We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm K. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials.

Nitrogen-doped graphitic nanoribbons (N x -GNRs), synthesized by chemical vapor deposition (CVD) using pyrazine as a nitrogen precursor, are reported for the fi rst time. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) reveal that the synthesized materials are formed by multilayered corrugated GNRs, which in most cases exhibit the formation of curved graphene edges (loops). This suggests that during growth, nitrogen atoms promote loop formation; undoped GNRs do not form loops at their edges. Transport measurements on individual pure GNRs exhibit a linear I -V (current-voltage) behavior, whereas N x -GNRs show reduced current responses following a semiconducting-like behavior, which becomes more prominent for high nitrogen concentrations. To better understand the experimental fi ndings, electron density of states (DOS), quantum conductance for nitrogen-doped zigzag and armchair single-layer GNRs are calculated for different N doping concentrations using density functional theory (DFT) and non-equilibrium Green functions. These calculations confi rm the crucial role of nitrogen atoms in the transport properties, confi rming that the nonlinear I -V curves are due to the presence of nitrogen atoms within the N x -GNRs lattice that act as scattering sites. These characteristic N x -GNRs transport properties could be advantageous in the fabrication of electronic devices including sensors in which metal-like undoped GNRs are unsuitable.

We report peculiar transport fingerprints at the secondary Dirac points created by the interaction between graphene and boron nitride layers. By performing ab initio calculations, the electronic characteristics of the moiré patterns produced by the interaction between layers are first shown to be in good agreement with experimental data, and further used to calibrate the tight-binding model implemented for the transport study. By means of a real-space order-N quantum transport (Kubo) methodology, low-energy (Dirac point) transport properties are contrasted with those of high-energy (secondary) Dirac points, including both Anderson disorder and Gaussian impurities to respectively mimic short-range and long-range scattering potentials. Mean free paths at the secondary Dirac points are found to range from 10 nm to a few hundreds of nm depending on the static disorder, while the observation of satellite resistivity peaks depends on the strength of quantum interferences and localization effects.