The generalized tight-binding model is developed to investigate the rich and unique electronic pr... more The generalized tight-binding model is developed to investigate the rich and unique electronic properties of AB-bt (bottom-top) bilayer silicene under uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, displays unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups, that is, there exist the sublattice-and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit irregular behavior corresponding to the critical points of the band structure. Moreover, the electric field can induce many LL anti-crossings. The main features of the LLs are uncovered with many van Hove singularities in the density-of-states and non-uniform delta-function-like peaks in the magneto-absorption spectra. The feature-rich magnetic quantization directly reflects the geometric symmetries, intra-layer and inter-layer atomic interactions, spin-orbital couplings, and the field effects. The results of this work can be applied to novel designs of Si-based nano-electronics and nano-devices with enhanced mobilities.
Concentration-dependent diverse magnetic and electronic properties of halogendoped silicene are i... more Concentration-dependent diverse magnetic and electronic properties of halogendoped silicene are investigated using the first-principles method. The optimal buckled geometric structures, atom-related energy bands, spin density distributions, spatial charge densities, and spin-and orbital-decomposed density of states strongly depend on the double-side and single-side adsorptions. The two-dimensional electronic structures are enriched by the competition between the significant halogen-Si bonds and weak sp 3 hybridizations. Such critical halogen-Si bonding is formed by the high charge transfer from Si atoms to halogen adatoms. The double-side adsorptions belong to the middle-gap semiconductors that become p-type metal at the 11% concentration, 1
The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic propert... more The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali- and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single $s-2p_z$ orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edge-localized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct...
A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene ... more A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in C-C, F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge distributions, and the orbital-projected density of states. The top-site F-C bonds, with the multi-orbital hybridizations, create the non-uniform buckled honeycomb lattice. There exist the C-, F- and (C, F)-dominated energy bands. Fluorinated graphene belongs either to the p-type metals (with/without the ferromagnetic spin arrangement) or to the large-gap semiconductors (without magnetism), depen...
The two-dimensional buckled honeycomb lattices system exhibits the rich Coulomb excitation spectr... more The two-dimensional buckled honeycomb lattices system exhibits the rich Coulomb excitation spectra, being dominated by the free carrier density, band structure, and transferred momentum (q). There are two kinds of plasmon peaks in the energy loss spectra, calculated from the random phase approximation. They are, respectively, revealed at low and middle frequencies. The former, which arises from the free carriers, belongs to acoustic mode. It's frequency depends on √ q at long wavelength limit. On the other hand, the latter is due to all the π-electronic collective excitations is an optical mode. Whether such plasmon can service is mainly determined by q. The frequencies and intensities of plasmon modes are very different among graphene, silicene, germanene, and Tin.
Physical chemistry chemical physics : PCCP, Jan 9, 2017
The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are in... more The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are investigated by the first-principles calculations. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement and edge structure. There exist C-C, C-F, and F-F bonds with multi-orbital hybridizations. Fluorine adatoms can create p-type metals or concentration- and distribution-dependent semiconductors, depending on whether the π bonding is seriously suppressed by the top-site chemical bonding. Furthermore, five kinds of spin-dependent electronic and magnetic properties cover the non-magnetic and ferromagnetic metals, non-magnetic semiconductors, and anti-ferromagnetic semiconductors with/without spin splitting. The diverse essential properties are clearly revealed in the spatial charge distribution, spin density, and orbital-projected density of states.
