Spectral and Temporal Optical Behavior of Blue-, Green-, Orange

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Article

Spectral and Temporal Optical Behaviour of Blue, Green, Orange, and Red Emitting CdSe Based Core/Gradient Alloy Shell/Shell Quantum Dots: Ensemble and Single Particle Investigation Results. Debjit Roy, Tapan Routh, Aswin Vijai Asaithambi, Saptarshi Mandal, and Prasun K. Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10051 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Spectral and Temporal Optical Behaviour of Blue, Green, Orange, and Red Emitting CdSe Based Core/Gradient Alloy Shell/Shell Quantum Dots: Ensemble and Single Particle Investigation Results. Debjit Roy, Tapan Routh, Aswin Vijai Asaithambi, Saptarshi Mandal, and Prasun K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) - Kolkata Mohanpur, West-Bengal, 741246, India. Section: Physical Processes in Nanomaterials and nanostructures

Supporting Information

ABSTRACT: Highly luminescent (photoluminescence quantum yield (PLQY) as high as 96%) CdSe based core/gradient alloy shell/shell (CGASS) quantum dots (QD) have been synthesized in ‘one pot’ using the reactivity difference between Cd and Zn precursors & Se and S precursors. This procedure is highly reproducible and quite useful for large scale synthesis. Upon photoexcitation these QDs show a multiexponential excited state decay behavior. Interestingly, with the growth of the shell the overall PL decay gets faster. All the decay traces have been fitted well with a three exponential decay function. Fitted decay traces reveal three different time constants, faster one of 1-4 ns, moderate one of 13-16 ns and slower one > 25 ns. With the growth of the shell amplitude for moderate time constant increases and that of slow time constant decreases consistently. The variation of PLQY could be correlated with the variation of amplitude of moderate time constant. Slow and moderate time constants have been shown to be associated with two mutually interdependent excited state decay channels and the competition between these two decay channels dictate the PLQY of these CGASS QDs. The moderate time constant is associated with electron-hole recombination process and slow time constant is associated with delayed emission from band edge due to interaction with the manifold of shallow traps. The increase in magnitude of amplitude of moderate decay is reflected in higher PLQY. PL decay of blue, green, orange, and red emitting CGASS QDs follow similar trend. This kind of uniform nature of PL decay of different colour emitting QDs is quite rare in literature and the fact that it has been observed in CGASS QDs perhaps hints towards novelty of these systems. At the single particle level these CGASS QDs are shown to be quite photostable without showing any blueing or bleaching for one hour or even longer even under air atmosphere. Thus, these CGASS QDs exhibit much improved optical behaviour in comparison to CdSe/ZnS Core/Shell QDs. Quite interestingly all four differently emitting CGASS QDs optically behave in a similar way even at the single particle level. KEYWORDS: Quantum Dot, Photoluminescence dynamics, Spectral and temporal correlation, Single particle spectroscopy, Blueing and Bleaching.

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The Journal of Physical Chemistry Introduction:

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Semiconductor quantum dots (QDs) which can be synthesized with simpler chemical processability and possess interesting optical properties like size dependent photoluminescence (PL) tunability across visible spectrum together with high photoluminescence quantum yield (PLQY), high molar extinction coefficient and superior photostability at single particle level are of very high demand as a potential nanostructure for technical applications as light harvester and emitter in optoelectronic devices like light emitting diodes, lasers, optical storage devices, solar cells, fluorescent labels and chemical sensors as well as bioimaging.1-10 Among different QDs those based on CdSe are the most extensively studied systems.11-13 But from application point of view these bare CdSe QDs are not at all suitable because of their sensitivity towards photo-oxidation of the surface which affects the PL QY badly.14,15 Generally the surface passivation of CdSe QDs has been done by shelling with a higher band gap semiconductor material like CdS or ZnS producing a core/shell (CS) QD system.16 This kind of shelling makes the PL of the core less sensitive to any changes in surrounding environment or surface and photo-oxidation. In this kind of (type I) heterostructure as the band gap of the core lies energetically within the band gap of the shell material, the shell band gap acts as a potential barrier for the photogenerated exciton and thus the latter remains confined mainly within the core and interact less with surface or surrounding environment.17 Thus, CdSe/CdS or CdSe/ZnS QDs are observed to be more stable at the ensemble level with respect to chemical degradation or photo-oxidation in comparison to bare CdSe QDs. These CS QD systems show 5070% PL QY.18-23 But single shelling material could not fulfill the requirements to form an ideal defect (interface related) free system as the single shell is unable to passivate the surface of the luminescent core crystallographically as well as electronically. With the development of advanced synthesis techniques core/shell/shell QDs have also been synthesized where appearance of double inorganic layer actually improve the PL efficiency.24,25 Near unity PLQY was indeed observed for CdSe/CdS/ZnS and CdSe/CdS/ZnCdS/ZnS QDs.23 The multishell structure allows for ‘stepwise’ change in lattice parameters from CdSe core to the outermost protective ZnS shell as well as provides an appropriate band alignment for effective electronic passivation could be maintained (Scheme 1). 24-27 However, the major problem with these core-multishell (CSS) system is the involvement of multiple steps during synthesis which restricts the formation of bright and stable QDs in large scale as well as severely lacks in reproducibility. Recently a new synthetic method has been developed which produces a CSS heterostructure where a gradient alloy shell could be synthesized in one pot.28-30 The single step - single pot synthesis offers large scale synthesis of QDs, whose emission is tunable across entire visible range in a reproducible manner by just varying the initial ratio of precursors. In this type of core/gradient alloy shell/shell (CGASS) QD the chemical composition of the shells over the core changed gradually from core to shell in comparison to discrete change in chemical composition in case of CSS QDs (Scheme 1).28-30 Thus, a smooth and gradual change in potential barrier from inner core to outer shell surface could be achieved in these CGASS QDs. Because of that, enhanced confinement of the photogenerated charge carriers could be achieved. As a result of this gradual smooth change of lattice parameters of the core and shell materials, lattice mismatch (mismatch between core/shell and different shelling materials) related strain gets released in a better manner compared to conventional CSS QDs having discrete stepwise change of the lattice parameters at the interfaces (Scheme 1).28-30

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Although these CGASS QDs offer superior optical properties and kinetic experiments have been performed in order to understand formation and characterization of these QDs,28,30 however, very little is known about the exciton dynamics of these CGASS QDs. Intraband relaxation and trapping of the charge carriers affect the exciton dynamics and PL efficiency significantly. Understanding the excited state dynamics of relaxation of the exciton after photo generation and their dynamic interaction with the trap states (surface or interface related defects), surface passivating and protecting ligands as well as surrounding environment play very crucial roles for these CGASS QDs towards optoelectronic applications as light harvester, converter (in solar cells), generator (in lasers) and detector etc.1-4 Excitons are short lived species (ps-ns time regime), so to probe exciton relaxation and recombination process, picosecond time resolved photoluminescence based Time Correlated Single Photon Counting (TCSPC) technique needs to be used. TCSPC can directly probe the variation of emission dynamics occurring due to the change of population of the transient species in different excited states after photo excitation. The heterostructured shell in CGASS QDs developed an alloyed interfacial structure (Scheme 1). So the analysis of the exciton dynamics of these newly developed CGASS QDs systems will help us understand the behaviour of the exciton in these heterogeneous systems. This understanding is of prime fundamental importance towards application of these CGASS QDs both in science and technology, especially in QD based photovoltaic applications. In this direction four different CGASS QDs emitting in four different wavelengths (blue, green, orange, red) have been synthesized (see Fig. 1). Here in this article steady state and picosecond time resolved photoluminescence dynamics of these four CGASS QDs have been reported and analyzed in detail. After the rigorous analysis it became possible to understand and interestingly correlate the excited state photoluminescence dynamics with steady state properties like PL QY. Although core/shell QDs have been shown to possess better photoluminescence properties in terms of PLQY and photostability at the ensemble level, however, from single particle experiment it has been shown that several core/shell QDs exhibit gradual decrease of emission maximum (commonly known as blueing)31-33 and finally bleaching of these QDs within minutes. This restricts the usage of so called better core/shell QDs for time dependent single particle measurements like single particle tracking. Thus, the quest for making QDs that will not show blueing or bleaching within an hour or even longer at the single particle level is very much active. In this article it has been shown that CGASS QDs are extremely photostable at the single particle level and these QDs do not show any blueing or bleaching for one hour or longer.

