Tunable White Fluorescent Copper Gallium Sulfide Quantum Dots

Apr 27, 2016 - In this sense, single-phased, doped QD down converters showing not only .... Thus, when recollected at the same wavelength with a much ...
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Tunable White Fluorescent Copper Gallium Sulfide Quantum Dots Enabled by Mn Doping Dae-Yeon Jo,†,# Daekyoung Kim,‡,# Jong-Hoon Kim,† Heeyeop Chae,‡ Hyo Jin Seo,⊥ Young Rag Do,§ and Heesun Yang*,† †

Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University (SKKU), Suwon 440-746, Korea ⊥ Department of Physics, Pukyong National University, Busan 608-737, Korea § Department of Chemistry, Kookmin University, Seoul 136-702, Korea ‡

S Supporting Information *

ABSTRACT: Fluorescence of semiconductor quantum dots (QDs) can be tuned by engineering the band gap via size and composition control and further doping them with impurity ions. Targeting on highly bright white-emissive I−III−VI -type copper gallium sulfide (Cu−Ga−S, CGS) host QDs with the entire visible spectral coverage of blue to red, herein, Mn2+ ion doping, through surface adsorption and lattice diffusion is fulfilled. Upon doping a distinct Mn emission from 4T1−6A1 transition successfully appears in white photoluminescence (PL) of undoped CGS/ZnS core/shell QDs and with varying Mn concentration a systematic white spectral evolution of CGS:Mn/ZnS QDs is achievable with high PL quantum yield retained. The origins of white PL of CGS:Mn/ZnS QDs that is well decomposed into three emission bands are appropriately assigned. The resulting single-phased, doped QDs are then employed as near-UV-to-white down converters for the fabrication of white light-emitting diodes (LEDs). Electroluminescent properties of white QD-LEDs depending on Mn concentration of CGS:Mn/ZnS QDs and forward current are also discussed in detail. KEYWORDS: Mn doping, copper gallium sulfide, single-phased quantum dots, white photoluminescence, down converters, white lighting devices

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INTRODUCTION Semiconductor quantum dots (QDs) can possess a wide range of tunability in band gap and consequent photoluminescence (PL) simply through their size and compositional control. PL of QDs can be further tailored by doping impurity ions, while their absorption characteristic remains unchanged. Cu+ and Mn2+ are the most common dopant ions particularly for group II−VI QD hosts such as ZnSe,1−9 CdS,10−13 ZnS,14,15 and ZnCdS.16−18 The emission of Cu-doped QDs stems from the radiative recombination of electrons from the host conduction band with holes trapped in Cu ions, thus yielding color-tunable emission with the size-dependent conduction band level shifting.2,12,13,18 On the other hand, Mn-doped QDs exhibit a much weaker dependency of emission energy on host QD size and composition since the radiative pathway is involved with © XXXX American Chemical Society

the energy transfer of electron−hole from host to localized Mn2+ ion and the subsequent electronic transition of Mn2+ dorbitals (4T1−6A1).1−6 Compared to those of undoped QDs with an excitonic PL, absorption and PL regions of such doped QDs are well separated. This large Stokes shift is of great importance for the application of fluorescent QDs to optoelectronic fields such as light-emitting diode (LED) and solar concentrator, since it affords the suppression of selfquenching due to reabsorption and/or Förster resonant energy transfer (FRET) between QD emitters.3−5 Besides, the emission of doped QDs is associated with inner core electronic Received: February 10, 2016 Accepted: April 27, 2016

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DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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white lighting devices with adjustable color temperature ranges of 3651−5351 K.

