Preparation of Few-Layered Wide Bandgap MoS2 with Nanometer Lateral Dimensions by Applying Laser Irradiation

: In this study, we report a new method for the quick, green, and one-step preparation of few-layered molybdenum disulﬁde (MoS 2 ) nanosheets with wide bandgap. MoS 2 nanosheets with small lateral dimension and uniform size distribution were synthesized for various applications. MoS 2 powder was synthesized using the hydrothermal method; then, thinned by applying laser irradiation with di ﬀ erent energies from 40 to 80 mJ. Transmission electron microscopy (TEM), X-ray di ﬀ raction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Vis absorption spectra, and photoluminescence (PL) spectra were applied for the characterization of the MoS 2 nanosheets in terms of morphology, crystal structures, and optical properties. The widest calculated bandgap 4.7 eV was for the sample under 80 mJ laser energy. The results conﬁrmed the successful preparation of highly pure, uniform, and few-layered MoS 2 nanosheets. Furthermore, it was possible to enhance the production rate of MoS 2 nanosheets (including nanosheets and nanoparticles) through laser irradiation. Thus, the present paper introduces a simple and green alternative approach for preparing few-layered MoS 2 nanosheets of transition metal dichalcogenides or other layered materials.


Introduction
Lately, few-layered materials along with two-dimensional materials have attracted the most attention due to their diverse properties and applications [1][2][3]. These materials exhibit completely different properties as compared with their bulk counterparts making them one of the best candidates for new generations of electronic and optoelectronic devices [4][5][6]. The application of few-layered materials has contributed to advances in the fabrication of ultrathin layered materials with high surface-to-volume ratio [7][8][9]. Transition metal dichalcogenides (TMDs) are in a form of MX 2 , where M and X denote transition metal and chalcogen atoms, respectively. The TMDs are important two-dimensional (2D) materials, which include about 60 members. The structures of these layered materials are hexagonally packed. Due to this property, the TMDs, such as MoS 2 , WS 2 , WTe 2 , TiS 2 , TaS 2 , ZrS 2 , or graphite are exfoliated to prepare the 2D materials. Unlike graphene which has no bandgap, the TMDs are natural semiconductors. Therefore, they are applied in electronic devices and optical devices. The mobility of 2D TMDs is much lower than graphene, which limits their applications in devices. Zhang's group prepared few-layer black phosphorus and characterized a few-layer black phosphorus field-effect transistor [10]. A wafer-scale MoS 2 was prepared by Yu et al. [11]. Novoselov et al. predicted some Characterization X-Ray diffraction (XRD) was utilized to study the structure of fabricated samples characterized by using X'Pert PRO (Malven Panalytical, Almelo ( Netherlands)), with Cu-Kα radiation, (λ = 0.154 nm). Fourier transforms infrared (FTIR) spectra were applied using a spectrophotometer (FTIR-410) (Jasco, Toku, Japan), from 500 to 4000 cm −1 . To determine the morphology and structure of the samples, transmission Electron microscopy (TEM) (Philips XL, Netherlands), was done. The bandgap of the samples calculated from UV-Vis optical absorption and transmission spectra (Lambda 750, PerkinElmer, Germany). The photoluminescence spectrum was recorded by a fluorescence spectrometer (PL, PerkinElmer LS 45, Germany).

Results and Discussion
The XRD of the MoS2 powder and few-layered MoS2 are shown in Figure 2. The existence peaks at 14.2°, 32.6°, 39.5°, 44.2°, 49.8°, and 58.3° are consistent with the (002), (100), (103), (006), (105), and (110) planes of hexagonal MoS2 structure (JCPDS NO. 37-1492). By increasing laser energy, the intensity of these peaks is reduced. The calculated lattice constants a = b = 0.315 nm, and c = 1.229 nm represent that all the samples have the crystallite nature of MoS2 materials. The (002) peak ascribing to a d-spacing of 0.62 nm illustrates the stacking of single layers, whereas peaks at higher angle are attributes of the crystallinity of the samples [37]. After laser irradiation, the intensity reduction of (002) peak proposed more discount in the thickness of MoS2 nanosheets [38,39]. It is considered that the (002) diffraction peak of the MoS2 powder, after laser irradiation, involves a shift to the lower angles and as a result, an increase in d-spacing indicating the destacking of MoS2 layers. It can be deduced with the enhancement in the laser energy the average grain sizes of the MoS2 nanosheets decrease. Therefore, the broadened diffraction peaks correspondingly confirm the decrease of the size of the MoS2 nanosheets, which is initiated by the laser irradiation. Characterization X-Ray diffraction (XRD) was utilized to study the structure of fabricated samples characterized by using X'Pert PRO (Malven Panalytical, Almelo (Netherlands)), with Cu-Kα radiation, (λ = 0.154 nm). Fourier transforms infrared (FTIR) spectra were applied using a spectrophotometer (FTIR-410) (Jasco, Toku, Japan), from 500 to 4000 cm −1 . To determine the morphology and structure of the samples, transmission Electron microscopy (TEM) (Philips XL, Netherlands), was done. The bandgap of the samples calculated from UV-Vis optical absorption and transmission spectra (Lambda 750, PerkinElmer, Germany). The photoluminescence spectrum was recorded by a fluorescence spectrometer (PL, PerkinElmer LS 45, Germany).

