First Principles Investigation on Structural and Optoelectronic Properties of newly designed Janus Lead Halides PbXY (X, Y = F, Cl, Br, I )

Inspired by the beauty in the asymmetry, we design a novel class of Janus structures PbXY (X,Y = F, Cl, Br, I) and propose it for the solar mediated photocatalytic water splitting hydrogen production as well as for the photovoltaic solarcell applications.These novel Janus structures show large modulation in layer thickness, bond lengths and bond angles due to asymmetry of two sides. Charge analysis shows that covalent bonding for less electronegative atoms (I and Br) and ionic bonding for more electronegative (Cl and F) atoms. Strong dual bonding like ionic one side and covalent other side is observed when heavy and lighter atoms are part of the same Janus structures. The as designed Janus structures show good dynamical stability through phonon calculations. Basic electronic structure using Generalised Gradient Approximation (GGA) reveal both direct and indirect nature of band gap with large tunability
varying from 2.5 to 3.5 eV. Such a large tunability of band gap can be exploitedfor multifunctional applications. Heyd-Scuseria-Ernzerhof (HSE) electronic structure calculations are performed for more accuracy and wider band gaps are predicted for these Janus structures. The calculated electron and hole effective masses show robust charge carrier dynamics. The orbital resolved electronic density of states (DOS) shows that the conduction band edge is composed of pz orbital of Pb atom. The partial charge density calculated at conduction band minimum (CBM) also support the result obtained from PDOS analysis. Breaking of centrosymmetry, covalent bonding along z-direction, polarization in the out of plane direction, the z-oriented orbitals of CBM all points that these materials are suitable for shift current generation. The calculated optical absorption spectra show that the Janus structures are suitable for visible light absorption. The calculated potential difference between the top and bottom layer show significant variation and maximum (1.02 eV) is observed for PbClF. Further, we show that combining both potential difference and HSE bandgap, valence band maximum (VBM) and CBM straddle the water redox potentials, thus making the Janus structures suitable for hydrogen evolution reaction (HER) and oxygen evolution reactions (OER) on the opposite sides of the Janus structures.