The list of surprising behaviours in “twisted bilayer” graphene (TBG) just keeps getting longer. The material – which is made by stacking two sheets of graphene on top of one another, and then rotating one of them so that the sheets are slightly misaligned – was already known to support a wide array of insulating and superconducting states, depending on the strength of an applied electric field. Now researchers in the US have uncovered yet another oddity: when TBG is exposed to infrared light, its ability to conduct electricity changes. According to Fengnian Xia of Yale University, Fan Zhang of the University of Texas at Dallas, and colleagues, this finding could make it possible to develop a new class of infrared detectors using these stacked carbon sheets.
A single layer of graphene consists of a simple repetition of carbon atoms arranged in a two-dimensional hexagonal lattice. In this pristine state, the material does not have an electronic bandgap – that is, it is a gapless semiconductor. However, when two graphene sheets are placed on top of each other and slightly misaligned, they form a moiré pattern, or superlattice. In this new arrangement, the unit cell of the 2D crystal expands to a huge extent, as if it were artificially “stretched” in the two in-plane directions. This stretching dramatically changes the material’s electronic interactions.
From magic angle to twistronics
The misalignment angle in TBG is critically important. For example, at a so-called “magic” misalignment angle of 1.1°, the material switches from an insulator to a superconductor (that is, able to carry electrical current with no resistance below 1.7 K), as a team at the Massachusetts Institute of Technology (MIT) discovered in 2018.
The existence of such strongly correlated effects – which were first theoretically predicted in 2011 by Allan MacDonald and Rafi Bistritzer of the University of Texas at Austin – kick-started the field of “twistronics”. In this fundamentally new approach to device engineering, the weak coupling between different layers of 2D materials, like graphene, can be used to manipulate the electronic properties of these materials in ways that are not possible with more conventional structures, simply by varying the angle between the two layers.
Infrared light affects TBG’s conductance
Xia and Zhang’s teams have now studied how TBG interacts with infrared light – something that has never been investigated before. In their experiments, they shone light in the mid-infrared region of the spectrum, with a wavelength of between 5 and 12 microns, onto samples of TBG and measured how the electrical conductance varied at different twist angles. They found that the conductance reached a peak at 1.81° and that the photoresponse of the material was much stronger compared to untwisted bilayer graphene. This is because the twist significantly enhances the interactions between light and the material and induces a narrow bandgap (as well as superlattice-enhanced density of states). They also found that this strong photoresponse fades at a twist angle of less than 0.5°, as the bandgap closes.
Further investigations by the team revealed that the TBG absorbs the incident energy of the photons from the infrared light. This increases its temperature, which, in turn, produces an enhanced photocurrent.
Ferromagnetism appears in twisted bilayer graphene
The results suggest that the conducting mechanism in TBG is fundamentally connected to the period of the moiré pattern, and the superlattice produced, which is itself connected to the twist angle between the two graphene layers, Zhang explains. The twist angle is thus clearly very important in determining the material’s electronic properties, Xia adds, with smaller twist angles producing a larger moiré periodicity.
Towards a new class of infrared detectors
The researchers, who report their work in Nature Photonics, now hope to find out whether they can combine photoresponsivity and superconductivity in TBG. “Can shining a light induce or somehow modulate superconductivity? That will be very interesting to study,” Zhang says.
The new work, which also involved Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan, could allow for the development of a new class of highly-sensitive infrared detectors based on graphene. Such detectors could potentially find applications in night vision, for example.
