Mechano-Optical Switching of a Single Molecule with Doublet Emission

Abstract

The ability to control the emission from single-molecule quantum emitters is an important step toward their implementation in optoelectronic technology. Phthalocyanine and derived metal complexes on thin insulating layers studied by scanning tunneling microscope-induced luminescence (STML) offer an excellent playground for tuning their excitonic and electronic states by Coulomb interaction and to showcase their high environmental sensitivity. Copper phthalocyanine (CuPc) has an open-shell electronic structure, and its lowest-energy exciton is a doublet, which brings interesting prospects in its application for optospintronic devices. Here, we demonstrate that the excitonic state of a single CuPc molecule can be reproducibly switched by atomic-scale manipulations permitting precise positioning of the molecule on the NaCl ionic crystal lattice. Using a combination of STML, AFM, and ab initio calculations, we show the modulation of electronic and optical bandgaps and the exciton binding energy in CuPc by tens of meV. We explain this effect by spatially dependent Coulomb interaction occurring at the molecule-insulator interface, which tunes the local dielectric environment of the emitter.

Keywords: AFM; CuPc; NaCl; STML; doublet; exciton; switching.

Figure 1. An experiment probing the manipulation of the adsorption geometry and its influence on the STML spectra.

(a-c) sequence of STM constant-current images of two CuPc molecules at 2ML-NaCl/Ag(111), subsequently switched from dynamic to steady state. Image parameters: 6 x 6 nm, -2.5 V, 1 pA (d) submolecular-resolution AFM image of the area in (c), taken in constant height mode with a CO-tip, image parameters 4.2 x 3.2 nm, 10 mV. (e) STML spectra of the molecules in (a-c), numbered correspondingly (1-5) and reference spectrum taken on a bare NaCl (6). Acquisition parameters for the spectra were -2.5 V, 50 pA, 60 s.

Figure 2. Analysis of the adsorption geometry in relation to the STML fingerprint.

(a) STM constant-current image of CuPc molecules in the steady and dynamic configurations and their registration with the NaCl lattice obtained with a CO-functionalized tip. Parameters 7.5 x 3.2 nm, -2.3 V, 1 pA. (b) Representative STML fingerprints of the dynamic (green) and steady (violet) configuration on 3 ML NaCl. (c-f) computationally optimized theoretical models of the steady (c,e) and dynamic geometries (d,f). (g) AFM constant-height frequency shift map with two different tip heights enabling submolecular resolution on the CuPc and atomic resolution on the NaCl substrate for registration. Parameters 1.7 x 2.8 nm, 25 mV.

Figure 3. Theoretical analyses of the excited and ground states and the impact of the substrate.

(a) Scheme of the simulated orbital energy level reordering upon a transfer of an electron (marked by red color) from HOMO to LUMO within an isolated CuPc molecule, showing the occupied and virtual levels near the Fermi level (E_F). The ground state is denoted as D₀, excited state as D₁. Corresponding orbital geometries are depicted in the same order for clarity. (b-g) Calculated charge redistribution isosurface plots and profiles, showing the depletion (blue) and accumulation (red) (at ±0.003 e^-/Å, respectively) of electrons in the CuPc / 2ML-NaCl systems in the dynamic (b,c) and steady (e,f) configuration. (d,g) Corresponding profiles of the electron density, obtained by integration in the directions parallel to the NaCl surface. Plots are superimposed onto the atomistic model for orientation.

Figure 4. Determination of the transport gaps by differential conductance spectroscopy.

(a) dI/dV spectra of one CuPc / 3ML-NaCl molecule in a dynamic state (green) and after stabilization (violet), taken at the positions above the molecule lobes, marked by arrows in the STM constant-current images in the insets. The spectra have been taken on the molecular lobes at various current setpoints (20 - 90 pA at -2.5 V). Normalized dI/dV plotted in logarithmic scale, corresponding to the HOMO (b) and LUMO (c).