Layer-by-layer assembled PMMA-SH/CdSe–Au nanocomposite thin films and the optical limiting property

Abstract

PMMA-SH/CdSe–Au nanocomposite thin films have been constructed by a layer-by-layer (LBL) assembly method. The LBL assembly process is carried out in a nonpolar solvent by the combination of photopolymerization and adsorption of CdSe–Au nanoparticles. Absorption spectra suggest that the LBL assembly is performed in a stepwise and uniform way. The optical, morphological, thermal, and optical limiting properties of the resultant PMMA-SH/CdSe–Au nanocomposite thin films are characterized by transmission, TEM, TGA and laser measurements. The LBL assembled PMMA-SH/CdSe–Au nanocomposite thin films exhibit good thermal stability, transparency, and optical limiting response to a 532 nm pulsed laser. The optical limiting threshold of the PMMA-SH/CdSe–Au nanocomposite thin film is 13 J cm⁻². This study provides a robust and efficient strategy for fabricating transparent polymeric thin films with laser optical limiting property.

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Introduction

Organic/inorganic nanocomposites with novel properties have attracted considerable attention from the research community due to the potential applications in the areas of opto-electronics,^1–3 sensing,⁴ thermoelectric,⁵ and biomedical devices.^6,7 To fabricate nanocomposites with evenly dispersed nanoparticles (NPs), numerous methods have been widely explored, including blending,⁸ in-site synthesis,⁹ in-site polymerization,¹⁰ and plasma chemical deposition.¹¹ Compared with these typical methods, layer-by-layer (LBL) assembly is considered to be a robust and versatile strategy for preparing the highly homogeneous organic/inorganic nanocomposite films with controlled thickness, composition, morphology and functionality onto various substrates.^12–14 Traditional LBL assembly was performed in aqueous solution via sequentially adsorbing of oppositely charged components.^13,15 Aqueous LBL nanocomposite films have been applied to a wide range of opto-electronic, Micro-Electro-Mechanical System (MEMS), and biomedical devices.^16–20 Nevertheless, it's not suitable for the direct buildup of nanocomposite films with uncharged NPs and polymers. Normally, additional ligand exchange is needed to transfer the NPs from nonpolar solvent to aqueous soultion,²¹ which will increase the complexibility of the LBL assembly process, and even deteriorate the properties of the resultant nanocomposite films.^22,23 Consequently, it's necessary to develop a facile and robust protocol for the growth of LBL films in nonpolar media.

Recently, several research groups have explored diverse strategies for incorporating functional NPs into nanocomposite films via LBL assembly in nonpolar media. Zhang et al. performed LBL assembly of Au/polyimide composite thin film in N,N-dimethylacetamide through the direct bounding of bare Au NPs to amine groups.²⁴ Lee et al. realized LBL assembly of charged particles in toluene via the assistance of surfactant AOT.²⁵ Cho et al. carried out LBL assembly of a series of nanocomposite thin films in nonpolar media employing photo-crosslinking, nucleophilic substitution, and ligand addition, respectively.^26–28 Very recently, we have demonstrated the LBL assembly of fluorescent free-standing nanocomposite thin film via the adsorption of CdSe–CdS NPs on thiol groups in toluene.²⁹ By the combination of spin-coating and photo-crosslinking, the nanocomposite film exhibits good transparency and fluorescent properties, which is significant for preparing optical devices. These results promote us to incorporate our recently achieved optical limiters, i.e. CdSe–Au NPs,³⁰ into nanocomposite thin films by implementing LBL assembly in nonpolar media to construct polymeric thin films with good optical limiting property, which will be prospective in fabricating novel optoelectronic devices.

In this study, we demonstrated a versatile and facile method to fabricate PMMA-SH/CdSe–Au nanocomposite thin films through LBL assembly in toluene. The LBL assembly was via the affinity interaction between CdSe–Au NPs and thiol groups in the photopolymerized PMMA-SH film. The LBL assembly was proved to be a stepwise and uniform process according to the absorption spectra. The nanocomposite thin films exhibit good transparency, thermal stability, and optical limiting property. The present work provides a robust strategy for the incorporation of uncharged NPs into thin films via LBL assembly in nonpolar media, and suggests the potential application in fabricating functional optoelectronic devices such as laser optical limiting devices.

