Quantum Dots for White Light-Emitting Diodes - ACS Publications

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Functional Inorganic Materials and Devices

Rational Control of Size and Photoluminescence of WS2 Quantum Dots for White LEDs Wenxu Yin, Xue Bai, Ping Chen, Xiaoyu Zhang, Liang Su, Changyin Ji, Haoming Gao, Hongwei Song, and William W. Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17966 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Applied Materials & Interfaces

Rational Control of Size and Photoluminescence of WS2 Quantum Dots for White LEDs

Wenxu Yin,1 Xue Bai,1,* Ping Chen,1 Xiaoyu Zhang,1 Liang Su,1 Changyin Ji, 1 Haoming Gao,1 Hongwei Song,1 William W. Yu1,2 1

State Key Laboratory of Integrated Optoelectronics and College of Electronic Science

and Engineering, Jilin University, Changchun 130012, China 2

Department of Chemistry and Physics, Louisiana State University, Shreveport, LA 71115,

USA

*Address correspondences to [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Transition metal dichalcogenides like tungsten disulfide (WS2) are luminescent but still bear low quantum yields and hardly meet the requirement of practical applications. Here, we developed a hot injection method to prepare highly luminescent WS2 quantum dots with tunable particle size in a noncoordinating solvent, some are even smaller than its Bohr-radius. The as-synthesized WS2 quantum dots exhibit a narrow size distribution and a high photoluminescence quantum yield of 32%, the highest record compared to the known reports. WS2 quantum dot layer was employed to fabricate white light-emitting diodes with a maximum brightness of 507 cd/m2, adjustable color temperatures from 4100 to 10000 K, and excellent color rendering index of 91. These results confirm the promising optoelectronic applications of WS2 quantum dots. Keywords:

tungsten

disulfide,

quantum

dot,

LED,

noncoordinating

solvent,

photoluminescenc

2


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Introduction Ever since the transition metal dichalcogenide (TMDC) monolayers were discovered with a direct bandgap,1-2 TMDCs have become a new class of semiconductors for optoelectronic applications.3-11 They possess tunable bandgaps and attractive optical phenomena such as indirect-to-direct bandgap crossover,12 valley-selective circular dichroism,13-15 and strong photocurrent responses.16-17 As one of the typical TMDCs, WS2 monolayers are normally characterized by several excitonic absorption peaks centered at 450, 550 and 625 nm, and one primary exciton emission band at 630 nm.18-19 However, their photoluminescence (PL) quantum yield (QY) is only around 10-4-10-2,8, 20-21

far below the values expected for a direct bandgap semiconductor. It has been found that with remarkable reduction in the lateral size of the

two-dimensional monolayers, WS2 quantum dots (QDs) display a significant increase in the direct transition energy (3.16 eV) compared to that of monolayers (2.1 eV).22 Unique properties such as excellent catalytic activity23 and high current density24 have also been observed from QDs due to their more edged atoms, larger surface area, and size confinement.9, 24-26 However, the highest QY for WS2 QDs is only 6.6%,27 still too low for practical optoelectronic applications. The main reason is that the current leading synthesis for WS2 QDs basically relies on exfoliation,24, 28-29 in which a large amount of defects such as vacancies or dangling bonds generated on QD’s surface.30 These surface defects act as the non-radiative electron-hole recombination paths and lead to the quenching of luminescence. Moreover, exfoliation is hard to rationally control the particle size.31 For instance, various sizes of WS2 QDs were obtained through separating the fragment from exfoliation procedure by centrifugation with different speeds,31 from which the WS2 QDs with broad size distributions were obtained. It is hindered to explore the size-dependent optical performances of WS2 QDs.

