Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have layered structures with excellent tribological properties. *
Since the energy difference between hexagonal-molybdenum ditelluride (2H-MoTe2) and distorted octahedral-molybdenum ditelluride (1T’-MoTe2) is very small among the transition metal dichalcogenides (TMDCs), MoTe2 becomes one of the most promising candidates for phase engineering. *
In the article “Comparative analysis of frictional behavior and mechanism of molybdenum ditelluride with different structures” Lina Zhang, Xinfeng Tan, Jianguo Jiao, Dan Guo and Jianbin Luo report that they found that the friction force and friction coefficient (COF) of 2H-MoTe2 were an order of magnitude smaller than those of 1T’-MoTe2 by the atomic force microscope (AFM) experiments. *
The friction difference between 1T’-MoTe2 and 2H-MoTe2 was further verified in molecular dynamics (MD) simulations. The density functional theory (DFT) calculations suggest that the friction contrast is related to the difference in sliding energy barrier of the potential energy surface (PES) for a tip sliding across the surface. The PES obtained from the DFT calculation indicates that the maximum energy barrier and the minimum energy path (MEP) energy barrier of 2H-MoTe2 are both smaller than those of 1T’-MoTe2, which means that less energy needs to be dissipated during the sliding process. The difference in energy barrier of the PES could be ascribed to its larger interlayer spacing and weaker Mo–Te interatomic interactions within the layers of 2H-MoTe2 than those of 1T’-MoTe2.
The obvious friction difference between 1T’-MoTe2 and 2H-MoTe2 not only provides a new non-destructive means to detect the phase transition by the AFM, but also provides a possibility to tune friction by controlling the phase transition, which has the potential to be applied in extreme environments such as space lubrication. *
Friction measurements were carried out using atomic force microscopy (AFM) under ambient conditions. *
NANOSENSORS™ PointProbe® Plus PPP-LFMR Silicon AFM probes which are especially designed for lateral or friction force microscopy and are optimized for a high sensitivity to lateral or friction forces with their very soft and thin AFM cantilevers, were used.
The normal and lateral force constants of the NANOSENSORS PPP-LFMR AFM probes were calibrated using the thermal noise method and improved wedge calibration method, respectively. *
In the lateral force mode, friction maps were measured with gradient loading applied to a square region of 200 nm × 200 nm with a scanning frequency of 2 Hz. *
By further reducing the scan range to 20 nm × 20 nm and increasing the scanning frequency to 20 Hz, the friction maps including atomic-scale stick–slip motion information were obtained, as shown in Figs. 3(a) and 3(f). *
*Lina Zhang, Xinfeng Tan, Jianguo Jiao, Dan Guo and Jianbin Luo
Comparative analysis of frictional behavior and mechanism of molybdenum ditelluride with different structures
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