Fabrication and Characterization of Cross-Linked Organic Thin Films with Nonlinear Mass Densities

Abstract

The preparation of urea (bonded) cross-linked multilayer thin films by sequential deposition of bifunctional and tetrafunctional molecular building blocks is demonstrated. Multilayer growth as a function of deposition cycles was inspected using UV-vis absorption spectroscopy. From infrared results, three characteristic infrared bands of amide I, amide II, and asymmetric νa(N-C-N) stretching confirmed the formation of polyurea networks by alternate dipping into solutions of amine and isocyanate functionality monomers. The deconvoluted component of the C 1s and N 1s spectra obtained by X-ray photoelectron spectroscopy shows clear evidence of stable polyurea networks. The enhancement of structural periodicity with film growth was demonstrated by grazing-incidence small-angle X-ray scattering measurements. The thin film near the substrate surface seems to have an amorphous structure. However, molecular ordering improves in the surface normal direction of the substrate with a certain number of deposited layers. Constant mass density was not observed with deposition cycles. The mass density increased up to 16% within deposited layers from proximate layers to those extending away from the substrate surface. This difference in the packing density might derive from the different degrees of cross-linking among layers proximate to the substrate surface and extending away from the substrate surface.

Multilayer Growth of Porphyrin-Based Polyurea Thin Film Using Solution-Based Molecular Layer Deposition Technique

“…All characteristic peaks confirmed the formation of urea linkage. 15,33,35,36,41 In addition, the out-ofplane γ-(N−H) vibration of pyrrole in free-base porphyrin was observed at around 730 and 800 cm −1 . A shoulder type υ-(N−H) band was also observed at around 965 cm −1 for in-plane N−H vibration of pyrrole.…”

“…15−18 As covalently bonded films possess much higher chemical and thermal stability (because of their chemical characteristics) 19−21 compared to the conventional MLD film fabrication based on noncovalent interactions such as Coulombic interactions 22,23 and hydrogen bonding. 24 To date, most MLD processes have been conducted through polymerization of volatile bifunctional monomers under vacuum conditions to prepare polyamide, 25−27 polyimide, 28,29 polyurethane, 30 and polyurea 15,31,32 based nanostructures. The urea coupling reaction offers a significant benefit to avoid the byproduct entrapment within the polymer film as no byproduct is produced in this reaction process.…”

“…37 Recently, our research group has demonstrated a cross-linked polyurea thin film using a different symmetrical bifunctional and tetrafunctional monomers. 15 In this work, the solution-based MLD technique is chosen to establish porphyrin-based covalent organic thin film through the urea coupling reaction between bifunctional 1,4-phenylene diisocyanate porphyrin-based polyurea multilayer assemblies. Systematic characterization of the sequential molecular growth was accomplished using UV−vis absorption spectroscopy and atomic force microscopy (AFM).…”

Precise Solution-Based Deposition of Ultrathin Covalent Molecular Networks by Layer-by-Layer Cross-Linking Polymerization of Tetra- and Bifunctional Amine/Isocyanate Pairs

“…This issue impedes the use of the network materials in applications such as membrane-based technologies that require their nonpowdery forms. Synthetic methods allowing fabrication of ultrathin films of covalent molecular networks, i.e., those with thicknesses smaller than a few tens of nanometers, have been sought for the development of ultrathin molecular separation membranes ,, and ultrathin coatings − with special chemical functionalities.…”

“…Because of the bifunctional monomers in each layer having the freedom to be tilted with respect to the film plane, the growth pattern of a [4 + 2] system may differ from that of the corresponding [4 + 4] system. A potential problem of using flexible bifunctional monomers in LBLCP is that the reactive functional group may be folded back and bound within its own molecular layer, resulting in a nonlinear or irregular growth of the film and thus difficulty in controlling the thickness of the film on the molecular level. ,− In addition, the reactivity of the amino and isocyanato groups may vary with the structure of the parent moiety. For example, aliphatic and aromatic amines (or isocyanates) have different levels of nucleophilicity (or electrophilicity).…”