3



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Here, a one-pot synthesis method to prepare WS2 QDs using “hot injection” reaction of tungsten hexachloride (WCl6) and thioacetamide (TAA) in a noncoordinating solvent was realized. The advantage of the synthesis is that the oleic acid (OA) and oleylamine (OLA) ligands are in situ linked on the surface of the WS2 QDs, controlling nanoparticle growth and stabilizing them. Thus, a rational control of the size, from 1 to 4 nm with narrow size distribution, was achieved. Moreover, the as-prepared WS2 QDs exhibit visible emission with a QY of 32%, much higher than the reported values, benefiting from an efficient surface passivation of QDs during the synthesis procedure.27 These WS2 QDs are applied for LEDs; the electroluminescence (EL) devices show bright white light emission and bias-controlled color temperature. Results and discussion We got an easy way to tune the size of WS2 QDs by only changing the reaction temperature. The QD size is larger with higher reaction temperature (Figure 1a-c). TEM images show a narrow size distribution (Figure 1g-i) and monodisperse for the as-synthesized particles. The average diameter is 1.4, 2.3 and 3.7 nm at the reaction temperatures of 255, 275 and 295 °C, respectively. The increase of QD size at the higher reaction temperature is mainly resulted from the fact that the particles will grow faster so the final particle size is bigger.32 The crystal lattices could be well recognized from HRTEM images (Figure 1d-f) indicating the high crystallinity. The lattice distances for different sizes of QDs were identical to be 2.05 Å, which is in good agreement with (006) plane of the 2H-phase WS2 crystal,26, 33 suggesting that crystalline WS2 QDs were formed. In addition, the blurred shade on the edge of QDs may be some amorphous states (Figure S1 and S2). The gradual brightening of SAED patterns (Figure S3) indicates the increased QD crystallinity with reaction temperature. The rational control of the QD size takes the advantage of the hot-injection approach. 34-35

4


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The XRD pattern of WS2 QDs gives peaks at 14.3˚, 28.8˚, 32.7˚, 33.5˚, 40.1˚ and 49.2˚ (Figure 2a) and they can be indexed to (002), (004), (100), (101), (103) and (105) crystal planes of 2H-phase WS2 crystal (JCPDS No. 84-1398), respectively.36 The Raman spectrum of WS2 QDs (Figure 2b) shows weak peaks at 413 and 356 cm-1 of A1g and E2g modes, respectively.37 Compared with the bulk WS2 in literatures, 9, 23, 38 the separation between A1g and E2g peaks of WS2 QDs is closer owning to the decreased number of layers from bulk’s material to few-layers of QDs, consisting with the previous reports.9, 23, 38

Two factors are responsible for the broad Raman peaks. On one hand, the signal for

TMDC QDs is very weak and uneasy to detect.39 On the other hand, there are some organic ligands on the surface of the QDs, which disturbs the signal and leads to the broad peak. The surface groups of QDs were characterized by FTIR spectroscopy (Figure S4). Intense IR vibrations peaks at 3460, 1722 and 1555 cm-1 are attributed to OH, C=O and COO stretching bands of carboxylic group.27 Also, the peak at 1647 cm-1 is attributed to C=C stretching, and a strong NH2 deformation peak presents at 3360 cm-1. Those results disclose that the surface of QDs is connected with long chain OA and OLA, which prevents QDs from overgrowing and aggregation. XPS was carried out to identify the element states in the as-synthesized WS2 QDs at reaction temperatures of 295 °C. The integral spectrum and elemental composition of the content is given in Figure S5. The high-resolution XPS spectrum for the tungsten element can be be fitted into four peaks (Figure 2c). Among them, the W 4f5/2 and W 4f7/2 peaks at 35.1 and 32.9 eV are assigned to W4+ in 2H-phase WS2;40-41 and the peaks at 34.3 and 32.1 eV could be assigned to W4+ of 1T-phase WS2. Similarly, in the high-resolution spectrum of S, the two deconvoluted peaks located at 163.7 and 162.6 eV are from S 2p3/2 and S 2p1/2 for S2- in 2H-phase WS2, and the other two signatures recorded at 163.1 and 161.8 eV are associated with the S2- of 1T-phase WS2 (Figure 2d). 5



