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Bilayer graphene
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"
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.
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
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.
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.
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 in bilayer systems have been shown theoretically to carry a supercurrent.
Pablo Jarillo-Herrero of MIT and colleagues from Harvard and the National Institute for Materials Science, Tsukuba, Japan, have reported the discovery of superconductivity in bilayer graphene with a twist angle of 1.1° between the two layers. The discovery was announced in Nature in March 2018. The findings confirmed predictions made in 2011 by Allan MacDonald and Rafi Bistritzer that the amount of energy a free electron would require to tunnel between two graphene sheets radically changes at this angle. The graphene bilayer was prepared from exfoliated monolayers of graphene, with the second layer being manually rotated to a set angle with respect to the first layer. A critical temperature of was observed with such specimens in the original paper (with newer papers reporting slightly higher temperatures).
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Bilayer graphene
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"
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.
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
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.
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.
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 in bilayer systems have been shown theoretically to carry a supercurrent.
Pablo Jarillo-Herrero of MIT and colleagues from Harvard and the National Institute for Materials Science, Tsukuba, Japan, have reported the discovery of superconductivity in bilayer graphene with a twist angle of 1.1° between the two layers. The discovery was announced in Nature in March 2018. The findings confirmed predictions made in 2011 by Allan MacDonald and Rafi Bistritzer that the amount of energy a free electron would require to tunnel between two graphene sheets radically changes at this angle. The graphene bilayer was prepared from exfoliated monolayers of graphene, with the second layer being manually rotated to a set angle with respect to the first layer. A critical temperature of was observed with such specimens in the original paper (with newer papers reporting slightly higher temperatures).