Modulation of Charge Transport through Single Molecules Induced by Solvent-Stabilized Intramolecular Charge Transfer.

The modulation of charge transport through single molecules can be established by using the intrinsic characteristics of molecules and the physical properties of their environment. Therefore, the impact of the solvent on the electronic properties of molecules in the junction and their charge transport behavior are of great interest. Here, for the first time, we focused on charge transport through dimethylaminobenzonitrile (DMABN). This molecule shows unique behavior, specifically noticeable electronic structure modulations in bulk solvents, e.g., dual fluorescence in a polar environment. Using the scanning tunneling microscopy break junction (STM-BJ) technique, we find an order of magnitude increase in conductance along with a second conductance value in polar solvents over nonpolar solvents. Inspired by the twisted intramolecular charge transfer (TICT) explanation of the famous dual fluorescence of DMABN in polar solvents, we hypothesize stabilization of twisted DMABN molecules in the junction in more polar solvents. Ab initio molecular dynamics (AIMD) simulations using density functional theory (DFT) show that DMABN can twist in the junction and have a larger dipole moment compared to planar DMABN junction geometries, supporting the hypothesis. The nonequilibrium Green's function with the DFT approach (NEGF-DFT) is used to calculate the conductance throughout the AIMD trajectory, finding a significant change in the frontier orbitals and transmission function at large internal twisting angles, which can explain the dual conductance in polar solvents in STM-BJ experiments.

Double Excitation Energies from Quantum Monte Carlo Using State-Specific Energy Optimization.

We show that recently developed quantum Monte Carlo methods, which provide accurate vertical transition energies for single excitations, also successfully treat double excitations. We study the double excitations in medium-sized molecules, some of which are challenging for high-level coupled-cluster calculations to model accurately. Our fixed-node diffusion Monte Carlo excitation energies are in very good agreement with reliable benchmarks, when available, and provide accurate predictions for excitation energies of difficult systems where reference values are lacking.

Correlated Energy-Level Alignment Effects Determine Substituent-Tuned Single-Molecule Conductance.

The rational design of single-molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here, we explore the relationship between chemical substituents and the conductance of metal-single-molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy-level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles" because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results but also within disparate molecular series reported in the literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.

First principles study of graphene on metals with the SCAN and SCAN+rVV10 functionals.

Integrating graphene into electronic devices requires support by a substrate and contact with metal electrodes. Ab initio calculations at the level of density functional theory are performed on graphene-fcc-metal(111) [Gr/M(111)] (M = Ni, Cu, Au) systems. The strongly constrained and appropriately normed (SCAN) and SCAN with the revised Vydrov-van Voorhis (SCAN+rVV10) functionals are relatively new approximations to the exchange-correlation (xc) energy shown to account for van der Waals (vdW) interactions which many non-empirical semi-local functionals fail to include. Binding energies and distances as well as electronic band structures are calculated with SCAN, SCAN+rVV10, Perdew-Burke-Ernzerhof (PBE), and PBE-D3 with and without Becke-Johnson damping, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), and optB86b-vdW. SCAN and SCAN+rVV10 succeed in describing chemisorption and physisorption in the Gr/Ni(111) system and physisorption in the Gr/Cu(111) and Gr/Au(111) systems. Incorrectly, the physisorption is found to be more favorable than chemisorption in the Gr/Ni(111) system with SCAN, but the result is reversed when the experimental bulk Ni lattice parameter is used as opposed to the SCAN calculated lattice parameter. The SCAN+rVV10 functional produces binding energies and distances comparable to those calculated using the random phase approximation as well as the experiment. The SCAN based functionals produce the highest spin magnetic moments in the bulk Ni and Gr/Ni(111) systems compared to the rest of the functionals investigated, overestimating the experiment by at least ∼0.18 µB. Also, in contrast to the rest of the functionals, the induced spin magnetic moment in graphene is found to be larger in magnitude in the physisorption region than the chemisorption region. The pristine graphene band structure is preserved in the physisorbed systems but with a shift in the Dirac point away from the Fermi energy causing graphene to become n-doped in the Gr/Cu(111) system and p-doped in the Gr/Au(111) system. Chemisorption occurs in the Gr/Ni(111) system where carbon pz states mix with the nickel d states causing a gap to form at the K point, destroying the Dirac point and conical dispersion.