Engineering the Self-Assembly Induced Emission of Cu Nanoclusters

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Engineering the Self-Assembly Induced Emission of Cu Nanoclusters by Au(I) Doping Jiale Liu,† Zhennan Wu,† Ye Tian,† Yanchun Li,§ Lin Ai,† Tingting Li,† Haoyang Zou,† Yi Liu,† Xindong Zhang,*,‡ Hao Zhang,*,†,∥ and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry and ‡State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China § Institute of Theoretical Chemistry, Jilin University, Changchun 130023, P. R. China ∥ Nanjing Haiyan Electric Technology Co. Ltd., Nanjing 211500, P. R. China S Supporting Information *

ABSTRACT: Aggregation-induced emission (AIE) and selfassembly induced emission (SAIE) effects have been employed to tune the emission properties of metal nanoclusters (NCs). However, it is still not possible to further enhance the photoluminescence quantum yields (PLQYs) and control the emission colors of the NCs using AIE and SAIE. On the basis of our recent work studying the contribution of Cu(I) defects in the SAIE of Cu NCs, in this article, Au(I) was doped into Cu NC self-assembled nanosheets (NSASs) to construct a more stable Au(I)-centered state. As a result, the PLQYs, emission stability, and tunability of emission colors of the Cu NSASs were significantly improved. Detailed studies reveal that the doped Au(I) induces a Au(I)−Cu(I) metallophilic interaction, which leads to a ligand-to-Cu−Au charge transfer, which facilitates the relaxation of excited electrons via a radiative pathway, thereby enhancing the emission intensity. The charge transfer from Cu to Au lowers the energy, thus leading to the red-shift of PL emission. Au(I) is likely doped into the Cu NSASs rather than in individual NCs, because 0.3% Au doping is enough to alter the emission properties. By mixing Au(I)-doped Cu NSASs with different emission colors (due to different Au doping percentages) as color conversion materials on commercially available 365 nm GaN chips, a white light-emitting diode prototype is fabricated. KEYWORDS: Cu nanocluster, self-assembly induced emission, two-dimensional material, Au doping, light-emitting diode

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INTRODUCTION Nanodots, including semiconductor quantum dots (QDs),1−3 perovskite QDs,4,5 metal nanoclusters (NCs), and so forth,6−10 are competitive phosphors for next generation illumination and display11−14 because of the requirement of light-emitting diodes (LEDs) with high color rendering and broad color gamut.14,15 II−VI CdSe/ZnS QDs have been commercially applied in LEDs,16 but the QDs contain Cd, which is highly toxic and environmentally unfriendly. I−III−VI and II−III−VI semiconductor QDs, such as AgInSe and CuInS2,17,18 are Cd free, but contain In, which is also toxic. Perovskite QDs are Cd free, but contain the heavy metal Pb.19 The exploration of alternative nanodot phosphors composed of earth-abundant and low toxicity elements is crucial for the development of the LED industry. In this scenario, Cu is a widely used metal element, and Cu NCs composed of a few to tens of atoms can be prepared via a colloidal chemistry approach.20−22 Cu NCs exhibit molecule-like emission properties owing to the gradual emergence of discrete electronic states.23−26 The emission properties of Cu NCs also depend on the size of the Cu cores, which is similar to the quantum size effect.27 However, the © XXXX American Chemical Society

emission intensity of Cu NCs is very weak and it is still difficult to tune the emission color. Recently, the investigation of the aggregation-induced emission (AIE) of metal NCs has opened the door to optimizing and adjusting the emission properties of metal NCs,28−32 and focuses on the interaction of metal NCs rather than the NC itself.30,31 With respect to the mechanism of AIE, the ligand-to-metal charge transfer (LMCT) and/or ligand-tometal−metal charge transfer (LMMCT) between the capping ligands and metal core induces a metal-centered triplet state, generating radiative relaxation.28,29 For individual NCs, the vibration and rotation of the ligands limit the radiative relaxation of excited states, which weakens the emission intensity of the NCs.28,29 For aggregated NCs, the vibration and rotation of the ligands are greatly suppressed, thus enhancing the emission intensity significantly.28−32 Although many works have reported the emission enhancement of metal Received: May 6, 2017 Accepted: July 10, 2017

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DOI: 10.1021/acsami.7b06371 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

(0.3%) is enough to optimize the emission properties, enhancing the photoluminescence quantum yield (PLQY) from 5.8 to 23.5% and tuning the PL peak position from 501 to 607 nm. A white LED (WLED) prototype is further fabricated by mixing the Au(I)-doped Cu NSASs with different emission colors (due to different Au doping percentages) as color conversion materials on a commercially available 365 nm GaN chip.