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In these results, the peaks assinged to 2H-phase are almost identical to that acquired from the WS2 bulk materials (Figure S6). In addition, it is known that the transition metal and chalcogen XPS characteristic peaks of the 1T-phase are identified at the lower binding energy side (~1.0 eV) with respect to the 2H-phase. Therefore, the QD sample contains a small amount of 1T-phase crystals. It is necessary to note that the 1T-phase with an octahedral coordination for the metal atom normally shows metallic properties, and no luminescence originated from this 1T-phase WS2.11 The QDs (synthesized at the reaction temperature of 295 °C) have a strong absorption in UV region and a broad absorption tail in blue and green regions (Figure 2e). The absence of feature absorptions in QDs is attributed to the excitonic feature.18 Under 365 nm excitation, intense blue PL was detected (inset of Figure 2e). For the excitation wavelengths of 300-340 nm, QDs display a broad excitation-independent emission band. Under the low-energy excitation of 360-460 nm, the emission band shifts obviously with the excitation wavelength (Figure 2f). Except for the conduction band minimum (CBM) and the valence band maximum (VBM) in WS2 QD, there are still some surface defect levels, which are from the organic molecules and/or some vacancies on the surface of QDs. The temperature-dependent PL measurements were applied to distinguish the PL origin. Under 340 nm excitation wavelength, an 11 nm emission band red-shift was observed when temperature increased from 20 to 100 °C (Figure S7). The PL peak red-shifting is corresponding to the temperature dependent semiconductor properties.42 Since the PL peaks under higher energy excitation wavelengths (λex≤340 nm) showed an excitation-independent behavior, exciton recombination governed the major emission.42 Contrarily, under 420 nm excitation wavelength, both the PL peaks (2 nm red-shift) at 20 and 100 °C only show negligible changes when the temperature increased (Figure S8), demonstrating

6


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these PL emissions under lower energy excitation wavelengths (λex ＞340 nm, Figure 2f) are controlled by the surface states. For WS2, there are direct and indirect gaps in the band structure irrespective of the WS2 thicknesses.18 When carriers are excited by higher energy wavelengths (λex≤340 nm), some of them will locate over the CBM and the others will contribute to the surface related emissions. As shown in the fitting of a PL spectrum (Figure S9), we can generally find three luminescent components in the broad excitation-independent emission band. Two components of them are centered at 390 and 440 nm (Figure 2f), respectively, and the energy difference between them (~370 meV) approximately matches the spin-orbital splitting of the valence band (400 meV).19 Another macroscopic peak at the longer wavelength is induced by surface defect states, showing the excitation-wavelength-dependent property. The size-dependent optical properties of WS2 QDs were further studied. The absorption spectra of the WS2 QDs with different size are shown in Figure 3a, and the optical bandgap are quantified from the Tauc plots of (αhν)2 versus hν (Figure S10). The bandgap value increases obviously for 1.4 nm QDs owning to the size confinement. The confinement could also be observed from excitonic emission band under 320 nm excitation,43 and a clear PL shift was observed in the PL of 1.4 nm QDs (Figure 3b and S11). But, a small shift could be detected for the surface-state related emission under 440 nm excitation. As calculated in the experimental section, the Bohr-radius of WS2 was determined to be 1.7 nm. The small Bohr-radius will lead to a weak quantum confinement effect, and thus induces the little size-dependent absorption and PL emission. To the knowledge of the authors, this is the first evident PL characteristics for size confinement effect of WS2 QDs. The PL QYs were measured for WS2 QDs (Table S2). For the QDs synthesized at 295 °C, the QY is 15%, 2.3 times of the reported highest QY (6.6%, Table S1), owing to a 7



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good surface passivation by OA and OLA.44 It is clear to see that the QYs for WS2 QDs increase with reaction temperature. This tendency is probably related to the improved crystallinity, which can be identified by the HRTEM images discussed above. The better crystallinity and fewer defects result in the reduction of the non-radiative recombination and subsequently enhance the QY. To improve the QY, the surface defects could be further efficiently reduced by providing more S atoms. Normally, the TMDC nanostructure’ surface has plenty of sulfur vacancies that induce non-radiative recombination and low QYs.45-46 The precursors’ molar ratio was adjusted from 1:2 to 1:3 and 1:4 for WCl6:TAA to increase sulfur content in QDs. As a result, the W/S molar ratios of corresponding products were detected as 1:1.3, 1:1.9 and 1:2.4 respectively by ICP-MS, but the profile and positions of absorption and emission spectra remain the same (Figure S12 and S13), and the emission intensity and QY increase (Figure 3c and Table S2). When tungsten achieves balance, the excess sulfur fills the sulfur vacancies to replace the adsorbed oxygen and hydroxyl, etc. and thus the QY is improved. With the increase of S, QDs had shorter emission decay time indicating that defects are better passivated by extra sulfur (Figure 3d). Although the faster decay time has a negligible change (Table S3), the relative ratio of the direct radiative recombination component increases from 41% to 51% and then 54% along with the S ratio increases. Not only the slower decay time decreased, but the trap states related recombination component decreased from 59% to 49% and then 46% along with the S ratio increase, clearly showing the increase of radiative recombination and decrease of non-radiative recombination as the result of S passivation. These results indicate that some of the trap states have been modified through changing the W/S ratio. With this treatment, a higher QY of 32% is achieved with W/S ratio of 1:2.4. This QY is 12.8 times comparing with 2.5% for W/S 1:1.3. Apparently the excess sulfur plays a crucial role to passivate WS2 QDs and ensures an efficient PL. 8


