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Solid solution quantum dots with tunable dual- or ultra-broadband emission for LEDs Krzysztof Gugula, Michael Entrup, Linda Stegemann, Stefan Seidel, Rainer Pöttgen, Cristian Alejandro Strassert, and Michael Bredol ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08190 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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ACS Applied Materials & Interfaces

Solid solution quantum dots with tunable dual- or ultra-broadband emission for LEDs Krzysztof Gugula,*,†, § Michael Entrup,¥ Linda Stegemann,‡ Stefan Seidel,§ Rainer Pöttgen,§ Cristian A. Strassert,‡ Michael Bredol† †

Department of Chemical Engineering, Münster University of Applied Sciences,

Stegerwaldstraße 39, 48565, Steinfurt, Germany ¥

Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-

Straße 10, 48149 Münster, Germany ‡

Physikalisches Institut and Center for Nanotechnology, Westfälische Wilhelms-Universität

Münster, Heisenbergstraße 11, 48149 Münster, Germany §

Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität

Münster, Corrensstraße 30, 48149 Münster, Germany

KEYWORDS alloyed quantum dots, co-doped, high Stokes-shift, color converter, defect chemistry.

ABSTRACT Quantum dots that efficiently emit white light directly or feature a “candle-like” orange photoluminescence with high Stokes-shift are presented. The key to obtaining these unique emission properties is through controlled annealing of the core Cu-In-Ga-S quantum dots in the presence of zinc ions and thus forming Zn-Cu-In-Ga-S solid solutions with 1 ACS Paragon Plus Environment


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different distribution of the substitution and dopant elements. The as-obtained nanocrystals feature excellent quantum yields of up to 82 %, limited or even eliminated reabsorption and a color rendering index of bare particles of up to 88 enabling the production of high quality white LEDs using a single color converter layer. Furthermore, the color properties can be tuned by changing the experimental conditions as well as by varying the excitation wavelength. The multicomponent luminescence mechanism is discussed in detail based on similar literature reports. White LEDs with unparalleled color quality and competitive luminous efficacies are presented herein.

1. INTRODUCTION Semiconductor quantum dots hold great promise in optoelectronics owing to their transparency, solution processability, high efficiency and tunable band gaps.1,2 With respect to the solid state lighting, retaining the matrix transparency using nanofillers allows for increasing the device efficiency thanks to reduced scattering in the converter layer, as compared to bulk inorganic phosphors.3,4 Modern white light emitting diodes (WLEDs) combine the red, green and blue emission components to produce white light with tunable color temperature by varying the relative phosphor ratios.5-7 Bilayer color converters can be stacked on a blue InGaN LED chip to obtain high color rendering index (CRI) values and warm-white colors.8-13 However, the resulting WLEDs may suffer from unstable color properties due to the red phosphor undergoing faster degradation under higher photon fluxes than the green one receiving less excitation power.14 Thus, color properties of the device vary during its lifetime. Zhao et al.15 addressed this issue by employing doped ZnSe nanocrystal blends with high Stokes-shifts, thus eliminating the reabsorption problem. They obtained a luminous efficacy (LE) of 43 lm/W using a 365 nm UV LED. Still, the necessity to synthesize

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three different batches of QDs would decrease the cost-effectiveness of such WLEDs considerably. Ideally, the use of a single converter layer emitting high quality white light with high quantum yields (QY) paired with a near-UV (NUV) LED (ca. 400 nm) could solve the device color stability and reproducibility issues. Degradation of either the phosphor material or the chip itself would not influence color quality much because the NUV portion of light has a minimal effect on the perception of colors by the human eye. In fact, several reports have already mentioned the successful synthesis of white-emitting QDs (WQDs). Namely, (Zn, Cd)Se16,17 and CdS:Mn18,19 nanocrystals were found to exhibit white light emission from a combination of band-edge and trap-state (surface or dopant) radiative components. Unfortunately, low quantum yields, the presence of toxic Cd and weak absorption in the NUV region prevented these materials to be effectively used in solid state lighting.20 More recently, Cd-free QDs were developed combining high quantum yields, broad emission bands and high Stokes-shifts for efficient color conversion. Notably, the preparation of Mn- and Cu- codoped ternary nanocrystals, showing efficient white emission and neutral-white colors when excited with a blue LED, was demonstrated.21,22 Also, Zhang et al.23 reported dual-emissive InP nanocrystals with a mixture of Cu-dopant and band-edge emissions through clever radial composition engineering. These reports prove the viability of semiconductor WQDs for application in next generation solid state lighting, combining CRI values exceeding 90 along with competitive lumen output. In this report, Zn-Cu-In-Ga-S nanocrystals with novel optical properties are presented. I-IIIVI/II-VI solid solution semiconductor QDs were synthesized using various annealing strategies, yielding white- or high Stokes-shifted “candle-orange” QDs (CQD). Substitution of In with Ga in ternary semiconductors has been exploited before for band gap engineering24-26 but no other changes in the spectral features of such semiconductor nanocrystals were



