Graphene oxide (GO) with an ideal two-dimensional structure, presents outstanding optical, electric, and mechanical properties which draws great attention in advanced information devices. Recently, GO-based films were found to show photochromic behavior, especially for the TiO2 involved system which has potential data processing capability, such as storing holograms. However, expanding spectral response range and increasing exposure sensitivity are still challenges for such a film, due to the limited photo-quantum efficiency in reduction reaction. Here, an innovative method of “Immersion-Dropping” technology is proposed to fabricate GO-based continuous films. We, for the first time, achieve colored holography from violet to yellow regions on GO/TiO2 nanocomposite films with introduction of weak acid molecules. A “diffraction self-enhancement” is observed. The obtained results benefit from the broadband photo-response of weak acid molecules and photo-triggered transferring of electrons in multi-channels. This work provides a research strategy for the large-capacity information storage and colorful display device.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Exploring a new way to store mass information efficiently and stably is highly desired with increasing data capacity in information technology . Compared with magnetic storage , semiconductor memory  and the traditional optical storage of “bit-wise” mode , holographic storage adopts the advanced recording mode of “page-wise”, which can realize 3D display and has been developed as a new generation of storage technology. The image storage format shows better performance in high-density, high-speed, low-power and long-life [5–8]. Especially, data storage density can be further enhanced by increasing optical recording dimensions such as inscribing information at different wavelengths . Therefore, selecting suitable media with multicolor response for wavelength multiplexing storage is essential. Recently, inorganic nanocomposite films attract much attention for their adjustable optoelectronic characteristics. For example, titania deposited with silver nanoparticles has been widely investigated for the presentation of multicolor photochromism based on localized surface plasmon resonance [10,11]. Polarization multiplexing holographic storage for the photochromic film was also realized [12–14]. However, the hologram readout stability is still hard to be controlled for the self-migration of silver nanoparticles.
Graphene which is consisted with a single layer of bonded-sp2 carbons with a two-dimensional honeycomb lattice presents outstanding electronic, thermal and mechanical properties [15–17]. The closely related compound, graphene oxide (GO), has a broadband spectrum response for its abundant energy levels, which is benefited from the wide size distribution of sp2 clusters [18,19]. Thus GO has an interesting property of tunable band gap, which opens up new applications in multi-wavelength-light-response devices. Irradiation of GO by sunlight or UV radiation can produce photo-generated electrons to participate in the reduction reaction from GO to reduced graphene oxide (rGO) . The associated production of holes in the sp2 clusters confined by sp3 carbon matrix can attack the water molecules adsorbed on the surface of GO sheets. The electrons initiate to dissociate oxygenated functional groups at the boundary of sp2 clusters and sp3 matrix because of the large enthalpy driving to expand electronic conjugation . Accordingly, the color of the carbon-based material changes from light-yellow to dark-brown [22–24]. Based on the photochromic phenomenon, athermally photoreduced GO can be used to realize colorful 3D images synthesized by a wavelength-multiplexed phase hologram and to extend the application of graphene materials in full-color three-dimensional display . While the photochemical reaction in GO sheets shows a clear dependence on the exciting photon energy. The pure GO film in air has limited visible spectrum response range of only less than 520 nm, and the photo-excitation efficiency at the green region is rather low . It is still a challenge to expand the spectral response range and increase exposure sensitivity. One of improved methods is to introduce TiO2 nanoparticles into GO sheets structures serving as a photocatalyst, however the optical excitation for the small amount of TiO2 is still limited [27,28].
In this paper, nanoporous titania films by dip-coating were used as effective matrixes with high loading capacity for GO nanosheets. Further, the inorganic framework also plays a supporting role in the formation of GO continuous films. Besides, small-sized molecules playing an electron donor role were adsorbed at the interface of GO/TiO2 to provide necessary electrons and hydrogen ions, which enhance holographic storage efficiency effectively from violet to yellow regions. Multicolor hologram storage was realized in the system, which puts a new way for functionalization of such carbon based films in optoelectronic devices.
