Atomically ordered materials are created from units of randomly arranged carbon atoms
Ioannis (Yannis) Petousis
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Left: Fullerenes (purple) and meta-xylene (blue) before the application of pressure. Right: Fullerenes collapse when pressure is applied, but meta-xylene molecules remain in position.

The intimately related terms entropy, order and disorder have intrigued some of the most prominent scholars of modern times, from the mathematician Constantin Carathéodory to the poet and writer Jorge Luis Borges. Without going deep into the subject, let us look at an everyday example and ask the question: can one's room be simultaneously tidy and messy? To the disappointment of many parents, the answer is yes! Imagine, for example, a room where shirts are inside the closet, linen inside the drawer, books on the shelf. However, shirts and linen are not nicely folded and stacked, books are not alphabetically ordered. Don't kids have a valid argument when they claim they, at least partly, tidied up their rooms?

It must have been similar experiences that led scientists at the Energy Frontier Research in Extreme Environments Center, EFree, to create an ordered or crystalline carbon material with disordered or amorphous building blocks.

In crystalline materials, atoms are arranged in regular patterns, while in those that are amorphous, no repeatable pattern can be observed. Interestingly, this is the first time that the aforementioned two states of matter have been combined at the atomic level. The resulting material is harder than diamonds; this is of particular importance for energy conversion and storage applications, because efficiency is often limited by the maximum pressure and temperature that device components can sustain. In addition, the material's electrical and light absorption properties, while not yet measured, are expected to be of importance to the electronics industry.

It is a carbon world! Carbon materials such as graphite (pencil lead), diamond, carbon nanotubes and fullerenes display a gigantic range of mechanical, electrical and electrochemical properties. For example, pencil lead is opaque, soft and electrically conductive while diamond is transparent, hard and an electrical insulator. Both materials consist of carbon and hence, their different behavior results from the arrangement of the atoms. The right combination of mechanical, electrical, electrochemical and light absorption properties is instrumental in enabling cheap and efficient energy devices such as solar cells and batteries. Therefore, scientists are constantly looking for new phases of carbon that will form the basis of novel materials and help solve the energy conundrum. In that respect, the efforts of the scientists at EFree could not be more focused. They are striving to discover new energy-related materials using extreme pressures and temperatures.

The recipe. In this particular case, they started with crystals made with fullerenes and meta-xylene, which are known to have interesting light absorption properties. Fullerenes are hollow, spherical arrangements of carbon atoms, usually around 60 or 70. Meta-xylene is an organic chemical, and its molecules are located in-between the fullerenes resulting in a periodic structure. In the experiment, when high pressure was applied, the fullerenes collapsed, the carbon atoms lost their regular arrangement and hence the clusters became amorphous. However, the meta-xylene molecules remained in position, resulting in a regular structure with amorphous building blocks. The effects were irreversible – when pressure was released, the material remained crystalline-amorphous. It is envisaged that by changing the size of carbon spheres or the meta-xylene to fullerene ratio, the periodic structure and hence the properties of the new material can be tuned to allow the optimal operation of different devices.

More Information

Wang L, B Liu, H Li, W Yang, Y Ding, SV Sinogeikin, Y Meng, Z Liu, XC Zheng and WL Mao. 2012. "Long Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks." Science 337:825-828. DOI: 10.1126/science.1220522

Acknowledgements

This work was supported by the Center for Energy Frontier Research in Extreme Environments, an Energy Frontier Research Center, funded by Department of Energy, Office of Science, Office of Basic Energy Sciences; HPCAT, Advanced Photon Source (APS) with support from the Office of Science, Basic Energy Sciences and the National Science Foundation; calculations used Oak Ridge Leadership Class Computing Facility, supported by the Office of Science, Advanced Scientific Computing Research, and university computing capabilities; and the National Natural Science Foundation of China and Program for New Century Excellent Talents in University (supported Jilin University research group to synthesize and characterize samples).

About the author(s):

Ioannis (Yannis) Petousis is currently a graduate student at Stanford University, School of Engineering. He is a Chartered Engineer with the Engineering Council in the UK and has, in the past, led engineering projects for deep water oil and gas producing facilities. He is a member of the Center on Nanostructuring for Efficient Energy Conversion.

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