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Consequently, the WS2 QDs were employed to fabricate LEDs by all-solution process. The emitting layer of WS2 QD film is placed inbetween the inorganic and organic charge transport layers. The cross-section SEM image of the LED (Figure 4a) shows a multilayer structure with patterned ITO, PEDOT:PSS (35 nm thickness), PVK (30 nm), QDs (15 nm), ZnMgO NCs (40 nm), and Ag. PEDOT:PSS is a buffer layer on ITO for stable and pinhole-free electrical conduction and to increase the anode work function. PVK is insoluble in hexane, so the spin-coating of the QD-hexane solution would not influence the PVK hole transport layer (HTL) nor mix any interlayer. The QD film‘s CBM and the VBM were confirmed to be −6.28 and -2.73 eV, respectively, by UPS (Figure S14). In order to get a better injection, ZnMgO NCs act as the electron transport layer (ETL) due to their CBM of −3.58 eV (Figure S15), which is closer to the CBM of WS2 QDs than that of ZnO NCs.47 The highest occupied molecular orbital (HOMO) of PVK was determined to be −5.8 eV.48 Therefore, the schematic diagram of the flat-band energy level of our QD-LED was achieved (Figure 4b). PVK’s deep HOMO energy level could reduce the hole-injection barrier from PEDOT:PSS to QDs and improve the hole injection into the active QD layer, and there are fewer electron-injection barriers in the interlayer of ZnMgO/QDs. Thus, electrons and holes could be both efficiently transferred to QD layer and recombined radiatively, resulting in an EL spectrum dominated by QDs. Figure 4c shows the current density–voltage and the luminance–voltage characteristics of QD-LEDs. The devices exhibit a turn-on voltage of 4.5 V and the maximum luminance of the QD-LEDs is 507 cd/m2 at 10 V. Though water-soluble WS2 QDs were once employed for LEDs, there were no data on brightness and efficiency.31 Thus, this is the first report on the brightness of WS2 QD-LEDs, which is benefiting from the as-prepared organic-soluble QDs that making the WS2 QD-layer with much better quality compared with that of water-soluble QDs, and indicates that the as-prepared WS2 QDs are extremely important in the fabrication of thin film optoelectronic devices.49-51 The QD-LED’s EL spectra indicate that the device spectra could cover the full 9



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range of visible light (Figure 4d). The EL peak position is different from the PL peak position, which can be assigned to the luminescence from defect level. The charge carriers are easier to inject into lower defect levels, leading to a small amount of high-energy blue light luminescence and a large luminescence component from the defect energy levels. With higher bias, the surface-related emission became obvious, because they could be efficiently realized at high current density compared to the band-edge recombination. Therefore, the LED’s Commission Internationale de Eclairage (CIE) coordinates and color temperatures could be varied from cool white to warm white by tuning the bias from 10 to 14 V without obvious brightness variations (Figure 4c and inset of Figure 4d). The standard CIE coordinates are (0.28, 0.28), (0.30, 0.31), (0.36, 0.38), and (0.38, 0.38) for 11, 12, 13, and 14 V, respectively, corresponding to the color temperature from 4100 to 10000 K (Figure S16). Besides, a color rendering index of 91 is achieved (Table S4). This single LED with variable color temperature but nearly the same brightness via adjusting voltage could be very useful for specialty applications, such as creating variable atmospheres in private spaces, galleries, museums, and hospitals. Conclusions Small WS2 QDs were synthesized by a hot injection approach. They exhibit size dependent optical property and a 32 % PL QY that is much higher than known records. This high QY results from the efficient surface passivation of QDs as well as the cooperation between the band-edge and surface-related transitions. The size confinement effect for the excitonic transitions is identified for the first time with the size comparable to the Bohr radius of WS2. White light WS2 QD-LEDs are realized with CIE coordinates of (0.33±0.05, 0.33±0.05), maximal CRI of 91, and maximal luminescence of 507 cd/m2. This is the best performance so far for WS2 QD-LEDs and reveals an excellent electroluminescence potential. 10


