Exploring Photon‐Upconverting Quantum Dots: From Strategic Design to Diverse Applications

Upconverting quantum dots (UCQDs) represent a class of emerging semiconductor nanocrystals that can convert long‐wavelength excitation photons into higher‐energy visible or ultraviolet emissions via an anti‐Stokes process, thereby overcoming the reliance of conventional quantum dots on short‐wavelength excitations. Owning to their unique physicochemical properties such as deeper tissue penetration, reduced photodamage, high signal‐to‐noise ratio and more efficient use of near infrared (NIR) photons, UCQDs are broadly applied in bioimaging, phototherapy, photodetection, solar cell, etc. This review firstly introduces the advantages of long‐wavelength excitable luminescence and the superior optical properties of UCQDs compared to their counterparts. It then summarizes state‐of‐the‐art approaches to construct UCQDs. Specifically, intrinsic mechanisms based on multiphoton absorption, phonon‐assisted upconversion and exciton interactions in quantum‐dot heterostructures are discussed. Hybrid systems are also covered, in which quantum dots act as sensitizers for triplet–triplet annihilation or couple with lanthanide‐doped upconversion nanoparticles. Subsequently, emerging applications of UCQDs in in vivo imaging, phototherapy, photodetection and solar cells are presented. Finally, current challenges and future perspectives for the development of UCQDs are provided as a conclusion.