Quantum-confined blue photoemission in strain-engineered few-atomic-layer 2D germanium

Abstract The indirect bandgap (0.67 eV) of bulk germanium (Ge) remains a major bottleneck towards its applications in optoelectronics, enabling poor optical features particularly photoluminescence. Obtaining desired optical functionalities, either by synthesizing few-atoms-thick two-dimensional (2D) germanium on silicon-based substrates, or by inducing an appreciable structural engineering in its crystal lattice, has long remained a formidable challenge yet to be mitigated. Herein, a facile vacuum-tube hot-pressing strategy to synthesize strain-engineered few-atomic-layer 2D germanium nanoplates (Ge-NPts) directly on fused silica substrate (SiO2) is developed. Leveraging from the unique mismatch between coefficient of thermal expansion of Ge and SiO2 substrate at elevated temperatures (700 °C), and under hydrostatic pressure (~2 GPa), a biaxial compressive strain of ~1.23 ± 0.06% in Ge lattice is engineered, causing a transition from indirect to direct bandgap with an ultra-large opening of 2.91 eV. Strained Ge nanoplates, consequently, display a remarkable 42-fold blue photoluminescence (at 300 K) compared to bulk Ge, accompanied by robust quantum-confinement effects, probed by the quantum-shift ~114 meV with decreasing thicknesses of Ge nanoplates.