Phosphine-Free Synthesis of Metal Chalcogenide ... - ACS Publications

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Phosphine-Free Synthesis of Metal Chalcogenide Quantum Dots by Directly Dissolving Chalcogen Dioxides in Alkylthiol as the Precursor Dong Yao, Wei Xin, Zhaoyu Liu, Ze Wang, Jianyou Feng, Chunwei Dong, Yi Liu,* Bai Yang, and Hao Zhang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China

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S Supporting Information *

ABSTRACT: Semiconductor quantum dots (QDs) are competitive emitting materials in developing new-generation light-emitting diodes (LEDs) with high color rendering and broad color gamut. However, the use of highly toxic alkylphosphines cannot be fully avoided in the synthesis of metal selenide and telluride QDs because they are requisite reducing agents and solvents for preparing chalcogen precursors. In this work, we demonstrate the phosphine-free preparation of selenium (Se) and tellurium (Te) precursors by directly dissolving chalcogen dioxides in the alkylthiol under the mild condition. The chalcogen dioxides are reduced to elemental chalcogen clusters, while the alkylthiol is oxidized to disulfides. The chalcogen clusters further combine with the disulfides, generating dispersible chalcogen precursors. The resulting chalcogen precursors are suitable for synthesizing various metal chalcogenide QDs, including CdSe, CdTe, Cu2Te, Ag2Te, PbTe, HgTe, and so forth. In addition, the precursors are of high reactivity, which permits a shorter QD synthesis process at lower temperature. Owing to the high quantum yield (QYs) and easy tunability of the photoluminescence (PL), the as-synthesized QDs are further employed as down-conversion materials to fabricate monochrome and white LEDs. KEYWORDS: quantum dots, phosphine-free synthesis, chalcogen precursors, metal selenide/telluride nanocrystals, light-emitting diodes

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INTRODUCTION

development of low-cost and environment-friendly QD synthesis routes is greatly welcomed.4,5,29 In general, colloidal synthesis of metal chalcogenide QDs relies on the reaction between metal precursors and chalcogen precursors.30,31 For example, the early methods for synthesizing CdSe and CdTe QDs usually use dimethylcadmium and organyl phosphine chalcogenide as the metal precursor and the chalcogen precursor, respectively.32−34 Later, the hazardous and inflammable dimethylcadmium is replaced by dissolving metal salts or metal oxide in organic solvents containing carboxylic or phosphonic acids, which greatly lowered the threshold of QD synthesis.35 However, the toxic, pyrophoric, unstable, and expensive alkylphosphines are still necessary for the preparation of soluble selenide or telluride precursors.15,30,36,37 To avoid the use of alkylphosphines, many efforts have been performed to dissolve elemental chalcogens in organic media.30,38−51 One direct method is heating up the Se powder in various organic solvents, for example, 1-octadecene (ODE), olive oil, oleylamine (OLA), and paraffin, near to the melting point of Se.43−45 Another strategy is the reduction of Se powder in

Quantum dots (QDs) have been intensively investigated as competitive candidates for fabricating light-emitting diodes (LEDs) due to the requirement of next-generation lighting and displays with wide color gamut and high color rendering.1−5 In comparison with other luminescent materials, QDs possess broad absorption spectra, narrow and tunable emission spectra, near unity photoluminescence quantum yields (PLQYs), good physicochemical stability, and solvent processing property.6−8 These advantages make QDs proper full-color solid-state luminescent materials for both the backlight unit of liquidcrystal displays and high performance lighting sources.9−15 Among various tested QDs, metal selenide/telluride QDs show strong and size-dependent PL emission covering the whole visible range and thus attract extensive attention in design and preparation of LEDs.16−20 Accordingly, great energy has been expended to produce high-quality metal selenide/telluride QDs in the past two decades.21−23 Particularly, the synthesis of QDs in colloidal solution becomes one of the most useful routes for producing high-quality QDs with high PLQYs and narrow PL emission because of its capability to tune the QDs’ compositions, sizes, and shapes in a broadening range.24−28 However, the industrial synthesis of QDs for LED applications still meets the challenge of high cost and severe pollution. The © 2017 American Chemical Society

Received: December 21, 2016 Accepted: March 2, 2017 Published: March 2, 2017 9840