The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic propert... more The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali-and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single s − 2p z orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edgelocalized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct spin configuration could be verified from the experimental measurements. The graphene-based systems, which are formed by the planar sp 2 bondings of carbon atoms, include layered graphites 1, 2 , few-and multi-layer graphenes 3, 4 , one-dimensional graphene nanoribbons (1D GNRs) 5, 6 and 1D carbon nanotubes (CNTs) 7, 8. From a geometric point of view, each GNR could be regarded as a finite-width graphene strip or an unzipped CNT. The 1D GNRs have stirred a lot of studies, mainly owing to the complex relations among the honeycomb lattice, the one-atom thickness, the finite-size quantum confinement, and the edge structure. They could be successfully synthesized by the various experimental techniques. The most common methods are to cut graphene, achieved by a metal-catalyzed cutting 9, 10 , oxidation cutting 11, 12 , lithographic patterning and etching 13, 14 , sonochemical breaking 6, 15 , and molecular precursors 16, 17. The available routes from the unzipping of multi-wall CNTs cover strong chemical reaction 18, 19 , laser irradiation 20 , metal-catalyzed cutting 21, 22 , plasma etching 23, 24 , hydrogen treatment and annealing 25 , unzipping functionalized CNTs by scanning tunneling microscope (STM) tips 26 , electrical unwrapping by transmission electron microscopy (TEM) 27 , intercalation and exfoliation 28 , and electrochemical unzipping 29. The other techniques involve chemical vapor deposition 30 and chemical synthesis 31. GNRs exhibit the feature-rich essential properties, such as, electronic structures 5, 32 , magnetic properties 32, 33 , optical spectra 34, 35 , and transport properties 36, 37. The electronic properties are diversified by changing by the ribbon width (W) 38, 39 , edge structure 38, 40 , edge-passivated dopants 41, 42 , adatom adsorptions 43, 44 , layer numbers 45 , stacking configurations 46 , surface curvatures 47, 48 , mechanical strains 49, 50 , electric fields 51-53 , and magnetic fields 32, 54, 55. GNRs are expected to be more potentially applicable in future nanodevices 15, 56, 57. In this work, the first-principles calculations are used to investigate the adatom-enriched electronic properties of the alkali-adsorbed GNRs. Whether the alkali adatoms can create the high free electron density will be explored in detail. GNRs, with or without hydrogen passivation at boundaries, exhibit the semiconducting behavior, as indicated from the theoretical predictions 13, 38, 39 , and the experimental measurements 17, 58-60. There are two typical types of achiral GNRs, namely, armchair and zigzag GNRs. The former have the diverse energy gaps (E g s) inversely proportional to widths 38, 39. Specifically, E g in the latter, which arises from the occupied and the unoccupied edge-localized energy bands, is induced by the anti-ferromagnetic spin configuration across the nanoribbon 5, 38. The strong dependence of E g on W has been confirmed by the electrical conductance 6, 13 and tunneling current 17, 58-60 measurements. The semiconductor-metal transition is revealed under a transverse electric field 5. The similar
The feature-rich electronic excitations of monolayer germanene lie in the significant spin-orbita... more The feature-rich electronic excitations of monolayer germanene lie in the significant spin-orbital coupling and the buckled structure. The collective and singleparticle excitations are diversified by the magnitude and direction of transferred momentum, the Fermi energy and the gate voltage. There are four kinds of plasmon modes, according to the unique frequency-and momentum-dependent phase diagrams. They behave as two-dimensional acoustic modes at long wavelength. However, for the larger momenta, they might change into another kind of undamped plasmons, become the seriously suppressed modes in the heavy intraband e-h excitations, keep the same undamped plasmons, or decline and then vanish in the strong interband e-h excitations. Germanene, silicene and graphene are quite different from one another in the main features of the diverse plasmon modes.
ABSTRACT The influences of modulated electric fields and modulated magnetic fields on low-energy ... more ABSTRACT The influences of modulated electric fields and modulated magnetic fields on low-energy Landau levels (LL’s) of monolayer graphene are investigated by the tight-binding model. The ratio of the uniform magnetic-field strength to the modulated field strength is arbitrarily chosen for discussions. Both modulated fields cause the LL’s to split into two twofold parabolic subbands; these subbands exhibit periodic oscillations and two kinds of band-edge states. However, the subband dispersions and oscillation amplitudes associated with electric and magnetic modulated fields behave differently. The main features of the electronic structure are reflected in the density of states, which presents pairlike and square-root divergent structures for both modulated field cases. The LL wave functions are strongly affected by both modulated fields, such as the broken symmetry, displacement of the center location, and alteration of the amplitude strength. These changes in the LL wave functions should be reflected in other physical properties, e.g., the optical selection rules. Furthermore, the dependence of the LL properties on the modulation strength and modulation period is also discussed in detail.
