First-Principles Study of Rectification in Bis-2-(5-ethynylthienyl)ethyne Molecular Junctions

Abstract

Using density functional theory (DFT) combined with the first-principles nonequilibrium Green's function (NEGF), we investigated the electron-transport properties and rectifying behaviors of several molecular junctions based on the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule. To examine the roles of different rectification factors, asymmetric electrode-molecule contacts and donor-acceptor substituent groups were introduced into the BETE-based molecular junction. The asymmetric current-voltage characteristics were obtained for the molecular junctions containing asymmetric contacts and donor-acceptor groups. In our models, the computed rectification ratios show that the mode of electrode-molecule contacts plays a crucial role in rectification and that the rectifying effect is not enhanced significantly by introducing the additional donor-acceptor components for the molecular rectifier with asymmetric electrode-molecule contacts. The current-voltage characteristics and rectifying behaviors are discussed in terms of transmission spectra, molecular projected self-consistent Hamiltonian (MPSH) states, and energy levels of MPSH states.

Electronic Transport of (SiC)n (n = 1–4) Molecular Chains via First-Principles Calculations

“…So scales of orbitals are larger under the negative bias, which is consistent with the result in Section . The asymmetric feature of the evolution of the MPSH eigenvalues reflect the built-in asymmetry in the chain and the unequal couplings of Au–Si and Au–C contacts . It is clearly seen that the degenerate HOMO and HOMO–1 states becomes nondegenerate with the higher bias which implies that the external electric field could combat and eliminate degeneracy of the system.…”

“…In fact, the transmission spectrum is associated with the orbitals of the chain in scattering region which have been broadened and shifted by the Au leads . Now, the modified molecular orbitals are gained by an efficient way ,, of projecting the self-consistent Hamiltonian of the molecular junction onto the linear chain and the apex Au atoms in consideration of their interaction. Afterward, the MPSH matrix is diagonalized and the related MPSH states are the eigenstates of molecular chain in a two-probe system without the self-energy of the leads.…”

“…From Figure , it can be seen that the currents at the positive voltage are much smaller than their negative counterparts, exhibiting the pronounced asymmetric I – V characteristics. This asymmetric feature stems from the built-in asymmetry in chain structure and the resulting unequal couplings at the contact …”

“…The unequal couplings of Au–Si and Au–C contacts lead to unequal overlap of electronic states in the MPSH states of the chains. The larger overlap from Au–C coupling provides a more favorable channel for the electronic transport than the relatively weaker overlap from Au–Si coupling from the other side . Thus, at negative (positive) voltage, the energy level of the chain elevates (lowers) with the chemical potential of the right electrode, leading to the increase (decrease) of the orbitals scales contributing to transport.…”

“…In quantum transport calculation, basis sets of SZP and DZP and their composite are commonly chosen in calculations. , For comparison, we additionally compute the conductance, transmission and current with different basis sets to test their effect on electronic transport results. For Si and C, the inclusion of polarization in basis set mixes the p and d orbitals, and s and p orbitals for Au.…”