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Alumina/polymer-coated nanocrystals with extremely high stability used as a color conversion material in LEDs
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 505714 (http://iopscience.iop.org/0957-4484/24/50/505714) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505714 (9pp)
doi:10.1088/0957-4484/24/50/505714
Alumina/polymer-coated nanocrystals with extremely high stability used as a color conversion material in LEDs Ju Yeon Woo, Jongsoo Lee and Chang-Soo Han1 School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Korea E-mail: [email protected]
Received 13 August 2013, in final form 23 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505714 Abstract The long-term stability of quantum dot (QD)-based devices under harsh environmental conditions has been a critical bottleneck to be resolved for commercial use. Here, we demonstrate an extremely stable QD/alumina/polymer hybrid structure by combining internal atomic layer deposition (ALD) infilling with polymer encapsulation. ALD infilling and polymer encapsulation of QDs synergistically prohibit the degradation of QDs in terms of optical, thermal and humid attacks. Our hybrid QD/alumina/polymer film structure showed no noticeable reduction in photoluminescence even in a commercial grade test (85% humidity at 85 ◦ C) over 28 days. In addition, we successfully fabricated a QD-based light-emitting device with excellent long-term stability by incorporating hybrid QD/alumina/polymer film as a color conversion material on light-emitting diode chips. S Online supplementary data available from stacks.iop.org/Nano/24/505714/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
detachments, and oxidation of the QDs [24]. The blinking phenomenon is considered to be due to the ionization of QDs by Auger recombination. Oxidation in particular is usually detrimental to the performance of QD-based devices due to the unwelcome changes it causes in the morphological, optical, and electronic properties of QD systems. A recent attractive alternative to encapsulation techniques is atomic layer deposition (ALD) [25, 26]. ALD is a stepwise chemical vapor deposition method offering excellent conformality on topologically complicated substrates, good reproducibility, sub-nanometer thickness control, uniformity over large areas, and relatively low growth temperatures [27–29]. ALD has been used to fill the pores in the films of PbS QDs with diameters as small as 7 nm [30]. Moreover, metal oxides deposited by ALD penetrate CdSe QD films, and improve film conductivity and carrier mobility [31]. ALD is suitable for depositing protective coatings on the surface of QDs, and ALD infilling can produce QD solids (QDs coated with ALD material) with greatly enhanced environmental stability. An improvement in the passivation afforded by the ALD surface coating can be achieved by increasing the amount of surface-coating material deposited, although this is expensive
The composite structures of semiconductor nanocrystal quantum dots (QDs) have attracted great interest for a variety of commercial applications, including photonic devices [1, 2], light-emitting diodes (LEDs) [3–6], biological labeling [7, 8], solar cells [9–11], and field-effect transistors (FETs) [12, 13]. However, a longstanding issue with stability and luminescence quantum efficiency under harsh environmental conditions hampers their suitability in commercial products. There have been numerous nanoparticle encapsulation approaches to solving this problem, based on organic ligands [14, 15], silica [16–18], carbon shells [19, 20] and amphiphilic polymers [21, 22], none of which effectively protects QDs from surface diffusion, oxidation, and degradation in complex environments. The lack of stability could stem from three main degradation phenomena as follows: decrease of the photoluminescence (PL), optical blue-shift, and blinking of QDs [23]. PL decay and blue-shift of QDs are usually explained by defect formation, ligand 1 Author to whom any correspondence should be addressed.
0957-4484/13/505714+09$33.00
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and time consuming. A surface coating of polymer can also provide more stability in the surrounding environment and for processing conditions like the overcoating of silicone as an encapsulation material onto the QD films. A key challenge is engineering a stable coating that will maintain the advantages of QDs and their electro-optical properties in complex environments. Here, we report a new strategy for creating ultra-stable QD films by combining ALD infilling with polymer encapsulation. Internal ALD infilling passivates defects, impurities, and localized charges in the vicinity of a QD, and acts as an atomic and molecular diffusion barrier to prevent thermal-induced QD degradation. Polymer encapsulation without direct contact with QDs acts as a secondary protective barrier to prevent the passage or diffusion of potentially deleterious species (e.g., oxygen, oxidizing agents, free radical species, and humidity) leading to photo- or humid-induced QD degradation from the external environment to the QD nanoparticles. We focus on the effects of humidity, ultraviolet (UV) light, and heat for QD films as a function of the ALD deposition thickness, polymer coating, and exposure time. Our hybrid QD/alumina/polymer film structure was the most stable of all films tested (bare QD, ALD infilling only, mixed QD/polymer, and hybrid QD/alumina/polymer), and did not show any decrease of PL or quantum yield (QY) for 28 days during very harsh environmental testing. Finally, by incorporating hybrid QD/alumina/polymer film as a color conversion material on LED chips, we obtained QD-LED-based light-emitting devices with excellent long-term stability. This technique presents many opportunities for developing the industrial applications of QD-based materials and devices.
were optimized to obtain uniform QD films 135 ± 15 nm thick. 2.3. ALD infilling Amorphous Al2 O3 was deposited using an ALD system (PLUS200, QUROS) with trimethylaluminum and O2 . The substrate temperature during the ALD deposition was 100 ◦ C and the operating pressure was about 0.1 Torr. The pulse time was 50 ms and the purge time was 10 s. Argon was used as a carrier gas with a flow rate of 50 sccm. Three levels of ALD deposition thickness were used: 5, 10, and 20 nm. 2.4. Preparation of hybrid QD/polymer film and its application to LEDs To prepare the hybrid QD/polymer film, we first mixed thermo-curable silicone resin parts A and B in a 1:4 weight ratio, and placed the mixture in a vacuum oven for 10 min at 1 bar to remove any bubbles. Silicone resin was then spun at 500 rpm for 120 s onto PET substrates with QD films after ALD treatment (10 nm deposition) using a spin coater (Ace-200, Dong Ah Trade Corp.) to form the polymer encapsulation structure. Finally, the nanocomposite films were thermally cured at 115 ◦ C for 2 h. The hybrid QD/polymer film was attached to blue LED chips using silicone sealant to fabricate the QD-based LED device. 2.5. Characterization Transmission electron microscopy (TEM) micrographs were obtained using a TEM system (Tecnai G2 F30 S Twin, FEI) at 300 kV. Cross-sectional TEM samples of QD films were prepared using a dual-beam focused ion-beam system (NOVA200, FEI). Energy dispersive x-ray spectroscopy (EDS) data were also acquired at various locations across the entire particle shown in the TEM image. Optical absorption, PL, and QY of the QD solution and QD films were characterized using a UV–vis spectrophotometer (SD-1000, Scinco), a fluorometer (Fluorolog, Horiba Jobin Yvon), and an absolute QY measurement system (C-9920-02, Hamamatsu) at room temperature. A temperature and humidity chamber (TH-ME-065, JEIO Tech) was used to study the effects of heat and moisture on QD films. For heat studies, QD films were placed inside an oven (OV-11, JEIO Tech) at 100 ◦ C. UV light soaking was performed on QD films mounted in a homemade testing box. An UV lamp was used to illuminate the films with 2 mW cm−2 of 352 nm light. Electroluminescence (EL) and luminance were evaluated in an integrating sphere using an LED spectra light measurement system (CSLMS LED 1060, Labsphere) at 4 V and 80 mA controlled by a Keithley 2400 digital source meter.
