As their heroic name suggests, superlattices have characteristics that are greater than the sum of their parts, combining two or more materials in organized layers. Superlattices made of inorganic compounds called perovskites hold the potential for engineering many different kinds of materials with applications in electronics and photonics. However, these materials are notoriously difficult to synthesize using conventional techniques. Now, researchers have found a new way to create self-assembling perovskites, and they characterized the process at Berkeley Lab user facilities.
The self-assembly process works by forming two-dimensional perovskite nanosheets, then stacking them, layer by layer, into nanocrystal superlattices. Researchers visualized the assembly process using transmission electron microscopy (TEM) at the Molecular Foundry, and then used the Advanced Light Source (ALS) for further characterization. At Beamline 7.3.3, grazing-incidence wide-angle x-ray scattering (GIWAXS) confirmed the crystal structure, which is difficult to determine by other means. The researchers then used in situ synchrotron-based small-angle x-ray scattering (SAXS) and ex situ small-angle x-ray diffraction (XRD) to monitor the self-assembly kinetics. These results demonstrated the layer-by-layer self-assembly behavior of the perovskites, becoming more ordered over time. In addition, TEM, SAXS, and XRD tracked how mechanical force can reverse the assembly; sonication “exfoliates” the superlattices back into their building blocks.
The layer-by-layer self-assembly process exhibited in this system is a simple, yet robust, approach to generating large-scale perovskite superlattices. The precision of this process offers tunability—adjusting the material’s properties by arranging the individual crystals. The properties of these perovskite superlattices could present a pathway to synthesize other two-dimensional superlattice nanomaterials for a variety of applications.
Y. Liu, M. Siron, D. Lu, J. Yang, R. dos Reis, F. Cui, M. Gao, M. Lai, J. Lin, Q. Kong, T. Lei, J. Kang, J. Jin, J. Ciston, and P. Yang, “Self-Assembly of Two-Dimensional Perovskite Nanosheet Building Blocks into Ordered Ruddlesden–Popper Perovskite Phase,” J. Am. Chem. Soc. 141, 13028–13032 (2019), doi:10.1021/jacs.9b06889.