Thermally Controlled Phase Transition of Low-Melting Electrode for Wetting-Based Spontaneous Top Contact in Molecular Tunnel Junction

Top contacts for molecular-scale electronic devices should exhibit reliable and reproducible electronic performance. This goal is challenging and difficult to achieve because metals are usually evaporated under high-energy conditions that easily damage delicate organic surfaces, and complicated nanofabrication processes are needed for achieving geometrically defined small contact areas. Soft top contacts that are made by users under ambient conditions can circumvent this problem but often show user-dependence. This paper describes that thermally controlled phase transition (TCPT) of low-melting (29.76 °C) electrode comprising gallium covered with a self-passivating oxide layer could be useful to form reliable, spontaneous (i.e., user-independent) top contacts over delicate ultrathin organic films such as self-assembled monolayers (SAMs). As a proof-of-concept, we demonstrate that the phase transition from solid to non-Newtonian liquid for gallium electrode is tuned under mild thermal conditions (room temperature to ∼50 °C), which does not damage the organic component and ensures conformal, geometrically defined contacts. The contact force predominantly depends on wetting of compliant liquid gallium onto SAMs, upon heating, not on user-pressure. Indeed, the TCPT-based large-area tunnel junctions on SAMs of n-mercaptoalkanoic acids yield markedly narrow dispersion of tunneling current density (σ_log|J| = 0.04-0.19) and tunneling attenuation coefficient (β = 0.92 ± 0.02n_C ^-1) consistent with the literature value. We envisage that our approach can be harnessed to accomplish liquid metal-based tunnel junctions without significant user-to-user variations and hence useful for reliable understanding of charge transport across molecules and practical applications.