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<\/p>\nArticle summary<\/p>\n
\nTwistronics studies what happens when two atomically thin materials, especially graphene, are stacked and rotated slightly, creating moir\u00e9 superlattices that can dramatically alter electronic properties. A small twist can transform materials into magnets, insulators, conductors or even superconductors.
\nThe field gained major attention after the discovery that bilayer graphene twisted to the \u201cmagic angle\u201d of 1.1\u00b0 develops flat electronic bands and becomes superconducting at very low temperatures. These strongly correlated electronic states continue to challenge existing theories and have revealed a range of unexpected quantum phenomena.
\nResearchers are now extending twistronics beyond graphene to materials such as halide perovskites, transition metal dichalcogenides and MXenes. These systems are exhibiting unusual behaviours, including altered exciton transport, exotic superconductivity and other novel electronic effects that could not be achieved in conventional crystals.
\nNew experimental tools, including quantum twisting microscopes that can vary twist angles in situ, are helping scientists explore previously inaccessible quantum states, such as fractional quantum Hall-like phases without strong magnetic fields. While practical applications remain distant, the research could ultimately aid the development of topological quantum computing and potentially guide the design of room-temperature superconductors.
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This summary was generated by AI and checked by a human editor<\/p>\n
Few chemical structures are as instantly recognisable as graphene. The orderly honeycomb construction of this two-dimensional carbon nanosheet has become very familiar to physicists and chemists alike.<\/p>\n
Yet two decades after its discovery, graphene continues to surprise. When two sheets of graphene are layered together, then twisted out of atomic alignment with the other, the familiar honeycomb appearance dissolves.<\/p>\n
As the twist angle changes, the complex patterns the two offset sheets create move from intricate mosaic-like arrangements to arrays of giant hexagons. In each case, the repeating nature of each sheet of graphene\u2019s structure remains, but translated into periodic pattern of far grander scale.<\/p>\n
The visual effect of this twist, known as a moir\u00e9 pattern, is not unique to two-dimensional nanomaterials. \u2018Moir\u00e9 patterns also appear at macroscale,\u2019 says materials chemist Yury Gogotsi from Drexel University in Philadelphia, US. When 2D nanomaterials are twisted into a moir\u00e9 superlattice structure, however, their properties also change, sometimes inverting completely.<\/p>\n
<\/p>\n
A tiny twist can turn a non-magnet into a magnet, or an insulator into a conductor. In the case of graphene, it can turn a conductor into a superconductor. \u2018When people discovered that you could take two layers of non-superconducting material, twist it by roughly one degree and it becomes superconductor, that captured the imagination,\u2019 says Gogotsi.<\/p>\n
In 2026, condensed matter physicists Eva Andrei, Pablo Jarillo-Herrero and Allan MacDonald were awarded the Kavli prize in nanoscience<\/a> for the work that led to the 2018 discovery of superconductivity in magic angle moir\u00e9 superlattice graphene<\/a>. The scene has only got wilder since, says Dahlia Klein from the University of Chicago, US, who was a PhD student in Jarillo-Herrero\u2019s MIT lab at the time of the 2018 discovery.<\/p>\n As more 2D nanomaterials have been combined and twisted, exotic new quantum phases of matter have emerged \u2013 some long hypothesised by theory, others never predicted and currently confounding explanation. \u2018With some of these phases, there is no existing framework to even describe it,\u2019 Klein says.<\/p>\n Chemists are increasingly joining the fun, twisting more complex 2D nanomaterials from covalent organic frameworks (COFs) to MXenes and measuring the effects. \u2018If you can exfoliate it, it is being twisted somewhere in the world right now,\u2019 says Klein.<\/p>\n Magic-angle graphene and flat bands<\/p>\n When Klein joined Jarillo-Herrero\u2019s lab as a graduate student in 2015 after completing her master\u2019s in chemistry, her focus was on growing 2D materials with novel properties. \u2018That work ended up with us being part of the team that discovered the first 2D magnets<\/a>,\u2019 Klein says. \u2018Now, of course, people are twisting 2D magnets and having their own fun, seeing if we can get magnetic textures that wouldn\u2019t exist on their own in a crystal.