Multiphoton Excitation and Defect-Enhanced Fast Carrier Relaxation

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C: Physical Processes in Nanomaterials and Nanostructures

Multiphoton Excitation and Defect-Enhanced Fast Carrier Relaxation in Few-Layered MoS Crystals 2

Junpei Zhang, Linhua Yao, Nan Zhou, Hongwei Dai, Hui Cheng, Mingshan Wang, Luman Zhang, Xiaodie Chen, Xia Wang, Tianyou Zhai, and Junbo Han J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00619 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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The Journal of Physical Chemistry

Multiphoton Excitation and Defect-Enhanced Fast Carrier Relaxation in Few-Layered MoS2 Crystals Jun-pei Zhang,

†

Wang,

†

Lin-hua Yao,

†

Luman Zhang,

†

‡

Nan Zhou,

Xiaodie Chen,

†

Xia Wang,

Han

†Wuhan

Hongwei Dai,

¶

†

†

Hui Cheng,

Tianyou Zhai,

‡

Mingshan

and Jun-bo

∗,†

National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, China, 430074.

‡State

Key Laboratory of Material Processing and Die and Mould Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China, 430074.

¶Wenhua

college, Wuhan, China, 430074.

E-mail: [email protected]

Abstract Femtosecond transient reection spectroscopy was employed to study the carrier dynamics in few-layered MoS2 crystals prepared by either a chemical vapor deposition (CVD) technique or a mechanical exfoliation method. Three decay processes were observed in samples prepared by both methods. The faster processes were approximately 300-500 fs and could be attributed to defect-assisted scattering. The slower processes varied from 1 ps to 250 ps and were related to carrier-carrier and carrier-phonon scattering. Comparative studies between the CVD and exfoliated samples with dierent 1

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thicknesses demonstrated that faster decay processes occurred in thinner samples, in which the interface and defects had stronger eects. Wavelength-dependent transient reection spectra demonstrated that a change in the sign of the signal occurred around the exciton absorption peaks, which could be attributed to competition of the stimulated emission and excited state absorption processes around the exciton absorption peaks. Our results are useful for understanding the dynamic behaviors of 2D materials, which are of particular importance for their applications in ultrafast optical devices and photonic devices.

INTRODUCTION Transition-metal dichalcogenide (TMD) materials have attracted broad interest due to their signicant properties in transports, valleytronics and optics. 17 Benetting from the weak van de Waals force between layers, TMD crystals can be exfoliated and engineered layer by layer. 8,9 MoS2 is a typical TMD semiconductor, whose energy band is located near visible and near infrared wavelengths, which aords great potential for applications in optical and photonic devices. 1016 Previous studies have demonstrated that the band gap of MoS2 crystals changes from an indirect band to a direct band as the thickness of the MoS2 crystals decreases from the bulk to the monolayer. 17,18 As a result, a dramatic enhancement in the photoluminescence (PL) has been observed due to the transition of the band gap from indirect to direct. 17 This transition has also been conrmed by theoretical calculations and scanning photoelectron microscopy measurements. 19,20 Numerous experiments, including PL, Raman, and transport measurements, have been performed to characterize the layer-dependent optical and electronic properties of the crystals. 10,2126 For semiconductors, the charge carrier dynamics play an essential role in determining their electronic and optical properties, which are of particular importance for their applications in ultrafast optical switching and ultrafast photodetection. 2730 Many techniques, including time-resolved photoemission spectroscopy, pump-probe transient absorption spectroscopy 2


