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Enzheng Shi Named as Innovator Under 35 by MIT Technology Review

12, 2022

Email: zhangchi@westlake.edu.cn
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MIT Technology Review named Westlake Prof. Enzheng Shi as a 2022 Innovator Under 35 Asia Pacific. He received the award at a Nov. 14 ceremony that also honored Westlake Prof. Rui Wang, who made MIT Technology Review’s global 2022 Innovator Under 35 list earlier this year, and was attended by Westlake Prof. Jiaxing Huang, chair professor of Materials Science and Engineering Chemistry.

Some 5,000 years ago, you could hear the rustle of jade being cut by the people of the Liangzhu culture. They would dip string into fine quartz sand, and then heave it back and forth across jade to polish the surface. The jade workshop functioned as a materials science lab for the people of Liangzhu, housing cutting-edge technology. Fast forward to the present day, where just 10 kilometers from the Liangzhu ancient ruins lies Westlake University. Westlake also works on state-of-the-art research in materials science: halide perovskite and carbon nanotubes.

Shi waited at the door with a warm smile, still visibly thrilled to be one of the Innovators Under 35 Asia Pacific. “Is materials science for losers? I don’t think so. It is so important that human history can be defined by the materials used at the time, like the Stone Age, Iron Age, and Information  Age,“ Shi said as he walked us into his office. As the visit to his lab continued, it was increasingly clear that materials were also a part of humanity’s  relationship with nature. In that sense, Shi’s lab was like a condensed natural history museum.

Shi’s lab, which sits on the top floor of a four-story alabaster building, has two rooms: one with sinks and a water purification system, and the other filled with glove boxes. Recently put into use, the lab looked clean and tidy.

”Do you prefer green tea or white?” Shi asked as he opened his well-organized fridge. Tea was part of his research life, and our conversation started at the surface.

The people of Liangzhu were obsessed with surfaces, as if they were part of human nature. In the movie 2001: A Space Odyssey, Stanley Kubrick and science fiction writer Arthur Clarke repeatedly cast light on the mysterious black alien monolith, which upset the gorillas. Only one was brave enough to touch it. At that precise moment, something in it changed, and its eyes started to shine with a different light.

Two-dimensional surfaces greatly influence our three-dimensional lives.

One of Shi’s research areas focuses on two-dimensional halide perovskite materials. He took the pile of paper on his desk as an example: Two-dimensional halide perovskite has layers and can be peeled to the nano degree. He then selected a piece of paper and drew the unique octahedral structure of monolayer perovskite.

Perovskite is the umbrella term for a compound structure. In 1839, German mineralogist Gustav Rose discovered natural calcium titanate (CaTiO3) in the Ural Mountains of Russia, in an ABX3 structure. In fact, 90% of the metallic elements in the periodic table can serve as calcium A or B ions in titanium ore.

“They serve as a scaffold,” Shi said.

Perovskite-type minerals may account for an estimated 38.5% of the total volume of the Earth in the depths of its crust. Just like silicon, which comprises 27% of all elements on the earth, ordinary sand can guide humans into the era of silicon-based intelligence.

This scaffolding makes a variety of constructions possible. In the 1970s, scientists developed the use of organic ions (methylamine or formamidine hydrogen) as A ions in the perovskite structure, lead as B cations, and chlorine, bromine, or iodine anions as X anions. Such perovskites have very good photovoltaic properties and can convert light energy into electricity.

People have high hopes for perovskite materials. Thanks to their flexibility, perovskite batteries can be painted on the outside of buildings to power houses, the same way we paint walls.

But it is far from perfect. Perovskite dissolves easily at room temperature, and is vulnerable to water and oxygen. This is why Shi had so many nitrogen-filled glove boxes in his labs – to keep water and oxygen out.

His breakthrough in stabilizing perovskites was published in Nature in 2020. But solar cells are just one of the applications of perovskite. Shi has enjoyed trying to apply materials science to areas few others have explored.  

So how do we stabilize perovskites? Shi pointed to the blue tape on his desk.

Before 2004, scientists believed that ideal two-dimensional materials couldn’t exist in a stable form.

Two researchers from the University of Manchester used adhesive tape to peel off graphite with a thickness of a single atomic layer, that is, graphene.