NCs via the AIE approach, the irregular aggregation of NCs still makes it difficult to achieve state-of-the-art preparation of metal NC materials with strong and full-color emission like semiconductor and perovskite QDs. Most recently, our group demonstrated the enhancement and control of Cu NC emission by forming compact and ordered self-assembly architectures, namely, self-assembly induced emission (SAIE).33 A relationship between the compactness of the assemblies and the emission properties was established. Such a self-assembly strategy can supply both an emission mechanism and controllability. On the one hand, the suppression of the vibration and rotation of ligands improves the radiative relaxation of the excited states of NCs in a similar way to common AIE. On the other hand, the flexibility of the selfassembly approach permits further control of the spatial distribution of assembled NCs, and therefore the inter-NC interactions. As a result, the emission color is potentially tunable. According to the emission mechanism, the emission of metal NCs depends both on the nature of the capping ligands and metal core.23−33 Many works have been done on suppressing the vibration and rotation of ligands and increasing the ratio of radiative relaxation within the field of AIE.28−33 However, the influence of the metal core on LMCT and/or LMMCT in AIE is usually ignored. Actually, even a slight deviation of the valence ratio of the metal core can significantly influence NC emission.34,35 In our previous work, Cu NCs self-assembly ultrathin nanosheets with a metal defect-rich surface were fabricated through an ethanol-induced approach.36 The presence of metal defects slightly increased the Cu(I)-toCu(0) ratio, which facilitated the radiative relaxation of excited electrons by influencing the LMMCT process. However, the ethanol-induced Cu(I)-centered surface defects were not efficient enough to generate strong emission. The resulting emission was also sensitive to environmental variation, because the Cu(I) defects were not stable enough. A more efficient and stable metal emission center was required. In this scenario, metal doping is a common method for modifying the emission properties of semiconductor QDs,37−39 perovskite QDs,40,41 and metal NCs by introducing an additional emission center.34,35 Au, Ag, and Cu are the most common components of fluorescent metal NCs.23−26,42−45 Many works have reported alloyed and doped metal NCs composed of Au−Ag,46−48 Au− Cu,49−52 and Ag−Cu.53 However, such NCs are usually individual structures. To the best of our best knowledge, the doping of a metal impurity into self-assembly architectures of metal NCs has never been reported. In addition, as Ag is not suitable for chlorine-rich systems,54 Au should be more suitable as the doped heteroatom. The doping of Au(I) is considered to induce a Au−Cu metallophilic interaction and influence the LMMCT process.52 Accordingly, the process of relaxation of excited electrons will also change. In this article, Au(I)-doped Cu NCs self-assembly nanosheets (NSASs) are prepared by introducing Au during the selfassembly of Cu NCs, and this strategy significantly enhances the emission intensity and leads to emission red-shift. The experimental results show that Au is doped into the whole Cu NSASs, rather than individual NCs. This doping induces a Au(I)−Cu(I) metallophilic interaction, thus facilitating the radiative relaxation of excited electrons by influencing the LMCT and/or LMMCT process of the Cu NCs. In addition, the influence of doped Au on the electronic structure of the Cu NCs generates an efficient and stable Au(I)-centered state. Au

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RESULTS AND DISCUSSION Au(I)-Doped Cu NSASs. The Cu NSASs were prepared in liquid paraffin (LP) and dibenzyl ether (BE) as the solvents using 1-dodecanethiol (DT) as the capping ligand at 50 °C. Different from our previous report,33 CuCl2 was first dissolved in tetrahydrofuran (THF) before being added to the reaction mixtures. The as-prepared Cu NSASs showed bright blue-green emission when excited by 365 nm ultraviolet light. The absolute PLQY was 5.8%. Au(I)-doped Cu NSASs were further prepared by adding HAuCl4 with specific mole percentages of 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, and 3%. With increasing Au amount, the emission peak at 501 nm started to decrease, whereas the peak at 607 nm increased (Figure 1a). The

