Optimization of the average figure-of-merit zT in medium-entropy GeTe-based materials via entropy engineering

Entropy engineering has emerged as an effective strategy for improving the figure-of-merit zT by decelerating the phonon transport while maintaining good electrical transport properties of thermoelectric materials.Herein,a high average zT of 1.54 and a maximum zT of 2.1 are achieved in the mid-entropy GeTe constructed by Ag,Sb,and Pb alloying.At room temperature,the mid-entropy GeTe tends to be a cubic structure.And the power factor is improved from 7.7μW·cm^(-1)·K^(-2) to 16.2μW·cm·cm^(-1)·K^(-2) due to the large increase in effective mass and the optimized carrier concentration.The increasing disorder created by heavy and off-centering Ag,Sb,and Pb atoms induces strong mass/strain fluctuations and phonon scattering to decelerate the phonon transport in GeTe.A low lattice thermal conductivity is obtained in the medium-entropy GeTe-based material.Moreover,a GeTe-based thermoelectric cooler is fabricated with the cooling temperature difference of 66.6 K with the hot end fixed at 363 K.This work reveals the effectiveness of entropy engineering in improving the average zT in GeTe and shows potential application of GeTe as a thermoelectric cooler.

Phase-pure two-dimensional Fe_(X)GeTe_(2) magnets with near-room-temperature Tc

Two-dimensional(2D)ferromagnets with out-of-plane(OOP)magnetic anisotropy are potential candidates for realizing the next-generation memory devices with ultra-low power consumption and high storage density.However,a scalable approach to synthesize 2D magnets with OOP anisotropy directly on the complimentary metal-oxide semiconductor(CMOS)compatible substrates has not yet been mainly explored,which hinders the practical application of 2D magnets.This work demonstrates a cascaded space confined chemical vapor deposition(CS-CVD)technique to synthesize 2D FexGeTe_(2) ferromagnets.The weight fraction of iron(Fe)in the precursor controls the phase purity of the as-grown FexGeTe2.As a result,high-quality Fe_(3)GeTe_(2) and Fe_(5)GeTe_(2) flakes have been grown selectively using the CS-CVD technique.Curie temperature(Tc)of the as-grown FexGeTe2 can be up to-280 K,nearly room temperature.The thickness and temperature-dependent magnetic studies on the Fe_(5)GeTe_(2) reveal a 2D Ising to 3D XY behavior.Also,Terahertz spectroscopy experiments on Fe_(5)GeTe_(2) display the highest conductivity among other FexGeTe_(2) 2D magnets.The results of this work indicate a scalable pathway for the direct growth and integration of 2D ternary magnets on CMOS-based substrates to develop spintronic memory devices.