Shell Gradient Alloy

Mar 31, 2017 - Display R&D Center, Samsung Display Co., Ltd., Yongin 446-711, Korea. ∥ Laboratory of Molecular Biophysics, National Heart, Lung and ...
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Highly efficient Blue-Emitting CdSe-derived Core/Shell Gradient Alloy Quantum Dots with Improved Photoluminescent Quantum Yield and Enhanced Photostability Junsang Cho,†,‡ Yun Ku Jung,†,§ Jin-Kyu Lee,† and Hak-Sung Jung*,†,∥ †

Department of Chemistry, Seoul National University, Seoul 151-747, Korea Department of Chemistry, Texas A&M University, College station, Texas 77843, United States § Display R&D Center, Samsung Display Co., Ltd., Yongin 446-711, Korea ∥ Laboratory of Molecular Biophysics, National Heart, Lung and Blood Institute, National Institutes of Health, 50 South Drive, Building 50, Bethesda, Maryland 20892, United States ‡

S Supporting Information *

ABSTRACT: Highly efficient blue-emitting CdSe-derived core/shell gradient alloy quantum dots (CSGA QDs) with photoluminescence quantum yield (PL QY) of ca. 90% have been synthesized through a facile “one-pot” approach. CdSe nuclei are initially formed as core and gradient alloy shells such as CdSexS1−x/ZnSeyS1−y simultaneously encapsulate the preformed CdSe core in an energy-gradient fashion eventually followed by coating with ZnS shells due to the faster precursor reaction kinetics of Cd and Se compared to analog of Zn and S. During the formation of core/shell structure, red-shifting of absorption/emission peaks followed by blue-shifting of analogues were observed due to the intradiffusion of sulfur anion to CdSe luminescent center. In this gradient architecture, interfacial lattice strain can be effectively alleviated, and thus high PL QY (ca. 90%) and enhanced photochemical stability can be achieved. The synthesized blue-emitting gradient alloy QDs with superior optical properties tunable in the range of 450−490 nm can be used for highly efficient blue-emitters and potentially applicable for the fabrication of white-light LEDs.

1. INTRODUCTION Colloidal semiconductor nanocrystals or quantum dots (QDs) have been of great interest for fundamental study and technical application in the past decades because of their unique optical properties such as broad absorption, narrow emission spectrum, and size-dependent emission tunability that result from the quantum confinement effect.1 These outstanding properties make QDs very attractive candidates in various field of applications such as light-emitting diodes (LEDs),2,3 solar cells,4−6 biolabeling,7 etc. Among group II−VI QDs with the chemical composition of M-E (wherein M = Cd, Zn and E = Se, S, Te), CdSe QDs have been most intensively studied because the spectral features and corresponding energy bandgap of these QDs can be easily controllable across the entire visible spectrum in the range of 400−700 nm by modulating the size and shape of semiconductors QDs.8 However, bare CdSe QDs themselves severely suffer from the surface oxidation and related photodegradation problems since nanosized materials are primarily sensitive to the surrounding environments and vulnerable to oxidation due to the increased surface to volume ratio.9,10 Therefore, there have been great challenges to maintain luminescent properties of QDs against photodegradation (or photooxidation) as well as to obtain high-quality QDs with enhanced photoluminescence and improved photostability. © 2017 American Chemical Society

Hence, a variety of strategies of surface passivation of the luminescent center of CdSe cores to maintain emission efficiency have been developed thus far: (1) surface protection by organic capping ligands1,8 and (2) surface coating with inorganic shells, which are wider bandgap materials.11−13 Organic ligands are generally too labile to protect the surface of QDs against chemical degradation eternally, whereas inorganic shells (wider bandgap materials than CdSe) interfacing with CdSe cores can have inherent lattice mismatch and create interfacial lattice strain problems between the core and shell because the lattice parameter of the core and shell are fundamentally different. However, inorganic shell materials are characteristically robust relative to dynamically surface-binding organic ligands that inorganic shell passivation provide physically and chemically more stable QDs’ structure. In this regard, much research effort has been devoted toward the development of the inorganic shell passivation (type I band alignment wherein valence and conduction bands of the core are entirely sandwiched by wider bandgap shell materials), leading to the development of QDs with discrete interface such as CdSe/ZnS core/shell QDs,11,14,15 CdSe/CdS/ZnS core/ Received: December 2, 2016 Revised: March 20, 2017 Published: March 31, 2017 3711

