Micrometer scale pore-interconnectivity in nanoporous ZIF-8 films with Zn enriched surface terminations

Abstract

Abstract ZIF-8 nanoporous films of variable thicknesses, which are proposed to be very low-k dielectric films for chip industries, have been deposited on Si substrate using simple solution chemistry method. Contrary to the earlier reports, ZIF-8 crystals number density on the substrate in early deposition cycles increases leading to higher coverage of the substrate. The growth of crystal size or film thickness occurs in later deposition cycles. The deposition of ZIF-8 crystals is highly random, however a preferential growth in 002 orientation is observed. Synchrotron based X-ray photoelectron spectroscopy has confirmed Zn enrichment at surface of the films compared to imidazole linkers. Depth dependent Doppler broadening spectroscopy measurements utilizing the positronium diffusion process have unequivocally confirmed that the pores in ZIF-8 films are interconnected at micrometer scale leading to their accessibility from the outer surface. This type of pore architecture along with the surface enrichment might be responsible for adsorbate selectivity and very high adsorption capacity of these films as reported in literature. On thermal annealing, the pore architecture of these films begins to collapse at a much lower temperature (~373 K) than the decomposition temperature (~700 K) of ZIF-8 powder.