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Experimental section Materials Tungsten hexachloride (WCl6, 99%), oleic acid (OA, 90%), oleylamine (OLA, 90%) and WS2 powder (99.9%) were attained from Aladdin Industrial. Thioacetamide (TAA, 99%) was obtained from J&K Scientific Ltd. Octadecene (ODE, 90%) and tetramethylammoniumhydroxide (TMAH) were obtained from Alfa Aesar. Magnesium acetate tetrahydrate (Mg(Ac)2 · 4H2O, 99%), poly(9-vinylcarbazole) (PVK, MW = 25000-50000), silver (Ag, 99.999%) and zinc acetate dihydrate (Zn(Ac)2 · 2H2O, 99.99%) were bought from Sigma-Aldrich. Poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was achieved from Clevios. Methanol (99.5%), ethanol (99.5%), acetone (99.5%), chlorobenzene (99.8%), toluene (99.5%) dimethyl sulfoxide (DMSO, 99.7%) and n-hexane (99.5%) were achieved from Beijing Chemical Factory. All the reagents were used directly without further purification. Preparation of WS2 QDs For a typical synthesis of 3.7 nm WS2 QDs, 1 mmol (0.396 g) WCl6, 10 mL ODE and 20 mL OA were added into a 100 mL three-neck flask. The mixtrue was heated to 150 °C for 15 min with vigorous stirring and N2. Then the mixture was heated to 300 °C, a clear mixture consisting of 0.22 g TAA and 5 mL OLA was rapidly injected into the hot solution, and the reaction was kept at 295 °C for 2 min. Then the solution was cooled down to room temperature. In order to remove the impurities and large particles, vast ethanol was added, and the mixture was centrifuged at 5000 rpm for 5 min. The supernatant was filtered by 0.22 μm Nylon filters. Finally, the as-synthesized WS2 QDs were precipitated with methanol and redispersed in toluene. The whole purification process repeated 2-3 times and the final product was completely redispersed in toluene. WS2 11



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QDs with particle sizes of 1.4 and 2.3 nm were prepared by adjustment of reaction temperature to 255 and 275 °C, respectively. WS2 QDs with different W/S ratios were achieved by the amount of TAA from 0.15 to 0.30 g. Synthesis of Zn0.9Mg0.1O Zn0.9Mg0.1O

(ZnMgO)

nanocrystals

(NCs)

were

synthesized

through

solution-precipitation method. Firstly, a clear mixture of 5 mmol TMAH and 10 mL ethanol soluion was obtained. The mixture was loaded to a cationic solution consisting of 0.30 mmol Mg(Ac)2·4H2O and 2.70 mmol Zn(Ac)2·2H2O dissovled in 30 mL DMSO. The solution was kept at room temperature for 60 minutes. In order to obtain the sediment of ZnMgO NCs, excessive acetone was added to the solution. The as-prepared ZnMgO NCs were completely redispersed in ethanol. Device Fabrication Substrates coated with indium tin oxide (ITO) were firstly treated by UV-ozone to increase the work function. PEDOT:PSS (~35 nm thickness) was spin-coated on ITO and anealed at 150 °C for 10 min. These substrates were then moved into a nitrogen-filled glovebox. PVK, WS2 QDs (made at 295 °C, 50 mg/mL) and ZnMgO were layer-by-layer spin-coated at 1000 rpm for 60 s. PVK layer (~30 nm), QD layer (~15 nm) and ZnMgO layer (~40 nm) were baked at 150 °C for 10 min, 180 °C for 30 min, and 100 °C for 10 min, respectively, before the next layer deposited. Finally, Ag cathode (~40 nm) was acquired by thermally evaporated through a line-patterned metal mask. Characterizations Selected area electron diffraction (SAED) patterns, transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) 12


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photographs were measured via FEI Tecnai F20 TEM. An IFS-66V/S FT-IR spectrophotometer was used to record the fourier transform infrared spectroscopy (FTIR). Power X-ray diffraction (XRD) pattern was collected by a Shimadzu diffractometer X-6100

with

a

Cu

K

radiation

(1.5406

Å).

