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Register a new userChiral properties of graphene h-BN hybrid systems
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The application of the chiral decomposition procedure to hybrid graphene h-BN systems revealed rules for the partition of the system into effective subsystems being bilayers plus monolayer in case the number of layers is odd. Three types of subsystems have been detected namely purely graphene bilayers and monolayers, mixed bilayers and pure h-BN monolayers depending on the hybrid composition. The effective parameters characterizing these chiral subsystems consist of the interlayer couplings and on-site potentials which shows the mechanism of compensation of the asymmetry introduced into the system by h-BN layers. For illustration, we provide a pedagogical overview about chiral tunneling in graphene subsystems (MLG, BLG) present in hybrid with one h-BN layer. We have established the parameter ranges for which the characteristic features in the spectrum are observed, such as Fabry-Perot resonances in the case of MLG and magic angles in the case of effective BLG. We also consider different hybrid stacking in order to indicate effective systems with the desired properties required in the electronic and spintronic applications.
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Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electro-chemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new hetero-interface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density $> 5 times 10^{13} cm^{-2}$ with mobility $> 10^3 cm^2/(Vs)$ in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.
We report on gate hysteresis in resistance on high quality graphene/h-BN devices. We observe a thermal activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375K. In order to investigate the origin of the hysteretic phenomenon, we design heterostructures involving graphene/h-BN devices with different underlying substrates such as: SiO2/Si and graphite; where heavily doped silicon and graphite are used as a back gate electrodes, respectively. The gate hysteretic behavior of the resistance shows to be present only in devices with an h-BN/SiO2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. Finally, we suggest a phenomenological model, which captures all of our findings based on charges trapped at the h-BN/SiO2. Certainly, such hysteretic behavior in graphene resistance represents a technological problem for the application of graphene devices at high temperatures, but conversely, it can open new routes for applications on digital electronics and graphene memory devices.
By atomistic modeling of moir{e} patterns of graphene on a substrate with a small lattice mismatch, we find qualitatively different strain distributions for small and large misorientation angles, corresponding to the commensurate-incommensurate transition recently observed in graphene on hexagonal BN. We find that the ratio of C-N and C-B interactions is the main parameter determining the different bond lengths in the center and edges of the moir{e} pattern. Agreement with experimental data is obtained only by assuming that the C-B interactions are at least twice weaker than the C-N interactions. The correspondence between the strain distribution in the nanoscale moir{e} pattern and the potential energy surface at the atomic scale found in our calculations, makes the moir{e} pattern a tool to study details of dispersive forces in van der Waals heterostructures.
High mobility single and few-layer graphene sheets are in many ways attractive as nanoelectronic circuit hosts but lack energy gaps, which are essential to the operation of field-effect transistors. One of the methods used to create gaps in the spectrum of graphene systems is to form long period moire patterns by aligning the graphene and hexagonal boron nitride (h-BN) substrate lattices. Here, we use planar tunneling devices with thin h-BN barriers to obtain direct and accurate tunneling spectroscopy measurements of the energy gaps in single- and bi-layer graphene-h-BN superlattice structures at charge neutrality (first Dirac point) and at integer moire band occupancies (second Dirac point, SDP) as a function of external electric and magnetic fields and the interface twist angle. In single-layer graphene we find, in agreement with previous work, that gaps are formed at neutrality and at the hole-doped SDP, but not at the electron-doped SDP. Both primary and secondary gaps can be determined accurately by extrapolating Landau fan patterns to zero magnetic field and are as large as $simeq$ 17 meV for devices in near perfect alignment. For bilayer graphene, we find that gaps occur only at charge neutrality where they can be modified by an external electric field. Tunneling signatures of in-gap states around neutrality suggest the development of edge modes related to topologically non-trivial valley projected bands due to the combination of an external electric field and moire superlattice patterns.
Being used in optoelectronic devices as ultra-thin conductor-insulator junctions, detailed investigations are needed about how exactly h-BN and graphene hybridize. Here, we present a comprehensive ab initio study of hot carrier dynamics governed by electron-phonon scattering at the h-BN/graphene interface, using graphite (bulk), monolayer and bilayer graphene as benchmark materials. In contrast to monolayer graphene, all multilayer structures possess low-energy optical phonon modes that facilitate carrier thermalization. We find that the h-BN/graphene interface represents an exception with comparatively weak coupling between low-energy optical phonons and electrons. As a consequence, the thermalization bottleneck effect, known from graphene, survives hybridization with h-BN but is substantially reduced in all other bilayer and multilayer cases considered. In addition, we show that the quantum confinement in bilayer graphene does not have a significant influence on the thermalization time compared to graphite and that bilayer graphene can hence serve as a minimal model for the bulk counterpart.
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