Investigation of Luminescence Enhancement and Decay of QD-LEDs

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Surfaces, Interfaces, and Applications

Investigation of luminescence enhancement and decay of QDLEDs: interface reactions between QDs and atmospheres Shang-Chieh Huang, Chang-Wei Yeh, Guan-Hong Chen, Meng-Chi Liu, and Hsueh-Shih Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18558 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

Investigation of luminescence enhancement and decay of QD-LEDs: interface reactions between QDs and atmospheres Shang-Chieh Huang, Chang-Wei Yeh, Guan-Hong Chen, Meng-Chi Liu and Hsueh-Shih Chen* Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT: We investigated the current unsolved problem of short-term enhancement and long-term decay of the luminescence intensity of quantum dots (QDs)-based light emitting diodes (LEDs) in applications to lighting and displays, and proved that the interface interaction between the QD surface and atmospheres plays a key role in the QD-LED operation process. It is suggested that the initial luminescence enhancement of QD-LEDs would be caused by the QD surface adsorbed species such as ligands and gas molecules rather than QDs themselves, while the luminescence decay is correlated to the interface reactions between QDs and photogenerated reactive oxygen species, which leads to formations of sulfate, hydroxide and oxide compounds after QDs are illuminated by 450-nm blue light in oxygen and water environments according to surface analysis and theoretic thermodynamics calculations. It was also found that involvement of water in the QD-LED operation can cause crystal merging of QDs possibly due to the surface sulfates in the presence of water.

Keywords: quantum dots, photo-stability, photo-oxidation, oxygen, moisture, water, LED

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INTRODUCTION Quantum dots (QDs) have attracted significant interests in optoelectronic and biomedical applications such as electroluminescence devices, 1 , 2 solar cells, 3 , 4 sensors 5 , 6 , bio-labeling 7 , 8 and recent promising wavelength-conversion QD light emitting diodes (QD-LEDs) 9 - 14 because of their tunable emission wavelengths, narrow emission bandwidth and high quantum efficiency.15,16 QDs are small nanocrystals with large specific surface area so the QD surface possesses a lot of structural defects, which generally act as the non-radiative recombination centers in the luminescence process.17 In order to improve the luminescence properties, few wide-bandgap ZnS or ZnSe atomic layers are generally grown to passivate the QD surface and to confine the carriers.18,19 On the other hand, Cd-based alloyed QDs with thick-shell have been developed to improve the material stability. Such structure reduces the interface defects and suppresses Auger recombination so the optical properties of QDs are improved in light emitting applications.20-23 Although a lot of improvements have been reported, unfortunately, the material stability and the long-term reliability for QD materials and devices (e.g., QD-LEDs) are still critical challenges in practical applications.24 The current QD-LEDs considered in either lighting or displays are based on the surface-mount technology (SMT), which would be first developed in early 2000s by us with a major LED manufacture which produced the first commercial QD-tube-based LCD TV in 2012. 25 , 26 Nowadays, QD-LEDs are still considered the target application form in wide-color-gamut LCD and lighting. However, none of announced QD-LEDs can successfully meet the current industrial requirements and reliability verifications, where the luminescence intensity of all devices either increases or decreases in different situations, no matter how stable QDs or QDLEDs have been claimed in literature. The material and device failure analytical results reported from the main LED and display companies and industry were diversified and the failure mechanism remains unclear. ACS Paragon Plus Environment

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The unsolved failure problem is considered to be a scientific subject to be urgently studied and solved. A general consensus of QD or QD-LED stability is thought to be caused by H2O and O2 as reported in previous studies.27-30 However, details and roles of water and oxygen on the luminescence intensity enhancement along with the intensity decay has not been clarified. Also, the interface reaction chemistry and thermodynamics is not thoroughly understood. In this study, we investigated the roles of oxygen and water in the PL intensity enhancement and decay of QDs illuminated by a standard commercial SMT blue InGaN chip with em = 450 nm. The PL intensity enhancement was confirmed to be relevant to the surface environment such as ligands rather than QD itself, while the PL intensity decay was related to the photo-oxidation products including sulfate, hydroxide and oxide compounds confirmed by the surface chemical analysis and theoretical thermodynamic calculation. A photo-oxidation model containing the reactive oxygen species (ROS) has been also presented to describe the photo-stability of QDs and extended to the intensity enhancement and decay of QDs in encapsulant resins in typical SMT QD-LEDs.

