Extending the Newns-Anderson model to allow nanotransport studies through molecules with floppy degrees of freedom

Abstract

-The Newns-Anderson model is ubiquitous in studies of the molecular transport in the presence of solvent (outer) reorganization. The present work demonstrates that intramolecular reorganization can also be significant for the transport through molecules with floppy degrees of freedom, for which the Newns-Anderson model can be extended. The expressions of the model parameters deduced from electronic structure calculations for (4, 4')-bipyridine (44BPY) quantitatively differ from those characteristic for outer reorganization due to strong intramolecular anharmonicities. These expressions can be utilized as input in transport calculations for 44BPY-based molecular junctions of experimental interest

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function

“…Lorentzian transmission (also known as the Newns-Anderson model 41,[70][71][72] provides an appropriate framework for analyzing all the experimental data on the transport through CP-AFM molecular junctions based on oligophenylene dithiols reported in this study. As already noted in the introduction, we adopt this model not because of its simplicity but rather because, for these junctions, it has a solid microscopic justification; the dominant role of the HOMO is visualized in Figure 2, which depicts the LUMOs much more distant from the Fermi energy and the (HOMO-1) orbitals are also sufficiently far away.…”

“…(6). Finally, we also mention possible reorganization effects; OPD4 is a molecule with three floppy vibrational degrees of freedom related to the three twisting angles of the relative rotations of the phenyl units.Studying the impact of reorganization of these floppy modes may be of interest, as revealed by the recent studies on bipyridine,51,71,72 although that molecule possesses only one floppy mode.ConclusionIn this paper, we have reported the results of an extensive investigation into transport and transport-related properties of oligophenylene dithiol molecules embedded in CP-AFM metal-molecule-metal junctions. Utilizing molecular species with up to n = 4 phenyl rings and electrodes of metals (Ag, Au, and Pt) having work functions varying across a broad range of ~1.4 eV, we were able to study the impact of n and on relevant junction properties.Charge transport occurs via non-resonant tunneling, as indicated by the exponential increase with n of the low-bias resistance R, the property most strongly affected by the molecular length.…”

A quantum chemical study from a molecular transport perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions

“…Table 1, 2, and 3 collect results on the lowest electron attachment energies for 4,4'-bipyridine (C 6 H 4 N 2 , 44BPY), 1,4-dicyanobenzene (NC-C 6 H 4 -CN, BDCN), and 4,4'-dicyano-1,1'-biphenyl (NC-(C 6 H 4 ) 2 -CN, 2BDCN). Table 1 presents very detailed numerical results for 44BPY, a showcase molecule 13,15,[46][47][48] , in order to illustrate the main issues, which we have encountered in calculations of electroaffinities for molecules utilized in molecular electronics. The EA-EOM-CCSD method predicts a weakly bound anion 44BPY •− (EA > ∼ 0).…”

High-resolution electrochemical STM of redox metalloproteins

“…The nature of the model yields normalised Gaussian distributions of width √ 2kT λ where λ is the reorganisation energy. The process of electron transfer might thus be viewed [26][27][28][29] as proceeding (for example) through an empty molecular level which momentarily enters the energy window between µ 1 and µ 2 . While the level is in this energy window, a degree of (coherent or incoherent) tunnelling can proceed, but when reorganisation of the protein occurs from the effect of the now (at least partially) reduced level, the level will relax (by an amount 2λ) to take it below the energy window.…”

“…Such methods should be extended to a broader range of proteins, linkers, and substrates in future. It is also worth noting that, if a protein molecule is intimately (covalently) linked to both the STM tip and substrate, strong electronic coupling of molecular levels to the continuum of metallic levels may also be important, as suggested by a number of authors [37,39,29,40].…”