Holography with photopolymers has found many important applications in photonics, which include holographic data storage , 3D display , self-written waveguides , photolithography , and neutron optics . In the past decade, studies of metallic nanoparticle-related physical phenomena led to an intriguing area of research. These include their unique optical properties in the visible spectral range such as surface plasmon resonance (SPR). It is known that holographic exposure in inorganic oxides/organic nanoparticle-dispersed photopolymer results in the periodic assembly of nanoparticles by the polymerization-driven mutual diffusion of the nanoparticles and monomer due to their imbalance of the chemical potentials in the bright and dark regions of an intensity-interference fringe pattern , , . As a result, a volume holographic grating with a large saturated refractive modulation amplitude (Δnsat) can be obtained , , . Vaia et al. also demonstrated holographic assembly of Ag metallic nanoparticles in photopolymer to achieve micrometer-scale holographic patterning . Goldenberg et al. also reported on holographic recording in photopolymer-dispersed with Au metallic nanoparticles (1.5–3 nm core diameter, 1–2 wt.% dispersion) . They obtained Δnsat as large as 0.0073 at a wavelength of 633 nm and a grating period of 980 nm. They speculated a role of Au nanoparticles during holographic recording in the promotion of the spatial segregation of monomer and an increase in Δnsat as follows: (a) thiolate groups adsorbed on the surfaces of Au nanoparticles participate in the photopolymerization process as they act as chain transfer agents, which could reduce the molecular weight of growing polymers; and/or (b) Au nanoparticles can possibly terminate generating free radicals via the electron-transfer reactions. Balan et al. synthesized a polymer nanocomposite containing Ag nanoparticles that were photogenerated in situ by the redox process of amine radical and silver cations under holographic illumination . This work provided an alternative method to manipulate the distribution of nanoparticles. Metallic nanoparticles such as Au, Ag, and Al were further used in poly(methyl methacrylate) based photopolymer systems for applications in holographic data storage ,  and ultrafast holography . Despite the wide uses of metallic nanoparticle-dispersed polymer nanocomposites as mentioned above, a role of metallic nanoparticles in holographic recording still remains unclear. In this paper, we report on Ag nanoparticle-dispersed polymer nanocomposites that provide large Δnsat with low loading of Ag nanoparticles (0.5–1.5 wt.% with respect to monomer). The Ag nanoparticles were synthesized with surface modification to ensure good solubility in the monomer blend. The effects induced by the inclusion of Ag on the photopolymerization process were quantified by the measurement of alkyl and Rose Bengal (RB) semioxidized radicals (RB˙+). The mechanism of the possible increase in Δnsat by the inclusion of Ag nanoparticles is discussed. In addition, the dark-field microscopy measurement was carried out in order to confirm the holographic assembly of Ag nanoparticles in the proposed polymer nanocomposites.
2 Materials and experimental section
2.1 Materials and preparation
A photoinitiator RB (0.7 wt.%) and a coinitiator N-phenylglycine (NPG, 1.4 wt.%), were added to the multifunctional monomer trimethylolpropane trimethacrylate (TMPTMA) to provide photosensitivity to green light (532 nm). All these three chemicals were obtained from Sigma Aldrich. Figure 1 shows a mechanism of photopolymerization in the Ag nanoparticle-dispersed polymer nanocomposite during exposure. When RB is exposed to green light, the ground state RB dye molecule absorbs a photon and is promoted to a singlet excited state molecule. The singlet state dye can undergo intersystem crossing into the more stable and longer-lived triplet state RB (3RB*). Then, 3RB* reacts with NPG to generate one initiation radical (aminoalkyl radical) and one reduced radical (RB˙−) through the photoinduced electron transfer process, followed by the subsequent proton transfer, and the decarboxylation , , ,  as shown in Figure 1. The initiation aminoalkyl radical acts as a primary radical to initiate a growing polymer through the propagation process. RB˙− acts as a radical scavenger to inhibit the polymerization but is also able to give rise to new initiating radicals , . 3RB* is also consumed by scavenging oxygen to generate one RB˙+ , . This oxygen scavenging process results in a reduction in the polymerization threshold at the onset of the grating buildup (vide infra). The chemical structures of the components used in our experiment are illustrated in Figure S1.