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observed. Herein, direct white emission from ternary/binary solid solution QDs with CRI values of up to 88 at 360 nm excitation wavelength and tunable color temperatures were obtained through high temperature annealing of the ternary nanocrystals in the presence of zinc ions. Moreover, by slightly changing the reaction kinetics, nanocrystals with efficient ultra-broadband emission and over 1.3 eV Stokes-shifts were obtained, never reported for ZnCu-In-Ga-S-based semiconductors before. WLEDs with these QDs exhibited excellent CRI values of up to 96, among the highest for single-color converter systems. 2. EXPERIMENTAL SECTION Materials: Copper (I) iodide (CuI, 99.99 %), indium acetate (In(OAc)3, 99.99 %), gallium acetylacetonate (Ga(acac)3, 99.99%), zinc stearate (ZnSt2, 12 % Zn basis), zinc acetate dihydrate (Zn(OAc)2·2H2O, 99.9 %) 1-dodecanethiol (DDT, 97 %), 1-octadecene (ODE, 90 %), oleic acid (OA, 90 %), were obtained from Sigma-Aldrich and used without further purification. The silicone resin (VT3601E) and the hardener were obtained from Peters GmbH. CuxInyGa1-yS2 core synthesis: QDs were synthesized using standard anaerobic techniques. The approach was adapted from the original report by Zhong et al.27 First, 24 mg CuI (0.125 mmol, x = 0.25),

15 mg In(OAc)3 (0.05 mmol,

y = 0.1),

165 mg Ga(acac)3

(0.45 mmol) and 5 mL of DDT were loaded into a 100 mL 3-neck flask and filled with argon. The mixture was heated to 120 °C for 15 min. QDs were then grown at 240 °C for 5 min to obtain the core crystals showing yellow color under ambient light. Other core QD variants were synthesized simply by changing the cation precursor stoichiometry and in the case when y ≥ 0.5, the synthesis was conducted at 230 °C to avoid QD precipitation. Annealing with ZnSt2 to obtain the WQDs: immediately after the core growth, a Znprecursor solution consisting of 12.65 g ZnSt2 (20 mmol, 40x excess), 10 mL DDT, 20 mL ODE and 10 mL OA dissolved at 120 °C was injected dropwise into the growth solution in 4 ACS Paragon Plus Environment

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three batches, 17 mL each, followed by annealing at 260 °C for 1 h after each batch. The reaction was left to cool down naturally below 100 °C and 50 mL hexane was injected to stabilize the solution. The resulting nanocrystals exhibited a neutral- to greenish-white emission color. The color temperature and absorption onset could be varied by changing the molar excess of ZnSt2 with higher CCT, the lower the amount of excess ZnSt2. Two-step annealing with Zn(OAc)2 and ZnSt2 to obtain the CQDs: The first step was performed using a zinc acetate precursor, consisting of 0.55 g Zn(OAc)2·2H2O (2.5 mmol), 2 mL OA, 5 mL ODE and 1 mL DDT dissolved at 160 °C under argon, by dropwise addition and subsequent annealing at 220 °C for 1 h. For the second annealing step, the ZnSt2 precursor, consisting of 1.58 g ZnSt2 (2.5 mmol), 1 mL DDT, 5 mL ODE and 4 mL OA dissolved at 130 °C under argon, was injected dropwise into the QD solution at 260 °C and left to react for an additional hour at this temperature. The solution was cooled down below 100 °C naturally and stabilized with 30 mL hexane. The resulting nanocrystals emitted broadband “candle-like” orange emission centered at 600 nm. The absorption onset could be varied by changing the annealing or core growth time. Purification: The growth solution diluted with hexane was first centrifuged at 6,000 rpm at 4 °C for 30 min to remove some excess acetates and stearates. Then the supernatant was washed three more times using hexane/acetone mixtures by centrifugation at 10,000 rpm for 10 min and finally redispersed in hexane for characterization and silicone encapsulation. Characterization: UV-Vis spectra were obtained using an Analytik Jena Specord 200 Plus spectrometer. Excitation, emission spectra and decay times were recorded on a PicoQuant FluoTime300 spectrometer equipped with a 300 W ozone-free Xe lamp, a 375 nm diode laser with pulse width < 80 ps and a R5509-42 NIR-photomultiplier tube. PL lifetimes were measured in Multichannel Scaling (MCS) or Time Correlated Single Photon Counting (TCSPC) mode when the lifetimes did not exceed ca. 5 µs. PLQYs were assessed via an