Figure 1. PL emission spectra (a) and the corresponding fluorescence images (b) of the chloroform dispersion of Cu NSASs with 0, 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, and 3% Au doping with 365 nm excitation.

apparent emission color changed from blue-green to yellow (Figure 1b). The optical images of the Cu NSASs are shown in Figure S1. The corresponding PLQYs are 5.7, 5.3, 9.8, 12.4, 18.3, 23.5, 23.2, and 20.4%. Obviously, the doping of Au leads to an enhancement and red-shift of the emission of the Cu NSASs. Because the Cu NSASs with 0.3% Au doping indicated the highest PLQY, the Cu NSASs without and with 0.3% Au doping were selected to study the doping dynamics and emission mechanism. The unchanged UV−vis absorption spectra imply that the composition of the Cu NCs does not significantly change (Figure S2). From matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), the relative molecular mass of the Cu NCs was calculated to be 1176, according to the reference peaks of cationization reagent trifluoroacetic acid sodium at B


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larger than the diameter of the Cu NCs. This indicates that the Cu NSASs are multilayered self-assembled architectures of NCs (Figure S6). Small angle X-ray diffraction (SAXRD) results indicated that the diffraction peaks of the Cu NSASs without and with 0.3% Au doping both appear at 2.50 and 2.61° (Figure 3), which correspond to distances of 3.53 and 3.38 nm. The

1200 and trifluoroacetic acid silver at 1285 (Figure 2). The composition of the Cu NCs was determined to be Cu9DT3,

Figure 2. MALDI-TOF mass spectra of Cu NSASs with 0 and 0.3% Au doping, which indicate the composition of Cu9DT3. Inset: computer-optimized structure of Cu9DT3 used to calculate the permanent dipole moment μ.

Figure 3. SAXRD patterns of Cu NSASs with 0 and 0.3% Au doping.

second and third ordered peaks were also observed around 5.02, 5.22° and 7.52, 7.85° for the Cu NSASs, and 4.99, 5.21° and 7.48, 7.83° for the 0.3% Au-doped Cu NSASs. The appearance of two SAXRD peaks means two kinds of NC distances, namely, intra- and interlayer. Because the interlayer NC distance is smaller than the intralayer distance,33 3.53 and 3.38 nm can be assigned to the intra- and interlayer NC distance, respectively. After Au doping, both the intra- and interlayer NC distances increase slightly, which proves the existence of Au in the interlayer and the intralayer of the Cu NSASs. As the size and composition of the Cu NCs are unchanged without and with 0.3% Au doping (Figures 2 and S3), the doped Au does not seem to form alloy NCs with Cu. To further study the position and status of the doped Au, the Cu NSASs were characterized by TEM mapping. As shown in Figure S7, Cu and Au elements are homogeneously dispersed in the Cu NSASs. Because each NC possesses nine Cu atoms and the percentage of doped Au is only 0.3%, it is impossible to form Au−Cu alloy NCs or Au NCs alone. So, Au must be doped across the whole Cu NSASs rather than in individual Cu NCs. The fluorescence microscopy images show the change of apparent emission color with Au doping (Figure S8). With tiny Au doping between 0.003 and 0.01%, blue and yellow emission is observed at the same time (Figure S8b−d). This means that the Cu NSASs are not fully doped. Only the part with Au doping can emit yellow emission. Too little Au cannot supply enough Au-related emission centers to alter the emission of the whole Cu NSASs. By increasing the level of doped Au to 0.03%, the blue emission almost disappears (Figure S8e), showing the domination of Au-related emission centers. The status of doped Au was further revealed by X-ray photoelectron spectroscopy (XPS). As shown in Figure S9b, the Au 4f7/2 and Au 4f5/2 peaks respectively appear at 84.3 and 88.0 eV, corresponding well with the characteristic peaks of Au(I).29 This means that the doped Au exists in the form of Au(I) ions. As determined by energy-dispersive X-ray spectroscopy (EDX) analysis, the real Au content is 0, 0.4, and 1.3% for the Cu NSASs with 0, 0.3, and 1% molar feed percent of Au (Figure S10). The slightly

