Sevak Khachadorian

Manipulation of the catalyst-support interactions for inducing nanotube forest growth

We show how an oxidative pretreatment of Fe, Co, or Ni growth catalyst on SiO2 support can be used to switch the growth mode of carbon nanotubes from tip growth to root growth, thus favoring the growth of dense, vertically aligned nanotube forests. The oxidative treatment creates a strong catalyst–support interaction at the catalyst–silica interface, which limits the surface diffusion and sintering of the catalyst nanoparticles and binds the catalyst to the SiO2 surface. This shows that the alignment and growth mode of nanotubes can be controlled, increasing the range of support materials giving dense nanotube forests.

Raman and Photoluminescence Spectroscopic Detection of Surface‐Bound Li+O2− Defect Sites in Li‐Doped ZnO Nanocrystals Derived from Molecular Precursors

We present a detailed study of Raman spectroscopy and photoluminescence measurements on Li-doped ZnO nanocrystals with varying lithium concentrations. The samples were prepared starting from molecular precursors at low temperature. The Raman spectra revealed several sharp lines in the range of 100-200 cm(-1), which are attributed to acoustical phonons. In the high-energy range two peaks were observed at 735 cm(-1) and 1090 cm(-1). Excitation-dependent Raman spectroscopy of the 1090 cm(-1) mode revealed resonance enhancement at excitation energies around 2.2 eV. This energy coincides with an emission band in the photoluminescence spectra. The emission is attributed to the deep lithium acceptor and intrinsic point defects such as oxygen vacancies. Based on the combined Raman and PL results, we introduce a model of surface-bound LiO(2) defect sites, that is, the presence of Li(+)O(2)(-) superoxide. Accordingly, the observed Raman peaks at 735 cm(-1) and 1090 cm(-1) are assigned to Li-O and O-O vibrations of LiO(2).

A Practical Approach for Applying Online Remote Experiments: OnPReX.

The development of Internet technologies stimulates the increase of online technology-supported education in universities. Online learning based on remote experiments is capable of diminishing the scantiness in practical courses. In this paper, we present online practical courses based on remote experiments (OnPReX). These courses consist of online experiments, interactive graphics and an online tutoring-system organised at the Berlin Institute of Technology (TUB). We developed two online practical courses, one focusing on classical and the other on modern physics, for bachelor and intermediate diploma engineering students. Our online practical courses based on remote experiments took place in two test phases during two semesters. These courses were evaluated and the results were used to improve the courses. Our online practical courses on remote experiments have successfully been integrated as electives at the TUB.

Deployment of remote experiments: The OnPReX course at the TU Berlin

The development of internet technologies leads to recent trends of online based education in universities. Remote experiments give the students the possibility to experience real physical situations and compare their experimental results with those of the physical models (simulation). Online learning based on remote experiments is capable of diminishing the scantiness in practical courses in the universities. In this work we present an online practical course based on Remote Experiments (OnPReX), interactive graphics and an online tutoring system organized at the Technische Universität Berlin. Furthermore we give background information about the development of our remote experiments. The remote experiments are real-life settings, designed and engineered at the solid-state physics department of the Technische Universität Berlin. We describe the development of two online practical courses, focusing on classical and modern physics, addressed at undergraduate engineering students. We ran the online practical course based on Remote Experiments in two test phases over two semesters and conducted an evaluation from which we improved technical and pedagogical aspects of the experiments and the course.

Spatially controlled growth of highly crystalline ZnO nanowires by an inkjet-printing catalyst-free method

High-density arrays of uniform ZnO nanowires with a high-crystal quality have been synthesized by a catalyst-free vapor-transport method. First, a thin ZnO film was deposited on a Si substrate as nucleation layer for the ZnO nanowires. Second, spatially selective and mask-less growth of ZnO nanowires was achieved using inkjet-printed patterned islands as the nucleation sites on a SiO2/Si substrate. Raman scattering and low temperature photoluminescence measurements were applied to characterize the structural and optical properties of the ZnO nanowires. The results reveal negligible amounts of strain and defects in the mask-less ZnO nanowires as compared to the ones grown on the ZnO thin film, which underlines the potential of the inkjet-printing approach for the growth of high-crystal quality ZnO nanowires.