Large-Area Atomically Thin MoS2 Nanosheets Prepared Using ...
Supporting Information
Large-Area Atomically Thin MoS2 Nanosheets Prepared Using Electrochemical Exfoliation Na Liu,† Paul Kim,† Ji Heon Kim,† Jun Ho Ye,† Sunkook Kim,‡ Cheol Jin Lee†,*
†
‡
School of Electrical Engineering, Korea University, Seoul 136-713, Republic of Korea
Department of Electronics and Radio Engineering, Kyung Hee University, Gyeonggi 446-701, Republic of Korea *
Address correspondence to [email protected]
S1
Table S1. Summary of electrolytes tested in exfoliation experiments: concentrations of solutions, intensity of working bias, and key results. Electrolyte
Concentration
Working bias
Experimental phenomena and results
(NH4)6Mo7O24
0.05 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
CH3COONH4
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
+20 V
Large amount of gas bubbles generated and some MoS2 flakes were exfoliated.
NaOH
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
HCl
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
Large amount of gas bubbles generated. Some thin MoS2 flakes were exfoliated, but inefficiently.
+20, +25, and +30
Large amount of gas bubbles generated and some thick MoS2
V
flakes were exfoliated.
NaCl
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
H2SO4
0.1 M
From +10 to +30 V
Large amount of gas bubbles generated, but no observable exfoliation.
0.5 M
–10, –15 and –20
Large amount of gas bubbles generated. Some small MoS2
V
flakes were exfoliated, but inefficiently.
+5 V
Large amount of gas bubbles generated, but no observable exfoliation.
+10 V
Large amount of gas bubbles generated and many thin MoS2 flakes were exfoliated.
+15 and +20 V
Large amount of gas bubbles generated. Many big and thick MoS2 flakes were exfoliated.
2.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
S2
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
Na2SO4
0.1 M
From +10 to +30 V
Large amount of gas bubbles generated, but no observable exfoliation.
0.5 M
+5 V
Large amount of gas bubbles generated, but no observable exfoliation.
+10 V
Large amount of gas bubbles generated and many thin MoS2 flakes were exfoliated (applied in this work).
+15 and +20 V
Large amount of gas bubbles generated. Many big and thick MoS2 flakes were exfoliated.
2.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
The chemical reactions at the anode proceeded as follows. (1)
Oxidation of water produced hydroxyl and oxygen radicals:
H O ∙ OH + H ∙ O + H (2)
Oxidation of water released oxygen: 2H O → 4H + 4e + O ↑
S3
1.6
Absorbance (a.u.)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
900
Wavelength (nm) Figure S1. UV-visible absorption spectra of MoS2 nanosheets dispersed in NMP solution indicate the repeatability of electrochemical exfoliation of MoS2. All of the spectra show two excitonic peaks around 615 and 675 nm, which suggest the existence of MoS2 nanosheets. The concentrations of the MoS2-nanosheet dispersions ranged from 0.007 to 0.014 mg·mL-1.
S4
(a)
Mo3d5/2
highly oxidized MoS2
Intensity (a.u.)
lowly oxidized MoS2 Mo3d3/2
S2p3/2 S2p1/2 S2s
x2
238
234
230
226 168
164
160
Binding energy (eV)
Figure S2. (a) X-ray photoelectron spectroscopy (XPS) characterization, (b, d) scanning electron microscopy (SEM) images, and (c, e) energy-dispersive X-ray spectroscopy (EDS) spectra of the surface of bulk MoS2 before and after experimental optimization. The extremely weak XPS peaks [blue line in (a)] and the eroded surface of bulk MoS2 crystal observed in (b) offer S5
evidence of a high degree of oxidation. The atomic percentage of oxygen, shown in (c), is about 30.38 at.%. Through experimental optimization, the red line in (a) shows typical Mo 3d and S 2p XPS peaks. In addition, one can clearly observe in (d) that the surface of bulk MoS2 crystal is very smooth. The EDS result shown in (e) also indicates that there is barely any oxygen.
S6
(b) 33
Count
30
20
17
10 3
2
1
2
20-30
30-40
40-50
50-60
0 1-10
10-20
Lateral size (µm)
Figure S3. (a) Optical microscope images of electrochemically exfoliated MoS2 nanosheets deposited on SiO2 substrate. (b) Statistical analysis of the lateral size of MoS2 nanosheets.
S7
(a)
(b) A1g
Intensity (a.u.)
1L 2L 3L 4L bulk
PL intensity (a.u.)
