Effect of Surface Stoichiometry on Blinking and Hole Trapping

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The Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals Erik Busby, Nicholas C. Anderson, Jonathan S. Owen, and Matthew Y. Sfeir J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08243 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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The Journal of Physical Chemistry

The Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals Erik Busby1,2, Nicholas C. Anderson2, Jonathan S. Owen2*, Matthew Y. Sfeir1* 1) Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, United States 2) Department of Chemistry, Columbia University, New York, NY 10027, United States

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ABSTRACT We measure the photoinduced carrier dynamics as the surface composition of CdSe nanocrystals is systematically varied from metal rich (~ 80% surface Cd) to nearly stoichiometric (~ 50% surface Cd). Using time-resolved optical spectroscopy, we determine that the luminescence lifetime is controlled by the rate of hole trapping at the newly exposed surface selenium atoms. However, the increased rate of the photoluminescence decay is not sufficient to explain the decreased photoluminescence quantum yield, and requires a growing proportion of nanocrystals in a dark or ‘OFF’ state to explain the data. A global kinetic model is proposed that relates the fraction of selenium sites to the rate of hole trapping. A linear relationship between the rate of hole trapping and the fraction of exposed Se sites (xSe) is observed within the range of accessible stoichiometries (xSe = 0.5 - 0.2). Extrapolation to higher surface cadmium fractions suggests that not all Se sites serve as effective hole traps. These results explain the strong nonlinear dependence of the fluorescence yield on the nanocrystal stoichiometry.


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INTRODUCTION An atomistic understanding of colloidal semiconductor nanocrystal (NCs) surfaces is complicated by the structural inhomogeneity of nanocrystals and the batch-to-batch variation afforded by modern synthetic preparations. As a result, a direct correlation between the surface structure and the electronic structure of NCs has been difficult to obtain. Nonetheless, the idea that NC surface structure and photophysical properties are strongly related is nearly as old as the field itself.1-2 In particular, photoluminescence intermittency3 between fluorescent “ON” states, weakly emissive “grey” states, and non-emissive “OFF” states is highly sensitive to interfacial structure.4-7 Recent single nanocrystal studies have shown that the photoluminescence quantum yield (PLQY), blinking dynamics, and fluorescence lifetime are highly complex and inhomogeneous within an ensemble of NCs.8-10 Although these properties can be measured with high spectral and temporal resolution,11 their structural origins are unknown, having been attributed to substitutional impurities, surface dangling bonds, and surface ligand electronic states.8, 12-14 The effect of surfaces is further complicated by the presence of facets, edges, and corners whose electronic structure depends on ligand binding and the underlying crystal symmetry. Even within a single nanocrystal, these structures will lead to inhomogeneous photophysics.9-10 Thus, a detailed understanding of NC photophysics requires a deep understanding of surface structure. While this detailed understanding is still being pursued, effective surface modification strategies have been developed for light emitting applications,4-6, 15-16 and to enhance inter-NC electronic coupling in field-effect transistors17-19 and optoelectronics.20-21 Among these methods, control over the “surface stoichiometry”, or the fraction of metal and chalcogenide surface atoms, has proven particularly important to NC PLQY. While it was recognized early on that


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adsorption of cadmium ions to colloidal cadmium sulfide nanocrystals increases the PLQY,1 only recently was surface metal enrichment recognized as a critical and common feature of many modern metal chalcogenide NCs.22-28 More recently, surface metal enrichment strategies have been utilized in NC-based thin film electrical devices, affecting both the mobility and the majority charge carrier species.26,29 In each case, precise control over the surface stoichiometry allows the NCs to be optimized for the desired application. In colloidal NCs, tuning the NC stoichiometry also influences the number and type of ligands needed to maintain the charge neutrality of the nanocrystal ligand complex.30 Practical methods to precisely control the NC stoichiometry, therefore, involve binding or displacement of metal surfactant anion complexes to and from the nanocrystals surface. For example, cadmium carboxylate complexes can be displaced from CdSe nanocrystal surfaces, and in this manner, the stoichiometry and ligand coverage can be precisely tuned (Figure 1a).27 Using this strategy we have prepared a series of CdSe nanocrystals with varying ligand coverage from a single nanocrystal batch, and studied the influence of their composition on exciton recombination. Using solution-phase ensemble optical measurements, we quantify the relationship between the surface stoichiometry and the photoluminescence lifetime, PLQY, and the ratio of nanocrystals in the bright “ON” and dark “OFF” states, in which a particle absorbs normally but relaxes via a fast non-radiative recombination process. This allows us to measure the inherent fluorescence intermittency without influences from substrate interactions that are inherent to single particle measurements. We unambiguously demonstrate that reducing the cadmium surface fraction leads to an increased rate of hole trapping and a non-linear decrease in the quantum yield for emission. The effect on the PL lifetime is much more modest, a discrepancy that can be resolved by accounting for a change in the fraction of nanocrystals in an “OFF” state.


