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

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.