3D-printed perovskite nanopixels for ultrahigh-resolution displays and multilevel anticounterfeiting
(Nanowerk Spotlight) The three-dimensional (3D) geometry of 3D-printed nanopixels can increase the emission brightness of display pixels, varying with the height of the pixels, and can be used to fabricate super high-resolution devices (see our previous Nanowerk Spotlight: "3D-printed nanoscale color pixels improve brightness control")
A group at Hong Kong University (HKU), lead by Prof. Ji Tae Kim, came up with the idea that perovskites can be used for implementing nanopixels with several advantages. Kim has been studying 3D printing of crystalline materials for years including perovskites (read more in our Nanowerk Spotlight: "3D-printing of perovskite nanostructures").
Three-dimensional (3D) printing of perovskite nanopixels. (a) Schematic showing the meniscus-guided crystallization process for the 3D printing of perovskites (fL = femtoliter). The process consists of (i) preparation of a nanopipette filled with a precursor ink, (ii) meniscus formation by pipette−substrate contact, (iii) meniscus-guided perovskite crystallization by upward movement of the pipette, and (iv) termination of crystallization by abruptly increasing the speed of pipette movement. This printing process can be used for different chemical compositions, enabling the fabrication of red (R), green (G), and blue (B) triple pixels. (b) Corresponding side-view real-time optical micrographs showing the printing process. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"During the implementation of nanoscale pixels with perovskites at HKU, we found saturated emission from the high-aspect-ratio nanopixels," Dr. Jaeyeon Pyo, a Senior Researcher at Korea Electrotechnology Research Institute (KERI), tells Nanowerk. "Prof. Kim suggested redesigning the project to further study the nanopixels' saturation behavior. This would enable us to demonstrate unique applications for the 3D printed perovskites."
Perovskites are promising materials for various optoelectronic applications. However, their applications have been limited to 2D systems due to the lack of suitable fabrication methods. In this work though, based on the accumulated knowledge of the nanoscale 3D printing of crystalline materials, the researchers were able to demonstrate perovskite 3D nanopixels for ultrahigh-resolution displays and multilevel anticounterfeiting.
Whereas Pyo's group previously showed that the emission intensity is increased with the height of 3D-printed nanopixels, in this study, they demonstrate that the increase can be saturated by the limited depth of field of the measuring optical system.
Pixel height, emission brightness, and spot size. (a−d) Three-dimensional-printed CH3NH3PbI3 nanopixels (red) with controlled heights of 1.5, 2.1, 3.3, 4.1, 4.8, 6.3, 7.7, and 8.7 µm: (a) side-view optical micrograph, (b) bottom-view wide-field photoluminescence image (top) and its corresponding intensity profile (bottom), (c) emission intensity vs pixel height, and (d) emission spot size vs pixel height. (e−h) CH3NH3PbBr3 (green) nanopixels and analysis corresponding to (a−d). (i−l) CH3NH3PbCl3 (blue) nanopixels and quantitative analysis corresponding to (a−d). (m) Saturation height vs emission wavelength and its correlation with the depth of field (DOF). (Reprinted with permission by American Chemical Society) (click on image to enlarge)
As the crystallization of perovskites is sensitive to the humidity in the air, the entire experimental setup was secured in a workbench under a humidity control.
"The saturation can be utilized for advanced applications" Pyo explains: "Uniformity of the brightness can be substantially improved by using the saturated regime. Previously, precise control of the height was required for flattening the brightness of the individual pixels. Instead, printing the pixels to the height over the saturation point can provide a uniform brightness without delicate individual height control."
"Furthermore" he adds, "information can be stored to the heights without apparent difference for the encryption. Because the difference of heights over the saturation point is indistinguishable by ordinary measurement, information can be securely stored to the heights that is accessible by a specific 3D measuring method."
Ultrahigh-resolution displays are one of the direct and immediate applications of this work. The patterned nanoscale perovskites can be used as a color filter in backlighted display devices.
In addition, the team envisions that their method can potentially be used in various security printing applications including currency, identification, and authentication. The system provides multilevel encryption of information based on 1) small dimension at the nanoscale that is accessible with a microscope; 2) fluorescent color that is accessible with a UV light; and 3) height difference over saturation point that is accessible with 3D measurement.
The challenge of using this multilevel encryption system for anticounterfeiting of currency notes lies in nanoscale patterning on paper – which is difficult due to the rough surface of paper. However, as the team demonstrated in a recent paper in Advanced Engineering Materials ("Nanoscale 3D Printing of Quantum Dots on Paper"), nanoscale 3D printing of quantum dots indeed is achievable on the rough surface of conventional office paper.
The researchers are now working on electrically driven lighting from the 3D printed systems.
"Going beyond the passive lighting from photoluminescence, electrically driven lighting shall open up the possibility of actively controllable lighting devices," Pyo concludes.