Thermal Transport And Thermoelectric Properties In Nanostructures

Recently, the studying of thermal transport properties in low dimensional materials has become one of the frontier subjects of condensed matter physics. The proposed concepts of phononics make people realized that the phonons can be controlled, like electrons, with a purpose. Meanwhile, the energy shortage and environmental pollutions promote the thermoelectric materials, which have functions of thermoelectric conversion, to be more focused. We have studied the thermal transport and thermoelectric properties in nanostructures as follows:(1) By adopting nonequilibrium molecular dynamics, geometry, stability, and thermal transport of graphene nanoquilts folded by hydrogenation are studied. The results show that both structure geometry and thermal conductivity are very sensitive to the adsorbed hydrogen ratio. For the multi-fold nanoquilts, they show increased thermodynamic stability than without hydrogenation ones. By varying the structure parameters, their thermal conductivities can be well modulated.(2) Heat dissipation is a very critical problem for designing nano-functional devices, including MoS2/Graphene heterojunctions. The thermal transport properties in MoS2/Graphene hybrid nanosheets are studied under various heating conditions. 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. Different transport mechanisms in the hybrid nanosheets are explained by phonon density of states, temperature distribution, and interlayer thermal resistance.(3) By using molecular dynamics, we have investigated the thermal transport in Si nanocones. The result show that R should not increase monotonically with geometric asymmetry. When θ < 90°, rectifier efficiency increases with the increase of angle; however, when θ > 90°, rectifier efficiency decreases with the increase of angle. 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.(4) By using nonequilibrium Green‘s function approach, the thermoelectric properties of hybrid MoS2/WS2 nanoribbons are investigated. The results demonstrate that hybrid MoS2/WS2 nanoribbons exhibit enhanced thermoelectric properties after hybridization. For the great reduction of kp and little changed electronic properties, the ZT peak has enhanced to 2.7 is about 1.8 times to MoS2 and 1.4 times to WS2, at 300 K. Moreover, the figure of merit can be enhanced further by modulating the structure periodic number. The ZT peaks of hybrid MoS2/WS2 nanoribbons are increased to 1.9 at 100 K, 4.1 at 300 K and 5.6 at 600 K, respectively.