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Register a new userOptical Probing of Electronic Interaction between Graphene and Hexagonal Boron Nitride
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Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few % decrease in the Fermi velocity (vF) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in vF and mechanical strain, but not by charge doping unlike graphene supported on SiO2 substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping.
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We investigate the adsorption of graphene sheets on h-BN substrates by means of first-principles calculations in the framework of adiabatic connection fluctuation-dissipation theory in the random phase approximation. We obtain adhesion energies for different crystallographic stacking configurations and show that the interlayer bonding is due to long-range van der Waals forces. The interplay of elastic and adhesion energies is shown to lead to stacking disorder and moire structures. Band structure calculations reveal substrate induced mass terms in graphene which change their sign with the stacking configuration. The dispersion, absolute band gaps and the real space shape of the low energy electronic states in the moire structures are discussed. We find that the absolute band gaps in the moire structures are at least an order of magnitude smaller than the maximum local values of the mass term. Our results are in agreement with recent STM experiments.
We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on SiO2 no diameter dependency is observed. In addition, QDs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T. Both results indicate a substantially reduced substrate induced disorder potential in graphene QDs on hBN.
We performed calculations of electronic, optical and transport properties of graphene on hBN with realistic moire patterns. The latter are produced by structural relaxation using a fully atomistic model. This relaxation turns out to be crucially important for electronic properties. We describe experimentally observed features such as additional Dirac points and the Hofstadter butterfly structure of energy levels in a magnetic field. We find that the electronic structure is sensitive to many-body renormalization of the local energy gap.
Chemically synthesized cove-type graphene nanoribbons (cGNRs) of different widths were brought into dispersion and drop-cast onto exfoliated hexagonal boron nitride (hBN) on a Si/SiO2 chip. With AFM we observed that the cGNRs form ordered domains aligned along the crystallographic axes of the hBN. Using electron beam lithography and metallization, we contacted the cGNRs with NiCr/Au, or Pd contacts and measured their I-V-characteristics. The transport through the ribbons was dominated by the Schottky behavior of the contacts between the metal and the ribbon.
We investigate the effect of surface acoustic waves on the atomic-like optical emission from defect centers in hexagonal boron nitride layers deposited on the surface of a LiNbO$_3$ substrate. The dynamic strain field of the surface acoustic waves modulates the emission lines resulting in intensity variations as large as 50% and oscillations of the emission energy with an amplitude of almost 1 meV. From a systematic study of the dependence of the modulation on the acoustic wave power, we determine a hydrostatic deformation potential for defect centers in this two-dimensional material of about 40 meV/%. Furthermore, we show that the dynamic piezoelectric field of the acoustic wave could contribute to the stabilization of the optical properties of these centers. Our results show that surface acoustic waves are a powerful tool to modulate and control the electronic states of two-dimensional materials.
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