Browsing by Author "Chen, Yu-Chang"

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Alkanethiol-based single-molecule transistors
(2008-12-01)
Ma, Chun-Lan
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Nghiem, Diu
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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 have investigated the transport properties of alkanethiol molecules in the two-terminal and three-terminal junctions by using first-principles approaches. We observe that states around the Fermi levels are introduced in the amino-substituted butanethiol junction. It leads to a sharp increase in the current, which is credited to the resonant tunneling. The current-voltage characteristics suggest that the amino-substituted butanethiol molecular junction may be a promising candidate for field-effect transistors.
Atomic-Scale Field-Effect Transistor as a Thermoelectric Power Generator and Self-Powered Device
(2011-08-04)
Liu, Yu-Shen
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Yao, Hsuan-Te
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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.
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.
Counting statistics in nanoscale junctions
(2011-01-03)
Liu, Yu-Shen
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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 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 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
;
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 Growth Temperature on Formation of Amorphous Nitride Interlayer between AlN and Si(111)
(2013-08-01)
Lin, Pei-Yin
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Chen, Jr-Yu
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Chen, Yu-Chang
;
Chang, Li
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材料科學與工程學系
The formation of an amorphous interlayer between AlN and Si(111), which may degrade the film quality, is studied by varying the substrate temperature from 860 to 1010 degrees C in metal-organic chemical vapor deposition with a preflow of trimethylaluminum. The microstructure and chemistry of the amorphous interlayer have been investigated using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Cross-sectional TEM examinations show that AlN is directly in contact with Si for growth at 860 degrees C. At higher growth temperatures, an amorphous interlayer can be formed even if an AlN layer has been previously deposited on Si, and its thickness increases with growth temperature. The XPS depth profile across the amorphous interlayer formed at 1010 degrees C shows that both Al and N exhibit similar distribution, which gradually decreases toward the Si substrate whereas the Si concentration has the opposite distribution. The composition of the amorphous interlayer is determined to consist of Al, Si, and N. (C) 2013 The Japan Society of Applied Physics
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
;
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.
Effects of isotope substitution on local heating and inelastic current in hydrogen molecular junctions
(2008-12-01)
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 investigate the inelastic features in the hydrogen molecular junction. We observe that local heating and inelastic current have significant isotope-substitution effects. The junction instability is also relevant to the isotope substitution. We predict that the HD junction has the smallest breakdown voltage compared with the H(2) and D(2) junction in the optimized geometry. The selection rule for modes that significantly contribute to the inelastic effects is related to the component of vibration along the direction of electron transport.
Efficient Packets Management over Wireless Heterogeneous Networks
(2009-01-01)
Chen, Yu-Chang
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Lai, Tai-An
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Cheng, Wei-Min
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資訊工程學系
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Department of Computer Science
With the wireless technology becoming more and more mature, uncountable numbers of wireless networks are used worldwide. Hence, QoS ( Quality of Service) has become the most important topic that have to be seriously considered, such as the jitter, the packet loss ratio and the transmission rate. In this paper, we propose an EPM (Efficient Packets Management) mechanism to improve the transmission quality in the wireless heterogeneous networks. The heterogeneous architecture which we propose is based upon the "WiMAX" (Worldwide Interoperability for Microwave Access) and the Wi-Fi with QoS specification (802.11e). We also propose a heterogeneous gateway named the Arbitrative Classify Gateway (AC-Gateway) in the wireless heterogeneous environment, and cooperate with our derived EPM mechanism to classify the streaming data from the media center, or the voice data from another base station (BS) according to the privilege. The packets will be scheduled efficiently and transmit correctly. Based on the architecture we propose, the transmission quality will be raised. Finally, we will give some powerful simulation results to prove our architecture works.
Exploration and characterization of the memcapacitor and memristor properties of Ni-DNA nanowire devices
(2017-09-01)
Chu, Hsueh-Liang
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Lai, Jian-Jhong
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Wu, Li-Ying
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Chang, Shen-Lin
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Liu, Chia-Ming
A 2-mu m-long Ni ion-chelated DNA molecule (Ni-DNA) was found for the first time to possess both memcapacitor and memristor properties; this Ni-DNA molecule is known as a dual memory circuit element (memelement). As a memelement, the state of impedance on Ni-DNA is proportional to the unit number of Ni ions containing a base pair complex (Ni-bp), which is determined by the previously applied external voltage. Interestingly, the impedances of Ni-DNA change in response to a change in the sweeping frequencies of the external bias. Our simulation results also indicate that changes in the effective resistance and capacitance of Ni-bp may be attributed to changes in the Ni ion redox species in the Ni-bp of a Ni-DNA nanowire. Therefore, the working mechanism of a nanowire-type memcapacitor and memristor is revealed. In summary, the Ni-DNA nanowire is shown to be a multi-dimensional memory device, whose memory state depends on the length of DNA and applied external voltages/frequencies.
First principles study of the electron density distribution in a pair of bare metallic electrodes
(2011-07-01)
Ma, Chun-Lan
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Chen, Yu-Chang
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Nghiem, Diu
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Tseng, Allen
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Huang, Pao-Chieh
Self-consistent calculations of electron density distribution from first principles for a series of semi-infinite metals show that the electron density almost drops to zero at 8.5 a.u. away from a metal surface. The electron densities in a series of bimetallic-electrode systems with a distance between the two electrodes of 21.7 a.u. are further investigated. Spin-polarized calculations of electron density for nonmagnetic and magnetic bimetallic-electrode systems are compared. Our work is helpful for first principles investigation of spin-dependent metal-molecule-metal tunneling junctions.
Heat dissipation in quasi-ballistic single-atom contacts at room temperature
(2019-12-10)
Tsutsui, Makusu
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Chen, Yu-Chang
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交大名義發表
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電子物理學系
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National Chiao Tung University
We report on evaluations of local heating in Au single-atom chains at room temperature. We performed onsite thermometry of atomic-scale Au junctions under applied sinusoidal voltage of variable amplitudes. The AC approach enabled to preclude electromigration effects for characterizing the influence of energy dissipations on the lifetime. We elucidated nonlinear increase in the effective temperature of the current-carrying single-atom chains with the voltage amplitudes, which was attributed to subtle interplay between electron-phonon scattering and electron-mediated thermal transport in the quasi-ballistic conductor. We also found that only 0.2% of the electric power contributed to local heating while the majority was consumed at the diffusive bank. The present findings can be used for thermal management of future integrated nanoelectronics.
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.
Programmable Redox State of the Nickel Ion Chain in DNA
(2014-02-01)
Chu, Hsueh-Liang
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Chiu, Shao-Chien
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Sung, Ching-Feng
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Tseng, Wellen
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Chang, Yu-Chuan
DNA is a nanowire in nature which chelates Ni ions and forms a conducting chain in its base-pairs (Ni-DNA). Each Ni ion in Ni-DNA exhibits low (Ni2+) or high (Ni3+) oxidation state and can be switched sequentially by applying bias voltage with different polarities and writing times. The ratio of low. and high oxidation states of Ni ions in Ni-DNA represents a programmable multistate memory system with an added capacitive component, in which multistate information can be written, read, and erased. This study also indicates that the biomolecule-based self-organized nanostructure can be used as a template for nanodevice fabrication.
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 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
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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
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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.
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
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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]
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]