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Photoluminescence Blinking from Single CdSeS/ ZnS Quantum Dots in a Conducting Polymer Matrix Nebras Al-Attar, Eamonn Kennedy, Gabrielle Kelly, and James H. Rice J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511734k • Publication Date (Web): 19 Feb 2015 Downloaded from http://pubs.acs.org on March 7, 2015

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The Journal of Physical Chemistry

Photoluminescence Blinking from Single CdSeS/ZnS Quantum Dots in a Conducting Polymer Matrix Nebras Al-Attar †, Eamonn Kennedy †, Gabrielle Kelly ‡, James H. Rice *† † ‡

NanoPhotonics Research Group, University College Dublin, Belfield, Dublin 4, Ireland. School of Mathematical Sciences, University College Dublin, Belfield, Dublin 4, Ireland.

ABSTRACT Quantum dot nanocrystals (NQDs) present within organic conducting (polymer) host environments form hybrid organic-inorganic materials that are applied in a range of technologies such as light emitting diodes or solar cells. Understanding hole-transport and exciton dynamics in these hybrid materials is central to device performance and efficiency. Integral to hole-transport is the understanding of multi-exciton processes such as charged excitons as well as neighbour-neighbour NQD interactions (on the nano and micro-metre length scales). Studied here are the photoluminescence dynamics of single alloyed NQDs in conducting (or insulating) polymer environments. We find that conducting polymers (through hole transport) affects the presence and dynamics of charged excitons relative to insulating environments. The presence of such charged excitons induces a change in blinking dynamics with a corresponding increase in photoluminescence correlation between neighbouring NQDs found using spatio-temporal statistical analysis. Understanding such phenomena advances the understanding of photoluminescence processes central to device design. INTRODUCTION Nanocrystal quantum dots (NQDs) possess a range of room temperature optical properties such as large quantum yield of emission, excellent photo-stability and tuneable emission frequencies. Photoluminescence intensity at the single NQD level (in many types of NQDs) exhibits a random switching between brightly fluorescing and none emitting states (blinking). One of the central mechanisms believed to be involved in this process is Auger recombination where electron-hole recombination energy is not transferred into a (radiative) photon but is transferred (non-radiatively) to a third charge 1,2. Auger decay in NQD nanomaterials is an effective non-radiative pathway due to a combination of the close proximity between interacting charges and the relaxation of momentum conservation. NQDs can be combined with polymers for inorganic/organic hybrid LEDs, photovoltaics or biosensing devices 3-6. Studies have shown that conducting/conjugated polymers/NQD composites can exhibit a type II band alignment where hole-transfer can occur from the photoexcited NQD to conducting or conjugated polymer 3-6. Understanding the hole and electron transfer of devices such as NQD photovoltaics is central to the optimization of efficient device operation, and offers exciting opportunities for better photovoltaic technology. In contrast, NQD LED and biosensing devices require surpression of hole-transfer from NQD to conducting/conjugated polymer in order for efficient radiative recombination to occur 3-6. Integral to hole-transport is the understanding of precursor (to hole-transfer) multi-exciton processes such as charged excitons as well as neighbour-

