Enhanced photoluminescence and thermal stability of zinc quinolate following complexation on the surface of quantum dots

Abstract

Reaction between colloidal ZnS nanocrystals (NCs) and 8-hydroxy quinoline (HQ) led to complexation on the surface of the NCs. The quantum dot complex (QDC), with ZnQ₂ attached to the surface of the NC, has a longer emissive lifetime, higher fluorescence quantum yield and enhanced thermal stability, making it a better LED material than ZnQ₂.

---

Interaction of colloidal quantum dots (Qdots) with chemically reactive species – being either functionalized on the surface or as independent entities – can lead to reversible and irreversible changes in their optical properties. This could involve electron, hole or energy transfer between the electronic state of the Qdot or the dopant in a doped Qdot and the internal or external reactive species.^1,2 A host of inorganic complexes, organic and biomolecules have been bridged to Qdots via organic or biomolecular ligands, which have been shown to be useful in biosensing, bioassay, photovoltaics and CO₂ conversion – based on their mutual chemistry.^3–7 Thus surface structure and composition of Qdots are as important as the core in determining their physical properties. For example, dangling bonds and surface trap states reduce the quantum efficiency of photoluminescence, which are addressed by passivation using organic and inorganic moieties. Additionally, luminescence quantum yield (QY) of Qdots can be improved via exchange of molecular and atomic ligands, following synthesis, thus allowing for applications in diverse environment and in targeted delivery.^8,9 On the other hand, the smallness of size and high surface free energy of Qdots make cation exchange facile, leading to generation of new crystals and crystals with complex morphology; whereas partial ion exchange can lead to superlattice formation.^10,11 Further, the cations present on the surface could be systematically removed or replaced leading to change in optical properties.^12,13

However, an important chemistry that received virtually no attention is reaction between Qdot and organic ligand, leading to new structure or species formation involving the Qdot. An ideal example would be complexation on the surface of the Qdot following reaction between the surface cation and suitable ligand. This may confer newer property to the Qdot, the complex or both. It is important to mention here that there are literature reports of incorporation of chelating ligand – at least partially – through exchanges with stabilizers. This method has been used for enhancement of optical properties of Qdots. However, generation of metal complexes on the surface of Qdots and their implications have not been clearly demonstrated.^14–18 We report that reaction between ZnS Qdot and 8-hydroxyquinoline (HQ) at room temperature led to formation of a new species – referred to herein as quantum dot complex (QDC) – involving Zn²⁺ and HQ on the surface of the Qdot. In that process, while the fluorescence of the Qdot was completely quenched, a new fluorescence attributable to the formation of the complex occurred. Interestingly, similar fluorescence could be observed when ZnQ₂ complex was added to the Qdot, possibly resulting in similar complex formation. Further investigations revealed significantly enhanced thermal stability of the QDCs over ZnQ₂, with retention of fluorescence in the former following heating at an elevated temperature.

Importantly, ZnQ₂ has been a preferred important inorganic compound for LED applications for its emissivity and stability.^19–22 Also, AlQ₃ is chosen for its excellent stability and high electroluminescence QY.^23–25 Recent literature suggest that Qdots with high QY could potentially be used for light emitting devices.²⁶ All of the above materials, although work well with reasonable QY there are drawbacks such as processing, thermal and optical stability, blinking and chemical degradation.^19–26 In that respect QDC may be able to overcome at least some of these problems and thus may become a material of preference, especially in light emitting applications.

Experimentally, the UV-vis absorption spectrum of cysteine stabilized 3.2 nm ZnS Qdots consisted of a peak at 308 nm, as is represented in Fig. 1A(a). The absorption spectrum of HQ consisted of a peak at 316 nm (Fig. 1A(b)). Further, the absorption spectrum of ZnQ₂ consisted of a peak at 380 nm (Fig. 1A(c)). On the other hand, when an aqueous dispersion of cysteine-stabilized 3.2 nm ZnS Qdots was treated with HQ (in MeOH), a second peak in the UV-vis absorption spectrum appeared at 361 nm, in addition to the original peak due to the Qdots at 308 nm (Fig. 1A(d)). The absorption spectrum of ZnQ₂-added Qdots (Fig. 1A(e)) had the same feature as that of HQ-added Qdots. The appearance of a new peak is indicative of interaction between the Qdots and HQ as well as ZnQ₂, leading to the formation of a new species. The identical absorption spectral peak in both the cases pointed toward formation of the same species. Further, when the samples were centrifuged and redispersed in water the absorption spectra remained the same, indicating that indeed there was product formation out of a reaction between the Qdots and HQ as well as ZnQ₂ (Fig. S1,†ESI).