2. Experimental details 2.1. Materials The 623 nm red-emitting CdSe/CdS/ZnS core–multishell QDs (Nanodot HE series, QD Solutions Co.) were used without modification. Trimethylaluminum (97%) and anhydrous solvents were used as received from Aldrich. The dual component thermal curable silicone polymer (OE-6630 A and B) was purchased from Dow Corning. InGaN-based blue LED chips (TKF 01B450 blue power die, λmax = 455 nm, non-epoxy molding packages) were purchased from Trikaiser. The silicone sealant (30SHM) was purchased from Okong. 2.2. QD film formation QD films were prepared by spray coating them onto polyethylene terephthalate (PET) substrates using a homebuilt spraying system. To avoid QD agglomeration, diluted QD dispersions (5 ml of 0.1 wt% QDs in hexane solution) were sprayed on the prepared PET substrates. The substrate was heated to about 80 ◦ C during the spraying process using a back heating pad to accelerate evaporation of the solvent. Spraying conditions such as compressed nitrogen pressure, nozzle diameter, and distance between substrate and nozzle
3. Results and discussion This study used well-synthesised 623 nm red-emitting CdSe/CdS/ZnS core–multishell QDs (figures S1(a) and (b) available at stacks.iop.org/Nano/24/505714/mmedia). 2
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Figure 1. (a) Schematics of the fabrication process of QD films infilled with alumina by low-temperature ALD. Cross-sectional TEM images of QD films (b) before and (c) after ALD treatment (10 nm alumina deposited at 100 ◦ C) (scale bar: 50 nm). (d) TEM image with high resolution (scale bar: 10 nm) and (e) Al element EDS mapping of QD films after ALD treatment. Elemental EDS profile of QD layers (f) with no ALD and (g) infilled with ALD alumina.
Densely packed, thin, solid QD films with high uniformity and homogenization were prepared by spray coating on PET substrates using a home-built spraying system (figure S1(c) available at stacks.iop.org/Nano/24/505714/mmedia). During film formation, the first exciton peak and PL of the QD films were slightly red-shifted relative to QD solutions due to the increased dielectric constant and substantial electronic coupling [32]. The surface must be coated because this fabrication is not robust. To overcome the photostability of QDs related to luminescent properties, we used low-temperature ALD of Al(CH3 )3 and O2 at 100 ◦ C to fill the internal pore network of these films with amorphous alumina (figure 1(a)). ALD infilling with an inorganic matrix offers a significantly more robust structure and also acts as a protective barrier layer to suppress atomic, molecular, and gas diffusion. Figures 1(b) and (c) are typical cross-sectional TEM images of QD films before and after ALD treatment (10 nm alumina deposition). The TEM analysis showed that no ALD QD films were densely packed, and that the coating thickness of 135 ± 15 nm was uniform in all areas. After ALD coating, the conformal and continuous amorphous alumina was deposited on the outermost surface of the QD films as well as inside the voids and pores of the QD films. Figure 1(d) shows cross-sectional TEM images with high resolution of QD films after deposition of 10 nm of ALD alumina. This TEM image clearly shows lattice fringes and a separate layer, the internal pore network of the QD films infilled with alumina and overcoating layer (∼10 nm thick)
of thin alumina covering the top of the QD films. Although Al2 O3 was deposited inside the voids and pores of QD films, the crystalline structure of the QDs was maintained. The EDS probe position and elemental intensity of the film constituents before ALD coating shown in figure 1(f) indicate that the core–multishell QD films were composed of Cd, Se, Zn, and S. As shown in mapping of aluminum elements and the EDS profile in QD films after ALD treatment (figures 1(e) and (g)), the presence of aluminum and oxygen indicated that Al2 O3 was present throughout the QD films and homogeneously deposited on the internal and external surfaces that were free of pinholes. Figures 2(a)–(c) show the QY change as a function of the time for ALD deposition thicknesses of 5, 10, and 20 nm, respectively, exposed to various environments for QD films in air. During the QD film fabrication, the QY of bare QD films was reduced to 55% from that of the QD in solution (74%); the lower QY was due mainly to surface oxidation and degradation. On the other hand, as the ALD deposition thickness increased after ALD infilling, the QY increased slightly from 55% to an asymptotic value of 67% due to exciton mobility enhancement via passivation of surface defects [25]. The QD films exposed to reliability test conditions of 85 ◦ C and 85% relative humidity, which are levels generally used for commercialization, showed a rapid and dramatic drop of QY in the first week of the test, followed by only minor changes thereafter (figure 2(a)). The main reason for this result is probably due to the 3
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Figure 2. QY versus time as a function of the ALD deposition thickness exposed to (a) 85 ◦ C heat and 85% relative humidity, (b) UV light (2 mW cm−2 at 352 nm), and (c) 100 ◦ C heat for QD films in air. The ALD deposition thickness is 5, 10, and 20 nm, respectively. PL emission spectra of the QD films as a function of time exposed to 100 ◦ C heat (d) without and (e) with ALD treatment (10 nm deposition).
action of moisture to quench the QY by direct interaction with its surface through energetic oxidative reactions; this process is complete within the first few days. This means that the ALD does not inhibit oxidation, or degradation in QD films due to imperfect infilling of the narrow interstitial spaces and defects of Al2 O3 layer leading to increased diffusion of moisture at elevated temperature through the amorphous ALD matrix to the surfaces of the QDs. The QY increased slightly as the ALD deposition thickness increased, but there was no significant improvement. For QD films exposed to UV light (2 mW cm−2 at 352 nm), the QY decreased gradually over time (figure 2(b)). UV illumination can promote surface redox reactions such as oxidation by generating electrons and holes capable of participating in chemical reactions at the QD surface [30]. This leads to degradation of surface passivating ligands and phase transformation by photo-oxidation. Furthermore, similar to the QY results under humidity, ALD infilling does not effectively stop either oxidation or degradation of QDs. On the other hand, ALD infilling is clearly effective at preventing thermal ageing (figure 2(c)). Whereas ALDinfilled QD films showed minimal changes in QY, that of bare QD films decreased significantly. This temperature-sensitive quenching is attributed to carrier trapping in defect states and easy detachment of ligands by thermal energy [33], and surface passivation through inorganic infilling can effectively suppress this process at high temperature. Figures 2(d) and (e) show the PL emission spectra of bare and 10 nm ALD-infilled QD films exposed to 100 ◦ C as a function of time. Bare QD films experienced rapid significant loss of PL within 28
days, which we interpret as permanent structural degradation caused by QD surface diffusion and oxidation, as expected. However, the thermal degradation of ALD-infilled films over time was much less than bare QD films. The PL intensity of bare QD films exposed to other environments (85 ◦ C, 85% relative humidity, and UV light) decreased significantly over time, and 10 nm ALD infilling improved the stability of QD films slightly (figure S2 available at stacks.iop.org/ Nano/24/505714/mmedia). Based on these results, we believe that the ALD infilling offers robust QD surface passivation, but does not inhibit QD surface diffusion, oxidation, or degradation in QD films due to imperfect infilling of the narrow interstitial spaces. Moreover, even after ALD infilling, harmful species can still migrate through the ALD matrix to the surfaces of the QDs, which can lead to oxidation, degradation, and a consequent drop in QY. Therefore, the fabrication of ultra-stable QD films requires further surface coating. A conventional coating process that incorporates QDs directly into a polymer affords some level of protection and functions to some extent as a barrier to the passage of potentially harmful species through the coated material. In particular, silicone polymers are generally used for LED encapsulants because of their thermal stability, transparency, and resistance to oxygen and UV light [34]. To study the effect of various environments on the stability of mixed QD/polymer films, we prepared QD films in which the QDs were embedded in a silicone polymer (see supplementary data available at stacks.iop.org/Nano/24/505714/mmediafor details). Figure 3(a) shows that the QY changes over time 4
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Figure 3. (a) QY versus time for mixed QD/polymer films in air. Black line is films exposed to heat and humidity; red line is films exposed to UV light alone; blue line is films exposed to heat alone. PL emission spectra of the mixed QD/polymer films as a function of time exposed to (b) 85 ◦ C heat and 85% relative humidity, (c) UV light, and (d) 100 ◦ C heat.
