Article Title: Inverse Rectification in Donor-Acceptor Molecular Heterojunctions

Authors:  Shannon K. Yee, Jibin Sun, Pierre Darancet, T. Don Tilley, Arun Majumdar, Jeffrey B. Neaton,  and Rachel A. Segalman

Journal: ACS Nano

Affiliation: University of California- Berkeley: Department of Mechanical Engineering, Department of Chemical and Biomolecular Engineering, Department of Chemistry. Molecular Foundry at Lawrence Berkeley National Lab. U.S. Department of Energy, ARPA-E.

You hear about silicon chips in cells phones and computers all the time- they work on the premise of “diodes”, which let electric current flow in one direction much better than the other. In silicon, diodes are made by putting in atomic impurities that alter the energy levels in adjacent regions to either donate or accept electrons (the one with the lower energy levels accepts electrons). When a voltage is applied, it moves the energy levels closer together, so it’s easier for electrons to move from the acceptor to the donor (from right to left in the image below, although this is a little counterintuitive as far as labeling goes). If you consider yourself an electron on the right side, who has to climb steps to the left, the steeper the steps are , the more challenging it is for you to flow in that direction. When current can flow in one direction much better than in the other direction, it’s called “rectification”.

In nanoscale electronics, sometimes things work differently, as Yee et al. found in this work. The authors made a molecular version of a diode, with a short, conjugated donor part (red) and a short, conjugated acceptor part (blue) of the molecule. The two regions are separated by a molecular unit (green) that keeps the two pieces physically close (via covalent bonds) but electronically separate. On either end are thiol (-S-H) groups. Thiols attach semi-covalently to gold (using hard/soft acid base theory thiolates are soft bases and gold is a soft acid- since they are both soft, they have good affinity). This means that this donor-acceptor molecule can be placed between two gold electrodes and the electronic behavior can be measured one molecule at a time!

Yee et al. used a clever sequence of steps to disperse these donor-acceptor molecules sparsely on a gold surface in a consistent orientation (acceptor bound to the surface) and then exposed the system to gold nanoparticles, which attached to the free thiol on the donor side of the molecule sticking up from the surface. A gold-coated (conductive) tip of an atomic force microscope was then used to contact the gold nanoparticle. By connecting the tip to the surface through a circuit, a voltage can be applied and the current can be measured through the molecule.

The data show that applying a negative voltage to the tip results in more current (electrons flowing left to right) than applying a positive voltage to the tip (electrons flowing right to left). This is surprising because it’s the exact reverse of what happens in a larger-scale system (in those systems, making the energy difference between the two materials smaller means electrons can flow much more easily through them). The authors propose (with support from some spectroscopy and some basic density functional theory calculations) that electrons moves through this molecule by tunneling through the molecule. The tunneling is facilitated by the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) being between the energy levels of the electrodes when a voltage is applied (yellow lines in the embedded picture).

This work is further proof that as electronics get smaller, chemists become more important! Molecules and methods of this sort can be used in the future to probe electronic behavior in molecules and at the interfaces between organic components. This type of work is increasingly important as inexpensive, solution-processable organic electronics continue to become more industrially valued.