Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures
Supporting Information
Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures Kevin Bogaert1, 2, Song Liu1, Jordan Chesin2, Denis Titow2, 3, Silvija Gradečak2, Slaven Garaj1, 4, 5* 1
Centre for Advanced 2D Materials and Graphene Research Centre, National
University of Singapore, 6 Science Drive 2, Singapore, 117546 2
Department of Materials Science and Engineering, Massachusetts Institute of
Technology, Cambridge, MA 02139 3
Department of Biology and Chemistry, Justus-Liebig-Univesity Giessen,
Heinrich-Buff-Ring 17-19,35392 Giessen, Germany 4
Department of Physics, National University of Singapore, 2 Science Drive 3,
Singapore, 117542 5
Department of Bioengineering, National University of Singapore, 9 Engineering
Drive 1, Singapore, 117575
Corresponding Authors *Email: [email protected]
1. Chemical vapor deposition growth of lateral heterostructures The schematic of the chemical vapor deposition (CVD) growth setup used in this work is shown in Figure S1. MoS2/WS2 heterostructures were grown in two subsequent CVD steps at ambient pressure using Ar as a carrier gas. A bare Si wafer substrate with 300 nm thick SiO2 layer was suspended above an alumina boat containing the metal precursor powder at the center of the CVD furnace. Sulfur powder (Sigma-Aldrich, 99.5%), the chalcogen precursor, was placed in another alumina boat and was initially outside of the furnace, at the upstream end of the
S-1
1-inch quartz tube. Once the furnace reached the growth temperature, the sulfur was introduced to the upstream of the furnace using magnets to reach approximately 200°C. In the first growth step, WS2 crystals were grown at 1100°C for 60 min using tungsten trioxide (WO3, Sigma-Aldrich, 99.9%) as the metal precursor with an Ar flow rate of 200 sccm. In the second growth step, the metal precursor was replaced with molybdenum trioxide (MoO3, Sigma-Aldrich, 99.5%), the growth temperature was reduced to 650-710°C and Ar flow rate to 25 sccm. Different growth temperatures and times were used in the second MoS2 growth, as summarized in Table S1.
Figure S1. Schematic of the CVD growth setup.
Table S1. Summary of the growth conditions used in this work. 1st WS2 growth
2nd MoS2 growth
Temperature
Growth time
Ar flow
Temperature
Growth time
Ar flow
(°C)
(min)
(sccm)
(°C)
(min)
(sccm)
650
15
680
30
710
30
Fig. 2 and 3 Fig. 4 and 5
1100
60
Fig. S4 and S8
200
25
2. Characterization and data analysis Optical images were recorded using a Leica DFC 450 C microscope. Atomic force microscopy (AFM) images were recorded with a Bruker Dimension Fastscan AFM
S-2
using a tapping mode. Raman and photoluminescence (PL) measurements were performed on a WITec Alpha 300 R confocal Raman microscope under ambient conditions at room temperature. A 532 nm laser was focused onto the sample using a 100× objective resulting in a spot size of ~1 µm in diameter. 1800 and 600 lines/mm grating were used in Raman and PL measurements, respectively. Scanning electron microscopy was carried out in a Helios Nanolab 600 Dual Beam Focused Ion Beam Milling System. Scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS) analysis was carried out in a JEOL 2010F field emission transmission electron microscope (TEM).
For the preparation of TEM samples, we used a poly(methyl methacrylate) (PMMA)-based wet transfer method.S1 The growth samples containing 2D crystals were spin-coated at 3000 rpm with PMMA (4 wt%, 950K molecular weight, in anisole) for 1 min followed by annealing at 180 °C for 2 min. The spin-coated samples were then placed in a potassium hydroxide solution (2M) for 2-3 h at 70 °C. Once the tape/PMMA/crystals released from the Si-substrate by etching of SiO2 layer, the tape was dipped carefully in deionized water to remove KOH residue and air-dried. The PMMA/crystal film was transferred onto a 400-mesh holey-carbon copper-grid (“TED Pella, INC”, No. 609). The aligned grid was placed directly on a hot plate at 105 °C for 10 min to ensure adhesion. To fully remove PMMA, the grid was placed in acetic acid for 24 h and rinsed with methanol.
Figure 3g and 4e of the main text were generated using a home-made MATLAB code. The expected wave numbers of the MoS2 and WS2 E and A modes were provided as inputs. For each hyperspectral pixel of the image, Gaussian curves were fitted to the data in a 10 cm-1 range centered at these expected values. The integral of the peak was approximated by calculating the height and width of the Gaussian curve. The intensity of the color assigned to each peak is equal to the corresponding peak integral at that pixel following normalization.
