Up-conversion hybrid nanomaterials for light- and heat-driven applications

Composites or hybrid materials offer diverse properties not achievable in pure materials. Here we critically review the interesting and controllable fluorescence and photothermal properties of diverse hybrid materials containing up-conversion nanoparticles (UCNPs). These hybrids couple plasmons, photonic crystals, bio-surfaces, and two dimensional (2D) materials to the UCNPs, offering optical non-linearity, and enable effective photo-electro-thermal control leading to new light and heat driven applications. Among the light driven applications, coupling of UCNPs with graphene and molybdenum disulfide (MoS₂) enables photodetectors with better photoresponse, and broader spectral range not accessible to individual components. Irradiated MoS₂ coupled-UCNPs is a new paradigm in resistive random access memory devices. Conjugation of graphene and perovskites, with the UCNPs, have led to novel optical limiting phenomenon and better solar cells. Examples of new opportunities offered by UCNPs in heat driven applications are photothermal water desalination using solar daylight and photothermal disintegration of fat droplets in obesity treatment. Phonons, manifesting as heat, can also be utilized to enhance fluorescence and translate to high sensitivity nanothermometers. This review covers fundamentals, and applications of the new UCNP-enabled class of hybrid materials in energy harnessing, light sources and detectors, memory devices, nanothermometers, desalination, intracellular pH sensing, and cancer theranostics.