Zurek considers the 60 carbon atoms forming the football-shaped truncated icosahedron of buckminsterfullerene an icon. The structure\u2019s discovery in 1985 won Harry Kroto, Robert Curl and Richard Smalley the 1996 Nobel prize in chemistry. Later, Smalley and scientists like Mildred Dresselhaus suggested stretching these buckyballs into tubes. Sumio Iijima and Toshinari Ichihashi at NEC Corporation in Tsukuba, Japan, found such carbon nanotubes when hunting buckyballs in soot with an electron microscope in 1991. Nanotubes boast tensile strength stronger than steel, and high electric and thermal conductivity.<\/p>\n
Both carbon allotropes caused great public interest. Zurek says nanotubes and buckminsterfullerene became superstars because \u2018they\u2019re just so beautiful and geometric\u2019. \u2018Everyone can relate to that aesthetic,\u2019 she adds.<\/p>\n
For centuries the only carbon allotropes humans knew were diamond and graphite. But in the 20th century scientists occasionally reported new and unusual forms, including these superstars. In 2004, the same year the Royal Society of Chemistry launched\u00a0Chemistry World<\/em>, scientists found two further new nanometre-sized allotropes of carbon, otherwise known as nanocarbons. One was carbon nanodots, fluorescent carbon nanoparticles which are less than 10nm in size, discovered when purifying nanotubes. The other, more famously, was graphene, most often thought of as single sheets of carbon atoms arranged in a honeycomb pattern. Often called a \u2018wonder material\u2019, graphene won its discoverers Andre Geim and Kostya Novoselov the Nobel prize in physics in 2010.<\/p>\n Today, scientists have predicted over 1600 different carbon allotropes, yet we know little about them. Have chemists been carried away by their romance with carbon? We might even have asked the same thing before theoretical possibilities proliferated. The challenges in controllably making graphene and carbon nanotubes mean they\u2019re hard to use widely. As well as being versatile, carbon seems to be fickle \u2013 but considering why it\u2019s fickle raises important questions.<\/p>\n Armed for attention<\/strong><\/p>\n How electrons arrange themselves around carbon atoms make a fascinating muse for chemists, Zurek explains. \u2018It\u2019s very versatile,\u2019 Zurek says. \u2018There could be an infinite number of forms of carbon.\u2019<\/p>\n Two electrons contained in carbon\u2019s 2s-orbital can join two in its 2p orbitals, forming three different types of hybrid orbitals, enabling single, double and triple bonds.\u00a0Kenichiro Itami<\/a>\u00a0from the Nagoya University in Japan, likens different types of bonds to people holding hands. Like most people, sp hybridised carbons have two arms. But sp2<\/sup>\u00a0hybridised ones have three, and sp3<\/sup>\u00a0hybridised ones have four. According to Itami, the important thing about these different carbon people is that they hold on tightly, even in many different arrangements, creating a wide variety of different angles between bonds. \u2018This is what makes carbon different from other elements,\u2019 he stresses.<\/p>\n Theoretical chemists explored single layers of sp2<\/sup>\u00a0hybridised carbons as early as 1947. Yet they thought that such layers would be unstable, according to\u00a0Ashok Keerthi<\/a>\u00a0from the University of Manchester, UK, and the nearby National Graphene Institute. \u2018The discovery of graphene in 2004 [was] a breakthrough,\u2019 he says. \u2018Its extraordinary properties were a monumental surprise\u00a0\u2013 its mechanical strength, electrical conductivity, thermal properties\u00a0\u2013\u00a0and all found in a material just one atom thick. It\u2019s opening new avenues for research, not only for carbon-based materials, but to many other two-dimensional materials.\u2019<\/p>\n Magic angle graphene is especially promising, says Zurek. In experiments, scientists have put sheets of graphene on top of each other, orientated at different angles. In the best-known example, scientists rotated the top sheet by 1.08\u00b0 compared to the bottom one. \u2018A few years ago, people did experiments showing that you can get exotic superconductivity,\u2019 Zurek says. Yet she notes that this research isn\u2019t widely talked about beyond scientific circles: \u2018The language that\u2019s used for it is not as easily accessible.\u2019 Meanwhile, the most inspiring forms of carbon can be difficult to access practically.<\/p>\n We were promised space elevators<\/strong><\/p>\n While graphite and diamond both occur naturally and can be increasingly effectively synthesised, other carbon forms are challenging. Itami notes that it\u2019s not easy to make nanotubes or graphene \u2018in a structurally uniform way\u2019. It\u2019s especially difficult for nanotubes, which can differ in structure, diameter and length and then can\u2019t be readily separated. That\u2019s greatly disappointing for science fiction fans, who might want to use nanotubes\u2019 exceptionally high tensile strength for a \u2018space elevator\u2019. Inspired by the 1979 Arthur C Clarke story\u00a0Fountains of Paradise<\/em>, this concept tethers a space station to Earth with a cable made of something like carbon nanotubes and uses a module to shift people and cargo between them. Graphene nanoribbons, which might be used to form novel composite materials or battery electrodes, suffer from a similar separation problem.