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neighbour NQD interactions (on the nano and micometre length scale) where the presence of charges within the host medium may lead to perturbation of exciton dynamics. Alloying NQDs alters the Auger efficiency rate through the smoothing of band edges between shell and core. This in turn reduces the spatial distance between the initial and the final state of the excited carrier during Auger recombination, which can affect the rate of this process 7-15. Alloyed NQDs can exhibit higher concentrations of charged excitons which are associated with reduced blinking dynamics and the observation of greater photoluminescence efficiency 7-10,13. In order to explore the potential for combining conducting polymers with alloyed NQDs for optical device design a greater understanding of the influence of conducting polymers on the NQD exciton dynamics is required. Studies of Auger decay processes in single NQDs have centred on the study of neutral (multi)exciton dynamics 16-20 as Auger dynamics of charged species (such as trion states) are challenging to create/control. Charged NQD (multi)exciton processes have been studied using electrochemical charge injection 7,23 and also using chemical treatments with reducing or oxidizing species 24. Alloyed NQDs photoluminescence involves charged excitons due to their electronic potentials resulting from the alloying of core and shell interface 9,10,22-25. Such studies reported that S-shell neutral excitons (X) and biexcitons (2X) states occur along with charged (or ionised) excitons created via Auger assisted ionization process using phonon-assisted tunnelling to form trapped states resulting in a higher ionization rate. Some of recombined carriers transfer their energy to other carriers potentially delocalizing one (and necessarily trapping the other) carrier from the formed biexciton, generating either a positive trion ((T1+) formed from two holes one electron) or a negative trion ((T1-) formed from two electrons one hole). Qin et al studied ITO based electrochemical control of the charged state of CdSeS/ZnS NQDs 7. The authors reporting that the T1- state to be shorter lived compared to X. This study is in line with other studies indicating that the trion states are shorter lived relative to neutral X 7,9,10,22-26. Park et al reported that a general property of NQDs was that the photoluminescence lifetime of T- is five times longer lived than T1+ and 2X is four times shorter lived than T1+ 9. Zang et al recently reported photoluminescence studies of charged/neutral exciton in a conjugated (conducting) polymer environment 16. The authors assessed the on/off-state dynamics in single NQDs and found that hole transfer from photoexcited (non-alloyed CdSe) NQDs to a conjugated polymer affects the photoluminescence blinking dynamics of a NQD in two distinct ways: it quenches the overall PL intensity and at the same time it increases the residence time of the NQD in the on-state 16 . We examine the exciton dynamics in single alloyed ZnSeS/ZnS probing the presence of charged and neutral multiexciton states and the influence of both insulating and conducting polymer environments on the NQDs photoluminescence dynamics. Single CdSeS/ZnS NQD time resolved and time-integrated photoluminescence blinking studies show that conducting polymer environments creates faster switching between bright and dim radiative and nonradiative states. We assign the presence of bright-on states to radiative recombination of neutral excitons and the presence of dim photoluminescence with intermediate intensity to charged excitons created by Auger based processes. Observed also are dark-off states which are assigned to Auger recombination based processes such as those yielding hot electrons. We study the effect of NQD interaction (in light of other studies 27-31) using spatio-temporal statistical analysis and find that conducting organic environments better support neighbourneighbour NQD blinking correlation when charged excitons are created. We interpreted the occurrence of observed correlation effects though the presence of charges (hole or electron) transferred to and conducted within the conducting polymer matrix.


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MATERIALS AND METHODS Materials. CdSeS/ZnS alloyed NQDs were spin coated onto pre-cleaned glass substrates or spin coated onto poly(methyl methacrylate (PMMA) or poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT). All materials were purchased from Sigma Aldrich. Samples were prepared with either a c.a. NQD-NQD separation distance of a few microns or with a larger separation distance of c.a. one hundred microns. The latter separation distance enabling single NQD studies while the former, smaller separation distance was used to assess the influence of neighbour-neighbour NQD interactions. Single NQD time-resolved fluorescence and fluorescence microscopy measurements were performed on a home-built confocal scanning stage microscope based on an inverted microscope (1.4 NA 100x oil objective) equipped with a piezo scanning stage and coupled to a diode-pumped solid-state laser system delivering 470 nm pulses of 63 ps width, at 80-2.5 MHz repetition rate range (Picoquant, Germany) 32,33. This laser is coupled with a single mode fibre optic with a collimated output. The average power at the sample was kept at around 100 nW. The fluorescence was collected in inverted format, spectrally separated from the excitation laser light by a dichroic beamsplitter, Notch and by a band-pass filter. The fluorescence signals were detected by a single photon counting avalanche photodiode SPAD (τ-SPAD Picoquant) and a time-analyser (TimeHarp260, PicoQuant). To confirm the presence of single NQDs fluorescence correlation spectroscopy was used. The fluorescence was split (50/50) by a non-polarising beam splitter cube and detected by two APDs to allow for anti-bunching measurements (forming a Hanbury Brown/Twiss configuration). This setup was designed to be modular, which enabled rapid switching between a Hanbury-Brown and Twiss configuration and the spectrometer pathway. This spectrometer was equipped EM-CCD and fibre optic coupled the SPCM (Perkin Elmer). Data acquisition and data analysis were performed with Symphotime 64 analysis software (Picoquant) which was used for decay trace fitting and histogram construction. Over 50 NQDs where studied for each environment. Blinking traces where recorded with observation time of >400 s with 20 ms bin times unless otherwise stated. RESULTS AND DISCUSSION Single quantum dot studies were first performed using single photon counting avalanche photodiode SPAD and a time-analyser methodology. Examination of the photoluminescence decay lifetime from a single NQD in PMMA was undertaken. Single NQD lifetimes for over 50 NQDs where recorded along with photoluminescence intensity over an 800 sec time window with a 20 ms bin time. Figure 1(a) (and also in Figure S1, supplementary information) show examples of the resulting photoluminescence intensity fluctuation/blinking trace. The recorded single NQD photoluminescence blinking intensity data-sets are divided into bright photoluminescence (on-state regions), regions of dim photoluminescence intensity (intermediate-states) and regions where the photoluminescence is lowest (off-state). Parallel to recording photoluminescence intensity fluctuations (blinking), photoluminescence decay traces were also recorded, (Figure 1b). A bi-exponential fit was applied to the photoluminescence decay traces in line with other studies 34,35. The bi-exponential fit contains one parameter for the sum of the radiative and nonradiative rate constants (kex) and a second parameter which contains the sum of kex and the fast (multiexciton) rate constant 34. Figure 1(c-d) illustrates the typical distribution of lifetimes for on-states, off-states and intermediatestates for a single NQD in PMMA polymer matrix environment.



