Suppressed Blinking under Normal Air Atmosphere in Toxic-Metal

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Suppressed Blinking Under Normal Air Atmosphere in Toxic Metal Free, Small Sized, InP Based Core/Alloy-Shell/Shell Quantum Dot Chayan K. De, Debjit Roy, Saptarshi Mandal, and Prasun K. Mandal J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01157 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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The Journal of Physical Chemistry Letters

Suppressed Blinking Under Normal Air Atmosphere in Toxic Metal Free, Small Sized, InP Based Core/Alloy-Shell/Shell Quantum Dot Chayan K. De†, Debjit Roy†, Saptarshi Mandal†, and Prasun K. Mandal*†‡ Department of Chemical Sciences, ‡Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West-Bengal, 741246, India.

†

ABSTRACT: Suppressed blinking has been reported in large sized (diameter ~14.1 nm) core/shell InP QDs, under reduced air environment. We report here suppressed blinking with ~four times smaller (diameter ~3.6 nm) core/alloy-shell/shell InP QD under ambient air atmosphere. has been obtained to be 0.65. About 26 % of the single QDs exhibit ON fraction >80%. Smaller ON exponent (1.19) magnitude in comparison to OFF exponent (1.45) signifies longer ON events are interrupted by smaller OFF events. ON event truncation time is ~ 1.5 times that of OFF event, signifying detrapping rate is much higher than the trapping rate. Interestingly detrapping rate/trapping rate (single particle level property), could be directly correlated to the PLQY (ensemble level property). An additional exponential term required to fit probability density distribution of ON event duration could be correlated with hole trapping, leading to extended ON times (> 60s). KEYWORDS: Small Sized CAS InP QD, High PLQY, Single particle spectroscopy, Suppressed blinking, Ambient air measurement, Long ON duration, Hole trapping.

Semiconductor

quantum dots (QDs) with size dependent tunable photoluminescence (PL) across a broad absorption range, together with exceptionally high photoluminescence quantum yield (PLQY) and superior photostability are quite suitable for wide range of applications like photovoltaics, optoelectronics, bioimaging etc.1-7 However, toxicity of most popular QDs, e.g. Cd-based II-VI QD or Pb-based IV-VI QDs poses serious concerns over the usage of these QDs. Very recently InP based III-V QDs are gaining noteworthy attention because of their much lower toxicity as compared to other popular QDs.8-12 Quite importantly, in comparison to Cd or Pb based QDs, InP based QDs possess larger Bohr excitonic radius that is suitable for better confinement of the exciton.13 Recently alloy-shelling have been reported to enhance the optical behaviour of CdSe based core/alloy-shell/shell QDs very significantly both at the ensemble and at the single particle level.14,15 The issue of low PLQY of InP core QDs 16,17 has been resolved by alloy shelling or multiple shelling with higher band gap material as in InP/ZnSeS18-21, InP/GaP/ZnS22, 23, InP/ZnSe/ZnS24-27 etc. Generally shelling of the core QD improves optical quality (enhanced PLQY and photostability) of QDs by providing better quantum confinement.24,28 Alloy-shelling achieves comparatively less lattice strain at the interface than conventional core-shell (CS) QDs.29 Lesser interfacial strain means reduced probability of formation of interfacial trap states.29,30 Most of the optical spectroscopic studies with InP based QDs have been performed at the ensemble level.10,18-23 There are

