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Christos S. Garoufalis - One of the best experts on this subject based on the ideXlab platform.
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Optical Gap and excitation energies of small Ge nanocrystals
Journal of Mathematical Chemistry, 2009Co-Authors: Christos S. GaroufalisAbstract:Using the density functional theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the Optical Gap and the oscillator strengths for several of the lowest, spin and symmetry allowed, electronic transitions of small Ge nanocrystals passivated by hydrogen. The largest nanoparticle has an approximate diameter of 2 nm. Our results show that the Optical Gap exhibits size dependence (due to quantum confinement) roughly similar to silicon nanoparticles. However, for this range of diameters, there is an indirect-to-direct transition in the spectra of Ge as the size of the nanocrystals decrease. The first allowed excitation (fundamental Optical Gap) of each germanium nanoparticle has relatively larger oscillator strengths compared to silicon. The diameter of the smallest Ge nanocrystal capable to emit in the visible region of the spectrum, is approximately 1.9 nm, compared to 2.2 nm for silicon nanocrystals.
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High accuracy calculations of the Optical Gap and absorption spectrum of oxygen contaminated Si nanocrystals.
Physical Chemistry Chemical Physics, 2005Co-Authors: Christos S. Garoufalis, Aristides D. ZdetsisAbstract:We report accurate high level calculations of the Optical Gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 A, the widening of the Gap due to quantum confinement facilitates the stabilization of SiO double bonds. For this type of bonding, the oxygen related states determine the value of the Optical Gap and make it significantly lower compared to the corresponding Gap of oxygen-free nanocrystals. For diameters larger than 20 A, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the Optical Gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.
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Real space Optical Gap calculations in oxygenated Si nanocrystals
Journal of Physics: Conference Series, 2005Co-Authors: Christos S. Garoufalis, Aristides D. ZdetsisAbstract:We have calculated the Optical Gap of small oxygenated Si nanocrystals with diameters in the range between 2 and 10 A using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), which includes partially exact Hartree–Fock exchange. The Optical Gap is obtained from the B3LYP HOMO-LUMO Gap by a well established correlation relation. Our results are in very good agreement with experimental measurements in oxygen containing samples, and confirm out earlier conclusions [Phys. Rev. Lett. 87 276402 (2001)] for the role of oxygen in the Optical Gap of Si nanocrystals.
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The Optical Gap of small Ge nanocrystals
Journal of Physics: Conference Series, 2005Co-Authors: Christos S. Garoufalis, M S Skaperda, Aristides D. ZdetsisAbstract:Using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the Optical Gap of small Ge nanocrystals passivated by hydrogen and with diameters between 2 and 20 A. Our results show that the Optical Gap exhibits a size dependence (due to quantum confinement) with many similarities as in the case of Si quantum dots. The diameter of the smallest Ge nanocrystal emitting in the visible region of the spectrum, is approximately 19 A. This critical dimension is smaller than the one found for the case of Si nanocrystals [Phys. Rev Lett. 87 276402 (2001)].
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Variation and adjustment of the Optical Gap of small Si nanocrystals by partial substitution of Si with Ge
Journal of Physics: Conference Series, 2005Co-Authors: Aristides D. Zdetsis, Christos S. Garoufalis, M S Skaperda, Emmanuel N. KoukarasAbstract:In order to adjust the optimum region of diameters of Si based nanocrystals, which can emit in the visible region of the spectrum, we have partially substituted in Si nanocrystals layers of Si atoms by similar layers of Ge atoms and calculated the Optical and HOMO-LUMO Gaps as a function of Ge concentration and of the size of the nanocrystals, up to about 20 A in diameter. For the calculation of the Optical Gap of SixGey:Hz nanocrystals as a function of x , y, and z, we have used the framework of time dependent density functional theory (TDDFT) with the hybrid nonlocal exchange-correlation functional of Becke, Lee and Yang (B3LYP). Our results show that by proper adjustment of x, y and z we can optimize either the range of diameters for a desired Gap, or the value of the Optical Gap for a given diameter.
