Revealing the doping effect of encapsulated lead halogenides on single-walled carbon nanotubes

We have investigated the electronic properties of single-walled carbon nanotubes (SWCNTs) filled with lead chloride, lead bromide (PbBr2) and lead iodide (PbI2). The filling of SWCNTs and formation of encapsulated one-dimensional nanocrystals of the salts were directly confirmed by high-resolution scanning transmission electron microscopy. The chemical composition of the encapsulated nanocrystals was proven by X-ray photoelectron spectroscopy (XPS). The electronic properties of the filled SWCNTs were characterized by Raman spectroscopy and XPS. All the three lead halogenides result in p-doping of SWCNTs and, consequently, there is a downshift of the SWCNTs' Fermi level due to the charge transfer towards the salts. The effect is stronger with metallic than semiconducting nanotubes. The efficiency of doping depends on the type of lead halogenide. The doping effect is the strongest for PbI2 and the weakest for PbBr2.