Highly Photoluminescent InP Based Core Alloy Shell QDs from Air

Article Cite This: J. Phys. Chem. C 2018, 122, 964−973

pubs.acs.org/JPCC

Highly Photoluminescent InP Based Core Alloy Shell QDs from AirStable Precursors: Excitation Wavelength Dependent Photoluminescence Quantum Yield, Photoluminescence Decay Dynamics, and Single Particle Blinking Dynamics Chayan K. De,† Tapan Routh,† Debjit Roy,† Saptarshi Mandal,† and Prasun K. Mandal*,†,‡ †

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West Bengal, 741246, India ‡ Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West Bengal, 741246, India S Supporting Information *

ABSTRACT: InP based quantum dots (QDs) are coming in a big way as an alternative to toxic Cd, or Pb based QDs. Unlike many literature reports in this work, green-yelloworange-red emitting highly photoluminescent (PLQY as high as 65%) and photostable InP/ZnSeS core/alloy shell quantum dots (CAS QDs) have been synthesized using a less toxic, airstable aminophosphine precursor (P(DMA)3). Unlike literature predictions in this paper, we show that green-yelloworange-red emitting InP based alloyed QDs can be prepared with InCl3 only. We report here the hitherto unobserved and quite interesting excitation wavelength dependent PLQY for all of these green-yellow-orange-red emitting InP based CAS QDs. PLQY increases monotonically with increasing excitation wavelength. Significant deviation of the PL excitation spectrum from the absorption spectrum has been observed in the shorter wavelength region. This observation is perhaps because the surface mediated nonradiative pathways predominate over radiative charge carrier recombination when excited at shorter wavelength. PL decay for these QDs generally follows a triexponential decay equation with the shortest lifetime of 3−10 ns, the moderate one with a lifetime of 24−30 ns, and the longest one with a lifetime > 60 ns. Moderate and long lifetimes have been shown to be associated with two mutually interdependent excited-state decay channels, and the competition between these two decay channels dictates the PLQY of these CAS QDs. The moderate lifetime has been shown to be associated with an electron−hole recombination process, and the long lifetime is associated with delayed emission from the band edge due to interaction with the manifold of shallow traps. Quite interestingly, amplitude of the moderate lifetime (dynamical property) has been observed to be correlated with the PLQY (spectral property). PL decay for all of these InP based CAS QDs has been observed to be excitation wavelength independent. However, PL decay gets slower with increasing monitoring wavelength. Thus, the presence of shallow trap states is evidenced. Single particle blinking dynamics of InP based CAS QDs has been investigated for the first time. We could achieve the lowest reported magnitude of the mON exponent for InP based QDs and the value is 1.19, which speaks about the much longer Ontimes or, in other words, superiority of our InP based CAS QD system in comparison to other reported InP based QDs, for example, InP core only, or InP/ZnS, InP/ZnSe/ZnS, InP/GaP/ZnS core/shell or core/shell/shell QD systems.

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INTRODUCTION Semiconductor quantum dots (QDs) especially II−VI QDs based on CdSe, owing to their unique optical properties like broad spectral absorption, high molar extinction coefficient, tunable photoluminescence (PL) emission with narrow bandwidth, high PL quantum yield (PLQY), and photochemical stability, have been considered as future material for nanoscience and nanotechnology.1−6 However, toxicity has been a major problem for group II−VI QDs as these QDs are © 2017 American Chemical Society

made up of the highly toxic elements like Cd, Hg, etc. The toxicity problem could be circumvented by the use of III−V and I−III−VI group QDs.7−13 Among all possible choices, InP based QDs could potentially be an alternative because of their low toxicity.7−11 However, bare core InP QDs are almost nonemissive (PLQY < 1%), highly sensitive toward photoReceived: November 16, 2017 Published: December 11, 2017 964