<\/p>\n Itami\u2019s team has taken steps towards resolving this \u2018mixture problem\u2019 in nanotubes through its work on single molecule nanocarbons. His team has synthesised carbon nanorings of cycloparaphenylene, made entirely of benzene rings connected by single carbon bonds, equivalent to a single strip sliced off a carbon nanotube. The amounts they have made are smaller than have been achieved for graphene, but are \u2018pretty close to the fullerenes\u2019, Itami says. The researchers have also used nanoring structures as templates to add atoms to,\u00a0making a tiny amount of whole nanotubes<\/a>.<\/p>\n Keerthi likewise works on a bottom-up synthesis method making uniform graphene nanoribbons from small molecule building blocks. \u2018We can control the width and edge structure very precisely, but the lengths vary from a few tens to hundreds of nanometres,\u2019 Keerthi says. While not completely uniform, the method can control nanoribbon edge structures, which defines their electronic and optical properties, he explains. That means nanoribbons can behave as a semiconductor, unlike larger graphene sheets, which are conductors.<\/p>\n Such progress on graphene is encouraging. Yet if we struggle with the most famous new carbon allotrope, which governments have invested billions into researching, what hope do the many less well-known ones have?\u00a0Davide Proserpio<\/a>\u00a0from the University of Milan, Italy, is among the best-placed people to ask.<\/p>\n How are we going to make this?<\/strong><\/p>\n With Artem Kabanov from Samara State Technical University in Russia, Proserpio tracks hypothetical three-dimensional carbon allotropes. In 2016, they set up the\u00a0Samara Carbon Allotrope Database (Sacada)<\/a>\u00a0after different publications predicted the same form of carbon. \u2018Too many hypothetical carbon allotropes have been rediscovered, or better recomputed, many times, so a database would help avoid such repetition,\u2019 explains Proserpio. But since they set up Sacada, the field has exploded, he adds. \u2018We started from about 250 allotropes in 2016, to reach, in February 2024, the huge number of 1661, all without considering 2D allotropes like graphene. New ones are proposed in the literature every month.\u2019<\/p>\n For most, their existence hasn\u2019t been tested. Itami calls this the \u2018unsynthesised problem\u2019. \u2018The question is, how are we going to make this?\u2019 he says.<\/p>\n Proserpio says that the proportion of hypothetical forms that have been made is \u2018tiny\u2019. \u2018There is much speculation, little solid evidence,\u2019 he says. \u2018If we could figure out ways to make them, they might have some interesting properties,\u2019 Proserpio adds. Scientists \u2018would love them, if they could be made\u2019. Among 2D allotropes there is evidence for graphdiyne, Proserpio notes. This version of graphene has four sp carbons along some edges that form two carbon\u2013carbon triple bonds.<\/p>\n Roald Hoffmann<\/a>\u00a0from Cornell University, US, a co-author on the paper unveiling Sacada, is concerned by the large number of unmade hypothetical carbon allotropes. \u2018What the explosion means is that theory and enumeration is much, much easier than real chemistry,\u2019 Hoffman tells\u00a0Chemistry World<\/em>. \u2018And there are a lot of underemployed theorists.\u2019<\/p>\n Carbon and on<\/strong><\/p>\n Further, Hoffmann calls the intriguing cyclocarbon allotropes\u00a0C16<\/sub><\/a>\u00a0and\u00a0C18<\/sub><\/a>\u00a0\u2013 rings made up of alternating single and triple bonds \u2013 as \u2018of little use in the lab or market\u2019. \u2018One has not come up with ways of making moles of them,\u2019 he says. The small rings will be \u2018very, very reactive\u2019, Hoffmann adds, so they can\u2019t be made in solid form at atmospheric pressure. Although visualising them, \u2018especially in reaction sequences, does give us fundamental mechanistic information\u2019.<\/p>\n In fact, abundant theoretical allotropes pose deeper questions that chemists might better spend their time considering. Hoffmann notes that the most stable structure of SiC is a diamond-like lattice, with alternating silicon and carbon atoms. SiC has over 100 allotropes that have been made. Why then, Hoffmann asks, does the supposedly versatile carbon have \u2018only two or so well-characterised three-dimensionally extended allotropes that have been made\u2019?<\/p>\n Meanwhile, Zurek points to two reasons for the allotrope explosion. One is that researchers have developed techniques by which to predict them, and repeatedly apply that principle. She would prefer that researchers publish the design principles instead. The second reason is that universities and funding agencies incentivise academics to publish papers. \u2018People have to meet various publication metrics, and that\u2019s one easy way to do it,\u2019 she says.<\/p>\n A possible solution would build on Hoffmann\u2019s suggestion, and perhaps further explain carbon allotropes\u2019 fickleness in being easy to predict but difficult to make. Zurek would like scientists to predict how to synthesise the allotropes, and how likely synthetic methods are to succeed. It would also be useful if their studies could prioritise which allotropes to study further. \u2018Many of the published works, I don\u2019t think are that revolutionary,\u2019 Zurek says. \u2018But then again, who knows? Maybe you need to do a lot to find one revolutionary candidate.\u2019<\/p>\n![\"\"](\"https:\/\/www.fie.undef.edu.ar\/ceptm\/wp-content\/uploads\/2024\/04\/533124_c0563532trilayer_magicangle_twisted_graphene_illustration_662514_crop.jpg\")
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