Metal/semiconductor nanocomposite systems with the ability of controllable holographic storage are fascinating for advancing information technology. Ag/TiO2 nanocomposite films present multicolor photochromism, which plays a key role in high-density optical memory. However, the film undergoes a reversible photo-redox reaction by the alternating action of visible and UV lights, which weakens the optical stability of stored information. To date, no effective method has been proposed to hinder the UV-erasure in the film. In this paper, the transferring behavior of electrons in a Schottky junction between Ag and TiO2 is inhibited effectively by introducing electron acceptors into the photochromic film. Plasmonic photo-dissolution is enhanced greatly, which is in accordance with the theoretical fitting based on the reversible photo-chemical reaction. Holograms can be written efficiently with high stability even under the destructive UV-irradiation, which are expected to be applied in an environmentally-stable photo-device.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Green storage devices dealing with mass information are desired in big-data era [1-2]. Holographic memory materials are thus fascinating that they enable huge leaps in capacity, access speed, long-term stability and high-performance display compared with magnetic materials, flash memory and memristor [3-4]. Among them, optical stable materials, such as lithium niobate (LiNbO3) crystals , play a key role in holography development. Recently, plasmonic metal-semiconductor nanocomposite media attract much attention due to the ability of effective light-energy conversion on the basis of plasmon-induced charge separation (PICS) [6–9]. For their outstanding performance of fast photo-chemical response and multi-dimension recording [10–13], several potential applications are exploited including image drawn [14-15], plotting, glass with specificity  and new-type photonic devices [17–19].
Especially, Ag/TiO2 nanocomposite films exhibit multicolor photochromism which can be applied in high-density optical memory [12, 20,21]. However, the recorded information based on PICS can be erased via the reversible electron transport between TiO2 and Ag nanoparticles (NPs) under UV excitation [14, 22–25], which weakens the stability of holographic storage in this kind of film. It is very important for such a nanocomposite system to modulate the storage ability from reversible recording to stable memory so as to develop multi-functional devices. Although stability of stored information in Ag/TiO2 films can be improved by optical manipulation , by now, no effective method has been proposed to prevent UV-excitation-induced information damage in respect of the designing of optoelectronic materials. As the charge separation and recombination mainly occur at the interface between Ag NPs and TiO2 film [7–9], modulation of electron transferring at interface is crucial for improving the storage stability and life.
In this paper, ultrasmall-sized separated charge-centers were introduced into the metal/semiconductor nanocomposite system in order to control the motive behavior of electrons at interface. The provided multi-electron transport channels are beneficial to enhance PICS efficiency. The fabricated film presents the property of anti-erasure of information, which can be stored with holographic images even under the direct illumination of UV light.
2. Fabrication and characterization of sample
2.1 Sample preparation
An anatase TiO2 nanoporous films were formed on glass slides by dip-coating from a solution of TiO2 NPs (STS-01, 0.4 mol/L, Ishihara Sangyo) and PEO20-PP070-PPO20 block copolymer (20 g/L) in an equi-volume water-ethanol mixture solvent by a sol-gel dip-coating method. The withdraw rate was 2 cm/s to fabricate smooth, uniform and transparent TiO2 film. The film was calcined at 450 °C for 1 h to remove the polymer. Subsequently, the TiO2 nanoporous film was immersed in anionic tungstophosphate (H3PW12O40, denoted as PW12) solution with the concentration of 0.016 mol/L for 5 h in order that PW12 molecules can be adsorbed on the surface of TiO2 sufficiently. Accordingly, the nitrogen-dried TiO2 nanoporous film with PW12 molecules was again immersed in the solution of 0.01 M silver nitrate (AgNO3 49ml) mixed with ethanol (1ml). Ag NPs were deposited on PW12-adsorbed TiO2 during UV-irradiation for 20 min at room temperature. Finally, the fabricated film was rinsed with deionized water, and irradiated with UV light for 5 min to reduce the residual Ag+ ions. During the process of UV-reduction, the sample color became brownish-gray gradually due to localized surface plasmon resonance (LSPR) absorption of deposited Ag NPs. The optical properties and morphology of the samples were characterized by UV-Vis spectrophotometer (UV-2600) and scanning electron microscope (SEM), respectively. The whole fabrication process is shown in Fig. 1.
2.2 Photo-electro-chemical experiment process
Photo-electro-chemical experiments were performed on PARSTAT 2273 potentiostat at room temperature. A typical three-electrode configuration was constructed with the composite films-assembled FTO glass as the working electrode, an Ag/AgCl as the reference electrode and the home-made platinum black as the counter electrode. The illumination was provided by a Hayashi LA-410 xenon lamp, of which light intensity was adjusted to 20 mW/cm2. The photo-electro-chemical measurements were done in a 0.5 M Na2SO4 electrolyte (pH = 5.8).
2.3 Optical setup for anti-UV holographic recording
The simplified scheme of the experimental apparatus is presented in Fig. 2. The diffraction grating was recorded with coherent s-polarized laser beams from a green laser (532 nm, 714 mW/cm2). The intersectional angle of the recording beams was fixed at 10°. The power density of the writing beams was the same and equal to 57 mW/cm2. A UV laser beam (360 nm, 71 mW/cm2) serving as erasing light source. A red laser (Changchun New industries Optoelectronics Tech. Co. Ltd.) generating 671 nm s-polarized light, was used as a probe source to monitor the holographic grating dynamics. The power density of the 671 nm laser was set as 7 mW/cm2 to minimize the destructive effect of readout radiation which in principle leads to photochemical reactions. The first-order diffracted signal was registered on a photo-diode interfaced with a computer. Diffraction efficiency of holographic gratings, taking Fresnel losses into account, can be defined as the ratio between intensities of the first-order diffracted beam and the incident beam after passing through the sample [27–29]. Besides, one of the writing beams was expanded by a beam expander after spatial filter, collimated to pass through a mask, and focused onto the center of the Ag/PW12/TiO2 nanocomposite film. The other beam was superimposed on the same spot as a reference beam. The reconstructed holographic images were collected by a CMOS video camera. The red laser (671 nm) was still used as probe sources to read holographic image.
3. Result and Discussion
3.1 Film morphology and UV-vis Absorption Spectra
Figures 3(a) and 3(c) show the surface and cross-sectional SEM images for the Ag/TiO2 film with PW12, respectively; while Figs. 3(b) and 3(d) correspond to the un-modified Ag/TiO2 film. The thicknesses of the two samples are similar (about 620 nm). The sample with PW12 indicates much smaller mean size of Ag NPs (about 14.7 nm) distribution than that of the sample without PW12 (about 21.2 nm). Such a size distribution difference may be ascribed to inhibiting the aggregation of Ag NPs under the UV-deposition by charge-centers. Cumulative volume fraction of the Ag/PW12/TiO2 film and Ag /TiO2 film are also obtained. Assisted with the electron acceptors, the plasmonic Ag NPs (< 30 nm) occupied a volume fraction of ~98%, as a highly desirable size distribution for efficient and fast photochromism [Fig. 3(e)], while a broad size-distribution of Ag NPs from 4 nm to 52 nm was formed when the noble-metal contacting with TiO2 directly [Fig. 3(f)]. Besides, the Ag NPs concentration of Ag/PW12/TiO2 is approximately 7.94 × 109/cm2, which is smaller than that of Ag/TiO2 film (~9.42 × 109/cm2).
In fact, the introduction of PW12 provides additional electron transport channel in the photo-catalytic deposition and electron-transfer processes [Fig. 4(a)]. The photo-generated electrons from TiO2 are dispersed and parts of them can be transferred to PW12 in UV-excitation , which retards deposition of Ag NPs effectively. The delay effect is verified again in UV-Vis absorption spectra of Ag/PW12/TiO2, and the Ag/TiO2 film as shown in Fig. 4(b). Absorbance value of the former (~0.95) reaches 70% of the latter (~1.38).
To investigate the photoelectron transformation of the PW12/TiO2 composite films, linear sweep voltammatry was used to obtain current-voltage (I-V) curves, which was conducted with the light source of UV light (350-400 nm). Dark currents for the two kinds of films are negligible. Figure 5(a) shows the photocurrent response of the PW12/TiO2 and the TiO2 films. It is easier to obtain positive current for the former than that for the latter when increasing potential vs. Ag/AgCl. The onset potentials for the two films are −0.5 V and −0.4 V vs. Ag/AgCl, respectively. The whole photocurrent curves can be divided into two stages. In the first stage from −0.8 V to −0.5 V vs. Ag/AgCl, both of the two films present a slow growth in photocurrent; while in the second stage (from −0.5 to 1.2 V vs. Ag/AgCl) the photocurrent increases sharply for PW12/TiO2 while it maintains a lower value for TiO2. A nearly 3-fold increase in the photocurrent of the PW12/TiO2 film is observed as compared to that of the TiO2 film at the potential of 0 V vs. Ag/AgCl. Due to the PW12 molecules are penetrated in the porous matrix of titanium dioxide, PW12 can be considered as an efficient electron scavenger to improve the inherent photochemical response of TiO2 film. These results demonstrate that PW12 is an important component for improving the photochemical performance of the TiO2 film. The electron-transfer processes are schematically shown in Fig. 5(b). UV-illumination induce band-excitation of TiO2 to generate electrons, which can be transferred effectively to PW12, forming a new kind of electronic transmission channel. The introduction of PW12 accelerates the transfer of electrons from the semiconductor film to the FTO electrode, and thus promotes the generation of carriers.
3.2. Modulation of reversible photochromism
To examine the electron-accepting role of PW12 in the photochromic process, the two kinds of films were alternately irradiated with 532 nm light (57 mW/cm2) and UV light (360 nm, 71 mW/cm2) for the period of 3 min. Absorption burning-hole at ~544 nm were observed after irradiation of the green light, for both of the two kinds of films [curve a1 in Fig. 6(a) and curve a2 in Fig. 6(b), respectively]. The hole was caused by photo-oxidation of Ag with O2 to lose the plasmon-based absorption. It is clearly shown that the absorption hole for the Ag/TiO2 film becomes shallower by switching from green to UV light irradiation, and can be recovered to the original level upon the second UV excitation [curves b2 and d2 in Fig. 6(b)]. That means the bleaching-coloring process was repeatable under multiple UV excitations for the Ag/TiO2 film. But for the case of Ag/PW12/TiO2 film, the absorption burning-hole can keep the original shape after the destruction of each UV irradiation [curves b1 and d1 in Fig. 6(a)]. The phenomenon is also related to the weakened reduction of Ag+ ions under UV excitation by PW12 molecules. The differential absorbance at 544 nm of Ag NPs versus cyclic index on these two kinds of media is presented in Figs. 6(c) and 6(d). With the help of PW12 molecules, the increased population of the reduced Ag NPs under UV excitation was inhibited greatly. The differential absorbance at 544 nm was only increased from −0.055 to −0.049 for Ag/PW12/TiO2 film after the irradiation of UV light, while it was enhanced from −0.085 to −0.050 for Ag/TiO2 film.
3.3. Nonvolatile holograms
To investigate the nonvolatile performance of the two kinds of nanocomposite films, the polarization holographic grating for (s + s) was erased by a single UV laser beam during several on/off cycles of the UV light irradiation. Figure 7(a) shows the diffraction efficiency as a function of time in several cycles for the nanocomposite film doped with PW12. It is clear that the recorded gratings cannot be erased completely. It needs to be pointed out that the photo-generated electrons from TiO2 are dispersed and only parts of them can be transferred to PW12 in UV-excitation. A small amount of electrons still could be combined with Ag+ ions, which leads to a decline in diffraction efficiency for Ag/PW12/TiO2 film. It was also found that the resident diffraction efficiency was accumulated with increasing the recording-erasure cycle times compared with traditional Ag/TiO2 film [Fig. 7(b)]. As the direction of the light field vibration in the polarization configuration (s + s) is perpendicular to the grating vector, the efficiency-accumulation mechanism based on the migration of Ag+ ions  is not suitable for the case. The only contributing factor to the increase of the residual diffraction efficiency may come from PW12. The efficient electron-collector of PW12 weakens the decrease of diffraction efficiency which, instead, is crucial for enhancing the nonvolatile holographic memory.
The excellent presentation of stored information depends on high diffraction efficiency in the holographic recording process. Thus the accumulated diffraction efficiency of Ag/PW12/TiO2 film improves the reading-out quality of the stored optical data. These two kinds of films were also applied to holographic image reconstruction in the same configuration as mentioned above. As shown in Fig. 8(a), it was found that the image cannot be erased completely in the Ag/PW12/TiO2 film after the first recording, and the brightness of the image was accumulated with increasing the recording-erasure cycles compared with the Ag/TiO2 film [Fig. 8(b)]. The nonvolatile property for the Ag/PW12/TiO2 film is agreed with the gradual increase of diffraction efficiency in Fig. 7(a). As for the Ag/TiO2 film, little residual holographic image can be found under the UV irradiation; in other words, this initialization behavior cannot be changed with the increase of cycles.
In fact, the holographic dynamics, under the co-irradiation of UV and coherent green lights, may also give support for the nonvolatile property of the sample with electron-acceptors. As shown in Fig. 8(c), diffraction efficiency increases continuously to 1.3% until 2000 s. However, the Ag/TiO2 film shows almost no optical response under the direct illumination of UV light. These results suggested that Ag/PW12/TiO2 film can shield UV light effectively in the holograms reconstruction. Based on the analysis above, a theoretical model, taking reversible photochromism of the nanocomposite film into account, is proposed as follows. The photochemical conversion between Ag and Ag2O may be represented by the following simplified expression:27-28],Eq. (1), the temporal evolution of NB in the bright regions can be given as,Eq. (8) with respect to NB and t yields a general solution given by:Figure 8(c) (solid lines) shows the kinetics descriptions according to Eq. (10), which agree well with the experimental results. The fitting results showed the rate constant of RA for Ag/PW12/TiO2 is 9.02 × 10−6/C, which is much higher than that for Ag/TiO2 film (3.65 × 10−7/C). Here, C is the final adjusted proportional factor. With the help of the electron-acceptors, charge-separation process was accelerated. Hence, the introduced PW12 molecules in the nanocomposite film play a positive role in the diffraction efficiency enhancement under the co-irradiation of visible and UV light.
The strategy for introduction PW12 in Ag/TiO2 films present an excellent anti-erasure performance of holographic memory in the visible region. Additionally, the efficiency of holographic recording at single wavelength is enhanced. After the irradiation of coherent visible lights for 675 s, the diffraction efficiency for the Ag/PW12/TiO2 film is 63% higher than that for the Ag/TiO2 film. As the PW12 molecules in Ag/TiO2 provide additional electronic transferring channel, the recording rate and the anti-eraser of reconstructed holograms were both enhanced. These improvements provide the feasibility for encrypted storage, showing great potential ability in information safety.
Ag/PW12/TiO2 nanocomposite films were successfully fabricated by UV-catalysis. Anti-erasure performance of the Ag/PW12/TiO2 benefits from the effective electronic transferring capability of PW12 molecules. Additionally, compared to the Ag/TiO2 film, the Ag/PW12/TiO2 film presents much better holographic efficiency and nonvolatile performance, which would be in favor of long-term memory. Theoretical fitting results also indicate that the introduction of dispersed electron acceptors in the metal/semiconductor film contributes to the enhancement of charge separation rate and efficiency. The results obtained from this study show that the appropriate combination with polyoxometalates can modulate optical response of such nanocomposite film, which is potentially useful in applications including photocatalysis, solar cell and nonvolatile optoelectronic devices.
National Natural Science Foundation of China (10974027, 31271442, 51372036, 61007006); the 111 project (B13013); the Fundamental Research Funds for the Central Universities (2412017FZ011); and the Natural Science Foundation of JiLin Province of China (20180101218JC).
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