ISSN 0018-1439, High Energy Chemistry, 2016, Vol. 50, No. 5, pp. 395–399. © Pleiades Publishing, Ltd., 2016. Original Russian Text © S.B. Brichkin, M.G. Spirin, S.A. Tovstun, V.Yu. Gak, E.G. Mart’yanova, V.F. Razumov, 2016, published in Khimiya Vysokikh Energii, 2016, Vol. 50, No. 5, pp. 417–422.
PHOTONICS
Colloidal Quantum Dots InP@ZnS: Inhomogeneous Broadening and Distribution of Luminescence Lifetimes S. B. Brichkin, M. G. Spirin, S. A. Tovstun, V. Yu. Gak, E. G. Mart’yanova, and V. F. Razumov Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia e-mail:
[email protected] Received January 12, 2016
Abstract— Indium phosphide colloidal quantum dots with a zinc sulfide shell, an average core diameter of 3 nm, a luminescence peak position of 600 nm, and a luminescence quantum yield up to 50% have been synthesized. By analyzing the stationary absorption and luminescence spectra in terms of the Kennard–Stepanov relationship, the values of homogeneous width and inhomogeneous broadening have been obtained, which determine the resulting width of the spectra: the corresponding full widths at half maximum (FWHM) were 31, 63, and 70 nm. From the value of inhomogeneous broadening and the sizing curve of indium phosphide, polydispersity of the synthesized particles has been estimated as 11%. Analysis of the luminescence decay kinetics has revealed three reproducible peaks with maxima near 4.35, 35 (main) and 200 ns in the lifetime distribution. It has been found that although repeated washing of the synthesized particles with methanol can decrease the quantum yield, the lifetime distribution observed remains constant, which in the context of the “blinking” effect indicates a very short luminescence decay time of the particles in the OFF-state. Keywords: colloidal quantum dots, absorption and luminescence spectra, luminescence quantum yield, luminescence lifetimes, blinking, indium phosphide DOI: 10.1134/S0018143916050064
Intense study of the spectral and luminescent properties of colloidal quantum dots (CQDs) has begun with the advent of the method of high-temperature colloidal synthesis [1], which permits obtaining semiconductor CQDs with a specified average size, high luminescence quantum yield, and low polydispersity, in some cases not exceeding 5% [2]. This is believed to be achieved by the possibility of separating in time the processes of nucleation and growth of nanoparticles in the course of synthesis. This method was first successfully applied to obtain CQDs of semiconductors of the AIIBVI type, and then was extended to other types of semiconductors (AIIIBV and AIVBVI). However, while maintaining the general idea of synthesis, the transfer of the method to other semiconductors often requires an individual approach to the selection of conditions of synthesis, of ligands and precursors. This situation occurs, in particular, for InP CQDs: the application to their synthesis of the conditions well proven to CdSe CQDs has not led to good results. Nucleation and growth of crystals in this system occur very slowly. Therefore, several days are required to obtain nanoparticles of desired size [3]. In addition, the high strength of the covalent bond in InP complicates the obtaining of monodisperse CQDs because of the difficulties associated with the separation of the stages of nucleation and growth of nanoparticles, which leads to
a broader size distribution of nanoparticles and, hence, broader luminescence bands. Usually a width of the luminescence band of InP CQDs at half maximum (FWHM) is greater than 50 nm [4, 5]. During the time elapsed since the first attempts of synthesis of InP CQDs, new precursors and ligands have been found, the conditions of the synthesis have been optimized, which at present permit obtaining InP CQDs with a good size distribution for a reasonable time, with control over the size of the obtained nanoparticles in a wide range [6]. However, optimization of the methods of obtaining InP CQDs with a high quantum yield is still an urgent task. One of the most effective methods of increasing the luminescence efficiency of InP CQDs is the growth of shells of semiconductors with a wider gap. This requires careful selection of regimes of synthesis and concentrations of reagents used. In this work, based on analysis of the latest most advanced research in the field of synthesis of InP CQDs, we present a relatively simple and effective method by which samples of indium phosphide CQDs with a zinc sulfide shell have been synthesized with an average core diameter of 3 nm, a luminescence quantum yield up to 50%, and a narrow size distribution. In addition, their spectral and luminescent characteristics have been analyzed in detail.
