RESUMO
A theoretical description of the electronic structure, optical spectrum and binding energy of a hydrogenic impurity in laterally coupled quantum discs, under applied electric and magnetic fields, is given within the framework of the effective-mass approach. Calculations are performed using the envelope-function formalism and a variational procedure, with the electric field applied in the coupling direction, the magnetic field along the growth direction, and the impurity at the center of the heterostructure. The results indicate that the anisotropy of the laterally coupled confinement potential leads to interesting relative extrema and anticrossings in the energy spectra, and that the infrared absorption spectrum is sensitive to the type of polarization and magnitude of external fields.
RESUMO
The transport properties of graphene nanoribbons with linear benzene-based molecules pinned at the ribbon edges are studied. The systems are described by a single pi-band tight-binding Hamiltonian and by using the Green functions formalism based on real-space renormalization techniques. Different configurations have been considered, such as two and three attached molecules separated by a variable distance d, and the case of a finite array of molecules attached to the ribbon in different geometries (one-side and alternated sequence). In the latter case the conductance behavior is compared with the case of a molecular superlattice-like structure. In these hybrid systems of ribbons with a large number of regular attached foreign structures, we have shown the formation of well-defined energy gaps for which the conductance is completely suppressed. These gaps can be tuned by varying the number, relative distance, and length of the attached molecules. An analysis is performed to understand the nature of the conductance gap and its relation with the foreign molecular structures, providing a mechanism to delineate novel molecular sensors.
Assuntos
Benzeno/química , Grafite/química , Modelos Químicos , Nanoestruturas/química , Simulação por Computador , Condutividade ElétricaRESUMO
A theoretical study of the electronic and optical properties of laterally coupled quantum dots, under applied magnetic fields perpendicular to the plane of the dots, is presented. The exciton energy levels of such laterally coupled quantum-dot systems, together with the corresponding wavefunctions and eigenvalues, are obtained in the effective-mass approximation by using an extended variational approach in which the magnetoexciton states are simultaneously obtained. One achieves the expected limits of one single quantum dot, when the distance between the dots is zero, and of two uncoupled quantum dots, when the distance between the dots is large enough. Moreover, present calculations-with appropriate structural dimensions of the two-dot system-are shown to be in agreement with measurements in self-assembled laterally aligned GaAs quantum-dot pairs and naturally/accidentally occurring coupled quantum dots in GaAs/GaAlAs quantum wells.
RESUMO
In this work we address the effects on the conductance of graphene nanoribbons (GNRs) of organic molecules adsorbed at the ribbon edge. We studied the case of armchair and zigzag GNRs with quasi-one-dimensional side-attached molecules, such as linear poly-aromatic hydrocarbons and poly(para-phenylene). These nanostructures are described using a single-band tight-binding Hamiltonian and their electronic conductance and density of states are calculated within the Green's function formalism based on real-space renormalization techniques. We found that the conductance exhibits an even-odd parity effect as a function of the length of the attached molecules. Furthermore, the corresponding energy spectrum of the molecules can be obtained as a series of Fano antiresonances in the conductance of the system. The latter result suggests that GNRs can be used as a spectrograph sensor device.
RESUMO
A theoretical study of the photoluminescence peak energies in InAs self-assembled quantum dots embedded in a GaAs matrix in the presence of magnetic fields applied perpendicular to the sample plane is performed. The effective mass approximation and a parabolic potential cylinder-shaped model for the InAs quantum dots are used to describe the effects of magnetic field and hydrostatic pressure on the correlated electron-hole transition energies. Theoretical results are found in quite good agreement with available experimental measurements for InAs/GaAs self-assembled quantum dots.
RESUMO
Electronic properties of straight carbon nanotubes under an external electric are investigated, following a single-π-orbital tight binding approximation. Metal-insulator transitions in metallic tubes and energy gap modulations in semiconducting ones were found due to the action of the electric field. Reductions in the tube symmetry operations induced by the field are manifested in the energy spectrum as a function of the angle determined by the field direction and equivalent in-plane atomic positions along the circumferential direction. We find that particular energies in the spectra exhibit a periodic oscillation with this dephasing angle. The range and position of those energies, as well the amplitude of the oscillation, can be properly manipulated by changing the strength and direction of the applied electric field.