Yale University researchers have developed a simple method for controlling the “doping” of carbon nanotubes (CNTs), a chemical process that optimizes the tubes’ properties. Reported April 29 in Nano Letters, the method could improve the utility of doped CNTs in a number of nanotechnologies and flexible electronics, including CNT-silicon hybrid solar energy cells. Led by André Taylor of the Yale School of Engineering & Applied Science and Nilay Hazari of Yale’s chemistry department, the researchers developed a method that uses organic compounds with a metal core — known as metallocenes — to produce two possible types of doped CNTs. A small amount of metallocenes in solution is deposited on the CNTs, which are then rotated at high speed. This simple “spin coating” process spreads the solution evenly across the surface of the CNTs, resulting in high doping levels that can improve electrical utility. Using the method, the researchers found that doping with electron-deficient metallocenes, such as those with a cobalt core, results in CNTs with more positively charged electron “holes” than available negatively charged electrons to fill those holes; these CNTs are known as “p-type” because of their positive charge. On the other hand, doping with electron-rich metallocenes, such as
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