Synthesis of WS1.76Te0.24 alloy through chemical vapor transport and its high-performance saturable absorption

Layered transitional metal dichalcogenides (TMDs) are drawing significant attentions for the applications of optics and optoelectronics. To achieve optimal performances of functional devices, precisely controlled doping engineering of 2D TMDs alloys has provided a reasonable approach to tailor their physical and chemical properties. By the chemical vapor transport (CVT) method and liquid phase exfoliation technique, in this work, we synthesized WS1.76Te0.24 saturable absorber (SA) which exhibited high-performance of nonlinear optics. The nonlinear saturable absorption of the WS1.76Te0.24 SA was also measured by the open aperture Z-scan technique. Compared to that of the binary component WS2 and WTe2, WS1.76Te0.24 SA has shown 4 times deeper modulation depth, 28% lower saturable intensity and a much faster recovery time of 3.8 ps. The passively Q-switched laser based on WS1.76Te0.24 was more efficient, with pulse duration narrowed to 18%, threshold decreased to 28% and output power enlarged by 200%. The promising findings can provide a method to optimize performances of functional devices by doping engineering.

Inspired by the history of Si semiconductor, the doping engineering appears to be the key to tailor physical and chemical properties of TMDs. Mixed chalcogenides or mixed metal elements of two different TMDs can control the band gap, such as WS 2× Se 2(1−x) 23 and Mo x W 1−x S 2 24 . However, it has been proven that only a few hundred milli-electron volts (meV) could be realized, i.e. 300 meV for WS 2x Se 2(1−x) and 170 meV for Mo x W 1−x S 2 solid solutions respectively. P. Yu et al. experimentally showed that 2H-WSe 2 and Td-WTe 2 can form stable layered WSe 2× Te 2(1−x) alloys 25 , with a phase transition from 2H-to-Td (x = 1 − 0.6 for 2 H structure; x = 0.5 and 0.4 for 2 H and 1 Td structures; x = 0 − 0.3 for 1 Td structure) controlled by the complete composition. The electronic structures changing from semiconducting to metallic enable wide tunability of the optical and electronic properties. Extraordinary physical properties of alloys are needed for in-depth study where the alloys showed some unique advantages compared to 2D binary TMDs, making them fundamentally and technically important in applications of optics and optoelectronics. One of the impressive physical properties of mono-and few-layer alloy is that TMDs display surprisingly excellent nonlinear optical properties, Y. Wang et al. studied the nonlinear optics properties of alloys of Bi 2 Te x Se 3-x with lower saturable intensity, deeper modulation depth 26 .
TMDs alloying still remains challenging resulted from the lattice mismatch of their parent counterparts. Here we synthesized WS 1.76 Te 0.24 alloy by doping Te 2− ions in WS 2 (2 H) structure. The nonlinear optics properties of WS 1.76 Te 0.24 SA were 4 times deeper modulation depth, 28% lower saturable intensity and a much faster recovery time of 3.8 ps compared to those of WS 2 and WTe 2 . To find out whether pulsed laser's performance can be promoted by alloying, passively Q-switched lasers were investigated based on WS 2 , WS 1.76 Te 0.24 and WTe 2 SAs at the wavelength of 1060 nm. We found that the passively Q-switched laser based on WS 1.76 Te 0.24 was more efficient, with pulse duration narrowed to 18%, threshold decreased to 28% and output power enlarged by 200%. The promising findings can provide a method to optimize performances of functional devices by doping engineering.

Methods
Synthesis and characterization of WS x te 2−x SAs. The WS 1.76 Te 0.24 monocrystalline was prepared by the chemical vapor transport (CVT) method with well-controlled temperature. There were two steps to obtain WS 1.76 Te 0.24 mono-crystalline. Firstly, WS 1.76 Te 0.24 polycrystalline was synthesized by heating a mixture of sulphur (Strem Chemicals 99.9%), tungsten (Strem Chemicals 99.9%) and tellurium (Strem Chemicals 99.9%) with stoichiometric amounts at 750 °C for 48 hours in an evacuated and sealed quartz ampoule (8 mm ID, 10 mm OD, 300 mm length). Considering the powerful exothermicity of the reaction, the mixture was slowly preheated to 750 °C for 12 hours to avoid explosion. Secondly, WS 1.76 Te 0.24 was grown by CVT method in a double zone furnace with as-prepared grinded polycrystalline powder and the transport agent was bromine (Sigma-Aldrich, 99.8%) at about 5 mg/mL. The procedure of growth was 72 hours in an evacuated and sealed quartz ampoule (8 mm ID, 10 mm OD, 300 mm length). Figure 1a showed a two-temperature zone tube furnace with well-controlled temperature. Throughout the growth process of WS 1.76 Te 0.24 , the raw material (T 2 ) and crystal growth zones (T 1 ) were kept at 1030 °C and 1010 °C, respectively. The parent components of WS 2 and WTe 2 were synthesized by the same method for the following contrast experiments. We prepared WS 2 , WS 1.76 Te 0.24 and WTe 2 SAs by liquid-phase exfoliation and spin-coating technique with the same parameters (sonication time, speed of centrifugation and spin-coating) for further comparison. First, the mixture of grinded 0.2 mg WS 1.76 Te 0.24 monocrystalline in 4 ml acetone solvent was sonicated in high power for 40 min. Only pure acetone was employed as the solvent to avoid introduction of extra impurities. Then, we collected the one-third top of the dispersions after the centrifugation at 2500 rpm for 10 min to remove the large sedimentations. Finally, we span and coated the dispersion on SiO 2 plate to obtain WS 1.76 Te 0.24 SA after the acetone was easily removed by volatilization in the air.
