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1,2-Ethanedithiol Treatment for AgIn5S8/ZnS Quantum Dot-Based Light Emitting Diodes with High Brightness Changyin Ji, Min Lu, Hua Wu, Xiaoyu Zhang, Xinyu Shen, Xiao Wang, Yu Zhang, Yiding Wang, and William W. Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16238 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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ACS Applied Materials & Interfaces

1,2-Ethanedithiol Treatment for AgIn5S8/ZnS Quantum Dot Light Emitting Diodes with High Brightness

Changyin Ji,†,# Min Lu,† Hua Wu,† Xiaoyu Zhang,† Xinyu Shen,† Xiao Wang,‡ Yu Zhang,†,* Yiding Wang, † and William W. Yu†,#,§,*

†

State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and

Engineering, Jilin University, Changchun 130012, China # College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‡

State Key Laboratory of Superhard Materials, and College of Physics, Jilin University,

Changchun 130012, China §

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

71115, United States

*E-mail: [email protected] (Y. Zhang), *E-mail: [email protected] (W. W. Yu).

KEYWORDS 1,2-ethanedithiol; ligand exchange; quantum dot; light emitting diode; AgIn5S8/ZnS

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ABSTRACT The surface organic ligands of the quantum dots (QDs) play important roles in the performance of QD electronic devices. Here, we fabricated low toxic AgIn5S8/ZnS QDs light-emitting diodes (QD-LEDs), and greatly enhanced the device efficiency through surface ligand exchange treatments. The oleic acid capped QDs were replaced with a shorter ligand 1,2-ethanedithiol, which was proved by the Fourier transform infrared spectrum measurement. The treated QD films became more compact with higher film mobility and shorter film photoluminescence lifetime. The more conductive QD films fabricated LEDs showed an external quantum efficiency over 1.52%.

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1. INTRODUCTION Colloidal quantum dots (QDs) are unique with many fascinating characteristics,1-6 such as high photoluminescence quantum yield (PL QY), easy solution processing, tunable emission and narrow emission bandwidth, all ensure them as very promising candidates in electroluminescence (EL) light-emitting diodes (LEDs) and display applications.7-12 Currently, CdSe QDs as the workhorse have been well studied.13-18 Despite of many advantages, the intrinsic toxicity of Cd leads to a doubtful future for their large-scale commercial applications. In order to solve this problem, I−III−VI QDs have been developed for replacing the toxic CdSe QDs. In the recent years, AgInS2 and CuInS2 (and their variations, abbreviated as AIS and CIS, respectively) QD-LEDs were reported, and these devices demonstrated great potential of such QDs for LED applications.19-25 AIS possesses most advantages of CIS for LED applications, such as wide-band emission and high QYs. More importantly, AIS inherits the high carrier mobilities of Ag chalcogenides,26 which is essential for LED performances. In the past a few years, quite a few AIS QD-LEDs were proposed. Different strategies have been utilized to enhance the performance of AIS QD-LED device. Xiang’s

group reported

AIS QD-LEDs with different ratios of Ag/In, and the devices showed a luminance of ~6 cd m-2.27 Pal’s group reported zinc-alloyed AIS (AIZS) QD-LEDs with a luminance of ~60 cd m-2.28 Following this work, Pal and coworkers also used ZnO and graphene oxide for carrier transport layer, and molybdenum oxide for hole injection layer to fabricate inverted AIS QD-LEDs, and realized a luminance of ~110 cd m-2.29 Meanwhile, Do and colleagues employed a double-shell structure to suppress the energy transfer of the AIZS QDs. These 3



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AIZS QD-LED devices showed a maximum luminance and an EQE of 999 cd m-2 and 0.49%, respectively.30 Though the brightness of AIS QD-LEDs has been improved significantly, they still have relatively poor EQEs.

