  A method based on the particle swarm optimization (PSO) algorithm is
presented to design quasi-two-dimensional (Q2D) materials. With this
development, various single-layer and bi-layer materials in C, Si, Ge, Sn, and
Pb were predicted. A new Si bi-layer structure is found to have a much-favored
energy than the previously widely accepted configuration. Both single-layer and
bi-layer Si materials have small band gaps, limiting their usages in
optoelectronic applications. Hydrogenation has therefore been used to tune the
electronic and optical properties of Si layers. We discover two hydrogenated
materials of layered Si8H2 and Si6H2 possessing quasi-direct band gaps of 0.75
eV and 1.59 eV, respectively. Their potential applications for light emitting
diode and photovoltaics are proposed and discussed. Our study opened up the
possibility of hydrogenated Si layered materials as next-generation
optoelectronic devices.
