Metal Nanoclusters/Polyvinyl Alcohol Composite Films as the Alternatives for Fabricating Remote-Type White Light-Emitting Diodes

Packing luminescent metal nanoclusters (MNCs) into polymers and fabricating novel MNCs/polymer composite materials is effective in obtaining high-performance light-emitting diodes (LEDs). Herein, water soluble Cu and Au nanoclusters are encapsulated in polyvinyl alcohol (PVA) by a casting method. The obtained MNCs/PVA composite films are highly emissive with triple primary colors, and inherit the merits of PVA, such as transparency, flexibility, machinability, stability and self-healing ability. By employing the MNCs/PVA composite films as down-conversions, remote type monochromic and white LEDs are fabricated. The white LEDs (WLEDs) exhibit a maximum color rendering index (CRI) of 86 with a Commission Internationale de l’Eclairage (CIE) color coordinate of (0.33,0.35). By varying the three MNCs/PVA film arrangement, the correlated color temperature (CCT) of the WLEDs is tuned from 5582 to 9490 K, which signifies the possibility of MNCs/PVA as alternative light-emitting materials for advanced illumination and display in the future.

Metal nanoclusters (MNCs), representatively, Au, Ag and Cu NCs, are one of the ideal candidate lighting materials [13][14][15][16][17]. Composed of several to hundreds of metal atoms, these MNCs possess sizes close to the Fermi wavelength of electrons and exhibit molecular-like optical properties, such as discrete energy levels and strong and tunable photoluminescence (PL) [18][19][20][21]. Additionally, what is even more important is that these MNCs are heavy-metal-free and can be easily obtained. Even so, MNC-based LEDs still have two drawbacks: (i) MNCs possess less PL stability than traditional light-emitting materials, especially when exposed to the stimulus such as heat, oxygen and moisture [22]; (ii) the conventional LED structure is to coat the light-emitting materials embedded in resins onto the UV chips. The direct contact to the UV chips results in continuous heat, and further leads to the phase separation, color inhomogeneity and efficiency deterioration [23][24][25][26][27]. Packaging the MNCs into polymers offers the possibility to overcome the above-mentioned shortcomings [28][29][30].

Preparation of Au-ATT NCs
Au-ATT NCs were synthesized according to the previous report with slight modification. In summary, 5 mL of HAuCl 4 ·4H 2 O (10 mmol/L) and 5 mL of ATT (80 mmol/L) were mixed and stirred for 2 min. Subsequently, 0.05 mol of NaOH solution was added to adjust the pH to weakly alkaline. After stirring in the dark at room temperature for 1 h, the obtained crude product was purified by dialysis. Then, 1 mL of Arg (40 mmol/L) was added to the as-prepared solution and the pH was adjusted to 10. The solution was stirred at 37 • C for 24 h until the Au-ATT NCs with bright green emission was obtained.

Preparation of Au-BSA NCs
To synthesize Au-BSA NCs, 5 mL of HAuCl 4 ·4H 2 O (10 mmol/L) and 5 mL of BSA (50 mg/mL) were mixed and stirred for 2 min. Subsequently, 0.5 mL of NaOH (1 mol/L) was added to adjust the pH to weakly alkaline. The reaction was maintained in a water bath at 37 • C for 24 h, and the obtained crude product was purified by dialysis.

Preparation of MNCs/PVA Composite Film
Totals of 0.5 mL of the as-prepared MNC aqueous solution (5~30 mg/mL) and 1.5 mL of the PVA aqueous solution (mass fraction 5~12%) were mixed at room temperature and stirred for 30 min. Subsequently, the mixture was centrifuged at 2000 rpm for 3 min to remove air bubbles. Then, the mixture was evenly dropped onto a cleaned glass slice and cured at room temperature.