DOI: 10.1021/acsami.6b16407 ACS Appl. Mater. Interfaces 2017, 9, 9840−9848

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ACS Applied Materials & Interfaces coordinative solvents, such as NaBH4, or alkylthiol reduction in the presence of OLA.46−48 Despite some achievements in preparing phosphine-free Se precursors to produce high-quality metal selenide QDs, the preparation of phosphine-free Te precursors still needs to be improved. Different from Se, Te is less reactive in organic solvents because of its high melting point (452 °C) compared with that of most organic solvents. In addition, Te is much harder to be reduced to Te2− with the consideration of the standard reduction potentials of elemental Se and Te. Li’s group developed an alternative route to solve this problem. They directly heat TeO2 in trioctylphosphine oxide (TOPO) up to 380 °C for 5 h to form the Te precursor. Although TOPO is greener and safer as a derivative of trioctylphosphine, the method is still not totally phosphine-free.49 Gupta’s group also developed a phosphine-free way to produce Te precursor. They heat the mixture of Te powder and NaBH4 in trioctylamine at 350 °C to form a soluble Te precursor.50 But the high reaction temperature makes this method so energy-intensive and time-consuming. Recently, our group reported an efficient way to produce phosphine-free Te precursors with high reactivity by employing NaBH4 as the reducing agent with the assistance of various alkylamides under relatively low temperature.51 However, the reactivity of the precursors is too high, thus lowering the controllability in QD synthesis. The methods using borohydride to reduce Na2TeO3 and TeO2 in aqueous media are established as well, which avoid the cost and safety issues brought by the use of alkylphosphines.52,53 But these methods only fit in the preparation of Cd-based QDs and fail to achieve QDs with ultrahigh PLQYs and narrow excitonic features because of the low reaction tempertaure limited by the boiling point of water.52,53 In this regard, the development of facile and feasible phosphine-free routes for synthesizing both selenide and telluride QDs is still challenging. Herein, we demonstrate a general phosphine-free method for synthesizing metal selenide and telluride QDs in organic solvents by foremost dissolving chalcogen dioxides in the alkylthiol. In this method, the phosphine-free Se precursor is prepared by dissolving SeO2 in dodecanethiol (DT) under sonication, while TeO2 is dissolved in DT at 100 °C under stirring to form the phosphine-free Te precursor. The SeO2 and TeO2 are reduced by DT to elemental Se0 and Te0 clusters, respectively, whereas DT is oxidized to disulfides. These chalcogen clusters further combine with disulfides, generating dispersible chalcogen precursors. The chalcogen precursors are suitable for synthesizing various metal chalcogenide QDs, including CdSe, CdTe, Cu2Te, Ag2Te, PbTe, HgTe, and so forth. Because of the proper reactivity, the precursors permit to synthesize QDs with good controllability at low temperature and in a short duration in comparison to the conventional phosphine routes. The as-synthesized monodispersed QDs with strong and tunable PL are utilized as down-conversion materials to fabricate monochrome and white LEDs.

Figure 1. (a) Photograph of vials containing 4 mmol of SeO2 in 8 mL of DT after sonication (left) and 4 mmol of TeO2 in 8 mL of DT after maintaining at 100 °C for 5 min (right). (b) Equation of the reaction between SeO2/TeO2 and DT. (c) Photograph of the vial containing 4 mmol of SeO2 dissolved in 8 mL of DT (left) and after addition of 0.5 mL of OLA (right). (d) Photograph of the vial containing 4 mmol of TeO2 dissolved in 8 mL of DT (left) and after addition of 0.5 mL of OLA (right).

1a). During the dissolution process, the chalcogen dioxide (EO2, E = Se, Te) is reduced by DT to elemental chalcogen (E0), and DT is oxidized into disulfides.49,54−58 The proposed sequence of the reaction is shown in Figure 1b. To validate the sequence, nuclear magnetic resonance (NMR) spectroscopy is executed (Figure 2). In comparison with DT, two new peaks

Figure 2. 1H NMR spectra of DT, Se precursor, and Te precursor in CDCl3. Inset: the magnified picture of the chemical shift. The peaks marked in the circles confirm the formation of didodecyl disulfide.