which was computer-optimized and is shown in the inset of Figure 2. On the basis of the computer-optimized structure, the permanent dipole moment μ of the Cu NCs was calculated to be 3.218 D, which is not very big because of the strong symmetry of the Cu NCs.55 As a result, the van der Waals attraction between the DT ligands on the Cu NCs acts as the main driving force for directing the self-assembly of Cu NCs with two-dimensional tendency. The MALDI-TOF MS peaks of the Cu NSASs with 0.3% Au doping are almost the same as those without Au doping, which confirms the unchanged composition of the Cu NCs. In addition, transmission electron microscopy (TEM) observation indicates that the sizes of the Cu NCs with and without 0.3% Au doping are both 1.8 ± 0.2 nm (Figure S3). These results mean that slight Au doping does not alter the structural properties of the Cu NCs in the Cu NSASs. Influence of Doped Au(I) on the Structure of the Cu NSASs. As mentioned above, the emission properties of the Cu NSASs clearly change after Au doping. Therefore, the doping dynamics of Au is important for understanding the emission mechanism. TEM images show that the morphology of the Cu NSASs converts from big nanosheets to small adhesive nanoribbons with slight Au doping (Figures S4 and S5). However, the self-assembly architectures are destroyed when the level of doped Au reaches 3%. As mentioned in previous studies,28,29 the self-assembly of Cu NCs suppresses the vibration and rotation of the capping ligand DT on the NCs, which enhances emission intensity. Despite the size of the Cu NSASs decreasing with slight Au doping, the Cu NSASs, in general, preserve their two-dimensional morphology. So, the advantage of self-assembly in enhancing NC emission is maintained. Because the atomic radius of Au is bigger than that of Cu,56 excess Au doping might destroy the order of Cu NCs in the Cu NSASs, thus lowering the emission intensity. This finding also proves the importance of structural integrity of NCs for SAIE. Atomic force microscopy (AFM) observation revealed the thickness of the Cu NSASs to be about 183 nm, which is much C


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Figure 4. Luminescence lifetimes of Cu NSASs with 0% (a) and 0.3% (b) Au doping at different wavelengths. Excitation (365 nm) was adopted. TRES of Cu NSASs with 0% (c) and 0.3% (d) Au doping with delay times of 0.2, 0.5, 1, 2, 3, 4, 6, 10, 15, 20, 30, and 50 μs at room temperature. The corresponding normalized TRES spectra are shown in (e) and (f).

Figure 5. Variation of the emission spectra of Cu NSASs with 0% (a) and 0.3% (b) Au doping from 77 to 298 K. Excitation (365 nm) was adopted. Insets: the fluorescence images of Cu NSASs excited by 365 nm ultraviolet light.

does not disappear, however, a new emission peak appears near 560 nm (Figure 4e). According to our previous study,36 this result is attributed to the Cu(I) defects on the surface of the Cu NSASs, because of the large surface area. However, the influence of Cu(I) defects on the whole emission of the Cu NSASs is relatively weak and negligible (Figure 4c), resulting in the low PLQY of 5.8% for the Cu NSASs. When 0.3% Au is doped, the Cu NSASs show a 4-fold emission enhancement and 100 nm emission red-shift (Figure 1). According to the emission mechanism of metal NCs, the doping of Au(I) may induce a Au(I)−Cu(I) metallophilic interaction,52 which leads to charge transfer from Cu to Au. This new recombination pathway facilitates the radiative relaxation of excited electrons. The strong influence of doped Au on the electronic structure of

higher content of Au than the feed percent is attributed to the higher reactivity of Au than Cu.54 Emission Mechanism of Au(I)-Doped Cu NSASs. Combining the long lifetime with the large Stokes shift and emission red-shift under low temperature (Figures 4a and 5a), the emission of Cu NSASs without Au doping was determined to be a typical metal-centered triplet state emitting through a Cu-centered LMCT and/or LMMCT process,33 which can be defined as a Cu-centered triplet state. The Cu-centered triplet state emission centers at about 500 nm with an emission lifetime of 1.62 μs (Figure 4a). Time-resolved emission spectroscopy (TRES) of Cu NSASs without Au doping showed that the PL emission peak red shifts with increasing delay time (Figure 4c,e). The emission peak of Cu NSASs around 500 nm D


