Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

Abstract

Charge carrier trapping is an important phenomenon in nanocrystal (NC) decay dynamics because it reduces photoluminescence (PL) quantum efficiencies and obscures efforts to understand the interaction of NC excitons with their surroundings. Particularly crucial to our understanding of excitation dynamics in, e.g., multiNC assemblies, would be a way of differentiating between processes involving trap states and those that do not. Direct optical measurement of NC trap state processes is not usually possible because they have negligible transition dipole moments; however, they are known to indirectly affect exciton photoluminescence. Here, we develop a framework, based on Marcus electron transfer theory, to determine NC trap state dynamics from time-resolved NC exciton PL measurements. Our results demonstrate the sensitivity of PL to interfacial dynamics, indicating that the technique can be used as an indirect but effective probe of trap distribution changes. We anticipate that this study represents a step toward understanding how excitons in nanocrystals interact with their surroundings: a quality that must be optimized for their efficient application in photovoltaics, photodetectors, or chemical sensors.quantum dot ͉ states ͉ electron transfer ͉ time-correlated single-photon counting ͉ fluorescence intermittency C olloidal semiconductor nanocrystals (NCs) are potentially useful in a variety of photoactive applications because of their widely tunable electronic band gaps and their ease of processability. Different from well-studied molecular fluorophores, the excited electronic states of NCs with diameters in the range 1 to 10 nm are examples of nanoscale excitons (1). To be used in solar photovoltaics, for example, photo-generated NC excitons must be able to dissociate and transfer charge carriers to their surroundings in a controlled fashion; however, this process is impeded in NCs by the considerable influence of surface-localized states that trap carriers (2). Perturbations due to traps are important because of the significant surface-to-volume ratios characteristic of small colloids (3, 4). Certain surface sites act as charge acceptors that dissociate excitons and therefore reduce the photoluminescence (PL) quantum yield. Changes in solvent or surface-bound coordinating ligands have been found to affect these surface traps and thereby influence steady-state (5-9) and time-resolved (10-12) NC PL. The ability of surface traps to act as charge acceptors makes them excellent model systems for elucidating exciton dissociation processes occurring on the NC surface. Properties intrinsic to NC excitons have been examined in detail (13-17) but comparatively little is understood about the interplay among intrinsic excitons and surface states. Here, we report investigations of CdSe/CdS/ZnS core/shell/shell NC ensemble PL and establish a quantitative model of interfacial trapping and its effect on NC PL dynamics.Colloidal CdSe NCs are coated with passivating ligands that sustain their dispersion in solution and minimize ...

Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

“…The trapping times (a) are plotted as a function of δ∆E (∆E = ∆E ht v or ∆E = ∆E sj c ) to account for possible variations in the trap depth ∆E ht v (or, equivalently, -however with the opposite sign -in the calculated value of ∆E sj c , see Figure 1), around its calculated position (δ∆E = 0), due to size/shape anisotropy in the sample and/or external causes (such as local electric fields). The matrix elements (b) are displayed as a function of the energy difference between the initial excitonic state E i (n) and the lowermost final excitonic state E f (1). The different regimes corresponding to the positive and negative values of E i (n) − E f (1) are schematically depicted by the cartoons.…”

“…However the details of the trapping dynamics in many materials are still subject of debate and intense research. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, in some cases it is even unclear whether it is the trapping of the electron and/or that of the hole that affects the fluorescence efficiency, as several non-radiative decay components have been observed with different magnitudes (sometimes differing by several orders of magnitude for the same material), prompting the suggestion that different types of traps must be present. This is the case of CdTe CQDs, where recent experimental studies have evidenced fluorescence decay curves that required at least a tri-exponential function to yield good agreement with the observed kinetics.…”

Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots

“…In fact the variation of the surface state distribution and density near the band-edge amongst single NCs was recently elucidated 19 , suggesting that it may be possible for some NCs to be almost free from the influence of surface states in the bandedge fine structure region.…”

Spontaneous Spectral Diffusion in CdSe Quantum Dots

“…In general, the surface of a NC is a complex interface between crystal surface atoms and molecular ligands that bind to individual surface sites in order to passivate dangling bonds. It is known that the surface coverage is not complete; 17 thus, a NC has many possible surface configurations, which in turn have different associated charge distributions. Thus, a reasonable model of SD is one involving photoactivated rearrangement of surface ligands causing discrete jumps 13 between different surface configurations in a complex potential landscape.…”