Results and Discussion
The XRD of the MoS 2 powder and few-layered MoS 2 are shown in Figure 2  . By increasing laser energy, the intensity of these peaks is reduced. The calculated lattice constants a = b = 0.315 nm, and c = 1.229 nm represent that all the samples have the crystallite nature of MoS 2 materials. The (002) peak ascribing to a d-spacing of 0.62 nm illustrates the stacking of single layers, whereas peaks at higher angle are attributes of the crystallinity of the samples [37]. After laser irradiation, the intensity reduction of (002) peak proposed more discount in the thickness of MoS 2 nanosheets [38,39]. It is considered that the (002) diffraction peak of the MoS 2 powder, after laser irradiation, involves a shift to the lower angles and as a result, an increase in d-spacing indicating the destacking of MoS 2 layers. It can be deduced with the enhancement in the laser energy the average grain sizes of the MoS 2 nanosheets decrease. Therefore, the broadened diffraction peaks correspondingly confirm the decrease of the size of the MoS 2 nanosheets, which is initiated by the laser irradiation.
The FTIR spectra of the samples are shown in Figure 3. The peaks below 700 cm −1 reveal the (Mo-O) deformation mode possibly caused by the sulfidation vibrations. The peaks between 750 and 1000 cm −1 indicate the most definite information on Mo-O coordination [40]. By increasing laser energy these peaks are more potent indicating stronger bonds of Mo-O. The absorption band between 1100 and 1650 cm −1 is ascribed to the stretching vibrations of the hydroxyl group and Mo-O vibrations. By reduction of MoS 2 layers after laser irradiation, these peaks are stronger. The Mo-S vibration peak is around 600 cm −1 , which is faded by increasing laser energy, meaning weaker bond of Mo-S. The C = C symmetric and asymmetric stretching vibrations are at 2890 and 2970 cm −1 , which show small changes by different laser energies [41]. After laser irradiation, the peak at 1600 becomes stronger, which means stronger Mo-S bound, which is the result of monolayer MoS 2 [42].  The FTIR spectra of the samples are shown in Figure 3. The peaks below 700 cm −1 reveal the (Mo-O) deformation mode possibly caused by the sulfidation vibrations. The peaks between 750 and 1000 cm -1 indicate the most definite information on Mo-O coordination [40]. By increasing laser energy these peaks are more potent indicating stronger bonds of Mo-O. The absorption band between 1100 and 1650 cm −1 is ascribed to the stretching vibrations of the hydroxyl group and Mo-O vibrations. By reduction of MoS2 layers after laser irradiation, these peaks are stronger. The Mo-S vibration peak is around 600 cm −1 , which is faded by increasing laser energy, meaning weaker bond of Mo-S. The C = C symmetric and asymmetric stretching vibrations are at 2890 and 2970 cm −1 , which show small changes by different laser energies [41]. After laser irradiation, the peak at 1600 becomes stronger, which means stronger Mo-S bound, which is the result of monolayer MoS2 [42].   The FTIR spectra of the samples are shown in Figure 3. The peaks below 700 cm −1 reveal the (Mo-O) deformation mode possibly caused by the sulfidation vibrations. The peaks between 750 and 1000 cm -1 indicate the most definite information on Mo-O coordination [40]. By increasing laser energy these peaks are more potent indicating stronger bonds of Mo-O. The absorption band between 1100 and 1650 cm −1 is ascribed to the stretching vibrations of the hydroxyl group and Mo-O vibrations. By reduction of MoS2 layers after laser irradiation, these peaks are stronger. The Mo-S vibration peak is around 600 cm −1 , which is faded by increasing laser energy, meaning weaker bond of Mo-S. The C = C symmetric and asymmetric stretching vibrations are at 2890 and 2970 cm −1 , which show small changes by different laser energies [41]. After laser irradiation, the peak at 1600 becomes stronger, which means stronger Mo-S bound, which is the result of monolayer MoS2 [42].  