Article pubs.acs.org/cm
Cite This: Chem. Mater. 2017, 29, 10206−10211
In Situ Fabrication of Flexible, Thermally Stable, Large-Area, Strongly Luminescent Copper Nanocluster/Polymer Composite Films Zhenguang Wang,†,‡ Yuan Xiong,‡ Stephen V. Kershaw,‡ Bingkun Chen,§ Xuming Yang,‡ Nirmal Goswami,∥ Wing-Fu Lai,⊥,# Jianping Xie,∥ and Andrey L. Rogach*,‡ †
The Key Laboratory of Life-Organic Analysis, Key Laboratory of Pharmaceutical Intermediates and Analysis of Natural Medicine, School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong 273165, P. R. China ‡ Department of Materials Science and Engineering & Centre for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR § Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China ∥ Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive, Singapore 117585 ⊥ School of Pharmaceutical Sciences, Health Science Center, Shenzhen University, Shenzhen 518060, P. R. China # Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR ABSTRACT: Composite polymer films incorporating luminescent copper nanoclusters are conveniently fabricated by solution processing of a precursor mixture of polyvinylpyrrolidone, poly(vinyl alcohol), a Cu2+ salt, and glutathione in water. The formation of Cu nanoclusters takes place through chemical reactions of the precursors in a hydrogel network, whose dehydration during the subsequent aging process leads to aggregationinduced emission enhancement. Large-area (25 cm × 32 cm), flexible, transparent, thermally stable, bright-orange-emitting films are realized, with the photoluminescence quantum yield reaching 30%. The emission of the films is attributed to a phosphorescence mechanism, taking place through ligand-to-metal charge transfer followed by a radiative relaxation over a triplet state. Cu-nanocluster-based composite films are employed as a lightemitting layer for down-conversion orange light-emitting diodes. alternative in this respect,14 in particular as it helps to avoid the phase segregation and deterioration of the PL QY. Recently reported examples include composite luminescent films of CdSe QDs synthesized in situ in a silica matrix,15 and lead halide perovskite QDs grown in situ in the framework of different polymers, such as polystyrene,16 polycarbonate,16 polyvinylidene fluoride,5 and cross-linked 5-aminovaleric acid.17 However, the use of heavy metals (Cd and Pb) and organic solvents for the production of such luminescent components of composite films raises potential environmental concerns. Here, we propose a strategy to fabricate composite polymer films with incorporated Cu NCs which are grown in situ in a 3D hydrogel network formed by cross-linked polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA) molecules in aqueous solution. This approach avoids the use of any heavy metals elements (Cd, Hg, and Pb) and toxic, expensive organic solvents, and allows us to fabricate large-area films (25 cm × 32 cm as demonstrated here) exhibiting bright-orange PL with QY up to
1. INTRODUCTION Polymer-based luminescent films are frequently used in sensors,1,2 solar concentrators,3,4 and light-emitting diodes (LEDs).2,5−7 Fabrication of such composite films often involves two processes: (i) synthesis of the luminescent component, such as an organic dye, semiconductor quantum dots (QDs), or metal nanoclusters (NCs) in solution, and (ii) dispersion of these in a monomer or a dissolved polymer, followed by polymerization/evaporation of the solvent.8−10 However, widespread application of this approach is in part hindered by several limitations: (i) sometimes poor dispersion of luminescent components in the polymer, leading to the phase segregation which makes it difficult to fabricate large-area, uniformly luminescent films; (ii) deterioration of the photoluminescence (PL) quantum yield (QY) of the luminescent component during the fabrication process of the film; (iii) poor PL stability, particularly when the films are exposed to heat, oxygen, or moisture.11−13 Another approach, namely, in situ fabrication of composite luminescent films, where the luminescent components are directly formed within the polymer matrix as a result of a chemical reaction or thermal processing, offers a useful © 2017 American Chemical Society
Received: October 9, 2017 Revised: November 15, 2017 Published: November 30, 2017 10206
DOI: 10.1021/acs.chemmater.7b04239 Chem. Mater. 2017, 29, 10206−10211
Article
Chemistry of Materials
hydrogels occurs, inducing the aggregation of Cu NCs inside the formed polymer network. GSH serves as both the reducing agent and a ligand for the formation of Cu NCs.20,21 Composite films were obtained by drop-casting of the precursor solution onto a glass substrate, followed by treatment in a vacuum oven at 40 °C for 10 h (Figure 1a). Transparent and free-standing films can be easily
30%, attributed to the aggregation-induced emission (AIE) enhancement. The resulting Cu NC/polymer composite films are flexible and show high thermal stability; they have been used to fabricate prototype down-conversion LEDs with orange emission.
