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Observation of trapped-hole diffusion on the surfaces of CdS nanorods

Abstract

In CdS nanocrystals, photoexcited holes rapidly become trapped at the particle surface. The dynamics of these trapped holes have profound consequences for the photophysics and photochemistry of these materials. Using a combination of transient absorption spectroscopy and theoretical modelling, we demonstrate that trapped holes in CdS nanorods are mobile and execute a random walk at room temperature. In CdS nanorods of non-uniform width, we observe the recombination of spatially separated electrons and trapped holes, which exhibits a t−1/2 power-law decay at long times. A one-dimensional diffusion–annihilation model describes the time-dependence of the recombination over four orders of magnitude in time, from one nanosecond to ten microseconds, with a single adjustable parameter. We propose that diffusive trapped-hole motion is a general phenomenon in CdS nanocrystals, but one that is normally obscured in structures in which the wavefunctions of the electron and trapped hole spatially overlap. This phenomenon has important implications for the oxidation photochemistry of CdS nanocrystals.

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Figure 1: Relationship between CdS NNR morphology and TA spectra.
Figure 2: TA bleach decay of electrons in CdS nanocrystals.
Figure 3: Recombination of a localized electron and trapped hole as a diffusion–annihilation process.

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Acknowledgements

This work was supported by the Air Force Office of Scientific Research under AFOSR award No. FA9550-12-1-0137 (synthesis), the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0010334 (TA measurements and analysis) and the National Science Foundation under CAREER Award No. CHE-1151151 (synthesis and theoretical modelling) and Award No. DMR-1410215 (theoretical modelling). J.K.U. and A.N.G. acknowledge support from National Science Foundation Graduate Research Fellowships under Grant No. DGE 1144083. We thank Y. Lu for assistance with TEM imaging.

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G.D. supervised the experiments and data analysis. J.D.E. directed the theoretical model development. J.K.U., A.N.G., M.B.W. and O.M.P. carried out the TA experiments. J.K.U. carried out the development of the diffusion model and data analysis. A.N.G. assisted in the development of data analysis. M.B.W. and O.M.P. carried out the syntheses. J.K.U., G.D. and J.D.E. wrote the manuscript.

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Correspondence to Joel D. Eaves or Gordana Dukovic.

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Utterback, J., Grennell, A., Wilker, M. et al. Observation of trapped-hole diffusion on the surfaces of CdS nanorods. Nature Chem 8, 1061–1066 (2016). https://doi.org/10.1038/nchem.2566

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