Scheme 1. Schematic representation of band alignment for CSS (a) and CGASS (b) respectively. Upper and lower edge of the rectangle represents the bottom of the conduction band and the top of the valence band and the band gap energy is indicated by rectangle.


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Experimental Section: Chemicals. Cadmium oxide (99.99%), selenium powder (99.5%), sulfur (99.98%, powder), zinc stearate (technical grade), trioctylphosphine (TOP, 90%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), were procured from Sigma Aldrich. Air and/or moisture sensitive chemicals were handled in a glove box and reactions were carried out with standard Schlenk line apparatus under nitrogen atmosphere. Synthesis. CGASS QDs with chemical composition gradient were synthesized in a single pot following conventional ‘Hot Injection’ approach.28,30 The emission wavelength of the QDs were tuned by varying the initial precursor ratios. Aliquots were withdrawn from reaction mixture at different intervals of time to characterize the shell growth. In this one pot synthesis after very rapid initial nucleation of CdSe core, shells of wider band gap materials namely CdS, ZnSe and ZnS were grown over the core epitaxially in accordance of their reactivity. Details of the synthesis of individual CGASS QDs have been mentioned below.

performed using time-correlated single-photon-counting module (Horiba Jobin Yvon IBH). Two picoseconds pulsed lasers (λex=377 nm, fwhm < 100 ps; λex=402 nm, fwhm < 100 ps) and a Nano LED (λex=340 nm, fwhm < 1 ns) with a repetition rate 1MHz were used as an excitation sources and an MCP photomultiplier tube (PMT) (Hamamatsu R3809U-50 series) was used as the detector. A nonlinear least squares iterative reconvolution procedure using IBHDAS6 (version 2.2) was employed to fit the decay curves using a suitable exponential decay equation. The quality of the fit was assessed from the χ2 values and the distribution of the residuals. Single Particle Measurements: Single particle time resolved intensity trace as well as spectrum were recorded in a home-build Total Internal Reflection Fluorescence (TIRF) and confocal microscopic setup. Details can be found in supp. info. Results and Discussion: Photophysical Properties of the QDs

Synthesis of Blue/Green QDs.

Synthesis of Orange/Red QDs. For the synthesis of orange QDs initial ratio of the metal precursors Cd to Zn was set to be 1:4 and Se to S ratio was 1:10. But here two different chalcogenide precursor solutions Se-TOP and S-TOP were prepared and added separately to the ‘hot’ metaloleate solution. S-TOP solution was added immediately after the addition of Se-TOP. For the synthesis of the red QDs the precursor ratios remain the same as orange QDs but S-TOP solution was added 30s after the addition of the Se-TOP solution. Ensemble Spectroscopic Measurements Steady State Measurements. Steady state absorption spectra were recorded in CARY Bio 300 UV-Visible Spectrophotometer. Corrected PL emission and excitation spectra were recorded with Fluoromax-3, Horiba Jobin Yvon spectrofluorimeter. PLQY for all the samples were calculated using the following equation,

Q = QR

ODR I n R2 OD I R n 2

Where Q, OD, I and n stands for quantum yield, optical density, integrated intensity, refractive index of solvents respectively. Subscript R refers to the reference. Time Resolved Measurements. Pico second time resolved PL decay measurements were

Steady state absorption, emission, excitation and picoseconds time resolved PL decay for CGASS QDs etc. experiments have been carried out. Not only different coloured QDs but also shell growth during the formation of QD of a particular colour have been studied by time resolved measurements. The correlation between variation of PL QY with the shell growth and corresponding PL decay dynamics have been probed into. Blue-Abs Green-Abs Orange-Abs Red-Abs Blue-Em Green-Em Orange-Em Red-Em

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For the synthesis of blue QDs 0.4 mmol of CdO was mixed with 8 mmol of zinc stearate (Cd to Zn ratio was 1:20) in 10 mL of ODE along with 33.6 mmol of OA. The mixture was degassed under vacuum at 140oC for 20 min, then filled with inert ultrapure N2 gas and then finally heated to 300oC to form a clear solution. To this ‘hot’ solution 0.4 mmol Se powder and 8 mmol S powder (Se to S ratio was 1:20) dissolved in 4mL TOP was added instantaneously. For the growth of the shell the temperature was set to be 280oC and aliquots were taken out at different time intervals to monitor the growth of the alloyed shell. The aliquots were dispersed in methanol to freeze the reaction and then purified by successive solvent extraction with hexane. The QDs were then dispersed in toluene for spectroscopic measurements. For green QDs similar method was followed except for the fact Cd to Zn as well as Se to S ratio was maintained to be 1:10.