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals including copper(II) nitrate [Cu(NO3)2], glutathione (GSH), polyvinylpyrrolidone (PVP; average molecular weight, 40 000), and poly(vinyl alcohol) (PVA) with a molecular weight of 10 000 were purchased from Sigma-Aldrich. Components of silicone resin OE-6551A and OE-6551B were purchased from Dow Corning Co. GaN LED chips of surfacemounted device type, with a peak emission wavelength centered at 380 nm, were obtained from EPILED Co., Ltd. 2.2. In Situ Fabrication of the Cu NC Composites Films. PVA was dissolved in distilled water at 80 °C at a concentration of 60 mg/ mL. A 6 mL portion of the aqueous solution of PVA was mixed with 1 mL of PVP (60 mg/mL), under stirring. A 2.5 mL portion of GSH (0.1 M) was added into the polymer mixture, and subsequently 0.5 mL of Cu(NO3)2 (0.1 M) was poured in. The mixture was dropped onto the surface of cleaned glass slides, which were then kept in vacuum at 40 °C for 10 h. 2.3. Characterization. Absorption and PL spectra of the films were recorded by using a Varian Cary 50 UV−vis spectrophotometer and Varian Cary Eclipse fluorescence spectrometer, respectively. Timeresolved PL lifetime curves at different excitation wavelengths were measured on a time-correlated single-photon counting setup. The absolute PL QY was determined by using an integrating sphere (Edinburgh Instruments) with its inner surface coated with BENFLEC, attached to the spectrofluorimeter FLS920P (Edinburgh Instruments). SEM images were obtained on an environmental scanning electron microscope (SEM, FEI/Philips XL30). TEM images were recorded on a Philips CM 20 microscope operating at 200 kV. An ESCALAB-MKII 250 photoelectron spectrometer (Thermo) was employed to obtain the X-ray photoelectron spectroscopy (XPS) data. TGA curves were recorded on an SDT-Q600 instrument, under nitrogen atmosphere. Parameters of LEDs such as color coordinates were measured using a spectroradiometer (Haas-2000, Everfine Co., Ltd.).
Figure 1. (a) Schematic illustration of the in situ fabrication method of Cu NC/polymer composite films. (b) SEM images. (c) TEM images. (d) XPS spectra of the composite films. (e) Transmittance spectra of the Cu NC/polymer composite film (black) and of the bare PVP/PVA film (red), inset shows the photograph of a film with a size bigger than an A4 sheet of paper. (f) PL (solid) and PLE (dotted) spectra of Cu NC/polymer composite film. (g) Photograph of an orange-emitting Cu NC/polymer composite film under UV light.
peeled off from the glass substrate after the evaporation of solvent. The morphology of the films was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. As shown in Figure 1b, uniformly distributed islandlike structures are observed on the SEM images, which can be attributed to a tropistic arrangement of the polymer chains during the formation of hydrogels with 3D structure, cross-linked by PVP and PVA, as well as during water evaporation in the subsequent aging process. With the zoom-in image of the islandlike structures using high-resolution TEM (HRTEM), small Cu NCs (average diameter of 2 nm) are observed (Figure 1c). The formation of Cu NCs in the film has been further demonstrated by X-ray photoelectron spectroscopy (XPS), which confirmed the existence of all the expected elements, including C, O, N, S, and Cu. For a study of the valence states of Cu, a high-resolution XPS spectrum of Cu was recorded (Figure 1d). The absence of satellite peaks at around 942 eV indicates the reduction of Cu2+.22−27 Two peaks at 932 and 952 eV can be assigned to Cu 2p3/2 and Cu 2p1/2 states of Cu(0)/ Cu(I), consistent with previously the reported literature data, which confirms the presence of Cu NCs in the composite film.23,28−30 Compared with the bare PVA/PVP films, composite Cu NC/polymer films maintain high transparency (higher than 80%) in the wavelength range 450−900 nm (Figure 1e). Intense orange emission (peaked at 600 nm) was observed, with a PL excitation (PLE) spectrum peaked at 330 nm (Figure 1f), which are characteristic optical properties of Cu NCs with AIE.21,31 For the establishment of optimized conditions for the
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Cu NC/Polymer Composite Films. A transparent precursor solution was obtained by codissolving PVP, PVA, a Cu2+ salt, and glutathione (GSH) in water. PVP and PVA are hydrophilic and biodegradable polymers, which have been approved by the USA Food and Drug Administration (FDA) for medical applications.18 Importantly, mixtures of PVP and PVA have been reported to form 3D hydrogels, which have been previously employed for drug delivery.19 Hydrogels of PVP/ PVA typically contain a high amount of water, providing space for water-soluble molecules or nanoparticles to be entrapped. Generation of these hydrogels has been achieved mainly by irradiation cross-linking or by repeated freezing and thawing.19 In this study, hot PVA solution was mixed directly with the solution PVP and other components (e.g., Cu2+ and GSH) for film formation. As the temperature of the mixture is reduced, there is also a reduction in the molecular motion of the polymer chains. Such intermolecular interactions between the hydroxyl groups of PVA molecules and the carbonyl groups of PVP chains have previously been reported to lead to small crystallite formation. These crystallites can function as crosslinks, enabling the formation of the 3D network structure. After the process of hydrogel formation, the aging process follows. In the course of the evaporation of water, dehydration of 10207
