Sung-Hoon Lee - Academia.edu (original) (raw)
Papers by Sung-Hoon Lee
Science (New York, N.Y.), Jan 9, 2015
Chiral edge states are the hallmark of two- and three-dimensional topological materials, but thei... more Chiral edge states are the hallmark of two- and three-dimensional topological materials, but their one-dimensional (1D) analog has not yet been found. We report that the 1D topological edge states, solitons, of the charge density wave system of indium atomic wires self-assembled on a silicon surface have chirality. The system is described by a coupled double Peierls-dimerized atomic chain, where the interchain coupling induces dynamical sublattice symmetry breaking. This changes its topological symmetry from Z₂× Z₂to Z₄ and endows solitons with a chiral degree of freedom. Chiral solitons can produce quantized charge transport across the chain that is topologically protected and controllable by the soliton's chirality. Individual right- and left-chiral solitons in indium wires are directly identified by scanning tunneling microscopy.
ACS nano, Jan 13, 2015
A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atom... more A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atoms are known to incorporate into the stepped Si(111) surface to form a Au-Si atomic wire array with both one-dimensional (1D) metallic and antiferromagnetic atomic chains. At a reduced density of Au, we find a regular array of Au-Si wires in alternation with pristine Si nanoterraces. Pristine Si nanoterraces impose a strain on the neighboring Au-Si wires, which modifies both the band structure of metallic chains and the magnetic property of spin chains. This is an ultimate 1D version of a strained-layer superlattice of semiconductors, defining a direction toward the fine engineering of self-assembled atomic-scale wires.
Using first-principles calculations of graphene having high- symmetry distortion or defects, we i... more Using first-principles calculations of graphene having high- symmetry distortion or defects, we investigate the chiral symmetry breaking in graphene as the source of gap opening. We identify that the gap opening by the chiral symmetry breaking in the honeycomb lattice is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in a linear lattice, the elemental 1D Dirac lattice,
The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energ... more The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energy calculations within the generalized gradient approximation. We find that water can adsorb molecularly on the down atom of the Si dimer, but a dissociative adsorption wherein OH ͑H͒ forms a bond to the down ͑up͒ atom of the Si dimer is more favored over the molecular adsorption ͑by 1.8 eV͒. The decay of the molecular state to the dissociative state occurs via a transition state with the energy barrier of only 0.15 eV. While the interaction between water molecules is repulsive, that between dissociated OH species is attractive by hydrogen bonding.
Physical Review Letters, 2000
Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poo... more Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poor environment. The discovered geometry is qualitatively different from the usual surface-dimer based reconstructions of III-V semiconductor (001) surfaces. The stability of the new structure, which has a c͑8 3 2͒ periodicity, is explained in terms of bond saturation and favorable electrostatic interactions between surface atoms. Simulated scanning tunneling microscopy images are in good agreement with experimental data, and a low-energy electron diffraction analysis supports the theoretical prediction. PACS numbers: 68.35.Bs, 61.14.Hg, 68.35.Md, 73.20.At III-V semiconductors play an increasing role in microelectronics, such as light-emitting diodes and high frequency, low noise devices for mobile phones, and are important candidates for the development of devices in the emerging field of spin electronics. The knowledge of the surface atomic structure is a prerequisite to achieve understanding and controlling of the surface or interface electronic properties. As we will show below, however, up to date analyses of surface structures of III-V semiconductors are hindered by some prejudice on the type of structures considered. For an example of the GaAs(001) surface, we show the existence of a new type of surface reconstruction.
Nano Letters, 2008
One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the e... more One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.
... Huaxiang Yin, Sunil Kim, Chang Jung Kim, Jae Chul Park, Ihun Song, Sang-Wook Kim, Sung-Hoon L... more ... Huaxiang Yin, Sunil Kim, Chang Jung Kim, Jae Chul Park, Ihun Song, Sang-Wook Kim, Sung-Hoon Lee, ... For a better circuit performance, the key process parameters such as gate insulator thickness (Tox) and channel film thickness (Tch), as well as the main plane geometry ...
