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ZnS quantum dots-hydrogel composites for warm white LEDs
Journal Pre-proof Highly luminescent water-soluble AgInS2/ZnS quantum dots-hydrogel composites for warm white LEDs Danlu Su, Le Wang, Min Li, Shiliang Mei, Xian Wei, Hanqing Dai, Zhe Hu, Fengxian Xie, Ruiqian Guo PII:
S0925-8388(20)30259-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.153896
Reference:
JALCOM 153896
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2019 Revised Date:
30 December 2019
Accepted Date: 16 January 2020
Please cite this article as: D. Su, L. Wang, M. Li, S. Mei, X. Wei, H. Dai, Z. Hu, F. Xie, R. Guo, Highly luminescent water-soluble AgInS2/ZnS quantum dots-hydrogel composites for warm white LEDs, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153896. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Danlu Su: Conceptualization, Methodology, Writing- Original Draft Le Wang: Validation, Data curation, Writing- Review & Editing Min Li: Resources, Visualization, Investigation. Shiliang Mei: Methodology, Writing - Review & Editing Xian Wei: Investigation, Writing - Review & Editing Hanqing Dai: Methodology Zhe Hu: Investigation Fengxian Xie: Supervision Ruiqian Guo: Project administration and Funding acquisition
Highly
luminescent
water-soluble
AgInS2/ZnS
quantum
dots-hydrogel composites for warm white LEDs
Danlu Sua,c, Le Wangb,c, Min Lib, Shiliang Meia,*, Xian Weia, Hanqing Daib, Zhe Hua, Fengxian Xiea, Ruiqian Guoa,b,*
a. Engineering Research Center of Advanced Lighting Technology, Ministry of Education; Institute for Electric Light Sources, Fudan University, Shanghai 200433, China. b. Academy for Engineering and Technology, Fudan University, Shanghai 200433, China. c. Authors contributed equally.
*. Corresponding author. Tel.: +86 19921250396 (S. Mei), +86 2155664588 (R. Guo). Email address: [email protected] (S. Mei), [email protected] (R. Guo).
Highlights ·
High-quality AgInS2/ZnS quantum dots with a quantum yield of up to 58.27% are synthesized via a facile microwave-assisted aqueous method.
·
Flexible luminescent films are obtained by embedding AgInS2/ZnS quantum dots in polyacrylamide hydrogel.
·
Water-soluble AgInS2/ZnS quantum dots-hydrogel composites are competitive color-conversion materials for warm-white LEDs.
ABSTRACT 1
Ternary
- -
quantum dots (QDs) have become promising color-conversion
materials for warm white light-emitting diodes (WLEDs) because of their broad emission, large stokes shift, high luminescence and low-toxicity. In this work, AgInS2/ZnS
(AIS/ZnS)
core/shell
QDs
were
synthesized
via
a
facile
microwave-assisted aqueous method. The PL emission can be tuned from 540 to 622 nm by varying the Ag/In ratio and the maximum quantum yield can reach 58.27%. Flexible luminescent films were obtained by embedding AIS/ZnS QDs in polyacrylamide hydrogel, which were combined with blue InGaN chips to fabricate remote-type warm WLEDs. The as-prepared LEDs exhibit a relatively high color rendering index (CRI) of 87.5 and a correlated color temperature (CCT) of 3669 K, indicating that water-soluble AIS/ZnS QDs are competitive color-conversion materials for warm WLEDs.
Keywords: AgInS2/ZnS; quantum dots; tunable emission; microwave-assisted; white LED; color-conversion materials
1. Introduction In recent few decades, quantum dots (QDs) have received extensive attention due to their remarkable optical properties such as broad absorption, highly bright luminescence and tunable emission [1-5]. Nevertheless, the toxicity of heavy metal elements in traditional II–VI group Cd-based QDs severely restricts the applications in commercial products [6]. In this case, ternary
- -
QDs with direct band gaps,
large stokes shift, long photoluminescence (PL) lifetime and low toxicity have been reported as a potential alternative to Cd-based QDs [7-9]. In addition to size-dependent features, the PL emission wavelength of the ternary alloyed QDs can be tuned via adjusting the chemical composition [10]. As a result,
- -
QDs such
as Cu-In-S or Ag-In-S (AIS) QDs have been widely applied in light emitting diodes (LEDs) [11, 12], solar cells [13, 14], chemosensors [15, 16], photocatalysis [17, 18] 2
and biomedical imaging [19-22]. Typically, commercial white LEDs (WLEDs) are made of blue InGaN chips and yellow-emitting Ce-doped yttrium aluminum garnet (Y3Al5O12: Ce3+, YAG: Ce) phosphors [23]. However, the lack of red emission results in the high correlated color temperature (CCT) and impaired color rendering index (CRI). AIS QDs with a broad emission band have become a competitive candidate as down-converted materials for the preparation of warm WLEDs [11,24-28]. In order to prepare AIS QDs on a large scale and at low cost, previous organic synthesized routes have been replaced by aqueous synthesis like microwave irradiation which is an efficient means to realize rapid and uniform heating [29-33]. Xiong et al. realized the synthesis of aqueous AIS /ZnS nanocrystals with a photoluminescence quantum yield (PL QY) of 40% by means of microwave-assisted approach [34]. The highest PL QY of AIS/ZnS QDs synthesized via a microwave-assisted method has reached 49% so far [35], but there is still room for further improvement. In addition, water-soluble QDs have been restricted by their poor compatibility with most polymer solutions, which makes it important to explore an effective method to apply water-soluble QDs to warm WLEDs [36-38]. Herein, high-quality AIS and AIS/ZnS QDs were synthesized via a microwave-assisted aqueous method. The as-prepared core and core/shell QDs were characterized by Ultraviolet-visible (UV-vis) spectrophotometry, PL spectroscopy and X-ray
diffraction
(XRD),
X-ray
photoelectron
spectroscopy
(XPS)
and
high-resolution transmission electron microscopy (HRTEM). By means of altering chemical composition, the AIS/ZnS QDs exhibited tunable emission ranging from 540 to 622 nm, among which the PL QYs of green, yellow and red QDs reached 49.22%, 58.27% and 56.21%, respectively. To prepare the variable-color luminescent film, the polychromatic water-soluble AIS/ZnS QDs were embedded in the polyacrylamide (PAAm) hydrogel (Scheme 1). The obtained QD/PAAm composite films have excellent flexibility and brightness. By combining a blue InGaN chip with the green and red luminescent films, a warm WLED with the CRI value of 87.5 and the CCT 3
value of 3669 K was successfully fabricated.
Scheme 1. Synthetic illustration for the preparation of AIS/ZnS QDs and QD/PAAm composite film for WLEDs.
2. Experimental section 2.1. Chemicals Indium (III) chloride (InCl3·4H2O, 99.9%), sodium sulfide (Na2S·9H2O, 98%), acrylamide
(C3H5NO,
99.9%),
N,N′-methylenebis(acrylamide)
ammonium
persulfate
(BAAm,
(APS, 99%)
99.99%), and
N,N,N',N'-tetramethylethylenediamine (TEMED, 99%) were purchased from Aladdin Chemistry Co. Ltd. Silver nitrate (AgNO3, 99.8%), zinc chloride (ZnCl2, 98.0%) and sodium hydroxide (NaOH, 96%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Glutathione (GSH, 99%) was purchased from Sahn Chemical Technology Co., Ltd. All the reagents were of analytical reagent grade and all aqueous solutions were prepared with deionized water. 2.2. Microwave-assisted synthesis of AIS and AIS/ZnS QDs AIS QDs were synthesized in aqueous solution using a microwave chemical reaction system (MCR-3) with controllable temperature units, whose heating time and temperature can be set as needed. With a power range of 0-1300 W and a working 4
frequency of 2450 MHz, the reaction system can heat the solution to 95 minutes and keep the error within 5
in 2
. In a typical experiment for the synthesis of the
core QDs with the Ag/In ratio of 1:10, 0.1 mmol AgNO3, 1 mmol InCl3 and 8 mmol GSH were mixed with 50 mL of deionized water in a 250 mL three-neck flask under magnetic stirring. The pH value of the mixture solution was adjusted to 8.5 by adding 2.0 M NaOH solution. Then 10 mL of freshly prepared Na2S solution (0.2 M) was added into the mixture, which was subsequently heated to 95
for an hour by
microwave irradiation. In the reaction system, Ag+, In3+ and S2- were combined in a specific proportion to form QDs with GSH as the capping agent. Ions like NO3-, Cl-, Na+ existed in solution in a free state and did not participate in the reaction. In order to synthesize the AIS/ZnS core/shell QDs, 10 mL of aqueous solution containing 1 mmol ZnCl2, 1 mmol Na2S and 1 mmol GSH was injected into the as-prepared core QDs solution followed by reaction for another 10 min. Under the stability of the ligand, Zn2+ and S2- epitaxially grow on the surface of the AIS QDs to generate a layer of ZnS shell whose thickness was controlled by repeating the injection of ZnS precursors solution. Afterwards the AIS/ZnS QDs were purified via adding excess ethanol and centrifuging to remove the ions that are not involved in the reaction. And then the AIS/ZnS QDs were re-dispersed in water or dried into powders for further characterizations. 2.3. Preparation of the QD/PAAm composite film PAAm hydrogel was obtained using free-radical polymerization of acrylamide monomer according to the previously reported method [39]. 3 g acrylamide monomer, 0.002 g BAAm and 0.02 g APS were dissolved in 6 mL AIS/ZnS QDs solution (5 wt %) in the air, followed by the addition of 20 µL TEMED under magnetic stirring for 10 s. Then the mixture was transferred into a circular mold with a diameter of 2 cm and a depth of 0.5 cm to form the film with a thickness of 0.5 mm. 2.4. Assemblage of AIS/ZnS QD-based LED devices To fabricate the QD-based LED devices, InGaN LED chips (λem=460 nm) were used as blue light sources. Besides, the prepared heterochromatic QD/PAAm hybrid 5
films were loaded on the chips. 2.5. Characterizations All the measurements were carried out at room temperature (RT). UV-vis absorption spectra and PL spectra of the purified QDs solutions were recorded by a Shanghai Lengguang 759S ultraviolet spectrophotometer and a Shanghai Lengguang F97XP fluorescence spectrophotometer, respectively. The XRD patterns of QDs powder were obtained by Bruker D8 Advance. The XPS analysis was implemented on an ESCALAB250Xi X-ray photoelectron spectrometer. The TEM and HRTEM images were obtained on a JEOL2100F transmission electro-microscope operating at an accelerating voltage of 200kV. The absolute PL QYs were measured by an integrating sphere on a FLSP920 spectrometer. And the luminous characteristics of LED devices are obtained by SPIC-200 spectral irradiance colorimeter.
3. Results and discussion 3.1. Fluorescence properties of AIS QDs Microwave radiation is an efficient heating method which can raise the temperature of the reaction system to 95
in 90 s. The temporal evolution of UV-vis
absorption and PL spectra of the as-prepared AIS core QDs when the ratio of Ag/In is 1:10 is demonstrated in Fig. 1. The broad absorption spectra (Fig. 1a) without obvious exciton absorption peaks conform to the typical characteristics of ternary compounds [40]. The optical band gap of direct-band-gap semiconductor can be calculated from the UV-vis absorption spectra based on Tauc’s relation, (αhν)1/m =A(hν-Eg), in which α is the absorption coefficient, m=1/2 for direct allowed transitions, A is a constant and Eg is the optical band gap [41]. It can be concluded that with the reaction time ranging from 15 to 120 min, the band gap of the AIS core slightly decreased from 3.07 to 3.00 eV due to increasing particle sizes (Fig. S1). Different from the traditional II-VI compound such as CdSe or CdTe whose PL emission properties are decided by their sizes [42,43], the ternary QDs have a wide 6
PL peak centered at 615 nm with a shoulder centered at 652 nm whose position is constant during the growing of the AIS core. And the stokes shift is greater than 200 nm, which indicates that the PL emission is engendered by donor-acceptor recombination related to various trap states instead of band-edge recombination [44, 45]. Nevertheless, the intensity of the PL emission is continuously improved with extending the reaction time, largely due to the increasing crystallinity of AgInS2. The PL QY of AIS QDs reached up to 29% after 60 min, which is higher than that of other aqueous AIS cores reported in previous work [45, 46].
Figure 1. (a) Evolutions of UV-vis absorption spectra and (b) PL spectra (λex=460 nm) of AIS QDs under different reaction time.
3.2. Characterization of AIS/ZnS QDs According to previous work [22], core/shell structure, composed by coating another material with a wider band gap such as ZnS on the core QDs, can distinctly enhance the optical properties of alloyed nanocrystals. The influence of ZnS shell thickness upon the PL spectra of AIS QDs with the Ag/In ratio of 1:10 is shown in Fig. 2a. After epitaxially growing the first ZnS shell, the emission peak shows a blue-shift from 615 to 595 nm, and the intensity almost doubles compared to AIS cores. The PL enhancement is due to the passivation of the surface-related defect states by ZnS. However, as the ZnS shell thickness increases, the PL enhancement becomes less obvious. Meanwhile, a continuous blue-shift of the PL wavelength can be observed during the coating of ZnS shell, which can be ascribed to the 7
diffusion of Zn2+ into the core lattice, namely cationic interdiffusion between the wider band gap ZnS shell (bulk Eg = 3.68 eV) and the AIS core (bulk Eg = 1.87 eV) leading to an increment of the band-gap energy [30,39]. As a result, an analogous alloyed Zn-Ag-In-S structure with a wider band gap was formed and the PL peak position wavelength and PL QY of AIS/ZnS(3) reached 578 nm and 58.27%, respectively. The formation of approximately alloyed structure can also be confirmed in the XRD patterns shown in Fig. 2b. The diffraction peaks of both AIS core and AIS/ZnS core/shell QDs are broadening owing to their small sizes and can be indexed to the tetragonal chalcopyrite crystal structure (JCPDS No. 25-1330). It is notable that the diffraction peaks of the AIS/ZnS QDs shift toward larger angles, approaching the bulk zinc blende ZnS phase (JCPDS No. 05-0566), which can be ascribed to the smaller lattice constant of ZnS relative to AIS and reveals the formation of core/shell structure rather than separate ZnS nanoparticles. The composition information of AIS and AIS/ZnS QDs with the Ag/In molar ratio of 1:10 can be obtained by XPS shown in Fig. 2c, manifesting the existence of Ag, In and S elements in the core QDs and additional Zn element in core/shell QDs. It can be calculated from the atomic percentage that the actual ratio of Ag/In in the AIS core is 1:4, larger than the ratio of the precursors, which can be explained by Pearson's hard-soft acid-base theory [47]. As a kind of soft base, S2- should be more likely to react with soft acid, such as Ag+. Since In3+ is a hard acid, it may have a lower reactivity with S2-. Therefore the ratio of Ag/In in the as-prepared core QDs is higher than that in the precursors. Nevertheless, after the coating of three layers of ZnS shell, the actual ratio of Ag/In dropped to 1:12, indicating that the diffused Zn2+ might preferentially substitute for Ag+ in the lattice structure.[32, 33] Fig. 2d demonstrates the PL decay curves of AIS and AIS/ZnS QDs with the emission peaks at 615 nm and 578 nm, which can be well fitted by a biexponential function: I(t) = B1 exp (-t/τ1) +B2 exp (-t/τ2), where τ1, τ2 represent the decay times of the PL emission and B1, B2 represent the relative weights of the decay 8
components at t = 0 [48]. And the fitted parameters are listed in Table 1. In light of previous researches about the electron-hole recombination mechanisms in the - -
alloyed semiconductors[31, 32, 34], τ1 is ascribed to the high-energy
radiative recombination of the surface trap states and τ2 is assigned to the low-energy donor–acceptor recombination, with the latter accounting for a larger proportion than the former. Surface-related defect states are relatively shallow and thus show faster decay lifetime than the intrinsic trap states. After coating ZnS shell, both τ1 and τ2 rise appreciably due to the passivation. The relative weight of τ1 with high energy emission increases obviously, according with the report that the contribution from the fast decay time component increases notably with decreasing the ratio of Ag/In [45]. As mentioned above, the addition of Zn2+ would displace some Ag+, resulting in the decrease of Ag element and the significant increase of B1/B2, which is also one of the reasons for the blue-shift of the PL spectra. As a result, the calculated average PL decay lifetime values of the AIS QDs and AIS/ZnS QDs are quite similar, 474.2 ns and 455.0 ns respectively.
Figure 2. (a) PL spectra (λex=460 nm) of AIS core QDs and AIS/ZnS core/shell QDs 9
with different shell thicknesses. (b) XRD patterns, (c) XPS survey spectra and (d) PL decay curves of AIS core QDs and AIS/ZnS core/shell QDs.
Table 1. Constants obtained from I(t) = B1 exp (-t/τ1) +B2 exp (-t/τ2) Samples
τ1 (ns)
B1 (%)
τ2 (ns)
B2 (%)
Lifetime (ns)
AIS
124
19.95
496
80.05
474.2
AIS/ZnS
172
36.55
510
63.45
455.0
The sizes and morphologies of AIS and AIS/ZnS QDs were further characterized by TEM and HRTEM images. As demonstrated in Fig. 3, the as-prepared QDs with nice monodispersity and high crystallinity possess spherical or subspherical shape. The average sizes of AIS and AIS/ZnS QDs are 2.70 nm and 3.07 nm respectively, revealing that the particle size increases during the coating process.
Figure 3. (a), (c) TEM images and (b), (d) HRTEM images of AIS core QDs and 10
AIS/ZnS core/shell QDs (inset: the particle size distribution of the corresponding QDs).
3.3. Influence of Ag/In molar ratio For most ternary and quaternary alloyed nanocrystals such as AIS QDs, PL emission is caused by defect-related donor–acceptor pair recombination rather than band-edge recombination, so their optical properties are composition-dependent rather than size-dependent [45,46]. The ratio of Ag/In is an important factor for tuning the fluorescence properties of AIS/ZnS QDs, which was adjusted by changing the amount of Ag precursors while keeping other experimental variables fixed in the preparation process. And according to the photoluminescence excitation (PLE) spectra at their respective emission peak wavelengths (Fig. S2), the optimal excitation wavelength is 460 nm for all AIS/ZnS QDs of different proportions. Figure. 4 demonstrates the absorption spectra, the normalized PL spectra, the PL QY and the digital photograph under UV irradiation of AIS/ZnS QDs with the molar ratio of Ag/In varied from 1:20 to 5:10. With the decrease of Ag content, both the absorption edge and PL spectra show obvious blue-shift. The emission peak position can be tuned from 622 to 540 nm excited by 460 nm, corresponding to the color tuned from red to green shown in Fig. 4d. As reported by previous reports, the conduction band minimum of AIS QDs is composed of hybrid orbitals of In 5p5s and S 3p, while the valence band maximum consists of hybrid orbitals of S 3p and Ag 4d [17, 32], which is lowered as the number of orbitals originating from Ag decreases, accordingly the band gap is widened. The absolute PL QY of AIS/ZnS QDs is also enormously affected by the Ag/In ratio, as shown in Fig. 4c. The optimal ratio giving the maximum emission intensity is 1:10 (Fig. S3), and the PL QY reaches 58.27%. It is also worth noting that at the ratio of 1:20 and 2:10, the PL QYs are also quite good, 49.22% and 56.21%, respectively. Therefore, the AIS/ZnS QDs with the Ag/In ratios of 1:20, 1:10 and 2:10 (PL peak located at 540, 578, and 600 nm) have the potential to prepare high-quality white 11
LED.
Figure 4. (a) UV-vis absorption, (b) PL spectra (λex=460 nm), (c) PL QYs and (d) digital photographs of AIS/ZnS QDs synthesized at different molar ratios of Ag/In.
3.4. Preparation of green, yellow and red luminescent QD/PAAm composite films and white LEDs In general, water-soluble QDs have poor compatibility with polymer solutions due to the existence of a large number of hydrophilic ligands on their surface, which is one of the challenges of limiting their application to LEDs. However, embedding polychromatic aqueous QDs into hydrogel which combines the optical properties of QDs with the mechanical properties of hydrogel not only provides a new solution, but also makes flexible LED devices possible [36-38]. In this report, PAAm hydrogels synthesized by free-radical cross-linking polymerization of acrylamide monomer and BAAm crosslinker in an aqueous solution containing the prepared AIS/ZnS QDs exhibit high transparency, good shape flexibility and excellent extensibility, as shown in Scheme 1. Homogeneous QD/PAAm hybrid films with various colors can be 12
obtained by controlling the component of AIS/ZnS QDs. The PL spectra of the composite films containing AIS/ZnS QDs with Ag/In ratio of 1:20, 1:10 and 2:10 excited by blue light at 460 nm are shown in Fig. 5a. The emission peaks are positioned at 565, 591 and 657 nm, and the PL QYs are 27.9%, 39.5%, and 36.5%, respectively. Figure. 5b shows the green, yellow and red hybrid films under 365 nm UV light. The flexible films exhibit uniformly distributed light emission, which demonstrates that the QDs are evenly dispersed in PAAm hydrogel and their PL emission are well-preserved.
Figure 5. (a) PL spectra (λex=460 nm) and (b) digital photographs (under UV irradiation) of green, yellow and red QD/PAAm composite films.
Since these hybrid films are suitable to be strongly excited by 460 nm, their PL performance can be examined by fabricating white LEDs with InGaN blue LED chips. Green (G), yellow (Y) and red (R) luminescent films are applied to be respectively combined with blue LED chips as color converters. The normalized luminescence spectra and the CIE chromaticity coordinates of the as-prepared LED at 20 mA are 13
delineated in Fig. 6a and b. The LED based on yellow composite film achieves white light emission with a CIE color coordinate of (0.349, 0.324), a CCT of 4393 K, and a CRI of 75.6. In order to achieve high quality warm-white light emission, green and red QD/PAAm hybrid films with the appropriate thickness are utilized to fabricate the LED device. It’s notable that the red film has to be placed under the green one to avoid reabsorption of green light. The luminescence spectrum of the RGB tricolor white LED covers the full visible-spectrum area from 430 to 750 nm, as demonstrated in Fig. 6c. The CIE color coordinate of the as-prepared warm WLED (Fig. 6d) is (0.392, 0.362), which falls in the white gamut and approaches to the Planckian locus. The CCT is 3669 K and the CRI reaches 87.5, which is a significant improvement compared to that of the LED device containing only yellow film, as shown in Table 2.
Figure 6. (a) Emission spectra and (b) CIE color coordinates of the LEDs based on green, yellow and red QD/PAAm films at a drive current of 20 mA. (c) Emission spectrum and (d) CIE color coordinate of the LED based on both green and red QD/PAAm films at a drive current of 20 mA. 14
Table 2. Optical properties of LEDs using various QD/PAAm composite films at a drive current of 20 mA CIE coordinate Samples x
CRI
CCT (K)
y
LED (G)
0.289
0.336
-
-
LED (Y)
0.349
0.324
75.6
4393
LED (R)
0.390
0.207
-
-
LED (G+R)
0.392
0.362
87.5
3669
4. Conclusions In summary, AIS core QDs and AIS/ZnS core/shell QDs have been successfully synthesized via a facile microwave-assisted method in aqueous solution with GSH as stabilizing agents. The PL QYs of core QDs and core/shell QDs are up to 29% and 58.27%, respectively. The PL wavelength can be conveniently tuned from 540 to 622 nm by adjusting the stoichiometric ratio of Ag and In. Green, yellow and red AIS/ZnS QDs are combined with PAAm hydrogel to fabricate highly luminescent hybrid films, among them the yellow film has been applied on commercial blue InGaN chips for a remote-type warm WLED with a CRI of 75.6 and a CCT of 4393 K. By combining red and green films, the warm WLED with a high CRI of 87.5 and a CCT of 3669 K is prepared, demonstrating the potential for WLEDs DEwith AIS/ZnS/PAAm as color-conversion materials.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC, No. 61675049, NSFC, No. 61377046, and NSFC, No.61177021) and Fudan University-CIOMP Joint Fund (FC2017-004).
22
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30259-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.153896
Reference:
JALCOM 153896
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2019 Revised Date:
30 December 2019
Accepted Date: 16 January 2020
Please cite this article as: D. Su, L. Wang, M. Li, S. Mei, X. Wei, H. Dai, Z. Hu, F. Xie, R. Guo, Highly luminescent water-soluble AgInS2/ZnS quantum dots-hydrogel composites for warm white LEDs, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153896. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Danlu Su: Conceptualization, Methodology, Writing- Original Draft Le Wang: Validation, Data curation, Writing- Review & Editing Min Li: Resources, Visualization, Investigation. Shiliang Mei: Methodology, Writing - Review & Editing Xian Wei: Investigation, Writing - Review & Editing Hanqing Dai: Methodology Zhe Hu: Investigation Fengxian Xie: Supervision Ruiqian Guo: Project administration and Funding acquisition
Highly
luminescent
water-soluble
AgInS2/ZnS
quantum
dots-hydrogel composites for warm white LEDs
Danlu Sua,c, Le Wangb,c, Min Lib, Shiliang Meia,*, Xian Weia, Hanqing Daib, Zhe Hua, Fengxian Xiea, Ruiqian Guoa,b,*
a. Engineering Research Center of Advanced Lighting Technology, Ministry of Education; Institute for Electric Light Sources, Fudan University, Shanghai 200433, China. b. Academy for Engineering and Technology, Fudan University, Shanghai 200433, China. c. Authors contributed equally.
*. Corresponding author. Tel.: +86 19921250396 (S. Mei), +86 2155664588 (R. Guo). Email address: [email protected] (S. Mei), [email protected] (R. Guo).
Highlights ·
High-quality AgInS2/ZnS quantum dots with a quantum yield of up to 58.27% are synthesized via a facile microwave-assisted aqueous method.
·
Flexible luminescent films are obtained by embedding AgInS2/ZnS quantum dots in polyacrylamide hydrogel.
·
Water-soluble AgInS2/ZnS quantum dots-hydrogel composites are competitive color-conversion materials for warm-white LEDs.
ABSTRACT 1
Ternary
- -
quantum dots (QDs) have become promising color-conversion
materials for warm white light-emitting diodes (WLEDs) because of their broad emission, large stokes shift, high luminescence and low-toxicity. In this work, AgInS2/ZnS
(AIS/ZnS)
core/shell
QDs
were
synthesized
via
a
facile
microwave-assisted aqueous method. The PL emission can be tuned from 540 to 622 nm by varying the Ag/In ratio and the maximum quantum yield can reach 58.27%. Flexible luminescent films were obtained by embedding AIS/ZnS QDs in polyacrylamide hydrogel, which were combined with blue InGaN chips to fabricate remote-type warm WLEDs. The as-prepared LEDs exhibit a relatively high color rendering index (CRI) of 87.5 and a correlated color temperature (CCT) of 3669 K, indicating that water-soluble AIS/ZnS QDs are competitive color-conversion materials for warm WLEDs.
Keywords: AgInS2/ZnS; quantum dots; tunable emission; microwave-assisted; white LED; color-conversion materials
1. Introduction In recent few decades, quantum dots (QDs) have received extensive attention due to their remarkable optical properties such as broad absorption, highly bright luminescence and tunable emission [1-5]. Nevertheless, the toxicity of heavy metal elements in traditional II–VI group Cd-based QDs severely restricts the applications in commercial products [6]. In this case, ternary
- -
QDs with direct band gaps,
large stokes shift, long photoluminescence (PL) lifetime and low toxicity have been reported as a potential alternative to Cd-based QDs [7-9]. In addition to size-dependent features, the PL emission wavelength of the ternary alloyed QDs can be tuned via adjusting the chemical composition [10]. As a result,
- -
QDs such
as Cu-In-S or Ag-In-S (AIS) QDs have been widely applied in light emitting diodes (LEDs) [11, 12], solar cells [13, 14], chemosensors [15, 16], photocatalysis [17, 18] 2
and biomedical imaging [19-22]. Typically, commercial white LEDs (WLEDs) are made of blue InGaN chips and yellow-emitting Ce-doped yttrium aluminum garnet (Y3Al5O12: Ce3+, YAG: Ce) phosphors [23]. However, the lack of red emission results in the high correlated color temperature (CCT) and impaired color rendering index (CRI). AIS QDs with a broad emission band have become a competitive candidate as down-converted materials for the preparation of warm WLEDs [11,24-28]. In order to prepare AIS QDs on a large scale and at low cost, previous organic synthesized routes have been replaced by aqueous synthesis like microwave irradiation which is an efficient means to realize rapid and uniform heating [29-33]. Xiong et al. realized the synthesis of aqueous AIS /ZnS nanocrystals with a photoluminescence quantum yield (PL QY) of 40% by means of microwave-assisted approach [34]. The highest PL QY of AIS/ZnS QDs synthesized via a microwave-assisted method has reached 49% so far [35], but there is still room for further improvement. In addition, water-soluble QDs have been restricted by their poor compatibility with most polymer solutions, which makes it important to explore an effective method to apply water-soluble QDs to warm WLEDs [36-38]. Herein, high-quality AIS and AIS/ZnS QDs were synthesized via a microwave-assisted aqueous method. The as-prepared core and core/shell QDs were characterized by Ultraviolet-visible (UV-vis) spectrophotometry, PL spectroscopy and X-ray
diffraction
(XRD),
X-ray
photoelectron
spectroscopy
(XPS)
and
high-resolution transmission electron microscopy (HRTEM). By means of altering chemical composition, the AIS/ZnS QDs exhibited tunable emission ranging from 540 to 622 nm, among which the PL QYs of green, yellow and red QDs reached 49.22%, 58.27% and 56.21%, respectively. To prepare the variable-color luminescent film, the polychromatic water-soluble AIS/ZnS QDs were embedded in the polyacrylamide (PAAm) hydrogel (Scheme 1). The obtained QD/PAAm composite films have excellent flexibility and brightness. By combining a blue InGaN chip with the green and red luminescent films, a warm WLED with the CRI value of 87.5 and the CCT 3
value of 3669 K was successfully fabricated.
Scheme 1. Synthetic illustration for the preparation of AIS/ZnS QDs and QD/PAAm composite film for WLEDs.
2. Experimental section 2.1. Chemicals Indium (III) chloride (InCl3·4H2O, 99.9%), sodium sulfide (Na2S·9H2O, 98%), acrylamide
(C3H5NO,
99.9%),
N,N′-methylenebis(acrylamide)
ammonium
persulfate
(BAAm,
(APS, 99%)
99.99%), and
N,N,N',N'-tetramethylethylenediamine (TEMED, 99%) were purchased from Aladdin Chemistry Co. Ltd. Silver nitrate (AgNO3, 99.8%), zinc chloride (ZnCl2, 98.0%) and sodium hydroxide (NaOH, 96%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Glutathione (GSH, 99%) was purchased from Sahn Chemical Technology Co., Ltd. All the reagents were of analytical reagent grade and all aqueous solutions were prepared with deionized water. 2.2. Microwave-assisted synthesis of AIS and AIS/ZnS QDs AIS QDs were synthesized in aqueous solution using a microwave chemical reaction system (MCR-3) with controllable temperature units, whose heating time and temperature can be set as needed. With a power range of 0-1300 W and a working 4
frequency of 2450 MHz, the reaction system can heat the solution to 95 minutes and keep the error within 5
in 2
. In a typical experiment for the synthesis of the
core QDs with the Ag/In ratio of 1:10, 0.1 mmol AgNO3, 1 mmol InCl3 and 8 mmol GSH were mixed with 50 mL of deionized water in a 250 mL three-neck flask under magnetic stirring. The pH value of the mixture solution was adjusted to 8.5 by adding 2.0 M NaOH solution. Then 10 mL of freshly prepared Na2S solution (0.2 M) was added into the mixture, which was subsequently heated to 95
for an hour by
microwave irradiation. In the reaction system, Ag+, In3+ and S2- were combined in a specific proportion to form QDs with GSH as the capping agent. Ions like NO3-, Cl-, Na+ existed in solution in a free state and did not participate in the reaction. In order to synthesize the AIS/ZnS core/shell QDs, 10 mL of aqueous solution containing 1 mmol ZnCl2, 1 mmol Na2S and 1 mmol GSH was injected into the as-prepared core QDs solution followed by reaction for another 10 min. Under the stability of the ligand, Zn2+ and S2- epitaxially grow on the surface of the AIS QDs to generate a layer of ZnS shell whose thickness was controlled by repeating the injection of ZnS precursors solution. Afterwards the AIS/ZnS QDs were purified via adding excess ethanol and centrifuging to remove the ions that are not involved in the reaction. And then the AIS/ZnS QDs were re-dispersed in water or dried into powders for further characterizations. 2.3. Preparation of the QD/PAAm composite film PAAm hydrogel was obtained using free-radical polymerization of acrylamide monomer according to the previously reported method [39]. 3 g acrylamide monomer, 0.002 g BAAm and 0.02 g APS were dissolved in 6 mL AIS/ZnS QDs solution (5 wt %) in the air, followed by the addition of 20 µL TEMED under magnetic stirring for 10 s. Then the mixture was transferred into a circular mold with a diameter of 2 cm and a depth of 0.5 cm to form the film with a thickness of 0.5 mm. 2.4. Assemblage of AIS/ZnS QD-based LED devices To fabricate the QD-based LED devices, InGaN LED chips (λem=460 nm) were used as blue light sources. Besides, the prepared heterochromatic QD/PAAm hybrid 5
films were loaded on the chips. 2.5. Characterizations All the measurements were carried out at room temperature (RT). UV-vis absorption spectra and PL spectra of the purified QDs solutions were recorded by a Shanghai Lengguang 759S ultraviolet spectrophotometer and a Shanghai Lengguang F97XP fluorescence spectrophotometer, respectively. The XRD patterns of QDs powder were obtained by Bruker D8 Advance. The XPS analysis was implemented on an ESCALAB250Xi X-ray photoelectron spectrometer. The TEM and HRTEM images were obtained on a JEOL2100F transmission electro-microscope operating at an accelerating voltage of 200kV. The absolute PL QYs were measured by an integrating sphere on a FLSP920 spectrometer. And the luminous characteristics of LED devices are obtained by SPIC-200 spectral irradiance colorimeter.
3. Results and discussion 3.1. Fluorescence properties of AIS QDs Microwave radiation is an efficient heating method which can raise the temperature of the reaction system to 95
in 90 s. The temporal evolution of UV-vis
absorption and PL spectra of the as-prepared AIS core QDs when the ratio of Ag/In is 1:10 is demonstrated in Fig. 1. The broad absorption spectra (Fig. 1a) without obvious exciton absorption peaks conform to the typical characteristics of ternary compounds [40]. The optical band gap of direct-band-gap semiconductor can be calculated from the UV-vis absorption spectra based on Tauc’s relation, (αhν)1/m =A(hν-Eg), in which α is the absorption coefficient, m=1/2 for direct allowed transitions, A is a constant and Eg is the optical band gap [41]. It can be concluded that with the reaction time ranging from 15 to 120 min, the band gap of the AIS core slightly decreased from 3.07 to 3.00 eV due to increasing particle sizes (Fig. S1). Different from the traditional II-VI compound such as CdSe or CdTe whose PL emission properties are decided by their sizes [42,43], the ternary QDs have a wide 6
PL peak centered at 615 nm with a shoulder centered at 652 nm whose position is constant during the growing of the AIS core. And the stokes shift is greater than 200 nm, which indicates that the PL emission is engendered by donor-acceptor recombination related to various trap states instead of band-edge recombination [44, 45]. Nevertheless, the intensity of the PL emission is continuously improved with extending the reaction time, largely due to the increasing crystallinity of AgInS2. The PL QY of AIS QDs reached up to 29% after 60 min, which is higher than that of other aqueous AIS cores reported in previous work [45, 46].
Figure 1. (a) Evolutions of UV-vis absorption spectra and (b) PL spectra (λex=460 nm) of AIS QDs under different reaction time.
3.2. Characterization of AIS/ZnS QDs According to previous work [22], core/shell structure, composed by coating another material with a wider band gap such as ZnS on the core QDs, can distinctly enhance the optical properties of alloyed nanocrystals. The influence of ZnS shell thickness upon the PL spectra of AIS QDs with the Ag/In ratio of 1:10 is shown in Fig. 2a. After epitaxially growing the first ZnS shell, the emission peak shows a blue-shift from 615 to 595 nm, and the intensity almost doubles compared to AIS cores. The PL enhancement is due to the passivation of the surface-related defect states by ZnS. However, as the ZnS shell thickness increases, the PL enhancement becomes less obvious. Meanwhile, a continuous blue-shift of the PL wavelength can be observed during the coating of ZnS shell, which can be ascribed to the 7
diffusion of Zn2+ into the core lattice, namely cationic interdiffusion between the wider band gap ZnS shell (bulk Eg = 3.68 eV) and the AIS core (bulk Eg = 1.87 eV) leading to an increment of the band-gap energy [30,39]. As a result, an analogous alloyed Zn-Ag-In-S structure with a wider band gap was formed and the PL peak position wavelength and PL QY of AIS/ZnS(3) reached 578 nm and 58.27%, respectively. The formation of approximately alloyed structure can also be confirmed in the XRD patterns shown in Fig. 2b. The diffraction peaks of both AIS core and AIS/ZnS core/shell QDs are broadening owing to their small sizes and can be indexed to the tetragonal chalcopyrite crystal structure (JCPDS No. 25-1330). It is notable that the diffraction peaks of the AIS/ZnS QDs shift toward larger angles, approaching the bulk zinc blende ZnS phase (JCPDS No. 05-0566), which can be ascribed to the smaller lattice constant of ZnS relative to AIS and reveals the formation of core/shell structure rather than separate ZnS nanoparticles. The composition information of AIS and AIS/ZnS QDs with the Ag/In molar ratio of 1:10 can be obtained by XPS shown in Fig. 2c, manifesting the existence of Ag, In and S elements in the core QDs and additional Zn element in core/shell QDs. It can be calculated from the atomic percentage that the actual ratio of Ag/In in the AIS core is 1:4, larger than the ratio of the precursors, which can be explained by Pearson's hard-soft acid-base theory [47]. As a kind of soft base, S2- should be more likely to react with soft acid, such as Ag+. Since In3+ is a hard acid, it may have a lower reactivity with S2-. Therefore the ratio of Ag/In in the as-prepared core QDs is higher than that in the precursors. Nevertheless, after the coating of three layers of ZnS shell, the actual ratio of Ag/In dropped to 1:12, indicating that the diffused Zn2+ might preferentially substitute for Ag+ in the lattice structure.[32, 33] Fig. 2d demonstrates the PL decay curves of AIS and AIS/ZnS QDs with the emission peaks at 615 nm and 578 nm, which can be well fitted by a biexponential function: I(t) = B1 exp (-t/τ1) +B2 exp (-t/τ2), where τ1, τ2 represent the decay times of the PL emission and B1, B2 represent the relative weights of the decay 8
components at t = 0 [48]. And the fitted parameters are listed in Table 1. In light of previous researches about the electron-hole recombination mechanisms in the - -
alloyed semiconductors[31, 32, 34], τ1 is ascribed to the high-energy
radiative recombination of the surface trap states and τ2 is assigned to the low-energy donor–acceptor recombination, with the latter accounting for a larger proportion than the former. Surface-related defect states are relatively shallow and thus show faster decay lifetime than the intrinsic trap states. After coating ZnS shell, both τ1 and τ2 rise appreciably due to the passivation. The relative weight of τ1 with high energy emission increases obviously, according with the report that the contribution from the fast decay time component increases notably with decreasing the ratio of Ag/In [45]. As mentioned above, the addition of Zn2+ would displace some Ag+, resulting in the decrease of Ag element and the significant increase of B1/B2, which is also one of the reasons for the blue-shift of the PL spectra. As a result, the calculated average PL decay lifetime values of the AIS QDs and AIS/ZnS QDs are quite similar, 474.2 ns and 455.0 ns respectively.
Figure 2. (a) PL spectra (λex=460 nm) of AIS core QDs and AIS/ZnS core/shell QDs 9
with different shell thicknesses. (b) XRD patterns, (c) XPS survey spectra and (d) PL decay curves of AIS core QDs and AIS/ZnS core/shell QDs.
Table 1. Constants obtained from I(t) = B1 exp (-t/τ1) +B2 exp (-t/τ2) Samples
τ1 (ns)
B1 (%)
τ2 (ns)
B2 (%)
Lifetime (ns)
AIS
124
19.95
496
80.05
474.2
AIS/ZnS
172
36.55
510
63.45
455.0
The sizes and morphologies of AIS and AIS/ZnS QDs were further characterized by TEM and HRTEM images. As demonstrated in Fig. 3, the as-prepared QDs with nice monodispersity and high crystallinity possess spherical or subspherical shape. The average sizes of AIS and AIS/ZnS QDs are 2.70 nm and 3.07 nm respectively, revealing that the particle size increases during the coating process.
Figure 3. (a), (c) TEM images and (b), (d) HRTEM images of AIS core QDs and 10
AIS/ZnS core/shell QDs (inset: the particle size distribution of the corresponding QDs).
