Carbon Nanotube-Bridged Graphene 3D Building Blocks for Ultrafast ...
SUPPORTING INFORMATION
Carbon Nanotube-Bridged Graphene 3D Building Blocks for Ultrafast Compact Supercapacitors Duy Tho Pham†,‡, Tae Hoon Lee†,‡, Dinh Hoa Luong†,‡, Fei Yao†, Arunabha Ghosh†, Viet Thong Le†,‡, Tae Hyung Kim†, Bing Li†,‡, Jian Chang†,‡ and Young Hee Lee†,‡* †
IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan Un
iversity, Suwon 440-746, Republic of Korea. ‡Department of Energy Science, Department of Phy sics, Sungkyunkwan University, Suwon 440-746, Republic of Korea. *Address correspondence to [email protected]
1
Figure S1. Schematic procedure for preparing ac-Gr/SWCNT films. -Step 1: Mix the GO solution with a CTAB-SWCNT solution. -Step 2: Stir the mixture for 1 hour at 250 rpm. -Step 3: Add a concentration-controlled KOH solution. -Step 4: Stir for 15 minutes and vacuum filter. -Step 5: Dry under vacuum at 90°C for 12 hours and thermally anneal (activation) at 800°C for 1 hour under an N2 atmosphere. -Step 6: Wash with acetic acid and DI water and dry under vacuum at 120°C for 12 hours.
2
Figure S2. Structural properties of the CTAB-SWCNTs and GO sheets. a) Zeta potential of the CTAB-SWCNTs (+55.9 mV) and b) GO sheets (-53.4 mV) measured at pH=7. c) Absorption spectrum for SWCNTs dispersed in water (0.3 mg ml-1) using CTAB (0.1%) as a surfactant. The inset shows the fluorescence spectrum using a laser wavelength of 532 nm. The distinct sub-band features indicate the individual SWCNT dispersion. d) The SWCNT length distribution. The broad peak ranging from ~50 nm to ~3.5 μm indicates the corresponding SWCNT lengths. The very sharp peak at ~1 nm indicates the SWCNT diameter. e) AFM image of GO sheets spincasted onto a SiO2 substrate. The inset shows the height profile of the GO sheet. The lateral GO sheet size ranged from ~500 nm to ~1 μm with a thickness of ~1 nm.
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Table S1. Zeta potentials and concentrations of the GO and CTAB-SWCNT solutions. Samples
pH Zeta Potential (mV) Concentration (mg ml-1)
GO
4
-51.4
0.5
GO
7
-53.4
0.5
CTAB-SWCNT
4
61.0
0.3
CTAB-SWCNT
7
55.9
0.3
4
Figure S3. TEM images of the ac-Gr/SWCNT film. a), b) Low-resolution images. c) Highresolution one. The dotted lines indicate the nanoscale pores which were formed via KOH activation. The white arrows indicate the graphene layers.
5
Figure S4. Characterization of another ac-Gr/SWCNT film. (GO/SWCNT mass ratio of 3/1 and 8M KOH). a) Top-view SEM image. b) EDS image taken from a). The inset shows the elemental analysis. A small amount of oxygen (4.5 wt %) and minor impurities including potassium, were detected. c) Cross-sectional SEM image showing the vertical SWCNT structure and d) a higher magnification image of the part outlined by the white box in c).
6
Figure S5. Schematic for the polarized Raman measurements. The inset shows the optical and SEM images of the sample cross-section.
7
Figure S6. Characterization of the SDS-SWCNT/Gr film. a) Cross-sectional SEM image. b) polarized Raman spectra. c) XRD pattern. SWCNTs were randomly disordered on the GO planar surfaces to reveal dangling SWCNTs at the edge, similarly to the CTAB-SWCNT/Gr sample. The G-band intensities in (b) were not significantly changed by the polarized angles, in contrast to the CTAB-SWCNTs on the graphene layers (Figure 2e). A large (002) peak near 23° in (c) indicates the graphene restacking occurred during the process reduction. These results demonstrate that the SDS-SWCNTs were not vertically aligned and ineffectively intercalated into the graphene layers, which was likely due to the similarity of the negatively charged head groups of SDS to those of the GO sheets. 8
The SDS-SWCNT/Gr film has a mass density of 1.48 g cm-3, which was obtained with an average thickness of 5.60 μm and measured area density of 0.83 mg cm-2.