Ag nanoparticles were prepared by means of a modified one-step synthesis method . AgNO3 powder (1.7g) (Beijing Chem. Work, Beijing, China) was added to a toluene solution (40 ml) and sonicated for 30 min to attain an Ag+ solution at a concentration of 0.25 mol/l. Decanoic acid (2.9 g) (Sigma Aldrich, St. Louis, MO, USA) was dropped into the above solution, and the mixture was stirred at a temperature of 80°C for 20 min. n-Butylamine (1.67 ml) (Sigma Aldrich, St. Louis, MO, USA) was then added dropwise into the solution at the rate of 2 drops/s and mixed well by stirring. After adding aqueous hydrazine (25 ml) as a reducing agent at a rate of 2 drop/s by vigorous stirring, the solution was then constantly stirred for 4 h. The product was washed by methanol (200 ml) and precipitated in 50 ml of acetone. After centrifuging at 8000 rad/min, resuspending in methanol, precipitating, and washing with acetone, the desired dark brown Ag nanoparticles were finally obtained. The size of Ag nanoparticles varied in the range of less than 150 nm, as shown in Figure S2A.
Ag nanoparticles were dissolved in xylene at a concentration of 4.0 mg/ml. The Ag nanoparticle solution was then added to the syrup containing TMPTMA, RB, and NPG at various concentrations in wt.% with respect to the monomer and stirred thoroughly in vacuum for 3 h at room temperature until xylene evaporated. Under this processing condition, Ag nanoparticles were homogenously dispersed in the monomer blend, as shown in Figure S2A. The mixture was then casted on a 20-μm loaded quartz substrate and sandwiched with another substrate. A series of low loadings of Ag nanoparticles (0–1.5 wt.%) in the monomer blend were carried out throughout our experiment. The refractive index of the photocured mixture varied from 1.497 to 1.496, which were measured by an Abbe refractometer, at a wavelength of 633 nm at an Ag nanoparticle concentration of 0–1.5 wt.%. This means that a possible contribution of the Ag nanoparticle into Δnsat is 0.001.
2.2 Holographic recording and measurements
We recorded an unslanted plane wave transmission grating at a recording intensity of 50 mW/cm2 by two mutually coherent beams from a diode-pumped solid-state CW laser (Coherent Verdi V5, λ=532 nm). A low intensity He-Ne laser beam (λ=633 nm) that was insensitive to RB was employed as a Bragg-matched probe beam to monitor the grating buildup dynamics. All of the beams were s-polarized. We measured the time-dependent diffraction efficiency of the unslanted transmission holographic grating η(t) defined as Id(t)/(Iin×F.C.), where Id is the first-order diffracted probe beam intensity, Iin is the input probe beam intensity, and F.C. is the Fresnel correction factor at two substrate-air boundaries . The effective thickness d was estimated by a least-squares curve fitting to the Bragg angle detuning data of η(∞) (≅ηsat) at the saturation using the β-value method (Uchida’s formalism) for an unslanted transmission grating without attenuation along the thickness direction . Values for Δnsat were extracted from ηsat with the help of Kogelnik’s formula at a Bragg-matched angle and d.
2.3 Morphology characterization
Morphology experiments were performed using an FEI SIRON200 field emission scanning electron microscopy (SEM) and an FEI Talos F200 transmission electron microscopy (TEM).
2.4 Spectral characterization
An Agilent G1103A spectrophotometer was used to determine the absorption dependence on Ag doping concentration for uncured samples loaded on quartz substrates.
2.5 Electron paramagnetic resonance
Concentrations of RB˙+ and alkyl radical at various concentrations of Ag nanoparticles were quantified by electron paramagnetic resonance (EPR) analysis. EPR spectra for a set of undoped and Ag nanoparticle-dispersed (0.5, 1, 1.5 wt.%) monomer blend were recorded by using a Bruker ER200-SRC-10/12 EPR spectrometer at different exposure times (5, 20, and 40 s). The corresponding concentrations of RB˙+ and alkyl radicals were extracted for each sample. A LED light source (532 nm) at an intensity of 50 mW/cm2 was used for curing the samples.
2.6 Dark-field microscopy
A Nikon DS-Fi2 microscopy combined with a fiber spectrometer (Ocean Optics USB-4000) was used for the dark-field imaging and spectral measurements to analyze uncured and grating recorded samples.
