Graphene, a widely studied 2D material, exhibits exceptional physical and electronic properties. Twisting two graphene monolayers at a small angle creates twisted bilayer graphene (tBLG), which forms a superlattice. The moiré pattern resulting from relative layer orientation has driven significant progress in graphene research, enhancing its optical and electrical properties, including superconductivity.
The mechanical, optical, and electronic properties of multilayer graphene structures can be tailored by altering the stacking order, interlayer spacing, and relative twisting angle (θ). Similarly, tBLG is fabricated by stacking two single-layer graphene sheets, synthesized by chemical vapor deposition (CVD), at a specific twisting angle.
The unusual stacking imparts various angle-dependent properties to tBLG.
Moiré patterns are generated by graphene-graphene interactions resulting from the relative layer orientations. These highly periodic patterns are responsible for the extraordinary optical and electronic properties of tBLG. Additionally, tBLG exhibits twisting angle-dependent Dirac spectra (similar to chirality dependence in carbon nanotubes), Fermi velocity, magnetoresistance oscillations, and quantum Hall effect.
Innovative methods are being explored to fabricate tBLG with small twist angles. For example, hexagonal boron nitride is utilized to obtain graphene layers with rotationally aligned crystal axes. tBLG is also prepared by cutting, rotating, and stacking a graphene layer through femtosecond laser micromachining and precise transfer.
Other methods for preparing tBLG films include controlled hydrophilic and hydrophobic boundary folding of single-layer graphene and vertical stacking.
Easy twisting and stacking of two graphene layers can result in a uniform and ordered moiré superlattice capable of exhibiting unusual superconductivity and correlations in tBLG. However, the twist angle becomes rigid after interlayer stacking. Alternatively, mechanical elastic strain can help control the electronic structure of tBLG by regulating the lattice spacing and symmetry.
A recent study reviewed various innovations in straining tBLG by in-plane and out-of-plane modes. It included the characterizations and calculations performed to quantitatively tune the strain-engineered electronic structures.
Another study demonstrated a topological superconducting state in tBLG depending only on the moiré minibands instead of the twist angle tuning. The method involved subjecting tBLG to induced Rashba spin-orbit coupling, s-wave superconductivity, and exchange field and is valid for 1.3 to 3 degrees twist angles. This approach could be feasible for developing a tBLG-based quantum computer.
Despite several research advances and potential applications, significant challenges remain in achieving controlled twisting of two graphene layers to fabricate and characterize tBLG. Additionally, precise stacking of bilayer graphene at the first magic angle of 1.1 degrees to observe superconductivity is tedious due to the intrinsic disruptions caused by strain and angular disorder.
Modifications in the twist angle significantly alter the spatial wavefunction distribution. While this allows engineering bandgaps in tBLG for electronic applications, the localized wavefunction at certain twist angles leads to a sudden reduction in carrier mobility. This can negatively influence the tBLG-based device performance. Hence, careful considerations are required to obtain preferred electronic characteristics and carrier mobility for ideal device functioning.
The ultrathin nature of tBLG leads to localized stretching or compression due to substrate deformation or thermal stresses during fabrication. Despite the positive impact of these localized strains on the physical properties of the material, their non-uniformity hinders practical device applications of the material.
The constant improvements in the synthesis, characterization, and electronic structure determination techniques are anticipated to accelerate the tBLG-related advances and applications. This, in turn, may lead to the advancement of “twistronics,” the fusion of “twist” and “electronics,” which exploits the electronic properties of layered materials like graphene changing with the twist angle.
The strain engineering of twisted 2D materials like tBLG can further expand “twistronics” and “straintronics” into the realm of “strain-twistronics.” Consequently, novel ferroelectric and optoelectronic devices with tunable characteristics and regulated performance may become a reality in the future.
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