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Figure 5b of the main text was generated by a similar home-made MATLAB code that identifies a Gaussian curve associated with the A exciton in the spectrum of each pixel and determines the energy of the peak center. The Gaussian curve is fitted to all data with an intensity value greater than half maximum. These energies were normalized and mapped such that the lowest calculated PL emission energy is shown in black and the highest energy is shown in white. Figure S6 maps the integrated intensity values from the Gaussian curves used to generate Figure 5b.
3. Stage I WS2 crystal characterization Figures S2a and S3 show representative Stage I WS2 crystals. In Figure S2b, Raman spectra from different crystal locations show similar features, indicating that this crystal is uniform. Figure S3 maps the Raman intensity ratio of the LA(M) vibrational mode with respect to the A vibration mode. This ratio scales with the inverse square of the average inter-defect distanceS2; as such, a lower intensity ratio corresponds to a larger inter-defect distance and therefore fewer defects. Our WS2 crystals have a median intensity ratio of 0.138 (top) and 0.112 (bottom). These compare favorably to values in literature of 0.14 and 0.12 for crystals grown by atmospheric pressure CVD, 0.20 for crystals grown by low pressure CVD, and 0.18 and 0.38 for exfoliated crystals.S2
Figure S2. (a) Optical image of a WS2 crystal grown at 1100 °C. Scale bar is 5 µm. (b) Raman spectra collected in the points marked in Figure S2a. The dashed lines show the wavenumbers of the A and E modes of WS2.
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Figure S3. (a) Raman maps showing the intensity ratio between LA(M) and A vibrational modes (174 and 417 cm-1, respectively) from two representative Stage I WS2 crystals. (b) PL emission peak maps of the same two crystals. Scale bar is 5 µm.
4. Heterostructure growth results Figure S4 shows a single crystal at Stage I (pure WS2) and Stage III (alloy). The size and thickness of the crystal do not change. The MoS2 Raman signal was detectable after the second growth step, which suggests that the Mo atoms diffuse from the edge to center and replace some of the W atoms, described as Stage III-H in Figure 1 of the main text. This edge-to-center directionality is shown again in Figure S8, where growth was terminated at Stage II and the MoS2 is found exclusively at the crystal edge. Figures S5 and S6 show TEM-EDS data from Stage III-L and Stage III-H crystals, respectively. In Figure S5, the ring spectrum shows strong W and S peaks and no Mo. We note that because the 2.29 keV Mo Lα line overlaps with the 2.31 keV S Kα line, the presence or absence of Mo was monitored by observing the intensity of the 17.44 keV Mo Kα line. The core spectrum shows strong Mo and S and a weak W signal (4 at. %). In Figure S6, both spectra show W, Mo, and S peaks, indicating that both regions are alloys. An EDS map could not be obtained for the alloyed crystal in Figure S7 due to low signal counts. The Cu peaks originate from the TEM grid and the K signal in Figure S5 is residual from the KOH solution used to etch the SiO2 substrate during the transfer procedure.
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Figure S4. Optical image of a WS2 crystal (a) and Raman intensity maps at the wave numbers corresponding to the E modes of WS2 (b) and MoS2 (c). Optical image of the same crystal after the subsequent 710°C MoS2 growth (d) and Raman intensity maps at the wave numbers corresponding to the E modes of WS2 (e) and MoS2 (f). AFM cross-sectional contour shows the height of the crystal to be 2.5 nm after the first and second growth. Scale bar is 5 µm.
Figure S5. (a) EDS map of a Stage III-L crystal. Yellow and purple correspond to W and Mo, respectively. Colored circles correspond to the locations of EDS scans. Scale bar is 5 µm. (b) EDS spectra of the ring (blue) and core (green) of a Stage III-L phase segregated crystal.
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Figure S6. (a) AFM height image and (b) PL intensity map of the crystal shown in Figures 4 and 5. Scale bar is 5 µm.
Figure S7. (a) Dark-field STEM image of a Stage III-H crystal, outlined with a dashed line. Colored circles correspond to the locations of EDS scans. Scale bar is 5 µm. (b) EDS spectra of the W-rich Stage III-H crystal edge (blue) and the Mo-rich crystal interior (green).
Figure S8. (a) Optical image of a lateral heterostructure after the second MoS2 growth at 710°C. Raman intensity maps of the E mode of MoS2 (b) and WS2 (c). MoS2 is
S-7
initially deposited at the activated WS2 edge area, described as Stage II in Figure 1 of the main text. Scale bar is 5 µm.
References: (1)
Her, M.; Beams, R.; Novotny, L. Phys. Lett. A 2013, 377, 1455-1458.
(2)
McCreary, A.; Berkdemir, A.; Wang, J.; Nguyen, M. A.; Elías, A. L.; Perea-López, N.; Fujisawa, K.; Kabius, B.; Carozo, V.; Cullen, D. A.; Mallouk, T. E.; Zhu, J.; Terrones, M. J. Mater. Res. 2016, 31, 931-944.
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Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures Kevin Bogaert1, 2, Song Liu1, Jordan Chesin2, Denis Titow2, 3, Silvija Gradečak2, Slaven Garaj1, 4, 5* 1
Centre for Advanced 2D Materials and Graphene Research Centre, National
University of Singapore, 6 Science Drive 2, Singapore, 117546 2
Department of Materials Science and Engineering, Massachusetts Institute of
Technology, Cambridge, MA 02139 3
Department of Biology and Chemistry, Justus-Liebig-Univesity Giessen,
Heinrich-Buff-Ring 17-19,35392 Giessen, Germany 4
Department of Physics, National University of Singapore, 2 Science Drive 3,
Singapore, 117542 5
Department of Bioengineering, National University of Singapore, 9 Engineering
Drive 1, Singapore, 117575
Corresponding Authors *Email: [email protected]
1. Chemical vapor deposition growth of lateral heterostructures The schematic of the chemical vapor deposition (CVD) growth setup used in this work is shown in Figure S1. MoS2/WS2 heterostructures were grown in two subsequent CVD steps at ambient pressure using Ar as a carrier gas. A bare Si wafer substrate with 300 nm thick SiO2 layer was suspended above an alumina boat containing the metal precursor powder at the center of the CVD furnace. Sulfur powder (Sigma-Aldrich, 99.5%), the chalcogen precursor, was placed in another alumina boat and was initially outside of the furnace, at the upstream end of the
S-1
1-inch quartz tube. Once the furnace reached the growth temperature, the sulfur was introduced to the upstream of the furnace using magnets to reach approximately 200°C. In the first growth step, WS2 crystals were grown at 1100°C for 60 min using tungsten trioxide (WO3, Sigma-Aldrich, 99.9%) as the metal precursor with an Ar flow rate of 200 sccm. In the second growth step, the metal precursor was replaced with molybdenum trioxide (MoO3, Sigma-Aldrich, 99.5%), the growth temperature was reduced to 650-710°C and Ar flow rate to 25 sccm. Different growth temperatures and times were used in the second MoS2 growth, as summarized in Table S1.
Figure S1. Schematic of the CVD growth setup.
Table S1. Summary of the growth conditions used in this work. 1st WS2 growth
2nd MoS2 growth
Temperature
Growth time
Ar flow
Temperature
Growth time
Ar flow
(°C)
(min)
(sccm)
(°C)
(min)
(sccm)
650
15
680
30
710
30
Fig. 2 and 3 Fig. 4 and 5
1100
60
Fig. S4 and S8
200
25
2. Characterization and data analysis Optical images were recorded using a Leica DFC 450 C microscope. Atomic force microscopy (AFM) images were recorded with a Bruker Dimension Fastscan AFM
S-2
using a tapping mode. Raman and photoluminescence (PL) measurements were performed on a WITec Alpha 300 R confocal Raman microscope under ambient conditions at room temperature. A 532 nm laser was focused onto the sample using a 100× objective resulting in a spot size of ~1 µm in diameter. 1800 and 600 lines/mm grating were used in Raman and PL measurements, respectively. Scanning electron microscopy was carried out in a Helios Nanolab 600 Dual Beam Focused Ion Beam Milling System. Scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS) analysis was carried out in a JEOL 2010F field emission transmission electron microscope (TEM).
For the preparation of TEM samples, we used a poly(methyl methacrylate) (PMMA)-based wet transfer method.S1 The growth samples containing 2D crystals were spin-coated at 3000 rpm with PMMA (4 wt%, 950K molecular weight, in anisole) for 1 min followed by annealing at 180 °C for 2 min. The spin-coated samples were then placed in a potassium hydroxide solution (2M) for 2-3 h at 70 °C. Once the tape/PMMA/crystals released from the Si-substrate by etching of SiO2 layer, the tape was dipped carefully in deionized water to remove KOH residue and air-dried. The PMMA/crystal film was transferred onto a 400-mesh holey-carbon copper-grid (“TED Pella, INC”, No. 609). The aligned grid was placed directly on a hot plate at 105 °C for 10 min to ensure adhesion. To fully remove PMMA, the grid was placed in acetic acid for 24 h and rinsed with methanol.
Figure 3g and 4e of the main text were generated using a home-made MATLAB code. The expected wave numbers of the MoS2 and WS2 E and A modes were provided as inputs. For each hyperspectral pixel of the image, Gaussian curves were fitted to the data in a 10 cm-1 range centered at these expected values. The integral of the peak was approximated by calculating the height and width of the Gaussian curve. The intensity of the color assigned to each peak is equal to the corresponding peak integral at that pixel following normalization.
S-3
Figure 5b of the main text was generated by a similar home-made MATLAB code that identifies a Gaussian curve associated with the A exciton in the spectrum of each pixel and determines the energy of the peak center. The Gaussian curve is fitted to all data with an intensity value greater than half maximum. These energies were normalized and mapped such that the lowest calculated PL emission energy is shown in black and the highest energy is shown in white. Figure S6 maps the integrated intensity values from the Gaussian curves used to generate Figure 5b.
3. Stage I WS2 crystal characterization Figures S2a and S3 show representative Stage I WS2 crystals. In Figure S2b, Raman spectra from different crystal locations show similar features, indicating that this crystal is uniform. Figure S3 maps the Raman intensity ratio of the LA(M) vibrational mode with respect to the A vibration mode. This ratio scales with the inverse square of the average inter-defect distanceS2; as such, a lower intensity ratio corresponds to a larger inter-defect distance and therefore fewer defects. Our WS2 crystals have a median intensity ratio of 0.138 (top) and 0.112 (bottom). These compare favorably to values in literature of 0.14 and 0.12 for crystals grown by atmospheric pressure CVD, 0.20 for crystals grown by low pressure CVD, and 0.18 and 0.38 for exfoliated crystals.S2
Figure S2. (a) Optical image of a WS2 crystal grown at 1100 °C. Scale bar is 5 µm. (b) Raman spectra collected in the points marked in Figure S2a. The dashed lines show the wavenumbers of the A and E modes of WS2.
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Figure S3. (a) Raman maps showing the intensity ratio between LA(M) and A vibrational modes (174 and 417 cm-1, respectively) from two representative Stage I WS2 crystals. (b) PL emission peak maps of the same two crystals. Scale bar is 5 µm.
4. Heterostructure growth results Figure S4 shows a single crystal at Stage I (pure WS2) and Stage III (alloy). The size and thickness of the crystal do not change. The MoS2 Raman signal was detectable after the second growth step, which suggests that the Mo atoms diffuse from the edge to center and replace some of the W atoms, described as Stage III-H in Figure 1 of the main text. This edge-to-center directionality is shown again in Figure S8, where growth was terminated at Stage II and the MoS2 is found exclusively at the crystal edge. Figures S5 and S6 show TEM-EDS data from Stage III-L and Stage III-H crystals, respectively. In Figure S5, the ring spectrum shows strong W and S peaks and no Mo. We note that because the 2.29 keV Mo Lα line overlaps with the 2.31 keV S Kα line, the presence or absence of Mo was monitored by observing the intensity of the 17.44 keV Mo Kα line. The core spectrum shows strong Mo and S and a weak W signal (4 at. %). In Figure S6, both spectra show W, Mo, and S peaks, indicating that both regions are alloys. An EDS map could not be obtained for the alloyed crystal in Figure S7 due to low signal counts. The Cu peaks originate from the TEM grid and the K signal in Figure S5 is residual from the KOH solution used to etch the SiO2 substrate during the transfer procedure.
S-5
Figure S4. Optical image of a WS2 crystal (a) and Raman intensity maps at the wave numbers corresponding to the E modes of WS2 (b) and MoS2 (c). Optical image of the same crystal after the subsequent 710°C MoS2 growth (d) and Raman intensity maps at the wave numbers corresponding to the E modes of WS2 (e) and MoS2 (f). AFM cross-sectional contour shows the height of the crystal to be 2.5 nm after the first and second growth. Scale bar is 5 µm.
Figure S5. (a) EDS map of a Stage III-L crystal. Yellow and purple correspond to W and Mo, respectively. Colored circles correspond to the locations of EDS scans. Scale bar is 5 µm. (b) EDS spectra of the ring (blue) and core (green) of a Stage III-L phase segregated crystal.
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Figure S6. (a) AFM height image and (b) PL intensity map of the crystal shown in Figures 4 and 5. Scale bar is 5 µm.
Figure S7. (a) Dark-field STEM image of a Stage III-H crystal, outlined with a dashed line. Colored circles correspond to the locations of EDS scans. Scale bar is 5 µm. (b) EDS spectra of the W-rich Stage III-H crystal edge (blue) and the Mo-rich crystal interior (green).
Figure S8. (a) Optical image of a lateral heterostructure after the second MoS2 growth at 710°C. Raman intensity maps of the E mode of MoS2 (b) and WS2 (c). MoS2 is
S-7
initially deposited at the activated WS2 edge area, described as Stage II in Figure 1 of the main text. Scale bar is 5 µm.
References: (1)
Her, M.; Beams, R.; Novotny, L. Phys. Lett. A 2013, 377, 1455-1458.
(2)
McCreary, A.; Berkdemir, A.; Wang, J.; Nguyen, M. A.; Elías, A. L.; Perea-López, N.; Fujisawa, K.; Kabius, B.; Carozo, V.; Cullen, D. A.; Mallouk, T. E.; Zhu, J.; Terrones, M. J. Mater. Res. 2016, 31, 931-944.
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