California [US], August 29 : NUS scientists have devised a technique to accurately control the alignment of supermoire lattices using a set of golden rules, paving the door for the advancement of next-generation moire quantum matter.
Moire patterns are created when two identical periodic structures are layered with a relative twist angle or when two different periodic structures are overlaid with or without a twist angle.
The twist angle is the angle formed by the two structures' crystallographic orientations. When graphene and hexagonal boron nitride (hBN), both layered materials, are placed on each other, the atoms in the two layers do not line up properly, resulting in a moiré pattern of interference fringes.
As a result, an electronic reconstruction is produced. Topological currents and Hofstadter butterfly states have been created using the moiré pattern in graphene and hBN.
When two moiré patterns are piled on top of each other, a new structure known as a supermoiré lattice is formed. When compared to standard single moiré materials, this supermoiré lattice broadens the range of adjustable material properties, allowing it to be used in a considerably broader range of applications.
Professor Ariando of the NUS Department of Physics led a research team that successfully achieved controlled alignment of the hBN/graphene/hBN supermoiré lattice. This approach enables the accurate placement of two moiré patterns on top of each other.
Meanwhile, the researchers developed the "Golden Rule of Three" to guide the use of their supermoiré lattice creation technique. However, creating a graphene supermoiré lattice has three major obstacles.
First, standard optical alignment relies heavily on graphene's straight edges, but finding a suitable graphene flake is time-consuming and labour-intensive.; Second, even with a straight-edged graphene sample, the likelihood of obtaining a double-aligned supermoiré lattice is 1/8 due to the uncertainty of its edge chirality and lattice symmetry.
Third, while edge chirality and lattice symmetry can be identified, alignment errors are frequently found to be considerable (more than 0.5 degrees) due to the physical difficulty of aligning two different lattice materials.
Dr. Junxiong Hu, the lead author of the research paper, said, “Our technique helps to solve a real-life problem. Many researchers have told me that it usually takes almost one week to make a sample. With our technique, they can not only greatly shorten the fabrication time, but also greatly improve the accuracy of the sample.”
The researchers use a “30° rotation technique” at the start to control the alignment of the top hBN and graphene layers. Then they use a “flip-over technique” to control the alignment of the top hBN and bottom hBN layers.
Based on these two methods, they can control the lattice symmetry and tune the band structure of the graphene supermoiré lattice. They have also shown that the neighbouring graphite edge can act as a guide for the stacking alignment. In this study, they have fabricated 20 moiré samples with an accuracy better than 0.2 degrees.
Prof Ariando said, “We have established three golden rules for our technique which can help many researchers in the two-dimensional materials community. Many scientists working in other strongly correlated systems like magic-angle twisting bilayer graphene or ABC-stacking multilayer graphene are also expected to benefit from our work. Through this technical improvement, I hope that it will accelerate the development of the next generation of moiré quantum matter.”
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