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For off-states, the lifetime distribution peaks at 2 ns with the lifetime distribution subsequently tailing off to c.a. 25 ns (see Figure 1(c-d)). Studies of the fluorescence decay lifetimes in CdSe NQDs for off-states reported that the lifetime of the off-state is affected by its duration in preceding on-states. Photoluminescence decay for off-states, measured after a preceding on-state, possesses decay lifetime ranges up to 25 ns with an average lifetime of 4 ns 34. This study reported that off-state dynamics are bi-exponential with short lived (e.g. 500 ps) decay process assigned to Auger recombination processes and longer lived decay processes (e.g. 4 ns) arising from fluctuating trapping of surface hole states. In addition it is noted that the presence of higher charged states (ex T2-) may also occur which are reported to possess a shorter lifetime than T1- 25. The on-state lifetime distribution shown in Figure 1(c,d)) shows a broadly Gaussian distribution in lifetimes peaking at 28 ns. The on-states potentially arise from X or 2X radiative recombination. Studies of on-states reported that 2X-states can be detected using g(2)(τ) second order photon correlation measurements 36. The normalized amplitude of the cascade feature is equal to the ratio of the 2X to X fluorescence quantum yields. g(2)(τ) photon correlation measurement of single NQDs in PMMA were recorded (example shown in supplementary information Figure S1). The presence of an intensity at time 0 can be identified and assigned to 2X and inspection of the g(2)(τ) plot indicates that features assignable to 2X are present with a g(2)(0) value indicating a 2X lifetime ~ 2ns (g(2)(0) ≈ 0.2). The lifetime of the X state for CdSeS/ZnS has been reported to be 33 ns 7. These values coincide with the observed on-state lifetime distribution peak at 28 ns (see Figure 1(c-d)). A small peak distribution can be seen at 2 ns in Figure 1(c-d) which can be assigned to the presence of weak 2X photoluminescence which possess a decay lifetime distribution and occurrence on the order of that expected from other studies of NQDs and from g(2)(0) measurements 7,36. Intermediate-state lifetime distributions were recorded (shown in Figure 2(c-d)). The distribution shows a peak in maximum occurrence centred at 9 ns. Studies have assigned intermediate photoluminescence intensity to charged excitons e.g. T1- and/or T1+ decay processes 7,9,10,21,23,25,26. Studies of alloyed CdZnSe/ZnSe photoluminescence are reported to be dominated by radiative recombination of T1+ state in a nanocrystal with a soft-confinement potential 10. Studies using thick-shell CdSe/CdS QDs with and without intentional alloying of the core/shell interface reported that the photoluminescence blinking arising from neutral X state was longer lived than either T1- or T1+ states, with T1- states longer lived (e.g. 17 ns) compared to T1+ states (e.g. 3 ns) 9. Electrochemical studies of single CdSeS/ZnS NQDs reported a lifetime of >1 ns for T1- states where the trion state is longer lived than the 2X state 7 . However the presence of the ITO surface used in the electrochemical measurements was noted to be potentially interacting with the NQDs resulting in a lowering of the NQD photoluminescence lifetimes. A small number of NQDs photoluminescence spectra exhibit specific spectral characteristics where two photoluminescence peaks are seen in place of a single photoluminescence peak. Figure 2(a) shows such photoluminescence spectra. The spectra show changes in photoluminescence intensity typical of blinking spectral intermittency. Summing the individual spectra exhibited two peaks, separated by 120 meV (Figure 2b). A spectral study of the fluorescence emission from single alloyed CdZnSe/ZnSe NQDs reported similar spectral features with peak to peak spacing of 164 meV which the ahtours assigned to a positive trion state 10. These peaks arise from charged excitons whose energies are


