Real-time nanomanipulations for tribological measurements of nanostructures
Nanoparticles (NPs) and nanowires (NWs) are now among the most important classes of materials in modern applied science, as they have demonstrated remarkable properties and have number of promising applications. In particular, fabrication of nanoelectromechanical systems (NEMS) requires precise control over positioning and behaviour of the NPs and NWs in various environments. Thus, deeper understanding of their mechanical properties is demanded. Commonly used tool for mechanical characterization of nanostructures is atomic force microscope (AFM) that features high precision and stability of the results but lacks in situ visual guidance.
In this work an experimental method of real-time measurements of tribological and mechanical properties of NPs and NWs using nanomanipulation technique inside a scanning electron microscope (SEM) is presented . Measurements are based on visually controllable manipulations of the nanostructures with a sharp AFM tip inside SEM using nanomanipulator.
Polyhedron and sphere-like metal (gold and silver) nanoparticles (NPs) were manipulated on an oxidized Si substrate to study the dependence of the static friction and the contact area on the particle geometry. To register the occurring forces a quartz tuning fork (QTF) with a glued sharp probe was used. Experimentally measured force is compared to static friction forces found by using various models. The effect of NP morphology on the nanoscale friction is discussed.
NWs of oxide materials (ZnO, CuO) are manipulated by a sharp tip and being elastically deformed. NW bending profile is preserved due to the balance between intrinsic elastic force, lateral friction force from the substrate and the force of external manipulator. The state of mechanical equilibrium is described with use of continuum mechanics based models and further employed to find frictional properties of the NWs.
In particular, maximal static friction force is estimated when nanowire is being pushed at one end and switches from partial to complete motion upon overcoming static friction . Static friction from the bent NW state can be also considered via a crack-based model . Kinetic friction is extracted from the profile of the nanowire being uniformly dragged at the midpoint. After being brought to rest after the manipulations, the nanowire causes redistribution of static friction force from the supporting surface which is calculated and compared with kinetic friction . Finally, a strong dependence of static friction on surface roughness is described .
 B. Polyakov, L. Dorogin, S. Vlassov, I. Kink, R. Lohmus // Tribological Aspects of In Situ Manipulation of Nanostructures Inside Scanning Electron Microscope; in “Fundamentals of Friction and Wear on the Nanoscale” (Springer Series "NanoScience and Technology", 2015), pp. 395-426.
 L. M. Dorogin, B. Polyakov, S. Vlassov, M. Antsov, R. Lõhmus, I. Kink and A. E. Romanov // Real-time manipulation of ZnO nanowires on a flat surface employed for tribological measurements: experimental methods and modeling; Physica Status Solidi (b) 250 (2013) 305–317.
 Sergei Vlassov, Boris Polyakov, Sven Oras, Mikk Vahtrus, Mikk Antsov, Andris Šutka, Krisjanis Smits, Leonid M. Dorogin, Rünno Lõhmus // Complex tribomechanical characterization of ZnO nanowires: nanomanipulations supported by FEM simulations; Nanotechnology (2016), in press.
 M. Antsov, L. M. Dorogin, S. Vlassov, B. Polyakov, M. Vahtrus, K. Mougin, R. Lohmus, I. Kink // Analysis of static friction and elastic forces in a nanowire bent on a flat surface: A comparative study; Tribology International 72 (2014) 31-34.
 B. Polyakov, S. Vlassov, L. M. Dorogin, P. Kulis, I. Kink, E. Gnecco and R. Lõhmus // The effect of substrate roughness on the static friction of CuO nanowires; Surface Science 606 (2012) 1393-1399.