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Abstract
Simple and effective methods are needed to incorporate two-dimensional functional materials with distinctive nonlinear optical (NLO) response into appropriate solid-state matrices while maintaining inherent functionalities for practical applications in optoelectronic fields. Here, ultrathin molybdenum disulfide (MoS2) nanosheets and lead silicate (PbO-SiO2) gel glasses were chosen as a representative guest dopant and mother matrix, respectively. The MoS2 was introduced into the PbO-SiO2 binary gel glasses by a simple sol-gel wet chemical technique to obtain transparent and s three-dimensional monolithic bulk materials. The presence of MoS2 in the gel glasses and the formation of binary gel glasses were confirmed by various techniques. The NLO and optical limiting (OL) performances were investigated by both open-aperture (OA) and closed-aperture (CA) Z-scan techniques on nano- and picosecond timescales with the use of a laser operating at 532 nm. Our results demonstrate that the NLO effect of MoS2/PbO-SiO2 binary gel glasses was greater than that observed for MoS2/SiO2 unary gel glasses because of enhanced third order nonlinear susceptibility effects induced by the heavy metal. The OA and CA Z-scan patterns suggested that the NLO response of the MoS2/PbO-SiO2 gel glasses is mainly attributed to their nonlinear absorption and nonlinear refraction. Remarkably, the extracted OL thresholds of the MoS2/PbO-SiO2 gel glasses were 12.4 and 7.8 times as great as those recently reported in a MoS2 suspension at nanosecond timescale and MoS2/PMMA organic glass at the picosecond timescale, respectively. The present results demonstrate the feasibility and versatility of MoS2/PbO-SiO2 silica gel glasses, as a new class of highly efficient NLO and OL materials that can be applied in the field of nonlinear optics.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Inspired by remarkable developments based on graphene, numerous novel two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), black phosphorus, few-layer MXenes, bismuthine, and topological insulators have attracted interest from researchers in the fields of electronics, photonics, and optoelectronics [1–3]. TMDs consisting of hexagonal layers of metal atoms (M) sandwiched between two layers of chalcogen atoms (X) with a MX2 stoichiometry are the most studied members of the 2D material family beyond graphene. Unlike the zero-band-gap of graphene, the electronic band structures of TMDs strongly depend on the number of layers, which is determined by the coordination and oxidation states of transition metal atoms. For example, previous studies have confirmed a transition from an indirect band gap (Eg = 1.29 eV) to a direct band gap (Eg = 1.90 eV) for MoS2 as the thickness of the MoS2 decreases to a monolayer. This change accounts for a more than a five-order of magnitude enhancement of the photoluminescence quantum yield observed in monolayer MoS2 [4,5]. Meanwhile, the peak position, intensity, line width or line shape of the spectrum may change significantly with the change of the number of two-dimensional material layers,, and therefore introduces some useful optical properties, such as ultrafast broadband optical response, large optical nonlinearities, strong excitonic effects, and high carrier mobility [6–9]. Hence, TMDs are attractive candidates for a host of emerging optical and photoelectrical devices.
As a representative TMD, 2D molybdenum disulfide (MoS2) has unique photoelectron properties, such as selective exciton formation in the Brillouin valley, super exciton binding energy, and a stress controllable band-gap width. Consequently, we have extensively investigated the nonlinear optical activities of these materials, including their ultrafast and broadband saturable absorption (SA) [10–14], two photon absorption (TPA) [15,16], high third-order nonlinear susceptibility [17,18], and optical limiting (OL) [19,20]. For example, Wang et al. first studied the saturable absorption characteristics of a few-layer MoS2 dispersion under a femtosecond laser at 800 nm, and found that MoS2 has a better saturable absorption response than that of graphene giving ultrafast nonlinear optical properties [19]. Their subsequent research found that the nonlinear absorption effect of transition metal sulfide MoS2 has a strong layer-number dependence [18]. Zhang et al. found that MoS2 semiconducting nanosheets have unique size-dependent nonlinear optical properties [10]. Cheng et al. reported that the nonlinear optical properties of MoS2 changed from saturated absorption to reverse saturated absorption with increasing power [12]. For convenience of practical applications, several studies have reported the successful incorporation of MoS2 nanosheets into polymer matrices, such as polyvinyl alcohol (PVA) [11] and polymethyl methacrylate (PMMA) [20], to overcome limitations in the liquid form, such as lack of flexibility, poor stability, high losses originating from absorption of solvents, and refractive index mismatch between the liquid and vessel. The obtained polymer-based solid-state composites maintain their inherent nonlinear optical (NLO) and OL effects; however, some obstacles remain, such as low melting points, low optical laser damage thresholds, and poor mechanical properties. Notably, certain applications involve high energy laser irradiation and some unexpected harsh environments. Compared with organic host matrixes, silica glass has excellent optical transparency, long-term thermal stability, good mechanical properties, and designable microstructures. Hence, silica is expected to be a better host material for obtaining solid-state composites, especially for applications in the fields of photonics and optoelectronics. Actually, several nanomaterials, from zero-dimensional carbon dots [21], to one-dimensional carbon nanotubes [22], and 2D graphene [23] and h-BN [24], have been encapsulated in solid-state three-dimensional macrostructures of sol-gels derived from organically modified silicate (ormosil) gel glass. Notably, in this host| the nanomaterial guests retained their inherent NLO and OL properties.
As an alternative, heavy-metal oxide glasses are potential candidates for applications in optoelectronics owing to their high third order nonlinearities [25–28], which are fundamental to the development of all-optical devices. Among heavy-metal oxide glasses, lead silicate glasses are of particular interest because of their robust mechanical resistance, good thermal stability and conductivity, large free volume, easy preparation, and high transparency from UV to NIR spectral ranges [29,30]. Notably, lead is incorporated into the glasses in the form of lead oxide, in large amounts, which results in high third-order nonlinear optical susceptibilities. For example, lead silicate glasses can be hosts for Au, Ag, and Cu nanoparticles incorporation and have demonstrated considerably enhanced third-order optical nonlinearities [31–33]. However, methods reported to obtain lead silicate glass have mainly focused on conventional melting-quenching techniques [31–35], which are limited by slow and uncontrolled cooling rates, the use of crucibles, high temperatures, and inhomogeneities related to density settling, and phase separation. In particular, high temperatures restrict the application of lead silicate glasses in the field of nonlinear optics because the encapsulated NLO and OL components lose their activities during the melting process. In addition, the high photo-induced NLO susceptibility of lead silicate glasses might result in unexpected OL activities. However, to the best of our knowledge, there have been no reports on practical applications on these systems as optical limiters, especially in combination with two-dimensional materials. Hence, there is a need to design and explore such 2D materials as high-performance optoelectronic devices.
Herein, we report an efficient and simple sol-gel technique to synthesize lead silicate binary glasses encapsulated with 2D MoS2 nanosheets, as illustrated in Fig. 1. This strategy might be readily extended to prepare other high-performance 2D materials, including other TMDs, black phosphorus, few-layer MXenes, bismuthine, and topological insulators for use in solid-state devices. Composite lead silicate binary inorganic composite gel-glasses, denoted as PbO-SiO2, were fabricated by hydrolysis and co-condensation of tetraethyloxysilane Si(OC2H5)4 (TEOS) and lead (II) acetate trihydrate [Pb(Ac)2]. The presence of MoS2 in the gel glasses and the formation of binary gel glasses were confirmed with the use of a variety of characterization techniques, including scanning electron microscope (SEM) imaging, Raman spectroscopy, and UV-Vis spectroscopy. Furthermore, detailed investigations of the NLO and OL properties of MoS2/SiO2-PbO gel glasses with both 532-nm nano- and picosecond-laser excitation were performed by open-aperture (OA) and closed-aperture (CA) techniques. The results are compared and mechanisms for NLO and OL properties are proposed. Importantly, the MoS2 embedded SiO2-PbO binary gel glass had excellent NLO and OL response both under ns- and ps-excitation. Our work demonstrates that MoS2 nanosheet doped SiO2-PbO binary gel glasses are promising candidates for a new generation of NLO and OL devices.
Fig. 1. Schematic diagram of synthesis of the MoS2/PbO-SiO2 gel glasses by sol-gel wet chemical method.
2. Experimental
2.1 Materials
All chemicals were used as received. Tetraethoxysilane [Si(OC2H5)4, TEOS], lead (II) acetate trihydrate [Pb(Ac)2], ethylene glycol methyl ether (C3H8O2) and 11.8 mL of glacial acetic acid (HAc), N,N′dimethyl formamide (DMF), and HNO3 were obtained from the Chinese Reagent Corporation (China, Shanghai) and were of analysis grade. 3-Glycidoxypropyltrimethoxysilane [CH2OCHCH2O–(CH2)3–Si(OCH3)3, GPTMS] and 3-aminopropyltriethoxysilane [NH2(CH2)3Si(OC2H5)3, APTES] were purchased from Sigma-Aldrich.
2.2 Preparation
2.2.1 Preparation of 2D MoS2 nanosheets
Ultrathin MoS2 nanosheets were prepared by modifying the lithium intercalation and an exfoliation method first reported by Joensen et al. [36]. Specially, 0.8 g of MoS2 was added to a 50-mL Schlenk tube under an Ar atmosphere, followed by addition of 25 mL n-BuLi hexane solution. The mixture was stirred at room temperature for 48 h and then allowed to settle. The supernatant of the mixture was removed and the residual black solid was collected. Then Ar-saturated water was introduced into the tube to avoid sputtering under the protection of Ar, because copious amounts of hydrogen gas are released at that moment. The MoS2 nanosheets suspension was sonicated for 1 h to complete the exfoliation process and the mixture was then centrifuged at 1500 rpm for 20 min. After centrifugation, the precipitates were discarded and the supernatant was centrifuged at 10000 rpm for 30 min. The black slurry was redispersed in water and centrifuged again until the aqueous dispersion became neutral. The final nanocomposite was dried through lyophilization.
2.2.2 Preparation of MoS2 doped SiO2-PbO binary gel glasses
Optically transparent binary inorganic composite gel-glasses were fabricated via hydrolysis and co-condensation of TEOS and Pb(Ac)2. The molar ratio of [TEOS + Pb(Ac)2]:ethanol:distilled water in the precursor was 1:5:5. The molar ratio of Pb(Ac)2/TEOS was selected to be 3:7 owing to our previous work, which has shown that this is the optimum ratio in terms of texture and optical transparency. Specifically, a solution of 6.7 mL of TEOS, 8.8 mL of ethanol, 2 mL of water, and the MoS2 suspension in DMF were mixed to obtain solution 1. Then DMF was introduced as a solvent and a drying control chemical additive at a ratio of 0.5 based on the volume ratio of ethanol. A 4.88-g portion of Pb(Ac)2 dissolved in 5 mL of C3H8O2 and 11.8 mL of HAc was used to form solution 2. After solution 1 and 2 were slowly mixed, the catalyst (HNO3) was added dropwise to promote hydrolysis and poly-condensation (pH = 2–3). The resulting clear and light black sol was continuously stirred for another 4 h at room temperature and the mixture was then divided into several equal volume parts, cast into polystyrene cells individually, sealed, and left to age and dry for 3 months. The transmittance of the obtained gel glasses was simply controlled by adjusting the quantity of MoS2 dispersed in DMF. The doping levels of MoS2 were 0, 2.13×10−5, 5.33×10−5, and 10.67×10−5 (mass ratio of MoS2 power to the composite gel glass), and the corresponding linear transmittance values were 71%, 69%, 66%, and 63%, respectively. The resulting sample was light black and the surface was smooth, allowing for NLO and OL performance testing without further processing.
For comparison, MoS2 doped unary SiO2 omosils gel glasses were also prepared to reveal the advantages of the NLO and OL behavior of binary SiO2-PbO gel glasses as host materials. Specifically, 15.0, 2.6, 1.4, 20.0, and 4.0 mL of TEOS, GPTMS, APTES, ethanol, and water, respectively, were mixed by ultrasonication for 30 min. Then, 5.0 mL of the DMF suspension containing certain amounts of MoS2 was gradually added to the mixture, which was ultrasonicated for 20 min. The mixture was divided into several parts of equal volumes, individually cast into polystyrene cells, sealed, and left to age and dry for several weeks to obtain the MoS2 doped unary SiO2 gel glasses.
2.3 Characterization
The morphology of 2D MoS2 nanosheets was observed by the JEM-2100 TEM (JEOL). The sample was dripped on the copper wire with a dropper and observed at an operating voltage of 200 kV. The surface characteristics of the MoS2/PbO-SiO2 binary composite gel glass were observed with a NOVA NANOSEM 450 (FEI, USA). The sample was dried at 120 °C for 24 h, and then the fresh surface was treated with a gold spray and observed at a working voltage of 20 kV. The structure of the sample was studied by Renishaw’s Invia confocal micro Raman spectrometer and measured on a slide. The laser excitation wavelength was 514 nm. Linear optical properties of the samples were analyzed with a Shimadzu UV-2600 UV spectrophotometer. In the test, the liquid samples were placed in a 10-mm quartz cuvette and the solid samples were fixed on the sample table for direct measurements.
2.4 Z-scan measurements
The OA and CA Z-scan technique [37] were applied to investigate the NLO and OL behavior of MoS2/SiO2 and MoS2/PbO-SiO2 binary composite gel glass with an output wavelength of 532 nm. A nanosecond-light source irradiated by a Dawa-S laser (Beamtech) was a Q-switched Nd:YAG pulsed laser system having a pulse width of 7 ns, a repetition frequency of 10 Hz, a beam waist radius of ω0 of 22 µm, and a Rayleigh length of 2.86 mm. The picosecond light source irradiated by a PL2250 laser (EKSPLA) was a Q-switched Nd:YAG pulsed laser system having a pulse width of 30 ps, a repetition frequency of 10 Hz, a beam waist radius of ω0 of 23 µm, and a Rayleigh length of 3.12 mm. Liquid samples were measured in 2-mm quartz cuvettes, whereas solid samples were placed on a mobile platform and clamped directly for testing. The thickness of the composites glasses is 2mm. The cuvettes or solid samples were mounted on a computer-controlled translation stage that shifted the samples along the z axis and all test procedures were conducted at room temperature. All the gel glasses were used for Z-scan experiments without further processing.
3. Results and discussion
3.1 Structural characterization of MoS2/PbO-SiO2 gel glasses
The morphology of the as-prepared MoS2 was characterized by TEM [Fig. 2(a)]. The MoS2 was a typical structure of a few-layer nanosheets, which was well dispersed in the suspension, confirming the high quality of the prepared liquid-phase exfoliated samples. For practical applications, the development of functional materials with OL properties should be incorporated into solid-state matrices by an effective approach to design and develop industrially applicable optical limiters. Here, ultrathin MoS2 nanosheets and lead silicate gel glasses were chosen as the representative guest dopant and mother matrix, respectively. The MoS2 was introduced into the PbO-SiO2 binary gel glasses by a simple sol-gel wet chemical technique to obtain transparent and stable three-dimensional monolithic bulk materials. Figure 2(b) shows an SEM image of a fractured surface of MoS2/PbO-SiO2 inorganic gel glasses at a doping level of 2.13×10−5. This structure was mainly composed of tightly compacted granules. The MoS2 was not detected in the SEM measurements owing to its low doping level and encapsulation of the MoS2 nanosheets by the gel glasses matrices.
Fig. 2. (a) TEM image of as-prepared ultrathin 2D MoS2 nanosheets and (b) SEM image of fractured surface of MoS2/PbO-SiO2 inorganic gel glasses with a doping level of 2.13×10−5.
Raman spectroscopy is a useful method for fingerprinting a material. The layer-dependent changes of vibrational structure were used to confirm the successful incorporation of MoS2 into the PbO-SiO2 binary gel glasses. As shown in Fig. 3(a), the Raman spectrum of the as-obtained MoS2 had two characteristic peaks centered at 384 and 407 cm−1, corresponding to the vibrational modes of E12g (in-plane vibration of molybdenum and sulfur atoms) and A1g (outer plane vibration of molybdenum atoms) [38], respectively. A peak frequency difference of 23 cm−1 was observed, and the positions of these two peaks were consistent with those reported for few-layer MoS2 [39]. The two characteristic bands of MoS2 were present in the MoS2/PbO-SiO2 gel glasses but red-shifted to 395 and 463 cm−1 [Fig. 3(b)], respectively. Previous investigations have confirmed that matrices have a positive influence on the Raman spectrum of MoS2, inducing a characteristic peak shift to a lower or higher frequency [40]. The observed red-shift is attributed to coupling between the MoS2 and PbO-SiO2 matrices. Another four characteristic peaks were also detected in the Raman spectrum of the MoS2/PbO-SiO2 gel glasses, namely, 938, 1037, 1345, and 1420 cm−1, respectively. The band at 938 cm−1 is attributed to a Si-O bond stretching mode with a weak coupling to Pb2+ [41], indicating that Pb2+ acts as a glass network modifier in our present work. This is reasonable because lead oxide affects the formation of glass structures in lead silicates glasses. Notably, Pb2+ can promote the formation of a network if the PbO concentration in the glass reaches approximately 50 mol% [42]. Otherwise, Pb2+ only modifies the glass network. In the precursors, the molar ratio of Pb(Ac)2/TEOS was 3:7, suggesting that PbO acts as a modifier in the current investigation. Therefore, the introduction of PbO into the vitreous SiO2 glass likely caused some Si-O bonds to break, which further confirmed the above Raman results. The most intense peak was centered at 1037 cm−1, and two weak bands at 1345 and 1420 cm−1 are attributed to Si-O-Si anti-symmetric stretching vibrations, -CH2 bending vibration and -COOH double tooth vibrations [43], respectively. All the above results indicate that MoS2 nanosheets were successfully incorporated into the lead silicates gel glasses.
Fig. 3. Raman spectrum of (a) as-prepared ultrathin MoS2 and (b) PbO-SiO2 and MoS2/PbO-SiO2 gel glasses.
3.2 Linear optical properties of MoS2/PbO-SiO2 gel glasses
Digital photographs of the MoS2/PbO-SiO2 inorganic gel glasses in Fig. 2 strongly suggest the successfully introduction of MoS2 nanosheets. The glasses had a light brown color, which became darker as the doping level was increased. Characters viewed through the glasses could be clearly visualized by the naked eye, suggesting that the inorganic gel glasses were highly transparent to visible light. None of the gel glasses had any visible sediments and all were free of cracks, confirming a homogeneous dispersion of the guest MoS2 dopants and good mechanical properties of the gel glasses, which are important features for practical applications.
The linear optical properties of the MoS2 suspension and MoS2/PbO-SiO2 gel glasses were further investigated by UV/Vis spectroscopy, as shown in Fig. 4(a) and (b), respectively. There were two absorption shoulder peaks in the range of 300–800 nm for the MoS2 nanosheets: one peak located at approximately 620 nm corresponds to the transition from the K point of the Brillouin zone; the other peak near 410 nm is attributed to a direct transition from the deep valence band to the conduction band [44]. Compared with 2H-MoS2, the excitonic features of the two peaks were relative weak, which we attribute to lithium intercalation-induced lattice distortion and a phase transition from 2H to 1T [45]. We note that the two characteristic bands were weakened after introduction into the PbO-SiO2 gel glasses. A similar phenomenon has also been reported in MoS2/PMMA organic glass, which was attributed to the low doping level of MoS2 in composite gel glasses and complete encapsulation by the glass matrices [46,47]. In the latter case, the solid-state environment influences both the transition from the K point of the Brillouin zone and the direct transition from the deep valence band to the conduction band in MoS2 structure consequently induces a rather weak characteristic absorption. Furthermore, the strong absorption between 300–400 nm caused by the matrix makes the absorption of MoS2 even less obvious. Despite these absorption features, these sol-gel derived MoS2 doped lead silicate glasses might be potential candidates for optoelectronic applications owing to their simple preparation, high transparency, and good mechanical properties.
3.3 NLO and OL properties of MoS2/PbO-SiO2 gel glasses
The NLO and OL performances of the atomically thin nanosheets of MoS2 in PbO-SiO2 solid-state gel glasses were investigated through a standard OA Z-scan technique, which has been extensively used to measure nonlinear absorption coefficients, nonlinear refractive indices, and nonlinear scattering effects of materials [37]. Figure 5(a) and (b) show the OA Z-scan results of MoS2/PbO-SiO2 binary gel glasses on nano- and picosecond timescales, respectively. For comparison, the results of the MoS2/SiO2 unary gel glasses are also included. Clearly, all the Z-scan curves showed a decrease in transmittance at the focal point of the laser where the input fluence was at its maximum, indicating a typical OL effect induced by the incident light. The measured OA Z-scan curve was fitted to the transmission equation for a third-order nonlinear process. The nonlinear extinction coefficient β was calculated by the equation [37]:
Fig. 5. OA Z-scan curves of MoS2/MoS2 (doping level is 11.10×10−5) and MoS2/PbO-SiO2 gel glasses (doping level is 10.67×10−5) at (a) nanosecond and (b) picosecond laser duration excitation. CA Z-scan of MoS2/MoS2 and MoS2/PbO-SiO2 gel glasses at (c) nanosecond and (d) picosecond laser duration excitation. Both the linear transmittance of the gel glasses is 63%. Black scattered squares indicate experimental data and the red solid line shows the curve of best-fit.
Fig. 6. OA Z-scan curves of MoS2/PbO-SiO2 gel glasses with different doping level at (a) nanosecond and (b) picosecond laser duration excitation. OL curves of MoS2/PbO-SiO2 gel glasses at (c) nanosecond and (d) picosecond laser duration excitation. Black squares indicate experimental data and red solid lines show the curves of best-fit.
Table 1. Third nonlinear parameters of linear transmittance (T%), nonlinear absorption coefficient (β), nonlinear refraction (n2), real part of third-order nonlinear susceptibility (χ(3)R), imaginary part of third-order nonlinear susceptibility (χ(3)I), and third-order nonlinear susceptibility (χ(3)) in MoS2/SiO2 (doping lever is 11.10×10−5) and MoS2/PbO-SiO2 gel glass (doping lever is 10.67×10−5) calculated with Z-scan theory.
To verify our assumption, we performed a CA Z-scan in both MoS2/SiO2 and MoS2/PbO-SiO2 gel glasses on nano- and picosecond timescales, and the results are shown in Fig. 5(c) and (d), respectively. The n2 values of both gel glasses were negative because of the self-defocusing effect. A symmetrical valley in the OA Z-scan curves generally implies the presence of nonlinear absorption (NLA) and/or nonlinear scattering (NLS). For gel glasses, the rigid solid environment prevented the formation of microbubbles and therefore eliminated NLS. Consequently, both the OA and CA Z-scan patterns indicate that the OL effects of the gel glasses stem mainly from NLA and nonlinear refraction (NLR). Combining the OA results in Fig. 6(a) and (b), the corresponding nonlinear parameters of MoS2/SiO2 and MoS2/PbO-SiO2 gel glasses at nano- and picosecond timescales are easily extracted by Z-scan theory and are summarized in Table 1. Both the n2 and χ(3) of the MoS2/PbO-SiO2 gel glasses respectively increased to be more than 2.2 and 11.9 times as great as those of MoS2/SiO2 gel glasses on nano- and picosecond timescales. Therefore, the above results further confirmed the excellent OL performance of the MoS2/PbO-SiO2 gel glasses, attributed mainly to the enhanced third order nonlinear susceptibility of the matrices.
Additionally, it is noted that there are quite large difference of the nonlinear parameters between nano- and picosecond ones. Generally, the medium absorbs the energy and converts it into thermal energy through non-radiative relaxation when irradiated by laser, and therefore a strong temperature radial gradient distribution forms along the laser propagation direction, resulting in local thermal expansion of the medium and acoustic waves [51]. Furthermore, the acoustic waves propagate through the medium and cause a gradient change in the density of the medium, resulting in a radial gradient change in the refractive index and then lead to thermally induced nonlinear refractive index. The time needed for establishment of thermally induced nonlinear effects is usually at the nanosecond regime. So the pulse width is less than the time of thermal induced nonlinearity when irradiated by 35 picosecond laser, and the thermal induced nonlinearity at picosecond is mainly resulted from linear absorption and much smaller than that result from NLA in the case to nanosecond. And therefore, the large nanoseconds nonlinear parameters can be ascribed to thermally induced nonlinear refractive index. The thermal induced nonlinearity can be further confirmed by the asymmetry of valley-peak curve in CA Z-scan results, as shown in Fig. 5(d). The asymmetry is attributed to the fact that the spatial distribution of the thermally-induced nonlinear refractive index doesn’t meet Gaussian distribution.
The OL effects of the MoS2/PbO-SiO2 gel glasses, were manifested by plotting the normalized transmittance versus the input energy density, as calculated from the corresponding open aperture Z-scan measurements in Fig. 6(a) and (b), as shown in Fig. 6(c) and (d), respectively. The position-dependent light fluence ${F_{in}}(z)$ at any position z can be calculated from the corresponding beam radius $\omega (z)$ and the input laser pulse energy ${E_{in}}$ [37]:
Table 2. Linear transmittance (T%), linear absorption coefficient (α0), nonlinear absorption coefficient (β), and OL threshold (FOL) in MoS2/PbO-SiO2 gel glass with different doping levels.
Remarkably, the achieved FOL for the MoS2/PbO-SiO2 gel glasses (T=63%) under nanosecond-laser excitation was 0.9 J/cm2, which is 12.4 times that of recently reported MoS2 suspended in N-methyl-2-pyrrolidone (11.16 J/cm2) with a similar linear transmittance [19] and 7 times that of covalently functionalized oxide graphene sheets incorporated into silica gel glass (6.3 J/cm2) [23]. Under picosecond-laser irradiation, the FOL for the MoS2/PbO-SiO2 gel glasses (T=63%) was 0.08 J/cm2, which increased to be 7.8 and 5 times that of MoS2/PMMA [47] and POSS modified MoS2/PMMA organic glasses [52], respectively. The above results indicate that the MoS2/PbO-SiO2 gel glasses are promising candidates for optical switching and optical limiter applications.
4. Conclusions
A simple sol-gel synthesis of transparent and stable lead silicate binary glasses encapsulating 2D MoS2 nanosheets is demonstrated. This method can be readily extended to fabricate novel high-performance 2D material-based solid-state devices. On the basis of data obtained by SEM, Raman spectroscopy, UV/Vis spectrum, the 2D MoS2 nanosheets were successfully incorporated into gel glasses matrices. The NLO and OL properties of the MoS2/SiO2-PbO gel glasses measured by both OA and CA Z-scan experiments under nano- and picosecond laser excitation at 532-nm. These results are compared and a mechanism suggested. Importantly, the MoS2 embedded SiO2-PbO binary gel glass had excellent NLO and OL response under both nano- and picosecond excitation, which was superior to that of MoS2/SiO2 unary gel glasses owing to enhanced third order nonlinear susceptibility induced by the heavy lead atoms. The observed NLO and OL performance of the MoS2/PbO-SiO2 gel glasses mainly originates from NLA and NLR. Owing to the good dispersion, the NLO activities can be optimized by adjusting the concentration of MoS2 nanosheets in the gel glasses matrix. Remarkably, the extracted OL thresholds of the MoS2/PbO-SiO2 gel glasses were respectively 12.4 and 7.8 times those of MoS2 suspension at nanosecond and MoS2/PMMA organic glass at picosecond. These excellent properties suggest the potential of binary SiO2-PbO gel glasses as effective host materials and suitability of the composite gel glasses for applications in protecting vulnerable eyes, sensitive optical instruments, and optoelectronic devices from laser beams. In addition, present work strongly confirms the feasibility of our strategy. And the preparation of other heavy-metal glasses, such as TeO2-SiO2, Nb2O5-SiO2, and Bi2O3-SiO2, doped with two-dimensional materials are carrying out in our lab, which would greatly promote and explore the practical application of heavy-metal composite glasses in the field of nonlinear optics.
Funding
Fujian Provincial Department of Science and Technology (2018L3001); Department of Education, Fujian Province (JZ160462); Overseas study program for key young teachers of China scholarship council (201809360003).
Disclosures
The authors declare no conflicts of interest.
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