Structure–Property Relationships in Lanthanide‐Doped Upconverting Nanocrystals: Recent Advances in Understanding Core–Shell Structures

Abstract

The production of upconverting nanostructures with tailored optical properties is of major technological interest, and rapid progress toward the realization of such production has been made in recent years. Ultimately, accurate understanding of nanostructure organization will lead to design rules for accurately tailoring optical properties. Here, the context of open questions still of general importance to the upconversion and nanocrystal communities is presented, with a particular emphasis on the structure–property relationships of core–shell upconverting nanocrystals. Although the optical properties of the latter have been thoroughly investigated, little is known regarding their atomic‐scale organization. Indeed, solving the atomic‐scale structure of such nanomaterials is challenging because of their intrinsic nonperiodic nature. Familiar concepts of crystallography are no longer appropriate; chemical and structural modulation waves must be introduced. To reveal the exact core–shell structures, innovative characterization techniques need to be applied and developed, as discussed herein. The continued development and application of structural characterization techniques will be vital to consolidate the currently incomplete link between atomic‐scale structure and upconversion properties. This will ultimately provide a valuable contribution to the emerging detailed guidelines on how to better design upconverting nanostructures to achieve given optical properties in terms of efficiency, absorption, spectral emission, and dynamics.

Composition and structure of magnetic high-temperature-phase, stable Fe–Au core–shell nanoparticles with zero-valent bcc Fe core

“…As a solution, a subshell division procedure was applied in this work for the determination of the composition of different regions within complex NPs, which is described in detail elsewhere. 23,24 Preconditions for this method are an EDXS detector with high signal-to-noise ratio, high quantum efficiency, and a small probe size (<0.5 nm) of the electron beam. Moreover, the NP shape is approximated to be spherical without distinctive surface roughness.…”

Combining HR-TEM and XPS to elucidate the core–shell structure of ultrabright CdSe/CdS semiconductor quantum dots

“…A still open question in this study remains a possible intermixing between core and shell (see SI, Figure S13) as discussed for many core/shell nanostructures [74][75][76] . Due to the structural coherence between CdSe and CdS, a sharp boundary between core and shell was not observed with HR-TEM; and the lateral resolution of elemental mapping is not sufficient to provide this information.…”

Lanthanide doping in metal halide perovskite nanocrystals: spectral shifting, quantum cutting and optoelectronic applications

“…Lanthanide doping in octahedral (perovskite) semiconductor NCs Figure 1 depicts Ln 3+ doping in widely studied crystal lattices from insulators to semiconductors. Ln 3+ doping in oxide and fluoride lattices, such as Y 3 Al 5 O 12 , NaYF 4 , and NaGdF 4 , has been successfully realized due to the higher coordination (coordination number, CN = 8 or 9) environment possible for Ln 3+ dopants [24][25][26] . However, a drawback of such host materials is their wide band gap, yielding both poor visible light absorption and poor charge transport.…”