ABSTRACT The Landau level (LL) spectra in rhombohedral graphite are calculated within the tight-b... more ABSTRACT The Landau level (LL) spectra in rhombohedral graphite are calculated within the tight-binding model without any perturbation expansion. All significant interlayer atomic interactions are included, up to next-nearest-neighbor graphene layers. The magnetic Hamiltonian matrix is derived and manipulated in a band-like form for numerical efficiency. The results of this diagonalization scheme manifest the effects of lattice symmetry and interlayer interaction in the specific stacking configuration. Three-dimensional character associated with the mirror symmetry of the ABC-stacking configuration is exhibited in the LL spectra.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010
The electronic and optical properties of monolayer and bilayer graphene are investigated to verif... more The electronic and optical properties of monolayer and bilayer graphene are investigated to verify the effects of interlayer interactions and external magnetic field. Monolayer graphene exhibits linear bands in the low-energy region. Then the interlayer interactions in bilayers change these bands into two pairs of parabolic bands, where the lower pair is slightly overlapped and the occupied states are asymmetric with respect to the unoccupied ones. The characteristics of zero-field electronic structures are directly reflected in the Landau levels. In monolayer and bilayer graphene, these levels can be classified into one and two groups, respectively. With respect to the optical transitions between the Landau levels, bilayer graphene possesses much richer spectral features in comparison with monolayers, such as four kinds of absorption channels and double-peaked absorption lines. The explicit wave functions can further elucidate the frequency-dependent absorption rates and the comple...
A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene ... more A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in CC , F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge distributions, and the orbital-projected density of states. The top-site F-C bonds, with the multi-orbital hybridizations, create the non-uniform buckled honeycomb lattice. There exist the C-, F-and (C, F)-dominated energy bands. Fluorinated graphene belongs either to the p-type metals (with/without the ferromagnetic spin arrangement) or to the large-gap semiconductors (without magnetism), depending on the 1
Essential properties of multilayer graphenes are diversified by the number of layers and the stac... more Essential properties of multilayer graphenes are diversified by the number of layers and the stacking configurations. For an $N$-layer system, Landau levels are divided into $N$ groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene the low-lying levels are related to surface states. The Landau-level mixing leads to anticrossings patterns in energy spectra, which are seen for intergroup Landau levels in AB...
Diverse magnetic and electronic properties of halogen-adsorbed silicene are investigated by the f... more Diverse magnetic and electronic properties of halogen-adsorbed silicene are investigated by the first-principle theoretical framework, including the adatom-diversified geometric structures, atom-dominated energy bands, spatial spin density distributions, spatial charge density distributions and its variations, and orbital-projected density of states. Also, such physical quantities are sufficient to identify similar and different features in the double-side and single-side adsorptions. The former belongs to the concentration-depended finite gap semiconductors or p-type metals, while the latter display the valence energy bands with/without spin-splitting intersecting with the Fermi level. Both adsorption types show the halogen-related weakly dispersed bands at deep energies, the adatom-modified middle-energy σ bands, and the recovery of low-energy π bands during the decrease of the halogen concentrations. Such feature-rich band structures can be verified by the angle-resolved photoemi...
The diverse structural and electronic properties of the Si-adsorbed and -substituted monolayer gr... more The diverse structural and electronic properties of the Si-adsorbed and -substituted monolayer graphene systems are studied by a complete theoretical framework under the first-principles calculations, including the adatom-diversified geometric structures, the Si- and C-dominated energy bands, the spatial charge densities, variations in the spatial charge densities and the atom- and orbital-projected density of states (DOSs). These critical physical quantities are unified together to display a distinct physical and chemical picture in the studying systems. Under the Si-adsorption and Si-substitution effects, the planar geometric structures are still remained mainly owing to the very strong C–C and Si–C bonds on the honeycomb lattices, respectively. The Si-adsorption cases can create free carriers, while the finite- or zero-gap semiconducting behaviors are revealed in various Si-substitution configurations. The developed theoretical framework can be fully generalized to other emergent...
