Only two diamonds are needed to make atomically thick materials thinner

On August 24, the international academic journal "Nature·Physics" published online the research results of researcher Xia Juan, Professor Wang Zenghui and collaborators of the University of Electronic Science and Technology of China "Interlayer of tungsten diselenide-molybdenum diselenide double-layer heterojunction Research on Strong Coupling and High Pressure Control. With the help of a diamond counter-anvil (DAC) device that can generate millions of atmospheres of pressure, they conducted extremely high-pressure research on two-dimensional heterojunction materials that are only one-thousandth of the thickness of cicada wings.

In the research, the scientists cleverly used the structural characteristics of the two-dimensional heterojunction, achieved efficient compression of nanomaterials with only atomic thickness, and observed a series of novel physical phenomena.

Heavier than Mount Tai: Diamond Anvil Technology

"The diamond anvil device has unique advantages in applying ultra-high pressure to tiny samples, and it is a very powerful experimental method." Xia Juan said.

So, how does the diamond counter-anvil device produce ultra-high pressure?

The main part of the diamond anvil device is two pointed-to-point diamonds (diamond anvils), and a gasket that encloses the very small space between the two diamond spires (also called anvil surfaces).

"When we push the two diamonds in the diamond anvil toward each other, the space between the diamond spires is sharply compressed. In addition to the sample, the space is also filled with a liquid pressure medium (such as silicone oil)." Xia Juan explained, Since the gasket is like a hoop, it tightly hoops the liquid pressure transmission medium so that it has nowhere to release, so the pressure in the space where the sample is located will rise sharply, thereby exerting a huge hydrostatic pressure on the sample, similar to The increasing sea pressure when diving into the deep sea.

The reporter learned that because the diameter of the diamond top anvil surface is very small, usually only a fraction of a millimeter, that is, the diameter of almost 4-8 hair strands, the pressure on the bottom plane of the diamond can be efficiently concentrated, so as to reach the top of the diamond. Very high pressure. The pressure of our daily life environment is 1 atmosphere, and the pressure of 10,000 meters under the sea is about 1,000 atmospheres. With this device, a high pressure environment of one million atmospheres can be easily realized.

So how big is the million atmosphere pressure? People often joked that "the pressure is high". Let's use Mount Tai as an example to estimate. The main peak of Mount Tai is 1450 meters high. When the density of the rock is three times that of water, it needs to bear the pressure from the mountain to more than 400 atmospheres when pressed under the mountain. Therefore, "heavier than Mount Tai" is actually far from enough to describe the pressure of diamond on the anvil.

Thin as a cicada's wings: a new two-dimensional heterojunction material

Two-dimensional materials are a new type of material that is currently receiving widespread attention. Its most notable feature is that it can be as thin as only an atomic level and still maintain excellent material properties. How thin is the "atomic level"? A typical cicada wing is a few micrometers thick (that is, about one-tenth of a hair strand), while the two-dimensional materials studied by physicists are generally nanometer-thick, even less than one-thousandth of a cicada wing. Therefore, "thin as a cicada wing" is actually far from enough to describe the thinness of two-dimensional materials.

So what is a two-dimensional heterojunction? "From a structural point of view, it can be understood as stacking different two-dimensional materials in a specific way to form a new two-dimensional material, which is similar to attaching two (or more) different'cicada wings' together. A new kind of "composite cicada wings" is formed." Wang Zenghui said.

For scientists, various two-dimensional materials are like Lego bricks: by choosing different two-dimensional materials and different stacking methods, various novel Lego works-two-dimensional heterojunctions can be constructed. This is equivalent to the possibility of artificially designing almost an infinite variety of new two-dimensional structures, and each structure may have different physical properties of the material. Therefore, in many research fields, two-dimensional heterojunctions are a type of very potential. New material structure.

With the weight of Mount Tai, the thin wings of the cicada

So, since the two-dimensional material is already thin at the atomic level, can it be further reduced in thickness? The expert gave a positive answer.

Xia Juan said: "This research is a bit similar to putting a "composite cicada wing" such as a two-dimensional heterojunction in the middle of a 10,000-ton hydraulic press, and using the extremely high pressure heavier than Mount Tai to make the two "cicada wings" fit together. Closer, so as to change the interaction between the two layers of "cicada wings", and observe the regulation of this process on the performance of the entire "composite cicada wings". It's just that our experiment is carried out at the nanometer scale."

The research team confirmed during the experiment that although the thickness of the two-dimensional heterojunction is already at the atomic level, it can still be further compressed by the pressure generated by the diamond on the anvil device due to its structural characteristics. When the environmental pressure of the sample increased to about 10,000 atmospheres, the researchers successfully observed a sudden change in the band structure and related physical properties of the two-dimensional heterojunction.

"Although this work is a very basic physical research, from an application point of view, carrying out research on the physical properties of new sensitive materials under high pressure is important for the development of new ultra-high pressure sensors, promoting the progress of my country's deep and deep sea detection technology, and accelerating shale The industrial development of modern energy strategies such as natural gas also has very important scientific significance and application value." Wang Zenghui said.

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