very few reports on the single particle spectroscopic results of the InP based QDs.31-35 Blinking dynamics has been reported for small sized core only InP QD.31 In this report31 percentage of ON fraction has been shown to be much less than that of OFF fraction and the exponent of ON time distribution is much higher than the exponent of OFF time distribution. Both these facts signify the poor optical property of the InP based core only QD which is justified by the low PLQY (30%). In order improve the optical quality of InP based QD, i.e. to suppress the blinking, two significant contributions have been reported very recently. 33,34In one report34 the experiments were performed in normal air atmosphere, whereas in the other report33 the investigation was performed in inert atmosphere. However, in both of these reports, researchers have worked with thick shell InP based QDs with diameter as high as 14.1 nm. Working in the special environment, e.g. reduced air atmosphere with the large and thick shelled InP based QD it has been shown that the percentage of ON-fraction increases up to 70 %.33 However, thick shell with large size (14.1 nm) and experiments performed in reduced air atmosphere condition critically hinder the usage of these thick shell InP based QDs. Hence it is quite necessary to achieve suppressed blinking in small sized InP based QDs (say, size less than 5 nm) and in experiments performed under normal air atmosphere. Toxic metal free, small size of QD and air atmosphere measurement are quite important from different applications point of view. To the best of our knowledge detailed blinking dynamical analyses for InP based CAS QD is non-existent in literature.

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Figure 1. TEM image (a), a variation of inter-planner spacing across the diameter (b), Ensemble absorption and PL emission spectra of green emitting InP based CAS QD. Inset shows the UV illuminated cuvettes with PLQY mentioned (c). Typical time-trace of single CAS QD particles with clear bimodal separation of ON and OFF states (d). Distribution of ON fraction (e) and OFF fraction (f) have been depicted.

In this current manuscript we report detailed analyses of reduced blinking in normal air atmosphere with small sized (diameter = 3.6 nm) toxic metal (e.g. Cd, Pb etc.) free InP based core/alloyshell/shell (CAS) QD. Synthesis and Characterization. Synthesis of InP based CS and CAS QDs have been depicted in the supp. info. (S1). These QDs have been characterized by TEM, PXRD and Energy-dispersive X-ray spectrometry (EDS) (Figure S1, S2 and S3). The presence of elements could be shown from EDS spectra. The fact that the alloy-shelling has happened could be shown unequivocally from the measurement of inter-planar spacing along the diameter of the single QDs.36From PXRD measurement of our InP based QDs lattice parameter has been calculated to be 5.61 Å for diffraction plane {111}. For pure ZnS, pure ZnSe and pure InP the values of lattice parameters are 5.41 Å, 5.67 Å and 5.86 Å respectively.28 Thus, experimentally observed magnitude of lattice parameter in our InP based QD lies in between pure InP, pure ZnSe and pure ZnS. Hence alloying among these has actually happened. However, more convincing proof could be given from the extremely sensitive inter-planar spacing values of single QDs. If core/shell heterostructure is formed then from centre of the QD till a certain distance the magnitude of inter-planar spacing would remain fixed for the core and then it will change abruptly when the shelling starts. However, if core/alloy shell is formed, then the value of inter-planar spacing would not vary drastically (as in case of Core/Shell QDs), instead, the magnitude of inter-planar spacing would change smoothly and gradually. Generally TEM images of InP based QDs are of poor quality. However, using Ni grid (instead of Cu grid) the quality of TEM images are slightly better. From the TEM image of our single InP based QDs (Fig. 1a), surface profile plot of variation of inter-planar spacing measured along the diameter of the QD has been shown (Fig. 1b). As can be seen from this figure the inter-planar spacing along the diameter of the QD does not change abruptly at any location from center along the diameter of the single QD. Instead, there is gradual and smooth change of inter-planar spacing from center along the