RESULTS AND DISCUSSION Photo-stability of QD particle film on blue LEDs in various atmospheres Figure 1 presents variations of relative light conversion efficiency (LCE, defined to be the ratio of QD emission peak area to reduced LED peak area, shown in Figure 1d) of ZnCdSeS/ZnS QD particle film illuminated by a blue LED (𝜆𝑒𝑚 = 450 nm) in various atmospheres (PL spectra can be found in Figure S1). In dry N2, relative LCE of QD film rapidly increases to ~ 160% in the first hour and then stabilizes at ~ 180% after illuminating for 24 h. No significant peak shift is observed during the illumination (Figure S2a in ACS Paragon Plus Environment

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Supporting Information). In dry O2, in contrast, the relative LCE decrease to ~ 65% in the first 24 h and then slowly decreases to ~ 44% after illuminating for 135 h. In wet N2 (N2/H2O, relative humidity = 85% at 25 oC), the relative LCE slightly increases to ~110% in the first 4 h and then linearly decreases to ~ 38% after illuminating for 135 h. In the case of wet O2 (O2/H2O, relative humidity = 85% at 25 oC), similar to that in O2, the relative LCE quickly decreases to ~ 55% in the first 24 h, and then decreases linearly, which resembles that in wet N2, but reaches zero after illuminating for 100 h. A calculated curve (blue dotted curve in Figure 1), which is the sum of the dry O2 and the wet N2 (O2 + N2/H2O) curves, exhibits a similar decay trend to that of the wet O2 (O2/H2O) curve, except an initial intensity increase, which is somewhat similar to the dry N2 case. This result implies that N2 has limited effect on the intensity decay. Also, it is possible that O2 and H2O are two individual roles in the intensity decay. On the basis of above experimental data, the results suggest that the initial intensity increase is neither directly related to O2 nor H2O, and the initial intensity reduction is dominated by O2, while the later intensity decrease is more related to H2O. In addition, a significant blue shift in the PL peak position together with apparent reduction in the blue light absorption are observed for QDs illuminated in both H2O-containing environments (wet N2 and wet O2) (Figure 1b/c). The phenomenon is ascribed to the photo-oxidation of the illuminated QDs, which oxidized the QD surface that reduces the semiconductor core size and light absorption ability (will be discussed later). Figure 1d schematically shows the QD-LED design and the LCE definition investigated in this study.


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Figure 1

ZnCdSeS/ZnS QDs illuminated by blue light (𝜆𝑒𝑚 = 450 nm) in dry N2, dry O2, wet N2 (N2/H2O) and wet O2 (O2/H2O)

atmospheres. (a) Variations of LCE of QDs during the illumination process. Blue dotted curve (O2 + N2/H2O) is the sum of the dry O2 and the N2/H2O data, which is provided for examining the effect of N2. (b) Variations of the PL peak position. (c) Reduction in the blue LED emission intensity (light absorption by QDs). (d) Estimation of LCE and illustration of the QD-LED studied in this study. QDs are directly deposited on a glass plate, followed by drying in inert gas. The QD glass plate is placed on the top of the plastic reflector containing a blue SMD InGaN LED chip. An air gap exists between the QD glass substrate and the chip.

In order to check whether the LCE reduction of QDs is caused by only atmosphere or not, a control study of QDs without blue light illumination was conducted and the results are summarized in Figure 2a. No significant LCE change is found for QDs without blue light illumination in all atmospheres no matter whether H2O is involved or not. Under the blue light illumination, the QD PL intensity obviously enhances in dry N2 and largely reduces in both oxygen and moisture atmospheres as those have shown above. The results clearly indicate that the gas molecules have considerable effects on QDs particularly exposed to blue light illumination. ACS Paragon Plus Environment