DOI: 10.1021/acs.jpcc.7b11327 J. Phys. Chem. C 2018, 122, 964−973

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The Journal of Physical Chemistry C oxidation due to lack of surface passivation.14−18 At the single particle level, it has been shown that, unlike CdSe core or CdSe/ZnS core/shell QDs which are prone to oxidation and hence get bleached within a minute,19−22 CdSe based alloyed QDs are not prone to photooxidation and thus are alive for almost an hour or longer.23 Applying similar reasoning, it is expected that the InP/ZnSeS type of core/alloy-shell (CAS) system would be better for superior optical properties than InP core and InP based core/shell systems.24−27 Generally, the InP core is synthesized using tris(trimethylsilyl)phosphine (P(TMS)3) as phosphorus precursor.14−17 However, P(TMS)3 is extremely toxic, exceptionally less stable in air, and highly expensive, which actually constrain large scale synthesis of InP based QDs.14−17,28 In our case, InP/ZnSeS CAS QDs have been synthesized using tris(dimethylamino)phosphine (P(N(CH3)2)3) as phosphorus precursor. (P(N(CH3)2)3) is much less toxic, stable in air, and even much cheaper compared to P(TMS)3.29−32 Since the precursor was less toxic, air-stable, and not so expensive, hence a large quantity of highly luminescent and highly stable green, yellow, orange, and red emitting InP based CAS QDs have been synthesized. For synthesis, we have modified the procedure described in the literature.30 It has been mentioned that, in order to synthesize green emitting InP based alloyed QD, InI3 has to be used30 and, in order to make yellow-orange emitting InP based alloyed QD, InBr3 has to be used.30 However, in this paper, we will show that green-yellow-orange-red emitting InP based alloyed QDs can be prepared with InCl3 only.

nonmetal has been confirmed by EDS measurement (Supporting Information). Optical behavior of these CAS QDs has been described below.

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RESULTS AND DISCUSSION Optical Studies of InP/ZnSeS CAS QDs. Photostability of these four QDs has been probed and also compared with bare CdSe and CdSe/CdS core/shell QDs (see the Supporting Information). After 24 h continuous UV (365 nm) irradiation, it has been observed that the orange and red InP/ZnSeS QDs are quite photostable, retaining ∼90% (or higher) PL intensity. However, photostability decreases for smaller-sized, shorter wavelength emitting QDs (green and yellow). For smallersized QDs, perhaps due to its high surface-to-volume ratio, surface cracks are easily formed.33 These cracks perhaps enable oxygen to reach the core and caused photo-oxidation.19 Because of photo-oxidation, PL intensity decreases. The green and yellow QD, with a smaller-sized core compared to the red one, has greater defect density at the core−shell interface and thus is less photostable compared to the red one. The steady state UV−vis absorption, PL emission and excitation spectra of very dilute solutions of greenish, yellow, orange, and red emitting InP based CAS QDs are shown in Figure 1 (for details of experimental condition, see the

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METHODS Synthesis and Structural Characterization of InP/ ZnSeS CAS QDs. In brief, these CAS QDs have been synthesized in one pot via a hot injection method (see Scheme 1). We have used oleylamine as ligand. However, stearate Scheme 1. Schematic Showing the Steps of InP/ZnSeS CAS QD Synthesis

anion (coming from Zn precursor (zinc stearate)) and dodecanethiol (which acts as an S precursor), and trioctylphosphine (used to dissolve S) could also be present at the surface of the as-synthesized QD and may act as ligands as well. We have used octadecene as the non-coordinating solvent. Details of the synthesis have been depicted in the Supporting Information. The size of these InP based QDs has been calculated from TEM (Supporting Information). The green emitting InP based CAS QD has the average diameter of 3.6 ± 0.03 nm. For yellow, orange, and red emitting InP based CAS QDs, the average diameter has been calculated to be 4 ± 0.03 nm, 4.6 ± 0.02 nm, and 4.9 ± 0.03 nm, respectively. Evidence of the alloy formation has been obtained from PXRD measurements which clearly show that an alloy structure has been achieved (Supporting Information). The presence of both metal and

Figure 1. UV illuminated QDs with PLQY value (A) and absorption, PL emission (λex = 400 nm), PL excitation spectra (B) of green (a), yellow (b), orange (c), and red (d) InP/ZnSeS CAS QDs, respectively.