diameter of the single QD. The magnitude of inter-planar spacing is highest at the centre (3.36 Å) and then gradually decreases to 3.12 Å at the edge of the QD. It is known in literature that interplanar spacing of d111 plane for InP is 3.38 Å, for ZnSe is 3.27 Å, and that for ZnS is 3.08 Å.19, 37 The inter-planar spacing (obtained from extremely sensitive single QD level TEM measurement) near the core is 3.36 Å which is in-between 3.38 Å (for InP) and 3.27 Å (for ZnSe). The inter-planar spacing we have obtained near the edge is 3.12 Å which is in-between 3.27 Å (for ZnSe) and 3.08 Å (for ZnS). Thus, the core is mostly InP and alloying has happened from the core till the edge of the QD. If we use these inter-planar spacing in Bragg's equation then we get the similar diffraction angle values obtained from PXRD. Hence from PXRD measurement (ensemble level measurement) as well as from extremely sensitive inter-planar spacing measurement (employing the TEM images of single QD level measurement) we could conclusively prove that alloy shelling has happened nearly from the core till the edge in our InP based QD and distinct core/shell interface does not exist in our InP based QD. As can be seen (Figure S1) the size of all these QDs are quite small (diameter less than 5 nm). Ensemble optical behaviour. Steady state UV-Vis absorption and PL emission of green emitting InP/ZnSeS CAS QD have been depicted in Fig. 1c. PLQY for this sample have been obtained to be 65%. Absorption and emission spectra of yellow, orange, and red emitting CAS QDs have been shown in supp. info. (Figure S4). PL decay of the green emitting CAS QD have been depicted in Supp. Info.(Figure S5 & Table S1) As can be seen from this figure PL decay follows triexponential decay function with lifetimes 4-5 ns, 25-27 ns and 60-63 ns. These values are similar to what has been reported in literature for InP based QDs. All the PL decays have been recorded with low laser power so that multiexciton generation can be neglected.

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Figure 2. Probability density distribution of ON event durations (a) and OFF event durations (b) of green CAS QD. Probability density distribution of ON event durations (c) and OFF event durations (d) of green, yellow, orange and red emitting InP based CAS QDs. PLQY of these four QDs have been plotted against detrapping rate/ trapping rate for all four CAS QDs (e) (see discussion portion for details).

Single particle PL microscopy. Single particle spectroscopy of the CAS QDs have been performed through microscopy employing our home built Total Internal Reflection Fluorescence (TIRF) microscope. Suitably diluted QDs embedded in PMMA matrix have been used to perform single particle spectroscopy. (See Supp. Info. Figure S6 for well separated single particle image and video). A typical single particle PL time-trace have been depicted in Fig. 1d. Binning time has been kept at 100 ms. For comparison of signal from single particle with the background we have also shown the intensity profile from the latter. As can be seen from this Fig. 1d OFF intensities from single particle matches quite well with that from the background. A well separated bimodal distribution of PL intensities from single particle have been obtained (Fig. 1d) the signal intensities being much higher than OFF or background intensities (Fig. 1d). Thus, it could be confirmed that the signal is definitely from the single particle. In order to separate ON from the OFF intensities the threshold value has been kept at 4σ above the peak intensity of the lower intensity (OFF or background) distribution (where σ is the standard deviation of the lower intensity distribution). Above this threshold any intensity is considered to be ON and below that it is considered to be OFF.38-40 A clear blinking behaviour with distinct ON and OFF events could be noticed (Fig. 1d). ON fraction distribution from ca. 200 single particles have been depicted in Fig. 1e and OFF fraction distribution have been depicted in Fig. 1f. As can be seen from Fig. 1e, peak for ON fraction distribution has been obtained at 0.65 meaning about 50% of the particles having ON fraction at 65 % and more. ~ 26% of the particles have ON fraction above 0.80 which is a remarkable achievement for InP based QDs with diameter less than 5 nm. A recent report with thick-shelled InP based QD (with size about four times w.r.t. our QD) have mentioned a peak ON time fraction to be just above 70%.33 Distribution of ON and OFF fraction for yellow, orange and red emitting InP based CAS QDs have been depicted in Supp. Info. Figure S7. The peak of ON fraction distribution have been shown to increase with increase in PLQY (Supp. Info. Figure S7) of all four InP based CAS QDs.