\u2019<\/p>\n Klein joined the magic angle graphene project toward the end of her PhD. The first hints that twisted graphene bilayers gained unusual electronic properties had come almost a decade earlier. Studying graphene using scanning tunnelling microscopy (STM), Andrei and her team at Rutgers University were the first to look not only at the moir\u00e9 structures in twisted graphene bilayers, but at how the twist affects the material\u2019s properties<\/a>.<\/p>\n As the atoms in the two graphene layers are twisted out of alignment, the material\u2019s electronic structure changes alongside the moir\u00e9 pattern, the team discovered. Most remarkably, at the magic twist angle of 1.1\u00b0, bilayer graphene entered the rare, fabled \u2018flat band\u2019 electronic state.<\/p>\n Whereas chemists generally think of electronic structure in terms of energy levels, condensed matter physicists think in electronic bands. Plotted as a graph with energy on the y-axis and momentum on the x, electronic bands are typically wavy, reflecting the dispersion in velocity of electrons moving through a crystal.<\/p>\n
<\/p>\n Graphene only has a flat band at\u00a01.1\u00b0, and that\u2019s where all the magic happens<\/p>\n
<\/p>\n A flat electronic band denotes the strange scenario that the electrons\u2019 velocity has slowed almost to nothing. \u2018The electrons are not zipping around the material like in a normal metal,\u2019 Klein says. \u2018Their kinetic energy is quenched, close to zero.\u2019 In this state, the electron\u2013electron Coulomb interactions dominate. And that\u2019s when the fun begins, she says.<\/p>\n \u2018In condensed matter, the smoking gun for interesting phases is a flat band,\u2019 says Klein. \u2018And the thing is, graphene doesn\u2019t have a flat band \u2013 it is only at this specific angle of 1.1\u00b0 that the top and bottom graphene layers hybridise to form this flat band, and that\u2019s where all the magic happens.\u2019<\/p>\n The form that magic takes is often a surprise. \u2018Theory has been very good at predicting where there will be flat bands, but poor at describing which of the many competing phases will win in these \u201cstrongly correlated\u201d regimes,\u2019 Klein says. When the Jarillo\u2013Herrero group succeeded in making bilayer graphene at the magic 1.1\u00b0 angle, they didn\u2019t know what properties might materialise.<\/p>\n \u2018The discovery of superconductivity was a true surprise \u2013 and there\u2019s still no consensus on the mechanism,\u2019 Klein says. Magic angle bilayer graphene\u2019s superconductivity, which arises below 1.7K, relates to the way electrons in the flat band form tightly bonded pairs, but the mechanism behind this pairing remains unclear<\/a>.<\/p>\n Even above the superconductivity critical temperature, magic angle bilayer graphene\u2019s properties puzzle. Its electrical resistance increases linearly with temperature over a much wider range than is observed in conventional metals. \u2018The same fingerprint appears in high-temperature superconducting cuprates, where it has resisted explanation for 40 years,\u2019 Klein says. \u2018Nobody predicted it here, and its origin is still debated.\u2019<\/p>\n Difficult to explain superconducting behaviour has also emerged in other \u2018twistronic\u2019 moir\u00e9 system 2D nanomaterial bilayers, including tungsten diselenide. Superconductivity signatures observed in twisted molybdenum ditelluride \u2018may point to a genuinely exotic pairing mechanism\u2019, Klein says.<\/p>\n Twisted perovskite bilayers<\/p>\n Twistronics was initially the preserve of condensed matter physics, but chemists have been paying attention. \u2018We noticed that most of the work on twisted 2D materials was done on very simple materials \u2013 graphene, and transition metal dichalcogenides such as molybdenum or tungsten diselenide \u2013 which are all based on the same type of hexagonal crystal lattice with six-fold symmetry,\u2019 says Letian Dou, a materials chemist at Emory University in Georgia, US. \u2018As chemists, we wondered, could other materials also give interesting properties?\u2019<\/p>\n Dou synthesises and studies the structure\u2013property relationships of halide perovskites, materials that have been in the solar cell research spotlight due to their strong interactions with light. For Dou, twisted bilayer 2D perovskites was an obvious avenue for exploration.<\/p>\n Just making high-quality single crystal 2D perovskite sheets large enough to stack, twist and study, is a challenge. \u2018Perovskite is intrinsically a 3D crystal, it\u2019s very hard to only promote growth on the in-plane direction,\u2019 says Dou. \u2018So, we took another approach<\/a>.