Nonequilibrium hot carriers formed near the interfaces of semiconductors or metals play a crucial role in chemical catalysis and optoelectronic processes. In addition to excitation by optical illumination, such hot carriers can also be generated due to electron tunnelling, a quantum-mechanical effect which allows the transport of electrons across a nanoscale junction between two conducting electrodes. Here we study electron tunnelling effects in electrically-driven plasmonic nanorod metamaterials containing up to 10^11 tunnel junctions per square centimeters and show that the generation of hot electrons makes the tunnel junctions highly reactive, facilitating strongly confined chemical reactions which can in turn modulate the tunnelling processes.
To form nanometer scale gaps for electron tunneling, an overgrown plasmonic nanorod metamaterial (Au nanorod assembly electrochemically grown in a substrate-supported, thin-film porous aluminum oxide template) was first ion-milled at an oblique angle (75º with respect to the normal to the sample surface) to make the embedded Au nanorods ~1 nm shorter than surrounding aluminum oxide. Then, a droplet of eutectic gallium and indium (EGaIn) was added onto the sample surface working as an upper electrode, forming millions of ~1-nm air gaps between the Au nanorod tips and the EGaIn. When a low voltage (2.5 V) was applied between the Au nanorods and the EGaIn, a strong light emission was observed from the substrate side (~ 4 mm2 in size) due to the radiative decay of plasmonic modes excited in the nanorod metamaterial by inelastic tunnelling electrons. With an increasing applied bias, the intensity of emission increases gradually, and is accompanied by a blue-shift of the cutoff wavelength (the energies of the emitted photons are always less than the energy of tunnelling electrons).
Apart from the excitation of plasmons and photons by the inelastic tunnelling electrons, hot electrons are generated simultaneously in the tips of Au nanorods by the elastic tunnelling electrons while leaving hot holes in the EGaIn. The large flux of energetic hot electrons makes the otherwise inert tunnel junctions highly reactive, facilitating the oxidation and reduction reactions in the junctions involving O2 and H2 molecules, respectively. These reactions are monitored either optically by changes in the intensity of light emission (~50%) resulting from the radiative decay of tunnelling-generated surface plasmons, or electrically via tunnelling current variations (~10%).
Electrically-driven plasmonic nanorod metamaterial with reactive tunnel junctions comprises a fertile platform merging photonics, electronics and chemistry at the nanoscale, opening up opportunities for developing electron tunnelling-based devices, such as sensors, light sources, nanoreactors, modulators and photodetectors.