Absorbance (a.u)

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0 360 400 440 480 520 560 600 640 680 720 Wavelength (nm)

Figure 1. Normalized absorption and emission spectra for different colour emitting CGASS QDs. UV-Vis absorption, PL emission and PL excitation spectra of orange colour emitting CGASS QDs have been shown in Fig 2. These are the different aliquots withdrawn at different times while synthesizing orange emitting QDs (for blue, green and red QDs see supp. info.). With increasing reaction time overall shape of the PL emission spectrum remains identical, although, at longer reaction time (30 minutes or higher) the PL emission spectra gets broadened, probably due to ‘Ostwald ripening’. Consistency in the shape and FWHM of the PL-emission spectra ensure the narrow size distribution of the as prepared QDs. Structural features were more prominent in PL excitation spectrum than absorption spectrum. Apart from minimum energy transition at least three other higher energy transitions could be clearly resolved and with the growth of the shell all these features got red shifted. Red shifting of absorption/excitation maxima is


The Journal of Physical Chemistry the result of larger extension of the electronic wave function of the core leaking out onto the shell material.25 This feature indicates perturbation of the core energy levels by shell energy levels. Photostability of these highly luminescent CGASS QDs (using the maximum PLQY sample in each set) have been investigated. These QDs are highly photostable and retains 80% (or higher) of their initial PL intensity even after 24 hours of continuous UV exposure. These CGASS QDs are as much photostable as conventional CdSe/CdS core/shell QDs (see supp. info.). 590 585 λ max (nm)

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580 575 570 565 560 0 10 20 30 40 50 60 Time (sec)

60min 30min 10min 5min 3min 1min 45s 15s

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Figure 2. Evolution of absorption, emission and PL-excitation spectra(dashed line) with consecutive growth of the gradiently alloyed concentric shells during the formation of orange QDs. Inset shows the change in PL emission maximum with time (for others colours see supp. info.). Examination of the PL decay dynamics would be very much useful in elucidating the nature of the exciton state and characterizing the competitive radiative and non-radiative recombination processes. Time resolved PL decay behaviour of these QDs would provide insight into the dynamics of the electron-hole recombination process with the shell growth. Shells were grown to diminish the effect of surface and interface related traps in reducing PL efficiency. Investigation of PL decay behaviour with successive growth of the gradient alloy shell would describe the effect of shelling over the dynamic interaction between exciton states and trap states. For all the CGASS QDs time resolved PL decay traces have been observed to be multiexponential in nature and could be successfully fitted with a three exponential function, eq 1, 3

I (t ) = I (0) * ∑ Ai e

−

t

τi

............eq.(1)

i =1

where I(0) and I(t) are the PL intensities at time 0 and t respectively. τi is the excited state life time of each component of PL decay, and Ai is the relative amplitude of that very component.

