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 (Eg) 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.