English

• 1. INTRODUCTION

  The two-dimensional (2D) layered materials have attracted considerable attention due to their diverse and potentially useful electronic and optical properties such as nonlinear optical (NLO) properties^{[1, 2]}. Among these emerging materials, 2D MoS₂ as the representative one owns the layered-dependent electronic structure evolution from an indirect band gap of 1.3 eV for the bulk to a direct gap of 1.9 eV for a monolayer^{[3, 4]}. Their atomic thickness together with peculiar electronic structure has endowed them with excellent optical properties such as strong saturable NLO absorption and moderate modulation depth^[5-7], which enabled their promising varied applications in optical switches, Q-switching and mode locking pulse lasers and other photonic devices^[8-10]. In addition, these optical properties of 2D MoS₂ could be specified by the dynamics of free carriers and bound excitons, providing a novel platform to investigate involved fascinating physical mechanisms^{[11, 12]}. Unfortunately, limited NLO response time obtained by the analysis of ultrafast carrier dynamics makes them uncompetitive in the devices based on ultrashort laser pulse generation^{[13, 14]}. Unlike typical semiconductor MoS₂, Bi₂Te₃ as a topological insulator possesses a narrow bandgap (~0.3 eV) bulk state and the Dirac-like linear dispersion band on the surface states^[15]. As a result, they exhibit broadband saturable NLO absorption and ultrafast carrier cooling rate, suitable for building ultrafast optical devices operated at ultrabroad wavelength^{[10, 16, 17]}. Nevertheless, this unique electronic structure also leads to their weak NLO absorption intensity and low modulation depth, hindering the stable mode-locking operation for ultrafast pulse laser^[18-20].

  Compared to individual counterparts, vertically stacking different layers with distinct band structures to form layered heterostructures with van der Waals (vdW) interaction offers a promising approach for optimized performance engineering^{[21, 22]}. Unique electronic properties of each individual layer might not be greatly perturbed by the interlayer weak vdW interaction, and the difference in work functions and the interfacial photophysics processes between different layers may enable the generated vertical heterostructures to exhibit tunable properties and novel physical phenomena. For instance, vertical MoS₂/WS₂, MoS₂/ReS₂, and MoS₂/graphene heterostructures showed great potentials in future optoelectronic applications owing to the fast charge transfer in their interfaces and diverse excitation and relaxation routes of photoexcited quasiparticles^[23-26]. Therefore, inspired by the great availability of heterostructure and the unique performance of MoS₂ and Bi₂Te₃, it is exciting to combine Bi₂Te₃ with MoS₂ and thus construct vertical MoS₂/Bi₂Te₃ heterostructures for further investigating their functional properties. Recently, significant photoluminescence quenching with fast charge transfer and ultrafast hot-carrier photovoltaic devices have been demonstrated in MoS₂/Bi₂Te₃ heterostructures^{[27, 28]}. However, relevant researches on 2D MoS₂/Bi₂Te₃ heterostructure and their nonlinear optical properties are quite limited.

  Here, we prepared vertical MoS₂/Bi₂Te₃ heterostructures by stacking 2D Bi₂Te₃ nanoplates on chemical vapor deposition (CVD)-grown MoS₂ flakes. The high crystallinity and optical quality of the obtained MoS₂/Bi₂Te₃ heterostructures were confirmed by transmission electron microscope (TEM), Raman and ultraviolet-visible (UV-Vis) absorption spectroscopies. Especially, we built Z-scan measurements with micro optical imaging and accurately determined the much higher nonlinear saturable absorption coefficient of (–1.3×10⁴)~(–2.2×10⁴) cm/GW in MoS₂/Bi₂Te₃ heterostructures than those of 2D Bi₂Te₃ flakes ((–7.0×10³)~(–9.0×10³) cm/GW). Additionally, ultrafast carrier dynamics following femtosecond laser 400 nm excitation were utilized to unravel more detailed photophysical process accounting for the NLO response in these samples. The CVD-grown monolayer (1L) MoS₂ flakes demonstrated the fastest carrier lifetime (~620 fs) than 2L, 3L and 4L counterparts, and provided an ideal material for the construction of 1L MoS₂/Bi₂Te₃ heterostructure. Expectedly, shorter carrier lifetime of ~440 fs was obtained on 1L MoS₂/Bi₂Te₃ heterostructures due to the charge transfer from the 1L MoS₂ to 2D Bi₂Te₃.