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In a multiexponential decay trace each component of decay, in general, should be associated with a decay channel. In such a case, variation of Ai could provide insight about the influence of each decay channel and hence the correlation between different decay channels which constitute the overall decay. In a complex system where multiple channels are associated to the excited state decay, it is not the overall nature of the decay trace rather individual time constants along with their relative amplitudes would provide better detailed picture of the overall decay dynamics. The effect of shelling for a particular (colour emitting) CGASS QD has been compared. Decay profile of orange CGASS QDs have been depicted in Fig 3. In general with the growth of the shells the overall decay gets faster (See Fig 3A.). Three different time constants have been observed, one faster component of 1-4 ns, a moderate component of 13-16 ns and a slowest component > 25 ns. The PL QY for differently shelled CGASS QDs have been calculated and it’s variation with time (with shell growth) is represented in Fig 3B. Interestingly the amplitude of moderate component increases and that of the slowest component decreases consistently with the growth of the shell (Fig. 3C). The amplitude of the fastest component does not change much. The consistent variation of amplitudes of moderate and slow time components (see Table 1) clearly indicates that PL decay dynamics is actually controlled by these two mutually interdependent decay channels. It is observed that the trend of variation of PL QY follows the same pattern as that of the variation of the amplitude of the moderate component of the PL decay. This observation necessarily points to the fact that the factor which is responsible for the enhancement of the emissive property of the QDs (PLQY), definitely influence the decay channel associated with the moderate time constant in the PL decay trace. Blue, green and red emitting CGASS QDs also behave in a similar fashion (see supp. info.). There are several previous literature reports regarding PL decay of bare CdSe QD as well as CdSe/CdS, CdSe/ZnS or Cdse/CdS/ZnS type of CS/CSS systems.18,27,34-40 In literature reports correlation between high PL QY and longer PL decay time has been a general trend. Our result is actually in contrary to the literature reports made by several other groups. In these literature reports (biexponential or multiexponential decay traces), the faster decay component was attributed to band edge electron-hole recombination and the slower decay as delayed emission from shallow traps as a result of localization of electron and hole near the band edge. Researchers have also mentioned the influence of photoinduced charged exciton decay which is generally very fast.34 In the present investigation correlation between two distinct time constants could be observed; where amplitude of moderate time constant increases and that of the slowest time constant decreases consistently with shell growth (see Fig 3C and Table 1). For a QD system, trapping of the photogenerated charge carriers remain in competition with the electron-hole recombination process which generates PL. Trapping of either of the charge carriers (electron or hole) actually converted the highly delocalized exciton state into a localized trap state present either at surface or interface. So any interaction of the exciton state with the traps will delay the electron-hole recombination process and the extent of delay actually dependent on the nature of the traps. The reason of delay was the poor overlap between charge carrier after trapping as one of the charge carriers remain in delocalized state and the other in the localized state.39


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Figure 3. PL decay trace for purified aliquots withdrawn at different at different time intervals during synthesis of orange QDs (A). A multiexponential decay behaviour was observed. All the decays were recorded following 377 nm excitation and monitored at respective emission maximum of the samples. (B) Variation of PL QY for different aliquots. (C) Variation of relative amplitudes of moderate (B2) and slower (B3) time component obtained from the triexponential fit of the decay trace. Table 1.Variation of excited state time constants and PL QY for aliquots withdrawn at different times during the formation of orange QDs. (λ λex = 377 nm). Time

PLQY

15s 45s 1min 3min 5min 10min 30min 60min

0.15 0.30 0.55 0.65 0.58 0.96 0.49 0.61

λ

mon

(nm) 562 570 576 587 589 591 590 584

τ

B

(ns) 1.42 1.52 2.11 2.41 2.05 3.79 1.55 2.40

4.50 4.69 3.67 3.32 4.76 2.46 4.77 2.97

1

τ

1

2

(ns) 12.98 13.50 15.60 15.80 13.81 16.03 12.88 15.49

An efficient shelling actually reduces the probability of interaction between the photogenerated charge carriers and the manifold of trap states. On this basis we could assign two excited state time constants responsible for two distinct competitive processes. Moderate time constant could be assigned to band edge electron-hole recombination as shelling efficiently increases it’s probability of occurrence, as evident from the amplitude variation of moderate component in the PL decay trace. The slower one could be assigned to be arising from band edge state due to its interaction with a manifold of shallow traps arising due to surface or interface related defects and it’s amplitude also decreases systematically with shelling (Fig. 3 & Table 1). These kind of assignment of decay processes are also supported by the trend of variation of PL QY with shelling as it exactly matches variation pattern of amplitude of moderate component (see Fig 3). This observation further confirms the fact that efficient shelling is actually responsible for enhanced charge carrier recombination probability and thus results into higher PL QY. Thus, it clearly indicates that a slower decay is not correlated with the larger PLQY, rather increase in the magnitude of amplitude of moderate decay (electron-hole recombination) is reflected in higher PLQY. We could obtain enhanced shelling in case of orange CGASS QD which is reflected in an enhanced PL QY (~ 96%).