Experimental section

Materials

1-Octadecene (ODE, tech. 90%), tetraoctylamine bromide (TOAB), and 3-mercaptopropionic acid (MPA, 99%) were purchased from Chemical Co., Ltd. Benzil and 2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone (BDMBP) were purchased from Sigma-Aldrich. Pemaerythritol-triacrylate (PE-3A) was purchased from Kyoeisha Chemical Co. Ltd., Japan. Highly oriented pyrolytic graphite (HOPG, grade ZYB) was purchased from MicroMasch Inc. Chloroauric acid (HAuCl₄·4H₂O, AR) was purchased from Shanghai July Chemical Co., Ltd. Selenium dioxide (SeO₂, AR) was purchased from Zhengzhou Kefeng Chemical reagent Co., Ltd. Anhydrous methanol (CH₃OH, AR), anhydrous ethanol (CH₃CH₂OH), sodium hydroxide (NaOH, AR), methyl methacrylate (MMA, AR), acetone (CH₃COCH₃, AR), toluene (C₆H₅CH₃, AR), myristic acid (MA, AR), oleic acid (OA, 90%), cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O, AR), anhydrous magnesium sulfate (MgSO₄, AR), sulfuric acid (H₂SO₄, 98%), methyl methacrylate (MMA, AR), 1,1,1-tris(hydroxymethyl) propane (TMP, AR) and dodecyl mercaptan (DDM, AR) were purchased from Sinopharm chemical reagent Beijing Co., Ltd. All of the chemical agents were used without further purification.

Synthesis of CdSe–Au nanoparticles

CdSe–Au nanoparticles (NPs) were synthesized according to a reported method.^30,31 First, CdSe nanocrystals (NCs) were synthesized and purified by the reported procedure.^32,33 In a typical synthesis, cadmium myristate (1 mmol, see ESI†), SeO₂ (1 mmol), and ODE (63 mL) were mixed in a 150 mL three-neck flask, and heated in the ambient condition to 240 °C. Three minutes after the temperature reached to 240 °C, 1 mL of OA was added dropwise into the reaction solution to stabilize the CdSe NCs. The resultant CdSe NCs were precipitated from the cooled reaction solution with acetone and dispersed in toluene. Second, CdSe–Au NPs were synthesized and purified according to the previously reported protocol.³⁰ Different amounts of Au^(I) toluene solution (see ESI†) were added to CdSe toluene solution under vigorous stirring. The reaction was completed by continuous stirring for 30 minutes. The resultant CdSe–Au NPs were precipitated using a large amount of ethanol and collected by centrifugation and decantation. The CdSe–Au NPs were then dissolved in toluene and used directly for further characterization and LBL assembly.

Preparation of photopolymerizable resin

Photopolymerizable resin containing thiol monomer was prepared by mixing trimethylolpropane tris(3-mercapto propionate) (denoted as trithiol, see ESI†), MMA, PE-3A, benzil, and BDMBP for 2 hours in the dark room.²⁹ The components of the photopolymerizable resin is shown in Table S1.†

Fabrication of PMMA-SH/CdSe–Au nanocomposite thin films

PMMA-SH/CdSe–Au nanocomposite thin films were prepared by using LBL assembly method, as shown in Scheme 1. First, transparent PMMA film containing thiol groups (PMMA-SH) was produced by the photopolymerization of the thiol-containing photopolymerizable resin based on the thiol–ene reaction.³⁴ The spin-coating of the photopolymerizable resin was carried out on a KW-4 spin-coater over 4000 rpm for 60 seconds. The photopolymerizable resin was polymerized by exposing to the UV light (320 W high-pressure Hg Arc lamp) for 4 minutes. Second, the PMMA-SH film was immersed in the toluene solution of CdSe–Au NPs (CdSe–Au NPs concentration: 0.8 g L⁻¹) for 30 minutes to complete the adsorption of the CdSe–Au NPs on the surface of the PMMA-SH film via the affinity interaction between CdSe and thiol groups. The PMMA-SH film was then removed from the CdSe–Au NPs solution, and rinsed with toluene and dried. Finally, PMMA-SH/CdSe–Au nanocomposite thin films were obtained by repeating the first and second step for several times.