marked with circles appeared in the NMR spectra of Se and Te precursors verifies the formation of disulfides.55−58 The mass spectrometry of DT, Se precursor, and Te precursor further certifies the formation of didodecyl disulfide in the chalcogen precursors (Figures S1−S3).47,48 Although the existence states of the reduced chalcogens are hard to be determined, the well-defined Tyndall effect of the transparent chalcogen precursors in the left panels of Figures 1c and 1d strongly indicates the formation of nanometer-sized chalcogen clusters. Because nanoscale E0 clusters possess a high specific surface area, the Se···S and Te···S nonbonding interactions between E0 clusters and the S of disulfides and remaining DT are capable to facilitate the formation of stable

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RESULTS AND DISCUSSION Chalcogen Precursor. Se precursor is prepared by dissolving 4 mmol of SeO2 in 8 mL of DT under vigorous stirring at room temperature (the same concentration as described in the Methods section), which produces a homogeneous light yellow solution (Figure 1a). Te precursor is prepared by dissolving 4 mmol of TeO2 in 8 mL of DT at 100 °C for 5 min, which generates an orange solution (Figure 9841


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ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Preparation Process of the Soluble Se/Te Precursor and Color Change of the Se/Te Precursor Solution after the Addition of OLA (Corresponding Photographs Are Shown in Figure 1c)

Figure 3. TEM images of the CdTe QDs synthesized with the Cd/Te molar feed ratio of 1.4/1 at 100 (a), 120 (b), 130 (c), 140 (d), 160 (e), and 185 °C (f). The reaction time is 10 min. The average diameters of the QDs are 2.2 (a), 2.6 (b), 2.8 (c), 3.2 (d), 3.3 (e), and 3.6 nm (f). Insets are the TEM size distributions. The scale bar is 50 nm.

and transparent solution.59−62 The chemical shifts (2.98 ppm for the Se precursor and 3.13 ppm for the Te precursor) in the NMR spectra provide direct evdience to confirm the nonbonding interactions between Se/Te and S (Figure 2).59,60 As a control, the raw Se or Te powder cannot be dissolved in DT directly under the same treatment mentioned above (Figure S4). Because of the large size, the specific surface area of raw Se or Te powder is much lower than that of E0 clusters. As a result, the nonbonding interactions between Se0/Te0 and S are negligible. To further verify the existence of the nonbonding interactions, an extra coordinating organic solvent (OLA) is added to the chalcogen precursors. The solution of Se

precursor turns from light yellow to dark red after the addition of OLA (right panel of Figure 1c). This phenomenon is attributed to the further reduction of Se0 clusters by the remaining DT and the subsequent formation of the Se−OLA complex, which is consistent with our previous results named alkylthiol-enabled Se powder dissolution in OLA (Scheme 1).47,48 Different from Se, Te precursor turns gray and black precipitates appear when OLA is added (right panel of Figure 1d). The black precipitates are identified to be hexagonal Te by X-ray diffraction (XRD) characterization (Figure S5). In addition, X-ray photoelectron spectroscopy (XPS) spectra of the black precipitates show two peaks at 583.2 and 572.9 eV, 9842


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Figure 4. UV−vis absorption (a) and PL emission (b) spectra of the CdTe QDs that are synthesized by altering the reaction temperature at 100, 120, 130, 140, 160, and 185 °C. The corresponding PL images excited by 365 nm UV light are shown in (c).

which further supports the valence state of Te0 (Figure S6).63 Because of the low reduction potential, Te0 cannot be further reduced by DT let alone coordinates with OLA to form the Te−OLA complex. After adding OLA, the intrinsic Te···S nonbonding interactions are completely damaged, which disturbs the total balance in the precursor solution and promote the aggregation of the small Te0 clusters. Since the solvent molecules are incapable to stabilize the aggregated Te0 clusters, black Te precipitates appear. The dissolution and precipitation processes are shown in Scheme 1. Figure S7 presents the NMR spectra of OLA, DT, and the chalcogen precursors mixed with OLA. The disappearance of the specific chemical shifts ascribed to Se···S and Te···S nonbonding interactions further confirms this conclusion.59,60 Synthesis of Cadmium Chalcogenide. In conventional alkylphosphine-assisted synthesis of metal chalcogenide QDs, chalcogens need to be cleaved from organyl phosphine chalcogenide precursors before reacting with metal precursors.64 This cleavage usually requires a high reaction temperature above 200 °C.32,35,64 With respect to the current phosphine-free chalcogen precursors, the chalcogens exist in the form of Se0 and Te0 clusters, dispersing in the solution via nonbonding interactions. Because of the high specific surface area, the phosphine-free E0 precursors can be easily reduced by organic molecules with unsaturated bonds, such as ODE, OA, and OLA, to form chalcogen-bridged compounds and further permit to synthesize metal chalcogenide QDs under low temperature.30,31,39