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determined by the difference between the trion annihilation energy and the excitation energy of the extra hole in its final state. Figure 2(c) shows a schematic diagram of electron and hole energy levels (as used in ref 10) along with the average spectral profile taken from Figure 2(a). The density of states for hole-states is higher than for electron states, as represented by solid lines in Figure 2(c) as reported in ref 10. The emitted photon (red, green and blue arrows) has been related to energy of the trion in which the energy of the extra hole can occupy one of many allowed levels after recombination depending on its effected mass. The observed effects are based on T1+ states (i.e. an electron with two hole-states within a soft confinement potential typical for alloyed NQDs). This observed spectra offers evidence for the presence of a positively charged trion state in the NQDs studied here. It is noted that while for ref 10 the authors reported the absence of blinking, we find that the NQDs here exhibit blinking dynamics (in line with the literature 6). As a consequence only infrequently was a shake-up process observed. Wang et al reported photoluminescence decay lifetime for alloyed CdZnSe/ZnSe for the charged T1+ state is 4 ns. Figure 2(d) shows the lifetime distribution from a single NQD exhibiting the shake-up process. The lifetime distribution is narrow with the NQD exhibiting a lifetime value of 24 ns. This indicates a long lived trion state, longer than the average measured (as shown in Figure 1 which indicates a trion lifetime of around 10 ns. This may be due to a long lived trion state at the tail end of the distribution shown in Figure 1 or the observed spectral features may arise from the presence of an aggregate of two NQDs. Inspection of Figure 2(a) shows changes in spectral profile as a function of time. The two peaks appear or disappear randomly. This may indicate that the hole-states are in different energy levels and that this process is random, exhibited by the appearance or absence of one/both of the two aforementioned bands. The absence in the occurrence of this spectral signature (for T1+) for the majority of NQDs observed (>50 NQDs) indicates that the majority of the NQDs are predominately not in T1+ states. Spectral diffusion can be seen to occur indicating that there are charges in the environment perturbing the exciton states resulting in small shifts in the emission intensity 36,37. These charges, if electrons, can create Stark like shifts in the NQDs energy levels which would result in the observed spectral shifts. This indicates that electrons (or holes) are present within the environment, arising from effects such as charge ionisation effects known to occur in NQDs 15. In order to further determine the environmental influence on these effects, NQDs in PEDOT environments were also undertaken. The photoluminescence intensity for single NQDs in PEDOT (compared to that in PMMA) shows a relatively reduced photoluminescence intensity (by c.a. 40/50 percent) coupled with a significantly greater switching between on, intermediate and off-states. Log-Log plots extracted from single NQD blinking in PEDOT and PMMA are shown in Figure 3(a-c). The log-log plot profile for all states (off, on and intermediate-states) in PEDOT environments shows quicker dwell times for NQDs in PEDOT relative to PMMA. Figure 1(e-f) shows plots for the fluorescence lifetimes for on, off and intermediate-states for NQDs in PEDOT (plotted using bi-exponential fits). The lifetime distribution over the three states shows that the distribution in lifetimes shifts to shorter lifetimes for NQDs in PEDOT relative to PMMA. For bi-exponential fits, NQDs in PEDOT show an off-state lifetime distribution that is broad with a much reduced occurrence compared to NQDs in PMMA. NQDs in PEDOT also show an off-state lifetime distribution that extends over 20 ns as in seen for PMMA but with reduced occurrence compared to on-states and intermediate-states. The lifetime distribution and relative occurrence for intermediate-states for NQDs in PEDOT



