Tunable, Bright, and Narrow-Band Luminescence from Colloidal

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Tunable, Bright, and Narrow-Band Luminescence from Colloidal Indium Phosphide Quantum Dots Parthiban Ramasamy, Nayeon Kim, Yeon-Su Kang, Omar Ramirez, and Jong-Soo Lee* Department of Energy Systems Engineering, DGIST, Daegu 42988, Republic of Korea S Supporting Information *

ABSTRACT: Synthesis of cadmium (Cd)-free quantum dots (QDs) with tunable emission and high color purity has been a big challenge for the academic and industrial research community. Among various Cd-free QDs, indium phosphide (InP) QDs exhibit reasonably good color purity with emission full width at half-maximum (fwhm) values between 45 and 50 nm for green and over 50 nm for red emission, which is not good enough, as values less than 35 nm are favorable in commercial display products. In this work, we present the synthesis of highly luminescent In(Zn)P/ZnSe/ZnS QDs with tunable emission from 488 to 641 nm and high color purity. We found that the addition of zinc during the conventional SILAR growth of shell (ZnSe or ZnS) deteriorated the absorption features of core InP QDs and resulted in broader emission line widths. We solved this issue by synthesizing Zn carboxylate covered In(Zn)P QDs in a single step and dramatically decreased the emission fwhm to as low as 36 nm with quantum yields (QYs) up to 67% for the green emitting QDs. We also demonstrate an effective successive ion layer adsorption and reaction method to continuously tune the InP QDs size from 1.6 to 3.6 nm with narrow size distribution. This enables us to tune the emission up to 641 nm with fwhm values less than 45 nm and QY up to 56% for red emission. This is the first report on the synthesis of InP QDs with such high color purity. In addition, the obtained QDs show exceptional stability under air (>15 days) and heat treatment (150 °C in air for 24 h). Given the difficulty in synthesizing size tunable InP QDs with narrow emission fwhm and high quantum yield, the results presented here are an important step toward the realization of Cd-free QDs as a feasible alternative in commercial display technologies.

1. INTRODUCTION The quest for color tunable and narrow-band light emitting materials has intensified since the human desire to see colors more vividly in displays has significantly increased. In this context, colloidal quantum dots (QDs) emerge as ideal candidates because of their precisely tunable narrow emission line widths, high photoluminescence (PL) quantum yields (QYs), and high stability. Among various QDs, only cadmiumbased QDs (CdSe) and metal halide perovskite QDs (CsPbX3, where X = Cl, Br, or I) have met the optical requirements to be applied in display technologies.1,2 However, the toxicity of the constituent elements (i.e., Cd and Pb) in both types of QDs and instability of perovskite QDs impose severe limitations for commercialization. Indium phosphide QDs are an attractive alternative owing to their lower toxicity and emission tunability ranging from visible to near-infrared region. Recently, significant improvements have been made in the optical properties of InP QDs and photoluminescent QYs: up to 85% was achieved for the core−shell-type InP QDs.3,4 Nevertheless, two major challenges need to be addressed to unlock the full potential of InP QDs in display applications. First, the size distribution in InP QDs needs to be improved, which in turn will contribute to narrow emission profiles and improve the color purity. Second, robust control over the particle size must be achieved. Recent mechanistic investigations suggested that the high reactivity of the organometallic © 2017 American Chemical Society

phosphorus precursor is the main reason for the observed difficulties in achieving good control over size and size distribution in InP QDs.5−7 Several methods such as employing less reactive phosphorus precursor, using of mixture of two different phosphorus precursors with different reactivities, magic sized clusters as single source precursor, continuous injection synthesis, and alloyed InxZnyP QDs have been attempted to improve the size distribution and size tunability but have generally ended up with limited success.7−13 Thus, developing robust synthetic methods to produce size tunable InP QDs with narrow emission line widths is highly desirable for practical applications. In this article, we present a systematically controlled synthetic method that overcomes the shortcomings of previous methods. Our approach allows the synthesis of InP QDs emitting across visible spectrum (488−641 nm) with emission full width at half-maximum (fwhm) as narrow as 36 nm. We employed three strategies to obtain size tunable InP QDs with narrow emission fwhm. First, we optimized the parameters that are critical to the synthesis such as indium/phosphorus ratio and phosphorus injection temperature to produce monodisperse InP QDs. Second, we developed an efficient shell Received: May 29, 2017 Revised: June 25, 2017 Published: August 4, 2017 6893

