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Similar to Characterization of CVD grown molybdenum disulfide monolayer and exfoliated Molybdenum Ditelluride flakes
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Characterization of CVD grown molybdenum disulfide monolayer and exfoliated Molybdenum Ditelluride flakes
- 1. Characterization of CVD grown MoS2 monolayer and exfoliated MoTe2 flakes Yan Yan1, Xiang Ji2, Lin Zhou2, Jing Kong2 Mount Holyoke College, Massachusetts Institute of Technology NSF DMR-1231319
- 2. Molybdenum disulfide (MoS2) • Mo atoms sandwiched between two sub- layers of S atoms by covalent bonds • MoS2 monolayer is 0.7 nm thick • Transition metal dichalcogenide (TMDC) used in novel electronic and optoelectronic devices • Chemical vapor deposition or exfoliation Chong et al., Nature Nanotechnology 2016 http://labs.cas.usf.edu/lamsat/facilities/images/facilit ies%20-%20deposition/CVD%201.JPG
- 3. Chemical Vapor Deposition (CVD) system for MoS2 growth • PTAS (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt) is a seed which initiate nucleation for CVD synthesis of MoS2. • S and MoO3 in the crucibles vaporize and diffuse bi-directionally and, with aid of the seed, react on the surface of wafers that are placed face- down right above the Mo crucible to produce MoS2 • The furnace heated to 600 ˚C, PTAS molecules diffuse across the surface
- 4. Factors that affect MoS2 nanocrystal features Growth temperature Mo: S ratio Seeding promoter
- 5. CVD growth temperature • 650 °C• 600 °C Increasing growth temperature induces formation of multilayer MoS2
- 6. Mo: S ratio • Mo: S = 1: 2• Mo: S = 1: 1 • Mo: S = 1: 5 • Mo: S = 1: 15 Wang et al. Chem. Mater. 2014, 26, 6371-6379 Mo : S >1:2 “chubby” triangle Mo : S =1:2 equilateral triangle Mo: S < 1:2 “skinny” triangle Too much sulfur induces formation of multilayer small triangles
- 7. Seeding promoter (PTAS) • Direct PTAS deposit (600 °C) • No seeding promoter (750 °C) • PTAS diffusion ( 600 °C) Pro: Seeding promoter is conducive to formation of large MoS2 nanocrystal and continuous film Con: Seeding promoter increases the amount of impurities deposited on the substrate
- 9. Molybdenum ditelluride (MoTe2) 1T’-MoTe2 • Monoclinic structure • Semi-metallic • High conductivity Ju Li et al. Science 2014, 346, 1344-1347 Sergey A. Medvedev et al. Nature Comm. 2016,7,11038 • Transition metal dichalcogenides • Different crystal structures, such as 2H-, 1T-, 1T’- and Td-type lattices • Chemical vapor deposition and exfoliation Topological field effect transistor- topological phase transition to realize fast on/off switching
- 10. Outline Polarization-resolved Raman spectra of 1T’-MoTe2 flake Thickness-dependent Raman spectra of exfoliated 1T’-MoTe2 flakes Thickness measurement of exfoliated flakes using AFM Exfoliation of 1T’-MoTe2
- 11. Exfoliation • 1. Separate MoTe2 flake using a sharp blade • 2. Place the flake on top of a piece of sticky tape • 3. Fold and unfold the tape repeatedly until a almost transparent layer of MoTe2 grains appear on the tape • 4. Stick the tape on top of a pre-cleaned silicon wafer • 5. Use a plastic tweezer to gently press the tape against the wafer • 6. Remove the tape gently and slowly
- 13. Thickness-dependent Raman spectra of exfoliated 1T’-MoTe2 flakes Thickness: 1: 2.8 nm ( 4 layers) 2: 3.0 nm ( 4 layers) 3: 3.0 nm ( 4 layers) 4: 5.3 nm (7 layers) 5: 3.7 nm (5 layers) 1 32 4 5
- 14. Thickness-dependent Raman spectra of exfoliated 1T’-MoTe2 flakes 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 1 (4L) 2 (4L) 3 (4L) 4 (7L) 5 (5L)
- 15. 1T’-MoTe2 flake used for polarization- resolved Raman test Anisotropic or isotropic? Anisotropy is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy Thickness ~ 6nm (8-9 layers)
- 16. Polarization-resolved Raman spectra of 1T’-MoTe2 flake 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 0 degree 30 degree 60 degree 90 degree 120 degree 150 degree 180 degree 210 degree 240 degree 300 degree 330 degree 360 degree
- 17. 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 0 degree 10 degree 20 degree 30 degree 40 degree 50 degree 60 degree 70 degree 80 degree 90 degree 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 90 degree 100 degree 110 degree 120 degree 130 degree 140 degree 150 degree 160 degree 170 degree 180 degree 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 180 degree 190 degree 200 degree 210 degree 220 degree 230 degree 240 degree 250 degree 260 degree 100 200 300 400 500 Intensity(a.u.) Raman shift (cm-1 ) 280 degree 290 degree 300 degree 310 degree 320 degree 330 degree 340 degree 350 degree 360 degree
- 18. Conclusion • Growth temperature, Mo: S ratio, presence of seeding promotor play important role in the feature of MoS2 monolayer • Exfoliated 1T’-MoTe2 flakes have different properties with different thicknesses • 1T’-MoTe2 displays a polarization-resolved raman spectrum with a 90 degree period cycle, indicating the material is anisotropic
- 19. Acknowledgement Lin Zhou Xiang Ji Weisun Leong Pin-Chun Shen Pingge He Yongjian Tang Everyone else in the lab Professor Jing Kong Professor Kathy Aidala • Mount Holyoke College • Massachusetts Institute of Technology • NSF Science Technology Center for Integrated Quantum Materials
Editor's Notes
- It will be interesting to show what I did during the summer
- Molybdenum disulfide MoS2 nanofilm disinfect water using sunlight. Small bandgap allows the material to adsorb visible light and generate electron-hole pairs that react with water to generate reactive oxygen species which can then kill 99.9% of bacterial in water. Chemical vapor deposition is the chemical process that is often used by semiconductor industry to produce thin film. It’s basically a furnace.
- Within this long tube, we place two cubicles that contain sulfure and molybdum troxide at a fixed distance. Then we place silicone wafers upside down on the Mo cubical. The lower side of the wafer is usually spin-casted a layer of seeding promotor called PTAS to promote MoS2 growth. The whole system is aired with Ar gas at a 9sccm flow rate and heated at around 600 degree. so both sulfur and molybdum trioxide can vaporize and diffuse, and react on the surface of the silicon wafer.
- By main job task is to take AFM images at different conditions and look for the crucial factors that affect the features of MoS2, and here are three main factors that we
- Here are the two AFM images of samples grown at different tempature. You can see a clear difference between the two growth tempatures, because at higher tempature, the features are much brighter and higher contrast and that means the scanned features are much thicker than the sample grown at lower tempature. Therefore, our conclusion
- The growth of the crystal face is determined by the
- 1T prim
- Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from alaser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information.
- Raman spectroscopy
- What this means is that we polarize the light that is shone onto the sample and see how that changes the signals layout of the spectrum. And if there is a periodic change in the spectrum, that means the 1T—moTe2 is anisotropic, or in other words, it has different properties in different directions.
- So by changing polarization lens every 30 degree and took a spectrum at that point, we were able acquire a series of data point, and by just looking at it, we can definitely spot a difference among the different spectra, especially the two peaks that are pointed by arrows. And it seems that there is a periodic pattern exists, and in order to identify the how many degrees consist a cycle
- We divide the spectra into a group of every 90 degree, and if you compare these graphs with each other, they are almost identical, so the pattern repeat itself every 90 degrees