Scientists at Rice University and Oak Ridge National Laboratory (ORNL) have found a way to control the growth of uniform atomic layers of the semiconductor molybdenum disulfide (MDS).
MDS is considered one of the materials at the core of an area of study around two-dimensional electronics. The goal of 2D electronics is to make working components and devices which are so small they would be invisible to the naked eye.
The research, which combined experimental and theoretical work, aimed to see if large, high-quality, atomically thin MDS sheets could be grown in a chemical vapour deposition (CVD) furnace, and then to analyse their characteristics.
The scientists hope to join MDS with graphene, which has no band gap, and hexagonal boron nitride (hBN), an insulator, to form field-effect transistors, integrated logic circuits, photodetectors and flexible optoelectronics.
In 2012, the researchers succeeded in making intricate patterns of intertwining graphene and hBN. The challenge then lay in finding a semiconducting solution, and then making the material play well with graphene and hBN.
MDS, unlike graphene and hBN, is not structurally flat. It is a stack, with a layer of molybdenum atoms between two layers of sulphur atoms.
The scientists predicted that MDS and carbon atoms would bind, and then together with hBN, form a novel, 2D semiconductor component.
Until recently, growing MDS in a usable form has been difficult. The use of sticky tape to pull layers from a sample, so famously successful with graphene, yielded inconsistent results. Early CVD experiments created MDS with overly small grains.
However, the scientists found they could control the nucleation of MDS grown in a furnace by adding artificial edges to the substrate, then taking the material between those structures.
With that obstacle overcome, the scientists then imaged the atomic structures using aberration-corrected scanning transmission electron microscopy. The atomic array can clearly be seen in the images and, more importantly, so can the defects that alter the material's electronic properties.
These images allowed the researchers to calculate the energies of a complex set of defects, and then find ways to combine the materials, not only in two-dimensional layers but also as three-dimensional stacks.
This means the very different materials with their respective electronic properties and band gaps can be put together in a new type of material that the scientists have dubbed van der Waals solids. They can essentially combine them in whatever stacking order they require.