N-Heterocyclic Olefins on a Silicon Surface.

The adsorption of N-heterocyclic olefins (NHOs) on silicon is investigated in a combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory study. We find that both of the studied NHOs bind covalently, with ylidic character, to the silicon adatoms of the substrate and exhibit good thermal stability. The adsorption geometry strongly depends on the N-substituents: for large N-substituents, an upright adsorption geometry is favored, while a flat-lying geometry is found for the NHO with smaller wingtips. These different geometries strongly influence the quality and properties of the obtained monolayers. The upright geometry leads to the formation of ordered monolayers, whereas the flat-lying NHOs yield a mostly disordered, but denser, monolayer. The obtained monolayers both show large work function reductions, as the higher density of the flat-lying monolayer is found to compensate for the smaller vertical dipole moments. Our findings offer new prospects in the design of tailor-made ligand structures in organic electronics and optoelectronics, catalysis, and material science.

Thermodynamics and electronic structure of adsorbed and intercalated plumbene in graphene/hexagonal SiC heterostructures.

Graphene-covered hexagonal SiC substrates have been frequently discussed to be appropriate starting points for epitaxial overlayers of Xenes, such as plumbene, or even their deposition as intercalates between graphene and SiC. Here, we investigate, within density functional theory, the plumbene deposition for various layer orderings and substrate terminations. By means of total energy studies we demonstrate the favorization of the intercalation versus the epitaxy for both C-terminated and Si-terminated 4H-SiC substrates. These results are explained in terms of chemical bonding and by means of layer-resolved projected band structures. Our results are compared with available experimental findings.

Tuning Gaps and Schottky Contacts of Graphene/Phosphorene Heterostructures by Vertical Electric Field and Strain.

We present a comprehensive study of the structural and electronic properties of a graphene/phosphorene (G/P) heterostructure in the framework of density functional theory, including van der Waals interaction in the exchange-correlation functional. While the G(4 × 1)/P(3 × 1) superlattice usually used in the literature is subject to a strain as high as about 7%, the in-plane strain could be drastically reduced to under 1% in the G(4 × 13)/P(3 × 12) heterostructure investigated here. Adapting the lattice constants of the rectangular lattices, the equilibrium configuration in the xy plane of phosphorene relative to the graphene layer is optimized. This results in an equilibrium interlayer distance of 3.5 Å and a binding energy per carbon atom of 37 meV, confirming the presence of weak van der Waals interaction between the graphene and the phosphorene layers. The electronic properties of the heterostructure are evaluated under different values of interlayer distance, strain and applied vertical electric field. We demonstrate that G/P heterostructures form an n-type Schottky contact, which can be transformed into p-type under external perturbations. These findings, together with the possibility to control the gaps and barrier heights, suggest that G/P heterostructures are promising for novel applications in electronics and may open a new avenue for the realization of innovative optoelectronic devices.

Ab Initio Study of Graphene/hBN Van der Waals Heterostructures: Effect of Electric Field, Twist Angles and p-n Doping on the Electronic Properties.

In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.