\u2019<\/p>\n The team developed an equilibrium-based process for exfoliating ultrathin sheets of methylammonium lead iodide perovskite from a crystalline precursor, triggered by adding a bulky organic surfactant. \u2018Once we remove that surfactant, we can make perovskite sheets with a few nanometres thickness, and stack them to study the coupling,\u2019 says Dou.<\/p>\n The team discovered unusual behaviours when probing the resultant moir\u00e9 perovskites with lasers. Using pump\u2013probe photoluminescence imaging, they generate excitons \u2013 excited-state electron\u2013hole pairs \u2013 in the material and studied their diffusion. \u2018Exciton mobility depended on the twist angle,\u2019 Dou says. \u2018Typically, excitons naturally diffuse \u2013 but in the 10\u00b0 moir\u00e9 material, they are frozen.\u2019 Light emission from these stationary excitons was unusually strong.<\/p>\n The next step to exploring moir\u00e9 perovskite properties<\/a> is to make more robust variants that could withstand electrical contact, Dou says. \u2018The first task is to make these materials more stable and more diverse,\u2019 he says. Methylammonium lead iodide perovskite is very unstable when cut down to ultrathin slivers. \u2018We are trying to make new structures that are intrinsically more stable,\u2019 he says.<\/p>\n The quantum twisting microscope<\/p>\n Since joining the University of Chicago to start her independent research group in early 2026, Klein has been building a custom cryogenic scanning probe microscope called a quantum twisting microscope (QTM). The instrument, which builds on the moir\u00e9 material microscopy techniques developed during her postdoc at the Weizmann Institute of Science in Rehovot, Isreal, will enable Klein and her team to twist 2D nanomaterial bilayers inside the microscope.<\/p>\n The technique exploits that relatively weak van der Waals interactions that stick graphene and related 2D nanomaterial bilayers together. Rather than place a pre-twisted bilayer moir\u00e9 superlattice sample into the instrument, a single layer sample of 2D nanomaterial is added. The second layer of 2D nanomaterial is adhered to the microscope\u2019s custom-made 3D-printed probe tip.<\/p>\n \u2018At the tip\u2019s apex is a 2D contact area a few hundred nanometres in size,\u2019 Klein says. Lowering the tip forms a localised bilayer where the apex meets the monolayer sample. Rotating the tip changes the angle between the two layers. \u2018I like to think of it like a kaleidoscope,\u2019 she says. \u2018Rather than having to make a bunch of samples with different twist angles, now we can twist in situ and try to image what\u2019s happening.\u2019<\/p>\n Escaping some of the constraints of conventional materials synthesis and imaging is partly what makes moir\u00e9 superlattice materials so compelling. Conventional materials have properties fixed by their constituent chemical bonds. \u2018The landscape that electrons move through is defined by the potential set by the atoms in the crystal lattice,\u2019 Klein says. \u2018You\u2019re stuck with that potential.\u2019<\/p>\n The hybrid superlattice structures that emerge in moir\u00e9 nanomaterials, however, offer a new way to play with electronic structure simply by adjusting the twist angle. \u2018These larger superlattice potentials are renormalising the electronic landscape that the electrons sit in \u2013 and so it gives you this totally new tunability beyond changing chemical bonds.\u2019<\/p>\n The much larger unit cell size of a moir\u00e9 superlattice, compared to conventional crystal structure, also lets researchers play with the numbers of electrons populating that landscape. There\u2019s no way you could add an extra electron per unit cell to a regular crystalline material where the unit cell might consist of just two atoms, Klein says. \u2018You can\u2019t add that much charge to a material, it wouldn\u2019t hold it.\u2019<\/p>\n But the superlattice structure of twisted moir\u00e9 materials features much broader-scale periodicity, where a unit cell might contain 10,000 atoms. \u2018It\u2019s easy to add one electron per 10,000 atoms,\u2019 Klein says. With a twist of the voltage knob on a scanning microscope, the full phase diagram of a moir\u00e9 material can be mapped.<\/p>\n Fractional quantum anomalous Hall states<\/p>\n In topological moir\u00e9 materials featuring flat bands, some of the strangest quantum phases behave as if they contain electrons that had been shattered into pieces and carry only a fraction of their usual single negative charge. Fractional variants of a phenomenon called the quantum Hall effect in moir\u00e9 materials are the first time this behaviour has been seen outside of a strong magnetic field.<\/p>\n \u2018Fractional quantum anomalous Hall states [FQAH], with no magnet needed, were predicted for twisted molybdenum ditelluride and then measured experimentally in 2023<\/a>,\u2019 says Klein. Soon after, the related fractional quantum spin Hall (FQSH) phase was seen<\/a> in the same material. \u2018The FQSH effect was predicted some years back, but no one had predicted that this material would host it,\u2019 Klein says. The finding, and the mechanism behind it, remains a topic of hot debate.<\/p>\n
