Crystalline Cd3As2

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Thermo-electric generators (TEGs) directly convert thermal into electrical energy and are the prime candidates for application in low-grade thermal energy/heat waste recovery. TEGs possess several critical advantages as they remain silent, highly reliable and lightweight, which make them uniquely suited for niche deployments in transportation, space satellites, electronics cooling and thermo-voltaic cells. With the efficiency in a few percent range, the efficiency-to-cost ratio remains poor, with the bottleneck traced back to the thermo-electric (TE) materials. PbTe and Bi2Te3, two key TE materials that are in commercial operation, have room temperature ZT of less than 1, whereas ZT exceeding 3 is required for a TEG to be economically viable. Until recently, the enhancement of ZT was mainly attained by introducing phonon scattering interfaces and rattling centers to slow heat transport. With thermal conductivity already approaching the fundamental limit of amorphous materials, further gain in ZT can be only realized by improving the thermo-electric power factor, S2, typically via costly band structure engineering or by means of quantum confinement. In this work, we instead focus on Cd3As2, a high performance 3D Dirac semimetal. Compared to other semimetals, Cd3As2 possess Dirac fermions that disperse linearly in k3-space and in turn, one of the largest electron mobilities known for crystalline materials, i.e. ~104-105cm2V-1s-1. The carrier back-scattering remains suppressed and the electrical transport is dominated by high-energy carriers that favorably affect the thermopower of Cd3As2. Since the power factor scales with the carrier mobility, μ, and weighted density-of-states effective mass, i.e. S2 ~ μ, and the lattice thermal conductivity is within amorphous limit for this material, Cd3As2 shows a strong potential for demonstrating high, device-favorable S and in turn ZT. A reduced temperature vapor-based crystallization pathway was developed to produce free standing, stochiometric 2D cm-size crystals in Cd3As2. In contrast to previous findings, the Seebeck coefficient is seen to initially rise linearly with T to reach a peak within ~ 300-400 K range which is followed by a decline with the shift in the trend attributed to the increase in the role of the ambipolar heat transfer as temperature increases.The primary mechanism of the minority carrier generation in our samples is attributed to phonon-assisted interband electronic transitions involving E and A1 bands. Possessing a very high room-temperature thermopowers of ~613 uV/K and amorphous limit thermal conductivity, bulk Cd3As2 is identified as another high promise phonon-glass electron-crystal TE material for development of next generation a high efficiency thermo-electric generators and refrigerators operating under normal conditions. Figure 1