DOI: 10.1021/acs.chemmater.7b02204 Chem. Mater. 2017, 29, 6893−6899

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Chemistry of Materials

ethanol and collected by centrifugation at 6000 rpm for 30 min. The QDs were washed three times by dispersion in hexane, followed by precipitation by addition of ethanol, and stored in hexane in a vial in a N2 filled glovebox. Caution. (TMS)3P should be handled in a moisture- and oxygenfree environment due to its highly pyrophoric nature. 2.2. Synthesis of InP/ZnSe/ZnS QDs. The InP core QDs were synthesized as mentioned above in Section 2.1. After they were maintained at 305 °C for 2 min, 0.1 mmol of zinc stearate in 1 mL of ODE was injected to the core and maintained at 300 °C for 10 min. Then, 0.15 mL of TOP-Se in 0.1 mL of TOP was injected to the flask and maintained at 300 °C for 15 min. Next, 0.1 mmol of zinc stearate in 1 mL of ODE was injected. After waiting for 10 min, 0.1 mL of TOP-Se in 0.1 mL of TOP was injected and maintained at same temperature for 15 min. One more layer of ZnSe was coated using the same amount of Zn stearate and TOP-Se used in previous step. Then, the mixture was cooled to RT, and 5 mL of 0.2 M Zn-oleate was added and maintained at 210 °C for 2 h. Then, 2.5 mL of 1-DDT was added and kept at 260 °C for 2 h to form outer ZnS shell. After cooling to RT, the QDs were precipitated with 50 mL of ethanol and collected by centrifugation at 6000 rpm for 15 min. The QDs were washed three times by dispersion in hexane followed by precipitation by addition of ethanol and stored in hexane in a vial in a N2 filled glovebox. 2.3. Synthesis of In(Zn)P/ZnSe/ZnS QDs Emitting at 535 nm (fwhm −38 nm). Indium acetate (0.15 mmol), zinc acetate (0.075 mmol), and palmitic acid (0.575 mmol) were mixed with 10 mL of ODE in a 50 mL three neck flask and fixed to a Schlenk line with a reflux condenser. The mixer was heated to 120 °C under the vacuum for 12 h (vacuum level reaching 150 mTorr). Then, the flask was refilled with N2 and cooled to RT. At this point, the mixture turned into a white turbid solution. Then a solution containing 0.1 mmol of (TMS)3P and 1 mL of TOP was injected into the flask. Following the injection, the mixture was heated to 305 °C (15 °C/min) and kept at that temperature for 2 min. Then, 0.15 mL of TOP-Se in 0.1 mL of TOP was injected to the flask and maintained at 300 °C for 15 min. Next, 0.1 mmol of zinc stearate in 1 mL of ODE was injected. After a 10 min wait, 0.1 mL of TOP-Se in 0.1 mL of TOP was injected and maintained at same temperature for 15 min. One more layer of ZnSe was coated using the same amount of Zn stearate and TOP-Se used in the previous step. Then, the mixture was cooled to RT, and 5 mL of 0.2 M Zn-oleate was added and maintained at 210 °C for 2 h. Then, 2.5 mL of 1-DDT was added and kept at 260 °C for 2 h to form the outer ZnS shell. After cooling to RT, the QDs were precipitated with 50 mL of ethanol and collected by centrifugation at 6000 rpm for 30 min. The QDs were washed three times by dispersion in hexane followed by precipitation by addition of ethanol and stored in hexane in a vial in a N2 filled glovebox. Note: The vacuum level of less than 150 mTorr and vacuum time of 12 h are key in synthesizing In(Zn)P/ZnSe/ZnS QDs with fwhm less than 40 nm. The high vacuum and long reaction time help to completely remove the water and/or acetic acid in the synthesis that is detrimental to the formation of high quality QDs.6,25 2.4. Synthesis of In(Zn)P/ZnS QDs Emitting at 488 nm (fwhm −35 nm). The same synthetic method as outlined in Section 2.3 with minor modifications was used, but 0.1205 mmol (35 μL) of (TMS)3P was used instead of 0.1 mmol, and TOP-Se was replaced with TOP-S. 2.5. Synthesis of In(Zn)P/ZnSe/ZnS QDs Emitting at 515 nm (fwhm −36 nm). The same synthetic method as outlined in Section 2.3 with minor modifications was used, but 0.525 mmol of palmitic acid was used instead of 0.575 mmol. 2.6. Synthesis of In(Zn)P/ZnSe/ZnS QDs Emitting at 550 nm (fwhm −40 nm). The same synthetic method as outlined in Section 2.3 with minor modifications was used, but 0.625 mmol of palmitic acid was used instead of 0.575 mmol. 2.7. Synthesis of Large InP QDs Using the SILAR Method. InP QDs 1.8 nm in size were synthesized following the procedure in Section 2.1. After synthesis, the mixture was cooled to 210 °C, and 1.5 mL of In(PA)3 was injected and kept at 210 °C for 30 min and cooled to RT. Then, a solution containing 0.1 mmol of (TMS)3P and 1 mL of

