A microscopic picture of surface charge trapping in semiconductor nanocrystals

Abstract

The current model for understanding trapping of charge carriers to the surface of semiconductor nanocrystals is inconsistent with experimental evidence indicating that carriers can thermally de-trap from surface sites. A proper understanding of the microscopic details of charge trapping would guide chemical design of the nanocrystal surface for applications such as charge transport, sensing, or photochemistry. This thesis presents a model of surface charge trapping in which transitions to surface state are governed by rates derived from semiclassical electron-transfer theory. In this picture, trapping to the surface induces a strong polarization in the nanocrystal, resulting in a trapped state with strong electron-phonon coupling via the Frölich mechanism. This trapped state then emits over a broad energy range due to a Franck-Condon vibronic progression. This model is shown to be consistent with the temperature-dependence of core and surface emission as well as the spectral properties of surface emission. The strong coupling of the surface state is validated by independent experiments, and the model is shown to hold promise for explaining the experimental data regarding the trapping of hot (excess energy) carriers.iii RésuméLe modèle prévalent concernant le piégeage des porteurs de charges à la surface de nanocrystaux semi-conducteurs est inconsistant avec certains résultats expérimentaux indiquant que les charges peuvent subir une relaxation thermique depuis les états de surfaces. Une bonne compréhension des aspects microscopiques du processus de piégeage des porteurs de charges permettrait de guider le design de la surface des nanocrystaux en vue d'applications comme le transport de charges, la détection ou la photo-chimie.Ce travail de doctorat propose un modèle du piégeage des charges où les taux de transition vers des états de surface sont basés sur la théorie semi-classique du transfert d'électrons. Dans ce modèle, le piégeage à la surface crée une forte polarisation dans le nanocrystal, ce qui résulte en un état piégé avec un grand couplage électron-phonon via le mécanisme de Fröhlich. Cet état piégé émet dans une large bande spectrale due à la progression vibronique de Franck-Condon. Le modèle proposé est consistent avec la dépendance en température du spectre d'émission du centre et de la surface ainsi que des propriétés spectrales d'émission de la surface. Le couplage fort de l'état de surface est validé par des expériences indépendantes et il est montré que le modèle est prometteur pour l'analyse d'expériences sur le piégeage des porteurs de charges chauds (ayant un excès d'énergie).iv

Role of PbSe Structural Stabilization in Photovoltaic Cells

“…It is also apparent that the GSB of the passivated NCs decays with a well-defi ned lifetime, which agrees well with the luminescence lifetime observed in other spectroscopic experiments on PbSe NCs. [93][94][95][96][97] In Figure 3 b, however, the corresponding GSB of the unpassivated NCs has a component that persists for longer than 10 µs. We summarize the kinetics for these bleach signatures in Figure 3 c. To interpret the differences in the TA signals, we consider the origin of the ground state bleach in this sample.…”

Ligand Surface Chemistry Dictates Light Emission from Nanocrystals

“…The semi-classical Marcus model proposed by our group can account for these changes in surface emission. (7,28) We have shown that the energy barrier for charge carriers from the core excitonic state to the emissive surface 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 potential explanation is that the Z-type bond changes the electronic environment of hole traps localized in their vicinity and thus increases the radiative recombination rate by increasing the wavefunction overlap of the localized hole and the core delocalized electron. In addition, it has been shown that the removal of Z-type ligands from surface chalcogenide sites leads to a higher susceptibility for adventitious adsorbates that form non-radiative trap complexes.…”

Anomalous photoluminescence enhancement due to hot electron transfer in core–shell Au–CdS nanocrystals

“…In the temperature range of 75 K through 175 K, within the experimental uncertainty, the PL intensity remains constant. While discussing the influence of surface charge trapping on the PL intensity, similar behavior is reported in case of CdS NCs and is attributed to the balance between charge carriers from the core and surface levels [25]. Comparison of the plot in Fig.…”