Comparison of DC and AC Transport in 1.5–7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic Ga–In versus Conducting Probe Atomic Force Microscope Junctions

Abstract

We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms.

Charge Transport through Self‐Assembled Monolayers of Monoterpenoids

“…We also studied as econd series of molecules: octanoic acid (5)a nd related unsaturated molecules (6)(7)(8)(9). We conclude from the set of molecules 1-9 that increasing the degree of unsaturation, but not the position of the C = Cb ond (6)(7)(8), influences the rate of tunneling through length-matched hydrocarbons. Figure 1c shows that compound 9 showed the only statistically significant difference in these log j J j values (log j J j= 1.2 AE 0.2).…”

“…We also studied as econd series of molecules: octanoic acid (5)a nd related unsaturated molecules (6)(7)(8)(9). We also studied as econd series of molecules: octanoic acid (5)a nd related unsaturated molecules (6)(7)(8)(9).…”

“…We examined the relative rates of charge transport by tunneling through SAMs of alkanoates on template-stripped silver, and varied the number of C = Cb onds from zero (completely saturated) to three (completely unsaturated). [7] JðVÞ¼J 0 ðVÞe Àbd ð1Þ Thef irst mechanism-coherent tunneling-has been studied by several groups,i ncluding that of Frisbie,w ho examined transport through SAMs of oligophenylene or polythiophene attached to gold by am etal-thiolate bond via conductive atomic force microscopy (cAFM), [8] and of Chiechi, who examined junctions of SAMs of molecules with fully conjugated oligophenylene groups. Tw ol imiting possibilities might contribute to this increase:i )T he average height of the tunneling barrier might decrease with unsaturation.…”

“…Twomodels commonly used to rationalize the observation that some molecules have significantly larger rates of tunneling than length-matched alkanes include i) coherent tunneling,w hich involves reducing the energy off-set between molecular frontier orbitals (the highest occupied molecular orbitals (HOMO) or the lowest unoccupied molecular orbitals (LUMO) and the work function of the electrodes), and ii) "superexchange tunneling," which involves molecular orbitals formed by non-bonding interactions among high-energy occupied orbitals. [7] JðVÞ¼J 0 ðVÞe Àbd ð1Þ Thef irst mechanism-coherent tunneling-has been studied by several groups,i ncluding that of Frisbie,w ho examined transport through SAMs of oligophenylene or polythiophene attached to gold by am etal-thiolate bond via conductive atomic force microscopy (cAFM), [8] and of Chiechi, who examined junctions of SAMs of molecules with fully conjugated oligophenylene groups. [9] These studies found that the rate of charge transport through fully conjugated molecules is up to factor of 10 3 larger than molecules of similar length with only partial or no conjugated orbital systems.M cCreery,B ergren, and co-workers,h owever,e xamined rates of charge transfer across as eries of different aromatic molecules and established that the rates changed only modestly (by af actor of ten) with large differences in their HOMO energies (> 2eV).…”

(Invited) Interface Engineering for Nanoelectronics

“…As the frequency increases, the |Z| decreases at a transition frequency where the AC bias frequency is too large for the molecular layer to respond. This is related to the thickness of the molecular layer assuming this is the separation of the parallel plates of a capacitor(31). Again, HF 6 H 13 SH appears different with a transition frequency at a higher value which could be caused by either a slightly thinner monolayer or a slightly less resistive molecular layer.…”