Supplementary Information Extraordinary room-temperature ...
Supplementary Information Extraordinary room-temperature photoluminescence in triangular WS2 monolayers Humberto R. Gutiérrez 1,†, Nestor Perea-López1, Ana Laura Elías1, Ayse Berkdemir1, Bei Wang 1, Ruitao Lv1, Florentino López-Urías 1#, Vincent H. Crespi1, Humberto Terrones1 and Mauricio Terrones1,2* 1
Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
2
Department of Materials Science and Engineering and The Materials Research Institute, 104 Davey Lab., The Pennsylvania State University, University Park, PA 16802, USA
† Present address: Department of Physics & Astronomy, University of Louisville, Louisville, KY 40292 USA. #On leave from Advanced Materials Department, IPICYT, Camino a Presa San Jose 2055, Col. Lomas 4a Sección, San Luis Potosí, México
1
Optical contrast of monolayered WS2 on Si/SiO2 substrates Figure S1 shows an optical image of WS2 islands on a Si substrate with a 300 nm SiO2 layer. The contrast is very clear. Similar contrast was found on Si substrates with a 285 nm SiO2 layer.
Figure S1. Optical image of WS2 monolayer islands on a Si/SiO2 wafers.
2
PL and Raman from WOx precursors Tungsten oxides could have very strong PL when excited with a high energy laser, such as 256 nm. This PL, however, is observed for a wavelength range between 300-450 nm, which is very different from the PL peaks we obtain for the WS2 triangular islands (between 620680 nm). Additionally, we performed Raman and PL spectroscopy using excitation laser lines 514 and 488 nm, and we could not find any PL signal for the WOx films. Furthermore, after sulfurization we could not observe any trace of WO3 by Raman spectroscopy.
Figure S2. Spectra collected with a 514 nm laser excitation wavelength. (a) Raman spectrum of bulk WO3 powder; (b) PL spectrum of bulk WO3 powder exhibiting no PL signal; (c) Raman spectrum of a deposited WOx film on a Si/SiO2 substrate, showing only Si peaks, and (d) PL spectrum of a deposited WOx film on a Si/SiO2 substrate.
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Position dependence of the WS2 monolayers PL FWHM and PL integrated intensity: We recorded several spectra using the extended mode of collection, at representative points of the WS2 islands, as shown in Figure 5 of the manuscript. Spots (P2 to P5) are marked in Figure 5f and plotted in Figure 5i of the manuscript. We analyzed the peak shapes and found a very small variation of FWHM along positions P2 to P5 (see figure S3). Figure S3b shows the FWHM variation along P2-P5, and no trend in FWHM values could be associated with the regions within the WS2 islands. The average FWHM for WS2 PL peaks is around 0.055 eV. In addition, the peak integrated intensity was found to be strongly dependent on the region within WS2 islands, and the highest integrated intensity values were found at the edges of the WS2 triangles (Figure S3c).
Figure S3. (a) Optical image of the WS2 triangular islands with position markers indicating P2-P5; PL spectra were acquired on these sites; (b) FWHM vs. position and no trend could be associated to the different locations; (c) Variation of PL peak area vs. position (the PL 4
peak area monotonically increases as the edge is approached), and (d) PL peak position, in which the PL photon energy monotonically decreases as the edge is approached. Comparison of the PL signal for WS2 and MoS2: We have been able to obtain both WS2 and MoS2; the MoS2 islands seem to be smaller.
Figure S4. PL spectra obtained from WS2 (a) and MoS2 (b) monolayer triangular islands, and (c) PL intensity mapping (intensity at the 680 nm peak) in one corner of a MoS2 island (inset at the top left corner is the optical image of the MoS2 island).
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The PL spectra from WS2 and MoS2 islands are very similar, in terms of having a very sharp excitonic peak. In the MoS2, however, it is always possible to observe a small shoulder at higher energies that could be associated to exciton B. We still do not fully understand the reason for observing this second peak that it is not observed in single layers exfoliated mechanically as reported previously. A possible explanation could be a contribution from second layer island formation. These islands could have lateral dimension of several atoms but not large enough to be observed by SEM and might appear as noise in an AFM scan. Further investigations are needed in that direction. Another observation to be made is that the PL efficiency is higher for WS2; notice that in figure S4 the Raman peaks are included for both cases and the PL intensity relative to the Raman is very high for WS2. This does not mean necessarily that WS2 is a better emitter, it could be only a problem of our synthesis approach that could work better for WS2 and the conditions for growing MoS2 islands has not been fully optimized. Further investigations are currently underway. The PL enhancement at the edges was also observed in the MoS2 islands (figure S4c), but not in all the islands and the enhancement factor was several orders of magnitude smaller when compared to WS2. In some MoS2 islands there was a PL enhancement at the edges but also at regions close to the center of the island.
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Comparison between MoS2 and WS2 monolayer PL FWHM: Figure S5 shows representative PL spectra from MoS2 and WS2 triangular monolayers exhibiting average FWHM values associated with both materials. PL peaks for MoS2 were found to be broader than those of WS2, although we are comparing two different materials and variations on the PL, the FWHM could be associated to different crystal qualities.
Figure S5. Normalized PL spectra for typical MoS2 and WS2 triangular islands. Besides the peak position, it is clear that MoS2 peaks are broader (FWHM ~70 meV) than WS2 (FWHM ~ 50 meV). Additional first principle calculations for different edge passivation:
Figure S6. PDOS adding the alpha and beta spins of WS2 nanorribbons passivated with oxygen in three different regions of the basic double cell used for the calculations. Left: Monoatomic oxygen satuarted edge, and Right: Molecular oxygen saturated edge. 7
Figure S7.- PDOS (as a sum of the alpha and beta spins) of a triangular island with one tungsten edge not saturated. (a) PDOS at the tungsten edge; (b) Molecular model showing with shaded regions where the PDOS has been considered; (c) PDOS at central region, and (d) PDOS at Sulfur-Tungsten edge. References for the Supporting Information (Figures S1-S5) S1. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.; Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Letters 2009, 9, (1), 30-35. S2. Regan, W.; Alem, N.; Aleman, B.; Geng, B. S.; Girit, C.; Maserati, L.; Wang, F.; Crommie, M.; Zettl, A., A direct transfer of layer-area graphene. Applied Physics Letters 2010, 96, (11), 113102. S3. Delley, B., An All-Electron Numerical-Method for Solving the Local Density Functional for Polyatomic-Molecules. Journal of Chemical Physics 1990, 92, (1), 508517. S4. Delley, B., From molecules to solids with the DMol(3) approach. Journal of Chemical Physics 2000, 113, (18), 7756-7764. S5. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C., First principles methods using CASTEP. Zeitschrift Fur Kristallographie 2005, 220, (5-6), 567-570. S6. Feng, M. et al. Strong photoluminescence of nanostructured crystalline tungsten oxide thin films. Applied Physics Letters 86, 141901 (2005).
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Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
2
Department of Materials Science and Engineering and The Materials Research Institute, 104 Davey Lab., The Pennsylvania State University, University Park, PA 16802, USA
† Present address: Department of Physics & Astronomy, University of Louisville, Louisville, KY 40292 USA. #On leave from Advanced Materials Department, IPICYT, Camino a Presa San Jose 2055, Col. Lomas 4a Sección, San Luis Potosí, México
1
Optical contrast of monolayered WS2 on Si/SiO2 substrates Figure S1 shows an optical image of WS2 islands on a Si substrate with a 300 nm SiO2 layer. The contrast is very clear. Similar contrast was found on Si substrates with a 285 nm SiO2 layer.
Figure S1. Optical image of WS2 monolayer islands on a Si/SiO2 wafers.
2
PL and Raman from WOx precursors Tungsten oxides could have very strong PL when excited with a high energy laser, such as 256 nm. This PL, however, is observed for a wavelength range between 300-450 nm, which is very different from the PL peaks we obtain for the WS2 triangular islands (between 620680 nm). Additionally, we performed Raman and PL spectroscopy using excitation laser lines 514 and 488 nm, and we could not find any PL signal for the WOx films. Furthermore, after sulfurization we could not observe any trace of WO3 by Raman spectroscopy.
Figure S2. Spectra collected with a 514 nm laser excitation wavelength. (a) Raman spectrum of bulk WO3 powder; (b) PL spectrum of bulk WO3 powder exhibiting no PL signal; (c) Raman spectrum of a deposited WOx film on a Si/SiO2 substrate, showing only Si peaks, and (d) PL spectrum of a deposited WOx film on a Si/SiO2 substrate.
3
Position dependence of the WS2 monolayers PL FWHM and PL integrated intensity: We recorded several spectra using the extended mode of collection, at representative points of the WS2 islands, as shown in Figure 5 of the manuscript. Spots (P2 to P5) are marked in Figure 5f and plotted in Figure 5i of the manuscript. We analyzed the peak shapes and found a very small variation of FWHM along positions P2 to P5 (see figure S3). Figure S3b shows the FWHM variation along P2-P5, and no trend in FWHM values could be associated with the regions within the WS2 islands. The average FWHM for WS2 PL peaks is around 0.055 eV. In addition, the peak integrated intensity was found to be strongly dependent on the region within WS2 islands, and the highest integrated intensity values were found at the edges of the WS2 triangles (Figure S3c).
Figure S3. (a) Optical image of the WS2 triangular islands with position markers indicating P2-P5; PL spectra were acquired on these sites; (b) FWHM vs. position and no trend could be associated to the different locations; (c) Variation of PL peak area vs. position (the PL 4
peak area monotonically increases as the edge is approached), and (d) PL peak position, in which the PL photon energy monotonically decreases as the edge is approached. Comparison of the PL signal for WS2 and MoS2: We have been able to obtain both WS2 and MoS2; the MoS2 islands seem to be smaller.
Figure S4. PL spectra obtained from WS2 (a) and MoS2 (b) monolayer triangular islands, and (c) PL intensity mapping (intensity at the 680 nm peak) in one corner of a MoS2 island (inset at the top left corner is the optical image of the MoS2 island).
5
The PL spectra from WS2 and MoS2 islands are very similar, in terms of having a very sharp excitonic peak. In the MoS2, however, it is always possible to observe a small shoulder at higher energies that could be associated to exciton B. We still do not fully understand the reason for observing this second peak that it is not observed in single layers exfoliated mechanically as reported previously. A possible explanation could be a contribution from second layer island formation. These islands could have lateral dimension of several atoms but not large enough to be observed by SEM and might appear as noise in an AFM scan. Further investigations are needed in that direction. Another observation to be made is that the PL efficiency is higher for WS2; notice that in figure S4 the Raman peaks are included for both cases and the PL intensity relative to the Raman is very high for WS2. This does not mean necessarily that WS2 is a better emitter, it could be only a problem of our synthesis approach that could work better for WS2 and the conditions for growing MoS2 islands has not been fully optimized. Further investigations are currently underway. The PL enhancement at the edges was also observed in the MoS2 islands (figure S4c), but not in all the islands and the enhancement factor was several orders of magnitude smaller when compared to WS2. In some MoS2 islands there was a PL enhancement at the edges but also at regions close to the center of the island.
6
Comparison between MoS2 and WS2 monolayer PL FWHM: Figure S5 shows representative PL spectra from MoS2 and WS2 triangular monolayers exhibiting average FWHM values associated with both materials. PL peaks for MoS2 were found to be broader than those of WS2, although we are comparing two different materials and variations on the PL, the FWHM could be associated to different crystal qualities.
Figure S5. Normalized PL spectra for typical MoS2 and WS2 triangular islands. Besides the peak position, it is clear that MoS2 peaks are broader (FWHM ~70 meV) than WS2 (FWHM ~ 50 meV). Additional first principle calculations for different edge passivation:
Figure S6. PDOS adding the alpha and beta spins of WS2 nanorribbons passivated with oxygen in three different regions of the basic double cell used for the calculations. Left: Monoatomic oxygen satuarted edge, and Right: Molecular oxygen saturated edge. 7
Figure S7.- PDOS (as a sum of the alpha and beta spins) of a triangular island with one tungsten edge not saturated. (a) PDOS at the tungsten edge; (b) Molecular model showing with shaded regions where the PDOS has been considered; (c) PDOS at central region, and (d) PDOS at Sulfur-Tungsten edge. References for the Supporting Information (Figures S1-S5) S1. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.; Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Letters 2009, 9, (1), 30-35. S2. Regan, W.; Alem, N.; Aleman, B.; Geng, B. S.; Girit, C.; Maserati, L.; Wang, F.; Crommie, M.; Zettl, A., A direct transfer of layer-area graphene. Applied Physics Letters 2010, 96, (11), 113102. S3. Delley, B., An All-Electron Numerical-Method for Solving the Local Density Functional for Polyatomic-Molecules. Journal of Chemical Physics 1990, 92, (1), 508517. S4. Delley, B., From molecules to solids with the DMol(3) approach. Journal of Chemical Physics 2000, 113, (18), 7756-7764. S5. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C., First principles methods using CASTEP. Zeitschrift Fur Kristallographie 2005, 220, (5-6), 567-570. S6. Feng, M. et al. Strong photoluminescence of nanostructured crystalline tungsten oxide thin films. Applied Physics Letters 86, 141901 (2005).
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