Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi level pinning at the molecule-metal interface

TL;DR

This study explores how molecular structure influences rectification in silicon-based diodes with self-assembled monolayers, revealing Fermi level pinning at the molecule-metal interface as a key factor affecting device behavior.

Contribution

It demonstrates that Fermi level pinning dominates the rectification properties regardless of pi-group variations, extending understanding of molecular diode mechanisms.

Findings

Rectification observed for all pi-groups tested.

Fermi level pinning at the molecule-metal interface limits rectification dependence on pi-group nature.

Analytical model extracts molecular orbital energies in resonance with electrode Fermi levels.

Abstract

We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We investigate the structure-performance relationships of these molecular devices and we examine to what extent the nature of the pi end-group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. For all the pi-groups investigated here, we observe rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (S. Lenfant et al., Nano Letters 3, 741 (2003)).The experimental current-voltage curves are analyzed with a simple analytical model, from which we extract the energy position of the molecular orbital of the pi-group in resonance with the Fermi energy of the electrodes. We report the experimental studies of the band lineup in these silicon/alkyl-pi conjugated molecule/metal junctions. We conclude that Fermi level pinning at the pi-group/metal interface is mainly responsible for the observed absence of dependence of the rectification effect on the nature of the pi-groups, even though they were chosen to have significant variations in their electronic molecular orbitals