Optical limiting performances of transitional metal dichalcogenides MX2 (M = V, Nb, Ta; X = S, Se) with ultralow initial threshold and optical limiting threshold

Chenglu Liang; Enze Wang; Xian Li; Jing Wang; Yijun Liu; Binyi Chen; Hongxiang Chen; Yang Liu; Xiangfang Peng

doi:10.3788/COL202220.021901

Abstract

The optical limiting performances of few-layer transitional metal dichalcogenides (TMDs) nanosheets in the VB group (

{VS}_{2}

{VSe}_{2}

{NbS}_{2}

{NbSe}_{2}

{TaS}_{2}

, and

{TaSe}_{2}

) were systematically investigated for the first time, to the best of our knowledge. It was found that these TMDs nanosheets showed a normalized transmittance in the range of 20%–40% at the input energy of

1.28 {GW / cm}^{2}

. Ultralow initial threshold F_S (

0.05 - 0.10 J / {cm}^{2}

) and optical limiting threshold F_OL (

0.82 - 2.23 J / {cm}^{2}

) were achieved in the TMDs nanosheets, which surpassed most of the optical limiting materials. This work showed the potential of TMDs beyond

{MoS}_{2}

in optical limiting field.

Keywords

initial threshold optical limiting optical limiting threshold transitional metal dichalcogenide

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^[1]. Effective optical limiting materials require the traits of low optical limiting threshold, high laser damage threshold, quick response, wide protection range of laser wavelength, and high laser protection efficiency^[2]. A low optical limiting threshold is a vital parameter for effective optical limiting materials. Lasers with energy intensity reaching only a few $mJ / {cm}^{2}$ would cause serious damage to organs or devices. However, most of the nonlinear optical limiting materials showed feeble limiting effects when the laser intensities were lower than $100 mJ / {cm}^{2}$ ^[3].

Transitional metal dichalcogenides (TMDs) emerged as a promising optical limiting material waiting to be explored^[4]. TMDs possess a two-dimensional (2D) layered structure bonding via Van der Waals’ force between the atomic layers. When the bulk TMDs were exfoliated to single/few-layer nanosheets, novel physical and chemical properties emerged in the atomic nanosheets^[5,6], which endowed them with intriguing functionality that cannot be found in the bulk counterpart^[7]. Among the TMDs, the VIB group metal dichalcogenides ${MoS}_{2}$ ^[7–9] and ${WS}_{2}$ nanosheets^[10–12] were the most widely explored in optical limiting performances for high-energy lasers due to the low initial threshold, tunable nonlinear optics (NLO) responses, and unique absorption ways dependent on the number of layers. Except for metal sulfides, other chalcogenides of Mo and W like ${WSe}_{2}$ , ${MoSe}_{2}$ , and ${MoTe}_{2}$ were also reported to exhibit optical limiting performances^[3,13,14]. In addition, the IVB (the fouth sub) group TMDs like ${ZrS}_{3}$ , ${ZrSe}_{3}$ , and ${TiS}_{2}$ were also explored as optical limiting materials^[15–17]. Our previous results demonstrated that the ${TiS}_{2}$ nanosheets showed better optical limiting performances^[17] than ${MoS}_{2}$ nanosheets and even better optical limiting performances than the benchmark $C_{60}$ in terms of normalized linear transmittance. The reported TMDs in optical limiting performances are mainly VIB (the sixth sub) and IVB group TMDs, which inspired us to explore the optical limiting performances of the VB (the fifth sub) group (V, Nb, and Ta) TMDs.

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 ( ${VS}_{2}$ , ${VSe}_{2}$ , ${NbS}_{2}$ , ${NbSe}_{2}$ , ${TaS}_{2}$ , and ${TaSe}_{2}$ ) 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 ( $F_{S}$ ) and even lower optical limiting thresholds ( $F_{OL}$ ) than the multi-wall carbon nanotubes (MWNTs) and $C_{60}$ ^[18–20], among which, the ${VSe}_{2}$ nanosheets showed the best optical limiting performances with the ultralow $F_{S}$ value of $0.05 J / {cm}^{2}$ , low $F_{OL}$ value of $0.9 J / {cm}^{2}$ , and normalized transmittance of 0.30 at $1.28 {GW / cm}^{2}$ . This work shows the potential of TMDs beyond ${MoS}_{2}$ in the optical limiting field.