transitions of dopant ions and thus is likely insusceptible to the lattice phonons and becomes thermally stable,2,4,5 which is in contrast to the emission of undoped counterparts where a strong coupling of delocalized excitons with lattice phonons occurs. Usually, in doped QDs, an excitonic PL of QDs is completely quenched and a single-dopant emission emerges instead. However, dual emission from a single QD was also obtainable, for example, by codoping ZnSe host QD with Mn and Cu ions,7 controlling Cu dopant concentration or shell thickness of InP:Cu/ZnSe core/shell,19 and devising a more complicated core/shell structure of InP:Cu/ZnS/InP/ZnS (dual components of Cu emission from InP:Cu core plus host emission from InP quantum well).20 In addition to the QD hosts aforementioned, the doping study on I−III−VI or its derivative QD compositions such as Cu−In−S (CIS),21−23 Zn−In−S (ZIS), 24−26 Cu−In−Zn−S (CIZS), and Ag−In−Zn−S (AIZS)27 has been recently fulfilled. For instance, PL of CIS:Mn QDs was spectrally extended compared to undoped ones, as a result of the superposition of host CIS defect emission along with Mn-related emission.22,23 In the case of ZIS host QD codoped with Cu and Mn ions, similar PL spectral extension through the combination of Cu and Mn emissive components and tunable spectral distribution through the variation of Mn-to-Cu concentration ratio was achievable.25 Then such doped QDs have been successfully applied as either single- or biphased color-converting emitters in the UV- or blue-pumping LED platform for the fabrication of a white lighting source.9,20,22,25,26 Among doped QDs enumerated above, bicolor-capable single-phased QDs of InP:Cu/ZnS/InP/ ZnS,20 CIS:Mn/ZnS,22 and ZIS:Cu,Mn/ZnS25 core/shell have been integrated as down converters with a blue InGaN blue LED chip, and the resulting white lights possessed high colorrendering index (CRI) values of 90, 83, and 95, respectively, enabled by their sufficient spectral coverage. Meanwhile, in the case of undoped QDs-integrated white LEDs, at least two kinds of different color-emitting QDs are required to secure a wide spectral distribution.28−31 In such undoped QD mixtures, however, efficient reabsorption/FRET between differently colored QDs simultaneously with substantial self-quenching between the same-colored QDs (due to small Stokes shift) would be highly probable, significantly limiting luminous efficacy of the device. In this sense, single-phased, doped QD down converters showing not only a wide emissive range but also a large Stokes shift can be regarded as promising emitting materials for high color-rendering, efficient white LED fabrication. Very recently, we developed the whole visibly covered, white fluorescent single-phased copper gallium sulfide (Cu−Ga−S, CGS) QDs with a high PL quantum yield (QY) and then applied for the fabrication of near-UV LED-based white lighting device.32 The resulting QD-LED exhibited good levels of CRI (82−84) and relatively cool white lights with correlated color temperatures (CCTs) > 5000 K. As a serial and more advanced study, herein, Mn doping into CGS host QD, an unprecedented attempt to the best of our knowledge, is implemented to tune a white spectral distribution. White spectrum of CGS QDs is successfully modulated upon Mn doping and systematic white spectral evolution of CGS:Mn QDs is achievable with varying Mn concentration with high PL QY retained. These white-color tunable CGS:Mn QDs are then utilized for the fabrication of high color-rendering (85−87),

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RESULTS AND DISCUSSION Doping of impurity ion into either a substitutional or an interstitial site of QD host has been challenging since the QD lattice tends to eject it outward, called self-purification,7,13 and thus effective doping cannot be usually expected simply from a synthetic approach where doping occurs simultaneously during host QD nucleation/growth stages. To improve the doping efficiency, therefore, various doping strategies including nucleation doping and growth doping, where the doping is temporally decoupled from nucleation and/or growth periods, have been devised, mainly targeting group II−VI QD hosts.2−4 In our synthesis, the incorporation of Mn dopant was achieved by surface adsorption and lattice diffusion. That is, Mn species were introduced after CGS QD growth, leading to the adsorption of Mn dopant onto QD surface, and such adsorbed Mn dopants would become buried by subsequent ZnS overlayer. Mn dopant initially residing at CGS QD surface would gradually diffuse into core lattice throughout subsequent ZnS shelling. Figure 1a shows a series of PL spectra (recorded with λexcitation = 370 nm) of CGS:Mn/ZnS QDs with different Mn concentrations of 0, 1, 1.8, 2.6, and 4 in Mn/Cu precursor molar ratio. For PL measurement highly dilute QD dispersions