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Langmuir shell/shell QDs,16 CdSe/CdS/CdZnS/ZnS core/multiple shells QDs,17−19 and gradient alloy QDs without abrupt interfaces such as CdSe x S 1 − x , 2 0 − 2 2 Cd y Zn 1 − y Se, 2 3 Cd1−x Znx S ySe 1−y ,24 and CdSe/CdSe xS 1−x /ZnSey S 1−y /ZnS core/shell gradient alloy (CSGA) QDs.25,26 Ultimately, in the gradient alloy structure, chemical composition gradually changes from inner CdSe core to outer shells without having a discrete interfacial separation between the core and shell.20,25,27 Thus, electronically photoexcited exciton (electron−hole pair) can be confined in the core effectively due to funnelling effect (shell components transfer charge carriers into core), and, crystallographically, the lattice mismatch problem can be successfully resolved because the lattice parameter continuously and progressively changes from the core to shell.25,27 For the fabrication of white-emitting LEDs based on QDs (QD-LEDs), the light-emitting QDs have to possess certain properties such as being stable against photo-, thermal- and chemical-degradation and retaining their initial high photoluminescence (PL) quantum yield (QY) for long-term operation.28,29 Highly efficient blue-, green-, and red-emitting QDs are thus desirable to be exploited as wavelength downconverters (converting UV light into visible light).29−32 Until recently, CdSe or CdSe-derived QDs have been comprehensively studied, and a great deal of synthetic chemistries for the preparation of green- and red-emitting CdSe-based QDs have been reported thus far. However, the synthesis of blue-emitting CdSe-derived QDs has still been a great challenge through the hot injection method because the chemical reaction is generally carried out at a high reaction temperature above 250 °C to induce the burst nucleation and following growth in order to obtain monodisperse and uniform nanoparticles with narrow size distribution.33 In addition, redshifting of the PL emission spectrum inevitably occurred for core/shell structure during the epitaxial growth of shell component onto the CdSe core because exciton wave function from the CdSe core can expand and thus penetrate to shell components after the formation of core/shell structure.11,14 Thus, it is no longer viable to synthesize small CdSe core QDs with an efficient blue emission at a high reaction temperature due to the very fast growth kinetics of the CdSe core. There are only limited reports published for the synthesis of blue-emitting CdSe-derived QDs. Č apek et al. reported the synthesis of CdSe/ZnS core/shell QDs with a PL emission wavelength below 500 nm, employing several synthetic steps including selective precipitation of the CdSe core.33 Recently, an ultrasmall CdSe with magic size was reported by Rosson et al. and Lawrence et al., but core CdSe core QDs alone are disadvantageous for the practical lighting application because of either low PL QY or low photochemical stability for long-term operation.34,35 Therefore, the synthesis of blue-emitting CdSederived QDs with a gradient structure ultimately overcoated with ZnS shells is strongly required to be exploited as the blueemitting component to achieve enhanced PL QY and high photochemical stability because the outermost ZnS shell induces better confinement of electron−hole wave function and funneling of exciton to core.36 Previously, we conducted a kinetics study on the formation of II−VI semiconductor nanocrystals and elucidated the formation mechanism of CSGA QDs, which have a composition of CdSe core with gradient alloy shell of CdSexS1−x/ZnSeyS1−y and final ZnS shell based on relative precursor reaction kinetics.26 Herein, we successfully demon-

strated the formation of blue-emitting CdSe-derived CSGA QDs with tunable emission in the range of 450−490 nm through a “one-pot” approach using a hot colloidal method, mostly developed thus far as well as very reproducible for QDs synthesis. We also investigated the chemical structure and correlated it with photophysical properties of synthesized blueemitting gradient alloy QDs. Most importantly, red-shifting of absorption/emission peaks followed by blue-shifting of analogues was observed due to intradiffusion of sulfur anion to the CdSe luminescent center, and thus the effective core size shrank, which has not been observed generally in the formation of CSGA QDs with type I band alignment. The PL QY of QDs was dramatically increased from 10% to 90% after interfacing the CdSe core with gradient alloy shells with wider bandgap components of group II−VI materials, and ca. 80% of original PL QY was retained over 24 h exposure to strong UV radiation. Highly efficient blue-emitting CdSe-derived CSGA QDs could be potentially utilized for white LED application as an efficient blue-emitter.

