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 (SiO 2 ) is developed. Leveraging from the unique mismatch between coefficient of thermal expansion of Ge and SiO 2 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. • Technological advancements of germanium in optoelectronics have remained restricted due to its indirect bandgap (0.67 eV). • Realization of Ge in ultrathin 2D crystals with strain-engineered lattice can exhibit remarkable property modulation. • A vacuum-tube hot-pressing (VT-HP) strategy to synthesize strain-engineered few atomic layer 2D Ge has been introduced. • Coefficient of thermal expansion mismatching causeda biaxial compressive strain of ~1.23 ± 0.06% in 2D Ge lattice. • An ultra-bright 42-fold blue photoemissionin 2D Ge was achieved due to an indirect to direct bandgap transition (~2.91 eV).