Controlling Film Formation and Host-Guest Interactions to Enhance the Thermoelectric Properties of Nickel-Nitrogen-Based Two-Dimensional Conjugated Coordination Polymers.

Abstract

Two-dimensional conjugated coordination polymers (cCPs) based on square-planar transition metal-complexes (such as MO4 , M(NH)4 , and MS4 , M = metal) are an emerging class of (semi)conducting materials that are of great interest for applications in supercapacitors, catalysis, and thermoelectrics. Finding synthetic approaches to high-performance nickel and nitrogen (Ni-N) based cCP films has been a long-standing challenge. Here, we develop a general, dynamically controlled on-surface synthesis method that produces highly conductive Ni-N-based cCP thin films and study the thermoelectric properties as a function of the molecular structure and their dependence on interactions with the ambient atmosphere. Among the four studied cCPs with different ligand sizes hexaminobenzene- and hexaaminotriphenylene-based films exhibit record electrical conductivity (100 - 200 S cm-1 ) in this Ni-N based cCP family, which is one order of magnitude higher than previous reports, and the highest thermoelectric power factors up to 10 μW m-1 K-2 among reported 2D cCPs. We study the transport physics of these films and show that depending on the host-guest interaction with oxygen/water the majority carrier type and the value of the Seebeck coefficient can be largely regulated. The high conductivity is likely reflecting good interconnectivity between (small) ordered domains with grain boundaries supporting disordered metallic transport. This article is protected by copyright. All rights reserved.