Opportunities from high-quality CVD graphene devices

Two-dimensional materials and van der Waals heterostructures stand at the core of current research in condensed-matter. A key feature of these systems is the possibility of engineering their electronic properties via controlled stacking, as well as tuning their response with external fields in gated devices. In this context, graphene multilayers offer extensive degrees of freedom, including their relative crystallographic alignment – so- called ‘twisting’. While extremely rare in natural graphite, twisted graphene layers can be abundantly synthesised via chemical vapor deposition (CVD) on metal catalysts. Although technologically relevant, this approach is commonly believed to yield low-quality crystals, i.e. unsuitable for fundamental research studies. However, recent advancements in the synthesis, transfer and encapsulation techniques can provide ultra-high mobility transport devices, opening up the opportunity of exploring the electronic properties of CVD-based unconventional stackings – including twisted graphene layers.
We will show that CVD graphene crystals can be successfully integrated in van der Waals heterostructures with outstanding electrical performances, including room-temperature mobility exceeding 10^5 cm^2V^{-1} s^{-1} and low-temperature mobility > 6×10^5 cm^2V^{-1} s^{-1}.
The quantum oscillation onset in such devices is observed at B<50mT, with evidence of the fractional quantum Hall effect measured in a commercial superconducting coil.
The optimized encapsulation and fabrication techniques are applied to twisted bilayer graphene samples with 30° interlayer rotation. This system realizes a dodecagonal quasicrystal without translational symmetry, and previous photo-emission experiments suggested the emergence of multiple Dirac cones replica at the Fermi level. Low- temperature magneto-transport measurements in dual-gated devices, however, shows 8- fold degenerate quantum Hall states and uncoupled-layers behaviour with large interlayer capacitance.
We will conclude by discussing ongoing experiments and describing possible developments of this research line.

Fabio Taddei (9038) - fabio.taddei@nano.cnr.it
Stefan Heun (9472) - stefan.heun@nano.cnr.it