When it comes to constructing tiny, next-generation technology, carbon’s many forms give it an edge over the competition.
Once confined to the realm of curious scientific speculation, carbon nanomaterials are now poised to enter the mainstream in a profound way. These tiny nanostructures are expected to penetrate almost every aspect of modern life – from health, to advanced computer technology, to fashion.
At this moment in time, the mere mention of the word ‘carbon’ seems to precipitate images of dusty grey plumes of smoke spilling from greasy chimney stacks. For years, carbon has become synonymous with pollution and greenhouse gas emissions. This is due to the prominence of carbon dioxide in popular media for the general public, who may now understandably baulk at the idea of using more carbon in their daily routines, or having carbon materials inserted into their body. It is long past due that someone exonerated carbon’s once good name.
Carbon exists in several different molecular arrangements, called allotropes. Allotropes of carbon should not be confused with carbon-based chemicals, which incorporate the atoms of other elements such as the greenhouse gas carbon dioxide. Allotropes of carbon are composed entirely of carbon atoms, just as gold ingots are composed entirely of gold atoms. Unlike gold, however, materials composed entirely of carbon vary profoundly in their appearance and physical properties.
For example, the carbon allotrope graphite is a soft, grey material with a metallic sheen that can carry an electrical current; whereas diamond, another carbon allotrope, is a transparent crystal, an electrical insulator, and the hardest naturally-occurring material ever discovered. To many, the dichotomy between graphite and diamond is trivial knowledge. But the world of carbon allotropes has long since expanded into very unfamiliar territory.
Laying it on thin
Researchers at the cutting edge of materials science have been experimenting with carbon allotropes so small that their physical dimensions are effectively meaningless in terms of human spatial perception. These nanomaterials are made of single sheets of graphite, called graphene, which consist of a repeating, interlocking hexagonal pattern of carbon atoms.
Graphene is currently the closest approximation of an entirely two dimensional object that can exist in any physical sense. It is only 140 picometers thick (the diameter of a single carbon atom). To put these dimensions in context: the relative height of graphene compared to a regular piece of paper is similar to that of the paper compared to a lighthouse.
Just as paper can be folded into three-dimensional shapes to produce new objects, there are carbon allotropes that consist of warped graphene surfaces. The first of these nanostructures to be discovered was buckminsterfullerene, first reported in 1985, which has the appearance of a seamless graphene sphere. Carbon nanotubes were discovered in 1991, and consist of sheets of graphene that have been rolled and joined at the edges to create a tube. Since these seminal reports, several derivatives of these allotropes have also been discovered, including onion-like fullerenes, carbon nanohorns, and bamboo-like carbon nanotubes.
These carbon nanomaterials can vary in size and complexity, but in general they are around one nanometer to one micrometer in overall size. Strikingly, this is comparable to the scale of the fundamental molecules of life, such as proteins (1-100 nanometers), DNA (2-3 nanometers wide), and physio-biochemical barriers such as lipid membranes (~5 nanometers deep). The similar size of carbon allotropes to natural, biological molecules is advantageous, as it allows these nanomaterials to be readily integrated into complex biochemical environments with minimal disruption to the systems being studied.
Carbon and the cure to everything
Carbon nanotubes have a myriad of peculiar attributes which can be exploited to create new and advanced technologies. In addition to their electrical conductivity, carbon nanotubes display a fluorescence profile. Fluorescent materials absorb short wavelengths of light, and then re-emit them as longer wavelengths: for example, the fluorescent watermarks featured on a standard Australian driver’s licence or passport. When viewed under a black light, the invisible short wavelength UV photons are absorbed by the dye and released at longer wavelengths, revealing the brightly coloured watermark.
Nanotubes absorb and emit light in a region of the electromagnetic spectrum called the near-infrared window. Significantly, biological tissue is transparent to near-infrared light in much the same way as it is transparent to X-rays. This allows researchers to image the organs and vasculature of living organisms, by administering a dose of non-toxic carbon nanotube solution, followed by irradiation of the whole animal with harmless infrared light.
This technique has many potential applications in medical science. For example, tumours can be easily located, providing valuable information to health care providers, without compromising the health of the patient through the use of radioactive or toxic contrast agents. While promising, this is not a one size fits all solution. Powerful imaging technologies that require administering radioactive materials, such as positron emission tomography, reveal a much greater level of detail than is achievable with carbon-based alternatives.
Many carbon allotropes are able to pass relatively freely through cell membranes, making them ideal vehicles for delivering a therapeutic payload into cells. The payload can be virtually anything: drugs and biopharmaceutical agents are obvious candidates, and whole DNA segments are also a possibility. In the latter case, the delivery of functional genes into the cells of patients offers hope for a permanent solution to genetic diseases.