2. RESULTS AND DISCUSSION 2.1. Chemical Structural Characterization of CSGA QDs. Highly efficient blue-emitting QDs with the gradient chemical composition of CdSe/CdSexS1−x/ZnSeyS1−y/ZnS were prepared according to the previously reported method using four different precursors (Cd-OA, Zn-OA, TOP-Se, and TOP-S).26 The metallic precursors of Cd and Zn with a stoichiometric ratio of 1:20 were treated with the equivalent ratio of Se and S at a reaction temperature of 280 °C. The initial nucleation of CdSe core and simultaneous growth of gradient alloy shells take place according to the precursor reactivity order as soon as the precursors started to decompose to monomers, which results in the formation of CdSe core with gradient alloy shells in a “one-pot”, as shown in Scheme 1. The Scheme 1. Formation Mechanism of CSGA QDs with CdSe Core, Gradient Alloy Shells, and Outer ZnS Shells

chemical composition of CSGA QDs varied from CdSe inner core to chalcogenide alloyed intermediate shells CdSexS1−x/ ZnSeyS1−y and outmost ZnS shells in the radial direction wherein the chemical composition gradually transformed from CdSe to ZnS, described in Figure 1.25,26 The energy value (eV) versus vacuum in Figure 1 is the bulk-limit value reported in previous publications.26,37 It was revealed from our previous kinetic study that the reactivity difference between Cd and Zn precursors was larger than that of Se and S precursors. Thus the formation kinetic rate of II−VI QDs was discovered in the order CdSe > CdS > ZnSe > ZnS due to the hard/soft-acid/ base (HSAB) principle. The oleate anion is regarded as a hard 3712

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(nλ = 2d sin θ). The XRD pattern for the aliquout taken at 10 s exhibited that the two different crystal phases of CdSe and CdS were concurrently present, suggestive of that the synthesized QDs presumably had core/shell configuration such as CdSe/ CdSexS1−x essentially having CdSe as the core constituent. At the very beginning of the reaction period, sole CdSe core QDs might be formed as nuclei but they could not be isolated in a practical manner for the structural characterization in consequence of very fast gradient alloy shell growth onto CdSe core. The zin-blende XRD reflection was shifted to higher 2θ values as a function of reaction time from 10 to 1200 s, demonstrating that the chemical configuration of CSGA QDs was transformed from the CdSe core before 10 s to each gradient alloy intermediate shell component of CdSexS1−x at 30 s, ZnSeyS1−y at 60 s, and eventually to ZnS from 180 s until the excess Zn and S precursors were depleted completely (shown in Scheme 1). The four different precursors reacted in the order of precursor reactivity in a “one-pot” process, resulting in the elimination of the discrete layer separation between the core and shell, creating rather a continuous and smooth transformation in chemical composition from core to shells (Figure 1). After the formation of CdSe nuclei, intermediate alloy shells promptly began to incorporate to the CdSe core as soon as the nucleation step of colloidal CdSe was completed and thus chemical composition was altered gradually in a gradient manner from CdSe core to CdSexS1−x, to ZnSeyS1−y, and to ZnS eventually. The XRD patterns became sharper as a function of reaction time, indicating that the crystallinity of synthesized QDs was significantly improved due to enlarged crystallite size for the elongated annealing time. The chemical composition of CSGA QDs was analyzed by ICP-AES in order to cross-confirm, as shown in Figure S1 and Table S1. The chemical composition of gradient QDs was Cd- and S-rich at the initial time of 10 s, which was consistent with XRD results since the intermediate shell CdSexS1−x already covered the surface of the core soon after the CdSe core was generated. The concentrations of Cd and Se quickly declined, whereas the analogues of Zn and S were increased at the reaction interval of 10−60 s. More reactive precursors such as Cd-OA and TOP-Se were consumed considerably faster at the early stage of reaction period relative to less reactive Zn-OA and TOP-S. The kinetic rate constant of Cd and Se was at least 1 order of magnitude higher than the analogues Zn and S from our previous kinetic study.26 The size and shape of QDs were measured using TEM, shown in Figure 3a,b. The size of QDs was increased from 1.5 to 8.3 nm by the introduction of gradient alloy shells to the CdSe core listed in Table S2 and particle size distribution histograms of QDs collected at different reaction time, shown in Figure S2. The core size of QDs can be calculated from the first excitonic peak of UV/vis absorption spectra;40 however, the core size might be overestimated in this approach because of the leakage of the electron and hole wave functions from core to shells.13,14 The synthesized QDs at 1200 s show quasispherical shape with a narrow size distribution with highly improved crystallinity. There was no discrete interfacial layer between core and shells for the synthesized QDs, indicating that gradient alloy shells grew epitaxially on CdSe core with very smooth and gradient chemical composition changing from CdSe core to ZnS shell. The lattice fringe of nanocrystals aligning to the (111) plane with zinc-blend crystal phase was clearly displayed in the inset of HR-TEM images with d-spacing value of 0.326 nm. The d-spacing value of the (111) plane of

Figure 1. Energy band position of CSGA QDs (in eV) from CdSe core to gradient alloy intermediate shells and to the outermost ZnS shells.

