Two-dimensional V-shaped PdI2: Auxetic semiconductor with ultralow lattice thermal conductivity and ultrafast alkali ion mobility

• A two-dimensional semiconductor monolayer PdI2 has been predicted from its van der Walls bulk material. • PdI2 monolayer exhibits an extraordinary in-plane negative Poisson ratio (-0.183). • Ultralow lattice thermal conductivity (0.23∼0.34 Wm-1 K-1) have been revealed in monolayer PdI2 at room temperature. • The V-shaped crystal structure makes PdI2 monolayer an excellent platform for alkali ions migration (0.08 eV to 0.152 eV). Using first-principles calculations, we predicted a multifunctional two-dimensional (2D) semiconductor PdI 2 from its van der Walls layered bulk. Monolayer or multilayer PdI 2 obtained by mechanical exfoliation exhibit high stability as freestanding 2D materials. Originating from its special corner-shared quasi-plane crystal structure, an amazing in-plane negative Poisson's ratio in monolayer PdI 2 has been confirmed. The layer dependent electronic and thermal properties have also been discussed in detail. Encouragingly, it has been found that strong phonon scattering which is induced by low symmetry will further reduce the lattice thermal conductivity in monolayer PdI 2 at room temperature. Besides, the monolayer PdI 2 is seen as an excellent platform for alkali ions migration with ultralow diffusion barriers, benefiting from its distinctive V-shaped crystal structure. Considering these interesting findings, we believe the 2D PdI 2 will be a promising candidate for future's low dimensional electron devices.