Expanding the Toolbox of Inorganic Mechanoluminescence Materials

CONSPECTUS: Mechanoluminescence is a process of light emission from materials in response to external mechanical actions. As mechanical energy is ubiquitously available in nature, mechanoluminescence can provide sustainable solutions to challenging problems in the fields of biology and optoelectronics as well as energy and environmental sciences. In particular, remote delivery of light through mechanoluminescence can also be achieved by noncontact forces from ultrasonic waves and magnetic fields, which enables noninvasive diagnosis and therapy in biomedical applications. Owing to the well-recognized merits, the research of mechanoluminescence has evolved into a highly interdisciplinary field, which in turn has inspired considerable interest in the development of novel mechanoluminescence materials and devices for boosting the emission performances such as brightness, repeatability, and spectral tunability. Mechanoluminescence was originally observed in processes associated with fracture and plastic deformation of materials, which are characterized by inefficient and nonreproducible light emissions. Therefore, current research is mainly directed to repeatable mechanoluminescence through elastic deformation for the continuous generation of light. Such elastico-mechanoluminescence is primarily observed in host materials with piezoelectricity and doped with activator ions such as manganese and lanthanide. Upon the application of mechanical stress, the piezoelectric hosts establish internal electric fields that trigger electronic transitions between the dopant-associated energy levels, eventually giving rise to light emissions from the dopant ions. On the basis of the learned knowledge, various host/dopant combinations have been synthesized and tested for mechanoluminescence in the past few years. Meanwhile, novel composite structures and devices incorporated with the mechanoluminescence materials have also been designed for an efficient light generation under different forms of mechanical actions, such as compressing, stretching, and rubbing. The significant progress in mechanoluminescence study has generated new opportunities for fundamental research and technological applications. To inspire and guide further investigations in this field, we systematically review emerging mechanoluminescence systems and their frontier applications. We focus on emerging host materials that have been identified to render bright mechanoluminescence with deliberately controlled emission characteristics, along with an analysis of the structure-property relationship in these materials. We also discuss innovative methodologies for translating mechanoluminescence into various cuttingedge device applications by precisely controlling the interaction of mechanoluminescence materials and their surrounding environments. We attempt to provide the rationale behind these developments, and simultaneously highlight future opportunities and challenges for mechanoluminescence.