Hierarchical assemblies of molecular frameworks—MOF-on-MOF epitaxial heterostructures

Functional, porous metal-organic frameworks (MOFs) have attracted much attention as a very flexible class of crystalline, porous materials. For more advanced applications that exploit photophysical properties, the fabrication of hierarchical assemblies, including the creation of MOF/MOF heterointerfaces, is important. For the manufacturing of superstructures with length scales well beyond that of the MOF pore size, layer-by-layer (lbl) methods are particularly attractive. These allow the isoreticular approach to be extended to superstructures with micrometer length scales, a range that is not accessible using conventional MOF design. The lbl approach further substantially extends the compositional diversity in MOFs. At the same time, the favorable elastic properties of MOFs allow for heteroepitaxial growth, even in the case of lattice misfits as large as 20%. While the MOF-on-MOF approach to designing multicomponent superstructures with synergistic multifunctionality can also be realized with sophisticated solvothermal synthesis schemes, the lbl (or liquid-phase epitaxy) approach carries substantial advantages, in particular when it comes to the integration of such MOF superstructures into optical or electronic devices. While the structure vertical to the substrate can be adjusted using the lbl method, photolithographic methods can be used for lateral structuring. In this review, we will discuss the lbl liquid-phase epitaxy approach to growing surface-anchored MOF thins films (SURMOFs) as well as other relevant one-pot synthesis methods for constructing such hierarchically designed structures and their emerging applications.