Scheme 1 Schematic representation of LBL growth of PMMA-SH/CdSe–Au nanocomposite thin films. n bilayers stand for the number of bilayer cycles.

Measurement and characterization

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEM 2100F (JEOL) instrument working at 200 kV. Absorption and transmittance spectra were collected using Shimadzu UV-2550 spectrometer. Photoluminescence (PL) spectra were evaluated using Hitachi F-4500 fluorescence spectrometer. The cadmium concentration was determined by a Varian 710-ES inductively coupled plasma spectroscopy (ICP-MS). Current–voltage curves of CdSe–Au NPs were obtained by using a NT-MDT-NTEGRA Spectra system (see ESI†). Thermogravimetric analysis (TGA) was measured by using a SDT Q600 system (TA instrument) with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The optical limiting response was measured using 532 nm laser beams produced by a Q-switched frequency-doubled Nd:YAG laser (8 ns, 10 Hz, Spectra Physics Inc., Quanta-Ray) (Fig. S1, see ESI†). The laser beam was focused by a lens with focal length of 300 mm and the laser energies were controlled by using a set of circular variable attenuators. The pulse energies before and after the sample were monitored by power meters P1 and P2 (Newport Optical Power Meter, Model 1916-R). The CdSe–Au toluene solution was contained in 1 cm thick quartz cells, while the PMMA-SH/CdSe–Au composite thin film was fixed vertically using a clamp. All of the measurements were conducted at room temperature.

Results and discussion

Absorption and PL spectra of the CdSe–Au NPs

Absorption and PL spectra of the CdSe–Au NPs are presented in Fig. 1. The absorption spectra of the CdSe–Au NPs can be described with three basic features: (i) with the increase of the Au(I)/CdSe ratio (Au/Cd ratio) from 0.6 to 0.85, the first excitonic peak is blue-shifted from 567 nm to 558 nm (Table S2, see ESI†). The blue-shift suggests a slight reduction in the size of the CdSe portion during the formation of CdSe–Au NPs. The size decrease of CdSe NCs in the formation of the CdSe–Au NPs has been observed in previously reported results.^30,35,36 (ii) The appearance of a tail on the red side of the CdSe seed gap. The red-tail on the absorption spectra was attributed to the formation of the small size Au clusters.^37,38 (iii) The second excitonic peak is eliminated after Au deposition. Fluorescence quenching was observed on PL spectra during the growth of Au clusters on the CdSe NC seeds. The fluorescence quenching was also confirmed by the photograph of the CdSe–Au NPs toluene solution under 365 nm UV lamp irradiation. Compared to the bright yellow photoluminescence of CdSe NCs under UV light, the CdSe–Au NPs exhibited negligible fluorescence under the same UV lamp for different Au/Cd ratios. The fluorescence quenching after the deposition of Au clusters on the CdSe NC seeds has also been observed in previous studies on similar systems.^35,39 The fluorescence quenching is mainly attributed to the electron transfer from CdSe to Au domains after photoexcitation, leading to the suppression of the recombination of the electron–hole pairs and opening up nonirradiative relax pathways.^40,41

Fig. 1 (a) Absorption (solid) and photoluminescence (short dot) spectra of CdSe NCs and CdSe–Au NPs with different Au/Cd ratio. Au/Cd ratio for 1#, 2#, 3# are 0.60, 0.72, and 0.85, respectively. Photographs of CdSe NCs and CdSe–Au NPs with different Au/Cd ratio under (b) ambient condition, and (c) UV lamp irradiation.