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is similar to those found for NQDs in PMMA with lifetime distribution peaking at 9 ns. The on-states exhibit a longer lived lifetime (for PEDOT compared to PMMA) with the distribution centring around 35 ns, noting that the relative occurrence of on-states is lower for NQDs in PEDOT compared to PMMA. A peak at around 2 ns (potentially assignable to neutral 2X states) which can be seen with greater occurrence for NQDs in PEDOT compared to the corresponding plot for PMMA. The NQDs studied here possess an emission peak at c.a. 670 nm with an absorption onset at c.a. 610 nm, PEDOT possess an emission band at c.a. 450 nm and an absorption band onset at c.a. 300 nm which results in no spectral overlap between PEDOT’s electronic absorption and NQD’s photoluminescence band/spectra, ruling out energy transfer from photoexcited NQDs to PEDOT. It is noted that a small onset of near-IR vibronic absorption intensity is observed which may potentially result in some energy transfer process. The absorption cross section of this is low, however. The probe laser (470 nm) will be absorbed only by the NQD resulting in selective photoexcitation of the NQD in the NQD/PEDOT environment. The potential for hole transport between the NQD to a conducting/conjugated polymer has been reported to occur under similar conditions 16,38,39. The driving force for hole-transport is mainly determined by the energy band offset between the HOMO levels of the NQD and PEDOT which are sufficiently spaced in the NQD/PEDOT hybrid material studied here for this to occur. The use of alloyed NQDs advances the potential for this to occur as studies on holetransfer rate in NQD/conducting-conjugated polymer hybrid materials demonstrated that in core/shell CdSe/ZnS NQDs the rate changes with shell thickness, with the shell acting as a tunnelling barrier towards hole transport 16,38,39. In alloyed NQDs such as those studied there is a greater potential for (electron) charge tunnelling occur 8,10 relative to hole-states. Probability distributions for on, off and intermediate-states ((P(ton)), (P(toff)) and (P(tintermediate)) are shown in Figure 3(a-c). It has been reported that hole transport effects both on and off-state time durations (intermediate states not reported) with a lowering of their dwell times (as seen also in Figure 3(a-c)), whereby the conducting polymer acts as a hole acceptor for the photoexcited NQD, quenching the photogenerated exciton by accepting a hole, generating a negative trion (e.g. T1-) that still emits 16. Note that the conducting polymer can accept a hole from the NQD when an electron is trapped thereby neutralizing the core 16,36,37 . Analysis of the probability distributions was undertaken using a modified power law model P(t) = b x t-m exp(-(t/t)n) (fitting are shown for each plot separately in supplementary information S5 for clarity). The changes in m are expressed as ∆mx (∆mx = mx(CDP) – mx(PMMA) where x = off, on or intermediate. mon (∆mx =0.1) and moff (∆mx =1.0) both increased while mintermediate (∆mx = -0.21) decreased for CDP relative to PMMA. This indicates that for NQDs in CDP there is a decrease in the on and off-states time, while the intermediate-state time is increased. It has been reported that for NQDs in conjugated polymers where hole-transfer has been reported, the NQD’s residence time in the on-state is increased thus decreasing the duration time in the off-state, where the study did not include the presence of intermediate states 16. The authors reported that for NQDs in conjugated polymer the polymer acts as a hole acceptor for the photoexcited NQD. This hole transfer process quenched the photo-generated exciton by generating a negative trion that still emits 16 . The HT rate for NQD can be estimated from single NQD photoluminescence data assuming photoluminescence lifetimes is given by τNQD = 1/(kr + knr ) for PMMA and τNQD = 1/(kr + knr + kHT) for CDP where kr, knr and kHT are radiative, non-radiative and hole-transfer rates respectively. As outlined in Figure 1 the overall photoluminescence lifetime for the system


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gets shorter for NQDs in CDP compared to PMMA shows that the radiative component for pn-states are slightly longer lived (noting that using a mono-exponential fitting routine the average lifetime gets shorter, see S5 supplementary information). kHT was estimated to be 0.9 x107 S-1 for NQDs in CDP. This value comparing with ranges of 1.1 x107 s-1 for ZnSe NQDs in conjugated/conducting polymer environments 16. In this study of ZnSe it was reported that the hole-transport rate been reported to be related to kHT ~ (4π2/h) x H2 x FC with H the donor–acceptor electronic coupling factor and FC = (1/4πλkBT)1/2 exp[-(∆G0 + l)2/4πλkBT] 16 where the Franck–Condon factor includes the reorganization energy (λ) and the free energy (∆G0) or the driving force for HT 16. H depends mainly on the donor–acceptor separation distance, while ∆G0 is determined by the HOMO and LUMO levels of donor and acceptor moieties. It has been reported that for NQDs in conjugated polymers the donor–acceptor separation is defined by the core-shell separation, which is ‘smeared’ in alloyed systems reducing this distance compared to core-shell ZnSe systems such as those reported in ref 16. This reduced separation distance potentially contributes to a fast hole-transfer rate when comparing core shell ZnSe 632 (kHT = 1.1 x107 S-1) emitting NQDs to alloyed ZnSeS NQDs here (kHT = 0.9 x107 S-1) which is approximately the same in value 16. However, the polymer types are different and it was also reported the driving force for hole-transport is also affected by the energy band off-set between the HOMO levels of the QD and conjugated/conducting polymer, which is related to the core size of the NQD, with smaller core sizes (in core shell systems) exhibiting a twice as fast hole transfer rate 16. Time-resolved single molecule photoluminescence studies of CdSe/ZnS/streptavidin NQDs showed that there is a broad distribution in the lifetimes from a single NQD suggesting that a simple “on/off” discrete state model is insufficient 40. The authors reported that the results were consistent with a model involving charged NQDs with time-dependent charge migration giving rise to the observed photoluminescence dynamics. Specifically, the authors reported that a broad distribution in emission intensity can be understood as resulting from a distribution of positive charge traps at the core-shell interface, or at the shell-streptavidin layer, which with time the charged NQD may reneutralize but is quickly photoionized under the excitation laser illumination, or the charge may persist but migrate among the localized trapping states via a thermally activated process 40. Studies of photoionization probability in CdSe/ZnS reported that roughly 1% of surface localized states lead to ionization 15. This effect may be greater for alloyed NQD systems where there is more effective tunnelling of charges into the shell state potentially reducing the ionisation energy. The use of PEDOT environments enables better conduction of any resulting charge (holes or electrons) potentially enabling stronger exchange of charges within the medium. In addition to the question of fluorescence blinking in NQDs, reports indicate that neighbourneighbour interactions occur between NQDs. Stefani et al 27 demonstrated that subsequent on and off-times of the luminescence blinking of semiconductor quantum dots (QDs) are correlated. Volkan-Kacso et al. 28 demonstrated that long-range correlations between consequent blinking events occur. Li et al 29 studied the fluorescence spectra from single CdSe core-CdS/ZnS and CdS/CdZnS/ZnS shell colloidal NQDs using cross correlation methods and found that the fluorescence events from two neighbouring NQDs were correlated up to distances of c.a. 1.5.micron. Hefti et al 30 applying Pearson product-moment correlation coefficient for neighbouring pairs of NQDs showed that the fluorescence trajectories of dots separated by up to ∼1 µm were correlated. Blinking rate enhancement was observed when nearby NQDs were in opposite emitting states. Qin et al 31 also applied cross correlation methods to NQDs and reported that when the distance between two NQDs is
400 nm ensures that there is no overlap of NQD PSF, enabling each NQD to be treated separately. The application of spatial-temporal statistics to assess any relationships in regard to blinking behaviour between neighbouring NQDs was undertaken. Empirical variograms were computed (see S3, supplementary information). NQD photoluminescence was divided into three parts – intermediate, on and off-state regions using maximum likelihood analysis (see supplementary information S4). This is not a sentence: Significant degree of spatial dependence occurs in the sample space or when the sampling unit dissipates into randomness when the variance terms of a temporally or in-situ ordered. Varogram analysis shows that only for intermediate-states does a correlation exists for NQD-NQD interaction as a function of distance, no correlation was seen for on vs the rest (e.g. off and intermediate-states) or for binary on vs off-states as outlined in supplementary information S3. Figure 4(a) shows the variogram for the intermediate-state vs on and off states (i.e. all other states). Confidence limits were computed using bootstrapping and the observed variogram was outside the limits indicating spatial structure in the data. The variogram indicates that covariance in the blinking data can be observed from NQD separation distances of 0 to c.a. 2 microns. The effect of environment was examined by comparing NQDs in PMMA to PEDOT. The blinking dynamics for NQDs in these environments was examined using semivariance methods. The resulting variograms are shown in Figure 4(b-c). The semi-variance for NQDs in PEDOT is a factor of c.a. 10 times higher compared to PMMA indicating


