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Enhanced Nonlinear Saturable Absorption of MoS/graphene Nanocomposite Films 2

Minmin He, Chenjing Quan, Chuan He, Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Sujuan Zhang, Jin Tao Bai, and Xin Long Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08850 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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The Journal of Physical Chemistry

Enhanced Nonlinear Saturable Absorption of MoS2/graphene Nanocomposite Films Minmin Hea, Chenjing Quana, Chuan Hea, Yuanyuan Huanga, Lipeng Zhua, Zehan Yaoa, Sujuan Zhanga,*, Jintao Baia, Xinlong Xua, b,*

a

Shaanxi Joint Lab of Graphene, State Key Lab Incubation Base of Photoelectric Technology and

Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China.

b

Guangxi Key Laboratory of Automatic Detecting Technology and Instruments, Guilin University of Electronic Technology, Guilin 541004, People’s Republic of China

ABSTRACT MoS2/graphene nanocomposite films are fabricated by vacuum filtering with liquid-phase exfoliated MoS2/graphene suspension. The nanocomposite films are characterized by Raman spectroscopy, UV-Vis spectroscopy, atomic force microscopy, indicating the optical films with a large scale and high optical homogeneity. The enhanced saturable absorption of MoS2/graphene nanocomposite films compare with pristine MoS2 film and graphene film are investigated using open-aperture Z-scan

* Corresponding author. Tel: +86 029-88303667. E-mail: [email protected] , [email protected] (X. Xu)

ACS Paragon Plus Environment


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technique with a femtosecond laser at 800 nm. The nonlinear absorption coefficient of MoS2/graphene nanocomposite film is ~ -1217.8 cm/GW, which is larger than that of MoS2 film (~ -136.1 cm/GW) and graphene film (~ -961.6 cm/GW) at the same condition. The imaginary part of the third-order nonlinear optical susceptibility of the (3) nanocomposite film can reach Imχ ~ 10-9 esu with a figure of merit ~10-14 esu cm,

low saturable intensity (~ 157.0 GW/cm2), and high modulation length (~ 32%). A coupling model is considered in order to understand the nonlinear absorption properties of MoS2/graphene nanocomposite films, which suggest the enhancement can be attributed to charge transfer between MoS2 and graphene. The results pave the way for the design of nonlinear optical properties with two-dimensional materials for good performance of optical switches or mode lockers based on saturable absorbers.


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1. Introduction Two-dimensional (2D) layered materials have excellent nonlinear optical properties due to their extraordinary electronic band structure properties1. These layered materials have covalent bond in-plane and van der Waals interaction between layers, which can be easily exfoliated by a liquid method2-3. Graphene and molybdenum disulfide (MoS2) are typical 2D layered materials，which demonstrate potential applications in optoelectronic and photovoltaic devices. Graphene as the earliest 2D material was obtained by mechanically exfoliated in 20044. Its carrier mobility can achieve up to 105 cm2/Vs at room temperature5. The ultrafast saturable absorption properties of graphene have been investigated in the femtosecond regime, which is due to the valence band depletion and conduction band filling

6-8

. What’s more, the

interband saturable absorption in graphene under femtosecond laser is due to the Pauli blocking9. Graphene-based saturable absorbers are broadband, which are favorable in different wavelengths for broadband mode-locked laser10-12. However, the zero band gap of graphene hinder its application in optical switching devices13. The discovery of graphene-like 2D materials14-15, such as transition metal dichalcogenides (TMDs) have attracted interesting due to their excellent band structures in visible region16. Monolayer MoS2 has a direct band gap of ~1.88 eV, while bulk MoS2 has an indirect bandgap of ~1.29 eV17. In 2013, Wang et al. demonstrated that the MoS2 nanosheets dispersion exhibits saturable absorption, resulting in the third-order nonlinear optical susceptibility Imχ(3) ∼ 10-15 esu, figure of merit (FOM) ∼10-15 esu cm18. In 2014, Wang et al. proved the nonlinear saturable absorption layered MoS2 films for both



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femtosecond and picosecond pulse laser at 532 nm and 1064 nm19. Recently, Zhang et al. also demonstrated nonlinear saturable absorption of monolayer and few-layer MoS2 films in the visible and near-infrared region20. The nonlinear response of MoS2 have many potential applications in Q-switched fiber laser, ultrafast photonics, and fast optical switching