2.1 Materials and films preparation
Anatase TiO2 films were prepared on glass slides by dip-coating from a solution of TiO2 nanoparticles (STS-01, 0.4 mol/L, Ishihara Sangyo) and PEO20-PP070-PPO20 (20 g/L) in an equal volume water-ethanol mixture solvent. The withdraw rate was 0.455 cm/s to fabricate smooth, uniform and transparent TiO2 films. And the film was annealed at 450 °C to remove the polymer, as shown in Figs. 1(a) and 1(b). Subsequently, the TiO2 nanoporous film was immersed in tannic acid (C76H52O46, denoted as TA) solution with the concentration of 6.67 × 10−4 mol/L for 3 h in order that TA molecules can be adsorbed on the surface of TiO2 sufficiently via its phenolic hydroxyl groups [Fig. 1(c)] [29,30]. Finally, GO solution of 0.25 mg/ml was dropped on the film at 40 °C to obtain GO/TA-TiO2 nanocomposite film with high transparency [Fig. 1(d)]. Four Chinese characters can be clearly observed through the film [Fig. 1(e)]. The whole process is noted as “Immersion-Dropping” method. Optical response property was characterized by UV-Vis spectrophotometer (UV-2600). The sample morphology was measured by atomic force microscope (AFM), presenting an excellent flatness of ± 6.2 nm with in the area of 10.89 µm2 [Fig. 1(f)].
2.2 Optical setup
Optical setup for holographic recording is shown in Fig. 2. Diffraction gratings were selectively recorded with two linearly s-polarized beams from blue-violet (403.4 nm, TOPTICA Photonics), blue, green and yellow (473 nm, 532 nm and 589 nm, respectively, Changchun New industries Optoelectronics Tech. Co. Ltd.). The intensities of the interfering beams were the same and equal to 5 mW. The angle between the writing beams was fixed at 5.5°. Grating period was calculated according to the equation Λ = λ/2sin(θ/2), where λ is the wavelength of writing beams. For example, Λ = 4.2 μm when the holographic recording at 403.4 nm. 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 first-order diffracted signal was registered on a photodiode faced 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 [31,32]. 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 GO/TA-TiO2 nanocomposite film. The other beam was superimposed on the same spot as a reference beam. The reconstructed holographic images from self-diffraction were collected by a CMOS video camera. The room temperature was set as 300 K, and the relative humidity 25%.
3. Results and discussions
3.1 Absorption spectra
UV-Vis absorption spectra of GO/TiO2 and GO/TA-TiO2 films are shown in Fig. 3(a). The in situ absorption spectra of the two kinds of nanocomposite films were also measured under monochromatic irradiation. Absorption spectral curves at different irradiation times under blue-violet excitation (403.4 nm, s-polarized, 5 mW) for GO/TiO2 and GO/TA-TiO2 films were obtained as shown in Figs. 3(b) and 3(c), respectively. The absorbance of the two films increases gradually in the spectral range of 350-900 nm, and the color of the sample changes from light-yellow to dark-brown in the near-UV irradiation. The reduction of GO sheets occurs. It was noticed that the GO/TA-TiO2 film shows an enhanced and expanded absorption band. Temporal evolution of absorbance for GO/TiO2 and GO/TA-TiO2 films was compared. Theoretical fitting to the absorbance at 671 nm with different irradiation times can be expressed as follows:Figs. 3(b) and 3(c) (dotted lines). Here, = 208.71 s and = 105.12 s were obtained. The absorbance variation can be calculated according to Eq. (2):Fig. 3(d) and Eqs. (3) and (4):Fig. 4.
The resulted conjugated sp2 regions will expand gradually and get in touch with each other. The insulating GO sheets will be converted to conductive reduced GO. Moreover, electrons from TA can be transferred via TiO2 conduction band to GO. The electron transformation from TA to TiO2 also provides a possible reduction channel for GO. The sufficient supplements of both H+ ions and electrons result in the fast and more effective reduction of GO sheets. Due to the wide absorption band of GO/TA-TiO2 films, the phenomenon of photochromism can occur at other wavelengths. In addition to the excitation of blue-violet light, significant changes of absorption spectra were also observed at 473 nm, 532 nm and 589 nm.
3.2 Holographic dynamics
The widened and enhanced absorbance from 350 nm to 900 nm after introduction of TA provides possibility for GO sheets reduction at other irradiating wavelengths than 403.4 nm. Diffraction efficiency dynamics (DED) for gratings recorded in GO/TiO2 and GO/TA-TiO2 films at 403.4 nm, 473 nm, 532 nm and 589 nm were all investigated, as shown in Figs. 5(a)-5(d), respectively. Here, a red laser beam (671 nm, s–polarized, 0.5 mW) was chosen to monitor the dynamics of holographic gratings inscribed by two coherent parallel (s-s) polarized beams from the violet to yellow regions with all the powers of 5 mW. For the holographic gratings of GO/TiO2 films, intensities of diffractive signals present a weak increasement. For the green beam (532 nm) recorded gratings, the diffraction efficiency tends to be saturated at 18000 s. Especially for that recorded at 403.4 nm and 473 nm, intensities of diffractive signals both decrease after the saturation point, from 2.06 × 10−2% to 5.43 × 10−3% at 403.4 nm and from 4.97 × 10−2% to 10−5% at 473 nm. Further, little intensity of diffractive signal was observed for GO/TiO2 films recorded at 589 nm. As shown in Fig. 3(a), GO/TiO2 films have low exposure sensitivity from violet to blue regions and almost no photo-response in the yellow region. While all the DED curves show significant improvement after the introduction of TA molecules in the composite film system, which present continuous growth. The difference of diffractive signal intensities between the two kinds of films becomes more apparent if prolonging recording time. It should be noticed that diffraction efficiency for GO/TA-TiO2 films still produce strong growth in the yellow region.
For the samples where holographic gratings can be formed, the change of color from light-yellow to dark-brown was all observed. So the whole holographic dynamics can be explained by wavelength-dependent GO reduction process, as shown in Fig. 6. In the bright regions of interference fringes, the electrons provided by light exciting TiO2 nanoparticles and TA molecules, participate in reduction from GO to rGO; while in the dark regions, GO sheets remain the original chemical structure. Thus the alternative distribution of GO and rGO results in the formation of carbon-based holographic gratings. The great photo-reduction degree of GO sheets contributes to the high contrast of holographic gratings of which diffraction efficiency can be increased accordingly.
Under the visible irradiation of the wavelength higher than 520 nm, the pure GO film can hardly release electrons . It was noticed that diffraction efficiency for GO/TiO2 films recorded with the green coherent lights was accumulated gradually to saturate value in about 18000 s which can be explained as self-reduction of GO sheets, while the self-reduction hardly occur under the yellow excitation. However, with the help of TA molecules, reduction reaction of GO sheets can occur under the 589 nm excitation. As the 403.4 nm and 473 nm excitations result in the production of electrons and H+ ions from TiO2 nanoparticles. The reduction of GO in TiO2 matrices can be accomplished at bright regions of holographic fringes with a shorter time. However, in the following stage the TiO2 nanoparticles at the bright regions will scatter the incident light to the dark regions where GO sheets can also be reduced, resulting in a decrease in grating contrast. In fact, TA-TiO2 nanocomposite system can also form holographic fringes. The diffractive signal was also observed for the two-body-system. However, the intensity of the diffractive signal in GO/TA-TiO2 films is still higher than the sum of that in GO/TiO2 and TA-TiO2 films. The electrons and H+ ions from TA molecules really make synergistic promotion of reduction of GO sheets and play a positive role in grating growth.
As discussed in the introduction part, non-destructive-readout of holographic gratings is essential for stable reconstruction of holograms. Accordingly, readout stability of holographic grating was carried out. After 1192 s, both of the writing beams (403.4 nm, 5 mW) were turned off and only the reading beam (671 nm, 0.5 mW) was used to illuminate the grating on the sample. As shown in Fig. 7, the diffraction efficiency of the GO/TiO2 film decreases slightly with the irradiation of red light. And the diffraction efficiency decreases by about 8% for ~9420 s. While for the GO/TA-TiO2 film, a “diffraction self-enhancement” effect was observed that the diffraction efficiency is enhanced about 13.82% after the red light irradiation in the same period.
The slight decrease for the former one may result from the scattering of the readout light and the both irradiation in dark and bright regions. However, it is quite different when TA molecules are introduced into the carbon-based system. It should be noticed that the electron-donor molecules still response to the red light. At the sole irradiation of the 671 nm light, TA can release a small amount of electrons and H+ ions. The sp3C region in GO has a wider band gap, and the bottom of the conduction band of the sp3C region is higher. So the photo-generated electrons of TA molecules cannot reduce the sp3C region directly and efficiently. The sp2 carbon cluster has a narrower band gap, which can capture electrons effectively and store them for reduction. The bright region of the grating has more sp2 carbon clusters than those in the dark regions. More amounts of electrons are stored for the reduction, so that the reduction of the bright region is much greater than that of the dark region. And the contrast of the grating is enhanced. The “diffraction self-enhancement” effect of the GO/TA-TiO2 film is obtained.
3.3 Multicolor holograms reconstruction
The efficient information storage and the broadened spectral response range of the GO/TA-TiO2 film provide possibility for multiple hologram recording and reconstruction, especially for colored hologram storage in GO/TA-TiO2 films. Here, four Chinese characters were stored with blu-ray (~405 nm), blue (473 nm), green (532 nm) and yellow (589 nm) lights, respectively. The colored holographic images were all captured by the COMS camera, as shown in Fig. 8. The electron donors play a key role in the efficient patterning of GO and rGO. Due to the adjustable photoconductivity of GO sheets, this research strategy will also promote the formation of complex conductive channels in single step, and make contribution for the development of multi-functional carbon-based electrical devices.
It needs to be pointed out that the obtained hologram requires a long period to be written. Oxygen in the environment plays a competitive role in photoinduced electron-transport of the nanocomposite system which is important for GO reduction. Because GO solution is dropped on the TiO2 nanoporous film modified with TA, insufficient interface contact between GO and TiO2 is unavoidable. In fact, the pure GO reduction consumes a long time. Moreover, since the wide size distribution of sp2 clusters in GO sheets, there is a problem of resonance matching of the nanocomposite film under visible excitation with a certain wavelength . Solving the issues mentioned above, the writing rate of the hologram can be increased. For example, creating inert atmosphere during the photochemical reaction can help to weaken the competitive effect of oxygen in electron transport. It is also possible to further improve the sample preparation process, to increase the contact area between GO and TiO2. Besides, selecting more suitable electron donors is beneficial for increasing the efficiency and the rate of light energy conversion.
We have shown that colored holography from violet to yellow regions can be achieved on GO/TiO2 nanocomposite films if introducing weak acid molecules with phenolic hydroxyl group. The continuous GO-based film with high exposure sensitivity was achieved by “Immersion-Dropping” method. The results present TA molecules play a key role in widening optical response range and increasing optical response ability for GO sheets. Accordingly, enhancement of hologram formation was demonstrated clearly based on photo-triggered transferring of electrons in multi-channels. This work indicates that electron-donors-modified GO/TiO2 nanocomposite films have potential application abilities in photocatalysis, large-capacity information storage and multicolor display devices. Utilizing the holographic exposure method, complex fringe structures can be obtained which is more convenient to form conductive channels of different periods in the carbon-participated microelectronic devices.
National Natural Science Foundation of China (10974027, 31271442, 51372036, 51732003, 61007006); The 111 Project (B13013); Fundamental Research Funds for the Central Universities (2412017FZ011); Natural Science Foundation of JiLin Province of China (20180101218JC).
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