Zhong-Xiang Xie

Enhancement of thermoelectric properties in graphene nanoribbons modulated with stub structures

The thermoelectric properties in graphene nanoribbons modulated with stub structures are studied using atomistic simulation of electron and phonon transport. The results show that the phonon transport is dramatically suppressed by the elastic scattering of the stub structure; while the thermopower S can be enhanced by a few times of magnitude. This leads to a strong enhancement of the figure of merit (ZT). Moreover, it is found that the enhancement of ZT can be effectively tuned by modulating geometric parameters of the stub and edge shapes, which offers an effective way to improve the thermoelectric performance of graphene nanoribbons.

Ballistic thermoelectric properties in graphene-nanoribbon-based heterojunctions

Ballistic thermoelectric properties in graphene-nanoribbon-based heterojunctions are investigated by using the nonequilibrium Greens function approach and the Landauer transport theory. The results show that the phonon thermal conductances have similar effects for the different heterojunctions, while the electron transport is highly sensitive to the geometry details of the heterojunctions. The fluctuation of electronic transmissions can strongly enhance the thermopower. We can obtain the high thermoelectric figure of merit ZT∼0.6 at room temperature T = 300 K and ZT∼0.9 at low temperature T = 100 K by optimizing the thermopower, together with suppression of phonon transport by mismatching interface structures.

Nonlinear phonon transport and ballistic thermal rectification in asymmetric graphene-based three terminal junctions

By using the nonequilibrium Green’s function and the Landauer transport theory, nonlinear phonon properties in asymmetric graphene-based three terminal junctions (AGTTJs) are investigated. Results show that AGTTJs exhibit pronounced nonlinear thermal rectifying behaviors, and the efficiency is efficiently tuned by increasing the asymmetric degree between the left and right terminals or modulating the central probe. The thermal rectifying mechanism is analytically explained by the schematic diagram. It is suggested that AGTTJs may be served as a good ballistic thermal rectifier.

Ballistic phonon thermal transport in a cylindrical semiconductor nanowire modulated with nanocavity

By developing the mode matching numerical technique, we investigate the ballistic phonon thermal transport through a cylindrical semiconductor nanowire modulated with a coupling nanocavity. It is found that the phonon transmission exhibits the periodical transmission properties in low frequency region. The resonant transmission and reflection behaviors of acoustic phonon modes at particular energy can be observed. In the limit T → 0, the thermal conductance approaches the universal quantum value π2kB2T/3h, and such a quantum is robust against all geometrical parameters. However, the thermal conductance exhibits nonmonotonic behaviors with increasing temperature and can be modulated by adjusting geometrical parameters of the nanocavity.

Ballistic thermoelectric properties in boron nitride nanoribbons

Ballistic thermoelectric properties (TPs) in boron nitride nanoribbons (BNNRs) are studied using the nonequilibrium Greens function atomistic simulation of electron and phonon transport. A comparative analysis for TPs between BNNRs and graphene nanoribbons (GNRs) is made. Results show that the TPs of BNNRs are better than those of GNRs stemming from the higher power factor and smaller thermal conductance of BNNRs. With increasing the ribbon width, the maximum value of ZT (ZTmax) of BNNRs exhibits a transformation from the monotonic decrease to nonlinear increase. We also show that the lattice defect can enhance the ZTmax of these nanoribbons strongly depending on its positions and the edge shape.

Ballistic thermal transport in a cylindrical semiconductor nanowire modulated with bridge contacts

Using the scattering-matrix method, we studied ballistic phonon transmission and thermal conductance at low temperatures in a cylindrical quantum wire with bridge contacts. The transmission coefficient exhibited a stepped profile, which became more evident as the bridge radius increased. When the dimensions of the bridge are identical to those of main wires, we observed a quantum platform of the thermal conductance, even in the presence of interface scattering. When the dimensions of the bridge are smaller than those of main wires, however, we could not observe the quantum platform. We also revealed other interesting physical properties, such as universal quantum thermal conductance and resonant transmission. A brief analysis of these results is given.