3 Results and discussion
Figure 2A shows a dependence of Δnsat on Ag nanoparticle concentrations. It can be seen that there exists the optimum Ag nanoparticle concentration (1 wt.%) maximizing Δnsat (0.0069). Note that each value is reproducible and was obtained as an average result following at least three separate exposures on different sample films. A photograph of a recorded grating at the Ag nanoparticle concentration of 1 wt.% is shown in Figure 2B. As mentioned in Subsection 2.1, in Ref. , the main possible contribution to a large Δnsat could be attributed to Au nanoparticles acting as both chain transfer agents and radical terminators. Our synthesized Ag nanoparticles were functionalized by a decanoic group, which did not act as chain transfer agents for acrylic polymerization. Moreover, if Ag nanoparticles were responsible for the radical termination mechanism, it would lower the polymer conversion and, therefore, reduce Δnsat , . Hence, the reason of Ag nanoparticle-induced large Δnsat in our experiment need to be further investigated.
Figure 2C illustrates the absorption spectrum of undoped and 1 wt.% Ag-doped samples. It can be seen that the inclusion of Ag nanoparticles results in a slight increase in the absorption in the visible spectral region. The absorption peak at ~560 nm is considered as the SPR, and scattering induced absorption by Ag nanoparticles embedded in the monomer blend. Ag nanoparticles bring effects on the photopolymerization process by its SPR during the holographic recording at a wavelength of 532 nm in this work , . Furthermore, we notice that a reduction in inhibition time arises by the inclusion of Ag nanoparticles, as shown in the inset of Figure 2D. The inhibition time reduces from 58 s to 24 s by doping 0.5 wt.% and then increasing to 28 s with a further increase in Ag nanoparticles. Interestingly, a similar observation on the trend of inhibition time varying with Au nanoparticle concentration was reported , although it was not discussed in detail. In the holographic photopolymerization process, the inhibition time usually depends on the consumption of inhibitors (oxygens) . The reduction in inhibition time implies that oxygen is consumed by Ag nanoparticles, so that the generation of 3RB* is facilitated. The increased radical generation could reduce the molecular weight of the growing polymer to promote the monomer diffusion from the dark to the bright illuminated regions and, thereby, increasing Δnsat. Hence, the inclusion of Ag nanoparticles in a radical photopolymerization system could induce two major effects on the increased radical generation enhanced by SPR and/or the reaction with oxygen.
In order to validate our proposed mechanism, a series of EPR measurements were carried out at varied Ag nanoparticle concentrations and exposure times. The measured EPR spectra, which indicate the characteristic peaks of RB˙+ and alkyl radicals, are illustrated in Figure S3. The radical concentration can be determined by CR×I/IR, where the CR is the reference radical concentration for RB˙+ and alkyl, I and IR are intensities of the characteristic EPR peak of the sample and the reference, respectively. The reference concentrations of RB˙+ and alkyl radicals were 1.254×106 and 1.000×106 spin/g, respectively. Tables 1 and 2 show the extracted concentrations of RB˙+ and alkyl radicals from EPR spectra. It can be seen that a reduction in RB˙+ occurs with increasing Ag nanoparticles. Namely, the amount of 3RB* that reacts with oxygen is reduced by Ag nanoparticles. The similar results obtained at 5 s and 20 s in 0.5 wt.% Ag nanoparticle dispersed case compared with the undoped case are considered as an influence by oxygen diffused from the air atmosphere. In the holographic recording experiment, sample films were covered by substrates to repel the replenishing oxygen from the surrounding air. However, samples in our EPR measurement were not covered, and substantial amounts of oxygen were able to diffuse into the samples at a relatively large rate compared with the case at low loading of Ag nanoparticles (0.5 wt.%) . Nevertheless, the observed reduction in RB˙+ concentration indicates that Ag nanoparticles suppress the reaction between 3RB* and oxygen. This leads to a reduction in the inhibition time as shown in Figure 2D and a possible increase in alkyl radicals.
