Enhanced Charge-Transport Behavior on PbS Nanocrystals Capped with Atomic Ligands

Colloidal nanocrystals (NCs) are promising materials with its unique electronic, magnetic and optical properties and therefore there have been tremendous research efforts. Among these application fields, especially in the case of electronic devices, the figure-of-merit parameter is charge carrier mobility because high mobility can guarantee prompt response of NCs-based devices against high frequency external signals. As-synthesized lead chalcogenide NCs are typically capped with insulating organic ligands such as oleylamine or oleic acid that impede the injection or extraction of charge carriers from individual NCs, resulting in very low charge carrier mobility. In order to enhance charge transport between adjacent NCs, long and insulating ligands have to be replaced with short and electronically transparent ligands. Traditionally, benzene dithiol or mercaptopropionic acid have been frequently used as such surface ligands to overcome low charge carrier mobility issues of colloidal NCs. However, the resulting devices usually rendered only limitedly enhanced mobility of < 0.1 cm²/Vs. Here we demonstrate the utilization of atomic ligands such as halide anion(X^-, X : Cl, Br, I) as an electrically transparent ligands for lead chalcogenide NCs, to realize high charge carrier mobility over 0.5 cm²/Vs. The origin of such improved charge transport behavior as a result of introducing atomic ligands is fully discussed together with various characterization results.