Anomalous lattice contraction and optical properties in GaN:Nd³⁺ nanocrystals

This study reports on the anomalous lattice contraction and strain-engineered optical properties of GaN:Nd³⁺ nanocrystals (0.5-3%). The materials were synthesized using the sol-gel method and annealed in an ammonothermal process, demonstrating good crystalline quality with a pure hexagonal wurtzite structure. Despite the mismatch between the ionic radii of Ga³⁺ (0.62 Å) and Nd³⁺ (0.98 Å), X-ray diffraction analysis revealed an anomalous lattice contraction. This unique feature, attributed to the Gibbs-Thomson effect in nanocrystallites, induces compressive strain that overrides steric hindrance. The structural strain was found to directly modulate the optoelectronic properties, resulting in a linear blueshift of the optical energy gap (E_g) from 3.187 eV to 3.213 eV. Furthermore, optical analysis showed that UV excitation leads to energy transfer, resulting in bands arising from the f-f transitions of Nd³⁺ ions in the near-infrared range. The optimal concentration was determined to be 2% for Nd³⁺. These findings highlight the potential of Nd-doped GaN nanostructures for the development of advanced, strain-engineered optoelectronic devices.