title: Charge percolation pathways guided by defects in quantum dot solids.
abstract: Charge hopping and percolation in quantum dot (QD) solids has been widely studied, but the microscopic nature of the percolation process is not understood or determined. Here we present the first imaging of the charge percolation pathways in two-dimensional PbS QD arrays using Kelvin probe force microscopy (KPFM). We show that under dark conditions electrons percolate via in-gap states (IGS) instead of the conduction band, while holes percolate via valence band states. This novel transport behavior is explained by the electronic structure and energy level alignment of the individual QDs, which was measured by scanning tunneling spectroscopy (STS). Chemical treatments with hydrazine can remove the IGS, resulting in an intrinsic defect-free semiconductor, as revealed by STS and surface potential spectroscopy. The control over IGS can guide the design of novel electronic devices with impurity conduction, and photodiodes with controlled doping.
authors.affiliation: †Applied Science and Technology Graduate Program, University of California at Berkeley, Berkeley, California 94720, United States.|‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Danylo
authors.initials: D
authors.lastname: Zherebetskyy
authors.affiliation: §Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States.
authors.forename: Noah D
authors.initials: ND
authors.lastname: Bronstein
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Sara
authors.initials: S
authors.lastname: Barja
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Leonid
authors.initials: L
authors.lastname: Lichtenstein
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: David
authors.initials: D
authors.lastname: Schuppisser
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Lin-Wang
authors.initials: LW
authors.lastname: Wang
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.|§Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States.|∥Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States.|⊥Kavli Energy NanoScience Institute, Berkeley, California 94720, United States.
authors.forename: A Paul
authors.initials: AP
authors.lastname: Alivisatos
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.|∥Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States.
authors.forename: Miquel
authors.initials: M
authors.lastname: Salmeron
keywords: Kelvin probe force microscopy; Quantum dot; charge percolation; charge transport; defect; in-gap states