  2. EXPERIMENTAL

  2.1 Preparation of the MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures

  Our 2D MoS₂ flakes with varied layer numbers were obtained by CVD according to the previously reported method^[29]. More specifically, the sulfur powders and electrochemical oxidized Mo foils as precursors were utilized to synthesize MoS₂. The electrochemical anodization of Mo foils (Alfa-Aesar, 0.025 mm) was performed at a current of 0.04 A for 10 minutes at room temperature. After that, oxidized Mo foils were arched on double-sided polished Al₂O₃ substrates at the center of the furnace. Sulfur powders (Alfa-Aesar, 99.999%, 1.5 g) were placed in the upstream and heated by heating belt at 190 ℃ when the temperature of the furnace reached 630 ℃. Then, the furnace was heated to 880 ℃ for 10 minutes and naturally cooled. To synthesize Bi₂Te₃ flakes, the solvothermal method was carried out^{[17, 30]}. 0.64 g polyvinylpyrrolidone (Aladdin, AR) was dissolved in 20 mL ethylene glycol to form a clear solution, followed by the addition of 0.221 g BiCl₃ (Aladdin, 99.99%), 0.21 g Na₂TeO₃ (Aladdin, 99.99%) and 0.56 g NaOH (Aladdin, AR). Next, the resulting precursor suspension was stirred for 1 h and then sealed in the autoclave (50 mL). Afterwards, the autoclave was heated to 180 ℃ for 6 h, followed by cooling to room temperature naturally. The obtained solution was centrifuged at 8500 r/min. Then, the obtained solid product was washed with deionized water and ethylene glycol, and finally dispersed in ethylene glycol. To construct MoS₂/Bi₂Te₃ heterostructures, 1 mL Bi₂Te₃ solution was dropped on the substrate with as-grown MoS₂ flakes. Additionally, the produced MoS₂/Bi₂Te₃ heterostructures were annealed at 200 ℃ in pure Ar atmosphere to enhance the coupling between the layers.

  2.2 Characterizations

  Optical images were captured with Olympus BX 53M microscope. Atomic force microscope (AFM) images were carried out with Bruker Dimension Icon. TEM experiment was performed with the Tecnai Talos F200i. Raman measurements were taken with Horiba-Jobin-Yvon Raman system at 532 nm laser, and the Si peak at 520.7 cm^-1 was used for calibration in the data analysis.

  The open aperture Z-scan and pump-probe techniques were employed with the homebuilt equipments. A diode-pump Yb medium femtosecond laser system with pulse repetition rate of 100 kHz, a center wavelength at 1030 nm, and a pulse width of ~190 fs was used as the excitation source. And an optical parametric amplifier (OPA) was also equipped to tune the varied wavelengths from 400 to 1500 nm. For Z-scan measurements, the wavelength of photoexcitation source was fixed on 400 nm. The samples on the double-sided polished Al₂O₃ substrates with ~0.5 mm thickness were mounted on a linear translation stage which could move near the focus to imitate the change of the laser intensity. Especially, the objective lens and camera were introduced in this system to observe and identify the samples as illustrated in Fig. 3a. Then, the spot of laser source was also focused on the targeted samples to realize micro-Z-scan characterization. For pump-probe measurements, the 400 nm laser was used to excite photocarriers in the samples and the OPA was utilized to generate probe beams with the wavelengths from 400 to 800 nm.