Probability density distribution of ON and OFF event durations for green emitting InP based CAS QDs have been plotted in Fig. 2a and 2b. The probability density values range up to six decades and the event durations range up to three decades. For all these green, yellow, orange and red emitting CAS QDs the probability density distribution of ON event durations (Fig. 2c) and OFF event durations (Fig. 2d) could be fitted with a truncated inverse power law plus exponential equation (eq. 1): −𝑚𝑚

𝑃𝑃𝑂𝑂𝑂𝑂 = 𝑎𝑎. 𝑡𝑡𝑂𝑂𝑂𝑂 𝑂𝑂𝑂𝑂 . 𝑒𝑒 −𝑘𝑘𝑂𝑂𝑂𝑂.𝑡𝑡𝑂𝑂𝑂𝑂 + 𝑏𝑏. 𝑒𝑒 −𝑗𝑗𝑂𝑂𝑂𝑂𝑡𝑡𝑂𝑂𝑂𝑂

......... eq. 1.

whereas, the probability density distribution of OFF event durations could be fitted with a truncated inverse power law equation (eq. 2): −𝑚𝑚

𝑃𝑃𝑂𝑂𝑂𝑂𝑂𝑂 = 𝑎𝑎. 𝑡𝑡𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 . 𝑒𝑒 −𝑘𝑘𝑂𝑂𝑂𝑂𝑂𝑂.𝑡𝑡𝑂𝑂𝑂𝑂𝑂𝑂

………..eq. 2.

The values for exponent for ON event and OFF event durations for green emitting CAS QDs (See Supp. Info. Figure S8) are 1.19 and 1.45 respectively. For yellow, orange and red emitting CAS QDs the plot as well as the magnitude of exponents have been depicted in Supp. Info. (Figure S9, Table S2 & Table S3) For most of the literature reports small sized (60 s which is quite an extraordinary achievement and hence almost non-existent in literature. Reported duration of ON events in small sized InP based QDs are much less. Long ON events obtained in our small sized InP based CAS QDs and experiments performed under normal air atmosphere are comparable to four times thick shell InP QDs when the experiments have been performed under reduced air atmosphere. Interestingly, probability density distribution of ON events have been fitted with truncated power law equation with additional exponential term. In literature additional exponential component has been assigned to photoinduced hole trapping.15, 49Generally if the hole gets trapped followed by electron trapping, the subsequent excitations would result in only radiative recombination of electron and hole i.e. long ON state without blinking.15 Similar results have been obtained in CdSe based CAS QDs.15 The analyses yielded an average time of the exponential process or in other words average time for hole trapping to be ~300-500 ms. Evidence of hole trapping has been reported in literature for CdSe based QDs.15 For CdSe based CAS QDs, the hole trapping time and detrapping time have been obtained to be 2-8 s and 200-300 ms.15 Hole gets trapped for a significantly long

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time, perhaps because of greater localization probability of the holes in CdSe QD. 15 Generally hole detrapping time is less than the hole trapping time.15 Perhaps that is the reason why we did not observe any hole detrapping (obtained from OFF event duration) as the binning time in our case is 100 ms. Single particle spectroscopic measurement with much smaller binning time needs to be performed. Such experiments are currently underway. Memory effect. In order to understand the optical behaviour of a InP based single CAS QD comprehensively it is necessary to know whether there is any memory effect in blinking. Although there are a lot of reports regarding blinking behaviour of QDs however there are very few studies regarding memory effect of QDs.15, 48, 50-52 In case of InP based QD so far memory effect has been analyzed using scatter plot of subsequent ON or OFF, but no correlation has been found.31 Memory between successive events contains microscopic information regarding dynamical excited state processes.15, 50 In the present case with InP based CAS QDs, in order to visualize the correlation between two successive events, a two dimensional (2D) joint probability distribution of ON and OFF event has been constructed (Fig. 3). 2D joint probability distribution analysis for event durations gives the correlation between event durations with a specific lag.53 The xand y-axes contain event durations (or logarithm of event durations) and the z-axis contains the probability of the occurrence of every x–y pair which is colour coded here.53 Fig. 3a represents a 2D joint probability distribution of immediate ON events and Fig. 3b represents the same for ON events which are sufficiently separated (fifty events apart in the present case). Then, the latter (Fig. 3b) has been subtracted from the former (Fig. 3a) and the result has been represented in Fig. 3c. Quite interestingly, the extracted 2D difference histogram has been found to be aligned across the diagonal (Fig. 3 (c)), indicating the existence of residual memory in ON event durations. Similarly corresponding 2D joint probability distributions have been made for OFF events (Fig. 3d, 3e and 3f corresponding to Fig. 3a, 3b and 3c, respectively). As can be seen from Fig. 3f, there remains significant memory between OFF events of similar time duration. Thus, as can be seen from Fig. 3 significant memory effect has been observed for green emitting InP based CAS QD. For yellow, orange and red emitting InP based CAS QDs also similar memory effect have been observed (Supp. Info. Figure S11). Thus, memory effect seems to be unequivocally present in our all InP based CAS QDs. Therefore, unlike the literature reports of core only InP based QDs for which memory effect was not observed, in case of our InP based CAS QDs significant memory effects has been observed in both ON and OFF events of similar time duration. Similar observation has been reported for CdSe based CAS QDs.15 In order to quantify the memory effect we have also calculated the Pearson’s logarithm coefficient for adjacent event durations. In all samples ON-ON and OFF-OFF events are positively correlated but ON-OFF event are negatively correlated (anti-correlated) (Figure S12 & Table S4). ON-ON correlation has been observed to have higher magnitude than OFF-OFF correlation. Thus, existence of memory in both ON and OFF events in case of InP based CAS QDs could be shown convincingly.