21-22

. However, most nonlinear optical measurements of 2D

materials are based on liquid environment. The experiments show that the solvent may arise influences on the inherent properties of samples, such as nonlinear scattering, which is due to the microbubbles generated by the incident laser21, 23. Hence, it is feasible to fabricate 2D films by vacuum filtration from the corresponding dispersion and to study their nonlinear optical properties in thin films24. Another amazing property due to the van der Waals interactions in 2D layered materials is that they can be reassembled into designer heterostructures with new or enhanced properties25. These heterostructures demonstrate the importance of synergistic effect when different 2D crystal materials are stacked together with weak van der Waals force. Graphene and analogous graphene crystal (such as MoS2) are assembled into novelty heterostructures, which have broadband response and tunable band structure characteristics26. The novel 2D physics appears in these heterostructures such as the absence of long-range order, 2D excitons, commensurate-incommensurate transition. These are due to the surface reconstruction, tunneling charge carriers, and built in electric filed. These assembly materials with atomic smooth interfaces and without the inter-diffusion of atom make them widely used in electronic and optoelectronic devices25,

27

. The peculiar features of the


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heterostructures guarantee unusual and new function for tunneling transistors28, resonant tunneling diodes29, and light emitting diodes30 etc. One of the easy ways to make the heterostructure is by liquid-phase exfoliated MoS2/graphene suspension, which will form the MoS2/graphene heterostructure in nanocomposites. The majority studies of MoS2/graphene nanocomposite focused on sensors31-32 or batteries

33

, but

scarce research in nonlinear optical properties. Recently, Jiang et al. reported nonlinear absorption properties and revealed the enhanced nonlinear saturable absorption of MoS2/graphene nanocomposite in liquid dispersions34. However, to avoid the optical scattering in dispersion effectively, the optical film is favorable. Furthermore, optical films approach the reality of applications. Additionally, relevant theoretical explanations for the saturable absorption of nanocomposites are also desirable. In this paper, we prepared MoS2/graphene nanocomposite films by vacuum filtration method with liquid phase exfoliation(LPE). The thickness of the nanocomposite can be controlled by the filtration volume of MoS2/graphene dispersions. In addition, we show that different MoS2/graphene nanocomposite films exhibit significant saturable absorption for femtosecond pulse at 800 nm with Z-scan technique, resulting in the nonlinear absorption coefficient of ~ -1217.8 cm/GW, which is larger than that for MoS2 (~ -136.1 cm/GW) and graphene (~ -961.6 cm/GW) at the same input power of 123 GW/cm2. The third-order nonlinear optical (3) susceptibility can reach Imχ ~ 10-9 esu and the figure of merit can reach ~10-14 esu

cm. Comparison with MoS2 films and graphene films, MoS2/graphene nanocomposite



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films demonstrates low saturable intensity (~ 157.0 GW/cm2) and high modulation length (~32%). A coupling model is considered in order to understand the nonlinear absorption properties of the nanocomposite films. The results suggest that the enhancement can be attributed to charge transfer between MoS2 and graphene. 2. Preparation of MoS2/graphene nanocomposite films We use LPE and vacuum filtration technique to fabricate MoS2/graphene nanocomposite films2-3,

35

. The MoS2 and graphite powders by half and half are

dispersed in N-methylpyrrolidone (NMP) with a concentration of 0.5 mg/ml. The solution is sonicated with 570 W for 1 hour in water bath by supersonic machine (Qsonica Q700). After high power sonication, the MoS2 and graphite powders are exfoliated to nanosheet effectively. Then the obtained suspensions are centrifuged at 6000 rmp/min for 30 minutes by a centrifuge (Centurion Scientific K241). After that, two-third of the above dispersion is used for vacuum filtering. Different supernatant volumes (10 ml, 20 ml, 30 ml, 40 ml) are evacuated and deposited onto a membrane with a pore size of 220 nm (Tianjin Jinteng Co. Ltd.) The large area of nanosheets attached on the membrane and formed a thin film after natural dry at room temperature. The film is transferred onto the glass substrate (Sail Brand 0.1 mm in thickness). Before the transfer process, the glass substrate is cleaned in acetone, ethyl alcohol (IPA), and deionized (DI) water for 15 minutes respectively by ultrasonic cleaner so that the surface is free of impurities. The film is cut into square and adhered on substrate with IPA, and pressed against the surface with 1 Kg weight for 2 minutes. Then adding acetone lightly until the membrane is dissolved. Sucking away the waste


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liquid with pipette after 20 minutes and repeating the step 5-6 times. This process of preparing film is similar to our previous works2, 35-36. Finally the MoS2/graphene film is transferred onto the substrate and dried at 40 ℃ for 30 minutes. The MoS2 films and graphene films were prepared in the same method.