  3. RESULTS AND DISCUSSION

  3.1 Structure characterization and spectroscopic properties of the obtained MoS₂/Bi₂Te₃ heterostructures

  The vertical MoS₂/Bi₂Te₃ heterostructure was schematically illustrated in Fig. 1a. The bottom CVD-grown MoS₂ monolayers exhibited the length in the range of ~30~50 μm confirmed by optical and AFM images (Figs. 1b and 1c and Supplementary materials, Fig. S1), which was suitable for surviving reliable micro-optical measurements involving following Z-scan and ultrafast transient absorption spectroscopy. For upper stacked Bi₂Te₃ flakes, the size and thickness were about 500~600 nm and 15~20 nm, respectively. And these Bi₂Te₃ flakes were dispersedly on 1L MoS₂ flakes with high coverage to form MoS₂/Bi₂Te₃ heterostructures. To further evaluate the crystallinity and structures of the as-prepared samples, we transferred the targeted samples to the holey carbon grids for TEM characterizations^[31]. TEM images captured on MoS₂/Bi₂Te₃ heterostructures showed triangular MoS₂ and hexagonal Bi₂Te₃ flakes (Fig. 1d), consistent with the AFM measurements. For the high resolution TEM (HRTEM) images, 2D MoS₂ and Bi₂Te₃ flakes demonstrated unambiguous lattice stripes with lattice spacing of 0.27 and 0.22 nm, assigned to their (100) and (110) planes, respectively (see the insets of Fig. 1d). All selected-area electron diffraction (SAED) patterns taken at varied regions of the pure MoS₂ monolayer exhibited only one set of hexagonally arranged diffraction spots, confirming its single crystalline nature over a large area (Fig. 1d and Supplementary materials, Fig. S2a). And the Bi₂Te₃ flake also displayed SAED patterns of only one hexagonally diffraction spot (Fig. 1d and Supplementary materials, Figs. S2b and 2c), indicating its good crystallinity. Combined both of them, the MoS₂/Bi₂Te₃ heterostructure shows two sets of clear hexagonal diffraction spots (Fig. 1d), indicating its high quality and undamaged preparation process.

  Figure 1

  

  Figure 1.  (a) Schematic of vertical MoS₂/Bi₂Te₃ heterostructure, (b) Optical images taken from monolayer MoS₂ flakes (upper) and their corresponding MoS₂/Bi₂Te₃ heterostructures, (c) Enlarged AFM image of a MoS₂/Bi₂Te₃ heterostructure.Inset: AFM image of the complete triangle MoS₂/Bi₂Te₃ heterostructure, (d) TEM image of a MoS₂/Bi₂Te₃ heterostructure, SAED patterns of the MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructure from left to right, respectively. Insets: the corresponding HRTEM images of MoS₂ and Bi₂Te₃

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  Next, the spectroscopic properties of MoS₂/Bi₂Te₃ heterostructures were investigated. We first conducted Raman spectra on these flakes and found five observable peaks as shown in Fig. 2a. The prominent diffraction peaks at ~60, ~101 and ~134 cm^-1 were indexed to the out-of-plane A_1g¹, the in-plane E_g² and the out-of-plane A_1g² vibration modes of 2D Bi₂Te₃, respectively^{[32, 33]}. Two characteristic peaks at ~382 and ~403 cm^-1 were associated with the in-plane E_2g¹ and out-of-plane A_1g vibration modes of 1L MoS₂, respectively^{[34, 35]}. The co-existence of the feature peaks of these two flakes further confirmed the successful preparation of high-quality MoS₂/Bi₂Te₃ heterostructures. Then, UV-vis absorbance spectra were utilized to study the linear optical response of MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures (Fig. 2b). The 2D Bi₂Te₃ flakes showed broad optical absorbance at 400~800 nm. Two characteristic peaks of ~623 and ~671 nm anticipated for 1L MoS₂ flakes were observed, assigned to its B (~2.0 eV) and A (~1.9 eV) excitonic transition^{[5, 36]}. The absorbance of MoS₂/Bi₂Te₃ heterostructure was higher than that of MoS₂ and Bi₂Te₃ only, suggesting their enhancement of the linear absorption.

  Figure 2

  

  Figure 2.  (a) Raman spectrum of a MoS₂/Bi₂Te₃ heterostructure, (b) UV-Vis spectra of MoS₂ flakes (purple), Bi₂Te₃ flakes (green) and MoS₂/Bi₂Te₃ heterostructures (orange)

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  3.2 Nonlinear optical properties of the obtained MoS₂/Bi₂Te₃ heterostructures

