The ability to capture and confine light in nano-structures in the form of so-called surface plasmons – collective surface charge oscillations at the metal-dielectric interface – is important in various areas of research including electronics and sensing. We are developing new platforms to study the interplay between electronics and plasmonics with the main emphasis on molecular electronic plasmonics. Here, one, or both, electrodes that contact the SAM may be a plasmonic resonator in the form of Ag or Au nanoparticle. The Figure below shows TEM and SEM images of some nanoparticles synthesized in our laboratories.

Tunneling charge transfer plasmons

Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. Quantum tunneling will occur predictively across two closely separated plasmonic resonators, which is indicated by the presence of a plasmon mode, the tunnelling charge transfer plasmon (tCTP). Molecular electronics is interesting in this contexts because the tunneling junction properties can be controlled at the molecular level potentially allowing for control over the plasmonic system.  We reported molecular electronic control over the tCTP mode in SAM-based tunneling junctions consisting of two plasmonic resonators (see Science2014343, 1496-1499.).

Fig 1: Quantum plasmon resonance controlled by molecular tunnel junctions. (a) The schematic of the molecular tunnel junctions consisting of two Ag nanocubes and a self-assembled monolayer between them. Two kinds of molecules (EDT, aliphatic 1,2-ethanedithiolates and BDT, 1,4-benzenedithiolates) with different barrier height were used. An electron beam is used to excite the plasmon mode of the molecular tunnel junction. (b) The EELS spectra of the molecular tunnel junction. The tCTP mode is observed in the infrared frequency range and controlled by the conductivity of the molecules. Because of the low barrier height of BDT molecules, the tCTP peak is blue shifted compared to the case of EDT molecules.

Molecular electronic plasmon sources

We developed an on-chip plasmon source based on molecular tunnel junctions (see Nature Photon. 2016, 10, 274 – 280). We found that plasmons in these junctions that originate from single, diffraction-limited spots, follow power-law distributed photon statistics, and have well-defined polarization orientations which are controlled by the tunneling direction defined by the tilt angle of the SAM or by simply changing the applied bias of the junction. Figure below shows the schematic of the SAM-based junctions with the EGaIn/Ga2O3 top electrode confined in a microfluidic network in contact with a SC12 SAM on template-stripped Au. These junctions excite both localized and propagating SPPs (Fig 2c-d). In the real plane image (Fig 2c), LSPs are characterized by diffraction limited emission spots and the SPPs by the unidirectional and un-diffracted emission spots around the boundary of the junction. In the BFP image (Fig 2d), the SPPs are shown as narrow arcs with specific wavevectors and labelled with the mode I (SPPs along the Au/SAM—Air interface,kSPP =1.01) and II (SPPs along the Au/SAM—PDMS interface, kSPP =1.47). Fig 2e shows the defocused plasmon emission image of Fig 2c which indicates the polarization orientation of the plasmon emission spots. Theoretical calculations confirm that the polarization orientation of the plasmon emission was ~30° with respect to the surface normal and remarkably close to the tilt angle of the SAMs. This relation implies that the plasmon excitation in the STJs occurs along the tilted back bone of the SAM. This observation was further confirmed by changing the tilt angle to ~10° by simply replacing the AuTS by and a AgTS bottom electrode. Using these junctions, plasmons could also be excited in plasmonic waveguides and bias-selective plasmon excitation was achieved with molecular diodes.

Fig 2: Molecular electronic plasmon sources. (a-b) Schematic of the on-chip molecular electronic plasmon sources based on SAM tunnel junctions. Real plane image (c), back-focal plane image (d) and defocused real plane image (e) of the plasmon sources.