Platinum Nanoparticles at Mica Surfaces

We report a systematic study of factors that influence the incidence of platinum (Pt) nanoparticles produced by the standard method of cutting mica sheets into shapes using a hot platinum wire. Related findings in prior literature are confirmed. Characterization by AFM (atomic force microscopy) shows that fewer particles were produced the thinner the Pt wire and that the density and size of particles were substantially less when using Pt wire not previously used to cut mica. In optimal cases, using 0.0125 in. diameter wire, we obtained ∼0.1% surface coverage; the mean particle height of particles was ∼2.3 nm, and the mean diameter, 15 nm. After detachment from a backing sheet, the particle abundance was reduced by a factor of roughly 2. For Pt wires of the same diameter, the abundance of particles as a function of mica thickness and location relative to the wire was studied. The identification of these particles as Pt was confirmed using a combination of TEM (transmission electron microscopy) and Rutherford backscattering (RBS). After applying normal loads relevant to conditions in the surface forces apparatus, up to 5 MPa, we conclude from AFM characterization that the particles were not deformed by normal load and that their indentation into the underlying mica substrates was negligible. Cutting mica with hot iridium (Ir) rather than Pt wire also produced nanoparticles. Finally, FECO interference fringes (fringes of equal chromatic order) between cleaved mica sheets in close contact were contrasted with and without nanoparticles present. Implications for surface forces measurements are discussed tentatively.