Researchers with the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the DOE’s Lawrence Berkeley National Laboratory (LBNL) grew a twisted multilayer crystal structure for the primary time and measured the structure’s key properties. The twisted structure could help researchers develop next-generation materials for solar cells, quantum computers, lasers and other devices.
“This structure is something that we have now not seen before — it was an enormous surprise to me,” said Yi Cui, a professor at Stanford and SLAC and paper co-author. “A brand new quantum electronic property could appear inside this three-layer twisted structure in future experiments.”
Adding layers, with a twist
The crystals the team designed prolonged the concept of epitaxy, a phenomenon that happens when one variety of crystal material grows on top of one other material in an ordered way — type of like growing a neat lawn on top of soil, but on the atomic level. Understanding epitaxial growth has been critical to the event of many industries for greater than 50 years, particularly the semiconductor industry. Indeed, epitaxy is a component of lots of the electronic devices that we use today, from cell phones to computers to solar panels, allowing electricity to flow, and never flow, through them.
So far, epitaxy research has focused on growing one layer of fabric onto one other, and the 2 materials have the identical crystal orientation on the interface. This approach has been successful for many years in lots of applications, similar to transistors, light-emitting diodes, lasers and quantum devices. But to seek out latest materials that perform even higher for more demanding needs, like quantum computing, researchers are looking for other epitaxial designs — ones that could be more complex, yet higher performing, hence the “twisted epitaxy” concept demonstrated on this study.
Of their experiment, detailed this month in Science, researchers added a layer of gold between two sheets of a conventional semiconducting material, molybdenum disulfide (MoS2). Since the top and bottom sheets were oriented otherwise, the gold atoms couldn’t align with each concurrently, which allowed the Au structure to twist, said Yi Cui, Professor Cui’s graduate student in materials science and engineering at Stanford and co-author of the paper.
“With only a bottom MoS2 layer, the gold is completely happy to align with it, so no twist happens,” said Cui, the graduate student. “But with two twisted MoS2 sheets, the gold is not sure to align with the highest or bottom layer. We managed to assist the gold solve its confusion and discovered a relationship between the orientation of Au and the twist angle of bilayer MoS2.”
Zapping gold nanodiscs
To check the gold layer intimately, the researcher team from the Stanford Institute for Materials and Energy Sciences (SIMES) and LBNL heated a sample of the entire structure to 500 degrees Celsius. Then they sent a stream of electrons through the sample using a method called transmission electron microscopy (TEM), which revealed the morphology, orientation and strain of the gold nanodiscs after annealing at the various temperatures. Measuring these properties of the gold nanodiscs was a essential first step toward understanding how the brand new structure could possibly be designed for real world applications in the long run.
“Without this study, we’d not know if twisting an epitaxial layer of metal on top of a semiconductor was even possible,” said Cui, the graduate student. “Measuring the entire three-layer structure with electron microscopy confirmed that it was not only possible, but in addition that the brand new structure could possibly be controlled in exciting ways.”
Next, researchers wish to further study the optical properties of the gold nanodiscs using TEM and learn if their design alters physical properties like band structure of Au. In addition they wish to extend this idea to try to construct three-layer structures with other semiconductor materials and other metals.
“We’re starting to explore whether only this mixture of materials allows this or if it happens more broadly,” said Bob Sinclair, the Charles M. Pigott Professor in Stanford’s school of Materials Science and Engineering and paper co-author. “This discovery is opening an entire latest series of experiments that we are able to try. We could possibly be on our approach to finding brand latest material properties that we could exploit.”
