Computational design of new polymorphs of two-dimensional semiconductor InSe with enhanced interlayer interaction

Atomically thin, two-dimensional InSe has attracted considerable attention due to its widely tunable band gap and high electron mobility. The intriguingly high dependence of band gap on layer thickness remains poorly understood, and is generally attributed to quantum confinement effect. We demonstrated via systemic first-principles calculations in our previous work that strong interlayer coupling may be mainly responsible for this phenomenon, especially in the fewer-layer region. The interlayer coupling was also found to be an essential factor influencing other thickness-dependent material properties such as indirect-to-direct band gap transitions, fan-like phonon frequency diagrams, carrier mobilities, etc. In this work, by combining global structure search algorithm and first-principles calculations, we strikingly discovered two new polymorphs of InSe consisting of the monolayer in point group D_3d, distinct from the known one in point group D_3h. The new polymorphs show thermodynamic and lattice dynamical stability, and large transition barrier to the existing phases. One newly discovered polymorph exhibits the enhanced interlayer coupling, manifested by the most tunable band gap and the highest electron mobility among all the InSe phases.