Abstract
Keywords
1. Introduction
With the wide application of high-intensity laser and related electronic equipments in sensor, communication, military, and medical fields, the development of optical limiting materials to protect optically sensitive organs or devices, such as human eyes and optical sensors, was crucial[
Transitional metal dichalcogenides (TMDs) emerged as a promising optical limiting material waiting to be explored[
In this work, the optical limiting performances of the few-layer VB group TMDs nanosheets were systematically explored for the first time, to the best of our knowledge. The VB group TMDs (, , , , , and ) were synthesized via solid state sintering followed by liquid phase exfoliation (LPE) to obtain the corresponding nanosheets with the thickness less than 3 nm and lateral size over 1000 nm. All of investigated TMDs nanosheets displayed satisfying optical limiting responses with relatively low optical limiting onsets () and even lower optical limiting thresholds () than the multi-wall carbon nanotubes (MWNTs) and [
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2. Experiments
2.1. Preparation of bulk and nanosheets 2D VS2, VSe2, NbS2, NbSe2, TaS2, and TaSe2
Bulk , , , , , and were synthesized via the solid state sintering method[
2.2. Material characterization
The thickness of the TMDs nanosheets was measured in a tapping mode by an atomic force microscope (AFM) (Dimension Icon). The UV spectrophotometer (UV-2600) was used to obtain the optical absorption spectra. The Raman spectrum was recorded by a Raman spectrometer (Invia Reflex) with laser of 532 nm as a laser source. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the chemical structure of the samples.
2.3. Z-scan measurement
The optical limiting performances of TMDs nanosheets were investigated using an open-aperture Z-scan technique. The Z-scan technique adopted a (Nd:YAG) laser system using a 532 nm laser with a laser pulse of 7 ns (10 Hz, Brio 640, Quantel, Les Ulis, France). The input/output flux of the laser pulse was recorded by an energy meter. The suspension of samples was kept in a 2 mm glass cuvette, and the cuvette was loaded on a computer-controlled mobile platform and moved along Z axis through the focal plane of a 100 mm focal length lens. The waist radius in the focal plane was 23 µm, and the input energy was in the range of 20–80 µJ. The input peak light intensity at the focus was in the range of .
3. Results and Discussion
The RD was conducted to characterize the crystal structure of bulk TMDs and TMDs nanosheets, as shown in Fig. 1. The peaks of values at 15.5°, 32.1°, 35.9°, 44.7°, 56.8°, and 67.3° corresponded to the (001), (100), (101), (012), (110), and (200) crystal planes, respectively, which were consistent with the standard [Inorganic Crystal Structure Database (ICSD) identification (ID) 45214][
Figure 1.XRD patterns of bulk TMDs and TMDs nanosheets. (a) VS2; (b) NbS2; (c) TaS2; (d) VSe2; (e) NbSe2; (f) TaSe2.
The direct observation of TMDs nanosheets was realized via the AFM and the AFM images are presented in Figs. 2(a)–2(f). The nanosheet structure of exfoliated TMDs was obvious. According to the height profiles [insets in Figs. 2(a)–2(f)] interpreted from corresponding AFM images, the thickness of nanosheets was about 6 nm, and the thickness of was about 4 nm. Other TMDs nanosheets possessed a thickness of 1–2 nm, while all of the TMDs nanosheets showed a lateral size over 1000 nm.
Figure 2.AFM images and corresponding height profiles (insets) of TMDs nanosheets obtained by LPE. (a) VS2 nanosheets; (b) NbS2 nanosheets; (c) TaS2 nanosheets; (d) VSe2 nanosheets; (e) NbSe2 nanosheets; (f) TaSe2 nanosheets.
An open-aperture Z-scan technique was used to examine the optical limiting performances of the TMDs nanosheets. A 532 nm laser source with a laser pulse of 7 ns was utilized. The normalized transmittance (the ratio of nonlinear to linear transmittance) was used to evaluate the nonlinear optical properties. A normalized transmittance value lower than one near the zero Z position means the existence of an optical limiting effect in materials. A lower value of normalized transmittance indicated the more enhanced optical limiting effect.
