ISSN 0018-1439, High Energy Chemistry, 2017, Vol. 51, No. 2, pp. 118–121. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.Yu. Gak, S.A. Tovstun, M.G. Spirin, S.B. Brichkin, V.F. Razumov, 2017, published in Khimiya Vysokikh Energii, 2017, Vol. 51, No. 2, pp. 126–130.
PHOTONICS
Influence of Alkanethiols on Fluorescence Blinking of InP@ZnS Colloidal Quantum Dots V. Yu. Gak*, S. A. Tovstun, M. G. Spirin, S. B. Brichkin, and V. F. Razumov Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail:
[email protected] Received September 13, 2016
Abstract⎯Luminescence decay curves have been measured for InP@ZnS colloidal quantum dots (CQDs) synthesized at different ratios between 1-octanethiol and 1-dodecanethiol. The luminescence lifetime distributions reflecting the ratios between the on, off, and grey states of CQDs have been calculated. With an increase in the portion of 1-octanethiol, the distribution is shifted toward the on states, which almost completely suppresses fluorescence blinking at all detection wavelengths. Keywords: colloidal quantum dots, luminescence spectra, thiols, luminescence lifetimes, blinking, indium phosphide DOI: 10.1134/S0018143917020047
INTRODUCTION Semiconductor colloidal quantum dots (CQDs) are characterized by a fluorescence blinking effect [1– 3], which consists in the spontaneous reversible transition of CQDs from the fluorescent on-state to the nonfluorescent off-state. In addition, CQDs can be also in some intermediate grey state with a very low luminescence quantum yield [4]. Blinking is an unwanted phenomenon, which lowers the luminescence efficiency of CQDs and their ensembles. A lot of efforts have been made to obtain non-blinking CQDs. According to the modified charging model of blinking [5], charge carrier (electron or hole) capturing in trap states located on the CQD shell surface is one of the main reasons for the appearance of the offtime intervals during which CQDs cannot emit a photon upon absorption of a quantum of light. One way to reduce the blinking effect is building up a thick shell made of a wide-bandgap semiconductor [6, 7], which creates a wide potential barrier and greatly reduces the probability for charge carrier tunneling from the CQD core to a trap in the shell. Another way is to decrease the number of traps in the shell by passivating its surface with appropriate ligands. By choosing the ligands, it is possible to control the process of passivation of surface states. As shown earlier [8], thiols can be effectively used as such ligands. For example, the introduction of βmercaptoethanol almost completely suppresses blinking of CdSe@ZnS CQDs. The subsequent studies [9] showed that the influence of thiols on the dynamics of blinking of CdSe@ZnS CQDs of the core@shell type could be more complicated. Depending on the condi-
tions of the introduction of thiols, both increase and decrease in the blinking effect can be observed in the experiment. The influence of alkanethiols (1-octanethiol, 1-dodecanethiol, 1-hexadecanethiol, and 1-octadecanethiol) as surface ligands on the blinking of CdSe@CdS CQDs was systematically studied [10], and it was found that 1-hexadecanethiol and 1-octadecanethiol were unable to effectively passivate the surface traps of CQDs. It was found in [11] that the use of 1-octanethiol as the sulfur precursor in the synthesis of the shell of CdSe@CdS CQDs also effectively suppressed fluorescence blinking (average proportion of the on-states reached 94%). On the other hand, the studies of fluorescence blinking have been mainly focused on CQDs of the AIIBVI type (CdSe and CdTe), and there are few works on fluorescence blinking of AIIIBV CQDs, in particular, indium phosphide InP [12, 13], which, in contrast to cadmium chalcogenide CQDs, consist of nontoxic elements and, hence, are promising for biological applications. The transfer of general ideas and methods of synthesis that are well proven for CdSe CQDs to InP CQDs requires an individual approach to the choice of synthesis conditions, ligands, and precursors [13]. In [14], we proposed a relatively simple and quite effective method for high-temperature colloidal synthesis of indium phosphide CQDs with a zinc sulfide shell in oleylamine medium, and record-breaking characteristics for InP@ZnS CQDs were attained: the luminescence quantum yield of 50% and the size distribution variance of 11%. 1-Dodecanethiol (DDT) served as a source of sulfur for the shell. At the same
118
INFLUENCE OF ALKANETHIOLS ON FLUORESCENCE BLINKING
