Tuning Ultra-Narrow Direct Bandgap in α-Sn Nanocrystals: A CMOS-Compatible Approach for THz Applications

α-Sn has recently been attracting significant interest due to its unique electronic properties. However, alternative strategies to the conventional epitaxial growth on InSb to stabilize it at room temperature and the ability to manipulate its bandgap are still a challenge. In this work, a complementary metal oxide semiconductor (CMOS)-compatible process employing microwave irradiation is used to synthetize α-Sn nanoparticles (NPs) of different size on a Si substrate. Morphological characterizations suggest the possibility to control the average Sn NPs size by means of a combined dewetting and coalescence process induced by the microwaves on Sn films. Transmission Electron Microscopy (TEM) and Synchrotron Radiation-Grazing Incidence X-ray Diffraction (SR-GIXRD) analyses confirm the stabilization of the α-Sn phase within an oxide shell, while X-ray Photoelectron Spectroscopy (XPS) measurements allow tracking the oxide shell evolution and reveal the opening of a bandgap. Optical investigation demonstrates unprecedented tunability of the ultranarrow bandgap energy of α-Sn between 64 and 137 meV (15-35 THz). The observed bandgap modulation with NPs size is consistent with a quantum confinement effect, which suggests the proposed approach as an effective strategy for tuning the α-Sn bandgap and broadening its potential for a CMOS-compatible integration in next-generation terahertz technologies.