1
E 2g
4L 3L 2L 1L bulk
360
380
400
420 -1
Raman shift (cm )
440
550
600
650
700
750
800
Wavelength (nm)
Figure S4. (a) Raman and (b) photoluminescence (PL) spectra of monolayer and few-layer MoS2 nanosheets as well as bulk MoS2 obtained from electrochemical exfoliation (excitation laser: 514.5 nm). The two dashed lines in (a) indicate the positions of the E12g and A1g peaks, respectively, for bulk MoS2. All of the PL spectra were normalized using the intensity of A1g Raman peaks.
S8
Figure S5. (a, d, g) Atomic force microscopy (AFM) topographies, (b, e, h) optical micrographs, and (c, f, i) Raman spectra (excitation laser: 514.5 nm) of electrochemically exfoliated MoS2 nanosheets. The colored spots in (b), (e), and (h) indicate the positions at which the Raman spectra were obtained.
S9
25
Counts
20 15 10 5 0 1
2
3
4
5
>5
Number of layers per sheet
Figure S6. Histogram showing the distribution of number of layers per sheet among 100 randomly selected MoS2 nanosheets. About 7% of these MoS2 nanosheets are monolayer and more than 70% of them comprised 2–5 layers.
S10
4+
3d
S 2p
5+
3d
S 2s
6+
3d
Mo Mo
Intensity (a.u.)
Mo
(c)
(b)
(a)
238
234
230
226 168
164
160
Binding energy (eV) Figure S7. XPS characterizations of MoS2 nanosheets (a) as-prepared and after annealing at (b) 200 °C and (c) 350 °C in N2 for 2 h.
Table S2. Atomic percentage of Mo4+, Mo5+, and Mo6+; ratio of population of Mo5+ and Mo6+ in as-prepared MoS2 nanosheets and MoS2 nanosheets that were annealed at 200 °C and 350 °C. MoS2
Mo4+ 3d
Mo5+ 3d
Mo6+ 3d
Mo5+ 3d:Mo6+ 3d
As-prepared
84.4 at.%
9.1 at.%
6.5 at.%
1:0.714
Annealed at 200 °C
84.1 at.%
9.8 at.%
6.1 at.%
1:0.622
Annealed at 350 °C
84.6 at.%
10.7 at.%
4.7 at.%
1:0.439 S11
Figure S8. (a) AFM topography and (b) spatial map of the difference between the Raman frequencies of the E12g and A1g modes (excitation laser: 523 nm) of the channel region of the MoS2 FET mentioned in Figure 6.
S12
Large-Area Atomically Thin MoS2 Nanosheets Prepared Using Electrochemical Exfoliation Na Liu,† Paul Kim,† Ji Heon Kim,† Jun Ho Ye,† Sunkook Kim,‡ Cheol Jin Lee†,*
†
‡
School of Electrical Engineering, Korea University, Seoul 136-713, Republic of Korea
Department of Electronics and Radio Engineering, Kyung Hee University, Gyeonggi 446-701, Republic of Korea *
Address correspondence to [email protected]
S1
Table S1. Summary of electrolytes tested in exfoliation experiments: concentrations of solutions, intensity of working bias, and key results. Electrolyte
Concentration
Working bias
Experimental phenomena and results
(NH4)6Mo7O24
0.05 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
CH3COONH4
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
+20 V
Large amount of gas bubbles generated and some MoS2 flakes were exfoliated.
NaOH
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
HCl
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
Large amount of gas bubbles generated. Some thin MoS2 flakes were exfoliated, but inefficiently.
+20, +25, and +30
Large amount of gas bubbles generated and some thick MoS2
V
flakes were exfoliated.
NaCl
0.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
H2SO4
0.1 M
From +10 to +30 V
Large amount of gas bubbles generated, but no observable exfoliation.
0.5 M
–10, –15 and –20
Large amount of gas bubbles generated. Some small MoS2
V
flakes were exfoliated, but inefficiently.
+5 V
Large amount of gas bubbles generated, but no observable exfoliation.
+10 V
Large amount of gas bubbles generated and many thin MoS2 flakes were exfoliated.
+15 and +20 V
Large amount of gas bubbles generated. Many big and thick MoS2 flakes were exfoliated.
2.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
S2
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
Na2SO4
0.1 M
From +10 to +30 V
Large amount of gas bubbles generated, but no observable exfoliation.
0.5 M
+5 V
Large amount of gas bubbles generated, but no observable exfoliation.
+10 V
Large amount of gas bubbles generated and many thin MoS2 flakes were exfoliated (applied in this work).
+15 and +20 V
Large amount of gas bubbles generated. Many big and thick MoS2 flakes were exfoliated.
2.5 M
+10 V
A few gas bubbles generated, but no observable exfoliation.
+15 V
A few gas bubbles generated. Some MoS2 flakes were exfoliated, but inefficiently.
The chemical reactions at the anode proceeded as follows. (1)
Oxidation of water produced hydroxyl and oxygen radicals:
H O ∙ OH + H ∙ O + H (2)
Oxidation of water released oxygen: 2H O → 4H + 4e + O ↑
S3
1.6
Absorbance (a.u.)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
900
Wavelength (nm) Figure S1. UV-visible absorption spectra of MoS2 nanosheets dispersed in NMP solution indicate the repeatability of electrochemical exfoliation of MoS2. All of the spectra show two excitonic peaks around 615 and 675 nm, which suggest the existence of MoS2 nanosheets. The concentrations of the MoS2-nanosheet dispersions ranged from 0.007 to 0.014 mg·mL-1.
S4
(a)
Mo3d5/2
highly oxidized MoS2
Intensity (a.u.)
lowly oxidized MoS2 Mo3d3/2
S2p3/2 S2p1/2 S2s
x2
238
234
230
226 168
164
160
Binding energy (eV)
Figure S2. (a) X-ray photoelectron spectroscopy (XPS) characterization, (b, d) scanning electron microscopy (SEM) images, and (c, e) energy-dispersive X-ray spectroscopy (EDS) spectra of the surface of bulk MoS2 before and after experimental optimization. The extremely weak XPS peaks [blue line in (a)] and the eroded surface of bulk MoS2 crystal observed in (b) offer S5
evidence of a high degree of oxidation. The atomic percentage of oxygen, shown in (c), is about 30.38 at.%. Through experimental optimization, the red line in (a) shows typical Mo 3d and S 2p XPS peaks. In addition, one can clearly observe in (d) that the surface of bulk MoS2 crystal is very smooth. The EDS result shown in (e) also indicates that there is barely any oxygen.
S6
(b) 33
Count
30
20
17
10 3
2
1
2
20-30
30-40
40-50
50-60
0 1-10
10-20
Lateral size (µm)
Figure S3. (a) Optical microscope images of electrochemically exfoliated MoS2 nanosheets deposited on SiO2 substrate. (b) Statistical analysis of the lateral size of MoS2 nanosheets.
S7
(a)
(b) A1g
Intensity (a.u.)
1L 2L 3L 4L bulk
PL intensity (a.u.)
1
E 2g
4L 3L 2L 1L bulk
360
380
400
420 -1
Raman shift (cm )
440
550
600
650
700
750
800
Wavelength (nm)
Figure S4. (a) Raman and (b) photoluminescence (PL) spectra of monolayer and few-layer MoS2 nanosheets as well as bulk MoS2 obtained from electrochemical exfoliation (excitation laser: 514.5 nm). The two dashed lines in (a) indicate the positions of the E12g and A1g peaks, respectively, for bulk MoS2. All of the PL spectra were normalized using the intensity of A1g Raman peaks.
S8
Figure S5. (a, d, g) Atomic force microscopy (AFM) topographies, (b, e, h) optical micrographs, and (c, f, i) Raman spectra (excitation laser: 514.5 nm) of electrochemically exfoliated MoS2 nanosheets. The colored spots in (b), (e), and (h) indicate the positions at which the Raman spectra were obtained.
S9
25
Counts
20 15 10 5 0 1
2
3
4
5
>5
Number of layers per sheet
Figure S6. Histogram showing the distribution of number of layers per sheet among 100 randomly selected MoS2 nanosheets. About 7% of these MoS2 nanosheets are monolayer and more than 70% of them comprised 2–5 layers.
S10
4+
3d
S 2p
5+
3d
S 2s
6+
3d
Mo Mo
Intensity (a.u.)
Mo
(c)
(b)
(a)
238
234
230
226 168
164
160
Binding energy (eV) Figure S7. XPS characterizations of MoS2 nanosheets (a) as-prepared and after annealing at (b) 200 °C and (c) 350 °C in N2 for 2 h.
Table S2. Atomic percentage of Mo4+, Mo5+, and Mo6+; ratio of population of Mo5+ and Mo6+ in as-prepared MoS2 nanosheets and MoS2 nanosheets that were annealed at 200 °C and 350 °C. MoS2
Mo4+ 3d
Mo5+ 3d
Mo6+ 3d
Mo5+ 3d:Mo6+ 3d
As-prepared
84.4 at.%
9.1 at.%
6.5 at.%
1:0.714
Annealed at 200 °C
84.1 at.%
9.8 at.%
6.1 at.%
1:0.622
Annealed at 350 °C
84.6 at.%
10.7 at.%
4.7 at.%
1:0.439 S11
Figure S8. (a) AFM topography and (b) spatial map of the difference between the Raman frequencies of the E12g and A1g modes (excitation laser: 523 nm) of the channel region of the MoS2 FET mentioned in Figure 6.
S12