Characterization
Transmission electron microscopy (TEM) was recorded on a JEM-2100F electron microscope (Hitachi, Tokyo, Japan). The dynamic light scattering was measured by a Zetasizer NanoZS (Malvern, Shanghai, China). UV-visible absorption spectra were recorded using a Shimadzu 3100 UV-vis spectrophotometer (Shimadzu, Tokyo, Japan). Fourier-transform infrared (FTIR) spectroscopy were performed with a VERTEX 80V FTIR instrument (Bruke, Karlsruhe, Germany). Photoluminescence spectroscopy was carried out on a Shimadzu RF-5301 PC spectrophotometer (Shimadzu, Tokyo, Japan). X-ray powder diffraction (XRD) investigation was performed on an Empyrean X-ray diffractometer (Malvern, Shanghai, China) using Cu K radiation (λ = 1.5418 Å). Atomic force microscope (AFM) tapping mode measurements were performed on a Bruke scanning probe microscope (Bruke, Madison, WI, USA) using a rotated tapping mode etched silicon probe tip. SEM micrographs were measured by JEOL-FESEM 6700F electron microscope (JEOL, Tokyo, Japan) with a primary electron energy of 3 kV. Before imaging, the samples were sputter-coated with 2 nm Pt. The luminescence decay curves were measured on Edinburgh FLS920 (Techcomp, Shanghai, China) equipped with an integrating sphere (excited at 365 nm). An FESEM 6700F electron microscope (JEOL, Tokyo, Japan) was used to observe the process of self-healing. The spectra of the LEDs were measured at room temperature in the open air with a Spectrascan PR-650 spectrophotometer (JADAK, New York, NY, USA) equipped with an integrating sphere. The CIE (Commission Internationale de L'Eclairage 1931) calorimeter system was used to identify the color of the light.

Preparation and Optical Properties of MNCs/PVA Solutions
To obtain the MNCs/PVA solutions, blue-emissive Cu-MMI NCs, green emissive Au-ATT and red-emissive Au-BSA NCs were foremost synthesized according to previous reports [37][38][39]. All the MNCs are well dispersed with uniform size distribution ( Figure S1). The average diameter of Cu-MMI, Au-ATT and Au-BSA NCs is 2.1, 2.6 and 2.4 nm, respectively. Desired amount of MNCs were then blended with PVA solutions to form homogenous MNCs/PVA solutions, where the MNCs mass fraction is 2.5%. Figure S2 illustrates the absorption and PL emission of the MNCs dispersed in aqueous solution and PVA solution. The photoluminescence emission quantum yields (PLQYs) of both the MNCs are measured to be 6.4, 2.5 and 5.3%, respectively. The MNCs/PVA composite solutions retain the optical properties of the MNCs. Only the spectra of Au-ATT NCs/PVA solution and Au-BSA NCs/PVA solution show a red shift of a few nanometers compared with those of the Au-ATT NCs and Au-BSA NCs aqueous solution, which could be attributed to interactions between the MNCs and PVA molecules and the electronic environment variation of the MNCs (Table S1). As shown in Figure S3 and Table S2, the PL lifetime of the MNCs/PVA solutions is significantly improved compared with the MNCs aqueous solutions. On the incorporation of PVA, due to the interactions, the vibration, rotation and torsion motion of the MNCs are restricted, which reduces the possibility of non-radiation paths and enhances the radiation attenuation, thereby extending the PL lifetime.

Structural and Optical Characterization of the MNCs/PVA Composite Films
MNCs/PVA solutions were dehydrated into films by a casting method. The obtained composite films are freestanding and can be easily peeled off from the substrate. The scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) reveals that all the MNCs/PVA composite films are composed of three main elements, Cu/Au, C and O and the MNCs are homogenously dispersed in the PVA matrix ( Figure 1). The average roughness of the Cu-MMI NCs/PVA, Au-BSA NCs/PVA and Au-ATT NCs/PVA films is 2.57, 1.86 and 1.11 nm, respectively, implying the smooth surface of the composite films ( Figure S4). The roughness of the Cu-MMI NCs/PVA film is a little higher than those of the Au-BSA NCs/PVA and Au-ATT NCs/PVA films, which can be ascribed to the higher degree of aggregation of Cu-MMI NCs in PVA [40][41][42].
non-radiation paths and enhances the radiation attenuation, thereby extending the P time.

