Surface Coating of Gradient Alloy Quantum Dots ... - ACS Publications

Article Cite This: Langmuir 2017, 33, 13040-13050

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Surface Coating of Gradient Alloy Quantum Dots with Oxide Layer in White-Light-Emitting Diodes for Display Backlights 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 Single Molecule 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: Recently, quantum dots (QDs) have been successfully developed as efficient color converters for lightemitting diodes (LEDs) display due to excellent optical properties of QDs. Herein, we demonstrate a new approach to form metal oxide layers (or metal oxide coating) on the exterior surface of gradient alloy QDs (the most advanced chemical architecture QDs developed thus far wherein the lattice parameter from the core to shell is changing in a gradient fashion) in order to improve the photochemical stability and photoluminescence efficiency. The resulting CdO-treated QDs are incorporated into polymer matrix films to fabricate a backlight unit as a part of display panel wherein CdO-treated gradient alloy QDs are utilized as color converters upon the blue-LED excitation. The fabricated 9.7 in. iPad 2 tablet liquid crystal display panel exhibited an excellent uniformity in terms of CIE chromaticity, luminance, and bright variation and superb durability test results (maintenance of ca. 110% brightness compared to initial value even after 3 weeks of operation). a distinctive interfacial separation.15−20 In this structure, the lattice-mismatch or-strain problems resulting from the different lattice parameters between the core and shell can be efficiently mitigated from the view of crystallography, and, electronically, exciton can be effectively confined into the emissive center, core, due to funneling effect (photoexcited exciton transferring from shell to core).17,18,21 The gradient alloy QDs which have the most advanced and developed chemical architecture thus far have exhibited the exceptionally high PL efficiency (ca. 90%) and enhanced photochemical stability (maintenance of 80% of initial emission even after long-term UV radiation).18,19 However, it is worth noting that gradient alloy QDs are not able to be eternally exonerated from the problems such as surface oxidation and corresponding PL quenching because the inherent chemical composition of final shells constitute sulfide (ZnS) instead of oxide.18,22 Recently, QDs have been rapidly developed and thus successfully commercialized as color converters in whiteemitting QD-LED backlights for the application of liquid crystal display (LCD) module due to their highly luminescent efficiency, negligible light scattering, and narrow emission.9,19

1. INTRODUCTION Semiconductor quantum dots (QDs) with unique optical properties derived from quantum confinement effect such as size-dependent photoluminescence (PL) emission across the visible spectrum, exceptionally high photoluminescence quantum yield (PL QY), and enhanced molar absorption coefficient have attracted much attention in various fields of applications such as light-emitting diodes (LEDs),1−3 solar cells,4,5 and fluorescent labels.6,7 In fact, core QDs alone only capped with organic-capping ligands exhibit low PL QY with broad emission originating from surface defect states.8 These QDs are readily degraded as a function of time due to a surface oxidation; therefore, maintaining the original optical properties of QDs over device fabrication as well as prolonged device operation periods still remains as a great challenge in quantum dot display technologies.9,10 The core/shell structure or the core with multishells QDs wherein core is encapsulated by shell with wide energy band gap materials can increase PL QY to some extent, but the core/shell heterostructures intrinsically possess the lattice-mismatch and -strain problems induced by interfacial separation between the core and shell.11−14 Recently, gradient alloy QDs have been designed and developed through the formation of gradient alloy shell layers on the emissive CdSe core wherein the lattice parameter changes in a gradient manner from the inner core to outermost shell without creating © 2017 American Chemical Society

Received: September 22, 2017 Revised: October 20, 2017 Published: October 23, 2017 13040

DOI: 10.1021/acs.langmuir.7b03335 Langmuir 2017, 33, 13040−13050

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Scheme 1. (a) Schematic Illustration of Morphological Evolution of Gradient Alloy QDs upon Additional Deposition of CdO on Top of GQDs (Top) and RQDs (Bottom) and (b) Fabrication of CdO-QD-Polymer Composite Films on PET Plate and Application of CdO-QD-Polymer Composite Films with Blue LED as BLU in 9.7 in. iPad 2

provides better surface passivation against oxidation and even improves PL QY due to the diminished density of surface defect states on the surface of QDs. Herein, we have demonstrated engineering the outermost surface of QDs by interfacing with CdO layers through the postsynthetic deposition of CdO via the thermal decomposition of Cd-oleate precursors.29,30 The cadmium molecular precursors are thermally decomposed and generated CdO monomers which are used for surface coating on outermost ZnS shells of gradient alloy QDs. Additional coating of QDs with CdO layer can successively passivate the surface of QDs which can effectively eliminate the surface defect states and reduce nonradiative relaxation; furthermore, it can preclude catastrophic degradation of luminescence by the surface oxidation. Consequently, CdO-treated QDs exhibited increased PL QY and improved photochemical stability which can be utilized as efficient color converters in white LEDs backlights for LCD display panel.31−33