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A cyclic light switch experiment is carried out to demonstrate the illumination effect on the QD PL intensity in dry N2 (Figure 2). The result clearly exhibits that PL enhancement occurs as the QDs are illuminated (e.g., 0－1 h, 2－3 h, and 4－5 h), while no obvious PL enhancement shows as the blue light is switched off. It is noted that a sudden intensity decrease occurs between 6 and 7 h. When the blue light illumination is carried out at 6 h, the LCE first exhibits a slight increase compared with the point at 5 h, but it then drops and keeps a lower LCE value for nearly 1 h until the last point measured at 7 h. The data are insitu collected by the spectrometer so the drop would not be caused by the instrumental error. Since the PL intensity is corresponding to the light illumination and the surface chemical composition of QDs in N2 has no significant change (will be shown later), the intensity changes are considered to be the surface environmental change such as ligand reorganization in the presence of nitrogen rather than the QD crystal itself.31-33 A further observation shows that the LCE appears to fluctuate relatively more in the 4－5 h period than does in the 2－ 3 h one, implying a dynamic change in the QD surface chemistry. Similar result is also observed for QDs kept in dark for a longer time (i.e., 7 days) and the PL enhancement is more obvious when the QDs is re-illuminated by blue light (Figure S3 in Supporting Information). Consequently, the PL intensity enhancement would be mainly related to the surface ligands. In the current study, the QDs are dispersed on a glass substrate and could be surrounded by gas molecules such as nitrogen. When QDs are under the light illumination, some of the photo-excited carriers would be delocalized to the surface region and would interact with the QD surface species including ligands and surrounded gas molecules. So the QD ligands could dynamically adjust themselves to relatively more stable configurations in the QD photo excitation-luminescence, i.e., photoluminescence process. Future study may be needed to investigate more details and relation between the PL characteristics and the QD ligand dynamics in the presence of nitrogen under blue light illumination. ACS Paragon Plus Environment

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Figure 2

(a) Relative LCE change of QDs with and without blue light illumination in various atmospheres. (b) Cyclic light switch

experiment to examine the PL enhancement of QDs illuminated by blue light in dry N2. The QD LCE are measured every 5 min during the illumination process. The light power is switched off in some periods of time as shown on the figure.

Microstructural analysis of QDs after blue light illumination in various atmospheres Transmission electron microscopy (TEM) and scanning transmission electron microscope in the annular dark-field (STEM-ADF) are employed to analyze the illuminated QDs in oxygen and moisture atmospheres, as shown in Figure 3. The average size of as-prepared QDs and illuminated QDs in dry O2, wet N2 and wet O2 cases are estimated to be 13.3 ± 1.2 nm, 13.3 ± 1.3 nm, 13.0 ± 1.2 nm and 12.5 ± 1.3 nm, respectively (Figure S5 in Supporting Information). For QDs illuminated in dry O2, the QDs show no significant change in size and morphology and look like those as-prepared QDs, but the illuminated ones have indistinct particle boundary and slightly irregular surface, indicating a lower crystallinity (Figure 3a/b). The phenomenon is reasonable since photo-oxidation would lead to some amorphization of the QD near-surface region, which strongly influences the QD PL properties.34 For QDs illuminated in wet N2 and wet O2, a slight size decrease is perceived and is attributable to further photo-oxidation of QDs (Figure 3c/d). In addition, a fused-like appearance is observed for QDs illuminated in both H2O-containing atmospheres, as highlighted by the red ACS Paragon Plus Environment

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squares in Figure 3c. Furthermore, in the presence of both H2O and O2, a merged structure clearly forms at the QDs surface, as shown by Figure 3d. Figure 3e gives a higher resolution image of the interface regions of selected merged particles. The interface reaction between two materials and their final interface structure are determined by both materials kinetics and thermodynamics.35,36 The coalesced part of the QDs (blue color region) has a clear lattice fringes with d-spacing of 0.36 nm extending from the QD matrix with that of 0.35 nm calculated from the fast Fourier transform (FFT) analysis, indicating the QD aggregation is not just a simple surficial contact. Instead, they merge with each other in the crystal growth level and form an epitaxiallike bridge. The interface regions are classified into several types based on the crystallinity and lattice misorientation. The orange regions present some lattice fringes misoriented about 3  4o adjacent to the QD matrix, while the middle red region is a relatively amorphous part. This phenomenon might be explained by incorporation of oxygen and hydrogen atoms into the QD near-surface region, which changes the surface composition and the crystal structure from Zn-S-Zn to Zn-SO4 and Zn-OH, as shown in Figure 3f, which will be further discussed in the surface compositional analysis later. Interestingly, the QD merging event is seemed to occur only in the presence of H2O. Considering quaternary ZnCdSeS or ZnS crystals and their related oxides such as ZnO, the sintering process should not occur in the room temperature even under the e-beam bombardment. For the other oxidized products, i.e., metal sulfate, it decomposes at around 700 oC, which does not possibly sinter at room temperature. Consequently, a possible explanation for the inter-merging phenomenon of QDs is the cause of sulfates, which could form hydrates and adsorb moisture and thus easily merge with each other to a certain extent when they physically contact with each other.


