Molybdenite (MoS2, molybdenum sulfide)
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Molybdenum disulfide (MoS2) is a transition metal dichalcogenide and is formed by two-dimensional (2D) graphene-like S-Mo-S layers. The bonding within the layers is typically covalent and much stronger than the van der Waals forces between the layers, while the layers are held together by weak noncovalent interactions. In each layer, a Mo atom is covalently connected to six sulfur atoms, while each sulfur atom is bonded to three Mo atoms. On one hand, MoS2-based catalysts are attractive because they are active for both methanation and water-gas shift reactions, allowing for the reaction to occur at low H2/CO ratios. On the other hand, as a semiconducting compound, MoS2 is characterized by its layered crystal structure. It has been of interest because the behavior of electrons in the layers is quasi-two-dimensional. Plus, the interaction between layers can be modified by incorporating impurities between the layers in a process known as intercalation.

Figure 2210a shows the dependence of the appearance of electron diffraction and Kikuchi patterns on the TEM sample thickness and the accelerating voltages of the incident electron beam for molybdenite crystals. Diffraction patterns can be observed in the region below Curve A, while Kikuchi patterns can be observed between Curves A and B. Both diffraction and Kikuchi patterns can be simultaneously observed in the region slightly below and above Curve A. In general, the Curve B gives the maximum thickness for which dislocations can be observable.

The dependence of appearance of diffraction and Kikuchi patterns on the TEM sample thickness and the accelerating voltages of the incident electron beam

Figure 2210a. The dependence of the appearance of diffraction and Kikuchi patterns on the TEM sample thickness and the accelerating voltages of the incident electron beam for molybdenite crystals. Adapted from [1]

Figure 2210b shows the sulfur and molybdenum peaks overlap in the EDS spectrum, but are separated and sharp in the WDS spectrum.

Comparison of the EDS and WDS spectra taken from MoS2

Figure 2210b. Comparison of the EDS and WDS spectra taken from MoS2.

Table 2210a. Properties of MoS2 compositions.

 
Parameter
Y2D (N/m)
118–141
Poisson’s ratio
~0.3
Indirect gap
~ 1.2 eV (bulk)
Dm (eV)
~11.7

Table 2210b. Structural properties of some MoS2 compositions.

Composition
Space group Point group
Lattice constant
(Å)
Note
Reference
Monolayer-MoS2
P-6m2 (187)        
MoS2-based
P63/mmc (194)   a = 3.15~3.16 and c = 12.3~12.58 with internal displacement parameter of z=0.12 MoS2-based catalysts are attractive because they are active for both methanation and water-gas shift reactions  
MoS2
  Used as dry lubricants due to their layered nature: atoms are strongly bonded within the same plane but weakly attached to sheets above and below by van der Waals force  
MoS2
Octahedral (D3d)   [2-6]
MoS2 with odd number of layers
Trigonal prismatic (D3h) Exhibit intrinsic piezoelectric property as a result of its lack of centrosymmetry in crystal structure [2-6]
MoS2 with even number of layers
D6h Piezoelectricity will vanish in centrosymmetrical MoS2 [2-6]
2H-MoS2
     
3R-MoS2
R3m (160)        
a: hexagonal lattice constant; c: out-of-plane lattice constant; z: internal displacement parameter.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[1] Uyeda, Ryozi; Nonoyama, Minoru, The Observation of Thick Specimens by High Voltage Electron Microscopy. Experiment with Molybdenite Films at 50-500 kV, Japanese Journal of Applied Physics, 6(5), pp. 557 (1967).
[2] Yang, R. S., Qin, Y., Dai, L. M. & Wang, Z. L. Power generation with laterally packaged piezoelectric fine wires. Nature Nanotechnol. 4, 34–39 (2009).
[3] Ong, M. T. & Reed, E. J. Engineered piezoelectricity in graphene. ACS Nano 6, 1387–1394 (2012).
[4] Li, Y. L. et al. Probing symmetry properties of few-layer MoS2 and h-BN by optical second-harmonic generation. Nano Lett. 13, 3329–3333 (2013).
[5] Wakabayashi, N.; Smith, H. G.; Nicklow, R. M. Phys. Rev. B 1975, 12, 659–665.
[6] Zhiming M. Wang, MoS2: Materials, Physics, and Devices, 2014.

 

 

 

 

 

 

 

 

 

 

 

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