Lewis base (electron pair donor), which can coordinate strongly to the hard Lewis acid Zn2+ cation (electron pair acceptor) compared to the soft Lewis acid Cd2+ cation, and thus Zn-OA is a thermodynamically more stable precursor than Cd-OA.38 In the same manner, tri-n-octyl phosphine is chemically coordinated to S strongly relative to Se and thus TOP-S decomposes at a higher temperature than TOP-Se. For the understanding of the formation mechanism of blue-emitting gradient alloy nanocrystals, aliquots at different reaction time intervals (10, 30, 60, 180, 600, and 1200 s) were acquired and characterized for the chemical composition, size, crystal structure, and photophysical properties of QDs. The crystallographic evolution of synthesized QDs at each reaction time was investigated using powder X-ray diffraction (XRD) as shown in Figure 2. All the XRD diffraction patterns

Figure 2. XRD patterns of isolated gradient alloy QDs at various reaction times at 10, 30, 60, 180, 600, and 1200 s, respectively, from bottom to top.

collected at various reaction times were well matched with cubic zinc-blend crystal phase of II−VI metal chalcogenide (ICDD (International Center for Diffraction Data) CdSe (PDF# 01-019-0191), CdS (PDF# 01-80-0019), ZnSe (PDF# 01-080-0021), and ZnS (PDF# 01-077-2100)). The gradient alloy QDs exhibited zinc-blende crystallographic phase instead of wurtzite phase because the reaction was conducted at 280 °C, wherein zinc-blende phase was thermodynamically more stable phase than wurtzite phase. It is known that the phase transition from zinc-blende to wurtzite occurs at 1020 °C in ZnS bulk.39 The zinc-blende XRD patterns were gradually moved to greater 2θ values as the reaction time extended, representing that the lattice parameter of QDs was diminished as a function of the reaction time according to Bragg’s equation 3713

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Figure 4. UV−vis absorption and PL emission spectra of QDs taken at different reaction times at 10, 30, 60, 180, 600, and 1800 s, respectively, from bottom to top (inset shows the illumination of synthesized QDs under excitation of 365 nm).

was increased as a function of reaction time as well as the wellknown phenomenon of exciton wave function leakage during the formation of type I core/shell QDs. The red-shift of exciton in absorption and emission spectra along with considerably enhanced PL QY are typically observed when shells grow on the band-edge luminescence of the CdSe core (Figure S2).27 This is associated with the leakage of exciton wave function because a thin shell component cannot strongly confine the exciton in the core.11,14 Incomplete blocking of the exciton wave function by shallow surface-covering of wider bandgap materials leads to the leakage of exciton wave function from the CdSe core into the shell and thus causes corresponding redshifting of the first exciton absorption and PL emission peaks during initial core/shell construction. After a reaction time of 60 s, an interesting phenomenon was observed in that the first exciton absorption peak along with the PL maximum emission peak were unexpectedly shifted to shorter wavelength (blueshifting) until the end of the reaction (1800 s of reaction time). The PL maximum emission peak was blue-shifted from 502 nm (at 60 s) to 485 nm (at 1800 s) by 17 nm. The blue-shift of the PL emission peak is the opposite trend of epitaxial growth of the ZnS shell on CdSe core (type I band alignment of core/ shell configuration). Rather, it is suggestive of cation or anion intradiffusion process from shell into core, resulting in an increment of the energy bandgap of band-egde luminescence and contraction of the effective size of the CdSe core.43−47 Therefore, this result suggests that the blue-shifting of the absorption/emission peak is originated from the ionic intradiffusion process of either cationic Zn2+ from the Zn-based intermediate shell or anionic S 2− from the Cd-based intermediate shell, which has wider energy bandgap materials compared to bare CdSe. The first exciton peak can move to the higher energy position during thermal annealing above 280 °C. It has been revealed that metallic Zn2+ with smaller atomic radii relative to S2− is further mobile and easy to diffuse into the core in core/shell conformation.43,45−47 However, with regard to the synthesis of CSGA QDs, the blue-shifting of the excitonic peak is probably due to the intradiffusion of S2− anion rather than Zn2+ cation because the very next intermediate shell to the CdSe core is the CdSexS1−x component.44 Considering the chemical structure of synthesized QDs of CdSe/CdSexS1−x/ ZnSeyS1−y/ZnS, the CdSe core is directly encapsulated by the CdSexS1−x intermediate shell, which has a greater influence on CdSe than Zn-based intermediate shells due to its spatially close proximity to core. In addition, blue-shifting might result