Specifically, white-LED backlights predicated on QDs for a display application have more benefits compared with conventional fluorescent lamp backlights such as high quantum efficiency, high color purity, and compact-size packaging.23 In spite of the aforementioned outstanding properties, QDs have yet some limitations compared with the highly stable inorganic bulk phosphors especially that photochemical stability of QDs has to be much more improved over a long period of device operation. In general, QDs show the degradation of PL emission and the emission-wavelength shifting upon a longterm light irradiation, which is associated with the generation of surface- or interface-related defect states originating from photooxidation or photobleaching of QDs.11,24,25 When QDs are interfaced with the exterior oxide layers, photochemical stability might be significantly improved because of the preclusion of surface oxidation.26,27 Initially surface oxidation starts to occur at the outermost layer of the QDs surface under light radiation, and oxidation can keep progress through oxygen diffusion into the inside of the QDs as a function of time, which has a deleterious impact on the PL performance.11,24 However, coating the QDs with oxide materials can efficiently alleviate the photooxidation problems, thereby providing a better longterm photochemical stability. Therefore, Lin et al. have reported the deposition of metal oxide on CdS or ZnS nanorods to prepare CdS/CdO and ZnS/ZnO heterostructure via the chemical vapor deposition method so that chemically more stable oxide shells can passivate the inner sulfide component, leading to the improved photoluminescence.28 In addition, the surface treatment with NaBH4 to bring about the surface oxidation can further improve the quantum efficiency of CdS nanocrystals.27 More recently, Subila et al. have investigated that higher PL QY of zinc-blende CdSe (ca. 37%) compared to wurtzite CdSe (ca. 4%) originated from the formation of surface CdO layer which can further preclude the formation of the surface defect states and induce better confinement of charge carriers.26 Overall, the literature on precedence suggested that surface oxide layer definitely

2. RESULTS AND DISCUSSION We have successfully developed to prepare CdO-coated gradient alloy QDs through postsynthetic thermal decomposition of Cd-oleate precursors in the presence of QDs such that CdO nanocluster was initially implanted as nuclei onto the surface crystal defect sites of present gradient alloy QDs having a higher surface free energy that induces the further growth of CdO layer (Scheme 1a). The CdO-treated QDs with enhanced PL QY and drastically increased photochemical stability were incorporated into polymer matrix to facilitate full color converter layers which are further sandwiched between backlight unit (BLU) and liquid-crystal layer, ultimately constituting the full LCD panel structure, as shown in Scheme 1b. We make further investigation of CdO-QDs in terms of optical properties with photochemical stability, detailed structural characterization, and ultimately fabrication of 9.7 in. iPad 2 tablet liquid-crystal display (LCD) panel using newly developed CdO-treated QDs as full color converter in BLU, 13041


