Colloidal PbS Nanosheets with Tunable Energy Gaps
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Colloidal PbS Nanosheets with Tunable Energy Gaps Zhoufeng Jiang , Simeen Khan1, Shashini Premathilake1,3, Ghadendra Bhandari1, Kamal Subedi1, Yufan He2, Matthew Leopold1, Nick Reilly1, Peter Lu2,3, Alexey Zayak1,3, Liangfeng Sun1,3 1 Department of Physics and Astronomy, Bowling Green State University, Bowling Green, OH 43403, U.S.A. 2 Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403, U.S.A. 3 Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, U.S.A. 1,3
ABSTRACT Ultrathin colloidal PbS nanosheets are synthesized using organometallic precursors with chloroalkane cosolvents, resulting in tunable thicknesses ranging from 1.2 nm to 4.6 nm. We report the first thickness-dependent photoluminescence spectra from lead-salt nanosheets. The one-dimensional confinement energy of these quasi-two-dimensional nanosheets is found to be proportional to 1/L instead of 1/L2 (L is the thickness of the nanosheet), which is consistent with results calculated using density functional theory as well as tight-binding theory. INTRODUCTION A big obstacle for developing colloidal quantum dots (QDs) based optoelectronic devices is the surface ligands. They are necessary for protecting the optical properties of the QDs, but significantly impede charge transport in QD films since they are typically organic insulators. To improve the charge transport in QD films, short organic ligands,1-3 inorganic ligands4 and atomic ligands5,6 have been developed to replace the original long organic ligands. Although they improve the mobility of the charge carriers in the QD films, the existing inter-QD spacing and the boundary of QDs still hinder the charge carrier transport. Making two-dimensional (2-D) nanosheets (NSs) can effectively reduce these hindrances,7,8 yet retain the tunable quantum confinement in one dimension. Besides the advantages of these 2-D materials on traditional optoelectronic devices, recent theoretical work also reveals that 2-D semiconductors are good candidate materials for spintronic devices. EXPERIMENT Our synthesis of PbS NSs follows a method similar to the one developed by Dogan et al9 and Schliehe et al7. It is based on a typical synthesis of PbS QDs,10 but a chloroalkane cosolvent is added during the reaction to enable 2-D attachments of the PbS nanocrystals. Finally the nanosheets are formed. The thickness of the nanosheets is determined by the size of the PbS nanocrystals formed at the beginning of the reaction. It is known that a higher reaction temperature results in larger PbS nanocrystals.11 In the synthesis of NSs, the thickness is controlled by the reaction temperature.12 A series of chloroalkane cosolvents with different boiling points – chloroform (61 0C), 1,2-dichloroethane (DCE) (84 0C), 1,1,2-trichloroethane (TCA) (110~115 0C), 1,2-Dichlorobutane (DCB) (125 0C) and 1,2,3-Trichloropropane (TCP) (156 0C) – are used to match each reaction temperature. The reaction temperature was typically set at 20 oC above the boiling point of each cosolvent,
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