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and the magneto-optical Kerr eect, have been used to investigate the dynamic behavior of TMD semiconductors. 3142 For MoS2 crystals, a decay process of 100 ps has been observed in atomically thin MoS2 akes, which is consistent with the PL lifetime of excitons. 32 Further studies have demonstrated that three dierent relaxation processes exist in suspended and supported MoS2 2D crystals. 33,42,43 The relaxation processes on the subpicosecond timescale are attributed to defect-assisted scattering, 33 while those on timescales of tens to hundreds of picoseconds originate from carrier-phonon interactions, valley scattering and direct recombination. 37,4447 In addition, electron spin polarization in monolayered MoS2 and MoSe2 has been observed to be on the timescale of several nanoseconds. 37 To date, extensive work has been performed to classify the dierent types of dynamic processes. However, for a given material the response times are closely related to the band gap structure, the quality of the surface, and the conguration of the pump and probe laser wavelengths. Therefore, it is of great importance to investigate and compare the layer- and defect-dependent dynamics of carriers of TMD materials at both resonant and non-resonant wavelengths, which would be very useful for understanding and engineering their optical and electrical performance. In this work, a dichromatic pump-probe setup with pump wavelengths of 392 and 785 nm (3.16 and 1.58 eV) and probe wavelengths from 610-695 nm (2.03-1.78 eV) was used to investigate the ultrafast dynamic responses of free carriers and excitons in MoS2 crystals prepared by both a CVD method and an exfoliation technique. The pump wavelength, probe wavelength, excitation light polarization and pump power dependencies of the optical dynamics were characterized and investigated. Transient reection signals (TRSs) with picosecond response times were obtained over a broad wavelength range, and a sign ip of the TRS was observed near the exciton absorption peaks. These observations are important for the application of TMD materials in ultrafast optical devices and photonic devices.

3



EXPERIMENTAL Preparation and characterization of MoS2 akes MoS2 akes were prepared by using both CVD and exfoliation methods. The CVD samples were deposited onto sapphire substrates in a CVD system by using MoCl5 (99.999%, Alfa) and S (99.9%, Alfa) powders as the precursors. 48 Then, the obtained samples were cleaned by ultrasound in ethanol, acetone, and isopropyl alcohol, separately to remove organic residue from the surface. The exfoliation samples were prepared on a SiO2 /Si substrate from MoS2 crystals (supplied by MTI corporation, KJMT group) by using scotch-tape as the exfoliation tool. The thickness and surface images of the MoS2 akes were measured by atomic force microscopy (AFM). The Raman spectra were obtained from a lab-made Raman setup, where a 532 nm continuous-wave laser was used as the excitation source, and four band lters (three band notch lter and one band pass lter, OptiGrate Corp.) were used. The transmission and reection measurements were performed by using a micro-optical setup equipped with a tungsten halogen lamp light source and a CCD-spectrometer.

TRS and SHG measurements of MoS2 akes The TRS measurements were carried out by using a lab-made micro-optical dichromatic pump-probe setup at the Wuhan National High Magnetic Field Center (Figure S1 in Supporting Information (SI)). A mode-locked Ti:sapphire laser (Mira900, Coherent, 76 MHz, 130 fs) was used as the fundamental laser source. Two laser branches were split from the fundamental laser, one was used to generate the 392 nm laser through a second harmonic generator, and the other was used to achieve a tunable laser from 610 nm to 695 nm by using an OPO (optical parametric oscillator). Either the 785 nm laser or the 392 nm laser was used as the excitation pump beam, while the 610-695 nm laser was used as the probe beam. Both the pump and probe lasers were focused onto the sample by using a 100× objective 4


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lens (Mitoyo, M Plan NIR HR, NA = 0.7). The second harmonic generation (SHG) measurements were obtained in the same setup as that used for the TRS measurements, where the 785 nm laser was used as the fundamental excitation, and a CCD spectrometer was used for recording the spectra of the second harmonic generation, stepping-motor controlled polarizers and half-wave plates were used to control the polarization of the excitation laser and generation outputs.

RESULTS AND DISCUSSION (b)

(a)

Ġ

SHG S5 10 μm

(c)

B

2.5

A

Raman Intensity (a.u.)

4

Absorption (a.u.)