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ACS Applied Materials & Interfaces the Cu NSASs leads to an efficient and stable Au(I)-centered state, which originates from the ligand-to-Cu−Au charge transfer and has an emission peak at about 600 nm. As Au is doped into the Cu NSASs, the charges first transfer from the ligand to Cu and then to Au. The charge transfer process lowers the energy. So, the emission spectra show a red-shift and longer lifetime after Au doping (Figure 4b). In comparison with the Cu NSASs without Au doping, the TRES of the Au-doped ones changes obviously in the range of 500−600 nm (Figure 4d). The normalized TRES indicates two emission states, a Cucentered triplet state and a Au(I)-centered state, which correspond to the emission peaks at 500 and 600 nm (Figure 4f). Because of the strong emission intensity of the Au(I)centered state, the contribution of Cu(I) defects on the emission was neglected. Although the Cu-centered triplet state emission is still strong, the Au(I)-centered state has a much longer emission lifetime. So, the Cu NSASs with 0.3% Au doping show yellow emission. It should be mentioned that both the NSASs without and with 0.3% Au doping possess multiple state emission at the same time. The apparent emission depends on which one is dominant. Because of the long emission lifetime and high emission intensity at 600 nm, the Cu NSASs with 0.3% Au doping show yellow emission centered at 600 nm. The increase in Au amount also makes the Au(I)centered state dominant, leading to the emission enhancement and red-shift (Figure 1). In addition, the asymmetry of the emission spectra of the Cu NSASs without and with Au doping further confirms the coexistence of multiple emission states (Figure S11). The Au(I) emission was further studied by temperature related emission spectra (Figure 5). As shown in Figure 5a, the red-shift of the Cu NSASs at low temperature indicates typical LMMCT-determined triplet state emission.33,36 Because the vibration and rotation of ligands are limited at low temperature, which suppresses the recombination of excited electrons via nonradiative relaxation pathways,28,29,33 both Cu- and Au(I)centered emission should increase at 77 K. However, the low temperature limits the intersystem crossing of excited electrons from the Cu-centered triplet state to the Au(I)-centered state via charge transfer.57 The emission intensity depends on both the radiative relaxation efficiency and the charge transfer from Cu to Au. So, the intensity of Cu-centered triplet state emission increases continuously, but the Au(I)-centered emission increases first and then decreases when the temperature changes from 77 to 298 K. As a result, the intensity of Cucentered emission from the Cu NSASs without Au doping increases continuously with decreasing temperature (Figure 5a). For the 0.3% Au-doped Cu NSASs, the Cu-centered emission at 500 nm also increases continuously, but the Au(I)centered emission at 600 nm increases first and then decreases when the temperature changes from 77 to 298 K (Figure 5b). According to the aforementioned emission mechanism, a schematic diagram of the excited state relaxation dynamics of the Au(I)-doped Cu NSASs is shown in Scheme 1. In all, the emission intensity of the Au(I)-centered state is much stronger than that of the Cu-centered triplet state and Cu(I) defects. The Cu NSASs with 0.3% Au doping indicate a 4-fold enhancement of PLQY up to 23.5%. In addition, the emission of the Au(I)-doped Cu NSASs is more stable than that of the Cu NSASs with a PLQY of 15.4% using ethanol to induce Cu(I) defects.36 The strong emission of the Au(I)doped Cu NSASs can be maintained for more than 6 months in an ambient environment both in solution and as a powder. In

Scheme 1. Schematic Diagram of the Excited State Relaxation Dynamics of Cu NSASs with Au Dopinga

a S0 and S1 represent ground state and excited singlet state. ISC represents the intersystem crossing process.

