Measuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope

TitleMeasuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope
Publication TypeJournal Article
Year of Publication2010
AuthorsHummon, MR, Stollenwerk, AJ, Narayanamurti, V, Anikeeva, PO, Panzer, MJ, Wood, V, Bulovic, V
JournalPhysical Review B
Volume81
Issue11
Date Published2010/03//
Abstract

We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage (IV) tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, (61.4 +/- 2.4) meV, and analysis of the DBTJ, show trap states similar to 1.09 eV below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.