Layer-by-layer assembly: from conventional to unconventional methods.

Layer-by-layer (LbL) assembly is a powerful means for fabricating multilayer thin films with controlled architecture and composition. This feature article discusses different types of methods for LbL assembly. On the one hand, some of the conventional LbL methods are introduced, which are driven by electrostatic interactions, hydrogen bonds, step-by-step reactions, sol-gel processes, molecular recognition, charge-transfer, stepwise stereocomplex assembly, and electrochemistry. On the other hand, some of the unconventional methods for fabricating of the building blocks which can not be assembled by conventional methods are also summarized. These unconventional methods usually involve the formation of supramolecular structures via one type of self-assembly. These structures can subsequently be used as building blocks in another type of self-assembly. To take advantage of these conventional and unconventional methods, a great number of building blocks can be fabricated into multilayer thin films with a defined sequence structure in a designed way. It has been demonstrated that LbL methods provide new horizons for surface molecular engineering.