Figure 1. (a) As-collected PL, (b) normalized PL relative to 473 nm emission component, and (c) absorption spectra of CGS:Mn/ZnS QDs with different Mn/Cu molar ratios. (d) Absorption versus PL of CGS:Mn/ZnS QDs with Mn/Cu = 4 showing a minimal spectral overlap. B

DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces with an identically adjusted optical density of 0.05 at 370 nm were used. Undoped QDs exhibited a white emission with an entirely visible spectral coverage of blue to red with a high QY of 75%. With a higher Mn/Cu ratio appreciable Mn2+ emission from a 4T1−6A1 transition increased monotonically without observing the concentration quenching within the range of Mn concentration chosen here, and simultaneously, a high energy emission component (473 nm) tended to be quenched. As seen clearly from normalized PL spectra relative to 473 nm emission component (Figure 1b), the white spectral distribution could be readily tuned from cool white to more reddish warm white by controlling the concentration of Mn dopant, while the absorption characteristic was insensitive to Mn doping (Figure 1c). For all Mn-doped QDs, PL excitation spectra monitored at a high (473 nm) and low energy emission wavelength (605 nm) were identical (Figure S1 of the Supporting Information), indicating that both emissions were derived from the same composition of CGS core and further signifying that the Mn dopant adsorbed on CGS core surface diffused into core lattice rather than the shell side throughout ZnS shelling. Incorporation of Mn dopant did not lead to a QY reduction, preserving excellent values of 75−78% regardless of Mn concentration. This indicates that the decreased high energy emission by Mn doping was compensated well by that much increased Mn emission, leading to roughly the same overall integrated emission areas, and further implying that Mn doping did not adjunctively create undesired new defect sites for nonradiative recombination. The fluorescence of I−III−VI type QDs originates from the nonexcitonic charge recombination via intragap defect states, leading to a substantially Stokes-shifted PL. The resulting spectral gap between absorption and PL would be beneficial in preventing the inter-QD self-quenching. However, the absorption−PL spectral overlap of I−III−VI QDs such as CIS/ZnS and CIZS/ZnS core/shell is in fact nontrivial28,29 due to the spectral intrusion of broad PL into the absorption region. Meanwhile, our CGS-based QDs possessed a minimal spectral overlap, that is, ∼0.4 eV in energy spacing, as shown from CGS:Mn/ZnS QDs with Mn/ Cu = 4 (Figure 1d), thus expecting an even lesser degree of selfquenching. PL of CGS:Mn/ZnS QDs with Mn/Cu = 1.8 could be well decomposed into three emission bands, peaking at 470, 590, and 610 nm, respectively, through a Gaussian function fitting (Figure 2a), and the fitting results for other Mn-doped QD samples are also found in Figure S2a−c. It is noted that we assume that Mn dopants were evenly distributed in all QDs, although it is also possible that the multiple emissions might result from the mixture of undoped and doped QDs. A 470 nm emission can be attributed to the radiative recombination of electron in a conduction band (CB) with hole trapped in Cu vacancy (VCu) as acceptor (Figure 2b), which is well accepted for other I−III−VI QD systems.33,34 It is conceivable that a broad 590 nm emission band, which enables a full white spectrum and has not been observable in other I−III−VI QDs, is associated with GaCu and/or a ZnS shelling-derived, newly generated defect state. Specifically, Zn ion reacting with CGS (or CGS:Mn) QD may substitute a pre-existing Cu+ site via cation exchange34 and/or reside at available VCu, presumably creating ZnCu site at/near the core surface.32 These defects as donor states would then participate in the radiative recombination with VCu acceptors (i.e., donor−acceptor pair (DAP) recombination) (Figure 2b). On the basis of the above peak fitting analysis, the integrated spectral contributions of the

