Other 2D Crystals
Graphene is a material that, due to its pure carbon structure, has properties that make it very special. What made the graphene literature explode was its “extraction” from 3D graphite. However, graphite is not the only material that is layered and has striking properties. There is an enormous number of other materials, not based on carbon, that are equally interesting from the basic science and applications point of view.
High temperature superconductors (HTC), for instance, are made out of weakly coupled CuO2 planes and have exotic electronic properties that are still object of controversy, almost 2 decades after their discovery. The strong Coulomb interactions in the Cu d-bands lead to Mott insulating phases and, upon doping, to high superconducting transition temperatures. One of the few points of agreement among researchers is the fact that the essential features of these materials are described by the 2D CuO2 planes. The 3D coupling only serves to stabilize long-range the magnetic and superconducting order that is observed in them. As in the case of graphene, this material can be exfoliated into isolated planes on top of SiO2 substrate (see Fig.1).
Fig.1. Crystal structure of different layered 3D materials and its 2D descendants obtained by exfoliation (adapted from K.S. Novoselov, D. Jiang, F. Schedin, T. Booth, V.V. Khotkevich, S.V. Morozov & A.K. Geim, “Two Dimensional Atomic Crystals” PNAS 102, 10451 (2005)).
Charge density waves (CDW) systems have been a source of research in the condensed matter community for a long time. In these materials, electron-electron and electron-phonon interactions lead to a new type of electronic order with the modulation of the electronic density, that is, an “electronic crystal” (another example is the so-called Wigner crystal that occurs at extremely low electronic densities). Perhaps, the most famous examples of those systems are the so-called transition metal dichalcogenides, (TMD) such as NbSe2, TaSe2, NbS2, or TaS2, which present CDW transition at high temperatures which is accompanied by a superconducting one at lower temperatures. CDW systems tend to be insulating with a gap at the Fermi surface but these materials are better metals in the CDW phase and the superconducting state emerges out of a strange metal, creating an interesting question on the nature of the competition between CDW and superconductivity.
MoS2 is another dichalcogenide which has layered structure but, unlike NbS2, is an insulator with a gap of order of 2 eV and hence not chemically reactive. This material can also be exfoliated (see Fig.1) and is commonly used as a lubricant (in analogous way to graphite). Because of its semiconducting properties, MoS2 can, in principle be used for DC transistors. However, so far, its electronic mobilities are much lower than graphene and its gap is too large for practical device applications. The electronic properties of this material maybe tailored by reduction of the gap and/or increase of the mobility with the use of Li. Li can be intercalated in the 3D material and it might be possible to do so in its 2D version.
The common theme among these systems is that they are layered and, in principle, can be exfoliated to their 2D form for studies. Obviously these are only a few examples but there are numerous others: layered manganites, such as La1-xSr1+xMn2O4, which are magnetic and present colossal magneto-resistance (CMR) and metal-insulator transition (MIT); layered titanates, such as Na2Ti3O7 or H2Ti3O7, which present strong photoelectric effects and can be used as ion exchangers, adsorbents, photo-catalysts, and catalyst supports; and layered cobaltates, such as NaCoO2 which present a fascinating number of many-body phenomena such as magnetism, and charge ordering, and LiCoO2, which are usually used in high performance lithium-ion batteries. All these materials are interesting on their own right, namely, they present correlated electronic states where charge, spin, orbital, valley, lattice degrees of freedom play an important role. These electronic states are stabilized by the weak interaction between layers but it is not know how these states will behave when the layers are actually decoupled. Thermal and quantum fluctuations are much stronger in 2D than in 3D and electronic states are very sensitive to those. The possibility of creating pure 2D strongly correlated electronic states is a dream of condensed matter researchers.
The most important lesson from the graphene story is probably this one: there is a universe of 2D crystals out there just awaiting to be studied. Each one of them has its own beauty and purpose. Paraphrasing Isaac Newton we can say that we are still in the infancy of a broad field and diverting ourselves with graphene, a material that looks more interesting than ordinary, whilst a great field of 2D crystals lay all undiscovered before us.