Browsing by Author "Liu, Yu-Shen"

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Atomic-Scale Field-Effect Transistor as a Thermoelectric Power Generator and Self-Powered Device
(2011-08-04)
Liu, Yu-Shen
;
Yao, Hsuan-Te
;
Chen, Yu-Chang
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電子物理學系
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Department of Electrophysics
Using first-principles approaches, we have investigated the thermoelectric properties and energy conversion efficiency of the paired metal-Br-Al junction. Owing to the narrow states in the vicinity of the chemical potential, the nano junction has large Seebeck coefficients such that it can be considered an efficient thermoelectric power generator. We also consider the nanojunction in a three-terminal geometry, where the current, voltage, power, and efficiency can be efficiently modulated by the gate voltages. Such current-voltage characteristics could be useful in the design of nanoscale electronic devices such as a transistor or switch. Notably, the nanojunction as a transistor with a fixed finite temperature difference between electrodes can power itself using the Seebeck effect.
Counting statistics in nanoscale junctions
(2011-01-03)
Liu, Yu-Shen
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Chen, Yu-Chang
;
電子物理學系
;
Department of Electrophysics
We present first-principles calculations for moments of the current up to the third order in atomic-scale junctions. The quantum correlations of the current are calculated using the current operator in terms of the wave functions obtained self-consistently within the static density-functional theory. We investigate the relationships of the conductance, the second, and the third moment of the current for carbon atom chains of various lengths bridging two metal electrodes in the linear and nonlinear regimes. The conductance, the second-, and the third-order Fano factors exhibit odd-even oscillation with the number of carbon atoms due to the full and half filled pi* orbital near the Fermi levels. The third-order Fano factor and the conductance are positively correlated.
Effect of Thermoelectric Cooling in Nanoscale Junctions
(2011-04-07)
Liu, Yu-Shen
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Hsu, Bailey C.
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Chen, Yu-Chang
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電子物理學系
;
Department of Electrophysics
We propose a thermoelectric cooling device based on an atomic-sized junction. Using first-principles approaches, we investigate the working conditions and the coefficient of performance (COP) of an atomic-scale electronic refrigerator where the effects of the phonon's thermal current and local heating are included. It is observed that the functioning of the thermoelectric nanorefrigerator is restricted to a narrow range of driving voltages. Compared with the bulk thermoelectric system with the overwhelmingly irreversible Joule heating, the 4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric refrigerator with the same thermoelectric figure of merit (ZT) due to suppressed local heating via the quasi-ballistic electron transport and small driving voltages. Quantum nature due to the size minimization offered by atomic-level control of properties facilitates electron cooling beyond the expectation of the conventional thermoelectric device theory.
Seebeck coefficient of thermoelectric molecular junctions: First-principles calculations
(2009-05-01)
Liu, Yu-Shen
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Chen, Yu-Chang
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電子物理學系
;
Department of Electrophysics
A first-principles approach is presented for the thermoelectricity in molecular junctions formed by a single molecule contact. The study investigates the Seebeck coefficient considering the source-drain electrodes with distinct temperatures and chemical potentials in a three-terminal geometry junction. We compare the Seebeck coefficient in the amino-substituted and unsubstituted butanethiol junctions and observe interesting thermoelectric properties in the amino-substituted junction. Due to the novel states around the Fermi levels introduced by the amino substitution, the Seebeck coefficient could be easily modulated by using gate voltages and biases. When the temperature in one of the electrodes is fixed, the Seebeck coefficient varies significantly with the temperature in the other electrode and such dependence could be modulated by varying the gate voltages. As the biases increase, richer features in the Seebeck coefficient are observed, which are closely related to the transmission functions in the vicinity of the left and right Fermi levels.
Seebeck coefficients in nanoscale junctions: Effects of electron-vibration scattering and local heating
(2011-01-25)
Hsu, Bailey C.
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Liu, Yu-Shen
;
Lin, Sheng Hsien
;
Chen, Yu-Chang
;
電子物理學系
We report first-principles calculations of inelastic Seebeck coefficients in an aluminum monatomic junction. We compare the elastic and inelastic Seebeck coefficients with and without local heating. In the low-temperature regime, the signature of normal modes in the profiles of the inelastic Seebeck effects is salient. The inelastic Seebeck effects are enhanced by the normal modes and further magnified by local heating. In the high-temperature regime, the inelastic Seebeck effects are weakly suppressed due to the quasiballistic transport.
Single-molecule refrigerators: Substitution and gate effects
(2011-05-23)
Liu, Yu-Shen
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Chen, Yu-Chang
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電子物理學系
;
Department of Electrophysics
Using a first-principles approach, we investigate the quantum cooling effects in single-molecule junctions. In comparison with the unsubstituted butanethiol single-molecule junction as a refrigerator, the amino-substituted butanethiol single-molecule junction shows significant enhancement in the coefficient of performance (COP). The enhancement is attributed to the appearance of new states in the neighborhood of chemical potentials due to amino substitution. The COP of butanethiol refrigerator can be improved further by the gate voltages. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3593379]
Thermoelectric Efficiency in Nanojunctions: A Comparison between Atomic Junctions and Molecular Junctions
(2009-11-01)
Liu, Yu-Shen
;
Chen, Yi-Ren
;
Chen, Yu-Chang
;
電子物理學系
;
Department of Electrophysics
Using first-principles approaches, we investigate the thermoelectric efficiency, characterized by the figure of merit ZT, in metallic atomic junctions and insulating molecular junctions. To gain insight into the properties of ZT, an analytical theory is also developed to study the dependence of ZT on lengths (l) and temperatures (T). The theory considers the combined heat current carried by electrons and phonons. We observe a characteristic temperature: T(0) = (beta/gamma(l))(1/2). When T << T(0), the electronic heat current dominates the combined heat current and ZT proportional to T(2). When T >> T(0), the phononic heat current dominates the combined heat current and ZT tends to a saturation value. Moreover, the metallic atomic junctions and the insulating molecular junctions have opposite trend for the dependence of ZT on lengths, that is, ZT increases as the length increases for aluminum atomic junctions, while ZT decreases as the length increases for alkanethiol molecular junctions.