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absolute method on a Hamamatsu Photonics C9920-02 equipped with a 150 W/CW Xenon light source, a photonic multi-channel analyzer, and an integrating sphere. Given PLQY results were measured from a silicone composite. The values in solution were lower presumably due to ligand-solvent interactions, reabsorption and/or insufficient absorption factors. XRD patterns were recorded using a Rigaku Mini Flex II Desktop diffractometer (λCuKα = 1.5406 Å). TEM pictures were obtained on a Zeiss Libra 200FE. Hydrodynamic diameters were obtained using a Malvern Zetasizer NanoZS. EDX spectra were measured on a Zeiss EVO MA10. A calibrated Avantes ULS2048XL spectrometer coupled with an integrating sphere was utilized to measure the properties of LED color converters. Color quality was assessed by the CIE (Commission Internationale de L'Eclairage 1931) colorimetry system. 3. RESULTS AND DISCUSSION Two methods to synthesize white- (WQDs) and “candle-orange”-emitting QDs (CQDs) were developed through applying a modified annealing procedure of the core Cu-In-Ga-S QDs. Figure 1a depicts the emission and excitation spectra of the WQDs obtained by annealing the core crystals at 260 °C with ZnSt2. Two broad emission bands and a long NIR tail could be clearly distinguished indicating the existence of multiple radiative recombination pathways. One band was centered at 485-500 nm (referred to as the cyan band), and the other at 600 nm (referred to as the red band). Increasing the amount of zinc ions during annealing (Zn/(In+Ga) ratio) caused a blue-shift of the cyan band, whereas the red band remained unchanged. Based on this observation, the cyan band can be attributed to deep-trap emission due the large Stokes-shift and its’ position being dependent on the valence and/or conduction band edges, which were in turn tuned through increasing the amount of ZnSt2 (annealing with Zn ions blue-shifted the absorption/excitation onset). Consequently, the red band emission must have originated from fixed intraband energy levels because its position remained 6 ACS Paragon Plus Environment

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unaffected by the band edges and could not be excited over ca. 500 nm (see Figure S1 for details). The emission components were deconvoluted using Gaussian fitting as depicted in Figure 1b but the fitted curves carry an intrinsic uncertainty due to the complex nature of the system under consideration and significant spectral overlaps. The red emission component became prominent when the Cu-Ga-S QDs were substituted with small amounts of indium and did not shape a distinct peak when the In:Ga ratio was higher than 1:9 as presented in Figure 1c and 1d. As expected for Cu-In-Ga-S-based nanocrystals, higher indium contents red-shifted the excitation spectra, meaning that solid solutions of Zn-In-Ga-S were formed with band gaps depending on the relative cation stoichiometry. Increasing the In:Ga ratio gradually changed the luminescence mechanism towards the one found in ordinary I-III-VI QDs, which may be connected with the higher In(OAc)3 reactivity relative to Ga(acac)3 and hence a more rigid, well crystallized core being formed that is then coated with a ZnS shell forming a type I band alignment. On the contrary, small, weakly crystallized Cu-In0.1-Ga0.9-S cores could be dissolved during high temperature annealing in the presence of Zn ions forming Zn-Cu-In-Ga-S doped/substituted QDs giving rise to the multicomponent emission. This hypothesis is supported by the QD electronic absorption spectra before and after annealing of the pristine Cu-Ga-S nanocrystals as presented in Figure 2a. Annealing of the Cu-In0.1-Ga0.9-S cores still blue-shifted the absorbance (Figure 2b) most likely due to the core band gap being larger than that of the Zn-Ga-In-S QDs. In Zn-Cu-Ga-S nanocrystals the absorption onset was red-shifted during annealing, which provides a clear proof of the dissolution of Ga3+ in ZnS and not Zn2+ in Cu-Ga-S. A control experiment conducted by heating up ZnSt2 in the presence of DDT showed that the synthesis of ZnS under the annealing conditions presented in this paper are insufficient to produce highly crystalline QDs that could participate in the emission process (see Figure S2 for details). Zn-Ga-S bulk crystals were earlier found to exhibit defect emission arising from DAP recombination



