Density control of self-aligned shortened single-wall carbon nanotubes on polyelectrolyte-coated substrates

Shortened single-wall carbon nanotubes (sSWCNTs) were self-aligned vertically on poly(diallyl dimethyl ammonium) chloride (PDAC) self-assembled surfaces (silicon wafer and ITO glass). The self-aligned sSWCNT layer density was measured by atomic force microscopy (AFM) in dependence of reaction time, pH and salt concentration. It was found that the density of self-aligned sSWCNTs was high for pH 4 and 7 at low salt concentrations. At pH 2, the density of vertically adsorbed sSWCNTs was lowest since salt formation close to the isoelectric point (IEP) of the sSWCNTs. For measurements at pH values higher than the IEP of sSWCNTs, with the lower the ionic strength, the higher density of self-aligned sSWCNTs on the substrate was observed. This was also independent of the substrate used. It is suggested that the observed density dependence on ionic strength may be caused by a screening effect of the counter ions. Zeta potential measurements and colloidal stability of sSWCNTs dispersions are reported. Data strongly suggest that the density control of sSWCNTs is caused by screening via counter ions.