Tunable Graphene Phononic Crystal

Abstract

In the field of phononics, periodic patterning controls vibrations and thereby the flow of heat and sound in matter. Bandgaps arising in such phononic crystals (PnCs) realize low-dissipation vibrational modes and enable applications toward mechanical qubits, efficient waveguides, and state-of-the-art sensing. Here, we combine phononics and two-dimensional materials and explore tuning of PnCs via applied mechanical pressure. To this end, we fabricate the thinnest possible PnC from monolayer graphene and simulate its vibrational properties. We find a bandgap in the megahertz regime within which we localize a defect mode with a small effective mass of 0.72 ag = 0.002 mphysical. We exploit graphene’s flexibility and simulate mechanical tuning of a finite size PnC. Under electrostatic pressure up to 30 kPa, we observe an upshift in frequency of the entire phononic system by ∼350%. At the same time, the defect mode stays within the bandgap and remains localized, suggesting a high-quality, dynamically tunable mechanical system.

Non‐Uniform Strain Engineering of 2D Materials

“…[28,29] This effect is dominant when the lifetime of the excitons is too short and can be avoided when the strained 2D material is a heterostructure and the lifetimes of the excitons are on the order of 1 ms. [44] Last, we introduced the graphene-based phononic crystal that can be tuned via strain. [24] Here, strain non-uniformity smears the phononic bandgap. Frequency tunabilty via strain in phononic system is currently still a new feature and needs careful exploration.…”

“…In Figure 2a, we show a recently considered graphene phononic crystal. [24] He-Ion lithography is used to cut a honeycomb lattice of holes creating the phononic order. [25] The unit cell of the artificial lattice and the corresponding Brillouin zone are shown in Figure 2b.…”

“…Finally, we consider the effect of non-uniform strain on another quasiparticle -the phonon, and investigate perturbed phononic crystal. [24] Upon electrostatic gating (described in sec. 3) the device deforms out of plane (a) and the strain distribution becomes increasingly non-uniform as shown in a line cut in Figure 6b.…”

“…[51,52] Yet this is only possible in very large samples due to spatial detection limits. [24] Another way is to measure transmission through a phononic sample. [15,16,53] To do so one needs an excitation on one side of the sample (e. g., via Surfaces Acoustic Waves or photo-thermally) and detects its motion on another side (e. g., interferometrical motion detection).…”

A Review on Graphene-Based Nano-Electromechanical Resonators: Fabrication, Performance, and Applications

“…The appearance of banded frequency, which was caused by the hybridization of mechanical modes, proved the formation of graphene phononic crystals. In addition, Kirchhof et al studied the local defect modes in graphene phononic crystals [92]. They fabricated a tunable monolayer graphene phononic crystal and localized a defect mode in the megahertz bandgap.…”