Figure 2. (a) PL spectral decomposition of CGS:Mn/ZnS QDs with Mn/Cu = 1.8 consisting of three emission bands from CB-to-VCu, DAP, and Mn2+4T1−6A1 transitions and (b) schematic illustration of the radiative pathways corresponding to the respective emissions.

respective emissive components from CGS:Mn/ZnS QDs with different Mn concentrations are summarized in Figure S2d. With increasing Mn/Cu ratio from 1 to 4, CB-to-VCu and DAP emissive components exhibited decreasing spectral contributions of 26 → 21% (i.e., ∼19% reduction) and 69 → 51% (i.e., ∼26% reduction), respectively, while Mn emission appreciably increased from 5 to 28%, indicative of competitive gain of Mn dopant emission at the expense of the former two. More quenching of DAP versus CB-to-VCu emission with increasing Mn dopant concentration may be understood by analyzing the respective PL decay behaviors of undoped CGS/ZnS QDs (Figure S3). Each PL decay curve recorded at 470 (i.e., CB-toVCu emission) and 590 nm (i.e., DAP emission) was well fitted with a triexponential function and the resulting average lifetimes (τav) were calculated to be 1.6 and 2.8 μs, respectively. Thus, it is conjectured that as Mn dopant increased, DAP component would be less competitive in PL gain due to its slower decay nature, leading to a higher probability of yielding to Mn emission, compared to the CB-to-VCu component. PL decay curves of Mn-doped QDs with Mn/Cu = 2.6 were also collected at different emission wavelengths. When monitored at 470 nm, the resulting τav (1.4 μs) was close to that of undoped QDs since CB-to-VCu emission was spectrally well separated from and thus not affected by Mn emission. Meanwhile, with monitoring at 604 nm, very long decay behavior with a τav of 2.9 ms, characteristic of intrashell d−d transition of Mn2+ ion,23,25,27 was observable (Figure 3). And an abnormally rapid decay feature in the vicinity of a very initial decay time was also noticeable. This bimodal profile stems from the perfect spectral C

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surface adsorption, with a majority of Mn species washed out after synthesis. Other elemental molar ratios relative to Cu were also tabulated in Table S1. These ICP analyses are also consistent with XRD results, where no secondary phases such as MnS were observable even for the samples synthesized with high Mn/Cu ratios. The QD sizes were indistinguishably the same regardless of Mn concentration, which could also be sensed from the identical full-width at half-maxima of reflection peaks (Figure 4a). The sizes of QDs were distributed in 3.8− 4.7 nm with a mean size of 4.2 nm, as shown in transmission electron microscopic (TEM) images (Figure S4) of representative QDs with Mn/Cu = 2.6. For the application of a series of Mn-doped QDs along with undoped ones to the fabrication of a white lighting device, a near-UV (393 nm) LED was chosen as a pumping source instead of blue LED since the excitation of those QDs by lower energy blue lights (440−460 nm) is negligible (Figure 1c). Sufficient amounts of the individual QDs as near UV-to-white down converters were packaged in LED mold to prevent UV photons from being transmitted outward. As shown in Figure 5a, electroluminescent (EL) spectra of white LEDs collected at

Figure 3. Time-resolved PL decay curves of CGS:Mn/ZnS QDs with Mn/Cu = 2.6 in different scales of milliseconds and microseconds (inset) at the same monitored emission wavelength of 604 nm (λexc= 355 nm).

overlap between Mn and DAP emission (Figure 2a), whose decay time scales are hugely different. Thus, when recollected at the same wavelength with a much shorter time scale (inset of Figure 3), the resulting τav was 2.7 μs, corresponding to DAP emission. A series of CGS/ZnS QDs with different Mn concentrations showed nearly the same X-ray diffraction (XRD) results (Figure 4a). Commonly, their reflection peaks were positioned between

Figure 4. (a) XRD patterns and (b) ICP-based actual Mn/Cu molar ratios of a series of CGS:Mn/ZnS QDs with different Mn concentrations.