Generally speaking, the poor EQE often results from two deficiencies of the QD active layer: the charge carrier mobility and the surface morphology.31-32 Surface ligands covering QDs play an important role in determining the electrical properties of the QD active layer and affect the performance of the fabricated QD-LEDs. The synthesized QDs are generally terminated with long-chain organic ligands to disperse them in organic solvents and avoid aggregation. However, after becoming solid films in LEDs, these long-chain organic ligands hinder the charge injection into QDs.19, 33 Furthermore, the internal charge carrier transport progress within the QD layer also becomes weak due to the long ligands. One method to improve the carrier injection and transport properties in the QD layer is to replace the long-chain ligands with short ones that can shorten the distance between QDs. For the purpose of enhancing electronic transportation between QDs, quite a few small molecules/ions have been used to passivate QDs, for example, metal chalcogenide complexes,34 chalcogenides, SCN− ion,35 halide ions,36 and halometallates.37 Here, we used 1,2-ethanedithiol (EDT) to replace the insulating oleic acid (OA) molecules that originally dispersed the AgIn5S8/ZnS (AIS/ZS) QDs, thus enhanced the mobility of AIS/ZS QD films. Besides, the EDT treatment also helped to form smooth and compact QD films. As a result, through optimizing the charge transport process, our LEDs achieved a peak brightness of 310 4


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cd m−2 and an EQE of 1.52%, which make them the best performing AIS QD-LEDs so far. An amber (yellow-orange) light emission has been observed under a voltage as low as 2.5 V, indicating that efficient and barrier-free charge injection into the QD emitters was realized.

2. EXPERIEMENT SECTION 2.1 Materials Zinc stearate (Alfa Aesar), zinc acetate (Sigma Aldrich), silver nitrate (AgNO3, Alfa Aesar), sulfur (S, Sigma Aldrich), indium acetylacetonate (In(acac)3, Sigma Aldrich), 1-octadecene (ODE, 90%, Aldrich), dodecanethiol (DDT, Aladdin), oleic acid (OA, 90%, Alfa Aesar), oleylamine (OLA, 70%, Aladdin), trioctylphosphine (TOP, 90%, Sigma Aldrich), 1,2-ethanedithiol (EDT, Alfa Aesar), sodium hydroxide (98%, Alfa Aesar), and polyethylenimine (PEI, Sigma Aldrich) were ordered from the respective companies; n-hexane, methanol, ethanol and toluene were bought from Sinopharm. All chemicals were used directly.

2.2 Synthesis of AgIn5S8 (AIS) core QDs. We synthesized the AIS QDs with a modified recipe.33 [Ag]/[In] molar ratio varied from 0.5 to 5; AgIn5S8 ([Ag]/[In]=1:5) possessing the highest EQE was synthesized by adding the mixture of AgNO3 (0.10 mmol), In(acac)3 (0.50 mmol), ODE (7.8 g) and OA (1.5 mmol) into a 50 mL flask. The mixture was vacuumed at 40 °C for 45 min. After purging, the system was refilled with N2 gas. When the temperature was adjusted to 90 °C, 4.0 mmol DDT was 5



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quickly injected into the mixture. Then the temperature was promoted to 120 °C, and 0.80 mmol S dissolved in OLA (4.0 mmol) was swiftly injected into the reaction mixture and heated for 3 min.

2.3 Synthesis of AIS/ZS core/shell QDs. A ZnS shell was immediately coated onto the AIS core in situ. A mixture of 0.4 mmol Zn stearate and 0.4 mmol S (dissolved in 4.0 mmol TOP) was injected into the solution of core QDs. Then the solution was heated for 2 h at different temperatures (120 °C to 220 °C) to obtain QDs with different emission wavelengths. The final product was purified by adding ethanol and centrifuged at 6500 rpm.38-40

2.4 Synthesis of ZnO Nanocrystals (NCs) ZnO NCs were prepared according to a literature method.41 Briefly, 0.44 g zinc acetate and 30 mL ethanol were added into a 100 mL flask and the mixture was heated to 75 °C under N2 atmosphere until it became clear. Then the mixture was cooled to 25 °C, 0.20 g sodium hydroxide (dissolved in 10 mL ethanol) was swiftly injected into the clear mixture under strong stirring for 1 h. Raw product of ZnO NCs was dispersed in hexane and purified by precipitation. Finally, the ZnO NCs were dispersed in ethanol and stored in N2 atmosphere.