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emission at λmax = 536 nm and red emission at λmax = 609 nm (hereinafter denoted shortly as GQDs and RQDs, respectively) and CdO-treated gradient alloy QDs with green emission at λmax = 541 nm and red emission at λmax = 612 nm (denoted as CdO-GQDs and CdO-RQDs, respectively). To directly observe the influence of CdO layer formation on the existing gradient QDs, the optical density at the first excitonic bandedge absorption for both unmodified QDs and CdO-treated QDs is intentionally adjusted as same, suggestive that the same number of QDs particles are dispersed in a solution in terms of emissive CdSe core. After QDs were treated with CdO layers, the PL maximum emission was slightly red-shifted to the longer wavelength from 536 to 541 nm for GQDs and 609 to 612 nm for RQDs. The slight red-shift after the formation of CdO layers was attributed to the partial leakage of electron and hole wave function from CdSe core throughout the CdO matrix.8,35 The effective dimension of exciton wave function of CdSe core may be increased to some extent after the deposition of CdO which brings about the delocalization of electron and hole wave functions; electron wave function has more probability of tunneling into CdO layer than analogue of hole. Moreover, it can decrease the confinement energy and result in the redshifting of the band-edge excitonic peak.35,36 More interestingly, CdO-treated QDs exhibited indeed amplified absorption cross section in the range of 400−500 nm due to the surface passivation of QDs with exterior CdO layers. The relatively thin CdO layers in the nanoscale regime have an increased energy band gap compared to the counterpart of bulk CdO (bulk state Eg = 2.35 eV) owing to quantum size effect,37 thereby indicating that the enhancement of absorption cross section is directly associated with the attachment of CdO to existing QDs.30 The UV−vis absorption of CdO nanocrystals with a band-edge absorption centered at 350 nm and extended broad shoulder ranging from 400 to 500 nm confirmed that CdO layers are indeed deposited onto the external surface of asprepared QDs, shown in Figure S1. The surface interfacing of QDs with the CdO layers resulted in the increased absorption cross section at 400−500 nm without any deterioration in the excitonic feature of luminescent CdSe core. The intensity at PL maximum emissions of both green and red CdO-QDs were enhanced significantly relative to untreated QDs, respectively. The absolute PL QY was also measured to observe the influence of the formation of CdO layers on luminescent efficiency of QDs, demonstrating that the absolute PL QY value was significantly increased from 59.5 to 72.6% for GQDs (increasing 1.22 times) and 55.2 to 63.1% for RQDs (increasing 1.14 times) after CdO treatment, listed in Table S1. The enhancement of PL QY may be directly connected with the increased absorption cross section at excitation wavelength (430 nm) as well as substantial elimination of surface-trap states by interfacing as-prepared QDs with additional CdO layers.26−28 The increased absorption cross section indicates that more visible light in the range of 400− 500 nm is additionally absorbed by CdO-QDs compared with unmodified QDs because of introduction of wide energy band gap CdO onto the exterior surface of gradient alloy QDs. However, in terms of energetic offset and relative band alignment of heterostructures of CdO-QDs where gradient alloy QDs are additionally interfaced by exterior CdO layers, both the valence band maximum (VBM) and the conduction band minimum (CBM) of CdO are located below analogue of ZnS.18,37 This strongly points out that the excited-state electrons transferring from the outermost CdO layers to the

showing an excellent uniformity in terms of CIE chromaticity, luminance, and bright variation and superb durability test results, which has not been achieved thus far. 2.1. Optical Properties and Photochemical Stability of CdO-QDs. The deployment of oxide layers as surface coating of QDs is a very advantageous postsynthetic surface passivation strategy for not only surface protection against surface oxidation but also maintenance of original PL QY as well as increase in PL QY.27,34 In order to effectively prevent the unfavorable quenching and degradation in PL emission induced by photooxdiation and photobleaching as a function of time, we intentionally modify as-prepared QDs (synthesized QDs redispersed immediately after precipitation/purification protocols) with surface CdO layer through the thermal decomposition of single molecular precursors using Cd-oleate.29,30 The detailed formation mechanism of CdO has been elucidated that Cd-oleate can undergo the decomposition to form CdO nanocluster at high temperature (200 °C). The formation mechanism of CdO has strongly motivated us that surface coating of existing QDs with CdO layers can be achievable via a single molecular precursor decomposition process wherein asprepared QDs are additionally overcoated with external oxide layer. A postsynthetic treatment of oxide layer for as-prepared QDs is a very convenient and strategically useful tool for (1) improving the PL QY, (2) enhancing the photochemical stability, and (3) preventing the PL degradation. The photophysical properties of as-prepared CdSe-based core/shell gradient alloy QDs and analogues after treatment of CdO, respectively, are compared and contrasted in Figure 1. It illustrates the spectra feature of UV−vis absorption along with PL emission for as-prepared gradient alloy QD with green

Figure 1. UV−vis absorption and PL emission spectra upon excitation of 430 nm of (a) GQDs and CdO-GQDs and (b) RQDs and CdORQDs. 13042


