Mechanical properties of carbon nanotubes grown by pyrolysis of ferrocene

Sammanfattning: Carbon nanotubes (CNTs) have drawn a lot of attention during the last decades due to its promising mechanical and electrical properties. Extensive research regarding the mechanical properties of CNTs has been carried out during the last decades. A lot of effort has been put into developing methods to properly characterize features such as Young’s modulus and the deformation processes of carbon nanotubes. A detailed knowledge of these properties is important for many of the suggested applications of carbon nanotubes.Here we have examined multiwalled carbon nanotubes (MWCNTs) grown by pyrolysis of ferrocene. In some cases the carbon nanotube contained an iron core or traces of iron in the core. The carbon nanotubes ranged from 20 nm to 65 nm in radius and 1000 nm to 4000 nm in length.An atomic force microscope (AFM) was used inside a scanning electron microscope (SEM) for in situ force measurements. The AFM cantilever was used to displace individual carbon nanotubes from their equilibrium positions. The forces used to displace the carbon nanotubes have been plotted against the displacements of the tubes to obtain the characteristic force-displacement curves. From the slope of these curves the spring constants of the carbon nanotubes have been found. Young’s modulus for each tube was derived from the spring constant and the tube dimensions.We found that Young’s modulus ranged from 7 GPa to 340 GPa with no observed dependence on the radius or the length. Previous works suggest that deformation processes such as rippling and buckling will drastically change the spring constant of the tubes when displaced. The maximum values of the applied forces in our measurements were smaller than those needed to push the tubes into the deformation stages. The relatively low values of Young’s modulus indicate that these tubes are rich in defects which dominate their mechanical behaviour.