B

2

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τ

ave

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χ

(ns) (ns) 45.61 37.53 49.89 31.55 1.11 50.73 37.98 44.58 30.84 1.12 58.41 39.72 38.53 30.63 1.03 72.36 36.17 24.32 24.57 1.07 68.93 27.51 26.31 19.63 1.13 81.91 30.93 15.63 19.96 1.08 62.70 30.44 32.53 22.47 1.13 71.32 40.23 25.71 27.37 1.08 However, the origin of the very fast time component could not be assigned with certainty. In literature reports different views in relation with the observed fast component could be found. Some research groups have referred this as a charged exciton recombination whereas some research groups have described this as a decay channel involving interaction with the dangling bonds present at the surface.27,34,41,42 As the magnitude of amplitude is quite small and no consistent variation of this amplitude, with consecutive shell growth could be obtained, at this point we could say that at least this decay channel is not significantly affected by the shell growth. Thus we would like to refrain from commenting on the origin of the very fast decaying component as more study is required to assign the origin of this decay channel. Femtosecond transient absorption study is currently underway for CGASS QDs to understand the origin of this ultrafast decay channel. Similar observations have been observed in other three (blue, green, red emitting) CGASS QDs (see supp. info.). It was quite interesting to note that PL decay of blue, green, orange, and red emitting CGASS QDs behave in a similar manner (Fig. 4 and Table 2) and this fact supports the reproducibility of the synthesis procedure as well as similar chemical nature of these type of CGASS QDs. This kind of uniform nature of PL decay of different colour emitting QDs is quite rare in literature and the fact that it has been observed in CGASS QDs perhaps hints towards novelty of these systems and this will help us to model nature of different colour emitting QDs in a uniform manner (see later).


The Journal of Physical Chemistry λem=510 Q.Y=0.47

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Figure 4. PL decay trace of blue, green, orange, and red emitting CGASS QD (with best PLQY in each set). λex = 377 nm. Table 2. Excited state time constants and PL QY for different coloured CGASS QDs at 377 nm excitation. λ

em

(nm) 510 532 591 619

PLQY

τ

B

0.47 0.59 0.96 0.71

(ns) 2.72 2.70 3.79 3.89

6.52 4.52 2.46 8.21

1

τ

1

2

(ns) 15.27 16.04 16.03 16.28

B

2

65.05 66.51 81.91 79.92

τ

3

(ns) 41.64 45.18 30.93 36.43

B

3

28.43 28.98 15.63 11.88

τ

2

ave

χ

(ns) 29.39 31.95 19.96 20.99

1.06 1.06 1.08 1.19

Table 3.Variation of excited state time constants for orange CGASS QDs (for the maximum PLQY sample) with excitation and monitoring wavelength λex (nm) 402

377

340

λmon (nm) 571 591 611 571 591 611 571 591 611

τ1 (ns) 3.85 3.79 5.41 3.14 3.79 4.68 4.08 5.27 6.58

B1 2.83 2.26 5.36 2.60 2.46 4.53 2.83 3.76 6.81

τ2 (ns) 16.31 16.08 17.04 16.08 16.03 16.66 16.31 16.81 17.38

The effect of excitation wavelength on the PL decay dynamics of these CGASS QDs has also been examined. With the variation of excitation wavelength no change in PL decay dynamics could be observed, particularly the electron-hole recombination time constant remains almost unchanged (Table 3). PL decay monitored at different energy across the PL spectrum did not show any observable change in the PL decay behavior (Table 3). This observation indicates that excited state dynamics of these CGASS QDs remains unaffected by the variation of excitation wavelength as well as the monitoring wavelength, which in turn suggests the existence of only one emitting state in the excited state. Detailed investigation & analyses and the interesting observation of uniform nature of PL decay dynamics of blue, green, orange, and red CGASS QDs could help us to model the PL dynamics of these CGASS QDs with a three level system (see Scheme 2). Moderate and slow lifetime components arises due to involvement of dynamic equilibrium between first exciton state (2) and shallow trap state (3) respectively. After initial excitation from ground state in which electron remains in the valence band (designated as (1) in Scheme 2) higher exciton states are created

