Permanent preservation of data is essential for massive information recording. Combination of semiconductor with plasmonic nanoparticles has been applied in multicolor display and high-density optical storage. However, bidirectional electron transfer occurs at the Schottky interface under UVA irradiation, resulting in reversible photochemical reaction, information erasure, low recording efficiency and writing rate. To address these issues, a novel Schottky heterostructure of Ag/Ta2O5 modified with alkali halide is developed to realize photoinduced one-directional electron transfer from metal to semiconductor. The recorded information in such a medium of KCl-Ag/Ta2O5 presents excellent holographic storage stability even under exposure of a strong UVA ray (360 nm, 385 mW/cm2). Meanwhile, grating growth rate and efficiency are significantly enhanced by optimizing Ag particle distance and Cl− anion loading amount. This work provides an important strategy for fast and persistent data storage.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In big data era, classified storage of massive information is the general trend. Electromagnetic storage platform exhibits high access speed but suffers from the poor ability of preserving long-term data. Persistent data storage requires the abilities of resisting ray damage, high density and low energy consumption. Holographic memory, as a new generation of data recording technology, meets the demand of large capacity and green storage [1–3]. Recently, holographic recording ability has been enhanced greatly on the platform of photochromic films [4,5]. For example, semiconductor/nano-Ag composites present multicolor photochromism [6,7]. Absorption band for the inorganic composite system covers ultraviolet, visible and near-infrared regions based on localized surface plasmon resonance (LSPR). The LSPR property can be further modulated by changing the size or shape of nanoparticles (NPs), as well as interparticle distance and environmental refractive index [8,9]. Under the resonant light irradiation, Ag NPs are oxidized to Ag+ ions with electron flowing from metal to semiconductor , which can be named as “photoinduced ionization of plasmonic particles” (PIPP). The conversion from light energy into electrical or electrochemical energy was applied to record hologram with visible light [11,12]. However, the memorized information will be erased via the reverse electron flow from semiconductor to Ag NPs under “ultraviolet radiation a” (UVA, 320 nm-400 nm) such as the 360 nm ray . The reverse photochemical reaction weakens the stability of optical storage in the film, and also reduces the possibility of long-data preservation. A key step to resolve the issue is to block the reduction path of Ag+ ions under UVA excitation. At present, several feasible ways have been proposed with impressive achievements: (i) Selecting orthogonal-circularly polarized lights (RCP + LCP) as writing source. Comparing with (s + s) mode of using two parallel linearly polarized lights, it provides a stronger electric field gradient force for the migration of Ag+ ions, and thus weakens photo-reduction in the resonant region . (ii) Adding electron acceptor layer (H3PW12O40) between Ag NPs and semiconductor. The ultra-small-sized acceptor molecules act as electron collectors to prevent electron backflow . (iii) Constructing specially-shaped Ag NPs such as regular nanocubic structures. Distal resonance of Ag nanocubes on regular porous oxide templates results in the electron backflow far from the initial oxidation path . However, the ability to resist UVA damage by the methods mentioned above is limited. A considerable number of photoelectrons from the oxide side are still involved in the reduction reaction of Ag+ ions, resulting in the decline of storage performance. It is urgent to develop permanent information storage of resisting UVA irradiation in a reliable optical medium.
As a newly emerging semiconductor, tantalum pentoxide (Ta2O5) has made great breakthroughs in many research fields. For instance, it is an excellent anti-reflective coating material in optical and photovoltaic devices due to its high refractive index and low chromatic dispersion [17,18]. Especially, the band gap of Ta2O5 is larger than 4.0 eV . The suitable position of conduction/valence bands makes it become an efficient photocatalyst . Then, one-directional hot electron transfer from metal NPs to Ta2O5 is possible under the action of visible or UVA light, naturally forming shield for the backflow process of electrons. Thus, information storage stability can be improved. However, Schottky interface plays a key role in writing information process. It is still difficult to accelerate the electron transfer when the contacting area between noble metal and semiconductor is limited. Efficient oxidation process depends on the proper living environment of silver ions. The type of anion ligand solution has been proved to influence the accuracy of spectral hole-burning of noble metal nanoparticles . Accordingly, surface modification of anion may be a feasible way to accelerate the PIPP process.