Supporting Information). PL emission spectra of four different InP/ZnSeS QDs have also been collected as a function of excitation wavelengths (see the Supporting Information). The PL emission maximum of these InP/ZnSeS CAS QDs was observed to be excitation wavelength independent. The excitation independent PL emission maximum could be anticipated from the monodispersity of our InP/ZnSeS sample. FWHMs of these spectra are 60−70 nm, indicating wide distribution of excited-state energy levels. Similar values of 965


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Figure 2. Excitation wavelength dependent PL efficiency/OD and PLQY of green (a), yellow (b), orange (c), and red (d) InP/ZnSeS CAS QDs, respectively.

obtained. Interestingly, PLQY has been observed to increase monotonically with increasing excitation wavelength for all four different QDs (Figure 2). Excitation wavelength dependent PLQY has been reported recently for the CdSe based QD;35−38 however, this is the first report of excitation dependent PLQY for InP based CAS QD. Please recall that the same observation was expected from the observed deviation of the PL excitation spectrum from the absorption spectrum.39 We have also calculated the PL efficiency over the excitation regime by simply dividing the normalized average PLE spectrum by the normalized absorption spectrum to track this excitation wavelength dependent PLQYs behavior (Figure 2). The PL efficiency profile matches quite well with the PLQY throughout the entire spectral region investigated. A similar report has been made for the CdSe based QD;35,40 however, this is the first report for InP based CAS QDs. QD loses its PL when the exciton follows nonradiative surface mediated decay pathways and does not come back to the band edge for PL. Our results indicate that, with increasing excitation/exciton energy, the surface mediated nonradiative pathways predominate over relaxation of the charge carrier to the emitting lowest energy states. Similar observation has been observed for CdSe based QDs.35−42 The high energy charge carrier perhaps undergoes nonradiative barrierless tunneling to the trap states of the surface passivating ligands of the QDs,35,41,42 and thus, contribute to the decrease of the PLQYs observed for excitations at shorter wavelength range. A sudden decrease in PLQY is observed at about 375 nm (or below) excitation. The 375 nm (3.31 eV) excitation is very high in terms of energy. For all the dots we have studied, this excitation creates an exciton ∼1 eV higher than the band gap. Therefore, we could safely assume that, at such a high energy, excitation continuum of high energy states sets in. Moreover, the shell material ZnS has the bulk band gap of 3.54 eV. Therefore, with 375 nm or shorter wavelength (or higher energy) excitations, we are creating excitons having energies similar or more than the shell band gap. As a result, excitations with 375 nm or shorter wavelength create excitons which are more prone toward surface trapping, and hence, we are getting

FWHM for InP based QDs have been reported in the literature.18,29−31,34 PL excitation (PLE) spectra of these QDs have been recorded as a function of monitoring wavelength (see the Supporting Information). Quite interestingly, PLE spectrum shows a strong deviation from the absorption spectrum, which is more prominent at the shorter wavelength range (Figure 1). We initially thought that ligands might absorb at shorter wavelength but will not emit and that is the reason behind the observed deviation of the PLE spectrum from the absorption spectrum. In order to make sure that the observed change is coming from the QD itself and not from the solvents or ligands, we recorded the absorption spectrum of the solvents alone and that of the ligands (SA, DDT, OLA, TOP), and both the solvents and the ligands showed very small absorbance in this region (see the Supporting Information). Hence, the contribution of ligand absorption toward the absorption spectrum of QD can be nullified. Thus, we could conclude that the observed deviation of the PL excitation spectrum from the absorption spectrum is characteristic of the QD. This observation, however, points to the fact that the QD will emit less because of enhanced nonradiative decay when excited at the shorter wavelength range in comparison to the longer wavelength range. In order to have a quantitative idea about how much less the emission would be when excited at the blue end in comparison to when excited at the longer wavelength end, we have plotted PL efficiency against the excitation wavelength for all green, yellow, orange, and red emitting InP based CAS QDs which is depicted in Figure 2. Please note that the OD at each excitation wavelength was kept quite low (∼0.05 or less). In order to have even better quantitative estimation, PLQYs have been calculated for varying excitation wavelength over the entire absorption spectrum. Maximum PLQY for each sample was observed at excitation just above the band edge and the corresponding maximum PLQY values for green, yellow, orange, and red QD have been measured to be ∼65, 45, 38, and 28%, respectively. It is quite encouraging to note that the PLQY of these InP based QDs as high as 65% could be 966


