Unraveling the Failure Modes of Molecular Diodes: The Importance of the Monolayer Formation Protocol and Anchoring Group to Minimize Leakage Currents

Abstract

The failure modes of molecular junctions have been only sporadically investigated, but they are important to understand so they can be prevented and molecular junctions can be optimized. Although it is commonly accepted that the performance and electrical characteristics of molecular tunneling junctions depend strongly on the supramolecular structure of the molecules inside the junctions, how sensitive molecular junctions are to small supramolecular changes and the factors that cause disorder are poorly understood for most types of molecular junctions. This paper shows that the performance of a large-area molecular diode depends on the choice of the anchoring group and solvent used for the formation of the self-assembled monolayer (SAM) in which both affect the surface coverage and the occurrence of disordered SAM domains. As a result, the rectification ratio was reduced by more than a factor of 10 across a series of molecular diodes, containing a diphenylacetylene backbone and a ferrocenyl terminus. The diodes failed because of leakage currents flowing across defective parts in the SAM lowering the rectification ratios, while the yield in working junctions or the electronic structure of the SAMs was remarkably insensitive to defects inside the SAMs. Thus, a judicious choice of anchoring group and solvent used in the SAM formation process can result in a 10-fold improvement in the performance of molecular diodes.

Thermal Stability of Alkanethiolate and Aromatic Thiolate Self-Assembled Monolayers on Au(111): An X-ray Photoelectron Spectroscopy Study

“…In addition, a binding energy of 161.1 eV manifests a noticeably stronger involvement of sulfur into the bonding with gold, correlating well with the atomically adsorbed sulfur model. Alternative assignments, which can be found in the literature, such as differently bound and laying-down molecules, − appear less realistic to us.…”

Amine-Anchored Aromatic Self-Assembled Monolayer Junction: Structure and Electric Transport Properties

“…The structure and order of SH-SAM are affected by the type of solvent, ,− which also has a more pronounced impact on the growth of NH 2 -SAM with weaker bonding. Therefore, we kept the concentration of amine molecules the same (0.50 mM) and studied the influence of the solvents (toluene, THF, hexane, DMF, acetone, and ethanol) on the quality of NH 2 -SAM.…”

“…An anchoring group is the pre-requested group for the assembly process of a SAM, governing the stability and structure of the SAM, , the key player for the across interface charge transport in SAM-based molecular devices, for example, it controls the molecular junction interface, determines the electron transport properties, − and even installs new functions in molecular junctions. ,, The most widely used anchor group for SAM is thiol, due to its robust metal–S covalent bond and good electronic coupling between molecules and electrode interfaces . The thiol SAM and charge transport in junctions have been extensively explored, − ,− and the effects of thiol group on the transport, such as through energy barriers and by coupling, have been extensively studied. ,− However, thiol-based molecular junctions face serious problems such as a broad conductance distribution due to large variability in Au–S binding geometry with very flexible contact angles or chain/plane rotation − and strong deformation of gold electrodes ,, as revealed by a single molecular junction study.…”

Solvent‐ and Temperature‐Dependent Assembly in Monolayer Films of a Ferrocene‐Naphthyridine Hybrid on HOPG

“…[15] The influence of solvent in controlling the quality and performance of molecular diode based on SAM of Fc derivative has been demonstrated. [16] Solvent controlled selection of adlayer pattern has been also shown for Fc derivatives at solid-air interface. [17] The studies related to the inclusion of conformational flexibility in the formation of adlayer patterns at the solid-air interface are scarce.…”

“…The microscopic arrangement of Fc derivatives is controlled at different types of solid‐liquid interfaces [15] . The influence of solvent in controlling the quality and performance of molecular diode based on SAM of Fc derivative has been demonstrated [16] . Solvent controlled selection of adlayer pattern has been also shown for Fc derivatives at solid‐air interface [17] .…”