Charge Transport and Directed Assembly of Nano-confined Porphyrin Molecules on Surfaces: Towards Molecular and Organic Electronics

Abstract

Presently, the aim of molecular/organic electronics is to incorporate organic materials into conventional silicon-based devices and complimentary metal-oxide semiconductor (CMOS) technology to expand the functionality of electronic devices, exploit bottom-up fabrication techniques, and to achieve nanoscopic dimensions. However, the success of molecular/organic electronics depends upon tailoring the function of devices by intentionally modifying the properties of the molecular components starting at the single molecule level and continuing to fabricate meaningful molecular structures.

Here, porphyrins were used as a platform to study the structure/electronic property relationship because they are candidates for molecule-containing devices and have vast chemical tunability. The structure of these porphyrins consists of a macrocycle attached to an alkanethiol tether that bonds to Au surfaces and were assembled within a dodecanethiol (C12) self-assembled monolayer (SAM). The physical morphology of the mixed SAM was examined and manipulated by Atomic Force Microscopy (AFM). The molecular-level morphology and the electronic properties were examined by Scanning Tunneling Microscopy (STM).

The porphyrin chemical structure was systematically perturbed and allowed to randomly self-assemble within a C12 SAM. Contrary to original expectations of molecular/organic electronics, chemical structure perturbations did not lead to variations in the electronic properties for single porphyrin molecules with consistent indications of tunneling dominated by the alkanethiol tether. To recover the ability to tune the electronic properties, assemblies large enough to stabilize charge were formed with indications of Coulomb Blockade. However, increasing variations in the relative conductance were observed for increasing assembly size. To interrogate this, the electronic properties were examined as a function of assembly duration and a stark difference in the current-voltage (I-V) characteristics attributed to subtle changes in nearest neighbor interactions.

To utilize the electronic properties of porphyrins found here in a pre-defined manner, a facile two-step method was developed to direct porphyrin assembly. First, the surface tether, pentanedithiol (C5DT), was controllably attached to the Au(111) surface in pre-defined geometries. Second, porphyrins were selectively attached to the C5DT by “click” chemistry. Employing this method, porphyrins built-up into a pi-stacked hierarchy on the C5DT domains and displayed the desired I-V characteristics.