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lived component of 1020 ns, and long-lived component of >25 ns. The short-lived component is reasonably assigned to the fast recombination of a bi-, multi-, and charged-exciton, which are dependent on the size of QDs and thickness of shells.34,39−41 The generation of multi- or charged-excitons cause the Auger recombination process in which exciton energy is transferred to promote an another exciton, which can reduce the PL efficiency of QDs and ultimately LED efficiency.42−45 The intermediate component is attributed to a band-edge exciton recombination process, whereas long-lived component is ascribed to a deep or shallow surface-trapped mediated process directly influenced by surface-capping ligands and surface defect state (or surface environment).39,40,46 Interestingly, the relative contribution and PL lifetime of intermediatelived component in CdO-GQDs substantially increased from 45 up to 76% and from 10.13 to 18.28 ns, indicating that the formation of CdO layers on the surface of GQDs can highly extend the charge carrier lifetime for inducing more radiative band-edge exciton recombination.34,47,48 The contribution from both short-lived and long-lived components drastically declined from 32 to 18% (former) and 23 to 6% (latter), respectively, indicating that formation of CdO layers can also effectively suppress the formation of multi- or charged-exciton states and charge trapping in the surface trap states, which are main nonradiative decay channels.49 For CdO-RQDs, the relative contribution of short-, intermediate-, and long-lived components does not change significantly, but the lifetime constant of intermediate component increased from 17.12 to 18.59 ns. The RQDs are less significantly affected by a CdO treatment compared with GQDs. The synthesized RQDs were probably well-passivated and had less lattice strains by the gradient alloy and final ZnS shells compared with GQDs, because the small effective size of green-emissive CdSe core had a larger lattice strain than larger red-emissive CdSe core in terms of the lattice strain between the core and gradient alloy shell. The increase in PL QY is primarily attributed to the decreased density of surface-related and interface-related trap states, which potentially decrease a radiative recombination and PL QY.26,27 The surface passivation (or additional coating) with CdO layers can substantially minimize above defect states, and thus charge carriers undergo the radiative recombination more favorably rather than the nonradiative recombination, simultaneously causing the elongated average PL lifetime and increased PL QY. The high PL efficiency and photochemical stability QDs over a long-term period of operation are strongly required especially for a display application.9 The CdO-treated QDs have the exterior CdO passivation layers, which can ultimately prevent gradient alloy QDs from photooxidation followed by photobleaching of QDs. Although gradient alloy QDs have the most advanced architecture such as gradient interface without discrete interfacial separation between the core and shell, asprepared gradient alloy QDs also tend to undergo photoinduced oxidation and PL bleaching because of the photochemical oxidation of external sulfide shells.18,22 In fact, the PL QYs of QDs drop to some extent as the chemical composition of QDs are indeed oxidized (when QDs are exposed to strong UV radiation). The photobleaching experiment was performed to contrast the photochemical stability between untreated QDs and CdO-QDs. The PL emission spectra for as-prepared QDs and CdO-QDs were monitored at different radiation time intervals under an atmospheric condition, shown in Figure 3 a and b. The PL intensity of both as-prepared GQDs and RQDs declined as a function of radiation time but reached a plateau at

emissive center CdSe core through the gradient architecture is thermodynamically unfavorable; electron transfer from CBM of CdO to ZnS is thermodynamically uphill. Therefore, PL enhancement is more associated with the elimination of surface trap states after surface coating with CdO. The PL decay dynamics were examined to gain more insight into the exciton state and better understanding of CdO coating with increased PL QY and their influence on the exciton decay kinetics (competitive radiative and nonradiative recombination processes), shown in Figure 2:38 The PL lifetimes of as-

Figure 2. PL decay trace and multiexponenetial decay fitting curve of (a) GQDs and CdO-GQDs and (b) RQDs and CdO-RQDs.

prepared QDs and CdO-QDs were measured to investigate the PL decay behavior with growth of CdO layers. In general, the PL lifetime is elongated for both CdO-QDs compared with untreated QDs from 18.3 to 25.3 ns for GQDs and from 23.8 to 27.2 ns for RQDs, listed in Table S2. The PL decay traces are fitted using a multiexponential function for both untreated QDs and CdO-QDs; a triexponential function yields the best fitting results. 3

I(t ) = y0 +

∑ Ai*e−t /τ

i

n=i

(1)

3

τave =

∑i = 1 Ai *τi2 3

∑i = 1 Ai *τi

(2)

where I(t) is the PL intensities at time t; y0 is PL intensity at time 0; Ai (i = 1−3) is the prefactor of each component of I; τi (i = 1−3) is the excited-state lifetime of each component; and τave is average lifetime. The individual lifetime components with three different time windows (or channels) can be found in PL decay kinetics: short-lived component of 15 ns, intermediate13043


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Figure 3. (a, b) Temporal change of PL emission spectra as a function of UV radiation time for (a) GQDs (left) and CdO-GQDs (right) and (b) RQDs (left) and CdO-RQDs (right); (c−f) temporal changes of PL maximum emission (red line) and normalized PL area intensity (blue line) as a function of UV radiation time for (c) GQDs, (d) RQDs, (e) CdO-GQDs, and (f) CdO-RQDs.