coating method to obtain InP-based core−shell QDs with high QYs and narrow fwhm. In general, the as-synthesized InP QDs exhibit relatively low photoluminescence QYs of less than 1%, and an overcoating of wide bandgap shell material is necessary to increase the QYs. Even though the shell coating strategy is an effective method to improve the QYs, its effect on the emission fwhm is often left unnoticed. We present a detailed investigation on how different shell coating methods affect the emission fwhm. The most common method to prepare core− shell QDs is the successive ion layer adsorption and reaction (SILAR) technique, in which the zinc and chalcogen precursors are injected successively to InP core and react at high temperature. We found that the addition of Zn precursors to the InP QDs generally results in the blue shift and broadening of the absorption spectrum, which increases the emission fwhm of core−shell QDs. We solved this issue by synthesizing Zn carboxylate covered In(Zn)P QDs in a single step and achieved narrow emission fwhm of around 36 nm and high QYs up to 67% after forming ZnSe/ZnS shell. Addition of Zn during the InP QDs synthesis has been reported by several groups to increase the QYs of InP QDs; however, it has not been effectively utilized to decrease the emission fwhm.4,13−24 Finally, we present an improved SILAR method to synthesize larger size InP QDs with narrow size distribution. Using SILAR technique, we continuously tuned the InP QDs size from 1.8 to 3.6 nm without affecting the size distribution, and this allowed us to precisely tune the emission up to 641 nm with narrow fwhm (99%), sulfur (99.998%), selenium pellets (99.999%), palmitic acid (>99%), oleic acid (90%), 1- dodecanethiol (>98%, 1-DDT), and 1-octadecene (90%, ODE) were purchased from Sigma-Aldrich. Tris(trimethylsilyl)phosphine ((TMS)3P) was purchased from SK Chemicals, Korea. Trioctylphosphine (97%, TOP) was purchased from Strem Chemicals. Hexane (HPLC grade) and ethanol (HPLC grade) were purchased from Samchun Chemicals, Korea. All chemicals were used without further purification. Preparation of Indium Palmitate (0.1 M In(PA)3). Indium acetate (1 mmol) and palmitic acid (3 mmol) were mixed with 10 mL of ODE in a 50 mL three-neck flask and fixed to a Schlenk line with a reflux condenser. The mixer was heated to 120 °C under vacuum for 12 h. Then, the flask was refilled with N2 and cooled to room temperature (RT) and stored inside a glovebox in a scintillation vial. Preparation of 0.2 M Zn-Oleate. ZnO (0.407 g) was mixed with 12.5 mL of oleic acid and 25 mL of ODE and heated to 120 °C under vacuum for 2 h. Then, the flask was filled with N2 and heated to 280 °C and kept for 1 h to form transparent clear solution. Finally, the mixer was cooled to RT and stored inside a glovebox in a scintillation vial. Preparation of TOP-Se (1 M). Selenium pellets (0.789 g) were dissolved in 10 mL of TOP at RT inside a nitrogen glovebox. Preparation of TOP-S (1 M). Sulfur (0.320 g) was dissolved in 10 mL of TOP at RT inside a nitrogen glovebox. 2.1. Synthesis of InP QDs. Indium acetate (0.15 mmol) and palmitic acid (0.45 mmol) were mixed with 10 mL of ODE in a 50 mL three-neck flask and fixed to a Schlenk line with a reflux condenser. The mixer was heated to 120 °C under vacuum for 12 h (vacuum level reaching 150 mTorr). Then, the flask was refilled with N2 and cooled to RT. At this point, the mixer turned into white turbid solution. Then, a solution containing 0.1 mmol of (TMS)3P and 1 mL of TOP was quickly injected into the flask. Following the injection, the mixture was heated to 305 °C (15 °C/min) and kept at that temperature for 2 min before cooling to RT. The QDs were precipitated with 50 mL of 6894