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Figure 3

STEM-ADF images of as-prepared ZnCdSeS/ZnS QDs and 450-nm blue light illuminated ones. (a) As-prepared. (b)

After illumination in dry O2. (c) After illumination in wet N2. (d) After illumination in wet O2. (e) A higher resolution image of the merged interface region. The interface regions between QDs are marked by different colors. The orange color exhibits lattice misorientation region at the QD surfaces (ϴ =3.23o and 4.03o). The middle red region is an amorphous part, while the blue region shows a clear lattice fringes with 0.36 nm d-spacing extended from the QD matrix, which is possibly (002) of ZnSO4/Zn(OH)2 (d = 0.355 nm) derived from oxidized (100) of a hexagonal QD that is supported by the surface composition analysis. The top-right inset is a FFT image of the interface region (blue region). (f) Molecular forms of oxidized ZnS (Zn-SO4 and Zn-SO4/Zn-OH). Zn links four SO4 tetrahedral groups, which could also be partly replaced by OH groups.

Surface composition analysis of QDs under illumination in various atmospheres Figure 4 presents X-ray photoelectron spectroscopy (XPS) spectra of QDs after illuminating by 450-nm blue light in dry N2 for 168 h. The Zn 2p, S 2p and Se 3d spectra are nearly the same for both the as-prepared QDs and the illuminated ones. The Zn 2p binding energies are around 1021.7 eV that is close to ZnS (Zn 2p ~ 1021.5 eV)37,38 and ZnSe (ZnSe Zn 2p ~ 1022.0 eV).39,40 The S 2p and Se 3d binding energies are 161.8 and 54.3 eV, which are assigned to ZnS (ZnS S 2p ~161.8 eV)39,40 and ZnSe (ZnSe Se 3d ~ 54.3 eV)41-43, ACS Paragon Plus Environment

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respectively. The shoulder peaks at ~ 166 eV would be from ZnSe 3p (ZnSe Se 3p3/2 = 160.1 eV, ZnSe Se 3p1/2 = 166.0).44 The XPS results indicate that QDs illuminated by a 450 nm InGaN chip in dry N2 have neither obvious oxidation nor surface composition variation, which are consistent with the above data of the long term illumination experiment (Figure 1). In addition, the O 1s peak position keeps nearly the same but the intensities at both shoulder sides slightly decrease, which could be connected to configurational change in those surface adsorbed organic compounds (O 1s C=O = 531 eV, C-O = 533 eV).45 In general, the chemical composition of QDs illuminated in dry N2 nearly remains unchanged. Combined with above PL enhancement data (Figure 1 and Figure 2), it is thought that the QD photo-brightening occurrence should be arisen from configurational variations of the surface ligands such as rearrangement and re-allocation caused by blue light photo-excitation.

Figure 4

XPS spectra for ZnCdSeS/ZnS QDs illuminated by blue LED (𝜆𝑒𝑚 = 450 nm) in dry N2 for 216 h. (a) Zn 2p. (b) S 2p. (c)

Se 3d. (d) O 1s. All curve intensities are normalized. The insets show the Gaussian fitting of the spectra.


