Temperature-dependent photoluminescence properties of Mn:ZnCdS quantum dots

Abstract

The thermal stability of quantum dots (QDs) is considered as one of the key factors for their applications in various devices, due to the inevitable current induced Joule heat. In this work, we report the temperature-dependent photoluminescence (PL) of Mn:ZnCdS QDs, with a designed structure of MnS/ZnS/CdS, in the temperature range from 80 to 480 K. It is found that the highest PL quantum yield (QY) of the Mn:ZnCdS QDs can reach up to 65% even at the high temperature of 360 K. Moreover, the optical performance exhibits an excellent thermal stability, since the close-packed Mn:ZnCdS QDs and the QDs/organic blend films can maintain 90% of their initial intensity at 400 K for a long time. Furthermore, the thermal quenching mechanism of Mn²⁺ ion emission is proposed. The results of the present work suggest that QDs with high PL QYs and enhanced thermal stabilities could be realized by blocking the nonradiative recombination centers with thick CdS shells.

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1. Introduction

Quantum dots (QDs) have been considered as a revolutionizing material to be applied in next-generation luminescent and photovoltaic devices, due to their size-dependent properties, flexible solution processing, and higher photochemical stability.^1–5 As compared to pure counterparts, Mn²⁺ ion-doped QDs have additional advantages of minimum self-quenching due to the large Stokes shift, long excited state lifetime, and enhanced thermal and chemical stabilities,^6–13 suggesting extensive applications in luminescent devices,^14,15 biological labels¹⁶ and QD-sensitized solar cells.^17,18 Especially in recent years, doping Mn²⁺ ions in QDs has been proven to be a powerful strategy to extend the lifetime of charge carriers to boost the efficiency of QD-sensitized solar cells.^17,18 Owing to numerous efforts toward persistent improvement in the controllable synthesis of Mn²⁺ ion-doped QDs^7,11,19–21 and deep understanding of their optoelectronic properties during the past decades,^{8–11,22–24} Mn²⁺ ion-doped QDs have become very promising and excellent candidate materials to be used in novel luminescent and photovoltaic devices with high performance.

Temperature-dependent photoluminescence properties of QDs are interesting for QD-based devices,^25–27 such as photovoltaics,²⁸ phosphor-converted light-emitting diodes (LEDs),^14,15 and luminescent concentrators.²⁹ These functional devices should be stable with high performance at elevated temperatures; however, the investigations on the thermal stability of QDs remain limited.^30,31 Furthermore, in the reported studies, significant thermal quenching of the QDs often occurs even below 300 K. For example, for Mn:ZnSe,³² Mn:CdS³³ and Mn:ZnS QDs,³⁴ which have a low PL QY of less than 30%, there is a greater than 50% loss of PL QY when the temperature is increased to room temperature. Although highly efficient Mn²⁺ ion emissions have been achieved in Mn:ZnS,³⁵ Mn:ZnSe,^7,19 Mn:ZnSeS QDs³⁶ and Mn:ZnCdS³⁷ in recent years, there are few works reported on the temperature-dependent optical properties of Mn²⁺ ion-doped QDs.^30,38

In this work, we reported the PL properties of Mn:ZnCdS QDs using time-resolved and temperature-dependent PL spectra in the range from 80 to 480 K. Nearly no evident variations in both the PL decay curves and integrated PL intensity of the Mn²⁺ ion emission were observed in the range from 80 to 360 K. The highest PL QY of 65% with an excellent thermal stability has been achieved in our synthesized Mn:ZnCdS QDs even at the high temperature of 360 K. In addition, the PL quenching mechanisms were explored by comparing the temperature-dependent integrated PL intensities and PL decay curves of Mn:ZnCdS QDs.