Table 2 shows that an increase in alkyl concentration arises with the inclusion of Ag nanoparticles compared with the undoped case. This result indicates that the Ag nanoparticle induced photopolymerization that enhances the generation of macroradicals via the electron transfer reactions . In the free radical polymerization system, the kinetic chain length related to the molecular weight of the formed polymer is defined as 
where kp and kt are the rate constants of propagation and termination, respectively, and [M] and [M˙] are the concentrations of monomer and growing macroradicals, respectively. An increase in macroradical concentration (i.e. alkyl radicals in the acrylic free radical polymerization process) results in a reduction in the molecular weight of the polymer and consequently facilitates the diffusion of monomer during holographic recording. However, the possible macroradical termination will increase with increasing their concentrations. Two major termination mechanisms in free radical polymerization, that is, combination and disproportionation terminations, involve the following two growing macro-radical reactions , :
where ktc and ktd are the rate constants of the combination and disproportionation termination processes, respectively, and Mn, Mm, and Mn+m represent terminated chains without any radical tip, i.e. dead polymers. As the percentage of the disproportionation termination is dominant in the termination process of (meth)acrylate polymerization , an increase in the macroradical termination at the initial stage of holographic recording will lead to a growth in the number of oligomers. We note that diffraction signals at the early recording stage are very low as a weak volume grating being recorded is continuously refreshed in the low viscous state of the photopolymer . Therefore, the inhibition time at high Ag nanoparticle concentrations (1 and 1.5 wt.%) becomes longer than that at low Ag nanoparticle concentrations (0.5 wt.%). In addition, the following electron-transfer reactions to affect the inhibition process may also occur :
The Ag nanoparticle could enhance the generation f RB˙− radicals through this reaction. As mentioned in Section 2.1, RB˙− is capable of inhibiting polymerization, i.e. it results in an increase in the inhibition time. On one hand, RB˙− can produce new initiating radicals, and therefore the increase in RB˙− may also lead to a reduction in molecular weight of the growing polymer because of the premature termination induced by its inhibition . Measured data shown in Tables 1 and 2 clearly show that the inclusion of Ag nanoparticles increases Δnsat due to an increase in macroradical generation that is able to reduce the molecular weight of polymer and to promote the migration of unreacted monomer.
In order to characterize a volume phase grating recorded in an Ag nanoparticle-dispersed polymer composite film, we examined its morphology by TEM measurement. Figure 3A illustrates a TEM image of the cross section of a recorded grating. It can be seen that Ag nanoparticles are more likely enriched in some particular areas. It can hardly confirm the periodic distribution of Ag nanoparticles through SEM and TEM in the polymer matrix because of its relatively small size compared with the grating period. We also performed the dark-field microscopy to find light scattering from embedded Ag nanoparticles. The strong scattering of uniform distributed Ag nanoparticles in monomer blend can be easily resolved using light-scattering dark-field microscopy, as shown in Figure 3B. The obtained scattering spectrum is also consistent with the results presented in Figure 2C. The redistribution of Ag nanoparticles is easily confirmed by comparing the dark-field images of holographic grating recordings without and with Ag nanoparticles, as shown in Figure 3C and D, as similar to the case of semiconductor CdSe quantum dot-dispersed nanocomposite gratings reported previously .
Volume holographic recording in an Ag nanoparticle-dispersed polymer composite film was demonstrated. We investigated an influence of varying Ag nanoparticle doping concentrations on Δnsat, the generation rate of macroradicals, and the scavenging of oxygen. We showed that a small loading of Ag nanoparticles (0.5–1.5 wt.%) results in a substantive increase in Δnsat. Our EPR analysis shows that the promotion of the mutual diffusion leading to an increase in Δnsat is attributed to the increased macroradical generation rate and the suppression of oxygen. This result shows a new mechanism of metallic nanoparticles involving free radical photopolymerization during holographic recording. The redistribution of Ag nanoparticles due to the polymerization-driven mutual diffusion was also confirmed using dark-field microscopy. Our results are important in understanding the role of metallic nanoparticles in free radical photopolymerization and holographic recording, and are very useful for both holography and polymer synthesis uses.
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0188).
About the article
Published Online: 2019-09-06
Funding Source: National Natural Science Foundation of China
Award identifier / Grant number: 61605006
Award identifier / Grant number: 61735002
Funding Source: Beijing Municipal Natural Science Foundation
Award identifier / Grant number: 4182013
Award identifier / Grant number: Z180015
J. G., D. W., and X. Z. would like to acknowledge the financial support by the National Natural Science Foundation of China (Grant Nos. Funder Id: http://dx.doi.org/10.13039/501100001809, 61605006, 61735002) and the Beijing Municipal Natural Science Foundation (Grant Nos. Funder Id: http://dx.doi.org/10.13039/501100005089, 4182013, Z180015).
Citation Information: Nanophotonics, Volume 8, Issue 10, Pages 1795–1802, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2019-0188.
© 2019 Xinping Zhang et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution 4.0 Public License. BY 4.0