Adhesive tape is usually just for parcels, but scientists could work magic with it. Single-layer graphene is transparent, with its carbon atoms organized into a firm honeycomb shape. In theory, graphene is 200 times stronger than steel, 1.6 times more electrically conductive than silver, and 13 times more thermally conductive than copper.

As a result, the two scientists won the Nobel Prize in 2010 and launched the era of research into two-dimensional materials.

Two-dimensional layered perovskite could also be peeled layer by layer like graphite. Shi introduced thiophene conjugated organic ligands into the structure of halogen perovskite. This would add a layer of hydrophobic protective film on the surface of the halogen perovskite. The organic ligand was able to isolate water and oxygen, effectively preventing a collapse of the perovskite scaffolding.

Crystallization is a common phenomenon in nature. Snowflakes, mineral crystals under the earth's crust, and even rock sugar in our kitchen are all grown crystals, and perovskite is no exception.

Shi’s lab has different devices for perovskite development: solutions, ball mills and high-speed centrifuges. These devices crystallize the power of nature and condense it into compact form.

The difference is, Shi needs to control the structure of the crystal growth. For example, they would experiment with growing two different kinds of 2D perovskites together to form an epitaxial concentric heterostructure or even a more complex superlattice. Shi showed us a sample of the material under the microscope. Under the ultraviolet light, different types of perovskites displayed different colors.

The colorful images revealed the heterojunction between the two different semiconductors, which was one of the most fundamental yet important subjects.

For example, the multiple quantum wells formed by different semiconductors are also a complex heterojunction. Lasers based on multiple quantum well structures have the advantages of a low threshold of current and stable performance. This is the key area of exploration in Shi’s lab.

While it’s true that most materials science research involves basic material experiments, it can also be key to solving challenges.

Shi posted in his office a quote from Karl Marx from the article Reflections of a Young Man on the Choice of a Profession:

If we have chosen the position in life in which we can most of all work for mankind, no burdens can bow us down, because they are sacrifices for the benefit of all; then we shall experience no petty, limited, or selfish joy, but our happiness will belong to millions, our deeds will live on quietly but perpetually at work, and over our ashes will be shed the hot tears of noble people.

Shi sees himself working for the good of humanity. His research into nano-semiconductor materials has helped develop new material preparation and processing technologies, reveal the corresponding relationship between material structure and properties, and guide the application of new materials, while broadening the boundaries of human knowledge.

Working in the lab is much like working in the kitchen, as there are also sinks, gloves, and equipment working at high pressure and temperatures. “It could be really relaxing to work on experiments, much like cooking,” Shi said.

A Ph.D. student was preparing for an experiment on carbon nanotubes at a corner of the lab. He cleaned the mortar repeatedly to prevent impurities while grinding the catalyst. Materials science involves many of these mundane but important details.

Like graphene, a carbon nanotube consists of rolled up sheets of single-layer carbon atoms that create a hollow structure. Carbon nanotubes can be produced by vapor deposition in a laboratory quartz tube at a high temperature of thousands of degrees. But the current challenge is that the length and orientation of carbon nanotubes are difficult to control. Like graphene, the material has high resilience and electrical conductivity.

Shi proposed a “filtering method” to control the carbon nanotubes produced from chemical reactions. This brand-new directional assembly strategy allows the randomly arranged carbon nanotubes to be assembled into a horizontal arrangement with high orientation, high density, and high capacity for conducting current.

The discovery of a new material property could change the direction of technological applications. With the help of carbon nanotube preparation technology, Shi used carbon nanotube-molybdenum disulfide heterojunctions to form transistor arrays. Such a horizontal array of super nanotube bundles could be used as electrode materials for micro and nano electronic devices, which could be smaller in size and generate less heat than metals.

"The Three-Body Problem described a super thin and strong rope that could cut the hull of a ship in the Panama Canal. If that exists, it would be a carbon nanotube material," Shi said, drawing a line in the air.

He believed that the current carbon nanotubes were strong enough, but scientists have yet to find a way to mass-produce long carbon nanotube materials. His current research focused on fiber failure, which tended to occur at the junction between different carbon nanotubes.

In his fantasy, carbon nanotubes could help us touch the sky. That reminded me of the Tower of Babel. In the painting, the Tower of Babel pierced the clouds and shined under the setting sun. It wasn’t finished, and our path of pursuit continues.