One doesn’t often associate flowers with nanotechnology. But defects in graphene do indeed resemble flower blossoms. What is behind this whimsical occurrence?
Let’s start with graphene. This fabulous material is a lattice of carbon, one atom thick. The carbon atoms are tightly packed into a six-sided repeating pattern that forms a honeycomb of surprising strength. Each carbon-carbon bond in graphene is 0.142 nm long. When you stack sheets of graphene together, you get graphite – the “lead” in lead pencils. To take up one millimeter of space, you would have to stack up three million sheets of graphene. Graphene could be very useful for integrated circuits (IC) because it has high carrier mobility and low noise. The first IC made of graphene was announced in 2011 by IBM.
Getting back to those flower-like defects, they can be made to occur by heating silicon carbide in an ultrahigh vacuum. Under these circumstances, some of the six-carbon rings are converted into five- or seven-carbon rings. Researchers have found a way to string together five- and seven-member rings to form closed loops within the honeycomb lattice. These defects look like flower blossoms when viewed by a scanning tunneling microscope. The mechanism behind the appearance of the defects seems to be that the high heat allows parts of the lattice to rotate, and when cooled, an alternative ring structure is formed.
So why bother putting defects into perfectly good graphene? It seems that by adding some defects to graphene, which is already tremendously strong, you introduce a little flexibility into the structure, making it more resistant to fracturing or tearing. If scientists can produce defects at will, they can create graphene with tailor-made properties for different applications. To take the flower analogy further, the defect is composed of six pairs of five- or seven-member rings, so they form a “bouquet”. There are seven possible different bouquet configurations, and each one would provide a unique set of electrical and mechanical properties.
Graphene is used to build carbon nanotubes. These are basically sheets of graphene rolled into cylinders. They can form very long “ropes” relative to their diameter. They are very good thermal conductors and have many unique properties. Nanotubes can also be multi-walled, which gives them added thickness. The quantum mechanical effects of the carbon-carbon bonding in nanotubes give them incredible strength. There are many possible applications for nanotubes. One promising area of research is the paper battery, a paper-thin sheet of cellulose to which carbon nanotubes have been added. The nanotubes function as electrodes, allowing the substance to conduct electricity. Batteries made of this material would be very energy efficient.