2.5 Fabrication of LED Devices The glass substrates coated with patterned ITO were successively cleaned with soap 6


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solution, double-distilled water, acetone, chloroform, toluene, ethanol, and finally treated with oxygen plasma for 15 min. A thin ZnO NC layer (~20 nm) was coated on the ITO substrate and annealed at 100 °C in air for 10 min. The substrate was moved into a N2 atmosphere glove box. A PEI (using a 0.4 wt% toluene solution) layer was coated onto the ZnO layer and annealed at 100 °C for 10 min. After this process, the work function of ZnO layer can be brought down to −3.9 eV.42 AIS/ZS QD layer (~15 nm total thickness) was fabricated by spin-coating at 2000 rpm. Then the substrate was treated with an EDT solution (0.02 vol% in acetonitrile) for 2 min, followed by rinse and spin for three cycles with acetonitrile. The film was annealed at 90 °C for 30 min. 4,4’-bis(carbazole-9-yl)biphenyl (CBP) and MoO3/Au electrode (~100 nm thickness) were coated onto the films with a shadow mask by thermal evaporation at a pressure of 10-6 mbar. The working area of the device was measured by the overlap areas of the cathode and anode which was 2 mm2.

2.6 Characterizations A JEM 2100F transmission electron microscope (TEM) was used to measure the high resolution

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images of AIS/ZS QDs. Carbon coated copper grids were used for the TEM

sample preparation. The microstructures of the samples were measured by using ﬁeld emission scanning electron microscope (SEM, JEOL JSM-6700). The absorption and photoluminescence (PL) spectra were recorded by Shimadzu UV-3600 spectrophotometer and Omni-λ300 monochromotor/spectrograph; X-ray diffraction (XRD) spectra were recorded by a Bruker D8 Advance XPert diffractometer (using Cu Kα 1.5406 Å) at a scan 7



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rate of 1°min-1 between 10°–70°. Ultraviolet photoelectron spectroscope (UPS) spectra were measured by using high resolution ultraviolet photoelectron spectroscope (PREVAC XPS/UPS System, R3000/VUV5K/MX-650). Bruker Tensor 27 spectrometer was employed to measure Fourier transform infrared (FTIR) spectra. A time-correlated single-photon counting (TCSPC) system was employed to measure the time-resolved PL spectra (Edinburgh Instruments). The highest repetition rate was 10 MHz (100 ns separation) and 1 µs separation was chosen to measure the decay curves to avoid the PL accumulation. Film thickness measurement was carried out on Dektak 150 profile system (Vecco). The curves of current–voltage–luminance of the AIS/ZS QD-LEDs were measured using a Keithley 2635 source meter connected with Newport 818-UV Si photodiode parallel with the light-emitting pixel at a fixed distance. The EL spectra were determined by an Ocean Optics QE65000 spectrometer. The LED brightness was recorded by measuring the number of photons that collected by the photodetector.

