Polyurea MLD using low melting point precursors and a possible method of testing suitability for protein crosslinking

Abstract

Molecular layer deposition (MLD) is a growing area of research and identifying ideal, easy-to-use precursors is an important goal in furthering the field. An ideal precursor has certain qualities: low cost, high vapor pressure, compatibility with a range of temperatures that allow for low temperature processes with sensitive substrates, reliable, predictable growth characteristics such as a linear relationship between film thickness and cycle count, and exponential saturation of the surface with precursor exposure characteristic of the self-saturating nature of the idealized MLD process. This work demonstrates an MLD process for depositing polyurea films using 2,4-toluene diisocyanate (TDIC) and ethylene diamine (ED), which can be favorably compared to the more common polyurea deposition of 1,4-phenylene diisocyanate (PDIC) and ED in terms of cost, vapor pressure, and low temperature performance. The process described here has nearly identical growth characteristics, making it an ideal stand-in for existing polyurea MLD applications. The similarities support the termination-diffusion model described in earlier work1, and the polyurea film is further assessed for its suitability for use in protein immobilization by demonstration of its free amine surface functionality and measurement of its zeta potential.