Many opportunities have been provided for nonlinear optical (NLO) devices by the rapid expansion of nanoscience and nanotechnology. A large number of nanomaterials have been shown to possess considerable NLO properties, which cultivate the design and construction of nanoscale optoelectronic and photonic devices , .
The nanocarbons from 3D carbon nanoparticles (graphite) to 0D fullerenes, to 1D carbon nanotubes (CNTs), and then to 2D graphene display significant NLO properties, which can be useful for optical applications. Graphene oxide (GO) comprises the 2D nanostructure of a combination of carbon and oxygen functional groups. GO possesses remarkable electronic, optical, and mechanical properties, and it is attractive for a wide range of technological applications. One possible route to exploit these functionalities is to use GO to create composite materials.
The synthesis of magnesium oxide (MgO)-GO nanocomposites with high-rate adsorption of methylene blue was reported by Heidarizad and Şengör, and they concluded that the high adsorption capacity of MgO-GO nanocomposites makes it a promising adsorbent for water and wastewater treatment . The hydrogen adsorption of magnesium-doped GO was studied using the density functional theory calculations . It has been found that hydroxyl can be reduced from the surface of GO by magnesium doping on the matter whether the hydroxyl exhibits an acidity or alkalinity.
Song et al. reported on the synthesis and NLO properties of reduced GO hybrid material covalently functionalized with zinc phthalocyanine (ZnPc) using the Z-scan technique at 532 nm with 4 ns laser pulses. They showed that RGO-ZnPc exhibits much larger NLO absorption coefficient and better optical limiting performance than those of individual GO, ZnPc, and GO-ZnPc hybrid. This is due to the combination of different NLO mechanisms containing the excited state absorption arising from numerous localized sp2 carbon configurations, two-photon absorption (TPA) from the sp3 domains, and saturable absorption (SA) from the sp2 clusters in the RGO moiety, the reverse SA (RSA) originating from the ZnPc moiety, and the contribution of the efficient photo-induced electron transfer or energy transfer process between ZnPc and RGO .
Mansour et al. considered the NLO properties of carbon-black suspensions and discussed the limitations and optimization of carbon-black suspension-based optical limiters for nanosecond pulsed laser using the Z-scan technique . Tutt and Boggess reviewed the NLO processes in various materials, such as fullerenes, organometallics, carbon-black suspensions, semiconductors, and liquid crystals, which can be used in passive optical limiting devices. Specifically, they considered the mechanisms of RSA, two-photon and free-carrier absorption, nonlinear refraction, and induced scattering of these materials. They reported that the fullerenes are RSA materials and may have application to optical limiting for the protection of eyes and sensor from harmful radiations . Hasan et al. reviewed the fabrication, characterization, incorporation, and operation of SWNT-polymer and graphene-polymer composites as saturable absorbers for the generation of ultra-short laser pulses through passive mode locking. In their research, CNTs showed ultrafast second- and third-order nonlinearities and SA in the near-infrared (NIR) region . In the other researches, graphene shows ultrafast carrier relaxation time and ultra-broadband resonate NLO response . Some research findings have shown that graphene represents weak optical absorption due to high transparency essence and low band-gap energy and it is not arbitrary for NLO applications . On the contrary, several works have reported on the NLO properties of GO and their results showed that this behavior is due to nonlinear scattering, TPA, and/or excited state absorption , , .
Up to now, few works have focused on the synthesis and investigation of optical, electrical, and thermal properties of MgO-GO. However, no work has yet been done on the NLO properties of MgO-GO. Here, we investigated the effect of temperature on the structural and NLO properties of MgO-GO nanocomposites. MgO-GO samples were synthesized using the hydrothermal method. Moreover, the morphological, structural, and linear optical properties of MgO-GO nanocomposites were also investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and UV-visible (UV-vis) spectroscopy. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of MgO-GO nanocomposites were calculated by the Z-scan technique and the effect of growth temperature was considered.
2 Materials and methods
2.1 Sample preparation
GO was synthesized using the process in Ref. . In the first step, 15 mg GO was dissolved in 20 ml distilled water (1 h). Another solution was prepared by the dispersion of 5 mg MgO in 30 ml ethanol and continuously stirred for 2 h at 80°C. Then, the two solutions were mixed. The suspension was then transferred into 200 ml Teflon-lined stainless autoclave and heated at 150°C for 6 h. The reaction system was cooled down to room temperature naturally, which afforded well-crystallized MgO-GO. The resulting powders were washed several times with distilled water and then dried in an oven at 60°C overnight to obtain the final product. The sample was labeled as MgO-GO1. The same processes were repeated at 180°C and 210°C instead of 150°C, and these samples were tagged as MgO-GO2 and MgO-GO3, respectively.
The structural properties of the MgO-GO sample were analyzed by a Philips X’Pert PRO Hola XRD with Cu Kα radiation with λ=1.5404 Å. The absorption spectrum of the MgO-GO sample was investigated using a Perkin-Elmer UV-vis spectrometer between 200 and 800 nm. FTIR spectra were obtained with a Jasco FTIR-410 spectrophotometer from 500 to 4000 cm−1. The SEM images of MgO-GO nanocomposites were acquired using a VEGA/TESCAN, XMU.
2.3 NLO characterization via the Z-scan setup
The schematic diagram of the Z-scan experimental setup for the measurement of the NLO parameters of MgO-GO samples is shown in Figure 1. The Z-scan technique is based on the spatial beam broadening and narrowing of the Gaussian beam in far field due to the presence of optical nonlinearity. A Q-switched Nd:YAG laser (Ekspla NL640 model, 532 nm, 10 ns, 200 Hz) was used as a light source. The sample was moved in the direction of light incidence near the focal spot of the lens. The radius of the beam waist ω0 was calculated to be 37 μm at the focal point. The Rayleigh length z0=πω02/λ was estimated to be 8.08 mm, much greater than the thickness of the sample, which is an essential prerequisite for Z-scan experiments. The intensity of the laser beam is 1.13×108 W/cm2 at the focal point. This setup was applied previously for the investigation of the NLO properties of Mg(OH)2 nanostructures .
3 Results and discussion
3.1 Structural properties
Figure 2 shows the XRD spectra of GO, MgO and MgO-GO samples synthesized at various temperatures. As shown in Figure 2, in MgO-GO2 and MgO-GO3 samples, the exfoliation and reduction of GO are revealed by the complete absence of the peaks at 2θ=10.01° and 8.79°, which are seen for GO and MgO-GO1 samples, respectively. A shift to low angle is observed for this peak in two mentioned samples. The partial shift of this peak and consequently increase in interlayer distance presumably is associated with the elimination of a few oxide groups of the GO.
A peak at 2θ=26.6° is observed in all samples, which correspond to the characteristic peak of hexagonal pristine graphite and shift to the larger angles by adding MgO to GO as well as increasing the temperature.
After doping GO with MgO, a broad peak has been observed in all nanocomposites. This peak for the MgO-GO1 sample is started from 2θ=12° to 2θ=33°. Also, the range of the wide peak is from 2θ=14° to 2θ=30° for the MgO-GO2 sample and from 2θ=16.5° to 2θ=29.5° for the MgO-GO3 sample. These peaks centered at 2θ=24° for all three samples. The appearance of this broad peak at 24° indicates the restoration of van der Waals interaction between the carbon frameworks on the graphene sheets through reduction , , , . In addition, the appearance of these peaks in all samples after the addition of MgO announces the presence of the amorphous phase of MgO, with almost no crystalline structure. It is clear that the broadening of these peaks decreased by increasing the growth temperature. As the growth of MgO is related to growth temperature, by increasing the temperature, MgO peaks appear on the XRD pattern and the improvement of the crystalline phase can be seen.
For comparison, the XRD pattern of the MgO sample is plotted in Figure 2 as well. As can be seen from the XRD spectra, there is no clear peak for MgO in the MgO-GO1 sample synthesized at 150°C. By increasing the growth temperature to 180°C and 210°C, MgO-GO2 and MgO-GO3 nanocomposites clearly exhibit the peaks assigned to MgO. The sharper peaks in the MgO-GO3 sample than in the MgO-GO2 sample show that growth temperature is an important factor for the fabrication of MgO-GO nanocomposites.
To estimate the crystallite size of MgO-GO nanocomposites, the Hall-Williamson plot was used . The crystallite size values of MgO-GO nanocomposites are listed in Table 1. In Table 1, the crystallite size of MgO-GO nanocomposites increased by increasing the growth temperature. Dreyer et al. showed that the functional groups such as carbonyl, hydroxyl, epoxide, and carboxyl at the GO surface may serve as nucleation sites that make possible the growth of nanomaterials on the GO surface . By increasing the growth temperature, the peaks related to GO become weaker, and in return, the peaks attributed to MgO becomes sharper; hence, the functional groups decreased as the temperature increased. Therefore, there are more nucleation sites for MgO in lower temperatures that lead to a smaller crystallite size .
Grain size and band-gap values of GO and MgO-GO samples synthesized at different growth temperatures.
|Crystallite size (nm)||2.71||4.92||9.64||40.75|
|Band gap (eV)||3.3–4.2||2.6–3||4.2–5.9||4.6–6|
3.2 Linear optical properties
The UV-vis spectra and Tauc plot of GO and MgO-GO samples are presented in Figure 3.
In Figure 3A (inset), the UV-vis spectrum of GO exhibited characteristic absorption bands with double absorption peaks: an intense absorption peak located at 230 nm and a weak one at 300 nm, which is marked by an arrow in Figure 3A (inset). The first peak is related to π-π* transition of the C=C bond and the latter one corresponds to n-π* transition of the C=O bonds of GO.
The UV-vis spectra and Tauc plot of MgO-GO1 sample are shown in the inset of Figure 3B. For the MgO-GO1 sample synthesized at 150°C, it can be seen that the weak peak located at 300 nm has disappeared as well as a red shift of the peak at 230 nm toward 265 nm due to the doping of GO with MgO. This red shift shows the reduction of GO and arises from the restoration of a π-conjugation network of reduced GO. In addition, the disappearance of the weak absorption peak at 300 nm vouches for the elimination of oxygen groups in reduced GO , , .
Figure 3C and D (inset) shows the UV-vis spectra and Tauc plot of the MgO-GO2 and MgO-GO3 sample produced by MgO doped-GO at 180°C and 210°C, respectively. These absorbance spectra exhibited a strong absorption in the region from 200 nm and decreased toward longer wavelengths. There is no trace of the absorption peak, which is observed in 265 nm for MgO-GO1, and it is vanished by increasing the growth temperature to 180°C as well as 210°C. As can be seen from the absorbance spectra, the peak is located at 230 nm for the MgO-GO1 sample that appeared as the shoulder for MgO-GO2 and MgO-GO3 samples marked by an arrow in Figure 3C and D (inset).
The absorption coefficient (α) of these samples can be calculated from UV-vis absorption data and use the formula of the Beer-Lambert law. α is used to calculate of band-gap energies, nonlinear absorption coefficients (β), and nonlinear refractive indices (n2) of these samples.
From the UV-vis data, the optical band-gap values were calculated using the Tauc relation, αhν=A(hν−Eg)n, where α is the absorption coefficient, hν is the energy (eV), Eg is the band-gap energy, and A is a constant. The parameter n is indicative of the nature of transition and for these samples is equal to ½.
The optical band-gaps of the four samples, GO and MgO-GO samples, were determined using a linear extrapolation to the Tauc plot as shown in Figure 3A–D. As the band-gap of GO is defined as a function of the oxidation degree or the contents of oxygen addends, it is not constant. Therefore, the amorphous GO sheet having a nonuniform oxidation level cannot exhibit sharp adsorption edges in the Tauc plot. Therefore, the optical band-gap of GO may be anywhere between semiconducting 1 eV and as high as 4.5 eV . From the Tauc plot shown in Figure 3A for the GO sample, the optical band-gap value can be measured in the range of 3.3–4.2 eV, which has good agreement with the values reported for GO in the literature , , .
In Figure 3B, for the MgO-GO1 sample synthesized at 150°C, the Tauc plot exhibits sharp adsorption edge and a narrow range for the band-gap value is evaluated as 2.6–3 eV.
A range of band-gap, from 4.2 to 5.9 and from 4.6 to 6, was calculated for the MgO-GO2 and MgO-GO3 samples synthesized at 180°C and 210°C, respectively. This wide energy band-gap values may be due to the uneven oxidation of sheets. The optical band-gap values for all samples are listed in Table 1.
As can be understood from the XRD results, MgO in the MgO-GO1 sample has an amorphous structure, and by increasing the temperature to 180°C and 210°C for the MgO-GO2 and MgO-GO3 samples, the MgO structure is more and more crystallized. The XRD results confirm the calculated optical band-gap values from the Tauc plot drawing. With a complete crystallization process of MgO, the samples tend to semiconductor behavior and the band-gap values increase.
On the contrary, the calculated band-gaps of MgO-GO nanocomposites in all three samples are significantly smaller than the wide band-gap energy 7.8 eV expected for the bulk, pure, crystalline MgO. Therefore, it seems that MgO as a result of addition to GO can be close to metallic conduction behavior .
The FTIR spectra for the GO and MgO-GO samples at different temperatures are shown in Figure 4. In Figure 4, several vibration frequencies are recorded in the GO sample, which indicates that the GO specimen has hydroxyl (-OH), carbonyl (-C=O), and epoxy (C-OC) groups. The spectrum of GO exhibits the presence of C-O (1050 cm−1), O-H (1412 cm−1), aromatic C=C (1640 cm−1), and C=O (1726 cm−1). The surface hydroxyl groups and water molecules with the main absorption band at 3400 cm−1 are assigned to the O-H group symmetry and asymmetry stretching vibrations . This peak almost disappeared after increasing the growth temperature, suggesting a possible reduction of GO by MgO. For the MgO-GO samples by the growth temperature increasing to 210°C, the C-O absorption peak vanishes. The absorption band at 860 cm−1 contributed to the characteristic absorption peak of cubic MgO, which increases with temperature enhancement. It can be seen that the intense characteristic vibration of MgO exists in the band ranging from 500 to 1000 cm−1 with the absorption peak at 860 cm−1, indicating the formation of cubic MgO . The Mg-O stretching is observed below 700 cm−1 during the heat treating process with different temperatures at 180°C and 210°C .
3.3 Morphological analysis
To clarify the morphological properties of the synthesized MgO-GO materials, the SEM analysis was performed. The morphology of MgO-GO nanocomposites was elucidated from SEM images at lower and higher magnifications as shown in Figure 5.
A panoramic view of samples synthesized at three different temperatures demonstrates that all three samples have a flake-like morphology, which confirms the removal of oxygen groups from the plane and the edges of reduced GO.
The SEM results indicate that the graphene sheets are exfoliated and decorated with MgO particles. Comparing the SEM images shows that, by increasing the growth temperature of MgO-GO samples, more MgO particles cover the surface of GO. The appearance of peaks corresponding to MgO in the XRD spectra with the increasing growth temperature confirms the SEM results. From the SEM images, it is clear that MgO particles are anchored nonuniformly on the surface of the reduced GO sheets.
3.4 NLO studies
The experimental setup for NLO measurements was shown in Figure 1 (Section 2). In the closed-aperture Z-scan experiment, the sample is transferred along the z-direction and the transmitted intensity is measured via an aperture in the far field. The normalized transmittance is plotted versus sample position z, which is measured with respect to the focal plane. From this curve, the sign of nonlinear refractive index n2 consequently self-focusing or defocusing the quiddity of the sample can be determined. The magnitude of n2 can be calculated from the closed-aperture curve as well as the following equations :
where I0 is the incident intensity at a focal point and Leff is an effective sample thickness, which is calculated as
where L is the actual thickness of the sample and the linear absorption α is measured from UV-vis data and the Beer-Lambert law for the MgO-GO samples. |∆φ0| in Eq. (1) is related to ∆φp−v as
where ∆φp−v is the normalized difference between the peak and valley in the transmittance T(z) and S is aperture’s linear transmission, which is 0.3 for the closed-aperture Z-scan experiment in this work.
In Figure 6, the normalized transmittance is plotted as a function of the z-direction for MgO-GO nanocomposites synthesized at the different growth temperatures. In Figure 6, the sign of n2 for all three samples is negative because the curves exhibited a valley after a peak. This situation demonstrates that all three samples act as self-defocusing materials .
Furthermore, the nonlinear refractive indices of the MgO-GO samples synthesized at 150°C, 180°C, and 210°C were obtained from the above equations in the order of the 10−12 cm2/W, as listed in Table 2.
NLO parameters of GO-doped MgO samples.
|Im χ3 (esu)||1.7×10−9||2.7×10−9||3.2×10−9||–|
|Re χ3 (esu)||8.3×10−11||9.8×10−11||1.1×10−10||–|
An open-aperture Z-scan experiment is carried out by the removal of the pinhole in Figure 1; thus, the transmitted beam arrives at the detector after passing the sample. The normalized transmittance is plotted versus the position of the sample (z). The presence of a peak or a valley in open aperture determined the NLO response of the sample to the light source. The magnitude of the nonlinear absorption coefficient β is calculated by fitting the experimental data to the following equation:
where q0(z,)=βI0Leff/(1+z2/z02) and Leff, β, I0, and z0 are the effective thickness sample, nonlinear absorption coefficient, laser intensity at the focal point, and diffraction length of the beam, respectively. The m parameter determines the order of the multiphoton processes; m=1 applies for TPA, m=2 for three-photon absorption (3PA), etc.
The open-aperture Z-scan curves are shown in Figure 7 for all the three MgO-GO samples. All the samples exhibit the transmittance with the minimum at z=0, which demonstrates RSA behavior for the MgO-GO samples synthesized at different temperatures. Using Eq. (4), the TPA curves are fitted to the experimental date that shows that the nonlinear absorption of these nanocomposites is predominantly due to the TPA mechanism. The solid curves in Figure 7 show the theoretical fitting of Eq. (4) to the experimental data. The nonlinear absorption coefficient is extracted by fitting the experimental data with Eq. (4) as a fitting parameter that is listed in Table 2 for easy comparison. The nonlinear absorption coefficient β of the MgO-GO1 sample prepared at 150°C is obtained in the order of 10−8 cm/W, whereas for two other samples it is found to be an order of magnitude larger than the β of the first sample and calculated in the order of 10−7 cm/W. Furthermore, the real and imaginary parts of χ3 are listed in Table 2. The imaginary part of χ3 is related to β as 
The real part of χ3 is related to n2 as
where in Eq. (5) and Eq. (6), n0 is refractive index, ε0 is the vacuum permittivity, λ is the laser wavelength and c is the light velocity in vacuum.
In Table 2, the Im χ3 is two orders of magnitude larger than the values for Re χ3, which means that the absorption effect is stronger than the refraction for the MgO-GO samples. An increase in the NLO absorption can occur due to the increase of optical nonlinear interaction between the radiations and the particles. Therefore, an increase in crystallite size leads to an increase in the multiple scattering; thus, it increases the effective interaction length and consequently the nonlinear absorption is increased .
The third-order optical nonlinearities χ3 are calculated using
In Table 2, the variations of growth temperature from 150°C to 210°C lead to an increase in β, n2, and |χ3| for MgO-GO nanocomposites.
To the best of our knowledge, there have been no reports of the investigation of the NLO properties of MgO-GO nanocomposites yet. Therefore, the comparison of the obtained results in this work with the same results was impossible. However, there are results for the NLO properties of other GO-based composites that show that the magnitude of the NLO parameters for MgO-GO nanocomposites is very satisfactory and that MgO-GO nanocomposites can be a promising candidate for NLO applications.
Biswas et al. studied the NLO properties of the GO-silver nanocomposite using the Z-scan technique via Q-switched Nd:YAG laser second harmonic radiation at 532 nm. They calculated β and n2 in the order of 10−7 cm/W and 10−12 cm2/W, respectively, which is in the same order of magnitude of β and n2 in this work . As silver and its composites have always been a good candidate for NLO applications, then the same order of magnitude for NLO parameters of MgO-GO and GO-Ag indicates that MgO-GO nanocomposites are a very promising candidate for NLO applications.
In this work, we have reported on the synthesis of MgO-GO nanocomposites by the simple hydrothermal method. The effect of growth temperature on the structural, linear, and NLO properties was investigated. XRD analysis, UV-vis spectroscopy, FTIR analysis, and SEM images confirmed the successful fabrication of MgO-GO nanocomposites.
The investigation of the NLO properties of MgO-GO nanocomposites under the excitation of nanosecond pulsed laser radiation at 532 nm wavelength is highlighted. The nonlinear absorption considerations show an increase in the TPA properties of MgO-GO nanocomposites with increasing growth temperature. The nonlinear absorption coefficients of MgO-GO nanocomposites are calculated in the order of 10−7 cm/W and they vary as much as an order of magnitude by an increase in growth temperature from 150°C to 180°C.
The closed-aperture Z-scan measurements show that the nonlinearity in MgO-GO samples is negative and the nonlinear refractive indices are in the order of 10−12 cm2/W and increase with increasing growth temperature.
These results suggest that MgO-GO nanocomposites may be a very promising nonlinear medium and can be open an avenue to graphene-based nonlinear photonics.
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