Two-Step “Seed-Mediated” Synthetic Approach to Colloidal Indium

Communication Cite This: Chem. Mater. 2018, 30, 3643−3647

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Two-Step “Seed-Mediated” Synthetic Approach to Colloidal Indium Phosphide Quantum Dots with High-Purity Photo- and Electroluminescence Parthiban Ramasamy,†,§ Keum-Jin Ko,‡,§ Jae-Wook Kang,‡ and Jong-Soo Lee*,† †

Department of Energy Science and Engineering, DGIST, Daegu 42988, Republic of Korea Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 54896, Republic of Korea

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‡

S Supporting Information *

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to synthesize larger QDs. Recently, Franke et al. successfully demonstrated the possibility to tune the size of InAs QDs with narrow size distribution by continuous injection synthesis and later tried to replicate the results for InP QDs but ended up with limited success.20 In this Communication, we present a two-step approach to synthesize highly monodisperse InP QDs (550 nm). In addition, the method was laborious and took nearly 7 h © 2018 American Chemical Society

Scheme 1. Schematic of “Seed-Mediated” Synthesis of Larger InP QDs and InP/ZnSe/ZnS QDs

continuously injecting Zn(In)-P complexes (Figure S3) as monomers source. Oleic acid capped InP QD “seeds” were synthesized following our previously reported method with minor modifications.1 The InP “seeds” have a decent size distribution with an average size of 1.9 nm, as evident from transmission electron microscopy (TEM) image and absorption spectrum (Figure S4a,b). InP QDs are known for their high sensitivity toward oxidation to form oxide or hydroxide covered surface and the presence of oxidized surface would be detrimental for further particle growth. We used X-ray photoelectron spectroscopy (XPS) to study the oxidation of phosphorus in the synthesized InP “seeds”, and results are shown in Figure S4c,d. The absence of oxidized phosphorus signal (∼135 eV) in Figure S4d shows the oxide free surface of the “seed” QDs. Next, the injection solution (Zn(In)-P complexes) was prepared by reacting Zn(In)-oleate with P(SiMe3)3 in 1-octadecene (ODE) at room temperature for 1 Received: May 15, 2018 Revised: May 25, 2018 Published: May 25, 2018 3643

DOI: 10.1021/acs.chemmater.8b02049 Chem. Mater. 2018, 30, 3643−3647

Communication

Chemistry of Materials

Figure 1. Optical and structural characterization of InP QDs grown by two-step synthesis. (a) UV−vis absorption spectra of differently sized InP QDs synthesized by externally supplying the monomers to the InP “seeds” (at 0.1 mmol/h, temperature 280 °C). The monomers concentrations were calculated based on initial indium amount. (b) Absorption peak position and relative size distribution represented by the HWHM of the first excitonic peak of InP QDs. (c) Change in InP QDs size and absorbance at 310 nm during the growth. (d) XRD patterns of InP “seeds” and 4.5 nm InP QDs. (e and f) TEM images and size distribution histograms of the InP QDs, Scale bar 50 nm.

Avoiding secondary nucleation events is the key to synthesis size tunable QDs. To check whether all precursor material added is incorporated solely into the growing InP QDs, we compared the change in QDs size with absorbance at 310 nm at different time of the synthesis. At a shorter wavelength region (310 nm), the extinction coefficient is relatively independent of the particle size and gives information about InP concentration in all the forms in the solution. As shown in Figure 1c, the QDs size increases linearly with absorbance at 310 nm, confirming that the injected precursors were exclusively added to the existing InP QDs and rules out secondary nucleation events that would have hindered the QDs growth and broadened the absorption spectra. The broad X-ray diffraction (XRD) peaks of the “seed” QDs became sharp after the reaction, which is characteristics of change in size (Figure 1d). TEM images also confirm the formation of monodisperse QDs by our method (Figure 1e,f). In the initial stages of the synthesis, the QDs maintained spherical shape of the “seeds” with size distribution less than 5%; however, at the end of the reaction, the shape changed from spherical to more tetrahedral with an increase in size distribution (9%). Next, we studied the effects of various reaction parameters such as role of “seed” QDs, injection speed and growth temperature in producing size tunable InP QDs with narrow size distribution. When the reaction was carried out in the absence of “seed” QDs (Zn(In)-P complexes were slowly injected to hot ODE), we observed the growth of larger particles; however, the size distribution was very broad (Figure S7). To probe the effect of precursor addition rate on the

