Lanthanides-doped near-infrared active upconversion nanocrystals: Upconversion mechanisms and synthesis

In the past few decades, although tremendous advancements have been witnessed by the progress in the fabrication of various kinds of nanomaterials, only those types of nanomaterials with unique features in terms of advantageous morphological attributes and physicochemical properties have become the area of focus towards their applicability in diverse fields. Among them, the lanthanide-doped upconversion nanocrystals (UCNCs) have garnered enormous interest, owing to their unique ability to transform near-infrared (NIR) light to short-wavelength light in the visible and ultraviolet (UV) range. Such UCNCs offer enormous additional advantages which are of particular interest in various applications. Although enormous efforts have been dedicated, in recent years, it is realized that the knowledge and understanding the chemistry of UCNCs is still insufficient, as the more people studied, the more unsolved puzzles are discovered about UCNCs. Therefore, chemistry, which plays a crucial role in regulating the luminescent behavior of UCNCs, needs in-depth elucidations to better understand the UCNCs and their upconversion processes. To facilitate future works and promote the development of UCNCs, in this review, various synthesis approaches and upconversion mechanisms are discussed, with a special focus given on the chemistry behind UCNCs and up-to-date studies.