Laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics have been observed after irradiation of single-crystalline indium phosphide ͑c-InP͒ with multiple linearly polarized femtosecond pulses (130 fs, 800 nm) in air. With an increasing number of pulses per spot, N, up to 100, a characteristic evolution of two different types of ripples has been observed, i.e., (i) the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines and (ii), in a specific pulse number regime ͑N =5-30͒, the additional formation of equally oriented ripples with a spatial period close to half of the laser wavelength. For pulse numbers higher than 50, the formation of micrometer-spaced grooves has been found, which are oriented perpendicular to the ripples. These topographical surface alterations are discussed in the frame of existing LIPSS theories.

This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.

Lateral force microscopy (LFM) is a variation of atomic/scanning force microscopy (AFM/SFM). It relies on the torsional deformation of the AFM cantilever that results from the lateral forces acting between tip and sample surface. LFM allows imaging of heterogeneities in materials, thin films or monolayers at high spatial resolution. Furthermore, LFM is increasingly used to study the frictional properties of nanostructures and nanoparticulates. An impediment for the quantification of lateral forces in AFM, however, is the lack of reliable and established calibration methods. A widespread acceptance of LFM requires quantification coupled with a solid understanding of the sources of uncertainty. This paper reviews the available experimental calibration methods and identifies particularly promising approaches.

The sensitivity to water vapour of one-, two-, and three-layer epitaxial graphene (1, 2, and 3LG) is examined in this study. It is unambiguously shown that graphene's response to water, as measured by changes in work function and carrier density, is dependent on its thickness, with 1LG being the most sensitive to water adsorption and environmental concentration changes. This is furthermore substantiated by surface adhesion measurements, which bring evidence that 1LG is less hydrophobic than 2LG. Yet, surprisingly, it is found that other contaminants commonly present in ambient air have a greater impact on graphene response than water vapor alone. This study indicates that graphene sensor design and calibration to minimize or discriminate the effect of the ambient, in which it is intended to operate, are necessary to insure the desired sensitivity and reliability of sensors. The present work will aid in developing models for realistic graphene sensors and establishing protocols for molecular sensor design and development.

No abstract

In an effort to expand the understanding of the mechanical properties of the polymeric interphase on a metal surface, a composite consisting of epoxy and copper was prepared and analyzed. Scanning force microscopy-based force modulation microscopy (SFM-FMM) was employed along with dynamic mechanical analysis (DMA) and energy dispersive X-ray analysis (EDX). Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resins were applied with amine curing agents. The samples were made taking advantage of electron beam lithography (EBL) in order to produce sharp edges of copper structures and a flat surface suitable for the SFM-FMM analysis, which was able to depict the stiffness within the interphase. It is considered significant information because the mechanical characteristic within the narrow interphase was revealed. Comparing with DMA and EDX, the stiffness information of SFM-FMM demonstrated a matching correlation and agreement in terms of preferential adsorption of the curing agent in the vicinity of the interface. The stiffness profiles of the two epoxy systems turned out to be different, and it shows the material dependence of the interphase characteristics.

No abstract

A detailed study of the mechanical interphase (IP) between an amine-cured epoxy and the amorphous thermoplastic polyvinylpyrrolidone (PVP) was performed. The amine curing agent was diaminodiphenylsulphone (DDS). With 170 °C, the curing temperature was close to the glass transition temperature of PVP. Using a depth-sensing indentation setup equipped with a Berkovich indenter, force penetration curves were measured at different positions on the cross-section of an epoxy/PVP/epoxy sandwich specimen. Profiles of the sample Young's modulus, Es, the hardness, H, and the plasticity index, ψ, were analysed as a function of the distance from the epoxy/PVP interface. Each of these quantities showed significant variations. Furthermore, the profile of the ratio between the reduced modulus of the tip–sample contact, Er, and the hardness is shown to reflect the profile of the plasticity index, that is the relative amount of plastic indentation work. A relationship is presented that allows the calculation of the plasticity index from the ratios Er/H and hf/hc, where hf and hc denote the final depth and the contact depth, respectively. Although the Young's modulus shows strong variations, the hardness appears to be the most sensitive parameter for IP property variations. Three different zones can be identified from the hardness profile, with respective widths of ∼21.3, 52.5 and 160.9 µm, which add up to a total IP width of ∼234.7 µm. Zone Z1 is located next to the PVP layer. The outer edge of zone Z2 is located at a position similar to the one of the amine depletion zone. The latter was detected by means of energy-dispersive analysis of x-rays and its width is ∼71 µm. Inspection of the residual imprints shows a larger plastic deformation for zone Z1, in agreement with the high values of the plasticity index and of the creep deformations. From these grounds, the different IP zones are discussed in terms of the diffusion processes taking place during the epoxy curing procedure. Zone Z1 is associated with bi-directional interdiffusion across the initial interface. Zone Z3 can be attributed to long-ranging effects reaching far beyond the amine depletion zone.