Color Displays: Perovskite optical gratings enable dynamic, tunable-color displays

Building on their prior work on using nanopatterned titanium dioxide metasurfaces to create microfluidic-based tunable-color displays that transition between colors on millisecond time scales, researchers at the Harbin Institute of Technology (Shenzhen, China) in collaboration with researchers at the National University of Singapore and Purdue University (West Lafayette, IN) are now developing solid-state color-tunable displays on nanosecond time scales using perovskite-based optical gratings.¹

In some ways similar to how many colorful living creatures in nature dynamically change color through the interaction of visible light with physical changes in external structural features, the displays created by the researchers are rapidly tunable through the interplay of nanostructured metasurface parameters and pump-light-dependent photoemission properties of the gratings themselves.

Material selection

Critical to creating structural color, the researchers fabricated a grating composed of a type of semiconductor called methylammonium lead halide perovskite (MAPbX₃) where MA = CH₃NH₃₊ and X = chlorine (Cl), bromine (Br), iodine (I), or their mixture. The high refractive index of MAPbX₃ perovskites ensure the Mie resonance of perovskite nanoparticles and therefore produces extrinsic structural color from the metasurface. As the grating period (p) varies from 405 to 280 nm and the gap (d) varies from 70 to 163 nm, the reflection peak blue-shifts from 672 to 458 nm, generating distinct colors from red to purple (see figure).

These MAPbX3 perovskites are also direct-bandgap semiconductors that can generate intrinsic emission colors, including photoluminescence (PL) and lasing. For example, for a grating with p = 325 nm and d = 100 nm, optical excitation with a 400 nm Ti:sapphire laser with pumping density of 14.83 μJ/cm² produces a broad PL emission peak with full-width half-maximum of 20 nm centered at 524 nm (green). Again, illumination of perovskite gratings with different dopants of Cl, Br, or I can produce emission from purple to red.

Unfortunately, these perovskites are only capable of static display without an external trigger. In this case, the excitation trigger is simultaneous illumination of the gratings with both white light and laser light, controlling the individual emission colors as well as the ratio of emission to reflection in the final color result. Experimentally, a nanostructure illuminated with white light at a certain power density produced a reflection peak at 625 nm, emitting a bright red color. But when the grating was also illuminated by an ultrafast Ti:sapphire laser, another peak appeared at 515 nm, transitioning from red to orange and finally chartreuse as the laser pump power increased.

By mixing the extrinsic structural color and intrinsic emission color, the color is tunable over a large gamut by simply modifying the external excitations—furthermore, the color changes are also reversible. The color gamut can be further enlarged by varying the stoichiometry of MAPbX₃, and the synergy between interlaced mechanisms allows color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment.

“This color-tunable display design principle can be readily extended to other materials such as GaN (gallium nitride), ZnO (zinc oxide), and CdS (cadmium sulfide),” says Shumin Xiao, professor at Harbin Institute of Technology. “Our design is a promising realization of in situ dynamic color nanoprinting that will empower advances in structural color, classified nanoprinting, augmented-reality devices, and barcode sensing.”

REFERENCE

1. Y. Gao et al., ACS Nano, Article ASAP (Aug. 16, 2018); https://pubs.acs.org/doi/10.1021/acsnano.8b02425.