Fermi Level Pinning and Orbital Polarization Effects in Molecular Junctions: The Role of Metal Induced Gap States

Abstract

Understanding the alignment of molecular orbitals and corresponding transmission peaks with respect to the Fermi level of the electrodes is a major challenge in the fi eld of molecular electronics. In order to design functional devices, it is of utmost importance to assess whether controlled changes in the electronic structure of isolated compounds are preserved once they are inserted in the molecular junctions. Here, light is shed on this central issue by performing density functional theory calculations on junctions including diarylethene-based molecules. It is demonstrated that the chemical potential equalization principle allows to rationalize the existence or not of a Fermi level pinning (i.e., same alignment in spite of a varying ionization potential in the isolated compounds), pointing to the essential role played by metal induced gap states (MIGS). It is further evidenced that the degree of level pinning is intimately linked to the degree of orbital polarization when a bias is applied between the two electrodes.

Impact of Anchoring Groups on Ballistic Transport: Single Molecule vs Monolayer Junctions

“…Figure 2 a. This finding is in agreement with several single molecule transport measurements, 69 − 72 where methyl spacers were shown to reduce the conductance by several orders of magnitude. 69 Similarly, Danilov et al 71 suggested a change of the transport mechanism from strong coupling with coherent tunneling for (−S) to weak coupling with sequential tunneling and Coulomb blockade behavior for (−CH 2 S) due to the insertion of the CH 2 spacer.…”

Understanding the Properties of Tailor-Made Self-Assembled Monolayers with Embedded Dipole Moments for Interface Engineering

“… , This indicates superior electric transport properties of the PmP-up and PPm-down monolayers, making these SAMs particularly promising for applications in organic electronics and photovoltaic devices. Such a stronger coupling in the absence of a methylene spacer has been found in various experiments , and is also consistent with the massively broadened transmission functions calculated for thiol-bonded vs methylthiol-bonded SAMs. , …”

Molecular Junctions: Control of the Energy Gap Achieved by a Pinning Effect

“…It is clear that the aim here is to screen the electric field, for both polarities. This trend was also observed in our previous work for a flat conjugated molecule with symmetric thiol anchoring groups. The screening concavity is actually more marked for the negative bias, which is well correlated with a lower γ efficiency.…”