Crystal Structures, Anisotropic Growth, and Optical Properties: Controlled Synthesis of Lanthanide Orthophosphate One‐Dimensional Nanomaterials

Abstract

The fundamental understanding of the relationship between crystal structure and the dynamic processes of anisotropic growth on the nanoscale, and exploration of the key factors governing the evolution of physical properties in functional nanomaterials, have become two of the most urgent and challenging issues in the fabrication and exploitation of functional nanomaterials with designed properties and the development of nanoscale devices. Herein, we show how structural and kinetic factors govern the tendency for anisotropic growth of such materials under hydrothermal conditions, and how the crystal structure and morphology influence the optical properties of Ln3+-doped nanocrystals. The synthesis of phase-pure and single-crystalline monoclinic, hexagonal, and tetragonal one-dimensional LnPO4 nanostructures of different aspect ratios by means of kinetically controlled hydrothermal growth processes is demonstrated. It is shown that the tendency for anisotropic growth under hydrothermal conditions can be enhanced simply by modifying the chemical potentials of species in the reaction solution through the use of carefully selected chelating ligands. A systematic study of the photoluminescence of various Eu3+-doped lanthanide phosphates has revealed that the optical properties of these nanophosphors are strongly dependent on their crystal structures and morphologies.

Geometric and electronic structure of lanthanide orthophosphate nanoparticles determined with X-rays

“…Lanthanide phosphate nanoparticles are an interesting class of wide band gap insulators because of their optical properties. Their colloidal solutions emit infrared, visible, and ultraviolet light [3][4][5][6][7] and, therefore, they can be used as components in lighting, displays, optoelectronic devices, lasers, and in long-lived dyes for biological labeling. 5 The lanthanide orthophosphates also display very high lattice energies and high resistance against oxidation and radiation damages, making bulk crystals of these materials a medium feasible even for nuclear-waste storage.…”

“…8 Thus, it is essential to understand how dimensionality influences the properties of lanthanide orthophosphates. Recent studies 9,7,10,6 have provided some insights on how crystal structure and kinetic factors determine the shape and size of these materials. However, an important issue to be addressed is the question of how morphology and geometric structure influence the electronic structure of these materials.…”

“…Besides, the intensity and the splitting of Eu optical transitions was shown to depend on the morphology and the crystal structure of the host lattice. 6 All these aspects of the optical properties address for a systematic and detailed study of the electronic structure of the whole series of lanthanide orthophosphate nanoparticles. As a structural probe we have used the variation of the oxygenlanthanide chemical bond length because it can be probed for the entire lanthanide orthophosphate series.…”

“…The reason for this is that bulk lanthanide orthophosphates have several polymorphic forms; hexagonal, tetragonal, and monoclinic phases. Yan et al 6 of the whole orthophosphate series in the nanosize ranges based on the same hydrothermal method. In the case of larger lanthanide ions ͑La, Ce, and Nd͒, they reported the orthophosphates to adopt the monoclinic structure and rodlike morphologies with widths of 20-30 nm and lengths up to 1 m. The lanthanide ions of intermediate size ͑Pr-Tb͒ preferred a partly hydrated hexagonal phase and highly anisotropic rodlike morphologies with widths of 10-100 nm and lengths of 0.3-3 m, while for the smaller Ln ions ͑Dy-Lu, and Y͒, the lanthanide phosphates were obtained in the tetragonal phase and spherelike smaller morphologies which showed a reduced tendency to grow along a certain direction.…”

Synthesis and study of anhydrous lanthanide orthophosphate (Ln = La, Pr, Nd, Sm) nanowhiskers

“…Nanoscale substances' properties, such as electronic, optical, chemical, thermal and mechanical properties [1][2][3][4][5][6] differ sufficiently from those of the bulk phase material. Usually, nanoparticles contain a significant quantity of substances adsorbed on their surface and the measurement of its own properties (in part, thermal and thermodynamic) is difficult.…”