decrease (figure S3(c) available at stacks.iop.org/Nano/24/ 505714/mmedia). Figures 3(b)–(d) show the PL emission spectra of the mixed QD/polymer films as a function of time exposed to the three different environments. Figure 3(b) shows that the PL of the mixed QD/polymer films was less sensitive to the humid environment than in the ALD infill case, which indicated that the polymer coating acted as a barrier to prevent moisture from contacting the QDs. The PL of the mixed QD/polymer films exposed to UV light remained extraordinarily stable over 28 days, decreasing by less than 10% over that time (figure 3(c)). This means silicone polymer did not stop the UV degradation thoroughly, but the superior stability of silicone polymer to UV light may be helpful in suppressing the degradation of the QDs. However, the PL of the mixed QD/polymer films exposed to 100 ◦ C decreased by more than 80% due to the unwelcome reaction (figure 3(d)). These side reactions probably originate from the release of oxidizing species and activated reactant at high temperature. These results indicate that while the mixed QD/polymer structure was useful for protecting the reactive QDs from potentially damaging environmental conditions, it was not suitable for preventing thermal-induced oxidation and degradation. Although each material does not effectively protect QDs from harsh environmental treatments, the combined alumina/polymer structure can protect QDs to such a degree that QD fluorescence remains highly stable.
as a function of three different environmental variables for mixed QD/polymer films in air. The initial QY of the mixed QD/polymer film decreased to about 43%, probably due to agglomeration in the QDs and different side reactions in the silicone resin at high temperature during film formation [35, 36]. Tracking the QY of the mixed QD/polymer films exposed to heat and humidity, and UV light alone over time showed a slight (10%) decrease in the QY, which indicated that the polymer coating offered greater stability in the presence of air, moisture, and photo-oxidation. The absence of any shift of optical absorption spectra of these QD films was also observed, confirming that the silicone polymer effectively prevents the passage or diffusion of potentially deleterious species from the external environment to the QD nanoparticles (figures S3(a) and (b) available at stacks.iop.org/Nano/24/505714/mmedia). In contrast, the QY of the mixed QD/polymer films exposed to 100 ◦ C showed a significant drop in the QY of more than 30% after 28 days. This was mainly due to a greater active release of oxidizing species in the polymer at high temperature that may degrade the QD and the remaining side reactions caused by the unreactive monomer, catalyst, and impurities related to curing temperature. Our optical absorption result also shows that heat exposure causes degradation of QDs, resulting in the excitonic peak blue-shifting considerably with exposure time due to the same reason as the QY 5
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Figure 4. (a) Schematic of the hybrid QD/alumina/polymer film structure. The film structure consists of the QD layers infilled with alumina (ALD infilling of the 10 nm) and a polymer layer (∼900 nm) on its external surface. Time traces of (b) the optical absorption spectra and (c) PL emission spectra of the hybrid QD/alumina/polymer films exposed to UV light. (d) QY versus time for hybrid QD/alumina/polymer films in air. Black line is films exposed to heat and humidity; red line is films exposed to UV light alone; blue line is films exposed to heat alone.
in air. The initial QY of the hybrid QD/alumina/polymer film was about 66% because of the trap passivation due ALD infilling, and negligible changes in QY occurred over 28 days. Hybrid QD/alumina/polymer films are thus the most stable in their surrounding environment and are suitable for applications such as LED-based light-emitting devices. A particularly attractive potential field of application for QD films is in the development of next-generation LEDs [37–39]. Compared to conventional phosphors, the use of QDs has potential significant advantages such as the ability to tune the emission wavelength, wide absorption properties, a narrow emission band, and low scattering [40]. This new approach of using hybrid QD/alumina/polymer films leads to a minor loss of quantum efficiency than when incorporating the quantum dots directly into an LED encapsulation medium (see figure 4(d)). Furthermore, the improved stability of QD films results in LEDs that are brighter, longer-lived, and less sensitive to various environmental conditions. Figure 5(a) shows the cross-section of an LED device structure using the hybrid QD/alumina/polymer film described in this study. To fabricate a red-emitting QD-based LED device, the hybrid QD/alumina/polymer film used as a color conversion material was attached to blue LED chips using silicone sealant. Figure 5(b) shows the display images of the fabricated hybrid QD/alumina/polymer-based red LED device and its
Figure 4(a) shows a schematic diagram of our new hybrid QD/alumina/polymer film structure. The structure consists of QD layers infilled by spraying with alumina (ALD infilling of 10 nm). The external surface is spin coated with about 900 nm of polymer. The internal ALD infilling provides both a robust QD film to be used in subsequent fabrication processes and a diffusion barrier to prevent thermal-induced QD oxidation and degradation. The further polymer encapsulation mainly eliminates, or at least minimizes, the passage or migration of deleterious species such as moisture, oxygen, and free radicals from the external environment to the QDs because it acts as a polymeric protective barrier. Also, discrete polymer encapsulation without direct contact with QDs can completely prevent thermal-induced side reactions. Figure 4(b) shows the optical absorption spectra of the hybrid QD/alumina/polymer films exposed to UV light as a function of time. No shift of optical absorption spectra of the QD films was observed, confirming that the hybrid QD/alumina/polymer film structure can completely inhibit QD surface diffusion, oxidation, and degradation without negatively affecting the optical behavior of QD films. Furthermore, the PL spectral maximum intensity did not change over time, which was consistent with the optical absorption spectra (figure 4(c)). Figure 4(d) shows the QY change over time as a function of three different environmental variables for hybrid QD/alumina/polymer films 6
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Figure 5. (a) Schematic illustration of cross-section of the LED device structure using the hybrid QD/alumina/polymer film. (b) Display images of an as-prepared hybrid QD/alumina/polymer-based red LED device (left) and the same LED device in operation (right). (c) The EL spectra as a function of time exposed to blue LED light for a forward current of 80 mA. (d) Luminous efficiency versus time exposed to blue LED light for three samples of QD-LEDs. Black line is bare films (no ALD); red line is ALD-infilled films (10 nm alumina at 100 ◦ C); blue line is hybrid QD/alumina/polymer films.
4. Conclusions
illumination characteristics. The hybrid QD/alumina/polymer film appeared pale orange and the emitted light was bright purplish-red due to the combination of the blue LED chip and red QD film. Figure 5(c) shows the EL spectra as a function of time exposed to blue LED light for a forward current of 80 mA. After 28 days, the hybrid QD/polymer-based red LED device continued shining brightly without any decrease of EL intensity and was more stable than a conventional QD-based LED device. Figure 5(d) shows the luminous efficiency change over time for three QD-LEDs exposed to blue LED light. The initial luminous efficiency using the bare QD film was about 21 lm W−1 , and a significant decrease in luminous efficiency occurred over time. The luminous efficiency increased when alumina was added using ALD infilling, but a decrease in luminous efficiency over time occurred in this case as well. On the other hand, the luminous efficiency of the hybrid QD/alumina/polymer LED device increased slightly and the luminous efficiency persisted for 28 days without change. Despite the harsh surrounding environment, our hybrid QD/alumina/polymer films greatly enhanced the long-term stability of the luminous efficiency of the LED devices that incorporated them (figure S4 available at stacks.iop.org/Nano/24/505714/mmedia). The most stable QD-based LED device can thus be achieved using hybrid QD/alumina/polymer film.
In summary, we have developed hybrid QD/alumina/ polymer film by combining ALD infilling with polymer encapsulation to create ultra-stable QD-based color conversion material. We have described the ageing behavior of QD films affected by humidity, UV light, and heat as functions of the ALD deposition thickness, polymer coating, and exposure time. We found that the hybrid QD/alumina/polymer structure serves both as a robust diffusion barrier to prevent thermal-induced QD oxidation and degradation, and as a polymeric protective barrier that inhibits the passage or migration of deleterious species. Furthermore, we found that our hybrid QD/alumina/polymer film structure was the most stable among all the films tested in the presence of humidity, UV light, and temperature, without any reduction of its electro-optical properties for 28 days. We used this hybrid QD/alumina/polymer film as color conversion material in QDbased LED devices to produce a successful QD/LED-based light-emitting device with excellent long-term stability. This technique has great potential for many industrial applications of QD-based materials and devices.