50% (former) and 60% (latter) of initial PL intensity after exposure for 15 h. Both untreated QDs exhibited an initial quick drop in PL emission intensity within 3 h, ascribed to the photobleaching of QDs because of photooxidation of external ZnS shells.22 The photoinduced lattice strain or distortion on the surface or interface of QDs can create more interfacial or surficial defect states, resulting in the preclusion of radiative recombination. The outermost ZnS shells might be oxidized initially within 3 h of exposure time which fortunately prevents further PL degradation owing to photoinduced formation of exterior oxide layers.22 However, it was strongly suggestive of long-term UV irradiation with a high photon energy that could lead to the formation of photoinduced interface- or surfacerelated defect states despite the gradient alloy QDs wherein lattice mismatch substantially decreased due to gradient chemical composition variation from core to shell.18,22 The gradient chemical architecture effectively alleviated the crystallographic lattice strains but was not able to release the photoinduced strain effectively. Moreover, intrinsic sulfide shell

was not free from surface oxidation and further oxide diffusion to the core.11 In addition, the PL maximum emission wavelengths of untreated QDs were slightly red-shifted for both GQDs and RQDs as a function of radiation time from 536 to 545 nm for GQDs and 610 to 615 nm for RQDs because of exciton leakage upon long-term photoexcitaiton and photooxidation.35 In stark contrast, both CdO-treated QDs exhibited drastically increased photochemical stability compared with untreated QDs without any emission-peak shifting in PL maximum emission wavelength. The PL maximum emission intensity of both CdO-QDs was maintained above 90% of the initial intensity after 15 h exposure, and the PL maximum emission wavelength was also maintained at the original position of 541 nm for GQDs and at 612 nm for RQDs under strong radiation of UV light even for 15 h. This strongly validates that the intentional coating of QDs with additional CdO layers is a very strategic and effective post-treatment method in order to obtain high-quality QDs with a highly enhanced photochemical stability.27,34 The serious problems 13044


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Figure 4. TEM images of (a) GQDs, (b) RQDs, (c) CdO-GQDs, and (d) CdO-RQDs and size distribution analysis (histogram) and Gaussian fitting curve for (e) GQDs (green) and CdO-GQDs (dark cyan) and (f) RQDs (red) and CdO-RQDs (pink); insets of (a) and (c) indicate the CBED of corresponding QDs.

gradient alloy QDs. It is suggestive that cubic phase of CdO (space group Fm3̅m) can be deposited on the zinc-blende (cubic) phase (space group F4̅3m) of as-prepared gradient alloy QDs owing to the same cubic crystal phase.28 In fact, the dspacing value along the (111) plane of CdO is known as 0.271 nm whereas the value of ZnS along the same plane is 0.312 nm demonstrating that the lattice parameter difference between ZnS and CdO is about 13.2% which is not tremendously different; the lattice mismatch between CdSe and ZnS is ca. 12% despite the formation of CdSe/ZnS core/shell QDs.18,28 The grain boundary between as-prepared QDs and deposited CdO layers was not observed because of thin layer formation of CdO epitaxially grown onto the gradient alloy QDs approximately 0.55−0.75 nm in thickness based on TEM observation. The CdO surface treatment of gradient alloy QDs could primarily alter the nanocrystals shape from quasispherical QDs to more spherical particles which is also evident on both bright-field and dark-field TEM images (Figures 4 and S2). It suggested that CdO layers were initially filled in the

related to photooxidation of surface of QDs which in turn resulted in the chemical composition degradation to oxide followed by PL degradation are effectively resolved by introducing CdO layers onto present gradient alloy QDs. In addition, the photoinduced lattice distortion might be relieved at CdO-QDs without any a deterioration of PL performance. This method can be widely generalized for the surface passivation and protection of other nanocrystals from the photooxidation or photobleaching. 2.2. Structural Characterization of Formation of Oxide Layer on the Gradient Alloy QDs. The morphology and dimension of both untreated and CdO-treated QDs were characterized by transmission electron microscope (TEM) images (Figure 4). After the formation of CdO layers on QDs, it was clearly discernible that the average diameter of QDs increased from 6.4 ± 0.9 to 7.9 ± 0.7 nm for GQDs and from 6.0 ± 1.3 to 7.1 ± 0.8 nm for RQDs. The increase in diameter of QDs after treatment of CdO strongly revealed that CdO layers were indeed formed on top of the surface of existing 13045


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Figure 5. (a) PL spectra of the fabricated CdO-QD-LEDs backlight, (b) digital photographs of substantialized a LCD tablet display panel with full color emissions (blue, green, red, and white), (c) CIE chromaticity coordinates of the fabricated LCD panel (blue, green, and red in a triangle) and white (center) compared with standard color RGB triangle (sRGB).