B2 87.33 84.04 83.32 86.20 81.91 82.66 86.51 87.18 82.90

τ3 (ns) 38.57 33.14 42.12 35.95 30.93 39.66 37.18 38.25 44.18

B3 9.54 13.60 11.31 11.20 15.63 12.81 10.65 9.06 10.28

τave (ns) 20.78 20.27 23.07 20.47 19.96 22.66 20.78 20.74 23.41

χ2 1.11 1.06 1.09 1.08 1.08 1.10 1.11 1.06 1.09

because electron and holes are formed at different energy levels of the conduction and valence band respectively. The higher energy exciton states relax back to ground exciton state nonradiatively in an ultrafast manner. The electron-hole recombination can only occur when the electron is in bottom of the conduction band and hole is in the top of the valence band. But this recombination could be delayed due to perturbation induced by the shallow traps present near both the band edges which can trap either of the charge carriers, as the trap state is energetically similar to interact with the first exciton state. Interaction between these two states is feasible and because of this interaction actually a distribution of population is created. Any factor that changes the relative population in trap states compared to first exciton state would change the population distribution. With the growth of the gradiently alloyed shell the number of traps got reduced so that the relative population of the charge carriers in the first exciton state increased and this was indeed reflected in increased amplitude of moderate component in the time resolved PL decay measurements. With the increase of band edge recombination and decrease of interaction with shallow traps, a concomitant enhancement of PLQY was noted.


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Scheme 2. Schematic represents processes involved in CGASS QDs photoexcitation dynamics. The population of the first excitonic state (2) was perturbed by the interaction with the shallow trap states (3) and electron-hole recombination was influenced.

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Figure 5. Intensity time trace of CGASS QD (orange emitting) at the single particle level (A); A typical PL emission spectrum (B); Time invariant PL emission maximum (C). These plots for other three CGASS QDs have been depicted in supp. info.


The Journal of Physical Chemistry Optical behaviour of CGASS QDs at the single particle level:

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We have also explored photoluminescence of these four differently emitting CGASS QDs at the single particle level. Although at the ensemble level several QDs have been reported to be a good optical emitter (for example PLQY, photostability and constant PL emission maximum) but most of them lacks in good optical behaviour in three respects at the single particle level. These are (i) blinking,43-47 (ii) blueing31-33 of emission maximum and finally (iii) bleaching of the QD. These three not so good properties restrict the usage of QDs at the single particle level. We were especially concerned about these last two properties. As these CGASS QDs have been shown to be optically superior in comparison to other core/shell QDs, we wanted to probe these CGASS QDs at the single particle level. If the QDs at the single particle level exhibit gradual shifting of the emission maximum towards lower wavelength with progress of time, it is known as 'blueing'. If the QD bleaches within short time (say a few minutes) then the signal magnitude in the single particle time trace would be similar to background level and no telegraphic signal would be observed. This kind of blueing and bleaching has been reported for core-shell CdSe based QDs.31 A typical time trace of orange emitting CGASS QDs at the single particle level has been depicted in Fig. 5A. As can be seen from this figure these CGASS QDs although possess near unity PLQY but it shows blinking. What can also be noticed from Fig. 5A is that these CGASS QDs possess a very long on times (sometimes as long as a few minutes). Several literature reports are there in which it was shown that CdSe based core/shell QDs exhibit constant blueing of emission maximum and finally bleaching of the QD.31-33 Thus, although CdSe/ZnS core/shell QDs are optically quite stable at the ensemble level, at the single particle level they exhibit blueing and ultimately bleaching within a few minutes. 31-33 Thus, although seems to be optically stable at the ensemble level, core/shell QDs are not optically stable at the single particle level. Quite interestingly from Fig. 5A it is evidenced that CGASS QDs are stable at least up to one hour or even more in air atmosphere. A typical single particle emission spectrum has been shown in Fig. 5B. These CGASS QDs at the single particle level quite interestingly does not show blueing of the emission maximum (Fig. 5C). Spectral diffusion is however noticed (Fig. 5C). Thus, although blinking could not be stopped, quite interestingly, a distinct blueing followed by bleaching of the QDs (reported in several literature reports) could be prevented. This could perhaps be because of the fact that the strain present at the core shell interface (because of steep potential barrier arising from drastic change of lattice parameters) of the core/shell QDs could be released in CGASS QDs because of alloyed interfacial structure (which in turn is because of a gradual smooth change of lattice parameters from Cd rich to Zn rich region).28-30 In case of core/shell QDs surface defects leading to leaching of oxygen to core and thereby oxidizing the core and hence reducing the size of the core was shown to be responsible for the blueing and finally bleaching of the QDs within a few minutes in air atmosphere.31 The same core/shell QDs shown not to exhibit blueing or bleaching under nitrogen atmosphere.31 However, in CGASS QDs as this kind of surface strain is released, thus, leaching of oxygen through surface to core is prevented. Hence oxidation of core is prevented and thus size reduction of core did not happen. Thus, we did not observe any blueing of PL emission maximum or

