Toward Printed Molecular Electronics: Direct Printing of Liquid Metal Microelectrode on Self-Assembled Monolayers

Direct printing of conductive material on ultrathin organic films such as molecular monolayers in a non-invasive and high-yielding manner can potentially offer a route to unprecedented merging of molecular electronics with printed electronics. This paper reports on an approach to fabricate arrays of molecular tunnel junctions via automated direct printing of liquid-metal microelectrode comprising eutectic gallium-indium onto self-assembled monolayers of n-alkanethiolates, as a proof-of-concept. The printing design enables production of 350 molecular tunneling junctions per chip of 2 cm × 2 cm template-stripped substrate and 2800 molecular tunneling junctions per batch of 4 in. diameter Si wafer, which exhibit high yields of working junctions (>80%). Printed molecular junctions have uniform geometrical contact area of ≈4.7 × 10³ µm² and guarantees reproducibility in tunneling performance. The junctions exhibit exponential decay of current density (J, A cm⁻²) as a function of molecular length, obeying the simplified Simmons model, and single, narrow log-normal distributions of current density (σ_log|J| = ≈0.2–0.5). Importantly, there are no significant differences in tunneling decay coefficient and charge injection current density not only between collections of junctions per printed electrode, but also between batches. Results reported herein promise high-throughput fabrication of molecular-scale devices via printed molecular electronics.