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stronger correlation. It was noted that for NQDs in a conducting environment, a more rapid change in blinking between the states is seen, compared to an insulating environment. Covariance in regard to blinking dynamics between NQDs is stronger for a conducting environment. Volkan-Kacso et al 28 reported that the multiple recombination centres model suggests that for every NQD there is a number of non-radiative recombination centres likely associated with surface traps. The fluctuation in the trapping rate results in quantum efficiency and fluorescence emission intensity fluctuations. The authors proposed that this model predicts long-range correlations between consequent blinking events. The physical reason for the long-range correlations reported has been associated with the hierarchy of the recombination centres switching times. Hefti et al 30 reported that NQDs separated by c.a. 1 µm, possessed correlation of on and off events. The authors suggesting that a triggering effect inherent in the blinking mechanics of NQDs prompts neighbouring dots to shift toward similar emission states. The authors applied a multiple charging model in which charge carriers, ejected from the NQD are perturbed by Coulomb interactions with neighbouring dots (multiply charged NQD with another NQD). PEDOT and PMMA possess different conductivities (150 S/cm and < 10-4 S/cm respectively 43,44 . This lends support to the movement of charge (electrons or holes) playing a role in the mechanism for the observed increases in blinking rates and variance of blinking behaviours between neighbouring NQDs. The mechanism outlined by Hefti et al 30 proposes that carriers, ejected from the NQD are perturbed by Coulomb interactions with neighbouring dots (multiply charged NQD with another charged NQD). Such a mechanism is enhanced by PEDOT potentially by providing an environment that supports the formation of charged NQDs. It is noted that variance is seen for NQDs in PMMA (albeit much weaker). PMMA is an insulator and does not support the relative movement of charge. Studies by Schmidt et al 21 noted that the environment was important in supporting the occurrence of blinking. The authors studying single CdSe/ZnS NQDs in polystyrene, polyvenylalcohol, and on silicon oxide, reported that blinking is effected by switchable nonradiative trapping, with the switching rate controlled by the environment. Induced/permanent polarisation of the molecules adjacent to NQDs create conditions for switching being potentially one such environment effect. It is noted here that PEDOT and PMMA possess similar refractive indexes (n20/D 1.334 and n20/D 1.49 respectively) 43.44. This indicates that the (induced) polarisation of the environment is not significant in the mechanism for increased blinking rates when the environment changes. However, conduction/movement of hole (or electron) charges is potentially important which is supported better with PEDOT compared to PMMA. It is noted that studies of NQDs exhibit power law with similar exponents 45 which were reported to provide evidence that blinking statistics are not swayed by environment-induced variations in kinetics. We find that the observed blinking can be understood via a modified power law (see Figure 3), where the power law exponents possess changes in values in line with reports by Zang et al 16. This study, in line with ours being for NQDs in a hole-transfer environment. Studies of charge trapping states in single NQDs capped with differently charged capping layers 46 showed that surface hole trapping rates effects the power law exponents.



