Supporting Information
Electron-Phonon Interactions in MoS2 Probed with Ultrafast
Two-Dimensional Visible/Far-Infrared Spectroscopy
Xunmin Guo‡, Hailong Chen‡, Xiewen Wen, and Junrong Zheng*
0.00
Experimental data
Fitting result
-OD
-0.05
-0.10
-0.15
370
380
390
400
410
-1
Frequency (cm )
Figure S1. Fitting the transient absorption change (TW = 2 ps) of Bulk MoS2 with the superposition
of two Gaussian line shapes in the frequency range of phonon mode E1u. The dots denote the
experimental data, and the solid line denotes Gaussian peak fit. The fitting result shows a sharp
positive peak and a negative peak with center frequencies at 382 cm -1 and 391 cm-1 respectively,
indicating the coupling shifts the frequency of phonon mode about 9 cm-1 higher. The peak
frequencies have an uncertainty of about 2~3 cm-1, due to the relatively low signal/noise ratio and
the irregular shapes of the samples
Estimate MoS2 temperature jump by excitation
With the following parameters:
Thermal capacity of bulk MoS2: 63.7 J/(mol∙K)
Density: 5.06 g/cm3
Mol mass: 160.07 g/mol
Excitation beam Area: 3.14*0.52 mm2
Coverage of MoS2 on the polyimide substrate: >50%
Thickness of the sample: >0.01mm
Energy absorbed by the sample: <1uJ
The temperature jump can be estimated as:
Tmax 
1 J 160.07
g
mol
J
g
63.7
 5.06 3  0.5    0.052  0.001cm3
mol  K
cm
 0.13( K )
-OD (normalized)
0.0
-0.4
2 m
~30 m
Large sheet
-0.8
-20
0
20
40
60
Time Delay (ps)
80
100
Figure S2. Carrier generation and recombination dynamics of MoS2 samples with different particle
sizes detected at 200 cm-1 with excitation at 800 nm. The generation dynamics is hardly affected by
the sample size. However, the recombination dynamics are dependent on the sample particle size. A
sample with larger particles has a slower recombination dynamics. We suspect that there are two
possible reasons responsible for the observation: (1) a small particle has more surface defected or
trapped states that can facilitate the charge recombination; and (2) a larger particle can have a
larger conjugation for carriers to diffuse and make the recombination more difficult, which is
probably less likely. The detailed mechanisms for the observed particle size dependent dynamics are
subject to future studies. The optical images of the samples are provided in fig.S3.
(A)
(B)
(C)
(D)
2mm
Figure S3. Optical images of samples with length and width of (A) about 2 microns with a thickness
of a few microns; (B) about 30 microns with a thickness of a few microns; (C) larger than 1mm; and
(D) a zoomed-out image of (C) with a thickness of tens of microns.
0.0
(A)
-OD (Normalized)
-OD
0.00
-0.02
-0.04
(B)
-0.2
-0.4
E1u peak
-0.6
-1
200 cm
-0.8
-1.0
-0.06
100
200
300
-1
Frequency (cm )
400
-20
0
20
40
60
80
100
Waiting Time tW (ps)
Figure S4. The excitation-induced signal of the bulk MoS2 measured at low excitation power, which
is about 35 J/cm2. The wavelength of the exciting pulse is centered at 800 nm. (A)
Excitation-induced negative absorption change of bulk MoS2 at the waiting time TW = 2 ps. Within
the experimental error (the signal to noise ratio noise is decreased because of much lower
excitation power), the spectral shape is almost the same as that shown in Fig.3B, i.e., among a
broadband negative signal, a sharp positive peak at around 380 cm-1 and a sharp negative peak at
around 390 cm-1 appear, indicating the excitation-induced frequency shift of the phonon mode E1u.
(B) Comparing the normalized waiting time dependent signal from the phonon mode E1u (black dots)
with that from photo-excited carriers (measured at 200 cm-1, red dots). The red curve is the single
exponential fit for the detected signal at 200 cm-1 with decay constant 22±3 ps. The result is also
similar as that measured at higher excitation power (see Fig.4D), i.e. after the excitation, the
intensities of signals from both of the photo-excited carriers and the phonon mode increase rapidly
within a couple of hundreds fs and then decay relatively slowly at the time scale of tens of ps,
indicating the appearances of the positive peak and the negative peak at 380~390 cm-1 are induced
by the electron-phonon coupling between the excitations of electrons and the phonon mode E1u of
MoS2.
Normalization of Excitation-Induced Transient Spectra
For the spot size of the excitation beam is not large enough to completely cover the entire
detection beam, the excitation-induced transient spectra can be approximately normalized in the
following manner.
Considering the focal spot diameter of a Gaussian beam is inversely proportional to the
frequency of the beam, the spot size of the detection beam on the sample is a function of the
frequency : r   1 , where r is the spot radius of the detection beam. For simplicity, the detection
beam can be treated as a uniform circular spots with the radius r, and the excitation beam is a
Gaussian beam, which overlaps with the detection beam on the sample. According to the intensity
distribution of the Gaussian beam, we have the average excitation intensity within the area of the
detection beam I (r )  [1  e2r
2
/ r02
] / r 2 , where r0 is the waist size of the excitation beam. Assuming
r0 equal to the spot radius of the detection beam at frequency 0, and considering r   1 , we have
I ()  [1  e20 / ]   2 . Since the signal we measured OD( ) is proportional to the average
2
2
intensity of the excitation beam, the excitation-induced transient spectra can be normalized with the
coefficient 1 / I ( ) .
Based on the above analysis, the normalized excitation-induced negative absorption change of
MoS2 at a waiting time TW = 2 ps is calculated (see Fig.S5), where we choose 0 =250 cm-1.
Comparing to the unnormalized spectrum, an opposite slope of the transient spectrum is obtained,
i.e., increasing absorption with lowering frequencies. For both of the unnormalized and normalized
transient spectra, the oscillation background shown in Fig.3B, which arises from the interference
introduced by the polyimide substrate, were removed with a cosine function by considering the
thickness (~30 m) and refractive index (~1.7) of the substrate.
0.00
unnormalized
normalized
-OD
-0.05
-0.10
-0.15
-0.20
100
200
300
400
-1
Frequency (cm )
Figure S5. Comparing the unnormalized and normalized excitation-induced negative absorption
change of MoS2 at a waiting time TW = 2 ps. The wavelength of the exciting pulse is centered at 800
nm.
According to eq. 1, we have
E ( )  E ( )
I ( )
OD( )  log(
)  log( 0
)
2
I 0 ( )
E0 ( )
2
 log(1 
2 Re  E0 ( )  E * ( ) 
E0 ( )
2
)
2 Re  E0 ( )  E * ( ) 
E0 ( )
2
Eq.S1
.
According to the literatures1, 2, we have the differential optical conductivity
 ( )  
E ( )
,
E0 ( )
Eq.S2
with the real part
 E ( ) 
Re   ( )    Re 