2. Experimental section

Similar to our previous work, the Mn²⁺ ion-doped ZnCdS QDs (Mn:ZnCdS QDs) were synthesized by the nucleation-doping strategy, which is described in detail in the supporting information (the representative synthesis procedure for the Mn:ZnCdS QDs, ESI†).³⁷ A preformed MnS nanocrystal (NC) core was initially coated with a ZnS buffer layer, and then further coated with a CdS shell. The middle ZnS buffer layer of ∼1 monolayer (ML) (calculation of the ZnS buffer layer thicknesses, ESI†) was introduced to reduce the local strains derived from the lattice mismatch between MnS and CdS to obtain a highly efficient Mn²⁺ ion emission. During the coating of ZnS/CdS shells on MnS cores, some Mn²⁺ ions would diffuse from the MnS core to the ZnS/CdS shell due to the high activity of the small MnS cores and the high coating temperature, which leads to the formation of Mn:ZnCdS interface layers. The as-synthesized QDs exhibit an extremely high Mn²⁺ ion emission with a QY of 70% (The PL quantum yield (QY) measurements, ESI†) at room temperature due to the fast energy transfer from the excitons in ZnCdS host to Mn²⁺ ions, which is similar to the Mn²⁺ ion-doped ZnCdS QDs (so-called Mn:ZnCdS QDs).

The inorganic/organic blend film of Mn²⁺ ion-doped QD/1,3,5-tris(N-phenylbenz-imidazol-2-yl) benzene (TPBI) (QD/TPBI blend film), which is the core unit for the highly efficient quantum dot LEDs, was deposited on quartz substrates from QD and TPBI mixed solutions by a spin coater. The morphologies of the Mn:ZnCdS samples were characterized by a transmission electron microscope (TEM) (JEM-2100F, JEOL, Japan). In the measurements of temperature-dependent PL spectra and decay curves, the samples were prepared by dropping QDs dispersed in chloroform on silicon wafer substrates, which were mounted in a Janis VPF-800 vacuum liquid nitrogen cryostat during measurement in the range from 80 to 480 K. The PL spectra and PL decays were recorded using a Horiba Jobin Yvon Fluromax-4P with a time-correlated single-photon-counting (TCSPC) spectrometer. A 150 W, ozone-free, xenon arc-lamp was used as the continuous excitation source, and a pulsed xenon lamp was utilized as the excitation source for PL decay measurements. The PL QYs were recorded using a Horiba Jobin Yvon Fluromax-4P with a quantum yield accessory, which can provide absolute QY values. Relative to calculation by comparing the solution to an organic dye, the absolute PL QY measurement can effectively reduce the experimental error.

3. Results and discussion

Fig. 1 shows the temperature-dependent PL spectra of Mn:ZnCdS QDs recorded in the range from 80 to 480 K. The average diameters of the as-synthesized Mn:ZnCdS QDs and the QDs without CdS shell coating (MnS/ZnS QDs) are ca. ∼4.8 nm and ∼2.8 nm, respectively (the inset in Fig. 1(a) and S1 in ESI,† respectively). Considering a thickness of 0.67 nm for 1 ML of the CdS shell in our case, the thickness of the CdS shell is estimated to be ∼3.0 MLs. The Mn:ZnCdS QDs show a strong PL centered at ∼570 to 590 nm when excited at 365 nm at various temperatures, which is confirmed to originate from a typical emission of Mn²⁺ ions due to its ⁴T₁ to ⁶A₁ transition.³⁷ The onset in the excitation spectra of the Mn²⁺ ion emission in the Mn:ZnCdS QDs is ∼420 nm,³⁷ which is between that of MnS/ZnS (∼325 nm)¹³ and MnS/CdS QDs (∼470 nm) with similar sizes, suggesting the formation of the ZnCdS alloy compound host due to the interdiffusion of Zn and Cd ions over the high temperature synthesis process. This indicates that, as compared to the Mn:ZnS QDs, the Mn:ZnCdS counterparts have narrower band gaps and more efficient absorption in the visible region. This could be a unique advantage to be utilized in QD-sensitized solar cells and QD-LED devices. Fig. 1(b) depicts the temperature dependence of the integrated PL intensities and the peak positions of Mn²⁺ ion luminescence. The integrated PL intensity almost remains almost constant when the temperature increases from 80 to 360 K (Fig. 1(b)), indicating that the PL QY of our Mn:ZnCdS QDs can remain up to ∼65% even at a high temperature of 360 K (∼70% at room temperature). The thermal stability of our Mn:ZnCdS QDs is superior to those of nondoped QDs reported before, which generally exhibit a durative strong quenching of the luminescence intensity from lower temperatures to elevated temperatures (e.g. CdSe/CdS/ZnS core–shell–shell QDs from 300 to 500 K,²⁶ CdTe QDs from 15 to 210 K,³⁹ and CdSe/ZnS core–shell QDs from 45 to 295 K (ref. 40)). This could be mainly ascribed to the thermally activated photoionization of the photoexcited carriers in QDs.²⁴ The PL intensity decreases rapidly with the increase of the temperature from 360 K and remains at 14% of its initial value at 480 K. In order to know whether the quenching is intrinsic or not, the temperature-dependent PL spectra of Mn:ZnCdS QDs in heating–cooling cycle processes were recorded, and the corresponding integrated PL intensities were plotted in Fig. 1(c). It seems that the PL intensity almost fully recovers upon cooling (remains ∼95% of its initial value), which suggests that the irreversible quenching process is absent. The reversible quenching suggests that the thermal quenching of the PL is intrinsic and the performance of the Mn:ZnCdS QDs is well retained even after high temperature treatment.²⁴ These results indicate that the Mn²⁺ ion-doped QDs are more robust than the nondoped counterparts, which can meet the requirements for applications in various functioning devices.

