Swedish Researchers Create New 3D Carbon Allotrope in the Lab
A team at the University of Gothenburg has synthesized diamondiyne, a porous carbon allotrope whose atoms are covalently bonded in all three dimensions, a feat previously achieved only with diamond.
Chemists at the University of Gothenburg in Sweden have synthesized a new form of carbon whose atoms are covalently bonded in all three dimensions, a structural property previously known only in diamond. The material, dubbed diamondiyne, was produced at relatively low pressure and forms a porous crystal whose properties remain under investigation.
Carbon is among the most versatile elements in the periodic table, able to bond in one, two or three dimensions and thus serve as a building block for structures with widely varying electronic properties. That versatility is visible in carbon's allotropes: fullerene, regarded as zero-dimensional; one-dimensional carbon nanotubes; and two-dimensional graphite and graphene. The search for further allotropes accelerated after graphene was first isolated in 2004. Researchers have since predicted 1,635 possible carbon structures, though most have never been made in the laboratory.
Among the few that have been realized is graphdiyne, an expanded version of graphene composed of carbon atoms with two kinds of hybridized electronic orbitals, sp2 and sp. Diamondiyne extends that logic into three dimensions. In its structure, sp3-hybridized carbons are connected by four sp-hybridized carbons that form two triple bonds, a unit known as a diyne group. The arrangement amounts to a carbo-mer of diamond.
Efforts to make diamondiyne began more than 30 years ago but stalled for two reasons. One of the material's building blocks, tetraethynylmethane, is unstable. And the carbon atoms in diamondiyne are linked by robust single carbon–carbon bonds and even stronger triple carbon–carbon bonds, making it difficult to form a crystal that is ordered in all three dimensions.
The Gothenburg team, led by physical chemist Karl Börjesson, overcame both obstacles with a cascade reaction whose kinetics they could control. The reaction takes place at a liquid–liquid interface, which ensures that monomers attach to a growing crystallite one by one. The liquid system consisted of tetra(triethylsilylethynyl)methane and CuF2. When fluoride ions from the latter react with the former, they produce tetraethynylmethane with exposed terminal alkyne groups. Copper ions then catalyse coupling between pairs of those terminal alkynes.
As the reaction proceeded, the researchers observed a film of diamondiyne crystallites growing perpendicularly out of the interface between the two liquids. Keeping the concentration of the starting compound relatively low further encouraged the molecule-by-molecule addition that produced the crystals. «To our knowledge, diamondiyne is the first allotrope besides diamond in which the carbon atoms form covalent bonds in all three dimensions,» Börjesson said.
The finished films measure around 5 square centimetres and range from several nanometres to tens of nanometres in thickness, depending on reaction time. Transmission electron microscopy confirmed that the crystal structure persists over volumes of 10 cubic nanometres. Liquid–liquid interfaces had previously been used to make graphdiyne and other two-dimensional materials; the new work extends the approach to a three-dimensional covalent framework.
The researchers had spent several years working with similar covalent organic frameworks, mainly focusing on carbon–carbon coupling reactions. «We then realized that we could use a similar synthetic methodology to make this new carbon allotrope,» Börjesson said. The team now hopes to grow larger crystallites so that the material's properties can be measured as accurately as possible. «We will need to do this to find out what we can use the material for,» he added. The findings were published in Angewandte Chemie.
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