Browsing by Author "Hsu, Bailey C."

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Band-Engineered Local Cooling in Nanoscale Junctions
(2017-02-15)
Hsu, Bailey C.
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Chen, Yu-Chang
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電子物理學系
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Department of Electrophysics
The stability and performance of nanoscale junctions are closely related to the local effective temperature. The local effective temperature is mainly caused by the competition between heating and cooling processes in inelastic electron-phonon scat-tering. Local cooling occurs when the rate of energy in cooling exceeds that in heating. Previous research has been done using either specific potential configuration or an adatom to achieve local cooling. We propose an engineer-able localcooling mechanism in asymmetric two-terminal tunneling junctions, in which one electrode is made of metal, whereas the other is made of a selectable bad-metal, such as heavily-doped polysilicon. The width of energy window of the selectable material, defined as the width covering all possible energy states counting from the conduction band minimum, can be engineered through doping. Interestingly, we have shown that substantial local cooling can be achieved at room temperature when the width of energy window of the low-density electrode is comparable to the energy of the phonon. The unusual local cooling is caused by the narrowed width of energy window, which obstructs the inelastic scattering for heating.
Effect of electron-vibration interactions on the thermoelectric efficiency of molecular junctions
(2012-07-11)
Hsu, Bailey C.
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Chiang, Chi-Wei
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Chen, Yu-Chang
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電子物理學系
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Department of Electrophysics
From first-principles approaches, we investigate the thermoelectric efficiency of a molecular junction where a benzene molecule is connected directly to the platinum electrodes. We calculate the thermoelectric figure of merit ZT in the presence of electron-vibration interactions with and without local heating under two scenarios: linear response and finite bias regimes. In the linear response regime, ZT saturates around the electrode temperature T-e = 25 K in the elastic case, while in the inelastic case we observe a non-saturated and a much larger ZT beyond T-e = 25 K attributed to the tail of the Fermi-Dirac distribution. In the finite bias regime, the inelastic effects reveal the signatures of the molecular vibrations in the low-temperature regime. The normal modes exhibiting structures in the inelastic profile are characterized by large components of atomic vibrations along the current density direction on top of each individual atom. In all cases, the inclusion of local heating leads to a higher wire temperature T-w and thus magnifies further the influence of the electron-vibration interactions due to the increased number of local phonons.
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
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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.
Effects of Current-Induced Forces on Pt-Benzene-Pt Single-Molecule Junctions
(2014-02-06)
Hsu, Bailey C.
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Amanatidis, Ilias
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Liu, Wei-Lin
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Tseng, Allen
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Chen, Yu-Chang
From first-principles approaches, we investigate the torque exerted by the current-induced forces in a highly asymmetric molecular junction, where the benzene molecule is directly connected to the platinum electrodes. We observe that the curved flow of the current streamline around the tilted molecule can induce a net torque, which tends to rotate the benzene molecule, much like the way a stream of water rotates a waterwheel. The magnitude and direction of the net torque are determined by the detailed current density flowing through the molecular junction. Such an asymmetric molecular junction could offer an experimentally practical system for the exploration of an electrically controllable single-molecule motor. The idea given above is also supported by a simple physical model based on the Landauer-Buttiker formula in the tight-binding framework.
Oscillatory and sign-alternating behaviors of the Seebeck coefficients in carbon monatomic junctions
(2013-09-24)
Hsu, Bailey C.
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Yao, Hsuan-Te
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Liu, Wei-Lin
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Chen, Yu-Chang
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交大名義發表
In the framework of density functional theory combined with the Lippmann-Schwinger formalism in scattering approaches, we calculate the Seebeck coefficients for carbon atom chains sandwiched between aluminum electrodes from first principles. The most striking feature is that the Seebeck coefficients are length dependent, and can be negative (n type) or positive (p type) for odd- or even-numbered carbon atom chains. Alternating n-type and p-type material properties of carbon atom chains are due to the full-and half-filled pi* -orbital states near the chemical potential.
Quantum Noise of Current in a 1,4-Benzenedithiol Single-Molecule Junction: First-Principle Calculations
(2018-08-23)
OuYang, Bin
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Hsu, Bailey C.
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Chen, Yu-Chang
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電子物理學系
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Department of Electrophysics
Shot noise refers to the autocorrelation of current related to the quantum statistics of discrete electrons. The study of shot noise can provide a deep understanding of the quantum fluctuation of current, owing to the interference between incident and reflected electrons. Electrons experience scattering when in the mesoscopic quantum point contact with materials of intermediate lengths. Thus, electron transport is typically incoherent and quantum channels are reformed in leads through adiabatic constriction. The starting point of the shot noise theory is based on a field operator constructed from incoherent wave functions in leads. However, electron transport is coherent in single-molecule junctions with length scales smaller than the dephasing length. In investigations regarding shot noise in systems with coherent electron transport, using a field operator constructed from coherent wave functions is natural. In this study, we investigated shot noise in a 1,4-benzenedithiol single molecule junction on the basis of coherent wave functions obtained self-consistently from first-principles approaches. The theoretical value of shot noise in our calculations is S approximate to 4.03 X 10(-26) A(2)/Hz at V-Bias = 0.01 V. This value is in good agreement with that obtained from a recent experimental measurement (S approximate to 4.37 X 10(-26) A(2)/Hz).
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
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Lin, Sheng Hsien
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Chen, Yu-Chang
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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.
Tailoring thermopower of single-molecular junctions by temperature-induced surface reconstruction
(2012-12-10)
Hsu, Bailey C.
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Lin, Chiung-Yuan
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Hsieh, Yau-Shian
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Chen, Yu-Chang
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電子物理學系
Recent experiments revealed that surface reconstruction occurs at around 300-400 K in the interface of C-60 adsorbed on Cu(111) substrate by scanning tunneling microscope techniques. To understand effects of such reconstruction on thermopower, we investigate the Seebeck coefficients of C-60 single-molecular junctions without and with surface reconstruction as a function of temperature at different tip-to-molecule heights from first-principles. Our calculations show that surface reconstruction can enhance or suppress Seebeck coefficients according to junctions at different tip heights. We further observe that the Seebeck coefficient of the junction at d = 3.4 angstrom may change from p- to n-type under surface reconstruction. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769814]