Supplemental Material
Quantum confinement effects across two-dimensional planes in MoS2
quantum dots
Z. X. Gan,1 L. Z. Liu,1 H. Y. Wu,1 Y. L. Hao,1 Y. Shan,1 X. L. Wu,1,* and Paul K. Chu2,*
1
Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics and Collaborative
Innovation Center of Advanced Microstructures, National Laboratory of Solid State
Microstructures, Nanjing University, Nanjing 210093, P. R. China.
2
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee
Avenue, Kowloon, Hong Kong, China.
1
(a)
a
320
340
360
380
400
420
440
460
480
500
P L in te n s ity (a .u .)
NMP
E x w a ve le n g th
(n m )
350
400
450
500
550
600
650
700
W a v e le n g th (n m )
(b)b
P L in te n s ity (a .u .)
N M P -W a te r
E x w a ve le n g th
(n m )
350
400
450
500
550
600
320
340
360
380
400
420
440
460
480
500
650
700
W a v e le n g th (n m )
Figure S1. (a) PL spectra acquired from N-methylpyrrolidinone (NMP). The pure NMP solvent
exhibits obvious PL. When the excitation wavelength is varied from 320 to 400 nm, the PL peak
appears at around 475 nm. As the excitation wavelength is increased, the PL peak is weak and
the position redshifts gradually. (b) PL spectra obtained from the NMP mixed with water
(volume ratio = 1:1). Owing to the red-edge effect,S1 namely the interactions between the
fluorescent NMP and water molecules, the PL peak shifts from 420 to 550 nm as the excitation
wavelength is varied from 320 to 500 nm. Usually the relaxation time of a polar solvent (∼10 ps)
is less than the fluorescence lifetime of organic dyes (several nanoseconds). Therefore, the
2
solvation process is normally completed before fluorescence emission. If the solvation dynamics
is not an order of magnitude shorter than the fluorescence lifetime, the fluorophore can be
emitted simultaneously to the excited state energy level, creating a time-dependent emission
energy. The time-resolved dynamics is related to the steady-state fluorescence by the time
integral of the time-dependent emission energy created by alignment of the solvent dipole with
the excited fluorophore. This phenomenon is called the “red-edge effect” which makes the
fluorescence peak dependent on the excitation wavelength.S1
100
0.27 nm
5 nm
Figure S2. TEM image of the MoS2 nanoflakes (NFs) acquired by simple bath sonication. Inset:
High-resolution TEM image. No PL can be detected from these NFs.
3
1000
MoS2 NF
a
408.6
(a)
383.4
800
600
400
Intensity (a.u.)
200
0
800
MoS2 QD
(b)b
408.8
383.5
600
400
200
0
360
370
380
390
400
410
420
-1
Raman shift (cm )
Figure S3. Raman spectra: (a) MoS2 NFs and (b) QDs. The frequency difference between the
E12g and A1g Raman modes is correlated with the film thickness, that is, 19 cm−1 for the
monolayer region and 28 cm−1 for bulk.S2-S4 Herein, the frequency difference of about 25 cm−1
implies that both MoS2 NFs and QDs contain a few layers (more than 3 layers). Hence, the
MoS2 QDs are approximately spherical. The effective mass approximation theory assumes the
particle to be spherical and that the motion of electron and hole may be described in terms of
their effective masses.S5 It cannot provide quantitative results for a layered structure, but instead
qualitative information about the size-dependent properties.S5
4
In te n s ity (a .u .)
(a)
Mo
Mo
Mo
4+
4+
3 d 5 /2
3 d 3 /2
S 2s
6+
236
NF
232
228
224
220
B in d in g e n e rg y (e V )
In te n s ity (a .u .)
(b)
240
Mo
4+
3 d 3 /2
QD
(blue)
6+
Mo
3 d 3 /2
6+
Mo
3 d 5 /2
238
236
Mo
234
4+
232
3 d 5 /2
230
228
B in d in g e n e rg y (e V )
In te n s ity (a .u .)
(c)
240
Mo
4+
3 d 3 /2
QD
(no color)
6+
Mo
3 d 3 /2
238
6+
Mo
3 d 5 /2
236
234
4+
Mo
232
3 d 5 /2
230
228
B in d in g e n e rg y (e V )
Figure S4. Mo 3d and S 2s core level XPS spectra obtained from (a) MoS2 NF, (b) blue QDs,
and (c) and colorless QDs.
The Mo 3d XPS spectrum of NFs in Fig. S4a reveals a doublet at 229.10 and 232.25 eV
corresponding to Mo4+ 3d5/2 and Mo4+ 3d3/2, an S 2s peak at 232.25 eV, and a weak peak of Mo6+
at ∼226.4 eV.S6, S7 Figs. S4b and S4c display the Mo 3d XPS spectra acquired from the blue and
colorless QDs, respectively. The two spectra both show two doublets of [(Mo4+ 3d5/2 and 3d3/2)
5
and (Mo6+ 3d5/2 and 3d3/2)]. XPS indicates that cutting the MoS2 alters the valence states of Mo
and the dangling bonds reconstruct to polysulfide or react with H2O and ethanol during the
ultrasonic treatment.S6,
S7
The Mo atoms on the QD are partially oxidized but there is no
essential difference in the valence states between the two types of QDs indicating that the surface
states do not alter significantly during storage.
(a)a
4 0 0 .0 k
Q D (b lu e )
In te n s ity (a .u .)
3 5 0 .0 k
O 1s
3 0 0 .0 k
2 5 0 .0 k
2 0 0 .0 k
M o 3d
C 1s
M o 3p
S 2p
1 5 0 .0 k
1 0 0 .0 k
5 0 .0 k
(b)
b
0 .0
1200
1000
800
600
400
200
0
B in d in g e n e rg y (e V )
3 0 0 .0 k
Q D (n o c o lo r)
In te n s ity (a .u .)
2 5 0 .0 k
O 1s
2 0 0 .0 k
1 5 0 .0 k
1 0 0 .0 k
C 1s
M o 3p
5 0 .0 k
0 .0
1200
1000
800
600
400
M o 3d
S 2p
200
0
B in d in g e n e rg y (e V )
Figure S5. Survey XPS data: (a) blue MoS2 QDs and (b) colorless MoS2 QDs.
6
Calculation of surface free energy
Figure S6. A large MoS2 slab (9.48×20×12.294 Å3) containing three unit cells in the x direction,
four in the y direction, and two layers along the z axis with the yellow and brown balls
representing S and Mo atoms, respectively.
The CASTEP package with default convergence tolerances of 1.0×10-5 eV for energy and 0.03
eV/Å for maximum displacement is used.
As shown in Figure S5, the large slab
(9.48×20×12.294 Å3) contains three unit cells in the x direction, four in the y direction, and two
layers along the z axis. The two upper rows are allowed to relax while the two lower ones are
fixed at the bulk geometry in order to simulate the bulk constraints. The calculation which is
performed at the Γ point with a cutoff energy of 400 eV is suitable for the prediction of the
electronic and structural properties of the MoS2 perfect surface.S8 The surface free energy can be
calculated by:S9
Esurf = [Eslab − n×Ebulk]/2A,
7
where Eslab is the energy of the slab containing n MoS2 units, Ebulk is the unit cell bulk energy,
and A is the area of the surface unit cell. Afterwards, the optimized geometrical structure is
employed to calculate the heat capacity of MoS2.
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