DOI: 10.1021/acs.chemmater.7b04239 Chem. Mater. 2017, 29, 10206−10211
Article
Chemistry of Materials PL intensity of the composite films, variations of the mass ratio of PVP to PVA and the molar ratio of Cu2+ to GSH have been studied. Under the ratio of PVP to PVA equal to 6:1, and Cu2+ to GSH equal to 1:5, an absolute PL QY of 30% was achieved under excitation wavelength of 385 nm, which is comparable to most of the luminescent films based on traditional semiconductor QDs.1 However, our Cu NC/polymer composite films are fabricated in aqueous solution, and use of any heavy or expensive metal elements and toxic organic solvents is avoided. Adding the proper amount of PVP was also vital for the ability to fabricate stretchable, free-standing films. This was confirmed by control experiments, where a small amount of PVP in the composite mixture results in viscous films which firmly adhered to the glass substrate, while a large amount of PVP leads to the formation of fragile composite films. At the optimal PVP/PVA ratio, free-standing flexible films were obtained, which could reach more than 300% of strain before tensile fracture with a tensile stress of 4 MPa. The in situ fabrication method can be easily adapted to the fabrication of large-area composite films, such as those with a size of 25 cm × 32 cm, bigger than A4 paper size (shown in Figure 1e, inset), which are characterized by bright and uniform orange emission (Figure 1g). 3.2. Monitoring of Dehydration-Induced AIE Processes. The process of in situ growth of Cu NCs in hydrogels was further investigated by applying PL spectra and PL decay measurements during the vacuum treatment of the precursor solution at elevated temperature. PL spectra and PL decay curves of the film treated for different time intervals are presented in Figure 2a,b, respectively. An increase of PL intensity, a blue shift of the PL maximum, and a prolonged average PL lifetime are observed with the increase of the heattreatment time. All of these findings support the occurrence of the AIE effect, where the freezing of intramolecular motions of ligands is the major factor influencing the luminescence efficiency.32 The aggregation process, caused by intermolecular interactions, such as π−π stacking, H-bonding, etc., hinders the flexibility of the ligands on the Cu NCs; as a result, the nonradiative decay channels caused by intramolecular motions are suppressed, and the luminescence intensity becomes enhanced.33,34 For the Cu NCs under study, those distributed within the hydrogel before the dehydration experience free movement of the surface thiolate ligands present on the NC surface, which means that the intramolecular motions of the ligands remain active, and the samples experience relatively low PL intensity. During the thermal treatment of the hydrogel, it becomes compact and rigid, which restricts the movement of the surface ligands thus resulting in the stronger luminescence. In addition, there is a possibility that the intercluster interactions become stronger through the formation of Cu(I)···Cu(I) bonds. It is well-understood in the noble metal (M) cluster literature that such clusters consist of a core built up from M(0) atoms surrounded by a shell consisting of M(I) atoms bound in turn to the ligands, so that their surface is dominated by the latter shell.35,36 Therefore, during the thermal treatment of the composite films, intercluster interactions through Cu(I)···Cu(I) bonds are a distinct possibility. The formation of such intercluster bonds imparts more rigidity in the aggregated state, so that again the PL intensity is enhanced. At the same time, simultaneous weakening of the intracluster Cu(I)···Cu(I) bonds would result in the PL shift toward higher energy, as observed for the system under study (Figure 2a). The AIE effect in the Cu NCs is summarized in the simplified Jablonski diagram shown in Figure 2c.37,38 The long
Figure 2. Evolution of PL spectra (a) and PL decay curves (b) of Cu NC/polymer composite films obtained with different thermal treatment times, as indicated on the frames. (c) Jablonski diagram illustrating the possible origin of the PL enhancement of Cu NC/ polymer composite films. S0, S1, S2, and T1 represent the ground state, the first singlet excited state, the second singlet excited state, and the first triplet state, respectively. (d) PL QY (black symbols) along with the trends in the radiative recombination lifetime (red squares) and nonradiative recombination lifetime (red triangles) of the Cu NC/ polymer composite films under different excitation energies. (e) PL spectra of Cu NC/polymer composite films under different excitation energies (as indicated on the frame, in eV).
