Transparent foamlike 2D networks of nitrogen-doped multiwalled carbon nanotubes obtained by self-assembly

Organizing carbon nanotubes into true monolayer structures is highly desirable, but this has been difficult due to their low solubility and tendency to aggregate. Here, we introduce a novel procedure to fabricate monolayers of nitrogen-doped carbon nanotubes (CN_x) using a Langmuir trough. Doped nanotubes disperse well in the subphase and slowly get trapped by the air/water interface, self-organizing in a remarkable two-dimensional foamlike network at low surface densities, as observed in situ by Brewster angle microscopy, with a thickness that corresponds to one layer. More compact films are easily obtained by compression. Atomic force microscopy (AFM) and scanning electron microscopy images of transferred films on solid substrates show that their foamlike network structure is preserved. The transparency of the films was determined by optical reflectance in the UV-vis region and their Raman spectra in different areas. The average thickness of these transferred films was measured by means of ellipsometry and AFM.