arXiv subjects

Zhongwei Zhang

Publications and source records attributed to Zhongwei Zhang.

43 records · Page 3Linked to original sources

arXiv2016

New record of high ZT found in hybrid transition-metal-dichalcogenides

The search for thermoelectrics with higher figures of merit (ZT) will never stop due to the demand of heat harvesting. Single layer transition metal dichalcogenides (TMD), namely MX2 (where M is a transition metal and X is a chalcogen) that have electronic band gaps are among the new materials that have been the focus of such research. Here, we investigate the thermoelectric transport properties of hybrid armchair-edged TMDs nanoribbons, by using the nonequilibrium Green's function technique combined with the first principles and molecular dynamics methods. We find a ZT as high as 7.4 in hybrid MoS2/MoSe2 nanoribbons at 800K, creating a new record for ZT. Moreover, the hybrid interfaces by substituting X atoms are more efficient than those by substituting M atoms to tune the ZT. The origin of such a high ZT of hybrid nanoribbons is the high density of the grain boundaries: the hybrid interfaces decrease thermal conductance drastically without a large penalty to electronic conductance.

physics.comp-ph↗

arXiv2016

A systematic investigation of thermal conductivities of transition metal dichalcogenides

The thermal conductivities of MoS2 and WS2 have been reported by some experimental and theoretical studies, however, the results are different from each other. Here, thermal transport properties of twelve types of single layer transition metal dichalcogenides (TMDs) MX2 (M = Cr, Mo, W; X = O, S, Se, Te) are investigated systematically, by solving Boltzmann transport equation based on first-principle calculations. After accurate considering the size effect and boundary scattering, we find that our calculations can fit the former experimental results well. Moreover, diverse transport properties in TMDs are revealed, and an abnormal dependence of thermal conductivity on atomic mass is observed. In most MX2 structures, the thermal conductivities decrease with the increase of mass of atom M or X. However, the thermal conductivities of sulfides MS2 and selenides MSe2 increase as M changes from Cr to Mo to W, which is contradictory to our traditional understanding. A detailed calculation indicates that the abnormal trend is originated from the rapid increase of phonon relaxation time. Our studies provide important data and effective mechanism for thermal transport in TMDs.

physics.comp-ph↗

arXiv2016

Electron and phonon properties and gas storage in carbon honeycomb

A new kind of three-dimensional carbon allotropes, termed carbon honeycomb (CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the experimental results, a family of graphene networks are constructed, and their electronic and phonon properties are calculated by using first principles methods. All networks are porous metal with two types of electron transport channels along the honeycomb axis and they are isolated from each other: one type of channels is originated from the orbital interactions of the carbon zigzag chains and is topologically protected, while the other type of channels is from the straight lines of the carbon atoms that link the zigzag chains and is topologically trivial. The velocity of the electrons can reach ~10^6 m/s. Phonon transport in these allotropes is strongly anisotropic, and the thermal conductivities can be very low when compared with graphite by at least a factor of 15. Our calculations further indicate that these porous carbon networks possess high storage capacity for gaseous atoms and molecules in agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2016

Phonon transport in single-layer Boron nanoribbons

Inspired by the successful synthesis of several allotropes, boron sheets have been one of the hottest spot areas of focus in various fields. Here, we study phonon transport in three types of boron nanoribbons with zigzag and armchair edges by using a non-equilibrium Green's function combined with first principles methods. Diverse transport properties are found in the nanoribbons. At the room temperature, their highest thermal conductance can be comparable with that of graphene, while the lowest thermal conductance is less than half of graphene's. The three boron sheets exhibit different anisotropic transport characteristics. Two of these sheets have stronger phonon transport abilities along the zigzag edges than the armchair edges, while in the case of the third, the results are reversed. With the analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Because all boron allotropes consists of hexagonal and triangular rings, many hybrid patterns can be constructed naturally without doping, adsorption, and defects. Our results are useful in materials and devices design using boron sheets for thermal management.

cond-mat.mtrl-sci↗

arXiv2016

Transition of thermal rectification in silicon nanocones

Current understanding of thermal rectification asserts that the rectification ratio (R), which measures the relative heat flux between two ends of a nanostructure, is determined by its geometric asymmetry. The higher the asymmetry, the higher the R. However, by using nonequilibrium molecular dynamics method we have calculated thermal transport in Si nanocones as an example, the results show that such an understanding may be incorrect and R may not increase monotonically with geometric asymmetry. Rather, R exhibits a sharp reverse when the vertex angle (θ) of the nanocone is approximately 90°. In other words, when θ > 90°, R decreases, rather than increasing. We show that this abnormal behavior is originated from a change in the thermal transport mechanism. At small θs, phonon transport is dominated by localized modes, especially for transport from tip to bottom. At large θs, however, these localized modes disappear, leading to R decrease.

cond-mat.mes-hall↗

arXiv2016

Geometry, stability and thermal transport of hydrogenated graphene nanoquilts

Geometry, stability, and thermal transport of graphene nanoquilts folded by hydrogenation are studied using molecular dynamics simulations. The hydrogenated graphene nanoquilts show increased thermodynamic stability and better transport properties than folded graphene structures without hydrogenation. For the two-fold graphene nanoquilt, both geometry and thermal conductivity are very sensitive to the adsorbed hydrogen chains, which is interpreted by the red-shift of PDOS. For the multi-fold nanoquilts, their thermal conductivities can be tuned from 100% to 15% of pristine graphene, by varying the periodic number or length. Our results demonstrated that the hydrogenated graphene nanoquilts are quite suitable to be thermal management devices.

cond-mat.mes-hall↗

arXiv2015

Thermal transport in MoS2/Graphene hybrid nanosheets

Heat dissipation is a very critical problem for designing nano-functional devices, including MoS2/Graphene heterojunctions. In this paper we investigate thermal transport in MoS2/Graphene hybrid nanosheets under various heating conditions, by using molecular dynamics simulation. Diverse transport processes and characteristics, depending on the conducting layers, are found in these structures. The thermal conductivities can be tuned by interlayer coupling, environment temperature and interlayer overlap. The highest thermal conductivity at room temperature is achieved as more than 5 times of that of single layer MoS2 when both layers are heated and 100% overlapped. Different transport mechanisms in the hybrid nanosheets are explained by phonon density of states, temperature distribution, and ITR. Our results not only could provide clues to master the heat transport in functional devices based on MoS2/Graphene heterojunctions, but also are useful to analyze thermal transport in other van der Waals hybrid nanosheets.

cond-mat.mes-hall↗