time, DDT, along with oleylamine, played the role of a stabilizing ligand passivating the ZnS surface. Analysis of the luminescence decay kinetics as a function of the detection wavelength showed the presence of a wide range of the lifetimes. In this work, to study the influence of alkanethiols with different lengths of the carbon chain on the fluorescence blinking effect of InP@ZnS CQDs, we took mixtures of 1-octanethiol (OT) and 1-dodecanethiol (DDT) in various ratios, which were both sulfur sources for the synthesis of the shell and surface trap modifiers passivating the ZnS surface. EXPERIMENTAL To obtain InP@ZnS CQDs, the following reagents were used: indium(III) chloride (99.995%, Acros), zinc chloride (anhydrous, 98%, Sigma–Aldrich), dodecylamine (98%, Aldrich), tris(dimethylamino)phosphine (TDMAP, 97%, Aldrich), 1dodecanethiol (98%, Aldrich), 1-octanethiol (≥98.5%, Aldrich), chloroform (99.5%, Sigma– Aldrich, with amylenes content of 0.01–0.02%), toluene (Acros Organics, 99.85%), and methanol (reagent grade, Khimmed). The synthesis of InP cores for InP@ZnS CQDs was carried out according to the procedure reported in [15]. For this purpose, a mixture of InCl3 (0.9 mmol), ZnCl2 (0.9 mmol), and dodecylamine (15.2 mmol) was placed in a reaction vessel and degassed at 110°C for 40 min, after which temperature of the mixture was raised to 220°C in an argon atmosphere, and the phosphorus precursor TDMAP (1.4 mmol) was added. The InP cores were synthesized at this temperature. The average InP core size was determined by the duration of the synthesis prior the moment of the addition of the sulfur precursors, DDT and OT mixture, needed for the growth of the ZnS shell. To obtain the particles with an average core diameter of 1.8 nm, a time of about 30 s after the introduction of TDMAP was required. The size of InP cores was estimated from the spectral data, using the expression reported in [16]. Building up the ZnS shell on CQDs was performed similarly to [14]: a mixture of thiols containing 5.3 mmol of DDT and 5.3 mmol of OT ([DDT] : [OT] = 1 : 1), 2.65 mmol of OT and 7.95 mmol of DDT ([DDT] : [OT] = 3: 1), or 7.95 mmol of OT and 2.65 mmol of DDT ([DDT] : [OT] = 1: 3) was introduced into the reaction vessel, the temperature was reduced to 200°C, and the mixture was held for 2 h. Then, the mixture was rapidly cooled by feeding water into the jacket of the reaction vessel and toluene (20 mL) was added into the reactor after reaching 110°C. It should be noted that the use of OT as a single sulfur source in the synthesis is fraught with difficulties due to a low rate of shell growth. The shell growth rate increases significantly in the presence of DDT. HIGH ENERGY CHEMISTRY
Vol. 51
No. 2
2017
119
The synthesis was followed by adding methanol (1 : 1 by volume), centrifuging (OPn-3, 3000 min–1), drying and subsequent washing two times the solid precipitate with chloroform, dispersing it in chloroform again, and filtering off using PTFE membranes with a pore diameter of 450 nm. The absorption spectra of test CQD solutions were measured on a Shimadzu UV-3101PC spectrophotometer and an Ocean Optics HR-2000 fiber-optic spectrometer. The luminescence spectra were recorded on an Ocean Optics USB2000-FLG fiber spectrofluorimeter upon excitation at 400 nm. The luminescence decay kinetics were measured on a PicoQuant FluoTime 200 time-resolved spectrofluorometer; a semiconductor laser with a wavelength λ = 400 nm and a pulse width of τ0 ≈ 75 ps was used as the excitation source. Processing the experimental luminescence decay kinetics and calculating the lifetime distributions were performed as described in detail in [17], using the Tikhonov regularization with the use of the O’Leary– Rust theorem for estimating the confidence intervals of the corresponding linear functionals. RESULTS AND DISCUSSION Figure 1 shows the luminescence spectra of InP@ZnS CQDs synthesized in this study. The luminescence decay curves were measured in a narrow (2 to 3 nm) interval of the fluorescence spectrum at five detection wavelengths: 480, 516, 553, 590, and 626 nm (shown in Fig. 1 by vertical arrows). The need for several detection wavelengths was due to the fact that the kinetics for different regions of the luminescence spectrum can differ because of its considerable inhomogeneous broadening in such systems [14]. The luminescence decay curves observed are not single-exponential; rather, they represent a superposition of a large number of exponents in a wide range of decay times on the order of 1–100 ns and are approximated in the form ∞
I ( t ) = a ( τ) e
∫
−t τ
d ln τ,
(1)
0
where I(t) is the luminescence decay kinetics and a(τ) is the probability density of distribution of the logarithm of the decay times [17]. Figure 2 shows the results of the calculations of a(τ) for different CQD samples and different detection wavelengths. It can be seen that for all the samples and detection wavelengths, the distribution of the logarithm of the decay times is bimodal. Two main broad peaks with maximums at τ ~ 55 ns and τ ~ 4 ns can be identified in the distribution. This kind of luminescence lifetime distribution is due to the effect of CQD fluorescence blinking. It is known [5, 18–20] that there is a direct correlation
120
GAK et al.
