Two-step phase manipulation by tailoring chemical bonds results in high-performance GeSe thermoelectrics

In thermoelectrics,phase engineering serves a crucial function in deter-mining the power factor by affecting the band degeneracy.However,for low-symmetry compounds,the mainstream one-step phase manipulation strategy,depending solely on the valley or orbital degeneracy,is inadequate to attain a high density-of-states effective mass and exceptional zT.Here,we employ a distinctive two-step phase manipulation strategy through stepwise tailoring chemical bonds in GeSe.Initially,we amplify the valley degeneracy via CdTe alloying,which elevates the crystal symmetry from a covalently bonded orthorhombic to a metavalently bonded rhombohedral phase by significantly suppressing the Peierls distortion.Subsequently,we incorporate Pb to trigger the convergence of multivalence bands and further enhance the density-of-states effective mass by moderately restraining the Peierls distortion.Additionally,the atypical metavalent bonding in rhombohedral GeSe enables a high Ge vacancy concentration and a small band effective mass,leading to increased carrier concentration and mobility.This weak chemical bond along with strong lattice anharmonicity also reduces lattice thermal conductivity.Consequently,this unique property ensemble contributes to an outstanding zT of 0.9 at 773 K for Geo.8oPbo.2oSe(CdTe)o.25.This work underscores the pivotal role of the two-step phase manipulation by stepwise tailoring of chemical bonds in improving the thermoelectric performance of p-bonded chalcogenides.

Metavalent bonding impacts charge carrier transport across grain boundaries

Understanding the mechanisms underpinning the charge carrier scattering at grain boundaries is crucial to design thermoelectrics and other electronic materials.Yet,this is a very challenging task due to the complex characteristics of grain boundaries and the resulting difficulties in correlating grain boundary structures to local properties.Recent advances in characterizing charge transport across grain boundaries are reviewed,demonstrating how the microstructure,composition,chemical bonding and electrical properties of the same individual grain boundary can be correlated.A much higher potential barrier height is observed in high-angle grain boundaries.This can be ascribed to the larger number density of deep trapping states caused by the local collapse of metavalent bonding.A novel approach to study the influence of the local chemical bonding mechanism around defects on the resulting local properties is thus developed.The results provide insights into the tailoring of electronic properties of metavalently bonded compounds by engineering the characteristics of grain boundaries.

Beam switching and bifocal zoom lensing using active plasmonic metasurfaces

Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators.While the larger counterparts typically rely on mechanical actuation,this can be undesirable in some cases on a microscopic scale due to inherent space restrictions.Here,we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6.In particular,we demonstrate beam switching and bifocal lensing,thus,paving the way for a plethora of active optical elements employing plasmonic metasurfaces,which follow the same design principles.

High-performance and stable AgSbTe_(2)-based thermoelectric materials for near room temperature applications

AgSbTe_(2)-based ternary chalcogenides show excellent thermoelectric performance at low-and middletemperature ranges,yet their practical applications are greatly limited by their intrinsic poor thermodynamic stability.In this work,we demonstrate that AgSbTe_(2)-based ternary chalcogenides can be stabilized for service below their decomposition threshold.A series of AgxSb_(2-x)Te_(3-x)(x=1.0,0.9,0.8 and 0.7)samples have been prepared by the melt-quenching method.Among them,phase pure Ag0.9Sb1.1Te2.1 is verified by comprehensive structural characterizations from macroscale by X-ray diffraction to microscale by energy-dispersive spectroscopy and then to sub-nanometer scale by atom probe tomography.This composition is further chosen for the stability investigation.The decomposition threshold of Ag_(0.9)Sb_(1.1)Te_(2.1)appears around 473 K.Below this temperature,the chemical compositions and thermoelectric properties are barely changed even after 720 h annealing at 473 K.The figure-of-merit(zT)value of Ag_(0.9)Sb_(1.1)Te_(2.1)below the decomposition threshold is very competitive for real applications even compared with Bi_(2)Te_(3-)based alloys.The average zT of Ag_(0.9)Sb_(1.1)Te_(2.1)at 300e473 K reaches 0.84,which is higher than most other thermoelectric materials in a similar temperature range,promising applications in miniaturized refrigeration and power generation near room temperature.