Figure 3. (a) TEM image of core/shell gradient alloy QDs obtained at different reaction times of 10, 30, 60, 180, 600, and 1200 s, respectively (inset shows magnified HR-TEM image at 1200 s), and (b) size profile of core and core/shell QDs as a function of time.

various II−VI metal chalcogenide of CdSe, CdS, ZnSe, and ZnS is known as 0.351, 0.335, 0.324, and 0.312 nm, respectively.41−43 The d-spacing value of synthesized QDs is mainly determined by the ZnS component due to high contribution of ZnS shells of ca. 3.4 nm thickness with initially used precursors’ stoichiometric ratio of Cd/Zn and Se/S of 1/20. The chemical structure of blue-emitting CSGA QDs was corroborated by XRD, ICP-AES, and TEM that the innermost core of gradient QDs consisted of CdSe because of higher precursor reactivity of Cd and Se compared to Zn and S, and alloyed intermediate layers of CdSexS1−x/ZnSeyS1−y could epitaxially incorporate onto the surface of CdSe core in a gradient manner eventually followed by an outermost ZnS shell. 2.2. Spectral Feature of Blue-Emitting CSGA QDs. The temporal change in the photophysical properties of CdSederived CSGA QDs was measured by UV−vis absorption and photoluminescence (PL) emission spectra, shown in Figure 4. The first exciton absorption peak and PL emission maximum peak appeared at a wavelength of 470 nm and at 485 nm, respectively, for the aliquot collected at 10 s. The spectral features were associated with CdSe core emission at early reaction period considering the correlation between the initial size of QDs and energy bandgap of materials. Moreover, the formation of CdSe was known as the fastest among various group II−VI QDs from the previous kinetic study.25,26 Until a reaction time of 60 s, the first absorption peak along with the PL maximum emission peak were progressively shifted to a longer wavelength (to 485 nm for the absorption peak and 505 nm for the emission peak) because the size of the CdSe core 3714

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Figure 5. (a) PL QY and (b) PL maximum wavelength (nm) and full width at half-maximum (fwhm in meV) as a function of reaction time for CSGA QDs.

important reason is ascribed to crystallographical gradient architecture, wherein lattice mismatching and interfacial lattice strain between the core and shell would be considerably released due to chemical composition variation in a gradient manner from core to shell without the formation of discrete interfacial separation (depicted in Figure 1). Accordingly, surface- and interface-related lattice defects (or trap sites) can be diminished effectively (which are the main causes of the nonradiative recombination pathway). Therefore, introduction of gradient alloy shells onto the luminescent CdSe core was a very effective way to enhance the PL QY.25−27 The full width at half-maximum (fwhm) of PL emission peak reflected the size distribution of synthesized nanocrystals (Figure 5b). The fwhm (meV) started with 160.9 meV at the early stage of reaction time (10 s) and became narrower to 152.3 meV with increasing reaction time (until 60 s), demonstrating that the size distribution of nanocrystals became narrower (focusing) in the epitaxial growth of Cd-based intermediate shell on CdSe core and after that became broader to 166.9 meV (defocusing) with the formation of Zn-based intermediate shell (until 180 s). The fwhm value continuously declined to 163.4 meV after 180 s, indicating that the size of nanocrystals was focused again through the thermal annealing process performed at 280 °C with uniform and monodisperse size nanocrystals. These are very consistent with TEM results. The emission wavelength of the synthesized gradient alloy QDs can be modified by modulating the ratio of Cd/Zn and Se/S in the range of 450−490 m (Figure S6). Increasing Zn and S precursor concentration resulted in the PL emission at the higher energy because of faster formation of gradient alloy shells, which could capture the growth of CdSe core at an even earlier time. However, when excessive Zn and S precursors were employed with stoichiometric ratio of 1/60 for Cd/Zn and Se/S, it does not lead to the effective formation of core/shell QDs with type I band alignment. The number of CdSe core nuclei formed is not enough to induce homogeneous nucleation for core formation followed by shell growth on core, but rather CdSe nuclei formation can compete with ZnS core formation due to increase in precursor concentration of Zn and S, resulting in the heterogeneous nucleation of CdSe with ZnS. The PL lifetimes were measured in order to compare the PL decay dynamics between the core with very thin shell (aliquot at 10s) and core/ shell gradient alloy structure (aliquot at 600s). The average PL lifetime of CdSe core with thin Cd-based alloy shell CdSe1−xSx obtained at 10 s is shorter than core/shell gradient alloy QDs