TEM images of the CdSe–Au NPs

TEM images were recorded to investigate the morphology of the CdSe–Au NPs, as shown in Fig. 2. Monodisperse CdSe NCs with the size of 3.5 nm were employed as the seeds (Fig. 2(a)). The deposition of Au domains on the CdSe NC seeds can be identified by the TEM images, in which the black points with enhanced contrast indicated the formation of Au clusters. The formation of the Au domains was further confirmed by the HR-TEM images, in which the crystal planes of the gray and black points were consistent with zinc blende (ZB) CdSe (International Center for Diffraction Data, no. 19-0191) and Au (International Center for Diffraction Data, no. 04-0784). Isolated Au NPs were not found in the purified sample, indicating the heterogeneous nucleation and growth of Au domains on the CdSe NC seeds. For CdSe–Au NPs with different Au/Cd ratio, only one Au domain was deposited on each CdSe NC, which is mainly ascribed to the small size of the CdSe NC seeds and nearly uniform activity on the surface of the monodisperse CdSe NC seeds. When the Au/Cd ratio increased from 0.6, 0.72 to 0.85, the size of the deposited Au domains changed from 2, 2.5 to 3.4 nm, which is plotted in Fig. S3 and Table S2.† In addition, TEM images also showed that the size of CdSe NC seed is slightly reduced after the deposition of Au domains, which is consistent with the results shown in the absorption spectra of the CdSe–Au NPs.

Fig. 2 TEM images of (a) CdSe and CdSe–Au NPs with Au/Cd ratio of 0.60 (b), 0.72 (c), and 0.85 (d), respectively.

Optoelectronic properties of the CdSe–Au NPs

The optoelectronic properties of the CdSe–Au NPs were evaluated by photoconductive atomic force microscope (PCAFM) with gold coated tip. The PCAFM measurement is shown schematically in Fig. 3(a). Dilute solution of CdSe–Au NPs was dropped on HOPG and dried with N₂. Then the optoelectronic measurements were carried out at room temperature in dark room using the NT-MDT-NTEGRA Spectra system (see ESI†). Fig. 3(b) presents the AFM topography of CdSe–Au NPs with a height of about 14.1 nm, indicating the aggregation of CdSe–Au NPs during the sample preparation. Typical current–voltage curves of CdSe–Au NPs under dark condition and laser irradiation are illustrated in Fig. 3(c). The current versus voltage characteristic is linear and symmetric within the measurement range. In dark condition, zero-bias current of CdSe–Au NPs was measured to be 33.5 pA. Upon exposing the CdSe–Au NPs to 532 nm laser irradiation at 0.059 MW cm⁻², zero-bias current increased to 45.7 pA, which was about 1.5 times larger than that in dark condition. The zero-bias current increase upon laser irradiation was mainly attributed to the light-induced charge carrier shift between CdSe domains and Au domains in CdSe–Au NPs. Following the laser irradiation, the subgap state in CdSe–Au NPs facilitated the excited electrons relaxing into the Au domains instead of recombining with the holes at the valence band, resulting in the enhanced current under laser irradiation.^42–44

Fig. 3 (a) Schematic illustration of the PCAFM measurement; (b) AFM topography of CdSe–Au NPs, inset shows a height profile of the CdSe–Au NPs; (c) current–voltage (I–V) curves of CdSe–Au NPs obtained by PCAFM measurement with and without laser irradiation, respectively.

Optical limiting properties of the CdSe–Au NPs

Optical limiting response of the CdSe–Au NPs was measured by using a Nd:YAG laser operating at 532 nm as excitation source. The linear transmittance of the CdSe NCs and CdSe–Au NPs solutions was adjusted to 70% at 532 nm. Fig. 4 plots the optical limiting curves of CdSe NCs and CdSe–Au NPs toluene solution at 532 nm. The input–output fluence curve of the CdSe NCs follows linear relationship, indicating that the CdSe NCs exhibit negligible optical limiting behaviour in the excitation range. Nevertheless, the input–output fluence curves of the CdSe–Au NPs exhibit a deviation from the linearity, and even a flat stage, which suggest the optical limiting response of the CdSe–Au NPs. Although all CdSe–Au NPs exhibited optical limiting response to 532 nm Nd:YAG laser, their optical limiting response is different when they are prepared with different Au/Cd ratio. The optical limiting response of the CdSe–Au NPs with high Au/Cd ratio is stronger than that with low Au/Cd ratio (Table S2, see ESI†). The optical limiting response of the CdSe–Au NPs was attributed to the combination of the CdSe NCs and Au clusters.^45–47 The enhanced optical limiting response of CdSe–Au NPs is mainly attributed to the surface plasmon response (SPR) of Au NPs and plasmon-exciton interaction in the CdSe–Au NPs. In this study, the SPR peak of Au NPs is around the excitation wavelength used for the laser optical limiting measurement, which leads to the absorption saturation and enhancement of optical limiting response.^48–50 Moreover, excited-state absorption or two-photon absorption will occur due to the exciton–plasmon interactions in the CdSe–Au NPs under laser irradiation,⁵¹ which is caused by the charge transfer between CdSe quantum dot and the surface plasmons of Au NPs.^52–54 The optoelectronic properties of the CdSe–Au NPs will render them applications in fabricating functional structures and devices.