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EXPERIMENTAL To obtain InP@ZnS CQDs, the following reagents were used: indium(III) chloride (99.995%, Acros), zinc chloride (anhydrous, 98%, Sigma–Aldrich), oleylamine (80–90%, Acros), tris(dimethylamino)phosphine (TDMAP, 97%, Aldrich), 1-dodecanethiol (DDT, 98%, Aldrich), zinc stearate (Zn(St)2, 10–12% Zn basis, Sigma–Aldrich), chloroform (99.5%, Sigma–Aldrich, stabilized by amylenes, 100– 200 ppm), and methanol (reagent grade, Khimmed). The synthesis of InP cores for InP@ZnS CQDs was carried out according to a published procedure [4]. For this purpose, a mixture of InCl3 (0.9 mmol), ZnCl2 (0.9 mmol), and oleylamine (15.2 mmol) was placed in a reaction vessel, degassed at 110°C for 40 min, after which the temperature of the mixture was increased to 220°C in argon atmosphere and TDMAP (0.25 mL, 1.4 mmol) was added. InP cores were synthesized at this temperature. The average size of InP cores was determined by the duration of the synthesis prior to the moment of addition of the sulfur precursor (DDT) needed for the growth of the ZnS shell. To obtain the particles with an average core diameter of 3.0 nm, the thiol was added 10 min after the introduction of TDMAP. The size of InP cores was estimated from the spectral data, using the empirical formula reported in [7]. To the growth of the ZnS shell on CQDs, DDT (10.6 mmol) was introduced into the reaction vessel, the temperature was reduced to 200°C, and the mixture was kept for 60 min. Then the temperature of the mixture was rapidly decreased to room temperature by feeding cooling water into the jacket of the reaction vessel. The absorption spectra of the solutions studied were measured on a Shimadzu UV-3101PC spectrophotometer and an OceanOptics HR-2000 fiber-optical spectrometer. The luminescence spectra were recorded on an OceanOptics USB2000-FLG fiberoptical spectrofluorimeter with excitation at 400 nm. The luminescence decay kinetics were measured on a FluoTime 200 (PicoQuantGmbH) spectrofluorometer. The lifetime distributions were obtained from these kinetics using Tikhonov regularization under the condition of non-negativity of the solutions by the method described in [8]. RESULTS AND DISCUSSION Synthesis and Postpreparation Treatment of InP@ZnS CQDs The method for preparing InP CQDs consists of three stages: synthesis of InP nanoparticles of a given size, growth of shells based on a wide bandgap semiconductor, and post-preparative treatment consisting in the removal of the initial reagents by successive deposition of CQDs and their washing. The first stage
of the synthesis of InP@ZnS CQDs, namely, preparation of InP cores, was carried out according to the method described in [4]. By selecting the time of the synthesis, InP CQD samples were obtained with an average diameter of 3 nm and a luminescence band maximum at 600 nm. The process of the growth of ZnS shells was substantially changed in comparison with the method described in [4]. After the introduction of DDT and reducing the temperature in the reaction vessel to 200°C, the growth process of InP cores was completed and the shell growth was initiated. Initially the InP cores do not luminesce, but, as ZnS shells are growing, luminescence intensity increases significantly, reaching a maximum. In the course of selection of the synthesis conditions, we concluded that too much DDT was introduced in the work [4], which led to a decrease in the size of InP cores due to their partial dissolution and a hypsochromic shift of the excitonic peak in the absorption spectra of 20–30 nm. Therefore, we optimized the amount of DDT added, which in the final version was 10.6 mmol. Figure 1 shows the time variation of the luminescence quantum yield in the course of the growth of ZnS shells on InP cores. It can be seen from Fig. 1 that the highest quantum yield value (0.45–0.5) was achieved within 60 min after the introduction of DDT. In the method described in the work [4], it is argued that in the course of the synthesis, the growth of a ZnS shell is slowed down due to the depletion of zinc ions. Hence, for the further formation of the shell, zinc stearate (Zn(St)2) was additionally introduced. However, in our synthesis, as can be seen from Fig. 1, the introduction of Zn(St)2 into the reaction medium not only leads to no increase in the quantum yield, but even reduces it. In addition, the presence of Zn(St)2 complicates the subsequent procedure of washing off CQDs from a large amount of excess components. Therefore, the process of building up a ZnS shell was carried out without the introduction of Zn(St)2, at 200°C for 60 min after the introduction of DDT. The final product of the synthesis, in addition to InP@ZnS CQDs coated with a shell of organic ligands, contains a large number of different components: solvent, free ligands, unreacted precursors, and reaction products. To remove all these excess reagents, CQDs were washed off. To do this, the resulting product was poured in a ratio of 1 : 1 by volume with methyl alcohol, in which CQDs are not soluble, and the mixture was kept on a water bath at 60°C for 5 min. The solution with impurities was poured from the sediment containing CQDs. The procedure was repeated twice. The sediment was dried from residue of methyl alcohol and then was dissolved in chloroform (3 mL) upon heating on a water bath at 60°C until obtaining a clear solution. To this solution, methyl alcohol was added in a ratio of 1 : 2 until appearance of turbidity caused by deposition of large aggregates of CQDs. The mixture HIGH ENERGY CHEMISTRY
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Fig. 1. Changing the luminescence quantum yield of InP@ZnS CQDs in the course of building up a ZnS shell. The average diameter of InP cores was 3.0 nm. The arrow shows the moment of addition of Zn(St)2.