from the intradiffusion from wider energy bandgap materials of CdSexS1−x instead of the much wider bandgap materials of ZnSeyS1−y because of the small exciton peak-shifting value of 17 nm. The thickness of CdSexS1−x intermediate shells can be regarded as relatively thin because only a limited amount of Cd precursors (0.2 mmol) were employed for the synthesis of Cdbased components, most of which were used for CdSe core synthesis, and the remainder were used for the CdSexS1−x shell. Thus only a limited amount of S2− anion from the Cd-based intermediate shell can migrate into the core, causing the small blue-shifting of 17 nm. Assuming that the blue-shift originates from the intradiffusion of Zn2+ cation into the core, it might be more rational that the blue-shifting occurs to even shorter wavelength with the elongated reaction time after the formation of the Zn-based intermediate shell as well as the ZnS shell. However, the blue-shifting was impeded after 1200 s of reaction time. We extended the reaction time even up to 3 h , but additional blue-shifting to shorter wavelength cannot be observed (Figure S3). Hence, the intradiffusion of S2− from the Cd-based intermediate shell triggered the small blueshifting of the first exciton peak during the thermal annealing at high reaction temperature of 280 °C, and after completion of the intradiffusion of S2− into the core CdSe, the PL maximum emission peak remains the same despite the extended reaction time with the fixed bandgap energy and effective size of CdSe core. The PL QY of QDs at different reaction times was monitored and calculated by comparing with standard organic dye, shown in Figure 5a and Figure S4.48 At the reaction time of 60 s, QDs exhibited very low PL QY of 30%. However, it was dramatically enhanced up to 90% at 180 s because of the formation of Znbased alloy shells, which can effectively confine exciton wave function in the core due to the funnelling effect wherein photoexcited exciton can be transferred from outer shells into the inner core in a gradient alloy structure.25,27 Afterward, the PL QY was retained at a similar level (ca. 90%) after the formation of ZnS shell from 180 s (Figure S4). The epitaxial growth of alloy intermediate shells and the outermost wider bandgap materials of ZnS shells can electronically block the tunnelling of the exciton to the shell and effectively funnel exciton from shells into the core, resulting in highly increased PL QY.25,27 Indeed, the exciton can relax mainly through the radiative recombination pathway instead of the competitive nonradiative pathway after exciton was funnelled into the core by the inherent core/shell gradient alloy architecture. Another 3715

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Figure 6. (a) PL spectra and (b) various optical properties (PL integrated area intensity, emission max., fwhm) of various blue-emitting CSGA QDs as a function of UV irradiation time, (c) PL spectra of white light fabricated by mixing blue-, green-, and red-emitting gradient alloy QDs in solution and, and (d) corresponding CIE chromaticity diagram.

value also maintain their original values of 480 and 31 nm, respectively, even after light exposure for 24 h, implying that the chemical structure of CSGA QDs is very stable against photo- and chemical-degradation and thus have an excellent photochemical stability. This is due to a significantly reduced lattice mismatch between the core and shell component, wherein the external photo- and thermal-strain and stress can be excellently relieved without forming interfacial lattice distortion or surface trap states. In addition, electron and hole wave-fucnctions are effectively confined and localized in the core component due to the funnelling effect, which can cause the radiative decay pathway to become more favorable.25,27 As-prepared blue-emitting core/shell gradient alloy QDs can be used as color-converting materials for generating a white light by mixing green- and red-emitting CSGA with an appropriate mass ratio of blue:green:red = 40:2:1, which also have very similar nanocrystal size of around 10 nm (Figure 6c,d and Figure S7). The blue-emitting QDs are much more required than green- and red-emitting QDs because of the reabsorption mechanism.50,51 In addition, the optical concentration of the first excitonic absorption peak of blueemitting CSGA QDs was considerably lower than that of greenand red-emitting CSGA QDs when using the identical mass of QDs dispersed in hexane because of very small effective size of CdSe core, shown in Figure S8. The inset digital photograph image of Figure 6c displayed bright white lights under UV excitation of 365 nm, demonstrating superior optical performance with high luminescence intensity. The Commission Internationale de l’Eclairage (CIE) color coordinates can be modulated by increasing amounts of blue-emitting QDs or redemitting QDs. The CIE coordinate can be adjusted from very bright white (0.32, 0.35) in the exact middle of the color