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2. Experiments

2.1. Preparation of bulk and nanosheets 2D VS₂, VSe₂, NbS₂, NbSe₂, TaS₂, and TaSe₂

Bulk ${VS}_{2}$ , ${VSe}_{2}$ , ${NbS}_{2}$ , ${NbSe}_{2}$ , ${TaS}_{2}$ , and ${TaSe}_{2}$ were synthesized via the solid state sintering method^[21]. Briefly, metallic elements of V, Nb, and Ta and corresponding chalcogenides (S/Se elements) with stoichiometric ratios were vacuum sealed in the quartz tubes with an inner diameter of 8 mm and a length of 120 mm. Using halogen ( $I_{2}$ , ${5 mg / cm}^{3}$ ) as the transfer agent, the sealed quartz tubes were heated to 550°C and maintained at this temperature for 5 h. Then, the tubes were further heated to 1000°C and maintained at this temperature for 3 days. After that, the system was cooled to room temperature, and the corresponding ${VS}_{2}$ , ${VSe}_{2}$ , ${NbS}_{2}$ , ${NbSe}_{2}$ , ${TaS}_{2}$ , and ${TaSe}_{2}$ were obtained. Bulk ${VS}_{2}$ , ${VSe}_{2}$ , ${NbS}_{2}$ , ${NbSe}_{2}$ , ${TaS}_{2}$ , and ${TaSe}_{2}$ were exfoliated by ultrasonication treatment. Briefly, 30 mg bulk powder was dispersed in a co-solvent of isopropyl alcohol $(IPA) / H_{2} O$ (1/1, in volume), and then the dispersion was sonicated for 90 min in a water bath. The sonicated suspension was centrifuged at 2000 r/min for 30 min to remove the unexfoliated bulk sediments. The as-obtained supernate was used for optical limiting tests.

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 $Nd : Y_{3} {Al}_{5} O_{12}$ (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 $0.32 - 1.28 {GW / cm}^{2}$ .

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 $2 θ$ 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 $1 T - {VS}_{2}$ [Inorganic Crystal Structure Database (ICSD) identification (ID) 45214]^[22]. The characteristic peak positions of other bulk TMDs powder matched well with their standard cards ${2 H - NbS}_{2}$ [Joint Committee on Powder Diffraction Standards (JCPDS) 38-1367], ${1 T - TaS}_{2}$ (JCPDS 02-0137), $1 T - {VSe}_{2}$ (ICSD ID 45215)^[22], $2 H - {NbSe}_{2}$ (JCPDS 65-7464), and $2 H - {TaSe}_{2}$ (JCPDS 65-3657), indicating a 2D layer structure in these TMDs. After LPE, only peaks in the $c$ -axis orientation, thus (001), (002), (003), and (004), remained, which proved the successful exfoliation of bulk 2D TMDs into nanosheets^[23].

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 ${VS}_{2}$ nanosheets was about 6 nm, and the thickness of ${NbS}_{2}$ 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) TaS₂nanosheets; (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 $0.32 GW / {cm}^{2}$ , $0.64 GW / {cm}^{2}$ , $0.96 GW / {cm}^{2}$ , and $1.28 {GW / cm}^{2}$ 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 $1.28 {GW / cm}^{2}$ .

Fluence ( $GW / {cm}^{2}$ )	Transmittance
Fluence ( $GW / {cm}^{2}$ )	${VS}_{2}$	${NbS}_{2}$	${TaS}_{2}$	${VSe}_{2}$	${NbSe}_{2}$	${TaSe}_{2}$
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

View all Tables

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, $C_{60}$ , was chosen to evaluate the optical limiting performances of our materials. The nonlinear transmittance of commercial $C_{60}$ was tested and compared with ${VSe}_{2}$ nanosheets, as shown in Fig. 4. The initial linear transmittances of $C_{60}$ and ${VSe}_{2}$ nanosheets were unified to be 65%. The summarized normalized transmittances of ${VSe}_{2}$ and $C_{60}$ at different input laser fluences were shown in Fig. 4(a). As the input energy increased, the normalized transmittance of ${VSe}_{2}$ nanosheets decreased continuously, similar with $C_{60}$ . However, the normalized transmittance of ${VSe}_{2}$ nanosheets was much lower than that of $C_{60}$ 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 F_S and F_OL values of C₆₀ and VSe₂ nanosheets.

