Optical Transitions and Excitonic Properties of Ge_1-xSn_x Alloy Quantum Dots

Using hybrid functional calculations and experimental characterization we analyze optical properties of 2-3 nm Ge_1-xSn_x alloy quantum dots, synthesized by colloidal chemistry methods. Hybrid functional theory, tuned to yield experimental bulk band structure of germanium, reproduces directly measured properties of Ge_1-xSn_x quantum dots, such as lattice constants, energy gaps, and absorption spectra. Time-dependent hybrid functional calculations yield optical absorption in good agreement with experiments, and allow probing the nature of the dark excitons in quantum dots. Calculations suggest a spin-forbidden dark exciton ground state, which is supported by the changes in the photoluminescence lifetimes with temperature and tin concentrations. The synthesis and theoretical understanding of Ge_1-xSn_x alloy quantum dots will add to the overall tool box of low to non-toxic, silicon compatible Group IV semiconductors with potential application in visible to near infrared optoelectronics.