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In the case of QDs illuminated in dry O2, the binding energy shifts to a higher energy position for both Zn 2p (+0.5 eV → 1022.2 eV) and S 2p (+7.6 eV → 169.4 eV), as shown in Figure 5. The Zn 2p peak is resolved to four convoluted peaks at 1021.5, 1022.2, 1022.5 and 1023.1 eV. The first two peaks are assigned to ZnS and ZnSe, respectively, while the other two are related to Zn oxidation, i.e., ZnO (ZnO Zn 2p = 1022.5 eV) and ZnSO4 (ZnSO4 Zn 2p = 1023.1 eV),46,47 respectively, as shown in the inset of Figure 5a. Formation of ZnSO4 is indeed observed from S 2p curve of the illuminated sample (red line in Figure 5b), as evidenced by the 169.4 eV peak, which chemically shifts +7.6 eV from the original ZnS peak at 161.8 eV due to increasing S oxidation state from 2 to +6, which also proves oxidation of the surface ZnS layer of QDs. Moreover, there is no obvious change in the Se 3d peak, which suggests that the inner part of QDs is barely oxidized and the oxidized layer (i.e., ZnO/ZnSO4) might be able to protect QDs from further oxygen attack. For O 1s spectra, the peak intensity of oxidized sample is much stronger than that of the as-prepared one (~ 8 times), indicating more oxidization (i.e., ZnO and ZnSO4) occurs as also shown by above Zn 2p and S 2p spectra. Based on the above results, oxidation of QD ZnS shell caused by blue light illumination leads to ZnO (ZnO O 1s = 531.4 eV) and ZnSO4 (ZnSO4 O 1s = 532.6 eV).48


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Figure 5

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XPS spectra for ZnCdSeS/ZnS QDs illuminated by blue LED (𝜆𝑒𝑚 = 450 nm) in dry O2 for 216 h. (a) Zn 2p. (b) S 2p. (c)

Se 3d. (d) O 1s and the as-prepared QD peak intensity is magnified ~ 8 times. All curve intensities are normalized. The insets show Gaussian fitting of spectra of the illuminated samples.

For wet N2 and wet O2 cases, similar XPS results are shown in Figure 6 and Figure 7. Larger shifts +0.7 V and +0.8 eV in the Zn 2p binding energy are observed for QDs in the wet N2 and wet O2 cases, respectively. The peak shifts are ascribed to formations of ZnO, Zn(OH)2 (Zn(OH)2 Zn 2p = 1022.6 or 1022.7 eV)42 and ZnSO4 compounds shown in Figure 6a and Figure 7a. In S 2p spectra in Figure 6b, a small peak at 169.4 eV is attributed to ZnSO4 (+ 7.4 eV shift from ZnS at 161.8 eV). The main difference between the wet N2 and wet O2 cases is that the sulfate peak intensity of wet O2 case is much stronger than that in the wet N2 case, indicating more serious oxidation occurs in the QD ZnS surface layer, which converts into ZnSO4 (Figure 6b and Figure 7b). The results also exhibit that the ZnS layer oxidation leads to similar products in both H2O and O2, except zinc hydride forms in the presence of water. In Se 3d spectra, a SeO2 peak arises up at 59.8 eV (SeO2 Se 3d = ACS Paragon Plus Environment

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59.8 eV) in the wet N2 case.49 Compared with the above dry O2 case (Figure 5c), where no SeO2 peak is observed, it indicates H2O could penetrate the surface ZnS layer and attack the inner core Se of QDs. Similar results are also observed for the wet O2 case, in which the illuminated QDs possess relatively higher SeO2 peak intensity than that in wet N2. The effect again demonstrates the oxidized QD surface layer caused by H2O is structurally looser or lower crystalline than that caused by dry O2, which may be explained by additional Zn(OH)2 compounds formed in the presence of water. So the lattice and crystal structure mismatch is severer for three different products (ZnO, Zn(OH)2 and ZnSO4) and the molecules could migrate to the deeper core part of QDs, as has been shown in Figure 3f. The O 1s peak is considered to be a combination of Zn(OH)2 (Zn(OH)2 O 1s = 532.0 eV)42 and ZnSO4 signals. In addition, a comparative study of QDs without illumination in wet O2 exhibits obvious change in XPS Zn 2p, S 2p, Se 3d and O 1s spectra, suggesting that oxidation of QDs in moisture and oxygen is triggered by the blue light illumination (Figure S6). Besides oxidation of the QD crystals, QD surface organic ligands (e.g., oleic acid, OA) would be also affected by H2O and O2 in the illumination process. Fourier transformed infrared (FTIR) spectra of QDs illuminated in various atmospheres are given in Figure S7. For QDs illuminated in dry O2, compared with the as-prepared QDs, an additional band arises up at around 1100-1200 cm-1 and is assigned to the C-O bonding, which could be evident for oxidation of OA ligands, as was also shown in previous studies.50 Similar oxidation data of OA ligands at 1100-1200 cm-1 are also measured for QDs illuminated in wet N2 and wet O2. Furthermore, some peaks at 620 cm-1, 983 cm-1 and 1100 cm-1 are related to sulfates consistent with the above XPS results (Figure 6b and Figure 7b).51 No extra peak is observed for QDs in dark in wet O2 (Figure S7b), revealing that the oxidation of the QD surface OA ligands would be also caused by the blue light illumination.