ACS Nano, 2011
Scanning tunneling microscopy (STM) and density functional theory (DFT) calculations were used to... more Scanning tunneling microscopy (STM) and density functional theory (DFT) calculations were used to investigate the surface morphology and electronic structure of graphene synthesized on Cu by low temperature chemical vapor deposition (CVD). Periodic line patterns originating from the arrangements of carbon atoms on the Cu surface passivate the interaction between metal substrate and graphene, resulting in flawless inherent graphene band structure in pristine graphene/Cu. The effective elimination of metal surface states by the passivation is expected to contribute to the growth of monolayer graphene on Cu, which yields highly enhanced uniformity on the wafer scale, making progress toward the commercial application of graphene.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equil... more Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equilibrium Green's function (NEGF) method. In this study, we characterize the effect of channel modulation by engineering dopant profiles, oxide thickness, and corner rounding ...
ACS Applied Materials & Interfaces, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
Physical Review B, 1999
The atomic structure of the As-covered GaAs(110) surface in one-monolayer coverage is studied usi... more The atomic structure of the As-covered GaAs(110) surface in one-monolayer coverage is studied using the pseudopotential density-functional theory. We compare the adsorption geometry of As/GaAs(110) with that of the well-studied isoelectronic Sb/GaAs(110) system and discuss the large difference in the desorption temperature found in both systems, based on the calculated adsorption energies. In addition, we calculate the As 3d and
Physical Review B, 2002
We use density-functional theory to describe the initial stages of Fe film growth on GaAs(001), f... more We use density-functional theory to describe the initial stages of Fe film growth on GaAs(001), focusing on the interplay between chemistry and magnetism at the interface. Four features appear to be generic: (1) At submonolayer coverages, a strong chemical interaction between Fe and substrate atoms leads to substitutional adsorption and intermixing.
E-beam lithography simulation is one of the effective tools for understanding the complex e-beam ... more E-beam lithography simulation is one of the effective tools for understanding the complex e-beam lithography process. In-house E-beam Lithography Simulator, ELIS, has been developed in order to analyze the mask CD errors. ELIS adopts the Monte Carlo method to accurately describe the electron scattering and energy deposition on the resist, and fits this result with more than two Gaussians to
Physical Review B, 2011
Carrier density and temperature-dependent resistivity of graphene grown by chemical vapor deposit... more Carrier density and temperature-dependent resistivity of graphene grown by chemical vapor deposition (CVD) is investigated. We observe in low mobility CVD graphene device a generic insulating behavior at low temperatures, and eventually a metallic behavior at high temperatures, manifesting a non-monotonic temperature dependent resistivity. This feature is strongly affected by carrier density modulation with the low-density samples exhibiting insulating-like temperature dependence upto higher temperatures than the corresponding high-density samples. To explain the temperature and density dependence of the resistivity, we introduce thermal activation of charge carriers in electron-hole puddles induced by randomly distributed charged impurities. Our observed temperature evolution of resistivity is then understood from the competition among thermal activation of charge carriers, temperature-dependent screening, and phonon scattering effects. Our experimental results are in good agreement with recent theories of graphene transport.
The Journal of Physical Chemistry Letters, 2014
One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photov... more One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.
![Research paper thumbnail of Ab initio investigations on the HOSO[sub 2]+O[sub 2]→SO[sub 3]+HO[sub 2] reaction](https://attachments.academia-assets.com/43771545/thumbnails/1.jpg)
](The Journal of Chemical Physics, 2000
HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosp... more HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO 2 ϩO 2 →SO 3 ϩHO 2 have been studied using Mo "ller-Plesset ͑MP2͒ and density functional ͑DFT͒ techniques with 6-31G** and triple-, quadruple-, and quintuple-quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.
Applied Physics Letters, 2013
ACS Nano, 2011
Using first-principles calculations of graphene having high-symmetry distortion or defects, we in... more Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been attributed usually to quantum confinement effects, can be understood with the chiral symmetry breaking.