was centrifuged (3000 rpm) for 20–60 min until separation of the sediment. The clear solution above the sediment, which contained impurities soluble in chloroform, was poured. The sediment was again dissolved in chloroform, and the procedure of deposition by centrifugation was repeated until achieving the desired purity of CQDs. Since the ligands in the shells of CQDs are in equilibrium with the free ligands in solution, in the course of decreasing the amount of the free ligands, a time should come when, as a result of washing, the ligands from the shells will begin to transfer to the free state and to expose the defects on the surface of CQDs. As a result, the CQD luminescence intensity should decrease. We studied this effect by controlling the change in the luminescence quantum yield after each wash. For this purpose, after each cycle of deposition and washing, samples were taken, the absorbance of the CQD solutions in chloroform at an excitation wavelength of 400 nm was adjusted to 0.1, and the CQD luminescence spectra were recorded. Figure 2 shows the results of these measurements. It can be seen from Fig. 2 that the position of the luminescence band maximum is virtually unchanged with washing, whereas the intensity decreases after each wash. Five washes lead to a significant drop in the luminescence intensity, which is caused by the beginning of the process of ligand removal from the CQD surface and exposure of the defects. Hence, the optimum degree of purification of InP@ZnS CQDs is achieved at two or three washes. Homogeneous Width and Inhomogeneous Broadening of Luminescence Spectra Figure 3 shows the absorption and luminescence spectra of InP@ZnS CQDs synthesized. In accordance with [9], on the basis of the analysis of these spectral data, it is possible to determine the homogeneous width and the inhomogeneous broadening, which determine the total width of the spectra. For HIGH ENERGY CHEMISTRY
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Fig. 2. Luminescence spectra of InP@ZnS CQDs in chloroform after 1, 2, 3, 4, and 5 washes.
this purpose, the fundamental Kennard–Stepanov relationship between absorption and luminescence spectra is used, which is formally similar to the Kirchhoff law determining the relationship between the emission and absorption abilities of bodies [10]. Assuming the Gaussian shape of the luminescence spectra and the excitonic absorption peak of CQDs, it follows from the Kennard–Stepanov relationship that they should have the same widths, and the difference between the positions of their peaks (Stokes shift) is equal to
( )
Δλ = hc δλ , k BT λ 2
(1)
where δλ is the standard deviation of the luminescence spectrum and the first excitonic maximum of the absorption spectrum, h is the Planck constant, c is the speed of light, kB is Boltzmann’s constant, T is the absolute temperature, and λ is the wavelength. Equation (1) permits, on the basis on the known Stokes shift, calculating the homogeneous width of the spectrum. Then, knowing the total width of the spectrum δλΣ, it is possible to calculate the inhomogeneous broadening δλ0:
δλ 20 = δλ 2Σ − δλ 2.