Fig. 1 (a) The temperature-dependent PL spectra of Mn:ZnCdS QDs recorded in the range from 80 to 480 K. The inset shows the typical TEM image of the QDs. (b) PL peak positions (black line) and integrated intensities (red line) of the Mn²⁺ ion emission. (c) Temperature-dependent integrated PL intensities of Mn:ZnCdS QDs in heating–cooling cycle processes. The black and red lines refer to the heating and cooling processes, respectively.

As shown in Fig. 1(b), the PL peak wavelength gradually shifts to a higher energy by 68 meV (from 589 to 572 nm) when the temperature increases from 80 to 480 K. The blue shift of PL is generally attributed to the thermal expansion of the host lattice induced by the increase of temperature.^10,41,42 However, in our case, the observed blue shift (∼68 meV) obviously shows a much stronger temperature dependence than those of Mn²⁺ ion-doped ZnS or CdS QDs (almost ∼5 fold larger¹⁰). Chen et al.¹⁰ have studied the tunable temperature dependence of Mn²⁺ ions' PL peak position in Mn²⁺ ion-doped CdS/ZnS core–shell QDs, which suggests that the larger local lattice strain within the dopant site could cause a stronger temperature dependence of the luminescence peak due to the enhanced local thermal expansion within the dopant site. In our case, for the as-synthesized Mn:ZnCdS QDs with a MnS/ZnS/CdS core–shell structure, although the localized nature of the Mn²⁺ ions is not very clear currently, there must be a local lattice strain within the dopant site since the CdS has a larger lattice parameter (5.82 Å) than ZnS (5.41 Å);^{10, 43} i.e., the localized lattice strain within the Mn:ZnCdS QDs causes the enhanced blue shift of the PL.^10,22

For practical applications, in addition to the high PL QY, the thermal stability plays a critically important role for the QDs to be used in devices at an elevated temperature. Fig. 2(a) and (b) show the PL spectra of the close-packed Mn:ZnCdS QDs and QDs/TPBI blend film, respectively, during thermal treatment at 400 K for 540 minutes. It is found that the peak positions and contours of the PL spectra from both samples are almost identical, suggesting the invariability of the luminescence origin of the Mn²⁺ ion emission. The PL intensities of the Mn:ZnCdS QDs decrease slightly, and then remain constant with increasing time during the thermal treatment at 400 K (Fig. 2(c)). The PL intensities could remain at ∼90% of their initial values after the long high-temperature heat treatment at 400 K for 540 minutes, clearly suggesting the excellent high-temperature stability of our synthesized Mn:ZnCdS QDs.

Fig. 2 (a) The evolution of PL spectra of the close-packed Mn:ZnCdS QDs at 400 K. (b) The evolution of PL spectra of the QDs/TPBI blend film at 400 K. (c) The integrated PL intensities of Mn²⁺ ion emissions as a function of time.