3500
2500 2000 1500 a(t)
Intensity, arb. units
3000
1000 500 0 400 450 500 550 600 650 700 750 Wavelength, nm
Fig. 1. Luminescence spectrum of InP@ZnS CQDs in chloroform after double washing (with absorbance D = 0.1). The vertical arrows indicate the wavelengths of the registration of the luminescence lifetimes: 480, 516, 553, 590, and 626 nm.
between the quantum yield Qi, the radiative rate constant γem, and the luminescence lifetime τi of a single CQD: (2) Qi = γ emτ i . It follows from Eq. (2) that the fluorescence lifetime distribution is related to the distribution of CQDs in the on- and off-states. A broad peak in the distribution with a maximum at ~55 ns corresponds to the radiative lifetime. Such a wide distribution of radiative times is explained by size distribution of CQDs and dispersion of luminescent characteristics of CQDs of the same size depending on their individual structure. The contribution of times of about 4 ns length apparently corresponds to strongly quenched luminescence of the “grey” CQDs [4]. Its intensity exceeds the background intensity, though is inferior to the emission intensity in the on-intervals. It can be seen that the “grey” luminescence is more pronounced at the edges of the spectral band. It was shown [21] that the length of the off-interval τoff is inversely proportional to grey-luminescence decay time tk:
1 ∝t . k τ off ( k )
(3)
It can be seen from Fig. 2 that with increasing the proportion of OT in the reaction mixture, the distribution is more and more shifted toward the on-states of CQDs. Such an influence of thiol ligands is due to their gradual decomposition at high temperature with releasing active sulfur [10] involved in the formation on InP cores of ZnS shells passivating surface defects, which decreases the probability for charge carrier trapping capturing in these traps. Furthermore, alkanethi-
1.0 0.8 0.6 0.4 0.2 1.0 0.8 0.6 0.4 0.2 1.0 0.8 0.6 0.4 0.2 1.5 1.2 0.9 0.6 0.3 5 4 3 2 1 0
(а)
(b)
(c)
(d)
(e)
0.1
1
10
100
1000
Fig. 2. Probability density a(t) of distribution of the logarithm of the luminescence decay times τ for InP@ZnS CQDs synthesized at [DDT] : [OT] = 3 : 1 (dotted curve), [DDT] : [OT] = 1 : 1 (dashed curve), and [DDT] : [OT] = 1 : 3 (solid curve), measured at different wavelengths within the luminescence band (see Fig. 1): (a) 480, (b) 516, (b) 553, (b) 590, and (e) 626 nm. The distributions are normalized to the height of the peak at τ ~ 55 ns. Luminescence was excited at a wavelength λ = 400 nm.
ols can bind to surface metal ions in the ZnS shell, ensuring its passivation and stabilization of the CQDs formed. The higher efficiency of OT than DDT in our experiments can be explained by the fact that the energy required to break the alkyl and thiol groups depends on the structure of the alkanethiol. Alkanethiols with a shorter chain easier decompose and release more active sulfur at the synthesis temperature [10]. In addition, relatively compact OT molecules are more mobile, effectively compete with DDT molecules for binding to the surface, and can form more dense coating of the surface by ligands, passivating greater amount of defects. The results presented above suggest that the structure of alkanethiols is very important in the synthesis of quantum dots, as shown by the values of their luminescence quantum yields (table). In addition, the use Luminescence quantum yields ϕ for InP@ZnS CQDs [DDT] : [OT]
ϕ
3:1 1:1 1:3
0.20 ± 0.02 0.31 ± 0.03 0.31 ± 0.03
HIGH ENERGY CHEMISTRY
Vol. 51
No. 2
2017
Proportion of on-luminescence
INFLUENCE OF ALKANETHIOLS ON FLUORESCENCE BLINKING
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
(а)
121
ACKNOWLEDGMENTS This work is supported by the Russian Foundation for Basic Research, project no. 16-03-00756. REFERENCES
(b)
460 480 500 520 540 560 580 600 620 640 Wavelength, nm
Fig. 3. Proportion of on-luminescence of InP@ZnS CQDs synthesized at [DDT] : [OT] = 3 : 1 (dotted curve), [DDT] : [OT] = 1 : 1 (dashed curve), or [DDT] : [OT] = 1 : 3 (solid curve), as measured at different wavelengths within the luminescence band after (a) double washing (see Fig. 2) and (a) single washing.