Among the family of metal chalcogenide QDs, CdSe and CdTe QDs are the most common and widely investigated.35−37 To validate that our phosphine-free chalcogen precursors are available for synthesizing metal chalcogenide QDs, CdSe and CdTe QDs are synthesized as the examples. A typical synthesis of CdSe QDs is executed by injecting the phosphine-free Se precursor into the solution containing cadmium acetate (Cd(Ac)2), oleic acid (OA), and ODE at 150 °C. The total reaction duration is 10 min (details in the Methods section). The transmission election microscopy (TEM) image is shown in Figure S8a, from which it can be seen that the CdSe QDs possess quasi-spherical shapes with narrow size distribution and the average diameter is 2.1 ± 0.3 nm. The XRD pattern in Figure S8c indicates that the products possess a cubic crystal phase.28 Figure S8b shows the UV−vis absorption and PL emission spectra of the as-synthesized CdSe QDs. The small Stokes shift indicates the well-ordered atomic array in their crystals structures. The narrow full width at half-maximum (fwhm) of the PL spectrum coincides well with the uniform size distribution of the QDs. In the CdSe QDs, the Cd-to-Se atomic ratio is 1/0.94, which is close to the feed ratio of 1/1 (Figure S8d). The slight excess of Cd derives from the Cd-rich surface of QDs.29,35 The XPS spectrum further reveals the valence state of the constituent Cd2+ and Se2− (Figure S8d).29,35 In comparison with CdSe, it is difficult to synthesize CdTe QDs on the basis of the existing phosphine-free routes.50,51 So, more efforts are devoted to synthesize CdTe QDs with our phosphine-free chalcogen precursors. CdTe QDs with different 9843


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ACS Applied Materials & Interfaces sizes are synthesized by tuning the reaction temperature at 100, 120, 130, 140, 160, and 185 °C (details in the Methods section). With increasing the reaction temperature, the diameters of the QDs increase. Figure 3 shows TEM images of CdTe QDs synthesized under different temperatures. The QDs appear as quasi-spherical particles with the average diameters increasing from 2.2 to 3.6 nm upon the temperature increment from 100 to 185 °C. Furthermore, the as-synthesized CdTe QDs exhibit the narrow size distribution, which are comparable to those obtained by non-phosphine-free precursors (insets of Figure 3).32,35 The sharp UV−vis absorption spectra of the QDs are in good agreement with the size dispersion (Figure 4a).32,35 Figure 4b indicates the PL emission spectra of the as-synthesized QDs with emission peaks in the range from 522 to 643 nm and narrow fwhm less than 30 nm (see details in Table 1). The PL image in Figure 4c shows the

