Organic–inorganic metal halide perovskites have emerged as a promising optoelectronic material with exceptional structure and property tunability. This new generation of functional materials possess excellent properties such as large optical absorption, long carrier diffusion length, high carrier mobility, and low-cost solution production process.
The research attention in metal halide perovskites has grown exponentially since 2009, when the use of organometal halide perovskites as visible-light sensitizers for photovoltaic cells was reported.
In the decade since, a rapidly growing body of research has utilized perovskites for applications as solar cells, photodetectors, light emitting diodes, and lasers.
In recent years, structure control of organic–inorganic metal halide hybrids has been explored to lower the dimensionality from 3D to 2D, 1D, and 0D at both morphological and molecular levels.
At the same time, fabrication methods based on inkjet printing emerged for patterning such perovskite micro- and nanostructures. However, these patterning techniques for perovskites are still limited to in-plane fabrication and alignment.
To overcome this limitation, researchers have developed a method to print perovskite nanostructures in three dimensions. The method exploits a femtoliter meniscus of precursor ink formed on a nanopipette to localize and guide solution-mediated perovskite crystallization in mid-air, enabling nanoscale and freeform 3D printing.
The researchers, led by a team from the Department of Mechanical Engineering at The University of Hong Kong, have published their findings in Advanced Materials.
As illustrated in the above figure, the perovskite crystallization inside the fL ink meniscus is driven by fast evaporation of DMF solvent. At a given steady-state geometry of the fL meniscus, as shown in the inset, the evaporative loss of solvent directly increases the concentration of perovskite solutes at the meniscus surface because the solutes do not evaporate into the air.
The researchers explain that steering the crystallization in three dimensions is based on moving the nanopipette with a motorized stage (see video below): « When an ink-filled nanopipette with a diameter of 600 nm approaches and physically contacts a Si substrate, fL-volume ink is wetted on the substrate, forming the meniscus at the pipette–substrate gap. CH3NH3PbI3 crystals immediately start to grow inside the ink meniscus. »
« As the pipette moves up (or the substrate moves down) with a constant pulling speed, the crystal growth is guided in three dimensions with the continuous ink supply from the pipette, » they describe the 3D-printing process. « The termination of the growth is conducted by abruptly increasing the pulling speed above a threshold speed, enabling in situ tailoring the dimensions of a freestanding perovskite nanowire. »
After the fabrication and characterization of freestanding CH3NH3PbI3 perovskite nanowires at a single-entity level, the team turned their attention to freeform 3D printing by omnidirectional meniscus guiding.
Production of mesh-shaped structures is a primary step toward layer-by-layer additive manufacturing process and by lateral guiding an ink meniscus using a nanopipette with 90° tilting the researchers demonstrated the reliable fabrication of perovskite mesh structures.
« The superior ability of the method to tailor such nanostructure’s dimensions with an on-demand, minimalist manner could contribute to scientific research for fundamentals of perovskite-based devices such as nanolasers, photodetectors, and metasurfaces, » the authors conclude. « The method can be generalized for production of freeform perovskite nanostructures with programmed chemical compositions if full-scale investigations of the printing parameters that influence the evaporation-induced crystallization are accomplished. »
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