STM verification of the reduction of the Youngʹs modulus of CdS nanoparticles at smaller sizes

We demonstrate the first STM evaluation of the Youngʹs modulus (E) of nanoparticles (NPs) of different sizes. The sample deformation induced by tip-sample interaction has been determined using current–distance (I–Z) spectroscopy. As a result of tip-sample interaction, and the induced surface deformations, the I-z curves deviates from pure exponential dependence. Normally, in order to analyze the deformation quantitatively, the tip radius must be known. We show, that this necessity is eliminated by measuring the deformation on a substrate with a known Youngʹs modulus (Au(111)) and estimating the tip radius, and afterwards, using the same tip (with a known radius) to measure the (unknown) Youngʹs modulus of another sample (nanoparticles of CdS). The Youngʹs modulus values found for 3 NPʹs samples of average diameters of 3.7, 6 and 7.5 nm, were E ~ 73%, 78% and 88% of the bulk value, respectively. These results are in a good agreement with the theoretically predicted reduction of the Youngʹs modulus due to the changes in hydrostatic stresses which resulted from surface tension in nanoparticles with different sizes. Our calculation using third order elastic constants gives a reduction of E which scales linearly with 1/r (r is the NPʹs radius). This demonstrates the applicability of scanning tunneling spectroscopy for local mechanical characterization of nanoobjects. The method does not include a direct measurement of the tip–sample force but is rather based on the study of the relative elastic response.