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Figure 3. Visualization of the correlations between two successive ON- and OFF-event durations. Two-dimensional joint probability distributions for two adjacent ON-event durations (a) [for two adjacent OFF-event durations (d)]. Two-dimensional joint probability dimensional joint probability difference histogram for ON event duration (c) OFF event duration (f) for green CAS QDs.

InP based QDs - Core/shell (CS) vs. Core/Alloy-shell (CAS). In order to understand the difference in optical behaviour between InP based CS and CAS QDs we have synthesized InP based CS QDs whose size, PL emission maximum, PLQY and PL decay match quite well with that of InP based CAS QDs described above (Figure 4a, 4b and Table 1). Thus, the ensemble level optical behaviour of InP based CAS and CS QDs match quite well, hence, these properties enable us to compare the effect of alloy-shelling on the single particle optical behaviour of CAS QD in comparison to CS QD. Comparison of their single particle level optical behaviour have been depicted in Figure 4c-4f. Green emitting InP based CAS QD with 65% PLQY has an average ON fraction of 0.65 (Figure 4c), whereas, the green emitting InP based CS QD with 62% PLQY has an average ON fraction of only 0.22 (Figure 4d). Although excitation beam wavelength and intensity remained same, but comparatively much lower average ON fraction for CS QD in comparison to CAS QD (both with similar PLQY) could perhaps be because of (biexciton formation mediated (less probable in our experimental condition) or charge tunneling (from the single exciton states) mediated) much higher degree of ionization in case of CS QD in comparison to CAS QD. Detailed experiments need to be performed in order to reveal the underlying mechanism. Thus, although InP based CS and CAS QDs behave similarly at the ensemble level, however InP based CAS QD exhibits much superior optical behaviour at the single particle level in comparison to InP based CS QD. Probability density distribution of ON events of InP based CS and CAS QDs have been compared in Figure 4e and the same for OFF even durations have been depicted in Figure 4f. Probability density distribution of ON and OFF events of both CS and CAS QDs could be fitted with a truncated inverse power

