The shortest carbon nanotube having one molecule high carbon is called “nanohoop”. It is a tiny ring of carbon, called cycloparaphenylene and will have impact on the development of faster electronic devices, more powerful sensors, and other advanced technologies.
Nanohoops
Single-walled carbon nanotubes (SWNT) can be thought of as built from component macro cycles, often called nanohoops. For example, cycloparaphenylenes can be the thought of as the precursor (at least in principle) of armchair SWNTs. To create chiral SWNTs, cycloparaphenylene-naphthalene and other acene substituted macro cycles can serve as appropriate precursors.
Nanohoops are macro cycles formed of aromatic rings linked in a 1, 4’ fashion. The nanohoops contain 3-24 repeat units. The strain energy of the nanohoops exponentially decreases with the number of building blocks n, and this strain strongly correlates with the bend angle at the ipso carbons.
Cycloparaphenylene synthesis offers a more targeted approach. This family of benzene-derived compounds forms the smallest possible carbon hoop structure, one molecule high. It also has a fixed diameter and orientation, the two variables that determine a nanotube’s electronic properties. Because of this, cycloparaphenylene molecules could possibly be used as seeds or templates to grow large batches of carbon nanotubes with precisely defined structures.
Carbon nanohoops have smaller optical absorption gaps and this counterintuitive trend, opposite to that expected from ordinary quantum confinement, reflects a large increase in electron-hole interaction strength with decreasing hoop diameter.
Silver nanohoops
Silver nanohoops are a metamaterial and exhibit an antisymmetric resonance that presents a highly negative real part of the permeability at visible wavelengths. The strength of this magnetic resonance is easily tunable through the inner radius of the nanohoops.
Synthesis
The hoop-shaped chain of benzene molecules had eluded synthesis, despite numerous efforts, since it was theorized more than 70 years ago. Their strained and distorted aromatic systems and radially oriented p orbitals have intrigued synthesis. The first synthesis and characterization ofcycloparaphenylene was demonstrated utilizing a novel aromatization reaction.
The heart of the synthetic challenge lies in overcoming the strain energy required to bend a string of benzene rings which normally resist bending into a hoop. The strain is considerable and increases with decreasing ring size: 5, 28, and 47 kcal/mol for hoops with 18, 12, and 9 benzene units, respectively.
Researchers used a strategy that involved the build- up of strain sequentially during the synthesis, using carefully selected small molecule precursors in combination with a cyclohexadiene molecule designed to provide the curvature and rigidity necessary for the ring to form. The strategy was successful and rings with 5, 8, and 14 benzenes were obtained in good yield (>35%).
Carbon nanotubes
Carbon nanotubes are hollow wires of pure carbon and can be semi conducting or metallic depending on their structure. Carbon nanotubes also feature extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). These features make carbon nanotubes ideal candidates for electron field emitters, white light sources, lithium secondary batteries, hydrogen storage cells, transistors, smaller computers, or tiny powerful sensors even to detect a single molecule and cathode ray tubes (CRTs).
In spite of this feature they have not yet penetrated much into the electronics or other sectors, because they are difficult to make with defined structures in large quantities. At present, they are produced in batches in laboratories, with only few nanotubes in each batch possessing the desired characteristics. This approach is inefficient for commercial applications. Hence scientists are working to improve and systematize the way carbon nanotubes are produced. In order to use the carbon nanotubes more widely and more effectively, it is necessary to implement a controlled growth of the carbon nanotubes with desired structural parameters.