Taking these factors into account, herein, we combine tantalum oxide with Ag NPs to construct Schottky heterojunction as one-directional electron transfer channel, which eliminates electron excitation in valence band of semiconductors caused by UVA radiation. We find that KCl-Ag/Ta2O5 heterostructure has the unique performance that recording efficiency by visible light remains stable under UVA irradiation. In addition, Cl− anion modification effectively regulates the size and particle spacing of Ag NPs. Accuracy and speed of spectral hole-burning, as well as recording rate and diffraction efficiency of the resultant KCl-Ag/Ta2O5 film are all improved, providing an effective strategy for high-efficient and ultra-stable optical memory in harsh environment.
Sol-gel method was selected to prepare Ta2O5 solution. F127 (0.30 g, Sigma, USA) as pore forming agent was added into ethanol solution (12.65 mL) to stir at 60 °C for 30 min until the polymer was fully dissolved. Tantalum pentachloride powder (0.60 g, Shanghai Macklin Biochemical Co. Ltd.) was dissolved in ethanol (12.65 mL) with stirring for 2 h. Two kinds of solution were then mixed with each other followed by adding deionized water (2.0 mL) with stirring for 2 h. After standing still at 300 K for 15 h, tantalum pentoxide precursor was obtained, which was used to be dip-coated on glass substrates with a rate of 0.455 cm/s. After annealing at 600 °C to remove the polymer, amorphous Ta2O5 nanoporous films (ST) were obtained . Ag NPs were magnetron-sputtered on the film for 15 s to obtain Ag/Ta2O5 heterostructures (SAT). KCl-Ag/Ta2O5 nanocomposite films (SKCl-AT) were prepared by immersing the Ag/Ta2O5 films into the KCl solution of 0.1 mol/L for 10 s. The resultant film was washed with ultrapure water for 5 s, followed by drying with a common air-gun. The sample (SKCl-AT) color changes from lavender to yellow after the treatment of KCl. The whole preparation process is shown in Fig. 1.
Optical setup for holographic recording is presented in Fig. 2. Coherent lights from a blue-violet laser (s-polarized, 405 nm, TOPTICA Photonics) were used to record diffraction gratings. The intersectional angle between the two recording beams was set as 15° and the writing beam power was the same and equal to 114 mW/cm2. A red laser beam (s-polarized, 671 nm, 14 mW/cm2, Changchun New industries Optoelectronics Tech. Co. Ltd.) was used as readout light to monitor holographic dynamics. A UVA laser beam (360 nm, p-polarized, 385 mW/cm2, Changchun New industries Optoelectronics Tech. Co. Ltd.) served as erasing light source. The first-order diffracted signal was registered on a photo-diode interfaced with a computer. The environmental temperature was set as 300 K, and the relative humidity was 40%.
3. Results and discussion
3.1 Film characterization and photochemical reaction
The morphologies of as-synthesized samples were investigated by a scanning electron microscope (SEM). As shown in Fig. 3(a), ST exhibits loose and porous microstructures. Spatially dispersed nanopores with different sizes provide sufficient anchoring sites for Ag NPs. From Fig. 3(b), Ag NPs with a nearly sphere shape can be observed to be arranged closely with the spatial distribution density of 1.2×1011/cm2, which are loaded on the surface of the Ta2O5 nanoporous film. Some of the small-sized Ag NPs aggregates with each other, which reduces the LSPR effect of Ag NPs. In Fig. 3(c), it can be found that the Ag NPs become more dispersed and smaller after KCl modification. The density of Ag NPs on the KCl-modified Ta2O5 nanoporous film is reduced to be 7.30×1010/cm2. At the same time, we selected 0.05 mol/L and 0.2 mol/L as the contrast concentration of KCl solution. For the concentration of 0.05 mol/L, the plasmonic particle distribution density is decreased slightly (1.0×1011/cm2) compared with that of SAT, and NP aggregation still exists. However, after treated with the KCl solution of 0.2 mol/L, the dispersion degree of Ag NPs is enhanced, whereas the silver content drops sharply with the distribution density of 4.0×1010/cm2. A transmission electron microscopy (TEM) was used to further investigate structural and morphological properties of SAT and SKCl-AT. As shown in Fig. 3(d), micro-interfaces between Ag and Ta2O5 were formed at multiple sites of SAT. The lattice spacing of Ag NPs is 0.25 nm, corresponding to the exposed (111) crystal face, as shown in Fig. 3(e). The micro-interfaces still exist in the SKCl-AT, but the size of Ag NPs is decreased obviously, as shown in Fig. 3(f). The lattice spacing of Ag NPs in SKCl-AT are 0.25 nm and 0.21 nm, corresponding to the exposed (111) and (200) crystal faces, respectively, as shown in Fig. 3(g). The TEM images also indicate that quite amount of Ag NPs are still loaded on the surface of Ta2O5 nanoporous films even by the KCl treatment.
In the regions as shown in Figs. 3(b) and (c), the diameter and spacing of the Ag NPs were statistically analyzed by the nanomeasure software. The cumulative volume fraction is defined as the ratio of the population of statistical Ag NPs to that of the total ones. The inset is the diameter distribution and cumulative percentage of Ag NPs corresponding to SAT and SKCl-AT. The Ag NP diameter distribution for SAT is obtained with the average value of 27.58 nm. The particle size below 40 nm occupies a considerable volume fraction of 90.25%. The Ag NP diameter distribution for SKCl-AT shows the average value of 20.53 nm. And the Ag NP size below 40 nm occupies a considerable volume fraction of 98%. In addition, the Ag NP spacing distribution for SAT and SKCl-AT are investigated with the average values of 8.12 nm and 14.88 nm, respectively. The statistical results confirm the fact that the distance between the adjacent plasmonic NPs increases after the surface modification of alkali halides.
UV-Vis absorption spectrum of ST is shown in Fig. 4(a). It can be found that the Ta2O5 film has almost no absorption in the visible and near-infrared regions. The optical band-gap is determined to be 4.1 eV by extrapolating a Tauc plot of (αhγ)n to the energy axis, where n=1/2 for an indirect band-gap material, α is absorption coefficient, h Planck’s constant and γ light frequency. It means that the electrons from Ta2O5 valence band cannot be excited by the light radiation with the wavelength longer than ∼300 nm. Figure 4(b) describes the UV-Vis absorption spectra of SAT and SKCl-AT, both exhibiting a rather wide absorption band from 330 nm to 900 nm, not only in visible region but also extending in UV region. Thus, utilization of multi-wavelength excitation is possible in the nanocomposite system. The Ag plasmon absorption peak appears near 500 nm in SAT, but moves to 430 nm in SKCl-AT. Moreover, the absorbance of SKCl-AT increases significantly below 475 nm, accompanied with the absorption decline above 475 nm. The spectral blue-shift after the alkali halide treatment means size tailoring of Ag NPs. This phenomenon may be ascribed to the effective dissolution of some large-sized Ag NPs in KCl solution, which leads to the decrease of absorbance at long wavelength. Commonly, the resonance peak of the small-sized particles locates in the short wavelength region. Thus, KCl modification increases the population of the small-sized Ag NPs, and enlarges the distance of the adjacent plasmonic particles, which are consistent with the observations of SEM and TEM.
As the alkali halide treatment was proved to increase the NP distance, the light response property of SKCl-AT may be quite different from that of SAT. Accordingly, the photochromic behavior of SAT and SKCl-AT were investigated by in-situ monitoring absorption spectrum under different laser beam irradiations. As shown in Fig. 4(c), two absorption bands of 320-440 nm and 650-850 nm for SAT are formed with prolonging the 405 nm (114 mW/cm2) irradiation time, while absorbance decreases obviously around 510 nm, resulting in the spectral hole-burning in green region. The absorbance increasement near 360 nm is much higher than that near 750 nm. Meanwhile, spectral hole-burning exhibits obvious change when switching different excitation wavelengths. The holes appear at 580 nm and 620 nm under the blue and green excitations, respectively. As shown in Fig. 4(d), the central position of spectral hole-burning deviates from the excitation wavelength and shows obvious red-shift. Commonly, plasmonic NPs contacting with the high-dielectric-constant substrate can induce the movement of LSPR spectrum towards longer wavelength . Besides, large-sized plasmonic particles also tend to resonate with red light . In our case, the deposited particles are so closely arranged that they even contact with each other. Accordingly, the LSPR peak position moves towards the red region. Quite differently, as shown in Fig. 4(e), with the 405 nm light irradiation, the absorbance of SKCl-AT decreases in the range of 360 nm −475 nm, and the most obvious decline is near 420 nm. At this time, the center wavelength of the spectral hole-burning for SKCl-AT (420 nm) is closer to the exciting light wavelength compared with that for SAT (510 nm). The absorbance of SKCl-AT increases significantly at 520 nm, but change little at 345 nm. Differential absorption spectra under the 405 nm, 473 nm and 532 nm irradiations for 30 min are shown in Fig. 4(f). At this time, the position of the spectral hole burning is well matched with the wavelength of the excitation light. The spectral burned hole by 405 nm light irradiation is deeper than that by 473 nm and 532 nm ones. Moreover, the burned hole shape of SKCl-AT is narrower and sharper than that of SAT, which may result from the large spacing of Ag NPs. Figure 4(g) shows temporal evolution of absorbance at the Surface Plasmon Resonance Peak (SPRP) with irradiation of the 405 nm monochromatic light. SKCl-AT also shows deeper burned-hole than SAT for the same irradiation period. Meanwhile, we also investigate the KCl-concentration-dependent photochromism for SKCl-AT. It was found that the spectral hole-burning of the SAT treated with the KCl solution of 0.1 mol/L was deeper than that treated with the KCl solutions of 0.05 mol/L or 0.2 mol/L after the 405 nm light irradiation for 30 min. Thus, 0.1 mol/L can be used as the optimal KCl concentration. In this way, small-sized and spatial-dispersed silver particles are obtained. With enlarging the distance between adjacent NPs, the absorption peak position for LSPR turns to be matched with the wavelength of the excitation light. The excellent resonance with external light field is reasonable, taking density and uniformity of Ag NPs into account. This phenomenon indicates that alkali halide treatment is conducive to the PIPP process. The photoelectronic transfer mechanism is presented in Fig. 4(h). Ag NPs can be oxidized directly to (AgCl2)− by Cl− ions when the samples are treated with KCl. Large-sized NPs are oxidized and then dissolved to form the small-sized ones which tend to be oxidized fast  and dissolved completely.
3.2 Holographic and anti-UVA erasure performance
As demonstrated above, the surface modification of KCl plays a key role in formation of dispersed and small-sized Ag NPs. The improved PIPP process may also enhance the copying ability of optical holographic fringes. Accordingly, diffraction dynamics of the holographic gratings recorded in SAT and the SKCl-AT are investigated. As shown in Fig. 5(a), the diffractive signal intensity from a red laser beam (671 nm) increases gradually when the coherent blue-violet lights irradiate SAT and the SKCl-AT. After recording for 1000 s, the diffraction efficiency of SKCl-AT is 5 times higher than that of SAT, which is due to the fast spectral hole-burning formation in SKCl-AT irradiated by the blue-violet light, and is consistent with the experimental results in Fig. 4. Then, the writing laser is turned off and the diffractive signal is detected only with the red light. In the following process of 1500 s, no decline of the readout signal for SAT and SKCl-AT is observed. After that, we continuously monitor diffraction efficiency of the gratings within 25 h in the dark environment. The diffraction efficiency of the gratings recorded in SAT remains unchanged with the increase of placement time, while the diffractive signal recorded in SKCl-AT in such an aging process even increases slightly compared with that at 1500 s, indicating the excellent holographic readout stability of the samples. To verify the ability to resist UVA erasure during holographic storage in the Schottky heterostructure, UVA-participated holographic recording is carried out, as shown in Fig. 5(b). At 1000 s, the writing beams (405 nm) are turned off and the UVA source (360 nm, 385 mW/cm2) is turned on to further stimulate the holographic grating. In this process, grating growth is still detected with the red light (671 nm). Diffractive signal of holographic gratings in SKCl-AT keeps constant in such a strong UVA environment, which shows excellent anti-UVA erasure performance. Interestingly, the diffraction efficiency of holographic gratings in SAT increases slightly with increasing UVA irradiation time. An obvious UV-enhancing effect in holographic recording is observed. As is known, the large-sized Ag NPs correspond to the resonance absorption peak at the long wavelength while the small-sized ones correspond to that at the short wavelength . The change of plasmonic particle size in bright fringes of holographic grating can be illustrated assisting by in-situ optical absorption spectra. A physical insight into anti-UVA erasing holographic storage is provided by the spectral absorbance at 671 nm versus irradiation time. The absorbance at 671 nm remained unchanged for SAT during the process of the 360 nm light irradiation alone, while the UVA radiation can induce continuous absorbance increasement in red region after the blue-violet light pre-irradiation, as shown in Fig. 5(c) (blue scattered dots). Commonly, the increase of holographic signal is closely related to the change of absorption coefficient or refractive index at the wavelength of detection light . Insufficient dissolution of the large-sized particles and secondary reduction result in the population increasement of small-sized Ag particles. The dissolved Ag+ ions can combine with non-resonant particle to generate the larger-sized particles. It corresponds to the phenomenon that the two absorbance bands emergence around 360 nm and 750 nm, respectively, as shown in Fig. 4(c). Here, under the 360 nm light irradiation, small-sized particles will be further consumed in the bright regions of the holographic fringes. Thus, the population of large-sized particles continues to be increased because of the combination of Ag+ ions and non-resonant particles. Therefore, the absorbance increases at 671 nm. However, in the dark regions of the holographic fringes, few of particles can resonate with the 360 nm light as the resonant region of Ag NPs obtained by magnetron sputtering is far from UVA. In this case, the 360 nm light plays little role in the particle growth in dark regions, and the absorbance at 671 nm remains unchanged. When the 405 nm light recorded grating in SAT is irradiated by UVA ray, the difference of resonance absorption at 671 nm becomes more obvious in the bright and dark regions, which leads to the slight increase of diffraction efficiency. Figure 5(d) shows absorbance at 671 nm versus irradiation time for SKCl-AT under the different excitation procedures which can further clarify the physical insight of the stable information storage in UVA environment. The absorbance at 671 nm remained unchanged either in the process of only 360 nm light irradiation or that of first blue-violet and then UVA irradiations. In the bright regions of the holographic fringes, PIPP occurs when SKCl-AT is irradiated by 405 nm light, and the Ag NPs of which the size is matched with the writing light are oxidized and dissolved sufficiently. Almost no ultra-small sized Ag NPs is generated, which can resonate with the 360 nm light [Fig. 4(e)]. For the dark regions, after KCl treatment, the ultra-small Ag NPs that may resonate with the UVA light on the surface of Ta2O5 are completely oxidized and dissolved by Cl− ions. Therefore, the 360 nm light radiation has little effect on the change of Ag NPs in the dark regions. PIPP processes in bright and dark regions of the SKCl-AT gratings are both inhibited at this time. The stored gratings in SKCl-AT can be readout stably in the UVA environment. Figure 6 summarizes temporal evolution of periodic distributions of the Ag NPs during the coherent 405 nm irradiations and then incoherent 360 nm light irradiations for SAT and SKCl-AT. The right parts of Fig. 6 just present optical microscope graphs for the persistent holographic gratings in SAT and SKCl-AT. Obviously, the grating contrast is enhanced via KCl modification.
Diffraction efficiency of holographic gratings is closely related to sample thickness. In order to evaluate the factors affecting the diffraction efficiency of the recorded gratings, we measured the sample thickness with a probe optical profilometer. The thickness of the Ta2O5 film is about 45 nm (ST). After loading Ag NPs, the thickness of the Ag/Ta2O5 nanocomposite film is about 95 nm (SAT). When the Ag/Ta2O5 nanocomposite film is treated with KCl solution, the thickness of the sample decreases to about 70 nm (SKCl-AT). However, the holographic grating period in SAT and SKCl-AT is ∼4 µm. Generally, a thick grating is formed when the sample thickness is more than ten times of the grating period . This result just shows that the grating we recorded belongs to the thin type one. In this case, Raman-Nath diffraction may occur, which exhibits weak angular and wavelength selectivity. Commonly, recorded gratings are a mixture of amplitude-type and phase-type gratings because optical excitation induces the changes of the absorption coefficient and refractive index. The diffraction efficiency of the thin gratings can be expressed as
In our experiment, the anisotropic photo-dissolution of Ag NPs results in the formation of phase grating . But more importantly, the absorption grating plays a dominant role in the light diffraction. According to our previous results of nano-Ag/semiconductor composite films, the diffraction efficiency of phase grating is ∼22% of that of absorption grating. The absorption-type holographic fringes are consisted with alternately distribution of nano-Ag and Ag2O. Herein, absorbance of nano-Ag by magnetron-sputtering is ∼0.25 at 420 nm. After photo-bleaching, the absorbance of the photosensitive layer decreases by 0.02. According to the theoretical predication, the diffraction efficiency can reach 6.25% if the absorbance of nano-Ag reaches the theoretical upper limit of 10 and the illuminated area is completely bleached with the absorbance value of 0. Optical bleaching is closely related to the writing light power. It is found that the diffraction efficiency increases exponentially with the increasement of the writing light power in a certain range. When the writing light power is increased from 3 mW to 10 mW, the diffraction efficiency of the grating is increased by about 5 times. Besides, our actual diffraction efficiency can be enhanced by increasing the loading amount of nano-Ag, i.e., increasing absorbance value of the photosensitive layer towards 10 as well as decreasing optical scattering on the sample surface. In addition, sensitivity (S) of photo-response media plays a key role in holographic recording, which is equal to the ratio of diffraction efficiency to the product of recording light intensity, exposure time and film thickness . Thus, the hybrid material with higher photo-sensitivity will produce larger diffraction efficiency when the recording light intensity and recording time are fixed. In our case, the media sensitivity is related to LSPR and PIPP. Expanding the contact area between metal and oxide-semiconductor, and increasing the thickness of porous oxide layer, can further improve the holographic formation response. As demonstrated above, KCl solution modification produces high-population and small-sized Ag NPs, improves the matching degree between plasmonic absorption band and recording wavelength, and enhances the LSPR ability via blue-violet light harvesting. More important, precise modulation of Ag particle population and Cl− anion concentration may result in desirable LSPR peak, which is expected to achieve UVA-resistant and fast holographic recording at different wavelengths. Further exploration and research seem to be promising.
Combination of Ag NPs with mesoporous Ta2O5 films is realized by sol-gel and magnetron-sputtering methods. The size and spacing of the plasmonic particles can be optimized by potassium chloride solution modification. It is found that Ta2O5 has a wide optical band gap and excellent electron transfer capacity. Multicolor photochromism is realized. Such a system has high-efficiency storage ability while ultra-strong UVA ray cannot damage the recorded holographic gratings. The clever-designed nanocomposite system provides excellent platform for the plasmonic holographic storage in harsh environment.
National Natural Science Foundation of China (11974073, 51732003, U19A2091); Overseas Expertise Introduction Project for Discipline Innovation (B13013); Natural Science Foundation of Jilin Province (20180101218JC); Education Department of Jilin Province (JJKH20201161KJ).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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