Near-infrared Single Particle Spectroscopy of Lead Sulfide/Cadmium Sulfide Nanocrystals

The detection and identification of single molecules/single nanocrystals by optical methods has been demonstrated over two decades ago by Moerner, Keller, Orrit, Bawendi and others. The impact of single particle methods on both life science and materials science research and related discoveries have been acknowledged this past year by Nobel Prize in Chemistry to W.E. Moerner of Stanford University for his pioneering work in low temperature single particle detection.

Colloidal nanocrystals or quantum dots (QDs) have appealing properties for photovoltaics (PVs) like high extinction coefficients, size-tunable bandgap and photoluminescence (PL), the ability to undergo multiple exciton generation. Combined with acceptor materials like carbon nanotubes, conductive polymers and/or transition metal oxides, QDs are explored as cost-effective, high performance active materials for PVs. Visible absorbing/emitting QDs like CdSe have been intensively studied with single particle optical methods, and they sum up the bulk (99.99%) of single nanocrystal optical studies. Challenges remain when it comes to the detection of optical signals from isolated QDs absorbing/emitting in near infrared (NIR). Low bandgap, NIR-emitting QDs like PbS have been so far the subject of just two single nanocrystal optical studies from Bawendi (MIT) and Krauss (Rochester) groups.

In this presentation I will highlight our recent time-resolved NIR single particle spectroscopy experiments addressing the dynamics of photoinduced charge transfer (CT) from core/shell PbS/CdS QDs to acceptor materials like TiO2 and to poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). In particular, I will show how single particle methods can unravel heterogeneity in (radiative) recombination of electron and hole in the PbS core following optical excitation, and how this heterogeneity is dramatically suppressed in the presence of charge transfer, either electron or hole. I will comment the findings for PbS/CdS QDs with our previous single particle work on high bandgap CdSe/ZnS QD [1-3], highlighting clear differences in single particle behavior between PbS and CdS originating from differences in their materials composition, e.g. difference in electron and hole masses.

1. H.Zang, P.K.Routh, R.Alam, M.Maye, M.Cotlet, Core size dependent hole transfer from a photoexcited QD to a conjugated polymer. Chem.Comm. 2014, 50, 5958-60.
2. Z.Xu, M.Cotlet, Quantum dot-bridge-fullerene heterodimers with controlled photoinduced electron transfer, Angew.Chem.Intl. Ed., 2011, 50(27) 6079-82.
3. Z.Xu, C.Hine, M.Maye and M.Cotlet, Shell-thickness dependent hole transfer in conjugated polymer/quantum dot hybrids, ACS NANO, 2012, 6(6), 4984-4993.