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involving i.e. Ga•Zn and (VZnGaZn)' neighboring defects.28,29 Because ZnS, Ga2S3 and In2S3 readily form mixed sulfides30,31, the localized center luminescence at 600 nm cannot involve In or Ga-related states, which would provide donor levels below the band gap instead.

Figure 1. Photoluminescence excitation (PLE) and emission (PL) spectra of the WQDs: a) depending on the molar excess of Zn with respect to Ga+In; b) fitted Gaussian spectral components for WQDs-40x-Zn; c) excitation and d) emission spectra of the Zn-Cu-In-Ga-S solid solutions with different In:Ga ratios.

Figure 2. Absorption spectra of core and annealed: a) Cu-Ga-S; b) Cu-In-Ga-S QDs. Through slightly modifying the reaction kinetics, the CQDs were obtained. Here, two annealing steps were performed by reacting the core QDs stepwise with Zn(OAc)2 and ZnSt2 at 220 and 260 °C, respectively. As a result, the cyan band was mostly suppressed yielding broadband “candle-like orange” emission with a Stokes shift on the order of 1.3-1.4 eV and a full width at half maximum (FWHM) of ca. 200 nm for nanocrystals absorbing up to ca. 8 ACS Paragon Plus Environment

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500 nm as presented in Figure 3a and 3b. The PL features resemble the Cu2+ related emission found in e.g. ZnS:Cu32,33 and ZnSe:Cu34,35 doped crystals. Although Cu(I) was used in the synthesis, oxidation to Cu(II) is possible during annealing in Cu-deficient CuxInS2 QDs. If we assume the quinary Zn-Cu-In-Ga-S system to be actually doped ZnS nanocrystals, band gap tuning would be achieved through the formation of Zn-Ga-In-S solid solutions with the quantum confinement effect being screened. We found out that the Cu2+ emission maximum can be shifted by changing the local chemical environment of the copper centers though modifying composition of the host solid solution. For example, Zn-Cu-Ga-S QDs shifted the red band towards green with a PL maximum at 565 nm as in Figure 3c and could be synthesized similar to the Zn-Cu-In-Ga-S WQDs, generating two emission bands (cyan and yellow) or as the CQDs, producing a single ultra-broadband PL signal centered at 565 nm. The shift of the localized band to 600 nm in Cu0.25-In0.1-Ga0.9-Zn-S solid solutions is not due to the presence of indium changing the electronic structure of the copper centers as such but rather due to different reaction kinetics of the Cu-In-Ga-S and Cu-Ga-S QDs during annealing. Pristine Cu-Ga-S QDs after the first annealing step (at 220 °C) also featured luminescence centered at 600 nm, albeit rather inefficient. Jo and Yang36 investigated the optical properties of Zn-Cu-Ga-S solid solutions and attributed the broadband cold-white emission to the ternary phase encapsulated in ZnS. We deem it unlikely that the ternary core would generate multicomponent emission upon passivation with ZnS, especially in view of the fact that the absorption onset was red-shifted after the ZnS coating (see also Figure 2a), which could not happen in ternary QDs possessing a lower band gap than the binary ones.



