In Situ Tuning the Reactivity of Selenium Precursor To Synthesize

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In situ tuning the reactivity of selenium precursor to synthesize wide range size, ultra-large-scale, and ultrastable PbSe quantum dots Linyuan Lian, Yong Xia, Changwang Zhang, Bing Xu, Lei Yang, Huan Liu, Daoli Zhang, Kai Wang, Jianbo Gao, and Jianbing Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04825 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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Chemistry of Materials

In situ tuning the reactivity of selenium precursor to synthesize wide range size, ultra-large-scale, and ultrastable PbSe quantum dots Linyuan Lian,1 Yong Xia,1 Changwang Zhang,1 Bing Xu,2 Lei Yang,2 Huan Liu,1 Daoli Zhang,1 Kai Wang,2 Jianbo Gao,3 and Jianbing Zhang*1,4 1

School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China 2 Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China 3 Department of Physics and Astronomy, University of Clemson, South Carolina 29634, United States 4 Shenzhen R & D Center of Huazhong University of Science and Technology, Shenzhen, Guangdong 518057, China

ABSTRACT PbSe quantum dots (QDs) have shown outstanding optoelectronic properties due to their extremely strong quantum confinement. However, the applications of PbSe QDs are substantially limited by their instability in air. Here, we developed a simple synthesis for PbSe QDs via in situ tuning the reactivity of selenium precursor. Due to in situ chloride passivation, the as prepared PbSe QDs showed excellent stability in air, demonstrated by the retention of absorption features and PL QYs for different sizes after heated at 80 oC in air. Furthermore, the ligand exchanged electronic coupled PbSe QD thin films also had excellent stability in air even for large particle sizes. In addition, the PbSe QDs showed high photoluminescence quantum yields due to hybrid organic (oleate) and inorganic (Cl-) passivation. Monodispersive large (1st exciton peak > 2100 nm) and extremely small (1st exciton peak < 750 nm) PbSe QDs were achieved by careful control of particle growth. As a heating up method, this new synthesis was easily scaled up to produce 23.5g PbSe QDs from one batch of reaction. The versatility of this new synthetic strategy was demonstrated by the synthesis of other metal selenide QDs, such as CdSe and ZnSe QDs.

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INTRODUCTION Lead chalcogenide (PbS, PbSe and PbTe) quantum dots (QDs) have become important optoelectronic and electronic materials due to strong quantum confinement, tunable bandgap in a large range, ease of synthesis, solution processability and efficient multiple exciton generation (MEG).1-3 Among the lead chalcogenide QDs, PbS are the most studied materials due to their acceptable stability and have been applied in various devices.1, 4, 5 Compared to PbS, PbSe QDs are less studied due to their air-sensitivity. However, PbSe QDs have excellent intrinsic optoelectronic properties which might lead to better device performance: (1) The exciton Bohr radius of PbSe is 46nm which is more than twice that of PbS (20nm).6 As a result, PbSe QDs exhibit much stronger quantum confinement than PbS QDs, which should induce more leakage of wave function in PbSe QDs and in turn stronger electronic coupling between QDs and better charge carrier transport in PbSe QD films.7 (2) PbSe QDs have better MEG performance than PbS QDs. Firstly, ultrafast spectroscopic experiments show that PbSe QDs have higher MEG efficiency.8 Secondly, QD solar cells with EQE above 100% have been observed in PbSe9, 10 QD devices but not yet in PbS QD solar cells. (3) There are fewer intragap states in electronically coupled PbSe than PbS QD films.11 Additionally, favorable band-like transport has been observed in PbSe QD films but not yet in PbS QD films.12 (4) PbSe QDs can form epitaxial connections between well-oriented proximal nanoparticles (2D superlattices)13, 14 while this kind of assembling is not achieved in PbS QDs. The confined-but-connected PbSe films show much better carrier mobility15 and higher optical absorption cross sections.16 (5) PbSe QDs have lower bulk bandgap and stronger quantum confinement than PbS QDs, as a result the bandgap of PbSe QDs can be tuned in a larger range which will find a wider range of applications. Therefore, PbSe QDs should exhibit better device performance than PbS QDs and would become ideal QD materials if the air-sensitivity can be addressed. However, large scale, wide range size synthesis while maintaining air-stability are the main challenges for PbSe QD device applications. The PbSe QDs degrade gradually demonstrated by the blueshift of absorption and photoluminescence (PL) peaks and quenching of PL intensity.17, 18 X-ray photoelectron spectroscopy (XPS) study shows that up to 50% of particle volume is transformed into PbO, SeO2, or PbSeO3 within 24 h when PbSe QDs solution is exposed to air under ambient conditions.18 Therefore, PbSe QDs would become ideal QD materials if above challenges can be addressed. In order to improve the stability of PbSe QDs, different postsynthetic strategies were developed. Pietryga et al. coated PbSe QDs with CdSe forming PbSe/CdSe QDs.19 Although the core/shell structure improves the stability substantially, the CdSe shell with a wider bandgap is a barrier for carrier transport. Bae et al. treated PbSe QDs using molecular chlorine (Cl2) to replace the surface Se with Cl atoms.20 The chloride passivated PbSe QDs dispersed in solvents show excellent stability indicating halide passivation is an