of the short-chain ligand extends the potential application of CQDs to consolidated systems of densely packed nanoparticles, in which nonradiative resonance energy transfer can occur [22]. It should be noted that the luminescence lifetime distribution may also depend on other factors including the postsynthetic treatment of CQDs. Figure 3 shows a change in the ratio of the area of the peak at ~55 ns to the total area of both peaks, that is, the proportion of the onstates, for different numbers of washing stages. However, as can be seen from Fig. 3, the general character of the dependence on the proportion of OT is retained. CONCLUSIONS In summary, colloidal quantum dots InP@ZnS have been synthesized using dodecanethiol and octanethiol. The luminescence decay curves for the resulting CQDs prepared have been measured. On the basis of their analysis, luminescence lifetime distributions reflecting the distribution of CQDs in the on, off, and grey states have been calculated. It has been shown that the most effective suppression of fluorescence blinking is achieved at the ratio of [DDT] : [OT] = 1 : 3. HIGH ENERGY CHEMISTRY
Vol. 51
No. 2
2017
1. Fernee, M.J., Tamarat, P., and Lounis, B., Chem. Soc. Rev., 2014, vol. 43, p. 1311. 2. Brichkin, S.B. and Razumov, V.F., Russ. Chem. Rev., 2016, vol. 85, no. 12, p. 1297. 3. Razumov, V.F., Phys. Uspekhi, 2016, vol. 186, no. 12, p. 1368. 4. Tenne, R., Teitelboim, A., Rukenstein, P., Dyshel, M., Mokari, T., and Oron, D., ACS Nano, 2013, vol. 7, no. 2013, p. 5084. 5. Osad’ko, I.S., Phys. Uspekhi, 2016, vol. 186, no. 5, p. 462. 6. Mahler, B., Spinicelli, P., Buil, S., Quelin, X., Hermier, J.-P., and Dubertret, B., Nat. Mater., 2008, vol. 7, no. 8, p. 659. 7. Chen, Y., Vela, J., Htoon, H., Casson, J.L., Werder, D.J., Bussian, D.A., Klimov, V.I., and Hollingsworth, J.A., J. Am. Chem. Soc., 2008, vol. 130, no. 15, p. 5026. 8. Hohng, S. and Ha, T., J. Am. Chem. Soc., 2004, vol. 126, no. 5, p. 1324. 9. Jeong, S., Achermann, M., Nanda, J., Ivanov, S., Klimov, V.I., and Hollingsworth, J.A., J. Am. Chem. Soc., 2005, vol. 127, no. 29, p. 10126. 10. Zhang, A., Dong, C., Liu, H., and Ren, J., J. Phys. Chem. C, 2013, vol. 117, no. 46, p. 24592. 11. 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, no. 5, p. 445. 12. Zan, F., Dong, C., Liu, H., and Ren, J., J. Phys. Chem. C, 2012, vol. 116, no. 6, p. 3944. 13. Georgin, M., Carlini, L., Cooper, D., Bradforth, S.E., and Nadeau, J.L., Phys. Chem. Chem. Phys., 2013, vol. 15, no. 25, p. 10418. 14. Brichkin, S.B., Spirin, M.G., Tovstun, S.A., Gak, V.Yu., Mart’yanova, E.G., and Razumov, V.F., High Energy Chem., 2016, vol. 50, no. 5, p. 395. 15. Song, W.-S., Lee, H.-S., Lee, J.C., Jang, D.S., Choi, Y., Choi, M., and Yang, H., J. Nanopart. Res., 2013, vol. 15, no. 6, p. 1750. 16. Xie, R., Li, Z., and Peng, X., J. Am. Chem. Soc., 2009, vol. 131, no. 42, p. 15457. 17. Tovstun, S.A., High Energy Chem., 2016, vol. 50, no. 5, p. 327. 18. Schlegel, G., Bohnenberger, G., Potapova, I., and Mews, A., Phys. Rev. Lett., 2002, vol. 88, no. 13, p. 137401. 19. Fisher, B.R., Eisler, H.-J., Stott, N.E., and Bawendi, M.G., J. Phys. Chem. B, 2004, vol. 108, no. 1, p. 143. 20. Zhang, K., Chang, H., Fu, A., Alivisatos, A.P., and Yang, H., Nano Lett., 2006, vol. 6, no. 4, p. 843. 21. Cordones, A.A., Bixby, T.J., and Leone, S.R., Nano Lett., 2011, vol. 11, no. 8, p. 3366. 22. Brichkin, S.B., Spirin, M.G., and Gak, V.Yu., Colloid J., 2014, vol. 76, no. 1, p. 6.
Translated by A. Tatikolov