Figure 1. (a) Illustration of fabrication procedures of MoS2/graphene nanocomposite films. (b) Photographs of MoS2/graphene nanocomposite film (1), graphene film (2), MoS2 film (3) on the substrates of 0.1 mm-thickness cover glass. (c) Photographs of MoS2/graphene nanocomposite films with different filtration volumes on cover glass. (d) AFM image of MoS2/graphene nanocomposite film. (e) Photographs of light spot on pure glass and MoS2/graphene nanocomposite film on cover glass. The MoS2, graphene, and MoS2/graphene nanocomposite films are fabricated in the same way for comparison. As shown in Figure 1(a), the fabrication procedures of MoS2/graphene nanocomposite films are exhibited. Figure 1(b) shows the photographs of MoS2/graphene nanocomposite film (1), graphene film (2), MoS2 film



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(3) with a filtration volume of 40 ml on the substrate of 0.1 mm-thick cover glass and the photographs of MoS2/graphene nanocomposite films with different filtration volumes on cover glass. We can control the suction volume of the solution so that the thickness of the films can be controlled. We can observe clearly that as the filter volume increases, the color of the films becomes deeper from left to right in Figure 1(c). The roughness and thickness of the MoS2/graphene nanocomposite film were characterized by atomic force microscope (AFM). We scanned the area (50 µm×50 µm) for the AFM characterization. The surface image shows most of the nanosheets are deposited evenly on the glass substrate in Figure 1(d). Inset shows the height of cross section with 15 µm, which is selected in Figure 1(d), indicating the average thickness of the MoS2/graphene nanocomposite film with 40 ml filtration volume is approximately 50 nm. For comparison, we also characterized the thickness of MoS2 and graphene film with 40 ml filtration volume onto glass substrate by AFM with the thickness approximately 65 and 50 nm in average for MoS2 and graphene film. The optical homogeneity of the optical film is particularly important for its optical characterization. As show in Figure 1(e), we characterized optical homogeneity of the optical film by a laser beam, revealing no manifest scattering when the beam passed through the pure glass with or without the films.

3. DISCUSSION AND RESULTS 3.1 Optical characterization of films


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Figure 2. (a) Raman spectrum of MoS2/graphene nanocomposite film. (b) UV-Vis absorbance spectra of MoS2/graphene nanocomposite film (red line), Graphene film (black line), and MoS2 film (blue line). (c) Absorption of the MoS2/graphene nanocomposite films with different filtration volumes. (d) Absorption as a function of filtration volume with MoS2/graphene nanocomposite dispersion at 800 nm. Raman spectroscopy further demonstrates the nanostructures of MoS2/graphene nanocomposite films. Figure 2(a) shows the Raman spectrum of MoS2/graphene nanocomposite film with typical peaks E12g (~385 cm-1), A1g (~409 cm-1), and D (~1352 cm-1), G (~1583 cm-1), 2D (~2717 cm-1), respectively. E12g mode is associated with an in-plane motion of Mo and S atoms, while the A1g mode is correlated with an out-of-plane vibration of Mo and S atoms37-38. D, G and 2D are typical Raman modes, which always correspond to graphene nanosheet. The D peak exists in spectrum due



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to edge effect39 as there are much small flakes stacked to form a thin film. The narrow G peak also certificates the existence of edge effect in MoS2/graphene nanocomposite film and the characterized 2D band implies the multi-layer graphene8. Figure 2(b) shows the UV-Vis absorbance spectra of MoS2/graphene nanocomposite film (red line), graphene film (black line), and MoS2 film (blue line) with 40 ml filtration volume, respectively. Two characteristic peaks anticipated for MoS2 film are observed. They are A (1.83 eV) and B (2.03 eV) excitonic transition arising from the K point of Brillouin zone3. The absorbance spectrum of MoS2/graphene nanocomposite film is higher than that of MoS2 and graphene only, which suggest the enhancement of the linear absorption. The absorbance spectra with different thickness of MoS2/graphene nanocomposite films are shown in Figure 2(c). The A and B excitonic response can be seen clearly and the absorbance increases with the increasing of filtration volume. It should be noted that there are absent of obviously exciton peak with 10 ml filtration volume of MoS2/graphene nanocomposite film, which may ascribe to symmetric deposition with low filtration volume. Figure 2(d) show the absorbance of MoS2/graphene nanocomposite films as a function of filtration volumes at the photon energy 1.55 eV ( λ =800 nm), which almost linearly agrees with the Beer-Lambert law.

3.2 Nonlinear optical properties of optical films The nonlinear absorption of our all materials are studied by a home-made open aperture Z-scan system40 as shown in Figure S1. The light source is a Ti: sapphire femtosecond laser and the laser is centered at 800 nm wavelength (photon energy 1.55 eV larger than the bandgap of MoS2 ~ 1.29 eV) with the repetition frequency of 1


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KHz. The incident laser was focused by a lens (f=175 mm) and can be assumed as Gaussian beam. The waist radius was measured to approximately 30 µm by a knife edge method, which is much shorter than the Rayleigh length. The detection was calibrated with a standard sample CS2 to ensure the precision of the Z-scan system. The CS2 solution is contained in a quartz cuvette with 1 mm optical path, and the nonlinear absorption coefficient is calculated as ~10-11 cm/W by fitting Z-scan curve of CS2, which is consistent with the previous report41. Figure 3 shows the typical open aperture Z-scan curve and represents the normalized transmittance for peak power density I0 ~123 GW/cm2. The absorption of sample under the intensive excitation consists of linear and nonlinear absorption, which can be expressed as: α(I) = α0 + αNL (I)I , where α 0 and α NL are linear and nonlinear absorption coefficients, respectively. According to the nonlinear absorption theory42, the Z-scan results can be fitted by a nonlinear absorption model, the normalized transmission Tnorm can be written as42: m