Figure 5. (a) A series of EL spectra and captured images, variations of (b) CIE color coordinates and (c) CRI−CCT of white LEDs integrated with CGS:Mn/ZnS QD down converters with different Mn concentrations (collected at a forward current of 20 mA).

those of tetragonal chalcopyrite CGS and cubic ZnS phases, which is typical of a layered structure in core/shell QDs.28,34 Actual Mn concentrations were assessed by an inductively coupled plasma optical emission spectrometer (ICP-OES). As shown in Figure 4b, with increasing Mn precursor added, actual Mn concentration increased in an almost linear fashion. But actual Mn/Cu molar ratios were much lower than nominal ones, specifically with the actual values of 0.04, 0.07, 0.09, and 0.14 for nominal ones of 1, 1.8, 2.6, and 4, respectively, implying that only a limited fraction of Mn species introduced after CGS core growth would participate in host dopant via

20 mA overall matched PL spectra (Figure 1a) with respect to peak position and peak-to-peak ratio, ascribable to the insignificant self-quenching enabled by the minimal absorption−PL spectral overlap aforementioned (Figure 1d). As sensed from such Mn doping-dependent EL spectral evolution, with increasing Mn concentration the Commission Internationale de l’Eclairage (CIE) color coordinates of white emission systematically shifted to the red region (Figure 5b), for example, from (0.336, 0.396) for undoped QD-based device to (0.395, 0.371) for the highest-doped QD-based one. A white spectral modulation through Mn doping and its concentration D

DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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QDs would still be valid in the present doped QDs. Meanwhile, Mn2+ emission that is associated with the inner core states of d−d transition nature and thus may not strongly couple with the phonons can possess a high thermal stability.2,4 This resistance of Mn2+ emission to thermal quenching may be mainly responsible for temperature-insensitive PL spectral shape (Figure S5b) by making up for the DAP emission that is more susceptible to thermal quenching. In this context, unequal peak-to-peak spectral ratios, that is, reducing spectral contribution of lower-energy emission, in white EL spectra with forward current variation (Figure 6) cannot be ascribed to different degrees of thermal quenching among three emissive components. It is well-known that Mn2+ emission becomes saturated via a ground-state depletion of Mn2+ ion due to its relatively long decay nature relative to excitation photon influx.35 Under this situation, an excited Mn2+ ion can be further excited to a higher electronic level through the spinallowed 4T1−4T2 transition, by which successively excited Mn2+ ion would lose some of its energy through the nonradiative relaxation to 4T1 level, followed by the sequential radiative transition from 4T1 to 6A1.36 Thus, as a forward current, that is, excitation photon flux, increases, the saturation of Mn2+ emission will become intensified, consistently abating a lowerenergy emission component of white EL. The LE decreased from 21.7 lm/W at 20 mA to 14.7 lm/W at 100 mA (Table S3), and this LE reduction with increasing forward current was consistent with other QD down-conversion LEDs, attributable mainly to the intrinsic efficiency drop of LED chip itself at a higher driving current and partly to the device operationinduced thermal emission quenching of QD down converters.28 Lastly, the device stability of the white QD-LED in Figure 6 was assessed by continuously operating it at 40 mA under ambient condition. A progressive EL quenching was observed with a prolonged operation, retaining only ∼19% in integrated EL emission after 20 h compared to the initial value (Figure S6). Such a significant reduction, a common observation in most of QD/resin composite-based devices, can be generally attributed to the photo-oxidative deterioration of QD surface.

