Csaba Vass

High-resolution, momentum-resolved photoelectron emission microscopy is employed to investigate ejected electrons extending all the way from the directly emitted primary photoelectrons to the slow secondary electrons resulting from the ensuing collision cascade. Excitation with unpolarised radiation from a helium light source enables the valence band to be mapped across the entire first surface Brillouin zone. The measured quasi-particle bands show very good quantitative agreement with state-of-the-art density functional theory supplemented by many-body corrections within the GW approximation, establishing a reliable reference for the initial-state electronic structure. Extreme-ultraviolet light of controlled polarisation from a high-harmonic generation source is used to access high-lying conduction-band states. The polarisation-dependent momentum distributions of directly photoemitted electrons reveal the roles of initial-state symmetry and final conduction-band states. In the low-energy secondary-electron regime we map out, for the first time, the momentum-space distribution of the quasi-bound in-plane resonance in the conduction band which acts as a doorway state for secondary-electron emission through the X-peak. Six-fold-symmetric intensity patterns at characteristic kinetic energies above vacuum, independent of photon energy and light polarisation, indicate cascading secondary-electron emission through preferential conduction-band channels. These nearly non-dispersive features arise from hybridised, sp2-derived σ∗ conduction-band states. By disentangling contributions from the valence bands, conduction bands mediating direct photoemission, and low-energy resonances populated by the slowing-down electron cascade, our results provide unprecedented experimental access to the valence-band electronic structure and multiple emission channels of bulk graphite, relevant also to other layered quantum materials.

The preparation of mixed boron-, carbon-, and nitrogen-containing 2D materials has been intensively studied to optimize their electronic properties. For this purpose, the decomposition of bis-BN cyclohexane (B2N2C2H12) was studied in this work on Ir(111) using electron diffraction as well as core and valence band photoemission. The underlying energetics has been disclosed by density functional theory calculations. The expectation that a precursor with all three elements in its ring would facilitate the formation of atomically mixed BCN films was not met. When the molecules were adsorbed at 50 °C, followed by subsequent annealing, the rupture of the six-membered rings started already at 275 °C. The preparation of the overlayer in the range of 650 °C – 740 °C primarily resulted in the formation of phase-separated h-BN and graphene domains. However, increasing the preparation temperature to 980 °C – 1110 °C considerably enhanced mixing. At the obtained N-deficient average compositions: (i) the average size of h-BN islands decreased much down to ~1 nm or below, and they were mostly terminated by B atoms; (ii) the area covered by extended h-BN domains decreased and disappeared; (iii) surplus B atoms were alloyed into graphene forming a 2D boron carbide matrix in which h-BN nanodomains are embedded; (iv) the edge region of h-BN nanoislands can be considered chemically a one-dimensional mixed BCN phase. Our results revealed that the presence of boron carbides should also be considered when studying BCN layers. For the first time, we experimentally characterized their band structure by momentum microscopy.