Nanoscale physics

Effect of size on thermal expansion of Ag and Cu nanoparticles
S. Bogatyrenko1, A. Kryshtal2
1 Faculty of Physics and Technology, Karazin Kharkiv National University of the NAS of Ukraine. 4 Svobody Sq., Kharkiv, 61022, Ukraine. 
E-mail: sib@univer.kharkov.ua
2 International Centre of Electron Microscopy for Material Science and Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology. Al. Mickiewicza 30, Kraków, PL-30059, Poland.

The work is devoted to the basic problems of nanophysics, namely it considers the effect of size on the physical and chemical properties of nanomaterials. We study the thermal expansion coefficient of Ag and Cu nanoparticles from the variation of their lattice parameter during in situ TEM heating.
Ag and Cu films with a mass thickness in the range of 0.72-25 nm were deposited on an amorphous carbon (a-C) substrate pre-heated to 100ºC. As a result, isolated flat islands of Ag and Cu uniformly covering a-C films were formed. The films under study were mounted on a nickel grid of the in-house holder for heating samples directly in the transmission electron microscope PEM-125K. In the first cycle, the samples were annealed at temperatures of 500ºC to bring the shape of the nanoparticles close to the equilibrium one and to avoid its variation in the subsequent measurement cycle.
In the second cycle, the morphology and crystalline structure of the metal films were investigated in the 20-500ºC temperature range. Selected area electron diffraction patterns from nanoparticle arrays with mean size varied in the range from 2 to 40 nm were registered in a transmission electron microscope at different temperatures. Diffraction ring sizes were measured using a free DiffTools plugin for Gatan Digital MicrographTM software and the lattice constant was calculated for each series of the samples with the accuracy of ±0.002 nm.
New quantitative data on the size dependence of the thermal expansion coefficients (CTE) of copper and silver nanoparticles were obtained. It was shown that the thermal expansion coefficient of Ag and Cu increases significantly with decreasing nanoparticle size (αCu = 30.3x10-6 K-1 for 2 nm-size particles, αAg = 32.8x10-6 K-1 for 2.7 nm particles). The pattern of the obtained CTE dependencies correlates well with the computer simulation data available in the literature.
This work was supported by the Ministry of Education and Science of Ukraine №0118U002027 and the Polish Ministry of Science and Higher Education (grant AGH no 16.16.110.663).