Atomic Layer Materials
By Jacky Wan
Atomic layer materials, or ALMs, are an exciting new class of materials that come in stacked sheets of atoms with very strong in-plane bonds, but much weaker out-of-plane bonds (see figure to the left). These layers can be isolated, resulting in sheets between one and a few atoms thick, and is as close to a truly two-dimensional system as we can achieve. Confining electrons to two dimensions give rise to exciting new physics which can be easily observed in shifts in electronic behavior as an ALM material is thinned down from a bulk sample. ALMs offer interesting physics due to their low dimensionality, and have been shown to be useful in creating highly efficient photodetectors, integrated circuits, and other opto-electronic devices.
Figure 1: Schematic of an atomic layer material heterostructure consisting of transition metal dichalcogenide layers.
The amount of different ALMs at this point is far too numerous to list, but far and away the most prominent one is graphene, which consists of a single sheet of carbon atoms arranged in a hexagonal lattice. Not only is it the thinnest membrane possible, it also displays incredible electrical, thermal, and structural properties. These properties are also highly thickness-dependent: monolayer graphene behaves much like a metal, although strictly speaking it is not. Bilayer graphene, on the other hand, has properties resembling a semiconductor. Another class of ALMs are the transition metal dichalcogenides (TMDs), which our lab heavily focuses on. These materials consist of one transition metal atom, such as tungsten and molybdenum, and two chalcogen atoms like selenium, sulfur, and tellurium. The TMDs are semiconductors, but once again, the electronic properties of these materials are thickness dependent. As TMDs are thinned down to monolayer thickness, they transition from an indirect to direct band gap semiconductor. This transition means electrons can now be excited efficiently with laser pulses and become free to conduct electricity. One of the most exciting aspects of ALMs is that they can be mixed and matched like atomic-scale Lego, allowing us to custom-build the tiniest electronics possible (one such construction is shown in the bottom right).
Figure 2: Optical images of an atomic layer material heterostructure, different layers are outlined in light blue (graphene), green (hexagonal boron nitride), and blue (graphene).
There are multiple ways to produce ALMs. The two most common methods are chemical vapor deposition (CVD) and mechanical exfoliation. In CVD, the desired base components such as carbon, tungsten, sulfur, are deposited on to a substrate in a controlled environment to allow the growth of the desired material. On the other hand, mechanical exfoliation starts with a bulk piece of the desired material and by using an adhesive tape we can gradually peel away the material, layer by layer. We can then transfer the exfoliated material from the adhesive on to a substrate such as a silicon wafer. Once these individual layers are generated, we can use different polymers to create a stamp to pick them up from the substrate. We can then align and place them on top of other ALMs to produce the desired heterostructures using a custom made, temperature controlled transfer microscope. It is essentially the same as assembling a sandwich, only in our case the sandwich is only microns across, and although it may be less filling in your stomach it is far more intellectually appetizing. Once these heterostructures are created, we can utilize many techniques such as scanning laser microscopy, and ultrafast optoelectronic probes, to observe novel electronic behavior in these heterostructures.