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represented by the respective relative amplitude. The intensity average PL decay lifetime (⟨τ⟩) has also been calculated for every PL decay using the following equation

a sudden decrease of PLQY. Although, not exactly the same but a similar result has been obtained for CdSe based QDs. Another interesting observation is the small reduction of PLQYs with the excitation at wavelengths a little higher than the absorption band edge. Under this excitation condition, the electron will be excited directly to the interface manifold trap states as this much energy is not sufficient to raise the electron to the conduction band. It is also possible that, at this condition, there is a higher propensity to interact and to pass to shallow trap states. This situation will lead to nonradiative decay and hence lower PL efficiency or lower PLQY. In order to have a significant understanding about the mechanism/pathway of excited-state PL decay, we have thoroughly investigated excited-state PL decay dynamics of all four InP/ZnSeS CAS QDs. A series of time-resolved PL decay profiles have been collected as a function of both excitation and PL monitoring wavelengths. Every PL decay

⟨τ ⟩ =

where Bs are the corresponding amplitudes of each lifetime τ. We strongly believe an insight about the origin of individual decay channel would provide better understanding of the PL decay dynamics.23 Each PL decay curve (see Figure 3) has been fitted with a triexponential decay function with the shortest lifetime of 3− 10 ns, the moderate one with a lifetime of 24−30 ns, and the longest one with a lifetime ∼ 60 ns and higher (see Table1). The relative amplitude of the fastest component (B1) and time constant do not change much. However, a consistent variation has been observed between the relative amplitude of the moderate (B2) and slowest (B3) component of the PL decay. Detailed investigation of PL decay dynamics has yielded an interesting correlation between PLQY and the amplitude of different decay amplitudes. The variation of the magnitude of B2, B3 (obtained from the triexponential fit of the PL decay) and PLQY for InP/ZnSeS QDs (with different PL emission maxima) for different excitation is plotted in Figure 4. It is quite evident that the PLQY and B2 component of the PL decay are highly correlated with each other for different excitation wavelengths as well as different QDs. This observation indicates that the decay channel associated with the B2 component is related to the radiative nature of electron−hole recombination. Thus, the hitherto unobserved correlation between steady state optical property (PLQY) and dynamical optical property (B2) could be established for InP based CAS QDs. Similar observation has been noted in the case of a CdSe based alloyed QD system.23 In order to obtain deeper insight into PL decay dynamics and hence the excited-state processes, PL decay has been recorded as a function of both excitation and monitoring wavelength for each of the four samples. PL decay for green emitting InP/ZnSeS CAS QDs, monitored at the respective PL maximum, has been noted to be excitation wavelength independent (Figure 5a). The average lifetime for PL decays with different excitation, but the same monitoring wavelength remains constant (Table 2). This observation signifies that similar types of PL decay channels are involved in this InP/ ZnSeS CAS QDs for different excitation wavelengths but the same monitoring wavelength (PL emission maximum). PL decay for the same green emitting InP/ZnSeS CAS QD has also been recorded for the same excitation wavelength but different monitoring wavelengths (Figure 5b). Interestingly, PL decay behavior for these InP based CAS QDs has been noted to be dependent on monitoring wavelengths. It was observed that the overall decay becomes slower as the monitoring PL wavelength increases. This is reflected in the increase of the magnitude of average lifetime as depicted in Table 3. In the

Figure 3. PL decay trace of green (a), yellow (b), orange (c), and red (d) InP/ZnSeS QDs. (λex = 408 nm for each case). PL has been monitored at the respective PL emission maximum. (for λex = 375 nm and λex = 340 nm, see the Supporting Information).

profile (Figure 3) could be fitted with a sum of three exponential decay functions mentioned below: 3

