Full length articleTransition from saturable absorption to reverse saturable absorption in multi-layered WS2 nanosheets
Introduction
Optical materials, in particular, nanomaterials having huge third-order nonlinearities and multiphoton nonlinear absorption (NLA) have attracted tremendous interests among the researchers due to their applications in different advanced optoelectronic devices, namely, optical switching, mode-locking, and optical limiting [1], [2], [3], [4], [5], [6], [7]. Among other nanomaterials, recently different two dimensional (2D) nanomaterials [e.g., graphene and transition metal dichalcogenides (TMDs)] have been considered to be as potential candidates for nonlinear optical (NLO) applications because of their close confinement of electrons in the 2D plane [1], [2], [3], [4], [5], [6], [7], [8], [9]. Extensive studies on TMDs have revealed that these nanomaterials are showing much better linear and NLO properties than those of the Graphene due to the transformation of indirect bandgap to direct bandgap nature on thinning of the layer numbers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Different TMDs (e.g., MoS2, MoSe2, WSe2, etc.) based 2D nanomaterials are such promising class of optical materials and because of their large nonlinearity they have been used in optical switching, Q-switching, mode-locking, and other optical limiting devices [6], [7], [8], [10]. The large third-order NLO responses in TMDs nanomaterials lead to different processes such as third harmonic generation (THG) and two-photon absorption (2PA) [8], [9], [10], [12].
In recent years, some reports on the study of NLO properties of a few-layer WNSs have been found in the literature. It has been reported that the NLA of WSe2, WS2, and MoS2 can be tuned from saturation absorption (SA) to reverse saturation absorption (RSA) by adjusting the excitation wavelength (λex) from the region below the bandgap to above the bandgap [9], [12]. Dong et al. [9] have given a theoretical model on the optical limiting properties of different layered TMDs nanosheets in N-methyl-2-pyrrolidone (NMP) at 532 nm pulsed laser excitation. They have observed that at low intensity, the matrix shows SA behavior and its behavior transfers to RSA with the increase of laser intensity. Bikorimana et al. [12] have investigated the NLO responses of 2D multilayer WS2 nanosheets (WNSs) and reported a similar type of behavior. The reports on investigation on the NLO properties of low dimensional WS2 nanosheet (i.e., monolayer, bilayer, trilayer, etc.) have sparsely been available in the literature [12], [13], [14], [15]. Also, the origin of such transformation from SA at low laser intensity to RSA at high intensity has not been clearly recognized so far in WS2. Moreover, in most of the earlier published results on the NLO properties of 2D WNSs, the samples have been synthesized by chemical vapour deposition or by any mechanical exfoliation techniques [12], [13], [14], [15]. Hence, to the best of our knowledge, there are no such reports showing the synthesis of thinned layer WNSs by simple hydrothermal treatment in relatively less time and studies on their NLO properties by Z-scan technique.
The trilayer of WNSs has been successfully synthesized through a relatively low temperature (200 °C) hydrothermal process in 12 h. It is observed that the variation of processing time is playing a major role in the formation of multilayer, penta-layer (2.50 nm), and trilayer (1.96 nm) WNSs. The NLO properties of thinned layer WNSs are investigated by using the OA Z-scan technique using ns pulsed (10 ns) laser excitation at wavelength of 532 nm. Interestingly, with the increase of laser excitation intensity, a transformation from SA to RSA is observed in trilayer WNSs and the saturation intensity (Is) in the case of SA is found to be ~0.53 ± 0.03 GW/cm2. It becomes evident from the UV–Vis. absorption and PL emission spectra of trilayer WNSs that the position of B excitonic peak is almost at the same wavelength as that of the used pump laser (532 nm). Therefore, radiation at 532 nm is directly absorbed activating one photon absorption (1PA) in trilayer WNSs. Subsequently this 1PA gives rise to SA behaviour in the sample at such low laser intensity. Further, with increase of laser intensity, the transformation of 1PA to two photon absorption (2PA) occurs due to more absorption at the two-photon wavelength of 266 nm. At an intensity of 0.93 GW/cm2, the trilayer of WNSs exhibits a large 2PA, and the highest 2PA coefficient (β2PA) is found out to be ~1.42 ± 0.01 cm/GW which is ~1.5 times higher than those of the other WNSs. Theoretically, we have simulated the normalized transmission spectra in the MATLAB environment, keeping all the parameters same. In the process, the value of β2PA for trilayer WNSs are determined with high accuracy. The calculated value of the imaginary part of the third-order NLO susceptibility (Im χ(3)) is ~4.24 × 10−13 esu. These outcomes as well as the properties of transition from SA to RSA in thinned layered of WNSs offer a great promise to explore the possibility of employing these materials in designing different optoelectronic devices.
Section snippets
Materials and methods
Initially, 13 mM of sodium tungstate dihydrate (Na2O4W, 2H2O); 62 mM of thiourea CH4N2S), and 63 mM of hydroxylamine hydrochloride (NH2OH, HCl) are mixed in a solution of 10 ml ethanol and 30 ml water. The resulting white suspension is continuously stirred at 60 °C for 1 h and 0.30 mM of CTAB is added to it. The mixture is stirred at 120 °C for 2 h. A homogenous solution is obtained, which is then transferred to a Teflon-lined stainless-steel autoclave. The autoclave is maintained at a
Characterizations and experimental results of synthesized samples
The XRD data of synthesized samples are analysed to determine the structural characteristics of synthesized WS2. Fig. 1a shows the XRD patterns of prepared S1, S2 and S3 samples and it is observed that all synthesized samples show some intense diffraction peaks at 13.88°, 28.65°, 32.86°, 33.74°, 36.00°, 39.40°, 44.50°, 49.50° and 55.50° which are systematically indexed to (0 0 2), (0 0 4), (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 0 4), (1 0 5) and (1 0 6) planes of the hexagonal (2H) phases of WS2 (JCPDS No.
Characterizations of synthesized samples
A detailed analysis of the XRD data reveals that synthesized samples are pure 2H-WS2 as the obtained peaks are well-matched with standard XRD database of 2H-WS2. The typical Raman spectra of synthesized WS2 samples show that under both the wavelengths of laser excitation, the E12g and A1g peaks exist. The lowest value of Δk is found to be that for S3 sample and it is ~63.0 cm−1 which is almost equivalent to trilayer WNSs [22]. It is quite obvious from the TEM and AFM images of synthesized
Conclusions
The thinned layered WNSs are successfully synthesized by simple hydrothermal treatment of 12 h at a relatively low-temperature of 200˚C. It is observed that for hydrothermal treatment of duration 6 h, a multilayer bulk WS2 nanosheets having roughness ~11.48 nm is achieved. But as the hydrothermal treatment time is increased to 9 h, the roughness of WS2 is reduced to penta-layer (~2.50 nm) WNSs. The NLO properties of those synthesized thinned layered WNSs are investigated by using the OA Z-scan
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors are very much thankful to CoE (Advanced Materials) & MHRD (TEQIP-III), NIT, Durgapur for the maintenance fellowship to SK. Authors are thankful to DST-FIST funded XPS facility at the Department of Physics and Meteorology, IIT, Kharagpur for XPS. Authors acknowledge the partial financial supports from Department of Science & Technology and Biotechnology, Govt. of West Bengal with the project Grant No. 332(Sanc.)/ST/P/S&T/16G-24/2018 dt.06.03.19. UC acknowledges partial financial supports
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