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Figure 6

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XPS spectra for ZnCdSeS/ZnS QDs illuminated by blue LED (𝜆𝑒𝑚 = 450 nm) in wet N2 for 216 h. (a) Zn 2p. (b) S 2p.

(c) Se 3d. (d) O 1s. All curve intensities are normalized. The insets show Gaussian fitting of spectra of the illuminated samples.

Figure 7

XPS spectra for ZnCdSeS/ZnS QDs illuminated by blue LED (𝜆𝑒𝑚 = 450 nm) in wet O2 for 216 h. (a) Zn 2p. (b) S 2p.

(c) Se 3d. (d) O 1s. The insets show Gaussian fitting of spectra of the illuminated samples.


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Thermodynamics of oxidation of QDs under blue light illumination Theoretical thermodynamic study of reactions between ZnS and atmospheres is conducted for revealing possible reaction pathways of the surface ZnS oxidation of QDs. Table S1 summarizes the free energies of formation of some possible products derived from ZnS oxidation.52,53 The calculated free energy of reaction of ZnS with O2 and/or H2O are listed in Table S2. The theoretical free energies of either [ZnS + O2] (QDs in dry O2) or [ZnS + O2 + H2O] (QDs in wet O2) are all negative, while that of [ZnS + H2O] is positive, meaning that ZnS oxidation is thermodynamically favored in oxygen-involved environment, as schematically shown in Figure 8a. Interestingly, the reaction free energies of [ZnS + H2O] are positive for all the compounds, indicating that oxidation of ZnS by H2O is not favored. However, the surface ZnS oxidation of QDs illuminated in wet N2/H2O and existence of ZnSO4 have been confirmed by the XPS analysis (Figure 6), which is inconsistent with the thermodynamic calculations. The consequence is understandable since the free energy of ZnS in a general state must be different from that in the photo-excited state. Upon the blue light illumination, photo-catalytic reactions would take place for semiconductors such as TiO2, ZnO, ZnS and CdSe. 54 - 56 Considering the reactive oxygen species (ROS) generated from the adsorbed O2 and H2O molecules at the QD ZnS surface by the blue light illumination, the ROS radicals (e.g.,

1





O2 /O− 2 and OH ) are much more

oxidative than either oxygen or water molecules. The free energies of possible reactions between ZnS and ROS species are summarized in Table S3. The calculated free energies of all the ZnS-ROS reactions are negative, showing that the ZnS oxidation by ROS radicals are thermodynamically favored, as schematically shown in Figure 8b. In other words, the blue light illumination activates the QD surface ZnS to an excited state transforming surface adsorbed O2/H2O into ROS species so they are able to proceed reactions, which could oxidize surface Zn and S to produce ZnSO4, as shown by above XPS analysis. Formation of ZnSO4 on ACS Paragon Plus Environment

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the QD surface should affect the coordination of OA ligands to the QD Zn or S sites and thus lead to QD aggregation. Meanwhile, the sulfate would form hydrates that could merge with each other in the presence of sufficiently adsorbed water, as exhibited in Figure 3e. Moreover, the theoretical values also exhibits that the free energies of ZnSoxygen reactions are generally more negative than those of ZnSwater reactions, indicating the ZnS oxidation by O2 is more feasible. Consequently, the O2 oxidation would be faster than the H2O oxidation for QDs, as has been shown in Figure 1a.

Figure 8

Various reaction paths and the corresponding reaction free energies of ZnSO4 converted from ZnS in the presence of H2O,

O2, or O2/H2O based on the theoretical thermodynamic calculation. (a) Reaction takes place in dark. (b) Reaction takes place under light illumination. The photo-generated ROS from adsorbed O2 and H2O are considered instead in the reaction. The data are summarized from Table S3.