The π-electronic structure of graphene in the presence of a modulated electric potential is inves... more The π-electronic structure of graphene in the presence of a modulated electric potential is investigated by the tight-binding model. The low-energy electronic properties are strongly affected by the period and field strength. Such a field could modify the energy dispersions, destroy state degeneracy, and induce band-edge states. It should be noted that a modulated electric potential could make semiconducting graphene semimetallic, and that the onset period of such a transition relies on the field strength. There exist infinite Fermi-momentum states in sharply contrast with two crossing points (Dirac points) for graphene without external fields. The finite density of states (DOS) at the Fermi level means that there are free carriers, and, at the same time, the low DOS spectrum exhibits many prominent peaks, mainly owing to the band-edge states.
Physical chemistry chemical physics : PCCP, Jan 21, 2015
This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multi... more This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multilayer graphenes are built from graphene sheets attracting one another by van der Waals forces; the magneto-electronic properties are diversified by the number of layers and the stacking configurations. For an N-layer system, Landau levels are divided into N groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene...
The generalized tight-binding model, with the exact diagonalization method, is developed to inves... more The generalized tight-binding model, with the exact diagonalization method, is developed to investigate optical properties of graphene in five kinds of external fields.
This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multi... more This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multilayer graphenes are built from graphene sheets attracting one another by van der Waals forces; the magneto-electronic properties are diversified by the number of layers and the stacking configurations. For an N-layer system, Landau levels are divided into N groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene the low-lying levels are related to surface states. The Landau-level mixing leads to anticrossings patterns in energy spectra, which are seen for intergroup Landau levels in AB-stacked graphene, while in particular, a formation of both intergroup and intragroup anticrossings is observed in ABC-stacked graphene. The aforementioned magneto-electronic properties lead to diverse optical spectra, plasma spectra, and transport properties when the stacking order and the number of layers are varied. The calculations are in agreement with optical and transport experiments, and novel features that have not yet been verified experimentally are presented.
The generalized tight-binding model is developed to investigate the rich and unique electronic pr... more The generalized tight-binding model is developed to investigate the rich and unique electronic properties of AB-bt (bottom-top) bilayer silicene under uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, displays unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups, that is, there exist the sublattice-and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit irregular behavior corresponding to the critical points of the band structure. Moreover, the electric field can induce many LL anti-crossings. The main features of the LLs are uncovered with many van Hove singularities in the density-of-states and non-uniform delta-function-like peaks in the magneto-absorption spectra. The feature-rich magnetic quantization directly reflects the geometric symmetries, intra-layer and inter-layer atomic interactions, spin-orbital couplings, and the field effects. The results of this work can be applied to novel designs of Si-based nano-electronics and nano-devices with enhanced mobilities.
Concentration-dependent diverse magnetic and electronic properties of halogendoped silicene are i... more Concentration-dependent diverse magnetic and electronic properties of halogendoped silicene are investigated using the first-principles method. The optimal buckled geometric structures, atom-related energy bands, spin density distributions, spatial charge densities, and spin-and orbital-decomposed density of states strongly depend on the double-side and single-side adsorptions. The two-dimensional electronic structures are enriched by the competition between the significant halogen-Si bonds and weak sp 3 hybridizations. Such critical halogen-Si bonding is formed by the high charge transfer from Si atoms to halogen adatoms. The double-side adsorptions belong to the middle-gap semiconductors that become p-type metal at the 11% concentration, 1
The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic propert... more The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali- and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single $s-2p_z$ orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edge-localized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct...
A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene ... more A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in C-C, F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge distributions, and the orbital-projected density of states. The top-site F-C bonds, with the multi-orbital hybridizations, create the non-uniform buckled honeycomb lattice. There exist the C-, F- and (C, F)-dominated energy bands. Fluorinated graphene belongs either to the p-type metals (with/without the ferromagnetic spin arrangement) or to the large-gap semiconductors (without magnetism), depen...