To understand the thermal quenching mechanism, another sample of Mn:ZnCdS QDs with a low PL QY of 13%, which has a similar MnS core and ZnS buffer layer to the first sample and with a thin CdS shell (the representative synthesis procedure for the Mn:ZnCdS QDs, ESI†), was studied for comparison. The thickness of the CdS shell can be estimated to be ∼1.0 MLs, since the size of the QDs is ∼3.4 nm (Fig. S1, ESI†). Their temperature-dependent integrated PL intensities are shown in Fig. 3. It seems that the Mn²⁺ ion emission intensity decreases quickly as the temperature increases from 80 to 480 K. Note that there is ∼88% loss in the PL intensity when the sample was heated up to 360 K as compared to that at 80 K, showing more temperature sensitivity than that of the QDs with a thick shell (with a high PL QY). To better understand the different thermal quenching issues in the doped QDs with various shell thicknesses, the PL decay curves of Mn²⁺ ions are displayed in Fig. 4. The curves were fitted by a bi-exponential function with two time components (τ_i) and weights (A_i). The amplitude-weighted lifetimes (τ_Mn(T)) were obtained by the following relation:

τ_Mn(T) = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₁) (1)

Fig. 3 The temperature-dependent PL spectra (a) and integrated PL intensities (b) of Mn:ZnCdS QDs with a PL QY of 13% from 80 to 480 K. The integrated PL intensities are normalized to that measured at 80 K.

Fig. 4 The temperature-dependent decay curves of Mn:ZnCdS QDs with a high PL QY of 70% (a) and low PL QY of 13% (b) from 80 to 440/400 K at an excitation wavelength of 365 nm.

Consequently, the temperature-dependent integrated PL intensities and lifetimes are plotted in Fig. 5. The results demonstrate that the τ_Mn(T) exhibits a similar temperature-dependent trend as that of PL intensity (I_(T)) for the doped QDs with a high PL QY (Fig. 5(a)), indicating the weak temperature-dependent efficiency of the energy transfer from the excitons to Mn²⁺ ions (Φ_ET(T)) according to the following equation:

I_(T) ∝ Φ_ET(T) × Φ_Mn(T) ∝ Φ_ET(T) × τ_Mn(T) (2)

where Φ_Mn(T) is the efficiency of the Mn²⁺ ion emission. The weak temperature dependence of the Φ_ET(T) indicates the thermal quenching is mainly due to the decrease of Φ_Mn(T) at elevated temperature. Moreover, the PL intensity of the doped QDs with a low PL QY drops faster than the PL lifetime once the temperature is over 180 K (Fig. 5(b)), suggesting that both the decrease of Φ_Mn(T) and drop of Φ_ET(T) are responsible for the PL thermal quenching. The different quenching mechanisms in the doped QDs with various shell thicknesses might be attributed to the lower density of nonradiative recombination centers in the QDs with high PL QY and the larger energy transfer rate (∼10¹¹ s⁻¹) than the Mn²⁺ ion radiative rate (10³ s⁻¹); i.e., the thermal stability of Mn²⁺ ion emission in QDs could be profoundly improved by increasing the shell thicknesses to decrease the density of nonradiative recombination centers.

Fig. 5 Temperature-dependent integrated PL intensities and average lifetimes of Mn:ZnCdS QDs with a high PL QY of 70% (a) and low PL QY of 13% (b). The integrated PL intensities and lifetimes are both normalized to those measured at 80 K.

It is generally considered that elevated temperatures could typically enhance the exciton–phonon coupling interactions in the host QDs for the doped QDs, which thus impact the energy transfer efficiency from the host to the doped ions. Namely, the energy transfer from the photoexcited exciton of the host to Mn²⁺ ions must compete with the nonradiative recombination resulting from exciton–phonon coupling interactions. Fortunately, the fast energy transfer rate (∼10¹¹ s⁻¹)⁴⁴ and the low density of nonradiative recombination centers in our doped QDs with a high PL QY together allow them a weaker temperature-dependent energy transfer efficiency. Furthermore, the Mn²⁺ ion emission from the inner-core d–d transition (⁴T₁ to ⁶A₁) in a tetrahedral coordination favors a lower sensitivity to the environment.⁴⁵ Thus, as compared to the pristine counterparts, the QDs doped with Mn²⁺ ions could be expected to have enhanced thermal stability.

