Charge Transport Physics in Coordination Nanosheets (CONASHs)

Abstract

Metal-organic frameworks (MOFs) have emerged as a versatile class of materials with intriguing properties, driven by their modular structure and tuneable functionalities. The insulating nature of most MOFs has hindered their applications which require highly conductive MOFs. A specific type of MOF distinguished by its two-dimensional (2D) π-conjugated structure called coordination nanosheet (CONASHs) exhibits significantly enhanced conductivity. This study delves into the unique charge transport characteristics of CONASHs. The investigation focuses on two exemplary coordination nanosheets, copper-benzenehexathiol (Cu-BHT) and nickel-based benzenehexathiol (Ni-BHT), presenting a detailed exploration of their charge transport properties. A comprehensive temperature-dependent transport study of Cu-BHT shows its remarkable conductivity exceeding 1500 S/cm at low temperatures and metallic transport nature. The integration of magneto-transport and thermoelectric characterizations reveals insights into its ambipolar charge transport behaviour. The simultaneous extraction of electron and hole mobilities contributes to a deeper understanding of its complex charge transport physics. Electrolyte gating transistors (EGTs) based on Cu-BHT demonstrate a gate-tuned metal-to-insulator transition. This study unveils a tuneable charge density, leading to a five-order-of-magnitude change in conductivity. The observed transition from ambipolar to unipolar transport under positive gate voltage illustrates the controllability of conducting channels, establishing EGTs as a potent tool for manipulating charge-dependent properties in coordination nanosheets. The investigation extends to Ni-BHT, a non-porous coordination nanosheet with a conductivity up to 112 S/cm. The exploration of temperature-dependent conductivity reveals an Efros–Shklovskii variable-range hopping transport, and the anomaly between Hall effect and thermoelectric coefficients is attributed to abnormal Hall effect in strongly disordered systems. The introduction of an EGT based on Lithium Perchlorate/Poly(ethylene oxide) electrolyte allows for the tunability of polarity, showcasing its potential for the manipulation of electronic states in CONASHs. This study establishes coordination nanosheets as a promising avenue for materials engineering, leveraging their unique π-conjugated structure for enhanced conductivity. The controllable tuning of charge transport properties through electrolyte gating opens up exciting possibilities for applications in electronic devices, underscoring the broader significance of investigating charge transport in CONASHs.