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bleaching of single QDs for one hour or even longer even under air atmosphere. Thus it is quite evident that these CGASS QDs exhibit much improved optical behaviour in comparison to CdSe/ZnS core/shell QDs at the single particle level and under normal atmospheric condition. Thus, these CGASS QDs which showed quite improved optical properties at the ensemble level exhibit quite inspiring results at the single particle level also. Interesting part is that all differently emitting (Blue to Green to Orange to Red) CGASS QDs exhibit similar behaviour even at the single particle level. Investigation of blinking dynamics of all these differently emitting QDs are currently underway and will be communicated soon separately. Conclusion: In conclusion, we have successfully synthesized four differently emitting (blue-green-orange-red) core/gradient alloy shell/shell QDs which show superior optical properties viz. PL QY. All four differently emitting could be shown to exhibit similar spectral as well temporal optical behaviour. Effect of gradiently alloyed shelling on PL QY and decay behaviour could be successfully explained. The excited state decay dynamics could be explained with a simple three level model. It is observed that first exciton state always maintained an equilibrium distribution with shallow traps and PL emission efficiency is dictated by this dynamic equilibrium. All the decay traces have been fitted well with a three exponential decay function. Fitted decay traces reveal three different time constants, faster one of 1-4 ns, moderate one of 13-16 ns and slower one > 25 ns. With the growth of the shell amplitude for moderate time constant increases and that of slow time constant decreases consistently. The variation of PLQY could be correlated with the variation of amplitude of moderate time constant. Slow and moderate time constants have been shown to be associated with two mutually interdependent excited state decay channels and the competition between these two decay channels dictate the PLQY of these CGASS QDs. The moderate time constant is associated with electron-hole recombination process and slow time constant is associated with delayed emission from band edge due to interaction with the manifold of shallow traps. The increase in magnitude of amplitude of moderate decay is reflected in higher PLQY. The gradient alloy nature of the shell imposes enhanced confinement of the photogenerated charge carriers and reduces interfacial strain in a smooth manner, thus effectively passivates the traps/defects and hence reduces interactions with the traps and thus reduces the probability of excited state deactivation by alternative pathways. In contrast to the literature reports the PLQY could be correlated to the amplitude of the moderate lifetime component. PL decay of blue, green, orange, and red emitting CGASS QDs follow similar trend. This kind of uniform nature of PL decay of different colour emitting QDs is quite rare in literature and the fact that it has been observed in CGASS QDs perhaps hints towards novelty of these systems. At the single particle level these CGASS QDs are shown to be quite photostable without showing any blueing or bleaching for one hour or even longer even under air atmosphere. Thus, these CGASS QDs exhibit much improved optical behaviour in comparison to CdSe/ZnS core/shell QDs. Quite interestingly all four differently emitting CGASS QDs behave in a similar way even at the single particle level.

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ASSOCIATED CONTENT *S Supporting Information Characterization of synthesized QDs, Steady state absorption, PL emission, time resolved PL decay, Time constants of PL decay, single particle measurement details, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT PKM thanks IISER-Kolkata for financial help and instrumental facilities. Support from the Fast-Track Project (SR/FT/CS-52/2011) of DST-India is gratefully acknowledged. DR thanks CSIR, and TR, AVA, SM thank IISER-Kolkata for respective Fellowship. PKM gratefully thanks Professor Dipankar Chattopadhyay, University of Calcutta for help in TEM measurement.

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Spectral and Temporal Optical Behavior of Blue-, Green-, Orange

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