Effect of a Dielectric Spacer on Electronic and Electromagnetic Interactions at Play in Molecular Exciton Decay at Surfaces and in Plasmonic Gaps

Abstract

The deposition of individual molecules, molecular networks, and molecular layers at surfaces is at the core of surface reactivity, energy harvesting, molecular electronics, and (single) photon sources. Yet, strong adsorbate−substrate interaction on metallic surfaces quenches the excited molecular states and harms many practical applications. Here, we theoretically address the role of a NaCl ionic crystal spacer layer in decoupling an adsorbate from the substrate and therefore changing the interplay between the competing decay channels of an excited molecule driven by electronic and electromagnetic interactions. A quantitative assessment of the corresponding decay rates allows us to establish the minimum thickness of the spacer required for the observation of molecular luminescence from the junction of a scanning tunneling microscope. Our work provides a solid quantitative theoretical basis relevant for several fields of nanotechnology where engineering of ionic crystal spacers allows for adsorbate charge manipulation, reactivity, and photon emission in nanocavities.

Adsorption properties of a paracyclophane molecule on NaCl/Au surfaces: a first-principles study

“…19 These factors could also affect the electron transport, electroluminescence, self-assembly, and magnetic properties of the molecule. 20,21 To this end, a detailed understanding of the interactions between adsorbed molecules and the decoupling substrate could supply valuable information for better control of the molecular properties.…”

Single-Molecule Time-Resolved Spectroscopy in a Tunable STM Nanocavity

“…11,14,19−22 In the μPL (far-field) experiments performed by irradiating these samples directly with a focused laser beam of 1.96 eV energy photons (schematically depicted in Figure 1a), we observe fluorescence signals only from the systems with NaCl decoupling layers, whereas the molecules on bare Ag(111) do not contribute (Figure S1) because of nonradiative quenching of the excitations to the substrate. 23 The MgPc signature in μPL appears as a single emission line at 1.90 eV with 4 meV full width at half-maximum (see Figure 1c) and can be attributed to the transition from the excited state to the ground state. In the case of ZnPc, a peak with a similar line width is observed at 1.91 eV, and another much narrower peak at a lower energy of 1.88 eV appears.…”

“…36 On the other hand, the contribution with a shorter exciton lifetime is possibly associated with molecules on 2 ML NaCl, where a stronger nonradiative quenching is expected to limit the lifetime. 23 The shorter-lifetime contribution to the total emission is significantly weaker in comparison with the longer-lifetime yield. The assignment of the dominant components of the signal to the molecules on 2−3 ML NaCl is supported by a control experiment performed on molecular aggregates (see Supporting Note 3).…”

“…For the PL experiments, we prepared systems with ultralow concentrations of MgPc and ZnPc species adsorbed on 2–3 ML NaCl/Ag(111) and on clean Ag(111). The morphology of the as-prepared NaCl/Ag(111) substrate consists of three different major landscapes, i.e., the 3 ML and 2 ML NaCl islands and the bare Ag(111). ,,− In the μPL (far-field) experiments performed by irradiating these samples directly with a focused laser beam of 1.96 eV energy photons (schematically depicted in Figure a), we observe fluorescence signals only from the systems with NaCl decoupling layers, whereas the molecules on bare Ag(111) do not contribute (Figure S1) because of nonradiative quenching of the excitations to the substrate . The MgPc signature in μPL appears as a single emission line at 1.90 eV with 4 meV full width at half-maximum (see Figure c) and can be attributed to the transition from the excited state to the ground state.…”

“…11,30 We have not observed any TEPL signal from the molecules adsorbed directly on Ag(111) (see Figure S1) due to the nonradiative quenching of their excited states by the substrate. 23 It has been reported that at very close tip−sample distances, in particular when the molecule is contacted or even lifted up with the tip, the Raman signal 31−34 can be dramatically enhanced on NaCl or bare metal substrates. However, these conditions were deliberately avoided in our study.…”