Structural and Optical Characterization of the MNCs/PVA Composite Films
MNCs/PVA solutions were dehydrated into films by a casting method. The ob composite films are freestanding and can be easily peeled off from the substrate. The ning electron microscopy (SEM) equipped with energy-dispersive X-ray spectro (EDS) reveals that all the MNCs/PVA composite films are composed of three ma ments, Cu/Au, C and O and the MNCs are homogenously dispersed in the PVA m ( Figure 1). The average roughness of the Cu-MMI NCs/PVA, Au-BSA NCs/PVA an ATT NCs/PVA films is 2.57, 1.86 and 1.11 nm, respectively, implying the smooth s of the composite films ( Figure S4). The roughness of the Cu-MMI NCs/PVA film is higher than those of the Au-BSA NCs/PVA and Au-ATT NCs/PVA films, which c ascribed to the higher degree of aggregation of Cu-MMI NCs in PVA [40][41][42]. To explore the interaction between MNCs and PVA in the composite films, radiation diffraction (XRD) is performed. For the pure PVA film, there is a broad d tion peak centered at 19.3 degrees, corresponding to the spacing of (101), which r the semi-crystalline characteristics of PVA ( Figure S5). The semi-crystalline natu dows PVA with a mass of free voids between the polymeric chains in the amor phase, where the MNCs could become trapped. Compared with pure PVA, the XR tern of MNCs/PVA composites show no appearance of new crystalline peak, but red shifts, revealing that the MNCs are well-dispersed in PVA, and does not chan basic structural configuration of PVA. The distance decrement of the adjacent PVA cules is ascribed to the interactions between the MNCs and PVA, probably with t droxyl groups [43,44]. The weak bonding effect in the MNCs/PVA composite is a firmed by the FTIR analysis. The IR spectra of the MNCs/PVA composite films ma the fundamental vibrational bands of PVA without the appearance of any additiona ( Figure S6). Comparatively, the characteristic stretching vibration peak of the hyd group in PVA at 2947 cm −1 shifts to 2942 in Cu-MMI NCs/PVA, 2942 cm −1 in Au NCs/PVA and 2938 cm −1 in Au-BSA NCs/PVA, respectively [45]. These results in To explore the interaction between MNCs and PVA in the composite films, X-ray radiation diffraction (XRD) is performed. For the pure PVA film, there is a broad diffraction peak centered at 19.3 degrees, corresponding to the spacing of (101), which reflects the semi-crystalline characteristics of PVA ( Figure S5). The semi-crystalline nature endows PVA with a mass of free voids between the polymeric chains in the amorphous phase, where the MNCs could become trapped. Compared with pure PVA, the XRD pattern of MNCs/PVA composites show no appearance of new crystalline peak, but slight red shifts, revealing that the MNCs are well-dispersed in PVA, and does not change the basic structural configuration of PVA. The distance decrement of the adjacent PVA molecules is ascribed to the interactions between the MNCs and PVA, probably with the hydroxyl groups [43,44]. The weak bonding effect in the MNCs/PVA composite is also affirmed by the FTIR analysis. The IR spectra of the MNCs/PVA composite films maintain the fundamental vibrational bands of PVA without the appearance of any additional peak ( Figure S6). Comparatively, the characteristic stretching vibration peak of the hydroxyl group in PVA at 2947 cm −1 shifts to 2942 in Cu-MMI NCs/PVA, 2942 cm −1 in Au-ATT NCs/PVA and 2938 cm −1 in Au-BSA NCs/PVA, respectively [45]. These results indicate that no covalent bonds form between the MNCs and PVA and the incorporation of MNCs hardly affects the basic PVA structure, which is consistent with the XRD result.
The MNCs/PVA composite films are so transparent that printed letters beneath the MNCs/PVA films can be clearly seen (Figure 2a-c). With a 0.5% mass fraction of MNCs, the transmission of the three MNCs/PVA exceeds 70% in both the ultraviolet and visible range. When the MNC mass fraction is increased up to 2.5%, Cu-MMI NCs/PVA films and Au-ATT NCs/PVA films achieve a transmission over 80% in the entire visible range, whereas the transmission of the Au-BSA NCs/PVA film remains over 70% in the range of 450 to 900 nm (Figure 2a-c). The minor absorption of visible light is beneficial for the fabrication of light-emitting diodes (LEDs). With the increment of the MNCs mass content, the PL intensity of the three MNCs/PVA composite films increase linearly, accompanied by the slight red shift of the PL peak position, which originates from the growing degree of MNCs aggregation (Figure 2d-f, Figures S1 and S7). The PLQYs of the MNCs/PVA films are 11.4, 10.4 and 9.9%, and the average PL lifetimes are 10.7, 11.2 and 11.8 µs, respectively, which are significantly higher and longer than those of the corresponding MNCs ( Figure S8). Based on the PLQYs and PL lifetimes, the radiative rate constant (k r ) and nonradiative rate constant (k nr ) of the MNCs and MNCs/PVA films are calculated (Table S3). After packing into the PVA films, the k r of MNCs is obviously promoted, whereas the k nr is inhibited. These phenomena are ascribed to the further vibration and rotation motion restriction of the MNCs in the PVA films.
hardly affects the basic PVA structure, which is consistent with the XRD result.
The MNCs/PVA composite films are so transparent that printed letters beneath the MNCs/PVA films can be clearly seen (Figure 2a-c). With a 0.5% mass fraction of MNCs, the transmission of the three MNCs/PVA exceeds 70% in both the ultraviolet and visible range. When the MNC mass fraction is increased up to 2.5%, Cu-MMI NCs/PVA films and Au-ATT NCs/PVA films achieve a transmission over 80% in the entire visible range, whereas the transmission of the Au-BSA NCs/PVA film remains over 70% in the range of 450 to 900 nm (Figure 2a-c). The minor absorption of visible light is beneficial for the fabrication of light-emitting diodes (LEDs). With the increment of the MNCs mass content, the PL intensity of the three MNCs/PVA composite films increase linearly, accompanied by the slight red shift of the PL peak position, which originates from the growing degree of MNCs aggregation (Figures 2d-f, S1 and S7). The PLQYs of the MNCs/PVA films are 11.4, 10.4 and 9.9%, and the average PL lifetimes are 10.7, 11.2 and 11.8 μs, respectively, which are significantly higher and longer than those of the corresponding MNCs ( Figure  S8). Based on the PLQYs and PL lifetimes, the radiative rate constant (kr) and nonradiative rate constant (knr) of the MNCs and MNCs/PVA films are calculated (Table S3). After packing into the PVA films, the kr of MNCs is obviously promoted, whereas the knr is inhibited. These phenomena are ascribed to the further vibration and rotation motion restriction of the MNCs in the PVA films.