3. RESULTS AND DISCUSSION Low-toxic, high QY AIS/ZS core-shell QDs were synthesized via a hot-injection one pot method.43 Figure 1(a, c) shows the absorption and PL spectra of amber and red AIS/ZS QDs, the emission peak wavelengths were 585 and 670 nm, respectively. The Stokes shift value was about 120 nm, such a large value is probably due to the donor-acceptor levels related radiative transitions and/or the surface defects related emission.44 Due to the more precise temperature control, the synthesized AIS/ZS QDs showed a full width at half maximum 8


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(FWHM) value of 91 nm, which is narrower than previous reports.44-45 The typical TEM images demonstrate that the average diameters of the amber and red AIS/ZS QDs are about 3.0 and 3.3 nm. And the QD particles are spherical with homogeneous size distribution (Figure 1b. d). The inset photographs are HR-TEM images with lattice fringe spacings of 0.319 and 0.322 nm, both corresponded to the AgIn5S8/ZnS (311) in XRD spectra and show the distinguished single crystalline structure of the AIS/ZS QDs.

The as-synthesized AIS/ZS QDs were capped with long-chain OA ligands. These ligands acted as insulating layers between QDs that militated efficient carrier transport when fabricated into films. Here, a layer of AIS/ZS QDs was deposited on a cleaned glass substrate by spin-coating and then treated with 0.1 M EDT in acetonitrile to replace the OA ligands. The EDT treatment is widely used in fabricating QD solar cells, and the best performing PbS QD solar cells up to now also employed EDT to treat the films as the p-type component for the active layer.42, 46-49 We extended this method for QD-LED fabrications.50 The PL QYs for amber and red AIS/ZS QD films were 31% and 35%, respectively. After EDT treatment, the PL QYs of the films slightly decreased to 26% and 29%, respectively. FTIR characterizations were performed to determine the quantity and type of ligands on the surface of resulting AIS/ZS QDs. Figure 2a shows the FTIR spectra of two samples before and after EDT treatment. The band emerging from 2500 to 3000 cm-1 is the C–H and S–H stretch region. The decrease in stretching intensities at 2843, 2924 and 2955 cm-1 of the powder spectrum corresponds to the removal of C–H bonds. EDT treated AIS/ZS QD 9



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powder is expected to have ~12% of the C–H groups comparing to the untreated QDs (33 and 4 C–H bonds in OA and in EDT, respectively) after ligand exchange. The disappeared S–H stretch signal of the EDT located at ~2550 cm-1 suggests that the formation of ethanedithiolate bound on the surface of single QD or two adjacent QDs.48

Figure 1. (a), (c), Evolution of the photoluminescence, absorption spectra of AIS/ZS core-shell QDs. TEM images of (b) 3.0 nm and (d) 3.3 nm AIS/ZS QDs, respectively. Insets: HR-TEM images of the AIS/ZS QDs.

Figure 2b shows the XRD diffraction patterns of AIS/ZS QD powders with and without EDT treatment. The peaks observed at 2θ of 27.3° (311), 45.8° (203) and 53.5° (620) become narrower and without position changes after EDT treatment. These peaks belong to cubic 10


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spinel AIS/ZS QDs (c-AgIn5S8/ZnS, JCPDS: 00-026-1477) according to the previous report.51 The narrower peaks of AIS/ZS QDs are related to the improved crystalline and packing tightness of the QDs after EDT treatment. During the ligand exchange, long-chain OA was replaced by short-chain EDT ligand. Thus the spacing of QDs becomes smaller, which is consistent with the morphology change of AIS/ZS QD films in SEM images.

SEM images in Figure 2c and 2d show the comparison of spin-coated AIS/ZS QD films before and after EDT treatment. We observed embossment on the surface of untreated QD films, which was originated from the OA covering of AIS/ZS QDs. For the EDT treated films, more compact and smooth surfaces were observed. AFM images clearly show the morphology evolution of AIS/ZS QD film surface before and after EDT treatment (Figure S1). For OA capped QD film, we failed to obtain a clear picture, which may be due to the coverage of OA ligands on the surface of the QD film. For EDT treated film, we found the QDs closely packed, with a root mean square (RMS) surface roughness value of 9.62 nm.

