posted on 2019-06-11, 00:00authored byWei Du, Yingmei Han, Hongting Hu, Hong-Son Chu, Harshini V. Annadata, Tao Wang, Nikodem Tomczak, Christian A. Nijhuis
Directional
excitation of surface plasmon polaritons (SPPs) by
electrical means is important for the integration of plasmonics with
molecular electronics or steering signals toward other components.
We report electrically driven SPP sources based on quantum mechanical
tunneling across molecular double-barrier junctions, where the tunneling
pathway is defined by the molecules’ chemical structure as
well as by their tilt angle with respect to the surface normal. Self-assembled
monolayers of S(CH2)nBPh (BPh
= biphenyl, n = 1–7) on Au, where the alkyl
chain and the BPh units define two distinct tunnel barriers in series,
were used to demonstrate and control the geometrical effects. The
tilt angle of the BPh unit with respect to the surface normal depends
on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle
of the alkyl chain is fixed at 30° and independent of n. For values of n = 1–3, SPPs are
directionally launched via directional tunneling through the BPh units.
For values of n > 3, tunneling along the alkyl
chain
dominates the SPP excitation. Molecular level control of directionally
launching SPPs is achieved without requiring additional on-chip optical
elements, such as antennas, or external elements, such as light sources.
Using the molecular tunneling junctions, we provide the first direct
experimental demonstration of molecular double-barrier tunneling junctions.