GeTe with rhombohedral-to-cubic phase transition is a promising lead-free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon-phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge Sb Te with optimized carrier concentration, a record-high figure-of-merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase-transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high-density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high-performance Pb-free thermoelectric materials.

Recently, layered two-dimensional ferromagnetic materials (2D FMs) have attracted a great deal of interest for developing lowdimensional magnetic and spintronic devices. Mechanically exfoliated 2D FMs were discovered to possess ferromagnetism down to monolayer. It is therefore of great importance to investigate the distinct magnetic properties at low dimensionality. Here, we report the wafer-scale growth of 2D ferromagnetic thin films of Fe 3 GeTe 2 via molecular beam epitaxy, and their exotic magnetic properties can be manipulated via the Fe composition and the interface coupling with antiferromagnetic MnTe. A 2D layer-by-layer growth mode has been achieved by in situ reflection high-energy electron diffraction oscillations, yielding a well-defined interlayer distance of 0.82 nm along {002} surface. The magnetic easy axis is oriented along c-axis with a Curie temperature of 216.4 K. Remarkably, the Curie temperature can be enhanced when raising the Fe composition. Upon coupling with MnTe, the coercive field dramatically increases 50% from 0.65 to 0.94 Tesla. The large-scale layer-by-layer growth and controllable magnetic properties make Fe 3 GeTe 2 a promising candidate for spintronic applications. It also opens up unprecedented opportunities to explore rich physics when coupled with other 2D superconductors and topological matters.

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Vertically stacking two dimensional (2D) materials can enable the design of novel electronic and optoelectronic devices and realize complex functionality. However, the fabrication of such artificial heterostructures in wafer scale with an atomically-sharp interface poses an unprecedented challenge. Here, we demonstrate a convenient and controllable approach for the production of wafer-scale 2D GaSe thin films by molecular beam epitaxy. In-situ reflection high-energy electron diffraction oscillations and Raman spectroscopy reveal a layer-by-layer van der Waals epitaxial growth mode.Highly-efficient photodetector arrays were fabricated based on few-layer GaSe on Si.These photodiodes show steady rectifying characteristics and a high external quantum efficiency of 23.6%. The resultant photoresponse is super-fast and robust with a response time of 60 μs. Importantly, the device shows no sign of degradation after 1 million cycles of operation. We also carried out numerical simulations to understand the underlying device working principles. Our study establishes a new approach to produce controllable, robust and large-area 2D heterostructures and presents a crucial step for further practical applications.

In this study, Bi was selected to dope PbTe nanocubes to develop high performance n-type PbTebased thermoelectric materials via nanostructure engineering using a facile solvothermal method. Bi dopants effectively improve the electrical transport properties of the as-sintered PbTe nanomaterials by tuning the carrier concentrations, achieving promising electrical conductivity and Seebeck coefficient which are comparable to bulk materials. Moreover, a low lattice thermal conductivity has been secured in the sintered Bi-doped PbTe nanomaterials, which is remarkably lower than its bulk counterparts. Extensive experimental and theoretical evidences reveal that such low lattice thermal conductivity is attributed to the enhanced phonon scattering by the high density of grain boundaries and dislocations, in turn, leading to an enhanced peak ZT ~1.35 at 675 K for n-type Pb 0.99 Bi 0.01 Te.