Supporting Information Colloidal Synthesis of Single-Layer MSe2 (M ...
Supporting Information Colloidal Synthesis of Single-Layer MSe2 (M=Mo, W) Nanosheets via Anisotropic Solution-Phase Growth Approach Wonil Jung,† Sujeong Lee,† Dongwon Yoo,† Sohee Jeong,† Pere Miró,‡ Agnieszka Kuc,‡ Thomas Heine,‡ and Jinwoo Cheon*,† †
Department of Chemistry, Yonsei University, Seoul 120-749, Korea Department of Physics and Earth Science, Jacobs University Bremen, 28759 Bremen, Germany
‡
*corresponding author: [email protected]
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Computational details All the systems were optimized using density functional theory (DFT) as implemented in the ADFBAND package.1-5 The exchange and correlation terms plus a dispersion correction were described using the generalized gradient approximation (GGA) in the scheme of Perdew-Burke-Ernzerhof (PBE-D3(BJ)) and the London dispersion corrections as proposed by Grimme with Becke-Johnson damping.6-8 Local basis functions (numerical and Slater-type basis functions of valence triple-ζ quality with one polarization function (TZP)) were used for all the atoms (core small). Relativistic effects were included through zeroth order regular approximation (ZORA).9-10 Table S1. Formation energies of the (0001), (112ത 0), and (101ത 0)a facets. The energies are in eV. Facet (0001) (112ത 0) (101ത 0) a
Formation energy per WSe2 -0.734 0.097 0.242
The (101ത 0) facets include two non-equivalent edge types.
For layered TMCs, the surface energy of the edges is significantly higher than the basal plane surface energy. To shed some light into the anisotropic growth patterns of WSe2 nanosheets, we evaluated the formation energies of given facets of WSe2. The formation energy of the facets (Eformation), as the major contribution factor of the surface energy, can be used to evaluate the stability of the facets. The formation energy of specific facet of WSe2 is defined as Eformation = {Efacet(WnSe2n) - nEatom(Wfcc) - nEatom(Segray)}/n where Efacet is the formation energy of WnSe2n with a specific facet structure, Eatom(Wfcc) is the formation energy of tungsten fcc crystal, Eatom(Segray) is the formation energy of gray structure selenium, and n is the number of WSe2 formula units in the calculated supercell. According to our DFT calculations with the structure modeling (Figure S1), the (0001) basal plane is found to be the most stable facet while the (112ത 0) and (101ത0) facets are 0.831 and 0.976 eV higher in energy per WSe2 formula unit, respectively (Table S1). Table S2. Binding energies of capping ligands to the (0001) facet. The energies are in eV. Capping ligand Methylamine Methyl alcohol Formic acid
Binding energy to (0001) facet per WSe2 -0.078 -0.060 -0.057
The binding energy (Ebinding) per supercell, is given by Ebinding = Efacet+ligand - Efacet - Eligand where Efacet+ligand is the total energy of a given facet and one attached molecule per supercell, Efacet is the energy of the facet, and Eligand is the energy of the capping ligand. The binding energy represents the enthalpy, and a negative sign indicates that the binding interaction is favorable.
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Figure S1. Studied facets: (0001), (112ത0), and (101ത 0) with methylamine, methyl alcohol, and formic acid. Red lines represent the periodic boundary conditions and the simulated supercell is highlighted in red.
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Synthetic details Single-layer WSe2 nanosheets: Tungsten hexacarbonyl (W(CO)6) precursor (0.02 mmol), trace amount of tetradecylphosphonic acid additive and oleic acid (10.5 mmol) were added to a 25 mL three-neck round-bottom flask under Ar atmosphere. The mixture was first heated to 330 °C at the heating rate of 8.5 °C/min under Ar flow, and diphenyl diselenide (Ph2Se2) precursor (0.04 mmol) dissolved in oleic acid (3.5 mmol) was injected subsequently. After 12 h, reaction mixture was cooled down to room temperature. After addition of excess toluene and butanol mixture (2:1 by volume), 200-400 nm single-layer WSe2 nanosheets were obtained by centrifugation, and the sample was dispersed in toluene. Single-layer MoSe2 nanosheets: Molybdenum hexacarbonyl (Mo(CO)6) precursor (0.02 mmol), trace amount of tetradecylphosphonic acid additive and oleic acid (10.5 mmol) were added to a 25 mL three-neck round-bottom flask under Ar atmosphere. The mixture was first heated to 350 °C at the heating rate of 9 °C/min under Ar flow, and diphenyl diselenide (Ph2Se2) precursor (0.04 mmol) dissolved in oleic acid (3.5 mmol) was injected subsequently. After 10 h, reaction mixture was cooled down to room temperature. After addition of excess toluene and butanol mixture (2:1 by volume), 200-500 nm single-layer MoSe2 nanosheets were obtained by centrifugation, and the sample was dispersed in toluene.