B1τ12 + B2 τ22 + B3τ32 B1τ1 + B2 τ2 + B3τ3

t

I(t ) = I(0) ∑ Bi e− τ i=1

where I(t) and I(0) represent the PL intensity of the sample at time t and initial time or time zero, respectively. τi represents the time of an individual PL decay component, and Bi is the relative amplitude of the respective component. Each and every individual time component of a multiexponential decay is associated with a separate PL decay channel, and the contribution of that very channel would be

Table 1. PLQYs and Excited-State Time Constants for Four CAS QDs with Different PL Maxima (Excitation at 408 nm) λem (nm)

PL QY

τ1 (ns)

B1

τ2 (ns)

B2

τ3 (ns)

B3

⟨τ⟩ (ns)

χ2

555 575 600 620

0.43 0.35 0.30 0.20

5.94 5.19 5.51 4.59

3.24 2.77 2.86 2.31

27.68 26.16 25.39 25.11

53.94 52.47 49.36 43.44

63.79 61.45 59.87 59.28

42.82 44.76 47.75 54.25

50.82 49.55 49.19 50.51

1.00 1.02 1.08 1.01

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Figure 4. Variation of PLQY, magnitude of B2 and B3 of triexponential PL decay for four CAS QDs with different PL maxima for different excitation wavelengths, viz., 340 nm (a), 375 nm (b), 408 nm (c).

Figure 5. PL decay trace for green QDs with different excitation wavelengths monitored at a fixed wavelength (a) and different monitoring wavelengths with a fixed excitation wavelength (b). Inset of (b) shows the variation of relative amplitudes of moderate (B2) and slower (B3) time constant of the triexponential decay fit.

Table 2. Excited-State Time Constants of Green CAS QDs with Varying Excitation Wavelengths λex (nm)

λem (nm)

τ1 (ns)

B1

τ2 (ns)

B2

τ3 (ns)

B3

⟨τ⟩ (ns)

χ2

340 375 408

555

8.64 5.06 5.94

1.77 2.15 3.24

26.36 26.80 27.68

46.55 51.82 53.94

59.64 62.40 63.79

51.68 46.03 42.82

50.02 50.68 50.82

1.00 1.04 1.00

literature, such long PL lifetime for QDs has been attributed to emission from trap states.43,44 As the monitoring wavelength increases, specially near the longer wavelength edge, the magnitude of τ3 increases, whereas the magnitudes of τ1 and τ2 almost remain constant. Additionally, the B3 (amplitude corresponding to τ3) magnitude has been observed to increase with a concomitant decrease in B2 magnitude (see inset of Figure 5b). We have observed a decrease in the magnitude of B1 as the monitoring wavelength increases. However, the overall contribution is less than 6%. In the literature, different views in relation with the observed fastest component have been reported for CdSe based QDs.43−48 Different views such as charged exciton recombination, decay channel involving interaction with the dangling bonds present at the surface, intraband relaxations, interaction with ligand mediated surface

states, etc., have been accounted for the observed fastest component. In our case, all the above-mentioned possibilities may happen. In order to identify the exact reason for the above-mentioned behavior, femtosecond pump−probe transient absorption measurements need to be done. Hence, we refrain from commenting on the origin of the fastest component. As the PL decay is monitored at the PL maximum magnitude of B2 is the highest. As the PL decay is monitored at the longer wavelength edge of PL emission, the magnitude of B3 increases. Thus, the decay channel associated with B2 could be referred to as radiative recombination of electron− hole from the excitonic state, whereas the decay channel associated with B 3 could be referred to as radiative recombination of electron−hole from the trap states. Similar

Table 3. Variation of Excited-State Time Constants for Green CAS QDs as a Function of Monitoring Wavelength λex (nm)

λem (nm)

τ1 (ns)

B1

τ2 (ns)

B2

τ3 (ns)

B3

⟨τ⟩ (ns)

χ2

408

505 530 555 580 605 630

3.17 4.12 5.94 5.50 4.60 4.30

5.92 2.95 3.24 1.94 1.48 1.40

24.33 26.70 27.68 26.79 26.00 25.90

48.63 52.16 53.94 45.23 32.82 19.25

62.60 64.13 63.79 63.99 69.76 88.29

45.55 44.88 42.82 52.83 65.70 79.36

51.15 51.79 50.82 54.06 62.82 84.08

1.08 1.06 1.00 1.04 1.01 1.07

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Figure 6. Single particle time trace of the green QD (a) with clear separation of ON and OFF intensity level (b). ON-time (c) and OFF-time (d) fractions of nearly 200 single particles have been shown. Probability density distribution plot of ON-times (e) and OFF-times (f) with the necessary fit have been shown. See text for details.