h. The precursor conversion was monitored using absorption and 1H nuclear magnetic resonance spectroscopy and confirmed the complete depletion of P(SiMe3)3 to form Zn(In)-P complexes (Figure S5). To grow larger InP QDs, Zn(In)-P complexes were taken in a syringe pump and slowly injected to an ODE solution containing InP QD “seeds”. Figure 1a shows the absorption spectra of differently sized InP QDs synthesized by slow and continuous injection (0.1 mmol/h) at 280 °C. Injection of Zn(In)-P complexes resulted in continuous growth of InP “seeds” with the absorption spectrum shifting from 490 to 650 nm, corresponds to a size increase from 1.9 to 4.5 nm. Besides the growth, we also observed significant improvement in the size distribution with absorption half-width at half-maximum (HWHM) decreases from initial value of 0.14 to 0.09 eV after 150 min of reaction (Figure 1b). As the reaction proceeds further, slight increase in HWHM value was observed. Absorption spectra in Figure S6 show that the size distribution obtained with our method is much superior to previously reported synthesis methods. By supplying the monomers continuously using syringe pump, our approach helps us to maintain constant monomer concentration in the growth solution and thereby produce size tunable InP QDs with narrow size distribution. In addition, the final QDs size is directly related to amount of precursors externally added at a particular growth temperature. This gives us a great control over the particle size, and by simply stopping the precursor supply we can isolate QDs with first excitonic peak difference of as low as 5 nm. 3644

DOI: 10.1021/acs.chemmater.8b02049 Chem. Mater. 2018, 30, 3643−3647

Communication

Chemistry of Materials growth of InP QDs, we varied the Zn(In)-P complexes injection rate from 0.05 to 0.3 mmol/h. Figures S8 and S9 show that injection rate plays a key role in determining the final size and size distribution. Slower growth with broader size distribution was observed for faster addition rates. This is attributed to the increased precursor concentration in the solution which leads to continuous and/or secondary nucleation events. TEM image in Figure S10 also confirms the presence of smaller QDs with larger one. We also found that the reaction temperature has more pronounced effect on the growth of the particles. When the reaction was carried out at temperatures less than 250 °C, very little growth of the InP “seeds” was observed (Figure S11a,b). Initially, we thought this might be due to the stability of Zn(In)-P complexes, which decompose slowly to produce indium and phosphorus monomers. However, when we measured the total concentration of InP in the growth solution, it increased linearly with time for all the reaction temperature, confirming the formation of new InP units (Figure S11c). A possible explanation for the suppressed growth at lower temperatures can be given based on the tight binding and high coverage of the oleate ligands on the surface of the InP QDs. For any crystal to grow continuously, monomers need to diffuse toward the surface and react on the surface of the growing crystal. However, the higher In−O bonding energy makes the oleate ligands less dynamic at lower temperatures and blocks the accessibility of monomers to the surface of growing InP “seeds”. As a result, the monomers start to nucleate separately in solution and this continuous nucleation events inhibited the growth and broadened the absorption spectrum of “seed” QDs. The as synthesized InP QDs exhibit very weak luminescence, hence the true benefit of our synthetic method can only be realized by synthesizing highly luminescent core−shell QDs with narrow emission fwhm. We synthesized InP/ZnSe/ZnS QDs that employ InP cores synthesized via our “seedmediated” synthetic approach. After forming the shell, the QDs exhibited bright luminescence that can be tuned over most of the visible window (Figure. 2). Most interestingly, the

Table 1. Selected FWHM and QY Values of InP/ZnSe/ZnS QDs PL peak (nm) fwhm (nm) QY (%)

533 37 65 ± 2

550 36 60 ± 3

580 36 58 ± 4

610 37 55 ± 2

625 39 45 ± 3

(HAADF) scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) elemental mapping, EDX spectrum of our red emitting core−shell InP/ZnSe/ZnS QDs are provided in Figures S12−S14. To demonstrate the potential of our QDs in next-generation display devices, we fabricated green and red quantum dot LED (QLED) devices (Figure 3a) with planar structure using spin-

Figure 3. (a) Schematic illustration of InP/ZnSe/ZnS based QLEDs structure. (b) Current density and luminance vs voltage of QLEDs. (c) PL spectra of QDs in solution, EL spectra and photographs of the QLEDs at 0.1 mA. (d) CIE coordinate of the QLEDs EL spectra under 0.1 mA.