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Figure 3. a) Absorbance and b) photoluminescence excitation and emission spectra of the CQDs with varying copper stoichiometry; c) emission spectra of the Zn-Cu0.25-Ga-S WQDs (1-step annealing) and of the CQDs (2-step annealing). HRTEM investigations, as presented in Figure 4a, 4b and 4c, revealed that the Cu-In0.1Ga0.9-S QDs have a typical diameter of ca. 3 nm, the WQDs 5-8 nm and the CQDs 4-5 nm and that the annealed crystals have a faceted quasi-spherical shape with no distinct differences between their morphologies. X-ray diffraction measurements (Figure 4d) showed that the core QDs are crystallized in the chalcopyrite CuGaS2 phase with peak broadening from small particle size and substitution with In. Reflections from the sphalerite ZnS structure became clearly observable after annealing, with the three main reflections indexed as 111, 220 and 311. CQDs were not monophasic as indicated by the appearance of the (200) reflection of the chalcopyrite superstructure around 33° also providing evidence that the modified over-coating procedure confined more I/III cations to the core. XRD particle sizes could not be reliably estimated due to peak broadening from substitution ions. The particle size distribution was characterized by Dynamic Light Scattering (DLS) measurements (see Figure S3 in SI). The obtained QD solutions were not agglomerated with reproducibly measured hydrodynamic diameters of 5.5-6.0±1 nm for CQDs and 7.0-8.5±2 nm for WQDs (depending on the amount of ZnSt2), larger than the diameters obtained from HRTEM pictures due to the presence of


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long-chain organic ligands. The polydispersity index (PDI) was about 0.05 indicating high quality monodispersed nanocrystals but typically the ternary/binary solid solutions are compositionally inhomogeneous causing broadening of the emission spectra, even if the particle size was proven to be narrow.37

Figure 4. a, b, c) TEM pictures of core Cu0.25-In0.1-Ga0.9-S QDs, WQD-40x-Zn and Zn-Cu0.25In0.1-Ga0.9-S CQDs, respectively; insets show high resolution images of single QDs, d) XRD patterns of core QDs, WQDs and CQDs with calculated patterns from the International Centre for Diffraction Data with the respective PDF-4+ reference numbers: 006-669 (CuGaS2), 0062561 (ZnS), 013-0169 (CuInS2). Although dual ZnS(e):Cu emission has been reported by other groups already33,38,39, no meaningful application in the field of optoelectronics was demonstrated most likely due to the weak efficiency of the Cu-related emission in bulk ZnS. In our case, the quantum yields of the WQDs were on the order of 50-70 % (higher for more Zn-rich QDs) and 75-82 % for the CQDs enabling applications in e.g. solid state lighting or wavelength conversion for solar cells. The higher CQD efficiency is likely attributable to suppressed reabsorption (PLQYs were measured from a concentrated silicone composite) and possibly to a different radial 11 ACS Paragon Plus Environment


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distribution of the cations. Assuming that the emission centers are located at the QD surface or that the passivating ZnS shell (undoped) is too thin, dangling bonds may quench the PL. A radial gradient distribution of the substitution and dopant elements was assumed to exist in the WQDs, which exhibited different emission properties depending on the excitation wavelength, thus allowing color temperature tuning simply by using an excitation source emitting in a different spectral range. The correlated color temperature (CCT) was varied between ca. 5750 and 6800 K (for WQDs-40x-Zn) with the CCT increasing with increasing excitation wavelength and CRI values between 82 and 88 (See Figure S4 and Table S1 for details). If we consider the core synthesis and subsequent annealing as a nucleation doping process, as described by Buonsanti and Milliron40, we can expect that a gradient of the dopant/substitution elements would be formed with decreasing Ga and In concentration with increasing distance from the nanocrystal core during coating with the host ZnS or an equilibrium of dopant/substitution elements at long enough annealing times and high temperature. It is important to note that such a gradient would also affect the electronic structure of the material and most importantly the positions of the conduction and/or valence bands causing the band gap to increase gradually towards the nanocrystal surface instead of forming a typical type I alignment found in ZnS-coated QDs.41 Because the ZnS shell now constitutes the host material, different excitation wavelengths have different penetration depths and consequently low energy photons are captured close to the core and high energy photons closer to the surface. To elucidate on the rates at which the core cations diffuse within ZnS, we need to consider their mobility and the precursor reactivity. Ga3+ being the smallest ion in the system42, is expected to diffuse with relative ease. Ternary nanocrystals preannealed with Zn2+ at lower temperature, were found to be more resistant to cation exchange during subsequent coating at higher temperatures.43 Thus, more cations would be confined to