statistics, we have recorded data from about more than 150 single dots (see the Supporting Information for details of single particle measurements). The ON-time fraction has been observed to be about 65% (Figure 6c), and the OFF-time fraction has been noted to be about 35% (Figure 6d). ON− OFF blinking dynamics has been performed, and the results are shown in Figure 6e,f. As can be seen from Figure 6e, the ON-time distribution exhibits deviation from linearity in a log−log plot (see the Supporting Information for detailed analysis models). ON- and OFF-time distribution for InP core52 and InP/ZnS55 QDs could be fitted with a simple power law equation. In the cases of InP/ZnSe/ZnS56 and InP/GaP/ ZnS56 QDs, the ON-time distribution could be fitted with a truncated power law, whereas the OFF-time distribution could be fitted with a simpler power law. In our case with the InP based CAS QD system, ON-time distribution could be fitted with a truncated exponential with the additional exponential equation reported previously57 and the OFF-time distribution could be fitted with a truncated power law equation. Thus, we can conclude that the blinking dynamics in our InP based CAS QD system is distinctly different from other InP based QDs, for example, InP core,52 InP/CdS,53,54 InP/ZnS,55 InP/ZnSe/ ZnS,56 and InP/GaP/ZnS56 based core/shell systems. An important parameter in blinking dynamics is the exponent of power-law distribution (known as mON and mOFF for ON and OFF, respectively). The higher the value of the mON exponent, the lesser is the longer ON-time probability. In other words, these QDs will be at ON-state for smaller duration. Thus, it is more desirable that the

observation has been noted for the other three (yellow, orange, red) InP/ZnSeS CAS QDs (see the Supporting Information). Thus, the influence of trap states in excited-state decay dynamics and hence the PLQY could be confirmed in these CAS QDs. Generally, in the literature, there are not many reports regarding monitoring wavelength dependent decay for QDs. To the best of our knowledge, monitoring wavelength dependent PL decay for QDs have earlier been reported for CdSe/ZnS core/shell QDs.43,44 We have observed a similar behavior, i.e., with increasing monitoring wavelength PL decay becomes slower. Following significant understanding of spectral and temporal optical behavior of InP based CAS QD at the ensemble level, we wanted to probe these InP based CAS QDs at the single particle level. Single particle investigations on CdSe based QDs are quite large in number;21,49,51 however, those on InP based QDs are quite rare in the literature.52−56 Results based on different types of InP based QDs, for example, InP core,52 InP/ CdS,53,54 InP/ZnS,55 InP/ZnSe/ZnS,56 and InP/GaP/ZnS56 based core/shell systems at the single particle level have been reported. However, single particle behavior of InP based CAS QD has not been reported. As the green emitting InP based CAS QD exhibits the best optical property, hence this particular QD has been used as a typical system to understand the blinking dynamics of InP based CAS QDs (see the Supporting Information). As can be seen from Figure 6a,b, there is distinct and significant difference between the ON- and OFF-state intensities. This means ON- and OFF-state intensities are distinctly resolved. In order to have sufficient 969