  To more precisely investigate the nonlinear absorption properties of as-prepared 2D MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures, we designed a homemade micro-Z-scan technique equipped with a microscopic imaging system as shown in Fig. 3a. In this system, optical microscope and camera were introduced to observe and locate the samples, which greatly facilitated the focus and irradiation of following photoexcitation on the targeted samples to realize micro-Z-scan characterization (Fig. 3b). Therefore, we could accurately compare the NLO properties of the same MoS₂ flake before and after constructing with 2D Bi₂Te₃. Figs. 3c and 3e showed the typical Z-scan curves of as-made 1L MoS₂, 2D Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures under the excited laser pulse of 400 nm (3.1 eV) with ~190 fs pulse width, respectively. We found that these MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures all exhibited obvious saturation absorption and the transmittance intensity gradually increased with the increase of incident excitation energy. In addition, compared with pure 2D MoS₂ and Bi₂Te₃, the MoS₂/Bi₂Te₃ heterostructures demonstrated larger transmittance and thus saturable intensity under the same input intensity toward the focus (Fig. 3f), indicating their enhanced light-matter interaction^[37]. This enhancement probably originated from the electron transfer from 1L MoS₂ to 2D Bi₂Te₃ flakes after photoexcitation due to the higher Fermi energy of MoS₂ than Bi₂Te₃^{[38, 39]}.

  Figure 3

  

  Figure 3.  (a) Schematic diagram of Z-scan system with optical imaging identification, (b) Optical image of a MoS₂/Bi₂Te₃ heterostructure with the laser spot irradiating, (c)~(e)Typical open aperture Z-scan curves of the monolayer MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures at 400 nm photoexcitation with different input powers, (f) Open aperture Z-scan curves of Bi₂Te₃, MoS₂ and MoS₂/Bi₂Te₃ heterostructures at the same input power of 5 μW and 400 nm photoexcitation, (g) The input power dependent nonlinear absorption coefficient (β) for the Bi₂Te₃ flakes and MoS₂/Bi₂Te₃ heterostructures

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  Furthermore, the nonlinear absorption coefficient (β) was adopted to quantitatively describe the NLO properties of these materials. The Z-scan results can be fitted by equation (1) according to a nonlinear absorption model^{[38, 40]}:

  $ T = \sum\limits_{m - 0}^\infty {\frac{{{{\left[ { - \beta {I_0}{L_{eff}}/\left( {1 + {z^2}/z_0^2} \right)} \right]}^m}}}{{{{\left( {m + 1} \right)}^{3/2}}}}} \approx 1{\text{ - }}\frac{{\beta {I_0}{L_{eff}}}}{{2\sqrt 2 \left( {1 + {z^2}/z_0^2} \right)}} $

  (1)

  where $ {L_{{\text{eff}}}} = \left( {1{\text{ - }}{e^{ - {\alpha _0}L}}} \right)/{\alpha _0} $ is the effective length of the sample; L and α₀ are the thickness of the sample and linear absorption coefficient, respectively; I₀ is the intensity at the focal point; z₀ and z are the diffraction length of Gaussian beam and the propagation distance, respectively. By fitting the Z-scan data, the estimated β fall in ranges of (–7.0×10³)~(–9.0×10³) cm/GW and (–1.3×10⁴)~(–2.2×10⁴) cm/GW in varied input powers for 2D Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures, respectively (Fig. 3g). The much larger β in MoS₂/Bi₂Te₃ heterostructures suggested that the construction of MoS₂/Bi₂Te₃ heterostructures could improve the main limitation of saturation absorption intensity for 2D Bi₂Te₃ and the obtained MoS₂/Bi₂Te₃ heterostructures demonstrated excellent NLO saturable performance.

  3.3 Transient optical response of the obtained MoS₂/Bi₂Te₃ heterostructures

  To further explore the NLO behavior and unravel the transient optical response of the as-prepared 1L MoS₂ flakes and their MoS₂/Bi₂Te₃ heterostructures, we performed femtosecond pump-probe spectroscopic technique to investigate the ultrafast carrier dynamics under pulse irradiation at 400 nm. Similar to micro-Z-scan measurement, the optical microscope system was also used for facilitating the change characterizations of the carrier dynamic for the same samples in pump-probe measurement (Figs. 4a and 4b). All the measurements in MoS₂ monolayers and their MoS₂/Bi₂Te₃ heterostructures displayed two distinct photobleaching features (negative absorption bands) derived from Pauli blocking, also indicative of their saturable absorption over the entire spectral range from 550 to 800 nm (Figs. 4c and 4d). The negative absorption appearing at ~620 (~2.0 eV) and ~680 nm (~1.8 eV) corresponded to the B- and A-exciton, respectively, in agreement with the previous observations^{[5, 36]}. And transient dynamics of photoexcited MoS₂ monolayers were mainly monitored at both A- and B-excitonic bleach positions, as shown in Fig. 4e. This bleach kinetics can be fitted on the basis of the three-exponential equation (2)^{[38, 41]}:

  $ \Delta A(t, \omega ) = a(\omega ) \cdot {e^{ - t/{\tau _1}}} + b(\omega ) \cdot {e^{ - t/{\tau _2}}} + c(\omega ) \cdot {e^{ - t/{\tau _3}}}$

  (2)

  Figure 4

  

  Figure 4.  (a, b) Optical images of a MoS₂ monolayer and the constructed MoS₂/Bi₂Te₃ heterostructure with the laser spot, (c, d) Representative transient absorption spectra of monolayer MoS₂ and the MoS₂/Bi₂Te₃ heterostructure at 400 nm photoexcitation, (e) Transient absorption kinetics of the MoS₂/Bi₂Te₃ heterostructure, (f) Comparison of transient absorption kinetics for the MoS₂ monolayer and its built MoS₂/Bi₂Te₃ heterostructure at ~620 nm. Inset: the enlarged decay curve recorded up to 5 ps. Solid lines in (e and f) represented the exponential fitting

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  where τ₁, τ₂ and τ₃ represent the carrier lifetime of the different response processes. The fast recovery time τ₁ could be mainly attributed to the exciton formation process for MoS₂ monolayers in picoseconds or subpicoseconds reported previously^[35]. The second τ₂ generally corresponded to the Auger recombination or exciton-exciton annihilation in the systems^[42]. τ₃ represented the typical inter-band relaxation time causing the slow-state relaxation in several hundred picoseconds^[43]. We primarily compared the recovery time within the shorter time scale (τ₁) for 1L MoS₂ in the absence and presence of 2D Bi₂Te₃ flakes. In the absence of Bi₂Te₃, the ~620 and ~680 nm bleaches in pure 1L MoS₂ recovered within ~860 and ~620 fs, respectively. For annealed MoS₂/Bi₂Te₃ heterostructure with strong coupling between the layers, the ~620 and ~680 nm bleaches exhibited faster recovery time of ~620 and ~440 fs (Fig. 4f), respectively, which made it more feasible for varied applications such as ultrafast pulse laser generation with narrow pulse width. By contrary, the MoS₂/Bi₂Te₃ heterostructure without annealing showed slower recovery time of ~1000 and ~950 fs in the ~620 and ~680 nm bleaches, respectively (Supplementary materials, Fig. S3). Combining these results and previous work^{[25, 28]}, we proposed that the reduced carrier lifetime could also be ascribed to the electron transfer from the 1L MoS₂ to 2D Bi₂Te₃ besides forming excitons, consistent with the results of micro-Z-scan.

  Based on the unique micro-imaging pump-probe technique and successful synthesis of single crystal CVD-grown MoS₂ flakes with varied layers, we further investigated the layer-dependent ultrafast carrier dynamics of 1L, 2L, 3L and 4L MoS₂ flakes and their MoS₂/Bi₂Te₃ heterostructures in details (Fig. 5a). As shown in Fig. 5b, both the A- and B-exciton bleaching peaks redshifted with the increase of layer numbers for pure MoS₂ flakes due to the gradually decreased bandgap evolution^[44]. The similar phenomena occurred in MoS₂/Bi₂Te₃ heterostructures (Fig. 5c and Supplementary materials, Figs. S4 and S5). Moreover, since the defect-assisted recombination at the surface became significant as the thickness was decreased, we observed that defect states-more 1L MoS₂ flakes demonstrated dramatically reduced carrier lifetimes than the defect states-less few-layer ones (Fig. 5d and Supplementary materials and Fig. S6), showing exactly the same trend as in the mechanically exfoliated MoS₂ flakes^{[45, 46]}. In addition, similar to 1L MoS₂/Bi₂Te₃, the heterostructures based on few-layer MoS₂ and 2D Bi₂Te₃ also possessed shorter carrier recovery time (Fig. 5d).