We investigate using first-principles calculations the atomic structure of the orthorhombic phase... more We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta 2 O 5 . Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of þ5 and À2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta 2 O 3 plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of Ta 2 O 5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive the collective relaxation of the O sublattice.
Science (New York, N.Y.), Jan 9, 2015
Chiral edge states are the hallmark of two- and three-dimensional topological materials, but thei... more Chiral edge states are the hallmark of two- and three-dimensional topological materials, but their one-dimensional (1D) analog has not yet been found. We report that the 1D topological edge states, solitons, of the charge density wave system of indium atomic wires self-assembled on a silicon surface have chirality. The system is described by a coupled double Peierls-dimerized atomic chain, where the interchain coupling induces dynamical sublattice symmetry breaking. This changes its topological symmetry from Z₂× Z₂to Z₄ and endows solitons with a chiral degree of freedom. Chiral solitons can produce quantized charge transport across the chain that is topologically protected and controllable by the soliton's chirality. Individual right- and left-chiral solitons in indium wires are directly identified by scanning tunneling microscopy.
ACS nano, Jan 13, 2015
A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atom... more A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atoms are known to incorporate into the stepped Si(111) surface to form a Au-Si atomic wire array with both one-dimensional (1D) metallic and antiferromagnetic atomic chains. At a reduced density of Au, we find a regular array of Au-Si wires in alternation with pristine Si nanoterraces. Pristine Si nanoterraces impose a strain on the neighboring Au-Si wires, which modifies both the band structure of metallic chains and the magnetic property of spin chains. This is an ultimate 1D version of a strained-layer superlattice of semiconductors, defining a direction toward the fine engineering of self-assembled atomic-scale wires.
Using first-principles calculations of graphene having high- symmetry distortion or defects, we i... more Using first-principles calculations of graphene having high- symmetry distortion or defects, we investigate the chiral symmetry breaking in graphene as the source of gap opening. We identify that the gap opening by the chiral symmetry breaking in the honeycomb lattice is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in a linear lattice, the elemental 1D Dirac lattice,
The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energ... more The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energy calculations within the generalized gradient approximation. We find that water can adsorb molecularly on the down atom of the Si dimer, but a dissociative adsorption wherein OH ͑H͒ forms a bond to the down ͑up͒ atom of the Si dimer is more favored over the molecular adsorption ͑by 1.8 eV͒. The decay of the molecular state to the dissociative state occurs via a transition state with the energy barrier of only 0.15 eV. While the interaction between water molecules is repulsive, that between dissociated OH species is attractive by hydrogen bonding.
Physical Review Letters, 2000
Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poo... more Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poor environment. The discovered geometry is qualitatively different from the usual surface-dimer based reconstructions of III-V semiconductor (001) surfaces. The stability of the new structure, which has a c͑8 3 2͒ periodicity, is explained in terms of bond saturation and favorable electrostatic interactions between surface atoms. Simulated scanning tunneling microscopy images are in good agreement with experimental data, and a low-energy electron diffraction analysis supports the theoretical prediction. PACS numbers: 68.35.Bs, 61.14.Hg, 68.35.Md, 73.20.At III-V semiconductors play an increasing role in microelectronics, such as light-emitting diodes and high frequency, low noise devices for mobile phones, and are important candidates for the development of devices in the emerging field of spin electronics. The knowledge of the surface atomic structure is a prerequisite to achieve understanding and controlling of the surface or interface electronic properties. As we will show below, however, up to date analyses of surface structures of III-V semiconductors are hindered by some prejudice on the type of structures considered. For an example of the GaAs(001) surface, we show the existence of a new type of surface reconstruction.
Nano Letters, 2008
One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the e... more One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.
... Huaxiang Yin, Sunil Kim, Chang Jung Kim, Jae Chul Park, Ihun Song, Sang-Wook Kim, Sung-Hoon L... more ... Huaxiang Yin, Sunil Kim, Chang Jung Kim, Jae Chul Park, Ihun Song, Sang-Wook Kim, Sung-Hoon Lee, ... For a better circuit performance, the key process parameters such as gate insulator thickness (Tox) and channel film thickness (Tch), as well as the main plane geometry ...
