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Ultrafast Dynamics in MoS2 Measured with Femtosecond Pump-Probe Technique

Introduction

Method and Result

Discussion

• Transition metal dichalcogenide (TMD) group • S-Mo-S structure with weak Van der Waals force between layers • Band gap Monolayer MoS2:Direct band-gap 2.0 eV Bulk MoS2: indirect band-gap 1.3 eV • High on/off ratio, high mobility • Suitable for light emitters photodetectors and solar cells • Future photonic and field-effect transistor (FET)

Five Samples: • Bulk MoS2 on quartz • Bulk MoS2 on silicon • Suspended bulk MoS2

• Monolayer MoS2 on silicon • Monolayer MoS2 on quartz

Bulk MoS2 attached to substrates directly by Van der Waals force Monolayer MoS2 grown by CVD, and transferred to different substrates.

BS

Ti: Sapphire 800nm, 35fs, 5kHz

BS HWP Polarizer

SHG

DL

chopper

Sample Signal Probe

ND filter

Iris

Iris

Filter

Iris

BS

lens

Balanced PD

Reference Probe

R

CCD

White light

Lock-in Amplifier

computer

Si

SiO2

Monolayer MoS2

Si Si

Quartz Quartz

1 2 3

4 5 6

Monolayer MoS2 transfer to Quartz

Suspended bulk MoS2 on TEM Grid with 100μm holes

Focus on sample Focus on TEM Grid

Femtosecond pump-probe setup (400nm pump / 800nm probe)

0.5 1.0 1.5 2.0 2.5 3.0

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(p

s)

Fluences (mJ/cm2)

Bulk MoS2 on quartz

Bulk MoS2 on silicon

Bulk MoS2 suspended

0.5 1.0 1.5 2.0 2.5 3.0

500

600

700

800

900

1000

1100

1200

1300

Fluences (mJ/cm2)

(p

s)

Bulk MoS2 on quartz

Bulk MoS2 on silicon

Bulk MoS2 suspended

Carrier lifetime

• Bulk MoS2: Shorter lifetime decreases with increasing pump fluences, and can be explained by carrier-carrier scattering. Longer lifetimes shows no apparent pump fluence dependence, can be explained by carrier-phonon and intervalley scattering.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

10

20

30

40

50

60

(p

s)

Fluences (mJ/cm2)

Monolayer MoS2 on quartz

• Monolayer MoS2: Two carrier lifetimes both increase with growing pump fluences. Carrier-carrier scattering and surface trapping states result in the fast decay. Slow decay can be explained by carrier-phonon scattering. Ref.3, Ref.5

Carrier-phonon scattering

Intervalley scattering

Pump

References

1.Radisavljevic, Branimir, et al. "Single-layer MoS2 transistors." Nature nanotechnology 6.3 (2011): 147-150. 2. Nature Nanotechnology 6, 135–136 (2011) 3. Shi H, Yan R, Bertolazzi S, et al. Exciton dynamics in suspended monolayer and few-layer MoS2 2D crystals[J]. ACS nano, 2013, 7(2): 1072-1080. 4. Ge S, Liu X, Qiao X, et al. Coherent longitudinal acoustic phonon approaching THz frequency in multilayer molybdenum disulphide[J]. Scientific reports, 2014, 4. 5. Sun D, Rao Y, Reider G A, et al. Observation of Rapid Exciton–Exciton Annihilation in Monolayer Molybdenum Disulfide[J]. Nano letters, 2014, 14(10): 5625-5629.

• Acoustic phonon lifetime do not vary much on different substrates. Ref.4

0 100 200 300 400 500 600 700-25

-20

-15

-10

-5

0

5

10

15

R

/R (

x 1

0-3)

Time Delay (ps)

0.47mJ/cm2 Exp

0.47mJ/cm2 Fit

1.06mJ/cm2 Exp

0.47mJ/cm2 Fit

1.87mJ/cm2 Exp

1.87mJ/cm2 Fit

2.66mJ/cm2 Exp

2.66mJ/cm2 Fit

0.0 0.2 0.4

Am

plitu

de (

a.u

.)

Frequency (THz)

0.47mJ/cm2

1.06mJ/cm2

1.87mJ/cm2

2.66mJ/cm2

38GHz

0 100 200 300 400 500 600 700

-1

0

1

2

3

4

5

2.66mJ/cm2

1.87mJ/cm2

1.06mJ/cm2

0.47mJ/cm2

R

/R (

x 1

0-3)

Time Delay (ps)

0 50 100 150 200 250 300-30

-25

-20

-15

-10

-5

0

5

R

/R (

x 1

0-3)

Time Delay (ps)

0.21mJ/cm2 Exp

0.21mJ/cm2 Fit

0.62mJ/cm2 Exp

0.62mJ/cm2 Fit

1.00mJ/cm2 Exp

1.00mJ/cm2 Fit

1.48mJ/cm2 Exp

1.48mJ/cm2 Fit

0 100 200 300 400 500 600-6

-4

-2

0

2

4

6

8

10

12

14

R

/R (

x 1

0-3)

Time Delay (ps)

0.21mJ/cm2

0.62mJ/cm2

1.00mJ/cm2

1.48mJ/cm2

Si under 1.00mJ/cm2

Carrier relaxation

Bulk MoS2 on quartz

Monolayer MoS2 on quartz Monolayer MoS2 on silicon

FFT of bulk MoS2 on quartz Coherent Acoustic Phonon

Acoustic phonon in bulk MoS2

Xianghai Meng1, Wenzhi Wu1,2, Avinash Nayak3 , Jung-Fu Lin4,Deji Akinwande3, Yaguo Wang1

1. Department of Mechanical Engineering, The University of Texas at Austin 2. Department of Electrical Engineering, Heilongjiang University, China, 3. Department of Electrical and Computer Engineering, The University of Texas at Austin 4. Department of Geological Sciences, The University of Texas at Austin

MoS2 FET Ref.2 Structure of layered MoS2 Ref. 1

Phonon lifetime in bulk MoS2

1 2-t/τ -t/τΔR=G (e +e )

R

Optical pump-probe technique

Photoexcited carrier and phonon dynamics

Electronic transport and optical property

Photonics and electronics

-100 0 100 200 300 400 500 600 700 800

-15

-10

-5

0

5

10

R

/R (

x 1

0-3)

Time Delay (ps)

0.47mJ/cm2 Exp

0.47mJ/cm2 Fit

1.06mJ/cm2 Exp

1.06mJ/cm2 Fit

1.87mJ/cm2 Exp

1.87mJ/cm2 Fit

2.66mJ/cm2 Exp

2.66mJ/cm2 Fit

Bulk MoS2 suspended

Pump

Carrier-carrier scattering and Trapping state

Carrier-phonon scattering