The initial threshold ( $F_{S}$ ), defined as the incident fluence at which the optical limiting effect starts, and optical limiting threshold ( $F_{OL}$ ), 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 $F_{S}$ and $F_{OL}$ 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 $F_{S}$ and $F_{OL}$ values were calculated from Fig. 4(b). The $F_{S}$ and $F_{OL}$ values of ${VSe}_{2}$ nanosheets were $0.05 J / {cm}^{2}$ and $0.9 J / {cm}^{2}$ , respectively, which were much lower than that of $C_{60}$ , as shown in Fig. 4(c). So, the ${VSe}_{2}$ nanosheets performed better optical limiting performance than $C_{60}$ . The $F_{S}$ and $F_{OL}$ 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 $F_{S}$ values of the investigated TMDs nanosheets were in the range of $0.05 - 0.10 J / {cm}^{2}$ , and the $F_{OL}$ values were between $0.82 and 2.23 J / {cm}^{2}$ . The ${VSe}_{2}$ nanosheets displayed the lowest $F_{S}$ and $F_{OL}$ values of $0.05 J / {cm}^{2}$ and $0.9 J / {cm}^{2}$ , respectively, which were much lower than that of the mainstream optical limiting materials, SnSe^[24], graphene oxide ZnS (GOZS)^[25], $C_{60}$ ^[26], ${MoS}_{2}$ ^[3], ${WS}_{2}$ ^[3], graphene^[3], CdS^[27], and MWNTs^[28], as compared in Fig. 5(b).

Materials	$T_{0}$ (%)	$F_{S}$ ( $J · {cm}^{- 2}$ )	$F_{OL}$ ( $J · {cm}^{- 2}$ )
${VS}_{2}$	65	0.10	0.95
${NbS}_{2}$	65	0.10	0.82
${TaS}_{2}$	65	0.10	1.16
${VSe}_{2}$	65	0.05	0.90
${NbSe}_{2}$	65	0.08	1.53
${TaSe}_{2}$	65	0.09	2.23

Table 2. Linear Transmittance ( $T_{0}$ ), Initial Threshold ( $F_{S}$ ), and Optical Limiting Threshold ( $F_{OL}$ ) of TMDs Nanosheets at 532 nm

View all Tables

Figure 5.(a) Relationship between the normalized transmittance of TMDs nanosheets and the input laser fluences. (b) Comparison of F_S and F_OL values for VSe₂ nanosheets in this work with other reported optical limiting materials at a laser wavelength of 532 nm.

The nonlinear absorption coefficient ( $β$ ) 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 $β$ values. Using equations to fit the experimental results can get the $β$ values. The obtained $β$ values were further fitted to analyze the linearity between the $β$ values and the incident laser energies to estimate the absorption methods^[29–33]. As shown in Fig. S1 of Supplementary Materials, the linearity between the $β$ values and irradiance intensity was calculated to be 88% for ${VS}_{2}$ , 88% for ${NbS}_{2}$ , 24% for ${TaS}_{2}$ , 83% for ${VSe}_{2}$ , 36% for ${NbSe}_{2}$ , and 13% for ${TaSe}_{2}$ , which indicated that both two-photon absorption (TPA) and reverse saturable absorption (RSA) existed in these samples. Besides, in order to explore the application potential of TMDs nanosheets in laser protection equipments and eliminate the influence of nonlinear scattering on optical limiting performance, ${NbS}_{2} / PMMA$ and ${NbSe}_{2} / PMMA$ composites were prepared to evaluate their optical limiting performance in solid state (in Fig. S2 of Supplementary Materials).

4. Conclusions

TMDs ( ${VS}_{2}$ , ${VSe}_{2}$ , ${NbS}_{2}$ , ${NbSe}_{2}$ , ${TaS}_{2}$ , and ${TaSe}_{2}$ ) 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 $F_{S}$ ( $0.05 - 0.10 J / {cm}^{2}$ ) and $F_{OL}$ ( $0.82 - 2.23 J / {cm}^{2}$ ) values, which surpassed the state-of-the-art $MWNTs / C_{60}$ . In addition, the TMDs nanosheets displayed a normalized transmittance in the range of 20%–40% at the input energy of $1.28 {GW / cm}^{2}$ . This work opened up the potential of TMDs beyond ${MoS}_{2}$ in the optical limiting field.

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