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


S5

3

S4 2

S3 1

S2 S1

0 580

(d)

A1g

1 E2g

2.0

D=25.8cm-1 1.5 1.0 0.5

S5

D=25.6cm-1 D=25.7cm-1

S4 S3

D=25.2cm-1 D=23.0cm-1

S2 S1

D

0.0 600

620

640

660

680

700

720

360

380

400

420 -1

Wavelength (nm)

Raman Shift (cm )

Figure 1: Optical image, absorption spectra and Raman spectra of MoS2 akes grown by the CVD method. (a) Schematic illustration of the optical measurement setup. (b) Optical 澺濝濛澢澔澥澔㗵㐺㜸⏂杻ⶐ 䐲咅僴⊛ micrograph of a MoS2 ake with a polarization-resolved SHG pattern. (c) Optical absorption spectra of samples S1-S5. (d) Raman spectra of samples S1-S5.

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Optical and TRS properties of MoS2 akes grown by the CVD method The MoS2 akes synthesized by the CVD method were tens of micrometers in size with dierent thicknesses and were characterized by AFM, SHG, absorption and Raman techniques before TRS measurement. Figure 1a shows a schematic of the optical microscopy setup for the absorption, Raman, SHG and TRS measurements, where a 100× objective was used to focus the light and collect the signals. Five samples with dierent thicknesses were selected for TRS characterization and were labeled S1-S5 (Figure S2 and Table S1 in SI). The shapes of all the samples were triangular, with thicknesses varying from 3 nm to 30 nm. Figure 1b shows the optical image of sample S5 as a representative example, whose shape was triangular with a thickness of 8 nm at the edge and 23 nm at the center. Within the gure, a typical polarization-dependent SHG pattern with SHG polarization parallel to the fundamental light is shown. The pattern shows six-fold symmetry, which agrees well with the results reported in the literature. 49 Figure 1c and 1d show the absorbance spectra and Raman spectra, respectively, for S1-S5. In the absorbance spectra, two absorption peaks corresponding to the A and B excitons could be observed at approximately 610 nm and 660 nm, respectively. The absorption peaks shifted to shorter wavelengths as the sample thickness decreased from 30 nm to 3 nm due to the quantum connement eect. In the Raman spectra, two Raman peaks at approximately 385 cm−1 and 405 cm−1 could be observed, which corresponded to the E12g and A1g vibration modes. As the sample thickness increased, the energy dierence between the two peaks increased from 23.0 cm−1 (3 layers) to 25.8 cm−1 (∼30-40 layers). This increase coincides well with the thickness-dependent energy dierences of MoS2 crystals. 21,26 Figure 2 demonstrates the dynamic characteristics of the MoS2 akes. Figure 2a presents the two typical decay curves obtained from the TRS measurements of S5. The curves with positive ∆T signal were attributed to photoinduced bleaching or stimulated emission (SE), while the curves with negative ∆T signal originated from photoinduced absorption including excited state absorption (ESA). 6


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1.0

(a)

30

λpump = 785 nm

S5

Absorption at S5

(b)

y = y0+A0exp(-x/15.201)

TRS (a.u.)

0.5

SE > ESA

λprobe = 680 nm

0.0 SE < ESA

0.5

λprobe = 665 nm y = y0+A0exp(-x/0.216) + A1exp(-x/15.978)

Slow Decay (ps)

20

0

10

20

30

40

0

0

10 500

SE < ESA

20

λpump = 785 nm

600

50

625

650

1000 700

675

Wavelength (nm)

Time (ps)

Ġ

5.0

(c)

S5

20

4.5 -5

4.0

25

15

0.4

0.6

0.8

1.0

Decay Time (ps)

90 Decay Time

1000

500

SE > ESA

10

30

1.0

TRS (10 )

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


Fast Decay (fs)

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(d)

ĵ

60

120

TRS at S5 30

150

10

2

3.5

Power (GW/cm )

0

180

S5

λpump = 785 nm

3.0

λprobe = 680 nm

0.4

0.6

0.8

330

210

2.5 1.0

300

240

2

270

Power (GW/cm )