The two-dimensional buckled honeycomb lattices system exhibits the rich Coulomb excitation spectr... more The two-dimensional buckled honeycomb lattices system exhibits the rich Coulomb excitation spectra, being dominated by the free carrier density, band structure, and transferred momentum (q). There are two kinds of plasmon peaks in the energy loss spectra, calculated from the random phase approximation. They are, respectively, revealed at low and middle frequencies. The former, which arises from the free carriers, belongs to acoustic mode. It's frequency depends on √ q at long wavelength limit. On the other hand, the latter is due to all the π-electronic collective excitations is an optical mode. Whether such plasmon can service is mainly determined by q. The frequencies and intensities of plasmon modes are very different among graphene, silicene, germanene, and Tin.
Physical chemistry chemical physics : PCCP, Jan 9, 2017
The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are in... more The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are investigated by the first-principles calculations. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement and edge structure. There exist C-C, C-F, and F-F bonds with multi-orbital hybridizations. Fluorine adatoms can create p-type metals or concentration- and distribution-dependent semiconductors, depending on whether the π bonding is seriously suppressed by the top-site chemical bonding. Furthermore, five kinds of spin-dependent electronic and magnetic properties cover the non-magnetic and ferromagnetic metals, non-magnetic semiconductors, and anti-ferromagnetic semiconductors with/without spin splitting. The diverse essential properties are clearly revealed in the spatial charge distribution, spin density, and orbital-projected density of states.
The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic propert... more The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali-and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single s − 2p z orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edgelocalized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct spin configuration could be verified from the experimental measurements. The graphene-based systems, which are formed by the planar sp 2 bondings of carbon atoms, include layered graphites 1, 2 , few-and multi-layer graphenes 3, 4 , one-dimensional graphene nanoribbons (1D GNRs) 5, 6 and 1D carbon nanotubes (CNTs) 7, 8. From a geometric point of view, each GNR could be regarded as a finite-width graphene strip or an unzipped CNT. The 1D GNRs have stirred a lot of studies, mainly owing to the complex relations among the honeycomb lattice, the one-atom thickness, the finite-size quantum confinement, and the edge structure. They could be successfully synthesized by the various experimental techniques. The most common methods are to cut graphene, achieved by a metal-catalyzed cutting 9, 10 , oxidation cutting 11, 12 , lithographic patterning and etching 13, 14 , sonochemical breaking 6, 15 , and molecular precursors 16, 17. The available routes from the unzipping of multi-wall CNTs cover strong chemical reaction 18, 19 , laser irradiation 20 , metal-catalyzed cutting 21, 22 , plasma etching 23, 24 , hydrogen treatment and annealing 25 , unzipping functionalized CNTs by scanning tunneling microscope (STM) tips 26 , electrical unwrapping by transmission electron microscopy (TEM) 27 , intercalation and exfoliation 28 , and electrochemical unzipping 29. The other techniques involve chemical vapor deposition 30 and chemical synthesis 31. GNRs exhibit the feature-rich essential properties, such as, electronic structures 5, 32 , magnetic properties 32, 33 , optical spectra 34, 35 , and transport properties 36, 37. The electronic properties are diversified by changing by the ribbon width (W) 38, 39 , edge structure 38, 40 , edge-passivated dopants 41, 42 , adatom adsorptions 43, 44 , layer numbers 45 , stacking configurations 46 , surface curvatures 47, 48 , mechanical strains 49, 50 , electric fields 51-53 , and magnetic fields 32, 54, 55. GNRs are expected to be more potentially applicable in future nanodevices 15, 56, 57. In this work, the first-principles calculations are used to investigate the adatom-enriched electronic properties of the alkali-adsorbed GNRs. Whether the alkali adatoms can create the high free electron density will be explored in detail. GNRs, with or without hydrogen passivation at boundaries, exhibit the semiconducting behavior, as indicated from the theoretical predictions 13, 38, 39 , and the experimental measurements 17, 58-60. There are two typical types of achiral GNRs, namely, armchair and zigzag GNRs. The former have the diverse energy gaps (E g s) inversely proportional to widths 38, 39. Specifically, E g in the latter, which arises from the occupied and the unoccupied edge-localized energy bands, is induced by the anti-ferromagnetic spin configuration across the nanoribbon 5, 38. The strong dependence of E g on W has been confirmed by the electrical conductance 6, 13 and tunneling current 17, 58-60 measurements. The semiconductor-metal transition is revealed under a transverse electric field 5. The similar
The feature-rich electronic excitations of monolayer germanene lie in the significant spin-orbita... more The feature-rich electronic excitations of monolayer germanene lie in the significant spin-orbital coupling and the buckled structure. The collective and singleparticle excitations are diversified by the magnitude and direction of transferred momentum, the Fermi energy and the gate voltage. There are four kinds of plasmon modes, according to the unique frequency-and momentum-dependent phase diagrams. They behave as two-dimensional acoustic modes at long wavelength. However, for the larger momenta, they might change into another kind of undamped plasmons, become the seriously suppressed modes in the heavy intraband e-h excitations, keep the same undamped plasmons, or decline and then vanish in the strong interband e-h excitations. Germanene, silicene and graphene are quite different from one another in the main features of the diverse plasmon modes.