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effective way to stabilize PbSe QDs. Jeong et al. improved the post-synthetic halide passivation by injecting halide salts to PbSe QDs growth solution after the formation of nanoparticles.21 Additionally, in situ passivation was also derived by including capping ligands in the nucleation and growth of PbSe QDs. Jeong et al. introduced phosphonic acid in the synthesis of PbSe QDs.22 The P-O- moieties on the surface render the PbSe QDs stable in solvents and in electronically coupled films. However, the long chain phosphonic acids remained on the QD surface after ligand exchange would limit the carrier transport in the PbSe QD films. Recently, Beard and Zhang reported cation exchange synthesis for stable halide passivated PbSe QDs,23, 24 however combination of cation exchange and quantized Ostwald ripening is needed for monodispersive large sized PbSe QDs.25 Therefore, a simple and large scale synthesis for monodispersive PbSe QDs with tunable sizes in a large range is highly desirable. In the present work, we developed a new method for the synthesis of ultrastable, wide range dot size, scale-up to > 20 g PbSe QDs. The complex of PbCl2 and oleylamine (OLA) was adopted as the Pb precursor for in situ chloride passivation. Diphenylphosphine selenide (DPPSe) and trioctylphosphine selenide (TOPSe) were used as the Se precursors. The Pb and Se precursors were mixed at room temperature and the growth of PbSe QDs was driven by heating the growth solution. The TOPSe with low reactivity was gradually converted to reactive DPPSe as the increase of temperature, promoting the growth and maintaining a narrow size distribution. Due to the in situ chloride passivation, the as prepared PbSe QDs showed high stability in air in the form of powder and high photoluminescence quantum yields (PL QYs). Furthermore, stable and electronically coupled PbSe QD thin films with the 1stexciton peak above 2000 nm were achieved for the first time. The size can be controlled in a large range, including extremely small dots and large dots with 1stexciton peak above 2100 nm, via temperature, the ratio of TOPSe:DPPSe or injection of additional Se precursor. Slow Ostwald ripening was observed in the new synthesis, facilitating the accurate size control of the PbSe QDs. As a heating up method, the new synthesis was easily scaled up to produce 23.5 g monodispersive PbSe QDs from one batch of reaction. The versatility of this new strategy was demonstrated by the applications in the synthesis of CdSe and ZnSe QDs.

EXPERIMENTAL SECTION Chemicals. PbCl2 (99.999%), CdO (≥99.95%), Zinc stearate (ZnO 12.5-14%), oleic acid (OA, tech. grade, 90%), 1-octadecene (ODE, tech. grade, 90%), trioctylphosphine (TOP, tech. grade, 90%), 1,2-ethanedithiol (EDT, 98%) were purchased from Alfa Aesar. Oleylamine (OLA, tech. grade, 70%), Diphenylphosphine (DPP, 95%), selenium powder (99.999%) were purchased from


