These materials scientists are a ‘power couple’ in the physical sciences

From rejected material to career-defining discovery.

  • Gemma Conroy

These materials scientists are a ‘power couple’ in the physical sciences

From rejected material to career-defining discovery.

5 May 2020

Gemma Conroy

Kenji Watanabe (left) Mark Zastrow

Kenji Watanabe (on the left) and Takashi Taniguchi synthesize high-quality hexagonal boron nitride (h-BN) crystals, which have important applications in the study of two-dimensional materials.

Two decades ago, Takashi Taniguchi had one goal: to produce a flawless piece of cubic boron nitride (c-BN), an ultra-hard material with a similar crystal structure to diamond, so he could explore the material’s potential as a semiconductor.

While Taniguchi spent hours trying to produce pure c-BN crystals without defects in his lab at the National Institute for Materials Science (NIMS) in Tsukuba, Japan, it was the by-products of his work that caught the attention of his research partner, Kenji Watanabe.

These tiny crystals of hexagonal boron nitride (h-BN) would soon become the material that Taniguchi and Watanabe are today renowned for. Since publishing their first paper on the material’s ultra-violet properties in 2004, the pair have worked together to produce high-quality h-BN crystals that are coveted by materials scientists around the world, as reported by Nature in August 2019.

Taniguchi says their crystals have made them “somehow famous” in the field of two-dimensional materials.

“Why Watanabe and Taniguchi? Because their crystal is the best,” Philip Kim, a condensed-matter physicist at Harvard University in Cambridge, Massachusetts told Nature.

Part of the story of their success is told by Dimensions data showing that the pair have appeared as co-authors on 1,123 papers and preprints, including 452 papers published in the 82 high-quality journals tracked by the Nature Index, since 2011.

In 2019 alone, they co-authored 127 physical sciences papers in Nature Index journals, more than any other pair of researchers in the field, based on co-publications. The majority of their Nature Index papers appear in Nano Letters, Physical Review Letters, and Physical Review B, which are among the most prominent physical sciences journals.

Below is Taniguchi and Watanabe’s co-authorship network in 2019 for physical sciences papers published in Nature Index journals. Zoom in and hover the mouse over the lines to view the number of co-publications between authors, as shown by link strength.

In-demand collaborators

Taniguchi and Watanabe’s signature crystals have made it possible for researchers to study how two-dimensional materials, such as graphene, conduct electricity. Their h-BN crystal sheets provide a flat, protective surface that supports one-atom-thick layers of graphene without interfering with the material’s electronic properties.

The pair supply samples of their crystals to several materials science labs internationally, and say this has helped them improve the quality of their product. The researchers have won several international patents for their h-BN synthesis methods.

“We learn a lot from the international collaborators who use our crystals in their work,” says Taniguchi. “This is what drives our research forward and keeps us motivated.”

Key to a ‘magic-angle’ graphene device

Taniguchi and Watanabe’s crystals have played a key role in the field of ‘twistronics’, a term coined in 2017 to describe study of how the electrical properties of two-dimensional materials can be changed by adjusting the angle of their layers. In ‘magic-angle’ graphene, for example, misaligning each layer by 1.1 degrees allows electrical currents to flow without resistance.

In 2019, h-BN crystals were a component in a magic-angle graphene device developed by an international team of researchers from the United States, Spain, and China. By making small tweaks to the voltage, the device can switch from being a superconductor to an insulator, with great potential for use in electronics.

The Nature paper about this development, which lists Taniguchi and Watanabe as co-authors, has accrued 45 citations since it was published in October.

Taniguchi attributes a large part of the success of his partnership with Watanabe to the fact that each brings a different perspective to the table. Taniguchi’s materials science background has made him an expert on crystal growth, while Watanabe’s knowledge of optical physics offers clues on how to accelerate and fine-tune the process.

Without Watanabe’s keen interest in h-BN early on, Taniguchi admits that his contributions to research on two-dimensional materials “would never have happened”.

Watanabe says that the material itself also keeps their partnership interesting. “Boron nitride always shows fascinating properties,” he says.

The network map was created using VOSviewer.