For decades, scientists have been investigating the potential of two-dimensional materials to transform our world. 2D materials have only one layer of atoms. Subatomic particles such as electrons can only move in two dimensions within them. This simple restriction can cause unusual electron behavior, imbuing the materials with “exotic” properties such as bizarre forms of magnetism, superconductivity, and other collective behaviors among electrons – all of which could be useful in computing, communication, energy, and other fields.
However, researchers have generally assumed that these exotic 2D properties exist only in single-layer sheets or short stacks. The so-called “bulk” versions of these materials, with their more complex 3D atomic structures, should behave differently. Or so they thought.
A team led by University of Washington researchers reports in Nature that it is possible to imbue graphite – the bulk, 3D material found in No. 2 pencils – with physical properties similar to graphite’s 2D counterpart, graphene. This breakthrough was not only unexpected, but the team believes its approach could be used to test whether similar types of bulk materials can also acquire 2D-like properties. If this is the case, 2D sheets will not be the only source of fuel for scientists to fuel technological revolutions. Bulk, 3D materials may be equally useful.
Stacking single layer on a single layer – or two layers on two layers – has been the focus for unlocking new physics in 2D materials for several years now. In these experimental approaches, that’s where many interesting properties emerge.
Matthew Yankowitz
“Stacking single layer on single layer – or two layers on two layers – has been the focus for unlocking new physics in 2D materials for several years now. In these experimental approaches, that’s where many interesting properties emerge,” said senior author Matthew Yankowitz, a UW assistant professor of physics and of materials science and engineering. “But what happens if you keep adding layers? Eventually it has to stop, right? That’s what intuition suggests. But in this case, intuition is wrong. It’s possible to mix 2D properties into 3D materials.”
The team, which also included researchers from Osaka University and Japan’s National Institute for Materials Science, adapted a common method for probing and manipulating the properties of 2D materials: stacking 2D sheets together at a small twist angle. Yankowitz and his colleagues created a twist angle of around 1 degree between graphite and graphene by layering a single layer of graphene on top of a thin, bulk graphite crystal. They discovered novel and unexpected electrical properties not only at the twisted interface, but also deep within the bulk graphite.
According to Yankowitz, who is also a member of the UW Clean Energy Institute and the UW Institute for Nano-Engineered Systems, the twist angle is critical in generating these properties. A moiré pattern is created when a twist angle between two 2D sheets, such as two sheets of graphene, alters the flow of charged particles like electrons and induces exotic properties in the material.
The twist angle induced a moiré pattern in the UW-led graphite and graphene experiments, with unexpected results. Despite the fact that only a single sheet of graphene atop the bulk crystal was twisted, researchers discovered that the electrical properties of the entire material differed significantly from that of typical graphite. And when they turned on a magnetic field, electrons deep in the graphite crystal adopted unusual properties similar to those of electrons at the twisted interface. Essentially, the single twisted graphene-graphite interface became inextricably mixed with the rest of the bulk graphite.
“Even though we were only generating the moiré pattern on the surface of the graphite, the resulting properties were bleeding across the entire crystal,” said Dacen Waters, a UW postdoctoral researcher in physics and co-lead author.
Moiré patterns generate properties that could be useful in quantum computing and other applications for 2D sheets. Inducing similar phenomena in 3D materials opens up new avenues for investigating unusual and exotic states of matter, as well as how to bring them out of the laboratory and into our daily lives.
“The entire crystal takes on this 2D state,” said co-lead author Ellis Thompson, a doctoral student in physics at the University of Washington. “This is a fundamentally new way to affect electron behavior in a bulk material.”
Yankowitz and his team believe their approach of generating a twist angle between graphene and a bulk graphite crystal could be used to create 2D-3D hybrids of its sister materials, including tungsten ditelluride and zirconium pentatelluride. This could unlock a new approach to re-engineering the properties of conventional bulk materials using a single 2D interface.