ABSTRACT The influences of modulated electric fields and modulated magnetic fields on low-energy ... more ABSTRACT The influences of modulated electric fields and modulated magnetic fields on low-energy Landau levels (LL’s) of monolayer graphene are investigated by the tight-binding model. The ratio of the uniform magnetic-field strength to the modulated field strength is arbitrarily chosen for discussions. Both modulated fields cause the LL’s to split into two twofold parabolic subbands; these subbands exhibit periodic oscillations and two kinds of band-edge states. However, the subband dispersions and oscillation amplitudes associated with electric and magnetic modulated fields behave differently. The main features of the electronic structure are reflected in the density of states, which presents pairlike and square-root divergent structures for both modulated field cases. The LL wave functions are strongly affected by both modulated fields, such as the broken symmetry, displacement of the center location, and alteration of the amplitude strength. These changes in the LL wave functions should be reflected in other physical properties, e.g., the optical selection rules. Furthermore, the dependence of the LL properties on the modulation strength and modulation period is also discussed in detail.
ABSTRACT The Landau level (LL) spectra in rhombohedral graphite are calculated within the tight-b... more ABSTRACT The Landau level (LL) spectra in rhombohedral graphite are calculated within the tight-binding model without any perturbation expansion. All significant interlayer atomic interactions are included, up to next-nearest-neighbor graphene layers. The magnetic Hamiltonian matrix is derived and manipulated in a band-like form for numerical efficiency. The results of this diagonalization scheme manifest the effects of lattice symmetry and interlayer interaction in the specific stacking configuration. Three-dimensional character associated with the mirror symmetry of the ABC-stacking configuration is exhibited in the LL spectra.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010
The electronic and optical properties of monolayer and bilayer graphene are investigated to verif... more The electronic and optical properties of monolayer and bilayer graphene are investigated to verify the effects of interlayer interactions and external magnetic field. Monolayer graphene exhibits linear bands in the low-energy region. Then the interlayer interactions in bilayers change these bands into two pairs of parabolic bands, where the lower pair is slightly overlapped and the occupied states are asymmetric with respect to the unoccupied ones. The characteristics of zero-field electronic structures are directly reflected in the Landau levels. In monolayer and bilayer graphene, these levels can be classified into one and two groups, respectively. With respect to the optical transitions between the Landau levels, bilayer graphene possesses much richer spectral features in comparison with monolayers, such as four kinds of absorption channels and double-peaked absorption lines. The explicit wave functions can further elucidate the frequency-dependent absorption rates and the comple...
A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene ... more A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in CC , F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge distributions, and the orbital-projected density of states. The top-site F-C bonds, with the multi-orbital hybridizations, create the non-uniform buckled honeycomb lattice. There exist the C-, F-and (C, F)-dominated energy bands. Fluorinated graphene belongs either to the p-type metals (with/without the ferromagnetic spin arrangement) or to the large-gap semiconductors (without magnetism), depending on the 1
Essential properties of multilayer graphenes are diversified by the number of layers and the stac... more Essential properties of multilayer graphenes are diversified by the number of layers and the stacking configurations. For an $N$-layer system, Landau levels are divided into $N$ groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene the low-lying levels are related to surface states. The Landau-level mixing leads to anticrossings patterns in energy spectra, which are seen for intergroup Landau levels in AB...