(2)
The application of this procedure to the data in Fig. 3 gives the following results: Δλ = 24.2 nm, δλΣ = 30 nm, δλ = 13 nm, δλ0 = 27 nm. Since FWHM of the Gaussian line is about 2.35 times larger than the standard deviation, the corresponding values of the homogeneous width and the inhomogeneous broadening of the spectrum are 31 and 63 nm, and the total spectral width is 70 nm. That is, in our case, the inhomogeneous broadening is twice as large as the homogeneous
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Fig. 3. Experimental (curve 1) absorption and (curve 3) luminescence spectra of InP@ZnS CQDs. Curve 2 shows the result of fitting the long-wavelength edge of the absorption spectrum with the Gaussian function. Curve 4 shows the calculated luminescence spectrum at zero inhomogeneous broadening. Horizontal arrows show the Stokes shift Δλ and the widths of the luminescence spectra (doubled standard deviations 2δλ and 2δλΣ). Vertical arrows indicate the wavelengths at which the lifetimes were measured: 530, 550, 600, and 660 nm.
width and, hence, gives the dominant contribution to the total width of the spectrum. There are at least two causes of inhomogeneous broadening. The first is the size distribution of CQDs, which, due to the quantum size effect, leads to broadening of the spectrum observed. The second cause stems from the fact that CQDs in themselves, even very close in size, can greatly differ in their spectralluminescent characteristics. One of these effects is fluorescence flickering (also called “blinking”), which is that one CQD can be in two states: luminescing (ON) or nonluminescing (OFF), with transitions between these states occurring randomly in a wide time interval of 10−6–101 s. The study of this effect is a subject of many works [11]. In addition, a certain intermediate state is possible, when CQD, when excited, emits a quantum of light, but with less probability than in the ON state. Hence, CQDs can be “light” (ON state), black (OFF state), and “gray” (a state with low luminescence quantum yield) [12]. Below we will discuss how this effect is manifested in InP@ZnS CQDs studied in this work. Calculation of Polydispersity of CQDs from Inhomogeneous Broadening The value of inhomogeneous broadening δλ0 = 27 nm obtained above was used to assess polydispersity of the particles synthesized. The necessary sizing curve of InP has been constructed by us on the basis of
fitting to the experimental data of the work [13] in the framework of the following equation: λmax/λ = 1 + d0/d, where d is the diameter of the particles, λ is the wavelength of the first excitonic maximum, and λmax and d0 are adjusted parameters found to be λmax = 1034.7 nm and d0 = 2.414 nm (428 < λ < 729 nm). For this sizing curve, the relationship between the polydispersity and the inhomogeneous broadening has the following form: λ ⎞ δλ 0 ⎛ (3) = ⎜1 − 0 ⎟ δ d , λ0 ⎝ λ max ⎠ d where δd/〈d〉 is the polydispersity of the particles (relative mean-square standard deviation of the diameter) and 〈λ0〉 is the average value of the position of the absorption peak of the individual particles in the ensemble. From this formula, for our particles, the polydispersity is found to be 11%. Note that this value may slightly differ from the true value due to the fact that the dimensional curve used does not allow for the presence of a ZnS shell on the particles. Analysis of Luminescence Decay Kinetics For InP@ZnS CQDs synthesized, the luminescence decay kinetic curves were measured at four observation wavelengths (530, 550, 600, and 660 nm) shown in Fig. 3 by vertical arrows. The need for several observation wavelengths is due to the fact that, because of significant inhomogeneous broadening, the kinetics for different spectral regions can vary. The curves obtained were used to calculate the distributions of the luminescence lifetimes in accordance with the formula ∞
I ( t ) = a ( τ) e
∫
−t τ
d ln τ,
(4)
0
where I(t) is the luminescence decay kinetics and a(τ) is the desired probability distribution density of the logarithm of the decay time [8]. Figure 4 shows the result of the calculations. This distribution is dominated by a broad peak with a maximum at ~35 ns. Given the high quantum yield, this peak apparently corresponds to the radiation time. At the edges of the luminescence band, especially at the short-wavelength edge, a large contribution is also made by times of the order of 4 ns. These times most likely correspond to strongly quenched luminescence of “gray” CQDs. Minor, but statistically noticeable contribution is also made by times of the order of 200 ns. Since these times are longer than the main radiation time of ~35 ns, it is logical to assume that they correspond to some trap luminescence. Note that the problem of obtaining the luminescence lifetime distributions from the kinetic curves is ill-posed. Therefore, the distributions obtained should be treated with caution. At least, they differ from the true distributions in that they are substantially smoothed, and the smoothing degree is greater, the HIGH ENERGY CHEMISTRY
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Fig. 4. Distributions of the logarithm of the luminescence decay times of InP@ZnS CQDs observed at different wavelengths within the luminescence band (see Fig. 3): (1) 530, (2) 550, (3) 600, and (4) 660 nm. The distributions were normalized to the peak height at ~35 ns. Luminescence was excited at a wavelength of 470 nm, with FWHM of the impulse response function (IRF) being 185 ps.