a and c). However, additional point diffractions newly appeared in CBED pattern, indicating that the crystallinity of CdO-QDs was much improved after introduction and coating of CdO on QDs through thermal annealing. 2.3. Fabrication of QD-LEDs Backlight for 9.7 in. iPad 2 LCD Display Panels. To exploit the CdO-GQDs and -RQDs with increased PL QY and the exceptionally enhanced photochemical stability as color converters for a display application, a preliminary test was performed by fabricating the white-LEDs wherein both green and red QDs were used as color converters.52 The white-LEDs were readily using CdOQD-polymer films which can be used as a new backlight unit (BLU) replacing with the old original BLU in an iPad 2 tablet upon excitation of blue-emitting InGaN LEDs. The CdO-QDpolymer composite films were prepared by blending CdO-QDs with polymer binder wherein CdO-QDs were homogeneously dispersed in the polymer matrix due to the hydrophobic van der Waals interaction between the long alkyl chain of surfacecapping ligands attached to the surface of CdO-QDs and polymer chain.53,54 Figure S6 illustrates the Fourier-transform infrared (IR) spectra for CdO-treated QDs with specific assignments of IR-active modes of surface-capping ligands such as C−H stretching at 2846 and 2920 cm−1, CO stretching at 1705 cm−1, and C−O stretching at 1602 cm−1 for oleic acid as well as N−H bending at 1602 cm−1 for n-octylamine, indicative of the coordination of n-octylamine and oleic acid on the surface of CdO-treated QDs. The homogeneous dispersion was achieved by using acryl resin polymer matrix wherein CdOQDs were embedded. Subsequently, the CdO-QD−polymer composite films were fabricated on polyethylene terephthalate (PET) polymer plate with the thickness of 50 μm and cured under UV radiation. The UV-curable photopolymer could encapsulate and solidify the CdO-QDs within polymer matrix through the photopolymerzation of monomer.54 The photoluminescence spectra of white-LEDs predicated on CdO-QDpolymer films exhibited white emission with a CIE value of 0.302 (Cx) and 0.307 (Cy) when CdO-QD-polymer composite films were excited by blue-LEDs (λem = 447 nm). The PL emission spectra of QD-LEDs backlight alone indicated that the combination of the blue-, green-, and red-emission resulted in the generation of white-light emission with high color purity (Figure 5a). Ultimately, the produced color in LCD panels originated from the color filters which are excited by the whiteemitting CdO-QD-LEDs backlight unit as shown in Figure 5b. The color filters could indeed convert to different colors such as blue, green, red, and white upon the excitation of QD-LEDs backlight (Figure S7); the white color shown in the photograph of Figure 5b was created by combination of RGB color filters

crystal surface defect states (not fully coordinated surface site of as-prepared QDs) once Cd-oleate precursors were readily decomposed to CdO monomers at high reaction temperature (200 °C). As a result, the exterior crystal defect states were completely filled by CdO and the size of CdO-treated QDs increased, as shown in TEM images (Figure 4). Afterward the formation of CdO nanocluster as a seed, CdO layers could start to form throughout the entire surface of as-prepared QDs at the elongated reaction time in the similar manner of layer-by-layer crystal growth.50,51 The detailed formation mechanism of CdO on gradient alloy QDs is still under investigation. The CdOQDs exhibited narrow size distribution compared to untreated QDs, shown in Figure 4e and f, which was very consistent with the variation in full width at half-maximum (fwhm) value of QDs and CdO-QDs in PL emission spectra, as shown in Figure 1. The fwhm value decreased from 35.3 to 32.4 nm for GQDs and from 37.9 to 33.0 nm for RQDs after CdO layers formation, providing direct evidence that QDs were encapsulated by CdO layers, leading to size focusing of CdO-QDs during the deposition of CdO layers. The chemical composition was indeed altered after CdO deposition, investigated by energy-dispersive X-ray spectroscopy (EDS) for the GQDs and CdO-GQDs as a representative example (Figure S3). The atomic concentrations of cadmium and oxygen were increased whereas analogues of zinc and sulfur were decreased after an introduction of CdO onto QDs. The atomic composition is listed in Figure S3 illustrating that CdO was effectively encapsulated onto the surface of gradient alloy QDs, thereby increasing the relative concentration of both Cd and O. Furthermore, the EDS color mapping for the elemental composition analysis clearly elaborated the presence of CdO layers on top of present gradient alloy QDs (Figure S4). It revealed that the Cd intensity was more strongly amplified than analogue of Se, indicating that Cd cation was not only existing inside of CdSe core but also outside of core−shell QDs (CdO layers) and O intensity was entirely overlapping with the QDs shown in STEM (Figure S4b−d). The high-resolution TEM images clearly displayed that the decomposition of Cd-oleate precursors does not induce a heterogeneous nucleation and growth of CdO nanocrystals but rather promotes the epitaxial growth on top of gradient alloy QDs, shown in Figure S5. The d-spacing parameter remained the same after a CdO treatment: 0.35 nm for green-QDs and 0.36 nm for red-QDs. This is indicative that CdO treatment did not alter the lattice parameters and crystal structure of existing gradient alloy QDs. After surface coating with CdO, crystal diffraction patterns of CdO were not directly observed in the convergent beam electron diffraction (CBED), shown in Figure 2 (insets of 13046


