Mechanical, Adhesive and Thermodynamic Properties of Hollow Nanoparticles
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Mechanical, adhesive and thermodynamic properties of hollow nanoparticles
U. S. Sch warz1 , S. A. Safran and S. Komura2
Department of Mate rials and In terfaces,Weizmann Institute, Rehov ot 76100, Israel Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam, Germany 2 Department of Chemistry, Tokyo Metropolitan University, T okyo 192-0397, Japan 1
Abstract When sheets of lay eredmaterial lik e C, WS2 or BN are restricted to nite sizes, they generally form single- and multi-walled hollow nanoparticles in order to av oiddangling bonds. Using contin uum approaches to model elastic deformation and van der Waals in teractions of spherical nanoparticles, we predict the variation of mechanical stability, adhesive properties and phase behavior with radius R and thickness h. We nd that mechanical stability is limited by forces in the nN range and pressures in the GPa range. Adhesion energies scale linearly with R, but depend only weakly on h. Deformation due to van der Waals adhesion occurs for single-walled particles for radii of few nm, but is quickly suppressed for increasing thickness. As R is increased, the gas-liquid coexistence disappears from the phase diagram for particle radii in the range of 1-3 nm (depending on wall thickness) since the in teraction rangedecreases like 1=R.
Introduction The prototypical hollow nanoparticle is the buckyball C60 , which crystallizes into fullerite. Many other morphologies for carbon sheets hav e been found since the early 1990s, including fullerenes C of varying size, multi-walled fullerenes (carbon onions ), and single- and multi-walled carbon nanotubes. Closed structures of carbon are formed in order to avoid dangling bonds at the edges of nite sized carbon sheets. This mechanism is generic for anisotropic layered material of nite size, and up to now more than 30 other materials hav e been prepared as hollow nanoparticles (see the reviews [1] and references therein). This includes the metal dichalcogenides MeX2 (Me = W, Se, X = S, Se), BN, GaAs and CdSe. Usually inorganic fullerenes lik e WS2 and MoS2 are multi-walled. In Fig. 1 we show transmission electron micrographs of WS2 -particles which were synthesized by solidgas reaction. Control of size and shape is rather dicult, as evidenced b ythe irregularly faceted shapes. Howev er,the methods used for the synthesis of hollow nanoparticles are developing very rapidly, and it is to be expected that control of size, thickness and shape will become muc hbetter in the future. Hollow nanoparticles combine covalent in-plane strength with exible out-of-plane bending of thin lms, which results in high mechanical stability. They interact b y van der Waals (vdW) forces, which for example determine the material properties of fullerite [2] and the phase behavior of buckyballs [3]. VdW interactions become even more important for larger fullerenes, carbon onions and inorganic fullerenes; these materials, therefore, feature a large degree of non-speci c adhesion. In particular, vdW adhesion can lead to n
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