Atomically Reconfigurable Single-Molecule Optoelectronics

Abstract

Deterministic control of excitonic properties is key to advancing nanoscale optoelectronic and quantum technologies and to understanding diverse physical, optical, chemical, and biological phenomena. At the molecular scale, these properties can be tuned through chemical modification, local-environment influence or charge-state manipulation. Yet, direct control of a molecule's transition dipole moment and its resulting light emission via atomic-scale structural modification has remained elusive. Here, using scanning tunnelling microscopy-induced luminescence, we show that a single structural parameter-the vertical displacement of the central metal atom in a planar phthalocyanine molecule on a decoupling layer-enables active tuning of the transition dipole, allowing either suppression or enhancement of emission. Exploiting this control, we realized a tunable homodimer switchable among three optical states: non-emissive, single-molecule-like emissive, and coupled states exhibiting subradiant and superradiant modes, directly revealing intermolecular dipole-dipole coupling. We further demonstrate a heterodimer in which resonant energy transfer can be turned on or off simply by controlling the acceptor's transition dipole moment. These findings not only establish atomic-scale displacement as a general strategy for optical molecular switching, but also demonstrate the reconfigurable engineering of excitonic interactions within molecular assemblies.

Figures (4)

• Figure 1: Single-molecule electroluminescence through plasmon enhancement.a, Schematic of STML experiment from different configurations of SnPc molecules. b, STM image depicting isolated single SnPc and ZnPc molecules adsorbed on 2M NaCl island on bare Ag(111) (image size: 12 $\times$ 8 nm$^2$; scanning parameters:$V = -2.3$ V and $I$ = 6 pA). Insets: showing molecular structure of ZnPc and SnPc. c, STML spectra acquired at the positions marked with the ‘colored dots’ in B ($V = -2.3$ V, $I$ = 20 pA, $t = 30$ s for single-molecules and $t = 120$ s for plasmon).
• Figure 2: Enhancing and suppressing the single-molecule light emission by the dipole engineering.a-d, STM images depicting two isolated single SnPc molecules on 2M NaCl (image size: 9 $\times$ 3 nm$^2$ ; scanning parameters:$V = 1$ V and $I$ = 6 pA). e, Bias spectroscopy of SnPc up and SnPc down acquired at the positions marked with the ‘colored dots’ in c, d. f, Schematic energy level diagram of electron redistribution in SnPc up to SnPc down, and the DFT calculated orbitals. g, STML spectra of SnPc up and SnPc down acquired at the positions marked with the ‘colored dots’ in c, d ($V = -2.3$ V, $I$ = 20 pA, and $t = 30$ s). h, i, Calculated HOMO→LUMO+1 and HOMO→LUMO transition charge densities in the SnPc up configuration. j, Calculated HOMO→LUMO transition charge density in the SnPc down configuration.
• Figure 3: Tunable coherent intermolecular dipole-dipole coupling SnPc-SnPc dimer system. STM images revealing a SnPc dimer in three configurations on 2 ML NaCl (image size: 4.5 $\times$ 4.5 nm$^2$; scanning parameters:$V = 1$ V and $I$ = 6 pA). a, SnPc down-SnPc down. b, SnPc down-SnPc up. c, SnPc up-SnPc up. d, STML spectra of SnPc dimer acquired at the positions marked with the ‘colored dots’ with conditions ($V = -2.3$ V, $I$ = 20 pA, and $t = 30$ s) SnPc up monomer light emission (Green) is also shown for comparison. e, Exciton band energy diagram of a molecular dimer. $E_{\text{mon1},e}$ and $E_{\text{mon2},e}$ excited-state energy of two isolated monomer; $E_{\text{mon},g}$ ground-state energy of an of an isolated monomer;$\Delta E_{\text{mon}}$ optical transition energy for an isolated monomer; $E_{\text{dim},g}$ ground-state energy of the dimer; $E_{\text{dim},g}^{1}$ and $E_{\text{dim},g}^{2}$ low-lying and high-lying excited-state energy of the dimer for a coupling modes with opposite phases; J exciton coupling strength. For simplicity we consider $\epsilon_{\mathrm{ex}}$ and $V$ are the same for both SnPc up monomers. f, Comparison between the measured and calculated STML spectrum of SnPc up-SnPc up dimer. Where the parameters are $\epsilon_{\mathrm{ex}} = 1.757~\mathrm{eV}$, $\epsilon_{p} = 1.9~\mathrm{eV}$, $V = 32.5~\mathrm{meV}$, and $J = 13.5~\mathrm{meV}$. g, Comparison between the calculated STML spectra of SnPc up monomer and SnPc up-SnPc up dimer.
• Figure 4: Tunable Energy transfer between donor ZnPc and acceptor SnPc molecules in real space. STM images revealing a ZnPc-SnPc dimer in two configurations on 2 ML NaCl (image size: 5 $\times$ 4 nm$^2$; scanning parameters:$V = 1$ V and $I$ = 6 pA). a, ZnPc-SnPc down dimer. b, ZnPc-SnPc up dimer. c, STML spectra of SnPc dimer acquired at the positions marked with the ‘colored dots’ with conditions ($V = -2.3$ V, $I = 20$ pA, $t = 60$ s for SnPc excitation and $V = -2.5$ V, $I = 100$ pA, $t = 30$ s for ZnPc and plasmon excitation ). d, Schematics of showing no energy transfer from ZnPc-SnPc down dimer configuration, if excitation is from ZnPc side. e, Schematics of showing energy transfer from ZnPc-SnPc up dimer configuration, if excitation is from ZnPc side. f, Schematics of showing no energy transfer from ZnPc-SnPc up dimer configuration, if excitation is from SnPc side.