In 2001, molecular nanowires millions of times tinier than a human hair were developed. Researchers discovered that nudging these tubes with a sharp tube can modify their ability to carry an electric current.
Video Courtesy of Cambridge University YouTube
The nanotube was nudged using the tip of an atomic force microscope. Similar to real space microscopy techniques, the AFM was used to make surface topography images by dragging a sharp tip over the folds and bumps on the surface of the structure.
A finely powdered metal nanoparticle array was arranged on a silicon dioxide substrate. The substrate was heated to a high temperature and methane was fed. Carbon was then infused into the metal particles, which acted as catalysts and carbon atoms were converted into honeycomb lattice nanotubes.
After attaching an electrode to a single nanotube across a silicon dioxide trench, the AFM tip was used for pushing the wire down into the trench while its electrical conductance is measured. The bending of the nanotube caused the flow of electricity to drop sharply.
The removal of the AFM tip results in straightening of the tube and electricity flow returns to normal. As the two sides of the tube are moved close to each other, carbon atoms form bonds across the tube interior. In a normal scenario, each carbon atom attaches to three other carbons leaving one free electron for electricity conduction. Forcing the walls together causes each carbon atom to bind to four instead of three carbon atoms. The electrical conductance is reduced due to the reduction in free electrons.
Squashing the AFM tip causes each atom to bond with other atoms changing the tube from an electrical conductor to an insulating structure similar to that found in diamonds. The removal of the AFM tip causes the dent to disappear. This high level of mechanical reversibility enables complete recovery of the electrical characteristics of the nanotube. Deformation of local nanotubes is a technique by which different functional components of nanotube transistors can be developed.
This can be used to make small electromechanical devices such as transducers for the conversion of mechanical movements to electrical signals. It can also be used in high-frequency telephone lines for voice and data, and on/off switches for nanoscale computer chips.
Molecular Copper Phthalocyanine Wires
The molecule copper phthalocyanine (CuPc) is an industrial pigment that gives blue colors to a number of daily use objects such as textiles or cars.
This molecule belongs to a family of polyaromatic molecules that can also behave as semiconductors.
In 2012 a research team from the London Centre for Nanotechnology produced highly directional nanowires of CuPc through a simple sublimation technique.
The wires attain lengths of several centimeters and lengths as low as 10 nm resulting in aspect ration close to that of carbon nanotubes. The wires have novel physical properties and exhibit optical absorption through a broader portion of the visible spectrum when compared to previous forms of CuPc.
These features can be used in organic solar cells by increasing the light absorption efficiency and enabling the separation of photoexcited states into charges at nanostructured interfaces between electron acceptors and donors.
X-ray and electron microscopy studies revealed that the molecules adopt a novel crystal structure with high electronic overlap along the long axis of the wires implying highly anisotropic electronic conductivity.
The molecule’s metallic centers bear unpaired spins that interact to form chains where the closest neighbors alternate between up and down orientations. Combining magnetic and semiconductor properties in flexible quasi-single dimensional objects can result in applications in molecular spintronics by generating novel ways for the storage and transmission of information.
Langmuir Blodgett Technique
In 2002 a team from the University of Chicago used the Langmuir – Blodgett technique for the construction of molecular nanowire structures having the charge transfer complex of a bis-tetrathiafulvalene substituted macrocycle and tetrafluorotetracyanoquinodimethane on mica substrates .
The dimensions of the nanowires transferred from a dilute aqueous potassium chloride subphase were 2.5 nm × 50 nm × 1 µm. The nanowires are oriented to specific directions corresponding to the potassium-ion array directions on the mica surface with six-fold symmetry.
The correlation between the substrate surface and the nanowires was observed when a dilute aqueous rubidium chloride subphase was used. When the subphase contained divalent cations, the correlation disappeared completely showing that the orientation of the molecular nanowires is based on the recognition of the monocation array on the mica surface. The nanowire structure presented is based on the crystal structure of a related complex. The nanowire conductivity was estimated to be 10- 3 S cm-1.
Specific T –shape junctions were formed by the nanowires suggesting they be used in future molecular nanoscale devices.
Molecular Nanowires from Discotic Liquid Crystals
In 2010, Scientists for the University of Leeds perfected a novel technique enabling the manufacture of molecular nanowires from thin strips of ring-shaped molecules known as discotic liquid crystals (DLCs). This may find application in light harvesting cells and low-cost biosensors that can be used to test water quality in developing countries.
DLCs are disk shaped molecules that hold promise for organic electronic devices. Controlling the alignment of DLCs is a difficult task and has been a key barrier to their use as molecular wires and in the LCD industry.
The Leeds team proposed a new technique using patterned surfaces to control alignment selectively enabling stacking of the piles neatly for the creation of molecular wires. Sheets of silicon or gold are printed with self-assembled monolayers that can be patterned with high and low-energy stripes.
When a liquid crystal droplet is applied to this patterned surface and heated, it spreads out like liquid fingers over the high-energy stripes and leaves the low energy regions bare. Professor Evans stated that within the stripes they found molecules that were arranged into hemi-cylindrical columns each measuring several microns having a high level of control over DLC alignment till date. The columns are better ordered when the stripes are narrower. The team expects that this amount of control could enable the development of a novel type of biosensor, which could detect anything that changes the surface properties.
It is very easy to switch between alignments by changing the surface properties, which is an interesting property as far as sensing devices are concerned. The team is testing the conductivity of these wires to use them for energy transfer in molecular systems. They are also exploring ways to polymerize the wires to render them stronger.
References and Further Reading