Schematic showing the three working directions of molecular electronic plasmonics.
Schematic showing the three working directions of molecular electronic plasmonics.

Individual molecules – or single layers of molecules – can act as components in electronic devices. Scientists have been exploring the concept of molecular electronics for decades in a bid to shrink devices to the nanoscale, but the effort is also opening up new functionalities and a better understanding of the behavior of electrons.

Now researchers are bringing molecular electronics ideas to the burgeoning field of plasmonics [T. Wang and C. A. Nijhius. Applied Materials Today 3 (2016) 73]. Surface plasmons are the collective oscillations of electrons at the interface between a metal and dielectric, and can confine electromagnetic fields such as light. As such, plasmonics enables optical and electronic functions at the nanoscale to be combined on the same device.

“Molecular electronic plasmonics bridges the fields of molecular electronics and plasmonics,” explains Christian A. Nijhuis of the National University of Singapore. “[It] utilizes the electronic properties of molecules to control and modulate surface plasmons.”

Building a molecular plasmonic system is relatively simple in theory: only an organic molecule and a plasmonic material such as a noble metal like gold or silver is required.

“The typical molecular electronic plasmonic system is essentially a molecular tunnel junction with the molecules sandwiched between two metallic electrodes that are plasmonic,” explains Nijhuis. “The molecule can be a single molecule or a self-assembled monolayer (SAM) and the metallic electrodes can be flat metallic surfaces or nanostructures such as nanoparticles.”

Molecular electronic plasmonics is making progress on three major fronts, says Nijhius. In the first of these, molecular electronics is being applied to quantum plasmonics. In this type of system, two closely-spaced plasmonic metal electrodes are linked by a SAM-based molecular tunnel junction. Plasmons are excited at the interface by light or an electron beam, inducing an electric field that drives quantum mechanical tunneling between the two electrodes. By varying the molecule used, the frequency of the output can be controlled.

Molecular junctions can also be used to excite plasmons, the properties of which again depend upon the molecule used. This type of plasmon excitation has been demonstrated inside a scanning tunneling microscope and more recently in actual on-chip devices. Conversely, the same type of molecular junction can also be used to detect plasmons.

“Molecular electronic plasmonics is important for both fundamental studies and practical applications,” Nijhuis told Materials Today.

It enables scientists to study the electrical properties of molecules and the mechanisms of charge transport, as well as explore quantum plasmon theories, he believes.

If it proves possible to create on-chip circuits of molecular electronic plasmon sources and detectors, there is the potential to create devices that can operate at very high frequencies.