Van der Waals Epitaxy of MoS2 Layers using Graphene as Growth ...
Supporting Materials
Van der Waals Epitaxy of MoS2 Layers using Graphene as Growth Templates Yumeng Shi, †, ‡, # Wu Zhou, &, │,# Ang-Yu Lu, ┴ Wenjing Fang, † Yi-Hsien Lee,┴, † Allen Long Hsu, Soo Min Kim, † Ki Kang Kim, † Hui Ying Yang,‡ Lain-jong Li, ┴ Juan Carlos Idrobo, │Jing
†
Kong†* †
Department of Electrical Engineering and Computer Sciences, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, USA ‡
Singapore University of Technology and Design, 20 Dover Drive Singapore 138682, Singapore
┴
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 11529, Taiwan
&
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
│
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge TN
37831-6064, USA *
To whom correspondence should be addressed. E-mail: (J. K.) [email protected]
Author Contributions: # These authors contribute equally.
1. CVD Synthesis Methods: 1.1 CVD-G synthesis: Single layer graphene was synthesized by using 25 µm thick Cu foils from Alfa Aesar (purity 99.9%). the Cu foils were placed in a quartz furnace and the growth chamber was pumped down to 10 mTorr using dry scroll pump. Hydrogen gas was introduced into the system with total pressure at ~300 mTorr and the temperature was raised to and kept at 1000 ºC for 20 mins to initiate Cu grain growth and remove residual copper oxide. Subsequently, methane gas was introduced with total pressure of ~700 mTorr to the growth chamber for graphene synthesis. The growth time was maintained for 10 to 30 mins. After the synthesis, the chamber was cooled down to room temperature at a cooling rate of ~ 100 ºC/min with hydrogen and methane gas. 1.2 MoS2 synthesis: Chemical vapor deposited graphene (CVD-G) on 25 µm thick Cu foil was placed into the quartz tube CVD chamber. The CVD-G was annealed at 400 ºC for 30 mins using 600 sccm Ar and 400 sccm H2 as the protection gas at ambient pressure to clean the graphene surface and reduce the oxygen species which may exists. After the thermal annealing, the growth chamber was cooled down to 20 ºC. When the desired temperature was reached, the growth chamber was pump down to ~10 mTorr by vacuum pump.
The MoS2 precursor
(NH4)2MoS4 was dissolved in DMF with a concentration of 100 mM. The DMF: (NH4)2MoS4 precursor was sealed in a quartz bubbler and carried by Ar gas at a typical gas flow rate of 10 sccm - 50 sccm with total pressure ranging from 200 mTorr to 2 Torr for precursor deposition. The supply of precursor last for 10 to 60 mins. After the precursor deposition, in order to remove the DMF residuals, the growth chamber was heated up to 50 ºC and pumped down to 10 mTorr for 30~60 mins. Subsequently, the temperature was raised from 50 ºC to 400 ºC at a heating rate of 5 ºC/min with Ar and H2 as the protecting gas. After the desired temperature is reached, the
sample was subsequently annealed at 400 ºC for an additional 1 hour. Finally, after the synthesis the growth chamber was cooled down to room temperature. 2. Transfer of the as-grown MoS2/Graphene hybrid to arbitrary substrates. Once the CVD synthesis was finished, the MoS2/Graphene hybrid was transferred by coating the film with a thin layer (~100 nm) of Poly[methylmethacrylate] (PMMA). After etching the underlying Cu foil with FeCl3 aquariums (or CE-100 Cu etchant from Transene Company INC.), the PMMA/ MoS2/Graphene film was transferred to DI water and was suspended on the surface of water to remove the etchant residue.
Subsequently, the film can be transferred to any
substrate or TEM grids for analysis and characterization. Finally, the top layer of PMMA can be removed by acetone or by directly annealing the samples in an Ar and H2 atmosphere at 400 °C for 2 hours. 3. Characterizations: Surface morphology of the samples was examined with commercial atomic force microscope (AFM, Veeco Icon). Raman spectra were obtained on a home-built Raman microscopic system equipped with a 532 nm laser. The Si peak at 520 cm−1 was used for calibration in the experiments. A field-emission transmission electron microscope (JEOL JEM-2010F, operated at 200 keV), equipped with an energy dispersive spectrometer (EDS) was used to obtain the information of the microstructures and the chemical compositions. Chemical composition was determined by X-ray photoelectron spectroscope (XPS, Phi V5000). XPS measurements were performed with an Al Kα X-ray source. The energy calibrations were made against the C 1s peak to eliminate the charging of the sample during analysis. The STEM imaging and EELS analysis were performed with a Nion UltraSTEM-100 located at Oak Ridge National Laboratory,
equipped with a cold field emission electron source and a corrector of third and fifth order aberrations.
1
The microscope was operated with a probe current of ~110 pA and at 60 kV
accelerating voltage, which is below the damage threshold of graphene. EEL spectra were collected using a Gatan Enfina spectrometer, with an energy resolution of 1 eV for 0.5 eV/channel dispersion. The convergence semi-angle for the incident probe was 31 mrad, with an EELS collection semi-angle of 48 mrad.
