Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

Abstract

We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation. This general method optimizes the synthesis with initial heating rates up to 100 °C/minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms, and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these nanocrystals (NCs) into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC's size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.

Fig. 1.

TEM images of the β-NaYF₄-based UCNPs. (A, D, G, J) NaYF₄: Yb/Er (20/2 mol%) UCNPs. (B, E, H, K) NaYF₄: Yb/Tm (22/0.2 mol%) UCNPs. (F, I) NaYF₄: Yb/Ho (20/2 mol%) UCNPs. (C, L) NaYF₄: Yb/Ce/Ho (20/11/2 mol%) UCNPs. All scale bars represent 100 nm.

Fig. 2.

Structural and optical characterization of the β-NaYF₄-based UCNPs. (A) Powder XRD patterns of the NaYF₄: Yb/Er (20/2 mol%) UCNPs with different shapes. The peaks are indexed according to the standard XRD pattern of β-NaYF₄ (JCPDS file number: 28-1192). Insets are the corresponding geometrical models. (B) HRTEM image of a spherical NaYF₄: Yb/Er (20/2 mol%) UCNP. (C) HRTEM image of a NaYF₄: Yb/Er (20/2 mol%) NR. (D) HRTEM image of a NaYF₄: Yb/Er (20/2 mol%) hexagonal nanoprism. (E) HRTEM image taken from the edge of a NaYF₄: Yb/Er (20/2 mol%) hexagonal nanoplate. (F) Room temperature upconversion emission spectra of the NaYF₄: Yb/Er (20/2 mol%) and NaYF₄: Yb/Tm (22/0.2 mol%) UCNPs dispersed in hexane. Inset: Photographs of the upconversion luminescence from the NaYF₄: Yb/Er (20/2 mol%) (left) and NaYF₄: Yb/Tm (22/0.2 mol%) (right) NR dispsersions under 980 nm diode laser excitation. (G) Room temperature upconversion emission spectra of the NaYF₄: Yb/Ho (20/2 mol%) and NaYF₄: Yb/Ho (20/1 mol%) UCNPs dispersed in hexane. Inset: Photographs of the upconversion luminescence from the NaYF₄: Yb/Ho (20/2 mol%) nanoprism (left), NaYF₄: Yb/Ho (20/2 mol%) NR (center) and NaYF₄: Yb/Ho (20/1 mol%) NR (right) dispersions under 980 nm diode laser excitation.

Fig. 3.

NaYF₄ (AR = 1.4) NR superlattices. (A) TEM image of a monolayer superlattice of NRs that are oriented parallel to the substrate. The upper right inset is the corresponding SAWED pattern and the lower right inset is the corresponding SAED pattern. Both patterns are acquired from an area of ∼6.5 μm². (B) TEM image of a double-layer superlattice of NRs that are oriented parallel to the substrate. The upper left inset is the high-magnification TEM image acquired from the same domain. The upper right inset is the corresponding SAWED pattern and the lower right inset is the corresponding SAED pattern. Both patterns are acquired from an area of ∼6.5 μm². (C) Optical micrographs of the NaYF₄ (AR = 1.4) NR superlattices observed with crossed polarizers. The scale bar represents 30 μm. (D) AFM image showing the domain boundaries of the NR superlattices.

Fig. 4.

NaYF₄ (AR = 2.0) NR superlattices. (A) TEM image of a monolayer superlattice of NRs that are oriented parallel to the substrate. The lower right inset is the corresponding SAED pattern acquired from an area of ∼2 μm². (B) TEM image of a double-layer superlattice of NRs that are oriented parallel to the substrate. The lower right inset is the corresponding SAED pattern acquired from an area of ∼2 μm². (C) TEM image of a monolayer of vertically aligned NR superlattices. The upper left inset is the high-magnification TEM image showing the hexagonally closed-packed array of NRs. The upper right inset is the corresponding SAWED pattern acquired from an area of ∼60 μm². (D) TEM image of a closed-packed hexagonally ordered array of vertically aligned NRs. The upper right inset is the corresponding SAWED pattern and the lower right inset is the corresponding SAED pattern. Both patterns are acquired from an area of ∼6.5 μm.

Fig. 5.

Hexagonal nanoprism and nanoplate superlattices. (A) SEM image of a monolayer superlattice of NaYF₄: Yb/Tm (22/0.2 mol%) hexagonal nanoprisms. The upper right and lower right insets are the high-magnification SEM and TEM images, respectively. (B) SEM image of the self-assembled superlattice of NaYF₄: Yb/Er (20/2 mol%) hexagonal nanoplates.