Ayaka Yamanaka

Electronic Properties of Carbon Nanotubes under an Electric Field

We study the electronic properties of carbon nanotubes under an electric field by investigating their electrostatic potentials, total energies, and energy gaps under a parallel electric field, based on the density functional theory. We find that, in capped carbon nanotubes, screening against the external electric field strongly depends on local atomic arrangement due to the inhomogeneous charge distribution arising from its bond alternation. In the case of armchair nanotubes, we find that the total energy and energy gap between the highest occupied and the lowest unoccupied states oscillate in triple periodicity of carbon nanotubes.

Electron injection into nearly free electron states of graphene nanoribbons under a lateral electric field

Using density functional theory with the effective screening medium method, we studied the electronic properties of graphene nanoribbons with zigzag and armchair edges under a lateral electric field. Our calculations showed that a nearly free electron (NFE) state emerges in the vacuum region outside the leftmost edge of the ribbons and shifts downward with increasing electric field. We also found that electrons are injected into this NFE state of graphene nanoribbons by the critical electric field, which is inversely proportional to the ribbon width.

Anomalous Electric-Field Screening at the Edge Atomic Sites of Finite-Length Zigzag Carbon Nanotubes

We studied the electronic properties of hydrogenated zigzag carbon nanotubes under an electric field, using first-principles total-energy calculations based on density functional theory. By investigating the electrostatic potentials at each atomic site under a parallel electric field, we found that the edge atomic sites of zigzag carbon nanotubes anomalously screen the electric field; the electrostatic potential rapidly oscillates near the edge C atomic sites. We also found that the penetration depth of the anomalous screening depends on the diameter of the nanotubes.

Energetics and Electronic Structure of h-BN Nanoflakes.

We studied the energetics and electronic structure of hexagonal boron nitride (h-BN) nanoribbons with hydrogenated and clean edges with respect to the detailed edge shapes using density functional theory. Our calculations showed that the stability of h-BN edges strongly depends on the edge termination. In the case of hydrogenated edges, the formation energy is constant for all edge angles ranging from armchair to zigzag, indicating that h-BN may exhibit rich variation in their edge atomic arrangements under static conditions. The hydrogenated h-BN nanoribbons are insulators with an energy gap of 4 eV irrespective of edge shape, in which the lowest branch of the conduction band exhibits nearly free electron states nature distributed in the vacuum region outside the ribbons. In contrast, the formation energy of h-BN nanoribbons with clean edges monotonically increases as the edge angle is changed from armchair to zigzag. Our analysis reveals that the increase of density of states at the Fermi level arising from dangling bond states leads to this monotonic increase of edge formation energy in h-BN nanoribbons with clean edges.

Structural dependence of electronic properties of graphene nanoribbons on an electric field

We studied the electronic properties of graphene nanoribbons with several edge structures under a parallel electric field using density functional theory with the effective screening medium method. Our calculations showed that the edge atomic sites of the nanoribbons with zigzag-shaped edges anomalously screen the electric field; the electrostatic potential oscillates rapidly, leading to over-screening in the vicinity of edge C atomic sites. We also found that the penetration depth of anomalous screening depends on the edge structure of nanoribbons.

Electronic structure modulation of graphene edges by chemical functionalization

Using the density functional theory with the effective screening medium method, we study the electronic properties of graphene nanoribbons with zigzag edges that are terminated by hydrogen and ketone, hydroxyl, carbonyl, and carboxyl functional groups. Our calculations showed that the work function and electronic structures of the edges of the nanoribbons are sensitive to the functional groups attached to the edges. The nearly free electron state emerges in the vacuum region outside the hydroxylated edges and crosses the Fermi level, indicating the possibility of negative electron affinity at the edges.

Electronic Properties of Capped Carbon Nanotubes under an Electric Field: Inhomogeneous Electric-Field Screening Induced by Bond Alternation

We study the electronic properties of capped carbon nanotubes under an electric field by investigating their electrostatic potentials, total energies, and energy gaps under a parallel electric field, based on the density functional theory with effective screening medium method. We find that, in the capped carbon nanotubes, screening against the external electric field strongly depends on local atomic arrangement due to the inhomogeneous charge distribution arising from its bond alternation induced by the pentagonal rings in the cap region. In the case of armchair nanotubes, we find that the relative permittivity and energy gap between the highest occupied and the lowest unoccupied states oscillate in triple periodicity in their units with respect to the length. The electric field induces the charge redistribution in which the charge accumulation and depletion only occur around the pentagonal rings at or vicinity of the top/bottom of the nanotubes.

Electronic structure and electric polarity of edge-functionalized graphene nanoribbons

On the basis of the density functional theory combined with the effective screening medium method, we studied the electronic structure of graphene nanoribbons with zigzag edges, which are terminated by functional groups. The work function of the nanoribbons is sensitive to the functional groups. The edge state inherent in the zigzag edges is robust against edge functionalization. OH termination causes the injection of electrons into the nearly free electron states situated alongside the nanoribbons, resulting in the formation of free electron channels outside the nanoribbons. We also demonstrated that the polarity of zigzag graphene nanoribbons is controllable by the asymmetrical functionalization of their edges.

Energetics of edge oxidization of graphene nanoribbons

On the basis of the density functional theory, we studied the geometries and energetics of O atoms adsorbed on graphene edges for simulating the initial stage of the edge oxidization of graphene. Our calculations showed that oxygen atoms are preferentially adsorbed onto the graphene edges with the zigzag portion, resulting in a large adsorption energy of about 5 eV. On the other hand, the edges with armchair shape are rarely oxidized, or the oxidization causes substantial structural reconstructions, because of the stable covalent bond at the armchair edge with the triple bond nature. Furthermore, the energetics sensitively depends on the edge angles owing to the inhomogeneity of the charge density at the edge atomic sites.

Influence of electric field on electronic states of graphene nanoribbons under a FET structure

We study the electronic properties of graphene nanoribbons with zigzag and armchair edges under a parallel electric field generated by two planar electrodes with a potential barrier simulating an insulating layer of electrodes in a FET structure using density functional theory combined with an effective screening medium method. Our calculations show that the nearly free electron (NFE) states strongly depend on the mutual arrangements of graphene nanoribbons with respect to the electric field. In contrast, the electronic energy bands associated with the π electrons are insensitive to the relative direction of the ribbon with respect to the external electric field. We also observe that the electric field concentration around the edges leads to the orientation dependence of the NFE states on the field.