Fig. 1 (A) UV-vis and (B) emission spectra of (a) cysteine capped ZnS Qdots (pH-6.6); (b) HQ; (c) ZnQ₂; (d) HQ added ZnS Qdots (pH-6.6) and (e) ZnQ₂ added ZnS Qdots (pH-6.6). For the emission spectral measurements, the excitation wavelength was set at 322 nm (Qdots) and 361 nm (others).

Fluorescence spectrum of HQ-treated Qdots consisted of a single peak at 500 nm with QY of 3.2%, when excited at 361 nm and that of 0.16% following excitation at 322 nm. ZnQ₂ treated Qdots had the same spectrum with QY of 3.0% and 0.14%, under excitation at 361 nm and 308 nm, respectively. Fluorescence spectrum of ZnQ₂ consisted of a peak at 550 nm (QY = 0.9%), while that of HQ had a weak peak at 520 nm (QY = 0.2%), both of them being excited at 361 nm (Fig. 1B and Table S1,†ESI). Further, when the dispersions were centrifuged and the solids so obtained were redispersed similar fluorescence spectra were obtained for the samples (Fig. S2,†ESI). Fluorescence excitation spectrum of the HQ as well as ZnQ₂ treated Qdots consisted of a single peak with maximum at 361 nm (Fig. S3,†ESI).

The above results indicated that interaction between HQ and ZnS Qdot led to the reaction resulting in possible complexation on the surface of the Qdot. Thus while the fluorescence due to the Qdot was quenched, the new fluorescence could be attributed to the formation of a complex on the Qdot (Fig. S4,†ESI). That addition of ZnQ₂ to Qdot also resulted in similar spectral behaviour further supported the formation of a complex on the surface, as ZnQ₂ upon functionalizing the surface could have produced similar structure. Control experiment with addition of water to the methanol medium containing ZnQ₂ complex resulted in weakening of emission without change in the peak wavelength, discounting the role of water only in changing the emission spectrum (Fig. S5,†ESI). The significant blue shift observed in the emission spectrum in the presence of Qdots could be due to the presence of S²⁻ ions on the surface of the Qdots. Moreover, there was no change in pH, during formation of QDCs from Qdots (pH 6.6), which ruled out the possibility of pH effect on the emission properties of ZnQ₂ complex. This was further substantiated by the observation that when solid Na₂S was added to ZnQ₂ (in MeOH) the peak at 550 nm blue-shifted to 525 nm (Fig. S6A,†ESI). The absorption spectrum was also blue-shifted from 380 nm to 368 nm in the presence of Na₂S (Fig. S6B,†ESI). It is important to mention here that when all of the above experiments were carried out with ZnS Qdots, which were synthesized without the use of any stabilizer, the results were similar, indicating that interactions between the Qdots and HQ or ZnQ₂ indeed led to complexation on the surface (Fig. S7,†ESI). Further, addition of benzoquinone (BQ) – an electron quencher – to ZnS Qdots led to the quenching of luminescence, which was recovered following addition of HQ. On the other hand, when BQ was added to QDC generated from either of the reaction there was no change in fluorescence (Fig. S8,†ESI). Further, BQ had no influence on the fluorescence of ZnQ₂ (Fig. S9,†ESI). The results clearly support the formation of a stable fluorescent complex on the Qdot, notwithstanding the presence of a quencher (BQ) of the Qdot. Time-resolved fluorescence measurements indicated that the average life-time of fluorescence increased under excitation at 375 nm (with respect to ZnQ₂ complex) and decreased under excitation at 308 nm (with respect to Qdots) following complexation (Fig. S10,†ESI). For example, the life-time for ZnQ₂ was found to be 2.5 ns, whereas HQ treated and ZnQ₂ treated samples had average life-time of 10.5 s and 11.5 ns respectively with excitation at 375 nm while reverse was observed with excitation at 308 nm (Table S2,†ESI). The increase in average life-time was commensurate with increase in QY as mentioned above. In addition, enhanced photostabilty of the QDCs formed from either of the reactions, with a fluorescence decrease rate (I/I₀) of 0.003% s⁻¹ versus 0.013% s⁻¹ for rhodamine 6G dye, indicated superior application potential of the new material (Fig. S11,†ESI). It may further be mentioned here that photostabilty of the QDC was as good as that of ZnQ₂ (0.004% s⁻¹), indicating preservation of photochemical stability of the complex even when present on the surface of the Qdot. (Table S3,†ESI).