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Chemistry of Materials TOP was injected into the flask. Following the injection, the mixture was heated to 305 °C (15 °C/min) and kept for 2 min. Additional InP layers were coated by repeating the same procedure. 2.8. Example of the Synthesis of InP/ZnSe/ZnS QDs Using SILAR-Grown InP Core QDs (Em: 621 nm, fwhm: 44 nm). Approximately 3 nm-sized InP QDs were synthesized following the method presented in Section 2.7 (5 layers). After the core synthesis, 0.25 mmol of zinc stearate in 1 mL of ODE was injected and maintained at 300 °C for 10 min. Then, 0.5 mL of TOP-Se was injected and maintained at same temperature for 15 min. Next, 0.25 mmol of zinc stearate in 1 mL of ODE was injected. After waiting for 10 min, 0.25 mL of TOP-Se in 0.0.25 mL of TOP was injected and maintained at same temperature for 15 min. One more layer of ZnSe was coated using the same amount of Zn stearate and TOP-Se used in the previous step. Then, the mixture was cooled to RT, and 10 mL of 0.2 M Zn-oleate was added and maintained at 210 °C for 2 h. Then, 5 mL of 1-DDT was added and kept at 260 °C for 2 h to form the outer ZnS shell. After cooling to RT, the QDs were precipitated with 50 mL of ethanol and collected by centrifugation at 6000 rpm for 30 min. The QDs were washed three times by dispersion in hexane followed by precipitation by addition of ethanol and stored in hexane in a vial in a N2 filled glovebox. Large Area Film Fabrication. The isolated green and red QDs were mixed with UV-curable resin and sonicated for 30 min followed by RT stirring for 24 h. Then, the mixture was coated on the flexible substrate and cured under UV light. Air Stability. The QDs in toluene were mixed with ODE (1:1 toluene:ODE) and kept in open air without any excitation. The PL QYs were measured every two days. Thermal Stability. The QDs were mixed with dried ODE and maintained at 150 °C in open air. The PL QYs were measured every 4 h.

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X-ray Diffraction (XRD). X-ray diffraction patterns were obtained by using a Rigaku MiniFlex 600 diffractometer equipped with a Cu Kα X-ray source (λ = 1.5418 Å). Samples for XRD analysis were prepared by depositing (drop casting) the purified QDs dispersed in hexane on a glass substrate. Transmission Electron Microscopy (TEM). TEM and high-resolution TEM (HRTEM) images were obtained using a Hitachi HF-3300 microscope operating at 300 kV. TEM samples were prepared by dropping the diluted QDs onto carbon coated 200 mesh copper grids. Procedure for Obtaining fwhm of UV−Vis Absorption Spectrum. The absorption spectra were fitted in MATLAB using the following equation:6 ⎛ −1(x − x)2 ⎞ 0 ⎟ f(x , σ ) = B + A exp⎜ 2σ 2 ⎠ ⎝

where B is equal to the energy of absorbance at 800 nm, A and x0 are equal to the absorbance and energy of the lowest energy electronic transition, respectively, and 2σ is the full width at half-maximum of the absorbance peak. An example of the fitted UV−vis absorption spectrum is given in Figure S1.

