Upconversion luminescent nanoheater based on NaGd(MoO4)2: Yb3+/Tm3+ nanocrystals: Surfactant-free solvothermal synthesis, upconversion photoluminescence and photothermal conversion

•Multifunctional upconversion luminescent nanoheater.•Surfactant-free solvothermal synthesis of NaGd(MoO4)2: Yb3+/Tm3+ nanocrystals.•Upconversion luminescence and the corresponding electronic transition processes.•Photothermal conversion slope efficiency up to 43.64 °C/W for laser irradiation of 10 s. Multifunctional upconversion luminescent nanoheaters are realized based on NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals synthesized by surfactant-free solvothermal method. The structure, morphology and composition of the nanocrystals are analysed by XRD, SEM, EDS, TEM, HRTEM and SAED techniques. Under 980 nm irradiation, NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals emit blue luminescence with emission peaks located in the blue and near-infrared spectral regions. The electronic transition processes for the upconversion luminescence are discussed based on the excitation power-dependent upconversion luminescence, luminescent dynamics and Yb3+/Tm3+ energy levels. The nanocrystals show efficient photothermal conversion after irradiated by 980 nm laser. The photothermal conversion slope efficiency for NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals irradiated by 980 nm laser for 10 s duration is 17.43 °C/W for 10 mol% Yb3+ doping and 43.64 °C/W for 15 mol% Yb3+ doping. Multifunctional luminescent nanoheater NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals exhibit both upconversion photoluminescence and photothermal conversion capabilities, and are promising candidates for temperature self-monitoring photothermal therapy or other biological applications.