Figure S1. Chemical composition analysis: (A) Mo (B) S and (C) carbon XPS spectra of the MoS2/graphene/Cu sample. (D) Survey spectrum of the MoS2/graphene/Cu sample. Fig. S1 shows the XPS spectra of the as-grown MoS2 flakes on CVD-G without removing the underlying Cu foil. As shown in Fig. S1 (A), the spectrum of Mo 3d orbit shows two peaks at 229.2 and 232.3 eV, which can be attributed to the doublet of Mo 3d5/2 and Mo 3d3/2. The binding energy for S 2p3/2 and S 2p1/2 are 162.0 and 163.3 eV, respectively, and they are shown in Fig. S1 (B). The carbon XPS peak obtained from the graphene substrate is shown in Fig. S1 (C). All the measured binding energies are consistent with the reported values for MoS2 crystals. 2
Figure S2. STEM analysis of MoS2. (A) A typical low magnification STEM ADF image of the loosely packed MoS2 nano-flakes on the MoS2 film; (B) A typical high magnification STEM ADF image taken between the MoS2 nano-flakes showing the formation of monolayer MoS2. The inset shows the FFT pattern from the marked region, which corresponds to the [001] zone axis of MoS2. (C) A typical electron energy loss spectrum (EELS) taken from the monolayer MoS2 film in between the nano-flakes, showing clear S and Mo signals. As shown in Fig.S2 (A), the MoS2 flakes display the highest contrast on the HAADF image, while regions with intermediate contrast can often be observed between these white flakes. The regions showing intermediate contrast were further identified to be mostly monolayer MoS2 films (Fig. S2 B), with occasional few layer regions. EELS taken from the monolayer MoS2 region (Fig. S2 C) indicates an elemental ratio of Mo/S close to 1:2.
Figure S3. EDS spectra taken from the crystals with hexagonal shape. The strong Cu peaks originate from the Cu TEM grids. The other elements (As and Fe) could be the impurities introduced by the Cu etchant during the transfer process.
Reference. 1. Krivanek, O. L.; Corbin, G. J.; Dellby, N.; Elston, B. F.; Keyse, R. J.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Woodruff, J. W. Ultramicroscopy 2008, 108, (3), 179-195. 2. Baker, M. A.; Gilmore, R.; Lenardi, C.; Gissler, W. Applied Surface Science 1999, 150, (1-4), 255262.
Van der Waals Epitaxy of MoS2 Layers using Graphene as Growth Templates Yumeng Shi, †, ‡, # Wu Zhou, &, │,# Ang-Yu Lu, ┴ Wenjing Fang, † Yi-Hsien Lee,┴, † Allen Long Hsu, Soo Min Kim, † Ki Kang Kim, † Hui Ying Yang,‡ Lain-jong Li, ┴ Juan Carlos Idrobo, │Jing
†
Kong†* †
Department of Electrical Engineering and Computer Sciences, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, USA ‡
Singapore University of Technology and Design, 20 Dover Drive Singapore 138682, Singapore
┴
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 11529, Taiwan
&
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
│
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge TN
37831-6064, USA *
To whom correspondence should be addressed. E-mail: (J. K.) [email protected]
Author Contributions: # These authors contribute equally.
1. CVD Synthesis Methods: 1.1 CVD-G synthesis: Single layer graphene was synthesized by using 25 µm thick Cu foils from Alfa Aesar (purity 99.9%). the Cu foils were placed in a quartz furnace and the growth chamber was pumped down to 10 mTorr using dry scroll pump. Hydrogen gas was introduced into the system with total pressure at ~300 mTorr and the temperature was raised to and kept at 1000 ºC for 20 mins to initiate Cu grain growth and remove residual copper oxide. Subsequently, methane gas was introduced with total pressure of ~700 mTorr to the growth chamber for graphene synthesis. The growth time was maintained for 10 to 30 mins. After the synthesis, the chamber was cooled down to room temperature at a cooling rate of ~ 100 ºC/min with hydrogen and methane gas. 1.2 MoS2 synthesis: Chemical vapor deposited graphene (CVD-G) on 25 µm thick Cu foil was placed into the quartz tube CVD chamber. The CVD-G was annealed at 400 ºC for 30 mins using 600 sccm Ar and 400 sccm H2 as the protection gas at ambient pressure to clean the graphene surface and reduce the oxygen species which may exists. After the thermal annealing, the growth chamber was cooled down to 20 ºC. When the desired temperature was reached, the growth chamber was pump down to ~10 mTorr by vacuum pump.
The MoS2 precursor
(NH4)2MoS4 was dissolved in DMF with a concentration of 100 mM. The DMF: (NH4)2MoS4 precursor was sealed in a quartz bubbler and carried by Ar gas at a typical gas flow rate of 10 sccm - 50 sccm with total pressure ranging from 200 mTorr to 2 Torr for precursor deposition. The supply of precursor last for 10 to 60 mins. After the precursor deposition, in order to remove the DMF residuals, the growth chamber was heated up to 50 ºC and pumped down to 10 mTorr for 30~60 mins. Subsequently, the temperature was raised from 50 ºC to 400 ºC at a heating rate of 5 ºC/min with Ar and H2 as the protecting gas. After the desired temperature is reached, the
sample was subsequently annealed at 400 ºC for an additional 1 hour. Finally, after the synthesis the growth chamber was cooled down to room temperature. 2. Transfer of the as-grown MoS2/Graphene hybrid to arbitrary substrates. Once the CVD synthesis was finished, the MoS2/Graphene hybrid was transferred by coating the film with a thin layer (~100 nm) of Poly[methylmethacrylate] (PMMA). After etching the underlying Cu foil with FeCl3 aquariums (or CE-100 Cu etchant from Transene Company INC.), the PMMA/ MoS2/Graphene film was transferred to DI water and was suspended on the surface of water to remove the etchant residue.
Subsequently, the film can be transferred to any
substrate or TEM grids for analysis and characterization. Finally, the top layer of PMMA can be removed by acetone or by directly annealing the samples in an Ar and H2 atmosphere at 400 °C for 2 hours. 3. Characterizations: Surface morphology of the samples was examined with commercial atomic force microscope (AFM, Veeco Icon). Raman spectra were obtained on a home-built Raman microscopic system equipped with a 532 nm laser. The Si peak at 520 cm−1 was used for calibration in the experiments. A field-emission transmission electron microscope (JEOL JEM-2010F, operated at 200 keV), equipped with an energy dispersive spectrometer (EDS) was used to obtain the information of the microstructures and the chemical compositions. Chemical composition was determined by X-ray photoelectron spectroscope (XPS, Phi V5000). XPS measurements were performed with an Al Kα X-ray source. The energy calibrations were made against the C 1s peak to eliminate the charging of the sample during analysis. The STEM imaging and EELS analysis were performed with a Nion UltraSTEM-100 located at Oak Ridge National Laboratory,
equipped with a cold field emission electron source and a corrector of third and fifth order aberrations.
1
The microscope was operated with a probe current of ~110 pA and at 60 kV
accelerating voltage, which is below the damage threshold of graphene. EEL spectra were collected using a Gatan Enfina spectrometer, with an energy resolution of 1 eV for 0.5 eV/channel dispersion. The convergence semi-angle for the incident probe was 31 mrad, with an EELS collection semi-angle of 48 mrad.
Figure S1. Chemical composition analysis: (A) Mo (B) S and (C) carbon XPS spectra of the MoS2/graphene/Cu sample. (D) Survey spectrum of the MoS2/graphene/Cu sample. Fig. S1 shows the XPS spectra of the as-grown MoS2 flakes on CVD-G without removing the underlying Cu foil. As shown in Fig. S1 (A), the spectrum of Mo 3d orbit shows two peaks at 229.2 and 232.3 eV, which can be attributed to the doublet of Mo 3d5/2 and Mo 3d3/2. The binding energy for S 2p3/2 and S 2p1/2 are 162.0 and 163.3 eV, respectively, and they are shown in Fig. S1 (B). The carbon XPS peak obtained from the graphene substrate is shown in Fig. S1 (C). All the measured binding energies are consistent with the reported values for MoS2 crystals. 2
Figure S2. STEM analysis of MoS2. (A) A typical low magnification STEM ADF image of the loosely packed MoS2 nano-flakes on the MoS2 film; (B) A typical high magnification STEM ADF image taken between the MoS2 nano-flakes showing the formation of monolayer MoS2. The inset shows the FFT pattern from the marked region, which corresponds to the [001] zone axis of MoS2. (C) A typical electron energy loss spectrum (EELS) taken from the monolayer MoS2 film in between the nano-flakes, showing clear S and Mo signals. As shown in Fig.S2 (A), the MoS2 flakes display the highest contrast on the HAADF image, while regions with intermediate contrast can often be observed between these white flakes. The regions showing intermediate contrast were further identified to be mostly monolayer MoS2 films (Fig. S2 B), with occasional few layer regions. EELS taken from the monolayer MoS2 region (Fig. S2 C) indicates an elemental ratio of Mo/S close to 1:2.
Figure S3. EDS spectra taken from the crystals with hexagonal shape. The strong Cu peaks originate from the Cu TEM grids. The other elements (As and Fe) could be the impurities introduced by the Cu etchant during the transfer process.
Reference. 1. Krivanek, O. L.; Corbin, G. J.; Dellby, N.; Elston, B. F.; Keyse, R. J.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Woodruff, J. W. Ultramicroscopy 2008, 108, (3), 179-195. 2. Baker, M. A.; Gilmore, R.; Lenardi, C.; Gissler, W. Applied Surface Science 1999, 150, (1-4), 255262.