3. RESULTS AND DISCUSSION We tested two different synthetic approaches to obtain monodisperse InP QDs. First was the hot-injection method, where the phosphorus precursor [(TMS)3P] is injected into a hot solution containing indium palmitate. This has been the preferred method to synthesis InP QDs.4,6−8,13,19,20 Second was the heating-up method, in which the precursors are mixed at room temperature and heated to a desired final temperature. Absorption spectra in Figures 1a and b show that the QDs prepared from heating-up method exhibit features more defined than those prepared from hot-injection method (the absorption fwhms of QDs synthesized from heating-up method are 56 and 66 nm for hot-injection, an 18% difference). The sharp features in the absorption spectra of QDs obtained using the heating-up method are an indication of narrow size distribution. Figure S2 shows the evolution of absorption spectra at different temperatures during the heating-up method. We observed the presence of small nanoclusters below 150 °C. Upon further heating, the absorption spectra continuously evolved to the red, and a sharp absorption peak at 480 nm was obtained at 305 °C. This demonstrates the intermediacy of clusters in the growth of uniform InP QDs under these conditions. In addition, even after maintaining at 305 °C for 3 h, no apparent change in the absorption spectra was observed (Figure 1b). This shows that the QDs growth was completed during the heating-up process, and no Ostwald ripening took place (which would have broadened the absorption peaks). However, in the case of the hot-injection method, the QD growth was mainly governed by Ostwald ripening, which resulted in broad size distribution (Figure 1a). Next, we studied the effect of phosphorus precursor concentration by varying the In:P ratio from 0.50 to 1. It was observed that increasing the phosphorus concentration resulted in blue shift of the absorption peak, and InP QDs with narrow size distribution were obtained for In:P ratio of 0.66 and 0.8 (Figure 1c). It should be noted that In:P ratio of either 0.5 or 1 has been widely used in the literature.16,18−20 Furthermore, we found that the addition of small amount (1 mL) of TOP along with phosphorus precursor is important to obtain narrow size distribution (Figure S3). TOP as a strong stabilizing ligand

CHARACTERIZATION

UV−vis Absorption Spectroscopy. Absorbance spectra of the QDs dispersed in hexane were recorded in 1 cm path length quartz cuvettes using a Cary 5000 UV−vis−NIR (Agilent Technologies) spectrophotometer. Photoluminescence (PL) Spectroscopy and Quantum Yield Measurement. PL spectra of QDs dispersed in toluene were recorded using a Cary Eclipse fluorescence spectrophotometer (λexc = 400 nm). PL QY. The quantum yields were measured by comparing the integrated PL intensities between primary dye solution and InP QDs at the same excitation wavelength using the following equation. QYQDs = QYdyeX(IQDs/Idye)X(Adye /A QDs)X(ηQDs /ηdye)2

where QYdye and QYQDs are the quantum yields of the standard dye and synthesized QDs, I is the integrated area of the PL spectrum, A is the absorbance value at excitation wavelength, and η is the refractive index of the solvent used. Coumarin 153, Coumarin 545, Rhodamine 101 inner salt, and Rhodamine 6G were used as standard dye, depending on their PL wavelengths. The QDs and dye solutions were prepared in hexane and ethanol, respectively. Time Resolved PL Spectra. The time-resolved PL spectra were measured using time-correlated single photon counting system (Picoquant, Fluotime 200). The PL emission from the samples was collected by a pair of lenses into the concave holographic grating of 1200 g/mm and detected by photomultiplier tube (PMT). The temporal resolution and repetition rates are 80 ps and 10 MHz, respectively. The samples were excited by with 375 nm pulses (LDH-P-C-375, 3 μW) at room temperature. 6895

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inferred that by optimizing shell coating method it is possible to further improve the luminescent properties of InP QDs. Consequently, we devised a one-pot method to synthesis Zn carboxylate covered InP QDs (In(Zn)P QDs) by adding Zn precursor during the core synthesis (Figure 2a). The added Zn

Figure 2. (a) Schematic of the synthesis of In(Zn)P/ZnSe/ZnS QDs. (b and c) Absorption and photoluminescence spectra of In(Zn)P/ ZnSe/ZnS QDs synthesized with different ratios of indium/palmitic acid and indium/phosphorus. Insets show the corresponding luminescence photographs of the QDs in hexane excited with a UV lamp.

Figure 1. Absorption spectra of aliquots taken during the growth of InP QDs in two different methods: (a) hot-injection and (b) heatingup (In:P ratio −0.66). (c) Absorption spectra of the heating-up method synthesized InP QDs with different concentrations of phosphorus precursor. (d) Absorption and photoluminescence spectra of InP/ZnSe/ZnS QDs with SILAR method grown ZnSe shell. No size selection process was used in this report. All of the absorption and photoluminescence spectra were recorded by diluting the crude reaction mixture with hexane.

