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Abstract
Heterostructures based on transition-metal dichalcogenide layered materials show great potential for various applications in nonlinear photonic devices such as optical switches, mode-locking lasers, and optical limiters. Herein, we report the design and synthesis of a hierarchical heterostructure of multi-wall carbon nanotubes (MWCNTs) decorated by three-dimensional molybdenum disulfide (MoS2). The unique MoS2/MWCNT heterostructure was successfully synthesized by a simple one-pot hydrothermal method, as confirmed by field emission scanning and transmission electron microscopies, X-ray diffraction, Raman spectrum, and X-ray photoelectron spectroscopy. The nonlinear optical (NLO) and optical limiting (OL) responses of the heterostructured MoS2/MWCNT and those of its individual components were investigated by the Z-scan technique at 532-nm with nano- and picosecond pulsed-laser sources. The NLO and OL properties of the MoS2/MWCNT heterostructure were improved compared with those of MoS2 and MWCNTs individually. The OL threshold of the heterostructured MoS2/MWCNT was 0.53 J/cm2, which is lower than or comparable to those of either common transition-metal dichalcogenides or graphene-like compounds. The NLO mechanisms are attributed to nonlinear absorption and nonlinear refraction at a picosecond timescale combined with nonlinear scattering induced by the MWCNTs at a nanosecond timescale. The improvements in NLO and OL performance are also attributed to photo-induced interfacial charge transfer between MoS2 and MWCNTs in the unique heterostructured MoS2/MWCNT. We report an efficient method of fabricating novel heterostructures with controllable NLO response. The unique morphology and excellent NLO properties obtained from these MoS2/MWCNT heterostructures show great potential for future optical and photonic applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
After the development of first laser in the 1960s, high energy lasers have become readily available and are widely used for various commercial, industrial, and military applications. In line with these developments, great interest has arisen in high performance nonlinear optical (NLO) materials capable of suppressing intense laser pulses to protect human eyes and sensitive sensors against potential damage from high power laser sources. Optical limiting (OL) is considered to be a practical means of protecting against lasers owing to a low limiting threshold, high damage threshold, fast response time, excellent transparency, and broadband spectral protection. Strong OL responses have been observed in a wide range of materials such as organo-polymers [1,2], carbon nanomaterials [3–5], organic dyes [6,7], two-dimensional materials [8–10], noble metal nanomaterials [11,12], and inorganic crystals [13,14]. The materials operate based on mechanisms including nonlinear absorption (NLA), nonlinear scattering (NLS), and nonlinear refraction (NLR).
Despite improvements in OL properties achieved through structural design strategies, there is no single OL material or mechanism, that meets all the requirements of an ideal optical limiter. However, combinations of two materials with large optical nonlinearities, arising from different nonlinear mechanisms, can increase NLO response beyond that predicted from the sum of those individual components [15,16]. Consequently, optically active materials and/or multiple nonlinear mechanisms are often hybridized. Many efforts have focused on combining NLO materials to fabricate composite OL materials suitable for practical applications. For instance, photo-induced electron transfer effects from covalently or noncovalently linked reverse saturable absorption (RSA) dyes (e.g. phthalocyanine, porphyrin) and carbon nanotube (CNT) composites have been widely studied, and these materials have demonstrated improved OL properties [17,18]. Decoration of graphene materials with metal particles is also an efficient strategy to obtain more efficient optical power limiters [19,20].
Layered transition-metal dichalcogenides (TMDs) offer an important building block for heterostructures with complementary NLO properties. TMDs show great potential for various applications in nonlinear photonic devices, such as optical switches, mode locking lasers, optical limiters, and logic circuits [21–24]. Improvements of NLO properties through hybridization of TMDs with other nanomaterials have recently been reported. Li and coworkers reported a nonlinear absorption coefficient for a graphene/WS2 heterostructure (9.7 × 104 cm /GW), which is much higher than that of WS2 alone (1.33 × 103 cm/GW) [25]. Zhang et al. reported an improved nonlinear absorption coefficient for heterostructured MoS2/carbon nanotubes [26]. Xu et al. noted that the nonlinear absorption coefficient of a MoS2/graphene film is approximately −1217.8 cm/GW, which is greater than those of the constituent MoS2 and graphene films (−136.1 and −961.6 cm/GW, respectively) [27]. Ouyang et al. prepared an NiS2/MoS2/poly(methyl methacrylate) (PMMA) organic glass and observed enhanced NLO and OL properties compared with NiS2/PMMA and MoS2/PMMA organic glasses [28]. Wang et al. synthesized nanocomposites of layered MoS2 and multi-walled carbon nanotubes (MWCNTs) with core-shell structures and found them to have improved third-order nonlinear optical performances under both femto- and nanosecond laser pulses over a broad wavelength range from the visible to the near infrared, compared with MoS2 and CNTs alone [26].
Despite extensive studies, there have been few reports on the NLO and OL effects of nano assemblies, such as hierarchical flower-like MoS2 nanoflakes, decorating MWCNTs. To date, a variety of synthetic strategies have been developed to prepare MoS2/CNT composites. In general, these methods can be divided into two kinds: step-by-step assembly and in situ synthesis [29–32]. Among these, hydrothermal approaches effectively generate high-performance materials with flexibility and controllability [33,34]. The homogeneous reaction environment enables high quality synthesis of MoS2/CNT composites through a simple one-step hydrothermal process. Construction of carbon-based metal sulfide hybrids is an efficient strategy for improving performance.
Herein, we used a one-pot hydrothermal technique to prepare a hierarchical three-dimensional MoS2/MWCNT heterostructure. The MWCNTs provide a skeleton, which is decorated by MoS2 nanoflakes with a flower-like structure. Successful synthesis of the composite was confirmed by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) observations. Further characterization was performed by X-ray diffraction (XRD), Raman spectrum, and X-ray photoelectron spectroscopy (XPS). The linear optical properties of the material were examined by diffuse reflectance ultraviolet–visible (DR UV–vis) absorption spectroscopy. Furthermore, the NLO and OL responses of the heterostructured MoS2/MWCNT composite together with those features of the individual MoS2 and MWCNTs were investigated by the Z-scan technique at 532-nm with a nano- and picosecond pulsed-laser source. The NLO and OL properties of the heterostructured MoS2/MWCNT composite were improved compared with those of MoS2 and MWCNTs alone. We explore the mechanisms of these properties in detail. Our results indicate that heterostructured MoS2/MWCNT has promise as a novel material for nonlinear optoelectronic devices.
2. Experimental
2.1 Reagents and materials
All chemicals were used as received. Sodium molybdate dihydrate (Na2Mo4·2H2O), thiourea (CH4N2S), and glucose (C6H12O6) were obtained from the Chinese Reagent Corporation (Shanghai, China) and were of analysis grade. Carboxyl MWCNTs (diameter > 50 nm, length = 0.5–2 µm) were purchased from XFNANO Materials Technology.
2.2 Preparation
A 20-mg portion of carboxyl MWCNTs and 60 mg of glucose were mixed into 60 mL of deionized water and ultrasonicated for 30 min. Subsequently, 0.6 g of Na2MoO4·2H2O and 0.8 g of CH4N2S were added to the mixture. After stirring for 30 min, the mixture was transferred to a 100-mL Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 °C and maintained at that temperature for 24 h. The resultant black product was filtered, washed with ethanol and water several times and dried at 80 °C for 12 h in a vacuum oven. Finally, the product was heated at 800 °C for 2 h in a N2 atmosphere at a heating rate of 1 °C/min. The resulting products are denoted MoS2/MWCNTs. For comparison, pure MoS2 nanosheets were also prepared by a procedure similar to that used for the MoS2/MWCNTs except for the absence of the MWCNTs and glucose.
2.3 Characterization
The morphologies of the MWCNTs and MoS2/MWCNT heterostructure were imaged with a FE-SEM (Nova Nano SEM450, FEI, USA) in conjunction with energy-dispersive X-ray spectrometry (EDS) to characterize the elemental composition. All samples were sputtered with gold prior to these observations. The detailed morphology of heterostructured MoS2/MWCNTs was further imaged with a TEM (JEM-2100). A droplet of sample was deposited on a copper grid and allowed to dry before observation at a working voltage of 200 kV. The crystallographic structures of the materials were analyzed by powder XRD (D8 Advance, Bruker, Germany) with Cu Kα radiation (40 kV, 40 mA, λ = 0.1542 nm). Raman spectra were acquired using a DXRxi spectrometer (Themo, USA) with excitation at 514 nm and data collection over the range of 500–3000 cm−1. XPS (Escalab 250, Thermo Fisher Scientific Co., USA) was performed with Al Ka radiation, and the binding energy of each element was calibrated to the 284.8 eV C 1s peak. DR UV–vis absorption spectroscopy was performed on a Lambda 950 spectrophotometer. A certain amount of barium sulfate (BaSO4) powder was flattened with a glass column before the experiment as a reflectance standard. A portion of the sample powder was placed onto a standard white plate followed by pressing and measuring the spectra.
2.4 Z-scan measurements
Open-aperture (OA) and closed-aperture (CA) Z-scan technique [35] were used to investigate the NLO and OL behavior of heterostructure MoS2/MWCNTs at an output wavelength of 532 nm. Specially, the nanosecond-light source was a Dawa-S laser (Beamtech), i.e., a Q-switched Nd:YAG pulsed-laser system having a pulse width of 7 ns, repetition frequency of 10 Hz, beam waist radius of ω0 of 22 µm, and Rayleigh length of 2.86 mm. The picosecond light source was a PL2250 laser (EKSPLA), i.e., a Q-switched Nd:YAG pulsed-laser system having a pulse width of 30 ps, repetition frequency of 10 Hz, beam waist radius of ω0 of 23 µm, and Rayleigh length of 3.12 mm. All samples were tested in 2-mm quartz cuvettes. Cuvettes were mounted on a computer-controlled translation stage that shifted the samples along the z axis. All test procedures were conducted at room temperature.
3. Results and discussion
3.1 Morphology and structure
The synthesis of the heterostructured MoS2/MWCNT by a one-pot hydrothermal method is shown in Fig. 1. CNTs are attractive substrates for hybrid structures owing to their large specific surface and the presence of surface oxygen groups, such as hydroxyls and epoxides, which provide reactive sites for chemical modification by conventional carbon surface chemistry [36,37]. In the hydrothermal synthesis, the MWCNTs act as carbon precursors and sodium molybdate and thiourea act as inorganic precursors. The hydrophilic functionalities (i.e., -OH, -COOH) on the external surface of the MWCNTs are negatively charged and can bind with positively charged Mo+ precursors by electrostatic interactions. During the hydrothermal process, the MoS2 nanoflakes tend to assemble and form hierarchical structures on the MWCNT backbone. The typical reaction for the formation of MoS2 can be expressed as [38]:
Our investigations revealed that the morphology of the deposited MoS2 nanoflakes depends on the mass percentage of added glucose. For addition of 5% glucose, the MoS2 nanoflakes self-assembled to form flower-like hierarchical structures that are evenly anchored to the MWCNTs. When the mass percentage of glucose was increased to 30%, coaxial core-shell MoS2/MWCNTs heterostructures were formed, as has been previously reported [39–42].
The morphology of the heterostructured MoS2/MWCNTs was characterized by FE-SEM imaging. Figure 2(a) shows representative FE-SEM images of the MWCNTs, which have a smooth surface and an average diameter of approximately 50 nm. The MWCNTs act as a scaffold for growth of the MoS2 nanosheets [43]. After the hydrothermal process, MoS2 nanoflakes with a uniform thickness of 8 nm crosslinked and assembled into flower-like clusters decorating the one-dimensional MWCNTs, as shown in Fig. 2(b) and 2(c). These structures were further investigated by TEM imaging. The TEM images in Fig. 2(d) and 1(e) show that the MWCNTs maintained a hollow structure with the MoS2 selectively encapsulating the MWCNT surface, forming a layer-stacking structure. Lattice spacings of 0.62, 0.23, and 0.32 nm were clearly identified in high magnification TEM images (Fig. 2(f)), respectively corresponding to the (002) and (103) planes of MoS2, and (002) planes of the MWCNTs. Energy-dispersive X-ray spectroscopy (EDS) mapping of the MoS2/MWCNT heterostructure (Fig. 2(h)) indicated the coexistence of Mo, S, and C.
Fig. 2. Typical FESEM images of (a) bare MWCNTs and (b), (c) heterostructured MoS2/MWCNT; (d), (e) TEM and (f) HRTEM images heterostructured MoS2/MWCNT; (g) EDX spectrum of heterostructured MoS2/MWCNT.
The crystal structures of MoS2/MWCNT heterostructure were characterized by XRD (Fig. 3(a)). The XRD of the MWCNTs are also shown for reference. The (002) peak at 26.1° in the XRD pattern of MWCNTs derives from the graphene layers of the MWCNTs. The heterostructured MoS2/MWCNT featured four additional diffraction peaks at 14.1°, 33.61°, 39.6° and 58.9°, which can be indexed to the (002), (100), (103), (110), and planes of hexagonal 2H-MoS2 (JCPDS No. 37-1492). The refined average lattice constants were: a = b = 3.1920 Å, c = 12.9361 Å, α= β = 90°, γ = 120°. In addition, the characteristic peak of the (002) grain plane at 14.1° was relatively sharp, indicating that the MoS2 preferentially grew along the c-axis and had good crystallinity [44].
Raman spectroscopy is an effective and non-destructive method that provides structural and electronic information, as well as insights into phase changes and/or chemical modification during synthetic procedures [45]. Figure 3(b) shows Raman spectra of MWCNTs, MoS2, and heterostructured MoS2/MWCNT, acquired with excitation at λexc = 514 nm at room temperature. For the pristine MWCNTs, D and G mode bands appeared at approximately 1348 and 1573 cm−1. The D band is attributed to the sp3 hybridized carbon atoms or defects in the sp2 hybridized network structure, and the G band arises from the structured graphene E2g vibration mode associated with sp2 hybrid carbon atoms [46]. Compared with the bare MWCNTs, the composite had slightly red-shifted features, and the intensity ratio of the D/G bands increased to ∼0.78 versus ∼0.61 for the bare MWCNTs. This shift is likely caused by strong interfacial interactions between MoS2 and MWCNTs, which is consistent with previous reports [47]. The Raman spectrum of the hydrothermally synthesized MoS2 had two characteristic low-energy peaks at 380.2 and 403.3 cm−1, which are assigned to the E12g in-plane and A1g out-of-plane vibration modes of MoS2 [48]. The number of MoS2 layers can be characterized from the peak frequency difference (ΔK) between E12g and A1g [49], and was 25.22 cm−1 in the present work, indicating that fewer than five layers of MoS2 nanoflakes grew on the surface of the MWCNTs. Furthermore, compared with pure MoS2, the E12g peaks were slightly red-shifted (∼0.6 cm−1) and the A1g peaks were blue-shifted (∼0.5 cm−1), which also can be attributed to the formation of interfaces between the MoS2 and MWCNTs.
X-ray photoelectron spectroscopy measurements were used to further characterize the composition and chemical bonding environment of heterostructured MoS2/MWCNT. As shown in Fig. 4(a), there were no peaks in the full survey spectrum except for S, Mo, C, and O, indicating that no other impurities were present in the heterostructured MoS2/MWCNT, in line with the EDS mapping results. The high-resolution C 1s peak (Fig. 4(b)) was fitted by three peaks, with binding energies of 284.8 and 285.9 eV corresponding to sp2 carbon-carbon double bonds (C = C) and sp3 carbon-carbon single bonds (C–C) of the MWCNTs, respectively, and a peak at 287.2 eV attributed to C = O–C coordination of the MoS2/MWCNTs [50]. Three peaks appeared at 531.8, 532.1, and 553.3 eV in the high-resolution spectrum of O 1s peaks (Fig. 4(c)), which are attributed to carboxyl or hydroxyl groups on the carboxylated MWCNT surfaces [51]. The high resolution XPS spectrum of the Mo 3d showed two peaks belonging to Mo 3d5/2 and Mo 3d3/2 at 229.8 and 233.0 eV [52], respectively, and two small peaks at 236.7 and 233.5 eV attributed to Mo6+ of Mo–O [52]. The formation of Mo6+ derives from surface oxidation of Mo4+ in air and the formation of Mo-O-C bonds with MoS2 and carbon [52]. In addition, the binding energy at 227.1 eV is attributed to S 2s. Two peaks at 162.7 and 163.9 eV appear in the high-resolution spectrum of S 2p (Fig. 4(d)), which correspond to S 2p3/2 and S 2p1/2 of E-MoS2 [53], respectively. The broad band at 169.3 eV corresponds to oxidized sulfur. The positions of the Mo 3d and S 2p lines in the heterostructured MoS2/MWCNTs are upshifted by ∼0.2 eV compared with those of pristine MoS2 (Fig. 4(e)). We attribute this shift to charge transfer from MWCNTs to MoS2.
Fig. 4. (a) XPS survey spectrum of heterostructured MoS2/MWCNT, (b)-(e) high-resolution spectrum of C1s, O1s, Mo3d, and S2p in heterostructured MoS2/MWCNT, (f) and (g) high-resolution spectrum of Mo3d and S2p in MoS2.
3.2 Linear optical properties
The linear optical properties of the MWCNTs and heterostructured MoS2/MWCNT were investigated with diffuse reflectance ultraviolet–visible (Fig. 5(a)). Consistent with previous reports, the pristine MWCNTs had broadband absorption from the visible to NIR range. For the heterostructured MoS2/MWCNT, the diffuse reflectance ultraviolet–visible, two characteristic absorption peaks at 623 and 674 nm arise from direct transitions from the valance to the conduction band at the K-point of the Brillouin zone of MoS2, known as B and A transitions, respectively. In addition, a broad absorption band centered at 416 nm was assigned to complex C and D transitions of MoS2. The band gap of the MWCNTs and heterostructured MoS2/MWCNT may be determined from the following equation [54]:
where α, h, υ, and Eg represent the absorption coefficient, Planck’s constant, frequency of light, and the band gap, A and n are constants, respectively. We set n to be 2 for the indirect bandgap. The band gap energy Eg values for the MWCNTs and MoS2/MWCNT heterostructure were determined from plots of (αhυ)2 versus hυ. Figure 5(b) shows the Tauc plots used to calculate the band gaps, from extrapolation of the linear region. The values of Eg were determined to be 1.42 and 1.32 eV for the MWCNTs and heterostructured MoS2/MWCNT, respectively. The narrowing of the bandgap in the heterostructure is attributed to interfacial interactions between the coating MoS2 and decorating MWCNTs. The above results confirmed that few-layer MoS2 nanoflakes were anchored to the MWCNT surfaces in line with the above the TEM, SEM, EDS, and Raman analysis.Fig. 5. (a) UV-Vis diffuse absorbance spectra of the MWCNTs and heterostructured MoS2/MWCNT. (b) Plotting of (a) in the (αhν)1/2∼ energy coordinate to evaluate the MWCNTs and heterostructured MoS2/MWCNT.
3.3 Nonlinear optical properties
The NLO properties of the MWCNTs, MoS2, and heterostructured MoS2/MWCNT were investigated at 532 nm with the Z-scan technique with nano- and picosecond laser pulses. Generally, the Z-scan method is used to investigate third-order NLO processes, including NLA, NLS, and NLR [35]. During the measurements, the incident and transmitted laser powers were monitored as the samples were moved along the propagation direction of the laser pulses. Figure 6(a) and 6(b) shows typical open-aperture (OA) Z-scan data for the samples subjected to nano- and picosecond laser pulses, respectively. All the Z-scan curves showed transmittance decreases when the samples were moved to the focus region, which is a typical feature of OL materials. The depth of these valleys reflect the OL ability. For both pulse widths, the valley in the transmittance curves of the heterostructured MoS2/MWCNT had the largest dip among the studied materials. Therefore, the heterostructured MoS2/MWCNT had more favorable NLO properties compared with the individual components.
Fig. 6. (a) OA Z-Scan curves of MWCNTs, MoS2, and heterostructured MoS2/MWCNT at (a) nanosecond (b) picosecond laser duration excitation. Black scattered squares indicate experimental date and the red solid line shows the curve of best-fit.
To obtain the nonlinear extinction coefficient β of the MWCNTs, MoS2, and MoS2/MWCNT heterostructure, the experimental data were analyzed by Z-scan theory, according to the following equation [35]:
Table 1. Third nonlinear parameters of 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 MWCNTs, MoS2,and MoS2/MWCNTs calculated with Z-Scan theory
Three mechanisms contribute to OL namely, NLA, NLS, and NLR. On the basis of different materials and absorption mechanisms, the former can be divided into: two-photon absorption (TPA), reverse saturable absorption (RSA), and free-carrier absorption (FCA). In some cases, both mechanisms operate in the same system to provide effective OL. The OL mechanism can be identified from the variation of β values with input laser energy density. In general, the β value will decrease as the input fluence increases for RSA process because of saturation of RSA whereas the β value will remain unchanged for a two-photon absorption process [55]. Here, an increase of β with input fluence over a nanosecond timescale (Fig. 7(a)) implies that, in addition to NLA from the MWCNTs and MoS2, the NLO performance is also influenced by NLS in the high-fluence regime for the heterostructured MoS2/MWCNTs. This is reasonable because MWCNTs have been reported to have strong OL effects, which arise from strong NLS owing to the creation of new scattering centers consisting of ionized carbon microplasmas and solvent microbubbles when subjected to nanosecond laser pulses [56]. As shown in Fig. 7(b), when the input intensity was increased, the β value remained almost constant. These results suggest that a TPA process contributes to the nonlinearity of the heterostructured MoS2/MWCNT over a picosecond timescale. Similar phenomena have been also observed in porphyrin functionalized single wall carbon nanotubes [57].
Fig. 7. NLA coefficient as a function of input intensities of the heterostructured MoS2/MWCNT at (a) nanosecond and (b) picosecond duration laser excitation.
The mechanisms of the OL behaviors of the samples were further studied by closed-apertures (CA) Z-scan method on nano- and picosecond timescales, as shown in Fig. 8(a) and 8(b), respectively. The n2 values of the three samples were negative because of the self-defocusing effect. Taken together with the OA results in Fig. 6(a) and 6(b), the corresponding nonlinear parameters of MWCNTs, MoS2, and heterostructured MoS2/MWCNT 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 heterostructured MoS2/MWCNT increased compared with those of the MWCNTs and MoS2 on nano- and picosecond timescales. Therefore, the excellent OL performance of the heterostructured MoS2/MWCNT may also be attributed to the enhanced third order nonlinearity.
Fig. 8. CA Z-Scan of MWCNTs, MoS2,and heterostructured MoS2/MWCNT at (a) nanosecond (b) picosecond laser duration excitation.Black squares indicate experimental date and red solid lines show the curves of best-fit.
In addition, it has been reported that the electron transfer between MWCNTs and MoS2 improves the light-matter interactions of the system, contributing to improved NLO response [26]. According to the XPS results and previous reports, electrons may be expected to transfer from the MWCNT core to the surrounding MoS2. Under laser irradiation (2.32 eV / 532 nm laser), electrons are excited from the ground-state to the excited state simultaneously for both MoS2 (1.29 eV) and MWCNTs (1.42 eV). These excited carriers not only relax from the excited state to the ground state, but also transfer from the MWCNTs to MoS2 reversibly. Thus, the enhanced NLO performance can also be attributed to photo-induced electron transfer and the coupling effect between MWCNTs and MoS2. Similar to the electron transfer process in other composites, such as graphene/porphyrin and CNTs/MoS2 described in previous reports [58,59]. In addition, the unique structure also enhances the NLO and OL performances.
In addition to the nonlinear extinction coefficient, the OL threshold, defined as the input fluence when the normalized transmittance drops below 50%, is another important parameter for evaluating NLO performance of a material and can be calculated from the corresponding OA Z-scan curves. The position-dependent light fluence $w = 2\pi /{L_m}$ at any position z can be calculated from the corresponding beam radium ${F_{in}}(z)$ and the input laser pulse energy ${E_{in}}$[35]:
where the beam radius is given byThe OL curves of the samples are shown in Fig. 9(a) and 9(b). Both curves show a similar trend, where the transmittance remains the same at low input fluence and then decreases as the fluence increases. This is a typical feature of OL materials, indicating that the heterostructured MoS2/MWCNT are potential OL candidates. The calculated OL threshold of the heterostructured MoS2/MWCNT at nanosecond, which deduced from the OL curve in Fig. 9(a), are comparable with the reported values of various TMDs and graphene like compounds (Table 2) tested under similar experimental conditions. Hence, this heterostructured MoS2/MWCNT is a novel NLO material with great potential in optical limiting applications.
Fig. 9. OL curves of MWCNTs, MoS2, and heterostructured MoS2/MWCNT at (a) nanosecond (b) picosecond laser duration excitation. Black squares indicate experimental date and red solid lines show the curves of best-fit.
Table 2. OL threshold of the heterostructured MoS2/MWCNT compared to the reported values of various TMDC’s and graphene like compounds
4. Conclusion
In conclusion, we have demonstrated a simple approach to synthesis of hierarchic three dimensional heterostructured MoS2/MWCNT, as an NLO material for OL applications. The morphology, composition, and structure of the materials were characterized by SEM, TEM, XRD, Raman spectrum, and XPS, revealing a flower-like MoS2 hierarchical structure assembled from nanoflakes adhered to the MWCNT surfaces. The NLO and OL performances of the heterostructures were investigated using both OA and CA Z-scan techniques with nano- and picosecond laser pulses at 532 nm. The heterostructure had better NLO and OL performances than those of either the MWCNTs or MoS2 alone. The onset OL threshold of the heterostructured MoS2/MWCNT under a picosecond pulse was 0.025 J/cm2, which is much lower than previously reported for such heterostructures. The enhanced NLO and OL performances are mainly attributed to NLA and NLS for nanosecond laser pulses and TPA for picosecond pulses, respectively. Moreover, light-induced electron transfer between MWCNTs and MoS2 contributes to the outstanding OL performance at both laser pulse timescales. The unique morphology and excellent NLO properties obtained from the rationally designed heterostructure show great promise for optical limiting applications.
Funding
Natural Science Foundation of Fujian Province (2020J01896).
Disclosures
There are no Conflicts to declare.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding authors upon reasonable request.
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