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Aladdin. Tetrachloroethylene (TCE, ≥98.5%), hexane (≥97%), and acetone (≥99.5%) were purchased from Sinopharm Chemical Reagent. All the chemicals were used as received. TOPSe and DPPSe (selenium powder dissolved in TOP and DPP respectively) were used as the molecular precursors. DPPSe was made by heating the DPP and Se powder at 120 oC for 30 min. Synthesis of PbSe QDs. In a typical synthesis, OLA (36 mL) and 9 mmol of PbCl2 were degassed under vacuum at 80 oC and heated to 140 oC under nitrogen and the temperature was maintained for 30 min until a white and turbid solution achieved. Then the suspension was allowed to cool to 30 oC, 110uL of 1M DPPSe diluted in 2 mL of ODE and 0.57mL of 1M TOPSe were injected sequentially to the lead precursor solution at 30 oC. Next, the reaction solution was reheated to 160 oC under vigorous stirring for the growth of the QDs. At various temperatures, 30 µL reaction solutions were withdrawn for the absorption measurements. The growth solution was maintained at 160 oC for 15 min and aliquots were withdrawn at different time intervals. The aliquots were dispersed in hexane with OA, washed one time using acetone, and dispersed in 3mL of TCE for the measurement of absorption spectra. Second injection of molecular precursors was needed to obtain larger PbSe QDs. After the growth solution was maintained at 160 oC for 15 min, 0.57 mL of 1M TOPSe and 110 µL of DPP diluted in 2 mL of ODE were injected sequentially to the growth solution, and the solution was maintained at 160 o C for another 15 min, resulting in PbSe QDs with the first exciton peak beyonds 2000 nm. If double the volume of the second injection reagents, 1.14 mL of 1M TOPSe and 220 µL of DPP, PbSe QDs with the first exciton peak beyonds 2100 nm were obtained. Purification of PbSe QDs. The reaction was quenched by a water bath when the desired size was achieved, and 20 mL of hexane and 20 mL of OA were added at 70 and 40 oC respectively, followed by vigorous stirring for 10 min. The purpose of addition of OA is replacing the weakly bound OLA. The unreacted lead precursors were precipitated from the raw solution by centrifugation after the addition of hexane and OA, and the precipitate was discarded. The supernatant solution was collected and precipitated by adding acetone. After centrifugation, the supernatant is discarded and the precipitated PbSe QDs was collected. Synthesis of CdSe and ZnSe QDs. The synthesis of CdSe and ZnSe QDs followed the same manner as that of PbSe QDs. In a typical synthesis of CdSe QDs, 0.256 g of CdO, 1.57 g of OA, and 12.5 g of ODE were loaded in a 50 mL round-bottom 3-neck flask, and then the mixture was degassed under vacuum at 80 oC and heated to 260 oC under nitrogen and the temperature was maintained for 20 min until the solution became colorless. Then the solution was allowed to cool to 30 oC, 110 µL of 1M DPPSe diluted in 2 mL of ODE and 0.57 mL of 1M TOPSe were injected sequentially to the cadmium precursor solution at 30 oC. Next, the reaction solution was reheated to 270 oC under vigorous stirring for the growth of the QDs. At various temperatures,


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reaction solutions were withdrawn for the absorption measurements. The synthesis of ZnSe QDs is the same with that of CdSe QDs except the Cd precursor (CdO and OA) was replaced by zinc stearate and the temperature was raised to 290 oC. Characterization. Optical absorption spectra were collected using a Shimadzu UV-3600 plus spectrophotometer. PL QYs were measured by an Ocean Optics QE65 Pro spectrometer equipmented with a Labsphere integrating sphere of 3.3-inch innerdiameter. TEM images were obtained using a FEI Technai G2 20 microscope with a LaB6 filament operated at 200 kV. XPS data were obtained on a Kratos AXIS-ULTRA DLD and the sample was prepared by drop casting QD solution on ITO coated glass. X-ray diffraction (XRD) patterns were recorded on a XRD-7000S diffractometer (Shimadzu). Calculation of dot size and particle concentration. The diameter and concentration of QDs were calculated using the relationship between the optical bandgap and QD size and the relationship between QD diameter and molar extinction coefficient at the 1stexciton peak given by Yu et al.26