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the core in the CQDs than the WQDs. The concentration of Cu with respect to the group III cations in either the CQDs or the WQDs was found dependent strictly on the applied annealing procedure, regardless of its input stoichiometry in the core crystals as indicated from EDX measurements (found in Table S2). The final I/III cation ratio was very similar in all CQDs despite varying the Cu stoichiometry 3-fold, which means that Cu is annealed out of the nanocrystals at high temperature up to an equilibrium concentration, which can be connected with the high mobility of copper ions in the zinc blende structure and the necessity for charge compensation between group I and III cations in a II-VI semiconductor host.44 A probable elemental distribution of the cations in the WQDs and CQDs is depicted in Scheme 1 based on the aforementioned predictions on cation diffusion and precursor reactivity. Large particle size of the WQDs and consequently a relatively low Ga concentration would enable the GaZn donor levels to participate in the luminescence with more efficient deep-trap emission closer to the core, whereas limited diffusion in the 2-step annealing would confine both group I and group III cations to the core more efficiently. However, an even higher concentration of Ga in the crystalline core, would in turn cause shrinkage of the ZnS band gap rather than provide donor levels. Now the upper localized state pertaining to the Cu2+ emission would be positioned near the band gap enabling efficient energy transfer and suppressing the Ga-related cyan band.

Scheme 1. Postulated substitution/dopant element radial distribution in a) WQDs, b) CQDs, taking into account differences in the average particle size between the two.



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The existence of deep trap- and Cu2+-related emission was reflected in the PL lifetimes in the quinary Zn-In-Ga-S:Cu system, which are presented in Table 1. The results were analyzed with respect to fractional intensities of the decay components (FIDCs). For WQDs, data was collected for different emission wavelengths – at the cyan band, red band and red tail. FIDCs were compared for the nearest neighbors (with respect to lifetime) between samples. Several decay components can be clearly distinguished in the WQDs, CQDs as well as ordinary ZnScoated ternary QDs: τ1 (150-600 ns), τ2 (1.3-2.2 µs), τ3 (3.5-6.7 µs) and τ4 (12-24 µs). Actual values and fractional amplitudes can be found in Figures S5 to S12 (as generated reports from the PicoQuant software). τ1 dominated in CuInS2/ZnS QDs and was therefore ascribed to DAP/deep-trap emission found in ordinary ternary nanocrystals. The terms DAP and deeptrap emission are used interchangeably because to date no consensus, as to the actual emission mechanism in ternary QDs, has been reached.45 τ2 become more pronounced at higher Ga/In ratios and can therefore be attributed to Ga-related emission and the cyan band. τ3 was found at its highest intensity around 600 nm and in the CQDs indicating the Cu2+ emission responsible for the red band. τ4 was more pronounced in the NIR tail and was also assigned to the copper-related band. τ2 was detected in the CQDs and was ascribed to residual deeptrap/DAP emission of the ternary core nanocrystals. The PL mechanism found in our nanocrystals bear similarities to an earlier model of Peka and Schulz46 for one-electron optical transitions in ZnS:Cu where they described that the 3d9 ground state of CuZn in the tetrahedral crystal field of the four S2– ligands splits into higher lying t2 and lower lying e levels. They 14 ACS Paragon Plus Environment

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associated the red emission to the recombination of an electron from a deep localized donor level at the Cu site and the IR emission to a transition from the t2 to e levels of Cu2+. Here, the cyan emission is considered to be due to Ga and In substitution in ZnS creating donor levels responsible for the deep-trap emission. The postulated energy levels participating in the multicomponent emission with their associated radiative decay constants τ1, τ2, τ3 and τ4 are depicted in Scheme 2. More experimental data is necessary to exhaustively characterize the optical properties found in the WQDs/CQDs. Here assumptions and simplifications were needed to elucidate the complex luminescence mechanism in the quinary Zn-Cu-In-Ga-S system.

