Uniform Surface Characteristics in Sequentially Polymerized Polyurea Films

In situ analysis of growth rate evolution during molecular layer deposition of ultra-thin polyurea films using aliphatic and aromatic precursors

Organic thin films formed by molecular layer deposition (MLD) are important for next-generation electronics, energy storage, photoresists, protective barriers and other applications. This study uses in situ ellipsometry and quartz crystal microbalance to explore growth initiation and growth rate evolution during MLD of polyurea using aromatic p-phenylene diisocyanate (PDIC) or aliphatic 1,6-hexamethylene diisocyanate (HDIC) combined with ethylenediamine (ED) or 1,6-hexanediamine (HD) co-reactants. During the first 10-20 cycles of growth, we show the growth rate can increase and/or decrease substantially depending on the substrate as well as the flexibility, length, and structure of the isocyanate and amine reactants used. The transition from initial to steady growth is attributed to a change in active surface site density as the growth proceeds, where the number of sites is determined by a balance between steric effects that block active sites, double reactions that consume multiple active sites, and precursor physisorption and sub-surface diffusion that create new active sites, where the extent of each mechanism depends on the precursors and deposition conditions. Results shown here provide useful insight into mechanisms needed to control growth of ultra-thin organic films for advanced applications.

Organic guest molecule induced ultrafast breathing of an epitaxially grown metal-organic framework on a self-assembled monolayer

We report epitaxially grown new two-dimensional metal-organic framework (MOF) thin films on a self-assembled monolayer (SAM). We fabricated these epitaxial thin-films using stepwise layer-by-layer seeding followed by solvothermal treatment. The MOF thin films exhibit ultrafast structural flexibility (through breathing) compared to their bulk samples upon uptake of organic guest molecules.

A Novel Organic "Polyurea" Thin Film for Ultralong-Life Lithium-Metal Anodes via Molecular-Layer Deposition

Metallic Li is considered as one of the most promising anode materials for next-generation batteries due to its high theoretical capacity and low electrochemical potential. However, its commercialization has been impeded by the severe safety issues associated with Li-dendrite growth. Non-uniform Li-ion flux on the Li-metal surface and the formation of unstable solid electrolyte interphase (SEI) during the Li plating/stripping process lead to the growth of dendritic and mossy Li structures that deteriorate the cycling performance and can cause short-circuits. Herein, an ultrathin polymer film of "polyurea" as an artificial SEI layer for Li-metal anodes via molecular-layer deposition (MLD) is reported. Abundant polar groups in polyurea can redistribute the Li-ion flux and lead to a uniform plating/stripping process. As a result, the dendritic Li growth during cycling is efficiently suppressed and the life span is significantly prolonged (three times longer than bare Li at a current density of 3 mA cm^-2 ). Moreover, the detailed surface and interfacial chemistry of Li metal are studied comprehensively. This work provides deep insights into the design of artificial SEI coatings for Li metal and progress toward realizing next-generation Li-metal batteries.