By: John Michael Pierobon
Twiggy is considered to be the first ultra thin supermodel. Supermodels of today are not any thinner because the materials and the manufacturing techniques that go into making supermodels have not changed. Unlike supermodels, flexible circuits are more flexible and thinner than ever before because the materials and the manufacturing techniques that go into making flexible circuits are rapidly improving.
These novel techniques, which are being developed all over the globe, are leading to new applications.
Carbon Nanotubes
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Allotropes are different structural modifications of an element. Allotropes of carbon include diamonds (which are bonded together in a tetrahedral lattice arrangement), graphite (which are bonded together in sheets of a hexagonal lattice), and fullerenes (which are bonded together in spherical, tubular, cylindrical or ellipsoidal formations).
Graphene is the basic structural element of several carbon allotropes including graphite, carbon nanotubes and fullerenes. Graphene is a one-atom-thick sheet of carbon with a honeycomb lattice structure. Many sheets of graphene stacked together are collectively called graphite.
Like supermodels, carbon nanotubes are long and slender. They are called carbon nanotubes because of their long, hollow structure, with the walls formed by one-atom-thick sheets of carbon (graphene). Carbon nanotubes have been constructed with a length-to-diameter ratio of up to 132,000,000:1. This ratio is significantly larger than for any other material.
Interesting Properties
Carbon nanotubes are nanowires. Nanowires have length-to-width aspect ratios of greater than 1,000 and have a diameter of the order of a nanometer (10-9 meter). Nanowires have many interesting properties, not seen in three-dimensional materials, which are useful in electronics, nanotechnology, optics, and other fields of materials science.
Structurally, carbon nanotubes have different properties in the axial and radial directions. In the radial direction they are relatively soft, but axially, carbon nanotubes are among the strongest materials found in nature. Carbon nanotubes have an extremely high tensile strength and modulus of elasticity.
Single walled carbon nanotubes (SWCNT) exhibit ballistic conduction. Ballistic conduction is the flow of electrons in a medium with negligible electrical resistivity over very long distances in a material. Ballistic conduction occurs when the mean free path of the electron is much bigger than the size of the nanowire through which the electron travels.
Metallic carbon nanotubes are ideal conductors even in the presence of scatterers. In theory, metallic carbon nanotubes can carry an electric current density of 4 × 109 A/cm2, which is three orders of magnitude above copper.
Transistors fabricated from SWCNT exhibit very high carrier mobility because of their ballistic conduction. Carrier mobility is an important parameter for semiconductor materials because higher mobility leads to better device performance.
Metallic carbon nanotubes are being studied for their applicability in integrated circuits because of their high thermal stability, high thermal conductivity, and large current carrying capacity.
Fabrication
Depending on their rolling angle and diameter, carbon nanotubes act as a semiconductor or be metallic. Finding the right balance between acting as a semiconductor or be metallic has proven to be tricky, but not as difficult as developing processes to mass-produce high quality carbon nanotube-based thin film transistors (TFT) on flexible and transparent substrates.
For years flexible TFT have been produced using a variety of semiconductor materials such as silicon and zinc oxide, which require vacuum deposition, high temperature curing, and complex transfer processes. Organic semiconductors have been rapidly developing, but organic semiconductors still have low carrier mobility and problems with their chemical stability. Carbon nanotube TFT have high carrier mobility and do not have problems with their chemical stability.
Fabrication techniques for carbon nanotube TFT can be divided into three categories: solid phase, liquid phase, and gas phase. For solid phase fabrication, carbon nanotubes supported on a solid and rigid wafer are detached from the wafer and transferred to a flexible substrate. For liquid phase fabrication, soot-like carbon nanotube material is first dispersed in liquid using powerful ultrasound to purify the materials and to separate the carbon nanotubes from each other, and then the solution is deposited on a flexible substrate by a liquid phase approach such as: nanoimprint, spin coating, gravure printing, dielectrophoresis, etc. For gas phase fabrication, a carbon nanotube thin film is formed in a chemical vapor, and using a filtration system, dry-transferred on to a flexible substrate.
Researchers in Finland, and in Japan, and in other parts of the world, are improving these processes which, in the future, will lead to the manufacture low-cost flexible devices such as electronic paper.
Applications
Smart phones and electronic readers such as Amazon's Kindle currently use electronic paper displays. At present, electronic paper is not low-cost, but electronic paper has the advantages of low power requirements as well as being light weight and flexible.
Tablet computers will become paper thin and flexible, while able to display a full range of colors that update quickly enough to accommodate video output. As the cost of electronic paper come down and the functionality improves, there will be more uses for flexible circuits based on SWCNT.
Small electrochemical sensors developed by NASA are using SWCNT to detect minute quantities of acetone, ammonia, benzene, methane, nitrogen dioxide, and toluene in the atmosphere.
In the medical device field, high-performance electronics embedded in flexible surfaces will lead to smaller and more comfortable wearable health monitoring devices. Flexible devices that bend and stretch like skin will soon become a reality.
Researchers at the University of Delaware are working on developing scalable, stretchable power sources for flexible circuits using carbon nanotube macrofilms, polyurethane membranes, and organic electrolytes. This could lead to tiny batteries that bend and stretch like human joints.
The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology.