Nanocomposite Dielectric Films with Interphasic Response from Olefin Metathesis of Functionalized HfO₂ Nanocrystals

Abstract

With the advent of the Internet-of-Things, flexible thin film electronics stand poised for explosive growth that will far surpass even the current boom of applications in displays, sensing, and haptics. However, the power-constrained edges of the Internet require highly energy-efficient modes of computing, which will entail operation at increasingly lower voltages. In addition to their inevitably low dielectric constants, polymer dielectrics often suffer from substantial polarization disorder, low breakdown voltages, ill-defined interfaces, and a propensity for charge trapping. As such, intense interest has focused on the design of hybrid nanocomposite dielectric layers, wherein high-κ dielectric nanocrystals are embedded within polymeric media. In this work, we describe a “grafting from” strategy wherein surface-passivating ligands of HfO2 nanocrystals bearing pendant olefin moieties are polymerized with norbornene monomers to yield dense hybrid nanocomposite dielectrics with an active interphase. The surface compatibilization approach reported here enables high solid loading of HfO2 nanocrystals within the polymeric matrix, minimizes void space, and engenders strong bonding of nanoscopic fillers with the continuous polymeric matrix, which is key to eliciting an interphasic dielectric response. Specifically, hydrothermally grown monoclinic HfO2 nanocrystals are functionalized with monolayers of allyltrimethoxysilane; the pendant allyl moieties are polymerized with norbornene through catalyzed olefin metathesis. Dense functionalized HfO2/polynorbornene nanocomposite films are obtained, which yield an effective permittivity as high as 11.3, thereby effectively harnessing the permittivity of the monoclinic HfO2 filler. The dielectric response is measured as a function of (a) surface functionalization; (b) HfO2 nanocrystal filler loading; and (c) film thickness and is best described by the Vo–Shi model of effectivity permittivity, which includes contributions from strong interphasic dipole polarization. The results demonstrate a generalizable strategy for eliciting interphasic dielectric response and for maximizing the contribution of high-κ dielectric fillers in nanocomposite thin films.