Fabrication and Low-Temperature Characterization of Phthalocyanine Molecular Tunnel Heterojunctions

We report on the fabrication and characterization of vertical tunnel heterojunctions in which sublimated phthalocyanine (Pc) films (H2Pc, CoPc, and MnPc) as thin as 1 nm are sandwiched between conducting bottom-layer base electrodes and top-layer soft-landing eutectic GaIn (EGaIn) electrodes. The heterojunctions can be reliably cycled to temperatures as low as 2 K and are remarkably robust, showing little evidence of pinhole shorts over a wide temperature range. Ultraslow sublimation of Pc powders (at rates of & ANGS;/min) onto temperature-regulated substrates for the most part produced ultrasmooth films with 100% coverage. As an example, atomic force microscopy in combination with X-ray diffraction showed that CoPc films with subnanometer roughness deposited onto conducting substrates had the crystalline ordering of lying-down planar molecules. The current-voltage (I-V) characteristics, obtained to temperatures as low as 2 K, reveal the onset of a subgap reduction in the density of states due to the appearance of superconductivity at T (c) = 6 K in the top EGaIn electrode. Together with the higher temperature fitting of a modified Simmons model, these behaviors provide incontrovertible evidence for direct quantum mechanical tunneling processes rather than thermally activated hopping through the transition-metal ions and associated satellite ligands of the Pc molecules in our heterojunctions. Our use of thermally stable soft landing EGaIn counter electrodes preserves the fragility of the underlying organic Pc molecules down to few-layer thicknesses at low temperatures where quantum properties can be exploited for molecular electronic applications.