Li-Ming Tang

Transition from insulator to metal induced by hybridized connection of graphene and boron nitride nanoribbons

A hybridized structure constructed by zigzag boron nitride nanoribbon and zigzag graphene nanoribbon is proposed, and their band structures and electronic transport properties are calculated by applying first-principles calculations. The results show that the band gap of the hybridized structure can be tuned and transitions from insulator to metal can be realized by changing the unit number of zigzag graphene nanoribbon. The currents with different spin polarization display different behavior.

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.

Coupling effect on phonon thermal transport in a double-stub quantum wire

By using the scattering matrix method, we investigate the thermal conductance in a double-stub quantum wire at low temperatures. The coupling effects between two stubs are analyzed in detail. It is found that the quantum structures exhibit oscillatory decaying thermal conductance with the width between two stubs at low temperature. A comparison between thermal conductances is made when stress-free and hard-wall boundary conditions are applied for acoustic modes, respectively. The result indicates that the behavior of the thermal conductance versus temperature is qualitatively different for the different types of boundary conditions.

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.

An important mechanism for thermal rectification in graded nanowires

In the quest for the origin of the different thermal rectifying behavior of two graded nanowires, we reveal the important role that standing waves play in the thermal transport properties of such graded structures. Evidence for the existence of standing waves is given from two angles, and one possible scenario of the origin of the standing wave is presented. The key point is that the formation of the standing wave, which greatly hinders the propagation of phonon waves, occurs only when the narrow end of the nanowire is at a higher temperature than the wide end, making the heat current flow preferably from the wide end to the narrow end.

Phonon wave interference in graphene and boron nitride superlattice

The thermal transport properties of the graphene and boron nitride superlattice (CBNSL) are investigated via nonequilibrium molecular dynamics simulations. The simulation results show that a minimum lattice thermal conductivity can be achieved by changing the period length of the superlattice. Additionally, it is found that the period length at the minimum shifts to lower values at higher temperatures, and that the depth of the minimum increases with decreasing temperature. In particular, at 200 K, the thermal conductivities of CBNSLs with certain specific period lengths are nearly equal to the corresponding values at 300 K. A detailed analysis of the phonon spectra shows that this anomalous thermal conductivity behavior is a result of strong phonon wave interference. These observations indicate a promising strategy for manipulation of thermal transport in superlattices.

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.

First-principles study of quantum confinement and surface effects on the electronic properties of InAs nanowires

We have used first principles methods to systematically investigate the quantum confinement effect on the electronic properties of zinc-blende (ZB) and wurtzite (WZ) InAs nanowires (NWs) with different orientations and diameters, and compared their electronic properties before and after pseudo-hydrogen passivation. The results show that the calculated carrier effective masses are dependent on the NW diameter, except for [110] ZB NWs, and the hole effective masses of [111] ZB NWs are larger than the electron effective masses when the NW diameter is ≥26 A. The band alignments of [111] ZB and [0001] WZ NWs reveal that the effect of quantum confinement on the conduction bands is greater than on the valence bands, and the position of the valence band maximum level changes little with increasing NW diameter. The pseudo-hydrogen passivated NWs have larger band gaps than the corresponding unpassivated NWs. The carrier effective masses and mobilities can be adjusted by passivating the surface dangling bonds.

Heat generated by electrical current in a mesoscopic system perturbed by alternating current fields

To analyze heat generation in a quantum dot coupled to normal leads and subject to an ac field, a formula is derived for the heat generated Q using the nonequilibrium Keldysh Greens function technique. The numerical results show that the external ac field influences significantly the heat generated. The threshold bias decreases to Vt=ω0−nω through photon absorption. Additional steps emerge from photon-assisted tunneling, and the width of the step is equal to the frequency of photon ω and the height increases with the magnitude of the ac field. For large V/ω, the Q/ω curves display many resonant peaks due to multi-photon absorptions and emissions. Moreover, a negative differential heat generation can be observed as ω>ω0.

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.