2023 In-House Research Highlights
Ab-Initio Prediction of Giant Stokes Shifts for Vacancies inMonolayer Phosphorene and Tin (II) Monosulfide

Project Summary: The two-dimensional semiconductors phosphorene and isoelectronic SnS have attracted extensive research interest for potential applications in optoelectronics, spintronics, catalysis, sensors, and energy conversion. We performed density functional theory (DFT) calculations to accurately compute formation energies, charge transition levels, and electronic properties of vacancies in both materials utilizing a correction scheme to ensure appropriate electrostatic boundary conditions for charged defects in 2D materials. Depending on the charge state of the vacancy, the easily distorted corrugated rectangular lattices of phosphorene and SnS undergo sizable relaxations and, in some cases, symmetry-breaking reconstructions, creating defect states within the gap that electrons can occupy at a lower energetic cost. Due to the significant atomic relaxations between the equilibrium defect structures in different charge states, optical charge transitions involving both types of vacancies exhibit sizeable Frank-Condon shifts with large Stokes shift of 0.3 eV for vacancies in phosphorene and giant Stokes shifts of over 1 eV for both cation and anion vacancies in SnS. These substantial shifts provide opportunities for increased efficiency in light emission diode, solar cell, and solar concentrator applications.

Published in Physical Review Materials 5, 124004 (2021), ibid. 6, 044003 (2022).

What Has Been Achieved: The electronic and optical properties due to the presence of charged vacancy defects introduced in equilibrium or during synthesis of two-dimensional phosphorene and tin monosulfide have been predicted. The results show that the 2D structure is easily distorted by the defects. These relaxations and reconstructions lead to large changes in the optical response during light adsorption and emission.

Importance of the Achievement: This is the first study demonstrates the possibility of achieving optical shifts in materials that are one to two orders of magnitude larger than other 2D and bulk materials. These effects can provide new opportunities for the design of light emission diodes, solar cells, and solar concentrators.

Unique Feature(s) of the MIP that Enabled this Achievement: The MIP mission in serving the user community with data on the effect of charged defects, dopants, and impurities on 2D materials properties directly underpins and motivates this sustained effort on applying ab-initio methods and charge corrections schemes to various relevant materials systems.

Publications: Biswas Rijal, Anne Marie Z. Tan, Christoph Freysoldt, and Richard G. Hennig, Charged vacancy defects in monolayer phosphorene, Physical Review Materials 5, 124004 (2021); Anne Marie Z. Tan, Maria A. Garcia, and Richard G. Hennig, Giant Stokes shift for charged vacancies in monolayer SnS, Physical Review Materials 6, 044003 (2022).

Acknowledgements: This work was supported by the National Science Foundation through the 2D Crystal Consortium − Materials Innovation Platform (2DCC-MIP) under the cooperative agreement DMR-1539916 and under Awards No. DMR-1748464 and No. OAC-1740251. Computational resources were provided by the University of Florida Research Computing Center. Part of the research was performed while the authors visited the Institute for Pure and Applied Mathematics (IPAM), which is supported by the National Science Foundation (Grant No. DMS-1440415).