PL lifetime (microseconds) and the large Stokes shift (>250 nm) suggest that the emission happens through a phosphorescence mechanism. Upon photoexcitation, the excited state rapidly relaxes to the first singlet excited state (S1), followed by a transition to the first triplet state (T1) through intersystem crossing. Both the S1 and T1 states exhibit significant charge transfer characteristics, where the intramolecular vibration and rotation of ligands on the Cu NCs lead to efficient nonradiative relaxation.37,39 After the formation of the composite film, the intramolecular motions of ligands are suppressed, reducing the chance of the nonradiative relaxation and resulting in an enhanced PL QY.31 3.3. Excitation-Energy-Dependent PL Properties. Excitation-energy-dependent PL QY is a rather often observed phenomenon for semiconductor QDs; in most reported cases, an increase in excitation energy is accompanied by a decrease in the PL QY,40,41 which has been ascribed to the predominance of nonradiative relaxation pathways taking place when the charge carriers relax down to the band edge from the high initial excitation energy levels.40 However, luminescent materials with high PL QY independent or weakly dependent on the excitation energy would be better suited for optoelectronic devices. The PL QY of the Cu NC/polymer composite films measured as a function of the excitation energy varying from 3.06 to 3.82 eV is presented in Figure 2d. Even though it also experiences a decreasing trend, namely, from 30% at 3.06 eV down to 24% at 3.82 eV, it appears as rather 10208
DOI: 10.1021/acs.chemmater.7b04239 Chem. Mater. 2017, 29, 10206−10211
Article
Chemistry of Materials
Table 1. PL Lifetimes (τ1−4, μs) and Fractions of the Emission Intensity (f1−4, %) Obtained from the Fitting of Experimental PL Decay Data by Multiexponential Functions for the Cu NC Composite Films, Under Different Excitation Wavelength, Together with Calculated Average PL Lifetime (τaverage, μs) excitation wavelength (nm) 325 335 345 355 365 375 385 395 405
τ1 ( f1) 4.4 3.6 3.3 2.2 3.1 2.5 2.8 3.2 3.2
(2.9) (1.1) (1.3) (0.7) (1.4) (0.5) (1.5) (2.5) (3.0)
τ2 ( f 2) 16.3 17.0 17.0 17.1 17.0 17.0 16.4 16.1 15.7
(77.5) (86.0) (85.9) (88.2) (88.8) (92.2) (86.7) (83.7) (83.5)
moderate as compared with the previously reported cases of the commonly used core-only (CdSe) and core−shell CdSe/ZnS semiconductor QDs. For further understanding of the mechanisms behind the minor excitation-energy-dependent PL QY of the Cu NC/polymer composite films, radiative and nonradiative PL lifetimes were calculated from the values of PL QYs and time-resolved PL decay measurements. The measured average decay times have a relatively flat excitation-energy dependence, with values ranging from 17.7 to 20.3 ns over the excitation wavelength range 325−405 nm (Table 1). The radiative lifetimes, as shown in Figure 2d, exhibit a slight increase (from 58.2 to 80.5 μs), and almost constant nonradiative lifetimes (around 26 μs) are observed with the increase of the excitation energy. The radiative lifetime trend is driven by the variation in the PL QY. One caveat must be raised: As the excitation wavelength is moved closer to the emission peak, in practice the lower wavelength tail of the peak overlaps slightly with weak scattering of the excitation light, tending to slightly inflate the estimate of the integrated PL for the measurements excited closest to the emission. By comparing the emission peak profiles, we estimate a worstcase overestimate of the PL QY in those instances of up to +2%, while PL QYs determined with lower excitation energies were unaffected. Nonetheless, most of the variation in the radiative lifetimes with excitation energy remains. Comparison of the normalized PL peak line shapes reveals a shift to longer wavelength of approximately 10 nm on raising the excitation energy from 325 to 405 nm (Figure 2e). The inference is that states accessed at higher excitation energies have more scope for coupling to surface adjacent ligands and other organic molecules, leading to a reduction in the PL QY. One factor in the increased surface coupling would be the slowing in the radiative rate which may be driven by longer relaxation times to the emissive state, allowing longer times for coupling to occur. 3.4. Thermal Stability of Cu NC/Polymer Composite Films. For optoelectronic applications, the stability of luminescent films which are exposed to elevated temperatures is a key consideration.8,20,42 The thermal stability of Cu NC/ polymer composite films was studied when heated in air at 85 and 100 °C. After heating for 96 h at 85 °C, only negligible changes of the PL intensity (