Acknowledgment This work is supported by QD phosphorous LED project (MOTIE), Center for Advanced Soft Electronics of Global Frontier (MSIP) in Korea. 7
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References [19]
[1] Sundar V C, Eisler H J and Bawendi M G 2002 Room-temperature, tunable gain media from novel II–VI nanocrystal–titania composite matrices Adv. Mater. 14 739–43 [2] Klimov V I, Ivanov S A, Nanda J, Achermann M, Bezel I, McGuire J A and Piryatinski A 2007 Single-exciton optical gain in semiconductor nanocrystals Nature 447 441–6 [3] Li Y, Rizzo A, Cingolani R and Gigli G 2006 Bright white-light-emitting device from ternary nanocrystal composites Adv. Mater. 18 2545–8 [4] Bae W K, Kwak J, Park J W, Char K, Lee C and Lee S 2009 Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient Adv. Mater. 21 1690–4 [5] Anikeeva P O, Halpert J E, Bawendi M G and Bulovic V 2009 Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum Nano Lett. 9 2532–6 [6] Bendall J S et al 2010 Layer-by-layer all-inorganic quantum-dot-based LEDs: a simple procedure with robust performance Adv. Funct. Mater. 20 3298–302 [7] Medintz I L, Uyeda H T, Goldman E R and Mattoussi H 2005 Quantum dot bioconjugates for imaging, labelling and sensing Nature Mater. 4 435–46 [8] Michalet X, Pinaud F F, Bentolila L A, Tsay J M, Doose S, Li J J, Sundaresan G, Wu A M, Gambhir S S and Weiss S 2005 Quantum dots for live cells, in vivo imaging, and diagnostics Science 307 538–44 [9] Koleilat G I, Levina L, Shukla H, Myrskog S H, Hinds S, Pattantyus-Abraham A and Sargent E H 2008 Efficient, stable infrared photovoltaics based on solution-cast colloidal quantum dots ACS Nano 2 833–40 [10] Choi J J et al 2009 PbSe nanocrystal excitonic solar cells Nano Lett. 9 3749–55 [11] Luther J M, Guo J, Lloyd M T, Semonin O E, Beard M C and Nozik A J 2010 Stability assessment on a 3% bilayer PbS/ZnO quantum dot heterojunction solar cell Adv. Mater. 22 3704–7 [12] Talapin D V and Murray C B 2005 Nanocrystal solids for nand p-channel thin film field-effect transistors Science 310 86–9 [13] Urban J J, Talapin D V, Shevchenko E V, Kagan C R and Murray C B 2007 Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2 Te thin films Nature Mater. 6 115–21 [14] Chan W C W and Nie S M 1998 Quantum dot bioconjugates for ultrasensitive nonisotopic detection Science 281 2016–8 [15] Qu L and Peng X 2002 Control of photoluminescence properties of CdSe nanocrystals in growth J. Am. Chem. Soc. 124 2049–55 [16] Selvan S T, Tan T T and Ying J Y 2005 Robust, non-cytotoxic, silica-coated CdSe quantum dots with efficient photoluminescence Adv. Mater. 17 1620–5 [17] Zhang T, Stilwell J L, Gerion D, Ding L, Elboudwarej O, Cooke P A, Gray J W, Alivisatos A P and Chen F F 2006 Cellular effect of high doses of silica-coated quantum dot profiled with high throughput gene expression analysis and high content cellomics measurements Nano Lett. 6 800–8 [18] Koole R, van Schooneveld M M, Hilhorst J, Donega C D, Hart D C, van Blaaderen A, Vanmaekelbergh D and Meijerink A 2008 On the incorporation mechanism of
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
[36]
8
hydrophobic quantum dots in silica spheres by a reverse microemulsion method Chem. Mater. 20 2503–12 Geng B Y, Du Q B, Liu X W, Ma J Z, Wei X W and Zhang L D 2006 One-step synthesis and enhanced blue emission of carbon-encapsulated single-crystalline ZnSe nanoparticles Appl. Phys. Lett. 89 03315 Bhattacharyya S, Perelshtein I, Moshe O, Rich D H and Gedanken A 2008 One-step solvent-free synthesis and characterization of Zn1−x Mnx Se@C nanorods and nanowires Adv. Funct. Mater. 18 1641–53 Dubertret B, Skourides P, Norris D J, Noireaux V, Brivanlou A H and Libchaber A 2002 In vivo imaging of quantum dots encapsulated in phospholipid micelles Science 298 1759–62 Pellegrino T, Manna L, Kudera S, Liedl T, Koktysh D, Rogach A L, Keller S, Radler J, Natile G and Parak W J 2004 Hydrophobic nanocrystals coated with an amphiphilic polymer shell: a general route to water soluble nanocrystals Nano Lett. 4 703–7 Garcia-Santamaria F, Chen Y, Vela J, Schaller R D, Hollingsworth J A and Klimov V I 2009 Suppressed Auger recombination in giant nanocrystals boosts optical gain performance Nano Lett. 9 3482–8 van Sark W G J M, Frederix P L T M, Heuvel D J V D, Gerritsen H C, Bol A A, Lingen J N J V, Donega C D M and Meijerink A 2001 Photooxidation and photobleaching of single CdSe/ZnS quantum dots probed by room-temperature time-resolved spectroscopy J. Phys. Chem. B 105 8281–4 Liu Y, Gibbs M, Perkins C L, Tolentino J, Zarghami M H, Bustamante J Jr and Law M 2011 Robust, functional nanocrystal solids by infilling with atomic layer deposition Nano Lett. 11 5349–55 Liu M, Li X, Karuturi S K, Tokb A I Y and Fan H J 2012 Atomic layer deposition for nanofabrication and interface engineering Nanoscale 4 1522–8 Knez M, Nielsch K and Niinisto L 2007 Synthesis and surface engineering of complex nanostructures by atomic layer deposition Adv. Mater. 19 3425–38 Liang X H, King D M, Li P and Weimer A W 2009 Low-temperature atomic layer-deposited TiO2 films with low photoactivity J. Am. Ceram. Soc. 92 649–54 Lambert K, Dendooven J, Detavernier C and Hens Z 2011 Embedding quantum dot monolayers in Al2 O3 using atomic layer deposition Chem. Mater. 23 126–8 Ihly R, Tolentino J, Liu Y, Gibbs M and Law M 2011 The photothermal stability of PbS quantum dot solids ACS Nano 5 8175–86 Pourret A, Guyot-Sionnest P and Elam J W 2009 Atomic layer deposition of ZnO in quantum dot thin films Adv. Mater. 21 232–5 Luther J M, Law M, Song Q, Perkins C L, Beard M C and Nozik A 2008 Structural, optical, and electrical properties of self-assembled films of PbSe nanocrystals treated with 1,2-ethanedithiol ACS Nano 2 271–80 Zhao Y, Riemersma C, Pietra F, Koole R, Donega C M and Meijerink A 2012 High-temperature luminescence quenching of colloidal quantum dots ACS Nano 6 9058–67 Jun S and Jang E 2013 Bright and stable alloy core/multishell quantum dots Angew. Chem. Int. Edn 52 679–82 Cao X, Li C M, Bao H, Bao Q and Dong H 2007 Fabrication of strongly fluorescent quantum dot-polymer composite in aqueous solution Chem. Mater. 19 3773–9 Jang E, Jun S, Jang H, Lim J, Kim B and Kim Y 2010 White-light-emitting diodes with quantum dot color converters for display backlights Adv. Mater. 22 3076–80
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[37] Lim J, Jun S, Jang E, Baik H, Kim H and Cho J 2007 Preparation of highly luminescent nanocrystals and their application to light-emitting diodes Adv. Mater. 19 1927–32 [38] Jang H S, Yang H S, Kim S W, Han J Y, Lee S G and Jeon D Y 2008 White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3 SiO5 :Ce3+ , Li+ phosphors Adv. Mater. 20 2696–702
[39] Kim K, Woo J Y, Jeong S and Han C-S 2011 Photoenhancement of a quantum dot nanocomposite via UV annealing and its application to white LEDs Adv. Mater. 23 911–4 [40] Nizamoglu S, Ozel T, Sari E and Demir H V 2007 White light generation using CdSe/ZnS core–shell nanocrystals hybridized with InGaN/GaN light emitting diodes Nanotechnology 18 065709
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Alumina/polymer-coated nanocrystals with extremely high stability used as a color conversion material in LEDs
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 505714 (http://iopscience.iop.org/0957-4484/24/50/505714) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505714 (9pp)
doi:10.1088/0957-4484/24/50/505714
Alumina/polymer-coated nanocrystals with extremely high stability used as a color conversion material in LEDs Ju Yeon Woo, Jongsoo Lee and Chang-Soo Han1 School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Korea E-mail: [email protected]
