Self-Focusing by Ostwald Ripening: A Strategy for Layer-by-Layer Epitaxial Growth on Upconverting Nanocrystals

Abstract

We demonstrate a novel epitaxial layer-by-layer growth on upconverting NaYF(4) nanocrystals (NCs) utilizing Ostwald ripening dynamics tunable both in thickness and composition. Injection of small sacrificial NCs (SNCs) as shell precursors into larger core NCs results in the rapid dissolution of the SNCs and their deposition onto the larger core NCs to yield core-shell structured NCs. Exploiting this NC size dependent dissolution/growth, the shell thickness can be controlled either by manipulating the number of SNCs injected or by successive injection of SNCs. In either of these approaches, the NCs self-focus from an initial bimodal distribution to a unimodal distribution (σ <5%) of core-shell NCs. The successive injection approach facilitates layer-by-layer epitaxial growth without the need for tedious multiple reactions for generating tunable shell thickness, and does not require any control over the injection rate of the SNCs, as is the case for shell growth by precursor injection.

Synthesis of β-Phase NaYF4:Yb,Er Upconversion Nanocrystals and Nanorods by Hot-Injection of Small Particles of the α-Phase

“…The fast release of monomers during the rapid dissolution of the -phase results in a monomer concentration so high that the particles of the less soluble -phase grow under the condition of monomer supersaturation. This leads to focussing of their particle size distribution as can be confirmed by numerical simulations based on the LSW-theory of Ostwaldripening [13,56,57].…”

“…During the last years, a large number of research papers was pub-lished on the synthesis of NaREF 4 nanocrystals (RE = rare-earth), since NaREF 4 and LiREF 4 are attractive host lattices for a variety of functional optical materials such as upconversion phosphors, [6][7][8][9][10][11][12][13][14] quantum-cutting phosphors, [15][16][17][18][19] laser materials, [20,21] and materials for solar cell applications [22][23][24][25][26]. A variety of synthetic methods has been described for producing solvent-dispersible sodium rare-earth tetrafluoride nanocrystals with controlled size, shape and capping ligand, including thermal decomposition, [7,[27][28][29][30][31][32] co-precipitation, [6,11,[33][34][35][36][37][38][39][40] or solvothermal methods [41][42][43][44][45][46][47]…”

808 nm driven Nd3+-sensitized upconversion nanostructures for photodynamic therapy and simultaneous fluorescence imaging

“…21 YbCl 3 ·6H 2 O (99.9%, 0.04 mmol), YCl 3 ·6H 2 O (99.9%, 0.445 mmol) and HoCl 3 ·6H 2 O (99.9%, 0.005 mmol) were dissolved in 3 mL of OA and 7.5 mL of ODE. The mixture was heated at 150°C under argon protection.…”

Current Advances in Lanthanide‐Doped Upconversion Nanostructures for Detection and Bioapplication

“…Veggel and co‐workers synthesized core–shell nanostructures through this method 129. First, they synthesized cubic (α) NaYF 4 NCs via thermal decomposition.…”