Suppressed Lattice Thermal Conductivity in Haeckelite Compounds for High-Performance Thermoelectric Applications.

Traditional semiconductors are known to exhibit excellent electrical properties but oversized lattice thermal conductivities, thus limiting their thermoelectric performance. Herein, we have discovered a low-energy allotrope of those traditional semiconductors. Compared with the wurtzite structure, the lattice thermal conductivity is reduced by more than five times in the haeckelite structure. This is attributed to the softening of acoustic phonon modes and concurrently enhanced anharmonicity in the haeckelite structure. Benefiting from the suppressed lattice thermal conductivity while retaining the excellent electrical properties of wurtzite structure, haeckelite compounds have been proven to be a novel category of high-performance thermoelectric materials. As an excellent representative, haeckelite CdTe exhibits a peak figure of merit approaching 1.3 at n-type doping and high temperature, which experiences a 3-fold improvement compared with its wurtzite counterpart. This work provides an alternative pathway of engineering the lattice thermal conductivities of traditional semiconductors toward superior thermoelectric properties.

Rational Design of Tetrahedral Derivatives as Efficient Light-Emitting Materials Based on "Super Atom" Perspective.

Traditional semiconductor quantum dots of groups II-VI are key ingredients of next-generation display technology. Yet, the majority of them contain toxic heavy-metal elements, thus calling for alternative light-emitting materials. Herein, we have explored three novel categories of multicomponent compounds, namely, tetragonal II-III2-VI4 porous ternary compounds, cubic I2-II3-VI4 ternary compounds, and cubic I-II-III3-V4 quaternary compounds. This is achieved by judicious introduction of a "super atom" perspective and concurrently varying the solid-state lattice packing of involved super atoms or the population of surrounding counter cations. Based on first-principles calculations of 392 candidate materials with designed crystal structures, 53 highly stable materials have been screened. Strikingly, 34 of them are direct-bandgap semiconductors with emitting wavelengths covering the near-infrared and visible-light regions. This work provides a comprehensive database of highly efficient light-emitting materials, which may be of interest for a broad field of optoelectronic applications.

Design Principle for Tetrahedral Semiconductors and Their Functional Derivatives: Cation Stabilizing Charged Cluster Network.

Colloidal quantum dots (QDs) of groups II-VI and III-V are key ingredients for next-generation light-emitting devices. Yet, many of them are heavy-element-containing or indirect bandgap, causing limited choice of environmental friendly efficient light-emitting materials. Herein, we resolve this issue by exploring potential derivatives of the parent semiconductors, thus expanding the material space. The key to success is the discovery of a principle for designing those materials, namely, cation stabilizing charged cluster network. Guided by this principle, three novel categories of cubic materials have been predicted, namely, porous binary compounds, I-II-VI ternary compounds, and I-II-III-V quaternary compounds. Using first-principles calculations, 65 realistic highly stable candidate materials have been theoretically screened. Their structural and compositional diversity enables a wide tunability of emitting wavelength from far-infrared to ultraviolet region. This work enriches the family of tetrahedral semiconductors and derivatives, which may be of interest for a broad field of optoelectronic applications.

Efficient Band-Edge Emission from Indirect Bandgap Semiconductor Quantum Dots upon Shell Engineering.

Environmentally friendly colloidal quantum dots (QDs) of groups III-V are in high demand for next-generation high-performance light-emitting devices for display and lighting, yet many of them (e.g., GaP) suffer from inefficient band-edge emission due to the indirect bandgap nature of their parent materials. Herein, we theoretically demonstrate that efficient band-edge emission can be activated at a critical tensile strain γc enabled by the capping shell when forming a core/shell architecture. Before γc is reached, the emission edge is dominated by dense low-intensity exciton states with a vanishing oscillator strength and a long radiative lifetime. After γc is crossed, the emission edge is dominated by high-intensity bright exciton states with a large oscillator strength and a radiative lifetime that is shorter by a few orders of magnitude. This work provides a novel strategy for realizing efficient band-edge emission of indirect semiconductor QDs via shell engineering, which is potentially implemented employing the well-established colloidal QD synthesis technique.