Apart from monochrome LEDs, the white LED (WLED) comprising three primary colors is also produced by taking full advantage of efficient optical excitation of the CdTe QDs. The WLED is composed of two CdTe QD emitters of green and red as the conversion layer and a commercial 3 × 3 area GaInN chip with blue emission centered at 450 nm (Figure S10). Figure 6a shows the highly bright emission of the resulting WLED. The three color points are located in CIE coordinates with the RGB coordinates of (0.68, 0.31), (0.28, 0.60), and (0.14, 0.03), respectively, which form an area color triangle. The area of the RGB triangle covers most of the NTSC color space (87%), which is superior to that of rare-earth phosphors (Figure 6b).66 Based on the colorimetry theory, the LED can emit any color in the triangle by mixing CdTe QDs with different emission colors under specific ratio. The black curve in Figure 6b indicates the variation of color temperature according to the color coordinates. Color temperature constants are the points on the black straight lines. These results indicate that the color temperature of LEDs can be tuned in a wide range, covering all color temperatures from the poles to the equator. Owing to the good PL properties of our CdTe QDs, the asprepared WLED exhibits narrow emission spectra and high luminous efficacy up to 31 lm/W at the coordinate (0.31, 0.30). The color temperature is 6936 K. Applicability of Phosphine-Free Te Precursor. To verify the applicability of our phosphine-free Te precursor, other telluride QDs are also synthesized. Figure S11a shows the TEM image of Cu2Te QDs, which is consistently shaped with the average diameter of 4.2 ± 0.4 nm. Different from CdTe QDs, Cu2Te QDs exhibit a good absorption in near-infrared region (Figure S11b). The XRD pattern in Figure S11c indicates the hexagonal structure of Cu2Te QDs.67 The composition of Cu2Te QDs is further investigated by EDX (Figure S11d). The Cu/Te atomic ratio is 2.0/1.0, which is in good agreement with the molar feed ratio. The XPS spectrum shows the valence state of each element in Cu2Te QDs (Figure S11e,f). Peaks at 932 and 951.7 eV appear in the Cu 2p spectrum, indicating the existence of Cu+.51,67 There are two strong peaks (572.1 and 582.6 eV) in the Te 3d spectrum, which is consistent with Te2− binding energy.51,67 The small peaks at around 576.2 and 586.6 eV can be attributed to the oxidation of a little Te on the surface of QDs.51,67 Apart from Cu2Te, telluride QDs such as Ag2Te, PbTe, and HgTe are also synthesized using the Te precursor (see details in Methods section). The size, structure, and composition of these QDs are revealed by TEM, UV−vis absorption spectra, XRD, EDX, and XPS (Figures S12−S14).51,67−70 These results further confirm that our Te precursor is applicable for synthesizing a variety of metal telluride QDs.

Table 1. Summary of the Peak Position, Fwhm, PLQYs, and the PLQYs after 1-month Storage under Room Light of the CdTe QDs Shown in Figure 3 PL peak position (nm)

fwhm (nm)

PLQYs (%)

PLQYs (%) after 1 month

522 561 579 606 624 643

32 30 28 30 29 28

31 36 47 33 38 44

30 36 48 33 38 44

color change of bright emissions from green to red. The assynthesized CdTe QDs show high PLQYs up to 47% without any overcoating treatment (Table 1). In addition, the CdTe QD solution is very stable. The PLQYs are almost unchanged after stored under room light for 1 month (Table 1). The CdTe QDs shown in Figure 4c are further characterized by XRD, XPS, and (energy-dispersive X-ray spectroscopy) EDX (Figure S9). The results reveal that the cubic CdTe QDs possess a Cdto-Te atomic ratio of 1.2/1.0, and the valence states of Cd and Te are +2 and −2, respectively.32,35,51 As a comparison, the PL emission of the CdTe QDs synthesized by the conventional alkylphosphine-assisted methods usually covers the visible region from green to red (510−670 nm) with the fwhm around 30 nm and the PLQY of 5−50%. This means that the PL properties of our CdTe QDs are comparable to those of previous ones.1,5,32 LEDs from CdTe QDs. Because of the good emission tunability and stability, narrow emission spectra, and high PLQYs, the as-synthesized CdTe QDs are further utilized as down-conversion materials to fabricate LEDs. For monochrome LEDs, CdTe QDs with green, yellow, and red emissions are respectively mixed with curable resin to form the conversion layer on the commercial GaInN chips (details in the Methods section).65 The emission of the GaInN chips centers at 365 nm. After energizing, the LED devices exhibit bright emissions of green, yellow, and red with the color coordinates of (0.28, 0.60), (0.51, 0.45), and (0.68, 0.31) (Figure 5). The corresponding emission spectra are shown in Figures 5c, 5f, and 5i, which are obviously narrower than those of rare-earth phosphor-based LEDs.66 The narrow emission spectra guarantee the high color purity, which is essential for the LEDs with display applications.5,24

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CONCLUSIONS In summary, we demonstrate a facile and feasible method to prepare phosphine-free chalcogen precursors by directly dissolving EO2 (E = Se, Te) in alkylthiols. During the preparation, EO2 is reduced to E0 clusters by alkylthiols, while the alkylthiols are oxidized to disulfides. The E0 clusters further combines with disulfides, thus generating dispersible chalcogen precursors. Because of the high reactivity, the phosphine-free chalcogen precursors are suitable for synthesizing high quality CdSe and CdTe QDs under mild conditions. The as-synthesized monodispersed QDs with strong and tunable PL emission are employed as down-conversion materials to fabricate monochrome and white LEDs. The 9844


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Figure 5. LED prototypes with green (a), yellow (d), and red (g) emission. Insets: photographs of the LEDs taken under sunlight. Corresponding color coordinates are (b) (0.28, 0.60), (e) (0.51, 0.45), and (h) (0.68, 0.31). Corresponding emission spectra are shown in (c), (f), and (i). The emission of the LED chips centers at 365 nm, and the operating voltage is 3.0 V.

