Electronic excitation spectra of metallic tip-substrate nanostructures from time-dependent density functional theory

Time-resolved nanoscopy has become a key route to observe electronic dynamics on their characteristic spatial and temporal scales. Among the newest techniques, which also inspired this work, is near-field optical tunneling emission (NOTE) microscopy, enabling access to ultrafast electronic processes with subnanometer spatial resolution. Motivated by the physical mechanisms underlying NOTE microscopy, this research set out to determine the excitation spectra of metallic tip–substrate nanostructures using TDDFT calculations and to take first steps toward identifying the modes that reside in the gap. After building the theoretical foundations up to TDDFT and defining how the excitation energies are computed from the time-dependent density, a workflow was established that produces consistent spectral responses within a justified parameter set. Starting from a small tip–substrate model, spectral shifts were tracked under systematic variations of tip and substrate size. This enabled the assignment of selected features to either component of the model and a qualitative observation of how confinement affects their positions. Motivated by the pursuit of gap-specific modes, distance sweeps were then performed from larger separations down to atomic distances, and features that emerge at small tip–substrate separations were successfully isolated across different metallic clusters. The overall junction-induced spectral behavior turned out to be consistent with the well-studied field of plasmonics. Results from this work clarify the path toward eventually simulating NOTE microscopy with nearly all of its crucial components: exact excitation energies for specific geometries, tunneling charge density, and emission power, similar to simulations already existing for other microscopy techniques.