coating and thermal evaporation processes. The J−V−L characteristics of both the green and red QLEDs are shown in Figure 3b. The green QLED exhibited turn-on voltage of 4.6 V and a maximum luminescence of ∼100 cd m−2 with a current efficiency of 0.07 cd A−1. The red QLED showed turn-on voltage of 3.7 V and a maximum luminescence of ∼230 cd m−2 with a current efficiency of 0.85 cd A−1. Figure 3c shows the EL spectra and the photograph of the green and red QLEDs operated at 0.1 mA. The green and red EL emission wavelengths were 532 and 628 nm with fwhm of 42 and 50 nm, respectively. The EL fwhm values are much superior than that of InP QDs-based LEDs reported previously (Table S1). The EL spectra showed red-shift and broadening compared with the PL spectra of QDs in solution, which is attributed to the interdot interactions in close packed solid films, dielectric dispersion and the electric-field-induced Stark effect. The Stark effect is also responsible for the Stark broadening of spectral lines by charged particles.21−24 The Commission Internationale de l’Eclairage (CIE1931) color coordinates of (0.293, 0.638), and (0.679, 0.317), corresponding to the emission peaks of 532, and 628 nm, respectively, are marked in Figure 3d. In conclusion, we have demonstrated a “seed-mediated” twostep synthesis method to achieve great control over the size and

Figure 2. Optical characterizations of InP/ZnSe/ZnS core−shell QDs. (a) Absorption and (b) photoluminescence spectra and photograph under UV illumination of the InP/ZnSe/ZnS QDs synthesized using differently sized InP QDs grown by “seed” mediated synthesis.

emission fwhm values were less than 40 nm for all emission wavelengths (Table 1). In particular, the green (533 nm) and red (625 nm) emitting QDs have an emission fwhm of 37 and 39 nm and QYs of 65 and 45%, respectively. To the best of our knowledge, the synthesis of size tunable InP QDs with such high color purity has never been reported previously. Detailed structural analysis such as XRD, high angle annular dark field 3645