As shown in Figs. 3(a)–3(f), the normalized transmittance values of the TMDs nanosheets were lower than one near the zero Z position in the tested input energy range, exhibiting a typical optical limiting effect. With the increase in input energy, the normalized transmittance of all the tested TMDs nanosheets decreased continuously, demonstrating an enhanced limiting effect. Specifically, the normalized transmittance values of the nanosheets at , , , and were summarized in Table 1. In addition, the normalized transmittance values of the investigated TMDs nanosheets were in the range of 0.23–0.34 at the input energy of .
Fluence ( | Transmittance | |||||
---|---|---|---|---|---|---|
0.32 | 0.53 | 0.56 | 0.55 | 0.51 | 0.58 | 0.64 |
0.64 | 0.32 | 0.36 | 0.45 | 0.40 | 0.41 | 0.42 |
0.96 | 0.28 | 0.28 | 0.38 | 0.35 | 0.34 | 0.37 |
1.28 | 0.23 | 0.23 | 0.25 | 0.30 | 0.32 | 0.34 |
Table 1. Normalized Transmittance Values of TMDs Nanosheets under Different Input Laser Fluences
Figure 3.Open-aperture Z-scan for (a) VS2 nanosheets, (b) NbS2 nanosheets, (c) TaS2 nanosheets, (d) VSe2 nanosheets, (e) NbSe2 nanosheets, and (f) TaSe2 nanosheets, with different incident laser pulse energies; solid line: theoretical fitting data.
The standard material, , was chosen to evaluate the optical limiting performances of our materials. The nonlinear transmittance of commercial was tested and compared with nanosheets, as shown in Fig. 4. The initial linear transmittances of and nanosheets were unified to be 65%. The summarized normalized transmittances of and at different input laser fluences were shown in Fig. 4(a). As the input energy increased, the normalized transmittance of nanosheets decreased continuously, similar with . However, the normalized transmittance of nanosheets was much lower than that of at similar input energies, which indicated a superior optical limiting performance.
Figure 4.Optical limiting performances of VSe2 nanosheets and C60 tested at the same conditions. (a) Normalized transmittance at different input laser fluences. (b) Relationship between the normalized transmittance and the input laser fluences. (c) The FS and FOL values of C60 and VSe2 nanosheets.
The initial threshold (), defined as the incident fluence at which the optical limiting effect starts, and optical limiting threshold (), defined as the input fluence point at which the normalized transmittance drops to 50%, are two vital parameters for optical limiting materials. A material possessing lower and has been pursued in optical limiting applications. The relationship between the normalized transmittance and the input laser fluences was shown in Fig. 4(b). The and values were calculated from Fig. 4(b). The and values of nanosheets were and , respectively, which were much lower than that of , as shown in Fig. 4(c). So, the nanosheets performed better optical limiting performance than . The and values of TMDs nanosheets obtained from Fig. 5(a) (the relationship between the normalized transmittance of TMDs nanosheets and the input laser fluences) are summarized in Table 2. The values of the investigated TMDs nanosheets were in the range of , and the values were between . The nanosheets displayed the lowest and values of and , respectively, which were much lower than that of the mainstream optical limiting materials, SnSe[
Materials | |||
---|---|---|---|
65 | 0.10 | 0.95 | |
65 | 0.10 | 0.82 | |
65 | 0.10 | 1.16 | |
65 | 0.05 | 0.90 | |
65 | 0.08 | 1.53 | |
65 | 0.09 | 2.23 |
Table 2. Linear Transmittance (
Figure 5.(a) Relationship between the normalized transmittance of TMDs nanosheets and the input laser fluences. (b) Comparison of FS and FOL values for VSe2 nanosheets in this work with other reported optical limiting materials at a laser wavelength of 532 nm.
The nonlinear absorption coefficient () was used to further reveal the optical limiting mechanism in TMDs sheets. A model containing saturation and reverse saturation was applied to analyze the experimental results to obtain the values. Using equations to fit the experimental results can get the values. The obtained values were further fitted to analyze the linearity between the values and the incident laser energies to estimate the absorption methods[
4. Conclusions
TMDs (, , , , , and ) in the VB group were successfully synthesized via solid state sintering, and the corresponding nanosheets were obtained by LPE. All of the tested TMDs nanosheets display satisfying optical limiting effects. The TMDs nanosheets possessed ultralow () and () values, which surpassed the state-of-the-art . In addition, the TMDs nanosheets displayed a normalized transmittance in the range of 20%–40% at the input energy of . This work opened up the potential of TMDs beyond in the optical limiting field.
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