Table 1. FIDCs of the WQDs and CQDs; QDs with In0.2-In1.0 stoichiometry were measured in TCSPC mode, WQDs and CQDs in MCS mode. QDs-λEm

τ1, % τ2, % τ3, % τ4, %

Cu0.25InS2/ZnS-550 nm

93

7

-

-

Zn-Cu0.25-In0.5-Ga0.5-S-550 nm

85

14

-

-

Zn-Cu0.25-In0.2-Ga0.8-S-550 nm

25

47

28

-

WQDs-Zn-Cu0.25-In0.1-Ga0.9-S-485 nm 11

36

37

16


30

49

17


29

46

20

WQDs-Zn-Cu0.25-Ga-S-480 nm

14

49

37

-

CQDs-Zn-Cu0.25-In0.1-Ga0.9-S-600 nm

-

15

62

23

Scheme 2. Hypothesized energy levels involved in the luminescence process in a) WQDs and b) CQDs, dotted gray arrows represent non-radiative transitions.



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Excellent CRI values of the obtained QDs enabled us to manufacture single-color converter LEDs with superior color quality using inexpensive NUV chips. The normalized spectra recorded at 20 mA forward current and the corresponding digital pictures along with the photometric quantities vs forward current of our prototype devices with 400 nm chips (LE = 1.4 lm/W) are presented in Figures 5a to 5k. Complete conversion of the UV excitation light was not viable due to multiple scattering occurring in highly concentrated silicone/QD layers and subsequent attenuation of the QD emission. Thus, the residual violet emission was intentionally left out but did not significantly affect the color stability at higher currents. The devices based on NUV LEDs featured very good color reproduction but somewhat low LE values of 14-17 lm/W for WQDs and 18-24 lm/W for CQDs at 20 mA (Figures 5e to 5h). The reason for that was two-fold. First, the efficiency of commercial NUV chips is generally lower than that of the blue LEDs. Secondly, despite high quantum yields of the QDs themselves, the necessity to convert all the excitation light into the usable red, green and blue spectral components means that the efficacy will suffer from the UV to blue conversion. Also the significant NIR band of the QD emission is mostly invisible to the human eye. Nevertheless, neutral-white WLEDs with unparalleled CRI values of 93-96 at 4000-4500 K using the WQDs and WLEDs with CRI values of 86-87 at 3000-4000 K using the CQDs were obtained. These devices exceeded the color quality of most single converter WLEDs presented to date47 and can also be obtained with varying color temperatures by changing the excitation wavelength. The CQDs were found to be significantly more efficient than the 16 ACS Paragon Plus Environment

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WQDs. On the one hand their efficacy was intrinsically lower due to the emission being largely offset from the maximum of the eye sensitivity function (555 nm). On the other hand, eliminated reabsorption led to an excellent conversion efficiency (CE) of up to 85 %, which is slightly higher than the corresponding QY of the QDs (ca. 82 %) but within typical measurement error using an integrating sphere (10 %) and is connected to different scattering profiles between the bare chip and the converter layer48. To increase the color quality of the CQD-based WLEDs, we used a 450 nm chip (LE = 15 lm/W) and thus enhanced all photometric properties of the device (Figure 5l) almost doubling the efficacy up to 43 lm/W and increasing the CRI to 93 with respect to a black-body radiator of 4600 K color temperature, while being close to the white point on the chromaticity diagram with (x,y) = 0.355, 0.345. Notably, by exciting the QDs near to the absorption onset, a higher relative intensity in the cyan-green region was obtained contributing to the excellent photometric properties. This pronounced green component could be attributed to emission from the Cu-In-Ga-S core, which was suspected to remain undissolved in the CQDs. While other authors have quoted LE values as high as 78 lm/W for QD color converters, they used blue chips with extremely high efficacies of up to 30 lm/W. Herein, the bare chip was only half as efficient. The QDs obtained here can therefore be considered state-of-the-art in single color converters for WLED applications. The prototype WQD-LED devices were tested for stability over 70 h illumination time. The respective graphs can be found in Figures S13a and S12b for silicone and silica matrices (aqueous QDs, prepared and immobilized according to our previous report49), respectively. The PL stability was remarkably improved by using an inorganic silica matrix underlining the importance of new matrix materials development for next generation LED devices. The decrease in emission signal from the “cyan band” was 4 % higher than that of the “red band” in silicone and 6.8 % higher in silica. The differences in degradation rates between deep-trap and dopant emission need to be taken into account when



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designing LED systems for general lighting, i.e. by mixing the WQD-LEDs and commercial warm-white LEDs, the increase in commercial LED CCT (degradation of the red phosphor) can be compensated by the decrease in WQD-LED CCT (degradation of the blue phosphor).