Received 13 August 2013, in final form 23 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505714 Abstract The long-term stability of quantum dot (QD)-based devices under harsh environmental conditions has been a critical bottleneck to be resolved for commercial use. Here, we demonstrate an extremely stable QD/alumina/polymer hybrid structure by combining internal atomic layer deposition (ALD) infilling with polymer encapsulation. ALD infilling and polymer encapsulation of QDs synergistically prohibit the degradation of QDs in terms of optical, thermal and humid attacks. Our hybrid QD/alumina/polymer film structure showed no noticeable reduction in photoluminescence even in a commercial grade test (85% humidity at 85 ◦ C) over 28 days. In addition, we successfully fabricated a QD-based light-emitting device with excellent long-term stability by incorporating hybrid QD/alumina/polymer film as a color conversion material on light-emitting diode chips. S Online supplementary data available from stacks.iop.org/Nano/24/505714/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
detachments, and oxidation of the QDs [24]. The blinking phenomenon is considered to be due to the ionization of QDs by Auger recombination. Oxidation in particular is usually detrimental to the performance of QD-based devices due to the unwelcome changes it causes in the morphological, optical, and electronic properties of QD systems. A recent attractive alternative to encapsulation techniques is atomic layer deposition (ALD) [25, 26]. ALD is a stepwise chemical vapor deposition method offering excellent conformality on topologically complicated substrates, good reproducibility, sub-nanometer thickness control, uniformity over large areas, and relatively low growth temperatures [27–29]. ALD has been used to fill the pores in the films of PbS QDs with diameters as small as 7 nm [30]. Moreover, metal oxides deposited by ALD penetrate CdSe QD films, and improve film conductivity and carrier mobility [31]. ALD is suitable for depositing protective coatings on the surface of QDs, and ALD infilling can produce QD solids (QDs coated with ALD material) with greatly enhanced environmental stability. An improvement in the passivation afforded by the ALD surface coating can be achieved by increasing the amount of surface-coating material deposited, although this is expensive
The composite structures of semiconductor nanocrystal quantum dots (QDs) have attracted great interest for a variety of commercial applications, including photonic devices [1, 2], light-emitting diodes (LEDs) [3–6], biological labeling [7, 8], solar cells [9–11], and field-effect transistors (FETs) [12, 13]. However, a longstanding issue with stability and luminescence quantum efficiency under harsh environmental conditions hampers their suitability in commercial products. There have been numerous nanoparticle encapsulation approaches to solving this problem, based on organic ligands [14, 15], silica [16–18], carbon shells [19, 20] and amphiphilic polymers [21, 22], none of which effectively protects QDs from surface diffusion, oxidation, and degradation in complex environments. The lack of stability could stem from three main degradation phenomena as follows: decrease of the photoluminescence (PL), optical blue-shift, and blinking of QDs [23]. PL decay and blue-shift of QDs are usually explained by defect formation, ligand 1 Author to whom any correspondence should be addressed.
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and time consuming. A surface coating of polymer can also provide more stability in the surrounding environment and for processing conditions like the overcoating of silicone as an encapsulation material onto the QD films. A key challenge is engineering a stable coating that will maintain the advantages of QDs and their electro-optical properties in complex environments. Here, we report a new strategy for creating ultra-stable QD films by combining ALD infilling with polymer encapsulation. Internal ALD infilling passivates defects, impurities, and localized charges in the vicinity of a QD, and acts as an atomic and molecular diffusion barrier to prevent thermal-induced QD degradation. Polymer encapsulation without direct contact with QDs acts as a secondary protective barrier to prevent the passage or diffusion of potentially deleterious species (e.g., oxygen, oxidizing agents, free radical species, and humidity) leading to photo- or humid-induced QD degradation from the external environment to the QD nanoparticles. We focus on the effects of humidity, ultraviolet (UV) light, and heat for QD films as a function of the ALD deposition thickness, polymer coating, and exposure time. Our hybrid QD/alumina/polymer film structure was the most stable of all films tested (bare QD, ALD infilling only, mixed QD/polymer, and hybrid QD/alumina/polymer), and did not show any decrease of PL or quantum yield (QY) for 28 days during very harsh environmental testing. Finally, by incorporating hybrid QD/alumina/polymer film as a color conversion material on LED chips, we obtained QD-LED-based light-emitting devices with excellent long-term stability. This technique presents many opportunities for developing the industrial applications of QD-based materials and devices.
were optimized to obtain uniform QD films 135 ± 15 nm thick. 2.3. ALD infilling Amorphous Al2 O3 was deposited using an ALD system (PLUS200, QUROS) with trimethylaluminum and O2 . The substrate temperature during the ALD deposition was 100 ◦ C and the operating pressure was about 0.1 Torr. The pulse time was 50 ms and the purge time was 10 s. Argon was used as a carrier gas with a flow rate of 50 sccm. Three levels of ALD deposition thickness were used: 5, 10, and 20 nm. 2.4. Preparation of hybrid QD/polymer film and its application to LEDs To prepare the hybrid QD/polymer film, we first mixed thermo-curable silicone resin parts A and B in a 1:4 weight ratio, and placed the mixture in a vacuum oven for 10 min at 1 bar to remove any bubbles. Silicone resin was then spun at 500 rpm for 120 s onto PET substrates with QD films after ALD treatment (10 nm deposition) using a spin coater (Ace-200, Dong Ah Trade Corp.) to form the polymer encapsulation structure. Finally, the nanocomposite films were thermally cured at 115 ◦ C for 2 h. The hybrid QD/polymer film was attached to blue LED chips using silicone sealant to fabricate the QD-based LED device. 2.5. Characterization Transmission electron microscopy (TEM) micrographs were obtained using a TEM system (Tecnai G2 F30 S Twin, FEI) at 300 kV. Cross-sectional TEM samples of QD films were prepared using a dual-beam focused ion-beam system (NOVA200, FEI). Energy dispersive x-ray spectroscopy (EDS) data were also acquired at various locations across the entire particle shown in the TEM image. Optical absorption, PL, and QY of the QD solution and QD films were characterized using a UV–vis spectrophotometer (SD-1000, Scinco), a fluorometer (Fluorolog, Horiba Jobin Yvon), and an absolute QY measurement system (C-9920-02, Hamamatsu) at room temperature. A temperature and humidity chamber (TH-ME-065, JEIO Tech) was used to study the effects of heat and moisture on QD films. For heat studies, QD films were placed inside an oven (OV-11, JEIO Tech) at 100 ◦ C. UV light soaking was performed on QD films mounted in a homemade testing box. An UV lamp was used to illuminate the films with 2 mW cm−2 of 352 nm light. Electroluminescence (EL) and luminance were evaluated in an integrating sphere using an LED spectra light measurement system (CSLMS LED 1060, Labsphere) at 4 V and 80 mA controlled by a Keithley 2400 digital source meter.