Fig. 4 Optical limiting response of CdSe NCs and CdSe–Au NPs with different Au/Cd ratios in toluene solution.

Layer-by-layer assembly of PMMA-SH/CdSe–Au nanocomposite thin films

PMMA-SH/CdSe–Au nanocomposite thin films were fabricated by the LBL assembly method using the CdSe–Au NPs with Au/Cd ratio of 0.85. The LBL assembly process of the PMMA-SH/CdSe–Au nanocomposite thin films was monitored by UV-vis absorption spectroscopy. Fig. 5 shows the UV-vis spectra of (PMMA-SH/CdSe–Au)_n nanocomposite thin film with different number of bilayer on a glass substrate. For comparison, the UV-vis spectrum of PMMA-SH film without CdSe–Au NPs is also plotted. It's clear that PMMA-SH film shows negligible absorption around 560 nm, while PMMA-SH/CdSe–Au nanocomposite thin films exhibit evident absorption band around 560 nm. As a result, the absorption band at 560 nm is assigned to the assembly of CdSe–Au NPs in the films, which is consistent with the absorption of pristine CdSe–Au NPs in toluene solution (Fig. 1(a)). Moreover, the linear increase of the absorbance at 560 nm with the increase of the number of bilayer suggested that an equal amount of CdSe–Au NPs were assembled in each cycle, indicating a stepwise and uniform growth process.

Fig. 5 UV-vis absorption spectra of PMMA-SH/CdSe–Au nanocomposite thin films with different number of bilayer cycles. Inset is the variation between the absorption at 560 nm and the number of bilayer for the (PMMA-SH/CdSe–Au)_n nanocomposite thin films.

TEM image of (PMMA-SH/CdSe–Au)₆ nanocomposite thin film

The internal structure of (PMMA-SH/CdSe–Au)₆ nanocomposite thin film is investigated by exploring the TEM image of the cross-section of the nanocomposite film. As shown in Fig. 6(a), one layer of inorganic NPs can be well recognized from the TEM image, indicating the segregation of the CdSe–Au NPs near the surface of spin-coated PMMA-SH film. The thickness of the inorganic NPs is determined to be 10–20 nm, which is similar to the traditional LBL assembly performed in aqueous solution.¹¹ Furthermore, the embedded CdSe–Au NPs were confirmed by the high resolution TEM image shown in Fig. 6(b). The lattice spacing of the NP with two domains can be recognized and determined to be 0.231 and 0.35 nm, which were consistent with the planes of Au (111) (International Center for Diffraction Data, no. 04-0784), and ZB CdSe (111) (International Center for Diffraction Data, no. 19-0191), respectively. It's worth noting that the CdSe–Au NPs are dispersed in the hydrophobic environment, which protects them from oxidative damage.

Fig. 6 (a) TEM image of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film. (b) HR-TEM image of the selected region shown in the red frame of (a). Inset of (b) is the HR-TEM image of the CdSe–Au NP in the red dashed frame.

Thermal property and transparency of (PMMA-SH/CdSe–Au)₆ nanocomposite thin film

Thermal stability of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film has been evaluated by the TGA analysis. As shown in Fig. 7(a), the decomposition temperature of the (PMMA-SH/CdSe–Au)₆ nanocomposite film is 334.5 °C (here the decomposition temperature is defined as the temperature of 10 wt% weight loss under nitrogen at a heating rate of 10 °C min⁻¹). TGA curves further indicate that 2.13 wt% of CdSe–Au NPs is incorporated in the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film through the LBL assembly. The superior thermal stability of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film is of great importance for practical applications.