acquired at 600 s, shown in Figure S5 and Table S2. As the gradient alloy shells encapsulated the core, PL lifetime was drastically increased 5-fold from 3.1 to 16.8 ns, suggesting that the nonradiative decay channel is largely suppressed and thus electron−hole pairs can live longer in a gradient structure, which eventually undergo radiative recombination.12 Moreover, the change in PL lifetime indicates that the formation of gradient alloy shells can induce a significant enhancement of the PL lifetime due to the intrinsic nature of chemical configuration, which makes exciton transfer toward the core, causing a substantial increase of the radiative recombination rate relative to nonradiative recombination. The epitaxial growth of alloy shells on luminescent CdSe core is an effective way to facilitate the more radiative recombination process and constrain the nonradiative recombination process. 2.3. Highly Photochemically Stabile Blue CSGA QDs and Applicabilty for White LEDs. One of the most important prerequisites for color-converting materials in white LEDs is associated with stability issues at the high excitation power. The photostability of the blue-emitting CSGA QDs was investigated under UV exposure for certain periods of time using a W-halogen lamp, shown in Figure 6a. The PL emission area intensity (integrating the area under the PL emission curve) was slightly diminished as a function of UV irradiation time, but it maintained 80% of its initial value even after 4 h of UV irradiation (Figure 6b). In addition, PL emission area intensity rather increased up to 85% after 24 h of UV irradiation because the outermost surface of gradient alloy QD might be oxidized to form the surface oxide layer, which can eliminate the possible surface trap emission, the one of the main exciton decay channel causing the diminution in PL QY.9,17,49 The PL maximum emission band position and fwhm 3716

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Langmuir ⎛ 1 − T ⎞⎛ ΔΦQDs ⎞ ST ⎟ PL QY = ⎜⎜ ⎟qST ⎟⎜ ⎝ 1 − TQDs ⎠⎝ ΔΦST ⎠

diagram to the left for bluish white (0.27, 0.32) and right for reddish white (0.36, 0.34). In other words, by controlling the amount of blue-, green-, and red-emitting core/shell gradient alloy QDs, various white light emissions can be generated with a various color temperature from 4392.7 to 9484.6 K.

(1)

where TST and TQDs are the transmittances at the excitation wavelength for the standard dye and QDs, respectively; ΔΦQDs and ΔΦST are integrated emitted photon flux (photons 3 s1) for QDs and the standard, respectively, upon excitation wavelength; and qST is the quantum yield of the standard dye. The PL lifetime measurement was accomplished using a fluorescent lifetime spectrofluorometer (Fluorocube), and PL decay traces were fitted using the following multiexponential eqs 2 and 3.

3. CONCLUSIONS Highly efficient blue-emitting CdSe-derived CSGA QDs have been synthesized using a hot colloidal method in a “one-pot” reaction. The gradient chemical structure of CSGA QDs was analyzed by XRD, ICP-AES, and TEM to evaluate the crystallographic, size, and chemical composition variation as a function of reaction time, and spectral evolution of synthesized QDs exhibited an initial red-shifting followed by blue-shifting of excitonic peaks strongly associated with the intradiffusion of S2− anion from the Cd-based intermediate shell of CdSe1−xSx to the luminescent CdSe core. Thus, the effective size of CdSe would be contracted and the band-edge luminescence shifted to higher energy. Due to the formation of gradient alloy shells on the CdSe core, CSGA QDs showed greatly improved PL QY from 10% to 90%, increased PL lifetime from 3.1 to 16.8 ns, and high photostability under long periods of UV radiation. The CSGA QDs have an effective architecture to induce the radiative recombination pathway instead of te nonradiative pathway due to reduced lattice mismatch and increased exciton funnelling effect. Highly stable and efficient blue-emitting CSGA QDs were employed as blue color converter materials with a UV LED to generate the bright white light emission by mixing with green- and red-GSGA QDs that are potentially applicable for superior white LEDs.

i=2

F(t ) = A 0 +

∑ Ai ·exp(−t /τi)