PL Stability of MNCs/PVA Composite Films
PL stability is an important parameter for the practical application of photovoltaic and optoelectronic devices. Generally, MNCs are sensitive to environmental stimulus, which need to be stored in the dark at 4 • C. Encapsulating MNCs into polymers can effectively improve the PL stability of MNCs to the environment. In Figure 3a, the MNCs/PVA composite films exhibit negligible PL intensity loss after stored in an open environment Nanomaterials 2022, 12, 204 6 of 11 (28 • C, humidity = 19%) for 14 days, and less than 20% PL intensity loss is recorded when the MNCs/PVA composite films are stored for 28 days, showing their good stability under ambient conditions. The MNCs/PVA composite films are heated to 80 • C in air to explore the thermal stability (Figure 3b). After heating for 10 h, more than 90% of PL intensity of the Au-ATT NCs/PVA and Au-BSA NCs/PVA composite films are preserved and the PL intensity of the Cu-MMI NCs/PVA composite film is even improved beyond the initial intensity. As shown in Figure 3c, after irradiation under 365 nm for 10 h, only minor changes in PL intensity of the MNCs/PVA composite films are recorded and more than 90% of PL intensity is preserved. Such good stability is attributed to the protection of PVA encapsulation to isolate the MNCs from the external environment, which is beneficial for the use of MNCs/PVA composite films in LEDs.

