodology is illustrated for a number of systems and, owing to the small computational overhead associated with its evaluation, is expected to be easily integrable into any modeling approach based on a structure's adjacency matrix.
Advances in Atom and Single Molecule Machines, 2012
This chapter showcases selected illustrations of various manifestations of nanoscale and molecula... more This chapter showcases selected illustrations of various manifestations of nanoscale and molecular electronic effects as investigated by quantum mechanical methods. The examples include results demonstrating (1) how graphitic nanoribbons can be assembled into multiterminal networks and the influence on electron transport; (2) how the position of a single embedded molecule can be modified to change the overall conduction state of a nanowire; (3) how carbon nanotubes can be assembled into complex covalent arrays and how these can be obtained experimentally; (4) how quantum interference can be understood as emerging from the presence of multiple levels of confinements in carbon nanorings; (5) how new functionality emerges at the nanoscale due to the interplay of magnetic, electronic, and structural properties of individual graphitic nanoribbons assembled into wiggle-like structures; and (6) how quantum chemical modeling can lead to the design of electrodes with enhanced interfaces for molecular coupling.
Journal of Computational Electronics, 2013
ABSTRACT The efficient calculation of the Green's function is a central issue for assessi... more ABSTRACT The efficient calculation of the Green's function is a central issue for assessing electronic transport at the nanoscale. In a near-to-equilibrium description, it can be obtained from a matrix inversion, combined with iterative algorithms developed in the 80s. However, this procedure becomes computationally challenging when dealing with very large systems. A set of algorithms (known as knitting and sewing) based on the recursive application of Dyson's equation were recently proposed, where the Green's function elements are obtained in a selective way and without the need of explicit matrix inversion, by including one matrix element at a time. Here we propose a variation of these algorithms adapted to parallel computing. The approach is based on the division of the system in a set of domains whose individual Green's functions are computed independently. The domains are then merged to yield the necessary elements of the Green's function for subsequent evaluation of the electronic transport properties. Promising scaling behavior is found, depending on the details of the domain decomposition.
Carbon, 2014
ABSTRACT We model the atomistic restructuring of different types of carbon nanoribbons as they ar... more ABSTRACT We model the atomistic restructuring of different types of carbon nanoribbons as they are irradiated and subjected to uniaxial stress. Time scales relevant to realistic experimental conditions are achieved with an original Monte-Carlo algorithm that enacts rare events in a stochastic manner using the structure adjacency information. We use a Hubbard model Hamiltonian to analyze the appearance of magnetic domains emerging from the concerted rearrangement of armchair edged sections into zigzag configurations. The self-consistent Hamiltonian is also used to compute electronic transport properties to establish the presence of spin-polarized current flowing across the initially non-magnetic nanoribbons.
Applied Sciences, 2014
We propose a graphene nanoribbon-based heterojunction, where a defect-free interface separates tw... more We propose a graphene nanoribbon-based heterojunction, where a defect-free interface separates two zigzag graphene nanoribbons prepared in opposite antiferromagnetic spin configurations. This heterospin junction is found to allow the redirecting of low-energy electrons from one edge to the other. The basic scattering mechanisms and their relation to the system's geometry are investigated through a combination of Landauer-Green's function and the S-matrix and eigen-channel methods within a tight-binding + Hubbard model validated with density functional theory. The findings demonstrate the possibility of using zigzag-edged graphene nanoribbons (zGNRs) in complex networks where current can be transmitted across the entire system, instead of following the shortest paths along connected edges belonging to the same sub-lattice.
Physical Review B, 2014
A combination of theoretical and computational methods is used to study the electronic properties... more A combination of theoretical and computational methods is used to study the electronic properties of threeterminal graphitic nanowiggles (3TGNWs). 3TGNWs consisting of three GNWs connected at 120 • to each other have been recently synthesized. Theory predicts that individual GNWs possess a broad set of electronic and magnetic properties. It follows that the presence of different magnetic states in single GNWs yields a rich set of possibilities in assembled GNWs. Here, we analyze twelve distinct magnetic states in one 3TGNW structure and determine that each state corresponds to unique electronic transport properties. We also show that these nanostructures possess attractive characteristics amenable to being exploited as basic building blocks for operative electronic devices.
Theoretical Perspectives, 2013
ABSTRACT This chapter reviews recent progress in describing electron transport in graphitic nanor... more ABSTRACT This chapter reviews recent progress in describing electron transport in graphitic nanoribbons (GNRs) from a theoretical and computational perspective and how works reported in the literature provide unique physicochemical insight applicable to the development of novel GNR-based materials and devices. The chapter first analyses the general quantum mechanical framework needed to evaluate electronic transport in nanostructured materials. The chapter then goes on describing the general properties of GNRs, including their transport and thermoelectric properties. Those properties are reviewed and discussed within the context of experimental feasibility.
