Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping

Abstract

Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Progress on lanthanide-based organic–inorganic hybrid phosphors

“…UC nanocrystals are luminescent nanomaterials that convert NIR excitation into visible emission (two low energy photons are ''added up'' to give one high energy photon). 20 Outlined examples comprehend the blue and green emission colours of the NaYF 4 : Yb,Er,Gd and NaYF 4 : Yb,Tm,Gd NPs, capped with OA 20 and the reddish emission colour of the NaYF 4 : Yb,Er NPs functionalised with diphosphonic acid, respectively. 18 Another stimulating report is the white light emission formed by the overlap of blue (450 and 475 nm, Tm 3+ ), green (545 nm, Ho 3+ ) and red (650 and 695 nm, Tm 3+ , Ho 3+ ) UC radiations of OA-grafted NaYF 4 : Yb, Ho, Tm nanorods.…”

Tailored Near‐Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super‐Exchange between Divalent Manganese Ions

“…It has hence been crucial to design (nano) materials that exhibit both emission and excitation of luminescence in the NIR region for in vitro and in vivo imaging applications 24, 25. For example, rare‐earth‐doped nanocrystals have attracted considerable interest in recent years as potential candidates for high‐resolution bioimaging because such nanocrystals may exhibit NIR upconversion (UC) emission upon excitation by a 976 nm laser diode (LD) 26, 27, 28, 29. However, the accompanying intrinsically strong VIS emission that is additionally occurring in these rare‐earth‐doped materials limits the emission penetration depth in biological tissue because of the tissue's low transparency for wavelengths below 600 nm 30…”

Transition Metal‐Involved Photon Upconversion

“…However, Mn 2+ substituted for Y 3+ in NaYF 4 can result in an extra F ‐ ion on the grain surface and induces transient electric dipoles with their negative poles pointing outward. This effect would substantially hinder the diffusion of the F ‐ ions required for crystal growth from the solution to the grain surface owing to charge repulsion, resulting in retardation of NaYF 4 nanocrystal growth 55, 56. Similar phase transformation phenomena were observed in NaLnF 4 :Yb/Er (Ln = Lu, Yb, Gd) UC nanoparticles,57 with simultaneous phase/size control and significantly enhanced UC intensity via Mn 2+ doping.…”

“…The hexagonal phase changes to the cubic phase after doping with a sufficient amount of Mn 2+ . Generally, dopant ions with larger ionic radii favour hexagonal structures, whereas smaller dopant ions tend to produce the cubic phase in the final products 56. In this case, the smaller Mn 2+ ion incorporated into NaYF 4 nanocrystals favours the formation of a pure cubic phase.…”