Anthony Salazar1,2,Hector Calderon3,Edy Cardona1,2,Diana Sánchez Ruiz1,Niharika Gupta1,Christian Kisielowski2,Junqiao Wu1,2,Oscar Dubon1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Instituto Politécnico Nacional3
Anthony Salazar1,2,Hector Calderon3,Edy Cardona1,2,Diana Sánchez Ruiz1,Niharika Gupta1,Christian Kisielowski2,Junqiao Wu1,2,Oscar Dubon1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Instituto Politécnico Nacional3
Until the early 1990’s the ZT of thermoelectric materials was believed to be restricted to unity. This meant that the thermodynamic efficiency was too low for mass production in competitive markets such as heating, ventilation and air conditioning and electronic cooling in computers. At that time seminal papers by L.D. Hicks and M.S. Dresselhaus made it clear that the dimensionality of materials can be used to increase boundary scattering of phonons reducing the lattice thermal conductivity. Reducing the lattice thermal conductivity is thought to be the critical factor in improving the ZT of materials. Besides dimensionality, alloying and structure disorder can also further reduce the lattice thermal conductivity of a material. Typically insulating amorphous solids and glasses have lower thermal conductivity than dense solids. However, in turbostratic structured layered crystals the thermal conductivity is shown to be as low as 0.05 W/mK. TEM analysis show that VLS grown MoS<sub>2</sub> nanowires have stacking faults starting at the metal catalyst and existing throughout the length of the wires. The lattice thermal conductivity and electrical conductivity of these materials were measured with the thermal bridge and four-point probe methods, respectively. MoS<sub>2</sub> nanowires show reduced thermal conductivity approaching what is reported in literature for the out of plane direction in bulk samples while the GaS<sub>1-x</sub>Te<sub>x</sub> nanowires demonstrate glass like thermal conductivity as low as 0.5W/mK. Additionally, MoS<sub>2</sub> nanowires demonstrate semiconducting behavior with high resistivity on the order of 10<sup>4</sup> ohm-cm while GaS<sub>1-x</sub>Te<sub>x</sub> nanowires have resistivity on the order of 10<sup>-2 </sup>ohm-cm. The reduction of thermal conductivity in MoS<sub>2</sub> nanowires is attributed to the stacking disorder and dimensionality, while the lattice thermal conductivity behavior in GaS<sub>1-x</sub>Te<sub>x</sub> nanowires can be attributed to alloying, dimensionality and possible stacking disorder. In conclusion, the VLS growth technique allows for the ability to introduce stacking disorder and grow nanowires with reduced lattice thermal conductivity. The influence of alloying and metal catalyst selection on stacking disorder will need to be considered in future studies of layered nanowire thermal conductivity.