Surface Morphology and Electrical Properties of Cu₃BTC₂ Thin Films Before and After Reaction with TCNQ

HKUST-1 or Cu₃BTC₂ (BTC = 1,3,5-benzenetricarboxylate) is a prototypical metal-organic framework (MOF) that holds a privileged position among MOFs for device applications, as it can be deposited as thin films on various substrates and surfaces. Recently, new potential applications in electronics have emerged for this material when HKUST-1 was demonstrated to become electrically conductive upon infiltration with 7,7,8,8-tetracyanoquinodimethane (TCNQ). However, the factors that control the morphology and reactivity of the thin films are unknown. Here, we present a study of the thin-film growth process on indium tin oxide and amorphous Si prior to infiltration. From the unusual bimodal, non-log-normal distribution of crystal domain sizes, we conclude that the nucleation of new layers of Cu₃BTC₂ is greatly enhanced by surface defects and thus difficult to control. We then show that these films can react with methanolic TCNQ solutions to form dense films of the coordination polymer Cu(TCNQ). This chemical conversion is accompanied by dramatic changes in surface morphology, from a surface dominated by truncated octahedra to randomly oriented thin platelets. The change in morphology suggests that the chemical reaction occurs in the liquid phase and is independent of the starting surface morphology. The chemical transformation is accompanied by 10 orders of magnitude change in electrical conductivity, from <10^-11 S/cm for the parent Cu₃BTC₂ material to 10^-1 S/cm for the resulting Cu(TCNQ) film. The conversion of Cu₃BTC₂ films, which can be grown and patterned on a variety of (nonplanar) substrates, to Cu(TCNQ) opens the door for the facile fabrication of more complex electronic devices.