Observation of trapped-hole diffusion on the surfaces of CdS nanorods James K. Utterback, Amanda N. Grennell, Molly B. Wilker † , Orion M. Pearce, Joel D. Eaves * and Gordana Dukovic * 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. M any novel properties of semiconducting nanostructures emerge through the confinement of electronic wavefunc- tions to small regions of space 1 . Cadmium-based chalco- genide nanocrystals are some of the most widely studied and used systems in nanoscience because synthetic control over particle shape and size allows one to manipulate both electronic energies and wavefunctions 1–3 . As a result of this tunability, there has been a growing interest in using colloidal Cd–chalcogenide nanocrystals for optoelectronic applications, such as solar energy conversion 4–8 . Such technologies require control over the generation, separation and extraction of photoexcited electrons and holes 6,8–11 . The dynamics of these two carriers can differ substantially 10,12–15 . To understand the principles that govern electron- and hole-relaxation dynamics in these complex systems, models based on fundamental physical phenomena are needed. However, the complicated shapes of experimentally measured excited-state decay curves are often elusive to simple kinetic models 14,15 . In CdSe and CdS nanocrystals, photoexcited holes rapidly and efficiently trap to localized states on the surface 10,12,13,15,16 . In nano- scale CdS, in particular, hole trapping occurs on a picosecond time- scale with >99% efficiency, so electrons primarily recombine with trapped rather than with delocalized holes 10,12,13,17 . Consequently, trapped holes play an integral role in excited-state dynamics 10,13,14,18 , and the ability to harvest them is critical for applications such as photovoltaics and solar photochemistry 17 . Despite their importance, remarkably little is known about the nature of the trap states and the dynamics of trapped holes 10,13,17,18 . The prevailing view is that the trapped holes are localized spatially, which suggests that delocalized electrons recombine with stationary holes 16–19 . There has not, however, been direct evidence in support of this picture of recombi- nation. If the energetically trapped holes were, instead, spatially mobile, the governing picture of their relaxation dynamics would fundamentally change. Here, using transient absorption (TA) measurements on the sub- picosecond to microsecond timescale in conjunction with theoreti- cal modelling, we provide evidence that trapped holes on CdS nanorod surfaces are not stationary. Instead, they execute a diffusive random walk at room temperature. In CdS nanorods of non-uniform width (non-uniform nanorods (NNRs)), excitation wavelengths can be chosen such that photoexcited electrons dis- sociate from trapped holes and localize to larger-diameter regions of the NNRs with lower quantum confinement. TA experiments that probe the relaxation of these localized electrons show a t -1/2 power-law decay over two orders of magnitude in time, which suggests a non-exponential recombination mechanism. In contrast, in CdS quantum dots (QDs) and NNRs when the electron and trapped hole are not separated spatially, the electron–hole recombi- nation is exponential. These observations motivate an analytical model for one-dimensional (1D) diffusion-limited electron–hole recombination in the NNRs that fits the electron decay over four orders of magnitude in time, from one nanosecond to ten micro- seconds, with only one adjustable parameter. We propose that the diffusive motion of trapped holes is a general phenomenon in CdS nanocrystals that is normally obscured in structures in which electron and trapped-hole wavefunctions remain spatially over- lapped during the measurement of their recombination dynamics. Finally, we illustrate how this fundamentally different picture of the behaviour of trapped holes may impact the photochemistry of CdS nanostructures. Results and discussion Power-law decay of spatially separated carriers. When a photoexcited electron is delocalized over an entire CdS nanocrystal, after hole trapping, the recombination behaviour is insensitive to the location of the trapped hole. To examine experimentally whether trapped holes can move on nanocrystal surfaces, we use nanostructures with excited states that exhibit spatial electron–hole separation and compare them with structures in which the photoexcited electron and hole are not separated. The synthesis of rod-shaped CdS nanocrystals results in a mixture of nanorods with uniform and non-uniform widths along their lengths 19 . These CdS NNRs provide a region in which Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, USA. † Present address: Department of Chemistry, Luther College, Decorah, Iowa 52101, USA. *e-mail: gordana.dukovic@colorado.edu; joel.eaves@colorado.edu ARTICLES PUBLISHED ONLINE: 11 JULY 2016 | DOI: 10.1038/NCHEM.2566 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 1 © 2016 Macmillan Publishers Limited. All rights reserved