  Figure 5

  

  Figure 5.  (a) Schematic diagram and optical images of 1L~4L MoS₂ flakes, (b) Time-resolved transient absorption spectra probed at 1ps for 1L~4L MoS₂ flakes of (a), (c) Heterostructures based on another 1L~4L MoS₂ flakes of Fig S5, (d) Statistics of the fast recovery time constant τ₁ for 1L~4L MoS₂ (purple) and 1L~4L MoS₂/Bi₂Te₃ heterostructures (orange), the yellow-colored left region and green-colored right region corresponded to the τ₁ values extracted from B- and A-exciton bleaching peaks

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  4. CONCLUSION

  In summary, we have successfully constructed high-performance MoS₂/Bi₂Te₃ heterostructures by drop-casting Bi₂Te₃ flakes on CVD-grown MoS₂ flakes. The obtained heterostructures maintained good crystallinity. Owing to our homemade open aperture Z-scan measurements with optical imaging, the 1L MoS₂/Bi₂Te₃ heterostructures were precisely determined to possess the enhanced saturable absorption and nonlinear absorption coefficient than 2D Bi₂Te₃. Furthermore, layer-dependent femtosecond transient absorption spectroscopy in CVD-grown MoS₂ unveiled the shortest carrier lifetime in 1L MoS₂ than 2L, 3L and 4L MoS₂. Based on these 1L MoS₂ flakes, 1L MoS₂/Bi₂Te₃ heterostructures exhibited faster carrier lifetime of ~440 fs due to the charge transfer from the 1L MoS₂ to 2D Bi₂Te₃. Our work provides a novel nonlinear material with superior saturable absorption properties and provides novel insight for the design of high-performance nonlinear materials by effectively combining the optical advantages of different 2D materials.

  
  
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  Figure 1  (a) Schematic of vertical MoS₂/Bi₂Te₃ heterostructure, (b) Optical images taken from monolayer MoS₂ flakes (upper) and their corresponding MoS₂/Bi₂Te₃ heterostructures, (c) Enlarged AFM image of a MoS₂/Bi₂Te₃ heterostructure.Inset: AFM image of the complete triangle MoS₂/Bi₂Te₃ heterostructure, (d) TEM image of a MoS₂/Bi₂Te₃ heterostructure, SAED patterns of the MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructure from left to right, respectively. Insets: the corresponding HRTEM images of MoS₂ and Bi₂Te₃

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  Figure 2  (a) Raman spectrum of a MoS₂/Bi₂Te₃ heterostructure, (b) UV-Vis spectra of MoS₂ flakes (purple), Bi₂Te₃ flakes (green) and MoS₂/Bi₂Te₃ heterostructures (orange)

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  Figure 3  (a) Schematic diagram of Z-scan system with optical imaging identification, (b) Optical image of a MoS₂/Bi₂Te₃ heterostructure with the laser spot irradiating, (c)~(e)Typical open aperture Z-scan curves of the monolayer MoS₂, Bi₂Te₃ and MoS₂/Bi₂Te₃ heterostructures at 400 nm photoexcitation with different input powers, (f) Open aperture Z-scan curves of Bi₂Te₃, MoS₂ and MoS₂/Bi₂Te₃ heterostructures at the same input power of 5 μW and 400 nm photoexcitation, (g) The input power dependent nonlinear absorption coefficient (β) for the Bi₂Te₃ flakes and MoS₂/Bi₂Te₃ heterostructures

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  Figure 4  (a, b) Optical images of a MoS₂ monolayer and the constructed MoS₂/Bi₂Te₃ heterostructure with the laser spot, (c, d) Representative transient absorption spectra of monolayer MoS₂ and the MoS₂/Bi₂Te₃ heterostructure at 400 nm photoexcitation, (e) Transient absorption kinetics of the MoS₂/Bi₂Te₃ heterostructure, (f) Comparison of transient absorption kinetics for the MoS₂ monolayer and its built MoS₂/Bi₂Te₃ heterostructure at ~620 nm. Inset: the enlarged decay curve recorded up to 5 ps. Solid lines in (e and f) represented the exponential fitting

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  Figure 5  (a) Schematic diagram and optical images of 1L~4L MoS₂ flakes, (b) Time-resolved transient absorption spectra probed at 1ps for 1L~4L MoS₂ flakes of (a), (c) Heterostructures based on another 1L~4L MoS₂ flakes of Fig S5, (d) Statistics of the fast recovery time constant τ₁ for 1L~4L MoS₂ (purple) and 1L~4L MoS₂/Bi₂Te₃ heterostructures (orange), the yellow-colored left region and green-colored right region corresponded to the τ₁ values extracted from B- and A-exciton bleaching peaks

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