Role of Boundary Layer Diffusion in Vapor Deposition Growth of ...
Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS Chun Li1, Liang Huang1, Gayatri Pongur Snigdha3, Yifei Yu1, and Linyou Cao*1,2 1
Department of Materials Science and Engineering, 2 Department of Physics, 3 Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695.
* To whom correspondence should be addressed. Email: [email protected]
1. Raman spectra of GeS source powder and as-grown nanosheet.
Figure S1. Raman spectra from the source materials of GeS power (blue curve) and resulting GeS nanosheets (red curve). Raman spectra were collected with a Renishaw 1000 Raman spectrometer. The peak position is consistent with the literature report.1
Figure S2. EDS spectrum collected in TEM from GeS nanosheets. Analysis for intensities of the Ge and S peaks demonstrates that the stoichiometry ratio of Ge: S is around 1.
10 µm
Figure S3. Magnified SEM image for the nanosheets grown at the very upstream edge of the substrate given in Figure 4A in the main text.
2. Analysis of Partial Pressure of GeS Vapor in the Synthetic Setup. At elevated temperature (400-500 oC in experiments), the source material of GeS powder sublimates into gaseous GeS molecules (instead of decomposing into Ge or S atoms). The evaporation flux (molar loss rate) per unit surface area under reduced pressure depends on the evaporation temperature T, and the different between equilibrium vapor pressure Pvap of GeS solid and the partial pressure Ppar of GeS vapor in the gas phase ΦGeS = (Pvap Ppar ) /(2πMRT) 0.5, where M is the molecular weight of GeS, and R is the molar gas constant. We approximately assume that the partial pressure Ppar of GeS vapor is uniform in the tube. Therefore, the partial pressure Ppar depends on the the total pressure Ptot and the ratio of the supplied of carrier gas Ar and GeS vapor, Ppar = Ptot (ΦGeSS)/( ΦGeS S + JAr), where S is the total surface area of the source materials and JAr is the molar flux of the carrier gas (mol/s). By combining these two equations, we can have the partial pressure Ppar as Ppar =
A 2 + 4Pvap Ptot − A 2
A = Pvap + Ptot +
2π MRT J Ar SGeS
Our analysis indicates that, in typical experiments, the partial pressure Ppar of GeS is always close to the vapor pressure Ppar, only also show mild dependence on the flow rate and the total pressure at the conditions in our experiments (5-30 sccm, 20-40 Torr). We can reasonably believe the partial pressure of GeS in the tube only depends on the temperature T applied to the source materials.
0.9
Total Pressure (Torr)
20
0.8 40
0.7 0.6
60
0.5 0.4
80 0.3 100
0.2 20
40 60 80 Flow rate (sccm)
100
Figure S4. Calculated partial pressure of GeS as a function of the flow rate (horizontal axis) of carrier gas and the total pressure (vertical axis) in the synthetic system. The partial pressure is plotted as a ratio with respect to the equilibrium pressure of GeS at the sublimation temperature of 450 oC, which is 0.186 Torr. 2
References (1)
Wiley, J. D.; Buckel, W. J.; Schmidt, R. L. Infrared reflectivity and Raman scattering in
GeS. Phys. Rev. B 1976, 13, 24892496. (2)
Mills, K. C. Thermodynamic data for inorganic sulphides, selenides and tellurides;
Butterworths, London, 1974.
Department of Materials Science and Engineering, 2 Department of Physics, 3 Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695.
* To whom correspondence should be addressed. Email: [email protected]
1. Raman spectra of GeS source powder and as-grown nanosheet.
Figure S1. Raman spectra from the source materials of GeS power (blue curve) and resulting GeS nanosheets (red curve). Raman spectra were collected with a Renishaw 1000 Raman spectrometer. The peak position is consistent with the literature report.1
Figure S2. EDS spectrum collected in TEM from GeS nanosheets. Analysis for intensities of the Ge and S peaks demonstrates that the stoichiometry ratio of Ge: S is around 1.
10 µm
Figure S3. Magnified SEM image for the nanosheets grown at the very upstream edge of the substrate given in Figure 4A in the main text.
2. Analysis of Partial Pressure of GeS Vapor in the Synthetic Setup. At elevated temperature (400-500 oC in experiments), the source material of GeS powder sublimates into gaseous GeS molecules (instead of decomposing into Ge or S atoms). The evaporation flux (molar loss rate) per unit surface area under reduced pressure depends on the evaporation temperature T, and the different between equilibrium vapor pressure Pvap of GeS solid and the partial pressure Ppar of GeS vapor in the gas phase ΦGeS = (Pvap Ppar ) /(2πMRT) 0.5, where M is the molecular weight of GeS, and R is the molar gas constant. We approximately assume that the partial pressure Ppar of GeS vapor is uniform in the tube. Therefore, the partial pressure Ppar depends on the the total pressure Ptot and the ratio of the supplied of carrier gas Ar and GeS vapor, Ppar = Ptot (ΦGeSS)/( ΦGeS S + JAr), where S is the total surface area of the source materials and JAr is the molar flux of the carrier gas (mol/s). By combining these two equations, we can have the partial pressure Ppar as Ppar =
A 2 + 4Pvap Ptot − A 2
A = Pvap + Ptot +
2π MRT J Ar SGeS
Our analysis indicates that, in typical experiments, the partial pressure Ppar of GeS is always close to the vapor pressure Ppar, only also show mild dependence on the flow rate and the total pressure at the conditions in our experiments (5-30 sccm, 20-40 Torr). We can reasonably believe the partial pressure of GeS in the tube only depends on the temperature T applied to the source materials.
0.9
Total Pressure (Torr)
20
0.8 40
0.7 0.6
60
0.5 0.4
80 0.3 100
0.2 20
40 60 80 Flow rate (sccm)
100
Figure S4. Calculated partial pressure of GeS as a function of the flow rate (horizontal axis) of carrier gas and the total pressure (vertical axis) in the synthetic system. The partial pressure is plotted as a ratio with respect to the equilibrium pressure of GeS at the sublimation temperature of 450 oC, which is 0.186 Torr. 2
References (1)
Wiley, J. D.; Buckel, W. J.; Schmidt, R. L. Infrared reflectivity and Raman scattering in
GeS. Phys. Rev. B 1976, 13, 24892496. (2)
Mills, K. C. Thermodynamic data for inorganic sulphides, selenides and tellurides;
Butterworths, London, 1974.