can be incorporated into the surface of the InP core during QDs growth and smoothen the lattice parameters at the interface between core and shell, facilitating the uniform coating of the ZnSe shell. In addition, it will also eliminate the negative effect of the Zn precursor addition to the presynthesized InP core. Figure S7a shows the absorption spectra of In(Zn)P QDs synthesized with different Zn:In ratios. With less Zn (Zn:In up to 0.5), no change in the absorption spectra was observed, and when Zn:In ratio exceeded 1, the absorption spectra blue-shifted and became broad (Figure S7b). The QYs of the In(Zn)P QDs increased while increasing the Zn amount and band edge luminescence become dominant (Figure S7c). XRD patterns in Figure S7d remained the same up to Zn:In ratio of 1 and slightly shifted toward higher angle for high Zn concentrations, showing the formation of alloyed structures.19 From the absorption and XRD data, we believe that no alloying takes place at low Zn concentrations and that the Zn carboxylates present mainly on the surface of the InP QDs.9,14 We chose Zn:In ratio of 0.5 for further studies because the absorption peak position and fwhm of In(Zn)P QDs obtained with this ratio is nearly the same as InP QDs. After synthesizing In(Zn)P QDs, we directly injected TOPSe at 305 °C and reacted for 15 min to form In(Zn)P/ZnSe core−shell QDs. The defect luminescence from In(Zn)P QDs completely disappeared, and bright band edge luminescence (535 nm) with fwhm of 38 nm was obtained (Figure S8). This further confirms that the added Zn carboxylate during the core synthesis was mainly present on the surface of the In(Zn)P QDs, which promoted the ZnSe shell growth. After forming additional ZnSe and ZnS shell, In(Zn)P/ZnSe/ZnS core−shell QDs with QY up to 71% were obtained (Figure 2b). The observed Stokes shift of 33 nm in both InP/ZnSe/ZnS and In(Zn)P/ZnSe/ZnS QDs further rules out the possibility of the presence of InZnP alloy in our core−shell QDs (Figures 1d and

helps to slow the nucleation and control the growth to improve the size distribution (Figure S4).5,26 Because the as-synthesized InP QDs are nonluminescent, we performed ZnSe shell growth on the InP QDs to improve the QYs. We tested several shell coating methods with various Zn precursors such as zinc stearate, zinc oleate, and zinc chloride. The Zn precursors were either directly injected to the crude InP QDs at 300 °C or at room temperature. In the case of room temperature injection, the reaction mixture was then heated to 300 °C for the TOP-Se injection. As shown in Figure S5 and Table S1, the addition of Zn precursors to the crude InP QDs resulted in blue or red shift and broadening of the absorption spectra. The extent of shift and broadening varied with different Zn precursors. When Zn carboxylates were used, the absorption spectra generally blue-shifted. As Stein et al. hypothesized, the presence of Zn carboxylate on the surface of the QDs can significantly localize the QDs LUMO and change its energy, which results in the blue-shift of the absorption.27 The reason for the broadening of the spectrum is not clear at this stage. In the case of ZnCl2 addition, the absorption spectrum was red-shifted. This may due to the removal of the stabilizing ligands on the surface of the QDs by the Cl− ions, which destabilized the QDs to merge and grow at high temperature. In addition, the defect luminescence from the InP core decreased, and band-edge luminescence became dominant due to the surface passivation by Zn carboxylates (Figure S6a). After the ZnSe/ZnS shell was formed, bright luminescence with QYs of more than 50% was achieved with emission fwhm ranging from 43 to 78 nm for different methods (Figure S6b). The fwhm value of 43 nm obtained using zinc stearate (Figure 1d) is close to the best values reported in literature.3,16,18 From these observations, we 6896

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Chemistry of Materials 2b).13 By changing the indium to palmitic acid or phosphorus ratio, we were able to synthesize In(Zn)P/ZnSe/ZnS QDs emitting at 488, 516, and 552 nm with fwhm of 35, 36, and 40 nm and QYs of 44, 67, and 60%, respectively (Figure 2c). To the best of our knowledge, the fwhm values obtained here are the narrowest for InP-based core−shell or alloy QDs. To investigate the carrier dynamics in our QDs, we carried out time-resolved photoluminescence (TRPL) measurements. As shown in Figure S9 the temporal decay of the QDs is biexponential in nature. The decay curves were fitted accordingly with the following function, and the fitting parameters are given in Table S2: IPL(t ) = A e−t / τ1 + Be−t / τ2, where A + B = 1

Figure 3. (a) Absorption and photoluminescence spectra of In(Zn)P/ ZnSe/ZnS QDs obtained in single large scale batch. Inset shows the photograph of the crude reaction product under UV irradiation. (b) Photograph showing the amount of final QDs obtained.