Figure

5. a-d) Electroluminescence/Photoluminescence

(EL/PL)

spectra,

e-h) the

corresponding photometric quantities vs LED forward current and i-k) digital pictures of the color converters on a 400 nm LED operated at 20 mA illuminating a rough white surface and colored objects in the inset, containing the WQDs-20x-Zn, WQDs-40x-Zn, CQDs with 600 nm and 565 nm peak emission, respectively; l) EL/PL spectrum of CQDs-600 nm stacked on a 450 nm LED operated at 20 mA; Color Quality Scale (CQS) values are given in parenthesis. 4 .CONCLUSIONS


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In this report, the synthesis of dual-emitting and ultra-broadband Zn-Cu-(In)-Ga-S solid solutions was presented. The PL signal could be deconvoluted into three components – cyan band (485-500 nm maximum), red band (600 nm maximum) and NIR band (700 nm maximum). By applying a modified annealing procedure, the cyan band vanished yielding Stokes-shifts on the order of 1.3-1.4 eV. Based on optical, morphological and compositional measurements, the cyan band could be attributed to either DAP or deep trap recombination involving i.e. GaZn donor states. The red band was ascribed to Cu2+ emission in the ZnS host. The position of the cyan band could be tuned by changing the zinc stoichiometry with respect to Ga+In, where Zn-rich QDs yielded blue-shifted emission. The relative PL intensity of both bands could be additionally tuned by changing the excitation wavelength thanks to a composition gradient in Zn-Cu-Ga-In-S QDs. Pristine WQDs emitted high quality neutralwhite light with CRI of up to 88, whereas the prototype WLEDs comprising a 400 nm chip and the QDs boasted CRI values of up to 96 for neutral-white devices and 86 for warm-white devices. Moreover, the prototype WLED with CQDs stacked on an efficient blue LED exhibited excellent properties overall (LE = 43 lm/W, CCT = 4600 K, CRI = 93). Owing to high quality direct white-light emission, the WQDs could serve as white emitters in electroluminescent devices, whereas the efficient CQDs with high Stokes-shifts could prove superior materials for wavelength conversion in solar cells.

ASSOCIATED CONTENT Supporting Information. Absorbance and excitation of the WQDs, DLS measurements, excitation-dependent PL spectra, EDX atomic ratios and PL decay data of the WQDs and CQDs, device lifetime measurements. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *Tel: +49-0255-19-62694 E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by a grant from the German Federal Ministry of Higher Education (FH ProfUnt 03FH028PX3) and an FH-Extra grant from the European Regional Development Fund and Bundesland NRW (no. 290010702). The authors acknowledge Dr. Piotr Cywiński and Prof. Ulrich Kynast for providing invaluable scientific input. REFERENCES (1) Anc, M. J.; Pickett, N. L.; Gresty, N. C.; Harris, J. A.; Mishra, K. C., Progress in Non-Cd Quantum Dot Development for Lighting Applications. ECS J. Solid State Sci. Technol. 2013, 2, R3071-R3082. (2) Zhang, Q.; Wang, C.-F.; Ling, L.-T.; Chen, S., Fluorescent Nanomaterial-Derived White Light-Emitting Diodes: What's Going On. J. Mater. Chem. C 2014, 2, 4358-4373. (3) Demir, H. V.; Nizamoglu, S.; Erdem, T.; Mutlugun, E.; Gaponik, N.; Eychmüller, A., Quantum Dot Integrated LEDs Using Photonic and Excitonic Color Conversion. Nano Today 2011, 6, 632-647. (4) Dai, Q.; Duty, C. E.; Hu, M. Z., Semiconductor-Nanocrystals-Based White Light-Emitting Diodes. Small 2010, 6, 1577-1588. 20 ACS Paragon Plus Environment

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