Carbon-Based Materials with Tunable Mechanical and Electronic Properties

Laser Synthesis of Diamond
The bipartite sodalite type clathrate structure, which consists of truncated octahedral "host" cages that trap strontium "guest" atoms, was synthesized under high-pressure and high-temperature conditions using a laser heating technique. Image is courtesy of Tim Strobel.

When someone hears the word diamond the first thing that comes to mind is often something related to jewelry. However, diamonds are actually mostly used for industrial purposes: 75% of the world’s supply of natural diamonds. Thermal conductivity, strength, hardness, light weight, and electron mobility of diamond (a measure of how rapidly the electrons pulled by an electric field can move through a metal) make it one of very few carbon compounds with special sp3 bonding.

While scientists have made many theoretical predictions over the years regarding the possible existence of these special compounds, only diamond and very few others exist. In other words, the number of compounds scientists have been able to synthesize does not come anywhere close to the number of theoretically predicted.

Zhu and Strobel, along with their team of individuals from Carnegie, the University of Chicago, and the U.S. Naval Research Laboratory helped to close this gap by synthesizing a new carbon-based material in the similar class to diamonds with tunable electronic and mechanical properties.

In its many forms, carbon is a basic building block of nature and is one of the most abundant elements throughout the universe. One-dimensional carbon-based materials such as polymers provide life’s basic building blocks. Two-dimensional carbon-based materials like graphene are useful for advanced technologies, including tribology. The number of three-dimensional carbon-based materials is very limited and includes diamond, lonsdaleite, and a few others. These three-dimensional materials are exceptionally useful for a wide range of uses due to their strong lightweight nature. While scientists predict that many other three-dimensional carbon allotropes (different forms in which an element can exist) should be possible to synthesize, it is not clear whether it will ever be possible to produce these materials in the lab.

The research team of Zhu and Strobel’s looked at clathrates, another three-dimensional bonded class of carbon materials. A clathrate is a structure in which water molecules bond under specific conditions and form complex networks of molecules in a cage-like structure that captures a guest atom. All of the atoms forming the cage structure are linked together with four-coordinate bonds.

Researchers have long been interested in clathrates and have repeatedly attempted to synthesize them over the last half-century. These structures theoretically may have many attractive properties, including shear and tensile strengths greater than the diamond’s ones.

There are many clathrates made up of elements other than carbon that are found in nature or have been synthesized by scientists. However, this is the first time a carbon-based clathrate has been synthesized.

The research team predicted and synthesized the first thermodynamically stable carbon-based clathrate. The atom trapped by this carbon-based structure is strontium. As a result, the synthesized material is metallic and conducts electricity. It is possible the material will exhibit superconductivity at extremely high levels of temperature.

A key feature of clathrates not found in other three-dimensional carbon-based materials is the ability to change their properties based on the type of atom trapped within the cages. The trapped atoms interact with the atoms forming the cage. Therefore, changing the trapped atom allows scientists to alter the properties of the material from a semiconductor to a superconductor. These changes at the same time do not affect the strength of the cage structure so the bonds remain extremely strong.

Further information: “Carbon-boron clathrates as a new class of sp3-bonded framework materials” Science Advances (2020), DOI: 10.1126/sciadv.aay8361

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