The formation of Zn²⁺–quinolato complex on the surface of the Qdots was further substantiated by FTIR spectroscopic results. The presence of C–C/C–N stretching, C–H bending peaks at 1605, 1577, 1500, 1468 and 1322 cm⁻¹, C–H out plane wagging at 822, 800 and 742 cm⁻¹ and in-plane ring deformation at 742, 642 and 605 cm⁻¹ in the QDCs indicated that HQ was successfully coordinated to the surface Zn²⁺ ion or ZnQ₂ was attached to the surface. The intensity ratio of the two prominent bands at 3333 cm⁻¹ and 1110 cm⁻¹ was measured to be 0.7, pointing toward the formation of a complex similar to the dihyrdate complex (Fig. S12 and expanded FTIR spectra in Fig. S26–S30 and Tables S4 and S5,†ESI). However, considering the observed role of surface S²⁻ ions a proposal could be made where a water molecule of the hexa-coordinated complex is replaced by S²⁻ ions, which is simultaneously bound to the surface of the Qdot via dangling bond.

Thermo-gravimetric analysis (TGA) showed weight loss (15–20%) for Qdots and QDCs up to 95 °C and there was no further change till 850 °C. These weight losses could be due to loss of surface adsorbed water. On the other hand, ZnQ₂ exhibited two weight losses – one at 120 °C, while the other major one at 450 °C, which could be due to decomposition of the sample. Differential scanning calorimetry (DSC) studies similarly showed that the major endothermic change observed for ZnQ₂ at 360 °C was absent for QDC obtained from both the sources, indicating superior stability of the complex on the surface of the Qdots (Fig. 2). It is plausible that the peak at 360 °C is due to melting of the ZnQ₂ crystal which was absent in the QDC, also supporting superior thermal stability of the complex when incorporated in the Qdots.

Fig. 2 (A) Thermo-gravimetric and (B) differential scanning calorimetric analyses of (a) ZnQ₂; (b) cysteine capped ZnS Qdots; (c) HQ added ZnS Qdots and (d) ZnQ₂ added ZnS Qdots.

The powder X-ray diffraction (XRD) pattern and high resolution transmission electron microscopy (HRTEM) analyses of QDCs generated from both the sources had the characteristics of as synthesized Qdots, which indicated that there was no significant structural changes of cubic ZnS Qdots (Fig. S13 and S14,†ESI). Further, TEM analysis results (Fig. S15,†ESI) revealed that the average particle size of the spherical structures remained unaffected following complexation.

A schematic representation in Fig. 3 captures the proposed formation of QDC, based on the reaction of Qdot with either HQ or ZnQ₂·2H₂O. The surface of Qdot is expected to have sufficient concentrations of Zn²⁺ and S²⁻ ions bound to the crystal via dangling bonds. Such Zn²⁺ ions would preferentially react with HQ forming QDC. In the process of complex formation, the ions may be removed from the surface and the whole complex would bind on the surface instead, also acting as a stabilizer for the Qdot. The facile removal of Zn²⁺ ions is commensurate with the literature reports of surface ion removal and ion-exchange reactions as mentioned above. The S²⁻ ions present on the surface may bind with ZnQ₂ along with another water molecule in the opposite axial position forming an octahedral complex on the surface. Similarly, ZnQ₂·2H₂O may lose one coordinating water molecule and bind with S²⁻ in order to form similar complex on the surface. The octahedral ZnQ₂·2H₂O complex with two water molecules in axial positions is stable at room temperature. Here, in both the QDCs, one of the axial positions of the complex may be occupied by S²⁻ – replacing a water molecule, while the sulphide ion itself would be bound to the Qdot through dangling bond, providing stability to the complex and the Qdot as well in exchange. On the other hand, the HOMO–LUMO band-gap energy of 3.26 eV of ZnQ₂.2H₂O changed to 3.43 eV in QDC formed from both the reactions. The role of S²⁻ in tuning of the energy gap of the complex is established by the observation of increase in the gap to 3.37 eV in the presence of the anion. The observation of significant change in the energy gap following the formation of QDC indicates role of Qdot in tuning the energy level of the complex. Further, the presence of a single excitation spectrum of QDC with the peak at 361 nm indicated that the formation of the complex led to quenching of fluorescence due to Qdot, while the new fluorescence is entirely due to the complex present on the surface of the Qdot, even though the absorption peak due to the Qdot was retained in the QDC.

