Preparation and thermoelectric properties of Mn<sub>3</sub>As<sub>2</sub>-doped Cd<sub>3</sub>As<sub>2</sub> nanostructures

Abstract

Cd<sub>3</sub>As<sub>2</sub>, especially its various nanostructures, has been considered as an excellent candidate for application in novel optoelectronic devices due to its ultrahigh mobility and good air-stability. Recent researches exhibited Cd<sub>3</sub>As<sub>2</sub> as a candidate of thermoelectric materials by virtue of its ultralow thermal conductivity in comparison with other semimetals or metals. In this work, at first <b>(</b> Cd<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>)<sub>3</sub>As<sub>2</sub> (<i>x</i> = 0, 0.05, 0.1) bulk alloys are prepared by high-pressure sintering to suppress the volatilization of As element, and then several kinds of Mn<sub>3</sub>As<sub>2</sub>-doped Cd<sub>3</sub>As<sub>2</sub> nanostructures are obtained on mica substrates by chemical vapor deposition (CVD), with bamboo-shoot-nanowire structure forming in a high-temperature region and films in a low-temperature region. Effects of Mn<sub>3</sub>As<sub>2</sub> doping on the crystalline structure, phase compositions, microstructures and thermoelectric properties of the Cd<sub>3</sub>As<sub>2</sub> nanostructures are systematically studied. Energy-dispersive spectrometer (EDS) analysis at various typical positions of the Mn<sub>3</sub>As<sub>2</sub>-doped Cd<sub>3</sub>As<sub>2</sub> nanostructures shows that the Mn content in these nanostructures is in a range of 0.02%–0.18% (atomic percent), which is much lower than the Mn content in <b>(</b> Cd<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>)<sub>3</sub>As<sub>2</sub> (<i>x</i> = 0, 0.05, 0.1) parent alloys. The main phases of these nanostructures are all body centered tetragonal <i>α</i> phase with a small amount of primitive tetragonal <i>α</i>′ phase. Doping results in the <i>α</i>″ phase and Mn<sub>2</sub>As impurity phase occurring. The Cd<sub>3</sub>As<sub>2</sub> film presents a self-assembled cauliflower microstructure. Upon Mn<sub>3</sub>As<sub>2</sub> doping, this morphology finally transforms into a vertical-growth seashell structure. In a high temperature region of the mica substrate, a unique bamboo-shoot-nanowire structure is formed, with vertical-growth bamboo shoots connected by nanowires, and at the end of these nanowires grows a white pentagonal flower structure with the highest Mn content of 0.18% (atomic percent) for all the nanostructures. Conductivity of the Cd<sub>3</sub>As<sub>2</sub> film and the bamboo-shoot-nanowire structure are ～20 and 320 S/cm, respectively. The remarkable conductivity enhancement can be attributed to higher crystallinity and the formation of nanowire conductive network, which significantly increase carrier concentration and Hall mobility. The Hall mobility values of the nanowire structures range from 2271 to 3048 cm<sup>2</sup>/(V·s) much higher than the values of 378–450 cm<sup>2</sup>/(V·s) for the films. The Seebeck coefficient for the bamboo-shoot-nanowire structure is in a range of 59–68 µV/K, which is about 15% higher than those for the films (50–61 µV/K). Although maximal power factor of the bamboo-shoot-nanowire structure is 14 times as high as that of the thin film, reaching 0.144 mW/(m·K<sup>2</sup>) at room temperature, this value is still one order of magnitude lower than the previously reported value of 1.58 mW/(m·K<sup>2</sup>) for Cd<sub>3</sub>As<sub>2</sub> single crystal.