Tuning Overbias Plasmon Energy and Intensity in Molecular Plasmonic Tunneling Junctions by Atomic Polarizability

Abstract

Plasmon excitation in molecular tunnel junctions is interesting because the plasmonic properties of the device can be, in principle, controlled by varying the chemical structure of the molecules. The plasmon energy of the excited plasmons usually follows the quantum cutoff law, but frequently overbias plasmon energy has been observed, which can be explained by quantum shot noise, multielectron processes, or hot carrier models. So far, clear correlations between molecular structure and the plasmon energy have not been reported. Here, we introduce halogenated molecules (HS(CH₂)₁₂X, with X = H, F, Cl, Br, or I) with polarizable terminal atoms as the tunnel barriers and demonstrate molecular control over both the excited plasmon intensity and energy for a given applied voltage. As the polarizability of the terminal atom increases, the tunnel barrier height decreases, resulting in an increase in the tunneling current and the plasmon intensity without changing the tunneling barrier width. We also show that the plasmon energy is controlled by the electrostatic potential drop at the molecule-electrode interface, which depends on the polarizability of the terminal atom and the metal electrode material (Ag, Au, or Pt). Our results give new insights in the relation between molecular structure, electronic structure of the molecular junction, and the plasmonic properties which are important for the development of molecular scale plasmonic-electronic devices.

Figure 1

(a) Schematic representation of the STJs with SC₁₂X (X = H, F, Cl, Br, or I) SAMs on different metal substrates (M = Ag, Au, or Pt). (b) Energy level diagram of the Pt-SC₁₂X//EGaIn junctions with zero bias, where X is hydrogen or halogen, as stated in panel a. The shapes of the tunnel barriers are indicated by the dashed lines which change as a function of X. (c) The shape of the electrostatic potential profiles across the Pt-SC₁₂X//EGaIn junctions (EGaIn stands for eutectic alloy of gallium and indium) with V_Bias to the EGaIn top electrode (in all of our experiments, the bottom electrode was grounded). (d) Interface energetics of the SC₁₂-bottom electrode interface. Φ_M and Φ_M-SAM are work functions of the bottom electrodes before and after SAM formation. The black arrows indicate the relative magnitude of the molecular interface dipoles.

Figure 2

(a) Plot of ΔΦ for SC₁₂H SAM on Ag, Au, and Pt. (b) Plot of ΔΦ for SC₁₂X (X = F, Cl, Br, and I) SAMs on Pt. (c) Plot of μ_bond for SC₁₂H SAM on Ag, Au, and Pt. (d) Plot of μ_mol,⊥ for SC₁₂X (X = F, Cl, Br, and I) SAMs on Pt.

Figure 3

J(V) characterization of the STJs with SC₁₂X SAMs at ±1.8 V on a Pt substrate. The error bars stand for the log-standard deviations.

Figure 4

Representative real plane emission images at −1.8 V with SC₁₂H (a), SC₁₂F (b), SC₁₂Cl (c), SC₁₂Br (d), and SC₁₂I (e) SAMs. (f) log₁₀ |J| and log₁₀ |I| at −1.8 V plot as a function of . The dashed lines are linear fits to the data, confirming the dependency of the light emission intensity and tunneling current on the barrier height.

Figure 5

(a) Light emission spectra recorded at −2.0 V bias on Pt substrates with different SAMs. (b) Plot of E_c as a function of voltage with SC₁₂X STJs. The dashed line in b indicates the quantum cutoff with hv = eV. (c, d) The correlation between the reduced photon energy ΔE and ε_r at different biases (c) and the average from the three biases (d), with error bars from the standard deviation. The lines are linear fits to data. These correlations confirm that α of the SAM terminal atoms changes the electrostatic potential profile of the junctions, tuning the excited plasmon energy.

Figure 6

(a) Light emission spectra recorded on different bottom electrodes (Au, Ag, and Pt) at −2.0 V bias with SC₁₂H junctions. (b) Emission spectra of SC₁₂H junction on Pt electrode collected at ±2.3 V.