Ballistic thermal conductance by phonons through superlattice quantum-waveguides

Ballistic thermal conductances (BTCs) by phonons through superlattice quantum-waveguides are investigated by using the scattering-matrix method and the elastic continuum theory. A comparison for the cylindrical model (CM) and the rectangular model (RM) is addressed. We find that for these two models, the quantum thermal conductance can be observed even when the superlattices exist in quantum-waveguides. At low temperature, BTCs for the CM and the RM present almost the same behaviors regardless of the periodic length of superlattices. However, at higher temperature, BTCs for the RM are larger than those for the CM stemming from lower cutoff frequencies of high order modes for the RM. We also find that BTCs undergo a noticeable transformation from the monotonic decrease to constant with increasing the periodic number of superlattices. A brief analysis of these results is given.

Tunability of acoustic phonon transmission and thermal conductance in three dimensional quasi-periodically stubbed waveguides

We investigate acoustic phonon transmission and thermal conductance in three dimensional (3D) quasi-periodically stubbed waveguides according to the Fibonacci sequence. Results show that the transmission coefficient exhibits the periodic oscillation upon varying the length of stub/waveguide at low frequency, and the period of such oscillation is tunably decreased with increasing the Fibonacci number N. Interestingly, there also exist some anti-resonant dips that gradually develop into wide stop-frequency gaps with increasing N. As the temperature goes up, a transition of the thermal conductance from the decrease to the increase occurs in these systems. When N is increased, the thermal conductance is approximately decreased with a linear trend. Moreover, the decreasing degree sensitively depends on the variation of temperature. A brief analysis of these results is given.

A local resonance mechanism for thermal rectification in pristine/branched graphene nanoribbon junctions

Using non-equilibrium molecular dynamics simulations, we investigate thermal rectification (TR) in pristine/branched graphene nanoribbon (GNR) junctions. The results indicate that the TR ratio of such junctions can reach 470% under small temperature bias, which has distinct superiority over asymmetric GNR and many other junctions. Moreover, the TR ratio decreases rapidly as the applied temperature bias increases. It seems to be against common sense that the TR ratio generally increases with temperature bias. Phonon spectra analyses reveal that the observed phenomena stem from the local resonance of longitudinal phonons in branched GNR region under negative temperature bias. Furthermore, the influence of ambient temperature, system length, branch number, and defect density is studied to obtain the optimum conditions for TR. This work extends local resonance mechanism to GNR for thermal signal manipulation.Using non-equilibrium molecular dynamics simulations, we investigate thermal rectification (TR) in pristine/branched graphene nanoribbon (GNR) junctions. The results indicate that the TR ratio of such junctions can reach 470% under small temperature bias, which has distinct superiority over asymmetric GNR and many other junctions. Moreover, the TR ratio decreases rapidly as the applied temperature bias increases. It seems to be against common sense that the TR ratio generally increases with temperature bias. Phonon spectra analyses reveal that the observed phenomena stem from the local resonance of longitudinal phonons in branched GNR region under negative temperature bias. Furthermore, the influence of ambient temperature, system length, branch number, and defect density is studied to obtain the optimum conditions for TR. This work extends local resonance mechanism to GNR for thermal signal manipulation.

Ballistic thermal transport by phonons in three dimensional periodic nanostructures.

Ballistic thermal transport properties by phonons in three dimensional (3D) periodic nanostructures is investigated. Results show that thermal transport properties in 3D periodic nanostructures can be efficiently tuned by modulating structural parameters of systems. When the incident frequency is below the first cutoff frequency, the quasi/formal-periodic oscillations of the transmission coefficient versus the periodic number/length can be observed. When the incident frequency is above the first cutoff frequency, however, these quasi/formal-periodic oscillations cannot be observed. As the periodic number is increased, the thermal conductance undergoes a prominent transition from the decrease to the constant. We also observe other intriguing physics properties such as stop-frequency gaps and quantum thermal conductance in 3D periodic nanostructures. Some similarities and differences between 2D and 3D periodic systems are identified.