∞

Tnorm =

∑ m=0

where L eff = (1 - e

-α 0 L

-α NL I0 Leff / (1 + z 2 / z 0 2 )    (m = 1, 2, 3) 3/ 2 (m + 1)

(1)

) / α 0 is the effective length of the sample, L is the thickness

of the sample. I0 is the intensity at focal point. z0 is the diffraction length of Gaussian beam. In general, our experimental data can be fitted well when m=2 in Equation (1).



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Figure 3. (a) Open-aperture Z-scan curves of MoS2/graphene nanocomposite film, graphene film, and MoS2 film with 40 ml. (b) Open-aperture Z-scan results of different filtration volume of MoS2/graphene nanocomposite films. As shown in Figure 3, the 2D layered films demonstrate nonlinear optical responses when they are excited by a femtosecond laser. Figure 3(a) shows the MoS2/graphene nanocomposite film exhibits strong saturable absorption property in open aperture Z-scan measurement. The total transmittance increases with the sample moving to focal point, and the transmission reaches the maximum at the focal point. That is, the materials can suppress transmission at low intensity light but allow transmission at high intensity light43. We have checked the substrate, which demonstrate no obvious nonlinear effect in this intensity. This suggests that the saturable absorption is completely from inherent responses of these 2D materials. It is obvious that with the hybridization of MoS2 and graphene, the saturable absorption is enhanced compared with the MoS2 and graphene. It is noted that the comparison is under the same conditions. All the experimental data can be fitted well by the Equation (1). What’s more, the beam radius is approximately 30 µm by fitting the Z-scan data. The value is consistent with that measured by a knife edge method. We can also calculate the


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nonlinear absorption coefficient αNL by the fitting results, and the imaginary part of third-order nonlinear susceptibility can be expressed as18: 10-7 cλn 2  Imχ (3) (esu) =   α NL 2  96π 

(2)

where, c is the velocity of the light in the vacuum, and λ is the wavelength of the incident laser. n is the linear refractive index, which can be calculated from the dielectric constant of MoS2 and graphene44. The figure of merit (FOM) is expressed as FOM = Imχ (3) / α0

, where α0 can be calculated from the absorbance spectra in Figure

(3) 2(b). At the same excitation intensity, Imχ and FOM of the MoS2/graphene

nanocomposite film at 800 nm are approximately -3.2×10-9 esu and approximately 5.8×10-14 esu cm, which is approximately nine times larger than the value of MoS2 and approximately one times larger than the value of graphene. Figure 3(b) shows the open aperture Z-scan curves of MoS2/graphene nanocomposite films with different thicknesses at the same input intensity. We can clearly notice that normalized transmittance of the sample increases along with thickness at the focal point. This phenomenon indicates that the thickness also affects the saturable absorption response of MoS2/graphene nanocomposite films, possibly due to more nanosheets are excited in the thick sample. The other MoS2/graphene nanocomposite films with different thicknesses have similar performance with 40 ml filtration volume in the supporting information Figure S2. The nonlinear optical parameters, including the nonlinear absorption coefficient, the imaginary part of third-order nonlinear susceptibility, saturation intensity, modulation depth etc., are summarized in table 1.



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αNL (cm/GW)

Imχ(3) (esu)

5.4×104

-1217.76

-3.2×10-9

65

1.3×104

-136.13

71

4.6×104

-961.57

materials

T (%)

MoS2/graphene

82

MoS2

Graphene

α0 (cm-1)

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Is (GW/cm2)

α0 L

5.8×10-14

157.0

32.2%

-0.3×10-9

4.6×10-14

347.6

9.8%

-2.4×10-9

3.1×10-14

166.1

27.4%

FOM (esu cm )

Table 1. Nonlinear optical parameters of the MoS2/graphene nanocomposite film, MoS2 film, and graphene film. From Table 1, we can see the value of Imχ(3) and FOM for MoS2/graphene film are larger than those of other 2D nanosheets, such as MoS2 -1.56×10-14 esu and ~1.47×10-15 esu cm in NMP, graphene -8.14×10-15 esu and 4.52×10-16 esu cm in NMP18, SnX2 (X=S,Se) ~8.7×10-11 esu45. The value of Imχ(3) for MoS2/graphene film is also larger than other hybrid nonlinear materials, such as Au-CdSe ~2.4×10-11 esu46, ZnO/graphene ~1.5×10-11 esu47. It also suggests that MoS2/graphene nanocomposite film has the largest Imχ(3) ~ -3.2×10-9 esu, modulation depth ~32% and the smallest saturation intensity IS ~ 157.0 GW/cm2 compared to pristine MoS2 film and graphene film at the same experimental condition. The 2D composite films possess higher nonlinear optical response, which suggests hybridization is one of the effective methods to design the nonlinear properties of 2D materials.