Fig. 7 (a) TGA curves and (b) transmittance spectra of the PMMA-SH film and (PMMA-SH/CdSe–Au)₆ nanocomposite thin film, respectively. Inset of (b) is the photo of PMMA-SH film and (PMMA-SH/CdSe–Au)₆ nanocomposite thin film, respectively.

Furthermore, we investigated the transparency of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film. Compared to the PMMA-SH film without CdSe–Au NPs, (PMMA-SH/CdSe–Au)₆ nanocomposite thin film remained transparent to the naked eyes, although it had brown colour due to the deposition of CdSe–Au NPs. The transmittance spectra of the PMMA-SH film and (PMMA-SH/CdSe–Au)₆ nanocomposite thin film are plotted in Fig. 7(b). The blank PMMA-SH film shows a transmission of 96.7% across the range of 450–700 nm, with a rapid transmittance drop between 300–450 nm due to the absorption of the residual photoinitiator and photosensitizer. After deposition of 2.13 wt% CdSe–Au NPs, the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film has the transmittance higher than 83% in the range from 450–700 nm. The decrease of the transmission is mainly attributed to the absorption of the embedded CdSe–Au NPs. The transmittance spectrum and photograph suggest that the transparency of the (PMMA-SH/CdSe–Au)₆ nanocomposite film is good in the visible range, which is of significant importance for the applications in fabricating optical functional films and devices.

Optical limiting response of (PMMA-SH/CdSe–Au)₆ nanocomposite thin film

Optical limiting response of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film at 532 nm was evaluated by using a Nd:YAG laser as excitation source. Fig. 8 shows the optical limiting curves of (PMMA-SH/CdSe–Au)₆ nanocomposite thin film and PMMA-SH film, respectively. For the PMMA-SH film, the output fluence increased linearly with the input fluence, indicating negligible optical limiting response. However, optical limiting action occurred for the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film due to the deviation of input–output fluence curve from linear relationship as the input influence over 13 J cm⁻². When we further increased the input fluence of the excitation laser beam, a flat stage emerged on the input–output fluence curve, indicating good optical limiting response of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film at 532 nm. It's obvious that (PMMA-SH/CdSe–Au)₆ nanocomposite thin film displayed optical limiting performance similar to that of pristine CdSe–Au NPs toluene solution (Fig. 4), confirming that the optical limiting properties of the CdSe–Au NPs are maintained in the solid state after LBL assembly. From the practical viewpoint, it's of great importance for PMMA-SH/CdSe–Au nanocomposite thin film to exhibit good optical transparency, optical limiting performance, and thermal stability. Consequently, the LBL assembly strategy is prospective for the fabrication of novel optoelectronic devices.

Fig. 8 Optical limiting response of the (PMMA-SH/CdSe–Au)₆ nanocomposite thin film.

Conclusions

In summary, we have demonstrated a facile and efficient method to construct PMMA-SH/CdSe–Au nanocomposite thin films by employing layer-by-layer assembly. The LBL assembly is performed by utilizing alternative photopolymerization and selective adsorption of CdSe–Au hybrid NPs. The LBL assembly procedure is carried out in nonpolar solvent, and found to be simple, reproducible, and stepwise. The PMMA-SH/CdSe–Au nanocomposite thin film exhibits good transparency, thermal stability, and optical limiting property. Through the LBL method, we successfully achieved the polymeric film with high visibility and good optical limiting response to 532 nm green laser. The threshold of the optical limiting response of the PMMA-SH/CdSe–Au nanocomposite thin film is 13 J cm⁻². This study would provide high prospective in realizing novel functional devices, such as laser optical limiting devices.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (Grant No. 51473176, 91323301, 61475164, 61205194, 61275171, 91123032 and 51003113), the National Basic Research Program of China (2010CB934103), and CAS-JSPS Joint Research Project (GJHZ1411) for financial supports.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02893d

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