When: November 29, 16:00
Where: Skoltech, room 303
ABSTRACT: Crystalline nanoparticles, including semiconductor particles and nanodiamonds, are actively investigated nowadays for applications in novel materials, quantum computing, biology and medicine. One of the important methods of nanoparticles study is Raman spectroscopy. In comparison with bulk material the Raman peak frequency in nanoparticles is downshifted and the peak is asymmetrically broadened. These effects are straightly related with the finite particle size. This opens up a possibility to use Raman spectroscopy for measuring nanoparticles sizes.
The standard tool for crystalline nanoparticles Raman spectra (RS) theoretical analysis is semiphenomenological phonon confinement model (PCM). It is based on the assumption of Gaussian decay of vibrations from the particle center to its boundary. Further analysis of this model shows that it contains some important flaws.
We propose a new model free of PCM disadvantages using the system microscopical parameters. The only adjustable parameter of the theory is the phonon linewidth. In order to obtain the particle vibrational eigenmodes we use the dynamical matrix method. Then, the bond polarization model is utilized to calculate the particle RS. With the usage of some averaging procedure, we also formulate our method in analytical form. The latter does not require cumbersome calculations but loses some information about the spectrum. In the framework of our theory we successfully describe the recent experimental data on nanodiamonds powders with sizes varying from 1 to 10 nm, while PCM is unable to fit them.
In the second part of the work we propose a simple method for obtaining nanoparticles RS. It is based on solving boundary value problem, which resembles Klein-Fock-Gordon equation. The results of this approach reproduce the numerical ones with the high accuracy. So, instead of solving dynamical matrix, one can solve simple Laplace equation. In the framework of this theory we investigate the question of particles shape influence on RS and we obtain some interesting results.
BIO: Oleg Utesov received bachelor degree in solid state physics from the St. Petersburg State Polytechnic University and master degree in condensed matter physics from the St. Petersburg Academic University in 2012. He gets PhD degree in theoretical physics in Petersburg Nuclear Physics Institute, Gatchina, in 2016. Today he is a researcher in theoretical physics division at St. Petersburg Nuclear Physics Institute and in the laboratory of nanobiotechnologies at St. Petersburg Academic University. His research interests include theory of spiral magnets and mutiferroics of spin origin, disordered bosonic systems, transport phenomena in semiconductors, vibrational properties of carbon nanostructures, and Raman spectroscopy of nanoparticles. The latter, supplied with the proper theoretical background, can be used for the nanoparticles arrays characterization, which is important for their usage in applications, e.g. novel materials, quantum computing, biology and medicine.