For the first announcement of “magic-angle” bilayer graphene’s newly discovered twist-tunable electronic properties last year, APS convened a special session in the atrium of the conference venue to accommodate the throngs of attendees eager to hear the details. Editors at Nature may have also considered convening a special section of their journal this week to accommodate the flash flood of announcements reporting evidence of a new kind of quantum optical behaviour in similar twist-tunable stacks of other types of 2D materials.

“We were actually surprised that we didn’t see the effects sooner,” says Xiaodong Xu, principle investigator at the Nanoscale Optoelectronics Laboratory at the University of Washington in the US, and a corresponding author on one of this week’s papers. No fewer than three papers reported experimental results indicating the presence of interlayer excitons trapped in the periodically dappled moiré potential field that results when two atomically thick layers of transition metal dichalcogenide (TMDs) are misaligned. Yet while Xu suggests the discoveries were in some ways due, he highlights how specific the conditions for observing interlayer trapped moiré excitons are, which may explain why they have only just been observed.

“It turns out the moiré effects can be obscured somewhat by an imperfect moiré pattern, which will lead to light emission similar to defects, and excess laser excitation power, which will provide a broad background,” Xu tells Physics World. “In fact, we have seen effects of moiré excitons several years back, but we just did not know what we were looking at and what the evidence for moiré excitons should be.  Once we started to excite the system with lower laser power and perform magneto-optical spectroscopy of samples with different twist angles, we began to realize that moiré-trapped excitons have been there all the time!”

The results highlight the “feasibility of engineering artificial excitonic crystals using van der Waals heterostructures for nanophotonics and quantum information applications” as  Fengcheng Wu at Argonne National Laboratory, Xiaoqin Li at the University of Texas at Austin and their colleagues propose in their report. In addition the work has fundamental significance, providing “an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition metal dichalcogenides,” according to the report by Feng Wang at the University of California, Berkeley, and co-authors.

Excitons meet 2D materials

An exciton is the quantum particle that arises as an electron couples to the quantum “hole” that is left behind when an electron is excited out of its band. They have been studied in many systems including 2D materials and TMDs. In fact the past decade has seen a surge in general in studies of TMDs – chemicals with the formula MX₂ where M is a transition metal, for example molybdenum or tungsten, and X is a chalcogen such as sulphur, selenium or tellurium – particularly in the form of atomically thin 2D materials where a single layer of the metal atoms is sandwiched between two layers of the chalcogen.

Xu’s group, is one of several that have studied the optical response of monolayer TMDs for many years. He describes how a monolayer TMD behaves as a quantum well. However in addition to spin, charge carriers in TMDs have an additional “valley” degree of freedom leading to quantum wells with coupled spin-valley physics. As a result, as far back as 2013, Xu’s group started pushing the idea of stacking two monolayers on top of each other, to mimic the double quantum well structures in other materials such as III-V semiconductors like GaAs/AlGaAs. In addition, they expected the atomically thin nature of these structures would make them very tunable.

“In our previous work, we realized that the system is a very exciting platform for manipulating excitons with long life time and spin-valley degrees of freedom,” say Xu. “In developing the understanding of our experimental results, our theory collaborator Wang Yao and his postdoc Hongyi Yu at University of Hongkong realized that there exists very interesting moiré exciton physics in the twisted heterobilayer system. Since then, we have been working on the experimental realization of these moiré effects.”

Different ways to peel an orange

Although all three papers report moiré excitons in 2D TMDs, there are some important distinctions in what each one demonstrates. Wu, Li and colleagues studied the response of MoSe₂/WSe₂ heterobilayers twisted at an angle of 1° and encapsulated in hexagonal boron nitride. Their photoluminescence studies with circularly polarized light revealed peaks in the spectra indicative of excitons at four distinct energies. Since the exciton “Bohr radius” – an analogue of an atomic radius – is much smaller than the period of the moiré pattern, they attribute the four quantized energy levels to lateral confinement imposed by the moiré potential. As further support for this model, when the twist angle is increased to 2° and the moiré pattern changes, the spacing of their exciton peaks also change and increase, although at significantly larger angles they disappear altogether. Their model also suggests an explanation for the co- and cross-circular emission from their structures.

The heterostructures in the work reported by Wang and colleagues – some of whom are also collaborators with Wu, Li and colleagues – are WSe₂/WS₂ encapsulated in hexagonal boron nitride. These experiments include few layer graphene contacts to observe the effects of tuning the carrier doping with an applied electric field. They observe emission peaks at three slightly higher energies that weaken and also eventually disappear as the angle between the layers is increased beyond 3°. They note that exciting the sample at any of these three emission peak energies leads to strong enhancement of the interlayer exciton emission at 1.409 eV, which they suggest indicates that the three peaks arise from the strongly coupled WSe₂/WS₂ heterostructure rather than from several separated domains. In addition, they observe strong blue shifts in the exciton energies as doping is increased, which affects all the peaks similarly. This effect is not observed in monolayers of the materials and cannot be explained by established electron–exciton interactions in monolayers.

Applying a magnetic field reveals yet more nuances in the behaviour of the moiré excitons as Xu and colleagues report in their study of MoSe₂/WSe₂ superlattices. They find equal and opposing shifts for cross and co-circularly polarized light depending on the magnetic field. They demonstrated that, these slopes, representing the g factors (a dimensionless magnetic moment), are determined by valley index pairing. They also show that the twist in the lattice can compensate for momentum mismatches in the excitons so that they radiatively recombine (known as Umklapp recombination). “We show that we can use g-factors  as a fingerprint to identify moiré excitons,” Xu adds. “This approach should be applicable to understand other optical responses.”

Read more

‘Twistronics’ tunes 2D material properties

Drawing on diverse expertise

So what brought all three reports at once? Xu suggests that advances in theory, greater availability of high-quality heterobilayer samples, and the mounting interest in moiré effects have all played a role. He highlights some of the factors that have made a difference for the Xu Lab in particular, such as the access to clean 3D bulk crystals provided by their long-term collaborators Jiaqiang Yan and David Mandrus at Oakridge National Laboratory, expertise acquired in the fabrication of structures with fine twist control as well as light emission polarization and g-factors measurements, and crucially the theoretical support from Yao to understand the measurements.

“We believe that the basic results are quite experimentally achievable in many other groups; it’s just a matter of making clean-enough samples with different twist angles and using the right experimental conditions (low excitation power, low temperature, magnetic field control, etc),” says Xu. He adds that the diverse expertise within this sector of the research community as highlighted in the differences in the reports this week, can help towards a deeper understanding of moiré excitons for both fundamental physics and potential applications. “It will be important for future progress in this new field to have productive cross-talk or collaboration between our groups and the community as a whole.”

Full details are reported in Nature at DOI: 10.1038/s41586-019-0975-z; DOI: 10.1038/s41586-019-0976-y; and DOI: 10.1038/s41586-019-0957-1