2. Experimental details 2.1. Materials The 623 nm red-emitting CdSe/CdS/ZnS core–multishell QDs (Nanodot HE series, QD Solutions Co.) were used without modification. Trimethylaluminum (97%) and anhydrous solvents were used as received from Aldrich. The dual component thermal curable silicone polymer (OE-6630 A and B) was purchased from Dow Corning. InGaN-based blue LED chips (TKF 01B450 blue power die, λmax = 455 nm, non-epoxy molding packages) were purchased from Trikaiser. The silicone sealant (30SHM) was purchased from Okong. 2.2. QD film formation QD films were prepared by spray coating them onto polyethylene terephthalate (PET) substrates using a homebuilt spraying system. To avoid QD agglomeration, diluted QD dispersions (5 ml of 0.1 wt% QDs in hexane solution) were sprayed on the prepared PET substrates. The substrate was heated to about 80 ◦ C during the spraying process using a back heating pad to accelerate evaporation of the solvent. Spraying conditions such as compressed nitrogen pressure, nozzle diameter, and distance between substrate and nozzle
3. Results and discussion This study used well-synthesised 623 nm red-emitting CdSe/CdS/ZnS core–multishell QDs (figures S1(a) and (b) available at stacks.iop.org/Nano/24/505714/mmedia). 2
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Figure 1. (a) Schematics of the fabrication process of QD films infilled with alumina by low-temperature ALD. Cross-sectional TEM images of QD films (b) before and (c) after ALD treatment (10 nm alumina deposited at 100 ◦ C) (scale bar: 50 nm). (d) TEM image with high resolution (scale bar: 10 nm) and (e) Al element EDS mapping of QD films after ALD treatment. Elemental EDS profile of QD layers (f) with no ALD and (g) infilled with ALD alumina.
Densely packed, thin, solid QD films with high uniformity and homogenization were prepared by spray coating on PET substrates using a home-built spraying system (figure S1(c) available at stacks.iop.org/Nano/24/505714/mmedia). During film formation, the first exciton peak and PL of the QD films were slightly red-shifted relative to QD solutions due to the increased dielectric constant and substantial electronic coupling [32]. The surface must be coated because this fabrication is not robust. To overcome the photostability of QDs related to luminescent properties, we used low-temperature ALD of Al(CH3 )3 and O2 at 100 ◦ C to fill the internal pore network of these films with amorphous alumina (figure 1(a)). ALD infilling with an inorganic matrix offers a significantly more robust structure and also acts as a protective barrier layer to suppress atomic, molecular, and gas diffusion. Figures 1(b) and (c) are typical cross-sectional TEM images of QD films before and after ALD treatment (10 nm alumina deposition). The TEM analysis showed that no ALD QD films were densely packed, and that the coating thickness of 135 ± 15 nm was uniform in all areas. After ALD coating, the conformal and continuous amorphous alumina was deposited on the outermost surface of the QD films as well as inside the voids and pores of the QD films. Figure 1(d) shows cross-sectional TEM images with high resolution of QD films after deposition of 10 nm of ALD alumina. This TEM image clearly shows lattice fringes and a separate layer, the internal pore network of the QD films infilled with alumina and overcoating layer (∼10 nm thick)
of thin alumina covering the top of the QD films. Although Al2 O3 was deposited inside the voids and pores of QD films, the crystalline structure of the QDs was maintained. The EDS probe position and elemental intensity of the film constituents before ALD coating shown in figure 1(f) indicate that the core–multishell QD films were composed of Cd, Se, Zn, and S. As shown in mapping of aluminum elements and the EDS profile in QD films after ALD treatment (figures 1(e) and (g)), the presence of aluminum and oxygen indicated that Al2 O3 was present throughout the QD films and homogeneously deposited on the internal and external surfaces that were free of pinholes. Figures 2(a)–(c) show the QY change as a function of the time for ALD deposition thicknesses of 5, 10, and 20 nm, respectively, exposed to various environments for QD films in air. During the QD film fabrication, the QY of bare QD films was reduced to 55% from that of the QD in solution (74%); the lower QY was due mainly to surface oxidation and degradation. On the other hand, as the ALD deposition thickness increased after ALD infilling, the QY increased slightly from 55% to an asymptotic value of 67% due to exciton mobility enhancement via passivation of surface defects [25]. The QD films exposed to reliability test conditions of 85 ◦ C and 85% relative humidity, which are levels generally used for commercialization, showed a rapid and dramatic drop of QY in the first week of the test, followed by only minor changes thereafter (figure 2(a)). The main reason for this result is probably due to the 3
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Figure 2. QY versus time as a function of the ALD deposition thickness exposed to (a) 85 ◦ C heat and 85% relative humidity, (b) UV light (2 mW cm−2 at 352 nm), and (c) 100 ◦ C heat for QD films in air. The ALD deposition thickness is 5, 10, and 20 nm, respectively. PL emission spectra of the QD films as a function of time exposed to 100 ◦ C heat (d) without and (e) with ALD treatment (10 nm deposition).
action of moisture to quench the QY by direct interaction with its surface through energetic oxidative reactions; this process is complete within the first few days. This means that the ALD does not inhibit oxidation, or degradation in QD films due to imperfect infilling of the narrow interstitial spaces and defects of Al2 O3 layer leading to increased diffusion of moisture at elevated temperature through the amorphous ALD matrix to the surfaces of the QDs. The QY increased slightly as the ALD deposition thickness increased, but there was no significant improvement. For QD films exposed to UV light (2 mW cm−2 at 352 nm), the QY decreased gradually over time (figure 2(b)). UV illumination can promote surface redox reactions such as oxidation by generating electrons and holes capable of participating in chemical reactions at the QD surface [30]. This leads to degradation of surface passivating ligands and phase transformation by photo-oxidation. Furthermore, similar to the QY results under humidity, ALD infilling does not effectively stop either oxidation or degradation of QDs. On the other hand, ALD infilling is clearly effective at preventing thermal ageing (figure 2(c)). Whereas ALDinfilled QD films showed minimal changes in QY, that of bare QD films decreased significantly. This temperature-sensitive quenching is attributed to carrier trapping in defect states and easy detachment of ligands by thermal energy [33], and surface passivation through inorganic infilling can effectively suppress this process at high temperature. Figures 2(d) and (e) show the PL emission spectra of bare and 10 nm ALD-infilled QD films exposed to 100 ◦ C as a function of time. Bare QD films experienced rapid significant loss of PL within 28
days, which we interpret as permanent structural degradation caused by QD surface diffusion and oxidation, as expected. However, the thermal degradation of ALD-infilled films over time was much less than bare QD films. The PL intensity of bare QD films exposed to other environments (85 ◦ C, 85% relative humidity, and UV light) decreased significantly over time, and 10 nm ALD infilling improved the stability of QD films slightly (figure S2 available at stacks.iop.org/ Nano/24/505714/mmedia). Based on these results, we believe that the ALD infilling offers robust QD surface passivation, but does not inhibit QD surface diffusion, oxidation, or degradation in QD films due to imperfect infilling of the narrow interstitial spaces. Moreover, even after ALD infilling, harmful species can still migrate through the ALD matrix to the surfaces of the QDs, which can lead to oxidation, degradation, and a consequent drop in QY. Therefore, the fabrication of ultra-stable QD films requires further surface coating. A conventional coating process that incorporates QDs directly into a polymer affords some level of protection and functions to some extent as a barrier to the passage of potentially harmful species through the coated material. In particular, silicone polymers are generally used for LED encapsulants because of their thermal stability, transparency, and resistance to oxygen and UV light [34]. To study the effect of various environments on the stability of mixed QD/polymer films, we prepared QD films in which the QDs were embedded in a silicone polymer (see supplementary data available at stacks.iop.org/Nano/24/505714/mmediafor details). Figure 3(a) shows that the QY changes over time 4
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Figure 3. (a) QY versus time for mixed QD/polymer films in air. Black line is films exposed to heat and humidity; red line is films exposed to UV light alone; blue line is films exposed to heat alone. PL emission spectra of the mixed QD/polymer films as a function of time exposed to (b) 85 ◦ C heat and 85% relative humidity, (c) UV light, and (d) 100 ◦ C heat.
