Researchers at the National Institute of Standards and Technology (NIST), along with Texas A&M and the University of Maryland, have successfully taken the first pictures of baby nanotubes when they were just a few atoms old. This collaborative study was carried out to better understand the factors that affect the formation of nanotubes. The results of the study have been published online in Nano Letters.
The pictures provide key insight regarding the origination and formation of carbon nanotubes and could pave the way for researchers to develop nanotubes that have the required properties. In order to grow the preferred carbon nanotubes, it is important to understand the growth process known as nucleation. To accomplish this, the nucleation process must be imaged as it happens, but this is a complex process as several fast-moving atoms are involved. This means, high resolution images should be taken as quickly as possible.
Given that high-resolution cameras are costly, NIST researchers slowed the rate of growth by reducing the pressure within an environmental scanning transmission electron microscope. Within the chamber of the microscope, under low pressure and high heat, the researchers observed that carbon atoms produced from acetylene fell down on tiny bits of cobalt carbide, at which point they adhere and form into graphene and ultimately surround the nanoparticle to grow into nanotubes.
As the carbon atoms form graphene on the surface of the cobalt carbide nanoparticle, they look for configurations that are energetically favorable. Although graphene has a hexagonal structure that resembles a honeycomb lattice, the shape of the nanoparticle pushes the carbon atoms to organize themselves into pentagonal shapes inside the honeycomb pattern. These pentagonal irregularities in the graphene structure enable the graphene to bend and turn into a nanotube.
Our observations showed that the carbon atoms attached only to the pure metal facets of the cobalt carbide nanoparticle, and not those facets interlaced with carbon atoms. The burgeoning tube then grew above the cobalt-carbon facets until it found another pure metal surface to attach to, forming a closed cap. Carbon atoms continued to attach at the cobalt facets, pushing the previously formed graphene along toward the cap in a kind of carbon assembly line and lengthening the tube. This whole process took only a few seconds.
NIST chemist Renu Sharma
The team is planning to determine the nanotubes’ chirality as they grow, and also intending to utilize metal nanoparticles with different surfaces to explore their adhesive properties and how they influence the diameter and chirality of the nanotubes.