Navegando por Assunto "Teoria molecular"

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Acesso aberto (Open Access)
Eletrônica molecular via método híbrido DFT/FGNE em anéis fenilas acoplados a eletrodos metálicos de nanotubos de carbono: a regra de conformação e quiralidade molecular
(Universidade Federal do Pará, 2011-11-11) SILVA JÚNIOR, Carlos Alberto Brito da; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
In this thesis, we investigate in detail the electronic transport properties, conformation and symmetry of zz9 SWCN structures attached to phenyl rings (2, 3, 4 and 5) under the influence of external electric field (voltage) via hybrid method DFT type B3LYP 6-311G (d, p) combined with NEGF and Group Theory. We found a good relationship between: 1 - the chiral index () by Group Theory and the law of cos2 (, dihedral angle) for the geometry under the influence of external electric field because  only depends on the atomic positions () of conformations, and is also strongly correlated with current passing through the system; 2 - normalized conductance (G / Go) is proportional to cos2 in the region of the gap (EHOMO-ELUMO), ie, in regions where the resonance occurs negative differential resistance (NDR) and 3 - the Fowler-Northeim (FN) plot Vmin displays that occurs when the tail of a resonant transmission peak voltage enters the window, that is, when these structures is an NDR, NDR as the number of the IV plot is associated with the number Vmin of the FN plot and can be explained by the model of molecular transport line 4 - the barrier height (EF - EHOMO and ELUMO - EF) as a function of molecular length, 5 - Vmin as a function of barrier height (EF - EHOMO ) and the molecular length. Thus, one implies that the molecular conformation plays a major role in determining the transport properties of the junction, suggests that the second law of cos2 is a more general applicability regardless of the nature of the electrodes; 3 serves as a spectroscopic tool and also to identify the molecule at the junction, 4 and 5 as the molecular length reaches a certain value (1.3 nm) the Vmin remains virtually unchanged. The results showed that the structural properties (geometric) undergo significant changes with increasing external electric field that are in good agreement with the values found in literature. The behavior of I-V curves and G/Go-V lose their linear dependence to give rise to a nonlinear behavior with the appearance of NDR. This point reveals the structural modifications suffered by the system. The I-V plot confirmed the statements that were made through the structural analysis for the system considered and showed how it gives the load flow analysis systems.
Acesso aberto (Open Access)
Transporte eletrônico e quiralidade molecular: um estudo de dispositivos orgânicos em sistemas de dois terminais
(Universidade Federal do Pará, 2010-06-18) SILVA, Shirsley Joany dos Santos da; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
In this work, we simulate the electronic distribution properties as charge in two molecular organic compounds, the Ponceau SS (PSS) and Oligo-(para)phenylene-vinylene (PPV), by means of ab initio calculations and the Nonequilibrium Green function (NEGF) method. These methods demonstrate equivalence to the description of molecular device. We make quantum calculations for the Hamiltonian derivative Hartree-Fock (HF) and obtained the current-voltage characteristics (I-V) for the two molecular structures. With the method NEFG, we model the transport through of electronics multilevel system obtaining the current by parameters that describe the resonances and the asymmetry of the system. In response the PSS demonstrated an asymmetric characteristic for direct and reverse polarization, the resonance is reached and confirm the device as a bi-directional molecular transistor. For the PPV also investigate the geometric properties through the connection between electronic transport and the degree of chirality calculated means chiral index that only depends on the atomic positions. Since, the structural properties of chiral molecules can induce an asymmetry in the electron transport, resulting in the grinding process. We demonstrated that corrent-voltage and dipole moment are proportional to the degree of molecular chirality. This result suggests that the electronic transport in this system can be exploited in assessing the degree of chirality.