<\/p>\n If we can understand these properties in a clean system, it could help us engineer an actual room temperature superconductor<\/p>\n
<\/p>\n This discovery of fractional materials that do not require a strong magnetic field potentially opens a pathway toward topological quantum computing. This mode of quantum computing has long been theorised, but until now there were no know materials that would permit the concept to be tried out.<\/p>\n Real world computers and devices remain a distant prospect. Moir\u00e9 superlattice material assembly remains at the artisanal phase, involving pristine layers of material carefully peeled from high-quality single crystals. It\u2019s a painstaking practice far removed from repeatable, scalable manufacture.<\/p>\n But today\u2019s pristine, bespoke moir\u00e9 systems offer a way to advance theory and understanding in related materials with real world promise \u2013 such as high temperature superconducting cuprates, which share many features with magic angle graphene. \u2018If we can understand these properties in a clean system engineered through moir\u00e9 heterostructures, it could help us engineer an actual room temperature superconductor using the design rules that we\u2019re learning,\u2019 Klein says.<\/p>\n Exploring chemical space to MXenes and COFs<\/p>\n Chemists are ideally positioned to develop methods that make moir\u00e9 superlattice materials easier to make \u2013 both to accelerate their study in the short term, and perhaps usher them closer to real world applications in the long run.<\/p>\n One angle of research is to improve synthesis techniques for high-quality thin film material growth. The second is to continue exploring novel 2D moir\u00e9 nanomaterials.<\/p>\n MXenes are some of the latest 2D nanomaterials to receive the twist treatment. These relatively complex materials consist of an early transition metal core, covered top and bottom with carbon or nitrogen and topped with surface-terminating functional groups such as halogens or hydroxyl groups.<\/p>\n This complexity lends MXenes some advantages over graphene, says Gogotsy, who co-led their discovery in 2011. \u2018They are true metals, they are highly tuneable, there are almost infinite number of compositions, we can really control the surface chemistry,\u2019 he says. But this complexity means large, high-quality MXene flakes are hard to make.<\/p>\n The first milestone in MXene moir\u00e9 superlattice research was recently reached, when a team at Trinity College Dublin in Ireland used drop-casting to generate randomly overlapping single-layer MXene flakes. They scanned over the resulting samples with STM hunting for moir\u00e9 patterns. It was a slow task, says Gogotsi, who collaborated on the project \u2013 but resulted in the discovery of three MXene moir\u00e9 patterns. One exhibited hints of unusual electronic behaviour.<\/p>\n \u2018Can it lead to superconductivity? We don\u2019t know yet,\u2019 Gogotsi says. Conventional MXene synthesis generates materials with a disordered mixture of functional groups on its surface. The key to maximising electronic coupling between twisted MXene bilayers probably lies in controlling surface functionalisation, Gogotsi says \u2013 an area of strong recent progress in MXene research<\/a>.<\/p>\n \u2018Nowadays we know how to make perfect MXene flakes with just single elements like chlorine or bromine on the surface,\u2019 Gogotsi says. Theory predicts these materials should possess novel electronic properties, he says. \u2018The challenge is to make those materials in sufficiently large flakes that one can twist them and do property measurements.\u2019 More dedicated time at the MXene lab bench beckons.<\/p>\n \u2018When you go to terra incognita and discover something new, you never know how useful it\u2019s going to be \u2013 but it\u2019s always interesting,\u2019 Gogotsi says. \u2018And when chemists, physicists, materials scientists, electrical engineers all work together, there may be some exciting things coming out of all these activities.\u2019<\/p>\n James Mitchell Crow is a science writer based in Melbourne, Australia<\/p>\n","protected":false},"excerpt":{"rendered":"Article summary
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