Optical Absorption in Hexagonal-Diamond Si and Ge Nanowires: Insights from STEM-EELS Experiments and <i>Ab Initio</i> Theory

Hexagonal-diamond (2H) group IV nanowires are key for advancing group IV-based lasers, quantum electronics, and photonics. Understanding their dielectric response is crucial for performance optimization, but their optical absorption properties remain unexplored. We present the first comprehensive study of optical absorption in 2H-Si and 2H-Ge nanowires combining high-resolution STEM, monochromated EELS, and <i>ab initio</i> simulations. The nanowires, grown <i>in situ</i> in a TEM as nanobranches on GaAs stems, show excellent structural quality: single crystalline, strain-free, minimal defects, and no substrate contamination, enabling access to intrinsic dielectric response. 2H-Si exhibits enhanced absorption in the visible range compared to cubic Si, with a marked onset above 2.5 eV. 2H-Ge shows absorption near 1 eV but no clear features at the direct bandgap, as predicted by <i>ab initio</i> simulations. A peak at around 2 eV in aloof-beam spectra is attributed to a thin 3C-Ge shell. These findings clarify the structure-optical response relationships in 2H materials.