The recent synthesis of one-dimensional van der Waals heterostructures — a type of heterostructure made by layering two-dimensional materials that are one atom thick — may lead to new, miniaturized electronics that are currently not possible.
Engineers commonly produce heterostructures to achieve new device properties that are not available in a single material. A van der Waals heterostructure is one made of 2D materials stacked directly on top of each other like a sandwich. The van der Waals force, which is an attractive force between uncharged molecules or atoms, holds the materials together. The one-dimensional van der Waals heterostructure produced by the researchers is different from the van der Waals heterostructures engineers have produced thus far.
It looks like a stack of 2D-layered materials that are rolled up in a perfect cylinder. In this way, the 2D materials still contact each other in a desired vertical heterostructure sequence. The research suggests that all 2D materials could be rolled into these one-dimensional heterostructure cylinders, known as heteronanotubes, which can work as extremely small diodes with high performance despite their size.
In regular, flat van der Waals heterostructures, confirming existence or absence of some layers can be done easily because they are flat and have a large area. This means a researcher can use various type microscopies to collect a lot of signal from the large, flat areas, so they are easily visible. When researchers roll them up, like in the case of a one-dimensional van der Waals heterostructure, it becomes a very thin wire-like cylinder that is hard to characterize because it gives off little signal and becomes practically invisible. In addition, in order to prove the existence of insulating layer in the semiconductor-insulator-semiconductor junction of the diode, one needs to resolve not just the outer shell of the heteronanotube but the middle one, which is completely shadowed by the outer shells of a molybdenum sulfide semiconductor.
To solve this, the team used a scattering scanning near-field optical microscope that can “see” the objects of nanoscale size and determine their material’s optical properties. A special method of analysis of the data, known as hyperspectral optical imaging with nanometer resolution, can distinguish different materials and test the structure of the one-dimensional diode along its entire length.