Figure 6. (a) LED prototype with white emission. (b) Corresponding color coordinate is (0.31, 0.30). Corresponding emission spectrum is shown in (c). The emission peak at 450 nm is from the GaInN LED chip, and the operating voltage is 9 V.

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devices exhibit narrow emission spectra and high luminous efficacy up to 31 lm/W. In addition, our phosphine-free chalcogen precursors are applicable for synthesizing a variety of metal chalcogenide QDs, which will facilitate the development of industrial synthesis of QDs via cheap and environmentfriendly routes.

METHODS

Materials. Selenium dioxide (SeO2, 99%) and tellurium dioxide (TeO2, 99.99%) were obtained from Aladdin Industrial Corporation, China. 1-Octadecene (ODE, 90%), oleic acid (OA, 90%), and oleylamine (OLA, 70%) were obtained from Aldrich. Dodecanethiol (DT, 98%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Cadmium acetate dihydrate (Cd(Ac)2·2H2O, 97%), cadmium chloride dihydrate (CdCl2·2H2O, 98%), copper chloride dihydrate (CuCl2· 9845


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ACS Applied Materials & Interfaces H2O, 99%), mercury chloride dihydrate (HgCl2·2H2O, 99%), silver nitrate (AgNO3, 99%), lead chloride (PbCl2, 99%), chloroform (99%), and ethanol (99.5%) were obtained from Beijing Chemical Reagent Ltd., China. All of the reagents were used as received. Preparation of Phosphine-Free Se Precursor. For the Se precursor, 1 mmol of SeO2 and 2 mL of DT were loaded in the threeneck flask. The flask was degassed by purging N2. The mixture was stirred vigorously in a N2 atmosphere until the SeO2 powder was dissolved. Finally, the solution turned light yellow. Preparation of Phosphine-Free Te Precursor. For the Te precursor, 1 mmol of TeO2 and 2 mL of DT were loaded in the threeneck flask. The flask was degassed by purging N2. The mixture was stirred vigorously and maintained at 100 °C in a N2 atmosphere until the TeO2 powder was dissolved. Finally, the solution turned orange. Synthesis of CdSe QDs. Typically, 8 mL of ODE, 1 mmol of Cd(Ac)2·2H2O, and 2 mL of OA were loaded in the four-neck flask. The flask was degassed by purging N2. Then the mixture was maintained under vacuum for 30 min at 100 °C to remove trace water. Afterward, the mixture was heated to 150 °C under N2 protection. The Se precursor was quickly injected at 150 °C. The reaction was maintained for 10 min and terminated by removing the heating source and cooling down the temperature of the flask with a water bath. Synthesis of CdTe QDs. Typically, 1.4 mmol of CdCl2·2H2O, and 10 mL of OLA were loaded in the four-neck flask. The flask was degassed by purging N2. Then the mixture was maintained under vacuum for 30 min at 100 °C to remove trace water. Afterward, the mixture was heated to a desired temperature (100, 120, 130, 140, 160, and 185 °C) under a N2 atmosphere. The Te precursor was quickly injected at the desired temperature. The reaction was maintained for 10 min and terminated by removing the heating source and cooling down the temperature of the flask with a water bath. Synthesis of Cu2Te QDs. A typical synthetic procedure of Cu2Te QDs was similar to that of CdTe QDs, except using CuCl2·2H2O rather than CdCl2·2H2O. The reaction temperature was 180 °C. The reaction duration was 15 min. Synthesis of Ag2Te QDs. Typically, 2 mmol of AgNO3, and 10 mL of OLA were loaded in the four-neck flask. The flask was degassed by purging N2. Then the mixture was heated to 180 °C. The solution turned brown at 90 °C, which meant the formation of Ag QDs. The Te precursor was quickly injected at 180 °C. The total reaction duration was 20 min. Synthesis of PbTe QDs. Typically, 4 mL of ODE, 1 mmol of PbCl2, 3 mL of OLA, and 3 mL of OA were loaded in the four-neck flask. The flask was degassed by purging N2. Then the mixture was maintained under vacuum at 100 °C for 30 min to remove trace water. Afterward, the mixture was heated to 150 °C under N2 projection. The Te precursor was quickly injected at 150 °C. The total reaction duration was 7 min. Synthesis of HgTe QDs. Typically, 1 mmol of HgCl2·2H2O and 10 mL of OLA were loaded in the four-neck flask. The flask was degassed by purging N2. Then the mixture was heated to 100 °C under N2 projection. The Te precursor was quickly injected at 100 °C. The reaction duration was 5 min. Purification. The purification was according to our previous report.47,48 Typically, 5 mL of chloroform was first added into the QD solution. Then the mixed solution was centrifugated at 3000 rpm for 3 min to discard the precipitates comprising of byproducts and unreacted precursors. Ethanol was added into the supernatant, and the solution was centrifugated at 8000 rpm for 5 min. The precipitates were collected and redispersed in 10 mL of chloroform. Fabrication of QD-LEDs. Two types of commercial InGaN LED chips were utilized as back-light source. All the chips were produced by Shen Zhen Hongcai Electronics Co., Ltd. The chips with operating voltage at 3 V and emission at 365 nm were used for preparing monochrome LEDs. The chips with operating voltage at 9 V and emission at 450 nm were used for preparing WLEDs. To prepare the down-conversion layer, CdTe QDs with different emission colors were premixed homogeneously with specific ratio and dried to powders. Then, the powders were mixed with the curable resin according to our previous method.48 The mixture was loaded covering the surface of the