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Figure 6. (a) Temporal variation of PL emission spectra of the fabricated BLU with the white emission and (b) corresponding the absolute and relative brightness as a function of turn-on operating time up to 3 weeks.

3. CONCLUSION In conclusion, we have elucidated the formation of CdO layers on gradient alloy QDs in terms of crystal structure, size, and elemental composition and optical properties. The CdO treatment on gradient alloy QDs resulted in the increased PL QY and high photochemical stability because of the formation of surface oxide layers and preclusion of corresponding surface oxidation. The PL decay dynamics was investigated, revealing that increased PLQY was strongly associated with effectively suppressing of nonradiative decay channel due to the reduced surface defect states. Ultimately, green- and red-emitting CdOtreated QDs with increased PL QY and enhanced photostability were successfully exploited as color converters in backlight unit upon excitation of blue-LEDs for the fabrication of the 9.7 in. full-color tablet LCD. Consequently, the fabricated display panel showed very good color uniformity in terms of CIE chromaticity, luminance, and bright variation and excellent durability test results (maintenance of ca. 110% compared to initial value even after 3 weeks of operation). We suggest that postsynthetic surface treatment with oxide layer can be generalized for surface treatment of other nanocrystals for inducing better photochemical stability and diminished surface defect states, especially crucial for light-emitting nanocrystals.

upon the excitation by QD-LEDs BLU. In addition, the area of standard RGB triangle was almost covered by fabricated LCD (ca. 85%) (Figure 5c). To verify the uniformity of fabricated QD-LEDs, the average values of the CIE chromaticity, luminance, and bright variation of fabricated LCD were calculated by randomly selecting nine different spots on display panel (Figure S8). The average values of CIE chromaticity coordinates were determined to be 0.3476 (Cx) and 0.3804 (Cy), and the luminance of LCD was measured as 343 Cd/m2. The fabricated LCD showed an excellent color uniformity and brightness. The difference between maximum and minimum value of Δu′ (variation in blue) and Δv′ (variation in red) compared to the value at the center of the LCD panel was determined to be 0.0027 (Δu′) and 0.0022 (Δv′), respectively, indicating the color purity was highly uniform throughout the entire panel. The durability test of fabricated white-emitting QD-LEDs BLU was carried out under the harsh condition of high temperature (60 °C) and high humidity (90%) for 3 weeks. The photochemical stability of commercial phosphors in the display market is well-known that the above 85% level of initial brightness value has to be maintained under the test condition where we performed the durability test.55 The initial absolute brightness value of fabricated white-emitting BLU was 3640 Cd/m2 which was increased fivefold compared to lone blueemitting LED (Figure 6). After 3 weeks of durability test, the absolute brightness value was even slightly increased up to 110% compared to initial brightness. The relative brightness, which was in comparison with the initial brightness value, clearly indicated that the relative value kept increasing for 1 week of test and was retained at nearly the same level for the remaining 2 weeks. The initial spike might be ascribed to the partial restoration fluorescence during short-period exposure to UV light, similar to the process called “photochemical annealing (or photoannealing)” or decline of self-quenching by rearrangement of QDs nanocrystals in the pliable polymer matrix as a function of time.56 The CIE (Commission Internationale de L’Eclairage) chromaticity coordinates were also maintained with minimal change compared with the initial value (Δx = 0.0087 and Δy = 0.0097). Our future works have to be an application of newly designed CdO-treated gradient alloy QDs to quantum dot LED display directly, which obviously provide a promising performance of display devices owing to increased photochemical stability and enhanced PL QY.

