Lone-pair electrons induced anomalous enhancement of thermal transport in strained planar two-dimensional materials

Strain engineering is one of the most promising and effective routes towards continuously tuning thermal transport due to the flexibility and robustness. However, previous studies mainly focused on quantifying how the thermal conductivity (kappa) is modulated by strain, while the fundamental understanding on the electronic origin has yet to be explored. In this paper, we establish a microscopic picture of the lone-pair electrons driving strong phonon anharmonicity in two-dimensional (2D) materials beyond the traditional three-dimensional (3D) systems. We provide solid explanation for the unexpectedly up to one order of magnitude enlarged kappa of a class of planar monolayers with bilateral tensile strain applied, which is in sharp contrast to the strain induced kappa reduction in graphene despite their similar planar structures. The anomalous positive response of kappa to tensile strain is attributed to the attenuated interaction between the lone-pair electrons and the bonding electrons of neighboring atoms, which reduces phonon anharmonicity and leads to the enlarged kappa. Our study uncovers the electronic origin of the strain modulated thermal transport, which would have great impact on future investigations related to energy nanotechnologies and applications.