Ho,3+ (or Tm3+ )-Activated Upconversion Nanomaterials: Anomalous Temperature Dependence of Upconversion Luminescence and Applications in Multicolor Temperature Indicating and Security

A novel type of composite nanomaterials between NaYF4:Yb3+, Ln3+ (Ln = Ho, Tm) nanowires and NaGdF4:Yb3+, Ln3+ nanoparticles is designed, and a visual color shift can be realized with a minor temperature rise (≈10 °C) or a continuous-wave 975 nm laser excitation. This is favorable for the development of an upconversion-based temperature indicating technology for real-time temperature monitoring and more secure anti-counterfeiting applications.