The goal of Eric Pop's team was to develop a manufacturing process to turn single-layer chips into practical realities.
The first atomically thin material was measured in 2004 when scientists observed that graphene – a material related to the "lead" in pencils – could be isolated in layers the thickness of a single carbon atom. The scientists who made this finding shared the 2010 Nobel Prize in Physics.
But the process used to make that discovery – the scientists lifted layers of graphene off a rock using sticky tape – was of no use in turning ultrathin crystals into next-generation electronics.
In the wake of the graphene discovery, engineers embarked on a quest to find similar materials and, more importantly, practical ways to fashion atomically thin switches into circuits.
It is on the issue of manufacturability where Pop's team made a big advance. They started with a single layer of material called molybdenum disulfide. The name describes its sandwich-like structure: a sheet of molybdenum atoms between two layers of sulfur. Previous research had shown that molybdenum disulfide made a good switch, controlling electricity to create digital ones and zeroes.
The question was whether the team could manufacture a molybdenum disulfide crystal big enough to form a chip. That requires building a crystal roughly the size of your thumbnail. This may not sound like a big deal until you consider the aspect ratio of the crystal required: a chip just three atoms thick but the size of your thumbnail is like a single sheet of paper big enough to cover the entire Stanford campus.
The Stanford team manufactured that sheet by depositing three layers of atoms into a crystalline structure 25 million times wider than it is thick. Smithe achieved this by making ingenious refinements to a manufacturing process called chemical vapor deposition. This approach essentially incinerates small amounts of sulfur and molybdenum until the atoms vaporize like soot. The atoms then deposit as an ultra-thin crystalline layer on a "handle" substrate, which can be glass or even silicon.
However, the researchers' job was not done. They still had to pattern the material into electrical switches and to understand their operation. For this, they made use of a recent advance led by English, who discovered that extremely clean deposition conditions are essential to form good metallic contacts with the molybdenum disulfide layers. The wealth of new experimental data available now in the lab has also enabled Suryavanshi to craft accurate computer models of the new materials and to begin predicting their collective behavior as circuit components.
"We have a lot of work ahead to scale this process into circuits with larger scales and better performance," Pop said. "But we now have all the building blocks."
Excerpted from Stanford School of Engineering news, "A team of engineers create a prototype chip a mere three atoms thick"