The TEM images of the MoS 2 samples were performed to investigate the structural deviations that occur after various laser irradiation energies ( Figure 4). Figure 4a is the image of MoS 2 powder before laser irradiation. In Figure 4b, the large area could be distinguished for 40 mJ laser energy. By increasing laser irradiation energy, decreasing of the number of layers is obvious. Figure 4c shows the image after 60 mJ laser energy. Figure 4d shows few-layered MoS 2 nanosheets with folding at the edge, which is a phenomenon that occurs in two-dimensional materials. The number of layers could be determined using this morphological deviation [43]. A single dark line at the folded edge could be related to a monolayer [44]. The TEM results, in this study, indicate that the prepared MoS 2 is few-layered with uniform size distribution. before laser irradiation. In Figure 4b, the large area could be distinguished for 40 mJ laser energy. By increasing laser irradiation energy, decreasing of the number of layers is obvious. Figure 4c shows the image after 60 mJ laser energy. Figure 4d shows few-layered MoS2 nanosheets with folding at the edge, which is a phenomenon that occurs in two-dimensional materials. The number of layers could be determined using this morphological deviation [43]. A single dark line at the folded edge could be related to a monolayer [44]. The TEM results, in this study, indicate that the prepared MoS2 is few-layered with uniform size distribution. To investigate the effect of laser energy on the absorption evaluation of MoS2, the few-layered MoS2 nanosheets were examined via UV-Vis spectroscopic technique by dispersing in water as a reference. Figure 5a shows the room temperature UV-Vis absorption spectra of the few-layered MoS2 nanosheets before laser irradiation and after 15 min laser irradiation under 40, 60, and 80 mJ energies. The UV-Vis absorption spectrum of MoS2 solution displays a clear broad peak located at 250 nm ascribed to the excitonic property of MoS2 nanosheets [45]. It is different from the 2D MoS2 To investigate the effect of laser energy on the absorption evaluation of MoS 2 , the few-layered MoS 2 nanosheets were examined via UV-Vis spectroscopic technique by dispersing in water as a reference. Figure 5a shows the room temperature UV-Vis absorption spectra of the few-layered MoS 2 nanosheets before laser irradiation and after 15 min laser irradiation under 40, 60, and 80 mJ energies. The UV-Vis absorption spectrum of MoS 2 solution displays a clear broad peak located at 250 nm ascribed to the excitonic property of MoS 2 nanosheets [45]. It is different from the 2D MoS 2 counterpart with large lateral dimensions, which display four peaks at 340, 430, 590, and 650 nm as a characteristic absorption band of the original bulk MoS 2 . For bulk MoS 2 , the peaks at 590 and 650 nm are ascribed to the K point of the Brillouin zone, and the peaks at 340 and 430 nm are ascribed to the direct transition from the deep valence band to the conduction band [26,46]. Nonetheless, these four characteristic absorption bands fade in the spectra of the as-prepared MoS 2 nanosheets. Only one peak is detected in the near-UV region (λ < 300 nm) ascribed to the excitonic property of MoS 2 [47]. The strong blue shift in the optical absorption is ascribed to the quantum confinement and edge effects [47,48] upon the reduction of the lateral size of the MoS 2 nanosheets to <50 nm. Thus, quantum confinement in layered d-electron materials such as MoS 2 paves the way to engineer the electronic structure of matter at the nanoscale. As the TEM results show, most of the as-prepared MoS 2 nanosheets are~30 nm and a strong blue shift is detected; this finding is in a good agreement with the results reported elsewhere [49,50].