λpump = 785 nm λprobe = 680 nm

澺濝濛澢澔澦澔⋙㯣撀 ⋙ὐ㈰濇澼澻⎍⋙↠䊈濈濆濇

Figure 2: TRS data of a MoS2 ake grown by the CVD method. (a) Typical TRS curves of S5 taken under a pump wavelength of 785 nm and probe wavelengths of 680 nm and 665 nm. (b) Probe wavelength-dependent TRS intensity and the corresponding decay time of S5. Alternating signals are observed near the exciton peaks. (c) The pump power-dependent TRS intensity and the corresponding decay time, the pump wavelength is 785 nm, and the probe wavelength is 680 nm. (d) Excitation polarization dependent TRS intensity and SHG for S5. The SHG signal shows six-fold symmetry.

7



As shown in Figure 2b, both decay behaviors could be observed in the probe wavelengthdependent TRS curves between 695 nm and 610 nm (1.78-2.03 eV) when the pump wavelength was xed at 785 nm. During the measurements, the pump power was xed at 5 mW (∼0.6 GW/cm2 ) while the probe power was xed at 200 µW (∼0.02 GW/cm2 ) for all probe wavelengths. For the data obtained near both the A and B exciton peaks, positive transient signals could be observed, while for the data obtained away from the exciton peaks the signals were negative (more data can be found in Figures S3-S4). Notably, longer decay times usually occurred near the exciton absorption peaks, which could be attributed to a decrease in the electron relaxation rate due to valley polarization. 37 For the measurements taken above, it should be noted that the pump photon energy was lower than energies of the probe and the exciton band gap. In this pump-probe conguration, no dynamic response was expected to be observed except for the optical Kerr eect (OKE). However, when a balanced optical detector was used and the initial output signal was set to zero, no obvious transient signal could be observed. This indicated that the OKE could be neglected. Accordingly, when the initial output value of the balanced detector was set to a small nonzero value or when an avalanche diode was used, a signicant transient signal could be obtained. This implied that multiphoton excitation was involved in the optical pump processes, where one photon was used to excite an electron from the valence band to the intermediate state (defect state or conduct band), while a second photon was used to excite the electron to higher energy levels. Detailed information will be discussed later. To characterize the optical and thermal stability of the MoS2 akes under laser irradiation, pump power-dependent TRS at a probe wavelength of 680 nm was performed, as shown in Figure 2c. As the pump power was increased from 0.2 GW/cm2 to 1 GW/cm2 , the intensity of the TRS increased linearly, which indicated that the sample was stable under irradiation at 1 GW/cm2 . The inset presents the pump power-dependent decay time; the decay time increased slowly from 12 ps to 24 ps, which can be attributed to the heating eect of the electrons by the pump laser. 50,51 Figure 2d shows the excitation polarization-dependent TRS 8


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and SHG. The SHG data exhibited a standard six-fold symmetry patterns, while the TRS intensity did not noticeably change with changing pump polarization, which further excluded second-order or third-order optical nonlinearity-induced TRS in the MoS2 akes under our experimental conditions.

Optical and TRS properties of MoS2 akes exfoliated from crystals (a)

(b)

P1

ɶ

(c)

P2

10 μm

(d)

P6

2.0 D=25.5cm-1

P5

-1

1.5

D=25.1cm

P4

-1

D=24.9cm

1.0 D=24.1cm-1

P3 P2

-1

0.5

D=22.6cm

-1

D=19.9cm

P1

D

0.0 380

300

A1g

1

E2g

390

400

410 -1

Raman Shift (cm )

420

200

1000

(e)

1.6

500 100

SE > ESA

0

0

100

P6 500 λpump = 785 nm

SE < ESA 200 300 600

625

650

675

Fast Decay Time (fs) TRS (a.u.)

2.5

Slow Decay Time (ps)

3.0

1000 700

Wavelength (nm)

(f)

80

P6

60 40

1.4

20

1.2

0.4

0.6

0.8

Decay Time (ps)

P6

~12.2 nm

~10.6 nm

~4.5 nm

P5

~12.9 nm

P3 P4

Raman Signal (a.u.)

