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Register a new userElectronic inhomogeneities in graphene: the role of the substrate interaction and chemical doping
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Added by
Andres Castellanos-Gomez
Publication date
2012
fields
Physics
and research's language is
English
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We probe the local inhomogeneities of the electronic properties of graphene at the nanoscale using scanning probe microscopy techniques. First, we focus on the study of the electronic inhomogeneities caused by the graphene-substrate interaction in graphene samples exfoliated on silicon oxide. We find that charged impurities, present in the graphene-substrate interface, perturb the carrier density significantly and alter the electronic properties of graphene. This finding helps to understand the observed device-to-device variation typically observed in graphene-based electronic devices. Second, we probe the effect of chemical modification in the electronic properties of graphene, grown by chemical vapour deposition on nickel. We find that both the chemisorption of hydrogen and the physisorption of porphyrin molecules strongly depress the conductance at low bias indicating the opening of a bandgap in graphene, paving the way towards the chemical engineering of the electronic properties of graphene.
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Theory of the electron spin relaxation in graphene on the SiO$_2$ substrate is developed. Charged impurities and polar optical surface phonons in the substrate induce an effective random Bychkov-Rashba-like spin-orbit coupling field which leads to spin relaxation by the Dyakonov-Perel mechanism. Analytical estimates and Monte Carlo simulations show that the corresponding spin relaxation times are between micro- to milliseconds, being only weakly temperature dependent. It is also argued that the presence of adatoms on graphene can lead to spin lifetimes shorter than nanoseconds.
We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene (Gr), a case of pure van der Waals (vdW) interaction, strengthens with $n$- and weakens with $p$-doping of Gr. Density functional theory calculations that include the vdW interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the vdW interaction is modified by changing the spatial extent of Grs $pi$ orbitals via doping.
We perform ab initio calculations that indicate that the relative stability of antiphase boundaries (APB) with armchair and zigzag chiralities in monolayer boron nitride (BN) is determined by the chemical potentials of the boron and nitrogen species in the synthesis process. In an N-rich environment, a zigzag APB with N-rich core is the most stable structure, while under B-rich or intrinsic growth conditions, an armchair APB with stoichiometric core is the most stable. This stability transition is shown to arise from a competition between homopolar-bond (B-B and N-N) and elastic energy costs in the core of the APBs. Moreover, in the presence of a carbon source we find that a carbon-doped zigzag APB becomes the most stable boundary near the N-rich limit. The electronic structure of the two types of APBs in BN is shown to be particularly distinct, with the zigzag APB depicting defect-like deep electronic bands in the band gap, while the armchair APB shows bulk-like shallow electronic bands.
The honeycomb lattice sets the basic arena for numerous ideas to implement electronic, photonic, or phononic topological bands in (meta-)materials. Novel opportunities to manipulate Dirac electrons in graphene through band engineering arise from superlattice potentials as induced by a substrate such as hexagonal boron-nitride. Making use of the general form of a weak substrate potential as dictated by symmetry, we analytically derive the low-energy minibands of the superstructure, including a characteristic 1.5 Dirac cone deriving from a three-band crossing at the Brillouin zone edge. Assuming a large supercell, we focus on a single Dirac cone (or valley) and find all possible arrangements of the low-energy electron and hole bands in a complete six-dimensional parameter space. We identify the various symmetry planes in parameter space inducing gap closures and find the sectors hosting topological minibands, including also complex band crossings that generate a valley Chern number atypically larger than one. Our map provides a starting point for the systematic design of topological bands by substrate engineering.
Defects in solid commonly limit mechanical performance of the material. However, recent measurements reported that the extraordinarily high strength of graphene is almost retained with the presence of grain boundaries. We clarify in this work that lattice defects in the grain boundaries and distorted geometry thus induced define the mechanical properties characterized under specific loading conditions. Atomistic simulations and theoretical analysis show that tensile tests measure in-plane strength that is governed by defect-induced stress buildup, while nanoindentation probes local strength under the indenter tip and bears additional geometrical effects from warping. These findings elucidate the failure mechanisms of graphene under realistic loading conditions and assess the feasibility of abovementioned techniques in quantifying the strength of graphene, and suggest that mechanical properties of low-dimensional materials could be tuned by implanting defects and geometrical distortion they leads to.
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