White light upconversion in NdOHCO3 to Nd2O3 nanocrystals: Structural and optical transition

In a previous report, some morphological, structural, and optical properties of NdOHCO3 to Nd2O3 transition were presented. This paper continues investigating this interesting inorganic nanomaterial, proposing the creation of the coordination complex [Nd(NH3)6]3+ ion during crystalline growth as a crucial step for the growth of NdOHCO3 nanocrystals. A concise experimental-theoretical model is detailed in order to understand the spherical-like crystalline growth observed. The orthorhombic and cubic phases for NdOHCO3 and Nd2O3 nanocrystal samples were identified from the X-Ray diffraction results. In addition, the average grain size was estimated for each sample: NdOHCO3 ~15.31–18.82 nm and Nd2O3 ~45.10–59.01 nm. Other optical parameters like absorbance, transmittance, and reflectance are examined. The Urbach energy (Eu) model allows us to investigate the degree of absorption. The Maxwell-Boltzmann statistical (MBS) model results fit well with the experimental photoluminescence (PL) spectroscopy data registered, implying that native intrinsic crystalline defects behave as free particles and the emission bands overlap 4f4s2→4f4s2 electronic intra-transitions. The PL spectra show two emission bands: Yellow ~3.10–1.55 eV and Red > 1.99 eV. Finally, the trap densities of NdOHCO3 (~1.08 × 1014 cm−3) and Nd2O3 (~9.64 × 1013 cm−3) were estimated.