Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure

Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by scanning Kelvin probe microscopy (SKPM) of surface potentials and CPDs across ultrathin (1–2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly­(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of −10 to +50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are ∼15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the performance of these interfaces in devices

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

Measuring the Thickness and Potential Profiles of the Space-Charge Layer at Organic/Organic Interfaces under Illumination and in the Dark by Scanning Kelvin Probe Microscopy

Scanning Kelvin probe microscopy was used to measure band-bending at the model donor/acceptor heterojunction poly­(3-hexylthiophene) (P3HT)/fullerene (C₆₀). Specifically, we measured the variation in the surface potential of C₆₀ films with increasing thicknesses grown on P3HT to produce a surface potential profile normal to the substrate both in the dark and under illumination. The results confirm a space-charge carrier region with a thickness of 10 nm, consistent with previous observations. We discuss the possibility that the domain size in bulk heterojunction organic solar cells, which is comparable to the space-charge layer thickness, is actually partly responsible for less than expected electron/hole recombination rates

AFM Probing of Polymer/Nanofiller Interfacial Adhesion and Its Correlation with Bulk Mechanical Properties in a Poly(ethylene terephthalate) Nanocomposite

The interfacial adhesion between polymer and nanofiller plays an important role in affecting the properties of nanocomposites. The detailed relationship between interfacial adhesion and bulk properties, however, is unclear. In this work, we developed an atomic force microscopy (AFM)-based abrasive scanning methodology, as applied to model laminate systems, to probe the strength of interfacial adhesion relevant to poly­(ethylene terephthalate) (PET)/graphene or clay nanocomposites. Graphite and mica substrates covered with ∼2 nm thick PET films were abrasively sheared by an AFM tip as a model measurement of interfacial strength between matrix PET and dispersed graphene and clay, respectively. During several abrasive raster-scan cycles, PET was shear-displaced from the scanned region. At temperatures below the PET glass transition, PET on graphite exhibited abrupt delamination (i.e., full adhesive failure), whereas PET on mica did not; rather, it exhibited a degree of cohesive failure within the shear-displaced layer. Moreover, 100-fold higher force scanning procedures were required to abrade through an ultimate “precursor” layer of PET only ∼0.2−0.5 nm thick, which must be largely disentangled from the matrix polymer. Thus, the adhesive interface of relevance to the strength of clay–filler nanocomposites is between matrix polymer and strongly bound polymer. At 90 °C, above the bulk PET glass transition temperature, the PET film exhibited cohesive failure on both graphite and mica. Our results suggest that there is little difference in the strength of the relevant interfacial adhesion in the two nanocomposites within the rubbery dynamic regime. Further, the bulk mechanical properties of melt mixed PET/graphene and PET/clay nanocomposites were evaluated by dynamic mechanical analysis. The glassy dynamic storage modulus of the PET/clay nanocomposite was higher than that of PET/graphene, correlating with the differences in interfacial adhesion probed by AFM

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