control also afforded a relatively wide tunability in correlated color temperature (CCT). A relatively cool white light (CCT of 5410 K) of undoped QD device became effectively warmer upon Mn doping, showing steadily decreasing CCTs from 5351 to 3651 K with increasing Mn spectral contribution (i.e., Mn concentration) (Figure 5c). Undoped QD-white device already possessed a high CRI of 83 due to their wide visible spectral coverage of blue to red, and Mn doping led to a slight enhancement of CRI to 85−87 (Figure 5c). Compared to undoped QD-white device with a luminous efficacy (LE) of 29.3 lm/W, doped QD ones possessed lower LEs of 21.7−24.2 lm/W, showing a decreasing tendency in LE with increasing Mn concentration. Bearing in mind that PL QYs were high for all QD samples independent of Mn doping, this disparity in LE among devices is associated primarily with a photopic eye sensitivity factor, that is, the spectral dependence of photometric quantities such as luminous flux (lm).28 The detailed EL quantities of the respective devices are also summarized in Table S2. A representative white LED integrated with CGS:Mn/ZnS QDs with Mn/Cu = 4 was further tested by varying forward currents of 20−100 mA. As noticed from Figure 6, the peak-to-

Figure 6. Forward current-dependent EL spectral evolution and variation of CIE color coordinates (inset) of white LED with CGS:Mn/ZnS QDs (Mn/Cu = 4).

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CONCLUSIONS The effective Mn doping into CGS host QDs was for the first time demonstrated by the doping strategy of surface adsorption and lattice diffusion. Undoped CGS/ZnS QDs exhibited a white emission with an entire visible spectral coverage of blue to red with a high QY of 75%. Upon Mn doping a marked Mn2+ ion emission of 4T1−6A1 transition appeared at the expense of CB-to-VCu and DAP emissions. With increasing dopant the Mn emission increased monotonically, thus producing conveniently tunable cool-to-warm white emissions. Moreover, Mn doping did not accompany a QY drop, preserving excellent values of 75−78% regardless of Mn concentration. A series of QDs doped with different Mn concentrations were then utilized as single-phased down converters by integrating them with near-UV LED for white LED fabrication. The resultant white lighting devices exhibited CRIs of 83−87 and LEs of 21.7−29.3 lm/W and particularly a wide CCT tunability of 3651−5410 K, depending on Mn doping and its concentration. Forward current-dependent EL spectral variation of doped QD-white LED was also investigated and explained by the saturation of Mn emission due to its long decay nature.

peak spectral ratio in the overall white EL was dependent on forward current, showing a seemingly increasing contribution of higher-energy (shorter-wavelength) emission peak and, accordingly, a slight movement of white point in CIE color coordinates to the left side (inset of Figure 6, Table S3) as a higher current was applied. In our previous investigation on undoped CGS/ZnS QD-based white LED, a similar forward current-dependent EL spectral variation was observed. And, on the basis of temperature-dependent PL study on undoped CGS/ZnS QDs, this was explained by a higher degree of thermal quenching of DAP recombination channel (i.e., lowerenergy emission) versus CB-to-VCu one (i.e., higher-energy emission) when QD down converters experienced heating accompanying LED operation.28 Analogously, temperaturedependent PL work on CGS:Mn/ZnS QDs (Mn/Cu = 4) in the form of epoxy composite was performed. Overall PL was thermally quenched gradually with increasing temperature (Figure S5a). Intriguingly, their normalized PL showed nearly the same spectral shape regardless of temperature (Figure S5b), being different from the case of the above undoped CGS/ZnS QDs. The presumption that the DAP emission is more thermally quenched relative to the CB-to-VCu one in undoped E

DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION

Research Article

ASSOCIATED CONTENT

S Supporting Information *

Synthesis of Mn-Doped Cu−Ga−S (CGS:Mn)/ZnS Core/Shell QDs. In a typical synthesis of CGS:Mn/ZnS QDs, 0.0625 mmol of Cu iodide, 0.5 mmol of Ga iodide, 0.5 mL of 1-dodecanethiol (DDT), and 5 mL of oleylamine (OLA) were placed in 50 mL of a three-neck flask, and this mixture was heated to 180 °C after the conventional degassing and purging procedures. At that temperature a sulfur (S) stock solution, which was obtained by dissolving 2 mmol of S powder into 2 mL of 1-octadecence (ODE) at 190 °C, was rapidly introduced into the above hot mixture and the reaction for the growth of CGS host QD was held for 5 min. Separately, four Mn dopant stock solutions with different concentrations were prepared by dissolving 0.1042, 0.1875, 0.2708, or 0.4716 mmol of Mn acetate in 1 mL of ODE and 1 mL of OLA at 180 °C and then 1.2 mL of the individual Mn stock solutions was taken, corresponding to Mn/Cu precursor molar ratios of 1, 1.8, 2.6, and 4, respectively. In preparing the Mn stock solution, we fixed the amounts of ODE and OLA, that is, 1 mL for each, and under this condition, a larger amount of Mn acetate than 0.4716 mmol was not completely dissolved. Therefore, the highest Mn/Cu ratio chosen here was 4. And each Mn stock solution was injected into pregrown CGS growth solution at 180 °C, and the reaction was held for 40−45 min for the adsorption of Mn species onto CGS QD surface. For successive ZnS shelling, the first ZnS stock solution, consisting of 4 mmol of Zn acetate dissolved in 2 mL of DDT, 2 mL of ODE, and 4 mL of oleic acid (OA), was slowly added into Mnadsorbed CGS core growth solution and the shelling reaction proceeded at 220 °C for 30 min. Subsequently, another ZnS stock solution, prepared by dissolving 8 mmol of Zn stearate in 4 mL of DDT and 8 mL of ODE, was added and this second shelling proceeded at 250 °C for 45−50 min. As-obtained CGS:Mn/ZnS QDs were subjected to conventional repeated purification cycles based on precipitation/redissolution with a hexane/ethanol combination and finally redispersed in chloroform. Fabrication of QD-LEDs. For the fabrication of white QD-LEDs, 3 mL of chloroform dispersion of CGS:Mn/ZnS QDs whose optical density was adjusted to be ∼3.0 at 370 nm was mechanically mixed with thermocurable epoxy resin/hardener (YD-128, Kukdo Chemical Industry, Seoul, Korea) of weight ratio of 1. Then chloroform included in the mixture was removed by heating it at 60 °C in a vacuum oven. A part of the resulting viscous CGS:Mn/ZnS QD paste was placed in the mold of a 50 × 50 mm2 surface-mounted device (SMD) type InGaNbased three-chip near UV LED (λemission = 393 nm, Taewon Semiconductor, Seoul, Korea), followed by a sequential thermal hardening process of 70 °C for 30 min and then 110 °C for 1 h. Characterization. UV−visible absorption and PL spectra of QDs were recorded using absorption spectroscopy (Shimadzu, UV-2450) and a 500 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd., Gyeonggi, Korea, Darsa Pro-5200), respectively. Absolute PL QY of a dilute QD dispersion was measured in an integrating sphere with an absolute PL QY measurement system (C9920-02, Hamamatsu). To collect time-resolved PL decay curves, the QD dispersion was excited by a pulsed Nd:YAG laser at 355 nm (Spectron Laser System SL802G). The emission was dispersed by a 75 cm monochromator (Acton Research Corp., Greer, SC, Pro-750) and multiplied by the photomultiplier (Hamamatsu Photonics R928), and the data was recorded with the LeCloy 9301 digital storage oscilloscope. Powder XRD (Rigaku, Ultima IV) using Cu Kα radiation was employed for the structural analysis of QDs. High-resolution TEM work was performed using a JEOL JEM-4010 electron microscope operated at an accelerating voltage of 200 kV. The actual compositions of QDs were analyzed with an ICP-OES (OPTIMA 8300, PerkinElmer). A series of EL data such as EL spectrum, LE, CCT, CIE color coordinates, and CRI of white QD-LEDs were obtained in an integrating sphere with a diode array rapid analyzer system (PSI Co. Ltd.).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01763. PL excitation spectra, PL spectral decomposition results, TEM images, ICP-based actual molar ratios and temperature-dependent PL spectra of Mn-doped QDs, time-resolved PL decay curves of undoped QDs, and summarized device performance values and operational stability of white QD-LEDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Y.). Author Contributions #

Both authors contributed equally to the paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068158) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1A6A1A03031833).

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DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b01763 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX