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Since the discovery of graphene in 2004, two-dimensional (2D) materials have been a hot topic in the community of condensed matter physics, which can be considered as the candidate for next-generation electronics and also as an important platform to demonstrate interesting physics.
Almost 20 years have gone by, what're the big things that happened in this field, for example in nanoelectronics, and what's the challenge for the future?
Actually, if you allow for quasi-2D systems, graphene has had a recent renaissance starting with the experimental discovery of correlated states in "magic angle" twisted bilayer graphene, which was originally predicted by Bistritzer and MacDonald. By stacking two graphene layers with a relative twist, new structures with very long periodicity can be generated. At certain "magic" angles, flat bands appear, where interaction effects can be strong and generate new interesting phases. More generally, such moiré materials offer a lot of tunability. I'm not sure if they're very useful for applications since at least graphene is very sensitive to the twist angle, but they do provide an exciting and tunable experimental platform to study Hubbard physics and topological states. An inherent challenge to the modeling of these systems is that the superlattice unit cells is large. Some others are outlined here.
There is also plenty of activity in transition metal dichalcogenides ("TMDs"), van der Waals magnets such as CrI$_3$, and MXenes. Here is a 2015 review on 2D materials beyond graphene. One of the practical challenges with 2Ds TMDs is a large contact resistance that so far seems to limit their usefulness for electronics. I'm not sure what the other challenges are. However, similar to graphene mentioned above, many of these materials are now being investigated in few-layer systems, where there may be interesting stacking-dependent behavior and moiré physics.
