This paper describes how the intensive (tunneling decay coefficient β and rectification ratio R) and extensive (current density J) properties of Ag−S(CH 2 ) n−1 CH 3 //GaO x /EGaIn junctions (n = 10, 14, 18) and molecular diodes of the form of Ag−S(CH 2 ) 11 Fc//GaO x /EGaIn depend on A geo , the contact area between the self-assembled monolayer and the coneshaped EGaIn tip. Large junctions with A geo ≥ 1000 μm 2 are unreliable and defects, such as pinholes, dominate the charge transport characteristics. For S(CH 2 ) 11 Fc SAMs, R decreases from 130 to unity with increasing A geo due to an increase in the leakage current (the current flowing across the junction at reverse bias when the diodes block current flow). The value of β decreases from 1.00 ± 0.06 n −1 to 0.70 ± 0.03 n −1 with increasing A geo which also indicates that large junctions suffer from defects. Small junctions with A geo ≤ 300 μm 2 are not stable due to the high surface tension of the bulk EGaIn resulting in unstable EGaIn tips. In addition, the contact area for such small junctions is dominated by the rough tip apex reducing the effective contact area and reproducibility significantly. The contact area of very large junctions is dominated by the relatively smooth side walls of the tips. Our findings show that there is an optimum range for the value of A geo between 300−500 μm 2 where the electrical properties of the junctions are dominated by molecular effects. In this range of A geo , the value of J (defined by I/A geo where I is the measured current) increases with A geo until it plateaus for junctions with A geo > 1000 μm 2 in agreement with recently reported findings by the Whitesides group. In this regime reproducible measurements of J can be obtained provided A geo is kept constant.

In molecular electronics, it is critical to minimize the sources that can result in defective electrodes, such as contaminations related to the fabrication process (photoresist and organic residues) or roughening of the electrode during etching, because these defects hamper the formation of well‐organized molecular structures. Junctions based on micropores are desirable as they are scalable, but micropores are not fabricated on ultrasmooth template‐stripped electrodes, and may suffer from stray capacitances and leakage currents across the insulating matrix. A method is reported to fabricate micropores in AlOx on template‐stripped Au based on a two‐step etch process so that the Au surface is not in direct contact with photoresistance during the fabrication process. These junctions do not suffer from stray capacitances or leakage currents, enable temperature variable measurements down to 8.5 K, have excellent current retention characteristics, and are stable for at least 2 months. By analyzing the normalized differential conductance curves and detailed comparison against junctions with cone‐shaped tips of EGaIn and EGaIn stabilized in a through‐hole in polydimethylsiloxane, how the surface roughness of top electrodes affects the effective contact area, influences the symmetry of the response of the junctions, and how the electrical characteristics scale with molecular length are established.

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH 2 ) n BPh (BPh = biphenyl, n = 1−7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45°when n is even and 23°when n is odd. The tilt angle of the alkyl chain is fixed at 30°and independent of n. For values of n = 1−3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.