Material characterization. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were adopted to learn the morphology of the WS 1.76 Te 0.24 nanoplates. TEM images in Fig. 1e-g showed the layered structures of WS 2 , WS 1.76 Te 0.24 and WTe 2 respectively, where the gray scale was directly proportional to the thickness. The observed well exfoliated nanoflakes with layered structure implied rigid mechanical property. The insets were the Selected Area Electron Diffraction (SAED) of WS 2 , WS 1.76 Te 0.24 and WTe 2 nanoflakes. The SAED showed that WS 2 , WS 1.76 Te 0.24 and WTe 2 were monocrystalline with 2 H, 2 H and Td phases, respectively. The diffraction of six-fold symmetry spots displayed the hexagonal lattice of WS 1.76 Te 0.24 nanoflake. EDS measurement was also produced to determinate the element ratio of the three samples as shown in Fig. 1h-j. It can be seen that the ratio of S to Te in WS 1.76 Te 0.24 is 1.76 to 0.24, indicating an efficient doping of Te ions in WS 2 framework. As shown in Fig. 1k-m, the atomic force microscopy (AFM) was carried out to measure the three SAs thicknesses. The thicknesses of WS 2 , WS 1.76 Te 0.24 and WTe 2 nanoflakes were about 15.6, 14.9, and 16.3 nm, corresponding to 23, 19 and 27 layers, respectively. To learn more about the WS 1.76 Te 0.24 alloy, EDS mapping was adopted shown in Fig. 1n. The green, red and yellow parts were the distribution of tungsten, sulphur, and tellurium element, respectively. The uniform doping of Te in WS 2 of WS 1.76 Te 0.24 was obtained. Furthermore, the WS 1.76 Te 0.24 SA with no additional dangling bonds is stable in the air. The high chemical stability was due to the substitution of atoms in the alloy TMDs.
Raman spectroscopy was employed to learn the detailed lattice vibration modes of WS 1.76 Te 0.24 affected by doping engineering where Te 2− replaced the S 2− in the WS 2 structure. The characterization was carried out by using a Jobin Yvon LabRam 1B Raman spectrometer with laser source at 532 nm. The comparison of the Raman fingerprints among the three samples in the range of 200-450 cm −1 is shown in Fig. 2a. In Fig. 2a, the characteristic peaks at 353.2 and 422.7 cm −1 were assigned as the in-plane (E 2g ) and out-of-plane (A 1g ) vibrational modes corresponding to WS 2 nanoflakes. For the Td-WTe 2 , the spectrum just showed the A 1 Raman mode at 217.8 cm −1 . The characteristic bands of WS 1.76 Te 0.24 showed the "two mode behavior" as the coexistence of vibrations of WS 2  The ellipsometer is a conventional method to measure the film's refractive index. However, ellipsometer has strict requirement on the samples for uniform surface, large size, and thin thickness. Due to its low spatial resolution, it is difficult to obtain refractive index of nanomaterials in nanometer size. Benefitting from the Lorentz-Drude model and Kramers-Kronig (K-K) relationship of the dielectric function 27 , we calculated the corresponding refractive index from the transmittance spectrum. Figure 2c shows the fitting curve of the reflectance spectra, where the refractive index parameters were obtained by the K-K relationship.