Figure S7. Raman spectra of the Gr/SWCNT, ac-Gr/SWCNT, GO and pure SWCNTs. The D/G band intensities were calculated as 0.79, 0.16, 0.10 and 0.13 for GO, pure SWCNT, Gr/SWCNT and ac-Gr/SWCNT, respectively. The D-band intensities were significantly reduced after annealing during activation. This process resulted in high electrical conductivity of the acGr/SWCNT and Gr/SWCNT films. 9
Figure S8. N2 adsorption/desorption analysis of the optimized ac-Gr/SWCNT and Gr/SWCNT samples. (a) Isotherm of the samples at 77 K. (b) The micropore and mesopore size distributions were estimated using the Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) methods, respectively.
Table S2. Pore properties of the Gr/SWCNT and optimized ac-Gr/SWCNT samples. All of the values were collected from the BET measurements.
Samples
Gr/SWCNT
Total BET
αs-plot
Micropore
External
Micropore
Mesopore
Total pore
surface
surface
surface
surface
volume
volume
volume
-1
-1
(mL g )
(mL g )
(mL g-1)
2
0.19
0.05
0.24
137
0.25
0.23
0.48
area
area
area
Area
(m2 g-1)
(m2 g-1)
(m2 g-1)
(m2 g-1)
471
455
453
652
565
428
acGr/SWCNT
10
Figure S9. Cross-sectional SEM images of the optimized ac-Gr/SWCNT film. a), b) and c) were taken from the same sample at different locations. The mass density of the optimized ac-Gr/SWCNT film was calculated using a measured area density of 0.6 mg cm-2 and an average thickness of 5.64 μm. The measurement was repeated several times with other optimized ac-Gr/SWCNT samples, and the mass density values were relatively similar. The measured mass density was 1.06 ± 0.02 g cm-3. The optimized acGr/SWCNT film exhibited a small Rs value of 4.5 Ω of 39,400 S m-1 was achieved.
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□-1 and thus a high electrical conductivity
Figure S10. The GO/SWCNT mass ratio optimization. a) XRD patterns, b) CV curves at 200 mV s-1 and c) gravimetric Cs in terms of scan rate for ac-Gr/SWCNT films with different ratios. d) The gravimetric specific capacitance was obtained at a scan rate of 200 mV s-1 as a function of GO/SWCNT mass ratio. When the CNT concentration was reduced to 4/1, a broad (002) peak reappeared, which indicates the restacking of the graphene layers after activation. A mass ratio of 3/1 exhibited the highest specific capacitance. Here, a 6M KOH solution was used to prepare all of the samples.
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Figure S11. KOH concentration optimization. a) Nyquist plots (the inset magnifies the highfrequency region), b) imaginary capacitance versus frequency plot, c) gravimetric specific capacitance as a function of the scan rate and d) gravimetric specific capacitance taken at a scan rate of 200 mV s-1 for ac-Gr/SWCNT films prepared with different KOH concentrations. All of the semicircles, relaxation times, and specific capacitances improved as the KOH solution concentration increased to 8M. The sample was severely damaged at KOH concentrations above 8M due to violent attacks on the carbon network by a huge amount of KOH. Here, the GO/SWCNT mass ratio was fixed to 3/1 when preparing all samples.
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Figure S12. Electrochemical performance of supercapacitors based on the optimized acGr/SWCNT films. a) and b) CV curves at different scan rates. c) CD curves at different current densities. d) Capacitance retention of 94.7% over 5,000 charge/discharge cycles at a current density of 1 A g-1. The inset shows CD curves at the same current density. e) Nyquist plots before and after cycling at a current density of 1 A g-1. The inset shows a magnified view of the high-frequency region.
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Figure S13. SEM images of the ac-Gr/SWCNT film. a), b) before and c), d) after 10,000 charge/discharge cycles at a current density of 10 A g-1. a), c) The top view and b), d) the crosssection. The morphology of both samples is similar, that demonstrated the high electrochemical stability of the sample.
