Interfacial Molecular Compatibility for Programming Organic-Metal Oxide Superlattices

Artificiallyprogramming a sequence of organic-metal oxidemultilayers (superlattices) by using atomic layer deposition (ALD)is a fascinating and challenging issue in material chemistry. However,the complex chemical reactions between ALD precursors and organiclayer surfaces have limited their applications for various materialcombinations. Here, we demonstrate the impact of interfacial molecularcompatibility on the formation of organic-metal oxide superlatticesusing ALD. The effects of both organic and inorganic compositionson the metal oxide layer formation processes onto self-assembled monolayers(SAM) were examined by using scanning transmission electron microscopy,in situ quartz crystal microbalance measurements, and Fourier-transformedinfrared spectroscopy. These series of experiments reveal that theterminal group of organic SAM molecules must satisfy two conflictingrequirements, the first of which is to promptly react with ALD precursorsand the second is not to bind strongly to the bottom metal oxide layersto avoid undesired SAM conformations. OH-terminated phosphate aliphaticmolecules, which we have synthesized, were identified as one of thebest candidates for such a purpose. Molecular compatibility betweenmetal oxide precursors and the -OHs must be properly consideredto form superlattices. In addition, it is also important to form denselypacked and all-trans-like SAMs to maximize the surface density ofreactive -OHs on the SAMs. Based on these design strategiesfor organic-metal oxide superlattices, we have successfullyfabricated various superlattices composed of metal oxides (Al-, Hf-,Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.