PL Stability of MNCs/PVA Composite Films
PL stability is an important parameter for the practical application of photovoltaic and optoelectronic devices. Generally, MNCs are sensitive to environmental stimulus, which need to be stored in the dark at 4 °C. Encapsulating MNCs into polymers can effectively improve the PL stability of MNCs to the environment. In Figure 3a, the MNCs/PVA composite films exhibit negligible PL intensity loss after stored in an open environment (28 °C, humidity = 19%) for 14 days, and less than 20% PL intensity loss is recorded when the MNCs/PVA composite films are stored for 28 days, showing their good stability under ambient conditions. The MNCs/PVA composite films are heated to 80 °C in air to explore the thermal stability (Figure 3b). After heating for 10 h, more than 90% of PL intensity of the Au-ATT NCs/PVA and Au-BSA NCs/PVA composite films are preserved and the PL intensity of the Cu-MMI NCs/PVA composite film is even improved beyond the initial intensity. As shown in Figure 3c, after irradiation under 365 nm for 10 h, only minor changes in PL intensity of the MNCs/PVA composite films are recorded and more than 90% of PL intensity is preserved. Such good stability is attributed to the protection of PVA encapsulation to isolate the MNCs from the external environment, which is beneficial for the use of MNCs/PVA composite films in LEDs.

Fabricability and Self-Healing Ability of the MNCs/PVA Composite Films
Due to the flexible and stretchable nature of PVA, it is easy for the MNCs/PVA composite films to be cast into any shapes with specific sizes. As shown in Figure 4a

Fabricability and Self-Healing Ability of the MNCs/PVA Composite Films
Due to the flexible and stretchable nature of PVA, it is easy for the MNCs/PVA composite films to be cast into any shapes with specific sizes. As shown in Figure 4a, a large square Au-BSA NCs/PVA film with a size of 16 cm × 16 cm, a coil of Au-ATT NCs/PVA composite film and a disk of Cu-MMI NCs/PVA composite film are fabricated. The three shaped MNCs/PVA composite films emit bright red, green and blue light, respectively, preserving their initial emission properties (Figure 4b).
Moreover, the MNCs/PVA films also inherit the self-healing properties of PVA. As illustrated in Figure 4c, to repair the 10 µm wide incision on the Cu-MMI NCs/PVA film, 1 µL of water is dropped onto the incision. The incision narrows immediately and completely disappears in 5 min without external heating. The addition of the trace water promotes the intermolecular hydrogen bonding between adjacent PVA molecules and thus inducing the self-healing process. Based on the self-healing ability, two separated slices of 1 cm × 1 cm Cu-MMI NCs/PVA film and Au-BSA NCs/PVA film are assembled through side-by-side arrangement and assistance of water, demonstrating the possibility of the MNCs/PVA films to build multicolor pixel arrays (Figure 4d-g) [46]. 1 μL of water is dropped onto the incision. The incision narrows immediately and completely disappears in 5 min without external heating. The addition of the trace water promotes the intermolecular hydrogen bonding between adjacent PVA molecules and thus inducing the self-healing process. Based on the self-healing ability, two separated slices of 1 cm × 1 cm Cu-MMI NCs/PVA film and Au-BSA NCs/PVA film are assembled through side-by-side arrangement and assistance of water, demonstrating the possibility of the MNCs/PVA films to build multicolor pixel arrays (Figure 4d-g) [46].

