Layer-by-layer growth technique. While the bulk powder form of MOFs is only of limited interest with regard to the fabrication of stacked layers, the layer-by-layer (lbl) or quasiepitaxial growth of SURMOFs offers a number of interesting options. Briefly, the lbl process proceeds by repeating a four-step cycle comprising the sequential immersion into the two solutions of the individual reactants, separated by rinsing with a solvent (typically ethanol) (Figure ). Thus, instead of mixing the reactants like in the conventional solvothermal bulk MOF synthesis, the metal or metal/oxo source is kept apart from the linker solution. Another important difference is that the lbl synthesis is carried out at temperatures much lower than these used in the solvothermal synthesis, in some cases growth already occurs at room temperature. To nucleate the growth of SURMOFs on the substrate, the exposed surface first needs to be functionalized with specific functional groups, thus, yielding -COOH, -OH or pyridyl-terminations. The first step typically is an exposure to the metal source (e.g., Cu-or Zn-acetate). Subsequently, the other steps (rinsing with ethanol, exposure to organic linker, rinsing with ethanol) are repeated until the desired thickness is reached. After synthesis, the SURMOF structure can be rigorously characterized by out-of-plane and in-plane x-ray diffraction. Figure . Layer-by-layer growth of a HKUST-I SURMOF on a quartz-crystal microbalance (QCM) crystal. From the frequency shift of the QCM substrate, the deposited mass can be determined in a quantitative fashion. Note that the growth is self-terminating, e.g., for the metal source the increase of mass saturates after a few minutes. Data are taken from Ref. [174].