Luminescence is a grand display in nature; it can be seen in the sky above or in insects to help them attract a mate. But it also plays a role in our daily lives—in solid-state lighting, sensors, displays, and bioimaging, for example. The ability to control the color and intensity of luminescence is a desirable feature, especially if it can be achieved in a fixed single-phase phosphor.

In a communication in Advanced Materials, Man-Chung Wong, Jianhua Hao, and co-workers from the Hong Kong Polytechnic University and the Hong Kong Polytechnic University Shenzhen Research Institute demonstrate remote and temporal tuning of luminescence intensity and wavelength in green- and blue-emissive piezophosphors through modulating magnetic field, achieving proof-of-concept devices including tunable white-light and red-green-blue full-color displays.

The laminate phosphor composite is composed of a layer of metal-ion-doped (Al, Cu) ZnS piezophosphors embedded into polydimethylsiloxane (PDMS) and an adjacent layer of soft ferromagnetic particles mixed into PDMS. When the frequency of a low magnetic field is remotely modulated, the ferromagnetic-particles layer generates a magnetostrictive strain that induces a piezophotonic effect—and thus light emission—in the neighboring piezophosphor layer. Increasing the excitation frequency of the magnetic field from 50 to 470 Hz causes a hypsochromic shift of the luminescence: the blue-emission intensity increases, while the green-emission is suppressed. Therefore, the emission spectrum can be reversibly tuned simply by alternating the magnetic field frequency. Additionally, increasing the root-mean-square magnetic field strength generally increases the emission intensity.

The researchers then demonstrate a tunable white-light and multicolor display by modulating the frequency of the magnetic field.  White-light emission is achieved by combining a blue-emissive magnetic-induced-luminescence (MIL) composite with a yellow fluorescence phosphor: cerium-doped yttrium aluminum garnet (Y₃Al₅O₁₂)_. Under a different modulation frequency of the magnetic field, tunable warm- to cold-white light is achieved. Similarly, a full-color display was shown using green- or blue-emissive MIL composites with a europium-doped, red-emitting phosphor ((Ca_1-xSr_x)S: Eu).

To find out more about the magnetic-field tuning of piezophotonic luminescence, please visit the Advanced Materials homepage.