In order to understand the incorporation mechanism by Te doping into WS 2 and the corresponding effect on optical nonlinear properties of 2D WS 1.76 Te 0.24 , we performed Z-scan measurement with a femtosecond laser (1060 nm, 175 fs) as excited source. The results of WS 2 , WS 1.76 Te 0.24 and WTe 2 SAs are showed in Fig. 2d. The increase of transmittance was easily observed with the increase of laser intensity, resulted from the nonlinear saturable absorption effect . The mechanism of saturable absorption can be explained as Pauli blocking principle in the conduction band. However, significant differences in saturable absorption efficiency and sensitivity among the three samples can be clearly distinguished in Fig. 2d. Based on the nonlinear optical theory, the transmittance is expressed in the form of 14 where A s is the modulation depth, A ns is the non-saturable components, I sat is the saturable intensity, and I is the incident light intensity. The data was fitted with the Eq. 1, the modulation depth and the saturable intensities were obtained as presented in  Fig. 3, the bandgap of WS 2 SA is 1.9 eV larger than the photon energy ( ω  ) of 1060 nm laser. The saturable absorption of WS 2 is resulted from the defect states. As shown in Fig. 3, under the excited light, the electrons in valence band of WS 2 are transferred to the defect states. The electrons in the defect states jump to the conduction band with one more photon each. Therefore, there are two platforms that the transition of one electron from valence band to conduction band of WS 2 and the transition requires two photons. Compared to that of WS 2 , the optical bandgap   www.nature.com/scientificreports www.nature.com/scientificreports/ of WS 1.76 Te 0.24 is 1.2 eV covering 1.0 µm, so the electrons directly transfer from valence band to the conduction band with one absorbed photon. In WTe 2 , the semi-metal characteristic makes it possess higher electron concentration in the conduction band. The higher concentration of electrons has stronger reflection on the excitation light as shown in Fig. 3. Therefore, compared to the binary component of WS 2 and WTe 2, WS 1.76 Te 0.24 has stronger photon absorption at 1.0 µm. The stronger photon absorption of WS 1.76 Te 0.24 can increase the number of absorbed photons to produce more electrons at the same laser intensity. Eventually, the saturable intensity of WS 1.76 Te 0.24 is lowered.
Suppose at a certain photon frequency, optical absorption satisfies I I ( ) Here the absorption coefficient α(I) is expressed as α α α . dz′ is the propagation distance in the sample. The third-order nonlinear optics susceptibility Imχ (3) can be expressed as here c is the speed of light, λ is the laser wavelength, n is the refractive index, the discrepancy caused by the linear absorption, namely figure of merit (FOM): 0 . Based on the model 2, we can obtain WS 1.76 Te 0.24 with α NL ~ −10 4 cm/GW, Imχ (3) ~10 −7 esu, FOM ~10 −14 cm• esu. Compared to previous works, the FOM of WS 1.76 Te 0.24 perform one order of magnitude larger than that of grapheme, graphene oxide, MoS 2 /NMP dispersions ~10 −15 esu cm 16 . That suggests a promising potential to achieve efficient nonlinear performance by alloying TMDs. However, one should note that FOM varies with the different experiment conditions such as the wavelength, pulse width and so on. For convincing comparison, we carried out the Z-scan on the same condition and the nanosheets were prepared by the same parameters of liquid-phase exfoliation and spin-coating technique. The results are shown in Table 1. It is unambiguous that the FOM value of WS 1.76 Te 0.24 SA was larger than those of WS 2 and WTe 2 SAs, which indicated the enhanced nonlinear performance of WS 1.76 Te 0.24.
The pump-probe system was adopted to study the carrier relaxation that reflects the optical response of materials. The undegenerated pump probe system is easy to align and the relaxation time is corresponded to the carrier-phonon coupling 28 . The ultrafast signal was measured using a Ti: Sapphire laser with pulse duration of 120 fs, repetition rate of 76 MHz, fluence of 200 µJ/cm 2 at 395 nm as the pump and the probe beam was at 790 nm with much lower fluence. The probe reflection was a function of the delay time that was detected by a Si photodetector and amplified by a lock-in amplifier. As shown in Fig. 2e, the ultrafast signal of WS 1.76 Te 0.24 flakes with absorption bleaching was obtained. The signal amplitude was as large as ~ 250%, implying excellent nonlinear optics property. Notably, the decay time was 3.8 ps fitted by a single exponential function. The decay time of WS 1.76 Te 0.24 was significantly shorter than 13 ps of WS 2 29 and 5 ps of WTe 2 6 , as a result of higher density of trapping states induced by Te doping in the nanoflake 30 . investigation of the WS 1.76 te 0.24 saturable absorption in passively Q switched laser at 1.0 μm.
