“We were initially dumbfounded by the formation of the centipedes,” Nick Kotov of the University of Michigan told nanotechweb.org. “The topology of the nanowires is very interesting - it could be exceptionally useful for the design of optically active and remarkably strong nanocomposites, due to the ‘Velcro’ effect.”
The researchers formed the centipedes by using a modified Stöber process to coat CdTe nanowires with silica. This technique employed the silane coupling agent mercaptopropyl(trimethoxysilane) as a surface primer to make the surface of the nanowires receptive to the coating. Ammonia acted as a catalyst, while tetraethyl orthosilicate (TEOS) provided a source of silica.
Kotov and colleagues made the CdTe nanowires by the self-assembly of individual CdTe nanoparticles. They used two capping ligands in the process - either mercaptoacetic acid, also known as thioglycolic acid (TGA), or mercaptosuccinic acid (MSA). Centipede-type structures only developed following the coating process for MSA-stabilized nanowires - they did not form for TGA-stabilized wires.
The MSA-stabilized CdTe nanowires had diameters of around 4.5-6.5 nm and were between several hundred nanometres and 10 microns long. The silica bristles that formed were about 30 nm long and several nanometres in diameter. Increasing the concentration of TEOS present during the coating process resulted in longer bristles, up to about 100 nm long.
The scientists believe that the silica coatings formed bristles because of non-uniform distribution of the stabilizer (due to phase separation) on the nanowire surface.
“In addition to the nanoscale Velcro materials mentioned, the bristled morphology of the centipedes can be very useful for sensor applications, because it provides a very large surface area with multiple attachment sites,” added Kotov.
Now, Kotov and colleagues plan to “develop a method for experimental visualization of non-uniform distribution of stabilizers on the nanoparticle and nanowire surface”.
The researchers reported their work in Nano Letters.