Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations.

A mechanistic understanding of adhesion in soft materials is critical in the fields of transportation (tires, gaskets, seals), biomedicine, micro-contact printing, and soft robotics. Measurements have long demonstrated that the apparent work of adhesion coming into contact is consistently lower than the intrinsic work of adhesion for the materials, and that there is adhesion hysteresis during separation, commonly explained by viscoelastic dissipation. Still lacking is a quantitative experimentally validated link between adhesion and measured topography. Here, we used in situ measurements of contact size to investigate the adhesion behavior of soft elastic polydimethylsiloxane (PDMS) hemispheres (modulus ranging from 0.7 to 10 MPa) on four different polycrystalline diamond substrates with topography characterized across eight orders of magnitude, including down to the Ångström-scale. The results show that the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. Further, the energy loss during contact and removal is equal to the product of intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively link the widely-observed adhesion hysteresis to roughness rather than viscoelastic dissipation. Figure 1: Comprehensive topography characterization for four rough nanodiamond surfaces. The surface topography was measured using a multi-resolution approach that combines transmission electron microscopy (rightmost region of the curves), atomic force microscopy (intermediate region), and stylus profilometry (leftmost region). The nanodiamond surfaces are designated using the following nomenclature: ultrananocrystalline diamond (UNCD) is shown in red; nanocrystalline diamond (NCD) in black; microcrystalline diamond (MCD) in green, and a polished form of UNCD (polished UNCD) in blue. AFM images (of 5-micron lateral size) are shown in the left inset; TEM images are shown in the right inset. More than 50 measurements for each surface are combined using the power spectral density, which reveals the contribution to overall roughness from different length scales (wavelengths). These comprehensive descriptions of surface topography enable the determination of true surface area and stored mechanical energy due to the topography, which are necessary to understand adhesion.The model for (Eq. 4) is applied to the measured data as shown in Fig. 3A using 1 = 25 ± 5 mJ/m 2 [29,33]. The best correlation between the experimentally measured work of adhesion and the predictions of Eq. 4 was obtained using the intrinsic work of adhesion of 37.0 ± 3.7 mJ/m 2 (R 2 = 0.67). The explicit accounting for the change in area of the soft surface led to improved model predictions; if we do not account for this change (calculations shown in Supplemental Section 4) the best fit to the measured data is significantly poorer (R 2 = 0.29).

Surface roughness affects the functional properties of surfaces, including adhesion, friction, hydrophobicity, biological response, and electrical and thermal transport properties. However, experimental investigations to quantify these links are often inconclusive because surfaces are fractal-like, and the values of measured roughness parameters depend on measurement size. Here, we demonstrate the characterization of topography of an ultrananocrystalline diamond (UNCD) surface at the angstrom scale using transmission electron microscopy (TEM), as well as its combination with conventional techniques to achieve a comprehensive surface description spanning 8 orders of magnitude in size. We performed more than 100 individual measurements of the nanodiamond film using both TEM and conventional techniques (stylus profilometry and atomic force microscopy). While individual measurements of root-mean-square (RMS) height, RMS slope, and RMS curvature vary by orders of magnitude, we combine the various techniques using the power spectral density and use this to compute scale-independent parameters. This analysis reveals that "smooth" UNCD surfaces have an RMS slope greater than 1, even larger than the slope of the Austrian Alps when measured on the scale of a human step. This approach of comprehensive multiscale roughness characterization, measured with angstrom-scale detail, will enable the systematic evaluation and optimization of other technologically relevant surfaces, as well as systematic testing of the many analytical and numerical models for the behavior of rough surfaces.

Bimetallic nanoparticles like Cu-Ni are particularly attractive due to their magnetic and catalytic properties; however, their properties depend strongly on the structure of the alloy i.e. mixed, core-shell or Janus. To predict the alloy structure, this paper investigates the size and shape effects as well as the surface segregation effect on the Cu-Ni phase diagram. Phase maps have been plotted to determine the mixing/demixing behavior of this alloy according the particle shape. Cu-Ni nanoalloy can form a mixed particle or a Janus one depending on the synthesis temperature. Surface segregation is also considered and reveals a nickel surface-enrichment. Finally, this paper provides a useful roadmap for experimentalists.