RESULTS AND DISCUSSION As we reported previously, the growth of PbSe QDs in the viscous PbCl2-OLA complex follows a diffusion controlled mode, facilitating the QD growth control to manage wide range size.23, 27-31 In addition, the chlorine containing precursor allows in situ chloride passivation, resulting in well passivated QDs.23, 30, 31 However, the reactivities of widely used Se precursors, TOPSe and tributylphosphine selenide (TBPSe), are too low to promote the nucleation and growth of PbSe QDs in the PbCl2-OLA complex.25 It was found that the second phosphine impurities in TOP, such as dioctylphosphine, are responsible for the nucleation of QDs in the traditional synthesis in ODE.32 Therefore, commercially available second phosphine, DPP, was added to promote the chemical yield.33-35 Experiments and theoretical calculation revealed that in the presence of DPP, TOPSe can be converted to DPPSe which is much more reactive than TOPSe.36, 37 Inspired by this work, we combined DPPSe and TOPSe as a Se precursor with high and tunable reactivity. The DPPSe reacts with PbCl2-OLA complex even at room temperature to form extremely small PbSe QDs. The Se in TOPSe is gradually released via converting to DPPSe as the increase of temperature, maintaining a high oversaturation which is critical for growth and size distribution of QDs.38, 39 In this way, all the precursors can be combined at room temperature and the growth will be promoted by simply heating the mixture. As indicated by the evolution of absorption spectrum in Figure 1a, a sharp exciton peak emerged even at 30 oC, which was not affected without the presence of TOPSe in a control experiment. This indicates


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the DPPSe is reactive enough to form monodispersive PbSe QDs even at room temperature. The 1st exciton peak red shifted continuously as the temperature increased from 30 to 160 oC. It is worth noting that the monodispersity was preserved in the growth demonstrated by the sharp 1st, 2ndand 3rdexciton peaks. The particle number remained constant in the early stage (30-70 oC), then decreased substantially from 70 to 110 oC, and keep almost constant in the rest stage of the growth (Figure 1b). The decrease of the particle number is a typical result of Ostwald ripening which usually leads to broadening of size distribution. The PbSe QDs grew in a diffusion controlled mode due to the high viscosity of the Pb precursor (PbCl2-OLA complex),31 and the dissolved particles released a large amount of Se precursor maintaining the oversaturation, as a result the size distribution remained narrow, which is consistent with previous observation.30, 31 Additionally, the decrease of particle number (30-70 oC) indicates the conversion of TOPSe to DPPSe in the presence of DPP is temperature dependent, i.e. the reaction did not occur at low temperatures (< ~70 oC). When the TOPSe started to be converted to DPPSe at temperatures above ~70 oC, the oversaturation accumulated resulting in the growth of PbSe QDs, maintaining of the size distribution and particle number (Figure 1b).

Figure 1. Evolutions of absorption spectrum (a), particle concentration and size (b) in the synthesis of PbSe QDs. It is interesting that the growth of PbSe QDs almost stopped when the temperature was maintained at 160 oC, demonstrated by the invariable particle size (Figure 1b) and size distribution (Figure 1a). This extremely slow Ostwald ripening was also observed in other QD


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syntheses recently.40, 41 As the Pb precursor is in excess, the “stop” of growth might be due to insufficiency/depletion of Se precursor. In order to address the “stop” mechanism, we study the growth dependence with Se precursor. Figure 2a shows the evolution of absorption spectrum after the injection of 0.57 mL TOPSe (1M) when the growth stopped. The position and shape (size distribution) of the 1st exciton peak were nearly preserved, demonstrating little effect of the injection of TOPSe. However, when 110 µL of DPP was injected, the 1st exciton peak redshifted for 80 nm in 10min and then the growth “stopped” again demonstrated by the unchanged peak (Figure 2b). When another 110 µL of DPP was injected subsequently, no change was observed (Figure 2b). When 0.57 mL of TOPSe (1M) and 110 µL of DPP were injected together, the 1st exciton peak redshifted for 255 nm and the growth “stopped” again (Figure 2c). These evolutions imply (i) TOPSe is sufficient in the present synthetic condition; (ii) the catalyst (DPP) was consumed in the growth; (iii) 110 µL DPP is adequate to convert the remaining TOPSe to DPPSe; (iv) excess DPP have no effect on the growth; (v) larger PbSe QDs can be prepared by injection of additional TOPSe and DPP. The growth process is schematically shown in Scheme 1. Although the “stop” of growth is due to insufficient reactive Se precursor, the mechanism of the maintenance of size distribution (peak shape) needs further investigation. In terms of growth control, the “stop” of growth facilitates accurate size control, i.e. desired size can be obtained by injection of a certain amount of additional Se precursor.

Figure 2. Evolutions of absorption spectrum after the injection of TOPSe (a), DPP (b), TOPSe and DPP (c) into the reaction solution when the growth “stopped” (160 oC-15 min).


