Band structure engineering of 2D materials using patterned dielectric superlattices

Abstract

The ability to manipulate electrons in two-dimensional materials with external electric fields provides a route to synthetic band engineering. By imposing artificially designed and spatially periodic superlattice potentials, electronic properties can be further altered beyond the constraints of naturally occurring atomic crystals. Here, we report a new approach to fabricate high-mobility superlattice devices by integrating surface dielectric patterning with atomically thin van der Waals materials. By separating the device assembly and superlattice fabrication processes, we address the intractable trade-off between device processing and mobility degradation that constrains superlattice engineering in conventional systems. The improved electrostatics of atomically thin materials allows smaller wavelength superlattice patterns relative to previous demonstrations. Moreover, we observe the formation of replica Dirac cones in ballistic graphene devices with sub-40 nm wavelength superlattices and report fractal Hofstadter spectra under large magnetic fields from superlattices with designed lattice symmetries that differ from that of the host crystal. Our results establish a robust and versatile technique for band structure engineering of graphene and related van der Waals materials with dynamic tunability.

Electrostatic superlattices on scaled graphene lattices

“…1 of Ref. 20. The bottom gate capacitance therefore shows a spatial modulation with a square lattice symmetry, as shown in the lower left inset of With the electrostatically simulated position-dependent back gate capacitance per unit area C bg (x, y), contributing carrier density n bg (x, y) = [C bg (x, y)/e]V bg , together with the uniform n tg , the resulting superlattice potential is given by U s (x, y) = − sgn[n(x, y)]hv F π|n(x, y)| with n = n bg + n tg , in order to set the global transport Fermi level at zero [46].…”

“…A more flexible approach to design artificial graphene superlattice structures for band structure engineering was pursued with the realization of electrostatic gating schemes [20]. To create an externally controllable periodic potential, the most intuitive way is to pattern an array of periodic fine metal gates on top of the graphene sample [42,43].…”

“…To compare with the resistance measurements reported in Ref. 20, we plot the inverse transmission 1/T as a function of n tg and V bg in the main panel of Figure 5(a), where most areas show high transmission (white regions correspond to low 1/T ). Along the diagonal dark thick line showing high 1/T values due to the main Dirac point, multiple satellite peaks can be seen when increasing |V bg | and hence the magnitude of the square superlattice potential, signifying the emerging multiple extra Dirac points due to the gate-controlled square superlattice potential.…”

“…In the following years, other exciting transport experiments have been reported [11][12][13][14][15][16][17], as well as a dynamic band structure tuning [18,19]. More recently, another approach for inducing a superlattice potential in graphene has been demonstrated by using patterned dielectrics [20]. Such a gate-tunable superlattice structure allows for the design of arbitrary superlattice geometries with defined periodicity but suffers from technical restrictions.…”

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…Furthermore, since the unique properties of 2D crystals stem from the periodicity of their atomic structure, free carriers and polariton species populating such materials are highly susceptible to the perturbation of the crystalline atomic structure. Therefore, properties of polaritons in 2D materials can be engineered at the quantum level via: (1) vertical stacking of vdW crystals, which leads to a hybridization of polariton species or control over the level of their anisotropy; (2) imposing a superlattice periodicity on 2D materials, which causes significant alteration on the polariton characteristics in nontrivial manner …”