Anisotropic, Nonthermal Lattice Disordering Observed in Photoexcited PbS Quantum Dots

Abstract

Given their nanoscale dimensions, colloidal semiconductor nanocrystals provide unique systems for investigating the dynamics controlling surface chemistry and fundamental issues regarding lattice reorganization upon changes in electron distribution. These systems are particularly amenable to ultrafast electron probes, offering an atomic level picture of the lattice reorganization involved following photoexcitation. Here, we study lead sulfide (PbS) quantum dots with ultrafast electron diffraction to characterize the atomic motions following high-intensity photoexcitation. Short-range nonthermal lattice distortions and increased atomic disorder were observed in PbS colloidal quantum dots ranging from 2.4 to 8.7 nm in size. These effects scaled inversely with size and were less pronounced in nanocrystals with a chloride-containing surface rather than only organic ligands, which is consistent with an effect arising at the surface. The anisotropic, nonthermal lattice disordering occurs preferentially along the (100) crystallographic directions, which could indicate an anisotropic distribution of localized charge between the differing lattice terminations of the {111} and {100} crystal facets. This is consistent with the larger anharmonicity for the lattice potential at lattice sites with reduced ligand coordination relative to the bulk, which has been shown to cause accelerated relaxation into dynamic and static surface trap sites. Through an exploration of quantum dot size and variation in surface termination, this work provides the missing structural details to advance our understanding and control of charge-carrier formation, trapping, and recombination processes in nanoscale semiconductor systems.

Auger Recombination and Carrier–Lattice Thermalization in Semiconductor Quantum Dots under Intense Excitation

“…Importantly, ln true( I I 0 true) vs the square of scattering vector s 2 displays a linear dependency (Figure d), which indicates that these changes in diffraction patterns arise mainly from the thermal effect of ultrafast lattice heating (see also Figure S5), namely the Debye–Waller (D–W) effect, apart from other possible photoinduced nonthermal effects. , This observation is consistent with that the lattice temperature deduced from the peak intensity change agrees very well with the value extracted from the peak position shift (Figures S6 and S7). In previous studies, an anomalous feature of the (400) Bragg peak in core/shell and PbS QDs within tens of picoseconds was observed and attributed to the hot carriers captured by the surface trap states. But no such feature was detected in our study because our pump photon energy is lower than the energy threshold (800 nm vs 400 nm) for such dynamical trapping .…”

“…But no such feature was detected in our study because our pump photon energy is lower than the energy threshold (800 nm vs 400 nm) for such dynamical trapping . Also, surface passivation by chlorine quenches the dynamic surface trapping …”

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

“…In this view, the observed photon energy may significantly underestimate the underlying energetic difference between the ground and excited electronic states . Further, the breadth of the trap emission would overwhelmingly arise from dynamical processes, so the effect of ensemble heterogeneity would be small. , Consensus remains to be found, and very recent reports emphasize the additional role of structural dynamism. ,,, Indeed, drawing on previous demonstrations, ,, we regard many spectroscopic observables as an ensemble- and time-average over the distribution of instantaneous configurations in size, shape, and surface within the ensemble . For instance, time-resolved measurements on core-only CdSe NCs have revealed that only a minority of NCs within the ensemble at a given instant undergo straightforward, monoexponential, radiative decay from band-edge states. , Instead, measurements on various CdSe nanostructures show partial suppression of prompt band-edge emission on single-picosecond time scales, consistent with pervasive but nonuniform hole trapping to states associated with the NC surface. ,, The key role of traps is supported by high-sensitivity transient fluorescence studies that observe substantial “delayed” PL consistent with a mixture of direct trap emission as well as the repopulation of band-edge states over widely distributed time scales. , Thus, it remains an open question whether sequential or correlated mechanisms for TET dominate when the photoexcited hole is rapidly trapped in a surface state. , …”

“…37,40 Consensus remains to be found, and very recent reports emphasize the additional role of structural dynamism. 13,33,37,41 Indeed, drawing on previous demonstrations, 13,42,43 we regard many spectroscopic observables as an ensemble-and time-average over the distribution of instantaneous configurations in size, shape, and surface within the ensemble. 43 For instance, time-resolved measurements on core-only CdSe NCs have revealed that only a minority of NCs within the ensemble at a given instant undergo straightforward, monoexponential, radiative decay from band-edge states.…”

“…This is because the usual technique of passivating the cores with an insulating, inorganic shell to reduce nonradiative decay can be counterproductive to efficient triplet transfer, due to the exponential dependence of Dexter transfer over bond-length distances. , However, as-synthesized, core-only CdSe NCs have complex photophysics even before functionalization with transmitter molecules, which we briefly review to facilitate the subsequent discussion of our results. First, following photoexcitation, core-only CdSe NCs generally exhibit a degree of excitonic emission from the lowest-lying bright states at the band edge. , Then, unshelled NCs often show an additional emission feature, which generally takes the form of a broad and red-shifted band , that can account for up to 80% of the total steady-state photoluminescence − (see also Figure S4). While the atomistic origins of this feature are uncertain, it is most prominent for the smallest particles, , can be modified or suppressed by shelling and surface treatments, ,, and is generally ascribed to surface hole trapping followed by radiative recombination. ,,, …”