Phd in PHYSICS

Thesis title: Tuning the electronic and vibrational properties of Nanoporous Graphene via alkali metal doping

This thesis investigates the electronic and vibrational properties of nanoporous graphene (NPG) under controlled alkali metal doping, providing a comprehensive understanding of its response in a metallic environment. Using advanced, multi-technique spectroscopic approaches in ultra-high vacuum conditions, the study successfully achieves a stepwise tuning over the Fermi level, allowing precise control of the charge transfer, unravelling many-body collective phenomena and a significant electron-phonon coupling in doped graphene. By employing NPG as a freestanding platform, the intrinsic structural, electronic and vibrational properties of graphene were preserved, avoiding any kind of interference from substrate interactions, and thus overcoming previous limitations from typical supported systems. Alkali metal doping was first investigated by means of potassium deposition, which demonstrated a stable and uniform adsorption process. The analysis of the electron spectral density of states close to the Fermi level, conducted by ultraviolet photoemission spectroscopy, revealed a progressive charge donation from the alkali metal to graphene, causing the partial occupation of the π∗ upper Dirac cone, with a near-rigid shift of the bands and without significant alterations in the electronic structure of the system. The increased charge density was correlated with the emergence of a charge-induced π∗-plasmon mode, attributed to charge density fluctuations in the upper Dirac cone, as directly probed by high-resolution electron energy loss spectroscopy. By correlating the plasmonic response with the conduction band occupation, the incremental tuning of the Fermi level allowed for precise tracking of plasmon energy as a function of charge carrier density. Additional insights into the plasmon mode dependency on the potassium content were achieved through the identification of the extrinsic π∗-plasmon excitation in the C 1s core-level photoemission spectra. Furthermore, the controlled vibrational response in potassium-doped NPG unveiled a strong electron-phonon coupling, revealing a substantial renormalization of phonon frequencies resulting from localized charge transfer. The correlation between the charge transfer associated with the plasmon in the upper Dirac cone and the Fermi level shift enabled the determination of the relevant electron-phonon coupling strength by Raman spectroscopy, which was found to be larger than the one typically provided by gate voltage doping systems. Finally, a comparative analysis of doping by two additional alkali metals, Cs and Na, revealed a significant charge transfer and π∗-plasmon formation provided by Cs-doping. The Cs-doped graphene showed a similar vibrational response to K-doped graphene, though with minor differences. In contrast, Na-doping resulted in minimal charge transfer due to its higher tendency for the formation of Na-Na atomic clusters, thereby not affecting the intrinsic graphene electronic and vibrational spectra. This comprehensive multi-technique approach expands our understanding of alkali metal doping, providing a unified perspective on the electronic and vibrational properties of graphene and highlighting potassium as the most effective candidate for chemical doping.