Bilayer graphene is a material consisting of two layers of graphene. One of the first reports of bilayer graphene was in the seminal 2004 Science paper by Geim and colleagues, in which they described devices "which contained just one, two, or three atomic layers" Structure Bilayer graphene can exist in the AB, or Bernal-stacked form, where half of the atoms lie directly over the center of a hexagon in the lower graphene sheet, and half of the atoms lie over an atom, or, less commonly, in the AA form, in which the layers are exactly aligned. In Bernal stacked graphene, twin boundaries are common; transitioning from AB to BA stacking. Twisted layers, where one layer is rotated relative to the other, have also been extensively studied. Quantum Monte Carlo methods have been used to calculate the binding energies of AA- and AB-stacked bilayer graphene, which are 11.5(9) and 17.7(9) meV per atom, respectively. This is consistent with the observation that the AB-stacked structure is more stable than the AA-stacked structure. Synthesis Bilayer graphene can be made by exfoliation from graphite or by chemical vapor deposition (CVD). In 2016, Rodney S. Ruoff and colleagues showed that large single-crystal bilayer graphene could be produced by oxygen-activated chemical vapour deposition. Later in the same year a Korean group reported the synthesis of wafer-scale single-crystal AB-stacked bilayer graphene Tunable bandgap Like monolayer graphene, bilayer graphene has a zero bandgap and thus behaves like a semimetal. In 2007, researchers predicted that a bandgap could be introduced if an electric displacement field were applied to the two layers: a so-called tunable band gap. An experimental demonstration of a tunable bandgap in bilayer graphene came in 2009. In 2015 researchers observed 1D ballistic electron conducting channels at bilayer graphene domain walls. Another group showed that the band gap of bilayer films on silicon carbide could be controlled by selectively adjusting the carrier concentration. Emergent complex states In 2014 researchers described the emergence of complex electronic states in bilayer graphene, notably the fractional quantum Hall effect and showed that this could be tuned by an electric field. In 2017 the observation of an even-denominator fractional quantum Hall state was reported in bilayer graphene. Excitonic Condensation Bilayer graphene showed the potential to realize a Bose–Einstein condensate of excitons. Electrons and holes are fermions, but when they form an exciton, they become bosons, allowing Bose-Einstein condensation to occur. Exciton condensates