3.3 Nonlinear optical enhancement mechanism


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Figure 4. (a) Energy-level diagram of MoS2/graphene nanocomposite film. The labels are described in the main text. (b) Normalized transmittance as functions of irradiance intensity for MoS2/graphene nanocomposite film (black circle), graphene film (red circle), MoS2 film (blue circle), respectively. To understand optical enhancement mechanism of MoS2/graphene nanocomposite film, we put forward energy level analysis of the nanocomposite. In fact, this method can be used to qualitatively analyze the nonlinear optical process. The three energy level model was proposed just as the previous report48 (please see the supporting information for more details). At the high excitation intensity, for layered graphene or MoS2, the population of the photon-generated carriers cause the states near the edge of band gap filled, blocking further absorption. However, for the donor-acceptor structure of the MoS2/graphene nanocomposite, the MoS2 can act as electron donor through interaction with graphene, the electrons can efficiently transfer from the valence band to the conduction band in both MoS2 and graphene as well as interband transitions between MoS2 and graphene34, 49. As shown in Figure 4(a), EF is the Fermi energy of MoS2, and it is slightly higher than the Dirac cone point of graphene34. σ0 and σ1 stand for ground-state and first excited state absorption cross-section of MoS2.

τ1 is the first excited state lifetime of MoS2. σ2 and σ3 present


- E 2 state and + E 2


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state absorption cross-section of graphene, respectively. τ4 is the + E 2 state lifetime of graphene. E is the photon energy of the excitation laser. τ2 and τ3 are the transition time from MoS2 to graphene and from graphene back to MoS2, respectively. We firstly use the previous model to analyze the absorption process of MoS2 and graphene only without consideration the interaction. Although some groups demonstrated that the steady-state approximation models in nanosecond regime48, 50 (for the quasi-state approximation, please see supporting information), the relaxation of excited carriers is generally smaller than the pulse width of femtosecond laser. After consideration of transition state of rate equations, the α(I) can be obtained from the rate equations of MoS2/graphene nanocomposite film. The propagation equation in the sample can be expressed as a function of input intensity I:

dI = -α(I)I dz '

(3)

For special MoS2/graphene nanocomposite film, the population density of the different energy-levels in MoS2 and graphene can be written as: N dN 0 -N σ I N = - 0 0 + 1 + -E 2 dt hν τ1 τ3

(4)

dN1 NσI N N = 0 0 - 1 - 1 dt hν τ1 τ2

(5)

dN -E 2 dt

= -

dN +E 2

N -E 2 σ 2 I hν

N -E 2 σ 2 I

+

N +E 2 τ4

N +E 2

-

N -E 2 τ3

(6)

N1 τ2

(7)

N = N0 + N1 + N -E 2 + N + E 2

(8)

dt

=

hν

-

τ4

+

N0 and N1 present population densities at ground and excited state of MoS2,


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respectively. N+E 2 and N -E 2 present population densities at electronic initial and final state of graphene after excited by a laser. N presents the total population density. Then the Equation (3) can be rewritten as:

dI = -σ 0 N 0 I - σ1 N1I - σ 2 N -E 2 I - σ 3 N +E 2 I dz '

(9)

Figure 4(b) demonstrates the normalized transmission as functions of input laser intensity for MoS2/graphene nanocomposite film, graphene film, and MoS2 film. According to Equation (4)-(9), the experimental data were fitted using four-order Runge-Kutta method for MoS2 (blue solid line in Figure 4(b)), graphene (red solid line in Figure 4(b)) and MoS2/graphene nanocomposite film (black solid line in Figure 4(b)). The saturable absorption of MoS2 and graphene film indicate that the absorption cross section of excited state is much less than that of ground state. From the fitting, we can get the ratio information of the carrier relaxation time between different energy levels, which is particularly important for the optical physics process. The electron transfer process in the nanocomposite material can be qualitatively analyzed by this model under the interaction of light and matter. The ratio of τ2 to τ1 is less than 1 (the value ~ 0.026), which suggests that most of the carriers in the excited state of MoS2 would transform to the excited state of graphene. The ratio of τ2 to τ3 is also less than 1 (the value ~ 0.83), which suggests that net charge transition from MoS2 to graphene would be stable. Furthermore, the ratio of τ3 to τ4 is much less than 1 (the value ~ 0.072), which also implies the excitation carriers in graphene would be stable to enhance the nonlinear response. Electrons can be excited from ground-state to excited state simultaneously under the laser (1.55 eV larger than the



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bandgap of few layer MoS2 ~1.29 eV) for both MoS2 and graphene. These excited carriers can not only relax from the excited state to the ground state, but also transfer from MoS2 to graphene reversely. However, carrier transfer between them plays a decisive role in the enhancement of nonlinear property of MoS2/graphene nanocomposite film. We think our model could also be used to describe the electron transfer process in other nanocomposites such as graphene/porphyrin and carbon nanotube/MoS2 in the previous report51-52. As the nanocomposite is easy to be engineered, the method would be useful for the new type design of saturable absorber for optoelectronics.