law plus exponential and truncated inverse power law respectively.(See Supp. Info. Figure S13).However, the magnitude of exponents of ON and OFF events varies quite drastically for both CS and CAS QDs. Magnitude on mON and mOFF for CAS QD have been noted to be 1.19 and 1.45 respectively. For InP based CS QD the magnitude on mON and mOFF have been noted to be 1.49 and 1.23 respectively. Thus, the relative magnitude of mON in comparison to mOFF is quite low for CAS QD, whereas the revere trend has been observed for CS QD. As has been discussed earlier smaller relative values of mON in comparison to mOFF means longer ON events are interrupted by shorter OFF events. Whereas, larger relative values of mON in comparison to mOFF means shorter ON events are interrupted by longer OFF events. Thus, we can conclude that in case of CAS QDs longer ON events and shorter OFF events are more probable, whereas the reverse is noted for CS QD. Thus, although CS and CAS QDs behave in a similar manner at the ensemble level, however, CAS QD exhibits far superior optical behaviour in comparison to CS QD at the single particle level. Additionally, ON and OFF event truncation time for CAS QD has been noted to be 8.13 and 5.71 s. Thus, the ratio of detrapping rate/trapping rate is 1.42 for CAS QD. For CS QD the magnitudes of ON and OFF event truncation time have been noted to be 8.34 and 16.41 s. Thus, the ratio of detrapping rate/trapping rate is 0.50 for CS QD. As has been discussed earlier higher relative magnitude for ON event truncation time in comparison to OFF event truncation time dictates optical superiority of the QD. Thus, near three times higher magnitude of (detrapping rate/trapping rate) in case of green emitting InP based CAS QD unequivocally establishes its optical superiority in comparison to CS QD at the single particle level (See Supp. Info. Figure S14).

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Figure 4. Absorption, PL spectra of green emitting CAS and CS QDs (a), and PL decay of green emitting CAS and CS QDs 4(b), ON fraction of green CAS QDs (c), green CS QD (d), ON probability density distribution (e) and ON probability density distribution (f) of green emitting CAS and CS QDs.

Table 1: Comparison of optical properties of green emitting CAS QD and green emitting CS QD. Sample

Size (nm)

PL max (nm)

PLQY (%)

(ns)

mON

mOFF

ON tc (s)

OFF tc (s)

RD/T

InP/ZnSeS CAS QD

3.58 ± 0.03

555

65

50.82

1.19 ± 0.01

1.45 ± 0.04

8.13 ± 0.41

5.71 ± 0.28

1.42

InP/ZnS CS QD

3.63 ± 0.01

555

62

43.58

1.49 ± 0.02

1.23 ± 0.01

8.34 ± 0.42

16.41 ± 0.82

0.50

Blinking model in our InP based CAS QD exhibiting suppressed blinking. Non-identical magnitude of power law exponents (mON and mOFF) has been invoked in available literature regarding blinking dynamics, especially in the competing charge tunneling (CCT) model. 54 It has been mentioned in the CCT model that in addition to electron tunnelling, a hole tunnelling process also occurs independently of each other and in a competing manner. This leads to the deviation of values of mON and mOFF from 1.5. Individual rates of electron and hole tunnelling can fluctuate with exponentially distributed probabilities.54 In our InP based CAS QD systems the CCT model can explain the observation of nonidentical power law exponents for both the ON- and OFF-event durations (mON = 1.19 and mOFF = 1.45 for green emitting CAS QD). A smaller magnitude of mON in comparison to mOFF indicates that longer ON-event durations are interrupted by comparatively smaller OFF-event durations. In our InP based CAS QD systems the holes get trapped for a significantly long time. As mentioned earlier, electron trapping followed by long duration of hole trapping leads to the observation of long ON-

event durations. The overall model has been depicted schematically in Fig. 5. Observation of memory in PL blinking and the presence of superimposed exponential processes (truncation plus additional exponential term) within the truncated power law framework suggest that more than one type of trapping process (electron and hole trapping) is involved in our InP based CAS QD.55 Following the CCT model, we believe that, for our InP based CAS QDs, observation of (1) a long hole trapping time and (2) long ON-event durations followed by short OFF-event durations (mON magnitude less than mOFF) are indicative of a suppressed non-radiative Auger recombination process for our InP based CAS QD system. For InP based QDs it has been shown that N,N,N, N-tetramethyl-p-phenylenediamine acts as a hole In our case we have used accepter.56 Tris(dimethylamino)phosphine (P(NMe2)3 to synthesize the InP based QDs, so there is a finite possibility of hole trapping in our InP based CAS QD.