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The Journal of Physical Chemistry C magnitude of the mON exponent be less. Similarly, the higher the value of the mOFF exponent, the lesser is the longer Offtime probability. In other words, these QDs will be at Off-state for a smaller duration. Thus, it is more desirable that the magnitude of the mOFF exponent be high. Although we do not want to discuss the blinking dynamics of our InP based CAS QDs here in detail (will be communicated latter), however, we want to make a few critical comments. In the blinking dynamics analysis, we have put a threshold ≥ 3 times standard deviation above the average OFF intensities distribution. We have obtained mON and mOFF values 1.19 and 1.45, respectively. An mON value of 1.19 is the lowest so far for InP based QDs. We have obtained an mOFF value of 1.45 which is greater than the value of mON. Such a small value of mON and comparatively larger mOFF signifies longer ON durations are more probable than longer OFF durations. This is in accordance with ∼65% ON-fraction in comparison to ∼35% OFF-fraction. It has been reported that, if the magnitudes of mON and mOFF are much less than 1.6, then no threshold dependence on blinking dynamics will be observed.58 In our case, the magnitude of both ON and OFF exponents are much less than 1.6. We have obtained a truncation time of about 8.1 s for ON-event durations and about 5.7 s for OFF-event durations. Thus, the ON-event truncation time is higher than that of the OFF event durations. Moreover, both truncation times are greater than 50 times than that of the binning time (100 ms) and much less than 1% of the total experimental time (about 900 s for each time trace). It has been reported in the literature that meaningful extraction of the data is possible only when truncation time is greater than 10 times the binning time and less than 10% of the total experimental time.50,58,59 This implies that the magnitudes of mON and mOFF are quite reliable. For the InP core,52 the magnitude of the mON exponent is ∼2 and the mOFF exponent magnitude is ∼1.5. For InP/ZnS55 QDs, the magnitude of the mON exponent is in between 1.3 and 1.7 and the mOFF exponent magnitude is in between 1.4 and 1.9. For InP/ZnSe/ZnS56 QDs, the magnitude of the mON exponent is 2.09 and the mOFF exponent magnitude is 1.76. For InP/GaP/ZnS,56 QDs the magnitude of the mON exponent is 1.36 and the mOFF exponent magnitude is 1.57. To the best of our knowledge, there is no published paper in which the magnitude of the mON exponent is less than 1.2 for InP system. In our InP based CAS system, the magnitude of the mON exponent is 1.19 and the magnitude of the mOFF exponent is 1.45. Thus, we could conclude here that alloying has improved the quality of InP based QD systems. Similar observation has been noted in the case of CdSe based QDs.23 In the individual time trace of our InP based CAS QD, we have observed dim/gray intensities in the blinking time trace. It has been mentioned in the literature that these states are due to a strongly, but not fully, quenched exciton state.60 It has been suggested that the sequence of hole and electron trapping could perhaps control the blinking behavior significantly.60 In our analysis of ON-event durations, the additional exponential term perhaps indicates the contribution from hole trapping. This kind of assignment is in accordance with literature reports.57,60 In order to verify our proposition, detailed blinking dynamics analyses on several InP based CAS QDs need to be performed. Experiments in this direction are over and the analyses are currently underway. Results of these analyses will be communicated soon. In the next three paragraphs, we would like to compare our results with what has been reported in the literature so far

related to InP based QDs. In most of the literature reports, P(TMS)3 has been used as phosphorus precursor.14−17,28 However, P(TMS)3 has its own problems to work with, as it is highly reactive under normal air atmosphere and hence requires an inert atmosphere to work with and P(TMS)3 is about 30 times more expensive than another phosphorus analogue P(DMA)3 that we have used.29−32 Moreover, using P(TMS)3, there are very few reports in which PLQY is above 60%.9,10,27 There are reports in which InP based alloyed QDs have been made, however, with PLQY only about 45%. There are a couple of reports in which InP based alloyed QDs have been made using P(TMS)3 and emitting from green to yellow to orange to red region; however, FWHMs of PL emission for those red emitting QDs are above 90 nm, which is quite broad.26,27 In this paper, InP based alloyed QDs have been made using air-stable, inexpensive P(DMA)3 instead of airsensitive, toxic, and expensive P(TMS)3, and the FWHM of our QDs is always less than 70 nm. To the best of our knowledge, there are three reports in which a non-P(TMS)3 phosphorus precursor (P(DMA)3 and P(DEA)3) has been used in order to prepare InP based QDs.29,30,32 None of these papers report an alloyed InP based QD as we have done. In one of these three papers, only a core has been synthesized.32 In the other report, an InP based core/shell QD has been prepared; however, the PLQY is only ∼50%.29 It has been opined that, in order to synthesize a green emitting InP based alloyed QD, InI3 has to be used and, for the synthesis of a yellow-orange emitting InP based alloyed QD, InBr3 has to be used.30 However, in this paper, we show that green-yellow-orange-red emitting InP based alloyed QDs can be prepared using InCl3 and we could achieve quite high PLQY (65%) for green emitting QDs using InCl3. In this paper, in addition to the above-mentioned points, we have performed a detailed PL decay dynamical analysis in order to understand the excited-state behavior in a rigorous manner. No such analysis has been reported in the literature for InP based QDs, specially InP based CAS QDs. We have also shown that these InP based alloyed QDs exhibit excitation wavelength dependent PLQY, and no such report is there in the literature for InP based QDs. Through detailed investigation and analyses, we could attribute processes responsible for multiexponential decay channels. From the single particle investigations point of view, we could achieve the lowest reported magnitude of the mON exponent and the value is 1.19, which speaks about the much longer ON-times or, in other words, the optical superiority of our InP based CAS QD system in comparison to the InP core, or InP/ZnS, InP/ZnSe/ZnS, InP/GaP/ZnS core/shell or core/shell/shell QD systems.