Physical Review Letters, 2011
Graphitic nanowiggles are periodic repetitions of non-aligned finite-sized graphitic nanoribbon d... more Graphitic nanowiggles are periodic repetitions of non-aligned finite-sized graphitic nanoribbon domains seamlessly stitched together without structural defects. These complex nanostructures have been recently fabricated (Cai et al, Nature 466, 470 (2010)) and are here predicted to possess unusual properties, such as tunable bandgaps and versatile magnetic behaviors. We used first-principles theory to highlight the microscopic origin of the emerging electronic and magnetic properties of the main subclasses of GNWs. Our study establishes a road-map for guiding the design and synthesis of specific GNWs for nanoelectronic, optoelectronic, and spintronic applications.
Physical Review B, 2013
Periodic repetitions of nonaligned and finite-sized graphene nanoribbon domains, known as graphen... more Periodic repetitions of nonaligned and finite-sized graphene nanoribbon domains, known as graphene nanowiggles, can be synthesized using a surface-assisted bottom-up approach. They have been predicted to possess unusual properties such as tunable band gaps and versatile magnetic behaviors. Here, a first-principles many-body Green's function approach within the GW approximation is used to accurately compute their band gaps, which are in the range 0.00-3.65 eV depending on geometry and magnetism. We also perform spin-polarized density functional theory calculations to demonstrate that the previously predicted complex spin states for free-standing nanowiggles are not significantly altered by the presence of the gold substrate on which they are synthesized.
Nanotechnology, 2014
The electronic transport properties of three-terminal graphene-based triangular patches are inves... more The electronic transport properties of three-terminal graphene-based triangular patches are investigated using a combination of semi-empirical tight-binding calculations and Green's function-based transport theory within Landauer's framework. The junctions are composed of a triangular structure based on armchair edged graphene nanoribbons. We show how details of the central region influence the resonant electronic transport across the triangular patches and highlight the unique features of the current flow as a function of geometry. These properties indicate an array of functionalities for the development of carbon-based complex nanocircuits and operational devices at the nanoscale.
Journal of Hazardous Materials, 2010
This work reports a theoretical study of nicotine molecules interacting with single wall carbon n... more This work reports a theoretical study of nicotine molecules interacting with single wall carbon nanotubes (SWCNTs) through ab initio calculations within the framework of density functional theory (DFT). Different adsorption sites for nicotine on the surface of pristine and defective (8,0) SWCNTs were analyzed and the total energy curves, as a function of molecular position relative to the SWCNT surface, were evaluated. The nicotine adsorption process is found to be energetically favorable and the molecule-nanotube interaction is intermediated by the tri-coordinated nitrogen atom from the nicotine. It is also predicted the possibility of a chemical bonding between nicotine and SWCNT through the di-coordinated nitrogen.
The Journal of Chemical Physics, 2008
We theoretically investigate the electronic charge transport in a molecular system composed of a ... more We theoretically investigate the electronic charge transport in a molecular system composed of a donor group (dinitrobenzene) coupled to an acceptor group (dihydrophenazine) via a polyenic chain (unsaturated carbon bridge). Ab initio calculations based on the Hartree-Fock approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. For small bridge lengths (n=0-3) we find a homogeneous distribution of the frontier molecular orbitals, while for n>3 a strong localization of the lowest unoccupied molecular orbital is found. The localized orbitals in between the donor and acceptor groups act as conduction channels when an external electric field is applied. We also calculate the rectification behavior of this system by evaluating the charge accumulated in the donor and acceptor groups as a function of the external electric field. Finally, we propose a phenomenological model based on nonequilibrium Green's function to rationalize the ab initio findings.
International Journal of Quantum Chemistry, 2006
A systematic study of the structural and electronic properties of a 1,2-dichlorobenzene (DCB) mol... more A systematic study of the structural and electronic properties of a 1,2-dichlorobenzene (DCB) molecule interacting with metallic single-wall carbon nanotubes is reported. The calculations were performed through ab initio methods using the SIESTA code. The interaction between DCB and nanotube is observed to depend on the diameter and it is larger for metallic nanotubes when compared with semiconducting. The binding
Applied Physics Letters, 2011
ABSTRACT
ACS Nano, 2012
Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of vary... more Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of varying chiralities. They are characterized by the presence of a number of possible different spin distributions along their edges and by electronic band-gaps that are highly sensitive to the details of their geometry. These two properties promote these experimentally observed carbon nanostructures as some of the most promising candidates for developing high-performance nanodevices. Here, we highlight this potential with a detailed understanding of the electronic processes leading to their unique spin-state dependent electronic quantum transport properties. The three classes of GNWs containing at least one zigzag edge (necessary to the observation of multiple-magnetic states) are considered in two distinct geometries: a perfectly periodic system and in a one-GNW-cell system sandwiched between two semi-infinite terminals made up of straight GNRs. The present calculations establish a number of elementary rules to relate fundamental electronic transport functionality, electronic energy, the system geometry, and spin state.