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Stability of colloidal WSe2 nanosheets
Figure S2. Stability of colloidal single-layer WSe2 nanosheets. (a-b) The photographs of single-layer WSe2 nanosheets dispersed in toluene (left) and 2 weeks after the dispersion (right): (a) Precipitation is not observed in the colloidal solution. (b) Visible laser pathway induced by Tyndall effect, shows even beam size through the solution, which confirms the no size change of colloidal nanosheets through aggregation. (c) TEM image of single-layer WSe2 nanosheets 2 weeks after synthesized. No aggregation is observed. (d) Elemental analysis of single-layer WSe2 nanosheets 2 weeks after exposure to the air. Oxygen is not detected in energy dispersive spectroscopy (EDS).
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Time-dependent experiments of WSe2 nanosheets Growth process of single-layer WSe2 nanosheets is investigated by TEM in time-dependent manner. In 5 minutes of reaction time, both amorphous and crystalline plate-like particles (~3 nm) are observed, which indicates that the nucleation process seems to occur at the very early stage of the reaction. After that, the lateral growth of single-layer nanosheets proceeds continuously as the time goes (i.e., ~14 nm at 0.5 h, ~85 nm at 3 h, and ~185 nm at 6 h). At 12 h, the WSe2 nanosheets with the lateral size of 200-400 nm are obtained without further growth afterward. Our observation indicates that the lateral size of the WSe2 nanosheets becomes gradually larger after the fast nucleation process within a few minutes.
Figure S3. TEM images of WSe2 nanosheets obtained at different reaction time after precursor injection: (a) 5 min, (b) 0.5 h, (c) 3 h, (d) 6 h, and (e) 12 h.
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XRD analysis of single-layer MoSe2 nanosheets
Figure S4. X-ray diffraction pattern (XRD) of single-layer MoSe2 nanosheets indexed to the reflection of bulk MoSe2 (red bars, JCPDS 29-0914). Only the peaks (black asterisk) that are not related to c-axis are observed in single-layer MoSe2 nanosheets.
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UV-Vis absorption spectra of MoSe2 nanosheets In the absorption spectra, bulk MoSe2 shows peaks at 827 nm and 725 nm that are assigned as excitonic peak A and B, respectively.11-12 The excitonic peak A and B shift to higher energy (790 nm and 693 nm) compared to the bulk. The behavior is similar to the thickness dependency of excitonic peaks observed in WSe2 nanosheets.
Figure S5. Absorption spectra of single-layer MoSe2 nanosheets (red line) and bulk MoSe2 (black line).
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References 1. Li, Z.; Chen, H.; Bao, H.; Gao, M. Chem. Mater. 2004, 16, 1391. 2. te Velde, G.; Baerends, E. J. Phys. Rev. B 1991, 44, 7888. 3. Wiesenekker, G.; Baerends, E. J. J. Phys. Cond. Matt. 1991, 3, 6721. 4. Franchini, M.; Philipsen, P.H.T.; Visscher, L. J. Comp. Chem. 2013, 34, 1818. 5. BAND2013, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com 6. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. 7. Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comp. Chem. 2011, 32, 1457. 8. Grimme, S. J. Comput. Chem. 2006, 27, 1787. 9. van Lenthe, E.; Baerends, E.J.; Snijders, J.G. J. Chem. Phys. 1993, 99, 4597. 10. van Lenthe, E.; Baerends, E.J.; Snijders, J.G. J. Chem. Phys. 1994, 101, 9783. 11. Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Leibig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.; de Vasconcellos, S. M.; Bratschitsch, R. Opt. Express 2013, 21, 4908. 12. Coehoorn, R.; Haas, C.; de Groot, R.A. Phys. Rev. B 1987, 35, 6203.
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Department of Chemistry, Yonsei University, Seoul 120-749, Korea Department of Physics and Earth Science, Jacobs University Bremen, 28759 Bremen, Germany
‡
*corresponding author: [email protected]
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Computational details All the systems were optimized using density functional theory (DFT) as implemented in the ADFBAND package.1-5 The exchange and correlation terms plus a dispersion correction were described using the generalized gradient approximation (GGA) in the scheme of Perdew-Burke-Ernzerhof (PBE-D3(BJ)) and the London dispersion corrections as proposed by Grimme with Becke-Johnson damping.6-8 Local basis functions (numerical and Slater-type basis functions of valence triple-ζ quality with one polarization function (TZP)) were used for all the atoms (core small). Relativistic effects were included through zeroth order regular approximation (ZORA).9-10 Table S1. Formation energies of the (0001), (112ത 0), and (101ത 0)a facets. The energies are in eV. Facet (0001) (112ത 0) (101ത 0) a
Formation energy per WSe2 -0.734 0.097 0.242
The (101ത 0) facets include two non-equivalent edge types.
For layered TMCs, the surface energy of the edges is significantly higher than the basal plane surface energy. To shed some light into the anisotropic growth patterns of WSe2 nanosheets, we evaluated the formation energies of given facets of WSe2. The formation energy of the facets (Eformation), as the major contribution factor of the surface energy, can be used to evaluate the stability of the facets. The formation energy of specific facet of WSe2 is defined as Eformation = {Efacet(WnSe2n) - nEatom(Wfcc) - nEatom(Segray)}/n where Efacet is the formation energy of WnSe2n with a specific facet structure, Eatom(Wfcc) is the formation energy of tungsten fcc crystal, Eatom(Segray) is the formation energy of gray structure selenium, and n is the number of WSe2 formula units in the calculated supercell. According to our DFT calculations with the structure modeling (Figure S1), the (0001) basal plane is found to be the most stable facet while the (112ത 0) and (101ത0) facets are 0.831 and 0.976 eV higher in energy per WSe2 formula unit, respectively (Table S1). Table S2. Binding energies of capping ligands to the (0001) facet. The energies are in eV. Capping ligand Methylamine Methyl alcohol Formic acid
Binding energy to (0001) facet per WSe2 -0.078 -0.060 -0.057
The binding energy (Ebinding) per supercell, is given by Ebinding = Efacet+ligand - Efacet - Eligand where Efacet+ligand is the total energy of a given facet and one attached molecule per supercell, Efacet is the energy of the facet, and Eligand is the energy of the capping ligand. The binding energy represents the enthalpy, and a negative sign indicates that the binding interaction is favorable.
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Figure S1. Studied facets: (0001), (112ത0), and (101ത 0) with methylamine, methyl alcohol, and formic acid. Red lines represent the periodic boundary conditions and the simulated supercell is highlighted in red.
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Synthetic details Single-layer WSe2 nanosheets: Tungsten hexacarbonyl (W(CO)6) precursor (0.02 mmol), trace amount of tetradecylphosphonic acid additive and oleic acid (10.5 mmol) were added to a 25 mL three-neck round-bottom flask under Ar atmosphere. The mixture was first heated to 330 °C at the heating rate of 8.5 °C/min under Ar flow, and diphenyl diselenide (Ph2Se2) precursor (0.04 mmol) dissolved in oleic acid (3.5 mmol) was injected subsequently. After 12 h, reaction mixture was cooled down to room temperature. After addition of excess toluene and butanol mixture (2:1 by volume), 200-400 nm single-layer WSe2 nanosheets were obtained by centrifugation, and the sample was dispersed in toluene. Single-layer MoSe2 nanosheets: Molybdenum hexacarbonyl (Mo(CO)6) precursor (0.02 mmol), trace amount of tetradecylphosphonic acid additive and oleic acid (10.5 mmol) were added to a 25 mL three-neck round-bottom flask under Ar atmosphere. The mixture was first heated to 350 °C at the heating rate of 9 °C/min under Ar flow, and diphenyl diselenide (Ph2Se2) precursor (0.04 mmol) dissolved in oleic acid (3.5 mmol) was injected subsequently. After 10 h, reaction mixture was cooled down to room temperature. After addition of excess toluene and butanol mixture (2:1 by volume), 200-500 nm single-layer MoSe2 nanosheets were obtained by centrifugation, and the sample was dispersed in toluene.
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Stability of colloidal WSe2 nanosheets
Figure S2. Stability of colloidal single-layer WSe2 nanosheets. (a-b) The photographs of single-layer WSe2 nanosheets dispersed in toluene (left) and 2 weeks after the dispersion (right): (a) Precipitation is not observed in the colloidal solution. (b) Visible laser pathway induced by Tyndall effect, shows even beam size through the solution, which confirms the no size change of colloidal nanosheets through aggregation. (c) TEM image of single-layer WSe2 nanosheets 2 weeks after synthesized. No aggregation is observed. (d) Elemental analysis of single-layer WSe2 nanosheets 2 weeks after exposure to the air. Oxygen is not detected in energy dispersive spectroscopy (EDS).
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Time-dependent experiments of WSe2 nanosheets Growth process of single-layer WSe2 nanosheets is investigated by TEM in time-dependent manner. In 5 minutes of reaction time, both amorphous and crystalline plate-like particles (~3 nm) are observed, which indicates that the nucleation process seems to occur at the very early stage of the reaction. After that, the lateral growth of single-layer nanosheets proceeds continuously as the time goes (i.e., ~14 nm at 0.5 h, ~85 nm at 3 h, and ~185 nm at 6 h). At 12 h, the WSe2 nanosheets with the lateral size of 200-400 nm are obtained without further growth afterward. Our observation indicates that the lateral size of the WSe2 nanosheets becomes gradually larger after the fast nucleation process within a few minutes.
Figure S3. TEM images of WSe2 nanosheets obtained at different reaction time after precursor injection: (a) 5 min, (b) 0.5 h, (c) 3 h, (d) 6 h, and (e) 12 h.
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XRD analysis of single-layer MoSe2 nanosheets
Figure S4. X-ray diffraction pattern (XRD) of single-layer MoSe2 nanosheets indexed to the reflection of bulk MoSe2 (red bars, JCPDS 29-0914). Only the peaks (black asterisk) that are not related to c-axis are observed in single-layer MoSe2 nanosheets.
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UV-Vis absorption spectra of MoSe2 nanosheets In the absorption spectra, bulk MoSe2 shows peaks at 827 nm and 725 nm that are assigned as excitonic peak A and B, respectively.11-12 The excitonic peak A and B shift to higher energy (790 nm and 693 nm) compared to the bulk. The behavior is similar to the thickness dependency of excitonic peaks observed in WSe2 nanosheets.
Figure S5. Absorption spectra of single-layer MoSe2 nanosheets (red line) and bulk MoSe2 (black line).
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References 1. Li, Z.; Chen, H.; Bao, H.; Gao, M. Chem. Mater. 2004, 16, 1391. 2. te Velde, G.; Baerends, E. J. Phys. Rev. B 1991, 44, 7888. 3. Wiesenekker, G.; Baerends, E. J. J. Phys. Cond. Matt. 1991, 3, 6721. 4. Franchini, M.; Philipsen, P.H.T.; Visscher, L. J. Comp. Chem. 2013, 34, 1818. 5. BAND2013, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com 6. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. 7. Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comp. Chem. 2011, 32, 1457. 8. Grimme, S. J. Comput. Chem. 2006, 27, 1787. 9. van Lenthe, E.; Baerends, E.J.; Snijders, J.G. J. Chem. Phys. 1993, 99, 4597. 10. van Lenthe, E.; Baerends, E.J.; Snijders, J.G. J. Chem. Phys. 1994, 101, 9783. 11. Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Leibig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.; de Vasconcellos, S. M.; Bratschitsch, R. Opt. Express 2013, 21, 4908. 12. Coehoorn, R.; Haas, C.; de Groot, R.A. Phys. Rev. B 1987, 35, 6203.
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