Upconversion (UC) luminescence of lanthanide ions (Ln³⁺) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln³⁺, transition metal (TM) ions, e.g., Mn²⁺, usually possess a single broadband emission due to its 3d⁵ electronic configuration. Wavelength-tuneable single UC emission can be achieved in some TM ion-activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln³⁺ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln³⁺ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d⁰ ion-containing hosts (d⁰ normally viewed as charged anion groups, such as MoO₆^6- and TiO₄^4-) may also have a strong influence on the electric dipole transition of Ln³⁺, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide-ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln³⁺ tuned by TM or d⁰ ions, and the UC of d⁰ ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

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