G A J Amaratunga - One of the best experts on this subject based on the ideXlab platform.
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evolution of sp2 bonding with deposition temperature in tetrahedral amorphous carbon studied by raman spectroscopy
Applied Physics Letters, 2000Co-Authors: Manishkumar Chhowalla, Andrea Ferrari, J Robertson, G A J AmaratungaAbstract:Two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature. The total sp3 fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The Optical Gap and resistivity show a similar, gradual transition. This indicates that the sp2 cluster size determines the Optical Gap, even when the sp2 content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the Gap.
Aristides D. Zdetsis - One of the best experts on this subject based on the ideXlab platform.
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High accuracy calculations of the Optical Gap and absorption spectrum of oxygen contaminated Si nanocrystals.
Physical Chemistry Chemical Physics, 2005Co-Authors: Christos S. Garoufalis, Aristides D. ZdetsisAbstract:We report accurate high level calculations of the Optical Gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 A, the widening of the Gap due to quantum confinement facilitates the stabilization of SiO double bonds. For this type of bonding, the oxygen related states determine the value of the Optical Gap and make it significantly lower compared to the corresponding Gap of oxygen-free nanocrystals. For diameters larger than 20 A, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the Optical Gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.
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Real space Optical Gap calculations in oxygenated Si nanocrystals
Journal of Physics: Conference Series, 2005Co-Authors: Christos S. Garoufalis, Aristides D. ZdetsisAbstract:We have calculated the Optical Gap of small oxygenated Si nanocrystals with diameters in the range between 2 and 10 A using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), which includes partially exact Hartree–Fock exchange. The Optical Gap is obtained from the B3LYP HOMO-LUMO Gap by a well established correlation relation. Our results are in very good agreement with experimental measurements in oxygen containing samples, and confirm out earlier conclusions [Phys. Rev. Lett. 87 276402 (2001)] for the role of oxygen in the Optical Gap of Si nanocrystals.
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The Optical Gap of small Ge nanocrystals
Journal of Physics: Conference Series, 2005Co-Authors: Christos S. Garoufalis, M S Skaperda, Aristides D. ZdetsisAbstract:Using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the Optical Gap of small Ge nanocrystals passivated by hydrogen and with diameters between 2 and 20 A. Our results show that the Optical Gap exhibits a size dependence (due to quantum confinement) with many similarities as in the case of Si quantum dots. The diameter of the smallest Ge nanocrystal emitting in the visible region of the spectrum, is approximately 19 A. This critical dimension is smaller than the one found for the case of Si nanocrystals [Phys. Rev Lett. 87 276402 (2001)].
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Variation and adjustment of the Optical Gap of small Si nanocrystals by partial substitution of Si with Ge
Journal of Physics: Conference Series, 2005Co-Authors: Aristides D. Zdetsis, Christos S. Garoufalis, M S Skaperda, Emmanuel N. KoukarasAbstract:In order to adjust the optimum region of diameters of Si based nanocrystals, which can emit in the visible region of the spectrum, we have partially substituted in Si nanocrystals layers of Si atoms by similar layers of Ge atoms and calculated the Optical and HOMO-LUMO Gaps as a function of Ge concentration and of the size of the nanocrystals, up to about 20 A in diameter. For the calculation of the Optical Gap of SixGey:Hz nanocrystals as a function of x , y, and z, we have used the framework of time dependent density functional theory (TDDFT) with the hybrid nonlocal exchange-correlation functional of Becke, Lee and Yang (B3LYP). Our results show that by proper adjustment of x, y and z we can optimize either the range of diameters for a desired Gap, or the value of the Optical Gap for a given diameter.
Dong-won Park - One of the best experts on this subject based on the ideXlab platform.