4. Conclusions

In summary, we have demonstrated the thermal behaviors of Mn:ZnCdS QDs in the temperature range of 80–480 K. The PL QY of the as-synthesized QDs can reach up to 65% even at a high temperature of 360 K. Moreover, both close-packed QDs and QDs/organic blend films exhibit excellent thermal stabilities at elevated temperatures. The thermal stability of Mn²⁺ ion emission in QDs could be profoundly improved by increasing the shell thicknesses to decrease the density of nonradiative recombination centers. Current work might represent a step forward in the practical applications of Mn²⁺ ion-doped QDs in optical devices, especially for those intended to work in high temperature environments.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (NSFC, Grant No. 61106066), 973 Program (Grant no. 2012CB326407) and Zhejiang Provincial Science Foundation (Grant no. LY14F040001).

References

1. A. Mews and J. Zhao, Nat. Photonics, 2007, 1, 683.
2. S. Coe, W.-K. Woo, M. G. Bawendi and V. Bulović, Nature, 2002, 420, 800.
3. Y. Shirasaki, G. J. Supran, M. G. Bawendi and V. Bulović, Nat. Photonics, 2012, 7, 13.
4. P. V. Kamat, J. Phys. Chem. Lett., 2013, 4, 908.
5. W. Zhang, Q. Lou, W. Ji, J. Zhao and X. Zhong, Chem. Mater., 2013, 26, 1204.
6. S. C. Erwin, L. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy and D. J. Norris, Nature, 2005, 436, 91.
7. N. Pradhan and X. Peng, J. Am. Chem. Soc., 2007, 129, 3339.
8. H.-Y. Chen, T.-Y. Chen and D. H. Son, J. Phys. Chem. C, 2010, 114, 4418.
9. H.-Y. Chen, S. Maiti and D. H. Son, ACS Nano, 2012, 6, 583.
10. H.-Y. Chen, S. Maiti, C. A. Nelson, X. Y. Zhu and D. H. Son, J. Phys. Chem. C, 2012, 116, 23838.
11. S. Acharya, D. D. Sarma, N. R. Jana and N. Pradhan, J. Phys. Chem. Lett., 2009, 1, 485.
12. S. Jana, B. B. Srivastava, S. Jana, R. Bose and N. Pradhan, J. Phys. Chem. Lett., 2012, 3, 2535.
13. J. Zheng, X. Yuan, M. Ikezawa, P. Jing, X. Liu, Z. Zheng, X. Kong, J. Zhao and Y. Masumoto, J. Phys. Chem. C, 2009, 113, 16969.
14. S. Nizamoglu, T. Erdem, X. W. Sun and H. V. Demir, Opt. Lett., 2010, 35, 3372.
15. P. Gu, Y. Zhang, Y. Feng, T. Zhang, H. Chu, T. Cui, Y. Wang, J. Zhao and W. W. Yu, Nanoscale, 2013, 5, 10481.
16. B. Lin, X. Yao, Y. Zhu, J. Shen, X. Yang, H. Jiang and X. Zhang, New J. Chem., 2013, 37, 3076.
17. P. V. Kamat, Acc. Chem. Res., 2012, 45, 1906.
18. J. Luo, H. Wei, Q. Huang, X. Hu, H. Zhao, R. Yu, D. Li, Y. Luo and Q. Meng, Chem. Comm., 2013, 49, 3881.
19. R. Zeng, M. Rutherford, R. Xie, B. Zou and X. Peng, Chem. Mater., 2010, 22, 2107.
20. J. Lin, Q. Zhang, L. Wang, X. Liu, W. Yan, T. Wu, X. Bu and P. Feng, J. Am. Chem. Soc., 2014, 136, 4769.
21. F. Huang, J. Zhou, J. Xu and Y. Wang, Nanoscale, 2014, 6, 2340.