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Scheme 1. The growth process in the synthesis of PbSe QDs. Therefore, three strategies can be adopted to control the size of PbSe QDs. For small sizes, the growth can be stopped at a certain temperature. The higher the final temperature, the larger the PbSe QDs (Figure 3 left). For medium sizes, the size can be tuned by changing the ratio of TOPSe : DPPSe as shown in Figure 3 (middle). In theses syntheses, the amount of TOPSe (0.57 mL) was kept the same while the amount of DPPSe was adjusted (80-165 µL) to maintain a consistent ratio of Pb : Se. The amount of DPPSe determines the number of seed particle, thus for a given amount of TOPSe the less DPPSe results in larger PbSe QDs. However, the size range that can be tuned by this strategy is limited (~1500-2000 nm for the 1st exciton peak) as even larger or smaller size is accompanied with worse size distribution. Additionally, in the accessible size range, smaller particles required a lower temperature because substantial Ostwald ripening occurred when smaller PbSe were heated at a relative high temperature. For larger sizes, additional TOPSe and DPP can be injected into the growth solution and the injected amount determines the final particle size (Figure 3 right).


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Figure 3. Three strategies for size control. (left) The size is controlled by the final growth temperature; (middle) the ratio of TOPSe : DPPSe (the amount of TOPSe is kept the same) determines the final size; (right) Additional TOPSe is injected for larger sizes (DPP is added to convert the TOPSe to reactive DPPSe).Synthetic details can be found in the experimental section and the absorption spectra can be found in the supporting information. By using the above three strategies, we obtained monodispersive PbSe QDs with a large range of sizes. Figure 4 shows the absorption spectra of PbSe QDs with a series of size and three typical TEM images. The high monodispersity is demonstrated by the sharp 1st and/or well distinguishable 2nd and 3rd exciton peaks and the TEM images. The monodispersity and a large range of size indicates the superiority of this new method. The high crystallinity is demonstrated by the high resolution TEM image and XRD pattern as shown in Figure S2. Notably, extremely small PbSe QDs with 1stexciton peak at 750nm were obtained for the first time. The size is estimated to be 2.1 nm using sizing curve given by Yu et al.26 PbSe QDs with such small size will show strong quantum confinement and might exhibit new optoelectronic properties. As a heating up method with controllability to release Se precursor, the new synthesis is capable of large scale production. By scaling up the amounts of precursors, we have managed to synthesize 23.5 g PbSe QDs from one batch of reaction as shown in Figure S3. It is worthy to note that the monodispersity was maintained in the large scale synthesis demonstrated by the sharp 1st, 2nd and 3rd exciton peaks.


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Figure 4. Absorption spectra of a large range of size of PbSe QDs and three typical TEM images. The as-synthesized PbSe QDs show excellent air stability in term of optical property. For lead chalcogenide QDs, the surface is exposed with (111), (110) and (100) crystalline faces.42 The (111) face is composed of Pb atoms, while the (100) and (110) face contains Pb and Se atoms. The ratio of crystalline faces depends on the particle size.43 Larger lead chalcogenide QDs have more (100) faces on the surface than small dots, thus large PbSe QDs are much more sensitive to air due to more Se atoms on the surface.44 Therefore, it is important to achieve stable PbSe QDs with different sizes, especially for large ones. Figure 5a shows the absorption spectra of fresh and heated (80 oC in air for 12h in the form of powder) PbSe QDs with four different sizes. No change is observed in all of the spectra, indicating excellent air-stability of these PbSe QDs. Additionally, the as-synthesized PbSe QDs show high PL QYs (Figure 5b), i.e. 28% for PbSe1230 (1st exciton peak position) and 8% for PbSe-1484. Both the PL QYs are in the highest level in the corresponding emission range.45 The PL QYs were maintained during the heating in air as shown in Figure 5b, confirming the excellent stability of these PbSe QDs. Furthermore, the ligand exchanged (1,2-ethanedithiol, EDT) large PbSe QD (1st exciton peak > 2000 nm) thin film also shows outstanding stability in air demonstrated by the unchanged absorption spectrum in 7 days in air (Figure 5c). The stable electronic coupling PbSe QD thin films would substantially facilitate its applications in electronic and optoelectronic devices.