smaller the signal-to-noise ratio. It is manifested in Fig. 4, in particular, that the lifetime distribution for the kinetics measured at the long-wavelength edge of the luminescence band is smoothed so, that the time peak at 200 ns is not noticeable. It was noted above that upon post-preparative washing of the synthesized particles from excess reaction products and starting reactants, a significant decrease in the quantum yield is observed (Fig. 2). To check whether this is due to changes in the luminescence lifetime distribution, the corresponding kinetics were measured. Figure 5 shows the distributions calculated on the basis of these kinetics. It can be seen from this figure that, in contrast to the quantum yield, the luminescence lifetime distribution does not almost change upon washing. Hence, we can conclude that the drop in the quantum yield upon washing is due to the transition of the particles to the OFF state (in the context of blinking), in which the luminescence decay time is much shorter than we can actually observe on our device. CONCLUSIONS Indium phosphide CQDs with a ZnS shell and a luminescence quantum yield of about 50% have been synthesized. Joint analysis of the stationary absorption and luminescence spectra of these particles permitted determining the homogeneous width and the inhomogeneous broadening of the spectra. Analysis of the luminescence decay kinetics as a function of the detection wavelength showed the presence of a wide range of the lifetimes. HIGH ENERGY CHEMISTRY
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Fig. 5. Distributions of the logarithm of the luminescence decay times of InP@ZnS CQDs observed at the peak of the luminescence band (600 nm) for the five successive stages of post-preparative washing: 1 (solid curve), 2 (dashed curve), 3 (dotted curve), 4 (dash-dotted curve), and 5 (short-dash curve) washes. The distributions were normalized to the peak height at ~35 ns. Luminescence was excited at a wavelength of 400 nm, with FWHM of the impulse response function (IRF) being 275 ps.
ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation, project no. 14-13-01426. REFERENCES 1. Murray, C.B., Norris, D.J., and Bawendi, M.G., J. Am. Chem. Soc., 1993, vol. 115, p. 8706. 2. Chen, O., Zhao, J., Chauhan, V.P., Cui, J., Wong, C., Harris, D.K., Wei, H., Han, H.-S., Fukumura, D., Jain, R.K., and Bawendi, M.G., Nat. Mater., 2013, vol. 12, p. 445. 3. Mičič, O.I., Curtis, C.J., Jones, K.M., Sprague, J.R., and Nozik, A.J., J. Phys. Chem., 1994, vol. 98, p. 4966. 4. Song, W.S., Lee, H.-S., Lee, J.C., Jang, D.S., Choi, Y., Choi, M., and Yang, H., J. Nanoparticle Res., 2013, vol. 15, p. 1750. 5. Lee, J.C., Jang, E.-P., Jang, D.S., Choi, Y., Choi, M., and Yang, H., J. Luminesc., 2013, vol. 134, p. 798. 6. Brichkin, S.B., Colloid J., 2015, vol. 77, p. 393. 7. Xie, R., Li, Z., and Peng, X., J. Am. Chem. Soc., 2009, vol. 131, p. 15457. 8. Tovstun, S.A., High Energy Chem., 2016, vol. 50, no. 5, p. 327. 9. Tovstun, S.A. and Razumov, V.F., High Energy Chem., 2015, vol. 49, p. 352. 10. Stepanov, B.I., Soviet Phys.–Dokl., 1957, vol. 2, p. 81. 11. Fernée, M.J., Tamarat, P., and Lounis, B., Chem. Soc. Rev., 2014, vol. 43, p. 1311. 12. Tenne, R., Teitelboim, A., Rukenstein, P., Dyshel, M., Mokari, T., and Oron, D., ACS Nano, 2013, vol. 7, p. 5084. 13. Mičič, O.I., Ahrenkiel, S.P., and Nozik, A.J., App. Phys. Lett., 2001, vol. 78, p. 4022.
Translated by A. Tatikolov
