High thermal conductivities and low dielectric permittivities are desired properties in next-generation dielectric layers, especially in high-power density chips. Further, tuning the on/off rates of thermal and electrical conduction from environmental field forces and stimuli is an important goal. Transport effects arising from 2D material parameters, including thickness, interlayer spacings, and channel lengths, are explored well in graphene and metal-organic framework (MOF) thin films.

2D covalent organic frameworks (2D COFs) are extended crystalline solids built from light, covalent building blocks. 2D COFs show relatively high thermal conductivities and electrical inertness suitable for dielectric layers. Additionally, their modular structures and incoporation into other 2D material heterostructures create a platform for studies on conductivity switching.

Despite these advantages, 2D COFs are conventionally prepared as insoluble, polycrystalline powers, limiting their integration in devices. A multidisciplinary effort recently reported that 2D COF thin-films have relatively high thermal conductivities (k = 1 W m^-1 K^-1) and ultra-low dielectric permittivities (k = 1.6) compared to other low density materials. Despite the high quality of these films, their surface roughnesses at varying thicknesses decreased the necessary precision for elucidating structurally relevant trends.

Here, we report the colloidal syntheses of boronate-ester and imine-linked COF thin-films with tunable thermal conductivities. Time-domain thermoreflectance measurements suggest thermal transport trends dependent on linkage type, pore functionality, environmental stimuli, and film thickness. Further, tunable electrical conduction arises from the incorporation of carbon nanotubes in the thin-films. Materials and chemical characterization, including atomic-force microscopy and grazing-incidence X-ray scattering, show high homogeneity, crystallite orientation, and functionality effects, leading to enhanced and tunable transport.

Dr. Austin M. Evans

Schmidt Postdoctoral Fellow, Northwestern University

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