Charge Carrier Injection Electroluminescence with CO-Functionalized Tips on Single Molecular Emitters

Abstract

We investigate electroluminescence of single molecular emitters on NaCl on Ag (111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force and light detection capabilities. Role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have a dramatic impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbital with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.

Mechano-Optical Switching of a Single Molecule with Doublet Emission

“…In Figure a both CuPc molecules manifest a 16-lobe pattern characteristic of molecules in a dynamic configuration arising from a bistable adsorption geometry. The distinctive STM appearance can be rationalized by the overlapping electronic structures of two chirally rotated adsorption configurations oscillating at a frequency well beyond the STM bandwidth . STML spectrum 1 in Figure e, typical for dynamic CuPc, taken at −2.5 V above a lobe consists of a sharp line at 1.9 eV on a plasmonic background (Spectrum 6).…”

Exciton-Trion Conversion Dynamics in a Single Molecule

“…The characteristic differences between the photon map contrasts generated by p-and s-wave tips can be understood in terms of their specific spatial modulation of the tunneling current. 25 At constant height, X and X + emission lines red-shift a few nm, which is apparent on a spectral map from a diagonal ZnPc cross-section and maps for each exciton at particular wavelengths (see Figure 2b−e and SI Figure S4). This may be explained in terms of photonic Lamb shift by strong coupling between the optical excitations of ZnPc and the highly confined plasmons and has been observed by tipenhanced plasmon absorption and photoluminescence.…”

“…X + map taken for comparison using a metal apex also displays the intensity clearly concentrated at the peripheral benzenes, confirming that the charge injection into them strongly promotes the transient state leading to the formation of trions. The characteristic differences between the photon map contrasts generated by p - and s -wave tips can be understood in terms of their specific spatial modulation of the tunneling current . At constant height, X and X + emission lines red-shift a few nm, which is apparent on a spectral map from a diagonal ZnPc cross-section and maps for each exciton at particular wavelengths (see Figure b–e and SI Figure S4).…”

Exciton–Photonics: From Fundamental Science to Applications

“…182 Generally, the transition dipole moment of the molecules is assumed to be a point dipole moment (PDM) in nature. Using tip-enhanced photoluminescence (TEPL), ZnPc molecules can be imaged up to sub-nanometer resolution (∼8 Å) 182,186,187 and nanobubbles in TMDCs up to 15 nm 188,189 with a plasmonic (Ag or Au) tip.…”