hν, according to the Tauc relation, (αhν) n = A (hν − Eg). In the Tauc equation, α is the absorption coefficient calculating from UV-Vis data according to the Beer-Lambert formula, hν is the photon energy in eV, Eg is the bandgap energy, and A is a constant. The symbol "n" indicates the nature of the electronic transition causing the absorption and can have the values 2, 1/2, 2/3, and 1/3 if the electronic transition is direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions, respectively. The extrapolation of the linear part of the UV-Vis spectrum to the point in which (αhν) 2 = 0, gives the optical energy bandgap of the samples (Figure 5b). The calculated bandgap energies are greater than that of monolayer MoS2 (1.9 eV) and bulk MoS2 (1.2eV) [26,52,53] and comparable to that of described monolayer MoS2 quantum dots (4.96 eV) [54]. The data are collected in Table 1. The slight difference in bandgap is ascribed to the size of nanosheets. Conventional semiconductors like silicon have a bandgap in the range 1−1.5 eV, whereas wide bandgap (WBG) materials have bandgaps in the range 2−6 eV such as SiC, GaN, and diamond [55,56]. Among them, gallium oxide (Ga2O3) has the largest band gap (~4.66 eV) showing the widest tunable spectral range as compared with those of the other WBG semiconductors. The MoS2 nanosheets prepared in this study,are in the range of WBG semiconductors. Among the samples, the one under 80 mJ laser energy has the widest bandgap (4.7 eV). Therefore, it is suitable to use in optoelectronic and electronic devices. The absorption spectra of all four samples exhibit shoulders ranging from about 280 nm to 340 nm attributed to the interband transitions from the occupied dz2 orbital to unoccupied dxy, dyz, dxz, and dx2-y2 orbitals [51]. From the UV-Vis data, it is possible to calculate the optical energy bandgap values of materials with a direct allowed bandgap by plotting the graph of (αhν) 2 versus hν, according to the Tauc relation, (αhν) n = A (hν − E g ). In the Tauc equation, α is the absorption coefficient calculating from UV-Vis data according to the Beer-Lambert formula, hν is the photon energy in eV, E g is the bandgap energy, and A is a constant. The symbol "n" indicates the nature of the electronic transition causing the absorption and can have the values 2, 1/2, 2/3, and 1/3 if the electronic transition is direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions, respectively. The extrapolation of the linear part of the UV-Vis spectrum to the point in which (αhν) 2 = 0, gives the optical energy bandgap of the samples (Figure 5b). The calculated bandgap energies are greater than that of monolayer MoS 2 (1.9 eV) and bulk MoS 2 (1.2 eV) [26,52,53] and comparable to that of described monolayer MoS 2 quantum dots (4.96 eV) [54]. The data are collected in Table 1. The slight difference in bandgap is ascribed to the size of nanosheets.
Conventional semiconductors like silicon have a bandgap in the range 1−1.5 eV, whereas wide bandgap (WBG) materials have bandgaps in the range 2−6 eV such as SiC, GaN, and diamond [55,56]. Among them, gallium oxide (Ga 2 O 3 ) has the largest band gap (~4.66 eV) showing the widest tunable spectral range as compared with those of the other WBG semiconductors. The MoS 2 nanosheets prepared in this study, are in the range of WBG semiconductors. Among the samples, the one under 80 mJ laser energy has the widest bandgap (4.7 eV). Therefore, it is suitable to use in optoelectronic and electronic devices. factors such as strain [62] or quantum confinement [63] have an effect on the shift. This phenomenon could also be caused by the presence of several trap states [52,64]. It is noticeable, after laser irradiation, there is a redshift in the PL peak of the samples and its intensity decreases. This property is attributed to the existence of polydispersity of MoS2 nanosheets [46,65], the hot PL from Brillouin zone's K point, and many trap states in MoS2 nanosheets as detected in graphene quantum dots [65]. Reduction of the intensity means a reduction in electron-hole recombination. The bandgap calculated by PL emission is always less than the original bandgap and is not the exact bandgap. The PL emission is mostly affected by the polarity of the solvent system and the changing solvent polarity causes a shift for the peaks. However, the absorption spectrum is much less affected by change in solvent polarity. Therefore, the bandgap is calculated from the absorbance spectrum. Bao et al. reported the change in PL peak position by the changing in excitation wavelength [66]. The PL spectra of MoS2 were measured at various excitation wavelengths. By increasing the excitation wavelength, the PL emission peaks shift to longer wavelengths. This phenomenon can also be due to the presence of several trap states, defects, and quantum confinements in layered material such as MoS2 [67].