To evaluate the mobility change of AIS/ZS QD films with different capping ligands, an electron-only device was fabricated and characterized. This device was in a structure of ITO/ZnO/PEI/AIS/ZS/Al. The curves of current density-voltage (J-V) of the AIS/ZS QD devices capped with OA or treated with EDT are showed in Figure 2e, demonstrating the significantly enhanced charge transport properties for the EDT treated QD films. The

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Mott-Gurney law and the space charge-limited current (SCLC) model was used to calculate the charge mobility µ: 9

ɛ ɛ μ 8 where ɛ and L stand for the dielectric permittivity and the thickness of the QD film; V stands for the bias voltage; and J stands for the current density. Due to the uncertainty of ɛ , we can qualitatively analyze the electron mobility of AIS/ZS film with and without EDT treated based on Figure 2e. The result shows that the electron mobility of AIS/ZS film can be largely improved after EDT treatment.

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Figure 2. (a) FTIR spectra of spin-cast AIS/ZS QDs film before and after EDT treatment. (b) XRD patterns of AIS/ZS QD powder before and after EDT treatment. The black line shows the (203) plane of cubic-AgIn5S8/ZnS QDs marked by an arrow,51 SEM image of AIS/ZS QD films (c) before and (d) after EDT treatment. (e) The space charge-limited current (SCLC) curves of AIS/ZS QDs. (f) PL decay curves of AIS/ZS QD films before and after EDT treatment; solid lines represent the fitting curves using double-exponential decay analysis (all the films were deposited on glass substrate).

Figure 2f demonstrates the PL decay curves of AIS/ZS QD films with and without EDT treatment. Since the removal of the insulating OA ligands will decrease the QD spacing, the wave functions of the electron and hole become overlapped , leading to a decrease in QD film PL lifetime. As shown in Figure 2f, after the EDT treatment, the average PL lifetime of the AIS/ZS QD films decreased from 307.53 to 273.76 ns. Furthermore, the influence of energy transfer from smaller to larger QDs become significant in the ensemble, which leads to the red shift of PL spectra of the QD film after EDT treatment (Figure S2).

In Figure 3a, the QD-LED device structure we employed here has a multilayer structure consisting of ITO/ZnO/PEI/AIS/ZS/CBP/MoOx/Au. ZnO, PEI, and AIS/ZS QD films were spinning coated on the glass substrates with ITO, while MoOx and Au layers were sequentially deposited by vacuum thermal deposition. Figure 3b shows the flat-band energy level diagram of each layer used in our AIS/ZS QD-LEDs. UPS measurement (Figure S3) 13



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was performed to EDT treated AIS/ZS films in order to map the QD’s kinetic energy. Tauc plot of an EDT treated AIS/ZS film on glass substrate showed a bandgap of 2.56 eV (Figure S4). Thus the calculated HOMO and LUMO values of QD films are -6.95 and -4.13 eV, respectively. The energy bands of other materials were taken from a reference.52 The ZnO film can work as an electron transport layer, at the same time, it can efficiently block the hole. The CBP layer helps to transport holes and block the reverse electrons. Through optimizing the thickness of these charge transport layers, we achieved a charge-transport balance, resulting in bright and efficient AIS/ZS QD-LEDs.

Figure 3. (a) Energy level and (b) device structrure of AIS/ZS QD-LEDs.

PEI is known for its ability in reducing conductor’s work function and ZnO film’s energy levels, which helps the electron inject into the QD layer. Figure 4a shows the curves of current density-voltage-luminance (J-V-L) of two amber AIS/ZS QD-LEDs, one is OA capped and another is EDT treated QD emitting layer. The turn-on voltage for the EDT treated QD-LED is as low as 2.5 V, and the current density is higher than another one (3−4 V), indicating an easier injection into and a better transportation within the emissive layers, 14


