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Thursday, 25 February 2021

Discovery

Carbon nanotubes were known before bucky balls – discovered in 1985 by Harry Kroto, Richard Smalley and Robert Curl. Yet eight years later, in 1993, Nature published two independent papers recording the ‘new’ breakthrough discovery of rolled up graphene tubes forming close-ended pipes. How does this make sense?

The question of who ‘discovered’ carbon nanotubes is difficult to give a simple answer to. Like many material discoveries, there is more than one level of known and unknown. Although the debate over which individual deserves the title ‘discoverer of oxygen’ cannot be firmly settled, our choice of answer forms part of the foundation by which we understand the nature, concept and goals of science as a field. And don’t forget, recognition can be career-making.
 
Riichiro Saito, saito@mgm.mit.edu, rsaito@ee.uec.ac.jp
 
The discovery of oxygen is most usually credited to the Englishman Joseph Priestley, who was the first to publish and so alert the rest of the world, in 1774. But there are two other contenders: Carl Wilhelm Scheele actually synthesised oxygen in Germany at least a year earlier, but delayed publishing his findings and so lost any proof of prior claim. Others argue that the French nobleman Antoine Lavoisier was in fact the first to discover “oxygen”. He certainly named it. But although Lavoisier did not repeat the synthesis of this gas and explore its properties until 1775, he was the first person to recognise oxygen as an element and thus underpin our modern understanding of the finding. Priestley and Scheele saw oxygen as “dephlogisticated air”, a substance able to absorb phlogiston from burning materials and let them stay aflame for longer. Because of this, they named it after the Greek for “fire-bringer”. Perhaps they “mis-discovered” it. Yet the problem with crediting Lavoisier with the discovery of oxygen is that our idea of what science is is constantly changing – and so our idea of what oxygen is may change too.

Other types of discoveries are tricky in different ways, such as recurrent “rediscoveries”, like high temperature superconductors. In the quest to find a material that can levitate a magnet above it at room temperature, each material which beats the last temperature record becomes a new triumph of science. This is like an athlete who beats the world speed record for her division. Every time a new speed or temperature is reached, that supersedes the last record and is the new “best”.


Runner: the new speed record is always the best one, and the last “best” loses it’s value every time a record is beaten. Dru Bloomfield
 
It may be easier to understand discovery as a process rather than event – contrary to popular perception. The discovery of carbon nanotubes is a good example of this.

If you type “Define: carbon nanotubes” into Google, it describes a cylindrical allotrope of carbon. There is a discrepancy here. When we talk about carbon nanotubes, we usually imagine single layer thick tubes – but this isn’t mentioned. What the definition does mention is the novel properties of carbon nanotubes – and this is what was discovered in the 1990s.

The discovery of bucky balls marked the starting flag for a race to uncover more and more exciting carbon allotropes. Bucky balls showed us something unexpected: that even the most unexciting, everyday element can be bizarre and useful in surprising ways. Dr. Richard Smalley, one of the discoverers of the buckminsterfullerene, even ‘predicted’ the formation of carbon nanotubes in 1990 – and pretty much drove out the scientists to find them.


Wikipedia via Mstroeck and Bryn C.

 
We can break “carbon nanotubes” into four different categories, attributing the discovery differently for each one.

1. Carbon tubes


Carbon nanotubes may have been discovered as long ago as 1889, when R. Bacon recorded the formation of narrow carbon tubes whilst he was decomposing methane. The following year, other researchers reported a similar finding to the French Academy of Sciences[1]. These may have been nanotubes – but no claims of nano-thickness walls accompanied the discoveries: even the best optical microscopes can only resolve down to micrometres, and there were no transmission electron microscopes about to do better. With such microscopes, the nanotubes would have looked like hairs. No claims of exciting properties were forthcoming either: samples simply weren’t pure enough.

2. Multi-walled carbon nanotubes


The first transmission electron microscopes (TEMs) were distributed by Siemens in 1939. These use accelerated electrons rather than light to image things down to almost the size of a single atom. Light has wavelengths between 400 and 700 nm, so can only image things bigger than this, whereas electrons may have wavelengths 100,000 times smaller. In fact, wavelength is tunable: it depends on how fast the electrons are moving – with a limit at the speed of light. Although early TEMs were not as high resolution as modern ones, it was through their use that the first nanoscale objects could be viewed.

In 1952, L.V. Radushkevich and V.M. Lukyanovich published the first reports of nano proportioned carbon tubes in a Soviet journal[2]. They measured their tubes at 50 nm in diameter and described their appearance, but the journal was in Russian, the Cold War was raging, and the paper was little read.

This may explain why Suomo Iijima was credited by much of the scientific world (and himself) as the unique discoverer of carbon nanotubes, when he rediscovered them in 1991. His experimental process after all was new, why not the products of it? His nanotubes were even smaller, and more varied – ranging from 3 to 30 nm in diameter, mostly straight, with multi-layers of walls and both ends of the tube closed.

But Iijima was not the even the first scientist to “rediscover” multi-walled carbon nanotubes: Roger Bacon of Union Carbide had reported straight, hollow carbon tubes built of graphic layers in the late 1950s[3], and John Abrahamson presented them at the 14th Biennial Conference of Carbon in 1979.

3. Single-walled carbon nanotubes


Doing further experiments to try to fill his carbon nanotubes with transition metals, Iijima did produce and first recognised single-walled carbon nanotubes, which may be readily distinguished from multi-walled carbon nanotubes because they are often curved and rarely more than 1-2 nm across. Even closer inspection reveals that only one layer of graphite surrounds the open tube. However, Iijima’s discovery was not unique: incidentally, a California team, attempting the same experiment, made exactly the same discovery, submitting their paper to Nature only 31 days later.

Although the recognition of single-walled carbon nanotubes was undoubtedly new in 1991, it may even be debated whether the discovery was new: they were not the first to be produced nor imaged. Morinobu Endo had published evidence of single-walled carbon nanotubes in both his doctoral thesis and a 1976 paper, although, perhaps not realising the distinction, he did not refer to them as single-walled until after 1993.

4. Carbon tubes with cool properties


Iijima also made one other major contribution to the field of carbon nanotubes: the production of structurally perfect nanotubes that exhibited exciting properties.

The open porous structure of graphene means that graphene-based materials are highly absorbent; they can filter gases or separate out harmful materials dissolved in wastewater. Extremely lightweight and extremely tough, carbon nanotubes can handle pressures 630,000 times higher than the weight of the atmosphere. Because of their strength, carbon nanotubes have been proposed for bullet proof vests, and their interesting electrical and conductive properties arising from delocalised electrons travelling down the nanotubes made them a recent favourite for invisibility cloaks.

References
why don't all references have links?

[1] Schützenberger P, Schützenberger L. Sur quelques faits relatifs à l’histoire du carbone, C R Acad Sci Paris 1890;111:774–8.
[2] Bond Energies. Organic Chemistry, Michigan State University.
[3] Bacon R, Bowman JC. Production and properties of graphite whiskers. Bull Amer Phys Soc 1957; 2:131.

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