Inductively

coupled

plasma

mass-spectrometry (ICP-MS) was operated on Thermo iCAP Qc. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB250 spectrometer. Ultraviolet photoelectron spectroscopy (UPS) spectra were measured with PREVAC system. UV–Vis absorption spectra were obtained on a Shimadzu UV-2550 spectrophotometer. A Hitachi F-4500 Fluorescence Spectrophotometer was applied to detect the PL spectra. The absolute emission QY values were obtained by a an integrating sphere installed in FLS980 spectrometer (Edinburgh) with a 450W Xe arc lamp. Raman spectra were measured on XploRA spectrometer equipped with a 514.5 nm argon laser. Time-resolution PL spectrum (TRPL) was obtained with a time-correlated single photon counting technique through an Edinburgh F920 and a picosecond laser as excitation source. The electrical characterization was carried out on a Keithley 2400 voltage-current source meter. The luminance and electroluminescence spectra of the devices were obtained through a Photo Research PR655. All measurements were executed under dark environment. Calculation of WS2 Bohr-radius The Bohr-radius of WS2 can be calculated as the following equation,



R B  rh 1 m e*  1 m h*



where  is the relative dielectric constant, and the rh is the Bohr radius of hydrogen atom.

m e* and m h* are the effective mass of WS2’s electron and hole.  , rh , m e* and m h*

13



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were adopted as 4.4, 0.0529 nm, 0.30 and 0.26, respectively. Thus, RB was calculated to be 1.7 nm. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions WXY, XB and WWY designed the experiments, interpreted the data and co-wrote the paper. WXY, XYZ, CYJ and HWS carried out the synthesis characterization, optical measurements and analysis. WXY, PC and LS fabricated the LED device and carried out the characterization of device. Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS This work was supported by NSFC (51772123, 11674127, 51702115, 11674126), Jilin Science Fund for Young Scholars (20170520129JH), China Postdoctoral Science Foundation (2017M611319), National Postdoctoral Program for Innovative Talents (BX201600060), BORSF RCS, and Institutional Development Award (P20GM103424). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXX/acsami.XXXXXXX. Characterization of QDs’ structure and luminescence, and LED electroluminescence data (PDF) 14


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Figure 1. TEM images of WS2 QDs prepared at different reaction temperatures of (a) 255 °C, (b) 275 °C, and (c) 295 °C; (d, e, f) the corresponding HRTEM images and (g, h, i) the size distributions.

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Figure 2. (a) XRD pattern of the QDs synthesized at reaction temperatures of 295 °C. (b) Raman spectrum of WS2 QDs synthesized at reaction temperatures of 295 °C. XPS surveys showing binding energies of (c) W and (d) S in WS2 QDs synthesized at reaction temperatures of 295 °C. (e) Absorption spectrum of WS2 QDs; inset: photographs of the QDs under sunlight (left) and 365 nm excitation (right). (f) Emission spectra of the QDs under excitation from 300 to 460 nm. The solvent for QD solution is toluene. 22


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Figure 3. (a) Absorption spectra of WS2 QDs. (b) Emission spectra of WS2 QDs under 320 and 440 nm excitation. (c) Emission spectra of WS2 QDs prepared by different molar ratios of WCl6 to TAA. (d) Emission decay curves of WS2 QDs monitored at 415 nm under 365 nm excitation. The solvent for QD solution is toluene.

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Figure 4. (a) Cross-section SEM image of a WS2 QD-LED. (b) Overall energy band diagram. (c) Current density and brightness vs driving voltage in N2 environment; inset is the LED’s emission. (d) EL spectra of the LED under different applied voltages; insets are CIE coordinates.

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Quantum Dots for White Light-Emitting Diodes - ACS Publications

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