Fig. 3 Schematic representation of quantum dot complex (QDC) formation on the surface of cysteine capped as well as uncapped ZnS Qdot following its reaction with either HQ (ligand) or ZnQ₂ (complex).

The octahedral ZnQ₂·2H₂O has its two axial positions occupied by H₂O. On the other hand, when the complex is attached to the surface of the Qdot, it is proposed that one of the H₂O molecules is exchanged with bonding to a surface dangling sulphide ion. This provides not only chemical stability to the complex but also change in its optical property such as blue shift of the emission maximum and increase in QY and thermal stability. This conclusion is further supported by the observation that when Na₂S was added to the ZnQ₂·2H₂O (in methanol) there was a blue shift in emission maximum accompanied by an enhancement in QY.

Finally, ZnQ₂·2H₂O is popular as an active material for light emitting diodes (LEDs).^19–22 Thus any potential application of the new QDC would require ease of processing and stability. We observed that the QDCs generated from both the processes could be turned into solid powder with the retention of their properties. Fig. 4A shows the optical micrographs of the powders of ZnQ₂·2H₂O and QDCs, which are brown under visible light and green under UV light, the characteristic fluorescent colour of the complex. In contrast, the colour of solid powder of the Qdots appeared colourless under white light and blue under UV light (Fig. S16,†ESI). More importantly, the green fluorescent colour of the free complex vanished upon heating for 10 min at 370 °C. On the other hand, the crystalline solid form with retention of fluorescence of QDCs could clearly be observed, indicating their superior thermal stability (Fig. 4B). It may be mentioned here that when the heat-treated solids were redispersed in methanol, the fluorescence spectra of the QDCs were retained (Fig. S17,†ESI); however, the fluorescence spectrum of the dispersion of the solid, obtained from heat treated complex, did not exhibit any characteristic fluorescence indicating loss of the structure.

Fig. 4 Microscopic images of solid samples (A) before and (B) after heating (at 370 °C) in presence of (1) white and (2) UV light (λ_ex = 350 nm); (a) ZnQ₂, (b) HQ added ZnS Qdots and (c) ZnQ₂ added ZnS Qdots.

In conclusion, a new material following complexation of Qdots leading to the formation of ZnQ₂ species on the surface has been reported. The high fluorescence QY, longer fluorescence lifetime and enhanced thermal stability of the QDCs are important indicators of their potential in light emitting and other applications. The role of surface ions in the formation and stability of the complex brings out a new chemistry which may usher in a new approach to inorganic complexes. Our observations of formation of similar complexes from reaction of HQ with Zn_xCd_1−xS and Mn²⁺ doped ZnS nanocrystals indicated generality of the approach and future prospects of a new field.

Author contributions

SB performed the experiments, contributed ideas and wrote the paper. SR performed the experiments, contributed ideas. AC conceived the idea, helped implement the experiments and wrote the paper.

Acknowledgements

We thank the Department of Biotechnology and Department of Science and Technology, Government of India for fund and development of facility. Assistances from CIF, IIT Guwahati, Rumi Khandelia, Ashim Malakar, Dr Devashis Chowdhury and Amaresh Kumar Sahoo are acknowledged. S.B. thanks the CSIR for fellowship (09/731(0115)/2011-EMR-I).

