Graphitic Surface Attachment by Single-Stranded DNA and Metal Nanoparticles
Condensed Matter Physics
Graphene and carbon nanotubes are extreme mechanical and electronic materials which have been the subjects of intense study and development since their discoveries. While many of their intrinsic properties have been discovered, their interactions with other materials are only beginning to be explored. The noncovalent binding of single-stranded DNA oligonucleotides to carbon nanotubes and graphene has been seen to give rise to effective gas sensors. We examine similar systems to each of these in turn, imaging carbon nanotubes decorated with single-stranded DNA in Transmission Electron Microscope, and performing X-ray reflectivity of a single-stranded DNA film on graphite. The TEM study shows that the DNA bunches up along tubes but does not tend to clump on single tubes. Helical wrapping is not seen on single tubes. X-ray reflectivity shows that DNA on a graphite surface forms an inhomogeneous layer around 1.6 nm thick. The differences between the various thicknesses of few-layer graphene are substantial though often underappreciated. These differences are highlighted in the system of several-nanometer metal particles on few-layer graphene flakes. We formed such particles by evaporation and annealing, then examined them in Scanning Electron Microscope. We found that gold nanoparticles were circular and experienced limited growth, with the radius varying as the number of layers to the 1/3 power. A theoretical explanation is given for this observation, based on an electrostatic interaction. This theory is also consistent with observations for titanium and silver nanoparticles. Ytterbium nanoparticles on graphene form instead into filaments. A related theory is presented showing that the same electrostatic interaction is capable of overcoming surface tension to deform particles from circularity.