22. A. Hazarika, A. Layek, S. De, A. Nag, S. Debnath, P. Mahadevan, A. Chowdhury and D. D. Sarma, Phys. Rev. Lett., 2013, 110, 267401.
23. R. Beaulac, P. I. Archer, X. Liu, S. Lee, G. M. Salley, M. Dobrowolska, J. K. Furdyna and D. R. Gamelin, Nano Lett., 2008, 8, 1197.
24. V. A. Vlaskin, N. Janssen, J. V. Rijssel, R. Beaulac and D. R. Gamelin, Nano Lett., 2010, 10, 3670.
25. C. E. Rowland, W. Liu, D. C. Hannah, M. K. Y. Chan, D. V. Talapin and R. D. Schaller, ACS Nano, 2014, 8, 977.
26. Y. Zhao, C. Riemersma, F. Pietra, R. Koole, C. M. Donega and A. Meijerink, ACS Nano, 2011, 6, 9058.
27. Q. Dai, Y. Song, D. Li, H. Chen, S. Kan, B. Zou, Y. Wang, Y. Deng, Y. Hou, S. Yu, L. Chen, B. Liu and G. Zou, Chem. Phys. Lett., 2007, 439, 65.
28. I. J. Kramer and E. H. Sargent, Chem. Rev., 2014, 114, 863.
29. N. D. Bronstein, L. Li, L. Xu, Y. Yao, V. E. Ferry, A. P. Alivisators and R. G. Nuzzo, ACS Nano, 2014, 8, 44.
30. X. Yuan, J. Zheng, R. Zeng, P. Jing, W. Ji, J. Zhao, W. Yang and H. Li, Nanoscale, 2014, 6, 300.
31. B. T. Diroll and C. B. Murray, ACS Nano, 2014, 8, 6466.
32. D. Norris, N. Yao, F. Charnock and T. Kennedy, Nano Lett., 2001, 1, 3.
33. M. Tanaka and Y. Masumoto, Chem. Phys. Lett., 2000, 324, 249.
34. W. Chen, A. G. Joly, J.-O. Malm, J.-O. Bovin and S. Wang, J. Phys. Chem. B, 2003, 107, 6544.
35. W. Zhang, Y. Li, H. Zhang, X. Zhou and X. Zhong, Inorg. Chem., 2011, 50, 10432.
36. R. Zeng, T. Zhang, G. Dai and B. Zou, J. Phys. Chem. C, 2011, 115, 3005.
37. S. Cao, J. Zheng, J. Zhao, L. Wang, F. Gao, G. Wei, R. Zeng, L. Tian and W. Yang, J. Mater. Chem. C, 2013, 1, 2540.
38. X. Yu, X. Peng, Z. Chen, C. Lian, X. Su, J. Li, M. Li, B. Liu and Q. Wang, Appl. Phys. Lett., 2010, 96, 123104.
39. G. Morello, M. D. Giorgi, S. Kudera, L. Manna, R. Cingolani and M. Anni, J. Phys. Chem. C, 2007, 111, 5846.
40. D. Valerini, A. Creti, M. Lomascolo, L. Manna, R. Cingolani and M. Anni, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 235409.
41. J. Suyver, J. Kelly and A. Meijerink, J. Lumin., 2003, 104, 187.
42. J. MacKay, W. Becker, J. Spaek and U. Debska, Phys. Rev. B: Condens. Matter Mater. Phys., 1990, 42, 1743.
43. A. Nag, S. Chakraborty and D. D. Sarma, J. Am. Chem. Soc., 2008, 130, 10605.
44. H.-Y. Chen and D. H. Son, Isr. J. Chem., 2012, 52, 1016.
45. K. P. Kadlag, M. J. Rao and A. Nag, J. Phys. Chem. Lett., 2013, 4, 1676.

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Footnote

† Electronic supplementary information (ESI) available: The representative synthesis procedure for the Mn:ZnCdS QDs; typical TEM images for MnS/ZnS QDs without CdS shells and the Mn:ZnCdS QDs with thin shell thicknesses; calculation of the ZnS buffer layer thicknesses; PL quantum yield (QY) measurements. See DOI: 10.1039/c4ra04402a

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