arXiv subjects

David M T Kuo

Publications and source records attributed to David M T Kuo.

30 records · Page 2Linked to original sources

arXiv2019

Superlattice nanowire heat engines with direction-dependent power output and heat current

Heat engines (HEs) made of low dimensional structures offer promising applications in energy harvesting due to their reduced phonon thermal conductance. Many efforts have been devoted to the design of HEs made of quantum-dot (QD) superlattice nanowire (SLNW), but only SLNWs with uniform energy levels in QDs were considered. Here we propose a HE made of SLNW with staircase-like QD energy levels. It is demonstrated that the nonlinear Seebeck effect can lead to significant electron transports for such a nanowire with staircase-like energy levels. The asymmetrical alignment of energy levels of quantum dots embedded in nanowires can be controlled to allow resonant electron transport under forward temperature bias, while they are in off-resonant regime under backward bias. Under such a mechanism,the power output and efficiency of such a SLNW are better than SLNWs with uniform QD energy levels. The SLNW HE has direction-dependent power output and heat current. In addition, the HE has the functionality of a heat diode with impressive negative differential thermal conductance under open circuit condition.

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arXiv2018

Optimizing thermoelectric efficiency of superlattice nanowires at room temperature

It is known that the figure of merit ($ZT$) of thin nanowires can be significantly enhanced at room temperature due to the reduction of phonon thermal conductance arising from the increase of boundary scattering of phonons. It is expected that the phonon thermal conductance of nanowires filled with quantum dots (QDs) will be further reduced. Here we consider a superlattice nanowire (SLNW) modeled by a linear chain of strongly coupled QDs connected to electrodes. We study the dependence of $ZT$ on the QD energy level ($E_0$) (relative to the Fermi level $E_F$ in the electrodes), inter-dot coupling strength ($t_c$), tunneling rate ($Γ$), and temperature $T$ in order to optimize the design. It is found that at room temperature the maximum power factor occurs when $(E_0-E_F)/k_BT \approx 2.4$ and $Γ=t_c$, a result almost independent of the number of QDs in SLNW as long as $t_c/k_BT <0.5$. By using reasonable physical parameters we show that thin SLNW with cross-sectional width near $3~nm$ has a potential to achieve $ZT\ge3$.

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arXiv2017

Heat diodes made of quantum dots embedded in nanowires connected to metallic electrodes

The quantum dot arrays (QDAs) embedded into inhomogeneous nanowires connected to metallic electrodes show an electron heat rectification effect, which is attributed to the thermal voltage arising from a temperature bias and the QDA with a broken spatial inversion symmetry. The staircase energy levels of QDAs can be controlled to the resonant and off resonant transports for electrons in the forward and backward temperature biases, respectively. The effect of electron Coulomb interactions on the rectification efficiency of heat diode is clarified by the case of double QDs. We find that it is important to reduce phonon heat currents for implementing a high efficient electron heat diode at high temperature.

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arXiv2017

Large enhancement in thermoelectric efficiency of quantum dot junction due to increase of level degeneracy

It is theoretically demonstrated that the figure of merit ($ZT$) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resulting from electron-electron interactions associated with the degenerate levels ($L$). We found that electrical conductance ($G_e$) as well as electron thermal conductance ($κ_e$) are highly dependent on the level degeneracy ($L$), whereas the Seebeck coefficient ($S$) is not. Therefore, the large enhancement of $ZT$ is mainly attributed to the increase of $G_e$ when the phonon thermal conductance ($κ_{ph}$) dominates the heat transport of QD junction system. In the serially coupled double-QD case, we also obtain a large enhancement of $ZT$ arising from higher $L$. Unlike $G_e$ and $κ_e$, $S$ is found almost independent on electron inter-dot hopping strength.

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arXiv2016

Thermoelectric efficiency of quantum dot molecules at a high temperature bias: the role of thermal-induced voltage

The nonlinear electron and heat currents of quantum dot molecules (QDMs) under a temperature bias are theoretically investigated, including all correlation functions arising from electron Coulomb interactions in QDMs. Unlike the case of double QDs, the maximum efficiency of serially coupled triple QDs (SCTQD) occurs in the orbital depletion regime owing to the interdot Coulomb blockade. The electron current in SCTQD shows a bipolar oscillatory behavior with respect to the variation of QD energy levels, whereas the heat current does not show such a behavior. This is mainly attributed to thermal-induced bias. In addition, we illustrate how the efficiency of SCTQD is influenced by the external load resistance, and phonon heat flow. Finally, a direction-dependent electron current driven by a temperature bias has been demonstrated for a SCTQD with staircase-like energy levels.