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Figure S14. CV curves under different potential windows at a scan rate of 5 mV s-1 of supercapacitors based on the optimized ac-Gr/SWCNT films. The neat EMIM BF4 ionic liquid (SigmaAldrich) which has the electrochemical window from 2.2 to +3.5 V was used as electrolyte. As seen above, within 4 V, the CV curves showed the nearly rectangular shapes. While, over 4 V, the current increased rapidly at high voltage, indicating electrolyte reaction or instability of electrolyte. The slight non-rectangular portion in CV and non-linear triangle in CD curves are shown even at low voltages and may originate from pseudo-capacitance contributed from oxygen-related functional groups that are present on SWCNTs and graphene surfaces. In some other works, instead of using the neat EMIM BF4 ionic liquid, this ionic liquid dissolved in some organic solvents (acetonitrile, propylene carbonate…) was used to improve the mobility
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of ions. This treatment can increase the capacitance but reduce the potential window due to the decomposed limit of organic solvents (normally less than 3.5 V).
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Table S3. Performance of the selected graphene-based supercapacitors.
Mass Materials
Type
loading (mg
Density (g cm-3)
Conductivity (S m-1)
-2
cm )
Gravimetric specific capacitance (F g-1)
Curved Graphene
powder
6.6
0.3
-
154
(ref.4)
Powder
2.5-4
0.36
500
166
1
1.25
~2500
260
10
1.25
-
209
0.036
0.048
1738
276
1.5
-
110
0.045
0.069
-
273
Powder
~4
0.4
303
231
Powder
~4.2
0.75
213
147
Powder
~10
1.15
-
154
EM-CCG
Hydrogel
(ref.5)
film
Laserscribed graphene (ref.12)
Volumetric
Measured
energy
energy
power
power
density
density
density
density
(Wh kg-1)
(Wh L-1)
(kW kg-1)
(kW kg-1)
1
85.6
25.7
1
-
5.7
70
25.2
-
250
0.1
88.2
110.3
-
~60
0.1
~68
85
-
~12.5
5
153.3
7.4
-
99
0.5
34.4
51.6
0.4
-
0.1
151.7
10.5
-
350
1
98
39.2
-
137
1.24
63
48
1
66
75
-
113
4.2
74
44
-
338
0.5
110.6
117.2
0.5
400
1
95.6
101.3
1
400
5
70.6
74.8
5.1
390
Current
Potential
density
window
(A g-1)
BMIM BF4, 4V
(ref.10) a-MEGO
Gravimetric
Electrolyte,
BMIM BF4, 3.5 V EMIM BF4, 3.5 V EIMI BF4, 3.5V
Maximum
Non freestanding,
EMIM BF4, 4V
flexible film
Chemically
Free-
bonded
standing,
0.15-
Gr/CNT
flexible
0.5
(ref.26)
film
TEABF4, 3V
Oriented graphene
Hydrogel
hydrogel
film
EMIM BF4, 4V
(ref.11) Porous 3D graphene
EMIM BF4, 3.5 V
(ref.14) Compressed a-MEGO
BMIM BF4, 3.5 V
(ref.15 ) Aligned-aMEGO
EIMI TFSI/AN,
(ref.16) as-MEGO (ref.17)
acGr/SWCNT (our work)
23
3.5 V EIMI Powder
1.3
0.59
-
174
TFSI/AN, 3.5 V
Freestanding, flexible
0.6
1.06
39401
199
EMIM BF4, 4V
film 172
127
EMIM BF4, 4V EMIM BF4, 4V
18
Figure S15. The proposed model for forming SWCNT pillars between graphene layers. Positively charged CTAB-SWCNTs easily attach to the negatively charged GO sheets via Coulombic interactions. Some may overlap when SWCNT lengths are long or the SWCNT density is high. In such cases, the SWCNTs can be bent due to the strong Coulombic repulsion between overlapping SWCNTs, which causes them to protrude from the surface. The next SWCNTs added then align parallel to the bent SWCNTs and preferentially orient as a liquid crystalline phase perpendicular to the graphene plane. This proposed model will be an open question in the future and discussed further by the community.
