Jul 20, 2023 0
The Future is Flat
by Jia Wang, Xia Hong
We live in a three-dimensional world. Even objects that seem completely flat, like a piece of paper, are actually three-dimensional, having thickness as well as length and width. Over the last two decades or so, however, scientists have discovered a wide variety of materials that are only a few atoms thick, making them effectively two-dimensional (2D). To give you a sense of just how thin these materials are, they are one hundred thousand times thinner than a piece of paper! But 2D materials aren’t only incredibly thin; they also exhibit exciting, bizarre, sometimes confusing properties.
For example, graphene—the OG 2D material, consisting of a single atomic layer of carbon atoms—is transparent to light, extremely lightweight, flexible, and strong, AND it conducts electricity and heat well. Graphene was discovered using exfoliation, the method shown in Fig. 1, in which scientists used Scotch tape to pick up atomic layers one or a few at a time, much like dead skin cells are removed when you exfoliate your skin. The discovery of graphene triggered the hunt for ever more exotic 2D materials. A major drawback of exfoliation, however, is that it yields only small flakes that have irregular shapes, sizes, and thicknesses. This makes them difficult to use for electronic applications, such as computers and smartphones, where millions of nanoscale devices are tightly packed together.
But we’ve figured out a new way to grow large, super-thin flakes of a 2D magnet chromium trichloride, or CrCl3. Now, here’s where things get cool—actually, here’s where things get hot! We start with CrCl3 powder, which we heat in order to evaporate it. We then use argon (an inert gas that doesn’t react with other things) to waft the vapor down a tube, where it recrystallizes on a surface to form solid 2D flakes of CrCl3 (Fig. 2). And these aren’t just any CrCl3 flakes! Unlike flakes produced through exfoliation, this method gives us regular-shaped flakes with uniform thickness and highly organized crystal structure. The flakes form hexagonal and triangular shapes (Fig. 3). We also created millimeter-long stripes that are only a few nanometers thick—this would be like a single human hair extending half the length of a football field! Even Rapunzel would be stunned!
Once we’ve made our thin flakes of CrCl3, we can use them to study cool physical properties like magnetism. This material is special because it’s a 2D layered antiferromagnetic material. Antiferromagnetic means that, like crayons in a brand new box of crayons, all the atoms in a given layer have their magnetic north poles pointing in the same direction, while the atoms in the neighboring layers have north poles pointing in the opposite direction (Fig. 4).
The small size of the CrCl3 flakes make it difficult to measure their magnetism using conventional methods, so we’ve come up with a neat way to do it. We make a very tiny magnetic sandwich consisting of a thin CrCl3 flake sandwiched by two electrodes. When we apply a magnetic field, something amazing happens—the north poles of all the atoms in the flake align with the magnetic field! This alignment allows the electrons to tunnel through the CrCl3 more easily, so the current flow through the sandwich increases until the tiny atomic magnets are fully aligned (Fig. 5).
The amazing properties of 2D magnets can produce new electronic devices that are small, lightweight, and can do exciting new tricks. Our contribution to this effort started with boiling off atoms, which we guided to form large crystalline flakes that became the filling in tiny sandwiches. Who knows, maybe one day you’ll discover the next big 2D material using an even more surprising technique!
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