The toxicity of nanomaterials for plants is an important issue in the agricultural field. Single-walled carbon nanotubes (SWCNTs) have a small diameter of about 2 nm, and they are toxic for single cells of plants. To reduce the toxicity of SWCNTs, there is a method of filling of SWCNTs. The filling of SWCNTs encompasses the loading of their the inner spaces with different substances. The physics of filled SWCNTs defines their toxicity. First, the microstructure of the filled SWCNT is an important point. Second, the electronic properties of the filled SWCNT is also a significant point. The microstructure and electronic properties of the filled substances are different from the bulk. They should be studied in order to reveal their differences and to model the properties of the filled nanomaterials. It is known that the band structures of the encapsulated substances with one-dimensional atomic structures are very different from three-dimensional band structures. These differences lead to new electronic structures with doping and Fermi-level variations. Here, cobalt iodide (CoI₂) was loaded in SWCNTs with diameter of 1.4 nm. The transmission electron microscopy showed the loading of cobalt iodide in the SWCNTs, and crystal formation. Raman spectroscopy, and X-ray photoelectron spectroscopy, proved the p-doping of the SWCNTs with cobalt iodide. The obtained information is needed to reduce the toxicity of nanomaterials for plants and reduce grass growing with nanomaterials in the agricultural field.