Dynamic band structure tuning of graphene moiré superlattices with pressure (1707.09054v2)

Abstract: Heterostructures of atomically-thin materials have attracted significant interest owing to their ability to host novel electronic properties fundamentally distinct from their constituent layers. In the case of graphene on boron nitride, the closely-matched lattices yield a moir\'e superlattice that modifies the graphene electron dispersion and opens gaps both at the primary Dirac point (DP) and the moir\'e-induced secondary Dirac point (SDP) in the valence band. While significant effort has focused on controlling the superlattice period via the rotational stacking order, the role played by the magnitude of the interlayer coupling has received comparatively little attention. Here, we modify the interaction between graphene and boron nitride by tuning their separation with hydrostatic pressure. We observe a dramatic enhancement of the DP gap with increasing pressure, but little change in the SDP gap. Our surprising results identify the critical role played by atomic-scale structural deformations of the graphene lattice and reveal new opportunities for band structure engineering in van der Waals heterostructures.

• The paper reveals that applying hydrostatic pressure nearly doubles the graphene Dirac point gap while leaving secondary gaps largely unaffected.
• The study quantifies a 5% reduction in interlayer spacing that correlates with a 9% increase in gate capacitance per gigapascal.
• The findings establish pressure as a precise tool for engineering band structures in van der Waals heterostructures, opening avenues for advanced nanoelectronic devices.

Dynamic Band Structure Tuning of Graphene Moiré Superlattices with Pressure

The paper investigates the modulation of graphene's electronic properties when encapsulated in hexagonal boron nitride (BN) under hydrostatic pressure. This heterostructure exploits the lattice mismatch between graphene and BN to form a moiré superlattice (MSL), significantly modifying graphene's band structure, specifically at the Dirac points (DP) and secondary Dirac points (SDP). The authors' focus lies on the understudied aspect of interlayer interaction strength, elucidated through pressure-induced changes in atomic spacing.

Key Results

1. Pressure-Dependent Band Gap Modulation: The researchers observed that applying hydrostatic pressure notably increased the primary DP gap while leaving the SDP gap relatively unaffected. Notably, the DP gap nearly doubled over the range of pressures applied, the highest gap ever demonstrated for monolayer graphene.
2. Dielectric Properties and Capacitance Variability: The compressive forces decreased interlayer spacing by approximately 5%, leading to a roughly 9% increase in gate capacitance per gigapascal. This paper linked the increase to both a reduced dielectric thickness and an enhanced dielectric constant of BN.
3. Band Structure Engineering: The findings highlight potential pathways for designing electronic properties in van der Waals (vdW) heterostructures through precise control of interlayer spacing, thus modulating the effective MSL potential.

Implications and Theoretical Considerations

These results suggest that apart from traditional rotational alignment, pressure acts as a crucial parameter for tuning electronic properties in vdW heterostructures. Specifically, the divergence between the pressure responses of the DP and SDP gaps suggests a complex interplay between electrostatic and structural factors, such as atomic-scale deformations that may influence the graphene lattice.

Theoretical Modeling

The paper incorporates theoretical modeling to bridge experimental observations and underlying mechanisms. The models indicate that the enhancement of the DP gap under pressure may arise from increased MSL coupling, facilitated by reduced interlayer separation. Interestingly, the modeling suggests that lattice deformations, such as in-plane strains and out-of-plane corrugations, could endow a large DP gap even in the absence of increased coupling, positing deformation as a primary influencer over traditional rigid lattice paradigms.

Future Directions

The research opens new routes for manipulating the electronic landscape in 2D materials and suggests potential application in devices where precise band gap control is critical, such as low-power digital logic and novel quantum devices. Further exploration into alternative approaches, such as strain-engineering and varying substrate materials, could unveil additional methods for customizing electronic properties in similar heterostructures.

In conclusion, the paper enhances the understanding of pressure effects on vdW heterostructures and serves as a foundation for further explorations into dynamic band structure engineering through novel manipulative techniques.