The synthesis and characterization of two-dimensional (2D) molecular crystals composed of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along the graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.

A Monte-Carlo-based simulated annealing process combined with ab initio calculations is employed to investigate electronic and structural properties of boron nitride (BN)-doped graphene, in a wide doping range. We find that, for a given BN doping concentration, the doping-induced band gap can vary over an order of magnitude depending on the placement of the B and N atoms. We propose an analytical tight-binding model that reproduces the dependence of the band gap on both the concentration and the morphology obtained in the ab initio calculations and provides an upper bound for the band gap at a given BN concentration. We also predict that the dependence of the band gap with applied tensile stress should be strong, nonmonotonic, and anisotropic, within the range of strain values attainable experimentally.

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Two-dimensional van der Waals heterostructures are attractive candidates for optoelectronic nanodevice applications. The charge transport process in these systems has been extensively investigated, however the effect of coupling between specific electronic states on the charge transfer process is not completely established yet. Here, interfacial charge transfer (CT) in the MoS/graphene/SiO heterostructure is investigated from static and dynamic points of view. Static CT in the MoS-graphene interface was elucidated by an intensity quenching, broadening and a blueshift of the photoluminescence peaks. Atomic and electronic state-specific CT dynamics on a femtosecond timescale are characterized using a core-hole clock approach and using the S1s core-hole lifetime as an internal clock. We demonstrate that the femtosecond electron transfer pathway in the MoS/SiO heterostructure is mainly due to the electronic coupling between S3p-Mo4d states forming the Mo-S covalent bond in the MoS layer. For the MoS/graphene/SiO heterostructure, we identify, with the support of density functional calculations, new pathways that arise due to the high density of empty electronic states of the graphene conduction band. The latter makes the transfer process time in the MoS/graphene/SiO/Si twice as fast as in the MoS/SiO/Si sample. Our results show that ultrafast electron delocalization pathways in van der Waals heterostructures are dependent on the electronic properties of each involved 2D material, creating opportunities to modulate their transport properties.

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In this paper, we address the challenge of identifying grain boundaries on the molybdenum disulphide (MoS2) surface at the nanometre scale using a simple self-assembled monolayer (SAM) decoration method. Combined with atomic force microscopy, octadecylphosphonic acid monolayers readily reveal grain boundaries in MoS2 at ambient conditions, without the need of atomic resolution measurements under vacuum. Additional ab initio calculations allow us to obtain the preferred orientation of the SAM structure relative to the MoS2 beneath, and therefore, together with the experiments, the MoS2 crystalline orientations at the grain boundaries. The proposed method enables the visualization of grain boundaries with sub-micrometer resolution for nanodevice investigation and failure analysis.

We report a first-principles study of edge-reconstructed, few-layered graphene nanoribbons. We find that the nanoribbon stability increases linearly with increasing width and decreases linearly with increasing number of layers (from three to six layers). Specifically, we find that a three-layer 1.3 nm wide ribbon is energetically more stable than the C60 fullerene, and that a 1.8 nm wide ribbon is more stable than a (10,0) carbon nanotube. The morphologies of the reconstructed edges are characterized by the presence of five-, six-, and sevenfold rings, with sp3 and sp2 bonds at the reconstructed edges. The electronic structure of the few-layered nanoribbons with reconstructed edges can be metallic or semiconducting, with band gaps oscillating between 0 and 0.28 eV as a function of ribbon width.

We present a first-principles study of structural, electronic, and transport properties of in-plane Gr:BN heterostructures in the form of graphene stripes embedded in a BN matrix. In our calculations, we consider carbon, nitrogen, and boron chemical potentials that are consistent with growth conditions (gas sources and temperatures) at either nitrogen-rich or boron-rich environments. Interestingly, we find that structures with excess of B atoms can be energetically more stable than structures with excess of N atoms even in N-rich growth conditions. The general trend is that N-rich growth conditions favor B/N stoichiometric heterostructures, while B-rich growth conditions favor heterostructures with excess of B atoms at the graphene/BN junctions, such that only B−C bonds occur at both edges of a graphene stripe region embedded in BN. We also investigate the dependence of magnetic properties and the band gap magnitudes of graphene stripe regions embedded in BN with several structural characteristics. We find that graphene stripes with only one bond type (either B−C or N−C) at the graphene/BN edges always present metallic behavior, with zigzag-oriented stripes of this type presenting large magnetic moments. Finally, we obtain the characteristic I−V curves for systems formed by junctions of two graphene stripes embedded in BN, one of them terminated by C−N bonds and the other terminated by C−B bonds. We find that systems of this type should present rectifying behavior.