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


1.0

2

Power (GW/cm )

P6 λpump = 785 nm

1.0 0.8

λprob = 680 nm 0.4

0.6

0.8

1.0

2

Power (GW/cm )

澺濝濛澢澔澧澔䝆䅈ᴋ㜸⏂䖅坩⺂⎍㑱㉯ Figure 3: Microscopy images, Raman spectra and transient transmission spectra of a fewlayered MoS2 ake exfoliated from a single crystal. (a) Optical microscope image of a fewlayered MoS2 ake on a SiO2 /Si substrate. (b) AFM image of the few-layered MoS2 ake shown in (a); the inset is the height information of the sample. (c) Polarization-dependent SHG. (d) Raman spectra of the sample at dierent sampling points. (e) Probe wavelength dependent transient signal intensity and decay time. (f) Pump power-dependent transient signal intensity and decay time. The pump wavelength is 785 nm, and the probe wavelength is 680 nm. In this section, the optical and dynamic properties of the MoS2 akes exfoliated from bulk crystals will be demonstrated for the purpose of comparison with those of the CVD-prepared

MoS2 akes. Figure 3a-c shows the optical image, AFM image and the SHG pattern of a MoS2 ake, from which six locations (labelled as P1-P6) with dierent thicknesses could be observed, which were approximately 3.0 nm, 3.5 nm, 4.4 nm, 6.7 nm, 10.5 nm and 12.9 nm, 9



respectively (from 4 to 20 layers; Figure S5a and Table S1 in SI). The SHG pattern also demonstrated six-fold symmetry, which agreed well with that of the CVD samples. Figure 3d shows the Raman scattering spectra of the ake at P1-P6. The Raman spectra appeared similar to those of the CVD samples, and the layer-dependent energy dierences between the E12g and the A1g modes coincided well with the literature values. 21,26 Figure 3e shows the probe wavelength-dependent TRS data from area P6. The pump wavelength was xed at 785 nm, while the probe wavelength ranged from 695 nm to 610 nm. The prole of the wavelength-dependent curve was similar to that of the CVD sample shown in Figure 2b, where the sign of the decay curve changed near the exciton peaks. However, one large dierence could be observed between the two dierent samples: the slowest decay time observed for the exfoliation sample was approximately 250 ps, which was almost 10 times slower than that of the CVD samples. This could be attributed to the higher quality of the exfoliation sample where the defect-assisted carrier-carrier and carrier-phonon scatterings are weaker. Figure 3f shows the power-dependent TRS at a pump wavelength of 785 nm and a probe wavelength of 680 nm. Similar to the results shown in Figure 2d, the TRS intensity increased linearly with increasing excitation power. The decay time uctuated at approximately 50 ps, indicating that thermal eects could be neglected for larger MoS2 akes.

Energy structure and carrier dynamic processes of the MoS2 akes From the TRS data demonstrated above for both the CVD and exfoliated samples, one problem remained to be explored. For the ve samples (S1-S5) grown by the CVD method and the six areas (P1-P6) selected from the exfoliated ake under excitation from a 785 nm laser, TRS data could only be obtained for the thicker samples (S3-S5, P5-P6). It was quite dicult to observe any signal from the thinner few-layered samples (S1-S2, P1-P4). One possibility was that the photon energy of the 785 nm pump laser (1.58 eV) was not ecient to excite the electrons from the valence band to the upper bands. Thus, transient 10


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25

(a)

λpump = 392 nm

-4

TRS (10 )

2.5

λprobe = 680 nm

2.0 S5

1.5 1.0

S4 S3 S2

0.5 0.0

Decay Time (ps)

3.0

S1

0

10

20

30

40


20

15

10

5

50

(b)

S1

S2

S3

S4

S5

Time (ps) 50

(c)


1.5 -4

P6

1.0 P5 P4 P3

0.5

P2 P1

0

10

20

30

40

(d)

λpump = 392 nm

40

Decay Time (ps)

2.0

TRS (10 )

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


λprobe = 680 nm

30 20 10 0 P1

50

P2

P3

P4

P5

P6

Time (ps)

Figure 4: (a) and (b) are the pump power-dependent pump-probe transient reection curves and the corresponding decay times for the exfoliated MoS2 samples under a xed probe wavelength of 680 nm. (c) and (d) are the layer-dependent pump-probe transient reection curves and the corresponding decay times for the CVD MoS2 samples.

11



spectroscopy experiments at a pump wavelength of 392 nm (3.16 eV) and a probe wavelength of 680 nm were performed. Figure 4 demonstrates the TRS data for both the CVD and exfoliated samples. Figure 4a and 4c are the decay curves for the CVD and exfoliated samples, respectively, where Figure 4b and 4d are the corresponding decay times. It was clear that transient processes could be observed for all the samples. These results are reasonable, since the photon energy of the 392 nm laser was sucient to directly excite the electrons from the valence band to the quasi-continuous band of the MoS2 materials. From the thickness-dependent decay time curves, it was possible to see that the decay time increased monotonically with increasing thickness. For the exfoliated samples, the decay time increased from 0.9 ps to 40 ps. For the CVD samples, the decay time increased from 12 ps to 21 ps. Considering that the freshly exfoliated samples were of a higher quality than the CVD samples, which developed numerous defects during the chemical deposition process, and thicker samples have smaller surface to volume ratios, it was easy to conclude that the shorter decay times observed in the CVD samples and thinner areas of the MoS2 ake were caused by an enhancement of electronphonon scattering at the defects. 38 For the TRS data obtained under 392 nm excitation, it is also worth noting that the sign of the TRS was always positive for probe wavelengths of 610-695 nm (Figure S5b-d). This could be attributed to a photoinduced bleaching eect of the ground state (GSB), where the single-photon excitation eciency (392 nm, 3.16 eV) was much higher than that of multiphoton excitation (785 nm, 1.58 eV). To further study the thickness-dependent decay behaviors of both samples under 785 nm excitation, the band gap energies of all samples were extracted from their absorption spectra obtained from either the reection spectra or transmission spectra. The results are shown in Figure 5a and 5b and Table S1 for the CVD and exfoliated samples, respectively. For the CVD samples, the band gap varied from 1.2 to 1.9 eV. For the exfoliated samples, the band gap varied from 1.5 to 1.8 eV. When compared to the energy of 785 nm pump laser, which is shown by a dashed blue line in the gure, it was clear that only samples S3-S5 and P5-P6 12


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2.0

(a)

Eg 785 nm Laser line

1.8

Energy (eV)

1.8

Energy (eV)

1.6 1.4

1.0

(b)

Eg 785 nm Laser line

1.7 1.6 1.5

1.2 S1

S2

S3

S4

P1

S5

(c)

P2

P3

P4

P5

P6

(d) C

C Defect assisted scattering ~ 300 fs

Defect assisted scattering ~ 300 fs

Carrier-carrier scattering ~ 2 ps Carrier-phonon scattering ~30 ps

A/B

Carrier-carrier scattering ~ 2 ps Carrier-phonon scattering ~ 100 ps

A/B

Intervalley scattering

Intervalley scattering

GS

Probe

Indirect recombination

~ 300 - 500 fs

Indirect Pump

Probe

~ 300 - 500 fs

Indirect Pump

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


Indirect recombination

GS

CVD Multilayered MoS2

Exfoliated Multilayered MoS2

Figure 5: (a) and (b) are the layer-dependent band gap energies for MoS2 prepared by CVD and mechanical exfoliation methods, respectively. (c) and (d) Schematics of the electron relaxation processes in the samples mentioned in (a) and (b).

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had band gap energies smaller than the excitation photon energy. This explains why the TRS could only be observed in some of the samples and was absent in the others. To demonstrate the dynamic behavior of MoS2 more clearly, schematics of the relaxation processes in the CVD samples and exfoliated samples are summarized in Figure 5c and 5d, respectively. In the optical conguration of our pump-probe setup, the measurement range of the decay time was within the timescale of hundreds of femtoseconds to hundreds of picoseconds (250s ps), so we only focus on the dynamic processes within this time window. From the TRS curves, at least three dierent dynamic processes were observed: defect-assisted scattering, carrier-phonon scattering, and exciton-exciton annihilation. 31,33,35,38,39,42,43,52 Details about the transitions and the corresponding decay times are indicated in Figure 5c and 5d and listed in Table 1. The fast component (300 fs) during both the rising/falling and decay processes originated from defect-assisted scattering. Although the true timescale of this process should be much faster than 100 fs, it can still be easily separated from the timescales of carrier-phonon (1-300 ps) and exciton-exciton ( 2 ps) interactions, which have timescales of a few picoseconds to hundreds of picoseconds. Thus far, all the dynamic processes have been addressed, and the signs of the dynamic processes can also be addressed. For all experiments, the dynamic processes can be summarized into three cases. (1) For the dynamics of MoS2 under a pump wavelength of 785 nm and probe wavelengths of 610-690 nm, ESA should be the dominant process, and the TRS signal should be negative. (2) However, when the probe wavelengths are close to the exciton peaks, the SE becomes dominant and ESA is suppressed. Thus, the sign of the TRS switched from negative to positive. (3) When the pump wavelength was changed to 392 nm, the signs of the dynamic curve probes at all wavelengths (610-690 nm) are always positive due to the GSB originating from the high eciency of single-photon excitation at 392 nm. Regardless, for each dynamic process, it was easily found that the overall decay time of the CVD akes was faster than that of the exfoliated akes because defects were readily introduced into the CVD samples during the growth process. 14


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Table 1: The decay time of the MoS2 samples. Pump Wavelength 392 nm 392 nm 785 nm 785 nm

Stage Rising Decay Rising Decay

CVD ∼ 500 fs ∼ 500 fs and 1-30 ps ∼ 300-550 fs ∼ 500 fs and 1-40 ps

Exfoliation ∼ 300 fs ∼ 500 fs and 1-100 ps ∼ 300-500 fs ∼ 300 fs and 1-250 ps

CONCLUSIONS MoS2 akes with thicknesses of 3 nm to 30 nm were prepared by CVD and exfoliation methods. A transient reection technique was used to investigate the dynamic behavior of the samples. Multiphoton excitation was observed when 785 nm was used as the excitation wavelength. At least three decay processes, including defect-assisted scattering, carriercarrier scattering and carrier-phonon scattering were observed. For both kinds of samples, the decay processes on a timescale of 300-550 fs were due to defect-assisted scattering. The processes on the timescale of several ps and longer could be attributed to carrier-phonon scattering. A comparison between the processes of the exfoliated MoS2 akes and the CVD akes demonstrated that the overall decay times of the exfoliated samples were slower than those of the CVD samples due to the higher quality of exfoliated surfaces and crystals. These results are helpful for understanding the dynamic behavior of MoS2 akes with dierent thicknesses and surface qualities, which is of particular importance for their application in ultrafast optical and photonic devices.

ASSOCIATED CONTENT Supporting Information Available Details of optical setups, AFM, TRS raw data and ttings.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Jun-bo Han: 0000-0002-5072-4897

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript.

Notes The authors declare no competing nancial interest.

Acknowledgement This work was supported by the National Scientic Foundation of China (11704138, 11404124), Natural Science Foundation of HuBei Province (2015CFB631) and China Postdoctoral Science Foundation funded Project (2017M612432) and the Fundamental Research Funds for the Central Universities (2018KFYXKJC011).

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Multiphoton Excitation and Defect-Enhanced Fast Carrier Relaxation

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