Application of MNCs/PVA Composite Films in Remote-Type LEDs
Based on the properties discussed above, the MNCs/PVA composite films are considered to be excellent down-converter alternatives for LED application. Since the MNCs/PVA composite films are free standing, a remote-type LED architecture is applicable to avoid the direct contacts between the down-converters and the GaN chip, minimize the operating temperature and reduce phase segregation between MNCs and PVA, thus improving the LED performance ( Figure S9). As shown in Figure 5a

Application of MNCs/PVA Composite Films in Remote-Type LEDs
Based on the properties discussed above, the MNCs/PVA composite films are considered to be excellent down-converter alternatives for LED application. Since the MNCs/PVA composite films are free standing, a remote-type LED architecture is applicable to avoid the direct contacts between the down-converters and the GaN chip, minimize the operating temperature and reduce phase segregation between MNCs and PVA, thus improving the LED performance ( Figure S9). As shown in Figure 5a To fabricate the WLED, triple-layer films are utilized together as the down-converter. The three composite films are arranged with a R-G-B sequence from bottom up, where R represents red-emissive Au-BSA NCs/PVA film, G represents green-emissive Au-ATT NCs/PVA film and B represents blue-emissive Cu-MMI NCs/PVA film. Figure 5g exhibits the photograph and spectrum of the WLED. The chromaticity coordinate of the WLED is (0.33, 0.35), which is close to the pure white light coordinate (0.33, 0.33). The color rendering index (CRI) and correlated color temperature (CCT) of the WLED is 86 and 5582 K, respectively, with a luminance of 89.36 cd/m 2 . Note that the low efficiency is mainly Nanomaterials 2022, 12, 204 8 of 11 due to the poor quality of the GaN chip. As is known to all, reabsorption exists between different emitters with absorption and emission overlaps. Based on the reabsorption effect, various composite film arrangements are attempted to tune the property of the WLED ( Figure S10). Apart from the R-G-B sequence, other five combinations based on the three MNCs/PVA films are list in Table S4. The emission of the WLEDs is tuned from pure white to cold white, with CCT ranging from 5582 to 9490 K (Table S4).  To fabricate the WLED, triple-layer films are utilized together as the down-converter. The three composite films are arranged with a R-G-B sequence from bottom up, where R represents red-emissive Au-BSA NCs/PVA film, G represents green-emissive Au-ATT NCs/PVA film and B represents blue-emissive Cu-MMI NCs/PVA film. Figure 5g exhibits the photograph and spectrum of the WLED. The chromaticity coordinate of the WLED is (0.33, 0.35), which is close to the pure white light coordinate (0.33, 0.33). The color rendering index (CRI) and correlated color temperature (CCT) of the WLED is 86 and 5582 K, respectively, with a luminance of 89.36 cd/m 2 . Note that the low efficiency is mainly due to the poor quality of the GaN chip. As is known to all, reabsorption exists between different emitters with absorption and emission overlaps. Based on the reabsorption effect, various composite film arrangements are attempted to tune the property of the WLED ( Figure S10). Apart from the R-G-B sequence, other five combinations based on the three MNCs/PVA films are list in Table S4. The emission of the WLEDs is tuned from pure white to cold white, with CCT ranging from 5582 to 9490 K (Table S4).

Conclusions
In summary, we succeeded in fabricating free-standing Cu-MMI NCs/PVA, Au-ATT NCs/PVA, Au-BSA NCs/PVA composite films by a simple casting method. The MNCs/PVA composite films are transparent and can emit three primary colors under 365 nm irradiation, respectively. Moreover, the MNCs/PVA composite films are easily processed into desired shapes and sizes, and exhibit excellent stability and self-healing ability. A remote-type WLED prototype with CIE coordinate of (0.33, 0.35) and high CRI of

Conclusions
In summary, we succeeded in fabricating free-standing Cu-MMI NCs/PVA, Au-ATT NCs/PVA, Au-BSA NCs/PVA composite films by a simple casting method. The MNCs/PVA composite films are transparent and can emit three primary colors under 365 nm irradiation, respectively. Moreover, the MNCs/PVA composite films are easily processed into desired shapes and sizes, and exhibit excellent stability and self-healing ability. A remote-type WLED prototype with CIE coordinate of (0.33, 0.35) and high CRI of 86 is further fabricated by employing the MNCs/PVA composite films as down-converters, the CCT of which could be tuned through various MNCs/PVA film arrangements. Due to the nontoxicity and ease of processing, this work may promote the practical application of the luminescent MNCs/PVA composite films as down-converter alternatives for lighting.