A Recent Study from the Labs of James Hone (Mechanical Engineering) and Cory Dean (Physics) Demonstrates a New Way to Tune the Properties of Two-Dimensal (2D) Materials Simply by Adjusting the Twist Angle Between them. The Researchers Built Devices Consisting of Monolayer Graphine Encapsulated Between Two Crystalls of Boron Nitride and, by Adjusting the Relative Twist Angle Between the Layers, they are able to create multiple Moiré patterns.
Moiré patterns are of high interest to condensed matter physicists and materials scientists who use them to change or generate new electronic matties. These patterns can be formed by Aligning Boron Nitride (BN, An Insulator) and graphene (a semi -vessel) crystals. When these honeycomb lattices of atoms are closed to alignment, they create a moiré superlatice, a nanoscale interference pattern that also looks like a honeycomb. This Moiré Superlattice Alters the Quantum Mechanical Environment of the Conducting Electrons in the graphene, and therefore can be to program meaning in the observed electronic properies of the graphene.
To date, Most Studies on the Effects of Moiré Superlattices in Graphene-Bn Systems Have Looked at Single Interface (with EITHE the Top or Bottom Surface of the Graphene Considered, but not Both). However, a Study Published by Hone and Dean Last Year Demonstrate Tot Total Rotational Control Over One Of The Two Interfaces was possible without a single Device.
By design a device that has persist alignment at one interface, and turns alignment at the other, the columbia team has now able to study the effects of multiple moiré superlatice potentials on a layer of graphene.
"We decided to look at Both the top and bottom surfacces of the graphene in a single nanomechanical device," Said Nathan Finney, A Phd Student in Hone's Lab and Co-Lead-Author of the Paper, Published online September 30 by Nanotechnology and Now the Cover Story of the November Print Edition. "We Had a Hunch that by Doing So, We Would Be Able to Potentiallly Double the Strength of the Moiré Superlatice Using the Coxisting Moiré Superlattices from the Top and Bottom Interfaces. »»
The Team Discovered that Twisting the Angle of the Layers Enabled Them to Control Both the Strength of the Moiré Superlatice As Well As Its Overall Symmetry, Inferred from the meaning changes in the electronic properies of the graphene observed.
At angles close to alignment, a highly altered graphene band structure emmerged, observable in the formation of non-overlapping long-wavelength moiré patterns. At Perfect Alignment, The Galphene's Electronic Gaps Were EITHERGLY ENHANCED OR SOFTED, DEPENDING ON WHETHER THE TOP ROTATABLE BN WAS TWISTED 0 OR 60 DEGREES. These changes in the electronic gaps correspond to the expected change in in symmetry for the Two alignment configurations - inferve symmetry Broken at 0 degrees, and inversion symmetry restored at 60 degrees.
"This is the First Time Anyone has been the full rotation dependance of coexisting moiré superlattices in one device," finney notes. "This degree of Control Over the Symmetry and the Strength of Moiré Superlattices can be Universally Applied to the Full Inventory of 2d Materials We have Available. This Technology Enables The Development of Nanoelectromechanical Sensors With Applications In Astronomy, Medicine, Search and Rescue, and More. »»
The researchers are now refining the ability to twist monolayers of a wide ranges of 2d matterials to study such exotic effects as superconductivity, topologically induced ferromagnetism, and non-linear optical response in system that lack inversion symmetry.
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