Researches on semiconductors, especially WBG semiconductors, have fabricated various power devices, which are reliable and performant enough to design high-efficiency level converters to match application requirements. The WBG semiconductors are significant scientifically and technologically, and they have become the main materials to be applied in high-performance optoelectronic and electronic devices. The WBG semiconductors have many benefits for The MoS 2 nanostructure has specific photoluminescence capabilities due to the quantum size effect [57,58]. By thinning the MoS 2 crystal to a monolayer, strong photoluminescence appears showing an indirect to direct bandgap transition in this d-electron system [59]. Figure 6 shows the PL spectrum of MoS 2 nanosheets solution excited at 400 nm. A strong sharp peak at 530 nm as a typical fluorescence spectrum of MoS 2 particles with nanometer scale lateral dimensions is observed [60]. The PL peak at 530 nm can be attributed to MoS 2 nanosheets with a lateral dimension up to a few tens of nm [61]. This is also supported by the TEM images showing the formation of few-layered MoS 2 nanosheets. The PL spectrum is different from the two distinguished peaks positioned at around 610 and 661 nm in 2D MoS 2 flakes with large lateral dimensions from the direct bandgap of the K point [26]. It is remarkably hard to explain a shift in the PL of the samples because many factors such as strain [62] or quantum confinement [63] have an effect on the shift. This phenomenon could also be caused by the presence of several trap states [52,64]. It is noticeable, after laser irradiation, there is a redshift in the PL peak of the samples and its intensity decreases. This property is attributed to the existence of polydispersity of MoS 2 nanosheets [46,65], the hot PL from Brillouin zone's K point, and many trap states in MoS 2 nanosheets as detected in graphene quantum dots [65]. Reduction of the intensity means a reduction in electron-hole recombination.
The bandgap calculated by PL emission is always less than the original bandgap and is not the exact bandgap. The PL emission is mostly affected by the polarity of the solvent system and the changing solvent polarity causes a shift for the peaks. However, the absorption spectrum is much less affected by change in solvent polarity. Therefore, the bandgap is calculated from the absorbance spectrum. Bao et al. reported the change in PL peak position by the changing in excitation wavelength [66]. The PL spectra of MoS 2 were measured at various excitation wavelengths. By increasing the excitation wavelength, the PL emission peaks shift to longer wavelengths. This phenomenon can also be due to the presence of several trap states, defects, and quantum confinements in layered material such as MoS 2 [67].
Researches on semiconductors, especially WBG semiconductors, have fabricated various power devices, which are reliable and performant enough to design high-efficiency level converters to match application requirements. The WBG semiconductors are significant scientifically and technologically, and they have become the main materials to be applied in high-performance optoelectronic and electronic devices. The WBG semiconductors have many benefits for optoelectronic and electronic purposes because of their large bandgap energy and they help devices operate at much higher temperatures, voltages, and frequencies [68]. The power electronic modules make by these materials are considerably more powerful and energy efficient as compared with those made from conventional semiconductor materials [69]. They are becoming popular in high voltage and high power applications such as the traction motors and inverters found in the automotive sector [70]. The nature of WBG energy is appropriate for absorbing or emitting ultraviolet (UV) light in optoelectronic devices. For optical absorption and emission of the UV lights, WBG materials are significantly involved. The prepared WBG few-layered MoS 2 nanosheets, in this study, are suitable for the mentioned applications.

Conclusions
A new, quick, green, and one-step method was introduced for preparing few-layered MoS 2 nanosheets by applying the laser irradiation. Uniform size distribution and a high production rate of few-layered MoS 2 nanosheets were achieved by solely adjusting the laser energy. Samples under 40, 60, and 80 mJ laser energy were fabricated. The calculation displayed the widest bandgap, i.e., 4.7 eV, was for the sample under 80 mJ laser energy. The as-prepared MoS 2 nanosheets exert strong quantum confinement and edge effects. MoS 2 nanosheets are a type of WBG semiconductor having minor, quicker, more reliable power electronic components with high efficiency. Increasing the temperature reduces the bandgap of a semiconductor, thereby affecting most of the semiconductor material parameters. The spacing of the electronic levels and the bandgap increases with decreasing particle size, therefore, they are more compact in size and, as a result, less design margin is required to allow for temperature variation, allowing designers to achieve better performance from these devices. To the best of our knowledge, this is the first time to apply this method to exfoliate the MoS 2 sheets.