New cancer treatments involving high doses of carbon allotropes are also in development. Tumours require a rich blood supply to sustain their accelerated growth, and develop their own blood vessels to survive. Tumour vasculature is poorly constructed and organized, and does not function nearly as well as the rest of the circulatory system. The abnormal blood circulation in cancer tissue results in a phenomenon called the enhanced permeability and retention effect. This means that some materials carried in the blood can accumulate in tumours, but are effectively cleared from healthy tissue.
Carbon allotropes are especially suited for selective accumulation in cancer cells. With the assistance of an infrared laser, the high concentration of carbon allotropes causes the cancer-ridden parts of the body to get hot, effectively destroying the diseased tissue, while leaving the surrounding healthy cells relatively unaffected. This technique is called photothermal therapy. Carbon nanotherapies are well established in animal models, and the day when these kinds of treatments are considered routine in human patients is steadily approaching. The US Food and Drug Administration is currently in discussions with Stanford University to approve nanotubes in colorectal cancer imaging, and a clinical trial for the delivery of genetic material using nanotube chaperones is currently underway.
Silicon’s impending defeat?
Carbon allotropes have also made significant advancements in nanoelectronic science, with carbon nanotubes in particular under intense investigation. Nanotubes have been produced with a length-to-diameter ratio of 1.32×108 to 1. This is roughly equivalent to the dimensions of a pencil long enough to stretch from Sydney to Brisbane. As the length of these materials is so great compared to their width, they are considered as one dimensional objects.
These quasi-one-dimensional wires also exist in perfect chemical purity. A standard copper wire is never 100% copper, as the presence of trace amounts of other materials is unavoidable during the manufacturing process. These trace impurities can have dramatic effects on the wire’s electrical behaviour.
Nanotubes and graphene have a very stringently defined chemical structure, and there is literally no room for trace impurities to become incorporated inside them. Early nanotube and graphene manufacturing was fraught with problems regarding the precise geometries of the carbon atoms in the material, causing these allotropes to develop kinks and other imperfections that negatively impacted their performance. The scale of manufacture was also limited to very small quantities suitable only for research purposes.
In partnership with Sungkyunkwan University, Samsung has overcome these challenges and is now mass-producing pristine graphene for use in televisions and smartphones with flexible displays. New techniques in nanotube mass production are also expected to support growing nanotechnology industries. The defect-free electronic structure of carbon allotropes imparts extremely high heat and electrical conductivity to these materials, and many new and exciting products may be just around the corner.
The ability of carbon allotropes to interact with light and their electrical behaviour have long promised to bring about the next generation of solar cells. There are several advantages to using carbon, rather than silicon, as the fundamental light-harvesting material in solar panels. In theory, carbon-based solar panel production is cheap and scalable, and delivers devices with higher energy returned on energy invested.
Unfortunately, despite massive collaborative efforts by some of the world’s leading research institutions, this technology is not yet a reality. For the past decade, carbon solar cell power conversion efficiency has been around 1%, whereas conventional silicon-based devices deliver around 15-20%. With these numbers, it seems unlikely that our houses will be adorned with carbon panels any time soon, but there is hope for the future. In 2014, researchers at the University of Kansas, Northwestern University, and MIT reported a new type of solar cell made from mixtures of different types of nanotubes and fullerenes. With this specific composition, the researchers reported power conversion efficiencies up to 3%. While this result represents a substantial leap forward for the field, there is still a long way to go before carbon solar cells can make a respectable stand against their silicon-based rivals.
Another attractive feature of carbon nanotube-based electronics is their flexibility – they can be bent, stretched and rolled without compromising their electrical properties. This makes them especially valuable to the imminent arrival of consumer electronic textiles: fabrics that have electronic components woven into them. Recent innovations in the construction of flexible batteries and microprocessor-like circuits from carbon nanotubes may allow for the fabrication of garments with fully-integrated computers. These clothes could potentially operate a number of nanoscale sensors for monitoring the health of the wearer. The manufacture of this technology is also aided by the fact that nanotubes can be made with modern printing technologies, making large-scale production of complex electronic textiles low cost.
While it might seem like we are approaching the limit of our technological capabilities, the arrival of mainstream carbon-based technology could mark the beginning of a new technological age. The consumer market is already hungry for devices that assist in healthy living: products that monitor basic health metrics such as physical exertion have found a comfortable customer base in developed countries.
As carbon-based technology ramps up, it’s likely that more advanced versions of such wearable devices will include features that allow users to monitor, for example, their blood glucose levels, hormones, and immune responses. It’s fun to speculate, but the reality is we won’t have to wait much longer to see what awaits us in this brave new nanotechnological world. It is natural to feel excited standing at the edge of the silicon age, but it’s sobering to remember that carbon nanotechnology has always been the dark horse. As any seasoned gambler will tell you, it’s too early to place betting odds just yet.
Edited by Bryonie Scott, Nicola McCaskill and Tessa Evans, and sponsored by Henry Le