Coordination-driven opto-electroactive molecular thin films in electronic circuits

Abstract

Surface engineering using controlled molecular structures, compositions, thicknesses, packing, and orientations is highly desirable for understanding nanoscale interfacial phenomena. The preparation of coordination-compound-based oligomer films with well-defined chemical structures, compositions, orientations, and thicknesses is crucial for ‘on-surface’ optoelectronic and electrical applications. We take advantage of the simple-yet-classic technique of layer-by-layer (LBL) construction to fabricate both homostructured Fe(II)/Co(II)-bis-terpyridyl (bis-tpy) oligomers and heterostructures. We exploit the electrochemical reduction method to prepare covalent template layers on which different transition metal ions and bis-terpyridine ligands are systemically assembled. Molecular thin films fabricated on transparent conducting oxides such as ITO substrates are utilized for optical, electrochemical, electrical, and electrical impedance spectroscopy studies to explore the possibilities of molecular-prototype devices. A gel electrolyte was placed between ITO/tpy-[Fe(bis-tpy)]₅ and ITO/tpy-[Co(bis-tpy)]₅ to mimic a molecular electronic device configuration to measure the current–voltage (I–V) response in a facile, cheap, fast, scalable, and clean-room-free approach. Electrical impedance spectroscopy was used to experimentally deduce charge-transfer resistance, contact resistance, and capacitance values, followed by circuit modeling. The circuit model was further validated via building a real electronic circuit using individual electrical components. Near-vertically-aligned molecular thin films could be suitable for various applications in optoelectronics and electrochromic and molecular electronics.