Colloidal quantum dots (QDs) have attracted interest as materials for opto-electronic applications, wherein their efficient energy use requires the understanding of carrier relaxation. In QDs capped by bifunctional thiols, used to attach the QDs to a surface, the relaxation is complicated by carrier traps. Using 2D spectroscopy at 77 K, we follow excitations in thiol-capped CdSe QDs with state specificity and high time resolution. We unambiguously identify the lowest state as an optically allowed hole trap, and identify an electron trap with excited-state absorption. The presence of traps changes the initial dynamics entirely by offering a different relaxation channel. 2D electronic spectroscopy enables us to pinpoint correlations between states and to easily separate relaxation from different starting states. We observe the direct rapid trapping of 1S3/2, 2S3/2, and 1S1/2 holes, and several competing electron relaxation processes from the 1Pe state.

Department of Chemical Physics Lund University PO Box 124 221 00 Lund Sweden

Department of Chemical Physics Lund University PO Box 124 221 00 Lund Sweden and Department of Chemistry Qena Faculty of Science South Valley University Qena 83523 Egypt

Department of Chemical Physics Lund University PO Box 124 221 00 Lund Sweden and Gas Processing Center College of Engineering Qatar University PO Box 2713 Doha Qatar

Department of Chemical Physics Lund University PO Box 124 221 00 Lund Sweden and Regional Centre for Special Optics and Optoelectronic Systems Institute of Plasma Physics Academy of Sciences of the Czech Republic Za Slovankou 1782 3 182 00 Prague 8 Czech Republic

Gas Processing Center College of Engineering Qatar University PO Box 2713 Doha Qatar

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Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy