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High-yielded ultrathin MoS2 nanosheets (UMS) with thickness below 4 nm were successfully synthesized by a simple, cost-effective and reproducible solid-state reaction method. Significant reverse saturable absorption and nonlinear refraction responses of the UMS were measured by the z-scan experiment under femtosecond pulses at 800 nm. The figure of merit is calculated to be ~2.52 × 10−15 esu cm. Furthermore, optical limiting (OL) effects of the UMS were observed with low threshold FOL ~44 mJ/cm2. These results reveal that solid-state reaction is a feasible method for the fabrication of optical nanomaterials used in nanophotonic devices including optical limiter, which can be expanded to prepare other two-dimensional nanomaterials.
© 2015 Optical Society of America
1. Introduction
Inspired by the research on graphene, two-dimensional (2D) nanomaterials in which strong covalent bonds in layers and weak van der Waals interaction between layers exist have become one of the most widely studied fields in nanoscience [1–6]. Layered molybdenum disulfide (MoS2) is one of the typical new 2D nanomaterials. Due to the electron motion confinement and the absence of interlayer perturbation, the energy band structure of MoS2 changes from the indirect to direct band-gap as the decreasing of the number of layer, resulting in the emergence of a series of photonic properties which cannot be observed in the bulk MoS2, such as visible photoluminescence, second and third harmonic generations, and ultrafast nonlinear absorption [7–10]. Nonlinear optical (NLO) response study is a fundamental and can contribute the developing of photonic and optoelectronic devices. Especially the research of two types of NLO materials, the saturable absorption (SA) material used in Q-switcher and mode-locker, and the reverse saturable absorption (RSA) material applied in optical limiter and optical switcher [9–13].
Recently, K. P. Wang et al. successfully exfoliated the 2D MoS2 nanosheets by liquid-phase method and observed the ultrafast SA for femtosecond (fs) pulses at 800 nm [10]. Afterwards, lots of researches are focused on the NLO properties of MoS2 nanosheets [12–17]. However, RSA properties of MoS2 nanosheets to fs pulses have rarely been investigated. With increasing use of powerful pulsed lasers (especially fs laser) for various applications, RSA materials used in the fs regime are the most recently developed laser protection materials owing to the function with blocking the transmittance at high incident intensity [18]. On the other hand, there have been a few methods developed for the preparation of 2D MoS2 nanosheets, such as chemical vapor deposition (CVD) [12], hydrothermal intercalation/exfoliation [19], liquid exfoliation [10]. However, they are still hard to realize the synthesis of large-scale 2D MoS2 nanosheets used in nanophotonic devices. The reproducibility and controllability of the CVD method are very poor with CVD needing a high temperature and an expensive set up. And the fabrication process is complicated. Due to chemical modification of the surface of MoS2, hydrothermal intercalation/exfoliation and liquid exfoliation methods usually use additives in the preparation process, such as, lithium ions, surfactants, or polymers, which are likely to alter the intrinsic physical properties of MoS2 [14]. What’s more, the high-yielded production of 2D MoS2 nanosheets is still highly challenging.
On the basis of the above key factors, solid-state reaction method for the synthesis of high-yielded UMS was applied in this work. This method is very simple, cost-effective, reproducible, and less detrimental to the intrinsic physical properties of MoS2 because of the absence of any additives [20]. The thickness, morphology and structure were characterized by transmission electron microscopy (TEM), Raman spectrum, absorption spectrum and photoluminescence spectrum, respectively. RSA and nonlinear refraction properties of the UMS were measured by the z-scan configuration under a fs laser excitation at 800 nm. Moreover, optical limiting (OL) effects of the UMS were observed with low threshold. The phenomena imply that solid-state reaction is a feasible method for the preparation of 2D materials used in nanophotonic devices including optical limiter.
2. Experimental
Raw materials of 0.20 g molybdenum trioxide (MoO3, Sigma-Aldrich) and 3.18 g thiourea (NH2CSNH2, Sigma-Aldrich) were first mixed homogeneously in an agate mortar, then fired in a covered corundum crucible at 780 °C for 1 h under the atmosphere of nitrogen. The resulting sample was cooled to room temperature and 0.2165 g black MoS2 was collected. The reaction routes could be expressed: 2NH2CSNH2→CS2 + H2NCH + 2NH3, 3CS2 + 2MoO3→2MoS2 + 3CO2 + 2S. The yield of MoS2 nanosheets is 97.4%. Then, the prepared powders were dissolved in ethyl alcohol under mild stirring to form a yellow-green solution. To remove large agglomeration, the MoS2 dispersions were centrifuged at 6000 rpm for 10 min, and the upper supernatant was collected.
X-ray diffraction (XRD) was performed on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.5418 Å) radiation. Microscopic images were obtained by TEM (Model JEM-2010; JEOL, Tokyo, Japan) and Raman spectrum was measured using a Raman spectrometer (RenishawinVia, Gloucestershire, UK) and a 785 nm laser as the excitation source. Optical absorption spectrum was recorded on a Perkin-Elmer Lambda-900 UV/vis/NIR spectrophotometer (Perkin Elmer, Waltham, MA). A photoluminescence spectrum was measured on a FS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., West Lothian, U.K.) equipped with a 450-W Xenon lamp. All the measurements were carried out at room temperature.
3. Results and discussion
XRD technique was carried out to investigate the crystalline structure of the sample. As shown in Fig. 1 (a), it clearly presents that all the diffraction peaks are congruent with the standard hexagonal 2H-MoS2 structure (JCPDS Card No. 65-1951), in which the diffraction peaks at 2θ = 14.0, 32.8, 39.4, 49.7, 58.5 and 68.6° can be unambiguously assigned to the (002), (100), (103), (105), (110) and (200) planes, respectively. No obvious peaks from the impurity phase are observed, and the sharp diffraction peaks indicates a good crystallinity of the as-prepared MoS2.
Fig. 1 (a) XRD pattern, (b) TEM and (c) HRTEM images of the MoS2 dispersions, The inset of (b) is SAED pattern, (d) Three-dimensional schematic representation of MoS2 structure, with the chalcogen atoms (S) in yellow and the molybdenum atoms (Mo) in blue.
Morphology and structure of the products were studied by TEM and high-resolution TEM (HRTEM). Figure 1(b) gives a low-magnification TEM image of the MoS2 dispersions, showing that the UMS with the lateral size below 500 nm were obtained. Selected area electron diffraction (SAED) pattern (inset of Fig. 1(b)) manifests that the UMS consist of several few-layers single-crystal MoS2 stacked in different orientations. The diffraction rings can be indexed to the reflections of hexagonal MoS2 (002), (100), (103) and (110) planes [20]. From HRTEM image of the folded edge, we can determine the layer number of layered material [21]. As directly evidenced from the folded edge in Fig. 1(c), the obtained MoS2 nanosheets contain 1-6 sandwiched S-Mo-S layers, mainly 2-3 layers, indicating that the thickness of the obtained MoS2 nanosheets is less than 4 nm. HRTEM image exhibits that the interlayer distance of (002) crystal plane of the MoS2 nanosheets is about 0.62 nm. Furthermore, Fig. 1(c) also apparently presents the lattice fringes with a spacing of 0.28 nm, which is in good agreement with d = 0.274 nm of the (100) planes of hexagonal MoS2 [22]. Three-dimensional schematic representation of MoS2 structure was displayed in Fig. 1 (d). In addition, from TEM and HRTEM images, distinct ripples and corrugations can be observed, revealing the ultrathin nature of MoS2 nanosheets [21, 23].
Raman spectrum was employed to confirm the atomic structural arrangement of MoS2. As shown in Fig. 2(a), two characteristic peaks at 385.0 and 409.1 cm−1 originate from the in-plane and an out-of-plane A1g vibrational modes of MoS2, respectively [13]. The mean frequency difference is calculated to be about ~24.1 cm−1, indicative of an average thickness of 1-6 monolayers, namely, below 4 nm (~0.62 nm for each monolayer). Moreover, the full-width-half-maximum of (~3.2 cm−1) and A1g (~2.9 cm−1) bands are consistent with the few-layers MoS2 nanosheets reported [10], also confirming that the UMS have been successfully synthesized.
Fig. 2 (a) Raman spectrum, (b) Optical absorption spectrum and (c) Photoluminescence spectrum of the MoS2 dispersions. the inset of (b) is photograph of the MoS2 dispersions.
An optical absorption spectrum of the MoS2 dispersions was illustrated in Fig. 2(b). It can be seen that four characteristic absorption bands located at 600-700 and 380-450 nm regions are in accordance with the general features of few-layers MoS2 nanosheets with hexagonal symmetry [1], such a phenomenon indicates that the UMS were dispersed in ethyl alcohol as the 2H-phase. The dual peaks at 612 and 664 nm (~1.87 eV) are due to the inter-band excitonic transition at the K point of the Brillouin zone, known as the B and A transitions, respectively. The separation of B and A results from the spin-orbit splitting of transitions at K [10, 22]. It is worth noticing that we can also deduce an average thickness of the obtained MoS2 nanosheets to be ~4 nm (equivalent to 6 monolayers) from the A exciton position according to a distribution for nanosheets with different thickness reported by Q. H. Wang et al [1]. The results further reveal that the very-few-layers MoS2 nanosheets were successfully prepared in this work. In addition, the absorption bands at 390 and 441 nm corresponding to the complicated C and D transitions between the higher density of state regions are also observed. The inset of Fig. 2(b) displays the characteristic yellow-green color of the MoS2 dispersions, similar to that in other reports [10, 22]. Noticeably, the UMS dispersions were highly stable, and exhibited no precipitation after being stored for some days under ambient conditions.
Monolayer MoS2 is the direct gap semiconductor where the lowest energy inter-band transition takes place at K [1]. Emission of photons with energy of 1.9 eV can occur as relaxation of excitons at K, whereas the bulk MoS2 do not exhibit photoluminescence due to the indirect gap [7]. Figure 2(c) gives photoluminescence spectrum of the MoS2 nanosheets dispersions under 532 nm excitation. Two characteristic emission peaks located at 638 and 658 nm are observed, which agree very well with those reported by [24] for mechanically exfoliated monolayer samples and [7] for chemically exfoliated few-layers MoS2 nanosheets. The observed photoluminescence is attributed to the intrinsic electronic properties of mono- and few-layers MoS2 nanosheets instead of chemical impurities or structural defects, that is, from the lowest conduction band to the highest spin-split valence band at K point. The above results verify that the UMS were obtained through a facile and reproducible solid-state reaction method in this contribution, agreeing well with the above TEM, Raman spectrum and absorption spectrum.
In order to characterize NLO response of the UMS, z-scan technique was applied and the experimental setup was shown in Fig. 3(a). The measurement system utilized a commercial Ti: sapphire fs laser (center wavelength λ = 800 nm, pulse width tp = 130 fs, and 1 kHz repetition rate). These laser pulses are ultrashort enough to minimize free carrier generation and exclude thermal effects on NLO performance [14]. In order to more precisely identify the measured data, CS2 solution contained in a cuvette (1 mm in thick) was used to calibrate. According the measured z-scan value, the third-order nonlinear refractive index of CS2 was calculated to be ~3.9 × 10−15 cm2/W, matching well with the reported result of ~(3.0 ± 0.6) × 10−15 cm2/W [25]. Therefore, our z-scan measurement platform is reliable.
Fig. 3 (a) Schematic diagram of the z-scan experimental setup, (b) OA z-scans of the MoS2 dispersions under the excitation of 130 fs pulses at 800 nm with different pulse powers, the solid line is the theoretical fitting curve from Eq. (1), (c) OL response of the UMS under different input fluences.
By replacing the standard CS2 benchmark with the UMS dispersions while the other parameters were kept constant, the open-aperture (OA) z-scan measurements on the UMS dispersions at different incident laser powers were conducted. As shown in Fig. 3(b), the RSA property of the UMS happened in ethyl alcohol when the incident power was increased up to 55 μW. As the input power enhanced over 300 μW, the UMS dispersions present pronounced RSA response, that is, the total transmission exhibit a decrease at positions close to the focus. In other words, the UMS can effectively hinder high intensity light but allow low intensity light to pass, which clearly indicate the fs OL effects. The OL curve in Fig. 3(c) was extracted from the corresponding OA z-scan results, which state the variation of the normalized transmittance versus input fluence. The OL threshold (FOL, defined as the incident fulence at which the normalized transmittance decreases to 50%) is an important parameter to evaluate the NLO property of a given material. Herein FOL values is determined to be ~44 mJ/cm2. It should be mentioned that no any other additives were added during sample preparation except the ethyl alcohol. Besides, the NLO response of pure solvent under the same conditions have been measured, indicating that much smaller nonlinearity compared to that of the UMS dispersions. Therefore, the observed NLO phenomenon shall be mainly attributed to the intrinsic of the UMS. The FOL of the UMS under fs laser pulses is lower or comparable than that of various materials [18, 26, 27]. Hence, the UMS is very promising for application in nanophotonic devices [14, 18].
Generally, when the incident photon energy larger than the band gap, semiconductors exhibit SA property caused by the free-carrier excitation from valence band to conduction band, as a consequence of Pauli-blocking. In contrast to the aforementioned performance, electrons/holes usually cannot be excited to the conduction/valence bands with the energy of photons lower than the band gap, unless two-photon absorption (TPA) happens [9, 10]. Because of such TPA, the transmission decreases when the incident intensity increases, resulting in the OA z-scan experimental curves are the lowest at the focal point. Of course, the origin of such phenomenon might also come from nonlinear scattering (NLS) mainly caused by the microbubble formation or excited-state absorption (ESA). However, ESA occurs under high pulse powers with the absorption cross section of exited state larger than that of ground state for laser [15]. In addition, the ultrashort (fs) pulses and relatively low pulse powers are not enough to form microbubble [14]. Hence, considering the relatively low pulse powers and the ultrashort pulses, ESA and NLS effects do not play a major role in here. As we all known, the multilayers MoS2 is an indirect semiconductor with a narrower band gap of 1.2 eV (1033 nm). With decreasing thickness, the band gap increases and calculations predict it to reach 1.9 eV (690 nm) for a single monolayer. Therefore, when excited by the fs laser beam at 800 nm (1.55 eV), the monolayer and multilayers MoS2 possess TPA and SA, respectively. RSA response in Fig. 3(b) is considered to be the domination of TPA of the mono- and very-few-layers nanosheets in MoS2 dispersions. It demonstrates that the as-prepared MoS2 are mostly the mono- and very-few-layers MoS2 nanosheets, namely, the UMS.
In order to further identify the mechanism of the RSA property, quantitative estimation of the nonlinear absorption coefficient β is conducted. β can be deduced by the equation [28],
Where q0(z) = βI0L(1 + x2), x2 = z2/z02, z0 and I0 are the diffraction length of the beam and the peak light intensity at focus, respectively. L is the effective thickness of sample. By fitting the OA z-scan data, we can find that β is weakly dependent on the input power. Importantly, the β is nearly constant when the input power higher than 300 μW, further indicating that a TPA determines the strong nonlinearity. The value of β is determined to be ~(8.05 ± 0.37) × 10−3 cm/GW. the imaginary of the third-order NLO susceptibility Im χ(3) is directly related to β, which can be derived from the formula [10],where c, λ, and n stand for the light speed in vacuum, wavelength of the laser pulses, and the refractive index, respectively. Hence, Im χ(3) is estimated to be ~(4.41 ± 0.20) × 10−15 esu (or ~(5.45 ± 0.25) × 10−40 cm4 V−3). To remove the effect caused by the linear absorption α0, we can employ the nonlinear figure of merit (FOM) as a criterion to evaluate the NLO property of the materials. FOM also helps us to compare our samples with the other reported nanomaterials, the larger FOM means the better NLO performance of the products. FOM for the third-order optical nonlinearity can be define as the following equation [10],Where α0 = 1.96 cm−1 was measured from absorption spectrum, FOM is calculated to be ~(2.25 ± 0.10) × 10−15 esu cm (or ~(2.78 ± 0.12) × 10−40 cm5 V−3). The value of FOM is larger than that of MoS2 ~1.23 × 10−15 esu cm, MoSe2 ~0.69 × 10−15 esu cm, MoTe2 ~1.45 × 10−15 esu cm [9], reduced graphene oxide ~0.36 × 10−15 esu cm and Au nanorods ~0.88 × 10−15 esu cm [10], and is comparable to that of carbon nanodots ~3.30 × 10−15 esu cm [18], graphene oxides ~4.20 × 10−15 esu cm and graphene ~5.00 × 10−15 esu cm [10]. Therefore, the larger FOM and lower FOL compared with various conventional OL materials indicate that the UMS synthesized by the solid-state reaction method can be potential materials used in nanophotonic devices, especially optical limiter.Furthermore, a typical closed-aperture (CA) z-scan of the MoS2 dispersions was shown in Fig. (a). In this trace, the effect of the nonlinear phase is of the same order of magnitude as the effect of RSA. To isolate the former, dividing the curve in Fig. 4(a) by the blue curve in Fig. 3(b) was performed, thereby obtaining the curve in Fig. 4(b). The trace has the typical shape of a z-scan measurement. The pre-focal valley and the post-focal peak imply a positive NLO refractive index n2, indicating the self-focusing effect in UMS. Fitting the trace by the formula mentioned in literature [29], the on-axis nonlinear phase shift at the focus ∆Φ (~0.93 rad) and n2 (~(0.907 ± 0.001) × 10−15 cm2/W) can be obtained. The real part of third-order NLO susceptibility Re χ(3) and n2 have the relation [9]: Re χ(3) = n02cn2/12π2, where n0 is the refractive index of the medium. Based on the equation, Re χ(3) is estimated to be ~(0.517 ± 0.0006) × 10−13 esu. Then, the absolute value of the third-order NLO susceptibility χ(3) of the UMS is determined to be ~0.519 × 10−13 esu according to the |χ(3)| = [(Re χ(3))2 + (Im χ(3))2]1/2 [30], corresponding to the second hyperpolarizability γ ~2.35 × 10−24 esu cm3. Hence, these researches not only confirm that the obtained UMS can be candidate materials applied in nanophotonic devices, but also further extend the understanding on the NLO properties of MoS2.
Fig. 4 (a) CA, and (b) CA/OA z-scans of the MoS2 dispersions under the excitation of 130 fs pulses at 800 nm with 457 μW pulse power.
4. Conclusion
We have successfully prepared high-yielded UMS through a simple, cost-effective and reproducible solid-state reaction approach. The UMS exhibit significant RSA and nonlinear refraction responses with FOM ~2.52 × 10−15 esu cm for fs pulses at 800 nm. Furthermore, OL effects with low threshold FOL ~44 mJ/cm2 were also observed. Therefore, solid-state reaction technology affords a feasible method for the fabrication of 2D MoS2 nanosheets used in the field of nanophotonics, which can be applied to synthesize other 2D nanomaterials.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grants no. 51132004, 51102096), Guangdong Natural Science Foundation (Grants no. S2011030001349, 1045106410104887).
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