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CONCLUSIONS In conclusion, green-yellow-orange-red emitting highly photoluminescent (PLQY as high as 65%) and photostable InP/ ZnSeS core/alloy shell quantum dots (CAS QDs) have been synthesized using a less toxic, air-stable aminophosphine precursor (P(DMA)3). It has been mentioned that, in order to synthesize a green emitting InP based alloyed QD, InI3 has to be used and, in order to make a yellow-orange emitting InP based alloyed QD, InBr3 has to be used. However, in this paper, we show that green-yellow-orange emitting InP based alloyed QDs can be prepared with InCl3 only. We report here the hitherto unobserved and quite interesting excitation wavelength dependent PLQY for all of these green-yellow970



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orange-red emitting InP based CAS QDs. Significant deviation of the PL excitation spectrum from the absorption spectrum has been observed in the short wavelength region. It could be shown that perhaps the surface mediated nonradiative pathways predominate over radiative charge carrier recombination at the shorter excitation wavelength range. PL decay curves of these green-yellow-orange-red emitting InP based CAS QDs follow a triexponential decay equation with the shortest lifetime of 3−10 ns, the moderate one with a lifetime of 24−30 ns, and the longest one with a lifetime > 60 ns. Quite interestingly, the amplitude of the moderate lifetime (dynamical property) has been observed to be correlated with the PLQY (spectral property). Moderate and long lifetimes have been shown to be associated with two mutually interdependent excited-state decay channels, and the competition between these two decay channels dictates the PLQY of these CAS QDs. The moderate lifetime has been shown to be associated with an electron−hole recombination process, and the long lifetime component has been shown to be associated with delayed emission from the band edge due to interaction with the manifold of shallow traps. PL decay for all of these InP based CAS QDs has been observed to be excitation wavelength independent. However, PL decay gets slower with increasing monitoring wavelength. Thus, the presence of shallow trap states is evidenced. Single particle blinking dynamics of InP based CAS QDs has been investigated. From the single particle investigations point of view, we could achieve the lowest reported magnitude of the mON exponent and the value is 1.19. Such a low value of mON speaks about the much longer ON-times or, in other words, superiority of our InP based CAS QD system in comparison to other reported InP based QDs, for example, InP core only, or InP/ZnS, InP/ ZnSe/ZnS, InP/GaP/ZnS core/shell or core/shell/shell QD systems.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11327.

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Article

Synthesis of CAS QDs, experimental details, synthesis procedure for different CAS QDs, characterization of synthesized CAS QDs, photostability, PL emission, time-resolved PL decay, time constant of PL decay, etc. (PDF) Movie (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Prasun K. Mandal: 0000-0002-5543-5090 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.K.M. thanks IISER-Kolkata for financial help and instrumental facilities. Support from the CSIR India, Project No. 01(2848)/16/EMR-II, is gratefully acknowledged. C.K.D. thanks INSPIRE, and D.R. thanks CSIR, and T.R. and S.M. thank IISER Kolkata for the respective Fellowship. 971


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Highly Photoluminescent InP Based Core Alloy Shell QDs from Air

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