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Zianni X.,Technological Educational Institute of Chalkida
Applied Physics Letters | Year: 2010

High optimal thermoelectric efficiencies are theoretically demonstrated in ballistic nanowires with diameter modulation. The physics underlying the good thermoelectric performance of diameter-modulated nanowires is the strong energy dependence of their transmission coefficients. It is shown that the thermoelectric efficiency is directly related to the geometry of the diameter modulation. It becomes evident that geometry optimization can lead to efficient thermoelectric devices based on modulated nanowires. © 2010 American Institute of Physics. Source


Zianni X.,Technological Educational Institute of Chalkida
Journal of Solid State Chemistry | Year: 2012

We report on calculations of the ballistic thermal conductance of nanowires with modulated width along their length. We discuss the effect of the shape of the modulation in the thermal conductance of the nanowires. The ballistic thermal conductance is determined by the phonon transmission coefficient. It is shown that the thermal conductance of the modulated wires is lower than that of the corresponding straight wires. The phonon conductance decreases with increasing number of modulating periods and saturates to the infinite superlattice value. It decreases below this value when the modulation profile is non-periodic. It is shown that the thermal conductance can be tuned by changing the shape of the modulation profile. This behavior could lead to structures of nanowires with enhanced thermoelectric efficiency. © 2012 Elsevier Inc. All rights reserved. Source


Themelis S.I.,Technological Educational Institute of Chalkida
Journal of Chemical Physics | Year: 2010

High lying doubly excited Wannier-ridge states of He and H- with symmetries 3Pe and 1Do are studied and energies and intrinsic characteristics of their wave functions are reported. Energies of these states associated with the hydrogenic threshold up to N=20 are presented and, wherever available, we compare them to other calculations. Proposed classification schemata for these states by approximate collective quantum numbers are examined. © 2010 American Institute of Physics. Source


Zianni X.,Technological Educational Institute of Chalkida
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The thermoelectric properties of single-quantum-dot (QD) devices have been studied theoretically taking into account the electron-phonon coupling in the QD. The thermoelectric transport coefficients and the thermoelectric efficiency have been calculated in the sequential tunneling regime. It is shown that the thermoelectric properties depend on temperature and on intrinsic properties of the QD: the electron energy spectrum, the phonon energy, and the electron-phonon coupling strength. Different regimes have been identified. In the weak electron-phonon coupling regime, it is explicitly shown that the interplay between quantum confinement and electron-phonon coupling determines the electron thermal conductance and the thermoelectric efficiency of the device. The figure of merit ZT decreases rapidly with increasing temperature and electron-phonon coupling strength. When the electron-phonon coupling is strong, it becomes evident that the thermoelectric coefficients and the thermoelectric efficiency depend primarily on phonons. © 2010 The American Physical Society. Source


Zianni X.,Technological Educational Institute of Chalkida
Journal of Electronic Materials | Year: 2010

The thermoelectric efficiency of a single-dot (e.g., nanocrystal or molecule) device has been studied theoretically. The transport coefficients, the power factor, and the figure of merit (ZT) of the quantum dot in the single-electron transistor configuration have been calculated in the sequential tunneling regime when Coulomb blockade is important. Very high values of ZT have been found in the quantum regime, and they have been explained by quantum confinement. An approximate analytical formalism has been derived for ZT as a function of the separation between the energy levels of the dot, ΔE, and the thermal energy, k B T. The electron-phonon coupling has been included in the calculations, and it is shown that it is important for realistic prediction of the thermoelectric efficiency of a quantum-dot device. It has been found that the thermoelectric efficiency decreases due to electron-phonon coupling. Distinct behavior has been obtained for weak and for strong electron-phonon coupling. Quantum confinement is the dominant mechanism determining the magnitude of ZT for weak electron-phonon coupling. For strong electron-phonon coupling, the energy conversion efficiency is determined by the phonon spectrum. In all cases, it has been found that the thermoelectric efficiency decreases rapidly with increasing temperature. © 2009 TMS. Source

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