decrease (figure S3(c) available at stacks.iop.org/Nano/24/ 505714/mmedia). Figures 3(b)–(d) show the PL emission spectra of the mixed QD/polymer films as a function of time exposed to the three different environments. Figure 3(b) shows that the PL of the mixed QD/polymer films was less sensitive to the humid environment than in the ALD infill case, which indicated that the polymer coating acted as a barrier to prevent moisture from contacting the QDs. The PL of the mixed QD/polymer films exposed to UV light remained extraordinarily stable over 28 days, decreasing by less than 10% over that time (figure 3(c)). This means silicone polymer did not stop the UV degradation thoroughly, but the superior stability of silicone polymer to UV light may be helpful in suppressing the degradation of the QDs. However, the PL of the mixed QD/polymer films exposed to 100 ◦ C decreased by more than 80% due to the unwelcome reaction (figure 3(d)). These side reactions probably originate from the release of oxidizing species and activated reactant at high temperature. These results indicate that while the mixed QD/polymer structure was useful for protecting the reactive QDs from potentially damaging environmental conditions, it was not suitable for preventing thermal-induced oxidation and degradation. Although each material does not effectively protect QDs from harsh environmental treatments, the combined alumina/polymer structure can protect QDs to such a degree that QD fluorescence remains highly stable.
as a function of three different environmental variables for mixed QD/polymer films in air. The initial QY of the mixed QD/polymer film decreased to about 43%, probably due to agglomeration in the QDs and different side reactions in the silicone resin at high temperature during film formation [35, 36]. Tracking the QY of the mixed QD/polymer films exposed to heat and humidity, and UV light alone over time showed a slight (10%) decrease in the QY, which indicated that the polymer coating offered greater stability in the presence of air, moisture, and photo-oxidation. The absence of any shift of optical absorption spectra of these QD films was also observed, confirming that the silicone polymer effectively prevents the passage or diffusion of potentially deleterious species from the external environment to the QD nanoparticles (figures S3(a) and (b) available at stacks.iop.org/Nano/24/505714/mmedia). In contrast, the QY of the mixed QD/polymer films exposed to 100 ◦ C showed a significant drop in the QY of more than 30% after 28 days. This was mainly due to a greater active release of oxidizing species in the polymer at high temperature that may degrade the QD and the remaining side reactions caused by the unreactive monomer, catalyst, and impurities related to curing temperature. Our optical absorption result also shows that heat exposure causes degradation of QDs, resulting in the excitonic peak blue-shifting considerably with exposure time due to the same reason as the QY 5
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Figure 4. (a) Schematic of the hybrid QD/alumina/polymer film structure. The film structure consists of the QD layers infilled with alumina (ALD infilling of the 10 nm) and a polymer layer (∼900 nm) on its external surface. Time traces of (b) the optical absorption spectra and (c) PL emission spectra of the hybrid QD/alumina/polymer films exposed to UV light. (d) QY versus time for hybrid QD/alumina/polymer films in air. Black line is films exposed to heat and humidity; red line is films exposed to UV light alone; blue line is films exposed to heat alone.
in air. The initial QY of the hybrid QD/alumina/polymer film was about 66% because of the trap passivation due ALD infilling, and negligible changes in QY occurred over 28 days. Hybrid QD/alumina/polymer films are thus the most stable in their surrounding environment and are suitable for applications such as LED-based light-emitting devices. A particularly attractive potential field of application for QD films is in the development of next-generation LEDs [37–39]. Compared to conventional phosphors, the use of QDs has potential significant advantages such as the ability to tune the emission wavelength, wide absorption properties, a narrow emission band, and low scattering [40]. This new approach of using hybrid QD/alumina/polymer films leads to a minor loss of quantum efficiency than when incorporating the quantum dots directly into an LED encapsulation medium (see figure 4(d)). Furthermore, the improved stability of QD films results in LEDs that are brighter, longer-lived, and less sensitive to various environmental conditions. Figure 5(a) shows the cross-section of an LED device structure using the hybrid QD/alumina/polymer film described in this study. To fabricate a red-emitting QD-based LED device, the hybrid QD/alumina/polymer film used as a color conversion material was attached to blue LED chips using silicone sealant. Figure 5(b) shows the display images of the fabricated hybrid QD/alumina/polymer-based red LED device and its
Figure 4(a) shows a schematic diagram of our new hybrid QD/alumina/polymer film structure. The structure consists of QD layers infilled by spraying with alumina (ALD infilling of 10 nm). The external surface is spin coated with about 900 nm of polymer. The internal ALD infilling provides both a robust QD film to be used in subsequent fabrication processes and a diffusion barrier to prevent thermal-induced QD oxidation and degradation. The further polymer encapsulation mainly eliminates, or at least minimizes, the passage or migration of deleterious species such as moisture, oxygen, and free radicals from the external environment to the QDs because it acts as a polymeric protective barrier. Also, discrete polymer encapsulation without direct contact with QDs can completely prevent thermal-induced side reactions. Figure 4(b) shows the optical absorption spectra of the hybrid QD/alumina/polymer films exposed to UV light as a function of time. No shift of optical absorption spectra of the QD films was observed, confirming that the hybrid QD/alumina/polymer film structure can completely inhibit QD surface diffusion, oxidation, and degradation without negatively affecting the optical behavior of QD films. Furthermore, the PL spectral maximum intensity did not change over time, which was consistent with the optical absorption spectra (figure 4(c)). Figure 4(d) shows the QY change over time as a function of three different environmental variables for hybrid QD/alumina/polymer films 6
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Figure 5. (a) Schematic illustration of cross-section of the LED device structure using the hybrid QD/alumina/polymer film. (b) Display images of an as-prepared hybrid QD/alumina/polymer-based red LED device (left) and the same LED device in operation (right). (c) The EL spectra as a function of time exposed to blue LED light for a forward current of 80 mA. (d) Luminous efficiency versus time exposed to blue LED light for three samples of QD-LEDs. Black line is bare films (no ALD); red line is ALD-infilled films (10 nm alumina at 100 ◦ C); blue line is hybrid QD/alumina/polymer films.
4. Conclusions
illumination characteristics. The hybrid QD/alumina/polymer film appeared pale orange and the emitted light was bright purplish-red due to the combination of the blue LED chip and red QD film. Figure 5(c) shows the EL spectra as a function of time exposed to blue LED light for a forward current of 80 mA. After 28 days, the hybrid QD/polymer-based red LED device continued shining brightly without any decrease of EL intensity and was more stable than a conventional QD-based LED device. Figure 5(d) shows the luminous efficiency change over time for three QD-LEDs exposed to blue LED light. The initial luminous efficiency using the bare QD film was about 21 lm W−1 , and a significant decrease in luminous efficiency occurred over time. The luminous efficiency increased when alumina was added using ALD infilling, but a decrease in luminous efficiency over time occurred in this case as well. On the other hand, the luminous efficiency of the hybrid QD/alumina/polymer LED device increased slightly and the luminous efficiency persisted for 28 days without change. Despite the harsh surrounding environment, our hybrid QD/alumina/polymer films greatly enhanced the long-term stability of the luminous efficiency of the LED devices that incorporated them (figure S4 available at stacks.iop.org/Nano/24/505714/mmedia). The most stable QD-based LED device can thus be achieved using hybrid QD/alumina/polymer film.
In summary, we have developed hybrid QD/alumina/ polymer film by combining ALD infilling with polymer encapsulation to create ultra-stable QD-based color conversion material. We have described the ageing behavior of QD films affected by humidity, UV light, and heat as functions of the ALD deposition thickness, polymer coating, and exposure time. We found that the hybrid QD/alumina/polymer structure serves both as a robust diffusion barrier to prevent thermal-induced QD oxidation and degradation, and as a polymeric protective barrier that inhibits the passage or migration of deleterious species. Furthermore, we found that our hybrid QD/alumina/polymer film structure was the most stable among all the films tested in the presence of humidity, UV light, and temperature, without any reduction of its electro-optical properties for 28 days. We used this hybrid QD/alumina/polymer film as color conversion material in QDbased LED devices to produce a successful QD/LED-based light-emitting device with excellent long-term stability. This technique has great potential for many industrial applications of QD-based materials and devices.