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While the experimentally accessible range of metal-enriched surfaces shows a hole trapping rate that is linear with the number of unpassivated Se sites, extrapolation to very low surface Se fractions suggest that the emission is largely insensitive to surface stoichiometry in the metalrich limit.

Figure 1. (a) Binding and displacement of cadmium carboxylate from CdSe nanocrystals with N,N,N’,N’-tetramethylethylenediamine

(TMEDA).

(b)

Absorption

(solid

lines)

and

photoluminescence (dotted lines) spectra with varied ligand coverage. (c) Time correlated single photon counting showing faster emission quenching in NCs with more stoichiometric surfaces. The instrument response is shown in grey. (inset) Time correlated single photon counting data is shown for longer delay times on log-log axes.


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EXPERIMENTAL SECTION Nanocrystal Synthesis Cadmium carboxylate-bound CdSe nanocrystals with a diameter of 3.7 nm (+/- 0.1 nm) were synthesized using 10 mmol of cadmium tetradecanoate and a 1:1 Cd/Se ratio by previously reported methods27, 31. Isolated nanocrystals were dissolved in benzene-d6. 1H nuclear magnetic resonance spectroscopy was used to measure the concentration of ligands (within an accuracy of 0.1 nm-2) by integrating the methyl peak versus an internal ferrocene standard as in Anderson et al.27 UV-Visible absorption spectroscopy was used to measure nanocrystal concentration32-33 and nanocrystal diameter33-34. The nanocrystal surface area was calculated using the diameter and assuming a spherical nanocrystal. The starting coverage of ligands for as synthesized nanocrystals was 1.65 cadmium carboxylates/nm2.

Reduction of Ligand Coverage Cadmium carboxylate was removed from the CdSe nanocrystals by L-type promoted Z-type displacement using N,N,N',N'-tetramethylethylene diamine (TMEDA) as we previously reported27. Nanocrystals were stirred with TMEDA for 1 hour and then precipitated three times with methyl acetate before being dried under vacuum for 1H NMR and UV-visible spectroscopies. The following surface coverages were achieved at different concentrations of TMEDA in d6-benzene: 1.51 nm-2 (5 mM), 1.45 nm-2 (10 mM), 1.22 nm-2 (50 mM), 1.16 nm-2 (0.1 M), 0.93 nm-2 (1.2 M), 0.52 nm-2 (1.5 M), and 0.25 nm-2 (2.0 M). 1H NMR spectroscopy showed no diamine was left after precipitation.


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Photoluminescence Photoluminescence was collected using a 1-MHz, 6-ps supercontinuum fiber laser as the excitation source. The laser output was filtered to give a 488±5nm bandwidth excitation pulse that was then focused onto the sample. The emission was then collected and focused into a 30cm spectrograph and detected with a liquid niotrogen cooled silicon CCD (for spectra) or an avalanche photodiode (for kinetics). Time-resolved photoluminescence measurements (TRPL) were taken using commercial time-correlated single photon counting electronics (PicoQuant, Picoharp 3000). Samples were measured under low fluence with inert atmosphere and stirring to avoid the effects of photocharging and oxidation. For analysis of the hole trapping rate constants, the total decay (ΓPL) is determined from the 1/e values in the TRPL (Fig. 1c). We have chosen to do this rather than fitting the full multiexponential decay kinetics since even the early time behavior is not strictly single exponential and since longer lived emission processes are related to the recombination of trapped carriers in shallow and deep traps35 and as such do not represent any of the terms in equation 4.

Transient absorption Transient absorption measurements were conducted using a commercial Ti:Sapphire laser (800nm, 100 fs, 3.5 mJ, 1 kHz) in a typical transmission pump-probe geometry. The excitation light was generated via an optical parametric amplifier (Light Conversion, TOPAS-C). Supercontinuum probe light was generated by focusing the laser fundamental into a sapphire plate. The resultant white light is split into a signal and a reference beam that are then detected on a shot-by-shot basis with a fiber-coupled silicon multichannel detector. For transient absorption measurements, samples were sealed under argon to avoid the effects of oxidation. Vigorous stirring and low excitation density ( < 0.01) were used to ensure that the results are independent of photocharging or multi-excitonic processes.


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RESULTS AND DISCUSSION Ensemble versus Bright State Quantum Yields We have measured the optical properties of CdSe NCs (diameter = 3.7 nm) as the surface is systematically tuned from metal enriched (1.65 cadmium carboxylate/nm2, 78% surface atom Cd, 22% surface atom Se) to a nearly stoichiometric composition (

Effect of Surface Stoichiometry on Blinking and Hole Trapping

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