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Figure 5. (a) Absorption spectra of fresh and heated (80 oC in air for 12h in the form of powder) PbSe QDs with four different sizes. (b) Evolutions of PL QYs of PbSe QDs with 1st exciton peak at 1230 nm and 1484 nm when heated at 80 oC. (c) Absorption spectra of fresh and aged EDT treated PbSe QD thin film. We attribute the high stability of these PbSe QDs due to Cl- passivation. Theoretical and experimental studies have shown that halide repairs the under-coordinated sites on the surface of lead chalcogenide QDs, rendering perfect hybrid surface passivation.21, 30, 31, 46 Because the chloride, which comes from PbCl2, is present in the reaction solution during nucleation and growth of QDs, surface reconstruction might allow for optimal surface passivation. Therefore, the in situ chloride passivation can result in better effect than those post-synthetic chloride treatments. For example, the EDT treated PbSe QD thin films here are stable in air while those made of PbSe QDs obtained by post-synthetic halide salt treatment degrade gradually in air (continual redshift of the 1stexciton peak).21 The Cl- content in the PbSe QDs synthesized by our new method is measured to be 42% (compared to Pb) by XPS, which is consistent with previous results. There is no change after the PbSe QD powder was heated at 80 oC for 12h in air as shown in Figure 6, i.e. no oxidized species was formed in the heating process, which accounts for the highly stable absorption spectra and PL QYs. On the contrary, the XPS spectra changed due to the increased contribution of PbO, Pb(OH)2, SeO2, SeO32- when the PbSe QDs synthesized by conventional method was stored in air in hexane.18


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Figure 6. XPS spectra of fresh and heated PbSe QDs. (Pb-X = Pb-oleate, Pb-Cl, PbO, Pb(OH)2) Finally, in order to demonstrate the versatility of the strategy of in situ tuning the reactivity of Se precursor, we extended the synthetic strategy to CdSe and ZnSe QDs. In both syntheses, the size distributions became better as the increase of temperature, demonstrated by the increasing sharpness of the exciton peaks as shown in Figure 7. Injection of additional Se precursor was also adopted to achieve a large range of size for CdSe QDs (Figure S4). Using the heating up method, the synthesis was easily scaled up to produce 3.87 g monodispersive CdSe QDs as shown in Figure S5.

Figure 7. Evolution of absorption spectrum in the synthesis of CdSe and ZnSe QDs via in situ tuning the reactivity of Se precursor.

CONCLUSION A strategy of in situ tuning the reactivity of Se precursor was developed and was successfully adopted to produce ultrastable PbSe QDs. DPPSe and TOPSe were both used as the Se precursor. The reactive DPPSe induced the formation of small nanoparticles while the TOPSe


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did not take part in the reaction at room temperature. As the increase of the solution temperature, the DPP (in DPPSe) was converted TOPSe to reactive DPPSe, promoting the growth of the PbSe QDs and maintaining a high oversaturation condition. PbCl2-oleylamine complex was used as the Pb precursor, allowing in situ chloride passivation. As a result, the as prepared PbSe QDs showed excellent stability in air and high PL QYs for different sizes. The 1st exciton peaks remained unchanged when the PbSe QDs powder were heated at 80 oC in air for 12h. Furthermore, the ligand exchanged electronic coupled PbSe QD thin films also showed excellent stability in air, facilitating their applications in different devices. Extremely slow Ostwald ripening was observed in the new synthesis, facilitating the accurate size control of the PbSe QDs. The growth was carefully controlled to produce a large range of sizes of PbSe QDs with the 1st exciton peak from 750 nm to above 2100 nm. As a heating up method, this new synthesis was easily scaled up to produce 23.5g PbSe QDs from one batch of reaction. Monodispersive CdSe and ZnSe QDs were also synthesized using this new strategy which might be efficient for other metal selenide QDs.

ASSOCIATED CONTENT Supporting Information. Additional absorption spectra of PbSe and CdSe QDs, a typical TEM image and XRD pattern of PbSe QDs. Absorption spectrum and a picture of the 23.5 g PbSe QDs and 3.87 g CdSe QDs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC Grant No. 51302096, 61571206, 51402148), the Hubei Provincial Natural Science Foundation of China (ZRMS2017000370), the Fundamental Research Funds for the Central Universities (No. 2017KFYXJJ039, 2015TS051), the Innovation Foundation of Shenzhen Government (JCYJ20160429182959405), and the Fundamental Research Funds of Wuhan City (no.


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