Graphitic nanosheets via two-dimensional polymerization enhancing organic all-optically controlled photorefractive performance
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All-optically controlled devices are the attractive subjects due to their future applications in all-optically controlled networks and computers. In this work, two-dimensional (2D) graphite nanosheets are synthesized to improve organic photorefractive performance in the all-optical operation, i.e., the photorefractive performance under zero electric field. The synthetic process is performed by 2D-polymerizing electrophilic and nucleophilic carbon species under solvothermal effect, where [:C≡C:]2− in calcium carbide is applied as nucleophilic carbon species and [C(Cl)] in carbon tetrachloride (CCl4) as electrophilic carbon species. The polycarbonization reaction consists of 1D horizontal growth for nanoribbon and 2D vertical growth for single-layer graphene. Then, single-layer graphene nanosheets are aggregated via π–π stacking effect into 2D graphite nanosheets being composed of multilayer graphene nanosheets. Notably, the self-template characteristics induce the synchronous and alternate ongoing of three above processes. Interestingly, when these 2D graphite nanosheets are doped into organic photorefractive devices, the all-optically controlled photorefractive performance is effectively improved by enhancing the gain coefficient of two-beam coupling about 46.4% from 80.1 cm−1 of control device to 117.2 cm−1 of nanographite-doped device. This is attributed to the excellent electric conductance of 2D graphite nanosheets, which strengthens the charge separation under the nonuniform light field inside organic photorefractive devices. Then, the stronger built-in electric field will be induced so that the orientation enhancement effect is increased in organic photorefractive devices. This work provides an effective and facile approach to improve organic all-optically controlled photorefractive performance with 2D graphitic nanosheets.
KeywordsOptical device Photorefractive Nanographite 2D-polymerization
All-optically controlled devices, which control the light using light without other external fields like electric field, are presently the attractive subjects [1-7] due to their potential applications in the future all-optically controlled networks and computers for all-optical signal processing, such as storage, amplification, interchange, combination and separation. They are deemed to have the potential to break through the limitation of electronic bottleneck in the traditional electronic devices , realizing the next generation of high-speed information transportation and high-capability information processing in the broader operating bandwidth. Among them, All-optically controlled photorefractive (AOC-PR) device without external electric field is an important photonic device for all-optical signal processing in the integrated photonic chips [8-15].
The PR effect is based on the first-order electro-optic effect, also known as Pockels effect or Kerr effect, which is the nonlinear change in refractive index in the noncentrosymmetric materials induced by the direct current (DC) field [16-20]. The PR processes consist of nonuniform light formation, exciton generation, charge-separated diffusion, built-in electric-field formation, refractive index modulation and optical signal processing. All these processes are generally performed under an external electric field. Through our continuous attempts over the years [8-10], the external electric field is regarded to be unnecessary for the PR effect, which is defined as all-optically controlled photorefractive effect. On the basis of above PR processes, it was found that the diffusion of separated charges is a key issue to strengthen the built-in electric field in the PR devices , which is an important factor to enhance the orientation enhancement effect in organic photorefractive devices.
It was well known that the charge diffusion depends on the electric conductance in organic photorefractive device [17, 20], which might be strengthened by excellent electric conductors. In this case, the 2D carbonaceous materials are able to be applied as the electric conductor to strengthen the built-in electric field in organic photorefractive devices [21-24], similarly to inorganic nanowires [25-31]. At the same time, the size of 2D carbonaceous materials is an important factor to guarantee the strengthening of built-in electric field, instead of quenching. As for the microscopic structure of photorefractive gratings, the size of 2D carbonaceous materials should be less than the photorefractive grating period \(\Lambda\), which could be calculated by \(\Lambda = \lambda /2\sin (\theta /2)\)  where θ is the interception angle of two interfering beams. Without question, the bottom-up chemical synthesis starting from organic building blocks should be an effective approach to control the size of 2D carbonaceous materials . To date, several bottom-up strategies about the chemical synthesis of 2D carbonaceous materials have been reported, such as solution synthesis , surface-mediated synthesis [34-37] and chemical vapor deposition (CVD) synthesis . However, the most striking synthetic approach is still the wet chemical mean  independent of ultra-high vacuum (UHV) despite the existing of strong aggregation derived from intermolecular π–π stacking .
In this work, to improve organic AOC-PR performance, the graphitic nanosheets are prepared as 2D carbonaceous materials through the 2D-polymerization reaction of nucleophilic substitution reaction of carbon-halogen bond with nucleophilic carbon species due to the formation of C–C bond. The excellent electric conductance of 2D graphitic nanosheets will strengthen the charge separation under the nonuniform light field inside organic photorefractive devices. As a result, the stronger built-in electric field will come into being so that the orientation enhancement effect in organic AOC-PR devices should be increased to afford the larger gain coefficient of two-beam coupling under zero electric field.
2 Experimental section
2.1 Instruments and characterization
The TEM images are achieved on JEM-2100 high-resolution transmission electron microscopy by JEOL Ltd., whose sample is fabricated by dropping the ethanol solution of 2D graphitic nanosheets on the formvar stabilized with carbon support films. The SEM images are observed from JSM-7500F field emission scanning electron microscopy by JEOL Ltd., whose sample is fabricated by dropping the ethanol solution of 2D graphitic nanosheets on the silicon wafer. The X-ray diffraction is measured at a scanning speed of 10°/min on a D/max 2500 XRD diffractometer (Rigaku) with Cu Kα radiation (0.1541 nm). The IR spectra are recorded on TENSOR II Fourier transform infrared spectrometer with an integrated platinum-ATR-accessory. The UV–Vis–NIR spectra are monitored on an ultraviolet–visible–near-infrared Cary5000 spectrophotometer by Agillent Technologies. The Mott–Schottky curves are tested on a RST5000 electrochemical workstation. The Raman spectra are recorded on a DXRxi Raman imaging microscope by ThermoFisher Scientific Corporation.
Calcium carbide (CaC2) is purchased from Sigma-Aldrich Chemical Corporation. Carbon tetrachloride (CCl4) is bought from Chinese chemical companies and purified to eliminate the impurity of carbon disulfide. The specific procedure is given below: Carbon tetrachloride (500 mL), potassium hydroxide (30 g) and ethanol (95%) are mixed in a 1000-mL round flask. After shaking for 30 min in the table concentrator, the solution is distilled and washed with distilled water. Then, the above procedure is performed again except that the amount of potassium hydroxide is reduced in half. The residual ethanol and water in the purified carbon tetrachloride are removed using anhydrous calcium chloride. After filtration, carbon tetrachloride is evaporated under reduced pressure and gathered through condensation. Finally, the purified carbon tetrachloride is kept under molecular sieve for use.
2.3 Synthetic procedure of 2D graphitic nanosheets
Calcium carbide and carbon tetrachloride are added at an equivalent mass ratio into a 50-mL autoclave, where [:C≡C:]2− is regarded to be two functionality nucleophilic monomer and CCl4 to be four functionality electrophilic monomer according to the principle of polymer chemistry. Then, the autoclave is sealed and heated to 250 °C in a salt bath. After the polycarbonization reaction is carried out for 10 h, the as-synthesized product is washed with a large number of hydrochloric acid (3 wt%). After filtrating and freeze-drying, the black solid powder is obtained.
2.4 Fabrication of photorefractive devices
Organic photorefractive composite film was fabricated by solution casting approach. Firstly, organic photorefractive compound CRA-CSN75-Cz25 and ethylcarbazole (ECZ) were respectively dissolved in THF at a concentration of 5 mg/mL. The photosensitizer (PC61BM) was also prepared as a THF solution. Afterward, the solutions of CRA-CSN75-Cz25, ECZ and PC61BM were mixed according to a certain volume proportion and filtrated by a PTEF filter with a pore of 0.2 µm. After a majority of solvent was evaporated, the solution was dropwise added on the ITO substrate, which was heated to 40 °C with a digital hot plate in order to speed up the solvent evaporation. The residual solvent was naturally evaporated under good ventilation. Subsequently, the solid sample was dried overnight in vacuum in order to remove the residual solvent. After that, another piece of ITO glass was covered on the solid sample at 70 °C, where the potential crystal will be eliminated. The photorefractive composite film was immediately pressed, while the film thickness was controlled through a spacer of 80 µm. Finally, the photorefractive device was suddenly cooled down to keep the photorefractive composite film in a good amorphous state. The fabricated devices were kept in the refrigerator for the photorefractive characterization.
Nanographite-doped photorefractive devices were also fabricated according to the above procedure except that 2D graphitic nanosheets were doped into organic photorefractive composites with a 0.3 wt% ratio before the solution was filtrated by a PTEF filter.
2.5 Photorefractive characterization
where d is the sample thickness, θin is the incident angle of Beam 1 inside the sample, β is the initial intensity ratio of beams after the sample in the absence of coupling, and γ0 is the beam-coupling ratio (I/I0). Here, I0 is the signal intensity without the pump beam. I is the signal intensity with the pump beam.
3 Results and discussion
3.1 Synthesis of 2D graphitic nanosheets
According to the conception of polymerizable functional groups of the monomers in polymer chemistry, the polymerization system of [:C≡C:]2− and [C(Cl)4] belongs to the (A2 + B4) synthetic strategy of hyperbranched or cross-linked polymer [41, 42] if [:C≡C:]2− is regarded as a two-functional monomer (one functionality per lone pair electrons) and CCl4 is considered to be a four-functional monomer (one functionality per C–Cl bond). If this polymerization system follows the conventional principles of polymer chemistry for hyperbranched or cross-linked polymer, the three-dimensional (3D) chemical structure will be achieved as depicted in Scheme S1. However, our experimental results are not entirely the case as mentioned above. Through our careful characterization and analysis, we confirmed that the polymerization of [:C≡C:]2− and CCl4 in this work follows the 2D planarly oriented polymerization for 2D carbonaceous materials with excellent electric conductance as described in Scheme S2. The 2D laminated structure is confirmed by the strong (002, 2θ = 26.5°) and weak (004, 2θ = 54.6°) diffraction signals [43, 44] in the XRD patterns of Fig. 1b, which are the characteristic signal of typical laminated carbonaceous materials. The sharp and shoulder signal at 2692 cm−1 in the Raman spectra of Fig. 1c indicated the 2D graphitic nanosheets mainly consist of multilayer graphene . In addition, the 2D graphitic nanosheets show the wide absorption property up to the near-infrared region as illustrated in Fig. 2b, which is derived from their 2D conjugated electronic structure. The negative flat band voltage of − 0.81 V in the Mott–Schottky curves of Fig. 2c indicates the electron-rich feature of 2D graphitic nanosheets. These results suggest the excellent absorption and electron-donating properties, which is propitious to the generation and diffusion of photo-generated carriers to strengthen the built-in electric field for the improvement in organic AOC-PR performance.
The above polymerization processes are also monitored by the HRTEM image. Figure 4d shows a lot of stripe-like carbonaceous structure, which are regarded to be some carbon species for nucleophilic [:C≡C:]2− and electrophilic C(Cl) to be incompletely put into the crystal lattice of graphene. These quasi-order carbon species are able to be observed more clearly in the magnified HRTEM image of Fig. 4e and very similar to the ordered carbon atoms in the graphene nanosheet as shown in the 3D model of Fig. 4f. When the carbon species is completely put into the crystal lattice of graphene further forming 2D graphitic nanosheets in Fig. 4c, the diffraction pattern of honeycomb-like hexagonal structure is illustrated in the inset of Fig. 4d. These results fully proved that 2D graphitic nanosheets are synthesized by 2D-polymerizing the electrophilic and nucleophilic carbon species as described in the diagrammatic sketch of Scheme 1 and the specific chemical route of Scheme S2.
3.2 Nanographite-doping photorefractive devices
In this work, the unique purpose to dope the 2D graphitic nanosheets into CRA-CSN75-Cz25/ECZ/PC61BM is to further improve its AOC-PR performance by strengthening the built-in electric field as shown in Fig. 7c, which might be produced by the more effective diffusion of photo-generated carriers owing to the excellent electric conductance of 2D graphitic nanosheets. As a result, the orientation action of organic photorefractive chromophores is increased, i.e., the orientation enhancement effect is strengthened. Notably, to avoid the quenching effect in this work, the size of 2D graphitic nanosheets is controlled to be less than the photorefractive grating period \(\Lambda\) of 1.2 µm, which is produced by the 633 nm laser in our photorefractive measurement.
Two-dimensional (2D) graphitic nanosheets are synthesized by 2D-polymerizing electrophilic and nucleophilic carbon species, to improve organic all-optically controlled photorefractive performance due to their excellent conductive characteristics. Herein, [:C≡C:]2− in calcium carbide is applied as nucleophilic carbon species and [C(Cl)] in carbon tetrachloride (CCl4) as electrophilic carbon species. When these 2D graphitic nanosheets are doped into organic photorefractive devices, all-optically controlled photorefractive performance without any external electric field is effectively improved because the built-in electric field is strengthened inside organic nanographite-doped photorefractive devices. It is attributed to the effective charge separation under the nonuniform light field caused by the excellent electric conductance of 2D graphitic nanosheets. As a result, the orientation enhancement effect in organic photorefractive devices is strengthened. Finally, the gain coefficient of two-beam coupling in the all-optical operation is enhanced about 46.4% from 80.1 cm−1 of control device to 117.2 cm−1 of nanographite-doped device. This work provides an effective and facile approach to improve organic all-optically controlled photorefractive performance by 2D graphitic nanosheets.
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC; Grant No. U1304212) and the Development Foundation for Distinguished Junior Researchers at Zhengzhou University (Grant No. 1421320043).
Yingliang Liu designed all the experiments, supervised the ongoing of all the experiments, and prepared the manuscript. Role: Conceptualization, Methodology, Formal analysis and investigation, Writing—original draft preparation, Writing—review and editing, Funding acquisition, Resources, Supervision. Weiwei Zuo synthesized the 2D graphitic nanosheets. Role: Methodology, Formal analysis and investigation. Mingming Li performed all the photorefractive experiments. Role: Methodology, Formal analysis and investigation. Juan Li supplemented and repeated a part of photorefractive experiments. Role: Methodology, Formal analysis and investigation. Shengang Xu and Shaokui Cao supervised the experimental ongoing and discussed about the experimental results. Role: Resources, Supervision.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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