Trapped-Hole Diffusion in Photoexcited CdSe Nanorods

Abstract

Surface charge-carrier traps are ubiquitous in colloidal semiconductor nanocrystals and fundamentally impact excited-state relaxation, making it critical to understand both their nature and their dynamics. Here, using photoluminescence upconversion and transient absorption spectroscopy, we study hole trapping and the dissociation between electrons and trapped holes in nonuniform CdSe nanorods and monitor their subsequent recombination dynamics. These recombination dynamics are described well with a diffusion–annihilation model wherein the trapped hole undergoes a random walk on the nanocrystal surface until it encounters the electron. This model fits the nonexponential excited-state decay over more than 7 orders of magnitude in time with a single adjustable parameter. The characterization of the spatial dynamics of trapped holes in CdSe nanostructures extends our fundamental understanding of excited-state dynamics in this important class of materials. The surface motion of trapped holes may have important implications for optoelectronic applications that rely on charge transport and charge transfer.

Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors

“…Therefore, the trapped hole population is further divided into “active” holes which can transfer to NPTZ, and “passive” holes which are unable to transfer during our observation time window due to kinetic barriers, such as the hole being localized at a trap state far from a NPTZ ligand. Since the surface coverage of NPTZ is only ∼20%, the holes trapped at sites which are not adjacent to a NPTZ ligand would need to undergo a series of slow hole hopping events to reach NPTZ ligands, which have been measured to occur over time scales longer than several nanoseconds. ,, The initial number of trapped holes, ⟨ N trap ⟩, and the fraction of active traps are fitting parameters for the model. The fraction of active traps falls around 50% and is held constant across excitation fluences to allow for the model to capture the plateau levels.…”

“…Since the surface coverage of NPTZ is only ∼20%, the holes trapped at sites which are not adjacent to a NPTZ ligand would need to undergo a series of slow hole hopping events to reach NPTZ ligands, which have been measured to occur over time scales longer than several nanoseconds. 38,51,52 The initial number of trapped holes, ⟨N trap ⟩, and the fraction of active traps are fitting parameters for the model. The fraction of active traps falls around 50% and is held constant across excitation fluences to allow for the model to capture the plateau levels.…”

Two-Dimensional Morphology Enhances Light-Driven H2 Generation Efficiency in CdS Nanoplatelet-Pt Heterostructures

“…Our simulation shows that, with the same rate constants for elementary steps and lateral size (same length for NRs and square NPLs), charge recombination in 2D NPLs is over 5-fold slower than 1D NRs regardless of fast ( k h /k X ≫ 1) or slow ( k h /k X ≪ 1) trapped hole diffusion (Figure S21 and Table S7). Our model is similar to a hole hopping model that has been applied successfully in previous studies of electron-trapped hole recombination in CdS and CdSe NRs. − Compared to 1D NRs, the 2D morphology requires many more random walk steps before the hole finds the recombination (Pt) site, leading to slower charge recombination. In our simulation, recombination can occur only at the Pt site to which the electron has been transferred, although there are several Pt particles per NPL (Figure c and d).…”

Role of Surface Morphology on Exciton Recombination in Single Quantum Dot-in-Rods Revealed by Optical and Atomic Structure Correlation

“…Early blinking models, for example, suggest that a wide range of trap-state recovery rates could result from distributions in back-tunneling rates of the trapped carrier that varies exponentially with the distance of trap sites around a QD emitter . Perhaps more interesting, recent transient absorption experiments on CdS and CdSe nanorods by Utterback et al , indicate that surface-trapped holes recombine with delocalized electrons in the bulk of the rod on time scales that depend on the spatial overlap of the wave function of the electron and trapped hole. Slow recombination times were attributed to diffusion of trapped holes that weakly overlapped with delocalized electrons in the nanorods.…”