Acknowledgment This work is supported by QD phosphorous LED project (MOTIE), Center for Advanced Soft Electronics of Global Frontier (MSIP) in Korea. 7
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References [19]
[1] Sundar V C, Eisler H J and Bawendi M G 2002 Room-temperature, tunable gain media from novel II–VI nanocrystal–titania composite matrices Adv. Mater. 14 739–43 [2] Klimov V I, Ivanov S A, Nanda J, Achermann M, Bezel I, McGuire J A and Piryatinski A 2007 Single-exciton optical gain in semiconductor nanocrystals Nature 447 441–6 [3] Li Y, Rizzo A, Cingolani R and Gigli G 2006 Bright white-light-emitting device from ternary nanocrystal composites Adv. Mater. 18 2545–8 [4] Bae W K, Kwak J, Park J W, Char K, Lee C and Lee S 2009 Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient Adv. Mater. 21 1690–4 [5] Anikeeva P O, Halpert J E, Bawendi M G and Bulovic V 2009 Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum Nano Lett. 9 2532–6 [6] Bendall J S et al 2010 Layer-by-layer all-inorganic quantum-dot-based LEDs: a simple procedure with robust performance Adv. Funct. Mater. 20 3298–302 [7] Medintz I L, Uyeda H T, Goldman E R and Mattoussi H 2005 Quantum dot bioconjugates for imaging, labelling and sensing Nature Mater. 4 435–46 [8] Michalet X, Pinaud F F, Bentolila L A, Tsay J M, Doose S, Li J J, Sundaresan G, Wu A M, Gambhir S S and Weiss S 2005 Quantum dots for live cells, in vivo imaging, and diagnostics Science 307 538–44 [9] Koleilat G I, Levina L, Shukla H, Myrskog S H, Hinds S, Pattantyus-Abraham A and Sargent E H 2008 Efficient, stable infrared photovoltaics based on solution-cast colloidal quantum dots ACS Nano 2 833–40 [10] Choi J J et al 2009 PbSe nanocrystal excitonic solar cells Nano Lett. 9 3749–55 [11] Luther J M, Guo J, Lloyd M T, Semonin O E, Beard M C and Nozik A J 2010 Stability assessment on a 3% bilayer PbS/ZnO quantum dot heterojunction solar cell Adv. Mater. 22 3704–7 [12] Talapin D V and Murray C B 2005 Nanocrystal solids for nand p-channel thin film field-effect transistors Science 310 86–9 [13] Urban J J, Talapin D V, Shevchenko E V, Kagan C R and Murray C B 2007 Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2 Te thin films Nature Mater. 6 115–21 [14] Chan W C W and Nie S M 1998 Quantum dot bioconjugates for ultrasensitive nonisotopic detection Science 281 2016–8 [15] Qu L and Peng X 2002 Control of photoluminescence properties of CdSe nanocrystals in growth J. Am. Chem. Soc. 124 2049–55 [16] Selvan S T, Tan T T and Ying J Y 2005 Robust, non-cytotoxic, silica-coated CdSe quantum dots with efficient photoluminescence Adv. Mater. 17 1620–5 [17] Zhang T, Stilwell J L, Gerion D, Ding L, Elboudwarej O, Cooke P A, Gray J W, Alivisatos A P and Chen F F 2006 Cellular effect of high doses of silica-coated quantum dot profiled with high throughput gene expression analysis and high content cellomics measurements Nano Lett. 6 800–8 [18] Koole R, van Schooneveld M M, Hilhorst J, Donega C D, Hart D C, van Blaaderen A, Vanmaekelbergh D and Meijerink A 2008 On the incorporation mechanism of
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
[36]
8
hydrophobic quantum dots in silica spheres by a reverse microemulsion method Chem. Mater. 20 2503–12 Geng B Y, Du Q B, Liu X W, Ma J Z, Wei X W and Zhang L D 2006 One-step synthesis and enhanced blue emission of carbon-encapsulated single-crystalline ZnSe nanoparticles Appl. Phys. Lett. 89 03315 Bhattacharyya S, Perelshtein I, Moshe O, Rich D H and Gedanken A 2008 One-step solvent-free synthesis and characterization of Zn1−x Mnx Se@C nanorods and nanowires Adv. Funct. Mater. 18 1641–53 Dubertret B, Skourides P, Norris D J, Noireaux V, Brivanlou A H and Libchaber A 2002 In vivo imaging of quantum dots encapsulated in phospholipid micelles Science 298 1759–62 Pellegrino T, Manna L, Kudera S, Liedl T, Koktysh D, Rogach A L, Keller S, Radler J, Natile G and Parak W J 2004 Hydrophobic nanocrystals coated with an amphiphilic polymer shell: a general route to water soluble nanocrystals Nano Lett. 4 703–7 Garcia-Santamaria F, Chen Y, Vela J, Schaller R D, Hollingsworth J A and Klimov V I 2009 Suppressed Auger recombination in giant nanocrystals boosts optical gain performance Nano Lett. 9 3482–8 van Sark W G J M, Frederix P L T M, Heuvel D J V D, Gerritsen H C, Bol A A, Lingen J N J V, Donega C D M and Meijerink A 2001 Photooxidation and photobleaching of single CdSe/ZnS quantum dots probed by room-temperature time-resolved spectroscopy J. Phys. Chem. B 105 8281–4 Liu Y, Gibbs M, Perkins C L, Tolentino J, Zarghami M H, Bustamante J Jr and Law M 2011 Robust, functional nanocrystal solids by infilling with atomic layer deposition Nano Lett. 11 5349–55 Liu M, Li X, Karuturi S K, Tokb A I Y and Fan H J 2012 Atomic layer deposition for nanofabrication and interface engineering Nanoscale 4 1522–8 Knez M, Nielsch K and Niinisto L 2007 Synthesis and surface engineering of complex nanostructures by atomic layer deposition Adv. Mater. 19 3425–38 Liang X H, King D M, Li P and Weimer A W 2009 Low-temperature atomic layer-deposited TiO2 films with low photoactivity J. Am. Ceram. Soc. 92 649–54 Lambert K, Dendooven J, Detavernier C and Hens Z 2011 Embedding quantum dot monolayers in Al2 O3 using atomic layer deposition Chem. Mater. 23 126–8 Ihly R, Tolentino J, Liu Y, Gibbs M and Law M 2011 The photothermal stability of PbS quantum dot solids ACS Nano 5 8175–86 Pourret A, Guyot-Sionnest P and Elam J W 2009 Atomic layer deposition of ZnO in quantum dot thin films Adv. Mater. 21 232–5 Luther J M, Law M, Song Q, Perkins C L, Beard M C and Nozik A 2008 Structural, optical, and electrical properties of self-assembled films of PbSe nanocrystals treated with 1,2-ethanedithiol ACS Nano 2 271–80 Zhao Y, Riemersma C, Pietra F, Koole R, Donega C M and Meijerink A 2012 High-temperature luminescence quenching of colloidal quantum dots ACS Nano 6 9058–67 Jun S and Jang E 2013 Bright and stable alloy core/multishell quantum dots Angew. Chem. Int. Edn 52 679–82 Cao X, Li C M, Bao H, Bao Q and Dong H 2007 Fabrication of strongly fluorescent quantum dot-polymer composite in aqueous solution Chem. Mater. 19 3773–9 Jang E, Jun S, Jang H, Lim J, Kim B and Kim Y 2010 White-light-emitting diodes with quantum dot color converters for display backlights Adv. Mater. 22 3076–80
Nanotechnology 24 (2013) 505714
J Y Woo et al
[37] Lim J, Jun S, Jang E, Baik H, Kim H and Cho J 2007 Preparation of highly luminescent nanocrystals and their application to light-emitting diodes Adv. Mater. 19 1927–32 [38] Jang H S, Yang H S, Kim S W, Han J Y, Lee S G and Jeon D Y 2008 White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3 SiO5 :Ce3+ , Li+ phosphors Adv. Mater. 20 2696–702
[39] Kim K, Woo J Y, Jeong S and Han C-S 2011 Photoenhancement of a quantum dot nanocomposite via UV annealing and its application to white LEDs Adv. Mater. 23 911–4 [40] Nizamoglu S, Ozel T, Sari E and Demir H V 2007 White light generation using CdSe/ZnS core–shell nanocrystals hybridized with InGaN/GaN light emitting diodes Nanotechnology 18 065709
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