  In this paper, we study theoretically the electron and spectral properties of
Ge1-xSnx systems, including alloys, cubic- and spherical quantum dots. The
single-particle electron and hole states are calculated within the sp3d5s*
tight-binding approach and used in further modeling of the optical properties.
We systematically study the interplay of Sn-driven indirect-direct band-gap
transition and the quantum confinement effect in systems of reduced
dimensionality. We demonstrate the regime of sizes and composition, where the
ground state in Ge1-xSnx quantum dot is optically active. Finally, we calculate
absorbance spectra in experimentally-relevant colloidal quantum dots and
demonstrate a satisfactory agreement with experimental data.
