Different chemical purification methods (oxidation in air followed by HCl etch and oxidation by HNO3) were investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). These recipes were compared with respect to cleanliness, structural defects, and their effect on the electronic properties. In particular, both p‐ and n‐type doping was observed in individual bundles after the HNO3 treatment. Our results imply that the quality of the material in terms of phase purity and structural integrity as well as the electronic properties depend critically on the purification technique.

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