(2)

i=1

⎛ (A ·τ ) + (A 2 ·τ2) ⎞ τave = ⎜ 1 1 ⎟ A1 + A 2 ⎠ ⎝

(3)

where A0 is the PL intensity at time zero; Ai is a weighting factor of each component; t is time; and τi is the lifetime of each component. A JEOL 2100 transmission electron microscope operating at an accelerating voltage of 200 kV was used to characterize the size, size distribution, and structure of the CSGA QDs. For the TEM sample preparation, 1−2 droplets of diluted solution were added on copper grids with carbon support and dried overnight. Powder X-ray diffraction (XRD) was performed with a New D8 Advance diffractometer (Bruker) in reflection geometry using Cu Ka radiation (1.5405 Å) in a 2θ range of 20−60 degrees. ICP-AES (inductively coupled plasma atomic emission spectrometer) measurement was achieved using a Shimadzu ICPS-7510. The photochemical stability test was performed using a W-halogen lamp (KANDOlite@, 200 W, 230 V, 118 mm lamp) with surface power density of 757 mW cm−2 at fixed distance of 15 cm from the lamp. Commission Internationale de l’Eclairage (CIE) color coordinates were determined using a CIE coordinate calculator developed by Patil added on MATLAB.

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4. EXPERIMENTAL METHODS 4.1. Methods and Chemicals. All air- and/or moisture-sensitive compounds were dealt with using standard Schlenk line techniques or in a glovebox under an inert gas atmosphere. CdO (Aldrich,), Zn(OAc)2 (Aldrich, 99.99+%), 1-octadecene (ODE; Aldrich, 90%), oleic acid (OA, Aldrich, technical grade 90%), sulfur shot (S; Aldrich, 99.98%), selenium shot (Se, Acros, 99.5%), and tri-n-octylphosphine (TOP; Aldrich, 90%) were used as received without additional purification. 4.2. Synthesis of the Blue-Emitting Core/Shell Gradient Alloy Quantum Dots (CSGA QDs). For synthesis of the blueemitting gradient alloy QDs, CdO (0.2 mmol), Zn(OAc)2 (4.0 mmol), OA (33.6 mmol), and ODE (10 mL) were used. They were placed in a 100 mL three-neck round-bottom flask equipped with a condenser and thermocouple adapter and heated to 180 °C under argon flow with vigorous stirring until all the solids in the flask were completely dissolved to make mixed metal-oleate (M-OA; M = Cd, Zn) precursors of Cd-OA and Zn-OA. The mixture was heated to 300 °C, and the chalcogenide stock solution containing Se (0.2 mmol) and S (4.0 mmol) dissolved in TOP 3 mL was swiftly injected into the reaction flask. The temperature was set to 280 °C for the following growth. A small aliquot was taken at different time intervals (10, 30, 60, 180, 600, and 1200 s, respectively) after the injection of chalcogenide source to the metal source. 4.3. Characterizations. UV−vis absorption (UV−vis) spectra for aliquots acquired at different times were recorded with a Scinco S3100, and the corresponding photoluminescence (PL) spectra were measured with a photoluminescence spectrometer, Jasco FP-6500. The PL QYs of QDs dispersed in hexane solution were measured and calculated by comparing with standard organic dye (9,10-diphenylanthrancene; PL QY of 91% in ethanol) with optical concentration of 0.03 under an excitation wavelength of 365 nm, as per the following eq 1.48

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04333. ICP-AES, UV−vis absorption, PL emission spectra (not normalized), PL lifetime measurement of blue-emitting gradient alloy QDs and TEM images, UV−vis absorption, PL emission spectra, and CIE diagram of blue-, green-, and red-emitting gradient alloy QDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hak-Sung Jung: 0000-0002-1391-3154 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by the Nano R&D Program through the National Research Foundation of Korea (NRF) financed by the Ministry of Education, Science, and Technology [20110019174] J.C. and Y.-K. J. appreciatively acknowledge the BK21 fellowship and H.-S. Jung was supported by a grant of the Korea Health Technology R&D project through the Korea Health Industry Development Institute (KHIDI), funded by 3717

DOI: 10.1021/acs.langmuir.6b04333 Langmuir 2017, 33, 3711−3719

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Langmuir

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the Ministry of Health & Welfare, Republic of Korea (HI15C0979).

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ABBREVIATIONS QDs, quantum dots; LEDs, light-emitting diodes; CSGA, core/ shell gradient alloy; CIE, Commission Internationale de l’Eclairage; PL QY, photoluminescence quantum yield; CdOA, cadmium oleate; Zn-OA, zinc oleate; TOP-Se, tri-noctylphosphine selenide; TOP-S, tri-n-octylphosphine sulfide; fwhm, full width at half-maximum

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