comparison, a rapid decline in emission intensity of the Cu NSASs with Cu(I) defects was observed when the Cu NSASs were separated from the solution. The poor emission stability can be attributed to the unstable Cu(I) defects in the altering environment. The electronic structure of Cu(I) defects is analogous to that of Cu(I) and Cu(0) in the core of the Cu NCs, which can undergo electronic delocalization and therefore lower the influence of the Cu(I) defects due to the little difference in redox potential between Cu(0) and Cu(I). Regarding the Au(I) heteroatoms in the Au(I)-doped Cu NSASs, they exist between the self-assembled Cu NCs. The big difference in the electronic structure and redox potential between Au(I) and Cu(0) makes electronic delocalization difficult. So, the doped Au(I) is more stable than the Cu(I) defects. In control experiments, other metals, such as Pb, Cd, Mn, Ag, and Zn, were introduced as the heteroatoms, but this did not alter the emission color of the Cu NSASs. This is due to the matching of the Cu and Au energy levels. LEDs from Au(I)-Doped Cu NSASs. To prepare LEDs with a series of monochromatic emission and white emission, more Au was added in the preparation of the Cu NSASs. As the Au percentage reached 80%, red emission centered at 655 nm was achieved (Figure S12). Then, the as-prepared Cu NSAS powders with 0, 0.3, and 80% Au doping were respectively mixed with polydimethylsiloxane (PDMS) precursors, coated on commercially available 365 nm GaN chips, and cured at 60 °C for 2 h to produce LEDs with blue-green, yellow, and red emission (Figure 6a−d). Their color coordinates (x, y) were determined to be (0.23, 0.40), (0.53, 0.45), and (0.59, 0.36) (Figure 6f). To produce a WLED, the Cu NSAS powders with 0, 0.3, and 80% Au doping were mixed in a weight ratio of 0.8:1:1.5. The emission spectra and color coordinates are indicated in Figure 6e,f. The device shows color coordinates of (0.33, 0.41), a color temperature of 5289 K, and a color rendering index of 86, which correspond to white light. E


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Figure 6. Optical, fluorescence, and corresponding LED images of Cu NSASs with 0% (a), 0.3% (b), and 80% (c) Au doping. (d, e) Emission spectra of the LEDs with blue, yellow, red, and white emission. (f) CIE chromaticity coordinates of the four LEDs. The inset in (e) is the optical image of the WLED.

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purchased from Sigma-Aldrich. Copper(II) chloride dihydrate (CuCl2· 2H2O, 99.0%, A. R.), LP, THF, acetone, and chloroform were all commercially available products and used as received without further purification. Ultraviolet GaN LEDs with an emission peak at 365 nm were purchased from Advanced Optoelectronic Technology Inc. The unpacked LEDs have a 1.42 mm × 1.42 mm chip. trans-2-[3-(4-tertButylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was purchased from Fluka. Preparation of Cu NSASs. The Cu NSASs were prepared in LP and BE as the solvents using DT as the capping ligand according to our previous method with slight modification.33 The difference is the initial dissolution of CuCl2 in THF before addition to the reaction mixtures. Specifically, 41.3 mg of CuCl2·2H2O was dissolved in 1 mL of THF and added into the mixtures of 6 mL of BE and 6 mL of LP at room temperature. Then, 2 mL of DT was added into the mixture and stirred at 50 °C for 10 h to produce the Cu NSASs. Preparation of Au-Doped Cu NSASs. HAuCl4·4H2O (1 mg) was dissolved in 10 mL of BE and 41.3 mg of CuCl2·2H2O was dissolved in 1 mL of THF. Then, 3 mL of the HAuCl4 solution and 1 mL of the CuCl2 solution were added into 6 mL of LP and 3 mL of BE. Finally, 2 mL of DT was added into the mixture and stirred at 50 °C for 10 h to produce the 0.3% Au-doped Cu NSASs. Au-doped Cu NSASs with other Au contents were prepared in the same way except the amount of Au was changed. Purification. At room temperature, 1 mL of pure or Au-doped Cu NSASs solution was washed and precipitated by adding 1 mL of chloroform and 2 mL of acetone. After that, the products were separated by centrifugation at 6000 rpm for 5 min. The purification process was repeated twice. Finally, the pure and Au-doped Cu NSASs were dispersed in chloroform.

