Amplification of Self-Sensitization at Nanoscale in NIR-Upconverting NaErF ₄ Core–Shell Crystals: Implications for Energy Applications

Enhancement of the self-sensitized near-infrared responsive (NIR)-upconversion (UC) process was examined on the core–shell (CS) type of NaErF4 crystals in the form of micrometer-sized particles mimicking bulk-dimension and nanosized particles via three approaches. Manipulation of the pristine NaErF4 core, inert shelling of the unmanipulated pristine core, and a combination of both core manipulation and inert shelling were undertaken. The effect of the third or combined process appeared to be synergistic, leading to an unprecedented augmentation of UC photoluminescence intensity when the core particle was taken at nanoscale. To understand the basis of such magnified optical property at nanoscale, synchrotron X-ray diffraction (SXRD) based on synchrotron source and theoretical modeling using density functional theory (DFT) were conducted. Assessment of local disorder by atomic pair distribution function (PDF) analysis and microstrain by Williamson–Hall’s analysis of SXRD revealed a relatively higher atomic-level disorder and microstrain existing in the manipulated (Li+ doped) nanosized core. The results obtained from DFT supported the structural feasibility of the Li+-doped engineered core and indicated a progressive increase in misfit strain with an increasing Li+ concentration. The experimental misfit strain calculation obtained by differential lattice parameter analysis from Rietveld refinement of SXRD confirmed a hyper-augmentation of the misfit strain, in the nanoscale domain, while no discernible difference of the same was observed in the microscale domain. For the first time, the extent of lattice mismatch was shown to get amplified at nanoscale for a CS crystal system using SXRD aided with DFT. The atomic-level study herein appeared to highlight the fundamental properties at nanoscale. Such engineered nanocrystals may be suitable for potential energy applications.