Browsing by Author "Rafailov, P. M."

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Chemical vapour deposition growth and Raman characterization of graphene layers and carbon nanotubes
(2016-01-01)
Lai, Y-C
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Rafailov, P. M.
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Vlaikova, E.
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Marinova, V.
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Lin, S. H.
Single-layer graphene films were grown by chemical vapour deposition (CVD) on Cu foil. The CVD process was complemented by plasma enhancement to grow also vertically aligned multiwalled carbon nanotubes using Ni nanoparticles as catalyst. The obtained samples were characterized by Raman spectroscopy analysis. Nature of defects in the samples and optimal growth conditions leading to achieve high quality of graphene and carbon nanotubes are discussed.
Chemical vapour deposition growth of graphene layers on metal substrates
(2014-01-01)
Lai, Y-C
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Yu, S-C
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Rafailov, P. M.
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Vlaikova, E.
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Valkov, S.
Graphene layers were grown by chemical vapour deposition (CVD) on Si wafers covered by a SiO2 substrate layer and a Ni interlayer, and on copper and nickel foil. The obtained graphene layers were characterized by Raman spectroscopy. The films grown on SiO2/Ni substrate and Ni foil comprise mainly multilayer defect-rich graphene, while those on Cu foil exhibit the spectroscopic fingerprint of relatively defect-free single-layer graphene due to the low carbon solubility in copper and the suitably chosen substrate position in a quasi-closed volume. Optimal growth conditions and the nature of defects in the layers are discussed.
Growth and characterization of LuVO4 single crystals
(2017-09-01)
Dimitrov, D. Z.
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Rafailov, P. M.
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Chen, Y. F.
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Lee, C. S.
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Todorov, R.
Large LuVO4 single crystals have been successfully obtained by high-temperature solution method. The structure details of these crystals are determined by X-ray crystallographic analysis and Raman spectroscopy. It is observed that the crystal consists of LuVO4 phase with trace amount of imperfections possibly due to oxygen vacancies. The optical quality of the crystal is assessed by Spectroscopic Ellipsometry (SE). The crystal shows higher than +0.2 birefringence in a large interval of wavelengths. (C) 2017 Elsevier B.V. All rights reserved.
Growth, composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals
(2016-07)
Tomov, V.
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Rafailov, P. M.
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Luo, Chih-Wei
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電子物理學系
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Department of Electrophysics
Multiferroic single crystals in the novel system Pb-Mn-Ni-Ti-O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6(3)cm. The dielectric and magnetic properties along with the temperature dependence of the c-lattice parameter have been studied in the temperature range 2 K - 500 ?. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z-axis respectively, thus confirming the ferroelectric-to-paraelectric phase transition.
Optical, electro-optical, electrical and dielectric characterization of nematic liquid crystal (E7) layers doped with graphene nanoparticles for electro-optics
(2019-01-01)
Marinov, Y. G.
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Hadjichristov, G. B.
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Rafailov, P. M.
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Lin, S. H.
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Marinova, V. M.
Thin (7 mu m) layers of nanocomposites from graphene nanoflakes (GrFs) dispersed at concentrations of 10(-3) wt.% into the nematic liquid crystal (NLC) E7 were characterized by various investigation techniques, such as Raman spectroscopy, impedance measurements and dielectric spectroscopy, as well as by electro-optical measurements (optical transmittance of the NLC layers versus the voltage of the applied external AC electric field). Conducting behaviour, dielectric permittivity and electric energy loss of our planar-aligned NLC layers at room temperature were analysed as a function of frequency in the range from 0.5 Hz to 1 MHz. The analysis of experimental data indicates that the molecular alignment through GrFs/NLC surface interactions is responsible for the reduction of the ionic conductivity of E7 NLC in the presence of GrFs. As compared to pure LC E7, this leads to improved characteristics for the studied nanocomposites, necessary in their practical applications in electro-optics.