DOI: 10.1021/acs.chemmater.8b02049 Chem. Mater. 2018, 30, 3643−3647

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Chemistry of Materials

(3) Koh, S.; Eom, T.; Kim, W. D.; Lee, K.; Lee, D.; Lee, Y. K.; Kim, H.; Bae, W. K.; Lee, D. C. Zinc−Phosphorus Complex Working as an Atomic Valve for Colloidal Growth of Monodisperse Indium Phosphide Quantum Dots. Chem. Mater. 2017, 29, 6346−6355. (4) Baquero, E. A.; Virieux, H.; Swain, R. A.; Gillet, A.; CrosGagneux, A.; Coppel, Y.; Chaudret, B.; Nayral, C.; Delpech, F. Synthesis of Oxide-Free InP Quantum Dots: Surface Control and H2Assisted Growth. Chem. Mater. 2017, 29, 9623−9627. (5) Bang, E.; Choi, Y.; Cho, J.; Suh, Y.-H.; Ban, H. W.; Son, J. S.; Park, J. Large-Scale Synthesis of Highly Luminescent InP@ZnS Quantum Dots Using Elemental Phosphorus Precursor. Chem. Mater. 2017, 29, 4236−4243. (6) Pietra, F.; De Trizio, L.; Hoekstra, A. W.; Renaud, N.; Prato, M.; Grozema, F. C.; Baesjou, P. J.; Koole, R.; Manna, L.; Houtepen, A. J. Tuning the Lattice Parameter of InxZnyP for Highly Luminescent Lattice-Matched Core/Shell Quantum Dots. ACS Nano 2016, 10, 4754−4762. (7) Altıntas, Y.; Talpur, M. Y.; Ü nlü, M.; Mutlugün, E. Highly Efficient Cd-Free Alloyed Core/Shell Quantum Dots with Optimized Precursor Concentrations. J. Phys. Chem. C 2016, 120, 7885−7892. (8) Park, J. P.; Lee, J.-J.; Kim, S.-W. Highly Luminescent Inp/Gap/ Zns Qds Emitting in the Entire Color Range via a Heating Up Process. Sci. Rep. 2016, 6, 30094. (9) Tessier, M. D.; Dupont, D.; De Nolf, K.; De Roo, J.; Hens, Z. Economic and Size-Tunable Synthesis of InP/ZnE (E = S, Se) Colloidal Quantum Dots. Chem. Mater. 2015, 27, 4893−4898. (10) Kim, S.; Kim, T.; Kang, M.; Kwak, S. K.; Yoo, T. W.; Park, L. S.; Yang, I.; Hwang, S.; Lee, J. E.; Kim, S. K.; Kim, S.-W. Highly Luminescent InP/GaP/ZnS Nanocrystals and Their Application to White Light-Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 3804− 3809. (11) Yang, X.; Zhao, D.; Leck, K. S.; Tan, S. T.; Tang, Y. X.; Zhao, J.; Demir, H. V.; Sun, X. W. Full Visible Range Covering InP/ZnS Nanocrystals with High Photometric Performance and Their Application to White Quantum Dot Light-Emitting Diodes. Adv. Mater. 2012, 24, 4180−4185. (12) Allen, P. M.; Walker, B. J.; Bawendi, M. G. Mechanistic Insights into the Formation of InP Quantum Dots. Angew. Chem., Int. Ed. 2010, 49, 760−762. (13) Gary, D. C.; Glassy, B. A.; Cossairt, B. M. Investigation of Indium Phosphide Quantum Dot Nucleation and Growth Utilizing Triarylsilylphosphine Precursors. Chem. Mater. 2014, 26, 1734−1744. (14) Franke, D.; Harris, D. K.; Xie, L.; Jensen, K. F.; Bawendi, M. G. The Unexpected Influence of Precursor Conversion Rate in the Synthesis of III−V Quantum Dots. Angew. Chem., Int. Ed. 2015, 54, 14299−14303. (15) Cossairt, B. M. Shining Light on Indium Phosphide Quantum Dots: Understanding the Interplay among Precursor Conversion, Nucleation, and Growth. Chem. Mater. 2016, 28, 7181−7189. (16) Harris, D. K.; Bawendi, M. G. Improved Precursor Chemistry for the Synthesis of III−V Quantum Dots. J. Am. Chem. Soc. 2012, 134, 20211−20213. (17) Joung, S.; Yoon, S.; Han, C.-S.; Kim, Y.; Jeong, S. Facile Synthesis of Uniform Large-Sized InP Nanocrystal Quantum Dots Using Tris(Tert-Butyldimethylsilyl)Phosphine. Nanoscale Res. Lett. 2012, 7, 93. (18) Gary, D. C.; Terban, M. W.; Billinge, S. J. L.; Cossairt, B. M. Two-Step Nucleation and Growth of InP Quantum Dots via MagicSized Cluster Intermediates. Chem. Mater. 2015, 27, 1432−1441. (19) Xie, L.; Shen, Y.; Franke, D.; Sebastián, V.; Bawendi, M. G.; Jensen, K. F. Characterization of Indium Phosphide Quantum Dot Growth Intermediates Using MALDI-TOF Mass Spectrometry. J. Am. Chem. Soc. 2016, 138, 13469−13472. (20) Franke, D.; Harris, D. K.; Chen, O.; Bruns, O. T.; Carr, J. A.; Wilson, M. W. B.; Bawendi, M. G. Continuous injection synthesis of indium arsenide quantum dots emissive in the short-wavelength infrared. Nat. Commun. 2016, 7, 12749. (21) Wang, H. C.; Zhang, H.; Chen, H. Y.; Yeh, H. C.; Tseng, M. R.; Chung, R. J.; Chen, S.; Liu, R. S. Cadmium-Free InP/ZnSeS/ZnS

size distribution of InP QDs. Oxide-free InP QDs with an average size of 1.9 were synthesized and used as “seeds” to grow larger InP QDs. The key step in our method is to avoid the secondary nucleation events while continuously injecting the monomers. We showed that this can be achieved by careful selection of monomer precursors and optimization of monomers injection rate and growth temperature. The high quality of the InP QDs enabled us to synthesis highly luminescent InP/ZnSe/ZnS QDs with tunable emission over the entire visible range with high color purity. We further demonstrated that green and red emitting QLEDs with narrow EL fwhm can be fabricated using our InP/ZnSe/ZnS QDs. The results presented here are an important step toward the realization of Cd-free InP QDs for the application in next generation display technologies.

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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.chemmater.8b02049. Detailed experimental details, UV−vis absorption spectra, TEM images, NMR spectra, XRD patterns, HAADFSTEM image and energy dispersive X-ray chemical map, EDX spectrum and size distribution calculation method (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J.-S. Lee. E-mail: [email protected]. ORCID

Parthiban Ramasamy: 0000-0001-5844-7196 Jae-Wook Kang: 0000-0002-1412-6179 Jong-Soo Lee: 0000-0002-3045-2206 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Human Resource Training Project for Regional Innovation (No. 2015035858) through the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) and the Ministry of Trade, Industry & Energy of Korea (No. 10052853). This work also partially supported by the Basic Science Research Program (NRF-2017R1A2B2001838) funded by the Ministry of Science, ICT & Future Planning. We thank Omar Ramirez for size distribution measurements. We thank Taehoon Cheon (CCRF, DGIST) for TEM characterizations.

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REFERENCES

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DOI: 10.1021/acs.chemmater.8b02049 Chem. Mater. 2018, 30, 3643−3647