Rectification Ratio and Tunneling Decay Coefficient Depend on the Contact Geometry Revealed by in Situ Imaging of the Formation of EGaIn Junctions

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 cone-shaped 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.