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arXiv2016

A heat engine made of quantum dot molecules with high figure of merits

The transport of electrons through serially coupled quantum dot molecules (SCQDM) is investigated theoretically for application as an energy harvesting engine (EHE), which converts thermal heat to electrical power. We demonstrate that the charge current driven by a temperature bias shows bipolar oscillatory behavior with respect to gate voltage due to the unbalance between electrons and holes, which is different from the charge current driven by an applied bias. In addition, we reveal a Lenz's law between the charge current and the thermal induced voltage. The efficiency of EHE is higher for SCQDM in the orbital depletion situation rather than the orbital filling situation, owing to the many-body effect. The EHE efficiency is enhanced with increasing temperature bias, but suppressed as the electron hopping strength reduces. The fluctuation of QD energy levels at different sites also leads to a reduction of EHE efficiency. Finally, we demonstrate direction-dependent charge currents driven by the temperature bias for application as a novel charge diode.

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arXiv2015

Quantum interference and structure-dependent orbital-filling effects on the thermoelectric properties of quantum dot molecules

The quantum interference and orbital filling effects on the thermoelectric (TE) properties of quantum dot molecules with high figure of merit are illustrated via the full solution to the Hubbard- Anderson model in the Coulomb blockade regime. It is found that under certain condition in the triangular QD molecule (TQDM), destructive quantum interference (QI) can occur, which leads to vanishing small electrical conductance, while the Seebeck coefficient is modified dramatically. When TQDM is in the charge localization state due to QI, the Seebeck coefficient is seriously suppressed at low temperature, but highly enhanced at high temperature. Meanwhile, the behavior of Lorenz number reveals that it is easier to block charge transport via destructive QI than the electron heat transport at high temperatures. The maximum power factor (PF) in TQDM occurs at full-filling condition. Nevertheless, low-filling condition is preferred for getting maximum PF in serially coupled triple QDs in general. In double QDs, the maximum PF can be achieved either with orbital-depletion or orbital-filling as a result of electron-hole symmetry. Our theoretical work provides a useful guideline for advancing the nanoscale TE technology.

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arXiv2015

Quantum interference and electron correlation in charge transport through triangular quantum dot molecules

We study the charge transport properties of triangular quantum dot molecule (TQDM) connected to metallic electrodes, taking into account all correlation functions and relevant charging states. The quantum interference (QI) effect of TQDM resulting from electron coherent tunneling between quantum dots is revealed and well interpreted by the long distance coherent tunneling mechanism. The spectra of electrical conductance of TQDM with charge filling from one to six electrons clearly depict the many-body and topological effects. The calculated charge stability diagram for conductance and total occupation numbers match well with the recent experimental measurements. We also demonstrate that the destructive QI effect on the tunneling current of TQDM is robust with respect to temperature variation, making the single electron QI transistor feasible at higher temperatures.

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arXiv2014

Long distance coherent tunneling effect on the charge and heat currents in serially coupled triple quantum dots

The effect of long distance coherent tunneling (LDCT) on the charge and heat currents in serially coupled triple quantum dots (TQDs) connected to electrodes is illustrated by using a combination of the extended Hurbbard model and Anderson model. The charge and heat currents are calculated with a closed-form Landauer expression for the transmission coefficient suitable for the Coulomb blockade regime. The physical parameters including bias-dependent quantum dot energy levels, electron Coulomb interactions, and electron hopping strengths are calculated in the framework of effective mass theory for semiconductor TQDs. We demonstrate that the effect of LDCT on the charge and heat currents can be robust. In addition, it is shown that prominent heat rectification behavior can exist in the TQD system with asymmetrical energy levels.

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arXiv2013

Heat rectification effect of serially coupled quantum dots

The nonlinear thermoelectric properties of serially coupled quantum dots (SCQDs) embedded in a nanowire connected to metallic electrodes are theoretically studied in the Coulomb blockade regime. We demonstrate that the electron heat current of SCQDs exhibit a direction-dependent behavior (heat rectification) in an asymmetrical structure in which the electron Coulomb interactions are significant. The phonon thermal conductivity of the nanowire is also calculated, which is used to estimate the phonon heat current. Finally, we discuss how to reduce phonon heat current to allow observation of electron heat rectification behavior in the SCQD junction system in low temperature regime ($\approx 3 K$).

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arXiv2010

Thermal rectification effects of multiple semiconductor quantum dot junctions

Based on the multiple energy level Anderson model, this study theoretically examines the thermoelectric effects of semiconductor quantum dots (QDs) in the nonlinear response regime. The charge and heat currents in the sequential tunneling process are calculated by using the Keldysh Green's function technique. Results show that the thermal rectification effect can be observed in a multiple QD junction system, whereas the tunneling rate, size fluctuation, and location distribution of QD significantly influence the rectification efficiency.

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arXiv2005

Binding energy of exciton complexes determined by the tunneling current of single electron transistor under optical pumping

We theoretically study the tunnelling current of a single electron transistor (SET) under optical pumping. It found that holes in the quantum dot(QD) created by optical pumping lead to new channels for the electrons tunnelling from emitter to collector. As a consequence, an electron can tunnel through the QD via additional channels, characterized by the exciton, trion and biexciton states. The binding energy of exciton complexes can be determined by the Coulomb oscillatory tunnelling current.

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