Dear all,
I have purchased a sample of flaky tungsten disulfide (WS2) and I'd like to depict more specifically (and more scientifically) which kind of WS2 it is. I found that you can call it 2H-WS2, but I am not completely sure if this is correct for this specific material (the supplier calls it "amorphous" which is clearly wrong). As far as I know, 2H describes the material's crystal structure, but I couldn't find any website explaining this denomination in detail. Moreover, I found a paper on 1T-WS2 and to be honest I have no clue how to discern the two. Can you help me?
Kind regards, Dietmar Haba
2H corresponds to 2 layers per H(exagonal) unit cell. 1T - one layer per (Trigonal) unit cell. Look for "polytype notations" and "polytype". In general you have to use diffraction methods (XRD, SAED, etc.). updated
2H corresponds to 2 layers per H(exagonal) unit cell. 1T - one layer per (Trigonal) unit cell. Look for "polytype notations" and "polytype". In general you have to use diffraction methods (XRD, SAED, etc.). updated
The prefix before the chemical formula is a notation for identifying polytypes in layered compounds. The number is the number of layers, the letter the crystal system (H for hexagonal, T for trigonal, other letters for other crystal systems). If there are more polytypes with the same number of layers and the same crystal system, additional number is added, e.g. 2M1 and2M2 in some micas.
How to identify the polytype? Most easily by diffraction. Each polytype has different lattice parameters and thus different diffraction pattern. Simple polytypes can be distinguished in powder diffraction patterns, in more complicated cases single-crystal x-ray diffraction or electron diffraction may be necessary. A mixture of several polytypes in the same sample is also not unusual.
I do not know the polytypism in WS2 well enough to suggest a detailed identification procedure in this case, but I would start by taking a powder diffraction pattern of the material and comparing it with the simulated patterns of the known WS2 polytypes.
Thanks a lot to both of you. This was very helpful. The article Polymorphism (materials science) on wikipedia covers this topic, although there is still not a lot information given there.
i have read out 3-4 papers, one of them is published by journal, the nature. they are suggesting the 2H has Hexagonal symmetry and is the trigonal prismatic whereas 1T has tetragonal symmetry and is octahedral.
Yan, J., Cruz, M. A. Dela, Cook, B., & Varga, K. (2015). Structural , electronic and vibrational properties of few-layer. Nature Publishing Group, (November), 1–13
Kim, J., Lee, Z., & Lee, Z. (2018). Phase Transformation of Two-Dimensional Transition Metal Dichalcogenides. 48(2), 43–48
Voiry, D., & Chhowalla, M. (2015). Phase engineering of transition metal dichalcogenides. Chemical Society Reviews
Max Nickolsky Dietmar Haba Lukáš Palatinus
You can do the XRD of your sample to determine the 2H or 1T phase of WS2. Usually 2H WS2 has 002 characteristics peak at 14.5 degree while 1T has about 9.4 degree.
Thanks
Dear Dietmar Haba,
In the 2H structure the metal atom coordination is trigonal prismatic and the material is semiconducting. In the 1T structure the metal coordination is trigonal antiprismatic (or octahedral) inferring to the material a metal character. The phase transition is triggered by the electrochemical incorporation of S vacancies (obtained by electrochemical etching), which on the one hand injects electrons into the framework of S–Mo–S and on the other hand facilitates the sliding of S planes. Density functional theory (DFT) calculations can show that O doping in the framework of S–Mo–S decreases the energy barrier for forming S vacancies and stabilizes the 1T-phase by occupying the 4d orbital of Mo. Calculations further show that the presence of S vacancies and O incorporation not only reduces the bandgap of MoS2. S vacancies are higher in 1T than 2H. 1T-MoS2> S vacancies > O doping >2H-MoS2. Raman Raman spectroscopies can be used to performance comparison of excitations, may useful in reporting localization(peak curve) to small islands in the film or includes domains across the majority of the film can be conducted. If the island is low means doping is less we will prefer 2H for semiconductors.
Hope this is helpful for you.
Ashish
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