STM verification of the reduction of the Young's modulus of CdS nanoparticles at smaller sizes A. Hazarika a,b,1 , E. Peretz a,1 , V. Dikovsky a,1 , P.K. Santra b , R.Z. Shneck c , D.D. Sarma b , Y. Manassen a, ⁎ a Department of Physics and the Ilse Katz Center of Science and Technology in the nm Scale, Ben-Gurion University of the Negev, P.O. Box 653 Beer-Sheva, 84105, Israel b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India c Department of Materials Engineering, Ben Gurion University of the Negev, P.O. Box 653 Beer-Sheva, 84105, Israel abstract article info Article history: Received 27 November 2013 Accepted 5 July 2014 Available online 15 July 2014 Keywords: Nanoparticles Tunneling current–distance spectroscopy The Young's modulus The Morse force We demonstrate the first STM evaluation of the Young's modulus (E) of nanoparticles (NPs) of different sizes. The sample deformation induced by tip-sample interaction has been determined using current–distance (I–Z) spec- troscopy. 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 Young's modulus (Au(111)) and estimating the tip radius, and afterwards, using the same tip (with a known radius) to measure the (unknown) Young's modulus of another sample (nanoparticles of CdS). The Young's modulus values found for 3 NP's 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 Young's modulus due to the changes in hydrostatic stresses which resulted from surface tension in nano- particles with different sizes. Our calculation using third order elastic constants gives a reduction of E which scales linearly with 1/r (r is the NP's radius). This demonstrates the applicability of scanning tunneling spectros- copy 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. © 2014 Published by Elsevier B.V. 1. Introduction The ability to synthesize, manipulate and characterize a wide variety of materials in the nanometric size domain has developed rapidly. Extensive research of nanoobjects such as nanoparticles, nanowires, nanotubes and superthin films have demonstrated different physical, chemical and mechanical properties from the corresponding bulk materials [1–6]. In particular, elastic moduli for the bulk modification are varied over five orders of magnitude depending on the material and can be used for its identification. On the nanometric scale the elastic moduli show complicated behavior. Increasing the role of the surface energy and stress in the nanoscaled systems influences their mechanical properties both directly and through structure transformations [7]; changes of electronic structure and magnetic interactions [6]. Surface passivation may also effectively change the elastic properties of nano- objects [6,8]. The outcome of this diversity is contradicting the ex- perimental results regarding the elastic moduli of the nanosystems as well as disagreement between theory and experiment. For many nanoobjects, even the tendency of change of their rigidity with the re- duction of the size is under question. For instance, Ref. [9] has reported a significant reduction of the bulk modulus in CdSe nanoparticles (NPs) within a size range of ~1–2 nm. Similarly, Ref. [10] has reported a de- crease in the modulus in ZnS NPs with the size decreasing below ~6 nm. In contrast, a very recent theoretical work [11] has suggested a substantial increase in the Bulk modulus of several systems, including CdSe NPs, with a decreasing size. It is well established that atomic force microscope (AFM) and scanning tunneling microscope (STM) equipped with an AFM head provide precise study of the short-range forces acting on the tip by the nanoobjects located on the surface. Know- ing the deformation and geometry of the nanoobject, we can determine its elastic moduli. For that we need to know in addition, the shape and sharpness of the STM and AFM tips. As a rule, the tip shape is not well defined. However, for one particular tip, with a particular (unknown) curvature, the tip will apply a well defined (unknown) force on the surface, which is reproducible and remains the same as long as the structure of the tip will not change. In this paper we show, how this fact is useful even without a direct force measurement. When the same tip is used in the experiment with two different samples, it is pos- sible to measure the relative elastic response of these two samples, and in this way, to use the Young's modulus of one of them to find the other. Surface Science 630 (2014) 89–95 ⁎ Corresponding author. 1 The first 3 authors have contributed equally to this work. http://dx.doi.org/10.1016/j.susc.2014.07.006 0039-6028/© 2014 Published by Elsevier B.V. Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc