Ge1−xSnx alloy quantum dots with composition-tunable energy gaps and near-infrared photoluminescence
Low-cost, less-toxic, and abundantly-produced Ge1−xSnx alloys are an interesting class of narrow energy-gap semiconductors that received noteworthy interest in optical technologies. Incorporation of α-Sn into Ge results in an indirect-to-direct bandgap crossover significantly improving light absorption and emission relative to indirect-gap Ge. However, the narrow energy-gaps reported for bulk Ge1−xSnx alloys have become a major impediment for their widespread application in optoelectronics. Herein, we report the first colloidal synthesis of Ge1−xSnx alloy quantum dots (QDs) with a narrow size dispersity (3.3 ± 0.5–5.9 ± 0.8 nm), a wide range of Sn compositions (0–20.6%), and composition-tunable energy-gaps and near-infrared (IR) photoluminescence (PL). The structural analysis of the alloy QDs indicates linear expansion of the cubic Ge lattice with increasing Sn, suggesting the formation of strain-free nanoalloys. The successful incorporation of α-Sn into crystalline Ge has been confirmed by electron microscopy, which suggests the homogeneous solid solution behavior of QDs. The quantum confinement effects have resulted in energy gaps that are significantly blue-shifted from bulk Ge for the Ge1−xSnx alloy QDs with composition-tunable absorption onsets (1.72–0.84 eV for x = 1.5–20.6%) and PL peaks (1.62–1.31 eV for x = 1.5–5.6%). Time-resolved PL (TRPL) spectroscopy revealed microsecond and nanosecond timescale decays at 15 K and 295 K, respectively, owing to the radiative recombination of dark and bright excitons as well as the interplay of surface traps and core electronic states. Realization of low-to-non-toxic and silicon-compatible Ge1−xSnx QDs with composition-tunable near-IR PL allows the unprecedented expansion of direct-gap Group IV semiconductors to a wide range of biomedical and advanced technological studies.