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Figure 5. A model showing the reasons behind normal ON and OFF durations along with long OFF and long ON durations in InP based CAS QDs.

Comparison of blinking behaviour of InP based CS and CAS QD systems. The origin of the suppressed non-radiative Auger recombination process in InP based CAS QD system can be correlated with the internal heterostructure of the CAS QD. The magnitude of lattice mismatch between InP and ZnSe is 3.4 % and the lattice mismatch between InP and ZnS is 7.7 %.28,57 In both of these type I QD systems the magnitude of confinement potential is much higher in ZnS than ZnSe which in turn is higher than InP. 13 Thus, from confinement potential point of view ZnS shelling should be better than ZnSe. However, lattice mismatch is higher in InP/ZnS than in InP/ZnSe. Hence attempts have been made to make thick shell InP/ZnSe based QDs to improve the optical behaviour.33, 34 Indeed they observed better optical behaviour in thick-shell CS QD, however, in the process the overall diameter of the thick-shell CS QD has become quite high >14 nm). 33, 34It has already been reported in the literature that alloy shelling of QD heterostructures have a much smoother and softer confinement potential in comparison to core/shell QDs.14 Moreover it has also been opined that lattice mismatch in CAS QDs systems would be intermediate in nature. 57 In our approach it is important to mention that unlike literature reports of metal alloy-shelling 58 in the present case we have employed non-metal alloy shelling between ZnSe and ZnS with the expectation that both the lattice mismatch and confinement potential barrier would be optimum in order to obtain improved optical behaviour keeping overall diameter quite small ( 60s has been observed for small sized InP based QDs and with measurements done under normal air atmosphere. Probability density distribution of ON event duration has yielded an additional exponential term which has been correlated with hole trapping which in turn results in extended ON times. Extensive verification of hole trapping in this direction


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needs to be made employing different kinds of hole trapping materials. Experiments in this direction are currently underway. Unlike literature reports with core only InP based QDs, significant memory effect has been observed in both ON and OFF event time distribution for CAS QDs. Observation of significant memory effect in PL blinking and the presence of superimposed exponential processes within the truncated power law framework suggest that more than one type of trapping process (electron and hole trapping) is involved in our InP based CAS QD. On comparing InP based CS and CAS QD of similar size, similar PL emission maximum, similar PLQY and similar PL decay it could be shown that CAS QD is optically far superior than CS QD at the single particle level. In a CAS QD system because of reduced lattice mismatch, smoothened confinement potential, the nonradiative Auger recombination rate has been reduced significantly in comparison to CS QD system. Hence, InP based CAS QDs are optically far superior than CS QDs. Unlike literature reports of suppressed blinking in case of thick shell (diameter ~14.1 nm) InP based CS QD in reduced air atmosphere (Nano. Lett. 2018, 18, 709- 716), in the current manuscript we report suppressed blinking with four times smaller (diameter ~3.6 nm) InP based CAS QD in ambient air atmosphere which is very crucial from different applications point of view. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthesis of CAS QDs, CS QD, TEM, EDAX and Size distribution, Experimental details for ensemble and single particle experiments. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS PKM thanks IISER-Kolkata for financial help and instrumental facilities. Support from the CSIR India, Project No. 01(2848)/16/EMR-II is gratefully acknowledged. CKD thanks DST-INSPIRE, DR thanks CSIR, and SM thanks IISER-Kolkata for respective Fellowship. REFERENCES 1. Alivisatos, A. P.; Gu, W. W.; Larabell, C., Quantum Dots as Cellular Probes. Annual Review of Biomedical Engineering 2005, 7, 55-76. 2. Klimov, V. I., Mechanisms for Photogeneration and Recombination of Multiexcitons in Semiconductor Nanocrystals: Implications for Lasing and Solar Energy Conversion. J. Phys. Chem. B 2006, 110, 16827-16845.

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Suppressed Blinking under Normal Air Atmosphere in Toxic-Metal

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