Researchers Demonstrate Monocrystals of Hexagonal Boron Nitride Assemble

Tiny steps can make a huge difference to scientists who are keen to develop large wafers of two-dimensional (2D) material.

Rice University researchers determined complementarity between growing hexagonal boron nitride crystals and a stepped substrate mimics the complementarity found in strands of DNA. The Rice theory supports experiments that have produced large, oriented wafers. (Illustration by Ksenia Bets)

Atom-sized steps in a substrate offer the means for 2D crystals growing in a chemical vapor furnace to unite in flawless rank. Researchers have recently noticed this occurrence, and currently, a Rice University team has a notion why it works.

Rice materials theorist Boris Yakobson and researcher Ksenia Bets supervised the construction of simulations that reveal atom-sized steps on a growth surface, or substrate, have the extraordinary ability to maintain monolayer crystal islands in alignment as they develop.

If the situations are ideal, the islands combine together to form a larger crystal without the grain boundaries so typical of 2D materials like graphene grown via chemical vapor deposition (CVD). That safeguards their electronic perfection and characteristics, which vary based on the material.

The theory put forth by Rice appears in Nano Letters, a journal of the American Chemical Society.

The study concentrated on hexagonal boron nitride (h-BN), aka white graphene, a crystal mostly grown via CVD. Crystals nucleate at different places on a flawlessly flat substrate material and not essentially in alignment with each other.

However, latest experiments have shown that growth on vicinal substrates—surfaces that seem flat but really have sparse, atomically small steps—can line up the crystals and assist them in joining into a single, uniform structure, as reported on arXiv. The report’s co-author and leader of the Korean team, Feng Ding, is an alumnus of the Yakobson lab and a present adjunct professor at Rice.

But the experimentalists do not reveal how it works as, Yakobson said, the steps are shown to be meandering and slightly misaligned.

I like to compare the mechanism to a ‘digital filter,’ here offered by the discrete nature of atomic lattices. The analog curve that, with its slopes, describes a meandering step is ‘sampled and digitized’ by the very grid of constituent atomic rows, breaking the curve into straight 1D-terrace segments. The slope doesn’t help, but it doesn’t hurt. Surprisingly, the match can be good; like a well-designed house on a hill, it stands straight. The theory is simple, though it took a lot of hard work to calculate and confirm the complementarity matching between the metal template and the h-BN, almost like for A-G-T-C pairs in strands of DNA.

Boris Yakobson, Materials Theorist, Rice University.

It was vague why the crystals fused into one so well until simulations by Bets, with the assistance of co-author and Rice graduate student Nitant Gupta, revealed how h-BN “islands” stay aligned while nucleating along perceptibly curved steps.

“A vicinal surface has steps that are slightly misaligned within the flat area,” Bets said. “It has large terraces, but on occasion there will be one-atom-high steps. The trick by the experimentalists was to align these vicinal steps in one direction.”

In chemical vapor deposition, a hot gas of the atoms that will develop the material is flowed into the chamber, where they rest on the substrate and nucleate crystals. h-BN atoms on a vicinal surface choose to rest in the crook of the steps.

They have this nice corner where the atoms will have more neighbors, which make them happier. They try to align to the steps and grow from there. But from a physics point of view, it’s impossible to have a perfect, atomically flat step. Sooner or later, there will be small indentations, or kinks. We found that at the atomic scale, these kinks in the steps don’t prevent h-BN from aligning if their dimensions are complementary to the h-BN structure. In fact, they help to ensure co-orientation of the islands.

Ksenia Bets, Researcher, Rice University.

Since the steps the Rice lab demonstrated are 1.27 angstroms deep (an angstrom is one-billionth of a meter), the growing crystals have little trouble conquering the boundary.

Those steps are smaller than the bond distance between the atoms. If they were larger, like two angstroms or higher, it would be more of a natural barrier, so the parameters have to be adjusted carefully.

Ksenia Bets, Researcher, Rice University.

According to the simulations, two growing islands that approach each other zip together effortlessly. Likewise, cracks that creep along steps easily heal as the bonds between the atoms are robust enough to overcome the small distance.

Any approach toward large-scale growth of 2D materials is worth chasing for an array of applications, according to the scientists. 2D materials like insulating h-BN, conductive graphene, and semiconducting transition metal dichalcogenides are all the focus of deep study by scientists worldwide. The Rice scientists expect their theoretical models direct the way toward large crystals of different kinds.

Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry at Rice.

The research was supported by the U.S. Department of Energy (DOE). Computer resources were given by the National Energy Research Scientific Computing Center, supported by the DOE Office of Science, and the National Science Foundation-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in collaboration with Rice’s Ken Kennedy Institute for Information Technology.

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