CONCLUSIONS In summary, we demonstrated the preparation of Au(I)-doped Cu NSASs by virtue of the emission dynamics of SAIE, which significantly improved the PLQY and emission stability. Au is likely doped across the whole Cu NSASs rather than in individual Cu NCs in the form of Au(I) ions. Therefore, 0.3% Au is enough to generate a 4-fold PL enhancement and 100 nm red-shift of the emission spectrum. The doping of Au induces an additional Au(I)−Cu(I) metallophilic interaction, which leads to a Cu to Au charge transfer, facilitating the relaxation of excited electrons via a radiative pathway. Because Au doping lowers the energy of the original Cu-centered triplet state by introducing a Au(I)-centered state, a red-shift of emission spectra is achieved. In comparison with the previously reported Cu(I)-centered defect state, the Au(I)-centered state is more stable. Thus, Au(I)-doped Cu NSASs can be separated from solution and applied as phosphors for fabricating LEDs. The current approach of doping a metal impurity into the whole NC self-assembled architecture rather than in individual NCs provides a new pathway for tuning the emission properties of metal NCs within the field of SAIE.

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METHODS

Materials. Chloroauric acid hydrate (HAuCl4·4H2O, Au mol % >47.8%) was purchased from Sinopharm Chemical Reagent Co., Ltd. DT (98%), trifluoroacetic acid sodium (C2F3NaO2, ≥99.0%), and trifluoroacetic acid silver (C2F3O2Ag, 98%) were purchased from Aladdin Chemistry Co. Ltd.. BE (98%) and PDMS precursors were F


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ACS Applied Materials & Interfaces Fabrication of LEDs from Au-Doped Cu NSASs. The emission peak of the GaN LED chip was at 365 nm under a working voltage of 4.0 V. In the preparation of the blue-green, yellow, and red color conversion layers, 0, 0.3, and 80% Au-doped Cu NSASs were respectively milled to fine powders and mixed with PDMS precursors in a ratio of 1:9 (w/w). Then, the mixtures were put into a vacuum chamber to remove bubbles. Finally, the mixtures were loaded on the top of LED chips and heated at 60 °C for 2 h to fabricate the bluegreen, yellow, and red LEDs. To fabricate the WLED, powders of 0, 0.3, and 80% Au-doped Cu NSASs were mixed with PDMS precursors in a ratio of 0.8:1:1.5:29.7 (w/w). Then, the white light color conversion layer was covered on the GaN LED chip in the same way. Characterization. UV−vis absorption spectra were measured using a Shimadzu 3600 UV−vis−NIR spectrophotometer. PL spectra, lifetimes, and time-resolved spectra were obtained using an Edinburgh FLS980. The absolute PLQYs were measured using an Edinburgh FLS920 equipped with an integrating sphere (excited at 365 nm). TEM images were obtained using a JEM-2100F electron microscope under an acceleration voltage of 200 kV with a charge-coupled device camera. An EDX detector coupled with TEM was applied for elemental analysis. XRD data was acquired on a PANalytical B.V.Empyrean Diffractometer using Cu Kα radiation (λ = 1.5418 Å). The emission colors and spectra of the LED devices were identified using the CIE (Commission Internationale de L’Echairage 1931) colorimetry system. Tapping mode AFM measurements were performed with a Bruker Nanoscope IIIa scanning probe microscope (Digital Instruments). The optical microscope images and fluorescence microscopy images were taken by a microscopy OLYMPUS BX51. MALDI-TOF mass spectra were recorded on a Bruker Autoflex speed TOF/TOF, where THF and DCTB were used as the solvent and matrix to dissolve the samples. XPS characterization was implemented using a VG ESCALAB MKII spectrometer with Mg Kα excitation (1253.6 eV).

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(2016YFB0401701), t he 973 Program of China (2014CB643503), and the Special Project from MOST of China.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06371. Additional optical images, absorption spectra, TEM images, element mapping images, fluorescence microscopy images, XPS analysis, EDX analysis, PL lifetime, and TRES spectra of pure and Au-doped Cu NSASs (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected]. Fax: +86 431 85193423 (H.Z.). ORCID

Yi Liu: 0000-0003-0548-6073 Hao Zhang: 0000-0002-2373-1100 Bai Yang: 0000-0002-3873-075X Author Contributions

H.Z. proposed and supervised the project. H.Z., J.L.L., Z.N.W., Y.L., X.D.Z, and B.Y. designed and performed the experiments and co-wrote the paper. Y.T., Y.C.L., L.A., T.T.L., and H.Y.Z. participated in most experiments. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (51425303), the National key research and development program of China G


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Engineering the Self-Assembly Induced Emission of Cu Nanoclusters

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