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Carbon Nanotube-Bridged Graphene 3D Building Blocks for Ultrafast Compact Supercapacitors Duy Tho Pham†,‡, Tae Hoon Lee†,‡, Dinh Hoa Luong†,‡, Fei Yao†, Arunabha Ghosh†, Viet Thong Le†,‡, Tae Hyung Kim†, Bing Li†,‡, Jian Chang†,‡ and Young Hee Lee†,‡* †
IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan Un
iversity, Suwon 440-746, Republic of Korea. ‡Department of Energy Science, Department of Phy sics, Sungkyunkwan University, Suwon 440-746, Republic of Korea. *Address correspondence to [email protected]
1
Figure S1. Schematic procedure for preparing ac-Gr/SWCNT films. -Step 1: Mix the GO solution with a CTAB-SWCNT solution. -Step 2: Stir the mixture for 1 hour at 250 rpm. -Step 3: Add a concentration-controlled KOH solution. -Step 4: Stir for 15 minutes and vacuum filter. -Step 5: Dry under vacuum at 90°C for 12 hours and thermally anneal (activation) at 800°C for 1 hour under an N2 atmosphere. -Step 6: Wash with acetic acid and DI water and dry under vacuum at 120°C for 12 hours.
2
Figure S2. Structural properties of the CTAB-SWCNTs and GO sheets. a) Zeta potential of the CTAB-SWCNTs (+55.9 mV) and b) GO sheets (-53.4 mV) measured at pH=7. c) Absorption spectrum for SWCNTs dispersed in water (0.3 mg ml-1) using CTAB (0.1%) as a surfactant. The inset shows the fluorescence spectrum using a laser wavelength of 532 nm. The distinct sub-band features indicate the individual SWCNT dispersion. d) The SWCNT length distribution. The broad peak ranging from ~50 nm to ~3.5 μm indicates the corresponding SWCNT lengths. The very sharp peak at ~1 nm indicates the SWCNT diameter. e) AFM image of GO sheets spincasted onto a SiO2 substrate. The inset shows the height profile of the GO sheet. The lateral GO sheet size ranged from ~500 nm to ~1 μm with a thickness of ~1 nm.
3
Table S1. Zeta potentials and concentrations of the GO and CTAB-SWCNT solutions. Samples
pH Zeta Potential (mV) Concentration (mg ml-1)
GO
4
-51.4
0.5
GO
7
-53.4
0.5
CTAB-SWCNT
4
61.0
0.3
CTAB-SWCNT
7
55.9
0.3
4
Figure S3. TEM images of the ac-Gr/SWCNT film. a), b) Low-resolution images. c) Highresolution one. The dotted lines indicate the nanoscale pores which were formed via KOH activation. The white arrows indicate the graphene layers.
5
Figure S4. Characterization of another ac-Gr/SWCNT film. (GO/SWCNT mass ratio of 3/1 and 8M KOH). a) Top-view SEM image. b) EDS image taken from a). The inset shows the elemental analysis. A small amount of oxygen (4.5 wt %) and minor impurities including potassium, were detected. c) Cross-sectional SEM image showing the vertical SWCNT structure and d) a higher magnification image of the part outlined by the white box in c).
6
Figure S5. Schematic for the polarized Raman measurements. The inset shows the optical and SEM images of the sample cross-section.
7
Figure S6. Characterization of the SDS-SWCNT/Gr film. a) Cross-sectional SEM image. b) polarized Raman spectra. c) XRD pattern. SWCNTs were randomly disordered on the GO planar surfaces to reveal dangling SWCNTs at the edge, similarly to the CTAB-SWCNT/Gr sample. The G-band intensities in (b) were not significantly changed by the polarized angles, in contrast to the CTAB-SWCNTs on the graphene layers (Figure 2e). A large (002) peak near 23° in (c) indicates the graphene restacking occurred during the process reduction. These results demonstrate that the SDS-SWCNTs were not vertically aligned and ineffectively intercalated into the graphene layers, which was likely due to the similarity of the negatively charged head groups of SDS to those of the GO sheets. 8
The SDS-SWCNT/Gr film has a mass density of 1.48 g cm-3, which was obtained with an average thickness of 5.60 μm and measured area density of 0.83 mg cm-2.
Figure S7. Raman spectra of the Gr/SWCNT, ac-Gr/SWCNT, GO and pure SWCNTs. The D/G band intensities were calculated as 0.79, 0.16, 0.10 and 0.13 for GO, pure SWCNT, Gr/SWCNT and ac-Gr/SWCNT, respectively. The D-band intensities were significantly reduced after annealing during activation. This process resulted in high electrical conductivity of the acGr/SWCNT and Gr/SWCNT films. 9
Figure S8. N2 adsorption/desorption analysis of the optimized ac-Gr/SWCNT and Gr/SWCNT samples. (a) Isotherm of the samples at 77 K. (b) The micropore and mesopore size distributions were estimated using the Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) methods, respectively.
Table S2. Pore properties of the Gr/SWCNT and optimized ac-Gr/SWCNT samples. All of the values were collected from the BET measurements.
Samples
Gr/SWCNT
Total BET
αs-plot
Micropore
External
Micropore
Mesopore
Total pore
surface
surface
surface
surface
volume
volume
volume
-1
-1
(mL g )
(mL g )
(mL g-1)
2
0.19
0.05
0.24
137
0.25
0.23
0.48
area
area
area
Area
(m2 g-1)
(m2 g-1)
(m2 g-1)
(m2 g-1)
471
455
453
652
565
428
acGr/SWCNT
10
Figure S9. Cross-sectional SEM images of the optimized ac-Gr/SWCNT film. a), b) and c) were taken from the same sample at different locations. The mass density of the optimized ac-Gr/SWCNT film was calculated using a measured area density of 0.6 mg cm-2 and an average thickness of 5.64 μm. The measurement was repeated several times with other optimized ac-Gr/SWCNT samples, and the mass density values were relatively similar. The measured mass density was 1.06 ± 0.02 g cm-3. The optimized acGr/SWCNT film exhibited a small Rs value of 4.5 Ω of 39,400 S m-1 was achieved.
11
□-1 and thus a high electrical conductivity
Figure S10. The GO/SWCNT mass ratio optimization. a) XRD patterns, b) CV curves at 200 mV s-1 and c) gravimetric Cs in terms of scan rate for ac-Gr/SWCNT films with different ratios. d) The gravimetric specific capacitance was obtained at a scan rate of 200 mV s-1 as a function of GO/SWCNT mass ratio. When the CNT concentration was reduced to 4/1, a broad (002) peak reappeared, which indicates the restacking of the graphene layers after activation. A mass ratio of 3/1 exhibited the highest specific capacitance. Here, a 6M KOH solution was used to prepare all of the samples.
12
Figure S11. KOH concentration optimization. a) Nyquist plots (the inset magnifies the highfrequency region), b) imaginary capacitance versus frequency plot, c) gravimetric specific capacitance as a function of the scan rate and d) gravimetric specific capacitance taken at a scan rate of 200 mV s-1 for ac-Gr/SWCNT films prepared with different KOH concentrations. All of the semicircles, relaxation times, and specific capacitances improved as the KOH solution concentration increased to 8M. The sample was severely damaged at KOH concentrations above 8M due to violent attacks on the carbon network by a huge amount of KOH. Here, the GO/SWCNT mass ratio was fixed to 3/1 when preparing all samples.
13
Figure S12. Electrochemical performance of supercapacitors based on the optimized acGr/SWCNT films. a) and b) CV curves at different scan rates. c) CD curves at different current densities. d) Capacitance retention of 94.7% over 5,000 charge/discharge cycles at a current density of 1 A g-1. The inset shows CD curves at the same current density. e) Nyquist plots before and after cycling at a current density of 1 A g-1. The inset shows a magnified view of the high-frequency region.
14
Figure S13. SEM images of the ac-Gr/SWCNT film. a), b) before and c), d) after 10,000 charge/discharge cycles at a current density of 10 A g-1. a), c) The top view and b), d) the crosssection. The morphology of both samples is similar, that demonstrated the high electrochemical stability of the sample.
15
Figure S14. CV curves under different potential windows at a scan rate of 5 mV s-1 of supercapacitors based on the optimized ac-Gr/SWCNT films. The neat EMIM BF4 ionic liquid (SigmaAldrich) which has the electrochemical window from 2.2 to +3.5 V was used as electrolyte. As seen above, within 4 V, the CV curves showed the nearly rectangular shapes. While, over 4 V, the current increased rapidly at high voltage, indicating electrolyte reaction or instability of electrolyte. The slight non-rectangular portion in CV and non-linear triangle in CD curves are shown even at low voltages and may originate from pseudo-capacitance contributed from oxygen-related functional groups that are present on SWCNTs and graphene surfaces. In some other works, instead of using the neat EMIM BF4 ionic liquid, this ionic liquid dissolved in some organic solvents (acetonitrile, propylene carbonate…) was used to improve the mobility
16
of ions. This treatment can increase the capacitance but reduce the potential window due to the decomposed limit of organic solvents (normally less than 3.5 V).
17
Table S3. Performance of the selected graphene-based supercapacitors.
Mass Materials
Type
loading (mg
Density (g cm-3)
Conductivity (S m-1)
-2
cm )
Gravimetric specific capacitance (F g-1)
Curved Graphene
powder
6.6
0.3
-
154
(ref.4)
Powder
2.5-4
0.36
500
166
1
1.25
~2500
260
10
1.25
-
209
0.036
0.048
1738
276
1.5
-
110
0.045
0.069
-
273
Powder
~4
0.4
303
231
Powder
~4.2
0.75
213
147
Powder
~10
1.15
-
154
EM-CCG
Hydrogel
(ref.5)
film
Laserscribed graphene (ref.12)
Volumetric
Measured
energy
energy
power
power
density
density
density
density
(Wh kg-1)
(Wh L-1)
(kW kg-1)
(kW kg-1)
1
85.6
25.7
1
-
5.7
70
25.2
-
250
0.1
88.2
110.3
-
~60
0.1
~68
85
-
~12.5
5
153.3
7.4
-
99
0.5
34.4
51.6
0.4
-
0.1
151.7
10.5
-
350
1
98
39.2
-
137
1.24
63
48
1
66
75
-
113
4.2
74
44
-
338
0.5
110.6
117.2
0.5
400
1
95.6
101.3
1
400
5
70.6
74.8
5.1
390
Current
Potential
density
window
(A g-1)
BMIM BF4, 4V
(ref.10) a-MEGO
Gravimetric
Electrolyte,
BMIM BF4, 3.5 V EMIM BF4, 3.5 V EIMI BF4, 3.5V
Maximum
Non freestanding,
EMIM BF4, 4V
flexible film
Chemically
Free-
bonded
standing,
0.15-
Gr/CNT
flexible
0.5
(ref.26)
film
TEABF4, 3V
Oriented graphene
Hydrogel
hydrogel
film
EMIM BF4, 4V
(ref.11) Porous 3D graphene
EMIM BF4, 3.5 V
(ref.14) Compressed a-MEGO
BMIM BF4, 3.5 V
(ref.15 ) Aligned-aMEGO
EIMI TFSI/AN,
(ref.16) as-MEGO (ref.17)
acGr/SWCNT (our work)
23
3.5 V EIMI Powder
1.3
0.59
-
174
TFSI/AN, 3.5 V
Freestanding, flexible
0.6
1.06
39401
199
EMIM BF4, 4V
film 172
127
EMIM BF4, 4V EMIM BF4, 4V
18
Figure S15. The proposed model for forming SWCNT pillars between graphene layers. Positively charged CTAB-SWCNTs easily attach to the negatively charged GO sheets via Coulombic interactions. Some may overlap when SWCNT lengths are long or the SWCNT density is high. In such cases, the SWCNTs can be bent due to the strong Coulombic repulsion between overlapping SWCNTs, which causes them to protrude from the surface. The next SWCNTs added then align parallel to the bent SWCNTs and preferentially orient as a liquid crystalline phase perpendicular to the graphene plane. This proposed model will be an open question in the future and discussed further by the community.
19
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