Digitally programmable perovskite nanowire-block copolymer composites

One-dimensional nanomaterials with highly anisotropic optoelectronic properties can be used within energy harvesting applications, flexible electronics and biomedical imaging devices. In materials science and nanotechnology, 3-D patterning methods can be used to precisely assemble nanowires with locally controlled composition and orientation to allow new optoelectronic device designs. In a recent report, Nanjia Zhou and an interdisciplinary research team at the Harvard University, Wyss Institute of Biologically Inspired Engineering, Lawrence Berkeley National Laboratory and the Kavli Energy Nanoscience Institute developed and 3-D printed nanocomposite inks composed of brightly emitting colloidal cesium lead halide perovskite (CsPbX₃, where X= Cl, Br, or I) nanowires.

They suspended the bright nanowires in a polystyrene-polyisoprene-polystyrene block copolymer matrix and defined the nanowire alignment using a programmed print path. The scientist produced optical nanocomposites that exhibited highly polarized absorption and emission properties. To highlight the versatility of the technique they produced several devices, including optical storage, encryption, sensing and full color displays. The work is now published on Science Advances.

The unique anisotropic optoelectronic properties of semiconducting nanowires arise from quantum and dielectric effects for broad ranging applications in electronics and photonics. New paths can be opened to assemble optoelectronic devices by precisely patterning 1-D nanomaterials into planar and 3-D structures. Compared to many types of semiconducting wires reported so far, cesium lead halide nanowires (CsPbX₃) with a perovskite crystal structure have offered several advantages for optoelectronic applications. The lead halide perovskite nanocrystals are ultra-bright and exhibit near-unity quantum yield without an encapsulating shell—in contrast to conventional, colloidal semiconducting nanocrystals with a core-shell structure.

Materials scientists can modify the halide composition and bandgap of perovskites to form bright and tunable emissions across the entire visible spectral range. As a result of the unique material properties and high quantum yields, perovskite nanowires have potential applications in optoelectronics, as active layers in liquid crystal display (LCD) backlighting, spectrum splitting, polarized photodetectors and optically pumped lasers. Researchers have explored several planar and 3-D patterning methods, including extrusion-based 3-D printing via direct ink writing (DIW) to form shape-morphing architectures composed of cellulose fibrils and aligned in a hydrogel matrix. However, general applications of DIW to pattern functional architectures in photonic devices still remain to be explored.

In the present work, Zhou et al. designed, printed and characterized polarized optical architectures composed of perovskite nanowire-filled block copolymer matrices. For this, they developed a nanocomposite ink with the perovskite nanowire bundles embedded in a cylindrical, microphase polystyrene-polyisoprene-polystyrene (SIS) block copolymer matrix. Using the proposed method, Zhou et al. expect other anisotropic materials including metals, semiconductors and block copolymers, and dielectric nanowires to be similarly programmably patterned.

The scientists formed different inks composed of nanowires by varying the SIS concentration to develop shear-thinning behavior and viscoelastic response required for DIW (direct ink writing). Using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements, they revealed the ordered hexagonal micro-domains of the SIS block copolymer filaments and revealed the printed SIS-CsPbBr₃ nanocomposites to be highly aligned along the print direction. This patterning method allowed for programmable nanowire orientation in the printed optical composites to influence their polarized and angular emission.

During direct ink writing, Zhou et al. generated print paths using G-code generated via MatLab, Slic3r and CIMCO and used glass nozzles to form nanocomposite architectures on glass coverslips. To demonstrate applications of digitally programmed polarization anisotropy in 3-D printed nanocomposites; Zhou et al. first engineered a 3-bit grayscale image of square shaped pixels (200 x 200 µm). Using the technique, the scientists achieved advanced patterned architectures to serve as optical memories for write once-read many (WORM) times devices of data storage.

The printed architectures can be used for security encryption in encryptable memories by printing multilayer architectures with different optical information stored in each layer. As a proof-of-principle, Zhou et al. patterned the letters "L-I-G-H-T' in a five-layer device in which they viewed each letter at different heights. They printed the letters "H' and "I' in random orientations and selectively viewed each letter in the transparent matrix by using a polarizer at the appropriate angle. Additionally, the scientists showed the possibility of encrypting an emission pattern—for instance, the letter H—by stretching the material. They envision the potential to create dynamic camouflage in skin-like material arrangements where different optical patterns emerge and disappear on mechanical stretching.

Thereafter, they extended the concept to mimic the RGB (red, green, blue) quantum dots that are widely used in color mixing. For this, Zhou et al. used anion-exchange reactions to obtain halide perovskites composed of red-emitting and blue-emitting nanowires and created tunable, multiplexed color displays using multi-material 3-D printing. Although perovskite nanowires are not yet optimally suited as materials for display applications, the work highlighted the ability to exert programmable control on the nanowire composition and alignment offered via digital assembly. Zhou at al. presented the tunable spectral responses of the multiplexed RGB array and its corresponding color range in the CIE (commission on illumination) chromaticity diagram to show the remarkably simple design offered by the printed displays to achieve color tunability.

Unlike LCDs that rely on conventional quantum dot color filters, the printed films in the present work used direct polarization photon downshifters, also known as "active" color filters. Zhou et al. intend to improve both nanowire synthesis and printing to achieve higher efficiencies for display applications.

In this way, Nanjia Zhou and co-workers showed that direct writing nanocomposite inks composed of perovskite nanowire-filled block copolymer matrices could pattern optoelectronic devices in numerous designs. They programmed the nanowire composition and alignment to create optical nanocomposites for applications in information storage, encryption, mechano-optical sensing and optical displays. The new findings will provide a pathway to rapidly design and manufacture functional devices from anisotropic building blocks encapsulated in soft polymer matrices.

© 2019 Science X Network