To identify whether the boosted saturable absorption effect of WS 1.76 Te 0.24 did favor in Q-switched laser, we set up a passively Q-switched Yb: Gd 2 SrAl 2 O 7 (Yb: GSAO) laser to investigate the performance of WS 2, WS 1.76 Te 0. 24 and WTe 2 SAs. In Fig. 4a, the schematic of experiment setup was shown. A laser diode of 976 nm was served as pump source, which was coupled in a fiber of a core diameter of 105μm and the numerical aperture of 0.22. A doublet lens was employed to focus the beam at 105 μm within the Yb:GSAO crystal. In a cooled down system, the Yb: GSAO gain medium was wrapped with indium foil and mounted in a copper holder with water-cooled at 21 °C. The 11 mm linear cavity composed of 1060 nm high reflectivity M1(R = −200 mm) and 18% transmittance plano M2. The as-prepared three samples on SiO 2 were inserted into the cavity serving as the saturable absorber. Figure 4b-d show the characteristics of average output power, repetition rates and pulse durations of the lasers on absorbed pump power variation. The absorbed pump power thresholds of WS 2, WTe 2, and WS 1.76 Te 0.24 were 1.02, 1.21 and 0.34 W , respectively. The threshold of the WS 1.76 Te 0.24 SA decreased to 28% due to the lower saturable intensity, as shown in Table 1. The output power was measured and calculated to be linearly correlated with the pump power. The maximum output powers of WS 2, WTe 2 and WS 1.76 Te 0.24 were 247.5, 152.8 and 350 mW , respectively. It is worth noting that WS 1.76 Te 0.24 SA achieved twice larger output power of WTe 2 . The repetition rate continuously increased from 108.2 to 195.2 kHz for WS 2 SA, and the repetition rate range of WS 1.76 Te 0.24 SA was 120.8 to 271.1 kHz, scope of WTe 2 varied from 112.2 to 170.8 kHz. As shown in Fig. 4d-f, pulse widths of 1.285 µs, 230 ns and 550 ns were obtained in 1.06 µm Q-switched lasers based on WS 2 , WS 1.76 Te 0.24 and WTe 2 SAs, respectively. As shown in Fig. 4e, the typical Q-switched pulse trains of WS 2, WS 1.76 Te 0.24 and WTe 2 were recorded by a 500 MHz bandwidth oscilloscope (Tektronix, DPO7054) through a high-speed detector (Thorlabs, DET10C/M), which confirms the stability of Q-switched operation. The narrowest pulse duration was obtained by WS 1.76 Te 0.24 SA that should be attributed to the much larger modulation depth than WS 2 and WTe 2 ( Fig. 2d and Table 1). The passively Q-switched laser based on WS 1.76 Te 0.24 narrowed pulse duration to 18%.The optical spectra of the Q-switched lasers were measured with an infrared optical spectrum analyzer (Yokogawa, AQ-6315A) with a resolution of 0.05 nm. The wavelength of WS 2, WS 1.76 Te 0.24 and WTe 2 were centered at 1061.13 nm with 0.12 nm full width at half maximum (FWHM), 1065.93 nm with 0.45 nm FWHM, and 1058.01 nm with 0.16 nm FWHM, respectively. Q-switched laser based on the WS 1.76 Te 0.24 SA can improve the key parameters as pulse width, slope efficiency and the average output power as listed in Table 2.

conclusion
In this work, we have experimentally demonstrated the enhanced nonlinear optical properties of WS 1.76 Te 0.24 by alloying WTe 2 and WS 2 . We synthesized ternary WS 1.76 Te 0.24 by CVT method. The SAED, EDS and Raman spectra showed good quality of the alloy WS 1.76 Te 0.24 nanosheets. The saturable absorption of WS 1.76 Te 0.24 at 1.06 µm was significantly more efficient than binary parents WTe 2 and WS 2 as evidenced by Z-scan and pump-probe results, (2019) 9:19457 | https://doi.org/10.1038/s41598-019-55755-x www.nature.com/scientificreports www.nature.com/scientificreports/ where WS 1.76 Te 0.24 SA showed 4 times deeper modulation depth, 28% lower saturable intensity and a much faster recovery time of 3.8 ps. The passively Q-switched laser based on WS 1.76 Te 0.24 was found more efficient, with pulse duration narrowed to 18%, threshold decreased to 28% and output power enlarged twice. The doping engineering SAs can improve the Q-switched lasers performance with lower energy consumption, narrower pulse width, and larger average output power. The promising findings can provide a method to optimize performances of functional devices by doping engineering.

Data availability
Data Availability For original data, please contact xiezhenda@nju.edu.cn.   Table 2. Experiment results of Q-switched lasers based on three SAs.