3.3. Influence of Ag/In molar ratio For most ternary and quaternary alloyed nanocrystals such as AIS QDs, PL emission is caused by defect-related donor–acceptor pair recombination rather than band-edge recombination, so their optical properties are composition-dependent rather than size-dependent [45,46]. The ratio of Ag/In is an important factor for tuning the fluorescence properties of AIS/ZnS QDs, which was adjusted by changing the amount of Ag precursors while keeping other experimental variables fixed in the preparation process. And according to the photoluminescence excitation (PLE) spectra at their respective emission peak wavelengths (Fig. S2), the optimal excitation wavelength is 460 nm for all AIS/ZnS QDs of different proportions. Figure. 4 demonstrates the absorption spectra, the normalized PL spectra, the PL QY and the digital photograph under UV irradiation of AIS/ZnS QDs with the molar ratio of Ag/In varied from 1:20 to 5:10. With the decrease of Ag content, both the absorption edge and PL spectra show obvious blue-shift. The emission peak position can be tuned from 622 to 540 nm excited by 460 nm, corresponding to the color tuned from red to green shown in Fig. 4d. As reported by previous reports, the conduction band minimum of AIS QDs is composed of hybrid orbitals of In 5p5s and S 3p, while the valence band maximum consists of hybrid orbitals of S 3p and Ag 4d [17, 32], which is lowered as the number of orbitals originating from Ag decreases, accordingly the band gap is widened. The absolute PL QY of AIS/ZnS QDs is also enormously affected by the Ag/In ratio, as shown in Fig. 4c. The optimal ratio giving the maximum emission intensity is 1:10 (Fig. S3), and the PL QY reaches 58.27%. It is also worth noting that at the ratio of 1:20 and 2:10, the PL QYs are also quite good, 49.22% and 56.21%, respectively. Therefore, the AIS/ZnS QDs with the Ag/In ratios of 1:20, 1:10 and 2:10 (PL peak located at 540, 578, and 600 nm) have the potential to prepare high-quality white 11
LED.
Figure 4. (a) UV-vis absorption, (b) PL spectra (λex=460 nm), (c) PL QYs and (d) digital photographs of AIS/ZnS QDs synthesized at different molar ratios of Ag/In.
3.4. Preparation of green, yellow and red luminescent QD/PAAm composite films and white LEDs In general, water-soluble QDs have poor compatibility with polymer solutions due to the existence of a large number of hydrophilic ligands on their surface, which is one of the challenges of limiting their application to LEDs. However, embedding polychromatic aqueous QDs into hydrogel which combines the optical properties of QDs with the mechanical properties of hydrogel not only provides a new solution, but also makes flexible LED devices possible [36-38]. In this report, PAAm hydrogels synthesized by free-radical cross-linking polymerization of acrylamide monomer and BAAm crosslinker in an aqueous solution containing the prepared AIS/ZnS QDs exhibit high transparency, good shape flexibility and excellent extensibility, as shown in Scheme 1. Homogeneous QD/PAAm hybrid films with various colors can be 12
obtained by controlling the component of AIS/ZnS QDs. The PL spectra of the composite films containing AIS/ZnS QDs with Ag/In ratio of 1:20, 1:10 and 2:10 excited by blue light at 460 nm are shown in Fig. 5a. The emission peaks are positioned at 565, 591 and 657 nm, and the PL QYs are 27.9%, 39.5%, and 36.5%, respectively. Figure. 5b shows the green, yellow and red hybrid films under 365 nm UV light. The flexible films exhibit uniformly distributed light emission, which demonstrates that the QDs are evenly dispersed in PAAm hydrogel and their PL emission are well-preserved.
Figure 5. (a) PL spectra (λex=460 nm) and (b) digital photographs (under UV irradiation) of green, yellow and red QD/PAAm composite films.
Since these hybrid films are suitable to be strongly excited by 460 nm, their PL performance can be examined by fabricating white LEDs with InGaN blue LED chips. Green (G), yellow (Y) and red (R) luminescent films are applied to be respectively combined with blue LED chips as color converters. The normalized luminescence spectra and the CIE chromaticity coordinates of the as-prepared LED at 20 mA are 13
delineated in Fig. 6a and b. The LED based on yellow composite film achieves white light emission with a CIE color coordinate of (0.349, 0.324), a CCT of 4393 K, and a CRI of 75.6. In order to achieve high quality warm-white light emission, green and red QD/PAAm hybrid films with the appropriate thickness are utilized to fabricate the LED device. It’s notable that the red film has to be placed under the green one to avoid reabsorption of green light. The luminescence spectrum of the RGB tricolor white LED covers the full visible-spectrum area from 430 to 750 nm, as demonstrated in Fig. 6c. The CIE color coordinate of the as-prepared warm WLED (Fig. 6d) is (0.392, 0.362), which falls in the white gamut and approaches to the Planckian locus. The CCT is 3669 K and the CRI reaches 87.5, which is a significant improvement compared to that of the LED device containing only yellow film, as shown in Table 2.
Figure 6. (a) Emission spectra and (b) CIE color coordinates of the LEDs based on green, yellow and red QD/PAAm films at a drive current of 20 mA. (c) Emission spectrum and (d) CIE color coordinate of the LED based on both green and red QD/PAAm films at a drive current of 20 mA. 14
Table 2. Optical properties of LEDs using various QD/PAAm composite films at a drive current of 20 mA CIE coordinate Samples x
CRI
CCT (K)
y
LED (G)
0.289
0.336
-
-
LED (Y)
0.349
0.324
75.6
4393
LED (R)
0.390
0.207
-
-
LED (G+R)
0.392
0.362
87.5
3669
4. Conclusions In summary, AIS core QDs and AIS/ZnS core/shell QDs have been successfully synthesized via a facile microwave-assisted method in aqueous solution with GSH as stabilizing agents. The PL QYs of core QDs and core/shell QDs are up to 29% and 58.27%, respectively. The PL wavelength can be conveniently tuned from 540 to 622 nm by adjusting the stoichiometric ratio of Ag and In. Green, yellow and red AIS/ZnS QDs are combined with PAAm hydrogel to fabricate highly luminescent hybrid films, among them the yellow film has been applied on commercial blue InGaN chips for a remote-type warm WLED with a CRI of 75.6 and a CCT of 4393 K. By combining red and green films, the warm WLED with a high CRI of 87.5 and a CCT of 3669 K is prepared, demonstrating the potential for WLEDs DEwith AIS/ZnS/PAAm as color-conversion materials.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC, No. 61675049, NSFC, No. 61377046, and NSFC, No.61177021) and Fudan University-CIOMP Joint Fund (FC2017-004).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: