Enhanced Photorefractive and Third-Order Nonlinear Optical

Jul 9, 2014 - The doping of graphene also improved the uniformity of alignments of 4-cyano-4′-pentylbiphenyl (5CB) in PDLC because the rod-shaped li...
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Enhanced Photorefractive and Third-Order Nonlinear Optical Properties of 5CB-Based Polymer-Dispersed Liquid Crystals by Graphene Doping Shulei Li,† Ming Fu,* Haiyan Sun, Yuqiong Zhao, Yongchuan Liu, Dawei He, and Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P. R. China ABSTRACT: The enhanced photorefractive and third-order nonlinear optical properties of graphene/graphene oxide (GO)-doped polymer-dispersed liquid crystals (PDLC) were studied by two-beam coupling and z-scan methods. A higher interior electric field will be produced according to the photorefractive performance because of the high electrical conductivity of graphene. The doping of graphene also improved the uniformity of alignments of 4-cyano-4′-pentylbiphenyl (5CB) in PDLC because the rod-shaped liquid crystals tended to align along the surface of sheet structures and the alternate positions of the hexagons on the graphene surface matched well with the alkyl chains of 5CB molecules. Higher third-order nonlinear refraction and absorption coefficient can be attributed to the geometric relationship between graphene and 5CB. The GO-doped samples exhibited higher photorefractive efficiency, third-order nonlinear refraction, and absorption coefficient than undoped PDLC samples, but lower values than the graphenedoped samples because of the nonconducting nature of GO and/or weaker surface matching.

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INTRODUCTION Graphene, one of the emerging nanocarbon materials, has attracted tremendous scientific and technological interest because of its unique physical and chemical nature, including an atomically two-dimensional structure,1 high carrier mobility,2 optical transparency,3 and a unique zero band gap.4 Pure graphene materials played an important role in applications, including nanoelectronics,5 plasmonics devices,6 electrodes,7 and touch screens.3,8 On the other hand, nanocarbon materials also played important roles as one of the components in hybrid materials used in sensors,9 dielectric materials,10 energy harvesting,11 storage,12 and reinforced materials.13 When carbon materials were doped into polymer matrices, remarkable enhancement in the strength,14 electrical conductivity,15 and thermal stability16 of the polymer composites was accomplished. In the optical field, nanocarbon-based materials also showed great promise for advanced nonlinear optical performance, such as optical limiters17−21 and second22 and thirdharmonic generation.23 Pure nanocarbon materials themselves including graphene possess unique nonlinear optical properties;24,25 however, it is other different doping effects of graphene that are of primary interest here. The doping of nanocarbons into other nonlinear optical materials would enhance their properties owing to specific interactions, large surface areas, and high electrical conductivity. Previous studies mainly focused on the covalent functionalization of graphene with oligothiophenes,26 fullerenes,25 NaYF4,27 and dyes,28 in which extra energy transfer and photonic states were provided for nonlinear optics. Besides the complex synthesis of new graphene-derived complex materials, doping © XXXX American Chemical Society

with graphene (without bonding) as one of the composites may also play important roles, resulting in advanced nonlinear optical performances because of the distinguished structure and properties of graphene. However, this has not been extensively studied yet. Polymer-dispersed liquid crystals (PDLC) are typical photorefractive materials based on the anisotropicity of liquid crystal (LC) molecules. LC molecules have their own anisotropic shape and dipole moment, making them extremely sensitive to the electromagnetic field, electric field, and structures the doping-induced geometries create in PDLC. Therefore, their nonlinear and photorefractive properties are closely related to the molecular alignment of LCs. Improved diffraction efficiencies and electro-optic effects by doping nanoparticles, such as CdSe,29−31 CdS,32,33 dye,34,35 and gold nanoparticles,36,37 have already been realized in recent years. Graphene has higher electric conductivity, better geometric morphology for LC molecular alignments, and lattice matching to alkyl chains,38−42 making graphene the most suitable candidate among all the other doping components. Therefore, graphene/graphene oxide (GO)-doped 4-cyano-4′-pentylbiphenyl (5CB)-based PDLC was developed to improve its photorefractive and third-order nonlinear optical properties. The mechanism for the enhanced property using graphene was elucidated. Received: May 29, 2014 Revised: July 3, 2014

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Figure 1. (a) Diffraction efficiencies of pure PDLC and PDLC doped with different concentrations (0.001, 0.005, and 0.01 wt %) of graphene or graphene oxides. (b) The enlarged diffraction efficiencies of pure PDLC and PDLC doped with different concentrations of graphene oxides.

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EXPERIMENTAL METHODS Fabrication of Graphene-Doped PDLC. The PDLC samples were fabricated by a solvent-induced phase separation method using different PDLC solutions, consisting of PMMA, nematic LCs (5CB), C60, and graphene/GO. PMMA (17.143 mg/mL) and 5CB (40 mg/mL) were dissolved in toluene at 70 °C using a magnetic stirrer. The content of C60 dopant was 0.3 wt % relative to the total mass of 5CB and PMMA. Graphene (diameter 0.5−2 μm, thickness 0.8−1.2 nm, Nanjing XFNANO Materials Tech Co.) or GO (diameter 1−5 μm, thickness 0.8− 1.2 nm, Nanjing XFNANO Materials Tech Co.) were dispersed in toluene for 4 h by an ultrasonic method to prepare 0.2 mg/ mL graphene and GO solutions, respectively. Six PDLC solutions with doping concentrations of 0.001, 0.005, and 0.01 wt % were prepared by adding appropriate amounts of graphene/GO solutions. The prepared PDLC solutions were placed in an ultrasonic oscillator for 2 h. Two pieces of cleaned indium−tin oxide (ITO)-coated glass were separated using 8 μm thick plastic film spacers. Finally, the PDLC samples were obtained after the pure PDLC or doped PDLC toluene solutions were infiltrated into the ITO substrate by capillary forces. Optical Characterizations. The photorefractive diffraction efficiencies were determined by a two-beam coupling setup, using an 8 mW He−Ne laser (632.8 nm) as the light source. An emitting p-polarized beam was obtained using a retardation plate (λ/2) and then divided into two beams with equal intensity by a beam splitter. The two laser beams had a fixed intersection angle of 12°. External electric voltages from 0 to 16 V were applied between two ITO slides by a direct current power supply. The third-order nonlinear optical properties of the PDLC samples were studied by standard closed- and openaperture z-scan methods. A He−Ne laser beam was divided into two beams with equal intensity. One beam acted as the probe, and the other beam acted as the reference. The automatically controlled testing platform for z-scan was mainly composed of two silicon photodiodes with 3 × 3 cm2 detection area measured by two multimeters (HIOKI 3238). The samples were loaded on a motorized translation stage along the zdirection. The measurements were automatically recorded using a computer, and the transmittances of materials were automatically calculated. All the experimental procedures were performed at room temperature.

on the energy transmission asymmetry between two polarized beams. Figure 1a shows the diffraction efficiencies (η) of the samples, GN PDLC 0.001, GN PDLC 0.005, GN PDLC 0.01 (0.001, 0.005, and 0.01 wt % concentrations of graphene in PDLC), GO PDLC 0.001, GO PDLC 0.005, and GO PDLC 0.01 (0.001, 0.005, and 0.01 wt % concentrations of GO in PDLC), and undoped PDLC, when different applied electric field intensities in the range 0−2 V μm−1 were applied. Figure 1b shows the enlarged diffraction efficiency curves of the samples PDLC, GO PDLC 0.001, GO PDLC 0.005, and GO PDLC 0.01, shown in Figure 1a. The diffraction efficiencies of pure PDLC and graphene/GO-doped PDLC increased with increasing electric field in the range 0−2 V μm−1. Undoped PDLC had the weakest improvement. The graphene-doped PDLCs (heavy continuous line in Figure 1a) have much higher efficiencies than pure PDLC and GO-doped PDLCs (dotted line). Moreover, the diffraction efficiencies of the graphenedoped PDLCs were enhanced more than that of the GO-doped PDLCs and pure PDLC under the same electric field intensity condition, when the electric field intensity increased from 0 to 2 V μm−1. Among the graphene and GO samples with different concentrations, GN PDLC 0.01 had the strongest diffraction efficiency (pink line with down triangle symbols) and the efficiency improved with increasing electric field. The efficiency and its improvement decreased with decreasing doping concentration. In fact, although not as high as graphenedoped PDLCs, the samples doped with GO (Figure 1b) still exhibited higher efficiencies and higher efficiency improvement with increasing electric field than pure PDLC. The energy transmission kinetics between two polarized beams arising from the photorefractive effect was also measured by the two-beam coupling setup, as shown in Figure 2. The signal beam was first amplified using the pump beam until a stable refractive index grating was formed. The same electric field, 1 V μm−1, was applied to the samples GN PDLC 0.001, GN PDLC 0.005, GN PDLC 0.01, and pure PDLC. The transmission intensity variations of the signal beams were normalized. The recorded grating time significantly increased with increasing concentration of graphene. Because the photorefractive effects can be attributed to the reorientation of 5CB molecules, the presence of graphene nanosheets may hinder the reorientation of 5CB. Therefore, the highest concentration of graphene afforded the longest stabilized grating time. The photorefractive effect is induced by photoinduced spacecharge fields by the interference of two laser beams. Photogenerated electrons and holes migrate by diffusion, producing an interior electric field between bright and dark

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RESULTS AND DISCUSSION The photorefractive properties of graphene and GO-doped PDLC were evaluated by two-beam coupling experiment based B

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electric field condition in two-beam coupling experiments because of its low clearing point of 35 °C. As shown in Figure 3b,c, the 5CB molecules would align equally in all the directions under homogeneous light conditions. When graphene particles were added to PDLC, the 5CB molecules nearby graphene may lie on the graphene surface, resulting in identical alignments, as shown in Figure 3c. However, the orientations of overall individual 5CB molecules and those adsorbed on graphene were still random. The light interference between the two incidence laser beams resulted in the realignment of 5CB molecules, as shown in Figure 3d,e. More uniform realignment of 5CB molecules can enhance variations of refractive index under interferential light condition. The 5CB molecules had a similar but not uniform orientation between the light and dark areas in PDLC, mainly perpendicular to the angular bisector of the two laser beams (Figure 3d). The 5CB molecules adsorbed on graphene may have a more uniform orientation (Figure 3e) than those without graphene because of the specific interactions between the 5CB molecules and graphene. A higher concentration of graphene may produce more 5CB molecules with uniform orientation, leading to enhanced photorefractive efficiency, as shown in Figure 1. GO has a similar sheetlike structure as graphene, contributing to the reorientation of 5CB molecules. However, the presence of extra surface functional groups, such as epoxides, hydroxyls, and carboxyl groups, prevents the molecular interactions between the GO and LC molecules. Therefore, the uniformity of the orientation of 5CB molecules adsorbed on GO is less than those adsorbed on graphene. Moreover, because of the poor electrical conductivity of GO, its doping effect on photorefractive efficiency was much weaker, even though higher concentrations of GO resulted in more uniform alignments and improved photorefractive efficiency. Because graphene synchronized orientations of nearby 5CB molecules, graphene and these molecules would rotate together under interferential light condition. A larger moment was needed during the rotation to slow down the final reorientation time (stabilized grating time). Therefore, increased grating times were observed for higher doped concentrations of graphene, as shown in Figure 2. Besides the second-order nonlinear optical property (photorefractive experiments), the third-order nonlinear optical property of graphene-doped PLDC was also investigated by the z-scan method at room temperature. Figure 4a,b shows the closed- and open-aperture z-scan results for the pure PDLC, graphene-doped PDLC, and GO-doped PDLC. All the samples showed reverse saturable absorption and self-focusing behaviors (positive refraction coefficient). The graphene-doped samples had higher refraction and absorption coefficients than the undoped and GO-doped PDLC samples. The above-mentioned coefficients of the GO-doped samples were higher than those of the undoped samples. Higher doping concentrations were related to the higher refraction and absorption coefficients. Because no electrical conductivity is involved for third-order nonlinear optical performance, the geometrical relationship between 5CB molecules and graphene/GO improved this performance. However, the 5CB molecules remained in the nematic state in the z-scan experiments, indicating a different 5CB/graphene relationship than that mentioned for the photorefractive effect. 5CB molecules had a uniform orientation in each LC domain. The orientations of different domains in the PDLC samples were random under natural light conditions.

Figure 2. Stabilized grating times of pure and graphene-doped PDLC with different graphene concentrations at an external electric field of 1 V μm−1 in two-beam coupling experiments.

interferential fingers. (An extra electric field is applied to the samples to increase the charge carrier separation efficiency.) Because the interior electric field was induced by the reorientation of the LC, the refractive index of PDLC changed. Because of the excellent electronic conductivity of graphene, the migration and separation rate of charge carriers increased in the graphene-doped PDLC sample. More charge carriers remaining in the laser interference result in a stronger interior electric field. Therefore, the diffraction efficiencies improved with increasing doping concentrations of graphene in PDLC under the same external electric field condition. Moreover, the rodlike LC molecules tend to align along the surfaces of nanostructures that graphene/GO can provide.39−41,43 Importantly, the alternate positions on the hexagons within the graphene surface and the alkyl chains of 5CB geometrically match well.40 Therefore, graphene has extraordinary geometrical interactions with 5CB molecules, as shown in the schematic in Figure 3a. Figure 3b−e shows the mechanism of photorefractive effects schematically when 5CB and graphene interact under interferential laser light condition. The LC, 5CB, worked in an isotropic state under the extra

Figure 3. Schematic illustration of the mechanism of 5CB alignment in pure PDLC and graphene-doped PDLC samples at room temperature under homogeneous light or interferential laser beam condition. (a) The geometrical relationship between single graphene and 5CB molecules. 5CB alignments in (b) pure PDLC and (c) graphenedoped PDLC under homogeneous light condition. 5CB alignments on (d) pure PDLC and (e) graphene-doped PDLC under two interferential laser beams condition. C

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Figure 4. (a, b) Third-order nonlinear optical properties of pure PDLC, graphene-doped PDLC, and graphene oxide-doped PDLC samples recorded by (a) closed- and (b) open-aperture z-scan curves. (c) Schematic illustration of the mechanism of graphene-doped PDLC sample with the laser energy focused on the center of the sample.

performance of the resulting composite. A higher interior electric field will be produced according to the photorefractive performance under interferential light conditions because of the high electrical conductivity of graphene existing in the sample. The geometrical relationship between graphene and 5CB molecules, including the preferential alignments of rod-shaped LCs on sheets and the epitaxial interactions between the hexagons on the graphene surface and the alkyl chains of 5CB molecules, may play important roles in the improvement of both photorefractive and third-order nonlinear optical properties. This study provided an efficient method for significantly improving the nonlinear optical performances of a PDLC system using small amounts of nanocarbon doping, which holds great promise for the development of advanced optoelectronic and energy storage materials and devices. Moreover, this study provides further physical and chemical insights into the relationship between graphene and LCs with respect to the orientation-related second/third-order nonlinear optical performances.

The light intensity significantly increased when a laser beam was focused onto the PDLC sample. The orientations of the domains may be redirected parallel to the transverse electric field (TE) directions of the laser beams, as shown in the center of Figure 4c, according to third-order nonlinear optical effects. Because the domain size of 5CB molecules was on the same order of magnitude as graphene, graphene can interact with the 5CB domains in two ways. Graphene can be located between the two LC domains, thus connecting and synchronizing the orientations of the two nearby domains (point M in Figure 4c). Alternatively, 5CB domain area may be enlarged by the doping of graphene (point N in Figure 4c). Both routes improve the uniformity of the orientation of 5CB molecules. Thus, the orientation-dependent third-order nonlinear refraction coefficient can be enhanced by the doping of graphene. The enhanced nonlinear absorption can also be attributed to the more uniform orientation of 5CB molecules. The doping of GO particles in PDLC would play the same role, even though they cannot orient 5CB molecules as efficiently as graphene. A lower improvement on the refraction and absorption coefficients is expected in the GO-doped samples than in the graphene-doped samples.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +861051688018. Tel: +861051688018. E-mail: mfu@ bjtu.edu.cn.

CONCLUSIONS In summary, the improvement in the photorefractive and thirdorder nonlinear optical properties of graphene/GO-doped PDLC samples was studied. Small amounts of graphene doping of a PDLC system significantly improved the nonlinear optical

Present Address †

No. 7th Research Institute, China Electronics Technology Group Corp., Guangzhou, P. R. China. D

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (grants nos. 91123025, 50902008, 61335006, 61378013), Beijing Higher Education Young Elite Teacher Project (no. YETP0574), and the National Basic Research Program of China (nos. 2011CB932700, 2011CB932703).

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