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Analysis of fluorescence decay traces from single CdSeS/ZnS NQDs shows that environments with higher conductivity (PEDOT relative to PMMA) create faster switching between radiative (on, and intermediate-states) and fewer non-radiative off-states. These results are interpreted using two neutral states, X and also 2X forming a bright on-state (see Figure 5). The bright state is dominated by radiative recombination of the neutral electronhole pair with X longer lived than 2X. The two other states are ionized states which occur via Auger assisted ionization process using phonon-assisted tunnelling to form trapped states resulting in a higher ionization rate 12. Some of recombined carriers transfer their energy to other carriers potentially delocalizing one (and necessarily trap the other) carrier from the formed biexciton, generating either T1+ (two holes one electron) or T1- (two electrons one hole). Any optical recombination action of electron-hole in T1- or T1+ should obey the selection rule of total spin angular momentum quantum number S=±1. Hence, the orbital quantum number (Mz) in the conductive band is 0. The total angular momentum, JC, in the conductive band is equal to the spin quantum number. However, the possible JV value in valance is ± ,± . the heavy holes have JV = ± whereas light holes have values band values ± . The ground state of the hole is the heavy hole with just two JV = ± , because light holes with projection JV = ± moved up in energy for valance band. The (T1-/T1+) emits a low intensity photoluminescence signal (low quantum yield). It is reported that T1- is longer lived than T1+ 9. This behaviour can be explained by a greater spectral density of valence vs conduction-band states and a much smaller effective localization radius of a hole compared to that of an electron; both of these factors increase the relative efficiency of the positive vs negative trion Auger pathway. The off-states are created by nonradiative Auger recombination whereby two electrons and a hole (left) recombine to form a “hot” electron been responsible for off-state which is short lived (picosecond timescales) 9. The faster switching rate between intermediate and on-states for NQDs in PEDOT may occur due to the hole transfer creating more efficiently T1- states. Reports on hole transport from NQDs to an acceptor reports that hole transport rate is on the order of 1x10-7 s this time frame enabling rapid creation of a charged exciton 38,39. Hole transport creates conditions that potentially favours the creation of T1- over the formation of T1+. The dynamics seen for NQDs in PEDOT can be assigned to T1-. Noting that T1+ is also present, however due to the lifetime distribution centring around 9 ns, this lifetime is closer to reported T1- states than T1+ states 9. The observation of blinking correlation for intermediate states may arise as follows: hole-states when transferred into the environment can interact via a columbic potential with surface (trapped) electrons in a (neighbouring) NQD resulting in a stabilisation and enhancement of the formation of T1- states, thereby enhancing the probability that such a state occurs within a given time frame. In conclusion, charged alloyed NQDs such as CdSeS/ZnS possess a soft confinement of electrons and holes, whereby the shell and core regions do not possess an abrupt boundary between the shell and core, but instead a more continuous boundary is present which enables charged exciton states to be efficiently created. We find that conducting polymers may be involved in hole transporting of changes from the NQD to the polymer. This affects the presence of T1+ charged exciton states with a significantly reduced rate of occurrence, while the T1- observation rate is increased. Correlations between the intermediate-states are observed and are assigned to dynamics involved in the creation of T1- states via the removal of a hole into the environment leading to potential columbic interactions between holetrapped electrons/T1- charged excitons. This detailed insight offers opportunities for controlling the type of charged exciton present through spatially selective and environmentally specific modifications of this process. In addition, we indicate a mechanism


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for the control of blinking dyanmics through the analysis of correlation effects between neighbouring NQDs, the occurrence of which for T1- charged exciton states opens up opportunities for novel NQD device designs. ACKNOWLDEMENT This publication has emanated from research conducted with the financial support of Science Foundation Ireland (SFI) PI grant and the DGPP which is funded under the Programme for Research in Third Level Institutions Higher Education Authority PRTLI Cycle5 and cofunded by the European Regional Development Fund. The Nanophotonics and Nanoscopy Research Group is supported by SFI grants 11/RFP.1/MTR/3151, 12/IP/1556. SUPPORTING INFORMATION Supporting Information is available for this paper. This information is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Single NQD blinking dynamics recorded using time-tag photoluminescence method (a) a section of photoluminescence intensity blinking trace (more detailed examples shown also in supplementary information, figure S1), (b) examples of decay traces taken from selected regions that possessed only intermediate or on or off-states. (c) and (d) Plot of occurrence vs average lifetime plot for a single NQD in PMMA, (e) and (f) Plot of occurrence vs average lifetime plot for a single NQD in PEDOT.