It is observed that the amplitude of the fast decay component (A) is the highest for InP core QDs (0.82), which is an indication of the presence of large number of surface defects where the trapped charge carriers recombine nonradiatively to give defect luminescence. On the other hand, the fast decay component is decreased to 0.55 for the In(Zn)P QDs, suggesting the zinc ions added in the starting of the synthesis effectively passivate the surface of InP QDs and decrease the defects.4,27 In the core−shell QDs, the fast decay component is further decreased to 0.13, and the slow decay component (B) is increased to 0.87. This shows that shell passivation efficiently removed the carrier quenching surface defects and increased the rate of radiative recombination, resulting in increased PL intensity. Figure S10a shows the XRD patterns of In(Zn)P and In(Zn)P/ZnSe/ZnS QDs. The major three peaks are indexed to the (111), (220), and (311) planes of the zinc blende structure. The peak shift between In(Zn)P and In(Zn)P/ZnSe/ ZnS QDs proves the presence of ZnSe/ZnS shell. TEM image of In(Zn)P/ZnSe/ZnS QDs is shown in Figure S10b. The core−shell QDs are uniform in size, and average size is around 4.2 nm. QDs synthesis methods often lack reproducibility due to the highly sensitive nature of emission wavelength and fwhm to the reaction conditions. Thus, we examined the reproducibility of our method by repeating the synthesis of 535 nm emitting core−shell QDs 10 consecutive times. The results (Figure S11) show that the emission wavelength (533 ± 2 nm) and fwhm (39 ± 1 nm) are highly reproducible. In addition to reproducibility, scalability is also an important parameter due to the requirement of large quantity of QDs in commercial display applications. Thus, heating-up method presented in our synthesis is highly suitable for large-scale synthesis because it overcomes the difficulties of mixing time and poor heat management associated with the typical hot-injection method.28 We verified the scalability of our method by scaling up the synthesis of green emitting QDs by a factor of 10. The reaction yielded 1.63 g of In(Zn)P/ZnSe/ZnS QDs with emission fwhm of 40 nm and QY of 64% (Figure 3), which are comparable to the values obtained from small scale synthesis (Figure 2a). These results highlight the advantage of our method to produce high quality InP QDS on a large scale. Another persisting problem associated with InP QDs is the synthesis of larger QDs with narrow size distribution. Previous attempts to synthesize larger size InP QDs with conventional methods have generally resulted in inhomogeneous size distribution. This is due to the highly reactive nature of the phosphine precursor, which is consumed entirely during the nucleation process and leaves less room for the homogeneous growth and increase the inhomogeneity in particle size. In

literature, most of the reported red emitting InP QDs synthesized using (TMS)3P have an emission fwhm above 60 nm.3,16−18,24,29 Our attempts to synthesize larger In(Zn)P QDs either by increasing the indium/palmitic acid ratio or decreasing the In:P ratio also resulted in broader size distribution. Hence, we developed a SILAR method to synthesize larger size InP QDs, where we alternatively injected indium palmitate and (TMS)3P to the presynthesized InP QDs (abs: 480 nm) with narrow size distribution. Initially, the presynthesized smaller InP QDs were reacted with indium palmitate at 210 °C for 30 min. This allows the functionalization of InP QDs surface with indium palmitate. Then, the mixture was cooled to RT for phosphine addition and maintained at 305 °C for 2 min. During this, the phosphine precursors are reacted with the indium on the surface of the QDs to form larger QDs. By repeating this SILAR cycle seven times, we were able to tune the InP QDs absorption peak from 480 to 615 nm (Figure 4a) corresponding to a size increase from 1.8 to 3.6 nm (Figure 4b). Most importantly, the SILAR grown InP QDs maintained the homogeneous size distribution of the initial seed InP QDs (Figure 4b and TEM images in Figure 4c) with standard deviation around 10%. The robust control over the InP QDs size and size distribution achieved here is much better than the previously reported syntheses of InP QDs via ripening process. Next, we used the SILAR method grown different sized InP QDs as cores to synthesize InP/ZnSe/ZnS core−shell QDs. After forming the shell, we were able to continuously tune the luminescence from 567 to 641 nm. More interestingly, all the QDs have emission fwhm around 44 nm and QYs over 50% (Figure 5a and b). This further confirms the narrow size distribution of InP core QDs obtained using SILAR method. The fwhm values obtained here are the narrowest for InP QDs emitting in the yellow to red region of the visible spectrum. Initial attempts to synthesize large In(Zn)P QDs via SILAR method were not successful. We believe that with further optimization of the SILAR method, it is possible to synthesize red emitting InP QDs with fwhm less than 40 nm. Photoluminescence stability of the QDs is an important parameter for commercial applications. We tested the stability of our green and red emitting QDs in air and heat. When kept in open air (Figure 6a), the QDs showed excellent stability and maintained their original QYs for 10 days and slightly decreased after 15 days (from 70 to 63% for green and 55 to 50% for red). 6897

