Growth of Thin, Anisotropic, π‑Conjugated Molecular Films by Stepwise “Click” Assembly of Molecular Building Blocks: Characterizing Reaction Yield, Surface Coverage, and Film Thickness versus Addition Step Number

We report the systematic characterization of anisotropic, π-conjugated oligophenyleneimine (OPI) films synthesized using stepwise imine condensation, or “click” chemistry. Film synthesis began with a self-assembled monolayer (SAM) of 4-formylthiophenol or 4-aminothiophenol on Au, followed by repetitive, alternate addition of terephthalaldehyde (benzene-1,4-dicarbaldehyde) or 1,4-benzenediamine to form π-conjugated films ranging from 0.6–5.5 nm in thickness. By systematically capping the OPI films with a redox or halogen label, we were able to measure the relative surface coverage after each monomer addition via Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, reflection–absorption infrared spectroscopy, and cyclic voltammetry. Nuclear reaction analysis was also employed for the first time on a SAM to calculate the surface coverage of carbon atoms after each stepwise addition. These six different analysis methods indicate that the average extent of reaction is 99% for each addition step. The high yield and molecular surface coverage confirm the efficacy of Schiff base chemistry, at least with the terephthalaldehyde and 1,4-benzenediamine monomers, for preparing high-quality molecular films with π conjugation normal to the substrate

Quantitative Surface Coverage Measurements of Self-Assembled Monolayers by Nuclear Reaction Analysis of Carbon-12

We report surface coverage measurements by Rutherford backscattering spectrometry (RBS) of self-assembled monolayers (SAMs) of both alkyl thiols and oligophenylene thiols on Au-coated mica, Si, and pyrolytic graphite. The ¹²C atom concentration was probed at 4.266 MeV ₂⁴He²⁺ primary beam energy, which enhances the ₂⁴He²⁺ scattering cross section by exciting ¹²C nuclear resonance states; this is a submode of RBS commonly referred to as nuclear reaction analysis (NRA). The surface coverage of ¹²C increased linearly with the number of ¹²C atoms in each SAM. The consistency of the ¹²C atom coverage values obtained by NRA was cross-checked by measuring the ³²S atom concentration by conventional RBS. From these data, we obtained an average coverage of 3.5 ± 0.2 molecules/nm² for both alkyl thiols and oligophenylene thiols on polycrystalline Au surfaces. The results show the utility of NRA for quantitative analysis of SAM coverage on Au

Exceptionally Small Statistical Variations in the Transport Properties of Metal–Molecule–Metal Junctions Composed of 80 Oligo­phenylene Dithiol Molecules

Strong stochastic fluctuations witnessed as very broad resistance (<i>R</i>) histograms with widths comparable to or even larger than the most probable values characterize many measurements in the field of molecular electronics, particularly those measurements based on single molecule junctions at room temperature. Here we show that molecular junctions containing 80 oligophenylene dithiol molecules (OPDn, 1 ≤ <i>n</i> ≤ 4) connected in parallel display small relative statistical deviationsδ<i>R</i>/<i>R</i> ≈ 25% after only ∼200 independent measurementsand we analyze the sources of these deviations quantitatively. The junctions are made by conducting probe atomic force microscopy (CP-AFM) in which an Au-coated tip contacts a self-assembled monolayer (SAM) of OPDs on Au. Using contact mechanics and direct measurements of the molecular surface coverage, the tip radius, tip-SAM adhesion force (<i>F</i>), and sample elastic modulus (<i>E</i>), we find that the tip-SAM contact area is approximately 25 nm², corresponding to about 80 molecules in the junction. Supplementing this information with <i>I–V</i> data and an analytic transport model, we are able to quantitatively describe the sources of deviations <i>δR</i> in <i>R</i>: namely, <i>δN</i> (deviations in the number of molecules in the junction), <i>δε</i> (deviations in energetic position of the dominant molecular orbital), and <i>δΓ</i> (deviations in molecule-electrode coupling). Our main results are (1) direct determination of <i>N</i>; (2) demonstration that <i>δN</i>/<i>N</i> for CP-AFM junctions is remarkably small (≤2%) and that the largest contributions to <i>δR</i> are <i>δε</i> and <i>δΓ</i>; (3) demonstration that δ<i>R</i>/<i>R</i> after only ∼200 measurements is substantially smaller than most reports based on >1000 measurements for single molecule break junctions. Overall, these results highlight the excellent reproducibility of junctions composed of tens of parallel molecules, which may be important for continued efforts to build robust molecular devices

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

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//GaO_<i>x</i>/EGaIn junctions having contact areas of 9.6 × 10² μm² (10⁹ nm²) 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 nm² 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//GaO_<i>x</i>/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