Abhishek Grewal
This thesis investigates novel single-molecule luminescence phenomena at their inherent, sub-molecular length scale. The microscopic understanding of luminescence processes will be crucial for the continued improvement of organic optoelectronic and semiconductor devices.In this work, a scanning tunneling microscope (STM) is combined with optical spectroscopy and Hanbury Brown and Twiss interferometry to image topography, electronically characterize, and analyze the vibrational mode excitations and emission statistics, all at the atomic scale. Moreover, the precise STM tip positioning allows tuning the coupling between the molecular exciton and the tip-induced plasmon with sub-molecular spatial control, which is key to the decay rate enhancement of molecular phosphors.A prototypical platinum phthalocyanine (PtPc) molecule, decoupled from a metal substrate by a thin film of NaCl, provides a model platform for examining STM-induced bipolar fluorescence, phosphorescence, and energy upconversion electroluminescence (UCEL). First, an STM investigation of only the ionic insulator NaCl demonstrates that both valence and conduction bands are of anionic Cl- character. This is relevant for the coupling between the molecule and the metal substrate. Theoretical considerations reveal a dominant contribution from low-lying molecular orbitals under certain conditions.Voltage and polarity of fluorescence onsets can be controlled using different metal substrates because their different work functions tune the energy alignment of the molecular orbitals with respect to the substrate Fermi level. For PtPc atop an Au(111) substrate, fluorescence is observed at both voltage polarities, even under tunneling conditions where UCEL occurs. This remarkable behavior allows the testing of different emission mechanisms and reveals new general guidelines for designing future optoelectronic devices. An analysis of photon emission statistics from a single molecule under UCEL conditions refutes previously proposed excitation mechanisms. It guides theory towards a novel mechanism, briefly introduced in the thesis.PtPc atop NaCl on Ag(111) exhibits fluorescence, and in this thesis, its phosphorescence is also observed by local excitation in the STM. Electronic characterization reveals a simultaneous voltage onset of both types of emission. Phosphorescence is also detected using tip-enhanced resonant photoexcitation of the singlet state. Since direct optical excitation of the triplet state is forbidden, this observation is the first demonstration of intersystem crossing in a tip-controlled single-molecule experiment. Exciton-plasmon coupling can be varied by fine positioning the tip in proximity to the molecule. This allows tuning the triplet emission intensity through control of the singlet lifetime. These findings are directly transferrable to highly topical phosphorescent OLEDs that employ plasmonic enhancement.The phenomenon of plasmonic antibunching is discovered for electron tunneling from a few monolayer thick C60 films to the STM tip. The layer thickness and the applied voltage control the sequential tunneling of charges by a Coulomb blockade mechanism. This mechanism eventually causes the emission of one photon at a time. Such a plasmonic source allows for a higher single-photon emission rate than an individual molecule. The design principles for other semiconducting films are discussed, suggesting a quest for similar nanostructured single-photon emitters.
EPFL2022