PL intensity enhancement/decay and photo-oxidation mechanism On the basis of above experimental and theoretical results and discussion, the PL intensity enhancement and decay of QDs upon blue InGaN chip light illumination shown in Figure 1 may be described by a proposed model illustrated in Figure 9. The PL enhancement of QDs illuminated in N2 would be caused by photoinduced rearrangement and/or reallocation of the surface ligands on the QDs, as shown by path a. For QDs illuminated in dry O2, the oxygen molecules adsorbed on the QD surface ZnS layer could be transformed into 

ROS (O− 2 and



1

O2 ) by photo-generated carriers delocalized to the surface. The O− 2 and

1

O2 then react

with the surface ZnS layer to form ZnO and ZnSO4 (path b). As the lattice and structural inconsistency exist ACS Paragon Plus Environment

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between ZnS and the products (ZnO- cubic or wurtzite, ZnSO4- orthorhombic), defects would form at the QD surface region during the oxidation and thus decrease the PL efficiency. The PL QY gradually declines and becomes stable after 48 h illumination, which is ascribed to protection of the dense surface ZnO/ZnSO4 layer preventing further attacks of oxygen. This may also explain that Se in the inner part of QDs is less oxidized as shown in the above XPS data. For QDs illuminated in N2/H2O (path c), the PL feature appears to a combination of that in pure N2 and H2O. A slight PL intensity enhancement in the initial stage would be contributed by the surface ligands rearrangement/reallocation similar to that of QDs in pure N2. Existence and adsorption of the H2O molecules on the QD surface would affect the surface ligand dynamics and depress the PL intensity enhancement. Similar ZnO/ZnSO4 products from ZnS oxidation in both O2 and N2/H2O are suggested, but Zn(OH)2 should form in the presence of H2O. The oxidized ZnS layer, i.e., ZnO/Zn(OH)2/ZnSO4 has more structural inconsistency so it is structurally looser compared with the ZnO/ZnSO4 one producing in dry O2. The looser layer allows constant oxidation toward the deeper core area that decreases the QD PL QY. In the case of QDs illuminated in O2/H2O (path d), the oxidation products are still similar to those in N2/H2O and the oxidation rate is even faster. The PL intensity reduction may be simply considered a combination of individual N2, O2 and H2O oxidation. Some interaction might exist between the components but would not dominate the oxidation process according to curve (O2 +N2/H2O)cal shown in Figure 1a. The surface ZnS is oxidized to a similar ZnO/Zn(OH)2/ZnSO4 loose layer. The oxidation reaction between ZnS and photo-generated ROS is the fastest in all the cases and can oxidize the inner core part of QDs. When QDs are spatially close to each other, the QD surface ZnSO4 can combine together in the presence of water since sulfate compounds easily absorb H2O and dissolve together. ACS Paragon Plus Environment

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Figure 9

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Illustration of PL intensity enhancement/decay and a proposed photo-oxidation mechanism of QDs illuminated in various

atmospheres. (a) N2. (b) O2. (c) H2O. (d) H2O and O2.

Photo-stability of QD-LED with QDs in encapsulants One of the most concerned problems of current QD-LEDs is the device stability and the reproducibility when QDs are encapsulated in various polymer resins in practical lighting applications. We have investigated LED encapsulation resins (poly(methyl methacrylate) (PMMA), epoxy and silicone) commonly used in the industry, as shown in Figure 10. According to the proposed photo-oxidation mechanism of QDs, oxygen and water molecules are the major cause of the QD oxidation under blue light illumination. Compared with other two resins, QDs encapsulated in epoxy are found to possess the best photo-stability ascribed to much lower oxygen and water permeability of epoxy (O2 permeability- epoxy/PMMA/silicone ~ 1/10/1000; H2O permeability- 1/10/200). The epoxy resins is thus recommended to be a suitable encapsulant instead of current silicones for QDs in practical applications of LEDs and displays.


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Figure 10


Photo-stability of QD-LEDs with QDs encapsulated in epoxy, PMMA and silicone resins. All experimental processes

were operated in atmosphere.