Notes and references

1. C. Wang, M. Shim and P. Guyot-Sionnest, Science, 2001, 291, 2390.
2. M. Shim and P. Guyot-Sionnest, Nature, 2000, 407, 981.
3. I. L. Medintz, T. Pons, S. A. Trammell, A. F. Grimes, D. S. English, J. B. Balnco-Canosa, P. E. Dawson and H. Mattoussi, J. Am. Chem. Soc., 2008, 130, 16745.
4. M. H. Stewart, A. L. Huston, A. M. Scott, A. L. Efros, J. S. Melinger, K. B. Gemmill, S. A. Trammell, J. B. Balnco-Canosa, P. E. Dawson and I. L. Medintz, ACS Nano, 2012, 6, 5330.
5. W. MA, L. Qin, F. Liu, Z. Gu, J. Wang, Z. G. Pan, T. D. James and Y. Long, Sci. Rep., 2013, 3, 1537.
6. S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka and T. Kajino, J. Am. Chem. Soc., 2011, 133, 15240.
7. I. L. Medintz, M. H. Stewart, S. A. Trammell, K. Susumu, J. B. Delehanty, B. C. Mei, J. S. Melinger, J. B. Balnco-Canosa, P. E. Dawson and H. Mattoussi, Nat. Mater., 2010, 9, 676.
8. J. Tang, K. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury and E. H. Sargent, Nat. Mater., 2011, 10, 765.
9. Z. Lingley, S. Lu and A. Madhukar, Nano Lett., 2011, 11, 2887.
10. D. H. Son, S. M. Hughes, Y. Yin and A. P. Alivisatos, Science, 2004, 306, 1009.
11. R. D. Robinson, B. Sadtler, D. O. Demchenko, C. K. Erdonmez, L. W. Wang and A. P. Alivisatos, Science, 2007, 317, 355.
12. R. Begum, S. Bhandari and A. Chattopadhyay, Langmuir, 2012, 28, 9722.
13. S. Bhandari, R. Begum and A. Chattopadhyay, RSC Adv., 2013, 3, 2885.
14. C. Lu, J. Gao, Y. Fu, Y. Du, Y. Shi and Z. Su, Adv. Funct. Mater., 2008, 18, 3070.
15. D. S. Bohle and C. J. Spina, J. Phys. Chem. C, 2009, 113, 14435.
16. L. Feng, C. Wang, Z. Ma and C. Lu, Dyes Pigm., 2013, 97, 84.
17. B. Liu, X. Lu, C. Tong, C. Wang, L. Feng, Y. He and C. Lu, RSC Adv., 2013, 3, 21298.
18. X. Lu, J. Yang, Y. Fu, Q. Liu, B. Qi, C. Lu and Z. Su, Nanotechnology, 2010, 21, 115702.
19. S. Sapochak, F. E. Benincasa, R. S. Schofield, J. L. Baker, K. K. C. Riccio, D. Fogarty, H. Kohlmann, K. F. Ferris and P. E. Burrows, J. Am. Chem. Soc., 2002, 124, 6119.
20. T. A. Hopkins, K. Meerholz, S. Shaheen, M. L. Anderson, A. Schimdt, B. Kippelen, A. B. Padias, H. K. Hall, N. Peyghambarian and N. R. Amstrong, Chem. Mater., 1996, 19, 344.
21. Y. Hamada, T. Sano, N. Fujita, T. Fujii, Y. Nishio and K. Shibata, Jpn. J. Appl. Phys., 1993, 32, L514.
22. N. Donze, P. Pechy, M. Gratzel, M. Schaer and L. Zuppiroli, Chem. Phys. Lett., 1999, 315, 405.
23. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
24. Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest and M. E. Thompson, Science, 1997, 276, 2009.
25. H. Aziz, Z. D. Popovic, N. Hu, A. Hor and G. Xu, Science, 1999, 283, 1900.
26. A. L. Rogach, N. Gaponik, J. M. Lupton, C. Bertoni, D. E. Gallardo, S. Dunn, N. L. Pira, M. Paderi, P. Repetto, S. G. Romanov, C. O'Dwyer, C. M. S. Torres and A. Eychmuller, Angew. Chem., Int. Ed., 2008, 47, 6538.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental Section, characterization, Fig. S1–S30, Tables S1–S5 and an additional explanation for expanded FTIR graphs. See DOI: 10.1039/c4ra03341h

---