Few of us have gone through childhood without the unpleasant experience of our skin peeling off after an injury. In the world of materials science, the tiny atoms that make up a material generally experience significant grief when they lose a few of their neighbors through the creation of a surface. These surface atoms start to gravitate more strongly towards their remaining neighbors, sometimes resulting in beautiful atomic-scale rearrangements.
Two-dimensional (2D) layered materials have taken the world of materials science by storm. Unlike other materials, one would expect that peeling off layers to create a surface in a layered material should not cause much grief to the surface atoms. A team led by Prof QUEK Su Ying from Department of Physics in NUS has found, however, that even in layered materials, surface atoms feel the loss of the missing neighboring layer enough to cling on more strongly to their remaining neighbors in the same layer. And these stronger surface interactions account for previously unexplained anomalous trends in the vibrational frequencies of these prototypical 2D materials.
Vibrational frequencies are routinely used to identify the thickness of 2D materials. For the vibration which involves adjacent layers moving against one another (see Figure). It was thought that the frequency should increase with thickness due to increased friction in thicker materials. By parameterizing high-level calculations into a simple physically-motivated model, the team found this to be indeed the case if the surface force constants are the same as in the bulk material. However, once the stronger surface force constants are accounted for, the frequency decreases with increasing thickness, in excellent agreement with experiments. Similar effects have since been found in other 2D layered materials.
These findings change the mindset that surface effects are not important in layered materials, and have implications for predicting vibrational frequencies and thermal properties in these systems.
Image shows vibrations in 2 layered, 3 layered and 4 layered 2D MoS2. Molybdenum atoms (red spheres) and Sulphur atoms (blue spheres) are bonded together by strong bond within each layer, but adjacent layers interact weakly. The blue shading in the background indicates the surface region where intralayer bonds are strengthened. (Image credit: LUO Xin and QUEK Su Ying)
References
1. Luo X, Zhao Y, Zhang J, Xiong Q, Quek SY. “Anomalous Frequency Trends in MoS2 Thin Films Attributed to Surface Effects.” Physical Review B. 88 (2013) 75320.
2. Zhao Y, Luo X, Li H, Zhang J, Araujo PAT, Gan CK, Zhang H, Quek SY. Dresselhaus MS, Xiong Q. “Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2.” Nano Letters. 13 (2013) 1007.
3. Sorkin V, Pan H, Shi H, Quek SY, Zhang YW. “Nanoscale Transition Metal Dichalcogenides: Structures, Properties and Applications.” Critical Reviews in Solid State and Materials Sciences. 39 (2014) 319.