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which is consistent with the FTIR and SCLC results. The peak luminance of the QD-LED device was also improved from 107 cd m-2 to 310 cd m-2, which is about 2-fold increase compared to ones employing original QD emissive layers. Devices using EDT treatments also got improved current efficiency (CE) and EQE. As shown in Figure 4b, the device using EDT treatment displays the maximum values of EQE of 1.52% and CE of 2.3 cd A-1 at 232 cd m-2. Meanwhile, the maximum values of EQE and CE were achieved to be 0.026% and 0.043 cd A-1 at 22.5 cd m-2, respectively, for the devices based on OA capped AIS/ZS QDs. The EQE of EDT treated device is much higher than the untreated one. The as prepared AIS/ZS QD-LEDs also showed good reproducibility; ~ 80% of the devices could give brightness over 280 cd m-2.

Figure 4. (a) Brightness and current density vs driving voltage of devices with (solid dot) or without (hollow dot) EDT treatment. (b) EQE and CE vs current density of devices with (solid dot) or without (hollow dot) EDT treatment.

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Figure 5a exhibits the PL spectrum of amber AIS/ZS QD solution and the EL spectrum of the device employing EDT treated amber AIS/ZS QD emitters. The EL peak of the QD centered at 620 nm is red-shifted by 20 nm compared with the solution PL spectrum (600 nm). This phenomenon is mainly attributed to two reasons: (1) electric field induced Stark effect; (2) decreased inter dot distance induced inter dot interactions in AIS/ZS film. The inset in Figure 5a shows a photograph of a working device emitting at 5 V. Figure 5b shows the Commission International de l’Eclairage

color coordinates of (0.5663, 0.3976). The PL

spectra of amber and red QD-LEDs with and without EDT treatment are shown in Figure S2. The EDT treatment can be used to fabricate other color QD-LEDs. We also demonstrate red AIS/ZS QD-LEDs employing the same device structure. The maximum values of luminance and EQE of the devices were 810 cd m-2 and 0.52% (Figure S5). The luminance of this red LED is the highest value among all reported AIS/ZS QD-LEDs.

Figure 5. (a) PL spectrum of amber AIS/ZS QD ﬁlm (black dot) and EL spectrum (red dot) of the LED using interface engineering, together with the photograph of a working device

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(the EDT treated device with an emitting area of 2 × 2 mm2) at an applied voltage of 5 V given as an inset. (b) CIE coordinates for the EL spectrum under an applied voltage of 8 V.

4. CONCLUSIONS In conclusion, we used EDT to replace the long OA ligand on AIS/ZS QD surface, which brought significant efficiency improvement to the QD films. The treated QD layers became more compactness and smoother, together with an increase in the film mobility. As a result, the device using EDT treated amber AIS/ZS QDs started to work at 2.5 V and showed maximum values of luminescence of 310 cd m-2 at 11.6 V and EQE of 1.52% at 8.3 V. The EQE was much higher than those of the device employing original amber AIS/ZS QDs (107 cd m-2 at 11.3 V and EQE of 0.026% at 8.0 V). Furthermore, the EDT treatment can be applied to other color emitting QD-LEDs and we demonstrated well-performing red AIS/ZS QD-LEDs employing the same device structure. We thus conclude that the EDT treatment is a promising approach to realize high performance, solution processed QD-LEDs.

■AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. Zhang), *E-mail:[email protected] (W. W. Yu). Author Contributions: C.Y. Ji and M. Lu contributed equally to this work. Notes: The authors declare no competing financial interest.

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■ ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (51272084, 61225018, 61475062,

61675086),

the Jilin Province Key Fund (20140204079GX),

the

BORSF

RCS/SURE/Endowed Professor programs, and the Institutional Development Award (P20GM103424).

■ SUPPORTING INFORMATION. AFM images, PL spectra, UPS data and Tauc plots of EDT treated and untreated AIS/ZS QDs. TEM and HR-TEM images of ZnO NCs. The curves of current density and brightness vs driving voltage of red AIS/ZS QD-LED devices with EDT treatment. These materials are available via the Internet at http://pubs.acs.org.

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