Comparative micro-Raman spectroscopy study of tellurium-filled double-walled carbon nanotubes

Tellurium-filled double-walled carbon nanotubes Te@DWNTs have been studied by Raman spectroscopy in the temperature interval from 300 to 700 K employing 785 nm excitation wavelength, and their spectra have been compared to those of pristine double-walled carbon nanotubes DWNTs. The DWNTs were synthesized by catalytic chemical vapor deposition. Assignment of the radial breathing modes and the tangential modes was done based on the one dimensional electronic energy band structure of carbon nanotubes. The tangential mode components of Te@DWNT are downshifted compared to those of pristine DWNT consistent with the proposed weakening of the carbon-carbon bond due to the introduced van der Waals interaction of the Te atoms with the DWNT. It was established that Te@DWNT can be unambiguously identified by the 30% temperature coefficient decrease of the G-band position

Temperature dependence of Raman scattering in filled double-walled carbon nanotubes

Four types of filled double-walled carbon nanotubes (DWNTs) (Se@DWNT; Te@DWNT; HgTe@DWNT; and PbI2@DWNT) have been studied by high-resolution transmission electron microscopy and micro-Raman spectroscopy in the temperature interval from 80 to 700 K employing 785 nm excitation wavelength. The temperature dependence of the dominant bands (D-band, G-band, and the (2D)-band) are analyzed in terms of the model developed by Klemens, Hart, Agraval, Lax, and Cowley and extended by Balkanski for anharmonic decay of optical phonons. The quasiharmonic frequencies and the anharmonicity constants were obtained from the temperature dependences of the analyzed Raman bands. The findings were compared to analogous study for empty DWNTs. The strength of the van der Waals interaction between the guest material and the carbon nanotube (CNT) estimated through the quasiharmonic frequencies was found to decrease in the following order: Se@DWNT; Te@DWNT; PbI2@DWNT, and HgTe@DWNT. In agreement with this, the anharmonicity due to the phonon–phonon interactions was found to decrease in the same order

Micro-Raman scattering of selenium-filled double-walled carbon nanotubes: Temperature study

Selenium-filled double-walled carbon nanotubes Se@DWNT have been studied by high resolution transmission electron microscopy HRTEM and micro-Raman spectroscopy in the temperature interval from 80 to 600 K employing 785 nm excitation wavelength. The temperature dependences of the dominant bands G-band and G-band are analyzed in terms of the model developed by Klemens Phys. Rev. 148, 845 1966, Hart et al. Phys. Rev. B 1, 638 1970, Cowley J. Phys. France 26, 659 1965 and extended by Balkanski et al. Phys. Rev. B 26, 1928 1983 for anharmonic decay of optical phonons. The findings were compared to analogous study for empty double-walled carbon nanotubes DWNTs. The DWNT interatomic force constant modification as a result of the presence of the Se atoms inside the tubes is revealed through larger anharmonicity constants describing the temperature dependences of the G-band and the inner tube tangential modes G-ban

Celebrating the 80th Anniversary of Professor Edward A. Friedman, Scientist and Educator

On September 29, 2015, the doyen of the Editorial board of Serdica Journal of Computing, Prof. Edward Friedman, celebrated his 80 th anniversary. This special issue is dedicated to the birthday boy in recognition of his numerous contributions: to science, to promoting effective strategies in integrating digital technologies with mathematics and science education, to adapting a software sys tem for the health treatment of rural patients in India and Sub-Saharan Africa, and most recently—to the development of new university courses on raising the awareness of the threat of nuclear terrorism

A new approach to the modeling and analysis of fracture through an extension of continuum mechanics to the nanoscale

The dissertation focuses on the analysis, through combined analytical and numerical techniques, of the partial differential equations arising from a new approach to modeling brittle fracture, based on extension of continuum mechanics to the nanoscale. The main part of this work deals with the analysis of several fracture models. Integral transform methods are used to reduce the problem to a Cauchy singular, linear integro-differential equation. It is shown that ascribing constant surface tension to the fracture surfaces and using the appropriate crack surface boundary condition, given by the jump momentum balance, leads to a sharp crack opening profile at the crack tip, in contrast to the classical theory of brittle fracture. However, such a model still predicts singular crack tip stress. For this reason a modified model is studied, where the surface excess property is responsive to the curvature of the fracture surfaces. It is shown that curvature-dependent surface tension, together with boundary conditions in the form of the jump momentum balance, leads to bounded stresses and a cusp-like opening profile at the crack tip. Further, an alternative approach, based on asymptotic analysis, which is suitable to apply in cases when the model includes a mutual body force correction term, is considered. The nonlinear nonlocal problem, resulting from the proposed model, is simplified which allows us to approximate the crack opening profile and derive asymptotic forms for the cleavage stress in a neighborhood of the crack tip. Finally, two possible fracture criteria, in the context of the new theory, are discussed. The first one is an energy based fracture criterion. Classically the energy release rate arises due to singular fields, whereas in the case of the modeling approach adopted here, a notion analogous to the energy release rate arises through a different mechanism, associated to the rate of working of the surface excess properties at the crack tip. Due to the fact that the proposed modeling approach allows us to fully resolve the stress in a neighborhood of the crack tip, without the customary singularity, a second fracture criterion, based on crack tip stress, is possible

Along the Traces of the Conic Sections

Based on a geometry problem by Ivan Salabashev we demonstrate an exploratory process leading to conic sections as a locus of points. We work for the purpose with the GeoGebra dynamic software making use of different objects’ representations for explorations related with conic sections

Tuning the electrical transport properties of double-walled carbon nanotubes by semiconductor and semi-metal filling

Manipulating the electrical properties of carbon nanotubes through semi-metal or semiconductor filling is of paramount importance in the realization of nano-electronic devices based on one dimensional composite materials. From low temperature electrical conductivity measurements of a network, of empty and filled double-walled carbon nanotubes (DWNT’s), we report a transition in electrical transport features from hopping to weakly activated conduction by HgTe filling and also semi-metallic conduction in selenium (Se) filled DWNT’s. Magneto-resistance (MR) studies of the filled DWNT’s show suppression of the hopping conduction and a signature of 3D weak localization for Se@DWNT’s at low temperatures and high magnetic fields. These results are discussed on the basis of strength of interaction between the filler material and the inner-walls of the host DWNT’s, which enhances the electronic density of states (DOS) in the material as well as the change in the property of the filler material due to constrained encapsulation

Achievements and Problems in the In-service Teacher Education in Inquiry Based Style

Report published in the Proceedings of the National Conference on "Education and Research in the Information Society", Plovdiv, May, 2015The paper deals with the experience of the authors in promoting the Inquiry Based Learning (IBL) in mathematics and science education within international and national projects. The emphasis is on in-service teacher education. Various types of activities and resources in support of all levels of IBL are considered, e.g. professional development courses, seminars, workshop, and performances; implementation of resources stimulating students to behave like working mathematicians. The first visible positive effects and potential problems in the implementation of IBL in the Bulgarian schools are discussed.Association for the Development of the Information Society, Institute of Mathematics and Informatics Bulgarian Academy of Sciences, Plovdiv University "Paisii Hilendarski