 E0 ( ) 
 E ( )  E * ( ) 
Re  E0 ( )  E * ( ) 
0
  Re 

.
2
2


E
(

)
E
(

)
0
0


 OD( )
Eq.S3
It means the real part of the photo-induced increase of THz conductivity in MoS2 samples has the
same line shape as the normalized excitation-induced negative absorption change shown in fig.S5,
and therefore, can be described roughly by the Drude formula.1
However, it does not mean the optical excitation enhanced absorption signal we measured is
from the free carrier Drude response. The above method of normalization is just a rough estimate.
In the calculations, we assumed the detection beam on the sample is a uniform circular spot.
However, the real situation is that, the intensity distribution of the detection beam generated by
THz-ABCD method can be very complicate and hard to predict. In addition, we also assumed that
the waist size of the excitation beam r0 is equal to the spot radius of the detection beam at frequency
0 =250 cm-1, which may not be accurate either. If we choose 0 =100 cm-1, the same slope of the
transient spectrum as the unnormalized one will be obtained, and then the localization rather than a
free carrier response may explain our experimental results better.
[1] C. Lui, A. Frenzel, D. Pilon, Y.-H. Lee, X. Ling, G. Akselrod, J. Kong, and N. Gedik, Phys. Rev.
Lett. 113, 166801 (2014).
[2] G. Jnawali, Y. Rao, H. Yan, and T. F. Heinz, Nano lett. 13, 524 (2013).