Calculation of Energy Level Alignment and Interface Electronic Structure in Molecular Junctions beyond DFT

Abstract

In atomistic simulations of molecular junctions, it is important to develop methods beyond density-functional theory (DFT) to describe the interface electronic structure and alignment of frontier molecular orbitals accurately. Here we describe a first-principles approach for molecular junctions that extends the DFT+Σ method, an approximate scheme based on self-energy corrections. The DFT+Σtot method presented here acts on junction states and introduces corrections to DFT-based molecular frontier orbitals not only on the molecular subspace but on the whole junction Hamiltonian. These self-energy corrections are scaled according to the molecular character of each junction wave function, a character which is given by projection coefficients between molecular orbitals and junction states. We illustrate this formalism in three paradigmatic weakly interacting single molecule junctions. Despite the weak metal/molecule interaction, calculated projection coefficients show metal/molecule hybridization resulting in molecular orbital features that can be significantly broadened over a wide energy range. Scaling of self-energy corrections to molecular levels seamlessly accounts for this hybridization and spectral delocalization. The resulting DFT+Σtot electronic structure brings molecular frontier orbital energies to values comparable to the G0W0 and WKM approaches. By considering the whole spectral distribution of molecular orbitals, DFT+Σtot shifts not just the main molecular peaks but modifies the whole junction electronic structure.

Cooperative Self-Assembly of Dimer Junctions Driven by π Stacking Leads to Conductance Enhancement

“…In both molecules, this σ resonance peaks is at −1.5 eV, but its coupling at the Fermi energy is overall lower in BI than in Im. The calculated conductance values of Im and BI are 6.4 × 10 –2 G 0 and 4.2 × 10 –2 G 0 , respectively, in reasonably good agreement with the experimentally determined values for the G1 peak listed in Table . ,, …”

“…The calculated conductance values of Im and BI are 6.4 × 10 −2 G 0 and 4.2 × 10 −2 G 0 , respectively, in reasonably good agreement with the experimentally determined values for the G1 peak listed in Table 1. 5,36,37 Next, we calculate the conducting properties of Im and BI dimers (2Im and 2BI) to investigate the experimentally determined G2 values. We built junctions with molecular dimers and optimized the interface geometry, where no restrictions are imposed on the geometry of the molecules.…”

Mechanically Tuned Thermopower of Single‐Molecule Junctions

“…[6,21] For PPD, we find 4.2 × 10 −2 to 8.0 × 10 −2 G 0 , very close to 4.6 × 10 −2 G 0 , [44] 4.3 × 10 −2 G 0 , [45] 2.4 × 10 −2 G 0 , [46] and 5.5 × 10 −2 G 0 . [47,48] The distance range shown in Figure 5 is consistent with the length of each molecule. For a given molecule, the spectra show relatively small changes over the different electrode separations.…”

“…[ 6,21 ] For PPD, we find 4.2 × 10 −2 to 8.0 × 10 −2 G 0 , very close to 4.6 × 10 −2 G 0 , [ 44 ] 4.3 × 10 −2 G 0 , [ 45 ] 2.4 × 10 −2 G 0 , [ 46 ] and 5.5 × 10 −2 G 0 . [ 47,48 ]…”

Development of Classical Force Fields for Interfaces between Single Molecules and Au

“…DFT is generalizable and widely applicable. Its computational cost makes metal–molecule interfaces with several hundreds of atoms widely accessible, , and, despite limitations in the calculation of molecular-level alignment, − yields an accurate picture of structural, thermal, and electronic properties of molecular junctions. , Most DFT studies, however, have focused on static properties of the junction, as determined by stable geometries obtained by calculations at zero temperature which minimize the forces or total energy. , Exploring the dynamics of metal–molecule junctions within DFT is a very onerous task because of the computational cost of simulations over sufficiently long timescales . For example, in the context of single-molecule conductance, where the molecule is placed between two nanosized electrodes, MD simulations using DFT have investigated the notoriously complex interface geometries of thiolate-linked molecules on Au. − However, these simulations are limited to a few picoseconds due to the computational costs of DFT.…”