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Figure 2. Blinking spectral trace from a single NQD in PMMA. (a) Spectral emission from a single NQD shown as a function of time. (b) Average spectra (c) plot of experimental data over-layed with schematic plot taken from ref 10. (d) Lifetime distributions for the NQD. Three different spectral regions are monitored.

Figure 3. (a-c) log-log plots for on, intermediate and off states for NQDs in PMMA and conductive (PEDOT) environments.


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Figure 4. (a) An empirical variogram of sum of PL intensity, the variogram was constructed up to all possible distances and also up to one half the maximum distance between the points. Confidence limits were computed using bootstrapping and the observed variogram was outside the limits indicating spatial structure in the data. (b-c) Empirical variogram of sum of PL intensity for intermed-states vs off + on-states (top) and on-states vs off+intermed-states (bottom). The variogram was constructed up to all possible distances and also up to one half the maximum distance between the points. The variograms for NQDs in (b) on CP and (c) on PMMA. The fitted lines need some comment.



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Figure 5. (a) Schematic representation of the band alignment for the negatively charged trion in CdSeS/ZnS nanocrystal. The electrons sit in a soft potential enabling an electron to tunnel to the surface forming a quasi type II structure. The transport of a hole resulting in a negative trion is shown as forming due to the band off-set between the confinement potentials of PEDOT and NQD. (b) The presence of the delocalised/surface bound electron can potentially interact with the positive hole charge residing on PEDOT. On-state formed from neutral exciton recombination. Off-state formed from electron ionization or (surface) trapped state converts which can convert back to the on-state by charge recombination, this state then can either form a positive trion or undergo hole transport. Hole receptors enable the formation of negative trion states.


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For Table of Contents Only



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Figure 1. Single NQD blinking dynamics recorded using time-tag photoluminescence method (a) a section of photoluminescence intensity blinking trace (more detailed examples shown also in supplementary information, figure S1), (b) examples of decay traces taken from selected regions that possessed only intermediate or on or off-states. (c) and (d) Plot of occurrence vs average lifetime plot for a single NQD in PMMA, (e) and (f) Plot of occurrence vs average lifetime plot for a single NQD in PEDOT. 254x190mm (96 x 96 DPI)


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Figure 2. Blinking spectral trace from a single NQD in PMMA. (a) Spectral emission from a single NQD shown as a function of time. (b) Average spectra (c) plot of experimental data over-layed with schematic plot taken from ref 10. (d) Lifetime distributions for the NQD. Three different spectral regions are monitored. 254x190mm (96 x 96 DPI)



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Figure 4. (a) An empirical variogram of sum of PL intensity, the variogram was constructed up to all possible distances and also up to one half the maximum distance between the points. Confidence limits were computed using bootstrapping and the observed variogram was outside the limits indicating spatial structure in the data. (b-c) Empirical variogram of sum of PL intensity for intermed-states vs off + on-states (top) and on-states vs off+intermed-states (bottom). The variogram was constructed up to all possible distances and also up to one half the maximum distance between the points. The variograms for NQDs in (b) on CP and (c) on PMMA. The fitted lines need some comment. 254x190mm (96 x 96 DPI)


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Figure 3. (a-c) log-log plots for on, intermediate and off states for NQDs in PMMA and conductive (PEDOT) environments. 254x190mm (96 x 96 DPI)



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Figure 5. (a) Schematic representation of the band alignment for the negatively charged trion in CdSeS/ZnS nanocrystal. The electrons sit in a soft potential enabling an electron to tunnel to the surface forming a quasi type II structure. The transport of a hole resulting in a negative trion is shown as forming due to the band off-set between the confinement potentials of PEDOT and NQD. (b) The presence of the delocalised/surface bound electron can potentially interact with the positive hole charge residing on PEDOT. On-state formed from neutral exciton recombination. Off-state formed from electron ionization or (surface) trapped state converts which can convert back to the on-state by charge recombination, this state then can either form a positive trion or undergo hole transport. Hole receptors enable the formation of negative trion states. 254x190mm (96 x 96 DPI)


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Photoluminescence Blinking from Single CdSeS ... - ACS Publications

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