ACS Nano, 2011
Scanning tunneling microscopy (STM) and density functional theory (DFT) calculations were used to... more Scanning tunneling microscopy (STM) and density functional theory (DFT) calculations were used to investigate the surface morphology and electronic structure of graphene synthesized on Cu by low temperature chemical vapor deposition (CVD). Periodic line patterns originating from the arrangements of carbon atoms on the Cu surface passivate the interaction between metal substrate and graphene, resulting in flawless inherent graphene band structure in pristine graphene/Cu. The effective elimination of metal surface states by the passivation is expected to contribute to the growth of monolayer graphene on Cu, which yields highly enhanced uniformity on the wafer scale, making progress toward the commercial application of graphene.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equil... more Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equilibrium Green's function (NEGF) method. In this study, we characterize the effect of channel modulation by engineering dopant profiles, oxide thickness, and corner rounding ...
ACS Applied Materials & Interfaces, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
Physical Review B, 1999
The atomic structure of the As-covered GaAs(110) surface in one-monolayer coverage is studied usi... more The atomic structure of the As-covered GaAs(110) surface in one-monolayer coverage is studied using the pseudopotential density-functional theory. We compare the adsorption geometry of As/GaAs(110) with that of the well-studied isoelectronic Sb/GaAs(110) system and discuss the large difference in the desorption temperature found in both systems, based on the calculated adsorption energies. In addition, we calculate the As 3d and
Physical Review B, 2002
We use density-functional theory to describe the initial stages of Fe film growth on GaAs(001), f... more We use density-functional theory to describe the initial stages of Fe film growth on GaAs(001), focusing on the interplay between chemistry and magnetism at the interface. Four features appear to be generic: (1) At submonolayer coverages, a strong chemical interaction between Fe and substrate atoms leads to substitutional adsorption and intermixing.
E-beam lithography simulation is one of the effective tools for understanding the complex e-beam ... more E-beam lithography simulation is one of the effective tools for understanding the complex e-beam lithography process. In-house E-beam Lithography Simulator, ELIS, has been developed in order to analyze the mask CD errors. ELIS adopts the Monte Carlo method to accurately describe the electron scattering and energy deposition on the resist, and fits this result with more than two Gaussians to
Physical Review B, 2011
Carrier density and temperature-dependent resistivity of graphene grown by chemical vapor deposit... more Carrier density and temperature-dependent resistivity of graphene grown by chemical vapor deposition (CVD) is investigated. We observe in low mobility CVD graphene device a generic insulating behavior at low temperatures, and eventually a metallic behavior at high temperatures, manifesting a non-monotonic temperature dependent resistivity. This feature is strongly affected by carrier density modulation with the low-density samples exhibiting insulating-like temperature dependence upto higher temperatures than the corresponding high-density samples. To explain the temperature and density dependence of the resistivity, we introduce thermal activation of charge carriers in electron-hole puddles induced by randomly distributed charged impurities. Our observed temperature evolution of resistivity is then understood from the competition among thermal activation of charge carriers, temperature-dependent screening, and phonon scattering effects. Our experimental results are in good agreement with recent theories of graphene transport.
The Journal of Physical Chemistry Letters, 2014
One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photov... more One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.
The Journal of Chemical Physics, 2000
HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosp... more HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO 2 ϩO 2 →SO 3 ϩHO 2 have been studied using Mo "ller-Plesset ͑MP2͒ and density functional ͑DFT͒ techniques with 6-31G** and triple-, quadruple-, and quintuple-quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.
Applied Physics Letters, 2013
ACS Nano, 2011
Using first-principles calculations of graphene having high-symmetry distortion or defects, we in... more Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been attributed usually to quantum confinement effects, can be understood with the chiral symmetry breaking.
We investigate using first-principles calculations the atomic structure of the orthorhombic phase... more We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta 2 O 5 . Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of þ5 and À2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta 2 O 3 plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of Ta 2 O 5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive the collective relaxation of the O sublattice.