chip and kept under vacuum to remove gas bubbles. After curing under UV-light for 2 min, the LEDs from CdTe QDs were fabricated. Characterization. 1H NMR measurement was performed on a Bruker Ultra Shield 500 MHz spectrometer. The NMR samples were dissolved in CDCl3 with tetramethylsilane. The GC-MS analysis experiment was performed on an Agilent 5975 instrument. Optical absorption spectra were characterized by a Shimadzu 3600 UV−vis− NIR spectrophotometer. Photoluminescent emission was tested on a Shimadzu RF-5301 PC spectrophotometer with the excitation wavelength of 400 nm. The absolute QYs was collected on Edinburgh FLS920 (excited at 400 nm) with a calibrated integrating sphere system. The TEM and selected area electron diffraction (SAED) images were characterized by a Hitachi H-800 electron microscope. The acceleration voltage was 200 kV. XRD patterns were obtained by a Siemens D5005 diffractometer. EDX measurement was carried out on FEI scanning electron microscope equipped with Oxford INCA Energy TEM 200. XPS was performed using a VGESCALAB MKII spectrometer with a Mg X-ray source.

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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/acsami.6b16407. Ion chromatogram, mass spectrum, XRD, XPS, and 1H NMR spectra of chalcogen precursors, TEM images, UV−vis absorption and PL emission spectra, XRD, EDX, and XPS of CdSe, CdTe, Cu2Te, Ag2Te, PbTe, and HgTe QDs, and the characterizations of the commercial GaInN chip (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax +86 431 85193423; e-mail [email protected] (Y.L.). ORCID

Yi Liu: 0000-0003-0548-6073 Bai Yang: 0000-0002-3873-075X Hao Zhang: 0000-0002-2373-1100 Author Contributions

H.Z. and Y.L. proposed and supervised the project. H.Z., D.Y., Y.L., and B.Y. designed and performed the experiments and cowrote the paper. D.Y., W.X., Z.Y.L., Z.W., J.Y.F., and C.W.D. participated in most experiments. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFB0401701), NSFC (51425303, 21374042), and the Special Project from MOST of China.

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REFERENCES

(1) Rogach, A. L.; Gaponik, N.; Lupton, J. M.; Bertoni, C.; Gallardo, D. E.; Dunn, S.; Li Pira, N.; Paderi, M.; Repetto, P.; Romanov, S. G.; O’Dwyer, C.; Torres, C. M. S.; Eychmüller, A. Light-Emitting Diodes with Semiconductor Nanocrystals. Angew. Chem., Int. Ed. 2008, 47, 6538−6549. (2) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.; Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012−1057.

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