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Chemistry of Materials

Figure 4. (a) Absorption spectra and (b) change in size and absorption fwhm of InP QDs synthesized using the SILAR method. The sizes of the QDs were calculated from the absorption data. (c) TEM images of InP QDs synthesized using the SILAR method. Insets show the size distribution histograms of QDs (>1500 measurements per sample) with standard deviation.

and heat. Fabrication of large area luminescent QD film is one of the key step toward the integration of QDs in commercial optoelectronic devices (i.e., QLED displays). As a proof of concept, we mixed our QDs with monomer and resin and fabricated uniform large area QD film (13 × 13 cm). When illuminated with UV lamp, the films produced bright and uniform luminescence (Figure 6b). Current studies are focused on the stability and integration of these films in lighting devices.

4. CONCLUSIONS In summary, we presented a systematically controlled synthetic method for In(Zn)P/ZnSe/ZnS QDs with tunable emission from cyan (488 nm) to red (641 nm) and high color purity. We showed that the heating-up synthesis rather than the widely used hot-injection method resulted in homogeneous growth and improved the size distribution in InP QDs. We showed that the conventional SILAR shell (ZnSe or ZnS) coating methods generally deteriorate the absorption features of InP core QDs, which contributes to the broadening of the emission lines. To eliminate the negative effect of SILAR shell growth method, we synthesized Zn carboxylate covered In(Zn)P QDs in a single step and achieved In(Zn)P/ZnSe/ZnS QDs with emission fwhm of 36 nm and QY up to 67%. By adopting a method similar to SILAR, we showed that the InP QDs size can be precisely tuned from 1.8 to 3.6 nm while maintaining narrow size distribution. This enabled us to tune the emission of InP QDs up to 641 nm with narrow fwhm (15 days) and under heat treatment (150 °C in air, 24 h).

Figure 5. (a) Absorption and photoluminescence spectra and (b) the corresponding fwhm and QYs of the InP/ZnSe/ZnS QDs synthesized using the SILAR method grown InP QDs (1−6 layers).

Figure 6. (a) Change in QY of green and red emitting QDs kept in the open at room temperature for 15 days. Inset shows the change in QY monitored for 24 h of aging at 150 °C in open air. (b) Photographs of large area (13 × 13 cm) QDs film with and without UV illumination.

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ASSOCIATED CONTENT

S Supporting Information *

The PL peak position and fwhm values remained unchanged. In addition, we dispersed the QDs in 1-octadecene and aged at 150 °C in open air for 24 h. No significant change in QY was observed. This proved the good stability of our QDs under air

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02204. 6898

DOI: 10.1021/acs.chemmater.7b02204 Chem. Mater. 2017, 29, 6893−6899

Article

Chemistry of Materials

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Evolution of absorption spectra at different temperatures during the heating-up method; absorption spectra of InP QDs synthesized in the presence and absence of TOP; absorption spectra of InP QDs before and after addition of different Zn precursors; PL spectra of InP QDs after addition of Zn and InP/ZnSe/ZnS QDs; characterization of In(Zn)P QDs synthesized with different Zn:In ratio; PL spectra of In(Zn)P QDs and In(Zn)P/ZnSe QDs; reproducibility test; TRPL data; and XRD pattern and TEM image of In(Zn)P/ZnSe/ZnS QDs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Parthiban Ramasamy: 0000-0001-5844-7196 Jong-Soo Lee: 0000-0002-3045-2206 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (Grant 2015035858) and the Ministry of Trade, Industry & Energy of Korea (Grant 10052853).

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DOI: 10.1021/acs.chemmater.7b02204 Chem. Mater. 2017, 29, 6893−6899