4. CONCLUSION In summary, MoS2/graphene nanocomposite films are obtained by controlling the filtration volume from the corresponding suspensions. The nonlinear absorption properties of MoS2/graphene nanocomposite films were analyzed with an open aperture Z-scan measurement with an ultrafast 800 nm laser. The MoS2/graphene nanocomposite films exhibited stronger saturable absorption compared with MoS2 and graphene films. Both nonlinear absorption coefficient and imaginary third-order susceptibility of the MoS2/graphene nanocomposite film are enhanced compared with those of MoS2 and graphene only. A coupling model is employed to understand the enhanced nonlinear absorption properties of the nanocomposite films. The results suggest the enhancement can be attributed to charge transfer from MoS2 to graphene. As it is easy to engineer the nanocomposite films, this work paves the way for designing new type of hybrid nonlinear materials with better saturable absorption


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property.

AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected] , [email protected] (X. Xu)

Supporting Information Experimental setups of open aperture Z-scan measurement, nonlinear optical properties of nanocomposite optical films, rate equations of MoS2 or graphene films, enhancement mechanism of MoS2/graphene nanocomposite film.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 11774288, 11374240, 61378040), Key Science and Technology Innovation Team Project of Natural Science Foundation of Shaanxi Province (2017KCT-01), and Guangxi Key Laboratory of Automatic Detecting Technology and Instruments (YQ17201).

REFERENCES (1)

Sun, Z.; Chang, H. Graphene and Graphene-like Two-Dimensional Materials in Photodetection: Mechanisms and Methodology. ACS Nano 2014, 8, 4133-4156.

(2) Zhao, Q.; Guo, Y.; Zhou, Y.; Yan, X.; Xu, X. Solution-Processable Exfoliation and Photoelectric Properties of Two-Dimensional Layered MoS2 Photoelectrodes. J. Colloid Interface Sci. 2017, 490, 287-293. (3) Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Cheminform 2011, 331, 568-571. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Sci. 2004, 306, 666-669. (5) Dawlaty, J. M.; Shivaraman, S.; Chandrashekhar, M.; Spencer, M. G.; Rana, F. Measurement of Ultrafast Carrier Dynamics in Epitaxial Graphene. Appl. Phys. Lett.2008, 92, 197. (6) Xing, G.; Guo, H.; Zhang, X.; Sum, T. C.; Huan, C. H. The Physics of Ultrafast Saturable Absorption in Graphene. Opt. Express 2010, 18, 4564-4573. (7) Kumar, S.; Anija, M.; Kamaraju, N.; Vasu, K. S.; Subrahmanyam, K. S.; Sood, A. K.; Rao, C. N.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

R. Femtosecond Carrier Dynamics and Saturable Absorption in Graphene Suspensions. Appl. Phys. Express 2009, 95, 183. (8) Wang, J.; Hernandez, Y.; Lotya, M.; Coleman, J. N.; Blau, W. J. Broadband Nonlinear Optical Response of Graphene Dispersions. Adv. Mater. 2010, 21, 2430-2435. (9) Bao, Q.; Han, Z.; Yu, W.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Ding, Y. T. Atomic-Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers. Adv. Funct. Mater. 2009, 19, 3077-3083. (10) Sun, Z.; Hasan, T.; Torrisi, F.; Popa, D.; Privitera, G.; Wang, F.; Bonaccorso, F.; Basko, D. M.; Ferrari, A. C. Graphene Mode-Locked Ultrafast Laser. ACS Nano 2010, 4, 803-810. (11) Chang, Y. M.; Kim, H.; Ju, H. L.; Song, Y. W. Multilayered Graphene Efficiently Formed by Mechanical Exfoliation for Nonlinear Saturable Absorbers in Fiber Mode-Locked Lasers. Appl. Phys. Express 2010, 97, 109. (12) Baek, I. H.; Lee, H. W.; Bae, S.; Hong, B. H.; Ahn, Y. H.; Yeom, D. I.; Rotermund, F. Efficient Mode-Locking of Sub-70-fs Ti:Sapphire Laser by Graphene Saturable Absorber. Appl. Phys. Express 2012, 5, 2701. (13) Meric, I.; Han, M. Y.; Young, A. F.; Ozyilmaz, B.; Kim, P.; Shepard, K. L. Current Saturation in Zero-Bandgap, Top-Gated Graphene Field-Effect Transistors. Nat. Nanotechnol. 2008, 3, 654-659. (14) Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766-3798. (15) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926. (16) Wang, Q. H.; Kalantarzadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (17) Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695-3700. (18) Wang, K.; Wang, J.; Fan, J.; Lotya, M.; O’Neill, A.; Fox, D.; Feng, Y.; Zhang, X.; Jiang, B.; Zhao, Q. Ultrafast Saturable Absorption of Two-Dimensional MoS2 Nanosheets. ACS Nano 2013, 7, 9260-9267. (19) Wang, K.; Feng, Y.; Chang, C.; Zhan, J.; Wang, C.; Zhao, Q.; Coleman, J. N.; Zhang, L.; Blau, W. J.; Wang, J. Broadband Ultrafast Nonlinear Absorption and Nonlinear Refraction of Layered Molybdenum Dichalcogenide Semiconductors. Nanoscale 2014, 6, 10530-10535. (20) Zhang, S.; Dong, N.; McEvoy, N.; O’Brien, M.; Winters, S.; Berner, N. C.; Yim, C.; Li, Y.; Zhang, X.; Chen, Z., et al. Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films. ACS Nano 2015, 9, 7142-7150. (21) Zhang, H.; Lu, S. B.; Zheng, J.; Du, J.; Wen, S. C.; Tang, D. Y.; Loh, K. P. Molybdenum Disulfide (MoS2) as a Broadband Saturable Absorber for Ultra-Fast Photonics. Opt. Express 2014, 22, 7249-7260. (22) Tsai, D. S.; Liu, K. K.; Lien, D. H.; Tsai, M. L.; Kang, C. F.; Lin, C. A.; Li, L. J.; He, J. H. Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments. ACS Nano 2013, 7, 3905-3911. (23) Dong, N.; Li, Y.; Feng, Y.; Zhang, S.; Zhang, X.; Chang, C.; Fan, J.; Zhang, L.; Wang, J. Optical


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets. Sci. Rep. 2015, 5, 14646. (24) Zhang, X.; Zhang, S.; Chang, C.; Feng, Y.; Li, Y.; Dong, N.; Wang, K.; Zhang, L.; Blau, W. J.; Wang, J. Facile Fabrication of Wafer-Scale MoS2 Neat Films with Enhanced Third-order Nonlinear Optical Performance. Nanoscale 2015, 7, 2978-2986. (25) Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499, 419-425. (26) Ebnonnasir, A.; Narayanan, B.; Kodambaka, S.; Ciobanu, C. V. Tunable MoS2 Bandgap in MoS2-graphene Heterostructures. Appl. Phys. Lett. 2014, 105, 031603. (27) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and Van Der Waals Heterostructures. Sci. 2016, 353. (28) Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Mishchenko, A.; Georgiou, T.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V., et al. Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures. Sci.2012, 335, 947-950. (29) Lin, Y. C.; Ghosh, R. K.; Addou, R.; Lu, N.; Eichfeld, S. M.; Zhu, H.; Li, M. Y.; Peng, X.; Kim, M. J.; Li, L. J. Atomically Thin Resonant Tunnel Diodes Built from Synthetic Van Der Waals Heterostructures. Nat. Commun. 2015, 6, 1-10. (30) Cheng, R.; Li, D.; Zhou, H.; Wang, C.; Yin, A.; Jiang, S.; Liu, Y.; Chen, Y.; Huang, Y.; Duan, X. Electroluminescence and Photocurrent Generation from Atomically Sharp WSe2/MoS2 Heterojunction p–n Diodes. Nano Lett. 2014, 14, 5590-5597. (31) Huang, K.-J.; Wang, L.; Li, J.; Liu, Y.-M. Electrochemical Sensing Based on Layered MoS2–Graphene Composites. Sens. Actuators, B 2013, 178, 671-677. (32) Zhang, K.; Sun, H.; Hou, S. Layered MoS2-Graphene Composites for Biosensor Applications with Sensitive Electrochemical Performance. Anal. Methods 2016, 8, 3780-3787. (33) Chang, K.; Chen, W. L-Cysteine-Assisted Synthesis of Layered MoS2/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720-4728. (34) Jiang, Y.; Miao, L.; Jiang, G.; Yu, C.; Xiang, Q.; Jiang, X. F.; Han, Z.; Wen, S. Broadband and Enhanced Nonlinear Optical Response of MoS2/Graphene Nanocomposites for Ultrafast Photonics Applications. Sci. Rep. 2015, 5, 16372. (35) Zhou, Y.; E, Y.; Ren, Z.; Fan, H.; Xu, X.; Zheng, X.; Lei, D. Y.; Li, W.; Wang, L.; Bai, J. Solution-Processable Reduced Graphene Oxide Films as Broadband Terahertz Wave Impedance Matching Layers. J. Mater. Chem. C 2015, 3, 2548-2556. (36) Yaohui, G.; Qiyi, Z.; Zehan, Y.; Keyu, S.; Yixuan, Z.; Xinlong, X. Efficient Mixed-Solvent Exfoliation of Few-Quintuple Layer Bi2S3 and Its Photoelectric Response. Nanotechnology 2017, 28, 335602. (37) Liebig, A.; Kloc, C.; Zahn, D. R. T.; BC6rner, J.; Albrecht, M.; Gordan, O.; BC6ttger, P.; Tonndorf, P.; Schmidt, R.; Bratschitsch, R. Photoluminescence Emission and Raman Response of Monolayer MoS2, MoSe2, and WSe2. Opt. Express 2013, 21, 4908-4916. (38) Bertrand, P. A. Surface-Phonon Dispersion of MoS2. Phys.Rev.B 1991, 44, 5745-5749. (39) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S., et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (40) Sheik-Bahae, M.; Said, A. A.; Van Stryland, E. W. High-Sensitivity, Single-Beam n(2) Measurements. Opt. Lett. 1989, 14, 955-957. (41) Ganeev, R. A.; Ryasnyansky, A. I.; Ishizawa, N.; Baba, M.; Suzuki, M.; Turu, M.; Sakakibara, S.;



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Kuroda, H. Two- and Three-Photon Absorption in CS2. Opt. Commun. 2004, 231, 431-436. (42) Ijaz, S.; Mahendru, A.; Sanderson, D. Sensitive Measurement of Optical Nonlinearities Using a Single Beam. IEEE J. Quantum Electron. 1990, 26, 760-769. (43) Wang, K.; Wang, J.; Fan, J.; Lotya, M.; O'Neill, A.; Fox, D.; Feng, Y.; Zhang, X.; Jiang, B.; Zhao, Q. Ultrafast Saturable Absorption of Two-Dimensional MoS2 Nanosheets. ACS Nano 2013, 7, 9260-9267. (44) Yan, X.; Zhu, L.; Zhou, Y.; Yiwen, E.; Wang, L.; Xu, X. Dielectric Property of MoS2 Crystal in Terahertz and Visible Regions. Appl. Opt. 2015, 54, 6732-6736. (45) Wu, J.-J.; Tao, Y.-R.; Wu, X.-C.; Chun, Y. Nonlinear Optical Absorption of SnX2 (X = S, Se) Semiconductor Nanosheets. J. Alloys Compd. 2017, 713, 38-45. (46) Sreeramulu, V.; Haldar, K. K.; Patra, A.; Rao, D. N. Nonlinear Optical Switching and Enhanced Nonlinear Optical Response of Au–CdSe Heteronanostructures. J. Phys. Chem. C 2014, 118, 30333-30341. (47) Solati, E.; Dorranian, D. Nonlinear Optical Properties of the Mixture of ZnO Nanoparticles and Graphene Nanosheets. Appl. Phys. B 2016, 122, 76. (48) Zawadzka, M.; Wang, J.; Blau, W. J.; Senge, M. O. Modeling of Nonlinear Absorption of 5,10-A2B2 Porphyrins in the Nanosecond Regime. J. Phys. Chem. A 2013, 117, 15-26. (49) Lu, S. B.; Miao, L. L.; Guo, Z. N.; Qi, X.; Zhao, C. J.; Zhang, H.; Wen, S. C.; Tang, D. Y.; Fan, D. Y. Broadband Nonlinear Optical Response in Multi-Layer Black Phosphorus: An Emerging Infrared and Mid-Infrared Optical Material. Opt. Express 2015, 23, 11183-11194. (50) O’Flaherty, S. M.; Hold, S. V.; Cook, M. J.; Torres, T.; Chen, Y.; Hanack, M.; Blau, W. J. Molecular Engineering of Peripherally and Axially Modified Phthalocyanines for Optical Limiting and Nonlinear Optics. Adv. Mater. 2003, 15, 19-23. (51) Xu, Y.; Liu, Z.; Zhang, X.; Wang, Y.; Tian, J.; Huang, Y.; Ma, Y.; Zhang, X.; Chen, Y. A Graphene Hybrid Material Covalently Functionalized with Porphyrin: Synthesis and Optical Limiting Property. Adv. Mater. 2009, 21, 1275-1279. (52) Zhang, X.; Selkirk, A.; Zhang, S.; Huang, J.; Li, Y.; Xie, Y.; Dong, N.; Cui, Y.; Zhang, L.; Blau, W. J., et al. MoS2/Carbon Nanotube Core–Shell Nanocomposites for Enhanced Nonlinear Optical Performance. Chem. - Eur. J. 2017, 23, 3321-3327.


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