We examine the relative contribution of ballistic and elastic cotunneling mechanisms to the charge transport through a single decanedithiol molecule linked to two terminal clusters of gold atoms. For this, we first introduced a conceptual model that permits a generalization of the Breit-Wigner scattering formalism where the cation, anion, and neutral forms of the molecule can participate with different probabilities of the charge transfer process, but in a simultaneous manner. We used a density functional theory treatment and considered the fixed geometry of each charge state to calculate the corresponding eigenvalues and eigenvectors of the extended system for different values of the external electric field. We have found that for the ballistic transport the HOMO and LUMO of the neutral species play a key role, while the charged states give a negligible contribution. On the other hand, an elastic cotunneling charge transfer can occur whenever a molecular orbital (MO) of the cation or anion species, even if localized in just one side of the molecule-gold clusters complex, has energy close to that of a delocalized MO of the neutral species. Under these conditions, a conduction channel is formed throughout the entire system, in a process that is controlled by the degree of resonance between the MOs involved. Our results indicate that while different charge transfer mechanisms contribute to the overall charge transport, quantum effects such as avoided-crossing situations between relevant frontier MOs can be of special importance. In these specific situations, the interchange of spatial localization of two MOs involved in the crossing can open a new channel of charge transfer that otherwise would not be available.
We have investigated the role that molecular orbitals (MOs) play in the electron transport through a single donor–acceptor molecule submitted to an external voltage applied through two metallic electrodes. Considering the weak coupling limit, in which level-broadening effects are negligible, we investigate the microscopic processes associated to the charge flow through the molecule by examining how the individual molecular levels actually respond to the external electric field. By taking into account the active role played by the MOs in the charge transport between the two electrodes, we have shown that an important contribution may arise in the situation of a field-induced “avoided-crossing” between neighboring energy levels, especially if the corresponding MOs are localized in different regions of the molecule. Conduction channels can be opened or closed as the result of “avoided-crossing” situations in which the spatial localization of the MOs considered changes between the acceptor and the donor opposite sides of the molecule. Our results indicate that the charge transfer between the electrodes is mainly dominated by noncoherent mechanisms involving hole transport through the uppermost occupied molecular orbitals. We suggest that these field-induced changes in the molecular environment may play a key role in the overall transport process and should be considered whenever actual measurements are being performed in single molecules.
In this work, we present a self-energy model based on the complex absorbing potential (CAP) method to calculate the transmission function through an extended molecule using scattering theory. Once the CAP mimics an infinite environment at the ends of a finite system, it can be used as a model for self-energy with a low computational cost. Moreover, the matrixes required for the transport calculation can be obtained from an ab initio calculation of some extended molecules in a single step using an adjustable model, thus taking into account changes in the electronic structure of the system. This approach was applied to study electron transport across a biphenyl molecular system for different torsion angles under an external applied electric field. The results obtained are in good agreement with the available theoretical and experimental results in the literature and provide an efficient approach, with a low computational cost method, for the interpretation of electrical transport at the molecular level.
Entender os fundamentos da transferência de carga em escala nanométrica é essencial para que possam ser projetados dispositivos à base de eletrônica molecular. Neste trabalho, implementamos para moléculas saturadas (isoladas e “estendidas”) um estudo teórico da estabilidade de algumas grandezas como a carga na molécula, o “gap HOMO-LUMO” e a densidade local de estados. Usando-se métodos ab initio no nível Hartree-Fock (HF) e da teoria do funcional da densidade (DFT), realizamos um estudo da taxa de convergência dessas grandezas com o aumento do tamanho do sistema. Os nossos resultados indicam uma estabilização relativamente rápida dessas grandezas com um número limitado de átomos de ouro na molécula estendida.
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