CONCLUSIONS We have confirmed that the intensity enhancement and decay of QDs is caused by QD surface interaction with atmospheres and could be extended to explain similar phenomena of QD-LEDs. The PL intensity enhancement of QDs illuminated by 450-nm InGaN blue light is observed only in the presence of N2, while the intensity decay occurs for QDs illuminated in the presence of either O2 or H2O. The PL intensity enhancement is attributable to rearrangement and/or reallocation of the QD surface ligands. On the other hand, the PL intensity decay is correlated to ZnSO4, Zn(OH)2 and ZnO produced from photo-oxidation of the QD surface ZnS layer reacting with photo-generated reactive oxygen species (ROS) converted from adsorbed oxygen and water molecules. The photo-oxidation products (i.e., ZnSO4, Zn(OH)2 and ZnO) could lead to structural defects such as misorientation and amorphization in the QD surface region due to inconsistency of the lattice structure and parameters so the PL intensity of QDs constantly decays under the blue light illumination. For practical QD-LED applications, an encapsulant with lower permeability of oxygen and water such as epoxy is recommended instead of silicone-based or polyacrylate-based materials.


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EXPERIMENTAL SECTION Sample preparation. ZnCdSeS/ZnS QDs used in the study possessed a low-Cd doping composition of Zn0.91Cd0.09Se0.67S0.33 overcoated with ZnS and the average diameter is ~ 11 nm.57,58 The photoluminescence wavelength was 525 nm and the quantum yield was estimated to be 60  11% (Figure S4 in Supporting Information). QD particle films were fabricated by drop-casting 5 μL QD-toluene solution with concentration of 20 mg/mL on glass substrates treated with UV-ozone. The QDs-glass substrates were then attached to a SMT InGaN chip (3006 chip, L  W  H = 3.0 mm  0.85 mm  0.6 mm, luminous flux 7.5 - 10 lm, em = 450 nm), where an air gap was kept to exclude the heating effect. This remote QD-LED type was also considered in our previous studies. For fabrication of standard QD-LEDs, an encapsulant resin was first mixed with QDs at 1 wt%, followed by dispensing on a SMT blue InGaN chip. The polymer resins used in the current study including poly(methyl methacrylate) (PMMA), epoxy and silicone polymers. PMMA was cured by a typical UV process, while both of epoxy and silicone were cured by a thermal process at 150 oC for 4 h. Photo-stability experiments. Photo-stability experiments of QD-LED were conducted in a glass chamber filled with various atmospheres. For dry O2 atmosphere, QD film and blue LED samples were placed in the glass chamber with continuous oxygen flow dehumidified. The humidity of dry O2 condition measured by a commercial humidity meter is below 20%RH. For wet N2 and wet O2 atmosphere, QD film and blue LED were placed in the glass chamber with continuous gas flow of nitrogen/oxygen bubbling through water. The humidity of wet condition is measured to be ~ 85%RH. All these experiments were performed under room temperature and the chamber was evacuated and refilled with the controlled gas for four cycles to a desired atmosphere condition. Then controlled gas continuously flew for 10 minutes before LED experiments were ACS Paragon Plus Environment

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carried out. For dry N2 atmosphere, QD film and blue LED was placed in a glove box with the oxygen and water concentration below 0.5 ppm. The illumination power of blue LED in all experiments was 130 W/m2. For the photo-stability of QD LEDs, QD LEDs were operated in ambient condition under room temperature. Characterization. Photoluminescence spectra of QD films and QD LEDs were investigated by fluorescence spectrometry. The size/size distribution and surface morphology of QDs were analyzed with sphericalaberration corrected field emission TEM (CS-TEM, JEOL JEM-ARM200FTH). The surface composition and chemistry of QDs was studied by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versaprobe II). Fourier transformed infrared spectra (FTIR) were obtained by Brucker Tensor 27. Inductively coupled plasma-mass spectrometer (ICP-MS) data were obtained by Agilent 7500ce.

ASSOCIATED CONTENT Supporting Information. TEM images, FTIR, XPS spectra and calculated thermodynamic data. (PDF)

AUTHOR INFORMATION Corresponding Author Hsueh-Shih Chen Email: [email protected] ORCID : 0000-0002-5715-0750

ACKNOWLEDGEMENT This work was supported by Ministry of Science and Technology (Taiwan) (contract nos. 102-2218-E007-013, 104-2623-E-007-007-ET, and 105-2119-M-007-031).


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Investigation of Luminescence Enhancement and Decay of QD-LEDs

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