Diverse magnetic and electronic properties of halogen-adsorbed silicene are investigated by the f... more Diverse magnetic and electronic properties of halogen-adsorbed silicene are investigated by the first-principle theoretical framework, including the adatom-diversified geometric structures, atom-dominated energy bands, spatial spin density distributions, spatial charge density distributions and its variations, and orbital-projected density of states. Also, such physical quantities are sufficient to identify similar and different features in the double-side and single-side adsorptions. The former belongs to the concentration-depended finite gap semiconductors or p-type metals, while the latter display the valence energy bands with/without spin-splitting intersecting with the Fermi level. Both adsorption types show the halogen-related weakly dispersed bands at deep energies, the adatom-modified middle-energy σ bands, and the recovery of low-energy π bands during the decrease of the halogen concentrations. Such feature-rich band structures can be verified by the angle-resolved photoemi...
The diverse structural and electronic properties of the Si-adsorbed and -substituted monolayer gr... more The diverse structural and electronic properties of the Si-adsorbed and -substituted monolayer graphene systems are studied by a complete theoretical framework under the first-principles calculations, including the adatom-diversified geometric structures, the Si- and C-dominated energy bands, the spatial charge densities, variations in the spatial charge densities and the atom- and orbital-projected density of states (DOSs). These critical physical quantities are unified together to display a distinct physical and chemical picture in the studying systems. Under the Si-adsorption and Si-substitution effects, the planar geometric structures are still remained mainly owing to the very strong C–C and Si–C bonds on the honeycomb lattices, respectively. The Si-adsorption cases can create free carriers, while the finite- or zero-gap semiconducting behaviors are revealed in various Si-substitution configurations. The developed theoretical framework can be fully generalized to other emergent...
The π-electronic structure of graphene in the presence of a modulated electric potential is inves... more The π-electronic structure of graphene in the presence of a modulated electric potential is investigated by the tight-binding model. The low-energy electronic properties are strongly affected by the period and field strength. Such a field could modify the energy dispersions, destroy state degeneracy, and induce band-edge states. It should be noted that a modulated electric potential could make semiconducting graphene semimetallic, and that the onset period of such a transition relies on the field strength. There exist infinite Fermi-momentum states in sharply contrast with two crossing points (Dirac points) for graphene without external fields. The finite density of states (DOS) at the Fermi level means that there are free carriers, and, at the same time, the low DOS spectrum exhibits many prominent peaks, mainly owing to the band-edge states.
Physical chemistry chemical physics : PCCP, Jan 21, 2015
This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multi... more This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multilayer graphenes are built from graphene sheets attracting one another by van der Waals forces; the magneto-electronic properties are diversified by the number of layers and the stacking configurations. For an N-layer system, Landau levels are divided into N groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene...
The generalized tight-binding model, with the exact diagonalization method, is developed to inves... more The generalized tight-binding model, with the exact diagonalization method, is developed to investigate optical properties of graphene in five kinds of external fields.
This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multi... more This article reviews the rich magneto-electronic properties of multilayer graphene systems. Multilayer graphenes are built from graphene sheets attracting one another by van der Waals forces; the magneto-electronic properties are diversified by the number of layers and the stacking configurations. For an N-layer system, Landau levels are divided into N groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene the low-lying levels are related to surface states. The Landau-level mixing leads to anticrossings patterns in energy spectra, which are seen for intergroup Landau levels in AB-stacked graphene, while in particular, a formation of both intergroup and intragroup anticrossings is observed in ABC-stacked graphene. The aforementioned magneto-electronic properties lead to diverse optical spectra, plasma spectra, and transport properties when the stacking order and the number of layers are varied. The calculations are in agreement with optical and transport experiments, and novel features that have not yet been verified experimentally are presented.
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