4. EXPERIMENTAL SECTION 4.1. Methods and Chemicals. All air- and/or moisture-sensitive compounds were handled with using the standard Schlenk line techniques or in a glovebox under inert gas atmospheres. CdO (Aldrich, 99.99%), 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-noctylphosphine (TOP; Aldrich, 90%) were used as received without any further purification. 4.2. Synthesis of Core/Shell Gradient Alloy QDs. The synthesis of green- and red-emitting gradient alloy QDs is performed according to previously reported methods.18 Briefly, the stoichiometric ratio of cadmium and zinc precursors (Cd:Zn = 1:10) is mixed with noncoordinating solvent of 1-octadecene (10 mL) and oleic acid (1 mL) in a three-necked round bottom flask (100 mL). The mixed precursors solution was heated up to 300 °C, and subsequently the mixed chalcogenide stock solution containing Se (0.4 mmol) and S (4.0 mmol) dissolved in TOP (3 mL) was quickly injected to the reaction flask and the temperature was set to 280 °C for followed growth. The synthesized QDs were collected by reprecipitation with excess ethanol, centrifugation, and redispersion with hexane which were repeated three times. The synthesis of red-emitting QDs nanocrystals is the same with the synthesis of green-emitting gradient 13047


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alloy QDs except for the timing of injection of sulfur source after injection of selenium source. 4.3. Synthesis of CdO Nanocrystals. 1 mmole of Cd(oleate)2 was loaded into a three-necked flask in the presence of 1-octadecene (10 mL) and octylamine (1 mL), and the mixture was heated up to 250 °C under vigorous stirring. When the colorless solution turned into opaque due to the formation of insoluble cadmium oxide (CdO) nanocrystals, the reaction temperature was kept for 30 min. 4.4. Surface Treatment of Gradient Alloy QDs with CdO Layers. The purified green- or red-emitting QDs in 1-ODE (500 mg/ 10 mL) were added to the solution containing Cd(oleate)2 (1 mmol) and octylamine (1 mL) in the three-necked round bottom flask under atmospheric conditions. The mixture was degassed and filled with Ar gas for 3 cycles using the Schlenk line and heated up to 200 °C and then maintained at this temperature for 30 min. The resulting solution was cooled to room temperature, and excess acetone was added to the solution to precipitate/collect the products through the precipitation/ centrifugation at 4000 rpm for 10 min. The precipitated products were redispersed in desirable nonpolar solvents for further device fabrication. 4.5. Fabrication of QD-Polymer Composite Films for QD-LED Backlight LCD Tablet Display. The synthesized red-emitting QDs (240 mg) and green-emitting QDs (600 mg), dispersed in toluene respectively, were blended into acryl polymer resin (15 g). The mixed QD-polymer solution was transferred to polyethyleneterephthalate (PET) polymer plates to form a QD-polymer composite film with thickness of ca. 50 μm and then cured under UV radiation of 365 nm (1200 mJ, 30 mW × 40 s). The fabricated liquid crystal display (LCD; LED-backlight) was composed of backlight unit (BLU) embedded with CdO-QD-polymer composite films, liquid crystal layer (LCL), and color filter which are substantialized in commercial platform (9.7 in. iPad 2 tablet display, 447 nm TG blue LED)). The detailed structure is in the following order from bottom to top: back plate/ reflector sheet/blue LEDs/light guide plate/QD-polymer film/polarizer/LCL/polarizer/protection cover/bezel. 4.6. Characterization. UV−vis absorption (UV−vis) spectra were monitored with a Scinco S-3100, and the corresponding photoluminescence (PL) spectra were recorded with a photoluminescence spectrometer, Jasco FP-6500. Absolute PL QYs were measured using absolute PL quantum yield spectrometer QE 1200, OTSUKA Electronics, and PL decay dyanmic and corresponding lifetimes were measured with the Fluorescent Lifetime Spectrofluorometer, Fluorocube. The photochemical stabilities of QDs and CdO-QDs were examined under the light radiation using a W-halogen lamp KANDO lite with 200 W, 230 V, 118 mm lamp, and 757 mW/cm2 at atmospheric conditions. The JEOL 2100 transmission electron microscope operating at an accelerating voltage of 200 kV was used to analyze the morphology, size, and crystallinity of the obtained gradient alloy QDs and CdO-treated QDs.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hak-Sung Jung: 0000-0002-1391-3154 Author Contributions ⊥

These authors equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J. Cho and Y. K. Jung appreciatively acknowledge the BK21 fellowship. 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 the Ministry of Health & Welfare, Republic of Korea (HI15C0979). The authors thank Yuwon Lee for helping to measure LED properties.

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

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03335. SEM, XRD, and UV−vis absorption of CdO nanocrystals; bright-field and dark-field STEM images of untreated and CdO-treated GQDs; TEM images along with EDS spectra for untreated and CdO-treated GQDs; highresolution TEM images for untreated QDs and CdOtreated QDs; uniformity in absolute brightness and color of LCD tablet panel; tables of PL properties (emission max, QY, and fwhm) and of PL decay traces kinetics fitting parameters (PDF) 13048


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