Ultrafast Nonlinear Optical Response of Two-dimensional MoS2/Bi2Te3 Heterostructure①

DONG Jun-Hao PAN Jia YE Chen-Yu WANG Ru HUANG Yong-Feng ZHENG Jing-Ying ZHAN Hong-Bing② WANG Qian-Ting②

a (School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, China)

b (College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China)

c (School of Electronic, Electrical Engineering and Physics,Fujian University of Technology, Fuzhou 350118, China)

d (Fujian Provincial Key Laboratory of Advanced Materials Processing and Application, Fuzhou 350118, China)

ABSTRACT Controlled stacking of different two-dimensional (2D) atomic layers hold great promise for significantly optimizing the optical properties of 2D materials and broadening their applications. Here, vertical 2D MoS2/Bi2Te3 heterostructures with high crystallinity and optical quality have been successfully constructed, through drop-casting 2D Bi2Te3 flakes on chemical vapor deposition (CVD)-grown MoS2 flakes. Based on our homebuilt micro Z-scan and pump-probe measurement, we precisely investigated and compared the nonlinear optical (NLO)performance of an individual micro-sized MoS2 flake before and after stacking 2D Bi2Te3 nanoplates. Moreover,layer-dependent ultrafast carrier dynamics of CVD-grown MoS2 flakes were also explored. Owing to the efficient charge transfer from the monolayer (1 L) MoS2 to 2D Bi2Te3, the 1L MoS2/Bi2Te3 heterostructure demonstrated excellent NLO performance with superior nonlinear saturable absorption coefficient and ultrashort carrier lifetime.Our work greatly enriches our understanding of 2D heterostructure and paves the way for designing new type of tunable 2D photonics materials by combining the optical advantages of different 2D materials.

Keywords: two-dimensional materials, nonlinear optics, ultrafast response;

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, 2DMoS2as 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 2DMoS2could 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, Bi2Te3as 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 MoS2and Bi2Te3, it is exciting to combine Bi2Te3with MoS2and thus construct vertical MoS2/Bi2Te3heterostructures 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/Bi2Te3heterostructures[27,28]. However, relevant researches on 2DMoS2/Bi2Te3heterostructure and their nonlinear optical properties are quite limited.

Here, we prepared vertical MoS2/Bi2Te3heterostructures by stacking 2DBi2Te3nanoplates on chemical vapor deposition(CVD)-grown MoS2flakes. The high crystallinity and optical quality of the obtained MoS2/Bi2Te3heterostructures were confirmed by transmission electron microscope (TEM),Raman and ultraviolet-visible (UV-Vis) absorption spectroscopies. Especially, we builtZ-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/Bi2Te3heterostructures than those of 2DBi2Te3flakes((–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) MoS2flakes demonstrated the fastest carrier lifetime (～620 fs) than 2L, 3L and 4L counterparts, and provided an ideal material for the construction of 1L MoS2/Bi2Te3heterostructure. Expectedly,shorter carrier lifetime of ～440 fs was obtained on 1L MoS2/Bi2Te3heterostructures due to the charge transfer from the 1L MoS2to 2DBi2Te3.

2 EXPERIMENTAL

2. 1 Preparation of the MoS2, Bi2Te3 and MoS2/Bi2Te3 heterostructures

Our 2DMoS2flakes 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 Al2O3substrates 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 Bi2Te3flakes, 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/Bi2Te3heterostructures, 1 mL Bi2Te3solution was dropped on the substrate with as-grown MoS2flakes. Additionally, the produced MoS2/Bi2Te3heterostructures 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-1was used for calibration in the data analysis.

The open apertureZ-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. ForZ-scan measurements, the wavelength of photoexcitation source was fixed on 400 nm. The samples on the double-sided polished Al2O3substrates 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/Bi2Te3heterostructure was schematically illustrated in Fig. 1a. The bottom CVD-grown MoS2monolayers exhibited the length in the range of ～30～50μm 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 followingZ-scan and ultrafast transient absorption spectroscopy. For upper stacked Bi2Te3flakes, the size and thickness were about 500～600 nm and 15～20 nm, respectively. And these Bi2Te3flakes were dispersedly on 1L MoS2flakes with high coverage to form MoS2/Bi2Te3heterostructures. 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/Bi2Te3heterostructures showed triangular MoS2and hexagonal Bi2Te3flakes (Fig. 1d),consistent with the AFM measurements. For the high resolution TEM (HRTEM) images, 2DMoS2and Bi2Te3flakes 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 MoS2monolayer 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 Bi2Te3flake 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/Bi2Te3heterostructure shows two sets of clear hexagonal diffraction spots (Fig. 1d), indicating its high quality and undamaged preparation process.

Fig. 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

Next, the spectroscopic properties of MoS2/Bi2Te3heterostructures 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-1were indexed to the out-of-planeA1g1, the in-planeEg2and the out-of-planeA1g2vibration modes of 2DBi2Te3, respectively[32,33]. Two characteristic peaks at ～382 and ～403 cm-1were associated with the in-planeE2g1and outof-planeA1gvibration 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/Bi2Te3heterostructures. Then, UV-vis absorbance spectra were utilized to study the linear optical response of MoS2, Bi2Te3and MoS2/Bi2Te3heterostructures (Fig. 2b). The 2DBi2Te3flakes showed broad optical absorbance at 400～800 nm. Two characteristic peaks of ～623 and ～671 nm anticipated for 1L MoS2flakes were observed, assigned to its B(～2.0 eV) and A (～1.9 eV) excitonic transition[5,36]. The absorbance of MoS2/Bi2Te3heterostructure was higher than that of MoS2and Bi2Te3only, suggesting their enhancement of the linear absorption.

3. 2 Nonlinear optical properties of the obtained MoS2/Bi2Te3 heterostructures

To more precisely investigate the nonlinear absorption properties of as-prepared 2DMoS2, Bi2Te3and MoS2/Bi2Te3heterostructures, 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 MoS2flake before and after constructing with 2DBi2Te3. Figs. 3c and 3e showed the typicalZ-scan curves of as-made 1L MoS2,2DBi2Te3and MoS2/Bi2Te3heterostructures under the excited laser pulse of 400 nm (3.1 eV) with ～190 fs pulse width,respectively. We found that these MoS2, Bi2Te3and MoS2/Bi2Te3heterostructures all exhibited obvious saturation absorption and the transmittance intensity gradually increased with the increase of incident excitation energy. In addition,compared with pure 2DMoS2and Bi2Te3, the MoS2/Bi2Te3heterostructures 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 MoS2to 2DBi2Te3flakes after photoexcitation due to the higher Fermi energy of MoS2than Bi2Te3[38,39].

Furthermore, the nonlinear absorption coefficient (β) was adopted to quantitatively describe the NLO properties of these materials. TheZ-scan results can be fitted by equation (1)according to a nonlinear absorption model[38,40]:Fig. 3. (a) Schematic diagram ofZ-scan system with optical imaging identification, (b) Optical image of a MoS2/Bi2Te3heterostructure with the laser spot irradiating, (c)～(e)Typical open apertureZ-scan curves of the monolayer MoS2, Bi2Te3and MoS2/Bi2Te3heterostructures at 400 nm photoexcitation with different input powers, (f) Open apertureZ-scan curves of Bi2Te3, MoS2and MoS2/Bi2Te3heterostructures at the same input power of 5 μW and 400 nm photoexcitation, (g) The input power dependent nonlinear absorption coefficient (β) for the Bi2Te3flakes and MoS2/Bi2Te3heterostructures

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 MoS2flakes and their MoS2/Bi2Te3heterostructures, 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 MoS2monolayers and their MoS2/Bi2Te3heterostructures 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 MoS2monolayers 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]:whereτ1,τ2andτ3represent the carrier lifetime of the different response processes. The fast recovery timeτ1could be mainly attributed to the exciton formation process for MoS2monolayers in picoseconds or subpicoseconds reported previously[35]. The secondτ2generally corresponded to the Auger recombination or exciton-exciton annihilation in the systems[42].τ3represented 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 MoS2in the absence and presence of 2DBi2Te3flakes. In the absence of Bi2Te3,the ～620 and ～680 nm bleaches in pure 1L MoS2recovered within ～860 and ～620 fs, respectively. For annealed MoS2/Bi2Te3heterostructure 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/Bi2Te3heterostructure 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 MoS2to 2DBi2Te3besides forming excitons, consistent with the results of micro-Z-scan.

Fig. 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

Based on the unique micro-imaging pump-probe technique and successful synthesis of single crystal CVD-grown MoS2flakes with varied layers, we further investigated the layer-dependent ultrafast carrier dynamics of 1L, 2L, 3L and 4L MoS2flakes and their MoS2/Bi2Te3heterostructures 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 MoS2flakes due to the gradually decreased bandgap evolution[44]. The similar phenomena occurred in MoS2/Bi2Te3heterostructures (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 MoS2flakes 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 MoS2flakes[45,46]. In addition, similar to 1L MoS2/Bi2Te3, the heterostructures based on few-layer MoS2and 2DBi2Te3also possessed shorter carrier recovery time(Fig. 5d).

Fig. 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

4 CONCLUSION

In summary, we have successfully constructed high-performance MoS2/Bi2Te3heterostructures by drop-casting Bi2Te3flakes on CVD-grown MoS2flakes. The obtained heterostructures maintained good crystallinity. Owing to our homemade open apertureZ-scan measurements with optical imaging, the 1L MoS2/Bi2Te3heterostructures were precisely determined to possess the enhanced saturable absorption and nonlinear absorption coefficient than 2DBi2Te3. Furthermore,layer-dependent femtosecond transient absorption spectroscopy in CVD-grown MoS2unveiled the shortest carrier lifetime in 1L MoS2than 2L, 3L and 4L MoS2. Based on these 1L MoS2flakes, 1L MoS2/Bi2Te3heterostructures exhibited faster carrier lifetime of ～440 fs due to the charge transfer from the 1L MoS2to 2DBi2Te3. 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 2Dmaterials.