Lanthanide-doped upconverting nanoparticles (UCNPs) are nanoscale colloidal light emitters that absorb multiple low-energy photons and emit a higher-energy photon. This optical property, known as upconversion luminescence (UCL), is enabled by the ladder-like energy levels of lanthanide ions doped within an inorganic host matrix. With their tunable optical properties and precise nanoscale size control, UCNPs have been used for applications such as biological imaging, 1 nano-thermometry, 2 super-resolution microscopy, lithography, 4 optical computing, and photodetection. The most common way to achieve efficient UCL from lanthanide ions is to embed them in inorganic host matrices such as NaYF4 and K2Pb2Cl5, whose low phonon energies reduce phononassisted non-radiative relaxation. UCL efficiency can be further enhanced by growing an undoped, epitaxial shell made of the same material, allowing the active lanthanide ions to be optically and chemically isolated from their environments. However, while the chemistry for growing high-quality epitaxial shells has been optimized for some UCNP host materials (e.g., NaYF4), the growth of epitaxial shells for other host materials (e.g., K2Pb2Cl5) can be more challenging and remains an active area of research. Non-epitaxial oxide shells have been grown around UCNPs, leading to increased thermal stability; 10 however, their high phonon energies typically do not enhance UCL and may even reduce it.