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Determination of effective Optical Gap in dye/TiO2 systems inspired by p-n junctions
Applied Physics Letters, 2015Co-Authors: Kyung-jun Hwang, Yonkil Jeong, Dong-won ParkAbstract:The effective Optical Gap and device current limits of dye-sensitized solar cells (DSCs) were investigated. Optical Gap determination was based on an approach that assumes the presence of a nanoscale p-n junction in the DSCs between the bulk TiO2 semiconductor and the dye-cluster with quantum size effect. On the basis of this approach, the effective Optical Gap of the dye-absorber was extracted from a relation between external quantum efficiency and photon energy. The short-circuit current density of the fabricated DSCs showed a current loss in the range from 3.7 to 5.1 mA cm−2 compared to the device current limit. This current loss can be mainly attributed to the light reflection of the window layer and the native charge-transfer loss by device imperfections, including subsidiary charge-transfer loss by a nanoscale Schottky junction between TiO2 and the electrolyte.
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determination of effective Optical Gap in dye tio2 systems inspired by p n junctions
Applied Physics Letters, 2015Co-Authors: Kyung-jun Hwang, Yonkil Jeong, Dong-won ParkAbstract:The effective Optical Gap and device current limits of dye-sensitized solar cells (DSCs) were investigated. Optical Gap determination was based on an approach that assumes the presence of a nanoscale p-n junction in the DSCs between the bulk TiO2 semiconductor and the dye-cluster with quantum size effect. On the basis of this approach, the effective Optical Gap of the dye-absorber was extracted from a relation between external quantum efficiency and photon energy. The short-circuit current density of the fabricated DSCs showed a current loss in the range from 3.7 to 5.1 mA cm−2 compared to the device current limit. This current loss can be mainly attributed to the light reflection of the window layer and the native charge-transfer loss by device imperfections, including subsidiary charge-transfer loss by a nanoscale Schottky junction between TiO2 and the electrolyte.
Andrea Ferrari - One of the best experts on this subject based on the ideXlab platform.
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Optical Gap in carbon nitride films
Thin Solid Films, 2003Co-Authors: Sandra E. Rodil, S. Muhl, S. Maca, Andrea FerrariAbstract:Abstract In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp 2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N 2 plasma source, N was introduced into sp 3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the Optical Gap initially decreases as the N content and the sp 2 fraction rises, but above a certain N quantity there is a level-off of the value, and the Gap then remains constant despite further increases in the fraction and clustering of the sp 2 phase. However, for a-CN films the Optical Gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band Gap is associated to an increase in the sp 2 fraction or its clustering. Here we discuss the conditions that lead to high Optical Gap in sp 2 -bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra.
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Highest Optical Gap tetrahedral amorphous carbon
Diamond and Related Materials, 2002Co-Authors: Kbk Teo, Andrea Ferrari, Giovanni Fanchini, Sandra E. Rodil, Jun Yuan, Jeff T. H. Tsai, Enzo Laurenti, Alberto Tagliaferro, John Robertson, William I. MilneAbstract:The deposition and characterisation of tetrahedral amorphous carbon (ta-C) with an E Optical Gap of 3.5 eV and Tauc Gap of 04 3 eV is presented. This is the highest Optical Gap reported in literature for ta-C and was directly measured using photothermal deflection spectroscopy (PDS) and UV-Vis spectrophotometry. Independent PDS, UV-Vis and electron energy loss spectroscopy (EELS) measurements confirmed the high Gap. A large Urbach slope of 600 meV was measured, which indicated that there are many tail states. Electron spin resonance (ESR) measurements confirmed that this material has a high spin density of 7.5=10 20 spinsycm . Post-deposition vacuum annealing of the samples to 500 8C resulted in a small increase of the Optical Gap, E ;3.6 3 04 eV, and a decrease of the defects to 2.7=10 spinsycm . Post-deposition annealing at this temperature did not significantly 20 3
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evolution of sp2 bonding with deposition temperature in tetrahedral amorphous carbon studied by raman spectroscopy
Applied Physics Letters, 2000Co-Authors: Manishkumar Chhowalla, Andrea Ferrari, J Robertson, G A J AmaratungaAbstract:Two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature. The total sp3 fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The Optical Gap and resistivity show a similar, gradual transition. This indicates that the sp2 cluster size determines the Optical Gap, even when the sp2 content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the Gap.
