Interfacial toughening in composite materials is reasonably well understood for static loading, but little is known for cyclic loading. Here, we demonstrate that introducing an interfacial molecular nanolayer at the metal-ceramic interface of a layered polymer-metal-ceramic stack triples the fracture energy for ~75–300 Hz loading, yielding 40% higher values than the static-loading fracture energy. We show that this unexpected frequency-dependent toughening is underpinned by nanolayer-induced interface strengthening, which facilitates load transfer to, and plasticity in, the polymer layer. Above a threshold interfacial bond strength, the toughening magnitude and frequency range are primarily controlled by the frequency- and temperature-dependent rheological properties of the polymer. These results indicate the tunability of the toughening behavior through suitable choice of interfacial molecular layers and polymers. Our findings open up possibilities for realizing novel composites with inorganic-organic interfaces, e.g., arresting crack growth or stimulating controlled fracture triggered by loads with specific frequency characteristics.

International audienceWe reveal the roles of moisture and temperature on the interplay between interfacial work of adhesion gamma(a) and metal plasticity gamma(p) for copper-silica interfaces modified with an organosilane nanolayer. We find that gamma(p) not equal 0 for interfaces with metal thicknesses h(cu) > 12 nm, and increases with ha, before it saturates at h(cu) similar to 165 nm. For a fixed h(cu), gamma(p) increases due to temperature-induced yield stress decrease despite a decrease in gamma(a) with temperature because of water-induced siloxane bond weakening. These findings should be valuable for understanding the fracture mechanics of, and designing, nanomolecularly-functionalized interfaces subject to thermomechanical and chemical stresses. (C) 2016 Elsevier B.V. All rights reserved

Controlling electrical transport across metal-thermoelectric interfaces is key to realizing high efficiency devices for solid state refrigeration and waste-heat harvesting. We obtain up to 17-fold increases in electrical contact conductivity Σ by inserting a mercaptan-terminated organosilane monolayer at Cu-BiTe and Ni-BiTe interfaces, yielding similar Σ for both metals by offsetting an otherwise 7-fold difference. The Σ improvements are underpinned by silane-moiety-induced inhibition of Cu diffusion, promotion of high-conductivity interfacial nickel telluride formation, and mercaptan-induced reduction of BiTe surface oxides. Our findings should enable incorporating nanomolecular layers with appropriately chosen terminal moieties in thermoelectric device metallization schemes without metal diffusion barriers.

We demonstrate that weak chemical interactions between untethered moieties in molecular nanolayers on metal surfaces can strongly influence the effective work function Φeff. Electron spectroscopy shows that nanolayers of mercaptan-anchored organophosphonates on Au and Pt decrease Φeff. The measured Φeff shifts correlate with the chemical state of phosphonic acid moieties, and scale with molecular length. These results are contrary to predictions of ab initio calculations of monolayer-capped surfaces, but are consistent with calculations of bilayer-capped surfaces with face-to-face hydrogen-bonded phosphonic acid moieties. Our findings indicate that intra-layer bonding and layering in molecular nanolayers can be key to tailoring heterointerfacial electronic properties for applications.

We demonstrate that inserting a mercaptan-terminated organophosphonic acid monolayer at a Cu-HfO2 interface can alter the metal work function by −0.1 eV ≤ΔΦ≤−0.4 eV. The electron spectroscopy measurements of valence band structure reveal that molecular length-induced changes in ΔΦ can exceed contributions from Cu-S and P-O-Hf bonding dipoles at the Cu-organophosphonate-HfO2 interfaces. The invariance of the organophosphonate monolayer thickness with molecular length indicates that the observed values of ΔΦ are due to differences in molecular configuration and monolayer morphology. These findings suggest that molecular length could be a knob for tuning the electronic properties of inorganic interfaces modified with a nanomolecular layer for applications.